tio2 based photocatalysts for environmental remediation ... · potential for abatement of...

TiO2 based photocatalysts for environmental remediation reactions

Ana Amorós Pérez

www.ua.es

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Grupo de Materiales Carbonosos y Medio Ambiente

Departamento de Química Inorgánica (Facultad de Ciencias)

Instituto Universitario de Materiales

TiO2 based photocatalysts for

environmental remediation reactions

Ana Amorós Pérez

Tesis presentada para aspirar al grado de

DOCTOR POR LA UNIVERSIDAD DE ALICANTE

MENCIÓN DE DOCTOR INTERNACIONAL

Doctorado en Ciencia de Materiales

Dirigida por:

María Ángeles Lillo Ródenas

Catedrática de Química Inorgánica

María del Carmen Román Martínez


Alicante, abril de 2019

María Ángeles Lillo Ródenas y María del Carmen Román Martínez,

ambas Catedráticas del Departamento de Química Inorgánica de la Universidad

de Alicante

CERTIFICAN QUE:

Dña. Ana Amorós Pérez, licenciada en Química, ha realizado en el

Departamento de Química Inorgánica de la Facultad de Ciencias de la

Universidad de Alicante, bajo nuestra dirección, el trabajo que lleva por título:

“TiO2 based photocatalysts for environmental remediation reactions”, que

constituye su Memoria para aspirar al Grado de Doctor con mención

Internacional por la Universidad de Alicante, reuniendo, a nuestro juicio, las

condiciones necesarias para ser presentada y defendida ante el tribunal

correspondiente.

Y para que conste a los efectos oportunos, en cumplimiento de la legislación

vigente, firmamos el presente certificado en Alicante, a 7 de marzo de 2019.

María Ángeles Lillo Ródenas


María del Carmen Román Martínez


Aerodinámicamente, el cuerpo de una

abeja no está diseñado para volar;

lo bueno es que la abeja no lo sabe.

Agradecimientos

Este trabajo no podría haberse realizado sin la colaboración de muchas

personas, que me han brindado su ayuda, sus conocimientos y su apoyo.

En primer lugar, quiero darles las gracias a mis directoras de tesis, María

Ángeles Lillo Ródenas y María del Carmen Román Martínez, por haber confiado

en mí para embarcarnos juntas en este proyecto. Gracias por vuestra dedicación,

orientación y esfuerzo, pero, sobre todo, gracias por vuestro interés en mi

formación como científica y como persona crítica.

Me gustaría dedicarle también unas palabras a Ángel Linares Solano. Ha

sido un verdadero placer poder coincidir con él en mis primeros años como

investigadora. Me considero muy afortunada por haber tenido la oportunidad de

trabajar juntos y le estaré eternamente agradecida por contagiarme su fascinación

por la ciencia.

Mis años en esta universidad me han llevado, además, a conocer grandes

compañeros entre los que, aunque no pueda mencionarlos a todos en estas

páginas, me gustaría destacar a algunos. A aquellos con los que compartí mis

primeros laboratorios, mis “quimicuxos”, gracias por darle sentido a todo. A mi

compañera, confidente y amiga Laura Cano. A Mohammed, por iniciarme. A Javi

Giménez, por divertirme. A Susana, por sus abrazos. A María José, por su

compañerismo. A Aroldo, Mónica y Guido, por su compresión y apoyo. A Javi

Català, por enseñarme y animarme siempre. A Vero, por entenderme. A Jaime y

Vicente, por hacerme reír. A Fátima, por ayudarme.

I would like to thank Nicolas Keller for receiving me and make me feel

very comfortable in his laboratory. Me gustaría agradecer también a todas

aquellas personas que hicieron que mi estancia en la Universidad de Estrasburgo

la recuerde con tanto cariño. En especial a Marisa, por regalarme tantos buenos

momentos.

Por supuesto, también me gustaría agradecer a todos lo que, fuera del

ámbito académico, me han hecho seguir creciendo. M’agradaria agrair a les

meues amigues i a les meues “Piules” l’oxigen i l’energia per seguir endavant,

especialment a Silvia, per fer que la nostra complicitat no tinga data de caducitat.

A la meua família, pel seu suport incondicional i per creure en mi. A ma mare,

per ser un exemple de constància, fortalesa i dedicació. A mon pare, per

ensenyar-me que, l’èxit és la constància en els propòsits. A la meua germana, per

transportar-me a un “mundo ideal”.

Finalmente, me gustaría darle las gracias a Alessio, por hacerme creer que

las casualidades no existen. Grazie per essere al mio fianco, per avermi insegnato

così tanto. Grazie per essere il mio più grande sostegno, la ragione delle mie

risate e la consolazione delle mie lacrime. Grazie per avermi reso 7 volte più forte,

per la prima Rhouckmouth e per avermi ricordato cosa c'è solo al primo piano.

Grazie per avermi fatto credere che tutto è possibile. Semplicemente… grazie per

esistere.

A todos vosotros, ¡muchas gracias!

Resumen

Con la creciente preocupación por el uso de recursos renovables y por

encontrar mejores formas de utilizar la energía solar, la fotocatálisis activada por

luz solar se ha convertido en una herramienta atractiva. En particular, la

fotocatálisis heterogénea es un proceso de gran potencial para la reducción de

contaminantes en fase gaseosa o líquida. Este método tiene ventajas

considerables sobre algunas tecnologías existentes: destruye los contaminantes

en lugar de transferirlos a otra fase; por lo general, conduce a la completa

mineralización de contaminantes orgánicos en CO2, H2O y, en algunos casos, a

productos como sales minerales inocuas; funciona en condiciones ambientales,

con cualquier tipo de sustrato, sin requisitos complejos de procesamiento y se

puede implementar fácilmente en aplicaciones acuosas y gaseosas.

El TiO2 se ha convertido en uno de los fotocatalizadores basados en

semiconductores más prometedores, ya que tiene varias ventajas, como su

estabilidad química, su alta resistencia a la corrosión fotoinducida y, además, es

un material abundante, barato, no tóxico y biocompatible. En los últimos años,

se han realizado muchos esfuerzos para mejorar la eficiencia de este

fotocatalizador, prestando atención a algunos de sus inconvenientes (baja

fotoactividad bajo radiación solar y alta tasa de recombinación de pares e‒/h+).

Para resolver estos inconvenientes, se está investigando en el proceso de síntesis

del TiO2, incluida la adición de otros elementos (metales o no metales) para

obtener materiales con mejores propiedades.

En esta Tesis Doctoral, se han seleccionado fotocatalizadores basados en

TiO2 y se han aplicado a tres reacciones de interés ambiental: i) descomposición

fotocatalítica de ácido acético, un contaminante frecuente en diferentes efluentes

líquidos que es bastante difícil de descomponer usando métodos convencionales

y cuya descomposición produce principalmente CH4, CO2 y H2; ii)

fotodegradación oxidativa de diurón, un herbicida común responsable de la

contaminación del agua; y iii) oxidación fotocatalítica de propeno a baja

concentración, un contaminante común en el aire que pertenece a la categoría de

compuestos orgánicos volátiles (COV), que se encuentra presente en el smog

fotoquímico y es una de las principales fuentes de contaminación en ambientes

interiores.

En primer lugar, la titania comercial P25 se ha modificado con especies

metálicas de transición (Cr, Co, Ni y Cu) incorporadas mediante el método

impregnación a partir de las disoluciones acuosas de los nitratos correspondientes.

El procedimiento de preparación también incluye un tratamiento térmico (500 ºC)

en argón para descomponer los nitratos, eliminar las impurezas y fortalecer la

interacción metal-TiO2. Las muestras preparadas se han caracterizado y probado

en las tres reacciones anteriormente comentadas. Los resultados indican que los

fotocatalizadores de P25 modificados no muestran actividad para la degradación

del diurón. En la fotodecomposición del ácido acético y en la fotooxidación del

propeno, la actividad fotocatalítica de los cuatro catalizadores que contienen

metales varía con la naturaleza de las especies metálicas y sigue una tendencia

similar en ambas reacciones. El efecto de la naturaleza de las especies metálicas

añadidas se basa principalmente en las propiedades electroquímicas de las

especies soportadas, siendo Cu/P25 (la muestra que contiene cobre) el catalizador

que proporciona mejores resultados.

En base a los resultados obtenidos se ha profundizado en el estudio del

cobre. Para ello se han preparado varias series de Cu/TiO2 por diferentes métodos,

variando el contenido en metal y la atmósfera del tratamiento térmico. La

influencia de las variables de síntesis en la eficiencia fotocatalítica se ha

analizado con detalle en las tres reacciones estudiadas.

El TiO2 se ha sintetizado mediante el método sol-gel, y se han incorporado

diferentes cantidades de Cu (0, 0.5, 1, 2, 5, 7 y 10 % en peso) mediante dos

métodos diferentes (in situ e impregnación), y luego han sido tratados

térmicamente a 500 ºC, en aire o argón. El fotocatalizador de Cu/TiO2 preparado

por el método in situ, tratado térmicamente en argón y con un 0.5 % en peso de

cobre proporciona los mejores resultados en la fotodecomposición del ácido

acético. Para la fotooxidación de propeno, la presencia de cobre parece ser

perjudicial. TiO2-aire es el catalizador con mayor conversión de propeno. En la

fotodegradación de diurón, las muestras Cu/TiO2 no son activas, solamente los

fotocatalizadores TiO2-Ar y TiO2-aire muestran actividad, pero inferior a P25.

Finalmente, el TiO2 ha sido modificado con un elemento no metálico,

carbón activado (AC). Los fotocatalizadores híbridos TiO2-AC se han preparado

por síntesis sol-gel de TiO2 en presencia de diferentes cantidades de un carbón

activado con morfología esférica, que se había obtenido previamente por

tratamiento hidrotermal de sacarosa. Se han preparado muestras con 0, 0.5, 1, 5

y 10 % en peso de AC y se han tratado térmicamente a 350 ºC en aire. Además,

se ha estudiado una serie de catalizadores de TiO2 tratados térmicamente a

diferentes temperaturas (350, 400, 450 y 500 ºC). La presencia de AC mejora la

actividad de los catalizadores en la fotodecomposición del ácido acético y en la

fotodegradación del diurón. Sin embargo, disminuye ligeramente la eficiencia en

la fotooxidación del propeno. Si se presta atención a la influencia del tratamiento

térmico, los resultados obtenidos han demostrado que el tratamiento térmico de

TiO2 a 500 ºC conduce al mejor catalizador para las tres reacciones estudiadas.

Summary

With the increasing concern in using renewable energy resources and

finding better ways to use solar energy, solar light-activated photocatalysis has

become an attractive tool. In particular, heterogeneous photocatalysis has a great

potential for abatement of pollutants in gas and liquid phases. This method has

several advantages over some other existing technologies: it destroys pollutants

rather than transferring them to another phase; it usually leads to complete

mineralization of organic pollutants, mainly leading to CO2 and H2O and, in some

cases, to other products, such as innocuous mineral salts; it can operate at ambient

conditions, with any type of substrate, without complex processing requirements

and it can be easily implemented in both liquid and gaseous applications.

TiO2 has emerged as one of the most promising semiconductor

photocatalysts, since it has several advantages, such as its chemical stability, with

high resistance to photo-induced corrosion and, in addition, it is an abundant,

cheap, non-toxic and biocompatible material. Over the last few years, many

efforts have been made to improve the efficiency of this photocatalyst by paying

attention to some of its drawbacks (low photoactivity under solar radiation and

high rate of e‒/h+ pairs recombination). To overcome these drawbacks, the

synthesis process is being investigated, including the addition of other elements

(metals or non-metals) to obtain materials with better properties.

In this PhD Thesis, photocatalysts based on TiO2 have been selected and

applied to three environmental remediation reactions: i) photocatalytic

decomposition of acetic acid, a frequent pollutant in different liquid effluents that

is quite difficult to decompose using conventional methods, and whose

decomposition mainly produces CH4, CO2, and H2; ii) oxidative

photodegradation of diuron, a common herbicide responsible of water pollution;

and iii) photocatalytic oxidation of propene at low concentration, a common air

pollutant belonging to the category of volatile organic compounds (VOCs),

which is involved in photochemical smog and is one of the major sources of

indoor air pollution.

Firstly, the commercial P25 titania has been modified with transition

metallic species (Cr, Co, Ni, and Cu) added by impregnation with aqueous

solutions of the corresponding nitrates. The preparation procedure also includes

heat treatment (500 ºC) in argon to decompose the nitrates, remove impurities

and to strengthen the metal–TiO2 interaction. The prepared samples have been

characterized and tested in the three studied reactions. The results indicate that

the modified P25 samples do not show activity for diuron degradation. In the

photodecomposition of acetic acid and in the photooxidation of propene the

photocatalytic activity of the four metal-containing catalysts varies with the

nature of the metallic species and follows a similar trend. The effect of the nature

of the added metallic species is mainly based on the electrochemical properties

of the supported species, being Cu/P25 (the sample that contains copper) the best

performing catalyst.

Once focusing on copper, a series of Cu/TiO2 have been prepared by

different methods. The copper content has also been varied, and the influence of

the synthesis variables on their efficiency in the three studied reactions has been

analysed.

TiO2 has been synthesized by sol-gel method, and different amounts of Cu

(0, 0.5, 1, 2, 5, 7 and 10 wt. %) have been prepared by two different methods (in

situ and impregnation), and then they have been heat treated at 500 ºC, either in

air or in argon. The Cu/TiO2 photocatalyst prepared by the in situ method, heat

treated in argon and with a 0.5 wt. % Cu content shows the best results for the

photodecomposition of acetic acid. For the photooxidation of propene, the

presence of copper seems to be detrimental, being TiO2-air sample the catalyst

with higher propene conversion. In the photodegradation of diuron, the prepared

Cu/TiO2 samples are not active, only TiO2-Ar and TiO2-air show activity, but

lower than P25.

Finally, TiO2 has been modified with a non-metallic element, carbon (AC).

Hybrid TiO2-AC photocatalysts have been prepared by sol-gel synthesis of TiO2

in the presence of different amounts of a spherical activated carbon, which had

been previously obtained by hydrothermal treatment of saccharose. Samples with

0, 0.5, 1, 5 and 10 wt. % AC have been prepared and heat treated at 350 ºC.

Moreover, a series of TiO2 catalysts heat treated at different temperatures (350,

400, 450 and 500 ºC) has been studied. The presence of carbon improves the

catalysts’ activity in the photodecomposition of acetic acid and in the

photodegradation of diuron. However, such carbon presence slightly decreases

the efficiency in the photooxidation of propene. If paying attention to the

influence of heat treatment, analysed in the TiO2 (T), the obtained results have

shown that TiO2 heat treated at 500 ºC is the best performing catalyst of the series

in the three studied reactions.

Index

0. Thesis structure 1

1. Introduction and objectives 5

1.1. Heterogeneous photocatalysis 6

1.1.1. General electronic structure of semiconductors 6 1.1.2. Heterogeneous photocatalysis mechanism 8 1.1.3. Experimental parameters in the photocatalytic

process 10

1.2. Titanium dioxide (TiO2) 12

1.2.1. Structure and properties of TiO2 12 1.2.2. Strategies for improving TiO2 photoactivity 14

1.3. TiO2 photocatalytic environmental applications 17

1.3.1. Photodecomposition of acetic acid 18 1.3.2. Oxidative photodegradation of diuron 20 1.3.3. Photocatalytic oxidation of propene 21

1.4. Objectives 23

1.5. References 23

2. Materiales, métodos y técnicas experimentales 35

2.1. Materiales 36

2.2. Métodos de preparación 36

2.2.1. Preparación de los catalizadores M/P25 37 2.2.2. Preparación de TiO2 38 2.2.3. Preparación de catalizadores Cu/TiO2 39 2.2.4. Preparación de catalizadores TiO2-AC 41

2.3. Técnicas de caracterización y análisis 42

2.3.1. Adsorción física de gases 42 2.3.2. Difracción de rayos X 46

2.3.3. Espectroscopia fotoelectrónica de rayos X 48 2.3.4. Termogravimetría 49 2.3.5. Microscopía electronica de barrido 50 2.3.6. Microscopía electrónica de transmisión 51 2.3.7. Espectroscopia ultravioleta visible 52 2.3.8. Espectrometría de masas 58 2.3.9. Carbono orgánico total 59 2.3.10. Cromatografía de intercambio iónico 60

2.4. Ensayos fotocatalíticos 62

2.4.1. Degradación fotocatalítica de ácido acético 62 2.4.2. Fotodegradación oxidativa de diurón 65 2.4.3. Oxidación fotocatalítica de propeno 68

2.5. Referencias 70

3. TiO2 modification with transition metal species (Cr, Co, Ni and Cu) 75

3.1. Introduction 76

3.2. Materials and methods 77

3.2.1. Preparation of M/P25 samples 77 3.2.2. Characterization 77 3.2.3. Photocatalytic activity measurements 78

3.3. Results 79

3.3.1. Textural and morphological properties 79 3.3.2. X-ray diffraction 81 3.3.3. UV-vis diffuse reflectance spectroscopy 83 3.3.4. XPS 84 3.3.5. Photocatalytic activity 87

3.4. Discussion 91

3.5. Conclusions 93

3.6. References 94

4. Cu/TiO2 photocatalysts 99


4.2. Materials and methods 100

4.2.1. Preparation of TiO2 101

4.2.2. Synthesis of Cu/TiO2 samples 101 4.2.3. Heat treatment of the samples 101 4.2.4. Catalysts’ characterization 102 4.2.5. Photocatalytic measurements 102

4.3. Results and discussion 103

4.3.1. Porosity characterization 103 4.3.2. XRD analysis 106 4.3.3. XPS analysis 108 4.3.4. TEM and SEM analysis 110 4.3.5. Study of the photocatalytic activity 112

4.4. Conclusions 116

4.5. References 117

5. TiO2-carbon hybrid photocatalysts 121


5.2. Experimental 123

5.2.1. Preparation of AC 123 5.2.2. Preparation of TiO2 and TiO2-AC photocatalysts 123 5.2.3. Characterization 123 5.2.4. Photocatalytic activity measurements 124

5.3. Characterization results 124

5.3.1. Determination of carbon content 124 5.3.2. Textural properties 128 5.3.3. XRD analysis 131 5.3.4. Scanning electron microscopy (SEM) 133

5.4. Photocatalytic activity results 134

5.4.1. Photocatalytic decomposition of acetic acid 134 5.4.2. Oxidative photodegradation of diuron 138 5.4.3. Photocatalytic oxidation of propene 147

5.5. Conclusions 150

5.6. References 151

6. General conclusions/Conclusiones generales 155

6.1. General conclusions 156

6.2. Conclusiones generales 160

Thesis structure

This Doctoral Thesis is structured in 6 chapters.

Chapter 1 presents a literature survey about heterogeneous photocatalysis

and the properties, characteristics and limitations of TiO2 as photocatalyst.

Strategies to improve the photocatalytic activity of titanium dioxide, specifically,

the incorporation of transition metals or carbon are also presented and discussed.

Some applications of TiO2 as photocatalyst are exposed, describing in detail three

reactions of environmental interest: the photocatalytic degradation of acetic acid

in liquid phase, the oxidative photodegradation of diuron in liquid phase and the

photooxidation of propene in gas phase. Finally, the objectives of the PhD Thesis

are also compiled in this chapter.

In Chapter 2 the materials and methods used in the photocatalysts

preparation are presented. The experimental techniques used, the devices and

experimental procedures used for the characterization of the prepared

photocatalysts are described. Moreover, the procedure carried out to perform the

photocatalytic tests (acetic acid and diuron in aqueous phase, and propene in gas

phase) are presented in detail. This chapter has been written is Spanish.

0

2 Chapter 0

In Chapter 3 commercial P25 modified with transition metal species (Cr,

Co, Ni and Cu) incorporated by impregnation method are studied. The

preparation procedure also includes a heat treatment (500 ºC) in argon. These

new materials were characterized by several techniques, such as N2 adsorption,

SEM, XRD, UV-vis DRS and XPS, and their photocatalytic activity was

investigated in the photo-degradation of aqueous acetic acid into CO2, CH4 and

H2, the oxidative photodegradation of diuron in aqueous phase, and in the gas

phase photooxidation of propene at low concentration. The overall performances

are discussed in terms of activity and selectivity as a function of the properties of

the photocatalysts.

The results of this chapter have been published in the journal Materials:

A. Amorós-Pérez, L. Cano-Casanova, A. Castillo-Deltell, M. A. Lillo-

Ródenas, M. C. Román-Martínez. TiO2 modification with transition metallic

species (Cr, Co, Ni and Cu) for photocatalytic abatement of acetic acid in liquid

phase and propene in gas phase. Materials 12 (2019) 1-18.

In Chapter 4 Cu/TiO2 photocatalysts are studied. The TiO2 materials were

prepared by sol-gel, and different amounts of copper (0, 0.5, 1, 2, 5, 7 and 10 wt.

%) were incorporated by two methods, impregnation and in situ. The post-

synthesis heat treatment was carried out using two different atmospheres (air and

Ar). The effect of copper amount, incorporation method and atmosphere heat

treatment are analysis from the point of view of the textural and crystalline

properties of the prepared materials. The influence of the synthesis variables on

their efficiency on the photodegradation of acetic acid, in liquid phase, the

oxidative photodegradation of diuron, in liquid phase, and the propene

photooxidation, in gas phase, is analysed.

These results have been published in the Catalysis Today journal:

A. Amorós-Pérez, L. Cano-Casanova, M. A. Lillo-Ródenas, M. C. Román-

Martínez. Cu/TiO2 photocatalysts for the conversion of acetic acid into biogas

and hydrogen. Catalysis Today 287 (2017) 78-84.

Chapter 5 addresses the study of properties, such as crystalline phase,

crystallite size and surface area, of two series of photocatalysts. On the other

hand, one series of TiO2 samples prepared by sol-gel method and heat treated at

different temperatures (350, 400, 450 and 500 ºC); and the other hand, a series of

TiO2-AC hybrid photocatalysts prepared by sol-gel using different amounts of a

spherical activated carbon (0.5, 1, 5 and 10 wt. %) obtained by the hydrothermal

carbonization of saccharose. The behaviour of the prepared photocatalysts is

studied in the photodecomposition of acetic acid, the oxidative photodegradation

of diuron, both in liquid phase and, the photooxidation of propene, in gas phase.

3 Thesis structure

The effects of the heat treatment temperature and of the amount of carbon on the

properties of the prepared TiO2 samples are analysed.

Finally, in Chapter 6, the most relevant conclusions of this Doctoral

Thesis are compiled in English and in Spanish.

Introduction and objectives

1.1. Heterogeneous photocatalysis

1.1.1. General electronic structure of solids and semiconductors

1.1.2. Heterogeneous photocatalysis mechanism

1.1.3. Experimental parameters in the photocatalytic process

1.2. Titanium dioxide (TiO2)

1.2.1. Structure and properties of TiO2 1.2.2. Strategies for improving TiO2

photoactivity

1.3. TiO2 photocatalytic environmental applications

1.3.1. Photodecomposition of acetic acid 1.3.2. Oxidative photodegradation of diuron 1.3.3. Photocatalytic oxidation of propene

1.4. Objectives

1

6 Chapter 1

1.1. Heterogeneous photocatalysis

Heterogeneous photocatalysis has gained acceptance as a viable

alternative to traditional biological, chemical and physical decontamination

technologies for water and air owing to advantages like non-selectivity, non-

toxicity, total removal of pollutant and by-products, and cost-effectiveness [1].

The IUPAC defines photocatalysis as the change in the rate of a chemical

reaction or its initiation under the action of ultraviolet, visible, or infrared

radiation in the presence of a substance, the photocatalyst, that absorbs light and

is involved in the chemical transformation of the reaction partners [2]. When such

process takes place at the interfacial boundary between two phases (solid/liquid,

solid/gas, liquid/gas) it is called heterogeneous photocatalysis [2].

Among the different categories of photocatalysts (such as semiconductor-

based composites, complex oxides, polyoxometalates and complex organic or

organometallic compounds) the inorganic semiconductors have been recognised

as the most successful ones in various applications [1].

1.1.1. Basics of the electronic structure of solids and semiconductors

To understand the electronic structure of semiconductors and their derived

properties (such as electric conductivity, magnetism and optical effects), it is

necessary to consider the interactions of electrons with each other and with

extended arrays of atoms or ions. The correlation of the electronic properties of

a solid with its structure can be established quite well using the band theory,

which is an extension of the molecular orbital theory.

When the atoms in a molecule interact, their atomic orbitals lose their

individuality and become molecular orbitals, according to the molecular orbital

(MO) theory. The number of molecular orbitals generated is always equal to the

number of combined atomic orbitals [3]. A simple general example is the case of

a diatomic Li2 molecule, in which each Li atom contributes one 2s orbital to the

production of two molecular orbitals: one bonding (σ2s) and other antibonding

(σ*2s) orbital. The electrons originally described as 2s1 electrons of the Li atoms

enter and half-fill these molecular orbitals. Bonding orbitals are at a lower energy

than the antibonding orbitals, so they fill the σ2s orbital and leave the σ*2s empty

(Figure 1.1a).

If this combination of Li atoms is extended to a third Li atom, three

molecular orbitals are formed, containing a total of three electrons. Again, the set

of molecular orbitals is half-filled. Thus, the MO theory of small molecules can

be extended to account for the properties of solids, which are aggregations of an

almost infinite number of atoms. When the system contains an enormously large

number (N) of atoms, a set of N molecular orbitals with an extremely small

7 Introduction and objectives

energy separation between consecutive levels is obtained. This collection of very

closely arranged molecular orbital energy levels is called “band” (Figure 1.1b).

Such a band is occupied by the valence electrons of the LiN material. If

each Li atom contributes one electron, then N electrons would occupy (at 0 K)

the N/2 molecular orbitals of lowest energy. The fully or partially occupied band

is called valence band (VB), and the highest energy occupied molecular orbital

is known as HOMO. The next-higher orbital is the lowest unoccupied molecular

orbital (LUMO), that belongs to the empty band called conduction band (CB)

(Figure 1.1b).

Figure 1.1. (a) Molecular orbital diagram for Li2 molecule and (b) band

formation of molecular orbitals (adapted from [4]).

The energies of valence and conductions bands depend on the energies of

the contributing atomic orbitals. Thus, the valence and conductions bands can be

close in energy (semiconductor solid), can overlap (conductor solid) or can be

far in energy (insulator solid). In semiconductors, there is a small energy gap

between the valence and conduction bands, and some electrons make the

transition between the two bands just by acquiring energy. Therefore, this gap is

associated with an amount of energy needed to transfer an electron from the

valence to the conduction band, which is called band gap energy (Eg).

Semiconductors are mainly classified into two categories: intrinsic and

extrinsic [5,6]. An intrinsic or pure semiconductor is an ideal semiconductor,

with no defects or impurities (Figure 1.2a). An extrinsic semiconductor is one in

which charge carriers are present as a result of impurities, either artificially added

(doping) or intrinsically present [7]. If the present impurities have extra valence

Li Li

σ2s

2s 2s

σ2s

*

Li2

En

erg

y

En

erg

y

2s

Li Li2 Li3 Li4 Li5 Li6 Li16 LiN… …

LUMO

HOMO

antibonding

bonding

antibonding

bonding

Val

ence

ban

dC

on

du

ctio

nb

and

(a) (b)

8 Chapter 1

electrons to share with adjacent atoms, they act as donors and the semiconductor

is called n-type (Figure 1.2b). If the present impurities cap trap electrons, they

withdraw electrons form the filled band, leaving holes which allow the remaining

electrons to move. This feature gives rise to p-type semiconductivity (Figure

1.2c).

Figure 1.2. The band structure for (a) intrinsic, (b) n-type and (c) p-type

semiconductors (based on [8]).

1.1.2. Heterogeneous photocatalysis mechanism

A photocatalytic reaction is initiated when a photon of energy (hν)

equalling or exceeding that of the band gap (Eg) of the semiconductor

photocatalyst is adsorbed, and a photoexcited electron (e‒) is promoted from the

filled valence band (VB) to the empty conduction band (CB). This results in a

hole (h+) formation in the VB, forming as a consequence an electron-hole pair (e‒

/h+) [9]. The e‒ and the h+ can recombine on the surface or in the bulk of the

particle in a few seconds, dissipating energy as heat, or they can be trapped in

surface states and they can react with donor (D) or acceptor (A) species (either

adsorbed species or close to the particle surface). This means that redox reactions

can take place at the surface of the photoexcited semiconductor photocatalyst.

The efficiency of a photocatalysts depends on the competition of different

interface transfer processes involving electrons and holes and their deactivation

by recombination [10–13]. A simplified diagram of the heterogeneous

photocatalytic processes occurring on an illuminated semiconductor particle is

displayed in Figure 1.3.

(b) (c)

En

erg

y

(a)

Density of states

En

erg

y

En

erg

y

Density of states Density of states


Figure 1.3. Photocatalytic mechanism. Schematic representation of the

processes occurring on the surface of an excited semiconductor particle (based on

[14]).

The heterogeneous photocatalytic process is a complex sequence of

reactions that can be expressed by the following set of simplified equations [15]:

semiconductor + hν → e‒ + h+ (1.1)

h+ + H2Oads → HO•ads + H+ (1.2)

H2O → OH‒ + H+ (1.3)

h+ + HO‒ads → HO•

ads (1.4)

h+ + Dads → D+ads (1.5)

HO• + Dads → Doxid (1.6)

e‒ + Aads → A‒ads (1.7)

E- E-e˗

h+e˗

h++

recombination

in the bulk

e˗

h+

h+ e˗+

recombination

in the surface

hν ≥ Egreduction

oxidation

(oxidant/acceptor)

AA ‒

D

(reductant/donor)

D+

VB

CB

h+

e˗

hν ≥ EgEg

En

erg

y

E-E-

10 Chapter 1

Note that D is usually the substrate to be oxidized and A is generally O2.

A great discussion exists about the oxidative pathway, which could take place by

direct h+ reaction (Equation 1.5) or by HO• radicals mediated reaction, in their

free or adsorbed form (Equation 1.6). The oxidative pathway leads, in many cases,

to complete mineralization of the D organic substrate to CO2 and H2O.

When the electron acceptor (A) is O2, it can be transformed into superoxide

radical anion (O2•‒) and can lead to additional formation of HO• [15], based on

the following set of reactions:

e‒ + O2ads + H+ → HOO• ↔ O2•‒ + H+ (1.8)

HOO• + e‒ + H+ → H2O (1.9)

2 HOO• → H2O2 + O2 (1.10)

H2O2 + O2•‒ → HO• + O2 + HO‒ (1.11)

H2O2 + hν → 2 HO• (1.12)

H2O2 + e‒ → HO• + HO‒ (1.13)

Water and carbon dioxide are the main oxidation products and, when the

organic substrate contains heteroatoms such as Cl, N, etc., a mineral acid is

usually also formed as a by-product (Equation 1.14) [16].

Organic waste photocatalyst, O2, hν ≥ Eg

⇒ CO2 + H2O + Mineral acid (1.14)

1.1.3. Experimental parameters in the photocatalytic process

Several factors can affect the photocatalytic reaction rates (such as light

intensity, light wavelength, temperature, catalyst amount, substrate concentration,

etc.). The light intensity is important because an excess of light promotes a faster

e‒/h+ recombination [15], the match between the radiation used and the band gap

energy value of the photocatalyst is critical to allow an efficient electron

promotion [17], and higher rates are usually achieved at higher temperature

because of the collision probability between substrate and photocatalyst increases

[18]. On the other hand, the amount of catalyst must be chosen so as to avoid

using an excess thereof and to ensure the total absorption of efficient photons

[19]. The kinetic regime depends on the pollutant concentration, showing, in

most of the cases, a Langmuirian behaviour [20]. Besides, in the case of reactions

in solution, the pH determines the surface charge on the semiconductor and the

speciation of the substrate to be transformed.


In addition to the operational parameters mentioned above, the

photocatalytic activity strongly depends on the photocatalyst properties.

Properties like crystalline structure, pore size, density of OH groups, surface

acidity, number and nature of trap sites (both in lattice and at surface), and

adsorption/desorption characteristics have shown to play an important role in

photocatalytic efficiencies [10]. Some of these properties are commented next.

⁖ Crystalline structure

The photocatalytic efficiency varies widely with the degree of crystallinity,

the crystalline phases present, the presence of defects and the crystal size of the

photocatalyst. These parameters mainly influence the production and lifetime of

the e‒/h+ pairs, and hence their availability for the surface redox reactions [21].

⁖ Surface area

As adsorption processes have a particular importance because the

photocatalytic reactions originate in the interface, a large surface area can be the

determining factor in certain photodegradation reactions [22]. However, powders

with a large surface area are usually associated with large amounts of crystalline

defects, which favour the recombination of electrons and holes, leading to a poor

photoactivity [23,24]. Recently, it has been reported that the photocatalytic

activity of amorphous materials is negligible, indicating that crystallinity is an

important requirement [25]. Then, surface area and crystallinity must be balanced

in order to obtain high photoactivities.

⁖ Particle size

Since the electron-hole recombination rate depends on the particle size,

this parameter is highly relevant for the photocatalytic efficiency [24]. It is well

known that in the nanometer-size range, the physical and chemical properties of

semiconductors are different from those of large particles. Small variations in

particle diameters lead to great modifications in the surface/bulk ratio, thus

modifying the significance of volume and surface e‒/h+ recombination.

