espace.library.uq.edu.aud4bf768/s4344402_final_the… · p a g e | i abstract advanced oxidation...

Perovskite Catalysts for Advanced Oxidation Processes (AOPs) in

Wastewater Treatment

Huihuang Chen

Master of Engineering

A thesis submitted for the degree of Doctor of Philosophy at

The University of Queensland in 2018

School of Chemical Engineering

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Abstract

Advanced oxidation processes (AOPs) are desirable to treat industrial wastewaters containing

dyes, mainly attributed to degradation by in-situ generated highly oxidizing hydroxyl radicals

(HO•). However, current AOPs generally require chemical additives or energy that hinder their

economic competitiveness. Metal oxides following a general formula ABO, including

perovskites, have been applied as heterogeneous catalyst for dye degradation under dark

ambient conditions without additional chemical/energy input. Nevertheless, significant

research gaps still exist associated with the sluggish degradation kinetics and structural stability

of catalysts in addition to a limited number of metal oxides reported to date as catalysts under

dark ambient conditions.

There is an array of metal oxides that can be used as catalysts for dye degradation under dark

conditions which remain unexplored. The research gaps in this theme include oxides of copper,

nickel and cobalt as B-site cation, whilst alkaline earth metals (Sr, Ca and Mg) in addition to

lanthanide elements (Ce and La) could be incorporate into the A-site, similar to perovskites.

These metal oxides can be formulated into binary (ABO), ternary (AA’BO or ABB’O) or

quaternary (AA’BB’O) compounds. This thesis therefore aims to understand the fundamental

correlation of physicochemical properties and catalytic performance of these metal oxide

compounds. It was hypothesized that the partial substitution of A’ or B’ in ABO3 confers

tuneable catalytic properties to enhance the degradation performance for Orange II (OII), which

is a major water pollutant from dye-related industries.

The first key contribution of this thesis is the successful preparation of CuO based catalysts

(AA’BO) as CaSrCuO (CSC) formed both metal oxide and perovskite phases. The varied Ca

content resulted in materials with different phases and CSC with high Ca content was very

effective, reaching 80% of OII (20 mg L-1) degradation in 10 min with 60% TOC removal after

120 min. CSC proved to be stable and maintained high performance up to nine cycles (75%).

CSC was very active in breaking down azo bonds of OII, thus generating electrons which

reacted with dissolved O2 in the solutions to yield reactive species (e.g., HO•) for further

degradation and mineralization.

The second contribution of this thesis is the replacement of Cu by Ni (again AA’BO) as

CaSrNiO (CSN), which formed a very active Ni2+ phase in the CSN metal oxide. It was found

that 97% OII was discoloured within 5min though TOC removal was low (~10%). The fast

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degradation was aided by electron donation from Ni2+ to O2, resulting in the formation of Ni3+

and reactive species (e.g., HO•). The Ni3+ proved to be catalytically inactive for OII

degradation under dark ambient conditions, and the catalytic performance of CSN decayed

rapidly under cycling testing.

The third contribution of this work was born out from a desire to stabilise the very active NiO.

Therefore, the research approach was to partially substitute Ni in CSN with Cu as AA’BB’O

compound to give CaSrNiCuO (CSNC). Indeed, CSNC was stable over 15 cycles with the

added benefit of maintaining a high OII degradation efficiency of 84% and good mineralization

(54% in 2h). Apart from electron transfer from the breakdown of azo bonds to O2 to generate

radical species, extra second electron transfer pathway was postulated from Ni2+/Cu1+ in

pristine CSNC to Ni3+/Cu2+ in the spent catalyst, where Cu2+ also proved to be an active phase

for the long-term catalytic stability.

The fourth contribution of this thesis focused on CoO as another B-site metal oxide and using

the A-site cations (Ba, Ca and Mg) and A’ site (Sr) as ASrCoO. These compounds resulted in

variable morphology, crystallite size, phases and catalytic performance. Ba and Ca was

partially substituted in the A-site and formed Ba0.5Sr0.5CoO3 and (Ca0.2Sr0.8)5Co4O12

perovskites, respectively, whereas Mg could not be incorporated, forming MgO and SrCoO

perovskite. These compounds reached high degradation efficiency though the degradation

kinetics was low (90% in 8h). A major finding here is that ASrCoO compounds proved to be

stable over 7 cycles, particularly for BaSrCoO with ~85% degradation efficiency over 7 cycles.

However, the catalytic performance of the non-substituted SrCoO for OII degradation decayed

at every cycle, thus confirming that the partial substitution as ASrCoO conferred superior stable

catalytic properties.

The current project designed, synthesized, characterized and evaluated a series of metal oxide

based heterogeneous catalysts. The reported catalysts herein dispensed the use of valuable

chemicals and energy input while demonstrating high catalytic activity and superior

recyclability for OII degradation under dark ambient conditions, which is of great importance

for potential practical applications. It is anticipated that they could be applied as efficient

alternative materials for low cost AOPs in the field of water treatment.

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Declaration by author

This thesis is composed of my original work, and contains no material previously published or

written by another person except where due reference has been made in the text. I have clearly

stated the contribution by others to jointly-authored works that I have included in my thesis.

I have clearly stated the contribution of others to my thesis as a whole, including statistical

assistance, survey design, data analysis, significant technical procedures, professional editorial

advice, financial support and any other original research work used or reported in my thesis.

The content of my thesis is the result of work I have carried out since the commencement of

my higher degree by research candidature and does not include a substantial part of work that

has been submitted to qualify for the award of any other degree or diploma in any university

or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been

submitted to qualify for another award.

I acknowledge that an electronic copy of my thesis must be lodged with the University Library

and, subject to the policy and procedures of The University of Queensland, the thesis be made

available for research and study in accordance with the Copyright Act 1968 unless a period of

embargo has been approved by the Dean of the Graduate School.

I acknowledge that copyright of all material contained in my thesis resides with the copyright

holder(s) of that material. Where appropriate I have obtained copyright permission from the

copyright holder to reproduce material in this thesis and have sought permission from co-

authors for any jointly authored works included in the thesis.

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Publications during candidature

Peer-reviewed papers

1. H. Chen, J. Motuzas, W. Martens, J.C. Diniz da Costa, Degradation of azo dye Orange

II under dark ambient conditions by calcium strontium copper perovskite, Appl. Catal.

B: Environ. 221 (2018) 691-700.

2. H. Chen, J. Motuzas, W. Martens, J.C. Diniz da Costa, Ceramic metal oxides with Ni2+

active phase for the fast degradation of Orange II dye under dark ambiance, Ceram. Int.

44 (2018) 6634-6640.

Publications included in this thesis

H. Chen, J. Motuzas, W. Martens, J.C. Diniz da Costa, Degradation of azo dye Orange II under

dark ambient conditions by calcium strontium copper perovskite, Appl. Catal. B: Environ. 221

(2018) 691-700.–Incorporated as Chapter 3.

Contributor Statement of contribution

Author Huihuang Chen (Candidate)

Conception and design (80%)

Analysis and interpretation (80%)

Drafting and production (80%)

Author Julius Motuzas




Author Wayde Martens




Author João C. Diniz da Costa




H. Chen, J. Motuzas, W. Martens, J.C. Diniz da Costa, Ceramic metal oxides with Ni2+ active

phase for the fast degradation of Orange II dye under dark ambiance, Ceram. Int. 44 (2018)

6634-6640.–Incorporated as Chapter 4.






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Manuscripts included in this thesis

H. Chen, J. Motuzas, W. Martens, J.C. Diniz da Costa, Effective degradation of Azo Dyes in

the Dark by Cu2+ Active Sites in CaSrNiCu Oxides, submitted to Journal of Environmental

Chemical Engineering–Incorporated as Chapter 5.


















H. Chen, J. Motuzas, W. Martens, J.C. Diniz da Costa, Degradation of Orange II dye under

dark ambient conditions by MeSrCuO (Me = Mg and Ce) metal oxides, submitted to Separation

and Purification Technology–Incorporated as Chapter 6.


Author Huihuang Chen (Candidate) Conception and design (80%)


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H. Chen, J. Motuzas, W. Martens, J.C. Diniz da Costa, Surface and catalytic properties of

stable Me(Ba, Ca and Mg)SrCoO for the degradation of Orange II dye under dark conditions,

submitted to Applied Surface Science–Incorporated as Chapter 7.


















Contributions by others to the thesis

Contributions by Julius Motuzas, Wayde Martens and João C. Diniz da Costa in the concept of

research work, experiment design, data analysis and interpretation as well as drafting & writing

in the advisory capacity.

Statement of parts of the thesis submitted to qualify for the award of another degree

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None.

Research Involving Human or Animal Subjects

No animal or human subjects were involved in this research.

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Acknowledgements

I would like to express my foremost gratitude to my advisors Prof. João C. Diniz da Costa, Dr.

Julius Motuzas and Dr. Wayde Martens for the continuous support, immense time and

knowledge contributions, invaluable guidance, motivation, encouragement and enthusiasm

throughout my PhD study. Joe, thank you for always being encouraging and for all the help

throughout my journey of the thesis. I sincerely acknowledge my committee members (Prof.

George Zhao and Prof. Suresh Bhatia) for their valuable reviews, precious time, constructive

criticisms and suggestions. Thank you all for making this project a possibility.

I would like to give my thanks to those who assisted me in diverse ways in the course of PhD

research including Simon Smart, David Wang, Dana Martens, Christelle Yacou, Guozhao Ji,

Liang Liu, Shengnan Wang, Guotong Qin, Hong Yang, Xiaozhen Zhang, Gianni Olguin

Contreras, Ben Ballinger, Nurehan Siti, Muthia Elma, Wenjing Wang, Charmaine Lamiel,

Rasmus Madsen, Yue Yuan and Nadine Elshof.

I also thank Dr. Barry Wood and Anya Yago for the training and technical support. Special

acknowledgements give to UQ staffs, especially Liam Bull, Siu Bit Iball, and June Nicholson,

Allan Duong, Martin Bull, Rik Taylor, Vicki Thompson, Steve Coombs for their technical

advices and supports.

My heartiest gratitude goes to my girlfriend (Lan Li) for your unconditional love,

understanding and encouragement, which makes me happy all the time. Hope you feel the same

way. I dedicated this thesis to my beloved family for your endless love and continuous support,

which is always the pillar of my strength throughout my life.

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Financial support

This research was supported by Australian Research Council, China Scholarship Council, and

UQ TUAP.

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Keywords

advanced oxidation processes (AOPs), perovskites, catalysts, heterogeneous catalysis, water

remediation, dark ambient conditions

Australian and New Zealand Standard Research Classifications (ANZSRC)

ANZSRC code: 090402, Catalytic Process Engineering, 40%

ANZSRC code: 090409, Wastewater Treatment Process, 40%

ANZSRC code: 091201, Ceramics, 20%

Fields of Research (FoR) Classification

FoR code: 0904, Chemical Engineering

FoR code: 0302, Inorganic Chemistry

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Table of Contents

Abstract ....................................................................................................................................... i

Publications during candidature ................................................................................................ iv

Publications included in this thesis ........................................................................................... iv

Manuscripts included in this thesis ............................................................................................ v

Contributions by others to the thesis ......................................................................................... vi

Acknowledgements ................................................................................................................ viii

Financial support ....................................................................................................................... ix

Keywords ................................................................................................................................... x

Table of Contents ...................................................................................................................... xi

List of Figures .......................................................................................................................... xv

List of Tables ........................................................................................................................... xx

List of Abbreviaitons .............................................................................................................. xxi

1 Introduction ........................................................................................................................... 1

1.1 Background ...................................................................................................................... 1

1.2 Scope and research contributions ..................................................................................... 2

1.3 Structure of thesis ............................................................................................................. 2

1.4 References ........................................................................................................................ 4

2 Literature review .................................................................................................................. 6

Abstract .................................................................................................................................. 6

2.1 Dye contaminated wastewater .......................................................................................... 6

2.2 AOPs for dye-containing wastewater treatment............................................................... 8

2.2.1 Available AOPS for dye-containing wastewater treatment ....................................... 8

2.2.2 Conclusions of current available AOPs ................................................................... 11

2.3 Catalysts for water remediation...................................................................................... 11

2.3.1 Non-perovskite catalysts.......................................................................................... 11

2.3.2 Perovskite catalysts.................................................................................................. 12

2.4 Conclusion and research gaps ........................................................................................ 17

2.5 References ...................................................................................................................... 18

3 Degradation of azo dye Orange II under dark ambient conditions by calcium strontium

copper perovskite ................................................................................................................... 30

3.1 Introduction .................................................................................................................... 30

3.2 Contributions .................................................................................................................. 30

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3.3 Abstract .......................................................................................................................... 31

3.4 Introduction .................................................................................................................... 31

3.5 Materials and methods ................................................................................................... 33

3.5.1 Synthesis and characterisation ................................................................................. 33

3.5.2 Catalyst evaluation .................................................................................................. 34

3.6 Results and discussion .................................................................................................... 35

3.6.1 Catalyst characterization .......................................................................................... 35

3.6.2 Catalyst performance ............................................................................................... 36

3.6.3 Role of CSC compounds and reaction mechanism ................................................. 40

3.7 Conclusions .................................................................................................................... 45

3.8 References ...................................................................................................................... 48

4 Ceramic metal oxides with Ni2+ active phase for the fast degradation of Orange II dye

under dark ambiance ............................................................................................................. 53

4.1 Introduction .................................................................................................................... 53

4.2 Contributions .................................................................................................................. 53

4.3 Abstract .......................................................................................................................... 54

4.4 Introduction .................................................................................................................... 54

4.5 Materials and methods ................................................................................................... 55

4.5.1 Materials, synthesis and characterization ................................................................ 55

4.5.2 Catalytic activity evaluation .................................................................................... 56


4.6.1 Characterization ....................................................................................................... 57

4.6.2 Reaction assessment ................................................................................................ 58

4.6.3 Degradation mechanism .......................................................................................... 62

4.7 Conclusions .................................................................................................................... 64

4.8 References ...................................................................................................................... 66

5 Effective degradation of Azo Dyes in the Dark by Cu2+ Active Sites in CaSrNiCu Oxides

.................................................................................................................................................. 70

5.1 Introduction .................................................................................................................... 70

5.2 Contributions .................................................................................................................. 70

5.3 Abstract .......................................................................................................................... 71

5.4 Introduction .................................................................................................................... 71

5.5 Experimental section ...................................................................................................... 73

5.5.1 Materials synthesis and characterization ................................................................. 73

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5.5.2 Catalysis................................................................................................................... 73


5.7 Conclusions .................................................................................................................... 83

5.8 References ...................................................................................................................... 84

6 Degradation of Orange II dye under dark ambient conditions by MeSrCuO (Me = Mg

and Ce) metal oxides .............................................................................................................. 87

6.1 Introduction .................................................................................................................... 87

6.2 Contributions .................................................................................................................. 87

6.3 Abstract .......................................................................................................................... 88

6.4 Introduction .................................................................................................................... 88

6.5 Experimental .................................................................................................................. 90

6.5.1 Materials and characterization ................................................................................. 90

6.5.2 Catalysis................................................................................................................... 91


6.6.1 Catalytic activity ...................................................................................................... 91

6.6.2 Structural and morphological analysis .................................................................... 94

6.6.3 Degradation mechanism .......................................................................................... 97

6.7 Conclusions .................................................................................................................... 99

6.8 References ...................................................................................................................... 99

7 Surface and catalytic properties of stable Me(Ba, Ca and Mg)SrCoO for the

degradation of Orange II dye under dark conditions ...................................................... 104

7.1 Introduction .................................................................................................................. 104

7.2 Contributions ................................................................................................................ 104

7.3 Abstract ........................................................................................................................ 105

7.4 Introduction .................................................................................................................. 106

7.5 Materials and method ................................................................................................... 107

7.5.1 Materials, synthesis and characterization .............................................................. 107

7.5.2 Catalysis................................................................................................................. 108

7.6 Results and discussion .................................................................................................. 109

7.6.1 Characterization of catalysts .................................................................................. 109

7.6.2 Catalyst activity ..................................................................................................... 112

7.6.3 Surface properties and reaction mechanism .......................................................... 114

7.7 Conclusions .................................................................................................................. 119

7.8 References .................................................................................................................... 120

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8 Conclusions and recommendations ................................................................................. 124

8.1 Conclusions .................................................................................................................. 124

8.2 Recommendations ........................................................................................................ 125

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List of Figures

Figure 2. 1: UV-vis spectrum of OII solution. Insert is tautomeric forms of OII. ..................... 7

Figure 2. 2: Available techniques for dye-containing wastewater treatment. ............................ 8

Figure 2. 3: ABO3 perovskite crystal structure. [68]............................................................... 13

Figure 2. 4: Typical elements in ABX3 perovskites. ............................................................... 13

Figure 2. 5: (a) UV-vis spectra evolution of MO in the presence of La4Ni3O10. Experimental

conditions: [catalyst] =1 g L-1, [MO] = 5 mg L-1, 650 rpm. ............................... 15

Figure 2. 6: Evolution of calcium-based perovskites synthesized via A-/B-site doping for dye

degradation. A/A’ and B/B’ represent A-site and B-site cations in ABO3

perovskites. ........................................................................................................... 16

Figure 3. 1: SEM images of CSC for x values of (a) 0, (b) 0.2, (c) 0.5, (d) 0.75 and (e) 1.0. . 35

Figure 3. 2: XRD patterns of CSC samples. ............................................................................ 36

Figure 3. 3: (a) Effect of Ca and Sr cation concentration in the A-site of CSC for the degradation

(±2 ppm) of OII. Experimental conditions: [OII]0=50 ppm, magnetic stirring, RT,

dark. (b) Cycling stability of CSC (x=0.75) sample. Experimental conditions:

[OII]0=20 ppm, magnetic stirring, RT, dark. ........................................................ 37

Figure 3. 4: (a). UV-vis absorption spectra evolution of OII with time and (b) TOC readings

for CSC (0≤x≤1). Experimental conditions: [OII]0=50 ppm, catalyst dosage=1 g L-

1, magnetic stirring, RT. ........................................................................................ 39

Figure 3. 5: (a) HPLC chromatograms and (b) UV-vis spectra of species found with reaction

times of 0, 1, 5 and 120 min. Experimental conditions: [OII]0=50 ppm, catalyst

dosage=1 g L-1, magnetic stirring, RT. ................................................................. 40

Figure 3. 6: Pristine and spent CSC high resolution XPS spectra of (a) Ca, (b) Sr, (c) Cu and

(d) O. ..................................................................................................................... 41

Figure 3. 7: TGA mass loss curves of OII, pristine CSC, blank CSC (water only) and spent

CSC (50 ppm of OII) samples............................................................................... 42

Figure 3. 8: FTIR spectra of OII, pristine CSC, spent CSC (50 ppm of OII) and physically

mixed samples of OII (10 and 50 ppm) with pristine CSC (see appendix Table A1

for band assignments). .......................................................................................... 43

Figure 3. 9: Degradation curves of OII (50 ppm) with no additive and OII (50 ppm) plus 2-

propanol (0.1 M) using CSC catalyst. Experimental conditions: [CSC]=1 g L-1,

magnetic stirring, RT and dark. ............................................................................ 44

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Figure 3.A 1: Representative nitrogen sorption isotherm of CSC samples (x=0.75). This

isotherm of Type II, is characteristic of dense particles due to the very low adsorbed

volume at low relative pressures. The increase of volume at p/po>0.75 is associated

with the inter-particle space. ................................................................................. 46

Figure 3.A 2: Effect of Ca and Sr cation concentration in the A-site of CSC for the degradation

(±2ppm) of OII for [OII]0 of (a) 10 ppm and (b) 100 ppm. Experimental conditions

magnetic stirring, RT and dark. ............................................................................ 47

Figure 3.A 3: Degradation of OII (10 ppm) without any catalyst after 120 min. Experimental

conditions: magnetic stirring, RT, dark. ............................................................... 47

Figure 3.A 4: pH measured as a function of time during the degradation of OII. ................... 48

Figure 3.A 5: Survey XPS spectra of OII, pristine and spent CSC. ........................................ 48

Figure 4. 1: SEM images of (a) raw and (b) spent CSN samples. ........................................... 57

Figure 4. 2: XRD patterns of (a) raw and (b) spent CSN samples........................................... 58

Figure 4. 3: XPS spectra of Ca 2p, Sr 3d and Ni 2p for both raw and spent CSN samples. ... 58

Figure 4. 4: (a) UV-vis absorption spectra of OII azo dye solution treated by CSN recorded at

different time intervals; (b) and (c) are the magnification of the UV region from

240 to 280 nm and from 300 to 410 nm, respectively. Benzene ring (P1), napthalene

ring (P2), azo form (P3) and hydrozone form (P4). Experimental conditions:

[OII]0=20 ppm, [CSN]=1 g L-1, magnetic stirring, RT, dark. ............................... 59

Figure 4. 5: (a) Effect of dye initial concentrations on degradation efficiency, and (b) reaction

cycle stability. Experimental conditions: [CSN]=1 g L-1, magnetic stirring, RT,

dark........................................................................................................................ 60

Figure 4. 6: (a) HPLC chromatograms of initial OII and treated OII after 2 h. (b) UV–vis

absorbance spectra of treated OII solution at the retention time of 1.1, 1.2 and 1.7

min. ....................................................................................................................... 60

Figure 4. 7: Mass loss of spent CSN (OII = 20 ppm) and blank CSN (exposed to water only)

samples. ................................................................................................................. 61

Figure 4. 8: Effect of reaction quenchers on OII degradation by CSN. Experimental conditions:

[OII]0=20 ppm, [CSN]=1 g L-1, [EDTA-2Na]=[KI]=[IPA]=[KBrO3]=0.10 M,

magnetic stirring, RT, dark. .................................................................................. 62

Figure 4. 9: Illustration of sensitization degradation mechanism for OII degradation over CSN

for [I] direct electron transfer and [II] nickel oxidation. ....................................... 63

Figure 4.A 1: Nitrogen sorption isotherm for adsorption (square) and desorption (circle). .... 65

Figure 4.A 2: XPS wide survey scan. ...................................................................................... 65

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Figure 4.A 3: The UV-vis absorbance spectra of OII in dark under magnetic stirring. .......... 66

Figure 5. 1: (a) SEM micrograph and (c) XRD patterns of CSNC (x=0.5). (b) N2 sorption of

CSNC (x=0.2, 0.5 and 0.8).................................................................................... 75

Figure 5. 2: (a) UV-vis spectra showing OII degradation as a function of time by CSNC (x=0.5),

(b) UV-vis spectrum variation of OII solution in the blank experiment.