Experimental investigations support the existence of an optimum particle size of

photocatalyst, for which photocatalytic oxidation rates of organic substrates are

maximized. According to some literature data, this value lies around 10 nm for

pure nanocrystalline TiO2 photocatalyst [24].

⁖ Band gap energy

The band gap energy of the semiconductor and the redox potential of the

adsorbates determine the reactions that can take place. The bottom energy level

of the conduction band is actually the reduction potential of photoelectrons, while

the top energy level of the valence band determines the oxidizing ability of

photoholes [22]. The semiconductor must have a band gap energy large enough

12 Chapter 1

to provide energetic electrons (Eg > 1.23 eV, typically > 2.0 eV) and at the same

time it must be small enough for an efficient absorption in the maing range of the

solar spectrum (Eg < 3.0 eV) [26]. Figure 1.4 shows the band gap energy of

several semiconductors and the standard redox potentials of water.

Figure 1.4. Band gap energies and band-edge positions with respect to the

vacuum level and NHE for selected semiconductors. The horizontal grey lines ( )

represent the conduction-band edges. The horizontal green lines ( ) represent the

valence-band edges. The two orange dashed lines ( ) indicate the water redox reaction

potentials (adapted from [27]).

1.2. Titanium dioxide (TiO2)

Ideally, a semiconductor to be used as photocatalyst should be: chemically

and biologically inert, photocatalytically stable, easy to produce and to use,

efficiently activated by sunlight, able to efficiently catalyse reactions, cheap, and

without risks for the environment or humans. Titanium dioxide (either as clusters,

colloids, powders or large single crystals) is close to be an ideal photocatalyst,

displaying almost all the above properties. TiO2 is an abundant and cheap

semiconductor, chemically stable, with high resistance to photo-induced

corrosion and, in addition, it is non-toxic and biocompatible. However, due to its

Eg (Eg = 3.2 – 3.0 eV) [22], TiO2 does not absorb visible light. Nonetheless, due

to its ability to generate e‒/h+ pairs in the presence of suitable UV radiation it has

been used since the 70s in heterogeneous photocatalytic reactions [28].

1.2.1. Structure and properties of TiO2

TiO2 is found in nature in three crystallographic forms: anatase, rutile and

brookite. The most stable form of TiO2 is rutile. However, anatase is generally

the most photoactive crystalline phase [29]. All three polymorphs can be readily

Ce

2O

32.4

eV

Cu

2O

2.2

eV

2.8

eV

In2O

3

LaT

i 2O

72.8

eV

BaT

iO3

3.2

eV

Nb

2O

53.4

eV

Ta

2O

54

.0 e

V

ZrO

25.0

eV

3.2

eV

Zn

O

TiO

23.2

eV

3.5

eV

NiO

3.4

eV

SrT

iO3 T

aO

N2

.5 e

V

C3N

42

.7 e

V

Ta

3N

52.1

eV

Ce

2S

32.1

eV

Sn

S2

1.7

eV

Cd

S2

.4 e

V

LaT

iO2N

2.1

eV

In2S

32

.0 e

V

Sb

2S

31.7

eV

3.6

eV

Zn

S

-2

-3

-4

-5

-6

-7

-8

-9

E v

svaccum

-2

-1

0

1

2

3

E v

sN

HE

H2O/H2

O2/H2O

oxide nitride chalcogenide


synthesised in the laboratory, and typically the metastable anatase and brookite

will transform to the thermodynamically stable rutile upon calcination at

temperatures exceeding ~ 600 ºC [30]. In all three phases, titanium (Ti4+) atoms

are coordinated to six oxygen (O2-) atoms, forming TiO6 octahedra [31]. Anatase

is made up of corner (vertex) sharing octahedra which form (001) planes resulting

in a tetragonal structure (Figure 1.5a). In rutile the octahedra share edges at (001)

planes to give a tetragonal structure (Figure 1.5b), and in brookite both edges and

corners are shared to give an orthorhombic structure (Figure 1.5c) [22,31,32].

The differences between the three crystalline phases give rise to different

properties, such as, for example, the density and the structure of electronic bands,

so that each of them has a different band gap energy: anatase 3.20 eV, rutile 3.00

eV and brookite 3.15 eV [33,34].

(a) (b) (c)

Figure 1.5. Representative crystalline structures of the three polymorphs of

TiO2: (a) anatase, (b) rutile and (c) brookite (based on [31]).

Among the different TiO2 powders commercially available, P25 from

Degussa is the most used in photocatalytic studies for organic compounds

decomposition. Commercial P25 is synthetized by the hydrolysis of titanium

tetrachloride (TiCl4) in the presence of hydrogen and oxygen at a temperature

above 1200 ºC (Equation 1.15) [35].

TiCl4 + 2 H2 + O2 → TiO2 + 4 HCl (1.15)

The obtained TiO2 is a mixture of anatase (70%) and rutile (30%). The

oxidation and reduction potentials of its valence and conduction bands are +2.95

V and -0.25 V [36], respectively, and it has an average crystal size in the range

14 Chapter 1

of 20-50 nm and a specific surface area about 50 m2/g [35]. P25 is used as

reference material in many works due to its good photocatalytic activity; however,

this material has the typical drawbacks of TiO2 and, in addition, presents a low

specific surface area. For that reason, researchers continue studying the

improvement of the TiO2 properties through the development of new synthesis

methods with the purpose of obtaining a material with high surface area, small

particle size, high crystallinity and low band gap value.

1.2.2. Strategies for improving the TiO2 photoactivity

The strategies adopted for improving the photocatalytic efficiency of TiO2

can be either textural modifications, such as increasing surface area and porosity,

or compositional modifications, by incorporation of atoms or compounds to the

TiO2 [31]. As reported in the literature, the overall efficiency of TiO2 has been

significantly enhanced by controlling the semiconductor morphology,

crystallinity and textural properties and these properties are highly influenced by

the preparation method.

1.2.2.1. Synthesis of TiO2

It is important to choose a cheap and simple preparation method that

allows obtaining TiO2 with high crystallinity and surface area, and small crystal

size. Keeping in mind the interest in developing improved titania materials, a

suitable preparation method should allow the incorporation during the synthesis

of elements/compounds to modify the TiO2 properties. The sol-gel method

satisfies all these needs and it has been widely used due to its simplicity, low cost,

low temperatures required, good control over the morphology size distribution,

purity and homogeneity of the obtained products. Besides, it allows the easy

incorporation of other elements/compounds in high loadings [22,37,38].

The sol-gel method is based on inorganic polymerization reactions and

includes four steps: hydrolysis, polycondensation, drying and thermal

decomposition. Hydrolysis of the precursors of the metal or non-metal alkoxides

takes place with water or alcohols but an acid or a base can be also added to

enhance the process. After the gel formation, the solvent is removed and then the

material is calcined to decompose the remains of the organic precursor [39]. The

schematic of the preparation process is shown in Figure 1.6. The size of the sol

particles depends on the solution composition, pH and temperature. By

controlling these factors, the size of the particles can be tuned [40].


Figure 1.6. Steps of the TiO2 photocatalysts preparation process by sol–gel

method (adapted from [41,42]).

1.2.2.2. Modified TiO2 photocatalysts

The optical and electrochemical properties of TiO2 can be enhanced by the

addition of metal and non-metal component(s). Furthermore, properties, such as

the surface and bulk crystal structure, particle size and morphology can also be

affected by this modification [43].

⁖ Modification with transition metals

The addition of transition metal species to TiO2 has shown to be useful to

reduce the photogenerated e‒/h+ recombination rate and, also, to improve the

photoresponse to the visible light [44–46]. The band gap and the electrochemical

properties of TiO2 can be modified when transition metal ions replace Ti (IV)

centers (substitutional doping), occupy interstitial sites (interstitial doping) or

form aggregates on the TiO2 surface [47]. Substitutional doping can occur if the

16 Chapter 1

difference between the atomic radii of the dopant and of Ti (IV) is less than 15%

[48] and it would cause a local distortion of the crystal structure that can affect

the crystallinity degree and the conditions required for phase transformation.

Doping (substitutional or interstitial) can also generate trap levels, which modify

the TiO2 band structure and allow the capture of the photo-induced e− in the CB,

leaving the h+ in the VB. This hinders the e−/h+ pairs recombination and, thus,

could promotes the photocatalytic efficiency [44,46,49,50]. Additionally,

metallic aggregates not chemically bonded to TiO2 can act as electrons

scavengers, also preventing the recombination of h+/e− pairs. On the other hand,

if the metal species has a work function (energy required for moving an electron

from a Fermi level to the local vacuum level [51]) higher than that of TiO2, it

provides a Schottky barrier that facilitates the transfer of electrons from the

semiconductor to the metal species. Thus, the metal particle serves as an electron

trap which prevents electron migration to the semiconductor, avoiding

recombination [43]. The ability of metal species to act as effective traps is related,

among others, with their electrochemical properties, the metal species

concentration and the intensity of the incident light [43,52,53]. Considering all

this, and with the purpose of enhancing the activity, the added transition metal

species should have a work function higher than that of TiO2.

Although noble metals (such as Pt, Pd or Au) have proved to be effective

to enhance the photocatalytic activity of TiO2 [54], they are unsuitable for large-

scale commercial use due to their limited availability and high cost. In contrast,

earth-abundant non-noble transition metal-based materials are promising

alternatives to enhance the TiO2 performance. For example, transition metals,

such as Cu, Co, Ni, Cr, Fe, Mn and V [44,46,55] have been reported to decrease

the TiO2 band gap and to extend its photo-response to the visible region. Among

the transition metals commonly used for the modification of TiO2, copper is very

promising due to its wide availability, low cost and high photocatalytic efficiency

in photo-oxidation reactions [56,57].

⁖ Modification with carbon

The modification of TiO2 with non-metallic elements, like nitrogen [58],

carbon [59], sulphur [60] and fluorine [61], has received intense attention and

has been proved to be another effective way to enhance photocatalytic activity of

TiO2 under visible light [62]. Among these, the use of carbon materials has

attracted considerable attention for this application because of their unique and

controllable structural and electrical properties [63]. Conventional carbon

materials (such as carbon black [64], activated carbons [65], graphite [66] and

novel ones (carbon nanotubes [67], fullerenes and graphene [68])) have been

used in photocatalysis. The presence of activated carbon (AC) in contact with


TiO2 can improve the photocatalytic efficiency of TiO2 because it: i) provides a

large surface area that allows to adsorb a wide range of organic compounds

[69,70] and, then, transfers them to the photoactive TiO2 through the AC-TiO2

interphase [71,72]; ii) hinders the crystal phase transformation from anatase to

rutile during heat treatments [73,74] and iii) is capable of increasing the lifetime

of the photogenerated e‒/h+ pairs, which enhances the generation of OH• radicals

[75,76]. Besides, if doping takes place, the carbon atoms can introduce localized

electronic states in the TiO2 band gap, extending the adsorption wavelengths

from UV to visible region [77]. Additionally, AC can be used as a TiO2 support,

which facilitates the recovery of the catalyst from solution [78], and it is a

material that can be produced by simple methods and from low cost precursors.

Different types of TiO2-AC hybrid materials, attending to the carbon-TiO2

interaction have been reported. On one hand, carbon can be incorporated without

modifiying the TiO2 structure. As examples, Araña et al. combined TiO2 with

activated carbon by simple physical mixture [72,79]; Torimoto et al., prepared

TiO2 loaded on an activated carbon support [69], whereas Tsumura et al.

synthesized TiO2 particles coated with carbon [80]. In these three examples, the

carbon is physically in contact with TiO2, being the positive role of carbon mainly

related with an enhancement of the adsorption of organic substrates. On the other

hand, carbon can be incorporated into the TiO2 lattice [81–83], and this can

modify both the physical and electronic structure of TiO2. Thus, in this TiO2-AC

hybrid materials an extension of the absorption wavelengths from UV to visible

region can take place, attributed to the introduction of localized electronic states

in the TiO2 band gap [43]. It is important to highlight that all these carbon-TiO2

interactions may be even co-present depending on the preparation conditions [84].

Hybrid TiO2-AC materials could be obtained using carbon containing Ti

precursors (such as titanium alkoxides) or adding external carbon precursors

during the TiO2 synthesis. Among the external carbon precursors usually used,

carbohydrates (such as cellulose, glucose, saccharose, fructose) have been object

of great interest for the preparation of spherical activated carbons because they

are cheap and environmentally friendly [85].

1.3. TiO2 photocatalytic environmental applications

The rapid development of civilization in the 19th century has occurred as a

result of the industrial revolution. However, the increase in urbanization and

industrialization over the years has degraded the quality of the environment. As

a consequence of traditional disposal practices of industrial waste materials, there

has been an increase in the toxicity of groundwater and air, which in turn is

affecting both human health and environment. Due to such malpractices, human

18 Chapter 1

society in the twenty-first century is facing major environmental issues, such as

global climate change and extended pollution, which are posing a great threat to

human health and ecosystem [86]. According to recent reports by World Health

Organization (WHO), nearly 3.7 million people have died globally due to

environmental pollution in the 21st century and nearly 92% of the world's

population lives in regions characterized by a dangerous level of air and water

pollution [87]. Hence, remediation of hazardous waste materials from water and

air has become a topic of high national and international priority.

To contrast this unsustainable trend, several countermeasures have been

taken for the remediation of pollutants, such as adsorption, biological and

chemical treatments. However, most of these means are still inefficient, time-

consuming, costly and require high energy inputs [11]. Hence, development of

new and eco-friendly processes for remediation of such toxic organic pollutants

is desirable. In this regard heterogeneous photocatalysis is among the most

promising solutions for the degradation of organic pollutants [9,11,88,89].

Indeed, heterogeneous photocatalysis provides a number of attractive features

that can be highly promising for treatment of air and water pollution: it destroys

pollutants rather than transferring them to another phase; it usually leads to

complete mineralization of organic pollutants into CO2, H2O and innocuous

mineral compounds; it can operate at ambient conditions, with any type of

substrate, without complex processing requirements; and can be easily

implemented in both aqueous and gaseous applications [15,90].

In this PhD Thesis, TiO2 based photocatalysts were prepared,

characterized and applied to the photocatalytic abatement of three representative

organic pollutants present in air or water: acetic acid, a typical pollutant present

in wastewater; diuron, a common herbicide that contaminates groundwater, and

propene, a volatile organic compound that is one of the major sources of indoor

air pollution, present in tobacco smoke. The acetic acid photodecomposition (in

liquid phase), the oxidative photodegradation of diuron (in liquid phase), and the

photocatalytic oxidation of propene (in gas phase) are explained next in more

detail.

1.3.1. Photodecomposition of acetic acid

Most of the organic matter that contaminates waters comes from urban

spills, livestock, agricultural or industrial activities. For example, about 75% of

suspended solids and 40% of filterable solids in a typical wastewater are of

organic nature [33]. Such organic compounds are transformed over time due to

the presence of microorganisms in the medium. These microorganisms first

hydrolyse the higher molecular weight compounds (polysaccharides, lipids,


proteins and nucleic acids), both dissolved and undissolved, forming the

corresponding oligomers and monomers (sugars, alcohols, fatty acids, amino

acids, glycerol, etc.). Then, these oligomers and monomers are converted into

volatile fatty acids, which will eventually decompose to acetic acid.

Acetic acid (HAc) (Figure 1.7) is a

recalcitrant compound quite difficult to

decompose using conventional methods due to

its high chemical stability and/or low

biodegradability [91]. In contrast, catalytic

photodegradation is an interesting technique

used for the degradation of such recalcitrant

contaminants, such as acetic acid. It also presents important advantages

considering the large volumes of pollutants-containing effluents, such as low cost,

and operation in mild operation conditions [92] and with different matrixes.

The first related work, published in the 1970s by Kraeutler and Bard,

studied the photodecarboxylation of acetic acid using Pt/TiO2 as catalyst [93–95].

This type of photodegradation follows the path known as “photo-Kolbe reaction”

(Equation 1.16), which leads to CH4 and CO2 as main products. This reaction

proved to be rather effective to depollute toxic organic acids through an

environmental process, and to convert them in valuable products that can be used

as an energy source [96].

CH3COOH → CH4 + CO2 (1.16)

Yoneyama et al. [97,98] detected, in addition to CO2 and CH4, C2H6 and

H2 during illumination of aqueous solutions of acetic acid and sodium acetate in

a batch reactor with Pt/TiO2. Moreover, these authors found that the crystalline

structure of TiO2 had an important effect on the photocatalytic activity i.e., the

anatase phase showed better activity than rutile. Later, the group of Morawski et

al. [99–101] confirmed in several subsequent studies the effectiveness of the

anatase crystal phase of TiO2, in agreement with Yoneyama et al. [97,102].

Morawski et al. also stated that the modification of TiO2 with metals could

provide better photocatalytic activity by decreasing the recombination rate of e‒

/h+ [99–101]. Wanag et al. [103] have studied the activity of graphitic carbon-

modified TiO2 samples in the acetic acid decomposition, observing that

adsorption also has an important role in photocatalysis process.

Figure 1.7. Acetic acid.

20 Chapter 1

1.3.2. Oxidative photodegradation of diuron

Intensive agriculture involves the use of large quantities of herbicides. The

presence of these substances in the environment is alarmingly increasing all over

the world. Since herbicides are usually persistent in soil and water, their presence

supposes a potential harmful risk for natural ecosystems [104]. In this regard,

water resources scarcity is an increasing problem in many areas of the Earth.

Hence, recycling aqueous effluents by new treatment methods is a challenge for

the scientific community. Although treating organic/inorganic pollutants by

means of different technologies (usually biodegradation) is quite often viable, a

variety of pollutants, such as some pesticides, are recalcitrant to conventional

biological treatments [105]. For this reason, more powerful oxidizing

technologies are required.

Organochlorides are some of the most popular and widely used pesticides.

They derive from chlorinated hydrocarbons. Their high physical and chemical

stability favour their persistence and slow biodegradability in the environment.

Diuron, N-(3,4-dichlorophenyl)-N,N-dimethyl-urea (Figure 1.8), is one of the

most commonly used herbicides. It belongs to the phenylamide family and to the

subclass of phenylurea. This herbicide inhibits the photosynthesis by preventing

oxygen production and blocking the electron transfer at the level of photosystem

II of photosynthetic microorganisms and plants [106]. This compound has been

used to control a wide variety of annual and perennial broadleaf and grassy weeds,

as well as mosses. It has been also used on non-crop areas such as roads, garden

paths and railway lines and on many agricultural crops such as fruit, cotton, sugar

cane, alfalfa and wheat [107].

Figure 1.8. Diuron, N-(3,4-dichlorophenyl)-N,N-dimethyl-urea, molecule.

However, dispersion of this compound in agriculture leads to pollution of

the aquatic environment by soil leaching [108,109], and it appears to be

moderately persistent and relatively immobile in water [110]. Its presence has

been reported in 70% of 100 rivers in 27 European Countries, with a maximum

concentration of 0.826 μg/L [111]. Because of its high toxicity, pollution due to

this substance is considered as a real environmental hazard. This is evidenced by


a law recently issued by European Commission that includes diuron in the list of

priority pollutants in water treatment policy [112].

The harmful presence of diuron in aqueous environments has pushed a

considerable research production, including advanced oxidation processes such

as heterogeneous photocatalysis [104]. Malato et al. have proposed a three-step

degradation pathway and have identified the main reaction by-products [113], in

agreement with previous works [114–116]. Thus, the Malato group proposed that

the degradation pathway in TiO2 photocatalysts would be initiated by the attack

on the aromatic ring by OH• radicals, that triggers a series of oxidation and

decarboxylation processes that eliminate alkyl groups and chlorine atoms. The

last step would involve oxidative opening of the aromatic ring, leading to small

organic ions and inorganic species [113].

In the last years, photocatalytic efforts have been focused on developing

more reliable and efficient photocatalysts, which can take advantage of the solar

spectrum more effectively [117]. Katsumata et al. showed that the photocatalytic

activity could be strongly enhanced after controlled platinization of TiO2

photocatalysts with an optimum 0.2 wt. % Pt loading [118]. De la Cruz et al.

proved that the presence of Sm+3 on TiO2 prepared by sol-gel method, reduce the

TiO2 band gap towards visible radiation region, leading to excellent results for

photocatalytic degradation of diuron under sun radiation [119]. Chusaksri et al.

reported that the degradation rate of diuron with 0.83 wt. % Au/TiO2

photocatalyst was 1.6-fold higher than with unmodified TiO2 [120]. Bamba et al.

studied the deposition of TiO2 prepared by sol-gel method on different carbon

materials, such as activated carbon, graphite and carbon aerogel, for the

photodegradation of diuron [70]. They observed that the TiO2/activated carbon

and TiO2/carbon aerogel samples were promising under sunlight irradiation,

attributing their good results to the predominance of anatase phase, high specific

surface area, large pore volume and their capacities to absorb in the visible-light

region.

1.3.3. Photocatalytic oxidation of propene

Rapid urbanization and industrialization contribute to the growing

emissions of volatile organic compounds (VOCs) into the environment. VOCs

are a large group of carbon-based chemicals that easily evaporate at room

temperature [121,122]. VOCs have been implicated as a major contributors to

photochemical smog, which can cause haze, damage to plants and animal life,

and eye irritation and respiratory problems for humans [123–125]. VOCs are

emitted from a wide range of outdoor and indoor sources. Outdoor sources

include, but are not limited to, chemical industries, paper production, food

22 Chapter 1

processing, paint drying, transportation, petroleum refineries, automobile

manufacturers, metal degreasing and textile manufacturers [126]. Indoor sources

include household products, office supplies, printers, insulating materials,

pressed woods, wood stoves, and leaks from piping [127–129].

Among the different VOCs, propene

(Figure 1.9) is considered a highly pollutant

molecule because of its high photochemical

ozone creativity potential (POCP) [130].

Moreover, it is present in vehicle emissions, in

many industrial applications, such as

petrochemical plants, foundry operations and

others [131], and it is one of the major sources of

indoor air pollution, due to is one of the principal components of tobacco smoke

[132,133].

Photocatalytic oxidation is one of the most effective and economically

feasible techniques for VOCs, especially in gaseous effluents with low organic

concentrations [126], such as propene gaseous streams.

So far, very few studies have been reported on the photocatalytic oxidation

of propene in gas phase at lower concentrations using TiO2 [131,133–137]. As

an example, Ouzzine et al. investigated the photocatalytic oxidation of propene

in gas phase at low concentration with TiO2 nanoparticles, prepared by sol-gel

method using two hydrolysis agents (acetic acid or isopropanol), and varying the

temperature of the heat treatment [136]. The most active photocatalyst was

obtained with acetic acid and without any additional heat treatment [136]. This

result was attributed to the following properties of the synthesized photocatalyst:

high surface area, high crystallinity, prevalence of the anatase phase, high pore

volume and high content of surface oxygen groups. Bouazza et al. studied the

effect of the humidity on the photooxidation of propene (100 ppmv) using TiO2-

based materials, concluding that the conversion of propene decreases in the

presence of humidity due to a possible competition between the adsorption of

propene and water vapour [137]. The same research group also investigated

hybrid photocatalysts for low-concentration propene abatement, particularly

TiO2/carbon [131,133,135] and TiO2/inorganic material [133]. These studies

showed that the activity depends mainly on two factors: the percentage of anatase

and rutile phases, being anatase the most active phase, and the type and features

of the additive [131,133,135]. Thus, as an example, they proved that the preferred

carbon-containing photocatalysts are those that contain carbon materials with

high surface area and especially high electric conductivity [130].

Figure 1.9. Propene molecule.


An exhaustive literature survey shows that metal-TiO2 photocatalysts have

never been studied before in the photocatalytic oxidation of propene at low

concentration, despite the potential positive results of this option.

1.4. Objectives

Considering the stated above, the general objective of this research is the

synthesis and characterization of nanostructured photocatalysts based on TiO2

modified with transition metals or activated carbon to be applied in the

photocatalytic abatement of pollutants, in liquid and gas phase.

More specifically, this Thesis will address the aims presented below.

⁖ Preparation of M/TiO2 (M = Cr, Co, Ni and Cu) photocatalysts by

impregnation using commercial P25 to study the effect of the metal specie

added.

⁖ Preparation of Cu/TiO2 photocatalysts by two methods (in situ and

impregnation), varying the copper content and the atmosphere of heat

treatment to study these variables.

⁖ Preparation of TiO2 photocatalysts by the sol-gel method using different

temperatures in the post-synthesis heat treatment, to study the effect of the

calcination temperature.

⁖ Preparation of TiO2-AC photocatalysts, using saccharose as activated

carbon precursor, with different AC contents, to analyse the carbon effect.

⁖ Characterization in detail of the prepared photocatalysts (textural

properties, morphology, crystalline phases and crystallinity and electronic

properties).

⁖ Application of the prepared photocatalysts to the abatement of three

pollutants: acetic acid photodecomposition (in liquid phase), oxidative

photodegradation of diuron (in liquid phase) and photocatalytic oxidation

of propene at low concentration (in gas phase).

⁖ Finding a clear relationship between the photocatalysts’ properties and

photocatalytic activity in each of the studied reactions.

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Materiales,

métodos y

técnicas

experimentales

2.2. Métodos de preparación

2.2.1. M/P25 2.2.2. TiO2 sol-gel 2.2.3. Cu/TiO2 2.2.4. TiO2-AC

2.3. Técnicas de caracterización y análisis

2.3.1. Adsorción física de gases 2.3.2. Difracción de rayos X 2.3.3. Espectroscopía fotoelectrónica de rayos X 2.3.4. Termogravimetría 2.3.5. Microscopía electrónica de barrido 2.3.6. Microscopía electrónica de transmisión 2.3.7. Espectroscopia ultravioleta-visible 2.3.8. Espectrometría de masas 2.3.9. Carbono orgánico total 2.3.10. Cromatografía de intercambio iónico

2.4. Ensayos fotocatalíticos

2.4.1. Degradación fotocatalítica de ácido acético

2.4.2. Fotodegradación oxidativa de diurón 2.4.3. Oxidación fotocatalítica de propeno

2

36 Capítulo 2

2.1. Materiales

En la Tabla 2.1 se presentan los reactivos químicos empleados en el

presente trabajo, indicando su fórmula química, estado físico, grado de pureza o

concentración y procedencia.

Tabla 2.1. Reactivos químicos utilizados.

Producto Fórmula

química

Estado

físico

Pureza o

concentración Procedencia

Tetraisopropóxido de

titanio C12H28O4Ti Líquido ≥ 97% Sigma-Aldrich

Ácido acético C2H4O2 Líquido ≥ 99% Sigma-Aldrich

Sacarosa C12H22O11 Sólido ≥ 99.5% Sigma-Aldrich

Dióxido de titanio

(Aeroxide P25) TiO2 Sólido ≥ 99.5% Evonik

Nitrato de cobre (II)

trihidratado Cu(NO3)2·3H2O Sólido 99% Panreac

Nitrato de cobalto

(II) hexahidratado Co(NO3)2·6H2O Sólido 98% Panreac

Nitrato de níquel (II)

hexahidratado Ni(NO3)2·6H2O Sólido 98% Panreac

Nitrato de cromo

(III) nonahidratado Cr(NO3)3·9H2O Sólido 97% Panreac

Diurón C9H10C12N2O Sólido ≥ 98% Sigma-Aldrich

Helio He Gas ≥ 99.999% Carburos

metálicos

Hidrógeno H2 Gas 500 ppmv Linde Group

Dióxido de carbono CO2 Gas 2000 ppmv Linde Group

Metano CH4 Gas 2000 ppmv Linde Group

Argón Ar Gas ≥ 99.999% Praxair

Aire Gas

Dióxido de carbono CO2 Gas 300 ppmv Linde Group

Propeno C3H6 Gas 100 ppmv Abelló Linde

2.2. Métodos de preparación

En esta Tesis Doctoral se han preparado fotocatalizadores basados en

dióxido de titanio usando diferentes métodos de síntesis. En primer lugar, se han

obtenido catalizadores constituidos por TiO2 comercial P25 modificados con

metales de transición (M/P25). En segundo lugar, se han preparado catalizadores

de TiO2 usando el método sol-gel. A partir de éstos se han sintetizado dos series

37 Materiales, métodos y técnicas experimentales

de fotocatalizadores modificados, por un lado, con cobre (Cu/TiO2), introducido

por dos métodos diferentes (impregnación e in situ) y, por otro lado, con carbón

activado (TiO2-AC). A continuación, se detalla el procedimiento experimental

seguido para la preparación de las tres series de fotocatalizadores mencionadas.

2.2.1. Preparación de los catalizadores M/P25

El dióxido de titanio comercial más usado en estudios de fotocatálisis para

la oxidación de compuestos orgánicos es el Aeroxide P25 de Degussa,

normalmente conocido como P25. Éste se produce mediante la hidrólisis del

tetracloruro de titanio (TiCl4) en presencia de hidrógeno y oxígeno a

temperaturas superiores a los 1200 ºC [1], como se muestra en la siguiente

reacción:

TiCl4 + 2 H2 + O2 → TiO2 + 4 HCl (2.1)

La preparación de la serie de catalizadores denominada M/P25 se ha

llevado a cabo modificando el TiO2 P25 comercial mediante impregnación con

una disolución de una sal del metal M (M = Cr, Co, Ni o Cu). Concretamente se

mezclaron 10 mL de agua desionizada con 2 g de P25; paralelamente se

disolvieron, en 5 mL de agua desionizada, las cantidades necesarias de

Cr(NO3)3·9H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O o Cu(NO3)2·3H2O para

obtener una carga de metal en el catalizador del 1 % en peso. La disolución de

cada sal se vertió sobre la suspensión de P25. La mezcla se agitó a temperatura

ambiente durante 2 h y se trató con ultrasonidos durante 30 min. Pasado este

tiempo, el exceso de disolvente se eliminó en una estufa a 80 ºC durante 24 h.

Todas las muestras obtenidas fueron sometidas a un tratamiento térmico

con el fin de eliminar las impurezas remanentes después de la síntesis [2] y

fortalecer la interacción del TiO2 con el metal [3]. El tratamiento térmico se llevó

a cabo en un reactor de cuarzo de lecho fijo en flujo de Ar (90 mL/min). La rampa

de calentamiento empleada fue de 5 ºC/min hasta alcanzar una temperatura de

500 ºC, la cual se mantuvo durante 2 horas. La nomenclatura usada para esta serie

de muestras es M/P25-Ar (M = Cr, Co, Ni o Cu) para los catalizadores

modificados y P25-Ar para el catalizador usado como referencia, el cual no

contiene metal. En el Esquema 2.1 se muestran los pasos del proceso de

preparación.

38 Capítulo 2

Esquema 2.1. Proceso de síntesis de la serie de catalizadores M/P25-Ar, dónde

M = Cr, Co, Ni o Cu.

2.2.2. Preparación de TiO2

En la segunda y tercera serie de fotocatalizadores sintetizados, Cu/TiO2 y

TiO2-AC, respectivamente, el dióxido de titanio ha sido obtenido mediante el

método sol-gel. Se ha seleccionado este método debido a su versatilidad,

reproducibilidad y facilidad en el control de las variables de síntesis. El proceso

sol-gel consiste en la hidrólisis del precursor de titanio que, por reacciones de

condensación subsiguientes, genera una suspensión de partículas (sol), cuya

agregación da lugar finalmente a la formación de un gel polimérico. Se ha

utilizado tetraisopropóxido de titanio (TTIP) como precursor de titanio, ácido

acético glacial (HAc) como agente de hidrólisis y agua destilada. La relación

molar empleada ha sido 1:10:350 (TTIP:HAc:H2O) [4].

En esta síntesis (Esquema 2.2), 9.3 mL de TTIP se han hidrolizado

utilizando 17.5 mL de ácido acético glacial a 0 ºC. A esta disolución se han

añadido 197.5 mL de agua destilada gota a gota, en agitación vigorosa durante 1

h. Posteriormente, la disolución se ha sometido a ultrasonidos durante 30 min y

se ha continuado la agitación magnética durante 5 h. La disolución se ha

mantenido después a 70 ºC durante 12 h en una estufa para dar lugar al proceso

de envejecimiento. El gel obtenido se ha secado a 100 ºC. Finalmente, el

fotocatalizador se ha reducido a polvo con ayuda de un mortero de ágata.

Mezclamos

P25 (2 g)

H2O (10 ml)

Añadimos

H2O (5 ml)

M(NO3)x

Ultrasonidos

30 min

Horno Ar500 ºC2 h

Estufa

80 ºC

24 h

P25-Ar

M/P25-Ar

1

2

Agitación

2 h3

4

5

6

Ar----

-


Esquema 2.2. Síntesis sol-gel del TiO2.

2.2.3. Preparación de los catalizadores Cu/TiO2

El dióxido de titanio obtenido mediante el método sol-gel se ha modificado

con diferentes cantidades de cobre (0.5, 1, 2, 5, 7 y 10 % en peso, nominal)

mediante dos metodologías diferentes (impregnación e in situ) y, posteriormente,

ha sido tratado térmicamente empleando dos atmósferas distintas (Ar y aire). A

continuación, se detallan los dos métodos de preparación empleados.

■ Impregnación

Se prepararon disoluciones de Cu(NO3)2·3H2O con las cantidades

apropiadas del precursor de Cu para que al impregnar 2 g de TiO2 con 5 mL de

las mismas se obtuvieran los siguientes porcentajes en peso (nominales) del metal:

0.5, 1 y 10 % en peso. La mezcla disolución-TiO2 se agitó durante 2 h y

posteriormente se sometió a ultrasonidos durante 30 min. Finalmente se secó en

una estufa a 80 ºC durante 24 h. La nomenclatura usada para nombrar esta serie

de muestras es la siguiente: Cu/TiO2imx; donde im indica que el método de

preparación es impregnación y x la carga nominal de Cu (0.5, 1 ó 10 % en peso)

(Esquema 2.3). Todos los catalizadores obtenidos fueron tratados térmicamente,

como se describe más adelante.

■ In situ

La incorporación de Cu al dióxido de titanio se llevó a cabo durante la

síntesis sol-gel de éste. Para ello, 5 mL de disolución acuosa de Cu(NO3)2·3H2O,

con la concentración adecuada del precursor del metal para obtener porcentajes

nominales de Cu de 0.5, 1, 2, 5, 7 y 10 % en peso, se añadieron gota a gota a la

mezcla formada por el TTIP y el HAc durante la síntesis sol-gel del TiO2.

Seguidamente, la cantidad restante de H2O (hasta alcanzar los 197.5 mL

necesarios para la síntesis) se adicionó gota a gota, y se continuó con el

Mezclamos

TTIP (9.3 ml)

HAc (17.5 ml)

Baño a 0 ºC

Añadimos

H2O (197.5 ml)

Gota a gota

Ultrasonidos

30 min

Estufa70 ºC, 12 h100 ºC, 12 h

Agitación

5 h

TiO2

1

2

Agitación

1 h

3

4

5

6

-----

40 Capítulo 2

procedimiento sol-gel como se ha descrito anteriormente. La nomenclatura de las

muestras obtenidas es la siguiente: Cu/TiO2isx; donde is indica que el método de

preparación es in situ y x la carga nominal de Cu (0.5, 1, 2, 5, 7 ó 10 % en peso)

(Esquema 2.4). Finalmente, los catalizadores se trataron térmicamente, como se

detalla a continuación.

Tanto las muestras preparadas por impregnación (Cu/TiO2imx) como las

obtenidas por el método in situ (Cu/TiO2isx) fueron sometidas a un tratamiento

térmico con el fin de aumentar la cristalinidad de las muestras [5], eliminar las

impurezas remanentes del proceso de síntesis [2] y fortalecer la unión metal-TiO2

[3]. El tratamiento térmico se realizó en un reactor de cuarzo de lecho fijo en

flujo (90 mL/min) de aire o de Ar. El horno se calentó a 5 ºC/min hasta 500 ºC y

esta temperatura se mantuvo 2 h. La nomenclatura de las muestras tratadas

incluye Ar o air para indicar la atmósfera del tratamiento empleada en cada caso.

Esquema 2.3. Procedimiento de preparación de los fotocatalizadores

Cu/TiO2imxAr y Cu/TiO2imxair, dónde im indica impregnación, x la carga nominal

de Cu (0.5, 1 ó 10 % en peso) y Ar o air la atmósfera del tratamiento térmico.

Esquema 2.4. Procedimiento de preparación de los fotocatalizadores

Cu/TiO2isxAr y Cu/TiO2isxair, dónde is indica in situ, x la carga nominal de Cu (0.5,

1, 2, 5, 7 ó 10% en peso) y Ar o air la atmósfera del tratamiento térmico.

Mezclamos

TiO2 (2 g)

H2O (10 ml)

Añadimos

H2O (5 ml)

Cu(NO3)2

Ultrasonidos

30 min

Horno Ar/aire500 ºC2 h

Estufa

80 ºC

24 h

1

2

Agitación

2 h3

4

5

6

-

---

-

Ar/

aire

Cu/TiO2imx-Ar

Cu/TiO2imx-air

Mezclamos

TTIP (9.3 ml)

HAc (17.5 ml)

Baño a 0 ºC

Añadimos

H2O (5 ml)

Cu(NO3)2

Gota a gota

Ultrasonidos

30 min


Agitación

5 h

Cu/TiO2isx-Ar

Cu/TiO2isx-air

1

2 Agitación

1 h

3

4

5

6

Añadimos

H2O (197.5 ml)

Gota a gota

7

Horno Ar/aire

500 ºC

2 h

8

Ar/

aire

-----


2.2.4. Preparación de los catalizadores TiO2-AC

Para sintetizar la serie de catalizadores TiO2-AC en primer lugar se llevó

a cabo la preparación del carbón activado (AC) mediante carbonización

hidrotermal de sacarosa. Para ello se preparó una disolución 1.6 M de sacarosa

empleando 12 g de sacarosa y 21 mL de H2O. Esta disolución se introdujo en un

autoclave de acero inoxidable recubierto de teflón y se sometió a un tratamiento

térmico a 180 ºC durante 12 h. Pasado este tiempo, el contenido del autoclave se

filtró y se lavó varias veces con agua desionizada hasta alcanzar un pH neutro en

el agua de lavado. El sólido lavado remanente en el filtro se recogió, se secó

durante 5 h a 110 ºC y se pesó para calcular el rendimiento del proceso de

carbonización.

El material obtenido mediante carbonización hidrotermal de la sacarosa se

activó con CO2 (flujo de 80 mL/min) en un horno horizontal, calentando a 5

ºC/min, hasta 800 ºC y manteniendo esta temperatura durante 10 h. El carbón

activado resultante se pesó para calcular el rendimiento del proceso de activación.

Esquema 2.5. Procedimiento de preparación del carbón activado mediante

carbonización hidrotermal de sacarosa y activación con CO2.

Para preparar los catalizadores TiO2-AC, el dióxido de titanio se sintetizó

mediante el método sol-gel (apartado 2.2.2.) durante el cual, con la adición de las

primeras gotas de H2O, se incorporó también la cantidad necesaria del AC para

obtener contenidos de carbón (nominales) en el catalizador de 0.5, 1, 5 y 10 %

en peso. Una vez incorporado el AC, se adicionó gota a gota la cantidad de agua

restante hasta alcanzar los 197.5 mL y se prosiguió con la síntesis sol-gel, tal y

como se detalla en el apartado 2.2.2. La nomenclatura utilizada para esta serie de

muestras es TiO2/ACX, donde X indica el porcentaje en peso de AC añadido

durante la síntesis.

Mezclamos en

Autoclave

Sacarosa (12 g)

H2O (21 ml)

Estufa

180 ºC

12 h

Estufa

110 ºC

5 h

Activación

CO2 80 ml/min

800 ºC

10 h

AC

Filtrado y

lavado

Con H2O

1 3 5

2 4

42 Capítulo 2

Todas las muestras obtenidas se han tratado térmicamente en una mufla en

atmósfera de aire, utilizando una rampa de calentamiento de 10 ºC/min hasta la

temperatura de tratamiento (350, 400, 450 ó 500 ºC), la cual se ha mantenido

durante 2 h. Las muestras tratadas se nombran TiO2/ACX (T) donde T es la

temperatura a la cual han sido tratadas.

Esquema 2.6. Preparación de los fotocatalizadores TiO2/ACX (T), donde X es

la concentración de AC en peso, y T la temperatura del tratamiento térmico.

2.3. Técnicas de caracterización y análisis

A continuación, se describen las distintas técnicas de caracterización

empleadas para determinar las principales propiedades de los fotocatalizadores

preparados. El estudio de estas propiedades permitirá interpretar mejor los

resultados de las reacciones fotocatalíticas llevadas a cabo. Además, se

presentarán las técnicas empleadas para analizar la evolución de dichas

reacciones.

La mayoría de los equipos empleados en este trabajo se encuentran en la

Universidad de Alicante, bien en los laboratorios del grupo de investigación

Materiales Carbonosos y Medio Ambiente del Departamento de Química

Inorgánica-IUMA (MCMA-UA), o bien en los Servicios Técnicos de

Investigación de la propia universidad (SSTTI-UA). Solamente algunas de las

técnicas y equipos utilizados se encuentran en los laboratorios del Institut de

Chimie et Procédés pour l'Énergie, l'Environnement et la Santé de la Universidad

de Estrasburgo (ICPEES-UniStra). Se hará uso de las siglas indicadas aquí para

especificar la localización de los equipos que a continuación se describen.

2.3.1. Adsorción física de gases

Las técnicas de adsorción física de gases son las más utilizadas en la

caracterización de sólidos porosos [6]. El fenómeno de adsorción ocurre cuando

Mezclamos

TTIP (9.3 ml)

HAc (17.5 ml)

Baño a 0 ºC

Añadimos

H2O (5 ml)

AC

Gota a gota

Ultrasonidos

30 min


Agitación

5 h

TiO2-ACX (T)

1

2 Agitación

1 h

3

4

5

6

Añadimos

H2O (197.5 ml)

Gota a gota

7

Mufla aire

350, 400,

450 ó 500 ºC

2 h

8

aire---

--


un gas (adsorbato) entra en contacto con un sólido poroso (adsorbente) en un

espacio cerrado, a una determinada presión y temperatura, y el sólido adsorbe el

gas incrementando su peso y disminuyendo la presión en el recipiente. Una vez

alcanzado el equilibrio, la presión se mantiene constante. La relación entre las

moléculas adsorbidas y la presión a temperatura constante se conoce como

isoterma de adsorción [7]. Una isoterma de adsorción aporta información sobre

el volumen de gas adsorbido a una determinada presión y permite calcular el área

superficial del sólido empleando un modelo determinado. La existencia de

características comunes entre las isotermas de algunos sólidos con propiedades

superficiales similares ha originado una clasificación de isotermas, propuesta por

la IUPAC [8]. Según ésta, pueden encontrarse isotermas de 6 tipologías

diferentes (Figura 2.1a). Existe, además, una clasificación de ciclos de histéresis

según su forma (Figura 2.1b).

Figura 2.1. Representación esquemática de (a) los seis tipos de isotermas de

adsorción y (b) los cinco tipos de ciclos de histéresis, según la clasificación de la

IUPAC [8].

El análisis de estas curvas mediante distintas teorías permite calcular

varios parámetros que definen las propiedades texturales de los sólidos [9–12].

Cabe destacar que, aunque pueden usarse distintos adsorbatos y condiciones de

adsorción, habitualmente se emplea la adsorción de N2 a -196 °C [12,13]. Sin

embargo, ésta presenta algunos inconvenientes para la caracterización de la

Presión relativa, P/P0

Can

tid

ad a

dso

rbid

a, n

(m

ol/

g)

IV(b)

V VI

IV(a)

II III

I(a) I(b)

Presión relativa, P/P0

Can

tid

ad a

dso

rbid

a, n

(m

ol/

g)

H1 H2(a) H2(b)

H3 H4 H5

(a)

(b)

44 Capítulo 2

porosidad más estrecha, debido a los problemas difusionales que se generan a

temperatura criogénica en los microporos de tamaños inferiores a 0.4 nm [11,14].

Como alternativa complementaria se emplea la adsorción de CO2 a 0 °C. Aunque

el N2 y el CO2 poseen dimensiones moleculares relativamente próximas (0.364 y

0.330 nm de diámetro cinético, respectivamente), la diferencia de temperatura

durante la adsorción (-196 ºC para el N2 respecto a 0 ºC para el CO2) implica una

cinética de adsorción más rápida para el dióxido de carbono, evitándose así los

problemas difusionales [10,11].

A continuación, se describen los parámetros más característicos obtenidos

a partir de las isotermas de adsorción, así como las ecuaciones necesarias para

llevar a cabo el cálculo.

⁖ Superficie específica BET.

La superficie específica (SBET) de los materiales estudiados se ha obtenido

mediante la teoría de Brunauer, Emmett y Teller (BET) [13]. Para ello, la

ecuación BET se aplica a los datos obtenidos a partir de la isoterma de

adsorción de N2 a -196 °C (Ecuación 2.1), la cual suele cumplirse en el

rango de presiones relativas (P/P0) entre 0.05 y 0.30 [7].

PP0

n (1-PP0

)=

1

nmC+

C-1

nmC(

P

P0

) (2.1)

En esta ecuación P es la presión del gas, P0 es la presión de vapor de

saturación, n es el número de moléculas de gas adsorbidas a una presión

relativa P/P0, nm es el número de moléculas necesarias para la formación

de una monocapa y C es un parámetro relacionado con el calor de

adsorción.

Al representar [P/P0]/[n(1-P/P0)] frente a P/P0 se obtiene una recta de cuya

ordenada en el origen y pendiente se determinan los parámetros

característicos de la ecuación BET: nm y C, respectivamente. A partir del

término nm se puede obtener el valor de la superficie específica del sólido

aplicando la siguiente expresión:

S = nmamNA10-21 (2.2)

dónde S es la superficie aparente del sólido adsorbente, expresada en m2/g,

am es el área media ocupada por la molécula de adsorbato, que para el caso

del N2 a -196 ºC es de 0.162 nm2 y NA es el número de Avogadro.


⁖ Volumen total de microporos (ø < 2 nm).

El volumen total de microporos se calcula aplicando la ecuación de

Dubinin-Radushkevich [15] a los datos de la isoterma de adsorción de N2

a -196 °C. Esta ecuación está basada en la teoría del potencial de Polanyi,

en la que se supone que la condensación del gas en los microporos ocurre

en forma de capas equipotenciales.

V

V0

= exp [(-1

(E0β)2) ∙ (RTln (

P0

P))

2

] (2.3)

En esta ecuación V es el volumen adsorbido a una presión P, V0 es el

volumen total de microporos del sólido, E0 es la energía característica,

dependiente de la estructura del poro, β es el coeficiente de afinidad,

característico del adsortivo y P0 es la presión de saturación del adsortivo a

la temperatura de trabajo [16].

Representando ln(V) frente a ln(P0/P)2 se obtiene el volumen total de

microporos (V0).

⁖ Volumen de mesoporos (2 < ø < 50 nm).

El volumen de mesoporos se ha estimado por diferencia entre el volumen

de N2 adsorbido a P/P0 = 0.9 y P/P0 = 0.2 (expresado como líquido) [17].

Mediante esta técnica existe una limitación práctica, sólo se determinan

los mesoporos con tamaño de 2 hasta 20 nm. Para tamaños mayores se

recomienda usar la porosimetría de mercurio [18].

⁖ Volumen total de poros.

Cuando la isoterma es de tipo IV y, por tanto tiene un límite de adsorción,

este límite se puede tomar como el correspondiente al volumen total de

poros y, en general, se puede determinar a partir de la cantidad de N2

adsorbida a una presión relativa de 0.99 [19], expresándose como volumen

de líquido.

La caracterización textural de los materiales del presente trabajo se ha

realizado mediante isotermas de adsorción/desorción de N2 a -196 °C y de

adsorción de CO2 a 0 °C. Se emplearon dos equipos automáticos de adsorción

física de gases de tipo volumétrico, AUTOSORB-6 y AUTOSORB-6B, que se

encuentran en los laboratorios de MCMA-UA (Figura 2.2). Para obtener las

isotermas de adsorción se emplearon aproximadamente 0.1 g de muestra, que

previamente se desgasificaron a vacío durante 4 h a 250 °C. El análisis de las

isotermas mediante los modelos BET y Dubinin-Raduskevich se ha realizado

mediante el software Autosorb 1 para Windows, proporcionado por

Quantachrome Corporation.

46 Capítulo 2

Figura 2.2. Equipos de adsorción física de gases AUTOSORB-6 y

AUTOSORB-6B.

2.3.2. Difracción de rayos X

La difracción de rayos X (DRX) está basada en la interacción de un haz de

rayos X con la nube electrónica de los átomos de un sólido cristalino cuyos

parámetros de celda son del orden de magnitud de la longitud de onda de la

radiación incidente. Parte de esta radiación es absorbida y, posteriormente

devuelta en forma de radiación dispersada en todas las direcciones del espacio.

Las distintas radiaciones dispersadas sufren fenómenos de interferencia que,

únicamente, son constructivas en direcciones muy bien definidas, dando lugar al

difractograma del cristal [20].

Las condiciones necesarias para que se produzca la difracción vienen

determinadas por la ley de Bragg [21]. Se asume que una sustancia cristalina se

basa en distintas familias de planos paralelos y equidistantes entre sí. Cada una

de estas familias tiene designado un índice de Miller (hkl) y un espaciado dhkl. Si

sobre estos planos incide un haz de rayos X monocromático, con una longitud de

onda λ, en una dirección que forma un ángulo θ con la superficie de los planos,

sólo se producirá difracción cuando el ángulo de incidencia, la longitud de onda

de la radiación y el espaciado de la familia de planos cumplan la relación de la

ley de Bragg (Ecuación 2.4):

λ = 2dhklsenθ (2.4)

siendo λ la longitud de onda del haz incidente y θ el ángulo al que aparece el

máximo de difracción. A partir de la ecuación de Scherrer (Ecuación 2.5) se

puede estimar el tamaño medio de los cristales. Así, esta ecuación relaciona el


tamaño medio de los cristales (B) con la anchura a mitad de altura del pico de

intensidad principal [22].

B = Kλ

βcosθ (2.5)

En esta ecuación β es la anchura del pico a media altura, expresada en radianes,

y K una constante cuyo valor es de 0.9 para las condiciones de operación

utilizadas [22]. La cantidad de cada fase cristalina presente en los materiales se

ha determinado utilizando el procedimiento reportado por Jensen y col. [23]. Para

ello se analiza el espectro de DRX de una muestra preparada a partir de una

mezcla compuesta por un 50% en peso de un material de referencia 100%

cristalino, CaF2, y un 50% en peso del TiO2 a estudiar. Los autores indican que

los porcentajes de anatasa (ACryst) y rutilo (RCryst) se pueden calcular a partir de

las siguientes ecuaciones:

ACryst =

AAnatase(101)

ACaF2(220) x 100

1.25

(2.6)

RCryst =

ARutile(110)

ACaF2(220) x 100

0.90

(2.7)

dónde AAnatase(101), ARutile(110) y ACaF2(220) son las áreas obtenidas de los picos del

difractrograma de rayos X de la mezcla TiO2/CaF2. Los parámetros 1.25

(Ecuación 2.6) y 0.9 (Ecuación 2.7) son la relación entre las áreas de los mismos

picos cuando se utilizan muestras de anatasa o rutilo 100% cristalinos,

respectivamente. El porcentaje de fase cristalina (WCryst) y fase amorfa (WAm)

presentes en las muestras de TiO2 se han determinado como indican las siguientes

expresiones:

WCryst = ACryst + RCryst (2.8)

WAm = 100 - WCryst (2.9)

Esta técnica ha sido empleada en el presente trabajo para identificar las

diferentes fases cristalinas, determinar el tamaño de cristal y cuantificar los

porcentajes de las fases cristalinas y de la fracción de TiO2 amorfo presentes en

los materiales objeto de estudio.

Las medidas de DRX se realizaron en el intervalo 2 = 6 - 80°, con una

velocidad de barrido de 2 °/min y utilizando la radiación CuKα, λ=1.54056 Å. El

equipo utilizado es un difractómetro Miniflex II Rigaku, 30 kV/15 mA que se

48 Capítulo 2

encuentra en los laboratorios del Departamento de Química Inorgánica de la UA

(Figura 2.3).

Figura 2.3. Equipo de difracción de Rayos X Miniflex II Rigaku.

2.3.3. Espectroscopia fotoelectrónica de rayos X.

La espectroscopia fotoelectrónica de rayos X (más conocida como XPS,

por sus siglas en inglés) es una técnica muy empleada en la caracterización

superficial de materiales debido a su bajo poder de penetración (entre 1 y 3 nm).

Esta técnica se basa en la medición de la energía cinética de electrones emitidos

desde niveles internos de los átomos que han sido excitados mediante la

aplicación de un haz de rayos X monoenergético en la superficie del material. Al

irradiar la muestra con un haz de rayos X, parte de la energía aportada se invierte

en liberar electrones, quedando los átomos superficiales parcialmente ionizados.

Este proceso se conoce como efecto fotoeléctrico [24]. El átomo así excitado

recupera su estado fundamental cuando los electrones de las capas superiores

pasan a ocupar los huecos generados en las capas más internas. Al incidir la

radiación X sobre la muestra, ésta liberará fotoelectrones con una energía cinética

determinada y característica de los elementos que la componen. El valor de esta

energía cinética permite determinar la composición elemental del material y la

concentración de cada elemento en él [25]. La velocidad de los electrones

emitidos se cuantifica mediante un espectrómetro. En un espectro típico de XPS

se representa el número de electrones detectados frente a la energía de enlace que

poseen dichos electrones.

El equipo empleado es un espectrómetro VG-Microtech Multilab 3000

equipado con un analizador de electrones semiesférico y una fuente de radiación

de Rayos X Mg-Kα 300W y se encuentra en STTII-UA (Figura 2.4). Antes de la

adquisición del espectro, las muestras se han mantenido en la cámara de análisis


hasta alcanzar una presión residual de 5·10-7 Nm-2. Los espectros se tomaron con

una energía de paso constante de 50 eV.

Figura 2.4. Equipo de espectroscopia fotoelectrónica de rayos X.

2.3.4. Termogravimetría.

La termogravimetría (TG) es una técnica de análisis térmico que se basa

en la determinación de las variaciones de masa de una muestra cuando ésta se

somete a un determinado programa de temperaturas, en una atmósfera

seleccionada. Es una de las técnicas de análisis térmico más utilizadas ya que

permite, entre otras cosas, determinar la estabilidad y/o descomposición térmica

de sustancias orgánicas e inorgánicas, polímeros, etc. [26].

Un equipo de análisis térmico se compone, de forma general, de un sistema

de control de gases que permite seleccionar la naturaleza de la atmósfera del

experimento, una balanza, un horno, un controlador de temperatura y un sistema

de adquisición de datos. Las medidas pueden realizarse tanto a temperatura

constante (experimento isotermo), enfriando o calentando a velocidad constante,

o en cualquier combinación de ellos. La atmósfera en la que se realiza el

experimento puede ser estática o dinámica (en flujo de gas) y, por tanto, es

posible descomponer especies en gas inerte (He, N2) o llevar a cabo reacciones

químicas utilizando una mezcla de gases reactiva (aire, O2). La velocidad de

calentamiento, el tamaño de partícula de la muestra, la cantidad de muestra, el

grado de empaquetamiento de la muestra, la atmósfera y el caudal del gas

empleado son factores que influyen en los resultados obtenidos y en la forma de

las curvas de TG.

Esta técnica permite detectar procesos que llevan asociados una variación

de masa tales como descomposición, sublimación, reducción, desorción,

50 Capítulo 2

adsorción o absorción. Las aplicaciones de esta técnica son innumerables,

habiéndose utilizado tanto para análisis cualitativos como cuantitativos [27].

En este trabajo la termogravimetría se ha empleado, principalmente, para

determinar la cantidad de carbón presente en la serie de muestras TiO2/AC. Para

ello, 10 mg de muestra se han calentado en una termobalanza a una velocidad de

10 °C/min en aire (50 mL/min) desde temperatura ambiente hasta 900 °C. El

equipo utilizado ha sido la termobalanza TG SDT Q600 (TA Instruments), que

se encuentra en los laboratorios de MCMA-UA (Figura 2.5).

Figura 2.5. Termobalanza TG-DSC SDTQ600 de TA Instruments.

2.3.5. Microscopía electrónica de barrido.

La microscopía electrónica de barrido (conocida habitualmente por sus

siglas en inglés, SEM, Scanning Electron Microscopy) permite obtener

información morfológica y topográfica de la superficie del sólido [28]. La imagen

de la superficie del material se forma al iluminar la muestra con un haz de

electrones de alta energía que origina señales que provienen de electrones

retrodispersados y/o electrones secundarios (electrones que escapan de la

muestra con energías inferiores a 50 eV) [29].

La emisión de electrones retrodispersados depende fuertemente del

número atómico de la muestra. Esto implica que dos zonas de la misma muestra

que tengan distinta composición se revelan con distinta intensidad, aunque no

exista ninguna diferencia de topografía entre ellas. Estos electrones generalmente

se encuentran a una pequeña distancia de la superficie y han recibido una

transferencia de energía mediante algún proceso de dispersión inelástica [28].

Esta dispersión inelástica puede, a su vez, generar rayos X como consecuencia

de la relajación de un átomo que ha sido excitado. Puesto que los rayos X


generados son característicos de cada átomo, es posible conocer la composición

elemental de las distintas partes de la muestra mediante espectroscopia de

fluorescencia de rayos X basada en dispersión de energía (EDX).

El equipo utilizado para obtener las imágenes SEM es un microscopio

JEOL JSM-840 (Figura 2.6a). Las imágenes SEM-EDX se realizaron con un

microscopio Hitachi S3000N y el espectrómetro de energía dispersiva de rayos

X utilizado fue el Bruker XFlash 3001 (Figura 2.6b). Ambos microscopios se

encuentran en los laboratorios STTII-UA.

.

Figura 2.6. (a) Microscopio electrónico de barrido (SEM) y (b) microscopio

electrónico de barrido con detector de rayos X (SEM-EDX).

2.3.6. Microscopía electrónica de transmisión.

La microscopía electrónica de transmisión (TEM, del inglés Transmission

Electron Microscopy) es una técnica de caracterización que también proporciona

(al igual que el SEM) información valiosa acerca de las características

estructurales, morfología y composición de las muestras.

Esta técnica consiste en irradiar una fina película de muestra con un haz

de electrones de densidad de corriente uniforme con una energía elevada (de 100

keV o superior). A diferencia de la microscopía electrónica de barrido, el análisis

de la muestra se realiza a partir de los electrones que atraviesan la muestra. Estos

electrones pueden sufrir dispersión al interaccionar con la muestra (elástica o

inelástica, en función de que se modifique o no su energía) o no experimentar

cambio alguno en su trayectoria. Los electrones dispersados elásticamente son

los responsables de la formación de las imágenes de difracción, los no

dispersados forman imágenes directas del material y los dispersados de forma

(a) (b)

52 Capítulo 2

inelástica son los responsables del ruido de fondo presente en toda imagen de

microscopía electrónica.

El microscopio electrónico de transmisión que se ha empleado en este

trabajo es un microscopio JEOL, modelo JEM-2010, que se encuentra ubicado

en los laboratorios STTII-UA (Figura 2.7).

Figura 2.7. Microscopio electrónico de transmisión JEOL modelo JEM-2010.

2.3.7. Espectroscopia ultravioleta visible.

La espectroscopia ultravioleta-visible (UV-vis) es una técnica que permite

la identificación, determinación y cuantificación de un amplio número de

especies, tanto orgánicas como inorgánicas, a partir de la medida de absorción de

radiación ultravioleta-visible del analito en disolución [30]. Además, una

variante de esta técnica, llamada espectroscopia UV-vis por reflectancia difusa,

se utiliza para la determinación de la energía de banda prohibida de los

semiconductores sólidos.

Experimentalmente, cuando la radiación electromagnética monocromática

de longitud de onda λ e intensidad I0 incide sobre las moléculas presentes en una

muestra, éstas solamente absorben una parte de la energía incidente (luz

absorbida). La energía restante puede sufrir varios procesos simultáneos: puede

atravesar la muestra sin variar su dirección (luz transmitida), puede atravesar la

muestra sufriendo cambios en su dirección (luz refractada), o puede no llegar a

penetrar en la muestra y ser reflejada (luz reflejada). Cuando la luz es reflejada

por una superficie lisa y pulida, ésta se refleja en una sola dirección (reflexión

especular). Si, por el contrario, la luz es reflejada por una superficie irregular,

esta luz se refleja en todas las direcciones (reflexión difusa) (Figura 2.8). La


extensión en la que ocurren estos fenómenos depende de la naturaleza del analito

(si se encuentra en disolución o en fase sólida).

Figura 2.8. Esquema de la medida de absorbancia de una disolución.

Cuando un analito en disolución es irradiado, los grupos cromóforos

(grupos funcionales de una molécula excitables por la absorción de radiación

luminosa UV-visible) presentes en él realizan transiciones electrónicas pasando

a un estado excitado (Figura 2.9). Cuando las moléculas vuelven al estado

fundamental, pierden la energía absorbida en forma de energía cinética sin emitir

radiación. El haz emergente de intensidad I tiene la misma longitud de onda que

el incidente [31] (Figura 2.8).

Figura 2.9. Regiones ultravioleta y visible del espectro y tipos de bandas de

absorción más frecuentes.

Luz incidente, I0

Fuente

de luz

Radiación

policromática

Monocromador

υ, I0

Detector

Muestra

υ, I

Luz

absorbida

Luz incidente

Luz reflejada

Reflexión difusa

Luz transmitida

Luz refractada

Luz dispersada

Reflexión especular

54 Capítulo 2

La intensidad de luz transmitida se cuantifica mediante la ley de Beer

(Ecuación 2.10) [32].

I = I0 e-ε cl (2.10)

dónde l es la longitud de paso o paso óptico de la celda, c la concentración de la

muestra y ε es el coeficiente de extinción molar a la longitud de onda determinada.

Su valor representa la probabilidad de absorción de la radiación.

El análisis de una muestra se realiza midiendo las magnitudes de

transmitancia o absorbancia, que se definen como:

⁖ Transmitancia (T).

La transmitancia o porcentaje de luz transmitida es una magnitud

adimensional cuyo valor depende de la concentración de soluto según una

relación de tipo exponencial (T = 10-ε cl).

T = I

I0

x 100 (2.11)

⁖ Absorbancia (A).

La absorbancia, expresada como log (I0/I) y determinada mediante la

ecuación de Lambert-Beer (Ecuación 2.12), permite definir el coeficiente

de extinción molar como la absorbancia de una disolución 1 M de soluto

con un paso óptico de 1 cm.

A = εcl (2.12)

La espectroscopia UV-vis se utiliza habitualmente en la determinación

cuantitativa de iones de metales de transición y compuestos orgánicos

conjugados en disolución. Esta técnica se ha empleado en el presente trabajo para

determinar la concentración de diurón. La molécula de diurón produce su

máximo de absorción a una longitud de onda de 246 nm como consecuencia de

la transición π → π* generada por el sistema de electrones π aromáticos de su

estructura [33]. El equipo empleado es un espectrofotómetro (Cary 100Scan

Varian) ubicado en los laboratorios de ICPEES-UniStra (Figura 2.10). Los

espectros de absorción han sido registrados en el intervalo de longitudes de onda

de 200-400 nm.


Figura 2.10. Espectrofotómetro UV-vis empleado para la cuantificación de

diurón.

Cuando el analito es un sólido, la reflexión difusa o reflectancia difusa

(Figura 2.8) cobra importancia. Éste fenómeno aporta información acerca de la

transición de electrones desde la banda de valencia (BV) a la banda de

conducción (BC), lo que permite el cálculo de la energía de banda prohibida (Eg)

en semiconductores. La espectroscopia UV-vis por reflectancia difusa se ha

utilizado en este trabajo para determinar la energía de banda prohibida (Eg) de

los fotocatalizadores.

El cálculo de Eg se ha llevado a cabo mediante 3 diferentes métodos: por

el método de absorbancia, considerando las transiciones permitidas directas

(método directo) y las transiciones permitidas indirectas (método indirecto).

⁖ Método de absorbancia.

En este método se representa la absorbancia frente a la longitud onda de la

radiación. La longitud de onda del borde de absorción se estima mediante

la intersección de la extrapolación de la recta de mayor pendiente con el

eje x (Figura 2.11a). La energía de la banda prohibida (Eg, en eV) se

obtiene a partir de la siguiente ecuación [34]:

Eg = 1239.8

λ (2.13)

dónde 1239.8 (en eV·nm) es el producto resultante de la constante de

Planck (h, en eV·s) y la velocidad de la luz (c, en nm·s-1) y λ (en nm) es la

longitud de onda del borde de absorción [35].

56 Capítulo 2

⁖ Método directo.

En él se representa (F(R)hν)2 (dónde F(R) es la función Kubelka-Munk de

la reflectancia, h la constante de Planck, en eV·s, y ν la frecuencia, en s-1)

frente a hν. El valor de Eg se obtiene a partir la extrapolación de la parte

lineal de la representación, que corresponde a una transición electrónica

directa (Figura 2.11b) [36].

⁖ Método indirecto.

En este caso, la Eg se obtiene empleando la función Kubelka-Munk de

reflectancia a partir de la extrapolación de la parte lineal de la

representación (F(R)hν)1/2 vs hν, que corresponde a una transición

electrónica indirecta (Figura 2.11c) [36].

R² = 0.9993

Absorbancia = -0.0088λ + 3.6702

λ (Abs = 0) ₌ 3.6702 ₌ 417.07 nm

0.0088....

Eg ₌ 1239.8 ₌ 2.97 eV

417.07.......

0

0.2

0.4

0.6

0.8

200 300 400 500 600 700

Ab

so

rban

cia

(u

nid

ad

es a

rbit

rari

as)

λ (nm)

R² = 0.9901

(F(R)hν)2 = 79.965hν - 268.54

Eg ₌ 268.54 ₌ 3.36 eV

79.965.........

0

20

40

60

80

100

120

2 3 4 5 6

(F(R

)hν)2

hν (eV)

R² = 0.9992

(F(R)hν)1/2 = 3.423hν - 10.175

Eg ₌ 10.175 ₌ 2.97 eV

3.423.........