Experimental conditions: [OII]0=20 ppm, [Catalyst] =1 g L-1, T=20 ℃, in the dark

and under constant stirring. ................................................................................... 75

Figure 5. 3: TGA plots of OII, blank CSNC and spent CSNC samples with airflow of 80 mL

min-1 and heating rate of 5 °C min-1. ..................................................................... 76

Figure 5. 4: (a) Effect of Cu and Ni cation concentration in CSNC for the degradation of OII

solution (C0=20 ppm); and (b) degradation of OII solution at varying initial

concentration (C0 from 20 to 35 ppm) by CSNC (x = 0.5). Experimental conditions:

[Catalyst] =1 g L-1, T=20 ℃, in dark and under constant stirring. ....................... 77

Figure 5. 5: Cycling test for CSNC (x = 0.5). Experimental conditions: [OII]0=20 ppm,

[Catalyst] =1 g L-1, T=20 ℃, in the dark and under constant stirring. Initial OII

concentration of 20 ppm and for each subsequent cycle 20 ppm was added to the

solution. ................................................................................................................. 77

Figure 5. 6: (a) HPLC chromatograms of OII solution degraded by CSNC (x=0.5), (b)

magnification of Fig. 4a in the retention time from 1 to 2 min, and (c) UV-vis

spectra of degradation products at 0, 30, and 120 min at the retention time of ca.

1.3 min. ................................................................................................................. 78

Figure 5. 7: Wide XPS spectra of pristine and spent CSNC and high-resolution XPS spectra of

Ca 2p, Sr 3d, Ni 2p and Cu 2p for pristine and spent CSNC. ............................... 79

Figure 5. 8: The relative amounts of Cu1+ and Ni2+ in raw and cycled CSNC (x=0.5). .......... 80

Figure 5. 9: Effect of radical quenchers on OII degradation by CSNC (x=0.5). Experimental

conditions: [OII]0=20 ppm, [CSNC]=1 g L-1, [EDTA-2Na]=[IPA]=0.10 M,

[KI]=20 mM, [KBrO3]= 10 mM, T=20 ℃, in the dark and under constant stirring.

............................................................................................................................... 81

Figure 5. 10: TOC results vs. time for CSNC (x = 0.5). Experimental conditions: [OII]0=20

ppm, [Catalyst] =1 g L-1, T=20 ℃, in the dark and under constant stirring. ........ 81

Figure 5. 11: Schematic illustration of OII degradation by CSNC catalyst. ............................ 82

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Figure 5. 12: Kinetics of OII degradation: 1/C versus time for different concentrations.

Experimental conditions: [Catalyst] =1 g L-1, T=20 ± 2℃, in the dark and under

constant stirring. .................................................................................................... 83

Figure 6. 1: Normalized OII concentrations (a) and corresponding UV-vis spectra of initial and

treated OII solutions after 30 min (c) by MeSrCuO (Me= Mg and Ce) and blank

sample SrCuO. (b) Normalized OII concentrations by MgO and CeO2 after 2h.

Experimental conditions: [OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, in the dark

and under stirring. ................................................................................................. 92

Figure 6. 2: (a) TOC removal efficiency of OII by Mg0.5Sr0.5CuO and Ce0.5Sr0.5CuO after 120 min

and (b) colour intensity variation of OII solution after treatment for 30 min by

MgSrCuO and CeSrCuO. Experimental conditions: [OII]0=20 ppm, [catalyst]=1 g L-1,

T=20 °C, in the dark and under magnetic stirring. .................................................... 92

Figure 6. 3: HPLC chromophores of OII solutions vs. retention time for (a) MgSrCuO and (b)

CeSrCuO. ............................................................................................................... 93

Figure 6. 4: UV-vis spectra of eluted species by (a) Mg0.5Sr0.5CuO and (b) Ce0.5Sr0.5CuO at the

retention time (RT) of 1.0 and 1.2 min. ................................................................... 94

Figure 6. 5: Cycling performance of MgSrCuO and CeSrCuO for OII degradation. Experimental

conditions: [OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, in the dark and under magnetic

stirring. ................................................................................................................... 94

Figure 6. 6: XRD patterns of as-prepared Mg0.5Sr0.5CuO and Ce0.5Sr0.5CuO samples. ........... 95

Figure 6. 7: SEM images and N2 adsorption-desorption isotherms (c) of as-prepared

Mg0.5Sr0.5CuO3-δ (a) and Ce0.5Sr0.5CuO3-δ (b). ...................................................... 95

Figure 6. 8: XPS survey spectra and high-resolution XPS spectra of Cu for pristine/spent in (a)

MgSrCuO and (b) CeSrCuO (b): and (c) Ce for pristine/spent CeSrCuO. ........... 96

Figure 6. 9: Radical quenching effect on OII degradation using (a) MgSrCuO and (b) CeSrCuO.

Experiment conditions: [OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, in the dark and

under constant stirring. ............................................................................................ 97

Figure 6. 10: Schematic presentation of potential reaction mechanisms involved in the

degradation of OII by MgSrCuO and CeSrCuO. .................................................. 99

Figure 7. 1: SEM images and N2 sorption isotherms of (a) BaSrCoO, (b) CaSrCoO, and (c)

MgSrCoO. ........................................................................................................... 109

Figure 7. 2: XRD patterns of nominal pristine (black line) and spent (red line) (a) BaSrCoO,

(b) CaSrCoO and (c) MgSrCoO.......................................................................... 110

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Figure 7. 3: Survey XPS spectra of pristine and spent catalysts and high-resolution XPS spectra

of pristine (black line) and spent (red line) catalyst for (a,b,c) BaSrCoO, (d,e,f)

CaSrCoO and (g,h,i) MgSrCoO. ......................................................................... 111

Figure 7. 4: (a) Normalized OII concentrations based on HPLC, (b) UV-vis spectra of initial

and treated OII solutions, and colour intensity variation of OII solution at 8h treated

by MeSrCoO catalyst. Experimental conditions: [OII]0=20 ppm, [catalyst]=1 g L-

1, T=20 °C, in the dark and under stirring. .......................................................... 112

Figure 7. 5: HPLC chromophores of OII solutions vs. retention time for (a) BaSrCoO; (b)

CaSrCoO; and (c) MgSrCoO. ............................................................................. 113

Figure 7. 6: UV-vis spectra of OII and eluted species by (a) BaSrCoO, (b) CaSrCoO and (c)

MgSrCoO. ........................................................................................................... 114

Figure 7. 7: TGA results of blank and spent samples and OII in flowing air. ....................... 115

Figure 7. 8: OII degradation kinetics by (a) BaSrCoO, (B) CaSrCoO and (c) MgSrCoO. ... 115

Figure 7. 9: Cycling performance of MeSrCoO perovskite catalysts for OII degradation.

Experiment conditions: [OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, each run is 8h,

in the dark and under constant stirring. ............................................................... 116

Figure 7. 10: OII degradation of several SrCoO, CoO and MgO. Experiment conditions:

[OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, each run is 8h, in the dark and under

constant stirring. .................................................................................................. 117

Figure 7. 11: Radical quenching effect on OII degradation using (a) BaSrCoO, (b) CaSrCoO

and (c) MgSrCoO. Experiment conditions: [OII]0=20 ppm, [catalyst]=1 g L-1,

T=20 °C, in the dark and under constant stirring. ............................................... 117

Figure 7. 12: Illustration of OII degradation by MeSrCoO catalysts at room temperature under

dark conditions. ................................................................................................... 119

P a g e | xx

List of Tables

Table 2. 1: Classification of dyes according to chemical structures with some examples. ....... 6

Table 2. 2: Non-perovskite heterogeneous catalysts for dye degradation in dark ambient

conditions. ............................................................................................................... 12

Table 2. 3: Cobalt-containing heterogeneous perovskite catalysts for dye degradation. ......... 14

Table 3. 1: OII degradation percentage using single and mixed metal oxides, and CSC samples.

Experimental conditions: [OII]0=10 ppm, catalyst dosage=1 g L-1, magnetic stirring,

RT. ........................................................................................................................... 38

Table 3. 2: Reaction product ions identified from OII degradation. ........................................ 40

Table 3.A 1: Orange II FT-IR bands assignment. .................................................................... 46

Table 4. 1: Reaction product ions identified from the degradation of OII after 2h ................. 61

Table 5. 1: Apparent reaction rates of OII degradation with different initial concentrations by

Ca0.5Sr0.5Ni0.5Cu0.5O3-............................................................................................. 83

Table 6. 1: Relative concentrations of Cu1+ to (Cu2++ Cu1+) in pristine and spent samples. ... 97

P a g e | xxi

List of Abbreviaitons

RhB Rhodamine B

MO Methyl Orange

MB Methylene Blue

MG Malachite Green

CR Congo Red

OII Orange II

RB-5 Reactive Blue 5

X-3B Reactive Brilliant Red X-3B

IPA Isopropanol

KBrO3 Potassium Bromate

AgNO3 Silver Nitrate

KI Potassium Iodide

EDTA Ethylenediaminetetraacetic Acid

EDTA-2Na Ethylenediaminetetraacetic Acid Disodium Salt

DABCO 1,4-Diazabicyclo[2.2.2]Octane

MMOs Mixed Metal Oxides

PS Persulfate

PMS Peroxymonosulfate

AOPs Advanced Oxidation Processes

SR-AOPs Sulphate Radical-Based AOPs

CWAO Catalytic Wet Air Oxidation

DOS Density of States

O•-

2 Superoxide Radical

OH- Hydroxyl Ions

•OH Hydroxyl Radical

•OOH Hydroperoxyl Radical

H2O2 Hydrogen Peroxide

1O2 Single Oxygen

ROS Reactive Oxidative Species

UV-Vis or UV/Vis Ultraviolet-Visible

TOC Total Organic Carbon

HPLC High Performance Liquid Chromatography

P a g e | xxii

LC-MS Liquid Chromatography-Mass Spectrometry

FTIR Fourier-Transform Infrared Spectroscopy

TGA Thermogravimetric Analysis

SEM Scanning Electron Microscopy

XRD X-ray Diffraction

XPS X-ray Photoelectron Spectroscopy

EDS or EDX Energy Dispersive X-ray Spectrometer

C h a p t e r 1 P a g e | 1

1 Introduction

1.1 Background

The disposal of wastewaters containing dyes from textiles, leather tanning and paper industries is of

environmental concern. Azo dyes account for up to 70% of the total consumed dyestuff with 10-15%

released into the effluent during the dyeing process [1, 2]. Azo dyes are non-biodegradable and toxic

materials in nature [3]. Apart from the notorious strong colour, azo dyes have a marked negative

impact on the eco-system of the receiving waterbody [4, 5]. As such, there is increasingly stringent

environmental legislation for disposing this type of effluent, and therefore driving research to develop

efficient and cost effective methods for treating these contaminated wastewaters [6]. Advanced

oxidation processes (AOPs) are promising methods compared with conventional physical methods

and biological methods. This is because AOPs can achieve the non-selective and complete

degradation of most organics [11] due to the strong oxidizing property of hydroxyl radicals that are

generated in situ via chemical oxidation using ozone/hydrogen peroxide/persulfate with or without

radiation assisted sources (e.g., light, thermal and ultrasound).

However, most developed AOPs generally require expensive oxidants like O3 and H2O2, persulfate

or light irradiation/energy sources [7], which hinders its application feasibility. Within the research

framework of AOPs, heterogeneous catalysis has provided significant improvements and central to

this development is materials research. Therefore, the key point to be addressed are the development

of suitable catalysts that can work under relatively mild conditions in order to achieve the high

oxidation efficiency and ease the harsh operation conditions [8-11]. Metal oxides and perovskites are

a group of functional materials and the physicochemical properties of interest can be rationally

tailored to improve catalytic properties. For instance, this can be achieved by carefully selecting A-

/B- site doping in the case of perovskites which have a general formula (ABO3) [12]. This tailoring

approach has been widely applied for the preparation of heterogeneous catalysts for dye degradation

in water in photocatalysis [13-15], sulphate radical-based AOPs [16], or photocatalysis assisted ozone

[17] or photocatalysis assised Fenton-like reaction [18].

Some perovskites have been applied as heterogeneous catalysts for dye degradation in the dark at

room/mild temperature and atmospheric pressure without the assistance of external stimulants like

irradiation or chemical input [19-22]. However, the reported perovskite-based catalysts tend to either

suffer from low degradation kinetics [19, 20] or endure structural instability [20-22] during the

reaction. Elevated temperature is still needed for Ce-doped SrFeO3 [23] to degrade OII in the dark at

atmospheric pressure, which adds extra cost associated in the treatment process. Besides, most of the

abovementioned reports ignored the recyclability, which is a vital indicator to evaluate the catalytic


performance of a “true” catalyst. Finally yet importantly, most reports failed to investigate the

degradation mechanism in terms of the interaction between organics and catalysts and the formation

of ROS. Hence, all these factors contribute to the research gaps that warrant further research on the

development of metal oxides and perovskite heterogeneous catalysts for dye-polluted wastewater in

dark ambient conditions.

1.2 Scope and research contributions

Based on the gaps in the existing literature, this thesis aims to develop novel metal oxide and

perovskite heterogeneous catalysts for the degradation of dyes at dark ambient conditions. Issues to

be addressed in this study are the inefficiency, instability and restricted operation conditions

associated with reported metal oxides and perovskite catalysts by tailoring the structure-function

relationship via rational selection of A-/B- site metal ions (A1-xAx’B1-yB’yO3). With this goal in mind,

a series of perovskites are synthesized based on literature survey and the effects of doping metals and

doping sites on the structure, morphology and catalytic performance is systematically investigated

using X-ray diffraction (XRD), nitrogen sorption, X-ray photoelectron spectroscopy (XPS), and

scanning electron microscopy (SEM). The structure-function relationship is interpreted by dye

degradation in the dark ambient conditions using UV-vis spectroscopy, total organic carbon analyser

(TOC), Fourier-transform infrared (FTIR), high performance liquid chromatography (HPLC) or

liquid chromatography-mass spectrometry (LC-MS).

The key contributions of this thesis are specified as follows:

A series of Cu-/Co- based perovskites were synthesised and applied as heterogeneous

catalysts in the dark ambient conditions without adding external chemicals and energy sources.

They exhibited high catalytic activity and excellent recyclability for OII degradation.

High degradation and fast kinetics were achieved using Ni containing metal oxide mixtures

as the changeable oxidation states of Ni facilitated the generation of reactive oxidative species

via providing additional electron transfer pathway.

The coexistence of Ni and Cu in CaSrNi1-xCuxO enhanced the recyclability and mineralization

ability for OII.

1.3 Structure of thesis

Chapter 1: Introduction

This chapter gives the background of the thesis and declares the scopes and key contributions to the

field of research.

Chapter 2: Literature Review


This chapter provides an overview of dye pollutants and recent progress on AOPs with the focus on

the application of metal oxide and perovskite-based heterogeneous catalysts for dye-containing

wastewater treatment. Reported heterogeneous catalysts were also discussed in the field of water

remediation with respect to the strategy to tailor the physicochemical properties related to catalytic

activity.

Chapter 3: Degradation of azo dye Orange II under dark ambient conditions by calcium

strontium copper perovskite

This chapter systematically studied the substitution effect of Sr for Ca in Ca1-xSrxCuO via correlating

materials properties and catalytic performance. The degradation experiment was carried out using OII

as a model pollutant in the dark at room temperature without adding any external stimulants. This

chapter is published in Applied Catalysis B: Environmental.

Chapter 4: Ceramic metal oxides with Ni2+ active phase for the fast degradation of Orange II

dye under dark ambiance

This chapter presented the active role of Ni2+ in metal oxides for degrading OII under dark ambiance

and the correlation between materials properties and catalytic features. This chapter is published in

Ceramics International.

Chapter 5: Effective degradation of azo dyes in the dark by Cu2+ active sites in CaSrNiCu oxides

This chapter reported that B-site cations (Cu/Ni) in Ca0.5Sr0.5Cu1-xNixO3 (x=0.2, 0.5 and 0.8) jointly

promoted OII degradation with the optimization ratio of 1:1. The combination of Cu and Ni as B-site

cations demonstrated higher TOC removal ability than the single composition. This chapter has been

submitted to Journal of Environmental Chemical Engineering.

Chapter 6: Degradation of Orange II dye under dark ambient conditions by MeSrCuO (Me =

Mg and Ce) metal oxides

This chapter proposed the synergistic effect of metal oxides on the catalytic degradation of organics

by comparing the role of single metal oxide and that of metal oxide composites. This chapter has been

submitted to Separation and Purification Technology.

Chapter7: Surface and catalytic properties of stable Me(Ba, Ca and Mg)SrCoO for the

degradation of Orange II dye under dark conditions

This chapter focused on A-site doping strategy using diverse metals to understand the doping effect

on the composition-functionality relationship comprehensively. It was found that doping different A-

site cations in SrCoO3 led to perovskites with differing morphology, grain size, and BET surface area,


which in turn affected its catalytic performance in terms of catalytic efficiency, stability, and

recyclability. This chapter has been submitted to Applied Surface Science.

Chapter 8: Conclusions and Recommendations

This chapter summarized all the findings from all the work as presented in the previous chapters and

recommended some future directions.

1.4 References

[1] H. Liu, G. Li, J. Qu, H. Liu, Degradation of azo dye Acid Orange 7 in water by Fe0/granular

activated carbon system in the presence of ultrasound, J. Hazard. Mater. 144 (2007) 180-186.

[2] R.G. Saratale, G.D. Saratale, J.-S. Chang, S. Govindwar, Bacterial decolorization and degradation

of azo dyes: a review, J. Taiwan Inst. Chem. Eng. 42 (2011) 138-157.

[3] R.V. Khandare, A.N. Kabra, A.A. Kadam, S.P. Govindwar, Treatment of dye containing

wastewaters by a developed lab scale phytoreactor and enhancement of its efficacy by bacterial

augmentation, Int. Biodeterior. Biodegrad. 78 (2013) 89-97.

[4] S.H. Lin, C.F. Peng, Treatment of textile wastewater by electrochemical method, Water Res. 28

(1994) 277-282.

[5] C.-L. Yang, J. McGarrahan, Electrochemical coagulation for textile effluent decolorization, J.

Hazard. Mater. 127 (2005) 40-47.

[6] M. Stylidi, D.I. Kondarides, X.E. Verykios, Visible light-induced photocatalytic degradation of

Acid Orange 7 in aqueous TiO2 suspensions, Appl. Catal. B: Environ. 47 (2004) 189-201.

[7] J. Li, C. Zhao, F. Lan, F. Chen, C. Teng, Q. Yan, J. Tang, An efficient CeGeO4 catalyst for

degradation of organic dyes without light irradiation at room temperature, Catal. Commun. 77 (2016)

26-31.

[8] F. Luck, Wet air oxidation: past, present and future, Catal. Today 53 (1999) 81-91.

[9] F. Arena, R. Di Chio, B. Gumina, L. Spadaro, G. Trunfio, Recent advances on wet air oxidation

catalysts for treatment of industrial wastewaters, Inorg. Chim. Acta 431 (2015) 101-109.

[10] J.A. Heimbuch, A.R. Wilhelmi, Wet air oxidation—a treatment means for aqueous hazardous

waste streams, J. Hazard. Mater. 12 (1985) 187-200.

[11] J. Fu, G.Z. Kyzas, Wet air oxidation for the decolorization of dye wastewater: an overview of

the last two decades, Chin. J. Catal. 35 (2014) 1-7.

[12] G. Zhang, G. Liu, L. Wang, J.T. Irvine, Inorganic perovskite photocatalysts for solar energy

utilization, Chem. Soc. Rev. 45 (2016) 5951-5984.


[13] H. Tavakkoli, D. Beiknejad, T. Tabari, Fabrication of perovskite-type oxide La0.5Ca0.5CoO3-δ

nanoparticles and its Dye removal performance, Desalination and Water Treatment 52 (2014) 7377-

7388.

[14] H. Shu, J. Xie, H. Xu, H. Li, Z. Gu, G. Sun, Y. Xu, Structural characterization and photocatalytic

activity of NiO/AgNbO3, J. Alloys Compd. 496 (2010) 633-637.

[15] L.F. Silva, O.F. Lopes, V.R. Mendonça, K.T. Carvalho, E. Longo, C. Ribeiro, V.R. Mastelaro,

An understanding of the photocatalytic properties and pollutant degradation mechanism of SrTiO3

nanoparticles, Photochem. Photobiol. 92 (2016) 371-378.

[16] X. Pang, Y. Guo, Y. Zhang, B. Xu, F. Qi, LaCoO3 perovskite oxide activation of

peroxymonosulfate for aqueous 2-phenyl-5-sulfobenzimidazole degradation: effect of synthetic

method and the reaction mechanism, Chem. Eng. J. 304 (2016) 897-907.

[17] C. Orge, J. Órfão, M. Pereira, B.P. Barbero, L.E. Cadús, Lanthanum-based perovskites as

catalysts for the ozonation of selected organic compounds, Appl. Catal. B: Environ. 140 (2013) 426-

432.

[18] B. Palas, G. Ersöz, S. Atalay, Photo Fenton-like oxidation of Tartrazine under visible and UV

light irradiation in the presence of LaCuO3 perovskite catalyst, Process Saf. Environ. Prot. 111 (2017)

270-282.

[19] J.-M. Wu, W. Wen, Catalyzed degradation of azo dyes under ambient conditions, Environ. Sci.

Technol. 44 (2010) 9123-9127.

[20] M. Sun, Y. Jiang, F. Li, M. Xia, B. Xue, D. Liu, Dye Degradation Activity and Stability of

Perovskite-Type LaCoO3-x (x= 0-0.075), Mater. Trans. 51 (2010) 2208-2214.

[21] J. Gong, C.-S. Lee, E.-J. Kim, J.-H. Kim, W. Lee, Y.-S. Chang, Self-generation of reactive

oxygen species on crystalline AgBiO3 for the oxidative remediation of organic pollutants, ACS Appl.

Mater. Interfaces (2017).

[22] M. Sun, Y. Jiang, F. Li, M. Xia, B. Xue, D. Liu, Structure, dye degradation activity and stability

of oxygen defective BaFeO3-x, Mater. Trans. 51 (2010) 1981-1989.

[23] M.L. Tummino, E. Laurenti, F. Deganello, A.B. Prevot, G. Magnacca, Revisiting the catalytic

activity of a doped SrFeO3 for water pollutants removal: Effect of light and temperature, Appl. Catal.

B: Environ. 207 (2017) 174-181.


2 Literature review

Abstract

This review examines the development of advanced oxidation processes (AOPs) for dye-containing

wastewater treatment, focusing on heterogeneous AOPs using perovskite-based catalysts. It begins

by describing the significance for dealing with dye pollutants in wastewaters in Section 2.1. Currently

available AOPs for dye pollutant degradation are addressed in Section 2.2. Common heterogeneous

catalysts for AOPs are discussed in Section 2.3 with an emphasis on perovskite catalysts in terms of

the concept and application for water remediation. A detailed literature survey is also made for

catalysts used under dark ambient condition in this section. Finally, a summary of major findings and

research gaps are discussed in Section 2.4.