0.0

0.5

1.0

1.5

2.0

2.5

3.0

2 3 4 5 6

(F(R

)hν)1

/2

hν (eV)

(a)

(b)


Figura 2.11. Cálculo de Eg por el método: (a) de absorbancia, (b) directo e (c)

indirecto para la muestra P25.

Los espectros de absorción UV-vis por reflectancia difusa de los

fotocatalizadores se han obtenido utilizando un espectrofotómetro Jasco V-670,

que se encuentra disponible en los laboratorios del Departamento de Química

Física de la UA (Figura 2.12). Este equipo está provisto de un accesorio llamado

esfera integradora; consiste en una esfera hueca recubierta en su interior de un

material altamente reflectante que envía la luz reflejada por la muestra al detector,

permitiendo el análisis de muestras sólidas. Como patrón de referencia se ha

utilizado sulfato de bario (BaSO4). Los espectros de absorción han sido

registrados en el intervalo de longitudes de onda de 200-800 nm.

Figura 2.12. Espectrofotómetro UV-vis utilizado para el estudio de la energía

de banda prohibida Eg de los fotocatalizadores.

R² = 0.9993

Absorbancia = -0.0088λ + 3.6702

λ (Abs = 0) ₌ 3.6702 ₌ 417.07 nm

0.0088....

Eg ₌ 1239.8 ₌ 2.97 eV

417.07.......

0

0.2

0.4

0.6

0.8

200 300 400 500 600 700

Ab

so

rban

cia

(u

nid

ad

es a

rbit

rari

as)

λ (nm)

R² = 0.9901

(F(R)hν)2 = 79.965hν - 268.54

Eg ₌ 268.54 ₌ 3.36 eV

79.965.........

0

20

40

60

80

100

120

2 3 4 5 6

(F(R

)hν)2

hν (eV)

R² = 0.9992

(F(R)hν)1/2 = 3.423hν - 10.175

Eg ₌ 10.175 ₌ 2.97 eV

3.423.........

0.0

0.5

1.0

1.5

2.0

2.5

3.0

2 3 4 5 6

(F(R

)hν)1

/2

hν (eV)

(c)

58 Capítulo 2

2.3.8. Espectrometría de masas.

El método más exacto y directo para determinar masas atómicas y

moleculares es la espectrometría de masas. En un espectrómetro de masas (Figura

2.13) se bombardea una muestra en estado gaseoso con un haz de electrones de

alta energía en el interior de una cámara que se encuentra en condiciones de alto

vacío (de 10-8 a 10-4 torr). Las colisiones entre los electrones y los átomos o

moléculas en estado gaseoso producen iones positivos (de masa m y carga z) los

cuales se aceleran al pasar entre dos placas con cargas opuestas. Los iones

acelerados son desviados, por un imán, en una trayectoria circular, cuyo radio

depende de la relación m/z. Los iones con menor relación m/z describen una curva

con menor radio que los iones que tienen una relación m/z mayor, de manera que

se pueden separar los iones con cargas iguales pero distintas masas. La masa de

cada ion (y por tanto del átomo o molécula original) se determina por la magnitud

de su desviación. Por último, los iones llegan al detector que registra la corriente

para cada tipo de ion, de manera que la intensidad de la corriente es directamente

proporcional al número de iones [37].

Figura 2.13. Diagrama esquemático de un espectrómetro de masas.

En esta Tesis Doctoral la espectrometría de masas se ha utilizado como

sistema de análisis de los productos de dos de las reacciones estudiadas: la

descomposición fotocatalítica de ácido acético en disolución acuosa y la

oxidación fotocatalítica de propeno en fase gas. En la primera de ellas, el

espectrómetro de masas se ha empleado para la identificación y cuantificación

de los principales productos gaseosos generados en la reacción (CH4, CO2 e H2).

Para ello, se ha utilizado el espectrómetro OmniStar, modelo GSD 300 O1, de

Pfeiffer Vaccum (Figura 2.14a). En la segunda, esta técnica se ha empleado para

identificar los productos de la reacción y medir la variación en la concentración

de propeno en la corriente de gas empleada. El equipo utilizado en este caso es

el OmniStar, modelo GSD 301 O1, de Pfeiffer Vaccuum (Figura 2.14b). Ambos


espectrómetros de masas se encuentran ubicados en los laboratorios de MCMA-

UA.

Figura 2.14. Espectrómetros de masas utilizados en el estudio de la actividad

fotocatalítica de: (a) la descomposición de ácido acético y (b) la oxidación de propeno.

2.3.9. Carbono orgánico total.

El carbono orgánico total (TOC por sus siglas en inglés: total organic

carbon) es la cantidad de carbono presente en compuestos orgánicos y se utiliza

frecuentemente para cuantificar la materia orgánica presente en disoluciones

acuosas.

En un análisis típico, se determina tanto el carbono total (TC, total carbon)

presente en la muestra como el carbono inorgánico total (TIC, total inorganic

carbon) para, finalmente por diferencia, obtener el TOC. Para determinar el TC

se agregan a la muestra ácido fosfórico y un oxidante (persulfato de sodio). La

mezcla se calienta a la vez que se irradia con luz ultravioleta para convertir el

carbono presente en la muestra en CO2. Por otro lado, para determinar el TIC la

muestra se acidifica con ácido fosfórico y se burbujea un gas de arrastre para

convertir el carbono inorgánico presente en CO2. En ambos casos, el CO2

generado se analiza con un detector infrarrojo y se convierte en concentración de

TC o TIC utilizando curvas de calibración (Esquemas 2.7a y 2.7b,

respectivamente). Finalmente, la concentración de TOC se obtiene sustrayendo

la concentración de TIC de la de TC (Esquema 2.7c) [38].

La medida del TOC se realizó para estudiar la desorción de diurón

previamente adsorbido en el fotocatalizador. Las medidas de TOC se realizaron

en un analizador Shimadzu TOC-L ubicado en los laboratorios de ICPEES-

UniStra (Figura 2.15).

(a) (b)

60 Capítulo 2

(a) Determinación de TC (carbono total)

aire

purificado CO2 generado

Muestra combustión detección

(b) Determinación de TIC (carbono inorgánico total)

ácido CO2 aislado

Muestra burbujeo detección

(c) Determinación de TOC (carbono orgánico total)

TOC = TC – TIC

Esquema 2.7. Procedimiento para determinar: (a) TC, (b), TIC y (c) TOC.

Figura 2.15. Analizador de carbono orgánico total (TOC).

2.3.10. Cromatografía de intercambio iónico.

La cromatografía de intercambio iónico (o cromatografía iónica) es una

técnica basada en las propiedades de carga de las moléculas que permite la

separación, identificación y cuantificación de iones y moléculas polares. Esto

hace que esta técnica sea ideal para el análisis de compuestos iónicos con un

amplio intervalo de pesos moleculares [39].

En cromatografía iónica, los iones del analito se introducen en la cabeza

de la columna cromatográfica empaquetada con una resina de intercambio iónico

adecuada (fase estacionaria). La elución se lleva a cabo después con una


disolución que contiene un ion (fase móvil) que compite con los iones del analito

por los grupos cargados de la superficie de la resina. Puesto que los cocientes de

reparto de los iones del analito difieren entre sí, durante la elución tiene lugar su

fraccionamiento. El tiempo que tarda una muestra en atravesar la columna y

alcanzar el detector se conoce como tiempo de retención y se utiliza para

identificar el compuesto [40].

Existen dos tipos de cromatografía de intercambio iónico dependiendo de

la carga iónica del analito: cromatografía de intercambio catiónico, en la cual la

molécula de interés está cargada positivamente y la fase estacionaria está cargada

negativamente; y de intercambio aniónico, en la que la fase estacionaria está

cargada positivamente puesto que el analito se basa en iones cargados

negativamente [41].

Uno de los aspectos atractivos de la cromatografía iónica es que la medida

de la conductividad proporciona un método general y adecuado para determinar

la concentración de las especies eluidas. La cromatografía iónica con detección

conductimétrica de iones eluidos de la columna es una técnica eficaz de

determinación cuantitativa de especies cargadas, tanto inorgánicas como

orgánicas.

En este estudio se ha empleado la cromatografía de intercambio iónico

para determinar los productos de descomposición del diurón (ácidos de cadena

corta y anión cloruro, principalmente). El cromatógrafo utilizado (Metrohm 790

IC) posee un detector de conductividad y se encuentra disponible en los

laboratorios de ICPEES-UniStra (Figura 2.16).

Figura 2.16. Cromatógrafo de intercambio iónico empleado para llevar a cabo

el análisis.

62 Capítulo 2

2.4. Ensayos fotocatalíticos

El comportamiento fotocatalítico de las muestras sintetizadas en este

trabajo fue evaluado a través de tres reacciones de interés medioambiental: la

degradación fotocatalítica de ácido acético (en fase acuosa), la fotodegradación

oxidativa de diurón (en fase acuosa) y la oxidación fotocatalítica de propeno (en

fase gas). Los tests fotocatalíticos de la primera y tercera reacción fueron llevados

a cabo en los laboratorios del grupo de Materiales Carbonosos y Medio Ambiente

de la Universidad de Alicante. En cambio, las medidas fotocatalíticas de la

degradación de diurón fueron realizadas durante una estancia de investigación en

los laboratorios del Institut de Chimie et Procédés pour l'Énergie,

l'Environnement et la Santé (ICPEES) y el Centre National de la Recherche

Scientifique (CNRS) de la Universidad de Estraburgo, en Francia.

2.4.1. Degradación fotocatalítica de ácido acético

El sistema experimental utilizado para estudiar la degradación

fotocatalítica de ácido acético se muestra en la Figura 2.17. Este montaje

experimental está constituido por los siguientes elementos:

⁖ Gases.

Para la calibración del sistema se ha empleado una bala de calibrado de

Linde Group con una mezcla de gases (CH4, CO2 e H2 en He, cuyas

concentraciones se detallan en la Tabla 2.1.). Durante la reacción se ha

usado como gas portador He o Ar (dependiendo del experimento) (véase

Tabla 2.1.).

⁖ Controlador de flujo.

Se utilizó un controlador de flujo Brooks Instruments tipo 0154 para

regular el caudal de los gases de calibrado y el gas portador.

⁖ Reactor.

Se empleó un reactor de cuarzo cilíndrico (Heraeus tipo UV-RS-2)

provisto de dos aperturas, para la entrada y salida de los gases. En el

interior del reactor se sitúan la disolución de ácido acético y el

fotocatalizador suspendido en ella, que se encuentran en constante

agitación, y la lámpara UV.

⁖ Lámpara UV.

Se utilizó una lámpara de mercurio de presión media (TQ-150, λmáx = 365

nm), que se encuentra situada en el interior del reactor (en contacto con la

disolución de ácido acético) y está rodeada por una camisa de refrigeración.


⁖ Refrigerante.

Por el interior de la camisa de refrigeración circula una corriente constante

de agua conectada a un sistema de refrigeración que la mantiene a 15 ºC,

de forma que la temperatura de la disolución de ácido acético se mantuvo

a 20 ºC.

⁖ Espectrómetro de masas.

La salida del reactor se encuentra acoplada a un espectrómetro de masas

(Balzers, Thermostar GSD 301 O1).

Figura 2.17. Imagen del sistema experimental utilizado en la degradación

fotocatalítica de ácido acético.

El procedimiento experimental llevado a cabo para realizar los tests

fotocatalíticos se detalla a continuación. En primer lugar, se prepararon 350 mL

de ácido acético 1 M y se vertieron en el interior del reactor junto con 0.350 g de

fotocatalizador. El reactor se acopló al sistema experimental, se introdujo la

lámpara en su interior y se conectó al sistema de refrigeración. Seguidamente se

purgaron las líneas y el reactor con una corriente de gas inerte (60 mL/min)

durante, aproximadamente, 1 hora. Después se conectó el sistema de

refrigeración y se encendió la lámpara UV, la cual se mantuvo encendida durante

12 h. Algunos experimentos se realizaron por duplicado con el fin de estudiar la

reproducibilidad. Además, también se realizaron experimentos en ausencia de

fotocatalizador para evaluar el fenómeno de la fotólisis. La Figura 2.18 presenta

un ejemplo de la medida de las señales de CH4, CO2 e H2 en los experimentos de

degradación fotocatalítica de ácido acético.

Espectrómetro

de masas

Controlador

de flujo

Reactor

Gases

Refrigerante Gases de calibrado

PC Lámpara UV

64 Capítulo 2

Figura 2.18. Variación de las señales de CH4, CO2 e H2 con el tiempo en los

experimentos de degradación fotocatalítica de ácido acético utilizando He como gas

portador.

Para cuantificar la eficiencia fotocatalítica de los catalizadores en la

degradación del ácido acético, la actividad fotocatalítica del proceso se determinó

como las cantidades de CH4, CO2 e H2 (en mmol) producidas durante 12 h, por

mol de ácido acético introducido en el reactor, de acuerdo con las siguientes

ecuaciones:

Producción de CH4=mmol CH4

mol HAc (2.14)

Producción de CO2=mmol CO2

mol HAc (2.15)

Producción de H2=mmol H2

mol HAc (2.16)

0 2 4 6 8 10 12 14

0

2x10-9

4x10-9

6x10-9

8x10-9

1x10-8

Oscuridad

Inte

nsi

dad

(unid

ades

arb

itra

rias

)

Tiempo (h)

H2 CH

4 CO

2

Luz

8x10-7

9x10-7

1x10-6

1x10-6

He

Inte

nsi

dad

(unid

ades

arb

itra

rias

)


2.4.2. Fotodegradación oxidativa de diurón

El sistema experimental utilizado para estudiar la fotodegradación

oxidativa de diurón se muestra en la Figura 2.19 y se compone de los siguientes

elementos:

⁖ Cámara solar.

La cámara solar empleada (Atlas, SUNTEST XLS+, 500 W/m2, lámpara

de arco de Xe NXE 2201) es un equipo de simulación ambiental solar de

1100 cm2 de área de exposición que cubre el espectro de emisión entre los

300 y 800 nm. En el interior de la cámara se sitúa, en constante agitación,

la disolución de diurón con el fotocatalizador suspendido.

⁖ Espectrofotómetro UV-vis.

El espectrofotómetro UV-vis empleado es el modelo Cary, 100Scan

Varian. La cantidad de diurón en disolución se determinó a partir de la

absorbancia a 246 nm.

Figura 2.19. Imagen del sistema experimental utilizado en la fotodegradación

oxidativa de diurón.

En los tests fotocatalíticos de degradación de diurón se empleó una

disolución de 10 mg/L de diurón (fue necesario agitar durante varios días en

oscuridad hasta que el diurón quedó totalmente disuelto). 100 mL de esta

disolución contenidos en un vaso de cuarzo con un imán agitador se colocaron

en el interior de la cámara solar (en oscuridad) sobre un agitador magnético. En

este momento se tomó una alícuota de disolución (blanco) la cual, tras filtrado,

se analizó mediante espectrofotometría UV-vis (Cbf). El valor de absorbancia

observado en el pico característico de diurón a 246 nm permite calcular la

concentración inicial de diurón en oscuridad. A continuación, se añadieron 0.1 g

Cámara solar

PC

Espectrofotómetro

UV-vis

66 Capítulo 2

de fotocatalizador a la disolución, se tomó una alícuota, se filtró y se analizó del

mismo modo. Este tipo de medida (en oscuridad) se realizó cada 30 min (C-90min,

C-60min, C-30min) hasta observar que el valor de la absorbancia se mantenía

constante (o variaba muy poco), indicando que se había completado el proceso

de adsorción de diurón en el fotocatalizador. El último valor de absorbancia

determinado permite calcular la concentración de diurón previa al encendido de

la lámpara (C0). Seguidamente se encendió la luz de la cámara solar y se tomaron

alícuotas de la disolución cada 5 min (C5min, C10min, C15min, etc.) para estudiar la

variación de la concentración de diurón con el tiempo. La reacción se detuvo

cuando el valor de la absorbancia era próxima a cero o permanecía prácticamente

invariable con el tiempo. En la Figura 2.20 se muestra un ejemplo de la variación

de los espectros de absorbancia en función del tiempo.

Figura 2.20. Espectros de absorbancia medidos en oscuridad y con luz a ciertos

intervalos de tiempo (a la izquierda) y evolución del máximo de absorbancia a 246 nm

en función del tiempo (a la derecha) en un experimento de degradación oxidativa de

diurón.

La cinética de Langmuir-Hinshelwood (L-H) se ha aplicado exitosamente

para describir reacciones sólido-gas y ha sido adaptada de forma satisfactoria

para describir reacciones sólido-líquido [42]. En particular, la ecuación de

Langmuir-Hinshelwood se ha usado frecuentemente para modelar la cinética de

fotodegradación del diurón con dispersiones de TiO2 bajo radiación UV-vis [43].

Sin embargo, solo con estudios cinéticos no es posible determinar si el proceso

200 250 300 350 4000.0

0.2

0.4

0.6

0.8

1.0

1.2

Luz

Abso

rban

cia

Longitud de onda (nm)

Cbf

C-90min

C-60min

C-30min

C0

C5min

C10min

C15min

C20min

C25min

C30min

Osc

uri

dad

-120 -100 -80 -60 -40 -20 0 20 40

0.3

0.4

0.5

0.6

0.7

Luz

Abso

rban

cia

Tiempo (min)

Oscuridad


de fotodegradación ocurre en la superficie del fotocatalizador o en la propia

disolución. Por lo tanto, aunque la isoterma de L-H se utilice en el modelado,

generalmente se acepta que la constante de velocidad del proceso y el orden de

reacción son solo “aparentes” [44]. Estos parámetros pueden emplearse para

describir la tasa de degradación del diurón, pero no tienen una realidad física y

no se pueden usar para identificar las reacciones que ocurren en la superficie del

fotocatalizador [44].

De acuerdo con el modelo cinético de L-H, la velocidad de reacción (r) es

proporcional a la fracción de área cubierta por el sustrato, diurón en este caso,

(θx). La expresión resultante es:

r =dC

dt= 𝑘rθx =

krKC

1+KC (2.17)

dónde kr es la constante de velocidad de la reacción, K es la constante de

adsorción del sustrato, C la concentración del mismo en el instante t, siendo t el

tiempo de irradiación. Al integrar la Ecuación 2.17 resulta la siguiente expresión:

ln (C0

C) + K(C0-C) = krKt (2.18)

dónde C0 es la concentración inicial de diurón. Cuando ésta es muy baja, la

Ecuación 2.18 puede aproximarse a una ecuación cinética de pseudo-primer

orden:

ln (C0

C) = k

' t (2.19)

dónde k’ es la constante de velocidad aparente de la reacción.

En este trabajo, la actividad de cada fotocatalizador en la fotodegradación

oxidativa de diurón se ha evaluado mediante la constante de velocidad aparente

(k’) del proceso. De acuerdo con la Ecuación 2.19, al representar gráficamente ln

(C0/C) frente a t, k’ puede determinarse de la pendiente de la recta obtenida. En

la Figura 2.21 se muestra, como ejemplo, el ajuste lineal obtenido para el

experimento cuyos resultados se han presentado en la Figura 2.20.

68 Capítulo 2

Figura 2.21. Ajuste lineal obtenido para la reacción mostrada en la Figura 2.20.

2.4.3. Oxidación fotocatalítica de propeno

El sistema experimental empleado para estudiar la actividad de los

fotocatalizadores en la oxidación fotocatalítica de propeno (C3H6) se muestra en

la Figura 2.22. Éste se compone básicamente de los elementos que se describen

a continuación.

⁖ Gases.

El experimento se llevó a cabo empleando una corriente de propeno de 100

ppmv en aire, utilizando dos caudales diferentes, 30 mL/min y 60 mL/min.

Además, se ha utilizado una bala de calibrado de CO2 de 300 ppmv en He

para cuantificar los productos de reacción.

⁖ Controlador de flujo.

Se utilizó un controlador de flujo másico automático Tylan RO-28 para

regular los caudales de los gases.

⁖ Reactor.

El fotocatalizador se situó sobre un lecho de lana de cuarzo que se

introdujo en el interior de un reactor vertical de cuarzo. El flujo de gas

atraviesa el reactor en sentido descendente.

0 5 10 15 20 25 30

0.0

0.2

0.4

0.6

0.8

ln(A

bso

rban

cia 0

/Abso

rban

cia)

Tiempo (min)

Equation y = a + b*x

Weight No Weighting

Residual Sum of Squares

0.00256

Pearson's r 0.99899

Adj. R-Square 0.99765

Value Standard Error

B Intercept 0 --

B Slope 0.02359 4.32653E-4


⁖ Lámpara UV.

La lámpara UV (Philips TL8W/05 FAM, λmáx = 365 nm) se encuentra

situada paralelamente al reactor de cuarzo a una distancia de separación de

aproximadamente 1 cm. Tanto la lámpara como el reactor están envueltos

por un cilindro de metacrilato cubierto de papel de aluminio.

⁖ Espectrómetro de masas.

La salida del reactor se encuentra conectada a un espectrómetro de masas

(Balzers, Thermostar GSD 301 O1).

Figura 2.22. Imagen del sistema experimental utilizado en la oxidación

fotocatalítica de propeno.

El procedimiento habitual para llevar a cabo un experimento de oxidación

fotocatalítica de propeno se inicia introduciendo 0.11 g de fotocatalizador sobre

el lecho de cuarzo en el interior del reactor. Seguidamente, el reactor se conecta

al sistema de forma paralela a la lámpara. Tras purgar el sistema con He, se hace

pasar una corriente de propeno de 100 ppmv en aire con un flujo de 60 mL/min.

Una vez se alcanza una concentración de propeno estable (aproximadamente 2

h), se enciende la lámpara UV y la iluminación se mantiene durante 2 h más,

aproximadamente, hasta que la señal de propeno alcanza un valor estacionario.

Pasado este tiempo, el flujo de propeno se reduce a 30 mL/min y se mantiene

durante un par de horas hasta alcanzar una señal estable. Algunos experimentos

se repitieron al menos dos veces para comprobar la reproducibilidad.

La conversión de propeno se calculó tal y como indica la Ecuación 2.20:

Conversión de C3H6 (%) =[C3H6]inicial - [C3H6]estacionario

[C3H6]inicial

x 100 (2.20)

PC

Reactor

Lámpara

UV

Espectrómetro

de masas

70 Capítulo 2

dónde [C3H6]inicial es la concentración inicial de propeno (100 ppmv),

[C3H6]estacionario es la concentración de propeno final estacionaria. La conversión

de propeno se calculó para los dos flujos utilizados (30 y 60 mL/min).

La Figura 2.23 muestra un ejemplo de la concentración de propeno en las

distintas etapas que componen el experimento tipo de actividad fotocatalítica

descrito anteriormente.

Figura 2.23. Variación de la concentración de propeno con el tiempo en las

distintas etapas de un experimento tipo de oxidación fotocatalítica de propeno con

flujos de 30 y 60 mL/min.

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0 1 2 3 4 5 60

20

40

60

80

100

120

140

160

C3H

6 (30 mL/min)

[C3H

6]

inicial

[C3H

6]

estacionario

C3H

6 (60 mL/min)

[C3H

6]

(ppm

)

Tiempo (h)

LuzOscuridad



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TiO2

modification

with transition

metal species

(Cr, Co, Ni and

Cu)

3.1. Introduction

3.2. Materials and methods

3.2.1. Preparation of M/P25 samples 3.2.2. Characterization 3.2.3. Photocatalytic activity

measurements

3.3. Results

3.3.1. Textural and morphological properties

3.3.2. X-ray diffraction 3.3.3. UV-vis diffuse reflectance

spectroscopy 3.3.4. XPS 3.3.5. Photocatalytic activity

3.4. Discussion

3.5. Conclusions

3

76 Chapter 3

3.1. Introduction

The addition of transition metal species to TiO2 catalysts has shown to be

useful to reduce the photogenerated e−/h+ recombination rate and, also, to

improve the photoresponse to the visible light [1–3]. The band gap and

electrochemical properties of TiO2 are modified when transition metal ions

replace Ti (IV) centers (substitutional doping), occupy interstitial sites

(interstitial doping) or form aggregates on the TiO2 surface [4]. In titanium

dioxide, substitutional doping can occur when the difference between the atomic

radii of the dopant and of Ti (IV) is less than 15% [5]. The introduction of dopants

in the TiO2 lattice causes local distortion of the crystal structure, thus altering the

crystallinity degree and phase transformation. Doping (substitutional or

interstitial) can also generate trap levels and modify the band gap, thus promoting

the photocatalytic efficiency [1,3,6,7]. Additionally, metallic aggregates not

chemically bonded to TiO2 can act as electrons scavengers, also preventing the

recombination of e−/h+ pairs.

Additionally, when a metal with a certain work function (energy required

for moving an electron from the Fermi level to the local vacuum level [8]) is put

in contact with a semiconductor of lower work function value, it provides a

Schottky barrier that facilitates the transfer of electrons from the semiconductor

to the metal. Thus, the metal serves as an electron trap, which prevents

recombination of e−/h+ pairs [9]. The ability of metal ions to act as effective traps

is related, among others, with their electrochemical properties, the metal

concentration and the intensity of the incident light [9–11]. Considering all this,

and with the purpose of enhancing the photocatalytic activity, the added

transition metal ions should have a work function higher than that of TiO2.

Although noble metals (such as Pt, Pd or Au) have proved to be effective

to enhance the photocatalytic activity of TiO2 [12], they are unsuitable for large-

scale commercial use due to their limited availability and high cost. In contrast,

earth-abundant non-noble transition metal-based materials are promising

alternatives to enhance the TiO2 performance. For example, transition metals,

such as Cu, Co, Ni, Cr, Fe, Mn and V [1,3,13] have been reported to decrease the

TiO2 band gap and to extend its photo-response to the visible region.

The use of non-noble transition metal/TiO2 systems for the photocatalytic

degradation of organic pollutants in the liquid and gas phase has been described

by several authors [1,3,6,7,14]. The incorporation of metal species to titania has

been often performed during the TiO2 synthesis [1,3,13,15], whereas relatively

few studies report the incorporation of metal species by impregnation over

previously synthesized TiO2 [6,14]. In this topic, the conclusions about the role

of the incorporated metal are sometimes controversial. For example, Di Paola, et

77 TiO2 modification with transition metal species (Cr, Co, Ni and Cu)

al. [14] studied the 4-nitrophenol photodegradation in aqueous suspension and

found a decrease of the TiO2 photoactivity when it contains Co, Cr, Cu, Fe, Mo

and V ions added by impregnation. In contrast, Tayade et al. [6] found that the

photocatalytic activity of Fe, Ni and Ag containing TiO2 in the degradation of

acetophenone and nitrobenzene in aqueous solution was higher than that of bare

TiO2, while the presence of Co or Cu led to lower photoactivity.

All these considerations prompted us to prepare a series of M/TiO2

photocatalysts (M = Cr, Co, Ni and Cu) by impregnation of the inexpensive,

versatile and commercial P25 titania (from Degussa). These new materials were

characterized by N2 adsorption isotherms, scanning electron microscopy (SEM),

X-ray diffraction (XRD), UV-visible diffuse-reflectance spectroscopy (UV-vis

DRS) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity

was investigated in the photo-degradation of aqueous acetic acid into CO2, CH4

and H2, in the oxidative photodegradation of diuron (in liquid phase) and in the

gas phase photooxidation of propene at low concentration. The overall

performances were discussed in terms of activity and selectivity as a function of

the properties of the photocatalysts.


3.2.1. Preparation of M/P25 samples

Degussa Aeroxide P25 TiO2 has been used as a raw material because of its

good activity in many photocatalytic reactions. Cr, Co, Ni, and Cu were

incorporated to P25 by impregnation as follows: P25 (2 g) was put in contact

with an aqueous solution (5 mL) of Cr(NO3)3·9H2O, Co(NO3)2·6H2O,

Ni(NO3)2·6H2O or Cu(NO3)2·3H2O of the appropriate concentration to obtain 1

wt. % metal loading. The mixture was stirred (2 h) and sonicated (30 min) and,

then, the solvent excess was removed (80 °C, 24 h).

All the samples were heat-treated in argon atmosphere (90 mL/min, 5

°C/min, 500 °C, 2 h) to decompose the nitrates, remove impurities and to

strengthen the metal–TiO2 interaction. The used nomenclature is M/P25-Ar (M

= Cr, Co, Ni or Cu). P25 was wetted and heat treated in the same conditions as

M/P25-Ar samples to use it as reference; this sample is named P25-Ar.

3.2.2. Characterization

Textural properties of the photocatalysts have been characterized by N2

adsorption–desorption at −196 °C (after degasification (250 °C, 4 h)). SEM and

SEM-EDX mapping were used to study the morphology of the samples.

Crystalline properties were characterized by XRD analysis. Diffuse reflectance

78 Chapter 3

spectra (DRS) were determined to calculate the band gap energy of the

photocatalysts. XPS spectra were obtained to study the electronic state of the

added metal species. Description of the techniques and of the experimental

conditions and procedures has been detailed in Chapter 2.

3.2.3. Photocatalytic activity measurements

The procedures to perform the photocatalytic activity measurements in the

three studied reactions is briefly presented next (for more details see Section 2.4).

⁖ Photocatalytic decomposition of acetic acid

Photocatalytic activity tests were performed in a cylindrical quartz reactor

equipped with a medium pressure mercury vapor lamp (λmax = 365 nm). 350

mL of 1 M acetic acid solution and 0.35 g catalyst were introduced in the

reactor, and after purging with He, the UV lamp was switched on and kept

working for 12 h. The outlet gases were analysed by mass spectrometry.

The photocatalytic activity was expressed as indicated in Equation 3.1.

Production of X=mmol X

mol HAc (3.1)

where X is CH4, CO2 or H2.

⁖ Oxidative photodegradation of diuron

The oxidative photodegradation of diuron was evaluated in a solar chamber

(Atlas, SUNTEST XLS+) to simulate solar light irradiation. In each

experiment, 100 mg of the photocatalyst were dispersed under stirring in 100

mL of a 10 mg/L diuron aqueous solution. The suspension was stirred in the

dark for 2 h (to allow a stabilization of the solution concentration due to

potential adsorption) and the light was switched on. Aliquots of the solution

were periodically analysed by UV-vis spectrophotometry and the diuron

concentration was monitored via the decrease of the intensity of the

absorption peak at λ = 248 nm. As diuron degradation is a pseudo first-order

reaction, the experimental results were fitted to a linear equation to obtain the

apparent kinetic constant.

⁖ Photocatalytic oxidation of propene

The experimental system used consists of a quartz reactor and a UV-A lamp

(λmax = 365 nm) placed parallel to the reactor. In each experiment 0.11 g

catalyst was placed in the reactor, and after purging with He, a stream of 100

ppmv propene in air was fed to the reactor. Once the propene concentration is

stable, the UV lamp is switched on and kept working until a stationary


propene signal is achieved. The outlet gases were analysed by mass

spectrometry.

Propene conversion was calculated using Equation 3.2:

Propene conversion (%) =Cinitial C3H6

- Cstationary C3H6

Cinitial C3H6

x 100 (3.2)

where CinitialC3H6 is the initial propene concentration, 100 ppmv, and

CstationaryC3H6 is the stationary propene concentration reached after a certain

irradiation time.

3.3. Results

3.3.1. Textural and morphological properties

Figure 3.1 shows the N2 adsorption–desorption isotherms of M/P25-Ar,

P25-Ar and P25 samples. It can be observed that the adsorption isotherms are

type IV, according to the IUPAC classification [16], with a hysteresis cycle

typical of mesoporous materials. In general, they are very similar, which implies

that P25 modifications do not significantly alter the porous texture.

Figure 3.1. N2 adsorption–desorption isotherms at –196 °C of the

photocatalysts.

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

350 P25

P25-Ar

Cr/P25-Ar

Co/P25-Ar

Ni/P25-Ar

Cu/P25-Ar

Vad

s N2 S

TP

(cm

3/g

)

P/P0

P25

P25-Ar

Cr/P25-Ar

Co/P25-Ar

Ni/P25-Ar

Cu/P25-Ar

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

350 P25

P25-Ar

Cr/P25-Ar

Co/P25-Ar

Ni/P25-Ar

Cu/P25-Ar

Vad

s N2 S

TP

(cm

3/g

)

P/P0

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

350 P25

P25-Ar

Cr/P25-Ar

Co/P25-Ar

Ni/P25-Ar

Cu/P25-Ar

Vads N

2 S

TP

(cm

3/g

)

P/P0

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

350 P25

P25-Ar

Cr/P25-Ar

Co/P25-Ar

Ni/P25-Ar

Cu/P25-Ar

Vads N

2 S

TP

(cm

3/g

)

P/P0

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

350 P25

P25-Ar

Cr/P25-Ar

Co/P25-Ar

Ni/P25-Ar

Cu/P25-Ar

Vads N

2 S

TP

(cm

3/g

)

P/P0

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

350 P25

P25-Ar

Cr/P25-Ar

Co/P25-Ar

Ni/P25-Ar

Cu/P25-Ar

Vads N

2 S

TP

(cm

3/g

)

P/P0

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

350 P25

P25-Ar

Cr/P25-Ar

Co/P25-Ar

Ni/P25-Ar

Cu/P25-Ar

Vads N

2 S

TP

(cm

3/g

)

P/P0

80 Chapter 3

The parameters calculated from the adsorption isotherms (Figure 3.1) are

collected in Table 3.1. All the M/P25 samples have similar textural properties,

but it can be mentioned that the surface area and micropore volume of Cr/P25-

Ar are slightly higher than for the rest of samples.