2.1 Dye contaminated wastewater

Approximately 7-10105 tons of dyes are produced annually with 280, 000 tons discharged [1] into

the environment via effluent wastewaters in industries (e.g., paper, printing, textiles, food and

cosmetics and leather). The textile industry is one of the greatest generators of liquid effluent

pollutants due to the high quantities of water used in the dyeing processes [2, 3]. Dye effluents have

posed a threat to human health and the ecosystem due to the toxic, carcinogenic, mutagenic, refractory

and non-biodegradable natures and the potential bio-accumulation in the food chain [4]. The very low

concentration of dye solution (<1 ppm) is highly visible thus decreasing dissolved oxygen

concentration and affecting the photosynthetic activity in aquatic systems by preventing the

transmission of light (reflection and absorption of sunlight), thus causing disturbance to the ecology

of the receiving waters [5]. Dyes mainly consist of three subsystems, namely chromophores (e.g. -

N=N-, responsible for the color of the dye), auxochromes (e.g. -OH, -SO3, responsible for the

intensity of the color) and a system of conjugated double bonds (aromatic systems) [1, 6, 7]. Dyes

can be classified in accordance with chemical structures, which have been listed in Table 2.1 with

some characteristic examples [7].

Table 2. 1: Classification of dyes according to chemical structures with some examples.

Types Examples

Azo dyes Orange II, acid red 2, disperse yellow 7, direct black

22

Anthraquinone dyes Disperse red 60, Reactive blue 19

Indigo dyes Indigo, tyrian purple, indigo carmine

Phthalocyanine dyes Phthalocyanine, direct blue 86, reactive blue 21

Sulfur dyes Sulphur black 1

Nitro and nitroso dyes Disperse yellow 1, acid yellow 24, picric acid


Azo dyes, which are characterized by one or more azo chromophore (-N=N-), approximately 70% of

all commercial dyes [8], making them the largest group of synthetic colorants released into the

environment [9]. The azo chromophore is generally attached to one or two aromatic groups.

Anthraquinone dyes are the second most important after azo dyes and possess brightness and good

fastness properties [10]. Indigo dye is an organic compound with a distinctive blue colour, almost

exclusively for dyeing denim jeans and jackets [11]. Phthalocyanine dyes are a class of macrocyclic

compounds possessing a highly conjugated π-electron system, intense absorption in the near-IR

region, and thermal and chemical stability [12, 13]. It demonstrates interesting optoelectronic

properties and can form coordination complexes with most elements[14, 15]. Sulfur dyes are

primarily used for dyeing textile cellulosic materials or blends of cellulosic fibers [16]. They have the

dullest range of colours amongst all synthetic dyestuff classes. Nitro and nitroso dyes have one or

more nitro or nitroso group conjugated with an electron-donating group via an aromatic system [7].

These dyes have minor commercial significance today but are of interest for their small molecular

structures.

Orange II (OII), widely used in industries, is an anionic monoazo acid dye resistant to UV or solar

light irradiation, temperature, microbial attack and common acids or bases [17-19]. OII represents

more than 15% of the world production of dyes used in the textile manufacturing industry [20]. Thus,

research on the development of efficient treatment processes for OII has attracted increasing interest.

OII exists in two tautomeric forms (insert of Fig. 2.1) in equilibrium aqueous solution with the

predominant form of hydrazone [21]. The UV-vis spectrum of OII solution exhibits 2 main bands at

430 and 485 nm, corresponding to transition of the azo and hydrazone forms, respectively, in addition

to two other peaks at 230 and 310 nm assigned to the benzene and naphthalene rings of OII [22]. The

evolution of absorbance intensity at 485 nm reflects the azo bond variation, which can be used to

determine OII concentration.

Figure 2. 1: UV-vis spectrum of OII solution. Insert is tautomeric forms of OII [21, 22].


2.2 AOPs for dye-containing wastewater treatment

Dye-containing wastewater treatment methods have undergone innovative and drastic changes over

the years. In order to unify the state of knowledge, identify research gaps, and stimulate new research

in the field of dye-containing wastewater remediation, this chapter analyses published research

pertaining to treatment processes for dye effluents using AOPs.

Figure 2. 2: Available techniques for dye-containing wastewater treatment.

AOPs, in a broad sense, refer to a set of chemical treatment procedures designed to remove

persistent/refractory organic materials in water by oxidation through reactions with hydroxyl radicals

(·OH) [23]. ·OH is the second strongest oxidizing agent known after fluorine which enables non-

selective and complete degradation of most organics [24]. It is highly unstable because of an unpaired

electron in the molecular structure and will extract an electron or hydrogen atom from the nearest

compound to convert into the more stable hydroxyl ion or water [25]. AOPs are called to fill the gap

between the treatability attained by conventional physical and biological treatments and the day-to-

day more exigent limits fixed by environmental regulations [26]. Some specific AOPs widely

reported are explained in detail below.

2.2.1 Available AOPS for dye-containing wastewater treatment

Catalytic wet air oxidation: Catalytic wet air oxidation (CWAO) is a thermal aqueous phase process

in which organic and inorganic substances are oxidized using gaseous oxygen (or air) at elevated

temperatures (125-320℃) and pressures (0.5-20MPa) [27, 28]. CWAO utilizes cheap, abundant non-

polluting natural oxygen (or air) to achieve the oxidation of organic substances, dispensing the use of

expensive oxidants like O3 and H2O2, persulfate. Thus, it is one of the most promising, economical

and technologically viable AOPs for the treatment of refractory organic pollutants in wastewater that

are too dilute to incinerate and too concentrated for biological treatment [29-31]. The key points to

be addressed are the development of highly stable and effective heterogeneous catalysts that reduce

harsh conditions (e.g., high temperature and pressure) as the oxidation efficiency can be largely


improved and the energy requirements can be greatly reduced by the use of heterogeneous catalysts

[32, 33]. Hence, the requirement of sustainable and efficient CWAO degradation of organics under

mild conditions is still a challenge nowadays and warrants further research on the rational design and

facile synthesis of novel catalysts.

Fenton-based reaction: Fenton reaction is based on an electron transfer between H2O2 and an

activator such as widely used ferrous iron (Fe2+) which acts as a catalyst. Iron (II) is thus oxidized to

iron (III), forming a hydroxyl radical and a hydroxide ion in the process (Eq. 2.1). Iron (III) is then

reduced back to iron (II) by another molecule of hydrogen peroxide, forming a hydroperoxyl radical

and a proton (Eq. 2.2). The net effect is a disproportionation of hydrogen peroxide to create two

different oxygen-radical species, with water (H++OH-) as a byproduct. Degradation of organic matters

is most effective at pH~3 for Fenton processes [34]. Drawbacks include pH adjustment before and

after treatment and the generation of large amounts of ferric hydroxide sludge in the neutralization

state [35, 36]. Other drawbacks include the fact that Eq. 2.2 is extremely slow and Fe (III) is a non-

active catalyst, leading to catalytic deactivation. In addition, utilization of H2O2 is a concern, which

is chemically instable and difficult in the storage and transport [37].

Fe2+ + H2O2 → Fe3+ +∙ OH + OH− 𝐸𝑞. (2.1)

Fe3+ + H2O2 → Fe2+ +∙ OOH + H+ 𝐸𝑞. (2.2)

Ozonation: Ozonation is quite effective in decolorizing textile wastewater [38]. Ozone is unstable in

water and the decomposition results in the generation of secondary oxidants (HO2-, O2

-, O3-, HO3

-,

and O- with principal OH) [39], which preferentially attack double bonds and destroy the

chromophores (i.e. N=N bonds) of dyes, leading to the complete discoloration [26]. However,

complete mineralization of organic compounds is generally not achieved and oxidative reactions are

relatively slow and selective [40]. In addition, the stability is affected by the presence of salts, pH and

temperature [41] and the short half-life period (~20 min) requires the continuous ozonation operation,

weakening the competitiveness of this method [42].

Sonolysis: Sonolysis is a relatively innovative AOPs based on the use of high-energy ultrasound to

realize the destruction of organic pollutants in water [43]. It is safe, clean, high penetrability in water

medium, and high degradation efficiency without generation of secondary pollutants [44]. Ultrasound

irradiation leads to the generation of acoustic cavitation that induces the formation of micro-bubbles.

High-energy consumption, non-uniform distribution of cavitation, long time treatment, inability of

mineralization, difficulty to operate at low frequency and scaling up are the major disadvantages [45-

47].

C h a p t e r 2 P a g e | 10

Electrochemical oxidation: Electrochemical oxidation converts pollutants into by-products [24] by

a direct anodic oxidation via direct electron transfer on the anode surface or by physisorbed or

chemisorbed OH generated by water discharge. Alternatively, the oxidation can be achieved by

mediated anodic oxidation in the bulk solution with the oxidizing reagents (e.g., •OH, OCl-) electro-

generated at the anode surface [48, 49]. Main advantages include environmental compatibility, and

easy automation and little or no consumption of chemicals and no sludge build up [48, 50, 51].

Drawbacks include high-electricity demand, pH regulation, the requirement of electrolytes for low

conductive wastewaters, and shortening lifetime of the electrodes by fouling due to the organic

material deposition on the surface [51].

Sulphate radical-based AOPs: The use of sulphate radical-based AOPs (SR-AOPs) has gained wide

attention as an innovative alternative to OH based AOPs in decontamination [37]. This is because

persulfate (PS) and peroxymonosulfate (PMS) are cheaper [52] and more stable than H2O2 and the

transportation and storage can be easily achieved. Generated SO•-

4 possesses comparable or even

higher oxidation potential [53] and are more selective [54, 55] than OH. The higher half-life period

of SO•-

4 than that of OH enables SO•-

4 to transport greater distances and better contact with target

compounds [37, 56]. Most importantly, the reactivity of SO•-

4 is pH independent while that of OH is

pH dependent [37]. This avoids the use of additional chemicals to adjust the pH of wastewater. The

challenges of employing SR-AOPs include the undesired formation of oxyanions, hazardous metal

ions, and harmful disinfection byproducts apart from the one-time use of PS/PMS reagents [57].

Photocatalysis: Photocatalysis is a process that utilizes light irradiation and the catalytic properties

of photocatalysts to carry out and/or accelerate certain chemical reaction [58]. It has shown a great

potential as a low-cost, environmental friendly and sustainable treatment technology [59] for water

purification as abundant solar energy can be utilized. Heterogeneous photocatalysis has been regarded

as a promising technology for the treatment of environmental pollutants present in aqueous domestic

and industrial effluents as it can be easily recycled and used. The photocatalysis is initiated upon light

adsorption (light energy≥ band gap) to generate charge carriers (i.g., electrons and holes). This

results in the formation of ROS (HO•, H2O2, O•-

2 et al.) that induce the subsequent degradation of

organics. Although many reports are available on photocatalytic degradation of environmental

organic pollutants using various types of photocatalysts, the catalytic performance up to now is still

far from satisfactory as reported oxide and sulphide semiconductor photocatalysts tend to suffer from

wide band gap, photoexcited carrier recombination or photo-induced corrosion, which limit their

photocatalytic ability and practical application.

C h a p t e r 2 P a g e | 11

2.2.2 Conclusions of current available AOPs

This review focuses on AOPs for dye degradation in environmental remediation, particularly for

catalyst-based AOPs. Although the abovementioned AOPs are widely reported for dye-contaminated

effluent treatment, they suffer from inherent limitations from the point view of economics and

applicability. They generally require H2O2, O3, PS/PMS, ultrasound, light irradiation, or pH

adjustment and increased temperature/pressure. Thus, it is ideal to develop efficient and robust

catalysts that can work in the dark at room temperature and atmospheric pressure without the addition

of external chemical and energy input. By doing so, it can greatly simplify the engineering design of

plants whilst cutting operating costs at the same time.

2.3 Catalysts for water remediation

Catalysts could be a key component to achieve dye degradation in dark ambient conditions without

external additives and energy input. Thus, this necessitates the design of highly active, stable, yet

cheap and earth-abundant heterogeneous catalysts. Materials of interest include metal oxides that

have long been used as catalysts in various catalytic processes. The limited reported catalysts for dye

degradation in this aspect are mostly metal oxides and reviewed in this section. Perovskites, as a kind

of unique metal oxides, are discussed separately in detail as well in order to provide guidance in

designing novel catalysts in the field of dye-containing wastewater remediation. Catalysts are

classified as non-perovskite catalysts and perovskite catalysts for convenience.

2.3.1 Non-perovskite catalysts

There are limited reports for simple metal oxides applied as catalysts for dye degradation in the dark

ambient conditions sparing the use of extra chemicals and energy up to date, which are summarized

in Table 2.1.

Dang et al. [60] reported the fast degradation of X-BR (90% in 8min) in the dark at 25 °C by Fe2O3.

The reaction started by the excitation of Fe2O3 to yield electrons. Oxygen accepted electrons to yield

·OH for dye degradation. The physicochemical properties and recyclability of Fe2O3 catalysts before

and after catalysts were not systematically studied. Ullah et al. [61] prepared CdSe-graphene

composite for MO and RhB catalytic degradation in dark ambience. Fast kinetics and relatively high

reusability were achieved. However, the authors did not rule out the possibility that the observed

removal of dyes might result from physical adsorption. This is particularly possible considering the

high surface area and large amount of graphene used. Metal oxide composite (Si/Ca/Mn/Ni/Cu) was

also applied for MB degradation in the dark at 35°C [62]. Thermocatalytic degradation and physical

adsorption contributed to the 82% MB removal in 30h.

C h a p t e r 2 P a g e | 12

Electron storage is another mechanism proposed to explain dye degradation in the dark. Take F-N-

W-codoped TiO2 as an example, W6+ as an electron acceptor stored electrons during photoexcitation

and then donated it to O2 to enable the thermo-oxidation activity for RhB decomposition [63] in the

dark. Piewnuan et al. [64] also reported the energy storage ability of V2O5 in TiO2/(TiO2-V2O5)/PPy

composite metal oxides for MB degradation in the dark. TiO2-V2O5 was charged under UV light and

then discharged in the dark to achieve MB degradation in the dark condition.

Dvininov et al. [65] firstly reported that platinum loaded semiconductor particles (Pt-HCa2Nb3O10)

could catalyze the reaction of air with MO in the dark, although the kinetics was sluggish. However,

the detailed degradation mechanism behind this interesting phenomenon is missing.

Table 2. 2: Non-perovskite heterogeneous catalysts for dye degradation in dark ambient

conditions.

Catalysts Dyes Efficiency Conditions Ref.

Fe2O3 X-BR 90% in 8 min 0.2 g L-1 catalyst; 20 mg L-1 dye; 25°C [60]

CdSe-Graphene MO/Rh

B

93% MO in 20

min; 76% RhB in

1h 1 g L-1 catalyst; 10-4 M dye; 25C [61]

SiO2/CaO/MnO/

NiO/CuO MB 82% in 30h 1 g L-1 catalyst; 40 mg L-1 dye; 35 C [62]

Pt-HCa2Nb3O10 MO 50% in 5h 1 g L−1 catalyst; 20 mg L−1 dye; 25 °C [65]

F/N/W-TiO2 RhB 92% in 2h 0.5 g L-1 catalyst; 30 mg L− dye; 60 °C [63]

TiO2/(TiO2-

V2O5)/PPy MB 20% in 6h 1 g L-1 catalyst; 10-5 M dye; 25C [64]

2.3.2 Perovskite catalysts

This section attempts to give a review on recent advances in perovskite catalysts for the abatement of

aqueous dye pollutants. It starts from the introduction of perovskite concept, followed by reviewing

applied perovskite catalysts in terms of A-/B-site cations. Special attention is given to perovskites

that worked in the dark, as this group of materials are preferred with respect to economic saving and

feasible operation.

Perovskites are ceramic-type materials with the general formula of ABO3. Larger A-site cations

(alkaline earth, rare earth or lanthanide metal) are on the corners of the lattice and have 12-fold

coordination with O (Fig. 2.3). Smaller B-site cations (transition metal) reside in corner-sharing

octahedral of O anions [66], The A/B site cation can be replaced by 90% of metal elements in the

Periodic Table into the crystal structure without destroying the matrix structure [67]. This allows the

thorough investigation of structure-function features by doping foreign cations with different

oxidation states or ionic radius. Physicochemical properties of perovskites would vary by A/B site

doping, thus opening experimental opportunities to expand the family of perovskite-based engineered

C h a p t e r 2 P a g e | 13

materials of interest by careful materials design [68-71]. In other words, multicomponent perovskites

prepared by partial substitution of metal cations allow researchers to explore and modulate crystal

structures and hence obtain desired functions [72-74]. A-/B-site cations are quite diverse. Typical A-

site cations are alkali metals (Na, K), alkaline earth metals (Mg, Ca, Sr, Ba) or lanthanide elements

(Ce, La). Transition metals (Co, Ni, Cu, Fe, Ti, Mn) generally constitute B-site.

Figure 2. 3: ABO3 perovskite crystal structure. [75]

Figure 2. 4: Typical elements in ABX3 perovskites.

Cobalt-based perovskite catalysts: Various types of cobalt perovskites were successfully

synthesized and applied as heterogeneous catalysts for dye degradation as summarized in Table 2.2.

For instance, LaCoO3 has been widely studied for the degradation of RhB [76, 77], RB-5 [78] and

MO [79] dyes via photocatalysis, ozonation, or sulphase radical based AOPs. Notably, Ca, Mg and

Ba can substitute A-site La to result in substituted perovskites that showed catalytic activity for other

dyes (e.g., CR [80], X-3B [81] and MG [82], respectively). However, all these reports on perovskite

for the degradation of dyes were not for the catalysis under dark conditions.The only single report

[83] using LaCoO3 for Mo degradation in the dark suffered from poor kinetics (10% degradation in

45h) that is restricted in practical application. Moreover, LaCoO3 was not stable after catalysis as the

degradation of MO by LaCoO3-x was due to the electron donation from dyes to the catalyst, resulting

in the reduction of Co3+ into Co2+.

C h a p t e r 2 P a g e | 14

Table 2. 3: Cobalt-containing heterogeneous perovskite catalysts for dye degradation.

Catalysts Dyes Efficiency Conditions Ref.

La0.5Ca0.5CoO3 CR 38% in 2h 0.2 g L-1 catalyst; 10 mg L-1 dye; 254nm [80]

LaCoO3

composite MB

50% in 30

min 10 g L-1 catalyst 25C; visible light [84]

LaCoO3 RhB 99% in 50

min

0.5 g L-1 catalyst; 2 mg L-1 dye; UV; pH 4; 35

C [76]

LaCoO3 RhB 95% in 1h 10 mg L−1 dye, 100 mg L−1 PMS, 100 mg L−1

catalyst; 30 °C [77]

LaCoO3 RB-5 100% in 15

min

0.7 g L-1 catalyst; 50 mg L− dye; 50 g m-3 O3;

PH 5.5 [78]

PrBaCo2O5+δ MB 100% in

15min

0.05 g L-1 catalyst; 10 mg L− dye; 0.75 gL-1

PMS; 25C [85]

Sm/Co-BiFeO3 MB 92% in 3h 1 g L-1 catalyst; 1 mg L− dye; sunlight [86]

Mg-LaCoO3 X-3B 93% in 20

min 0.6 g L-1 catalyst; 40 mg L-1 dye; 25C; visible

light [81]

CaCoO3 Acid

Red 97% in 3h 4 g L-1 catalyst; pH 2; 50 ml L-1 H2O2 (10%) [87]

SrFe0.5Co0.5O3- MO 95% in 2h 2 g L-1 catalyst; 10 mg L-1 dye; H2O2; UV light [88]

La1-xBaxCoO3 MG 97% in 1h 5g L-1 catalyst; 10 mg L-1 dye; >420 nm [82]

LaCoO3 MO 65% in 2h 0.1 g L-1 catalyst; 100 mg L-1 dye; >410 nm [79]

LaCoO3 MO 10% in 45h 1 g L-1 catalyst: 20 mg L-1 dye; in the dark [83]

Copper-based perovskite catalysts: The interaction of copper with other metals can modify the

redox properties of the materials, which is the origin of the improved catalytic activities [89-93].

Intensive effort has been devoted to explore high catalytically active Cu-containing perovskites for

water remediation [77, 94, 95]. The presence of copper in the perovskite catalysts enhanced the

photocatalytic degradation ability by forming synergistic effect between Cu and other components

in the compounds [96, 97]. The redox pair of Cu2+/Cu1+ at the catalyst surface is the key factor

responsible for the catalytic reaction [98]. Cu-based perovskites have been used for dye-containing

wastewater treatment in Fenton-like reaction and photocatalytic reaction. However, to the best of the

author’s knowledge, no report exists in terms of its application in dye degradation in the dark ambient

conditions.

Nickel-based perovskite catalysts: Nickel is usually used as co-catalyst/catalysts to obtain a high

catalytic activity via facilitating the charge carrier separation [99-101]. Nickel containing catalysts

have been widely under research in the last decades in the field of dye-containing wastewater

treatment. Based on the existing form of Ni in the catalysts, Ni-containing catalysts can be grouped

into Ni-doped perovskite catalysts [102-104], Ni-based perovskite catalysts [105-109], and perovskite

catalysts combined with Ni species [110, 111].

C h a p t e r 2 P a g e | 15

Nickel-containing perovskite La4Ni3O10 [112] demonstrated catalytic degradation ability for MO in

the dark ambient condition without any external agents. The characteristic UV-vis absorption peak at

464 nm almost disappeared after 8.5h, suggesting the degradation of MO (Figure 2.5a). The newly

developed peak at 244 nm further ascertained MO was indeed degraded instead of adsorption.

Electron donation from MO into the catalyst initiated the reaction, followed by electron transfer and

the reaction between dissolved O2 and donated electrons to form ROS, which led to MO degradation.

Figure 2. 5: (a) UV-vis spectra evolution of MO in the presence of La4Ni3O10. Experimental

conditions: [catalyst] =1 g L-1, [MO] = 5 mg L-1, 650 rpm [112].

Calcium-based perovskite catalysts: Calcium titanate (CaTiO3) was the first minerals found in

perovskite-structure family and was named “perovskite” after the Russian mineralogist Lev Perovski.

Since then, Ca has been an attractive cation in the A site of perovskites for water remediation. CaTiO3

has been studied for MO [113] and MB [114] abatement in water via photocatalysis. The replacement

of foreign metal ions (Fe [115], Co [87], Cu [116], Bi [117]) for Ti in CaTiO3 was also investigated

for photocatalytic/Fenton-like degradation of dyes. Besides, calcium-doping strategy was also

conducted in the A-site of perovskites and demonstrated superior photocatalytic activity than undoped

ones [118-121]. For example, La was partially substituted by Ca in LaFeO3 to obtain La1-xCaxFeO3,

which enhanced visible light absorption and improved the visible light photocatalytic activity for MB

[119]. The substitution of Sr in A-site of SrFeO3 with Ca led to higher photocatalytic activity toward

MB than SrFeO3 [120]. Ca-containing layered semiconductor Pt-HCa2Nb3O10 [65] even

demonstrated catalytic activity for MO degradation in the dark ambient conditions without adding

any additives and energy input.

C h a p t e r 2 P a g e | 16

Figure 2. 6: Evolution of calcium-based perovskites synthesized via A-/B-site doping for dye

degradation. A/A’ and B/B’ represent A-site and B-site cations in ABO3 perovskites.