Table 3.1. Textural properties of the prepared materials.

SBET

a

(m2/g)

VDR N2b

(cm3/g)

Vmesoc

(cm3/g)

VTd

(cm3/g)

Øe

(nm)

P25 60 0.02 0.12 0.17 12

P25-Ar 58 0.02 0.48 0.53 36

Cr/P25-Ar 73 0.03 0.37 0.43 23

Co/P25-Ar 62 0.02 0.31 0.35 23

Ni/P25-Ar 53 0.02 0.34 0.37 28

Cu/P25-Ar 63 0.02 0.33 0.38 24

aSBET: BET surface area calculated from N2 adsorption data. bVDR N2: micropore volume calculated by applying the Dubinin-Radushkevich equation to

the N2 isotherms. cVmeso: mesopore volume calculated by the difference of the amount of nitrogen adsorbed at

P/P0 = 0.9 and P/P0 = 0.2. dVT: total pore volume determined from the amount of nitrogen adsorbed at P/P0 = 0.99 in

the N2 isotherms. eØ: average pore diameter calculated from the N2 adsorption isotherms by the Barret-Joyner-

Halenda (BJH) method.

SEM analysis did not reveal noticeable morphological differences neither

between the M/P25-Ar samples and P25-Ar, nor among the M/P25-Ar samples.

Figure 3.2 shows the SEM images of P25-Ar and Cu/P25-Ar samples as an

example. The SEM-EDX mapping image of Cu/P25-Ar (Figure 3.3) shows the

uniform distribution of the Cu species on TiO2.

(a)

(b)


(c)

(d)

Figure 3.2. SEM images of (a, b) P25-Ar and (c, d) Cu/P25-Ar samples with

10 μm scale bar (a, c) and 1 μm scale bar (b, d).

Figure 3.3. SEM-EDX mapping for Cu/P25-Ar sample: Ti (red) and Cu

(green).

3.3.2. X-Ray diffraction

Figure 3.4 shows the XRD patterns obtained for P25, P25-Ar and M/P25-

Ar samples. All of them present the characteristic peaks of anatase (2θ values of

25.3° (101), 37.8° (004), 48.0° (200), 54.5° (105), 55.0° (211), 62.7° (204), 70.4°

(116), and 75.2° (220)) and rutile (2θ values of 27.5° (110), 36.1° (101) and 54.4°

(211)) [17]. No characteristic peaks of transition metal oxides have been

identified in the M/P25-Ar XRD patterns. This can be explained by the low metal

loading, because the metal oxide particles are highly dispersed, and/or because

the metal ions have been partially introduced into the TiO2 structure.

82 Chapter 3

Figure 3.4. X-ray diffraction patterns of the photocatalysts.

Table 3.2 summarizes the average crystallite size of anatase and rutile and,

also, the contribution of crystalline and amorphous TiO2 in each photocatalyst.

The addition of transition metal species does not modify in a relevant way the

TiO2 crystalline structure, the phases distribution or the crystallite size of any of

the phases. The Cr/P25-Ar sample presents the smallest rutile average crystallite

size, which could be related to its slightly higher specific surface area (see Table

3.1).

Table 3.2. Proportion and average crystallite size of anatase (A) and rutile (R),

and proportion of amorphous TiO2 in the studied catalysts.

Crystalline TiO2

(%)

Amorphous TiO2

(%)

Average crystallite size

(nm)

A R A R

P25 73 14 13 22 28

P25-Ar 77 11 12 19 32

Cr/P25-Ar 76 13 11 19 25

Co/P25-Ar 74 14 12 17 30

Ni/P25-Ar 76 12 12 20 32

Cu/P25-Ar 76 12 12 19 31

A = Anatase, R = Rutile.

10 20 30 40 50 60 70 80

Cu/P25-Ar

Ni/P25-Ar

Co/P25-Ar

Cr/P25-Ar

P25-Ar

P25

A (

220)

A (

215)

A (

204)

A (

116)

A (

211)

A (

004)

A (

105)

A (

200)

R (

101)

R (

110)

Inte

nsi

ty (

arbit

rary

unit

s)

2 (º)

A (

101)

A: Anatase

R: RutileCu/P25-Ar

Ni/P25-Ar

Co/P25-Ar

Cr/P25-Ar

P25-Ar

P25


3.3.3. UV-vis diffuse reflectance spectroscopy

Figure 3.5 compiles the UV-vis diffuse reflectance spectra (DRS) of P25,

P25-Ar and M/P25-Ar samples. P25-Ar has the lowest absorption in the visible

region (> 400 nm), and P25 and the most of the M/P25-Ar samples show close

adsorption values in this region. Moreover, the absorption edge for the M/P25-

Ar samples is shifted to the visible range (423–436 nm), which results in a

decrease in their band gap energy values. The absorption edge of the studied

photocatalysts increases as follows: P25-Ar < P25 < Ni/P25-Ar < Cu/P25-Ar <

Cr/P25-Ar < Co/P25-Ar (see Table 3.3). The bottom part of Figure 3.5 presents

a photograph of the prepared samples, showing that they are coloured, in

agreement with the presence of metal oxides.

Figure 3.5. Up: UV-vis diffuse reflectance spectra for bare P25, P25-Ar and

for M/P25-Ar (M = Cr, Co, Ni and Cu) catalysts. Bottom: photograph of the prepared

photocatalysts.

200 300 400 500 600 700 800

P25

P25-Ar

Cr/P25-Ar

Co/P25-Ar

Ni/P25-Ar

Cu/P25-Ar

Ab

sorb

ance

(ar

bit

rary

un

its)

Wavelength (nm)

P25-Ar Cr/P25-Ar Co/P25-Ar Ni/P25-Ar Cu/P25-Ar

P25

P25-Ar

Cr/P25-Ar

Co/P25-Ar

Ni/P25-Ar

Cu/P25-Ar

84 Chapter 3

The band gap (Eg) values of the photocatalysts have been calculated by the

absorbance method and also considering the direct allowed transitions (direct

method) and the indirect allowed transitions (indirect method). The three

calculated values, presented in Table 3.3, show a similar trend, being those

calculated by the absorbance and indirect methods quite close, and lower than

those calculated by the direct method. Furthermore, it can be observed that the

Eg values of the M/P25-Ar samples are lower than those of P25-Ar.

Table 3.3. Absorption edge wavelength and calculated Eg values for P25, P25-

Ar and M/P25-Ar photocatalysts.

Absorption edge

wavelength (nm)

Eg1

(eV)

Eg2

(eV)

Eg3

(eV)

P25 417 2.97 3.36 2.97

P25-Ar 403 3.08 3.53 3.11

Cr/P25-Ar 436 2.85 3.40 2.86

Co/P25-Ar 436 2.84 3.40 2.88

Ni/P25-Ar 422 2.93 3.41 2.95

Cu/P25-Ar 426 2.91 3.39 2.92

1 Absorbance method. 2 Direct allowed transitions (direct method). 3 Indirect allowed transitions (indirect method).

As reported in the literature [6,7,14], the presence of transition metal

species, even in samples prepared by impregnation, might introduce new intra

band gap states in the TiO2 structure, which could enhance their photoefficiency

in the visible region. This behaviour can be mainly attributed to two phenomena:

i) the new energy levels would be located below the conduction band edge of

TiO2, leading to a decrease of the TiO2 band gap (what implies a shift to the

visible part of the spectrum); and ii) these energy levels would also act as e− traps

that would decrease the e−/h+ recombination rate [1].

3.3.4. XPS

P25-Ar and the M/P25-Ar catalysts were analysed by XPS in order to

study their surface composition and the oxidation state of the metal cations.

Figures 3.6a,b show, respectively, the obtained Ti 2p3/2 and O 1s XPS spectra.

The binding energy (B.E.) of Ti 2p3/2, about 458.6 eV (Figure 3.6a), indicates the

presence of titanium in an octahedral anatase network [18], which is consistent

with Ti (IV). In the case of the Cr/P25-Ar sample, the B.E. is slightly lower than


in the rest of the samples what, according to the literature, can be attributed to

the presence of some Ti (III) on the surface [19].

The O 1s spectra (Figure 3.6b) show two contributions, one at about 529.8

eV, ascribed to lattice O-2 in crystalline TiO2 [20], and a second one at 531.5 eV,

that can be due to O- ions located in defect sites related to grain boundaries [20].

It can be observed that the intensity of the peak at 531.5 eV is slightly higher in

Cr/P25-Ar, due to the presence of a larger amount of oxygen vacancies. The

differences between the Cr/P25-Ar sample and the rest of M/P25-Ar catalysts,

revealed by XPS, are consistent with the previously commented XRD and porous

texture results. The presence and nature of the added transition metal species do

not modify the O 1s peaks typical of bare TiO2 [21].

Figure 3.6. Deconvoluted XPS spectra for: (a) Ti 2p3/2 and (b) O 1s in P25-Ar

and in the M/P25-Ar photocatalysts.

Figure 3.7 shows the Cr 2p3/2, Co 2p3/2, Ni 2p3/2 and Cu 2p3/2 deconvoluted

spectra obtained.

460 458 456

Ti 2p3/2

Ti 2p3/2

Ti 2p3/2

Binding energy (eV)

Inte

nsi

ty (

arbit

rary

unit

s)

Ti 2p3/2

Ti 2p3/2

P25-Ar

(a)

Cr/P25-Ar

Ni/P25-Ar

Co/P25-Ar

Cu/P25-Ar

534 532 530 528

Binding energy (eV)

O 1s

O 1s

O 1s

O 1s

O 1s

Inte

nsi

ty (

arbit

rary

unit

s)

P25-Ar

(b)

Cr/P25-Ar

Ni/P25-Ar

Co/P25-Ar

Cu/P25-Ar

(a) (b)

86 Chapter 3

Figure 3.7. XPS deconvoluted spectra of: (a) Cr 2p3/2 in Cr/P25-Ar, (b) Co

2p3/2 in Co/P25-Ar, (c) Ni 2p3/2 in Ni/P25-Ar and (d) Cu 2p3/2 in Cu/P25-Ar.

The XPS spectra of Cr 2p3/2 in Cr/P25-Ar (Figure 3.7a) can be

deconvoluted into two contributions: at 576.6 eV, assigned to Cr (III) species,

and at 578.6 eV, which is attributed to Cr (VI) [22]. The Co 2p3/2 XPS spectra

(Figure 3.7b) shows a main asymmetric peak located at 780.7 eV and a satellite

peak positioned at 786.5 eV. These patterns reveal that Co/P25-Ar sample only

contains Co (II) [23]. The Ni 2p3/2 spectra (Figure 3.7c) shows a main peak at

855.6 eV, ascribed to Ni (II) in an oxygen environment [24], a minor peak at

857.3 eV that corresponds to Ni (III) species [25], and a satellite peak, at 861.3

eV, that supports the presence of divalent nickel [24]. Finally, the Cu 2p3/2 XPS

spectrum (Figure 3.7d) shows two contributions at 932.4 eV and 934.0 eV, that

can be assigned, respectively, to Cu (I) and Cu (II) [20,26]. The Cu (II) state also

leads to the shake-up satellite peaks at 940.8 and 943.6 eV [26].

Table 3.4 shows the binding energy values and the identified oxidation

states for the metal species, with the estimated proportions, in each photocatalyst.

580 575

(a) Cr 2p3/2

Inte

nsi

ty (

arbit

rary

unit

s) Cr 2p3/2

Binding energy (eV)

790 785 780

Inte

nsi

ty (

arbit

rary

unit

s)

Binding energy (eV)

(b) Co 2p3/2

Co 2p3/2

Satellite peak Co 2p

3/2

865 860 855

(c) Ni 2p3/2

Inte

nsi

ty (

arbit

rary

unit

s)

Binding energy (eV)

Satellite peakNi 2p

3/2

Ni 2p3/2

945 940 935 930

Binding energy (eV)

Inte

nsi

ty (

arbit

rary

unit

s)(d) Cu 2p

3/2

Cu 2p3/2

Satellite peak

Cu 2p3/2


It can be mentioned that, in some cases, the oxidation state of the M species is

different from those of the metal ions in the salts used for impregnation. This can

be explained by the developed M-TiO2 interaction, being Cu species the most

affected by this interaction.

Table 3.4. Binding energies and predominant oxidation states of the metal

transition ions in photocatalysts determined from XPS spectra.

Binding energy (eV) Identified metal

oxidation states

Proportion1

(%) Ti 2p3/2 O 1s M 2p3/2

P25-Ar 458.6 529.8 - - -

Cr/P25-Ar 458.4 529.7 576.6

578.6

Cr (III)

Cr (VI)

74

26

Co/P25-Ar 458.6 529.9 780.7 Co (II) 100

Ni/P25-Ar 458.6 529.8 855.7

857.3

Ni (II)

Ni (III)

66

34

Cu/P25-Ar 458.7 529.9 932.4

934.1

Cu (I)

Cu (II)

86

14

1 Data obtained from the characteristic XPS peak areas for each oxidation state of the metal

species. This proportion is referred to the total metal content determined by XPS.

The metal content on the catalyst’s surface has been calculated from XPS

data and a value around 3 wt. % was obtained for all the M/P25-Ar samples.

Considering that the nominal (and very likely actual) metal loading is 1 wt. %,

this reveals a certain surface enrichment in metal species on the more external

surface of the photocatalysts.

3.3.5. Photocatalytic activity

The photocatalytic performance has been evaluated in the liquid phase

diuron photodegradation and acetic acid photodecomposition and in the gas

phase photooxidation of propene. The results obtained in these three reactions

are presented and discussed next.

3.3.5.1. Oxidative photodegradation of diuron

The obtained results show that the prepared M/P25-Ar samples are not

active in the diuron photodegradation. It seems that, in this case, the presence of

the metal completely hinders the activity of P25 for this reaction. A revision of

the literature has shown that in samples prepared by impregnation of platinum

salts on P25 the optimal metal loading for the photocatalytic degradation of

diuron is around 0.2 wt. % [27], whereas for metals incorporated during ZnO

88 Chapter 3

synthesis, optimum values of 0.05 and 0.7 wt. % are obtain for Cu and Ag,

respectively [28,29]. The authors propose that at higher metal loading, the size

of the metal particles increases and, in this case, the photogenerated charge

carriers recombine more easily. To take into account this observation, Cu/P25-

Ar photocatalysts with 0.01 and 0.1 Cu wt. % were prepared and tested. They

displayed photoactivity, with k’ values of 25.0·10-3 and 23.4·10-3 min-1,

respectively. However, their activity is far from that of P25 (k’ = 63.4·10-3 min-

1).

These results leave different ways of improvement open, such as the

synthesis of photocatalysts with higher surface area and the incorporation of the

metal by different methods, among others.

3.3.5.2. Photocatalytic decomposition of acetic acid

Figure 3.8 plots the amount of CH4, CO2 and H2 (in mmol per mol of acetic

acid (HAc)) generated during 12 h in the different catalytic tests performed.

Because the amount of H2 generated is very low, the comments on the obtained

results will be based on the production of CH4 and CO2. It must be remarked that

the amounts of CH4 and CO2 produced by photolysis are small compared to those

produced in presence of the photocatalysts.

Figure 3.8. CH4, CO2 and H2 produced after 12 h with the photocatalysts and

in photolysis test (in absence of photocatalyst).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

mm

ol

gen

era

ted

/mo

l H

Ac

CH4

CO2

H2

CH4

CO2

H2


Methane formation during the photocatalytic decomposition of HAc is

assumed to follow the so-called photo-Kolbe reaction (Equation 3.3),

CH3COOH + h+ → CH3• + CO2 + H+ (3.3)

the main products of which, CH4 and CO2 [30–33], are ideally produced in

CH4/CO2 molar ratio equal to 1. However, it is interesting to note that in the

obtained results, the amount of CO2 produced is higher than the amount of CH4

(being thus the CH4/CO2 ratio lower than one). This means that some other

reactions, besides the photo-Kolbe one, have to be taken into consideration.

Mozia, et al. [30] suggested that the excess of CO2 could be originated by the

oxidation of acetic acid by O2, as shown in Equation 3.4:

CH3COOH + 2 O2 → 2 CO2 + 2 H2O (3.4)

Therefore, since in our experiments no oxygen was present in the system,

the involvement of photogenerated oxygen should be considered [34,35]:

OH• + OH• → H2O2 (3.5)

H2O2 + 2 h+ → O2 + 2 H+ (3.6)

H2O2 + OH• → H2O + HOO• (3.7)

HOO• + OH• → H2O + O2 (3.8)

HOO• + H+ + e‒ → H2O2 (3.9)

2 H2O2 → 2 H2O + O2 (3.10)

Considering that in the experiments carried out, O2 is formed by some of

the reactions indicated above, it could be assumed that both the photo-Kolbe

(Equation 3.3) and the oxidation reaction (Equation 3.4) occur simultaneously. If

both reactions would occur up to the same extent, the theoretical CH4/CO2 ratio

would be 0.30. However, the measured CH4/CO2 ratio ranges between 0.47 and

0.77, meaning that with the tested catalysts and conditions, the photo-Kolbe

reaction is predominant, prevailing acetic acid degradation over its oxidation.

Data of Figure 3.8 show that the heat treatment of P25 in Ar seems to be

detrimental as the catalytic activity decreases. However, the M/P25-Ar

photocatalysts produce more CH4 and CO2 than P25-Ar, confirming a clearly

positive effect of the metal. Besides, it can be observed that the photocatalytic

activity is significantly influenced by the nature of the added metal. Concerning

the methane output, the photocatalytic efficiency of the M/P25-Ar samples

follows the trend: Cu/P25-Ar >> Ni/P25-Ar > Cr/P25-Ar ≈ Co/P25-Ar.

The acetic acid conversion was calculated considering the obtained

amounts of CH4 and CO2 in 12 h. The estimated acetic acid conversion varies

90 Chapter 3

between 0.01 % and 0.13 % for all of the samples, which corresponds,

respectively, to 0.10 and 1.28 mmol of acetic acid converted per gram of

photocatalyst in 12 h.

3.3.5.3. Photocatalytic oxidation of propene

Figure 3.9 shows the propene conversion values obtained with the

different photocatalysts and in the blank (photolysis) experiment, tested at 30 and

60 mL/min. The mass scan carried out to determine oxidation compounds in the

outlet stream reveals that CO2 is the only oxidation product. Also, quantification

of the produced CO2 has allowed to perform a carbon balance which confirms

that total mineralization of propene takes place, according to the following

reaction (Equation 3.11), and in agreement with the literature [36,37].

2 C3H6 + 9 O2 → 6 CO2 + 6 H2O (3.11)

Photodegradation of propene by photolysis is negligible compared with

the photocatalytic oxidation. The activity order of the investigated photocatalysts

follows the same trend in both flow-rates tested (P25 >> P25-Ar > Cu/P25-Ar >

Ni/P25-Ar > Cr/P25-Ar ≈ Co/P25-Ar), which can be considered as a proof of

reproducibility. As expected, propene conversion is higher when the used flow

is 30 mL/min because the contact time in longer and/or lower amount of propene

molecules must be oxidized per unit of time.

Figure 3.9. Propene conversion (at 30 and 60 mL/min) obtained without

catalyst, with P25, P25-Ar and M/P25-Ar photocatalysts.

0

10

20

30

40

50

60

Pro

pen

e c

on

vers

ion

(%

)

30 mL/min

60 mL/min


3.4. Discussion

As mentioned above, the M/P25-Ar samples are not active for diuron

photodegradation, but they are active for acetic acid decomposition and propene

oxidation. The activity in these two last mentioned reactions depends on the

nature of M. The M/P25-Ar samples can be ordered according to their activity

following a similar trend in both reactions: Cu/P25-Ar > Ni/P25-Ar > Cr/P25-Ar

≈ Co/P25-Ar. Because of the similarity in the textural properties (Table 3.1),

crystallinity and TiO2 phase distribution (Table 3.2) of the investigated

photocatalysts, it can be concluded that the differences in activity found are not

related with the mentioned properties. Thus, it must be considered that the

photocatalytic activity depends on the nature of the incorporated metal species.

As the metals have different electrochemical properties, their ability to influence

the photocatalytic behaviour of titania can be different. The ionic radius and the

work function are relevant parameters that would determine their behaviour.

Table 3.5 lists the ionic radii [38] and the work functions [39] of each M species

identified by XPS in the M/P25-Ar samples.

The ionic radius of Cr (III) is similar to that of Ti (IV), and this could let

Cr (III) to replace Ti (IV) in the TiO2 lattice. Such a substitution in M/TiO2

samples, has been previously reported in samples also prepared by impregnation

and heat-treatment at about 500 °C [14]. Moreover, the ionic radius of Cr (VI) is

smaller than that of Ti (IV), so Cr (VI) species could also be incorporated in the

TiO2 structure. The incorporation of Cr species in the TiO2 lattice might introduce

some defects and oxygen vacancies (that could explain the slightly different

properties of Cr/P25-Ar catalyst, revealed by N2 adsorption, XRD and XPS). All

the other metals must be located either in the network interstices or as aggregates

on the TiO2 surface. The latter is the most plausible option since no significant

changes in the crystalline structure of the M/P25-Ar samples compared to P25-

Ar have been observed.

As indicated in the introduction of this chapter, electron transfer can occur

from lower work function materials to higher work function ones. In the case of

the M/P25-Ar samples, the metal work function must be higher than that of TiO2

to achieve an efficient photocatalytic behaviour. The metal species can, thus,

behave as efficient traps for photogenerated electrons, preventing the e‒/h+

recombination and, consequently, improving the photocatalytic activity [40].

92 Chapter 3

Table 3.5. Ionic radii and work function values for the predominant oxidation

states of the transition metal ions in the photocatalysts.

Identified metal oxidation states1 Ionic radius2 (Å) Work function3 (eV)

Ti (IV) 0.75 5.4 ± 0.2

Cr (III) 0.76 5.0 ± 0.2

Cr (VI) 0.58 6.8 ± 0.2

Co (II) 0.88 4.6 ± 0.2

Ni (II) 0.83 6.3 ± 0.2

Ni (III) 0.74 4.9 ± 0.1

Cu (I) 0.91 4.9 ± 0.1

Cu (II) 0.87 5.9 ± 0.1

1 Data obtained from XPS. 2 Obtained from reference [38]. 3 Obtained from reference [39].

The activity results obtained show that Cr/P25-Ar and Co/P25-Ar are the

samples with the lowest photocatalytic activity in both reactions. In the case of

the Co catalyst, it can be due to the lower work function of Co (II) ions with

respect to that of Ti (IV). This would not allow Co (II) ions to capture electrons

in an efficient way [6,9,39]. As the Cr species present in Cr/P25-Ar have a high

work function, their low activity must be related to a different property. As

commented, it seems that the Cr (III) ions partially substitute Ti (IV) in Cr/P25-

Ar and probably because of that, the catalyst loses activity. In contrast, Ni/P25-

Ar and Cu/P25-Ar show the highest performances in both reactions. This can be

explained considering that these samples contain, respectively, Ni (II) and Cu (II)

ions, that have higher work function than Ti (IV). Another advantage of copper

is that, as reported in the literature [31,41–43], the conduction band (CB) edges

of Cu2O and CuO species are less negative than that of TiO2, while the valence

band (VB) edges of Cu2O and CuO are less positive than that of TiO2. This

implies that in the Cu-P25-Ar sample, Cu2O and CuO introduce energy levels

close to both the CB and VB of TiO2, which can act as effective e− and h+ traps.

Besides, the coexistence of the two Cu species generates electron transfer in

cascade, from more positive energy values to less positive ones. This way of

electron transfer may result in a better e‒/h+ separation, associated to higher

photoactivity [31].

In the photodecomposition of acetic acid, Cu/P25-Ar is clearly more active

than the rest of M/P25-Ar photocatalysts. Besides, as mentioned above, all the

M/P25-Ar samples are more active than P25-Ar. On the other hand, in the gas

phase oxidation of propene, Ni/P25-Ar and Cu/P25-Ar show a similar behaviour,


being bare P25-Ar more active than the M/P25-Ar photocatalysts. This indicates

that the course of the two reactions seems to depend differently on the catalyst

properties. Literature works compared the photoactivity of TiO2 systems in the

abatement of pollutants in liquid and gas phase, and found that the

photocatalysts’ behaviour was different in the two phases [44,45]. This can be

related with either the own reaction mechanism, or to differences in experimental

conditions, like the reaction media (liquid or gas phase), the reactor configuration

or the substrate concentration. For example, as indicated in the literature [46–49],

in the liquid phase, the solvent (usually water) helps to remove reaction

intermediates and products from the catalyst surface, avoiding or hindering

deactivation, whereas in fixed-bed reactors like the one used for propene

oxidation, the light distribution becomes a limiting factor [50]. Probably, the gas

phase photooxidation of propene is a rapid process occurring mainly on the

titania surface. It seems that, in gas phase, when the metal species are present,

the reaction rate decreases (i.e. by a decrease of the TiO2 exposed surface) and

this is also influenced, as mentioned above, by the metal's nature. In liquid phase

reactions diffusion limitations likely make the whole reaction slower and, thus,

the pathway in which the supported metal species are involved is not limiting the

reaction. In this case, the positive effect of the metal species as electron traps

prevails.

3.5. Conclusions

A series of M/TiO2 photocatalysts (M = Cr, Co, Ni, Cu) were prepared by

impregnation of the commercial P25 titania followed by heat treatment in Ar at

500 °C. P25 was heat treated in the same conditions to use it as reference. The

characterization results have shown that the textural properties and the

crystallinity of the M/P25-Ar photocatalysts are very close to those of bare P25-

Ar, and only sample Cr/P25-Ar shows slight differences. The XRD and SEM

studies allow to conclude that the metal species are highly dispersed on the TiO2

surface. The XPS analysis indicated that excepting Co, the rest of the metals are

present in more than one oxidation state and that Cr could have partially been

incorporated in the TiO2 structure, also supported by the atomic radii values.

The prepared M/P25-Ar samples were not active for photocatalytic diuron

degradation reaction but they were active for acetic acid decomposition and

propene oxidation. The photoactivity of the M/P25-Ar photocatalysts follows the

same trend in these two reactions: Cu/P25-Ar > Ni/P25-Ar > Cr/P25-Ar ≈

Co/P25-Ar. To explain the differences between catalysts, the work functions and

the ionic radii of the incorporated metal ions have been considered to play a role.

Although the relative activity order of the metal-containing photocatalysts is the

94 Chapter 3

same in both reactions, P25-Ar shows better activity than Cu/P25-Ar in propene

oxidation, whereas Cu/P25-Ar is more active than P25-Ar for acetic acid

degradation. This can be explained considering that the rate-determining steps

are significantly different in gas and liquid media.

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Cu/TiO2

photocatalysts

4.1. Introduction


4.2.1. Preparation of TiO2 4.2.2. Preparation of Cu/TiO2 samples 4.2.3. Heat treatment of the samples 4.2.4. Catalysts’ characterization 4.2.5. Photocatalytic measurements

4.3. Results and discussion

4.3.1. Porosity characterization 4.3.2. XRD analysis 4.3.3. XPS analysis 4.3.4. SEM and TEM analysis 4.3.5. Study of the photocatalytic activity

4.4. Conclusions

4

100 Chapter 4

4.1. Introduction

Copper, and in particular, copper oxides have been successfully employed

to improve the photocatalytic properties of TiO2 [1–14]. As copper oxides are

semiconductors with a narrow band gap (CuO, cupric oxide, 1.4 eV; Cu2O,

cuprous oxide, 2.2 eV [15]), they can serve as electron mediators and extend the

absorption to the long wavelength region [16]. However, the effect/s of copper

species on the TiO2 properties has not been fully clarified yet and, besides, it is

strongly dependent on the way in which copper is incorporated in the TiO2 host

material.

Copper-containing TiO2 materials have been prepared by techniques such

as impregnation [1,7–9], sol–gel [4,10,11], mechanical alloying [2],

simultaneous magnetron co-sputtering [12], electrochemical methods [13], etc.

Depending on the preparation method, copper species could replace Ti (IV) sites,

could be located in the interstitial sites or may segregate on the TiO2 surface [14]

with different consequences on the physicochemical properties of the final

photocatalyst (texture, crystallinity, electronic structure and light absorption

ability). Moreover, such Cu species can be present in different oxidation state

and this would also affect the final properties [3].

In the previous chapter, M/P25-Ar (M = Cr, Co, Ni, Cu) photocatalysts

were synthesized by impregnation and heat treated at 500 ºC in Ar. Among them,

Cu/P25-Ar sample proved to be the best performing one in the photodegradation

of acetic acid and in the photooxidation of propene.

Considering this, the aim of this chapter is to synthesize Cu/TiO2

photocatalysts by different methods and using different amounts of copper to

study in detail the role of this metal in the physicochemical properties of the

photocatalysts. TiO2 was prepared by sol-gel and different amounts of copper

were incorporated (using Cu(NO3)2) either during the TiO2 sol-gel synthesis (in

situ incorporation) or by impregnation of the synthesised TiO2. Post-synthesis

heat treatment at 500 ºC was carried out using two different atmospheres: Ar or

air. The photocatalytic efficiency of the prepared materials was measured in the

photocatalytic decomposition of acetic acid, the oxidative photodegradation of

diuron (both in aqueous phases) and the photooxidation of propene (in gas phase).


The preparation methods of Cu/TiO2 photocatalysts, the characterization

techniques used and the procedure for the photocatalytic measurements are

detailed in Chapter 2. All these aspects are addressed briefly next.

101 Cu/TiO2 photocatalysts

4.2.1. Preparation of TiO2

Nanosized TiO2 particles were prepared by sol-gel in a procedure that

comprises the following steps [17]: 1) 9.3 mL tetraisopropoxide (TTIP) were

mixed with 17.5 mL glacial acetic acid (HAc) at 0 ºC, 2) 197.5 mL distilled water

were added dropwise under stirring for 1 h (the TTIP:HAc:H2O molar ratio was

1:10:350), 3) the solution was ultrasonicated for 30 min, 4) solution stirring

continued for 5 h until a clear suspension of TiO2 nanocrystals was formed, 5)

the suspension was aged in an oven at 70 °C for 12 h and, finally 6) the resulting

gel was dried at 100 °C and the solid crushed into a fine powder [18]. A certain

amount of this TiO2 was heat treated as specified in Section 4.2.3.

4.2.2. Synthesis of Cu/ TiO2 samples.

The two methods used to prepare the Cu/TiO2 samples are described below.

⁖ Impregnation (im)

TiO2 (2 g) was mixed with an aqueous solution of Cu(NO3)2·3H2O (5 mL) of

the appropriate concentration to obtain Cu loadings of 0.5, 1 and 10 wt. %.

The mixture was stirred for 2 h, and then it was heated at 80 ºC for 24 h. The

nomenclature used is Cu/TiO2imx (x = 0.5, 1 or 10). The obtained solids were

heat treated as described in Section 4.2.3.

⁖ In situ (is)

An aqueous solution of Cu(NO3)2·3H2O (5 mL) with the appropriate

concentration to obtain Cu loadings of 0.5, 1, 2, 5, 7 and 10 wt. % was

introduced dropwise in the step 2 of the TiO2 sol-gel synthesis. Then, the

remaining amount of distilled water (up to 197.5 mL) was added dropwise.

The rest of the synthesis steps remain as described in Section 4.2.1. The

nomenclature for these samples is Cu/TiO2isx (x = 0.5, 1, 2, 5, 7 or 10). The

catalysts were heat treated as described in the next section.

4.2.3. Heat treatment of the samples

The heat treatment was performed to increase the crystallinity of TiO2 [19],

to remove impurities [4] and to strengthen the metal-TiO2 interaction [20]. The

treatment was carried out in a fixed bed quartz reactor (5 ºC/min, 500 ºC, 2 h) in

air or Ar (90 mL/min). The samples name includes Ar or air to indicate the

atmosphere of the heat treatment. The calcination temperature, 500 ºC, was

selected in order to have a large amount of anatase crystalline phase [19,21] and

to remove the nitrates.

102 Chapter 4

4.2.4. Catalysts’ characterization

The prepared photocatalysts were characterized using the following

techniques: N2 adsorption-desorption at -196 ºC, X-ray diffraction, XPS analysis,

TEM and SEM-EDX mapping.

A CuO sample obtained by calcination of Cu(NO3)2·3H2O (10 ºC/min, 500

ºC, 1 h) was analysed and used as reference in the XPS analysis. XPS

measurements have been done using a pass energy of 50 eV in all samples, with

the exception of Cu/TiO2is7-Ar, Cu/TiO2is5-Ar and Cu/TiO2is2-Ar for which the

pass energy was 20 eV.

4.2.5. Photocatalytic measurements


Photocatalytic tests were performed in a cylindrical quartz reactor with a

medium pressure mercury vapor lamp (λmax = 365 nm). 350 mL of a 1 M acetic

acid solution and 0.35 g catalyst were introduced in the reactor. After purging

with He, the UV lamp was switched on and kept working for 12 h. The outlet

gases were analysed by mass spectrometry.