Strontium-based perovskite catalyst: Strontium-based perovskites are another group of perovskite

materials that have attracted the interest of scientific community recently for organic dye pollutant

elimination. SrTiO3 is the most popular one because of its high catalytic activity and high chemical

and photochemical stabilities [114, 122]. Foreign metal doping in A-/B- site [104, 123-126], non-

metal doping [127-130], composites with noble metals [131-133]/graphene[134]/g-C3N4[129]/metal

oxides [135, 136] have been employed to extend the photo absorption range and enhance the catalytic

performance of SrTiO3. It is noteworthy that SrFeO3- was capable of degrading Acid Orange 8

rapidly under dark ambient conditions without any external stimulants [137]. Very recently,

Tummino et al. [138] partially substituted Sr by Ce in SrFeO to investigate the effect of light and

temperature on the catalytic activity of Ce-doped SrFeO. It was found that it could catalytically and

selectively degrade OII without irradiation in the 55-80 °C temperature range by the thermo-induced

hydroxyl radicals. However, RhB could not be degraded under the identical conditions. Sun et al.

[139] replaced Sr by Mn in SrTiO3 and the resultant materials were examined to be catalytic active

for RhB degradation in dark without any external stimulants due to the reducibility of Mn4+ to Mn3+.

The obtained catalyst degraded approximately 50% RhB after 4h reaction.

Barium-based perovskite catalyst: To date, barium-based perovskites have been widely applied as

heterogeneous photocatalysts for dye abatement in water [140-145]. BaFeO3-x [146] has been

demonstrated to be active for MO degradation in the dark at room temperature. BaFeO3-x attracted

CO2 in aqueous solution to form CO32-, followed by the reaction with Ba2+ to form BaCO3. Then A-

site and O-site defects are generated. Labile Fe4+ was reduced to Fe3+ with the concurrent oxidation

of MO.

C h a p t e r 2 P a g e | 17

Synthesis methods

Synthesis methods significantly influence its structure, size and potential application of perovskite

materials [147-149]. The diverse available innovative processing methods allow one to prepare

desired perovskites by controlling the preparation temperature, morphology, homogeneity and

reproducibility. These methods include conventional solid-state reaction, sol-gel method,

hydrothermal method, combustion synthesis, sonochemical method, co-precipitation method and

microwave-assisted method. Among the various methods, the combination of sol-gel chemistry and

further annealing (i.e., citric acid sol-gel method) allows the incorporation of dopant cations into the

functional perovskite structure [150], which is widely employed to synthesize perovskite catalysts for

water remediation.

2.4 Conclusion and research gaps

This review clearly demonstrates the urgency of treating dye-containing wastewaters (particularly

azo dyes) from the industry. Orange II, a kind of azo dyes, is widely used in industries and resistant

to degradation under natural conditions. AOPs are considered as the most promising and highly

competitive innovative methods for wastewater treatment containingorganic pollutants utilizing high

temperature and pressure (CWAO), solar energy (photocatalysis), O3 (ozonation), PS/PMS or H2O2

(Fenton-like reaction). However, current AOPs are chemically and energetically intensive processes,

which are undesirable for cost-effective and large-scale water treatment. In this regard, the

development of novel heterogeneous catalysts that can work in the dark without the use of external

stimulants is a gap observed in the literature.

Therefore, endeavor should be directed to the development of highly effective and stable

heterogeneous catalysts that can dispense the use of expensive chemicals and improve the harsh

conditions for dye-containing wastewater treatment. Recently, metal oxides, particularly perovskites,

have been considered with the aim to design highly active and stable catalysts for dye degradation in

dark ambient conditions, though the number of papers reported are limited. Through careful materials

design by varying metal ions in the perovskite formula, its physicochemical characteristics of interest

can be tailored in order to minimize reaction barriers. Some materials have been applied as

heterogeneous catalysts for dye degradation in dark ambient conditions up to date.

Nevertheless, there are still knowledge gaps in the literature that warrant further research in this field.

Firstly, the reported catalysts either show sluggish degradation kinetics or suffer from structural

instability. Thus, the dependence of catalytic performance (activity, stability and recyclability) on the

component (A/B-site cation) with respect to structure-functionality needs to be correlated. Secondly,

there is still inconsistency in the knowledge about the degradation mechanism of dyes in dark ambient

C h a p t e r 2 P a g e | 18

conditions. Therefore, the interaction between organics and catalysts and the detailed formation

mechanism of ROS should be researched to unravel the degradation process. Thirdly, the specific

role of single metal oxide catalyst and the potential synergistic effect between multiple metal oxide

catalysts are worthy probing in order to provide a deeper insight into the catalysts and its catalytic

performance under dark conditions.

This thesis hypothesizes the A/B-site cations from perovskites affect the catalytic performance of

AA’BB’O materials in terms of catalytic activity, stability and recyclability by changing the structure-

morphology features. With this in mind, this project adopts Co, Ni and Cu as the B-site cation in the

AA’BB’O formula. Previous research has proved that perovskites with Co and Ni as B-site cations

achieved dye degradation in dark ambient conditions without any additional chemical/energy input.

As the benign synergistic effect between Cu and other components are widely reported in dye

degradation, it is anticipated that Cu as B-site cation could contribute to the dark ambient degradation

of dyes synergistically with other elements. Meanwhile, the effect of A-site cations is also

investigated using divalent alkaline earth metals (Mg, Ca, Sr, and Ba) and multivalent lanthanide

metal (Ce) to correlate it with the catalytic performance. This research thesis also aims to unravel the

degradation process from the initial interaction between organics and catalyst, to the generation of

ROS, and further to the analysis of degradation products and catalysts after reaction. The synergy

between multiple heterogeneous catalysts is also discussed. Overall, all the knowledge gaps and

hypothesis formed the basis of this PhD thesis.

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3 Degradation of azo dye Orange II under dark ambient conditions by calcium strontium

copper perovskite

3.1 Introduction

As mentioned in Section 2.3.1, copper-based perovskite catalysts are widely reported for dye-

containing water remediation due to the catalytic nature of Cu. However, there are no reports

regarding Cu-containing catalysts that worked in dark ambient conditions. Hence, Chapter 3 aims to

synthesize and characterize Cu-based perovskites as an alternative heterogeneous catalyst for the

degradation of organic pollutants in the dark at room temperature and atmospheric pressure. This

study systematically investigate the effect of the relative content of calcium and strontium on the

catalytic activity of resultant materials (CaxSr1-xCuO3) using Orange II (OII) as the organic model

pollutant.

3.2 Contributions

We published Chapter 3 in Applied Catalysis B: Environmental. This chapter is wholly my own work

with the exception of the contribution by Prof. João C. Diniz da Costa, Dr. Julius Motuzas and Dr.

Wayde Martens.

C h a p t e r 3 P a g e | 31

Degradation of Orange II under dark ambient conditions by calcium strontium copper

perovskite

Huihuang Chena, Julius Motuzasa, Wayde Martensb, João C. Diniz da Costaa*

aThe University of Queensland, FIM2Lab – Functional Interfacial Materials and Membranes

Laboratory, School of Chemical Engineering, Brisbane Qld 4072, Australia.

bScience and Engineering Faculty, Queensland University of Technology, Brisbane, Qld 4000,

Australia.

* Correspondent author: +61 7 3365 6960 (Tel), [email protected] (email).

3.3 Abstract

This work investigates the effect of calcium strontium copper (CSC) based catalysts for the

degradation of an azo dye orange II (OII) under dark conditions without the addition of peroxides or

ozone. CSC were synthesized via a combined EDTA-citric acid complexation method. The resultant

catalyst was composed of perovskite and metal oxide phases, however, the perovskite phase was the

most active for degradation of OII. The content of Ca and Sr in the A-site of the perovskite structure

was varied whilst the B-site was Cu rich. CSC compounds with higher Ca content in the A-site were

slightly more effective at degrading OII. The degradation kinetics under dark conditions was fast with

up to 80% of OII being degraded within 10 min. TOC results showed that the degradation was only

partial as more than 60% of the organic carbon remained in the solution, supported by the formation

of by-products determined by HPLC. The remainder of the carbon were found to be adsorbed on the

surface of the spent CSC catalyst, as by-products of the reaction and OII molecules. The CSC catalyst

proved to be effective for breaking -N=N- bonds from solutions containing low (10 ppm) to high (100

ppm) OII concentrations. This reaction produced electrons which generated radical species, including

hydroxyl radicals as confirmed by 2-propanol, which further degraded OII and its by-products.

Keywords: dark conditions, orange II degradation, perovskite, heterogeneous catalysis

3.4 Introduction

The disposal of wastewaters containing organic pollutants from textile industries is of global

environmental concern. Worldwide approximately 280,000 tons of textile dyes are discharged in

industrial effluents every year [1, 2]. Azo dyes, which contain -N=N- bonds, are widely used by the

textile industries, accounting for over 50% of all commercial dyes [3]. These dyes are non-

biodegradable and toxic [4–6], whilst affecting water transparency even at small concentrations (1

ppm L-1) [7], limiting the penetration of sunlight necessary for aquatic photosynthesis [8]. For this

reason, textile wastewater emissions have been scrutinised by non-governmental agencies and the

mailto:[email protected]

C h a p t e r 3 P a g e | 32

public resulting in environmental protection agencies around the world developing stringent

guidelines to control their emissions and have legislated hefty penalties for non-compliance. To

address this problem, many processes based on physical, biological and chemical methods, have been

developed and trialled. In general, physical methods (e.g. flocculation, adsorption and membrane

filtration) suffer from secondary pollution as they merely transfer pollutants downstream without

achieving a real degradation of target pollutants [9]. As such, physical processes still require

downstream processing of wastewaters. Bio-treatment of azo dyes is usually considered ineffective

due to the resistance of the dyes to aerobic degradation [10, 11]. Biodegradation requires the use of

living organisms under strict process conditions (e.g., pH and temperature) [12], which tend to be

greatly affected in textile wastewaters due to the toxicity of most commercial dyes [13]. Additionally,

azo dyes undergo reductive cleavage through anaerobic biological treatment and potentially generate

toxic aromatic amines [14, 15].

Advanced oxidation processes (AOPs), in which highly reactive oxidative species (e.g. HO•, O·-

2 ,

HOO• and HO•

2 [16]) are utilized, have been widely studied as an alternative way of treating organic

pollutants [17]. AOPs can be categorized into photo- and dark- oxidation procedures. Photocatalysis,

especially using TiO2 catalysts, is widely known as an efficient method to remove and destroy organic

pollutants under light irradiation. For instance, Cd-doped TiO2 degraded 95% of OII after 2 h when

illuminated with visible light [18]. Photocatalysis however is generally energy intensive and greatly

inhibited in the absence of light illumination. Dark oxidation (e.g., ozonation, ultrasonication and

Fenton process) are alternative methods, dispensing the need of light radiation, by requiring reactants

such as O3 and/or H2O2. The heterogeneous Fenton-like reaction has proved to be attractive, with the

production of new catalysts such as iron oxides compounded with graphene oxide [19], Cu sulfate

[20], and vanadium titanate [21]. Compounds containing graphene oxide have been found to greatly

improve the degradation efficiency of OII, though Cu sulfate iron oxide proved to deliver the best

performance reaching ∼100% OII degradation in 60 min. These reactions do however need H2O2 to

generate the powerful HO• radicals for the destruction of organics. A more interesting approach is

the degradation of organic pollutants in wastewaters without the aid of external stimulants or light

illumination. In this way, there is no need for extra costs associated with the consumption of chemical

products or energy. Recently, perovskites have attracted the attention of the research community due

to their ability to oxidize and remove organic compounds from wastewater in dark ambient conditions.

Perovskites are crystalline ceramics with a cubic structure described by the general formula ABO3,

where the partial substitution of the cation sites (A or B) with other cations (A’ or B’) results in AxA’1-

xByB’1-yO3-δ compounds. Rummino et al. [22] showed that Sr0.85Ce0.15FeO3-δ perovskite-type mixed

oxide degraded orange II and rhodamine B as model pollutants without light irradiation. Initial reports

C h a p t e r 3 P a g e | 33

using BaFeO3-δ showed it took 5 days to remove 50% of methyl orange [23]. By replacing Ba in the

A site with Sr and forming a perovskite SrFeO3-δ, acid orange 8 (150 μM) was degraded in 60 min

[24]. These results give a clear indication that the cation used in the perovskite compound plays a

major role in the catalytic degradation of organics in wastewaters. There are a number of cations that

can be used as doping rare earth or earth alkaline elements at the A site and transitional metals at the

B site. A transitional metal of interest is Cu, with recent reports for oxygen systems showing

outstanding performance in Ba0.5Sr0.5Co0.8Cu0.2O3-δ (BSCC) [25], or in binary Co0.8Cu0.2Ox [26]

compounds. Indeed, Cu has been used as a cation in perovskite compounds and tested for the

degradation of organic compounds in solutions. However, these Cu containing perovskites were

tested using hydrogen peroxide in a heterogeneous Fenton-like reaction. Examples include Cu doped

LaTiO3 perovskite for the degradation of rhodamine B [27] and LaCuO3 perovskite for the

degradation of phenol [28].

Therefore, this work investigates the performance of novel perovskites in the degradation of dyes

under dark conditions and without any external stimulants (i.e. no ozone or no hydrogen peroxide).

We fully substituted Fe in the B-site with Cu, and in the A-site Sr was partially or fully substitute

with Ca, in a series of CaxSr1-xCuO3-δ (CSC) with x varying from 0, 0.25, 0.5, 0.75 to 1. OII dye was

the model organic pollutant tested as it is widely used in the textile industry. The CSC perovskites

were fully characterized to understand their materials properties; and tested from low (10 ppm) to

high (100 ppm) OII aqueous concentrations. The relationship between the materials properties and

catalytic performance is discussed and an OII degradation mechanism using CSC perovskite under

dark condition is proposed.

3.5 Materials and methods

3.5.1 Synthesis and characterisation

All chemicals were used as received including strontium nitrate (98% Alfa Aesar), copper (II) nitrate

hemi (pentahydrate) (98% Alfa Aesar), calcium nitrate tetrahydrate (> 99.0% Chem-Supply Pty Ltd),

aqueous ammonia solution (30% Chem-Supply Pty Ltd), citric acid monohydrate (99.5% Chem-

Supply Pty Ltd) and ethylenediamine tetraacetic acid (EDTA, 99% Sigma-Aldrich). The target

organic pollutant was OII also known as Acid Orange 7 (Molecular formula: C16H11N2NaO4S, Mw:

350.32 g mol-1, λmax = 485 nm, > 85% SigmaAldrich). CSC (CaxSr1-xCuO3-δ, x = 0, 0.25, 0.5, 0.75

and 1) was prepared via a combined EDTA-citric acid complexation method. The molar ratios of total

metal ions, EDTA, citric acid, and ammonium hydroxide were kept at 1:1.1:2:10. Initially, ammonia

aqueous solution was added to a beaker containing EDTA under stirring until EDTA was completely

dissolved and a clear solution was formed. In a separate beaker, stoichiometric quantities of nitrate

salts and citric acid were dissolved in deionized water to obtain a mixture solution. Subsequently, the

C h a p t e r 3 P a g e | 34

EDTA solution was added into the mixture solution under stirring to get the desired solution, which

was heated while magnetically stirred to evaporate most water until a viscous fluid was attained. This

final mixture was initially sintered in atmospheric air in a furnace up to 450 °C with a dwell time of

8 h utilizing heating and cooling rates of 5 °C min-1. A second sintering step was applied up to 1000 °C

using the same dwell time and ramping rates. The crystalline phase of the ground samples was

characterized by X-ray powder diffraction (XRD, D8 Advance, Bruker, USA) with Cu-Kα radiation

(λ = 1.5406 Å) at 40 kV and 40 mA by step scanning in the range of 10° ≤ 2θ ≤ 100°. X-ray

photoelectron spectroscopy (XPS) was performed on X-ray photoelectron spectrometer (Kratos Axis

ULTRA) equipped with monochromatic Al Kα (hv = 1486.6 eV) radiation and calibrated internally

by carbon deposit C 1s binding energy (BE) at 284.8 eV to quantitatively analyse the chemical

composition of samples and chemical state of elements. The powder morphology was analysed using

a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS).

Nitrogen adsorption at 77.4 K on degassed samples at 150 °C was carried out using a Micromeritics

TriStar 3000 to determine BET surface area. The remaining TOC in the sample solutions was

monitored using a Shimadzu TOC-Vcsh Total Organic Carbon Analyzer. Mass loss analysis was

performed using a TGA-DSC 1 Thermogravimetric Analyser (Mettler Toledo) by heating the samples

from 28 to 800 °C at a heating rate of 5 °C min-1 under air atmosphere at a flow rate of 80 mL min−1.

The infrared absorption spectra were recorded using a Fourier transform infrared (FTIR)

spectrophotometer (IRAffinity-1, Shimadzu).

3.5.2 Catalyst evaluation

The catalytic degradation of OII over dispersed CSC was conducted in a 250 mL beaker, in which

200 mg of the catalyst was suspended in 200 mL of OII aqueous solution in dark at room temperature

without any external stimulant. The dispersion of CSC in water was achieved by magnetic stirring

during the degradation process. At designated intervals, ∼5 mL of reaction suspension was sampled

and filtered using 0.45 μm Milipore syringe filters. The samples were analysed using a UV–vis

spectrophotometer (Evolution 220, Thermo Fisher Scientific) to determine the degradation yield (d)

of OII at its characteristic wavelength d = (A0–At)/A0, where At is the absorbance of treated OII

solution at time t and A0 is the absorbance of the initial OII solution. Recyclability of CSC was

evaluated using OII (20 ppm). At the time interval of 15 min, 5 mL reaction mixture was sampled

and analysed by UV–vis spectrometer. After each cycle (1 h reaction), 20 mL 2000 ppm OII stock

solution was added to the remaining reaction mixture (180 mL) to compensate the consumed OII and

to maintain the 200 mL reaction mixture volume. 2-propanol (Alfa Aesar, 0.10 M) mixed in an OII

solution was used as hydroxyl radical (HO·) scavenger. A fully computer controlled HPLC system

(UltiMate 3000, UHPLC+ focused, Thermmo Scientific) comprising a quaternary solvent delivery

C h a p t e r 3 P a g e | 35

pump, diode array and fluorescence detectors and an auto sampler was used to follow OII

concentration-time profiles. Potential products were separated on an Eclipse XDB-C8 5 μm, 150 mm

× 4.6 mm column using 70:30 aqueous solution of ammonium acetate (20 mM): acetonitrile as an

isocratic mobile phase at 1 mL min-1 and ambient temperature. The injection volume was 20 μL and

detection was achieved with the diode array detector set at 253 nm. Degradation products were

determined using liquid chromatography-mass spectrometry (LC–MS) system. A LC system

(UltiMate 3000, UHPLC+ focused, Thermmo Scientific) was equipped with a C18 column (4.6 mm

× 150 mm × 5 μm) and coupled online to an electrospray ionisation mass spectrometer (ESI–MS,

Thermo Fisher Scientific Orbitrap Elite).

3.6 Results and discussion

3.6.1 Catalyst characterization

Fig. 3.1 shows the representative SEM images of CSC samples. It is observed that all samples are

composed of non-homogeneous particles of differing sizes. Particle sizes ranged from less than 1 μm

to as large as 10 μm. Many of the smaller particles are interconnected via necks to large particles, an

effect of the sintering temperature up to 1000 °C. It is also noted a number of irregular shaped pores

with sizes varying up to 10 μm. The pores tend to be small and well packed in the case of smaller

particles, whilst large pores are observed between larger particles. BET surface analyses (Appendix

Fig. 3.A1) indicates that all these samples are essentially constituted of dense solid particles with

similar surface area of ∼0.3 m2 g-1.

Figure 3. 1: SEM images of CSC for x values of (a) 0, (b) 0.2, (c) 0.5, (d) 0.75 and (e) 1.0.

Fig. 3.2 displays the XRD patterns of all CSC samples containing perovskites structures in addition

to single metal oxides. The XRD patterns allocate to perovskites display peaks similar to

Ca0.3Sr0.7CuO2 and Ca5Sr9Cu24O41 (JCPDS PDF # 48-0140 and 48–1501, respectively). The CSC

samples are characterized by crystal phases with orthorhombic unit cells. In the case of CSC (x = 0)

the most intense peaks at 2θ 25.16, 25.45, 29.75, 31.61, 32.85, 34.35, 37.23, 37.82, 44.07, 44.30,

46.41, 50.32, 51.05 and 52.26 were attributed to a strontium copper oxide phase (PDF2#01-083-0266).

Also observed are low intensity peaks at 2θ 26.95, 28.43, 30.71, 38.15 and 42.33 assigned to strontium

hydroxide (PDF2#01-071-2365), whilst minor phases with peaks at 2θ 30.71, 31.05 and 33.35

ascribed to strontium copper oxide (PDF2#00-048-1496). As the amount of Ca increases in the

ternary metal oxides CSC (0.25 ≤ x ≤ 0.75), a CSC perovskite phase (PDF2#00-048-1506) at 2θ

C h a p t e r 3 P a g e | 36

16.92, 21.65, 24.88, 25.693, 30.02, 31.46, 32.73, 34.04, 37.11, 37.75, 43.98, 46.00, 46.30, 51.38 and

52.05 became prominent. It is also interesting to note that as the Ca content increases, the peaks with

the highest intensities start shifting to higher 2θ angles. The equimolar A site mixture (x = 0.5)

exhibits two crystalline CSC phases, a major (PDF2#00-048-1509) and a secondary (PDF2#01-085-

2489) phase.

Figure 3. 2: XRD patterns of CSC samples.

Further increase in calcium to x = 0.75 resulted in the formation of CaO at 2θ 31.77, 36.91 and 52.78

as the major phase (PDF2#01-070-5490). Further, a secondary phases appeared at 2θ 14.37, 27.83,

31.84, 34.42, 35.58, 43.425, 45.386, 46.28, 46.99 and 54.65 for a perovskite Ca1.6Sr0.4CuO3

(PDF2#00-048-1482) compound and at 2θ 17.87, 33.62 and 49.9 for calcium hydroxide (PDF2#00-

044-1481). Finally, the samples containing calcium and copper only (x = 1) were composed by three

major phases: calcium copper oxide (PDF2#00-034-0282) at 2θ 14.49, 35.24, 44.39, 46.38, 47.38,

48.14 and 56.45, calcium oxide (01-070-4068) at 2θ 31.77, 36.91, 52.78 and copper (I) oxide

(PDF2#01-071-3645) at 2θ 36.44, 42.32 and 61.42. Small amounts of calcium hydroxide (PDF2#00-

044-1481) at 2θ 32.29, 37.36 and 53.93; and copper (II) oxide (PDF2#01-073-6023) at 2θ 35.67 and

38.92 were also observed in the XRD pattern. All these results clearly indicate that a complex

compound was formed and that the CSC samples contain a mixture of perovskites, metal oxides and

metal hydroxides.