The photocatalytic activity was expressed as indicated in Equation 4.1.

Production of X =mmol X

mol HAc (4.1)

where X is CH4, CO2 or H2.


The oxidative photodegradation of diuron was performed under simulated

solar light irradiation (Atlas, SUNTEST XLS+). In each experiment, 100 mg

of the photocatalyst were dispersed under stirring in 100 mL of a 10 mg/L

diuron aqueous solution. Aliquots of the solution were periodically analysed

by UV-vis spectrophotometry and the variation of the diuron concentration

was monitored via the intensity of the absorption peak at λ = 248 nm. As

diuron degradation is a pseudo first-order reaction, the experimental results

were fitted to a linear equation to obtain the apparent kinetic constant.




catalyst was placed in the reactor. A stream of 100 ppmv propene in air was

fed to the reactor, once the propene concentration is stable, the UV lamp is


switched on and kept working until a stationary propene signal is achieved.

The outlet gases were analysed by mass spectrometry.

Propene conversion was calculated using Equation 4.2:

Propene conversion (%) =Cinitial C3H6

- Cstationary C3H6

Cinitial C3H6

x 100 (4.2)

where CinitialC3H6 is the initial propene concentration, 100 ppmv, and

CstationaryC3H6 is the stationary propene concentration reached after a certain

irradiation time.

4.3. Results and discussion

4.3.1. Porosity characterization

Figure 4.1 shows the obtained N2 adsorption–desorption isotherms and it

can be observed that all of them are type IV, according to the IUPAC

classification [22], with a hysteresis cycle typical of mesoporous materials. All

the prepared materials exhibited H2 type hysteresis loops, while P25 gives rise

to an adsorption isotherm with a H3 type hysteresis loop [22].

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

Vad

sN2S

TP

(cm

3/g

)

P/P0

P25

TiO-Ar

TiO-air

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

P/P0

Vad

sN2S

TP

(cm

3/g

)

Cu/TiO2im0.5-Ar

Cu/TiO2im0.5-air

Cu/TiO2is0.5-Ar

Cu/TiO2is0.5-air

P25

TiO2-Ar

TiO2-air

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

P/P0

Vad

sN2S

TP

(cm

3/g

)

Cu/TiO2im0.5-Ar

Cu/TiO2im0.5-air

Cu/TiO2is0.5-Ar

Cu/TiO2is0.5-air

Cu/TiO2im0.5-Ar

Cu/TiO2im0.5-air

Cu/TiO2is0.5-Ar

Cu/TiO2is0.5-air

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

Vad

sN2S

TP

(cm

3/g

)

P/P0

P252

TiO2-Ar

TiO2-air

(a) (b)0

104 Chapter 4

Figure 4.1. N2 adsorption–desorption isotherms at –196 °C of the synthesized

photocatalysts and P25.

Table 4.1 summarizes the determined BET surface areas. The samples are

presented in two sets: Set 1 includes the Cu/TiO2 samples with 0.5, 1 or 10 wt.

% Cu prepared by the two mentioned methods and heat treated in air or Ar and,

Set 2 is a series of Cu/TiO2isxAr samples with x = 0.5, 1, 2, 5, 7 and 10 that has

been selected with the purpose of studying the effect of the copper content in

more detail.

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

P/P0

Vad

sN2S

TP

(cm

3/g

)

Cu/TiO2im1-Ar

Cu/TiO2im1-air

Cu/TiO2is1-Ar

Cu/TiO2is1-air

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

P/P0

Vad

sN2S

TP

(cm

3/g

)

Cu/TiO2im10-Ar

Cu/TiO2im10-air

Cu/TiO2is10-Ar

Cu/TiO2is10-air

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

P/P0

Vad

sN2S

TP

(cm

3/g

)

TiO2-Ar

Cu/TiO2is0.5-Ar

Cu/TiO2is1-Ar

Cu/TiO2is2-Ar

Cu/TiO2is5-Ar

Cu/TiO2is7-Ar

Cu/TiO2is10-Ar

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

P/P0

Vad

sN2S

TP

(cm

3/g

)

Cu/TiO2im1-Ar

Cu/TiO2im1-air

Cu/TiO2is1-Ar

Cu/TiO2is1-air

Cu/TiO2im1-Ar

Cu/TiO2im1-air

Cu/TiO2is1-Ar

Cu/TiO2is1-air

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

P/P0

Vad

sN2S

TP

(cm

3/g

)

Cu/TiO2im10-Ar

Cu/TiO2im10-air

Cu/TiO2is10-Ar

Cu/TiO2is10-air

Cu/TiO2im10-Ar

Cu/TiO2im10-air

Cu/TiO2is10-Ar

Cu/TiO2is10-air

(c) (d)

TiO2-Ar

Cu/TiO2is0.5-Ar

Cu/TiO2is1-Ar

Cu/TiO2is2-Ar

Cu/TiO2is5-Ar

Cu/TiO2is7-Ar

Cu/TiO2is10-Ar

(e)


Table 4.1. Main textural parameters, binding energy (Cu 2p3/2) and Cu (II) amount.

Set Sample SBET

a

(m2/g)

Anatase

crystallite

sizeb

(nm)

Cu 2p3/2 B.E. (eV)c Cu (II)

amountd

(%)

CuO

contribution

Cu2O and/or Cu

contribution

1

P25 60 22 - - -

TiO2-Ar 139 8 - - -

TiO2-air 150 9 - - -

Cu/TiO2im0.5-Ar 154 8 934.1 932.2 0.26

Cu/TiO2im0.5-air 127 10 934.0 932.0 0.21

Cu/TiO2is0.5-Ar 153 8 934.1 932.1 0.14

Cu/TiO2is0.5-air 125 10 933.8 932.0 0.26

Cu/TiO2im1-Ar 151 8 933.0 932.0 25.85

Cu/TiO2im1-air 104 10 933.8 931.9 55.65

Cu/TiO2is1-Ar 150 9 933.0 931.9 15.10

Cu/TiO2is1-air 135 9 933.3 931.9 0.72

Cu/TiO2im10-Ar 15 20 934.4 932.6 76.77

Cu/TiO2im10-air 92 10 934.4 932.4 45.34

Cu/TiO2is10-Ar 130 10 934.3 932.4 44.27

Cu/TiO2is10-air 84 10 934.5 932.5 47.34

CuO - - 933.7 - -

2

TiO2-Ar 139 8 - - -

Cu/TiO2is0.5-Ar 153 8 934.1 932.1 0.14

Cu/TiO2is1-Ar 150 9 933.0 931.9 15.10

Cu/TiO2is2-Ar 147 9 934.5 932.2 36.50

Cu/TiO2is5-Ar 148 9 933.9 932.0 49.48

Cu/TiO2is7-Ar 138 10 934.5 932.5 40.80

Cu/TiO2is10-Ar 130 10 934.3 932.4 44.27

aSBET: BET surface area calculated from N2 adsorption data. bDetermined by Scherrer’s equation (Equation 2.5). cDetermined by the method reported by Biesinger et al. from XPS data [23]. dDetermined by XPS data.

Regarding the samples of Set 1, it can be observed that the surface area of

the synthesized TiO2 is larger than that of P25, and sample TiO2-air has slightly

higher surface area than TiO2-Ar. Likely, air causes the oxidation of some

impurities present in the material and favours the conversion of unreacted TTIP

to TiO2. Comparing the Cu/TiO2 samples with bare TiO2 treated in the same

atmosphere, it can be observed that most Cu/TiO2-Ar samples have slightly larger

106 Chapter 4

surface area than TiO2-Ar. Contrarily, all Cu/TiO2-air samples have lower

surface area than TiO2-air, possibly because the heat treatment in air could

transform more effectively the Cu precursor into Cu oxides that would, block

some porosity. For the same reason, Cu/TiO2-Ar samples have larger surface area

than Cu/TiO2-air samples (except for the Cu/TiO2im10 sample). In the case of

the Cu/TiO2im0.5, Cu/TiO2im1, Cu/TiO2is0.5 and Cu/TiO2is1 the heat treatment

in argon leads to a higher surface area than the treatment in air (independently of

the preparation method). In the case of samples containing 10 wt. % Cu, the

tendencies are not so clear, being sample Cu/TiO2 is10-Ar the one with the largest

surface area.

Comparing samples with different Cu loading it can be observed that, in

general, the surface area decreases when the Cu content increases. This trend is

clearly observed for the series of Cu/TiO2isx-Ar samples (see Set 2 in Table 4.1).

4.3.2. XRD analysis

The obtained XRD patterns (Figure 4.2) show the peaks corresponding to

anatase (25.3º ((101) plane), 37.8º ((004) plane), 48.0º ((200) plane), 53.9º ((105)

plane) 55.1º ((211) plane), 62.7º ((204) plane), 70.4º ((116) plane) and 74.5º

((220) plane) [24,25]), while no rutile peaks are found. This confirms that the

heat treatment at 500 ºC leads to the desired TiO2 phase.

Table 4.1 shows that the crystallite size of all the synthesized samples is

smaller than that of P25. Regarding the effect of the synthesis variables, it can be

observed that: i) excepting samples with 10 wt. % Cu, the samples heat treated

in air have crystallite size higher than those treated in Ar, and ii) the Cu/TiO2

samples have larger crystallite size than the bare TiO2 samples heat treated in the

same atmosphere. Therefore, the presence of copper has an influence on the

development of the crystal structure. In this sense, literature states that the

formation of CuO likely generates a large number of defects as oxygen vacancies

in the TiO2 network, which act as nuclei for new crystals, accelerating the

crystallization process and leading to larger crystals [26].

The obtained XRD patterns do not show any characteristic peaks of copper

oxides (which would appear at 2 theta values: 36.65º and 61.36º for Cu2O cuprite

[27], and 35.75º and 38.95º for the tenorite structure CuO [28,29]), probably

because copper oxides are very well dispersed with a very low particle size. Only

samples Cu/TiO2is2-Ar, Cu/TiO2is5-Ar, Cu/TiO2is7-Ar and Cu/TiO2is10-Ar

present peaks corresponding to metallic copper (2 theta values: 43.47º, 50.69º

and 74.67º for face-cubic centered Cu (0) [30]). This peak is not observed in

samples Cu/TiO2is0.5-Ar and Cu/TiO2is1-Ar, prepared in the same way but with


a much lower Cu content (see Figure 4.2b), probably because of the low Cu

content.

Figure 4.2. X-ray diffraction patterns of the (a) Set 1 of photocatalysts and P25

and for (b) Set 2 of photocatalysts (see Table 4.1).

10 20 30 40 50 60 70 80

Inte

nsi

ty (

arbit

rary

unit

s)

2 (º)

Cu/TiO2is10-air

Cu/TiO2is10-Ar

Cu/TiO2im10-air

Cu/TiO2im10-Ar

Cu/TiO2is1-air

Cu/TiO2is1-Ar

Cu/TiO2im1-air

Cu/TiO2im1-Ar

Cu/TiO2is0.5-air

Cu/TiO2is0.5-Ar

Cu/TiO2im0.5-air

Cu/TiO2im0.5-Ar

TiO2-air

TiO2-Ar

P25

A (

101)

A (

004)

Cu

0 (

111)

A (

200)

Cu

0 (

200)

A (

105)

A (

211)

A (

204)

A (

116)

A (

220)

Cu

0 (

220)

A (

215)

A: Anatase

R: Rutile

Cu0: metallic copper

R (

110)

R (

101)

10 20 30 40 50 60 70 80

Cu/TiO2is10-Ar

Cu/TiO2is7-Ar

Cu/TiO2is5-Ar

Cu/TiO2is2-Ar

Cu/TiO2is1-Ar

Cu/TiO2is0.5-Ar

TiO2-Ar

A: Anatase


2 (º)

Inte

nsi

ty (

arb

itra

ry u

nit

s)

A (

101)

A (

004)

Cu

0 (

111)

A (

200)

Cu

0 (

200)

A (

105)

A (

211)

A (

204)

A (

116)

A (

220)

Cu

0 (

220)

A (

215)

10 20 30 40 50 60 70 80

Inte

nsi

ty (

arb

itra

ry u

nit

s)

2 (º)

Cu/TiO2is10-air

Cu/TiO2is10-Ar

Cu/TiO2im10-air

Cu/TiO2im10-Ar

Cu/TiO2is1-air

Cu/TiO2is1-Ar

Cu/TiO2im1-air

Cu/TiO2im1-Ar

Cu/TiO2is0.5-air

Cu/TiO2is0.5-Ar

Cu/TiO2im0.5-air

Cu/TiO2im0.5-Ar

TiO2-air

TiO2-Ar

P25

A (

101)

A (

004)

Cu

0 (

111)

A (

200)

Cu

0 (

200)

A (

105)

A (

211)

A (

204)

A (

116)

A (

220)

Cu

0 (

220)

A (

215)

A: Anatase

R: Rutile


R (

110)

R (

101)

Cu/TiO2is10-air

Cu/TiO2is10-Ar

Cu/TiO2im10-air

Cu/TiO2im10-Ar

Cu/TiO2is1-air

Cu/TiO2is1-Ar

Cu/TiO2im1-air

Cu/TiO2im1-Ar

Cu/TiO2is0.5-air

Cu/TiO2is0.5-Ar

Cu/TiO2im0.5-air

Cu/TiO2im0.5-Ar

TiO2-air

TiO2-Ar

P25

Cu/TiO2is10-Ar

Cu/TiO2is7-Ar

Cu/TiO2is5-Ar

Cu/TiO2is2-Ar

Cu/TiO2is1-Ar

Cu/TiO2is0.5-Ar

TiO2-Ar

(a)

(b)

108 Chapter 4

4.3.3. XPS analysis

Figure 4.3 shows the Cu 2p3/2 spectra obtained for the different Cu/TiO2

photocatalysts and for the CuO reference. The 2p3/2 level of CuO is characterized

by a shake-up satellite peak due to multiplet splitting that appears at a binding

energy about 9 eV higher than the 2p3/2 peak [31,32]. The satellite peak arises

when the emitted photoelectron loses part of its kinetic energy to excite a valence

electron to an unoccupied d orbital of Cu. The presence of the satellite peak is,

therefore, an indication of the existence of d9 partially filled orbitals in Cu

fundamental state and thus, it appears only when Cu (II) species are present. The

XPS spectrum of the CuO reference presents a main signal located at 933.7 eV

and a satellite peak with maxima at 941.3 and 943.8 eV, in agreement with the

presence of Cu (II).

In general, the main signal can be deconvoluted into two contributions,

indicating different oxidation states of copper. The contribution at B.E. around

934 eV can be assigned to Cu (II) while the one at B.E. close to 932 eV can be

assigned to Cu (I) (B.E 932.1 eV [33]) and/or to Cu (0) (B.E 932.4 eV [34]). Thus,

this information indicates that all samples contain Cu (I) or Cu (0) and thus, in

samples that according to XRD contain Cu (0), the presence of Cu (I) species in

non crystalline or small crystal size compounds cannot be discarded. In the case

of catalysts with 0.5 wt. % Cu and some of those with 1 wt. % Cu, the satellite

peak is almost not observed, probably because the signal intensity is very low.

But the fact that the main peak shows two contributions seems to indicate that

these samples also contain copper in different oxidation states. Therefore, Cu (II)

and Cu (I) species coexist in most samples. In the samples prepared by in situ

method and treated in Ar the presence of Cu (0), forming crystals of sufficiently

large size, have been detected by XRD. Thus, Cu/TiO2is-Ar samples certainly

contain Cu (II) and Cu (0) and, possibly Cu (I), although as mentioned before the

presence of Cu (I) cannot be fully assured.

The percentage of Cu (II) in the different samples was determined by the

method reported by Biesinger et al. [23], in which the ratio of the peak area of

the satellite and in the main peak are calculated and compared with the same ratio

determined in the XPS spectrum of the CuO reference.


Figure 4.3. Cu 2p3/2 deconvoluted spectra of the (a) Set 1 and (b) Set 2 of Cu/TiO2

samples and CuO.

925930935940945950

Inte

nsi

ty (

arb

itra

ry u

nits)

Binding energy (eV)

CuO

Cu/TiO2is10-air

Cu/TiO2is10-Ar

Cu/TiO2im10-air

Cu/TiO2im10-Ar

Cu/TiO2is1-air

Cu/TiO2is1-Ar

Cu/TiO2im1-air

Cu/TiO2im1-Ar

Cu/TiO2is0.5-air

Cu/TiO2is0.5-Ar

Cu/TiO2im0.5-air

Cu/TiO2im0.5-Ar

925930935940945950

Inte

nsi

ty (

arbitra

ry u

nits)

Binding energy (eV)

CuO

Cu/TiO2is10-Ar

Cu/TiO2is7-Ar

Cu/TiO2is5-Ar

Cu/TiO2-is2-Ar

Cu/TiO2is1-Ar

Cu/TiO2is0.5-Ar

Main peakCu 2p3/2Satellite peak

Cu 2p3/2

Main peakCu 2p3/2

Satellite peakCu 2p3/2

(Intensity x 0.5)

(Intensity x 0.5)

(a)

(b)

933.7

933.7

CuO

Cu/TiO2is10-air

Cu/TiO2is10-Ar

Cu/TiO2im10-air

Cu/TiO2im10-Ar

Cu/TiO2is1-air

Cu/TiO2is1-Ar

Cu/TiO2im1-air

Cu/TiO2im1-Ar

Cu/TiO2is0.5-air

Cu/TiO2is0.5-Ar

Cu/TiO2im0.5-air

Cu/TiO2im0.5-Ar

CuO

Cu/TiO2is10-Ar

Cu/TiO2is7-Ar

Cu/TiO2is5-air

Cu/TiO2is2-Ar

Cu/TiO2is1-Ar

Cu/TiO2is0.5-Ar

110 Chapter 4

Data included in Table 4.1 show that all samples contain Cu (II).

Comparison of the Cu (II) content shows that, in general, samples prepared in

situ and treated in Ar contain a lower Cu (II) percentage. Comparing the different

preparation conditions, samples with lower Cu (II) percentage are Cu/TiO2is-Ar

ones and, generally, increasing Cu loading increases the Cu (II) percentage;

however, from 5 wt. % copper content, the Cu (II) percentage remains practically

constant. Thus, it is also worth mentioning that the Cu (II) percentage in samples

with 0.5 wt. % Cu is very low.

4.3.4. TEM and SEM analysis

Figure 4.4 shows representative TEM images of samples: Cu/TiO2is10-Ar

(Figure 4.4a), Cu/TiO2is10-air (Figure 4.4b), Cu/TiO2im10-Ar (Figure 4.4c) and

Cu/TiO2im10-air (Figure 4.4d). These samples consist of the aggregation of

nanoparticles with a mean crystal size of about 10 nm, excepting TiO2im10-Ar,

in which particles of about 20 nm are observed. The dark spots in the high

contrast TEM images indicate the uniform distribution of the copper particles in

the TiO2 structure. In samples with copper loading lower than 5 wt. %, copper

particles are not observed in the TEM images (not shown) probably because the

Cu species are highly dispersed.

Figure 4.5 depicts the SEM-EDX mapping images of samples Cu/TiO2is2-

Ar (Figure 4.5a) and Cu/TiO2is10-Ar (Figure 4.5b,c). Figure 4.5a shows that

copper is uniformly dispersed on the TiO2 surface. On the other hand, Figure

4.5b,c shows that copper (either metal or oxide) is forming larger particles. The

size of the copper aggregations varied in the range of 0.4-1.2 μm. Such defined

copper particles have been also observed in sample Cu/TiO2is7-Ar.

(a)

(b)


(c)

(d)

Figure 4.4. TEM images of (a) Cu/TiO2is10-Ar, (b) Cu/TiO2is10-air, (c)

Cu/TiO2im10-Ar and (d) Cu/TiO2im10-air photocatalysts.

(a)

(b)

(c)

Figure 4.5. SEM-EDX mapping for (a) Cu/TiO2is2-Ar and (b, c) Cu/TiO2is10-

Ar samples: Ti (red) and Cu (green).

112 Chapter 4

4.3.5. Study of the photocatalytic activity

4.3.5.1. Photocatalytic decomposition of acetic acid

Figure 4.6 compiles the photocatalytic activity results obtained with P25,

TiO2 and the 0.5 wt. % Cu/TiO2 samples. It shows that the prepared

photocatalysts are more active than P25, in agreement with their larger surface

areas and lower crystallite sizes. TiO2-air is more active than TiO2-Ar but, on the

contrary, Cu/TiO2-Ar catalysts are more active than Cu/TiO2-air ones. According

to these data, the higher activity can be related with the larger surface areas,

which are also related with crystallite sizes (note that for similar crystalline phase

contents the lower the crystallite sizes, the larger the surface areas).

Regarding the preparation method, Cu/TiO2is catalysts are more active

than Cu/TiO2im, although they have similar surface area and crystallite size

(Table 4.1). These results emphasize that activity depends on a combination of

properties including surface area, crystallinity and crystallite size, but other

parameters such as copper-TiO2 interaction and the distribution of copper

oxidation states seem to play a relevant role. Thus, the different activity of

samples prepared by impregnation or in situ methods could be due to a different

interaction between Cu species and TiO2, being this probably more effective in

the is-samples. On the other hand, samples heat treated in Ar are more active than

those prepared in air, and this would indicate that the treatment in Ar leads to a

more suitable composition, with a higher content of photocatalytically active

copper species. Sample Cu/TiO2is0.5-Ar is the most active among the tested

samples (Figure 4.6). Considering this, it can be concluded that the in situ

preparation method and the heat treatment in Ar are the most suitable. XRD data

show that the Cu/TiO2isx-Ar photocatalysts contain Cu (0) while XPS reveals

the presence of Cu (II) and probably of Cu (I) species. The higher photocatalytic

activity of these samples allows to assume that the coexistence of the three copper

oxidation states: Cu (0), Cu (I) and Cu (II) has a positive effect. This can be

explained considering that Cu (I) and Cu (0) can give electrons to the oxygen

absorbed on the surface of catalyst, thus accelerating the interfacial electron

transfer [33], and Cu (II) can trap excited electrons, inhibiting the electron/hole

pair recombination.


Figure 4.6. CH4, CO2 and H2 produced after 12 h in the presence of different

prepared photocatalysts and commercial P25.

In order to analyse the effect of the copper loading, the activity of the

Cu/TiO2isx-Ar catalysts (x = 0.5, 1 and 10) is compared in Figure 4.7. It can be

observed that an increase in the copper content leads to a reduction in the amount

of H2, CO2 and CH4 produced. Note that with the Cu content increase, the surface

area of these samples decreases, their crystallite size increase, and probably the

active sites of TiO2 are covered by copper particles. In addition, lower Cu loading

could be related to a higher metal dispersion, as reported in the literature [35] and

as was observed in the SEM images. Thus, a low copper loading seems to be

beneficial for a high photocatalytic activity.

In order to ensure the reproducibility of the obtained results, in several

cases the activity test was repeated with a different portion of sample. Figure 4.7

shows that the production of CH4, CO2 and H2 in the two different tests is quite

similar, what reveals that the catalytic experiments performed are highly

reproducible.

0.0

0.5

1.0

1.5

2.0

2.5

mm

ol

gen

era

ted

/mo

l H

Ac

Series1

Series2

Series3

CH4

CO2

H2

114 Chapter 4

Figure 4.7. CH4, CO2 and H2 produced after 12 h in the presence of Cu/TiO2isx-

Ar (x = 0.5, 1, 10) catalysts in two photocatalytic tests. A repeated test performed with

a new portion of sample is indicated with (2) in the sample’s name.

4.3.5.2. Oxidative photodegradation of diuron

The obtained results show that only the two samples that do not contain

copper are active in this reaction, with apparent kinetic constant of: 5·10-3 min-1

for TiO2-Ar and 14·10-3 min-1 for TiO2-air. However, these values are clearly

lower than those obtained with P25 (63·10-3 min-1). Although samples TiO2-Ar

and TiO2-air have a high specific surface area (139 and 150 m2/g, respectively),

they have also a high percentage of amorphous TiO2 (22 and 28 %, respectively)

while P25 contains only 13 % amorphous TiO2. As commented before, the

crystalline structure is an important factor related to the photoefficiency.

Therefore, the low k' values obtained for the TiO2-Ar and TiO2-air may be related

to their poor crystallinity.

The obtained results indicate that the presence of copper in TiO2 is

detrimental for the photocatalytic oxidation of diuron. Some researchers have

previously observed that if the metal loading is higher than an optimum value

[37–40], the photocatalytic activity is negatively affected. Sakthivel et al. studied

0.0

0.5

1.0

1.5

2.0

2.5

mm

ol

gen

era

ted

/mo

l H

Ac

Series1

Series2

Series3

CH4

CO2

H2


M/TiO2 (M = Pt, Au or Pd) photocatalysts for the photo-oxidation of acid green

16 in aqueous solution [37] and they found an optimum metal loading of 0.8, 0.8

and 0.05 wt. % for Pt, Au and Pd, respectively. According to this, it has been

considered that the Cu loading of the Cu/TiO2 photocatalysts of this work is too

high (even the 0.5 wt. %). An excess of Cu particles might cover active sites of

the TiO2 surface thereby reducing its photocatalytic efficiency.

On the other hand, Sakthivel et al. [37] have reported that the efficiency of

M/TiO2 systems depends on the metal electron affinity and work function, what

allows a favourable contact with the TiO2 semiconductor. Thus, the efficiency of

Pt/TiO2 and Au/TiO2 photocatalysts is higher than that of Pd/TiO2 because Pt and

Au have a higher electron affinity and work function. Considering that the

optimal metal loading for Pt, Au and Pd are determined by their electron affinity

values (2.128, 2.309 and 0.558 eV/atom, respectively [41]), it can be assumed

that in the case of copper (whose electronic affinity is 1.228 eV/atom [41]) the

optimum load would be less than 0.5 wt. %.

According to this, it would be interesting to study in a future work the

activity of a series of Cu/TiO2 samples with high crystallinity and low copper

content (lower than 0.5 wt. %).

4.3.5.3. Photocatalytic oxidation of propene

All the prepared photocatalysts have been tested in the photooxidation of

propene at low concentration (100 ppmv). The series of Cu/TiO2-xAr

photocatalysts show poor activity in this reaction (propene conversion is less than

17 % (at 30 mL/min flow rate)). These samples have a dark grey colour, probably

due to remaining carbonaceous residues that have not been oxidized during the

thermal heat treatment in Ar. This can cause high UV light absorption, reducing

the photocatalytic efficiency of the material. This is in agreement with previously

published results [42].

Figure 4.8 shows values of propene conversion, obtained with the two

flow-rates tested, for P25, TiO2-air and the Cu/TiO2x-air samples. It can be

observed that these samples (with the exception of the two Cu/TiO210-air

samples) are active. The same tendency is found with the two flow-rates tested,

what can be considered as a proof of the reproducibility.

According to Figure 4.8 the activity order of the photocatalysts is the

following: TiO2-air > P25 > Cu/TiO2is0.5-air > Cu/TiO2im0.5-air > Cu/TiO2is1-

air > Cu/TiO2im1-air >> Cu/TiO2is10-air ≈ Cu/TiO2im10-air.

116 Chapter 4

Figure 4.8. Propene conversion (at 30 and 60 mL/min) obtained with P25, TiO2-

air and Cu/TiO2x-air photocatalysts.

Propene conversion is higher with the TiO2-air sample than with P25, what

can be due to its higher surface area (150 vs 60 m2/g). However, the Cu/TiO2-air

samples are less active than P25 in spite of having larger surface area. And the

activity decreases as the copper loading increases. Therefore, it seems that the

presence of copper is detrimental for the photocatalytic oxidation of propene.

This negative effect can be explained taking into account that the copper oxides

present in the samples cause a greenish coloration and can absorb part of the UV

light; and considering that the copper species partially cover the TiO2 surface,

reducing the exposed active surface. This is supported by the fact that the samples

in which copper has been incorporated by the in situ method are more active than

the analogous ones prepared by impregnation as in the second case the copper

species are expected to be mostly located on the TiO2 surface.

4.4. Conclusions

Cu/TiO2 catalysts with different copper contents (0.5, 1, 2, 5 and 7 and 10

wt. %) have been synthesized by sol-gel, using impregnation and in situ methods

to incorporate copper, and they have been submitted to a heat treatment at 500

°C, either in air or in argon. These catalysts have been tested in the photocatalytic

0

10

20

30

40

50

60

70

80

90

Pro

pen

e c

on

vers

ion

(%

)

30 mL/min

60 mL/min


decomposition of acetic acid, the oxidative degradation of diuron and the

photocatalytic oxidation of propene.

In general, the Cu/TiO2-Ar samples have larger surface area than Cu/TiO2-

air samples, all the prepared photocatalysts contain only anatase but the Cu/TiO2

samples have larger crystallite size than the bare TiO2 samples heat treated in the

same atmosphere. Cu (II) and Cu (I) species coexist in most samples and

Cu/TiO2is2-Ar, Cu/TiO2is5-Ar, Cu/TiO2is7-Ar and Cu/TiO2is10-Ar samples

seem to contain also metallic copper. Copper particles are highly dispersed in

Cu/TiO2isx-Ar samples with x ≤ 5 wt. %, but relatively large particles are present

on the TiO2 surface for higher copper content.

The prepared photocatalysts are more active than commercial P25 in the

decomposition of acetic acid. Cu/TiO2isx-Ar samples give rise to the best

photocatalytic efficiency, being Cu/TiO2is0.5-Ar most active sample one,

probably because it has the largest surface area, relatively small crystallite size

and highly-dispersed copper species. The Cu/TiO2 photocatalysts are not active

in the oxidative photodegradation of diuron, probably because the copper

particles reduce the exposed active surface. In the photocatalytic oxidation of

propene, the TiO2-air catalyst showed the best results. The incorporation of

copper results detrimental for this reaction, as in the photodegradation of diuron

reaction.

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Carbon. 67 (2014) 104–118.

TiO2-carbon

hybrid

photocatalysts

5.1. Introduction

5.2. Experimental

5.2.1. Preparation of AC 5.2.2. Preparation of TiO2-AC

photocatalysts 5.2.3. Characterization 5.2.4. Photocatalytic activity measurements

5.3. Characterization results

5.3.1. Determination of carbon content 5.3.2. Textural properties 5.3.3. XRD analysis 5.3.4. Scanning electron microscopy (SEM)

5.4. Photocatalytic activity results

5.4.1. Photocatalytic decomposition of acetic acid

5.4.2. Oxidative photodegradation of diuron 5.4.3. Photocatalytic oxidation of propene

5.5. Conclusions

5

122 Chapter 5

5.1. Introduction

In recent years, much effort has been directed towards improving TiO2

photocatalytic efficiency. In Chapters 3 and 4, the TiO2 modification with

transition metal ions has been studied in order to improve its photocatalytic

efficiency. However, the modification of TiO2 with non-metallic elements has

received intense attention and has been demonstrated to be another effective way

to enhance photocatalytic activity of TiO2 in the visible light region [1]. The use

of carbonaceous materials has attracted considerable attention because of their

unique and controllable structural and electrical properties [2]. It has been stated

that the presence of activated carbon (AC) in contact with TiO2 synergistically

improves its photocatalytic efficiency due to several reasons. The AC: i) provides

a large surface area that allows to adsorb a wide range of organic compounds

[3,4] and, then, transfers them to the photoactive TiO2 through the AC-TiO2

interphase [5,6]; ii) hinders the crystal phase transformation from anatase to rutile

during heat treatments [7,8]; and iii) is capable of increasing the lifetime of the

photogenerated e‒/h+ pairs, what enhances the generation of OH• radicals [9,10].

Additionally, AC can be used as support for TiO2 which facilitates the recovery

of the catalyst from solution [11]. Besides, AC can be produced by simple

methods and from low cost precursors.

The photoactivity of the TiO2 and of the TiO2-AC hybrid photocatalysts,

depends on their physicochemical parameters, such as crystal structure,

crystallite size and surface area [12,13]. And, in turn, they depend, among others,

on the temperature of the post-synthesis heat treatment, required to transform

amorphous TiO2 to crystalline phase [14]. In the TiO2-AC samples the amount

of containing carbon is also a relevant parameter.

Among all types of activated carbons, the spherical activated carbons have

interesting advantages due to their smooth surface, good fluidity and high

mechanical strength over the powdered and the granular activated carbons

[10,15,16]. Carbohydrates (such as cellulose, glucose, saccharose, fructose) have

been object of great interest as precursors for the preparation of spherical

activated carbons because they are cheap and environmentally friendly [15]. To

obtain spherical carbons from carbohydrates, hydrothermal treatment has been

one of the preferred methods in recent years [15,17].

Considering the ideas presented above, this chapter addresses the study of

the properties, such as crystalline phase, crystallite size and surface area, of some

TiO2 and TiO2-AC photocatalysts and their effect on the photocatalytic behaviour.

The following two series of catalysts have been prepared by sol-gel. On the one

hand, a series of pure TiO2 samples that have been submitted to a post-synthesis

123 TiO2-carbon hybrid photocatalysts

heat treatment at 350, 400, 450 and 500 ºC; and on the other hand, a series of

TiO2-AC hybrid photocatalysts containing different amounts of a spherical

activated carbon, previously synthesized from saccharose by hydrothermal

synthesis, that have been heat treated at 350 ºC. The behaviour of the prepared

photocatalysts is studied in three reactions of environmental interest: 1)

photodecomposition of acetic acid (HAc), in liquid phase; 2) oxidative

photodegradation of diuron, in liquid phase and, 3) photooxidation of propene,

in gas phase.