3.6.2 Catalyst performance

The degradation of OII was initially studied by varying the cation concentration CaxSr1-xCuO3-δ in the

A-site (0≤x≤1). Fig. 3.3a shows that all compositions were very effective in a fast degradation of

OII solution of 50 ppm. Similar performance was also observed for processing OII solutions of 10

and 100 ppm (Appendix Fig. 3.A2). The CSC samples containing a higher amount of Ca (x = 0.75

and 1) in Fig. 3.3a gave the best results with ∼80% degradation in 10 min, and ∼95% in 60 min. It is

C h a p t e r 3 P a g e | 37

interesting that this rapid degradation was achieved under dark conditions without the addition of

ozone or peroxide. The samples containing high amounts of Sr (x = 0 and 0.25) also gave fast initial

sharp degradation though reaching a maximum OII degradation efficiency of ∼90% at 60 min. The

performance of the equimolar sample (x = 0.5) was similar to the Sr rich samples. In principle, these

results indicate that Ca in the A-site of the CSC structure was more active for the degradation of OII

under dark conditions. Long-term stability testing in Fig. 3.3b shows that the CSC sample first cycle

of 85% OII degradation efficiency reduced in subsequent cycles and became stable at ∼75% from

cycle 5.

Figure 3. 3: (a) Effect of Ca and Sr cation concentration in the A-site of CSC for the degradation

(±2 ppm) of OII. Experimental conditions: [OII]0=50 ppm, magnetic stirring, RT, dark. (b)

Cycling stability of CSC (x=0.75) sample. Experimental conditions: [OII]0=20 ppm, magnetic

stirring, RT, dark.

However, the XRD patterns showed that the CSC compounds are actually complex mixtures of

perovskite, metal oxides and hydroxides. In order to verify which phases are the most active, the

degradation of OII was carried out for single and mixed metal oxides. Table 3.1 displays the

concentration of OII after 120 min. The single and mixed metal oxides showed OII degradation up to

50%. Subsequently, we acidified the degraded solutions with 0.5 M HCl and found that the OII

concentration increased to 0.63 ≥ C/Co ≥ 0.87. This is related to the pH effect of OII in the solution,

which affects the OII absorbance intensity measure at 485 nm. Interestingly, the subsequent

acidification of OII solutions tested using CSC samples showed small to almost non-existent changes

in the OII concentration. Therefore, these results strongly suggest that the active phase in CSC

compound is the perovskite phase. Further, the OII aqueous solution without any catalyst was also

tested, showing no degradation (Appendix Fig. 3.A3). Therefore, the OII degradation is solely

attributed to the catalytic properties of CSC compound.

C h a p t e r 3 P a g e | 38

Table 3. 1: OII degradation percentage using single and mixed metal oxides, and CSC samples.

Experimental conditions: [OII]0=10 ppm, catalyst dosage=1 g L-1, magnetic stirring, RT.

Samples 2h 2h+acid

CaO 0.52 0.84

SrO 0.58 0.73

CuO 0.69 0.79

CaO+SrO 0.61 0.87

CaO+CuO 0.48 0.67

SrO+CuO 0.47 0.63

CaO+SrO+CuO 0.50 0.71

CSC (x=0) 0.19 0.27

CSC (x=0.25) 0.26 0.30

CSC (x=0.5) 0.16 0.24

CSC (x=0.75) 0.16 0.16

CSC (x=1) 0.14 0.14

Although the CSC perovskite phase is the most active, it is interesting that the samples containing Ca

(x = 0.75 and 1) were more active in the degradation of OII (Fig. 3.3a). Further analyses of the XRD

patterns (Fig. 3.2) clearly indicate a major difference between CSC samples. The samples with high

Sr content (x = 0 and 0.25) were made mainly of a major perovskite phase with minor oxide phase.

In contrast, the samples with a high amount of Ca (x = 0.75 and 1) had two major phases of perovskite

and oxides, in addition to hydroxides. Therefore, the improved performance of the CSC samples with

high Ca content could be attributed to a compounding effect of double oxides and pH. In the first

instance, the presence of CaCuO resulted in 0.86 of OII degradation (Table 3.1), thus aiding in the

overall performance. This type of effect has also been observed for CaNiO for the degradation of

methylene blue using visible light [29]. The pH effect is also contributory as calcium oxide has a

propensity to form calcium hydroxide in aqueous solution, thus increasing the pH. The pH of the

samples containing Ca were slightly higher than those containing Sr (Appendix Fig. 3.A4). This is in

line with reports for the degradation of indigo carmine using calcium oxide and visible light, where

degradation efficiency increased from 95.4% for pH 9–10 to 100% for pH 12 [30].

The OII solutions were decolorized during testing, given a clear indication of the destruction of azo

bond -N=N- [31]. OII exists in two tautomeric forms in equilibrium aqueous solution [32] and the

hydrazone form is predominant in aqueous solution [33, 34]. The UV-vis spectrum of OII solution in

the visible region in Fig. 3.4a exhibits two main bands at 430 and 485 nm, corresponding to transition

C h a p t e r 3 P a g e | 39

of the azo and hydrazone forms, in addition to two other peaks at 230 and 310 nm assigned to the

benzene and naphthalene rings of OII molecules [35]. Fig. 3.4a also shows the UV-vis absorption

evolution of OII degradation as a function of time. The initial band for OII is the base line at time 0

min. Within the first 5 min, all bands significantly decreased, whilst almost disappearing at 20 min.

This confirms the OII degradation rates in Fig. 3.3 at 50 ppm. Simultaneously, two new bands started

to appear at 254 and 344 nm which can be attributed to byproducts. In order to confirm the presence

of byproducts, TOC analyses in Fig. 3.4b clearly indicates that the degradation of OII was partial, as

more than 64% of carbon remained in the solution after 120 min. Of interest, it is noted that the

samples containing a higher concentration of Sr (CSC x = 0 and 0.25) had the lowest TOC readings,

whilst the equimolar sample (CSC x = 0.5) was almost ineffective in degrading OII with a TOC

reading as high as 92%.

Figure 3. 4: (a). UV-vis absorption spectra evolution of OII with time and (b) TOC readings for

CSC (0≤x≤1). Experimental conditions: [OII]0=50 ppm, catalyst dosage=1 g L-1, magnetic

stirring, RT.

In order to understand further the by-product formation observed in the UV–vis spectra in Fig. 3.4a,

a HPLC analysis was carried out of OII solutions degraded to 120 min. The resulting chromatograms

and UV-vis spectra are presented in Fig. 3.5a and b, respectively. The chromatogram of OII (sample

at time 0 min) has a major peak at retention time (tR) 2.5 min and a minor peak at 2.02 min. The

analysis of the solution after 1 min treatment revealed that the peaks corresponding to initial OII

remained almost the same though at lower intensity. This clearly indicates that OII degradation

occurred very quickly in the first min of reaction. There is also a minor peak appearing at 1.3 min.

As the reaction proceeds further to 5 and 120 min, the OII major peak at 2.5 min retention time

reduces significantly in intensity, while the new peak at 1.3 min retention time slightly increases in

intensity as seen in the UV-vis spectra (Fig. 3.5b). These results confirm the breakdown of OII

C h a p t e r 3 P a g e | 40

molecules and the generation of by-products which were further analysed by LC-MS technique. Table

3.2 lists a number of smaller molecular by-products formed during the degradation of OII.

Figure 3. 5: (a) HPLC chromatograms and (b) UV-vis spectra of species found with reaction

times of 0, 1, 5 and 120 min. Experimental conditions: [OII]0=50 ppm, catalyst dosage=1 g L-1,

magnetic stirring, RT.

Table 3. 2: Reaction product ions identified from OII degradation.

Ion Number m/z [M-H]- Elemental formula (ion)

1 200.98 C7H5SO5-

2 191.04 C10H7O4-

3 121.03 C7H5O2-

4 165.02 C8H5O4-

5 172.01 C6H6NO3S-

6 172.99 C6H5O4S-

7 157.00 C6H5O3S-

8 173.02 C10H5O3-

9 327.04 C16H11N2O4S-

3.6.3 Role of CSC compounds and reaction mechanism

The surface chemistry of the CSC catalysts was analyzed by XPS in order to understand the structural

evolution prior and after catalytic testing. The survey XPS spectra (Appendix Fig. 3.A5) indicate that

the surface of pristine and spent CSC were composed of Ca, Sr, Cu, O and adventitious C. High

resolution spectra were deconvoluted as displayed in Fig. 3.6. The Ca 2p XPS narrow spectrum in

C h a p t e r 3 P a g e | 41

Fig. 3.6a shows double peaks at 346.8 eV and 350.3 eV attributed to Ca 2p3/2 and Ca 2p1/2 components,

respectively [36]. The XPS narrow spectrum for the Sr 3d (Fig. 3.6b) with peaks at 133.2 and 135.3

eV corresponds to Sr2+ [37]. The Cu 2p3/2 peak at ∼933.8 eV in Fig. 6c is accompanied by two

satellites at ∼941.8 to ∼944 eV, which are assigned to Cu2+, and the peak at ∼932.8 eV to Cu1+ [38].

Considering the relative intensity of Cu 2p spectra for both pristine and spent catalyst, Cu presented

mainly as Cu2+ in both samples. This clearly indicates that oxidation of Cu was not significantly

affected as is the case with Fe in the Fenton reaction.

The binding energy of O 1s electrons in oxygen bonded to metal cations is one of the most informative

parameters with respect to the oxide structure [39]. The O 1s oxygen spectra were deconvoluted into

three distinguishable peaks (Fig. 3.6d). The peak at ∼533 eV was attributed to trace amount of oxygen

coming from carbonates [40]. The two peaks at the binding energies of ∼531 eV and ∼529 were

assigned to adsorbed and lattice oxygen atoms, respectively [41, 42]. The O 1 speak at ∼529 eV was

very similar for both pristine and spent catalyst, implying no changes in the lattice oxygen. However,

the intensity of adsorbed oxygen at ∼531 eV increased after the OII degradation process. This result

indicates that oxygen containing species (O or OH) were present on the spent CSC surface which are

derived from the catalytic process or from the formation of metal hydroxides as determined by XRD

patterns (Fig. 3.2).

Figure 3. 6: Pristine and spent CSC high resolution XPS spectra of (a) Ca, (b) Sr, (c) Cu and (d)

O.

C h a p t e r 3 P a g e | 42

Under the testing conditions in a heterogeneous catalytic reaction, mass transfer of OII as the reactant

from the fluid phase to contact the surface of the solid CSC catalyst was facilitated by the constant

stirring of the solution. Upon contact, the reactant adsorbed on the surface of the solid catalyst

followed by reaction as represented by Eq. (3.1) and Eq. (3.2) below, respectively. TOC results (Fig.

3.4b) clearly indicate that the majority of carbon (∼70%) remained in the solution after 120 min

testing. Although the catalyst proved to be very efficient for OII degradation based on UV-vis

measurements (Fig. 3.3a and Fig. 3.4a), the fate of 30% carbon needs to be ascertained if this was in

fact attributed to partial mineralization or adsorption.

The TGA results in Fig. 3.7 show that the blank CSC sample (exposed to water for 2 h) resembles

the pristine CSC sample, though it shows a higher mass loss. This was attributed to the hydroxylation

of metal oxides (i.e. CaO + H2O → Ca(OH)2) as the pristine CSC sample was not exposed to water.

The spent and blank CSC samples have similar patterns up to 260 °C, where the mass loss of the

spent CSC sample became higher than the blank CSC sample all the way to 550 °C. The final mass

loss difference between the blank and spent CSC sample is ∼2% per gram, equivalent to 20 mg. As

the spent CSC sample was tested for a concentration of 50 ppm and 30% could not be accounted for

as per TOC results, which is equivalent to 15 ppm or 15 mg. Within the experimental error of this

work, in addition to the hydroxylation of metal oxides which may be slightly different in both blank

and spent CSC, these results strongly suggest that the non-accounted 30% TOC value was definitely

due to adsorption. However, the spent CSC sample mass loss profile differs from the OII profile, thus

given a clear indication that other compounds were adsorbed on the surface of the CSC catalyst after

the reaction.

Figure 3. 7: TGA mass loss curves of OII, pristine CSC, blank CSC (water only) and spent CSC

(50 ppm of OII) samples.

C h a p t e r 3 P a g e | 43

In order to further understand the adsorption of compounds on the CSC catalyst, the FTIR results in

Fig. 3.8 show that the spent CSC samples has several minor intensity peaks which can be correlated

to the major intensity peaks in the spectrum of OII. At the same time, the pristine and spent CSC

samples spectra resemble each other though the slightly broader doublet band 1454 and 1424 cm-1

[43] increased in intensity for the spent sample, together with the absorption band at 870 cm-1. These

bands are assigned to carbonates, indicating the increase in carbonate formation during reaction. The

spectrum of the spent CSC sample was also compared to the spectra of the physically mixed OII (10

and 50 ppm) with pristine CSC. However, the spectrum of the spent CSC sample does not entirely

resemble the spectra of the physically mixed samples. It is observed that the intensities at 1232 and

1253 cm-1 (linked to chromophore parts (i.e., -N-N- and -C-N-) of hydrazone form of OII, respectively)

[44,45] are negligible compared with physical mixture of the pristine CSC + OII (10 ppm).

Furthermore, the intensities of nearly all peaks (1450, 1553, 1568, 1596 and 1620 cm-1) attributed to

C=C aromatic skeletal vibrations [46] also decreased. Considering that the non-accounted 30%

carbon from the TOC results are attributed to adsorption, equivalent to 15 ppm in the spent CSC

sample, these results may suggest the adsorption of OII was lower than 10 ppm, and that other

compounds derived from the reaction as by products also adsorbed on the surface of the spent CSC

sample.

Figure 3. 8: FTIR spectra of OII, pristine CSC, spent CSC (50 ppm of OII) and physically mixed

samples of OII (10 and 50 ppm) with pristine CSC (see appendix Table A1 for band

assignments).

As the tested solution was clear after testing, these results strongly suggest that the CSC catalyst was

effective in breaking the conjugation in the OII molecules, particularly the -N=N- bonds, which are

the most labile [47]. The breakdown of -N=N- bonds generate electrons (Eq. (3.2)). The perovskite

C h a p t e r 3 P a g e | 44

phase of the complex CSC compound was ascertained to be the most active phase in the degradation

of OII. As several by-products were formed as determined by HPLC (Fig. 3.5) and LC-MS (Table

3.2) measurements, then the fate of electrons in this reaction must be ascertained. Perovskites have

good electron conductivity [48], particularly Cu, which is an electron conducting material. Therefore,

to accept electrons CSC must also donate electrons to maintain electrical neutrality. The electrons

generated in this reaction have the propensity to form superoxide radical anion O·-

2 (Eq. (3.3)) in

aqueous solutions. This process may have led to further reactions such as the generation of

hydroperoxyl radical HO•

2 (Eq. (3.4)), which further reacted to obtain H2O2 (Eq. (3.5)), HO• and OH-

(Eq. (3.6)). Recently, Turmino et al. [22] reported the presence of hydroxyl radical species (HO•) by

using 2-propanol as a radical scavenger in the degradation of OII under dark conditions and using

SrCeFe perovskites.

To confirm the presence of hydroxyl radicals in this work, 2-propanol was mixed with OII to make a

solution prior to adding the CSC catalyst. Fig. 3.9 shows a reduction in the OII degradation for the

mixture containing 2-isopropanol as compared to the OII solution without additives. These results

confirm that hydroxyl radicals were generated, thus explaining the formation of several by-products.

Further, the hydroxyl radicals can react with OII/OII+ to generate a series of byproducts (Eq. (3.7)).

However, the hydroxyl radicals can be formed only when there is the supply of electrons by the

breaking down of the azo bonds (Eq. (3.2)). Once this reaction is completed, then the supply of

electrons ceased and likewise the generation of hydroxyl radicals.

Figure 3. 9: Degradation curves of OII (50 ppm) with no additive and OII (50 ppm) plus 2-

propanol (0.1 M) using CSC catalyst. Experimental conditions: [CSC]=1 g L-1, magnetic

stirring, RT and dark.

OII + CSC → CSC(OII) Eq. (3.1)

CSC(OII) ↔ CSC + OII+ + e− Eq. (3.2)

C h a p t e r 3 P a g e | 45

e− + O2 → O2−∙ Eq. (3.3)

O2−∙ + H+ ↔ HO2

∙ Eq. (3.4)

2HO2∙ → H2O2 + O2 Eq. (3.5)

H2O2 + e− → HO ∙ +OH− Eq. (3.6)

HO ∙ +𝑂𝐼𝐼 𝑂𝐼𝐼+⁄ →∙∙∙→ 𝑏𝑦 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 Eq. (3.7)

3.7 Conclusions

CSC compounds contained a complex structure of perovskite and metal oxide phases. These

compounds were very effective in breaking down an azo dye molecule OII under dark ambient

condition from low 10 ppm to high 100 ppm concentrations. Samples containing a higher content of

Ca showed slightly higher degradation than those containing a high content of St in the A-site of the

perovskite structure. The CSC catalyst proved to reach long-term stability after 9 cycles of testing.

Further, the perovskite phase of the CSC compound was more active in the degradation of OII than

metal oxides. The reaction kinetics were very fast, and within 60 min more than 82% of OII was

degraded. However, the degradation was characterized by the breakdown of -N=N- bonds evidenced

by clear solutions instead of reddish-yellow solutions containing OII. The degradation of OII was

partial only, leading to formation of by-products as ascertained by HPLC and LC-MS. TOC

accounted for more than 60% of the carbon, whilst the remainder of the carbon was found to be

adsorbed on the surface of the spent CSC catalyst. The CSC compound delivered a combined catalytic

effect under dark conditions, by effectively breaking down OII molecules whilst partially adsorbing

by-products of the reaction and OII molecules. The surface of the CSC catalyst was very active, thus

reacting with OII and generating by-products and electrons. The electrons played a major role in the

formation of hydroxyl radicals, which further degraded OII and/or its by-products. Once the

breakdown of -N=N- bonds ceased, then the reaction reached completion, the supply of electrons

stopped and hydroxyl radicals were no longer generated.

Acknowledgements

H. Chen gratefully acknowledges for the China Scholarship Council and The University of

Queensland scholarships. Authors thank the Australian Research Council (ARC) financial support

(FT130100405). J.C. Diniz da Costa thanks the support from the ARC via the Future Fellowship

Program (FT130100405).

Appendix

C h a p t e r 3 P a g e | 46

Table 3.A 1: Orange II FT-IR bands assignment.

Figure 3.A 1: Representative nitrogen sorption isotherm of CSC samples (x=0.75). This

isotherm of Type II, is characteristic of dense particles due to the very low adsorbed volume at

low relative pressures. The increase of volume at p/po>0.75 is associated with the inter-particle

space.

C h a p t e r 3 P a g e | 47

Figure 3.A 2: Effect of Ca and Sr cation concentration in the A-site of CSC for the degradation

(±2ppm) of OII for [OII]0 of (a) 10 ppm and (b) 100 ppm. Experimental conditions magnetic

stirring, RT and dark.

Figure 3.A 3: Degradation of OII (10 ppm) without any catalyst after 120 min. Experimental

conditions: magnetic stirring, RT, dark.

C h a p t e r 3 P a g e | 48

Figure 3.A 4: pH measured as a function of time during the degradation of OII.

Figure 3.A 5: Survey XPS spectra of OII, pristine and spent CSC.

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C h a p t e r 4 P a g e | 53

4 Ceramic metal oxides with Ni2+ active phase for the fast degradation of Orange II dye under

dark ambiance

4.1 Introduction

It is generally believed that B-site cations determine the catalytic activity largely. As nickel can

facilitate the generation of ROS to enhance catalytic activity (Section 2.3.2) and CaSrCuO oxides

demonstrated high catalytic activity and recyclability for dye degradation in the dark ambient

conditions (Chapter 3), the aim of Chapter 4 is to probe the effect of Cu complete substitution in

CaSrCuO by Ni on the catalytic degradation ability towards OII.

4.2 Contributions

Chapter 4 has been published in Ceramics International. This chapter is wholly my own work with

the exception of the contribution by Prof. João C. Diniz da Costa, Dr. Julius Motuzas and Dr. Wayde

Martens.

C h a p t e r 4 P a g e | 54

Ceramic metal oxides with Ni2+ active phase for the fast degradation of Orange II dye under dark

ambiance





Australia.


4.3 Abstract

Ceramic metal oxides based on calcium strontium nickel (CSN) were synthesized via a combined

EDTA-citric acid complexation method and evaluated using Orange II (OII) as the model pollutant

under dark conditions, without adding any external stimulants. The CSN catalyst was characterized

by a very fast reaction reaching 97% OII degradation within 5 min. The surface of CSN metal oxides

proved to be very active toward the breakdown of the -N=N- azo bond of OII. A second electron

generating pathway was found as Ni2+ phase in the CSN catalyst oxidized to Ni3+ for the spent catalyst.

Both electron generating pathways resulted in the formation of hydroxyl radicals (OH·) as determined

using radical quenchers. Hydroxyl radicals were responsible for the formation of several intermediate

products. The Ni2+ phase was very active contrary to the Ni3+ phase which could not degrade OII.

The fast degradation kinetics of OII using CSN in dark at room temperature was attributed to the

double electron generation pathways.

Keywords: Ceramics, Ni2+ phase, OII degradation, dark condition, fast reaction

4.4 Introduction

Water pollution by discharged dyes from the textile industry is a major global environmental concern,

with serious sustainability implications for society. Many dyes are hazardous to the environment and

are toxic to living organisms, directly or through their absorption or reflection of sunlight entering

the water [1–3]. Dyes usually have poor biodegradability and good resistance to UV or visible light

irradiation [4, 5]. Therefore, various physical, chemical and biological processes are used to treat dye-

containing wastewater. These include adsorption [6], coagulation/membrane nanofiltration [7],

electrochemical [8], microbiological fuel cell [9], ozonation [10], Fenton reaction [11–13] and

photocatalysis [14–16]. However, these processes usually involve complicated procedures, or are

economically unfeasible or limited in terms of operation conditions [17]. Thus, the development of


C h a p t e r 4 P a g e | 55

fast, simple and effective methods to remove dyes from dye-contaminated wastewaters is of vital

environmental significance.

Recently, metal oxides became desirable as they can oxidize and remove organic compounds in the

dark without adding external stimulants and energy. For instance, many metal oxides have been

reported to degraded dyes in the dark such as BaFeO3-δ degraded 80% methyl orange in 30 h [18],

SrFeO3-δ degraded 98% acid orange 8 in 10 min [19] and La4Ni3O10 degraded ~97% methyl orange

in 3.5 h [20]. Hence, metal oxides are attractive as their operation under dark conditions simplifies

the engineering configuration of textile wastewater plants, whilst reducing operational costs by

dispensing the need of catalytic stimulants such as peroxides in the heterogeneous Fentonlike reaction

or energy in photo-catalysis using UV/visible light irradiation.

Chen et al. [21] reported very recently on a metal oxide CaxSr1−xCuO compound which could degrade

60% and 90% OII (20 ppm) in 10 and 60 min under dark conditions, respectively. Based on this

general formula, there is an array of metal oxides that can be replaced, so opening a window of

research opportunities to develop more efficient metal oxide-based catalysts. One of the metal ions

of interest is Ni, which is low-cost, non-noble metal catalyst widely used for industrial applications.