5.2. Experimental

The preparation of the spherical activated carbon and of the two series of

photocatalysts, the characterization techniques employed, and the procedures

used to perform the photocatalytic tests are summarized below (for more details

see Chapter 2).

5.2.1. Preparation of AC

The AC has been prepared by hydrothermal carbonization of saccharose

followed by activation with CO2.

5.2.2. Preparation of TiO2 and TiO2-AC photocatalysts

TiO2 samples were prepared by the sol-gel method using titanium

tetraisopropoxide (TTIP) as Ti precursor and then, they were submitted to a post

synthesis heat treatment in a muffle at different temperatures (350, 400, 450 and

500 ºC). The samples were named as TiO2 (T), where T is the temperature of the

heat treatment. The non-treated sample is named TiO2 (WT), where WT means

without treatment. The commercial titania P25 (Degussa) has been used as

reference.

To prepare the TiO2-AC materials a certain amount of the prepared AC

was added during the TiO2 sol-gel synthesis. The purpose was to obtain samples

with 0.5, 1, 5 and 10 wt. % AC (nominal contents). The catalysts were submitted

to a post-synthesis heat treatment in a muffle (350 °C). This series of samples

was named as TiO2-ACX (T), where X refers to the theoretical amount of added

carbon and T is the temperature of the heat treatment, in ºC.

5.2.3. Characterization

The prepared photocatalysts have been characterized by thermogravimetry

analysis, N2 adsorption-desorption at -196 ºC, SEM and X-ray diffraction.

124 Chapter 5

5.2.4. Photocatalytic activity measurements


Photocatalytic tests were performed in a cylindrical quartz reactor with a

medium pressure mercury vapor lamp (λmax = 365 nm). 350 mL of 1 M acetic

acid solution and 0.35 g catalyst were introduced in the reactor. After purging

with Ar, the UV lamp was switched on and kept working for 12 h. The outlet

gases were analysed by mass spectrometry.


The oxidative photodegradation of diuron was performed under simulated

solar light irradiation. In each experiment, 100 mg of the photocatalyst were

dispersed under stirring in 100 mL of a 10 mg/L diuron aqueous solution. The

suspension was stirred in the dark for 2 h (to allow a stabilization of the

solution concentration due to potential adsorption) and the light was switched

on. Aliquots of the solution were periodically analysed by UV-vis

spectrophotometry and the diuron concentration was monitored via the

decrease of the intensity of the absorption peak at λ = 248 nm. As diuron

degradation is a pseudo first-order reaction, the experimental results were

fitted to a linear equation to obtain the apparent kinetic constant.




catalyst was placed in the reactor. A stream of 100 ppmv propene was fed to

the reactor, once the propene concentration is stable, the UV lamp is switched

on is switched on and kept working until a stationary propene signal is

achieved. The outlet gases were analysed by mass spectrometry.

5.3. Characterization results

5.3.1. Determination of the carbon content

Figure 5.1 shows the results of the thermogravimetry (TG) and the

differential thermogravimetry (DTG) analysis of the series of TiO2 (T) and TiO2

(WT) samples. The DTG curves were determined in order to unambiguously

establish the temperature range in which the weight losses occur.

The TG curve of the TiO2 (WT) sample (Figure 5.1a) exhibits weight loss

in four temperature intervals. In the first region (below 120 ºC) the weight loss

(1.6 wt. %) corresponds to H2O desorption [18]. The second weight loss (3 wt.

%), from 120 to 270 ºC, is attributed to the vaporization of acetic acid remaining


from the synthesis and to the beginning of the decomposition of

tetraisopropoxide residues [19]. The third weight loss, occurring in the range

from 270 to 400 ºC, is the most pronounced (6.8 wt. %) and could be related to

the complete decomposition of organic residues and to the removal of different

forms of chemisorbed OH‒ groups, indicating the beginning of the TiO2

formation [18,19]. The fourth weight loss, the smallest (0.2 wt. %), is observed

from 550 ºC and could be attributed to phase transformation from amorphous to

anatase TiO2 [20]. The DTG curves (Figure 5.1b), show that the four endothermic

peaks are located, approximately, at 80, 180, 336 and 630 ºC.

In the case of the TiO2 (T) samples, the TG and DTG curves also show the

four weight losses commented above, but much less pronounced.

Figure 5.1. (a) TG and (b) DTG curves for TiO2 (WT) and TiO2 (T) samples.

Table 5.1 summarizes the quantification of these weight losses at the four

commented temperature intervals. It can be observed that, in general, as the

temperature of the post-synthesis heat treatment increases, the weight losses

become smaller.

85

90

95

100

Wei

gh

t (%

)

0 200 400 600 800-0.12

-0.08

-0.04

0.00

dm

/dT

(%

/ºC

)

Temperature (ºC)

TiO2

TiO2 (350)

TiO2 (400)

TiO2 (450)

TiO2 (500)

(a)

(b)

TiO2 (WT)

TiO2 (350)

TiO2 (400)

TiO2 (450)

TiO2 (500)

0 200 400 600 800-0.12

-0.08

-0.04

0.00d

m/d

T (

%/º

C)

Temperature (ºC)

TiO2

TiO2 (350)

TiO2 (400)

TiO2 (450)

TiO2 (500)

126 Chapter 5

Table 5.1. Weight loss (wt. %) in the four temperature intervals.

Weight loss (wt. %)

T range (ºC)

Sample 25-120 120-270 270-400 550-700

TiO2 (WT) 1.6 3.0 6.8 0.2

TiO2 (350) 0.8 1.3 0.5 0.3

TiO2 (400) 0.7 1.0 0.2 0.3

TiO2 (450) 0.6 0.8 0.2 0.3

TiO2 (500) 0.3 0.4 0.1 0.0

Figure 5.2 shows the TG and DTG profiles for the series of TiO2-ACX

(350) samples (with X = 0, 0.5, 1, 5 and 10). According to the TG curve (Figure

5.2a) three small weight losses took place below 460 ºC, that in the DTG curve

(Figure 5.2b) correspond to peaks at, approximately, 80, 180 and 330 ºC. They

are equivalent to those observed in Figure 5.1 and are due to the evaporation of

chemisorbed water, the elimination of organic residues remaining from the

synthesis and the removal of chemisorbed hydroxyl groups, respectively. The

main weight loss occurs from 450 to 650 ºC (DTG peak located at 580 ºC) and is

likely due to the combustion of carbon and the removal of oxygen-containing

groups [21]. As commented above, the phase transformation from amorphous

TiO2 to anatase, also occurs in this temperature range. However, this weight loss

is small in comparison to that due to the carbon combustion.

Thus, the carbon content of each sample has been calculated by subtracting

the weight loss corresponding to the transformation of amorphous TiO2 to

anatase determined for the TiO2 (350) sample (0.2 wt. %) from the weight loss

observed between 450 and 650 ºC in the TG profiles (Figure 5.2a).

Table 5.2 summarizes the quantification of the commented four weight

loss steps and the calculated carbon content. As expected, the weight loss in the

first three temperature intervals does not vary from one sample to another, and

the last weight loss increases as the nominal carbon content increases.


Figure 5.2. (a) TG and (b) DTG curves for TiO2 (350) and TiO2-ACX (350)

samples.

Table 5.2. Weight loss (wt. %) in the four temperature intervals and

calculated carbon content.

Weight loss (wt. %) C content

(wt. %)

T range (ºC).

Sample 25-120 120-270 270-400 450-650

TiO2 (350) 0.8 1.3 0.5 0.2 0.0

TiO2-AC0.5 (350) 0.9 1.3 0.4 0.6 0.4

TiO2-AC1 (350) 1.0 1.2 0.3 0.7 0.5

TiO2-AC5 (350) 0.8 1.4 0.4 3.8 3.6

TiO2-AC10 (350) 1.1 1.3 0.6 8.5 8.3

85

90

95

100

Wei

gh

t (%

)

TiO2 (350)

TiO2-AC0.5 (350)

TiO2-AC1 (350)

TiO2-AC5 (350)

TiO2-AC10 (350)

0 200 400 600 800-0.12

-0.08

-0.04

0.00

dm

/dT

(%

/ºC

)

Temperature (ºC)

TiO2 (350)

TiO2-AC0.5 (350)

TiO2-AC1 (350)

TiO2-AC5 (350)

TiO2-AC10 (350)

(a)

(b)

TiO2 (350)

TiO2-AC0.5 (350)

TiO2-AC1 (350)

TiO2-AC5 (350)

TiO2-AC10 (350)

0 200 400 600 800-0.12

-0.08

-0.04

0.00

dm

/dT

(%

/ºC

)

Temperature (ºC)

TiO2 (350)

TiO2-AC0.5 (350)

TiO2-AC1 (350)

TiO2-AC5 (350)

TiO2-AC10 (350)

128 Chapter 5

5.3.2. Textural properties

Figure 5.3 shows the N2 adsorption-desorption isotherms of the TiO2 (WT),

TiO2 (T) and P25 samples. It can be observed that the adsorption isotherms are

type IV, according to the IUPAC classification [22], with a hysteresis cycle

typical of mesoporous materials. As expected, the adsorption capacity of the

prepared photocatalysts decreases with increasing the heat treatment temperature.

Also, it can be clearly observed that the isotherms of the TiO2 (WT) and TiO2 (T)

samples is type H2, according to the IUPAC classification [22]. This kind of

hysteresis loop is typical of mesoporous materials, and is indicative of the

presence of cylindrical capillaries with narrow and wide necks, and a certain

contribution of “ink bottle” or bottle-type pores [23]. The isotherm of P25

presents a H3 type hysteresis, that is usually given by non-rigid aggregates of

plate-like particles forming slit-like pores [23].

Figure 5.3. N2 adsorption-desorption isotherms at -196 ºC of P25 and the TiO2

(T) and TiO2 (WT) samples.

Table 5.3 summarizes the textural properties determined from the

adsorption isotherms shown in Figure 5.3. In general, the prepared samples

exhibit larger surface areas and pore volumes than the commercial P25 (except

for TiO2 (500)). Regarding the effect of the heat treatment temperature, as the

temperature increases (from TiO2 (WT) to TiO2 (500)), the BET surface area

decreases (from 296 to 45 m2/g), as well as the mesopore volume (from 0.17 to

0.07 cm3/g) and the total pore volume (from 0.32 to 0.09 cm3/g), while the

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

Vad

s N2 S

TP

(cm

3/g

)

P/P0

V/g

V/g

V/g

V/g

V/g

V/g

P25

TiO2 (WT)

TiO2 (350)

TiO2 (400)

TiO2 (450)

TiO2 (500)


average pore diameter increases (from 4 to 8 nm). This results from crystallite

size growth and sintering of the mesopores [24,25].

Table 5.3. Textural properties of P25, TiO2 (WT) and TiO2 (T) samples.

SBET

a

(m2/g)

VDR N2b

(cm3/g)

Vmesoc

(cm3/g)

VTd

(cm3/g)

Øe

(nm)

P25 60 0.02 0.12 0.17 12

TiO2 (WT) 296 0.11 0.17 0.32 4

TiO2 (350) 144 0.05 0.18 0.25 7

TiO2 (400) 98 0.04 0.13 0.18 7

TiO2 (450) 72 0.05 0.10 0.15 8

TiO2 (500) 45 0.02 0.07 0.09 8

aSBET: BET surface area calculated from N2 adsorption data. bVDR N2: micropore volume calculated applying the Dubinin-Radushkevich equation to the

N2 isotherms. cVmeso: mesopore volume calculated as the difference between the amounts of nitrogen

adsorbed at P/P0 = 0.9 and P/P0 = 0.2. dVT: total pore volume determined from the amount of nitrogen adsorbed at P/P0 = 0.99 in

the N2 isotherms. eØ: average pore diameter calculated from the N2 adsorption isotherms by the Barret-Joyner-


Figure 5.4 shows the N2 adsorption-desorption isotherms of the AC

obtained from saccharose and the TiO2 (350) and TiO2-ACX (350) samples and

of the AC obtained from saccharose. It can be observed that the AC exhibits type

I isotherm according to the IUPAC classification [22], characteristic of

microporous structures. The prepared photocatalysts are mesoporous, with type

IV adsorption isotherms and H2-type hysteresis loops and show a similar

adsorption capacity.

The parameters calculated from the adsorption isotherms presented in

Figure 5.4 are collected in Table 5.4. From these calculated data it can be noted

that the synthetized AC is essentially microporous with a specific surface area of

491 m2/g and total micropore volume (VDR N2) of 0.25 cm3/g. As expected, the

incorporation of AC to TiO2 slightly improves the surface area of the final

photocatalyst. In general, as the carbon content increases (from 0 to 8.3 wt. %),

the surface area and the micropore volume increase (from 144 to 173 m2/g and

from 0.05 to 0.07 cm3/g, respectively), but, the mesopore volume and the average

pore diameter decrease (from 0.18 to 0.15 cm3/g and from 7 to 5 nm, respectively).

130 Chapter 5

Figure 5.4. N2 adsorption-desorption isotherms at -196 ºC for AC, TiO2 (350)

and TiO2-ACX (350) samples.

Table 5.4. Textural properties for AC, TiO2 (350) and TiO2-ACX (350)

samples. The carbon content calculated from TG data has been also included.

C content

(wt. %)

SBETa

(m2/g)

VDR N2b

(cm3/g)

Vmesoc

(cm3/g)

VTd

(cm3/g)

Øe

(nm)

AC - 491 0.25 0.02 0.27 2

TiO2 (350) 0.0 144 0.05 0.18 0.25 7

TiO2-AC0.5 (350) 0.4 154 0.06 0.18 0.27 7

TiO2-AC1 (350) 0.5 151 0.06 0.15 0.25 7

TiO2-AC5 (350) 3.6 164 0.06 0.16 0.26 6

TiO2-AC10 (350) 8.3 173 0.07 0.15 0.25 5

aSBET: BET surface area calculated from N2 adsorption data. bVDR N2: micropore volume calculated applying the Dubinin-Radushkevich equation to the N2

isotherms. cVmeso: mesopore volume calculated as the difference between the amounts of nitrogen

adsorbed at P/P0 = 0.9 and P/P0 = 0.2. dVT: total pore volume determined from the amount of nitrogen adsorbed at P/P0 = 0.99 in the

N2 isotherms. eØ: average pore diameter calculated from the N2 adsorption isotherms by the Barret-Joyner-


0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500V

ads N

2 S

TP

(cm

3/g

)

P/P0

V/g

V/g

V/g

0.750006129597198

0.429556029882604

V/g

AC

TiO2 (350)

TiO2-AC0.5 (350)

TiO2-AC1 (350)

TiO2-AC5 (350)

TiO2-AC10 (350)


5.3.3. XRD analysis

Figure 5.5 compiles the XRD patterns of P25 and of the prepared TiO2

(WT) and TiO2 (T) samples. The 2θ values of the TiO2 crystalline phases

characteristic peaks are [26]:

⁖ Anatase (A): 25.3° (101), 37.8° (004), 48.0° (200), 54.5° (105), 55° (211),

62.7° (204), 70.4° (116) and 74.5° (220).

⁖ Rutile (R): 27.5° (110), 36.1° (101) and 54.4° (211).

⁖ Brookite (B): 25.3° (120), 25.7° (111) and 30.8° (121).

Figure 5.5 indicates that the commercial P25 contains anatase and rutile,

while the prepared photocatalysts contain only anatase.

Figure 5.5. XRD patterns for P25 and TiO2 (WT) and TiO2 (T) samples.

The quantification of crystalline and amorphous TiO2 in each sample was

performed following the procedure reported by Jensen et al. [27], which is

described in detail in Chapter 2, Section 2.3.2. In the mentioned method, the TiO2

crystallinity is determined by doing the XRD analysis of a 50/50 (wt./wt.)

TiO2/CaF2 mixture and of a 100% crystalline CaF2 sample. Since the TiO2

samples studied in the present chapter only contain anatase, the amount of

crystalline titania equals the amount of anatase (determined by Equation 2.8,

being RCryst = 0). The mean crystallite sizes have been determined by the Scherrer

equation (Equation 2.5). Table 5.5 summarizes the amount of crystalline and

P25

TiO2

TiO2 350

TiO2 400

TiO2 450

TiO2 50010 20 30 40 50 60 70 80

A (

215)

A (

220)

A (

116)

A (

204)

A (

211)

A (

105)

A (

200)

A (

004)

R (

101)

R (

110)

Inte

nsi

ty (

arbit

rary

unit

s)

2 (º)

A (

101)

P25

TiO2 (WT)

TiO2 (350)

TiO2 (400)

TiO2 (450)

TiO2 (500)

132 Chapter 5

amorphous titania and the crystallite size for the commercial P25, and the TiO2

(WT) and TiO2 (T) samples.

Table 5.5 shows that the prepared samples contain a higher fraction of

amorphous titania than P25, which is consistent with their larger surface area. All

the prepared TiO2 samples present high crystallinity, although they are less

crystalline than P25. Even the TiO2 (WT) sample (that has not been thermally

treated) shows high crystallinity (62 %). It is important to mention that the

anatase content of the TiO2 (T) samples is higher than that of P25. As expected,

with the increase in the heat treatment temperature, an increase in the anatase

content (from 62 to 78 %) and crystallite size (from 6 to 16 nm) are observed.

This agrees with the BET surface areas showed in Table 5.3, since a lower surface

area is associated with higher crystallinity and/or larger crystallite size.

Table 5.5. Crystalline and amorphous TiO2 contents and crystallite size for P25

and the TiO2 (WT) and TiO2 (T) samples.

Crystalline TiO2

(%)

Amorphous TiO2

(%)


(nm)

A R A R

P25 73 14 13 22 28

TiO2 (WT) 62 - 38 6 -

TiO2 (350) 76 - 24 9 -

TiO2 (400) 76 - 24 10 -

TiO2 (450) 77 - 23 14 -

TiO2 (500) 78 - 22 16 -


Figure 5.6 compiles the XRD patterns for TiO2 (350) and the TiO2-ACX

(350) samples. Considering the 2θ values of the TiO2 crystalline phases

characteristic peaks commented before, as expected, only 2θ values typically for

anatase phase are observed for TiO2-AC photocatalysts. The XRD pattern of the

synthesised AC (not shown) corresponds to an amorphous material.

The calculated amounts of crystalline and amorphous titania (obtained by

the Jensen et al. method [27]) and the crystallite sizes (obtained by the Scherrer

equation (Equation 2.5)) are summarized in Table 5.6. The TiO2-ACX (350)

samples have slightly lower anatase content and crystallite size than the TiO2

(350) sample and thus, it can be concluded that the incorporation of AC does not

have a significant influence on the crystallinity of TiO2 samples treated in this

way.


Figure 5.6. XRD patterns for AC and for TiO2-ACX (350) (X = 0, 0.5, 1, 5 and

10 wt. %) samples.

Table 5.6. Crystalline and amorphous TiO2 contents and crystallite size for

TiO2-ACX (350) (X = 0, 0.5, 1, 5 and 10 wt. %).

Crystalline TiO2

(%)

Amorphous TiO2

(%)


(nm)

A R A R

TiO2 (350) 76 - 24 9 -

TiO2-AC0.5 (350) 74 - 26 9 -

TiO2-AC1 (350) 74 - 26 8 -

TiO2-AC5 (350) 73 - 27 8 -

TiO2-AC10 (350) 74 - 26 8 -


5.3.4. Scanning electron microscopy (SEM)

Figure 5.7 shows the SEM images obtained for AC and TiO2-ACX (X = 0,

0.5, 1, 5 and 10 wt. %) samples prepared without heat treatment. It can be

observed that the obtained AC presents a quite defined spherical morphology.

Regarding the TiO2-ACX samples, in those with lowest carbon content (nominal

0.5 and 1 wt. % AC) the presence of AC cannot be observed. However, in TiO2-

AC5 and TiO2-AC10 samples, the spherical morphology of the AC is clearly seen.

It can be noted that the AC does not mix intimately with the TiO2 crystals, which

10 20 30 40 50 60 70 80

Inte

nsi

ty (

arb

itra

ry u

nit

s)

2 (º)

TiO2 350

TiO2-AC0.5 350

TiO2-AC1 350

TiO2-AC5 350

TiO2-AC10 350

A (

10

1)

A (

00

4)

A (

20

0)

A (

10

5)

A (

21

1)

A (

20

4)

A (

11

6)

A (

22

0)

A (

21

5)

TiO2 (350)

TiO2-AC0.5 (350)

TiO2-AC1 (350)

TiO2-AC5 (350)

TiO2-AC10 (350)

134 Chapter 5

explains why its presence does not modify the crystallinity of the TiO2-AC

samples. It seems that the AC spheres are covered by a TiO2 layer and also TiO2

particles.

(a) (b) (c)

(d) (e) (f)

Figure 5.7. SEM images of: (a) AC, (b) TiO2 (WT), (c) TiO2-AC0.5 (WT), (d)

TiO2-AC1 (WT), (e) TiO2-AC5 (WT) and (f) TiO2-AC10 (WT).

5.4. Photocatalytic activity results

The results obtained in the study of the liquid phase photodecomposition

of acetic acid and the diuron oxidative photodegradation, and in the gas phase

photooxidation of propene are presented and discussed next.

5.4.1. Photocatalytic decomposition of acetic acid

Figure 5.8 plots the production of CH4, CO2 and H2 during 12 h for some

photocatalysts: TiO2 (350), TiO2-AC0.5 (350) and commercial P25. Data of an

experiment done without photocatalyst to check photolysis is also included. From

the plotted results, it can be observed that the amount of H2 produced is very

small, and so, it will not be considered it in the further discussion of the samples

photocatalytic activity. It can be observed that the produced amounts of CH4 and

CO2 in the blank experiment are very small compared to those produced in the

presence of photocatalyst. So, it can be concluded that the amount of acetic acid

degraded by photolysis is negligible.


Figure 5.8. CH4, CO2 and H2 produced after 12 h without photocatalyst and

using the photocatalysts: P25, TiO2 (350) and TiO2-AC0.5 (350).

As mentioned in previous chapters, the HAc is photodegraded following

the so-called photo-Kolbe reaction (Equation 5.1), producing mainly CH4 and

CO2 [28,29]. It should be considered that CO2 can also be formed by oxidation

of HAc (Equation 5.2). Both reactions can occur simultaneously in an equivalent

extension and, in case that this occurs; the theoretical CH4/CO2 ratio would be

0.3. Thus, when the measured CH4/CO2 ratio is higher than 0.3 this means that

the photo-Kolbe reaction is predominant, while when the CH4/CO2 ratio is

smaller than 0.3 the acetic acid oxidation prevails over its photodegradation.

CH3COOH + h+ → CH3• + CO2 + H+ (5.1)

CH3COOH + 2 O2 → 2 CO2 + 2 H2O (5.2)

In order to determine the relative importance of these two reactions the

CH4/CO2 ratio in each photocatalytic test has been calculated and it is higher than

0.3. For P25, the CH4/CO2 ratio is 0.4, for all prepared samples, except for TiO2

(WT), this ratio is higher than 0.4, while for TiO2 (WT) the CH4/CO2 ratio is 0.33.

This indicates that in most of the cases the photo-Kolbe reaction predominantly

occurs, only for TiO2 (WT) the photodegradation and oxidation of acetic acid

occur almost in an equivalent extension.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

mm

ol genera

ted/m

ol H

Ac

.

.

.

CH4

CO2

H2

136 Chapter 5

With the aim of being able to compare all the prepared samples that have

been tested in different periods of time, the amounts of CH4 and CO2 generated

in the different catalytic tests are expressed in relative values respect to amount

produced using the P25 titania photocatalyst (Equations 5.3 and 5.4).

X (CH4) = mmol of CH4 produced with synthetized TiO2 samples

mmol of CH4 produced with P25 (5.3)

X (CO2) = mmol of CO2 produced with synthetized TiO2 samples

mmol of CO2 produced with P25 (5.4)

Figure 5.9 plots the relative production of CH4 and CO2 (X (CH4, CO2))

during 12 h for the different photocatalysts tested: i) the series of TiO2 (WT) and

TiO2 (T) samples, ii) the series of TiO2-ACX (350) samples, and iii) the

commercial P25. Figure 5.9 also contains the standard deviations calculated from

the repetition of some experiments with a new portion of material.

Figure 5.9. CH4 and CO2 produced after 12 h for each sample expressed as

indicated in Equations 5.3 and 5.4.

It can be observed that most of the prepared samples produce more

methane and less CO2 than P25. Only sample TiO2 (WT) is clearly less active

0.0

0.5

1.0

1.5

2.0

2.5

X (

CH

4, C

O2)

P25

TiO

2-A

C10

(350

)

TiO

2-A

C5

(350

)

TiO

2-A

C1

(350

)

TiO

2-A

C0.

5 (3

50)

TiO

2 (5

00)

TiO

2 (4

50)

TiO

2 (4

00)

TiO

2 (3

50)

TiO

2 (W

T)

CH4

CO2


than P25, and in contrast with the rest of photocatalysts, with this sample the

relative production of CO2 is higher than the relative production of methane.

To explain the differences in products distribution it is necessary to take

into account that the crystallinity degree, the distribution of crystalline phases

and the potential presence of surface crystal defects, strongly affect the e‒/h+

separation lifetime [30]. Negligible or low photocatalytic activity has been

reported for amorphous titania, attributable to an easy recombination of

photoexcited electrons and holes in the amorphous structure [31]. Hence, the

higher content of amorphous titania in the TiO2 (WT) sample can explain the

lower CH4 production obtained using this catalyst. In the case of the TiO2 (T)

samples (with a higher anatase content (Table 5.5)), as the temperature of heat

treatment increases, the amorphous TiO2 fraction decreases (Table 5.5) and, in

general (with the exception of TiO2 (400)), the catalysts produce more CH4, and

less CO2. The increase of the lifetime of the e‒/h+ pairs in the samples with higher

crystallinity (the ones that were heat treated at high temperatures (450, 500 ºC))

would favour the photo-Kolbe reaction (Equation 5.1), predominating acetic acid

photodegradation respect to its oxidation (Equation 5.2).

Regarding the series of TiO2-ACX (350) samples, the activity data

presented in Figure 5.9 show that: i) all the prepared materials produce higher

CH4 amounts than P25 and ii) they produce higher amounts of CH4 and CO2 than

the TiO2 (350) photocatalyst. The presence of AC is considered to be beneficial

because, according to previous works [4,5,7,32–34], it seems to reduce the rate

of the e‒/h+ pairs recombination and can adsorb HAc facilitating its contact with

TiO2.

Only slight differences between the samples with different carbon content

have been found, and among them, sample TiO2-AC1 (350) seems to be the most

active. Although TiO2-AC5 (350) and TiO2-AC10 (350) photocatalysts only

contain 3.6 and 8.3 wt. % of AC, respectively, they have a dark grey colour, and

this implies a significant light adsorption. Li et al. proved that when the TiO2-AC

photocatalysts contain more than 5 wt. % AC part of the light is absorbed by the

activated carbon surface [35]. This means that increasing the amount of carbon

can be beneficial due to the advantages mentioned above but it also has

drawbacks related to intense light absorption. Because of that, it seems that there

is an effective optimum amount of carbon (around 1 wt. %), and a further increase

of it leads to a decrease of photocatalytic efficiency.

Figure 5.10 plots the production of CH4 and CO2 as a function of time for

a portion of TiO2-AC1 (350) sample tested during 12 h. It can be observed that

the evolution of these products is essentially constant over time, and the catalyst

138 Chapter 5

maintains its activity for 12 hours. This behaviour has been observed in all tested

samples. To study the catalysts stability, an additional experiment was conducted

for 40 h using a new portion of the TiO2-AC1 (350) sample. The results are

shown in Figure 5.10. It can be seen that, the reaction rate is maintained constant

for 40 h, meaning that the catalyst is not deactivated after this time. These results

also reveal the reproducibility of the performed catalytic tests.

Figure 5.10. Time evolution of the produced CH4 and CO2 respect to initial

HAc for TiO2-AC1 (350) sample tested for 12 and 40 h.

5.4.2. Oxidative photodegradation of diuron

As explained in Chapter 2, Section 2.4.2, the photocatalytic degradation of

diuron has been performed under simulated solar light irradiation. Prior to

irradiation, the suspension of the photocatalyst (100 mg) in the diuron solution

(100 mL, 10 mg/L) was stirred in dark for 2 h to ensure that the adsorption-

desorption equilibrium is established. The diuron concentration was monitored

by UV-vis spectrophotometry via the intensity of the absorption peak at λ = 248

nm [36].

Figure 5.11 shows the evolution of the diuron concentration versus time

when P25, and the TiO2 (WT) and TiO2 (T) samples are used as catalysts and

also in a blank experiment to evaluate photolysis.

0 10 20 30 400.0

0.5

1.0

1.5

2.0

2.5

3.0

TiO2-AC1 (350) 12 h

TiO2-AC1 (350) 40 h

mm

ol

CH

4/m

ol

HA

c

Time (h)0 10 20 30 40

0.0

0.5

1.0

1.5

2.0

2.5

3.0 TiO2-AC1 (350) 12 h

TiO2-AC1 (350) 40 h

mm

ol

CO

2/m

ol

HA

c

Time (h)


Figure 5.11. Diuron evolution over time for commercial P25, TiO2 (WT) and

TiO2 (T) photocatalysts.

Let us remind that C is the concentration of diuron at time t, while Cbf is

the initial diuron concentration (before adding the photocatalyst). The plot of

C/Cbf versus time is divided in two zones: the first 2 hours (presented in x-axis

with negative numbers) correspond to dark conditions and, from the time

identified as 0, the data correspond to the irradiated system. It can be observed

that, in general, the relative concentration of diuron decreases during the darkness

period. This is due to the adsorption of diuron on the catalyst surface. When the

light is turned on, the relative concentration of diuron decreases at much faster

rate, because diuron photodegradation is occurring, and probably adsorption, as

well. Therefore, it must be recalled that during the experiments two phenomena

can simultaneously occur: diuron adsorption (in darkness and under illumination)

and diuron photodegradation (only under illumination).

Data of Figure 5.11 show that in absence of catalyst no photodegradation

is observed, and thus, photolysis can be neglected in these experimental

conditions. The TiO2 (WT) catalyst shows an important adsorption but a

negligible photocatalytic degradation of diuron. In contrast, the series of TiO2 (T)

samples are active. In general, diuron adsorption is higher for the samples treated

at lower temperature while the photoactivity seems to be similar for all of them.

-120 -100 -80 -60 -40 -20 0 20 40

0.2

0.4

0.6

0.8

1.0

1.2

light

Without catalyst

P25

TiO2

TiO2 (350)

TiO2 (400)

TiO2 (450)

TiO2 (500)

C/C

bf

Time (min)

dark

Photolysis

P25

TiO2 (WT)

TiO2 (350)

TiO2 (400)

TiO2 (450)

TiO2 (500)

140 Chapter 5

The P25 titania shows also some adsorption and the fastest diuron

photodegradation.

The amount of adsorbed diuron mainly depends on the textural properties

of the samples. This explains that the amount of adsorbed diuron increases when

increasing the BET surface of the photocatalysts. Figure 5.12 plots the percentage

of diuron adsorbed in darkness and the SBET of each sample. It is observed as

these two parameters keep a close relationship.

Figure 5.12. Percentage of adsorbed diuron in darkness and SBET values for

each sample.

As it is known, the efficiency of titania in photocatalytic reactions is

influenced by many factors, such as amount of anatase [37], particle size [38]

and surface area [38]. As commented before, the easier recombination of e‒/h+

pairs in amorphous titania has been reported to explain its poor or negligible

photocatalytic [31]. Thus, the highest activity of P25 is likely related with the

low amount of amorphous titania, whereas the high content of amorphous titania

in sample TiO2 (WT) could explain its low photoactivity. The activity of the TiO2

(T) samples is between that of P25 and TiO2 (WT) and this is agreement with

their degree of crystallinity (lower than in P25 but higher than in the TiO2 (WT)

sample). Therefore, the results in Figure 5.11 suggest that the photoefficiency is

related with the degree of crystallinity (or the amount of amorphous titania) of

the photocatalysts. This result should be remarked because it reveals the

0

10

20

30

40

50

S

BE

T (

m2/g

)

Diu

ron

ad

sorp

tio

n i

n d

arkn

ess

(%)

P25

TiO

2 (W

T)

TiO

2 (3

50)

TiO

2 (4

00)

TiO

2 (4

50)

TiO

2 (5

00)

0

100

200

300


importance of determining the amorphous and crystalline contributions, what is

seldom reported, being the attention usually paid to the distribution of crystalline

phases (anatase, brookite and rutile) in the crystalline fraction of titania.

The rate of diuron disappearance from solution, considering both

adsorption and photodegradation, under illumination can be evaluated by the

apparent constant (k’) of the process, considering that it follows a pseudo first

order kinetic behaviour (see Chapter 2, Section 2.4.2). The plot of ln(C0/C) versus

time (C0 is diuron concentration at 0 time and C is diuron concentration at t time)

shows a linear trend and its slope represent k’ (see Chapter 2, Equation 2.19).