The use of Ni has been demonstrated as LaNiO3 and as LaNiO3/graphene composites for photo-

degradation for methyl orange [22] and acid red [23], or as Ni(II)-substituted Bi2VO5.5 for the

degradation of 4-SPPN under visible light irradiation [24]. Most importantly, Ni-containing

La4Ni3O10 [20] and La2NiO4 [25] have shown the potential to degrade organics in the dark.

Therefore, this work investigates the catalytic effect of a metal oxide compounds containing Ca, Sr

and Ni for the degradation of OII, a dye which contains azo groups (R1‒N=N‒R2) and represents

~60–70% of the global production of azo dyes [26, 27]. The compound of interest is Ca0.5Sr0.5NiO

(denoted as CSN) metal oxides. The CSN compound was fully characterized to understand its

material make up. Further, the CSN compound was assessed for OII degradation from 10 to 40 ppm

in the dark at room temperature, including determination of intermediate products. Finally, a reaction

mechanism is proposed based on the CSN materials characteristic and degradation reaction

performance.

4.5 Materials and methods

4.5.1 Materials, synthesis and characterization

Calcium nitrate tetrahydrate, citric acid monohydrate, aqueous ammonia solution and

ethylenediaminetetra acetic acid disodium salt dehydrate (EDTA-2Na) were purchased from Chem-

Supply Pty Ltd. Strontium nitrate, and isopropanol (IPA) were obtained from Alfa Aesar.

Ethylenediamine tetra-acetic acid (EDTA) and potassium iodide (KI) was purchased from Ajar

C h a p t e r 4 P a g e | 56

FineChem Pty Ltd. Nickel (II) nitrate hexahydrate, potassium bromate (KBrO3), and orange II were

supplied by Sigma-Aldrich. All reagents were at least of analytical grade and used as received. The

molar ratio of total metal ions: EDTA: citric acid: ammonium hydroxide was constant set at 1:1.1:2:10.

In a typical synthesis procedure, two separate solutions were prepared. Solution I contained ammonia

aqueous, which was added slowly to a beaker containing EDTA under magnetic stirring until full

dispersion and formation of a transparent solution. Solution II contained stoichiometric amounts of

nitrate salts and citric acid dissolved into deionized water. Then solution I was dispersed into Solution

II under vigorous magnetic stirring. Subsequently, the resultant mixture was heated to evaporate most

water to attain a viscous fluid. The viscous fluid was initially sintered in atmospheric air in a furnace

up to 450 ℃ for 8 h with heating and cooling rates of 5 °C min-1. A second sintering step was applied

up to 1000 ℃ using the same dwell time and ramping rates. The obtained powder was milled and

stored for further use. The powder morphology was analysed using a scanning electron microscope

(SEM) equipped with an energy dispersive X-ray spectroscopy (EDS). Powder X-ray diffraction

(XRD) was used for phase identification using an X-ray diffractometer (XRD, D8 Advance, Bruker,

USA) with Cu-Kα radiation (λ=1.5406 Å, 40 kV, 40 mA, 10 ≤ 2θ ≤100). Surface chemical

components and valence states were characterized by X-ray photoelectron spectrometer (XPS)

(Kratos Axis ULTRA) equipped with monochromatic Al Kα (hν=1486.6 eV) radiation.

4.5.2 Catalytic activity evaluation

The catalytic activity of CSN was evaluated using orange II (OII > 85% Sigma-Aldrich) in an aqueous

solution at 20 ppm as the model pollutant. The reaction was initially carried out by dispersing CSN

in OII solution with CSN dosage of 1 g L-1 in the dark at room temperature without external reagents

or energy input. The suspension was stirred continuously and at given time intervals, 5 mL aliquots

were taken out and filtered using 0.45 µm Milipore syringe filters. Evolution of OII concentration

was monitored by measuring the adsorption intensity at λ=485 nm using a UV-vis spectrophotometer

(Evolution 220, Thermo Fisher Scientific). A blank test was performed in the absence of CSN under

otherwise identical conditions. To clarify the reactive species involved in the degradation of OII by

CSN, several trapping experiments were performed. KBrO3 (0.10 M) was used as an electron

scavenger, IPA (0.10 M) and KI (0.10 M) to scavenge hydroxyl radicals (OH•). In addition, a

chelating agent (EDTA-2Na (0.10 M)) was added into the reaction suspension to investigate whether

the degradation occurred on CSN surface or not. Degradation products were determined using liquid

chromatography-mass spectrometry (LC-MS) system. A LC system (UltiMate 3000, UHPLC+

focused, Thermmo Scientific) was equipped with a C18 column (4.6 mm × 150 mm × 5 µm) and

coupled online to an electrospray ionisation mass spectrometer (ESI-MS, Thermo Fisher Scientific

Orbitrap Elite). Total organic carbon (TOC) was measured using a Shimadzu TOC analyzer (model

C h a p t e r 4 P a g e | 57

TOC-VCSH). The mineralization degree of OII was calculated using the TOC values of OII solutions

(20 ppm) before and after reaction (120 min).


4.6.1 Characterization

Both raw (i.e., pristine as-prepared) and spent (i.e., tested for degradation) CSN consist of

agglomerate particles of micrometer size (Fig. 4.1). Raw CSN clearly shows the formation of necks

between smaller and larger particles (Fig. 4.1a), an effect of sintering metal oxides at high

temperatures up to 1000 °C. The spent CSN (Fig. 4.1b) shows particles of the same size as the pristine

sample, though the surface became rough with lots of small particles protruding around each larger

particle. This result suggests that the morphological features of the CSN sample slightly changed after

reaction. The nitrogen sorption of CSN samples displayed isotherms of type III (Appendix Fig. 4.A1),

a characteristic of non-porous materials. Essentially, the BET surface area was ~1 m2 g-1, in line with

micron sized particles visually observed in the SEM images in Fig. 4.1.

Figure 4. 1: SEM images of (a) raw and (b) spent CSN samples.

The XRD patterns for both raw and spent CSN are depicted in Fig. 4.2. These patterns are

characterized by sharp and intense peaks, thus demonstrating the crystal structure of CSN metal

oxides. The intense peaks at 2θ 37.1, 43.0, 62.7, 75.2 and 79.2° are attributed to cubic NiO (PDF#01-

073-1523) while low intense peaks at 18.0, 28.6, 34.0, and 47.0° are ascribed to Ca(OH)2 (PDF#00-

044-1481). Peaks at 14.2, 19.3, 24.2, 26.3, 28.2, 31.4, 31.8, 39.0 and 40.3° are assigned to

Sr(OH)2·H2O (PDF#00-028-1222) in the raw sample. The spent CSN sample is represented by two

phases, one of which was (Sr, Ca)CO3 (PDF#44-1421) with the peaks at 2θ 25.6, 29.4, 36.2, 44.1, 47

and 50°. The remaining peaks were assigned to patterns of rhombohedral NiO phase (PDF#01-089-

7390) with main peaks at 37.1, 43.0, 62.7, 75.2, 79.2, 79.3, 79.5, 79.6 and 94.9. These results suggest

changes in the phase of the CSN catalyst after reaction.

C h a p t e r 4 P a g e | 58

Figure 4. 2: XRD patterns of (a) raw and (b) spent CSN samples.

Ca, Sr, Ni and O were observed in the XPS survey spectra of both raw and spent CSN (Appendix Fig.

4.A2). Fig. 4.3 shows that the peaks for both Ca and Sr remained unchanged before and after reaction,

thus suggesting that these elements were not affected during the degradation of OII. However, Ni was

affected as changes were observed in the peaks of the raw and spent CSN samples. The two main

peaks at ~873.6 eV and ~856.2 eV were assigned to Ni 2p1/2 and Ni 2p3/2, respectively [28]. Ni 2p

spectra (Fig. 4.3c) consist of two spin-orbit doublets of Ni 2p and two shake up satellites denoted as

“Sat”. The Ni 2p3/2 XPS spectra were fitted into two sub-peaks centered at ~854.3 and ~856.2 eV,

which pertain to Ni2+ and Ni3+ ions, respectively [29]. It is clearly observed that the Ni2+ peaks

comparatively to Ni3+ peaks reduced significantly in intensity after OII degradation. This result

strongly suggests that Ni2+ in the raw CSN oxidized to Ni3+ in the spent CSN.

Figure 4. 3: XPS spectra of Ca 2p, Sr 3d and Ni 2p for both raw and spent CSN samples.

4.6.2 Reaction assessment

The UV-vis spectra of the OII solutions prior and after reaction are displayed in Fig. 4.4. The

absorbance peak at 485 nm assigned to azo groups [30] almost fully disappeared within 5 min, leading

to the fast discoloration of OII. This implies that the -N=N- bonds in the azo group in OII was almost

C h a p t e r 4 P a g e | 59

fully degraded by the CSN catalyst. However, other compounds of OII did not seem to be degraded.

For instance, two emerging peaks observed at 200 and 254 nm might be associated with the generation

of sulfanilamide and 1-amino-2-naphtol [31]. The peak at 254 nm, which is attributed to aromatic

intermediates, intensified with time before remaining practically constant by the end of experiment

(Fig. 4.4a), indicating that aromatic intermediates remained in the solution. A new absorbance peak

at 344 nm appeared at first and then decreased with time, indicating further degradation of

intermediates compounds (Fig. 4.4c). The OII did not degrade without the CSN catalyst under dark

conditions (Appendix Fig. 4.A3). Further, OII solutions obtained after the reaction with CSN samples

were acidified and resulted in non-significant (Appendix Fig. 4.A4) changes in the OII degradation.

Therefore, all the degradation results in this work are entirely attributed to the catalytic effect of CSN.

Figure 4. 4: (a) UV-vis absorption spectra of OII azo dye solution treated by CSN recorded at

different time intervals; (b) and (c) are the magnification of the UV region from 240 to 280 nm

and from 300 to 410 nm, respectively. Benzene ring (P1), napthalene ring (P2), azo form (P3)

and hydrozone form (P4). Experimental conditions: [OII]0=20 ppm, [CSN]=1 g L-1, magnetic

stirring, RT, dark.

The effect of OII degradation was investigated by varying the OII concentration from 10 to 40 ppm.

Fig. 4.5a shows a very fast degradation of OII within 5 min independently of its concentration. It is

interesting to observe that at low OII concentrations of 10 and 20 ppm, OII degradation was almost

complete close to 100%. However, the degradation efficiency reduced from ~90 to ~75% as the OII

concentration increased from 25 to 40 ppm, respectively. These results suggest that the CSN catalyst

started losing its catalytic activity. Therefore, CSN samples were recycled to determine their stability.

Fig. 4.5b clearly indicates that CSN catalysts lose their activity at every subsequent cycle. This is

attributed to the oxidation of nickel from Ni2+ to Ni3+ as verified in the XPS analysis (Fig. 4.3). In

other words, Ni2+ is the active phase. The loss of the active phase is common in iron oxides in the

heterogeneous Fenton like reaction [32, 33], which is extensively used for the degradation of OII.

C h a p t e r 4 P a g e | 60

Figure 4. 5: (a) Effect of dye initial concentrations on degradation efficiency, and (b) reaction

cycle stability. Experimental conditions: [CSN]=1 g L-1, magnetic stirring, RT, dark.

HPLC chromatograms of initial and treated OII solutions for 2 h are displayed in Fig. 4.6. OII has a

typical peak with the retention time tR of 1.7 min and the combined peaks at 1.1 and 1.2 min are

related to the coexisting of other compounds in OII powder. Fig. 4.6a clearly shows that the peak at

1.7 min decreased significantly after 2 h of reaction. However, the intensity of the combined peaks at

1.1 and 1.2 min increased concurrently, indicating the corresponding compounds were accumulating.

It is noteworthy that the relevant intensity of these peaks varied after the reaction as observed in Fig.

6b and c, thus indicating changes in their concentration after 2 h reaction. Table 4.1 lists a number of

intermediate products, which were identified by LC-MS for OII degradation sample after 2 h reaction.

These results confirm the degradation of OII and the generation of a number of intermediates in the

presence of CSN under dark conditions at room temperature.

Figure 4. 6: (a) HPLC chromatograms of initial OII and treated OII after 2 h. (b) UV-vis

absorbance spectra of treated OII solution at the retention time of 1.1, 1.2 and 1.7 min.

C h a p t e r 4 P a g e | 61

Table 4. 1: Reaction product ions identified from the degradation of OII after 2h

Ion number m/z [M-H]- Elemental Formula (ion)

1 200.9863 C7H5SO5–

2 191.0351 C10H7O4–

3 121.0294 C7H5O2–

4 165.0195 C8H5O4–

5 172.0072 C6H6NO3S–

6 172.9915 C6H5O4S–

7 156.9964 C6H5O3S –

8 173.0245 C10H5O3–

9 343.0392 C16H11N2O5S–

10 171.0160 C16H10N2O5S2–

11 315.0331 C16H11O5S–

12 327.0444 C16H11N2O4S–

The fate of non-accounted carbon by TOC was further investigated to ensure if this value was

associated with mineralization or adsorption on the CSN catalyst. The TGA results in Fig. 4.7 show

that the blank CSN sample (exposed to water for 2 h but no OII) follows similar mass loss as the

spent CSN sample. The final mass loss difference between the blank and spent CSN sample is ~2.2%

per gram, equivalent to 22 mg. The spent CSN sample (1 g L-1) was tested for a concentration of 20

ppm and 10% could not be accounted for as per TOC results, which is equivalent to 2 ppm or 2 mg.

Hence, a fraction of the compounds containing carbon did actually adsorb on the surface of the CSC

catalyst.

Figure 4. 7: Mass loss of spent CSN (OII = 20 ppm) and blank CSN (exposed to water only)

samples.

C h a p t e r 4 P a g e | 62

4.6.3 Degradation mechanism

Based on the above results, the degradation of OII took place via two electron transfer mechanisms.

The first electron transfer mechanism (I) is associated with the contact of the OII molecule with the

CSN catalyst (Eq. (4.1)). Due to the catalytic activity of the surface of the CSN particle, this causes

the breakdown of weakest bonds in OII which are -N=N- azo group, thus generating electrons (Eq.

(4.2)). This is supported by disappearance of the UV-vis spectra absorbance peak at 485 nm in Fig.

4.4, responsible for the discoloration of the solution. The second electron transfer mechanism (II) is

associated with the oxidation of Ni2+ in the raw CSN to Ni3+ in the spent CSN. This is evidenced by

the XPS results in Fig. 4.3, and the oxidation of Ni2+ to Ni3+ donated an electron (Eq.(4.3)).

Eqs. (4.1)-(4.3) suggest that the degradation mechanism of OII is dominated by the surface properties

of CSN. However, they do not explain the number of intermediates identified in Table 4.1, which

require reaction with radicals for their formation. Recently, Tummino et al. [34] reported the presence

of hydroxyl radical species (HO∙) for the degradation of OII under dark conditions by metal oxides

containing SrCeFe oxides. In order to elucidate the reaction pathways in this work, further

experimental work was carried out using a number of radical quenchers to determine the reactive

species/radicals. These included a chelating agent (EDTA-2Na), an electron scavenger (KBrO3), and

OH· scavengers (KI and IPA). Fig. 4.8 shows that EDTA-2Na dramatically suppressed OII

degradation. This result indicates that EDTA-2Na chelated on the surface of OII so blocking the

contact of OII with the CSN surface. In other words, confirming that OII degradation occurred on the

CSN surface. KBrO3 also reduced the degradation of OII thus indicating that electrons were generated

during the degradation reaction. Likewise, OII degradation reduced in the presence of KI and IPA,

thus confirming the generation of OH.

Figure 4. 8: Effect of reaction quenchers on OII degradation by CSN. Experimental conditions:

[OII]0=20 ppm, [CSN]=1 g L-1, [EDTA-2Na]=[KI]=[IPA]=[KBrO3]=0.10 M, magnetic stirring,

RT, dark.

C h a p t e r 4 P a g e | 63

Based on the above trapping experimental results, the reactive species responsible for the production

of intermediates and/or further OII degradation can be elucidated. The electrons generated by the

brake down of -N=N- azo group and those donated by the oxidation of bivalent nickel ions have the

propensity to form superoxide radical anion O•-

2 (Eq. (4.4)) in aqueous solutions. This leads to the

subsequent generation of hydroperoxyl radical HO•

2 (Eq. (4.5)), H2O2 (Eq. (4.6)), and finally hydroxyl

radical HO∙ and OH-(Eq. (4.7)). Hydroxyl radical is responsible for further reactions and the

formation of several intermediates (Eq. (4.8) and Table 4.1). Fig. 4.9 schematically illustrates the

reaction pathways of OII degradation using CSN catalysts under dark conditions at room temperature.

Figure 4. 9: Illustration of sensitization degradation mechanism for OII degradation over CSN

for [I] direct electron transfer and [II] nickel oxidation.

OII + CSN → CSN(OIIads) Eq. (4.1)

CSN(OIIads) ↔ OII+ + CSN + e− Eq. (4.2)

Ni2+ → Ni3+ + e− Eq. (4.3)

𝑒− + 𝑂2 → 𝑂2−∙ Eq. (4.4)

𝑂2−∙ + 𝐻+ ↔ 𝐻𝑂2

∙ Eq. (4.5)

2𝐻𝑂2∙ → 𝐻2𝑂2 +𝑂2 Eq. (4.6)

𝐻2𝑂2 + 𝑒− → 𝐻𝑂 ∙ +𝑂𝐻− Eq. (4.7)

C h a p t e r 4 P a g e | 64

𝐻𝑂 ∙ +𝑂𝐼𝐼/𝑂𝐼𝐼+ → ⋯ → 𝑏𝑦𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 Eq. (4.8)

In addition, the Ni2+ phase of CSN proved to be very active as 97% of OII (20 ppm) was degraded

within 5 min. The extra generation of electrons by the oxidation of Ni2+ to Ni3+ also promoted the

formation of reactive species. For comparison purpose, the CSN degradation kinetics was much faster

under dark condition than other advanced catalysts. For instance, TiO2 reached 66% degradation (OII

= 35 ppm) in 30 min under UV irradiation [35]. TiO2-graphene nanocomoposite degraded 81% of

OII (20 ppm) in 90 min under solar irradiation [36]. AgCl@Ag@TiO2 delivered 98% OII (20 ppm)

degradation in 20 min under visible light [37] and graphene oxide/Fe1−xZnxOy catalysts under UV

irradiation and Fenton reaction achieved 72% OII (35 ppm) in 180 min [38]. Apart from fast

degradation kinetics in this work, it must be noted that the Ni2+ phase of CSN was very active under

dark conditions, a major advantage over other catalysts, which required light irradiation or peroxide

in the heterogeneous Fenton-like reaction.

4.7 Conclusions

This work shows that the Ni2+ phase of ceramic base catalysts containing calcium strontium nickel

(CSN) metal oxides was very efficient in degrading a pollutant textile dye (OII). The surface of the

CSN catalyst proved to be very active by rapidly degrading up to 97% OII within 5 min. The fast

reaction kinetics was attributed to electron generation from the breaking down of azo groups and from

the oxidation of Ni2+ to Ni3+. The generation of electrons in aqueous solution led to several reactions

and very importantly to the formation of powerful hydroxyl radicals, thus breaking down OII

molecules into many intermediate products. Whilst the reduced Ni2+ phase in CSN proved to be very

active in degrading OII, the oxidized Ni3+ phase lost its catalytic activity, thus warranting further

research to stabilize the Ni2+ phase. Nevertheless, it is noteworthy that CSN degraded OII under dark

ambient conditions without the aid of external stimulants. This is contrary to the majority of the

advanced oxidation processes employing Fenton-type catalysis or photocatalysis, which require

chemical additives and energy input. Apart from simplifying the engineering scale up design of

wastewater process plants from the textile industry, active ceramic metal oxides potentially reduce

operating costs by dispensing the need of chemical additives such as peroxide and energy. On a life

cycle basis, these achievements are desirable as the treatment plant environmental foot print is

reduced.

Acknowledgements



C h a p t e r 4 P a g e | 65



Appendix

Figure 4.A 1: Nitrogen sorption isotherm for adsorption (black square) and desorption (red

circle).

Figure 4.A 2: XPS wide survey scan.

C h a p t e r 4 P a g e | 66

Figure 4.A 3: The UV-vis absorbance spectra of OII in dark under magnetic stirring.

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5 Effective degradation of Azo Dyes in the Dark by Cu2+ Active Sites in CaSrNiCu Oxides

5.1 Introduction

As discussed in Chapter 4, the recyclability and TOC removal ability of CaSrNiO oxides can be

further improved. As CaSrCuO (Chapter 3) showed relatively high catalytic activity and

mineralization as well as recyclability. It can be expected that the combination of Ni and Cu in the B

site of CaSrNiCuO (CSNC) would result in beneficial synergy in the catalytic performance. Thus,

this chapter systematically investigates the effect of Ni/Cu coexistence on the catalytic behaviour of

CaSrNiCuO materials. Besides, the relative content of Ni and Cu was varied to optimize the catalytic

activity of Ca0.5Sr0.5NixCu1-xO (x=0.2, 0.5 and 0.8). We also studied the degradation mechanism and

reaction kinetics involved in OII degradation in this chapter.

5.2 Contributions

Chapter 5 has been submited to the Journal of Environmental Chemical Engineering. This chapter is

wholly my own work with the exception of the contribution by Prof. João C. Diniz da Costa, Dr.

Julius Motuzas and Dr. Wayde Martens.

C h a p t e r 5 P a g e | 71

Effective degradation of Azo Dyes in the Dark by Cu2+ Active Sites in CaSrNiCu Oxides





Australia.


5.3 Abstract

Metal oxides containing calcium strontium nickel and copper (CSNC) were investigated for the

degradation of an azo dye Orange II (OII) pollutant under dark conditions and without any external

stimulant. The most active catalyst contained equimolar concentration of Ni and Cu. The reaction

proceeded rapidly reaching 50% OII degradation within the first 5 min and 95% by 2 h with 54% of

the organic carbon mineralized. The pristine CSNC catalyst containing Ni2+/Cu1+ underwent partial

oxidization to Ni3+/Cu2+. Despite this oxidation change, the CSNC catalyst remained stable for over

15 cycles, and Cu2+ proved to be an active reaction site as the Ni3+ phase is not catalytically active.

The degradation mechanism under dark conditions occurs by OII contacting the catalytic surface of

CSNC where -N=N- azo bonds were broken and electrons were donated. Concomitantly, more

electrons were donated by the oxidation of Ni2+/Cu1+. Electrons led to the formation of reactive

species (O2−, HO2

and OH•) and hydrogen peroxide, as confirmed by the partial mineralisation of

OII and radical trapping experiment.

Keywords: metal oxides, dark catalysis conditions, Cu2+ active phase, orange II

5.4 Introduction

Water pollution caused by the discharge of azo dyes from textile industries has been of major concern

due to impacts to human health and the ecosystem [1-4]. In order to avoid these impacts, advanced

oxidation processes (AOPs) are highly attractive to degrade organic compounds in textile waste water

[5] due to the action of highly reactive radical species (e.g. HO•, O2·-, O2, HOO• and HO2

·) [6]. Within

the research framework of AOPs, heterogeneous catalysis has provided significant improvements and

central to this development is materials research. For instance, heterogeneous photocatalysis using

TiO2 has been widely investigated [7-9] accompanied by substantial research efforts such as

composite semiconductors and surface sensitization [10] to improve catalytic performance. Another

approach is using heterogeneous Fenton-like catalysts such as iron oxides [11-13], which produce

powerful oxidative radicals when contacted with peroxides [14, 15]. However, iron oxides are


C h a p t e r 5 P a g e | 72

unstable [16] due to the oxidation of the active phase Fe2+ to Fe3+ and research effort has been focused

on improving long term stability by compositing iron oxide with graphene oxide [17] as an example.

In both cases, there is a need of external stimulants such as light in photocalysis, and chemical

reactants such as peroxides for the Fenton reaction. These external stimulants add complexities to the

operation of reactors and additional operating costs due to the energy and/or chemical input.

A more desirable approach is to operate reactors without external stimulants, which would imply in

simplifying engineering design in addition to savings in capital and operating costs. One of the most

promising approaches is dark catalysis where energy and chemical inputs are absent. Recently, mixed

metal oxides have shown to have efficient catalytic properties for the degradation of organics in

wastewaters [18]. Initial reports showed that BaFeO3 could degrade 50% of methyl orange under dark

conditions in 5 days [19]. While this initial attempt showed an extremely low degradation rate,

subsequent work showed that the replacement of Ba with Sr resulted in a much more active SrFeO3

compound, leading to much faster degradation of acid orange 8 (150 μM) in 60 min [20]. A major

advantage of mixed metal oxides is the flexibility to form an array of compounds. By taking this

approach, Tummino et al. [21] partially substituted Sr with Ce to form Sr0.85Ce0.15FeO3, a compound

that degraded orange II (OII) and rhodamine B under dark conditions.

Therefore, the partial substitution and/or replacement of cations in a mixture of mixed metal oxides

offers a range of opportunities to optimise and enhance the catalytic performance. Many of these

partially substituted compounds have shown high catalytic activity such as Cu for oxygen transport

[22] or as Cu-doped perovskites in the Fenton-like reaction [23, 24]. Very recently, Chen et al. [25]

reported that CaxSr1-xCuO3 perovskite was effective in breaking down the azo bonds of OII dyes

reaching 80% degradation in 10 min (OII = 50 ppm) under dark conditions, though mineralisation

was low at 30%. In a subsequent work, Chen et al. [26] replaced Cu with Ni, and reported even faster

OII (20 ppm) degradation rates of 97% in 5 min also under dark conditions. The compound CaSrNi

oxide did not form a perovskite and remained as a metal oxide. Further, this compound became

unstable upon recycling due to the oxidation of nickel active phase Ni2+ to the non-active phase Ni3+.

As the unstable Ni proved to have very fast degradation rates, it is therefore important to stabilize the

Ni active phase. A potential candidate is Cu, which also proved to have fast degradation rates.

Therefore, this work investigates the partial substitution of Ni with Cu in the B-site of a perovskite

compound, to form Ca0.5Sr0.5NixCu1-xO3- (CSNC). A detailed investigation of the catalytic activity

of CSNC towards OII degradation was carried out in the dark at room temperature without external

oxidants. The CSNC catalyst was fully characterised and its reactivity correlated to materials

properties.

C h a p t e r 5 P a g e | 73

5.5 Experimental section

5.5.1 Materials synthesis and characterization

Calcium nitrate tetrahydrate, citric acid monohydrate and aqueous ammonia solution were purchased

from Chem-Supply Pty Ltd. Strontium nitrate, copper (II) nitrate hemi (pentahydrate) and isopropanol

(IPA) were obtained from Alfa Aesar. Ethylenediamine tetra-acetic acid (EDTA) and potassium

iodide (KI) were purchased from Ajar FineChem Pty Ltd. Nickel (II) nitrate hexahydrate, orange II

(OII), ammonium acetate, potassium bromate (KBrO3) and acetonitrile were supplied by Sigma-

Aldrich. All chemical reagents and solvents were of analytical or HPLC grade and used as received

without further purification. High quality water obtained from a Millipore Milli-Q system was used

for HPLC analysis.

The CSNC as Ca0.5Sr0.5NixCu1-x oxides were prepared via an EDTA/citric acid complexing method.

Briefly, appropriate amounts of nitrate salts and citric acid were dissolved in water to make a mixed

solution followed by the addition of transparent EDTA-ammonia solution under vigorous magnetic

stirring. EDTA and citric acid served as the complexing agents. The molar ratios of total metal ions,

EDTA, citric acid, and ammonium hydroxide were kept at 1:1.1:2:10. Mild heating of the obtained

solution induced the gelation. The gels were pre-calcined at 450 ℃ for 8 h with heating and cooling

rates of 5 ℃ min-1. Subsequently, the calcined powder was sintered up to 1000 ℃ using the same

dwell time and ramping rates. The resultant perovskite powder was ground and stored for further use.

Structural properties of the compound were determined by N2 sorption at 77 K on Micromeritics

Tristar 3020. The samples were degassed at 200 ℃ overnight under vacuum to remove any adsorbed

components. X-ray diffraction (XRD) was carried out to analyse the crystals on a Rigaku Smartlab

X-ray diffractometer using a filtered Cu Ka radiation (45 kV, 200 mA with a step size of 0.02° and

speed of 4° min−1, 10 ≤ 2θ ≤ 100, λ=1.5406 Å). The surface morphology was viewed by means of a

scanning electron microscope (SEM) JEOL JSM-7001F. Surface elemental and chemical state

information were determined via an X-ray photo-electron spectroscopy (XPS: Kratos Axis ULTRA)

using an Al K𝛼 X-rays (hν=1486.6 eV) radiation.

5.5.2 Catalysis

The catalytic activity of CSNC was evaluated by using OII as a model pollutant. In a typically

catalytic reaction, CSNC powder (1 g L-1) was dispersed into a 250 mL flask containing 200 mL OII

solution (20 ppm; pH 6.8) to start the reaction. Alumina foil was used to cover the flask to keep the

dark condition throughout the reaction time (120 min). An aliquot of 5 mL suspension was sampled

at different time intervals and filtered using 0.22 µm Milipore syringe filters. The UV-vis spectra of

the treated OII solutions were recorded to determine the degradation percentage. A blank experiment

C h a p t e r 5 P a g e | 74

was conducted without CSNC under otherwise identical conditions. Cycling experiments were

performed to evaluate the recyclability of CSNC using OII (20 ppm). After each cycle (2h), 5 mL

treated OII suspension was taken out to determine the degradation percentage. Meanwhile, 5 mL of

OII stock solution was added to maintain the constant volume and the total OII concentration of 20

ppm. All catalytic experiments were carried out in the dark at at room temperature (~20 °C) under

continuous stirring. Quenching tests as a diagnostic were carried out with the appropriate reactive

scavengers to investigate the contributory roles of generated reactive species. KI (20 mM) and IPA

(100 mM) were used as hydroxyl radical (OH) scavengers. KBrO3 (10 mM) was used to trap potential

involved electrons. EDTA-2Na (100 mM) is an excellent chelating agent which can chelate the

transition metal ions (e.g., Ni, Cu) to suppress the formation of reactive species by replacing the active

groups [27].

The UV-vis spectroscopy of filtrates was recorded using a UV-Vis spectrophotometer (Evolution 220,

Thermo Fisher Scientific). The total organic carbon (TOC) value was measured to evaluate the

mineralisation degree using a Shimadzu TOC analyser (model TOC-VCSH). A fully computer

controlled HPLC system (UltiMate 3000, UHPLC+ focused, Thermmo Scientific) comprising a

quaternary solvent delivery pump, diode array and fluorescence detectors and an auto sampler was

used to follow OII concentration-time profiles. Potential products were separated on an Eclipse XDB-

C8 5 μm, 150 mm × 4.6 mm column using 70:30 aqueous solution of ammonium acetate (20 mM):

acetonitrile as an isocratic mobile phase at 1 mL min-1 and ambient temperature. The injection volume

was 20 μl and detection was achieved with the diode array detector set at 485 nm.


SEM imaging shows that the as-prepared CSNC (x=0.5) samples consisted of large dense particles

(~40 µm) with relatively small micro-sized particles randomly distributed on the surface (Fig. 5.1a).

The CSNC samples were characterised by very low surface areas of less than 2 m2 g-1 (Fig. 5.1b).

This is in line with the SEM image in Fig. 5.1a, giving a clear indication of the non-porous nature of

the samples with a widely distributed particle sizes ranging from 0.5 to 40 µm. The XRD diffraction

patterns (Fig. 5.1c) display sharp and intense peaks, indicating the highly crystalline nature of CSNC

(x=0.5). Peaks at 2 14.2, 31.7, 34, 35.2, 43.5, 45, 46, 47.3, 54, 57.2, 59.6, 64, and 66.3 are attributed

to perovskite CaSrCuO3 (JCPDS 00-048-0138), whilst a second phase with peaks at 37, 43.1, 62.5,

74.9 and 79 can be indexed to NiO (JCPDS 04-005-9695). No other impurity peaks were observed

in the XRD patterns. The Scherrer equation was adopted to calculate the average grain size using the

three strongest peaks in each XRD pattern, which was 48.9 and 47.6 nm for CaSrCuO3 and NiO,

respectively. Although the mixture has a Goldshmidt number of 0.86, and is within the tolerance

C h a p t e r 5 P a g e | 75

value of 0.8 to 1, these results suggest that Ni was not incorporated into the B-site of the perovskite

structure. Hence, the resultant material is a mixture of metal oxide and perovskite.

Figure 5. 1: (a) SEM micrograph and (c) XRD patterns of CSNC (x=0.5). (b) N2 sorption of

CSNC (x=0.2, 0.5 and 0.8).

The degradation of OII is displayed in the UV-Vis spectra in Fig. 5.2a. The OII prior to exposure to

CSNC catalyst at t = 0 min is characterized by several bands with an intense peak at 485 nm

(hydrazine), a shoulder at 430 nm (azo), and several aromatic rings at 310, 260 and 230 nm [28]. As

the catalytic reaction progresses, there is a significant reduction of the intense peak at 485 nm and the

shoulder at 430 nm, thus suggesting the degradation of the hydrazine and azo components of the OII

molecule. On a similar basis, the intensity of the aromatic peaks also reduced. As the reaction

progressed further to 120 min, the majority of the peaks decreased in intensity and became almost

undetectable, giving a clear indication that the reaction was reaching completion. This was

accompanied by the visual inspection of the samples which changed from a brown colour at t = 0 min

to a clear colour at t = 120 min. For comparative purpose, blank experiments (Fig. 5.2b) without

catalysts resulted in no degradation of OII, thus confirming the catalytic activity of CSNC in Fig.

5.2a.

Figure 5. 2: (a) UV-vis spectra showing OII degradation as a function of time by CSNC (x=0.5),

(b) UV-vis spectrum variation of OII solution in the blank experiment. Experimental conditions:

[OII]0=20 ppm, [Catalyst] =1 g L-1, T=20 ℃, in the dark and under constant stirring.

20 µm20 µm20 µm20 µm20 µm

10 μm

(a) (c)

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

5

Qu

an

tity

Ad

so

rbed

(cm

³/g

ST

P)

Relative Pressure (p/p°)

Ni=0.2

Ni=0.5

Ni=0.8

(b)

20 40 60 80 100

Inte

nsit

y (

a.u

.)

()

200 300 400 500 600 7000.0

0.5

1.0

1.5

2.0

Ab

so

rban

ce (

a.u

.)

Wavelength (nm)

OII

Treated AO7 after 120 min

(a) (b)

C h a p t e r 5 P a g e | 76

TGA analysis of OII, blank CSNC (exposed to water for 2h), and spent CSNC (after catalysis for 2h)

samples was conducted to ensure that the decrease of OII in Fig. 5.2 was due to degradation reaction

instead of adsorption. Fig. 5.3 shows that blank and spent CSNC shared similar mass loss trend and

the final mass loss difference up to 790 °C was negligible for blank CSNC (17.2%) and spent CSNC

(16.1%). Further, the changes of mass loss of the blank and spent samples do not follow the OII mass

loss trend. Therefore, these results confirm that adsorption was of OII on spent CSNC was negligible.

The minor final difference in mass loss for the blank and spent CSNC samples may be attributed to

minor physicochemical differences (e.g., hydroxylation) between the sample exposed to catalysis and

the sample exposed to water only.

Figure 5. 3: TGA plots of OII, blank CSNC and spent CSNC samples with airflow of 80 mL

min-1 and heating rate of 5 °C min-1.

The degradation of OII was initially investigated by varying the ratio of Ni (x) and Cu (1-x) as

displayed in Fig. 5.4a. It is observed that all samples delivered very fast degradation during the first

5 min of reaction. At this stage, the reaction activity became low for the CSNC catalyst with the

highest (x=0.8) or lowest (x=0.2) content. However, the catalyst with the equimolar (x=0.5) content

of Cu and Ni continued to degrade OII quite rapidly reaching 70% degradation in 20 min. From there

on, the reaction slowed down up to 120 min, reaching 95% degradation of OII. The reduction of

degradation is attributed to the depletion of OII dye, resulting in mass transfer limitations for the OII

molecules to contact the CSNC particles. These results suggest that there is a balance between the

synergistic effect of Cu and Ni, where excess of any of these cations in the catalyst is detrimental,

while the equimolar ratio delivers the best performance. Similar catalytic effect was also reported for

the Fenton-like reaction using equimolar ratio of iron and copper [29].

C h a p t e r 5 P a g e | 77

Figure 5. 4: (a) Effect of Cu and Ni cation concentration in CSNC for the degradation of OII

solution (C0=20 ppm); and (b) degradation of OII solution at varying initial concentration (C0

from 20 to 35 ppm) by CSNC (x = 0.5). Experimental conditions: [Catalyst] =1 g L-1, T=20 ℃,

in dark and under constant stirring.

Based on these results, the equimolar CSNC (x=0.5) was chosen as the optimal catalyst for further

studies. Fig. 5.4b shows that the CSNC (x=0.5) catalyst degradation rate was similar as a function of

the concentration of OII dye, again reached OII degradation values around 95%. In practice, it is

important to evaluate the stability of catalysts in heterogeneous catalysis. With that goal in mind, 15

consecutive experiments were carried out using the same catalyst. Results in Fig. 5.5 show that the

CSNC maintained a catalytic activity of an average of ~ 90% (±5) over the 15 cycles of reaction,

indicating CSNC has good catalytic stability for OII degradation.

Figure 5. 5: Cycling test for CSNC (x = 0.5). Experimental conditions: [OII]0=20 ppm, [Catalyst]

=1 g L-1, T=20 ℃, in the dark and under constant stirring. Initial OII concentration of 20 ppm

and for each subsequent cycle 20 ppm was added to the solution.

0 20 40 60 80 100 120

0.0

0.2

0.4

0.6

0.8

1.0

C/C

0

Time (min)

Cu: Ni

0.8: 0.2

0.5: 0.5

0.2: 0.8

0 20 40 60 80 100 120

0.0

0.2

0.4

0.6

0.8

1.0

C/C

0

Time (min)

20 ppm

25 ppm

30 ppm

35 ppm

(a) (b)

C h a p t e r 5 P a g e | 78

Figs. 5.6a shows the HPLC chromatograms of OII solutions degraded at 0, 30 and 120 min. At 0 min,

there was a major peak centred at 2.4 min and a minor peak at 1.8 min. Both peaks belonged to OII

molecules. The peak intensity at 2.4 min significantly reduced with time due to the destruction of -

N=N- bonds in OII molecules, which is in consistence with UV-vis results (Fig. 5.2). The peak at 1.8

min also decreased with time accompanied by a shift to lower retention time. A new peak belonging

to the formation of byproducts at the retention time of 1.3 min appeared and intensified with time as

seen in Fig. 5.6b. Fig. 5.6c shows the UV-vis spectra of eluted species at the retention time of 1.3 min

before reaction (t=0). The UV-vis spectra are almost a flat line, indicating no compounds were eluted

at this specific retention time. However, distinct new UV-vis peaks appeared at wavelengths below

500 nm for treated OII samples after 30 min and 120 min, thus suggesting that byproducts were

generated during the reaction.

Figure 5. 6: (a) HPLC chromatograms of OII solution degraded by CSNC (x=0.5), (b)

magnification of Fig. 4a in the retention time from 1 to 2 min, and (c) UV-vis spectra of

degradation products at 0, 30, and 120 min at the retention time of ca. 1.3 min.

To shed further light on the properties of the CSNC catalyst, pristine and spent samples were analysed

by XPS. The survey XPS spectra (Fig. 5.7) shows the presence of Ca, Sr, Ni, Cu, O and adventitious

C. The high resolution XPS spectra of Ca 2p, Sr 3d, Ni 2p and Cu 2p in pristine and spent CSNC are

deconvoluted as also displayed in Fig. 5.7. The double peaks at 347.6 and 351.0 eV are attributed to

Ca 2p3/2 and Ca 2p1/2, respectively [30]. The XPS spectrum for Sr 3d has an asymmetrical profile,

decomposed into two separate peaks related to Sr 3d3/2 (135 eV) and Sr 3d5/2 (133.3 eV) [31]. The

XPS spectrum of Ni 2p shows the characteristic of Ni2+ (854.4 eV) and Ni3+ (856.2 eV) and one

satellite peak (denoted as Sat.) at 861.3 eV [32]. Similarly, the characteristic XPS spectrum of Cu 2p

has peaks assigned to Cu1+ (932.9 eV) [33] and Cu2+ (934.6 eV) [34] as well as a broad satellite peak.

It is observed that the deconvoluted peak areas for Ni2+ and Cu1+ reduced while the Ni3+ and Cu2+

increased for the spent samples in comparison to pristine samples. These results strongly suggest that

200 400 600 800

Ab

so

rba

nc

e (

a.u

.)

Wavelength (nm)

0 min

30 min

120 min

0 1 2 3

1.8

No

rma

lise

d in

ten

sit

y

Retention time (min)

0 min

30 min

120 min

1.3

2.4

1.0 1.5 2.0

0 min

30 min

120 min


(a) (b) (c)

C h a p t e r 5 P a g e | 79

the CSNC catalyst underwent an oxidation process during the degradation of OII dyes under dark

conditions.

Figure 5. 7: Wide XPS spectra of pristine and spent CSNC and high-resolution XPS spectra of

Ca 2p, Sr 3d, Ni 2p and Cu 2p for pristine and spent CSNC.

It is interesting that the CSNC (x=0.5) catalyst was stable for 15 cycles (Fig. 5), though the catalyst

is oxidised during the OII degradation. Recently, Chen et al. [26] reported Ni3+ was not effective for

OII degradation under dark condition. Therefore, the role played by the oxidised Cu2+ must be

ascertained. To address this critical point, the CSNC catalyst was analysed using XPS for each one

of the 15 stability cycles. Fig. 5.8 shows the ratios of Ni2+/(Ni2++Ni3+) and Cu1+/(Cu1++Cu2+) based

on the areas of deconvoluted peaks for each cycle. The ratios decreased significantly in the first cycle,

thus suggesting a significant charge effect change due to the oxidation process and forming Ni3+ and

Cu2+. The oxidation process progressively slowed up to cycle number 6. From cycle 6 to 15, the

oxidation process became negligible and the catalyst no longer underwent oxidation and achieved

steady state. Recently, Ni2+ was very effective in degrading OII, though Ni3+ proved to be non-

354 352 350 348 346 344

Inte

ns

ity

(a

.u.)

Ca 2p

2p3/2

351.4 eV

347.9 eV

Pristine

Spent

2p1/2

Binding Energy (eV)

138 136 134 132 130

Inte

ns

ity

(a

.u.)

Sr 3d

3d5/2

3d3/2

135 eV

133.3 eV

Pristine

Spent

Binding Energy (eV)

864 860 856 852

Ni 2p

Inte

nsit

y (

a.u

.)

Ni2+

Ni3+

2p3/2

Sat.

2p3/2

Binding Energy (eV)

Pristine

Spent

948 945 942 939 936 933 930 927

Cu 2p

Inte

nsit

y (

a.u

.)

Cu2+ Cu

1+

2p3/2

Sat.

Binding Energy (eV)

2p3/2

Pristine

Spent

C h a p t e r 5 P a g e | 80

reactive [26] under similar experimental conditions as in this work. Therefore, these results clearly

indicate that Cu2+ is counter acting the non-reactive Ni3+, whilst maintaining the OII degradation at

the same level as the pristine catalyst. In other words, Cu2+ within the perovskite structure is very

effective in degrading OII dyes. This is in line with findings for Cu2+/Cu+ surface species in Fe3-

xCuxO4 also for dye degradation [35], though the latter used a Fenton electrocatalysis process which

differs from this work where there is no external stimulants and the reaction is carried under dark

conditions.

0 2 4 6 8 10 12 14 160.00

0.05

0.10

0.15

0.20

0.25

0.30

Cycle Number

Cu

1+/(

Cu

1++

Cu

2+)

0.00

0.05

0.10

0.15

0.20

0.25

Ni2

+/(

Ni2

++

Ni3

+)

Figure 5. 8: The relative amounts of Cu1+ and Ni2+ in raw and cycled CSNC (x=0.5).

Based on the XPS result discussion above, the oxidation of Ni2+ to Ni3+and Cu1+ to Cu2+ are

represented by Eqs. 5.1 and 5.2. As these cations are part of the CSNC (x=0.5) catalyst, it can be

ascertained that the electrons were released as represented in Eq. 5.3. Further, the discoloration of

OII observed visually and the reduction of the azo bands (Fig. 5.2) suggested the destruction of -

N=N- azo bonds, which are the most vulnerable part in OII for oxidative attack [36, 37], thus

generating OII+ and releasing electrons as per Eq. 5.4. In principle, the release of electrons can form

reactive species. For instance, electrons can directly reduce O2 to yield the superoxide radical (O-

2 ,

Eq. 5.5), followed by the generation of a series of reactive species (Eqs. 5.6-5.8). Finally, the formed

reactive species leads to the degradation and mineralization of OII molecules by contacting the

catalyst surface (Eq. 5.9).

To ensure that radical species were formed, a number of radical quenchers were tested for the

degradation reaction of OII also under dark conditions. Fig. 5.9 shows that EDTA-2Na, a chelating

agent, dramatically suppressed OII degradation by 97 % (±2). This means that EDTA-2Na chelated

onto the surface of the catalyst, and as a result OII could not contact the CSNC (x=0.5) surface.