Figure 5.13 shows the obtained apparent constant (k’) for TiO2 (WT), the series

of TiO2 (T) samples, and P25, and also for the blank experiment. The results are

analogous to those presented in Figure 5.11. Diuron is not degraded by photolysis;

the apparent constant is also close to 0 min-1 for TiO2 (WT), whereas the TiO2 (T)

samples have similar k’ values, and the highest k’ is obtained for P25. However,

although the k' value is the same for the test carried out without photocatalyst and

with the TiO2 (WT) sample, the final concentration of diuron in solution is not

the same due to in the case of TiO2 (WT) a significant diuron absorption in

darkness takes place (that supposes a 20 % diuron removal).

Figure 5.13. Apparent kinetic constant (k’) values determined in a test without

catalyst and using P25, TiO2 (WT) and TiO2 (T) samples.

0

20

40

60

80

100

k'·1

0-3

(m

in-1)

E1

% adsorbed in dark

Efectividad a t=10min

Efectividad a t=30min

P25

with

out

cata

lyst

TiO

2 (W

T)

TiO

2 (3

50)

TiO

2 (4

00)

TiO

2 (4

50)

TiO

2 (5

00)

142 Chapter 5

Figure 5.14 shows the evolution of the diuron concentration with time, in

dark (2 h left to study adsorption) and under illumination, for the TiO2-ACX (350)

and TiO2 (350) samples. It can be seen that, the amount of adsorbed diuron in

dark conditions increases notably with the carbon content. In the samples with

nominal 5 and 10 wt. % AC, the adsorption process does not reach the

equilibrium during the 2 hours in dark conditions. So, the adsorption process is

likely still occurring in an important extension during illumination.

Figure 5.14. Variation of the diuron concentration with time in dark and

illumination conditions for TiO2 (350) and TiO2-ACX (350) samples.

Figure 5.15 presents the calculated apparent constants (k’) for TiO2 (350)

and TiO2-ACX (350) samples. As the elimination of diuron is strongly related

with the adsorption phenomena and since the AC provides porosity, which

favours diuron adsorption, k’ increases with the activated carbon content in the

prepared materials. The apparent constant value is highly influenced by the extent

of the adsorption that occurs in dark conditions because the diuron concentration

at the moment of switching on the light (C0) depends on such adsorption process.

-120 -100 -80 -60 -40 -20 0 20 400.0

0.2

0.4

0.6

0.8

1.0

1.2

C/C

bf

Time (min)

Abs/Abs0

Abs/Abs0

Abs/Abs0

Abs/Abs0

Abs/Abs0

dark lightTiO2 (350)

TiO2-AC0.5 (350)

TiO2-AC1 (350)

TiO2-AC5 (350)

TiO2-AC10 (350)

-120 -100 -80 -60 -40 -20 0 20 400.0

0.2

0.4

0.6

0.8

1.0

1.2

C/C

bf

Time (min)

Abs/Abs0

Abs/Abs0

Abs/Abs0

Abs/Abs0

Abs/Abs0

dark light


Figure 5.15. Apparent kinetic constant (k’) for TiO2 (350) and TiO2-ACX (350)

(X = 0.5, 1, 5 and 10 wt. %) samples.

To study the effect of the diuron concentration at time 0 (C0) in the k’ value,

the following experiments using TiO2-AC10 (350) catalyst were carried out: 0.1

g photocatalyst were added to a diuron solution (100 mL of 10 mg/L of diuron),

the mixture was stirred in dark for a certain period of time (1 min, 30 min, 120

min and 360 min) and then, the light was turned on. Both, in dark and

illumination conditions the concentration of diuron was monitored at certain time

intervals.

Figure 5.16a shows the variation of the relative diuron concentration with

time, in darkness and under illumination. It can be concluded that the longer the

time that the sample remains in dark, the larger the adsorption extent, and the

lower the rate of diuron photodegradation. Figure 5.16b plots the first-order

linear transforms: ln(C0/C) as a function of time. k’ can be obtained from the

slope of the linear plot (see Equation 2.19, Chapter 2).

0

20

40

60

80

100

TiO

2 (3

50)

k'·1

0-3

(m

in-1

)

TiO

2-A

C0.

5 (3

50)

TiO

2-A

C1

(350

)TiO

2-A

C5

(350

)TiO

2-A

C10

(350

)

144 Chapter 5

Figure 5.16. (a) Variation of the diuron concentration with time using the TiO2-

AC10 (350) sample, in dark (for 1, 30, 120 or 360 minutes) and illumination conditions

and (b) first-order linear transforms of the diuron photodegradation (from light

conditions data of Figure 5.16a).

Table 5.7 compiles the k’ values calculated from the slopes of the lines

shown in Figure 5.16b. It can be observed that k’ increases with the increase in

the time spent by the sample in darkness, reaching the maximum value of 0.078

min-1 when such a time is 120 min.

Table 5.7. Apparent constant values (k’) obtained for the TiO2-AC10 (350)

photocatalyst kept in dark conditions for different times prior to illumination.

Sample Time in darkness (min) k' (min-1)

TiO2-AC10 (350)

1 45·10-3

30 54·10-3

120 78·10-3

360 34·10-3

-360 -300 -240 -180 -120 -60 00.0

0.2

0.4

0.6

0.8

1.0

1.2 TiO

2-AC10 (350) 1 min

TiO2-AC10 (350) 30 min

TiO2-AC10 (350) 120 min

TiO2-AC10 (350) 360 min

C/C

bf

Time (min)0 10 20 30

0.0

0.5

1.0

1.5

2.0

ln (

C0/C

)Time (min)

1 min

30 min

120 min

360 min

Equation y = a + b*x

Weight No Weighting

Residual Sum of Squares

0.006 0.07756 0.01958 0.01238

Pearson's r 0.99935 0.99427 0.99545 0.98466

Adj. R-Square 0.99848 0.98667 0.98791 0.95941

Value Standard Error

1 min Intercept 0 --

1 min Slope 0.0452 6.65527E-4


30 min Slope 0.05431 0.00238


120 min Slope 0.07817 0.00432


360 min Slope 0.03356 0.00343

(a) (b)

-360 -300 -240 -180 -120 -60 00.0

0.2

0.4

0.6

0.8

1.0

1.2 TiO

2-AC10 (350) 1 min

TiO2-AC10 (350) 30 min

TiO2-AC10 (350) 120 min

TiO2-AC10 (350) 360 min

C/C

0

Time (min)

TiO2-AC10 (350) 1 min in dark conditions





Comparing the data related to diuron degradation rate under illumination

presented in Figures 5.16a and Figure 5.16b it can be mentioned that, as expected,

the fact that the initial diuron concentration of the illuminated period (C0) is

different for each sample, causes that the removal rates shown in Figures 5.16a

and 5.16b to be different.

Considering the relevance and extent of diuron adsorption on the

photocatalysts, additional experiments were conducted to study the potential

diuron/organic matter desorption and subsequent oxidation. The total organic

carbon (TOC) of the solution was measured for that purpose.

Tests similar to those reported in Figure 5.14 but up to a complete removal

of diuron from solution were performed using samples TiO2-AC1 (350) and

TiO2-AC10 (350). After recovering the catalysts from the solution, they were

mixed with 100 mL ultrapure water, the suspension was irradiated, and the TOC

of the solution was periodically measured. Figure 5.17 shows the time evolution

of TOC. It can be observed that in the case of TiO2-AC1 (350), the TOC value

increases in the first 30 minutes and then decreases, while in the case of TiO2-

AC10 (350) the TOC increases up to 180 min and then decreases. This reveals

that diuron or other organic compounds are being desorbed from the catalysts

and that, as expected, the amount of adsorbed species is larger in TiO2-AC10

(350). The TOC decrease after the registered maxima likely indicates that the

photooxidation of the dissolved organic compounds takes place.

These results open the possibility of using illumination to regenerate

exhausted photocatalysts in order to proceed to their reutilization.

146 Chapter 5

Figure 5.17. Evolution of TOC for the TiO2-AC1 (350) and TiO2-AC10 (350)

samples reused in ultrapure water.

Previous results have reported that short-chain organic acids, such as acetic,

formic and oxalic acids, together with chloride and nitrate ions, appear as final

products of the photocatalytic degradation of diuron [39,40] (Figure 5.18).

Figure 5.18. Oxidation products from diuron photodegradation (based on [40]).

Ion chromatography was used to analyse the final degradation products,

focusing on the compounds and ions previously referred to. Table 5.8 shows, as

an example, the concentration (in ppm) of acetic, formic and oxalic acids and

chloride ion detected at the end of a catalytic test carried out with the TiO2-AC10

(350) photocatalyst. The finding of these final products agrees with previous

results from the literature and corroborate the oxidation ability of the hybrid

photocatalyst.

0 50 100 150 200 2500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

TO

C (

mg

/L)

Time (min)

TiO2-AC10 (350)

TiO2-AC1 (350)

Ring open reactions

CO2 H2O


Table 5.8. Amounts of by-products detected by ion chromatography at the end

of the photocatalytic carried out with the TiO2-AC10 (350) sample.

Sample Acetic acid

(ppm) Formic acid

(ppm) Oxalic acid

(ppm) Chloride

(ppm)

TiO2-AC10 (350) 0.24 0.06 1.12 0.30

5.4.3. Photocatalytic oxidation of propene

The TiO2 (T) photocatalysts were also tested in the photocatalytic

oxidation of propene at low concentration in gas phase. The obtained results are

presented in Figure 5.19. As commented and explained in previous chapters,

photolysis in the used conditions is negligible and the same trend in the catalytic

activity of the tested samples is found with the two propene flow-rates used, what

remarks the reproducibility of the results. It can be observed that all the prepared

catalysts are more active than the P25 titania, what can be explained by the higher

surface area and higher anatase content of the prepared photocatalysts.

Figure 5.19. Propene conversion (at 30 and 60 mL/min) in a blank test and for

P25 and the TiO2 (T) samples.

Previous works have proved that a large anatase phase content is

favourable for this reaction [41]. By increasing the heat treatment temperature

0

20

40

60

80

100TiO

2 (5

00)

TiO

2 (4

50)

Phot

olys

is

TiO

2 (3

50)

Pro

pen

e co

nver

sion (

%)

60 mL/min

30 mL/min

P25

148 Chapter 5

from 350 to 450 °C the activity improves 12 % and a further increase of such a

temperature produces almost no improvement. This fact can not be explained

considering properties of the TiO2 (T) samples like their surface areas (as SBET

decreases when the heat treatment temperature increases) or their crystallite sizes

(increasing the heat treatment temperature leads a crystallite size increase). TG

analysis has shown that during the heat treatment some organic compounds

remaining from the TiO2 synthesis are removed (Figure 5.1 and Table 5.1). The

elimination of these organic compounds would allow propene and light to more

easily access the photoactive portion of the material. Thus, it can be considered

that propene conversion increases with the heat treatment temperature because

of the removal of remaining organic compounds and, as consequence, the

available active surface is higher.

Figure 5.20 compiles the propene conversion values obtained with P25,

TiO2 (350) and the TiO2-ACX (350) (X = 0.5, 1, 5 and 10 wt. %) samples. It can

be observed that the carbon containing samples are less active than TiO2 (350).

Among the TiO2-ACX (350) photocatalysts, those with less carbon content (0.5

and 1 wt. %) are slightly more active. Likely the detrimental effect of the carbon

presence is related to the fact that carbon can cover the active sites of TiO2,

decreasing the surface of TiO2 that is exposed to light, thus decreasing the

photocatalytic efficiency. According to this, TiO2-AC10 (350) sample would

expected to be less efficient than TiO2-AC5 (350), since its carbon content is

higher. However, for this sample the propene conversion is higher, and it

probably happens because the process of propene adsorption begins to be

significant, that is propene is partially removed by adsorption.

As the samples with the lower carbon content show better activity, a new

series of samples prepared with catalyst TiO2-AC0.5 heat treated at different

temperatures was also studied. The textural and crystalline properties, and the

carbon content of this new series of samples are collected in Table 5.9 and their

propene conversion values are included in the Figure 5.20.

Surprisingly, the TiO2-AC0.5 (T) samples, with a quite a low carbon

content (0.42, 0.13 and 0.02 wt. % for TiO2-AC0.5 (350), TiO2-AC0.5 (450) and

TiO2-AC0.5 (500), respectively) are less active than the analogous TiO2 (T)

samples (Figure 5.19) in spite of their higher surface area (see Table 5.9). This

can be explained because the TiO2-AC0.5 (T) samples have less anatase and are

more amorphous than the analogous TiO2 (T) samples and also, because, as

commented above, carbon can be responsible of decreasing the exposed TiO2

surface. As expected, an increase of the heat treatment temperature of the TiO2-

AC0.5 (T) samples, leads to an increase of propene conversion: from 350 to 450

°C, the activity increases 12 %, while upon a further increase to 500 ºC, it is


almost equal. As explained before, this can be related with the elimination of

remaining synthesis residues by the heat treatment.

Table 5.9. Textural and crystalline properties and carbon content of TiO2-

AC0.5 (T) samples.

SBET

a

(m2/g)

VDR N2b

(cm3/g)

Vmesoc

(cm3/g)

VTd

(cm3/g)

Amorphous

TiO2 (%)

Average

crystallite

size (nm)

C

content

(wt.%)

TiO2-AC0.5 (350) 154 0.06 0.18 0.27 26 9 0.42

TiO2-AC0.5 (450) 67 0.02 0.08 0.11 24 12 0.13

TiO2-AC0.5 (500) 37 0.01 0.06 0.08 25 18 0.02

aSBET: BET surface area calculated from N2 adsorption data. bVDR N2: micropore volume calculated applying the Dubinin-Radushkevich equation to the N2

isotherms. cVmeso: mesopore volume calculated as the difference between the amounts of nitrogen adsorbed

at P/P0 = 0.9 and P/P0 = 0.2. dVT: total pore volume determined from the amount of nitrogen adsorbed at P/P0 = 0.99 in the

N2 isotherms.

Figure 5.20. Propene conversion (at 30 and 60 mL/min) for P25, TiO2 (350),

TiO2-ACX (350) (X = 0.5, 1, 5 and 10 wt. %) and TiO2-AC0.5 (T) (T = 450 and 500

ºC) samples.

0

20

40

60

80

100TiO

2-A

C0.

5 (5

00)

TiO

2-A

C0.

5 (4

50)

TiO

2-A

C10

(350

)

TiO

2-A

C5

(350

)

TiO

2-A

C0.

5 (3

50)

TiO

2-A

C1

(350

)

TiO

2 (3

50)

Pro

pen

e co

nv

ersi

on

(%

)

60 mL/min

30 mL/min

P25

150 Chapter 5

5.5. Conclusions

This chapter deals with the study of TiO2 photocatalysts synthetized by

sol-gel method and heat treated at different temperatures, and TiO2-AC

photocatalysts also prepared by sol-gel with the addition of different amounts of

a synthesized AC (by hydrothermal carbonization of saccharose followed by

activation with CO2 at 800 ºC).

The surface area of the TiO2 samples decreases when the heat treatment

temperature increases and, it increases with the carbon content. The synthesized

samples contain only anatase and, as expected, the anatase content and the

crystallite size increase with the heat treatment temperature. The incorporation

of AC did not affect the TiO2 crystallinity as it does not mix intimately with the

TiO2 crystals.

In the photocatalytic decomposition of acetic acid, all the samples

produced more CH4 than CO2 than P25 and the CH4/CO2 ratio values indicate

that, in general, the photo-Kolbe reaction is predominant over the oxidation. The

TiO2-ACX (350) photocatalysts produce larger amounts of CH4 and CO2 than

bare TiO2 (350). Among them, sample TiO2-AC1 (350) is the one that produced

the highest CH4 amount. It important to note that all the photocatalysts maintain

their reaction rate during, at least, 40 h.

In the photodegradation of diuron, P25 is more active than the TiO2 (T)

and TiO2-ACX (350) (X = 0.5, 1 and 5 wt. %) samples. For the TiO2-AC10 (350)

a high diuron removal was measured due to the importance of the adsorption

process. It has been proved that k’ increases with the increase of the adsorption

time in dark; however, the disappearance rate on diuron under illumination

increases with the C0 values. On the other hand, TOC values and ion

chromatography data showed that diuron adsorbed onto the photocatalyst can be

desorbed and then oxidized.

All the prepared catalysts have shown be more active than P25 in the

photocatalytic oxidation of propene, what can be explained by their higher

anatase content and higher surface area. By increasing the heat treatment

temperature, propene conversion increases, and this has been attributed to the

removal of remaining organic compounds what leaves a higher exposed TiO2

surface. The TiO2-ACX (350) samples are less active than bare TiO2 (350),

probably due to their higher amorphous content. In addition, the carbon presence

could adsorb part of the light thus shielding the photoactive material.


5.6. References

[1] A.J. Shi, B.X. Li, C.R. Wan, D.C. Leng, E.Y. Lei, Hybrid density functional

studies of C-anion-doped anatase TiO2, Chem. Phys. Lett. 650 (2016) 19–28.

[2] R. Leary, A. Westwood, Carbonaceous nanomaterials for the enhancement of

TiO2 photocatalysis, Carbon. 49 (2011) 741–772.

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General conclusions/

Conclusiones generales

6.1. General conclusions

6.2. Conclusiones generales

6

156 Chapter 6

6.1. General conclusions

In the present Doctoral Thesis, different series of TiO2-based

photocatalysts have been prepared and characterized: i) commercial P25

modified with 1 wt. % (nominal) of transition metals (Cr, Co, Ni or Cu)

incorporated by impregnation followed by heat treatment at 500 ºC in Ar

atmosphere; ii) TiO2 photocatalysts synthetized by sol-gel method containing

different Cu percentages (0, 0.5, 1, 2, 5, 7 or 10 wt. %, nominal), which had been

incorporated by two different methods (in situ or impregnation), and then heat

treated at 500 ºC using two different atmospheres (Ar or air); iii) TiO2

photocatalysts prepared by sol-gel, without post-synthesis heat treatment and

heat treated in air at 350, 400, 450 and 500 ºC; and iv) TiO2 photocatalysts

modified with different amounts of AC-derived from saccharose (0, 0.5, 1, 5 and

10 wt. %, nominal) by in situ method and heat treated at 350 ºC in air.

The prepared photocatalysts have been used in three reactions for

pollutants’ removal: i) photodegradation of acetic acid into CH4, CO2 and H2; ii)

oxidative photodegradation of diuron and iii) photooxidation of propene at low

concentration.

From the study of the incorporation of different transition metals to P25

TiO2 the following conclusions have been obtained:

■ The crystalline and textural properties of TiO2 have not been affected after

the metal incorporation and, in general, the added metal species are highly

dispersed on the TiO2 surface.

■ Cr species could have partially been incorporated into the TiO2 structure

due to the Cr (III) and Cr (VI) radii are similar or lower, respectively,

compared to that of Ti (IV).

■ The nature of the metallic species in M/P 25-Ar samples influences their

activity for acetic acid and propene abatement. The photoactivity of the

M/P25-Ar samples follows in these two reactions the same trend: Cu/P25-

Ar > Ni/P25-Ar > Cr/P25-Ar ≈ Co/P25-Ar.

■ The low activity of the Co/P25-Ar is explained because Co (II) has lower

work function than Ti (IV), whereas in the case of Cr/P25-Ar, the

incorporation of Cr species into the TiO2 lattice seems to be detrimental

for the catalytic efficiency.

■ Cu/P25-Ar and Ni/P25-Ar are more efficient because these metal species

have work function values higher than Ti (IV). In the case of Cu/P25-Ar

the presence of Cu (II) and Cu (I) species, is considered to favour trapping

e‒ and h+ in an effective way.

157 General conclusions/Conclusiones generales

■ P25-Ar shows better activity than Cu/P25-Ar in propene oxidation,

whereas Cu/P25-Ar is more active than P25-Ar for acetic acid degradation.

This difference is probably related with the fact that the metal species

reduce the exposed titania surface and, in gas phase reactions, this has an

important effect, decreasing the photocatalytic efficiency.

■ M/P25-Ar samples are not active for the diuron photodegradation,

probably because the metal content is too high for this application and it

reduces the exposed titania surface.

The study based on the preparation of TiO2 photocatalysts modified with

Cu using different methods has allowed us to conclude that:

■ The Cu/TiO2 photocatalysts are more active than P25 in the acetic acid

photodegradation.

■ The presence of Cu seems to be beneficial for this reaction, being the in

situ incorporation method and the heat treatment in Ar the procedures that

give rise to the coexistence of the different copper species (Cu (II), Cu (I)

and Cu (0)) and thus to the highest photocatalytic efficiency.

■ The most active sample for acetic acid decomposition is Cu/TiO2is0.5-Ar

because it has the largest surface area, relatively small crystallite size and

highly-dispersed copper species.

■ In propene photooxidation P25 shows higher activity than the prepared

Cu/TiO2 photocatalysts; copper species can adsorb part of the UV light

and/or copper species could be located on TiO2 surface, which reduce the

exposure surface of titania, being that detrimental for the gas phase

reactions.

■ Catalysts prepared by in situ method are more active for propene oxidation

than those prepared by impregnation. The copper species incorporated by

impregnation are mainly located on TiO2 surface, blocking its porosity and

reducing the exposed titania surface.

■ The prepared Cu/TiO2 samples are not active for the diuron

photodegradation probably due to the metal content of the samples is too

high, reducing the exposed titania surface.

From the study of the effect of the temperature of the post heat treatment

after TiO2 preparation by sol-gel method, the following conclusions can be

extracted:

158 Chapter 6

■ As expected the heat treatment leads to a decrease of the SBET due to the

crystalline size growth and sintering of the mesopores. As the heat

treatment temperature increases, so do the anatase content and crystallite

size.

■ As the heat treatment temperature increases, the catalysts produce more

CH4 in the acetic acid photodegradation what has been explained by the

increase in the degree of crystallinity.

■ The series of TiO2 (T) catalysts is more active than P25 both in the

decomposition of acetic acid and in the oxidation of propene, what can be

explained by their large anatase content and higher SBET values.

■ P25 is more active than the TiO2 (T) series in the photocatalytic

degradation of diuron.

■ The temperature of post-synthesis heat treatment clearly affects to the

capacity of the TiO2 catalysts for diuron adsorption, but it does not

determine their efficiency for the photocatalytic degradation of diuron.

■ Propene conversion increases with the heat treatment temperature increase,

probably due to such heat treatments can affect the composition of the

photocatalyst, efficiently removing organic residues from TTIP precursor,

and leading to a greater number of active sites on the TiO2 surface.

From the study of the effect of activated carbon incorporation in the TiO2

samples, the following conclusions have been extracted:

■ The incorporation of AC to TiO2 improves the textural properties of the

photocatalysts but does not affect the TiO2 crystallinity.

■ In the acetic acid photodegradation, the TiO2-ACX (350) series produces

larger CH4/CO2 ratios than P25.

■ The obtained CH4/CO2 ratios, above 0.3 for all TiO2-ACX (350) samples,

indicate that the photo-Kolbe reaction is predominant over the oxidation.

■ The presence of a low amount of AC in the photocatalysts is beneficial for

acetic acid photodegradation, being TiO2-AC1 (350) the most active

sample. Higher carbon content leads to an efficiency decrease, possibly

because carbon could adsorb part of the light thus shielding the photoactive

material.

■ Regarding diuron elimination, only TiO2-AC10 (350) leads to higher

removal rate (k’) than P25. This is because the adsorption process is

especially important in this sample.


■ Adsorbed diuron can be desorbed and then oxidized, which is useful for

the reuse of catalysts.

■ The TiO2-ACX catalysts are less active than bare TiO2 for propene

conversion, probably because of the reduction of the exposed TiO2 on the

surface of the hybrid photocatalyst.

In general, it can be concluded that, the photocatalytic efficiency depends

on several parameters. Among them, surface area, crystal phase, crystallite size

and the percentage of crystalline TiO2 have a very important effect. Furthermore,

the presence of additives plays an important role on the electrochemical

properties of the final material and therefore on its photocatalytic efficiency.

Although the photodegradation pathway of each studied pollutant is

different, it has been observed that, in general, the presence in too high amounts

of the additive (Cr, Co, Ni, Cu or AC) seems detrimental to its photocatalytic

efficiency due to the reduction of the active sites of the TiO2.

6.2. Conclusiones generales

En la presente Tesis Doctoral, se han preparado y caracterizado diferentes

series de fotocatalizadores basados en TiO2: i) P25 comercial modificado con 1

% en peso (nominal) de metales de transición (Cr, Co, Ni o Cu) incorporados por

impregnación seguido, de un tratamiento térmico a 500 ºC en atmósfera de Ar;

ii) fotocatalizadores de TiO2 sintetizados por el método sol-gel incorporando

diferentes porcentajes de Cu (0, 0.5, 1, 2, 5, 7 o 10 % en peso, nominal) mediante

dos métodos diferentes (in situ o impregnación), y tratados térmicamente a 500

ºC utilizando dos atmósferas diferentes (Ar o aire); iii) fotocatalizadores de TiO2

preparados por sol-gel, no tratados y tratados térmicamente en aire a 350, 400,

450 y 500 ºC; y, por último, iv) fotocatalizadores de TiO2 modificados con

diferentes cantidades de carbón activado derivado de sacarosa (0, 0.5, 1, 5 y 10

% en peso, nominal) por el método in situ y tratados térmicamente a 350 ºC en

aire.

Los fotocatalizadores preparados se han utilizado para la eliminación de

tres contaminantes: i) fotodegradación de ácido acético a CH4, CO2 e H2; ii)

fotodegradación oxidativa de diurón y iii) fotooxidación de propeno a baja

concentración.

Del estudio de la incorporación de diferentes metales de transición al TiO2

P25 se obtuvieron las siguientes conclusiones:

■ Las propiedades cristalinas y texturales del TiO2 no se han visto afectadas

después de la incorporación del metal y, en general, las especies metálicas

añadidas están bien dispersas en la superficie del TiO2.

■ Las especies de Cr pueden haberse incorporado parcialmente a la

estructura del TiO2 debido a que los radios iónicos de Cr (III) y Cr (VI)

son similares o inferiores, respectivamente, a los de Ti (IV).

■ La naturaleza de las especies metálicas en las muestras M/P25-Ar influye

en su actividad para la degradación de ácido acético y la fotooxidación de

propeno. La fotoactividad de las muestras sigue, en estas dos reacciones,

la misma tendencia: Cu/P25-Ar > Ni/P25-Ar > Cr/P25-Ar ≈ Co/P25-Ar.

■ La baja actividad del Co/P25-Ar es debida a que el Co (II) posee una

función de trabajo inferior a la del Ti (IV), mientras que en el caso de la

muestra Cr/P25-Ar, la incorporación (parcial) de las especies de Cr a la

red de TiO2 parece ser perjudicial para su eficiencia catalítica.

■ Cu/P25-Ar y Ni/P25-Ar son más eficientes porque estas especies metálicas

tiene valores de función de trabajo más altos que Ti (IV). En el caso de

Cu/P25-Ar, se considera, además, que la presencia de las especies Cu (II)

y Cu (I) favorece la captura efectiva de e‒ y h+.


■ P25-Ar muestra mayor actividad que Cu/P25-Ar en la fotooxidación de

propeno, mientras que Cu/P25-Ar es más activo que P25-Ar para la

degradación del ácido acético. Esta diferencia probablemente esté

relacionada con el hecho de que las especies metálicas reducen la

superficie de la titania expuesta y, en las reacciones en fase gaseosa, esto

tiene un efecto importante, al disminuir la eficiencia fotocatalítica.

■ Las muestras de M/P25-Ar no son activas para la fotodegradación de

diurón, probablemente porque el contenido en metal es demasiado alto

para esta aplicación y reduce la superficie de titania expuesta.

El estudio basado en la preparación de los fotocatalizadores de TiO2

modificados con Cu utilizando diferentes métodos nos ha permitido concluir que:

■ Los fotocatalizadores de Cu/TiO2 son más activos que P25 en la

fotodegradación del ácido acético.

■ La presencia de Cu parece ser beneficiosa para esta reacción, siendo el

método de incorporación in situ y el tratamiento térmico en Ar los

procedimientos que dan lugar a la coexistencia de las diferentes especies

de cobre (Cu (II), Cu (I) y Cu (0)) y, por lo tanto, a la eficiencia

fotocatalítica más alta.

■ La muestra más activa en la descomposición del ácido acético es

Cu/TiO2is0.5-Ar porque tiene el área superficial más elevada, un tamaño

de cristal relativamente pequeño y, en ella, las especies de cobre se

encuentran muy bien dispersas.

■ En la fotooxidación de propeno, P25 muestra una actividad más alta que

los fotocatalizadores Cu/TiO2preparados. Esto es debido a que las especies

de cobre pueden adsorber parte de la radiación o podrían ubicarse en la

superficie de la titania, reduciendo su superficie de exposición, lo cual es

perjudicial para las reacciones en fase gas.

■ Los catalizadores preparados por el método in situ son más activos en la

oxidación de propeno que los preparados por el método impregnación. Las

especies de cobre incorporadas por impregnación están ubicadas

principalmente en la superficie del TiO2, bloqueando su porosidad y

reduciendo la superficie de titania expuesta.

■ Las muestras Cu/TiO2 no son activas en la fotodegradación de diurón,

probablemente debido a que su contenido en metal es demasiado alto,

reduciendo así la superficie de titania expuesta.

166 Chapter 6

A partir del estudio del efecto de la temperatura del tratamiento térmico

realizado después de la síntesis de TiO2 por el método sol-gel, se pueden extraer

las siguientes conclusiones:

■ Como cabría esperar, el tratamiento térmico conlleva una disminución de

SBET debido al crecimiento del tamaño cristalino y la sinterización de los

mesoporos. A medida que aumenta la temperatura del tratamiento térmico,

también lo hace el contenido en anatasa y el tamaño de los cristales.

■ Conforme aumenta la temperatura del tratamiento térmico, los

catalizadores producen más CH4 en la fotodegradación de ácido acético,

lo cual es debido al aumento del grado de cristalinidad.

■ La serie de catalizadores de TiO2 (T) es más activa que P25, tanto en la

descomposición de ácido acético como en la oxidación de propeno, lo que

puede explicarse por su gran contenido en anatasa y sus valores de SBET

más elevados.

■ P25 es más activo que la serie TiO2 (T) en la degradación fotocatalítica de

diurón.

■ La temperatura del tratamiento térmico afecta claramente a la capacidad

de adsorción de diurón de los catalizadores TiO2, pero no determina su

eficiencia para su degradación fotocatalítica.

■ La conversión de propeno aumenta con el aumento de la temperatura del

tratamiento térmico, probablemente debido a que dichos tratamientos

térmicos pueden afectar a la composición del fotocatalizador, eliminando

eficazmente los residuos orgánicos del precursor de TTIP y conduciendo

a un mayor número de sitios activos en la superficie del TiO2.

Del estudio del efecto de la incorporación de carbón activado en las

muestras de TiO2 se han extraído las siguientes conclusiones:

■ La incorporación de AC al TiO2 mejora las propiedades texturales de los

fotocatalizadores, pero no afecta a la cristalinidad del TiO2.

■ En la fotodegradación de ácido acético, la serie TiO2-ACX (350) produce

relaciones CH4/CO2 más elevadas que las que produce P25.

■ Las relaciones CH4/CO2 obtenidas, superiores a 0.3 para todas las muestras

TiO2-ACX (350), indican que la reacción de foto-Kolbe predomina sobre

la oxidación del ácido acético.

■ La presencia de una baja cantidad de AC en los fotocatalizadores es

beneficiosa para la fotodegradación del ácido acético, siendo la muestra

TiO2-AC1 (350) la más activa. Un mayor contenido en carbón conduce a


una disminución de la eficiencia fotocatalítica, posiblemente debido a que

el AC podría absorber parte de la luz.

■ Respecto a la eliminación de diurón, solo la muestra TiO2-AC10 (350)

conduce a una velocidad de eliminación mayor que en el caso de P25. Esto

es debido a que el proceso de adsorción es especialmente importante en

esta muestra.

■ El diurón adsorbido se puede desorber y luego oxidar, lo cual es útil para

la reutilización de los fotocatalizadores.

■ Los fotocatalizadores de TiO2-ACX son menos activos que el TiO2 para la

fotooxidación de propeno, probablemente debido a la reducción de la

cantidad de TiO2 expuesto en la superficie del fotocatalizador híbrido.

En general, se puede concluir que la eficiencia fotocatalítica depende de

varios parámetros. La superficie específica, la composición cristalina, el tamaño

de los cristales y el porcentaje de TiO2 cristalino tienen un efecto sustancial.

Además, la presencia de aditivos desempeña un papel importante en las

propiedades fisicoquímicas del material final y, por lo tanto, en su eficacia

fotocatalítica.

Aunque el mecanismo de fotodegradación de cada contaminante estudiado

es diferente, se ha observado que, en general, la presencia en cantidades

demasiado altas de aditivo (Cr, Co, Ni, Cu o AC) parece ser perjudicial para su

eficiencia fotocatalítica debido a la disminución de los sitios activos del TiO2 por

el elemento adicionado.

La verdadera ciencia enseña, sobre

todo, a dudar y a ser ignorante.

Miguel de Unamuno.

tio2 based photocatalysts for environmental remediation ... · potential for abatement of...

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