Consequently, OII degradation reaction did not occur. In other words, this result confirms that surface

C h a p t e r 5 P a g e | 81

reaction is the primary mechanism for the degradation of OII. An electron scavenger (KBrO3) was

also tested, showing a reduction in the degradation of OII. This result therefore confirms that electrons

were generated during the OII degradation reaction as per Eqs. 1-4. Likewise, the bubbling of N2

supressed OII degradation, indicating that O2 also played a role in OII degradation. Finally, the use

of OH• scavengers (KI and IPA) also resulted in the reduction of OII degradation, thus confirming

the generation of OH• as per Eq. 8. Further confirmation of the generation of OH• can be seen in

TOC results in Fig. 5.10. For instance, the normalized remaining TOC in treated OII solution

gradually decreased with time and a final ~54% mineralisation of OII was achieved after 120 min.

This is attributed to the presence of OH• radicals, which is in line with recent reports that the formed

OH resulted in OII degradation under dark conditions using SrCeFe oxides [21].

0 20 40 60 80 100 120

0.0

0.2

0.4

0.6

0.8

1.0

Time (min)

No quencher

EDTA-2Na

KI

IPA

KBrO3

N2

C/C

0

Figure 5. 9: Effect of radical quenchers on OII degradation by CSNC (x=0.5). Experimental

conditions: [OII]0=20 ppm, [CSNC]=1 g L-1, [EDTA-2Na]=[IPA]=0.10 M, [KI]=20 mM,

[KBrO3]= 10 mM, T=20 ℃, in the dark and under constant stirring.

Figure 5. 10: TOC results vs. time for CSNC (x = 0.5). Experimental conditions: [OII]0=20 ppm,

[Catalyst] =1 g L-1, T=20 ℃, in the dark and under constant stirring.

C h a p t e r 5 P a g e | 82

𝑁𝑖2+ → Ni3+ + e− Eq. (5.1)

Cu1+ → Cu2+ + e− Eq. (5.2)

CSNC(Ni2+, Cu1+) → CSNC (Ni3+, Cu2+) + 2e− Eq. (5.3)

OIICSNC→ 𝑂𝐼𝐼+ + e− Eq. (5.4)

e− + O2 → O2− Eq. (5.5)

O2− + H+ → HO2

Eq. (5.6)

2HO2 → O2 + H2O2 Eq. (5.7)

H2O2 + e− →∙ OH + OH− Eq. (5.8)

HO ∙ +OII/OII+ → ⋯ → byproducts+ CO2 Eq. (5.9)

Cu2+ + e− → Cu1+ Eq. (5.10)

Fig. 5.11 displays a schematic representing the reaction mechanism pathways. Upon contacting the

CSNC (x=0.5) surface, the weak azo -N=N- bonds are broken where electrons are generated (Eq. 5.4).

Both nickel and copper donate electrons and are oxidised (Eqs. 5.1 and 5.2), whilst the generated

electrons produce radical species (Eqs. 5.5-5.6) and peroxide (Eq. 5.7). This further leads to the

production of HO• radical species (Eq. 5.8), and consequently the formation of byproducts and

mineralisation as per Eq. 5.9. The continuous degradation of OII over 15 cycles (Fig. 5.5) for the

CSNC catalyst did not occur for the same catalyst without copper (i.e., calcium strontium nickel –

CSN) where the oxidation of Ni2+ resulted in the unreactive Ni3+

[26]. As Cu2+ in the CSNC (x=0.5)

proved to be very active, then a further mechanism may be occurring to explain the cycling stability

(Fig. 5.5). Copper has a propensity to accept and donate electrons. As the oxidation reaction (Eq. 5.2)

occurs, then reverse reaction is bound to take place where Cu2+ accepts an electron from OII, thus

reducing to Cu1+ as per Eq. 5.10. Hence, the redox reaction keeps occurring between Cu2+ (Eq. 5.10)

and Cu1+ (Eq. 5.2), concurrently with the generation of electrons from the breakdown of azo bonds

(Eq. 3). Again this reaction pathway leads to the continued formation of OH radical and the partial

(i.e., ~54%) mineralization of OII.

Figure 5. 11: Schematic illustration of OII degradation by CSNC catalyst.

C h a p t e r 5 P a g e | 83

The data for OII degradation in Fig. 5.4b fitted well (R2>0.991) to a pseudo second-order kinetics

(Fig. 5.12). The apparent rate constants decreased with the increase in OII initial concentrations from

0.0271 to 0.0106 ppm-1 min-1

(Table 5. 1), similar to trends reported elsewhere [38]. This is attributed

to the decrease in the number of active sites on the catalyst surface due to surface covering with OII

molecules. As surface coverage is directly proportional with the initial concentration of OII [39], then

the decreased active sites per OII molecules on the catalyst surface led to the reduced rate constants.

Figure 5. 12: Kinetics of OII degradation: 1/C versus time for different concentrations.

Experimental conditions: [Catalyst] =1 g L-1, T=20 ± 2℃, in the dark and under constant

stirring.

Table 5. 1: Apparent reaction rates of OII degradation with different initial concentrations by

Ca0.5Sr0.5Ni0.5Cu0.5O3-.

Concentrations (ppm) 20 25 30 35

Apparent rate constants (ppm-1min-1 g-1) 0.0271 0.0149 0.0120 0.0106

5.7 Conclusions

The CSNC oxides produced in this study exhibited high discoloration and mineralization rates for

OII under dark ambient conditions without light illumination nor any other additional stimulants. The

surface of CSNC proved to be very active showing fast 50% degradation of OII within 5 min. This

was attributed to the breakdown of -N=N- bonds with the generation of electrons. The CSNC catalyst

was stable over 15 cycles, reaching ~95% OII degraded and ~54% mineralized within 2 hours of

reaction. The pristine sample contained a high amount of Ni2+/Cu1+, which oxidised to Ni3+/Cu2+

during the reaction through oxidation. Electrons were responsible for the formation of reactive species

(O2−, HO2

and OH•) and hydrogen peroxide, as confirmed by the partial mineralisation of OII and

radical trapping experiment. The Cu2+ proved to be very active for the degradation of OII as Ni3+ is

non-reactive. The improved mineralisation of OII by CSNC as compared to the same catalyst without

C h a p t e r 5 P a g e | 84

Cu confirmed the synergetic effect played by Ni and Cu in the CSNC compound. Therefore, CSNC

could be an efficient alternative material as a novel advanced oxidation technology for low cost

treatment of wastewaters containing textile dyes.

Acknowledgement





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Costa, Copper oxide-perovskite mixed matrix membranes delivering very high oxygen fluxes, J.

Membr. Sci. 526 (2017) 323-333.

[23].L. Zhang, Y. Nie, C. Hu, J. Qu, Enhanced Fenton degradation of rhodamine B over nanoscaled

Cudoped LaTiO3 perovskite, Appl. Catal. B: Environ. 125 (2012) 418-424.

[24].T. Soltani, B.-K. Lee, Enhanced formation of sulfate radicals by metal-doped BiFeO3 under

visible light for improving photo-Fenton catalytic degradation of 2-chlorophenol, Chem. Eng. J. 313

(2017) 1258-1268.

C h a p t e r 5 P a g e | 86

[25].H. Chen, J. Motuzas, W. Martens, J.C. Diniz da Costa, Degradation of azo dye orange II under

dark ambient conditions by calcium strontium copper perovskite, Appl. Catal. B: Environ. 221 (2018)

691-700.

[26].H. Chen, J. Motuzas, W. Martens, J.C. Diniz da Costa, Ni2+ active phase of metal oxides for the

fast degradation of orange II dye under Dark Ambiance, Ceram. Int. (2018)

https://doi.org/10.1016/j.ceramint.2018.01.071

[27].A. Karadag, B. Ozcelik, S. Saner, Review of methods to determine antioxidant capacities, Food

Analyt. Meth. 2 (2009) 41-60.

[28].H. Park, W. Choi, Visible light and Fe (III)-mediated degradation of acid orange 7 in the absence

of H2O2, J. Photochem. Photobiol. A: Chem. 159 (2003) 241-247.

[29].A.F. Rossi, R.C. Martins, R.M. Quinta-Ferreira, Composition effect of iron–copper composite

catalysts in the Fenton heterogeneous process efficiency and cooxidation synergy assessment, Ind.

Eng. Chem. Res. 53 (2014) 15369-15373.

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J. Rare Earth 26 (2008) 511-514.

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512-521.

C h a p t e r 6 P a g e | 87

6 Degradation of Orange II dye under dark ambient conditions by MeSrCuO (Me = Mg and

Ce) metal oxides

6.1 Introduction

The introduction of foreign metal cations can allow for the generation of metal oxide composite that

might synergistically improve the catalytic performance compared to the parent component. As

mentioned in Chapter 3, SrCuO is efficient in degrading OII in the dark ambient conditions. Thus,

the equimolar substitution of Sr in SrCuO by Mg and Ce is employed in this chapter to probe the

possible synergistic effect on the catalytic ability of resultant compounds.

6.2 Contributions

Chapter 6 has been submitted to Separation and Purification Technology. This chapter is wholly my

own work with the exception of the contribution by Prof. João C. Diniz da Costa, Dr. Julius Motuzas

and Dr. Wayde Martens.

C h a p t e r 6 P a g e | 88

Degradation of Orange II dye under dark ambient conditions by MeSrCuO (Me = Mg and Ce) metal

oxides





Australia.


6.3 Abstract

This work investigates the catalytic performance and materials property of ternary metal oxides as

Me0.25Sr0.25Cu0.5O (Me= Mg and Ce). The catalysts were tested for the heterogeneous degradation of

orange II (OII) azo dye in aqueous solution in the dark at room temperature without any other reagents

or energy/light irradiation. The MrSrCuO degraded more than 97% OII in the first 30 min, higher

than that of binary metal oxides SrCuO (90.5%) or single metal oxides CeO2 (23.6%) and MgO

(23.3%). The degradation pathway predominantly occurred by OII contacting the catalyst surface and

resulting in the breakdown of azo bonds and the generation of electrons. Partial mineralization of OII

dye occurred thus confirming the generation of radical species. Hence, electrons reacted with

dissolved O2 in the OII aqueous solution, leading to the consecutive formation of reactive radical

species (e. g. , O2−, HO2

, H2O2,• OH). Radical trapping experiment confirmed the presence of radical

species. XPS analysis revealed part of Cu2+ underwent irreversible reduction after reaction due to the

electron donation from OII with the accompanied increase in Cu1+ species in the spent samples.

Cycling tests confirmed the reduction of the catalytic activity, clearly indicating that Cu2+ was the

active phase in MeSrCuO for the degradation of OII dye.

Keywords: OII degradation; dark conditions; ternary metal oxides; copper oxides.

6.4 Introduction

Water pollution is a global environmental challenge with 17-20% of industrial water pollution related

to effluents such as textile dyeing [1]. Of particular concern is the impact associated with the

discharge of azo dyes on the ecological balance [2]. To address this problem, heterogeneous catalysis

in advanced oxidation process (AOPs) has been widely studied including photocatalysis [3-6] and the

Fenton-like reaction [7-10]. Recently, heterogeneous dark catalysis has been attracting the attention

of the research community. Dark catalysis is a process carried out in complete darkness and dispenses

the need for light and energy input needed in photocatalysis or chemical additives like peroxide in


C h a p t e r 6 P a g e | 89

the Fenton reaction. The rational of using dark catalysis is to simplify the engineering design of

wastewater plants in the textile industry whilst reducing operation costs through reducing the energy

and chemicals required to degrade the dyes.

The number of reports on heterogeneous dark catalysis in the literature is limited. Nevertheless, a

catalytic material of interest is metal oxides. In these materials, it is very important to understand the

role played by each metal oxide in the catalytic process. For instance, initial reports by Sun et al. [11]

showed that a binary mixed metal oxide BaFeO3 degraded methyl orange, though the reaction was

very slow as it took 5 days to reduce the dye concentration by 50% only. Subsequently, Leiw et al.

[12] reported improved degradation kinetics using SrFeO3 which degraded acid orange 8 in 60 min.

Hence, by replacing Ba with Sr in MeFeO3, where Me represents a metal cation, the catalytic

performance of the catalyst greatly improved. Recently, Tummino and co-workers [13] reported that

by adding a third metal oxide CeO to SrFeO3, thus forming SrCeFeO3 which degraded orange II (OII)

and rhodamine B dyes without light irradiation. These examples show that metal oxides can be

effective as binary or ternary metal oxide mixtures as catalysts to degrade wastewaters containing

textile dyes.

The above examples are based on metal oxides containing FeO3, which is a catalyst extensively used

in the heterogeneous Fenton-like reactions [14-17]. Therefore, another strategy to enhance the

performance of catalysts is to change the FeO3 base metal oxide by another. For instance, Chen et al.

changed to NiOx as the base metal oxide and reported fast degradation of OII dyes using CaSrNiO3

[18]. There is a large number of base metal oxides that can be used for dark catalysis, but one that

has interesting properties is CuO. Apart from having outstanding properties for oxygen systems as

BaSrCoCuO3 [19], LaTiCuO3 [20] and LaCuO3 [21] degraded rhodamine B and phenol using

peroxide in a Fenton-like reaction, accordingly. In addition, CuO can also degrade OII under visible

light as composite metal oxides with ZnO [22] and layer double hydroxides [23]. Very recently, Chen

and co-workers demonstrated that CuO based metal oxides were very efficient in the degradation of

OII dye under dark conditions as CaSrCuO3 [24]. An interesting aspect of using metal oxides for the

catalytic degradation of dyes in wastewaters is the formation of radical species, including hydroxyl

radicals [13], which are responsible for breaking down organic molecules.

As metal oxides are emerging catalysts for dye degradation applications, it is of interest to investigate

the performance of other metal oxides in CuO based catalysts mixed with other metal oxides. Such

example catalysts would be binary mixtures SrCuO3 in view of the improved performance of SrFeO3.

It may also be advantageous to use ternary mixtures such as MeSrCuO3 where Me is a third metal

cation. For instance, Mg metal oxides such as MgFeO degraded methylene blue [25], whilst

C h a p t e r 6 P a g e | 90

MgCoFeO3 degraded rhodamine B [26], both in photocalysis. Another metal oxide of interest is CeO2

which showed higher catalytic activity as CeMoO3 for the degradation of methylene blue under dark

conditions [27]. CeO2 based metal oxide catalysts also improved the mineralization rate in the

ozonation of C. I. Reactive Blue 5 dye solution compared to single ozonation (no catalyst) [28, 29].

The research into ternary metal oxides is warranted due to possible synergistic effects for the

degradation of dyes under dark conditions.

Therefore, this work investigates the performance of Me0.25Sr0.25Cu0.5O as ternary metal oxide

mixtures where the role of Me (Mg and Ce) is studied in terms of composition-functionality and

catalytic degradation of OII. The catalytic activity of the synthesized materials for OII degradation

was evaluated in the dark, at room temperature without any other reagents or energy/light. The

intrinsic physiochemical properties and the catalytic activity were correlated to a number of materials

properties derived from spectroscopy analysis such as XRD, SEM and XPS.

6.5 Experimental

6.5.1 Materials and characterization

All chemicals were of either HPLC or analytical grade and used as received. Cu(NO3)2·2.5H2O,

Ba(NO3)2 and Sr(NO3)2 were purchased from Alfa Aesar. Mg(NO3)2·6H2O, Ce(NO3)3·6H2O, citric

acid monohydrate, ethylenediaminetetraacetic acid disodium salt dehydrate (EDTA-2Na) and

aqueous ammonia solution (30% v/v) were supplied by Chem-Supply Pty Ltd..

Ethylenediaminetetraacetic acid (EDTA) and potassium iodide (KI) were procured from Ajar

FineChem Pty and silver nitrate and Orange II (OII, max=485 nm) from Sigma-Aldrich. Distilled

water was used as the solvent to prepare all solution samples.

Me0.25Sr0.25Cu0.5O (as MeSrCuO) were prepared by an EDTA-citric acid sol-gel method [16]. Briefly,

stoichiometric nitrate salts, ammonium hydroxide, and citric acid were dissolved into distilled water

under magnetic stirring prior to heating treatment for the formation of viscous sol-gel, which was

further sintered to obtain the desired solid sample. A blank sample containing no Me (Mg and Ce)

was prepared as Sr0.5Cu0.5O (as SrCuO) using the same procedure as described above for catalytic

performance comparison purpose. A field emission scanning electron microscope (FE-SEM, JEOL

JSM-7001F) was used to examine the morphology and microstructure features. X-ray diffraction

(XRD) patterns were obtained using X-ray powder diffraction (D8 Advance, Bruker, USA) with Cu

K radiation (=1.5406 Å) to determine the crystal structure. BET specific surface area was

calculated based on nitrogen adsorption-desorption data on a Micromeritics TriStar 3000 apparatus.

X-ray photoelectron spectroscopy (XPS) measurements were performed in an ultrahigh vacuum on

X-ray photoelectron spectrometer (Kratos Axis ULTRA) with a monochromatic Al K source

C h a p t e r 6 P a g e | 91

(hv=1486.6 eV) to analyze the chemical composition and chemical valence of pristine and spent

catalysts. All XPS data were analyzed using CasaXPS software and corrected using the maximum of

the adventitious C 1s signal at 284.8 eV.

6.5.2 Catalysis

The catalytic activity of as-synthesized materials was evaluated using OII azo dye as the model

organic pollutant to simulate industrial dye-containing wastewater. The tests were performed in the

dark at 20 °C without any other reagent and/or energy or light irradiation. The dark environment was

maintained using the Erlenmeyer flask (250 mL) covered with aluminum foil. The reaction was

initiated by adding a certain amount of as-prepared materials into OII solutions under continuous

magnetic stirring. Catalyst dosage was 1 g L-1 and initial dye concentration was 20 mg L-1. An aliquot

of the reaction suspension was periodically taken and filtered using a 0.22 m Milipore syringe filter

for optical spectroscopy analysis. A double beam UV-vis spectrophotometer (Evolution 220, Thermo

Fisher Scientific) was used to determine the concentration of solution samples with time. The

analytical wavelength selected for optical absorbance measurement was 485 nm. Solid samples were

collected on a filter for further characterization. Blank test was conducted in the presence of SrCuO

catalysts under otherwise identical conditions. For cycling experiments, concentrated stock OII was

repeatedly added to maintain the initial OII concentration (20 ppm) before each run. The total organic

carbon (TOC) of treated solutions was determined at the end of experiment using a Shimadzu TOC-

Vcsh Analyzer. HPLC technique was used to follow OII concentration-time profiles.


6.6.1 Catalytic activity

Fig. 6.1 shows the catalytic activity of MeSrCuO and the blank sample SrCuO. Both Mg and Ce

containing catalysts exhibited superior catalytic activity (97%) for OII degradation than that of the

blank sample SrCuO (90.5%) (Fig. 6.1a). Hence, by adding Mg or Ce to the SrCuO, the catalytic

activity improved. This is supported by the single metal oxide results (Fig. 6.1b), showing that MgO

and CeO2 reached 22% and 35% OII reduction in 120 min reaction. It is noteworthy that the majority

of the OII degradation occurred within the first 5 min, a clear indication of very fast kinetics as

compared to conventional Fenton-like iron oxide catalyst which take up to 30 min to reach 62% [30]

and 90% [31] degradation. The OII degradation is confirmed by UV-vis results in Fig. 6.1c, where

the absorbance intensity of almost all characteristic peaks of OII decreased after 30 min. For instance,

there are five characteristic absorption bands in the UV-Vis spectrum of OII solution with a high

intensity band hydrazone at 485 nm and a shoulder azo at 430 nm, in addition to benzene ring at 230

nm, naphthalene ring at 310 nm [32, 33] and aromatic amines at 254 nm [34]. There was a significant

reduction in the absorbance intensity at 485 nm, indicating the destruction of the azo group. However,

C h a p t e r 6 P a g e | 92

a peak at ~ 260 nm remained after reaction, suggesting the presence of organic compounds such as

by-products of the degradation of OII.

Figure 6. 1: Normalized OII concentrations (a) and corresponding UV-vis spectra of initial and

treated OII solutions after 30 min (c) by MeSrCuO (Me= Mg and Ce) and blank sample SrCuO.

(b) Normalized OII concentrations by MgO and CeO2 after 2h. Experimental conditions:

[OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, in the dark and under stirring.

TOC test was conducted to evaluate the mineralization ability of both catalysts toward OII (Fig. 6.2a).

Despite the fast degradation kinetics of OII (Fig. 6.1a), TOC removal during OII reaction was minor

(<12%). This is in line with the UV-vis results in Fig. 6.1c which show that organic compounds

remained after the reaction. These results strongly suggest that azo bonds (-N=N-) were broken during

the reaction, as evidenced by the visual colour change from darkish yellow for the initial OII solution

to clear for the treated solution (Fig. 6.2b) as reported elsewhere [35]. In other words, there is a clear

indication that both catalyst are very effective in breaking the azo bonds and generating by-products.

Figure 6. 2: (a) TOC removal efficiency of OII by Mg0.5Sr0.5CuO and Ce0.5Sr0.5CuO after 120 min

and (b) colour intensity variation of OII solution after treatment for 30 min by MgSrCuO and

CeSrCuO. Experimental conditions: [OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, in the dark and

under magnetic stirring.

0 20 40 60 80 100 120

0.0

0.2

0.4

0.6

0.8

1.0

MgSrCuO

CeSrCuO

SrCuO

C/C

0

Time (min)200 300 400 500 600 700

0.0

0.5

1.0

1.5

2.0

Ab

sorb

an

ce (

a.u

.)

Wavelength (nm)

MgSrCuO

CeSrCuO

SrCuO

OII

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

C/C

0Time (min)

MgO

CeO2(a) (c)(b)

(b)

MgSrCuO CeSrCuO0.0

0.2

0.4

0.6

0.8

1.0

TO

C/T

OC

0

Materials

OII

MgS

rCu

O

CeS

rCu

O

(a) (b)

C h a p t e r 6 P a g e | 93

The decay of OII concentration by both catalysts was followed by HPLC as a function of time (Fig.

6.3). All eluted species had retention times under 2 min. The peak at the retention time of 1.8 min

corresponding to the parent compound OII diminished sharply on the HPLC curves of treated OII

solution, in line with the trends observed in Fig. 6.1b. With the rapid depletion of OII, byproducts

were detected at the retention time of 1.1 and 1.2 min. The UV-vis spectra of eluted species in treated

OII solutions after 15 min (MgSrCuO) and 30 min (CeSrCuO) are shown in Fig. 6.4a and Fig. 6.4b,

respectively. It can be seen that new distinct UVvis spectra are present in treated OII solutions,

indicating the formation of byproducts after the reaction. This agrees with UV-vis results (Fig. 6.1)

and TOC result (Fig. 6.2a). Overall, combining the results of UV-vis spectra, TOC and HPLC analysis,

OII was readily decomposed by doped SrCuO in the dark at 20 °C with the synchronous generation

of byproducts due to the cleavage of azo bonds, leaving organic carbon unmineralized by the end of

reaction. The above results indicate that the destruction of the conjugated structure of OII is the major

degradation step.

0 1 2 3 4 0 1 2 3 4

1.2

1.1

1.1

1.8

0

5

10

15

20

25

30

Inte

nsi

ty (

a.u

.)


0

5

10

15

(a)1.8

1.2

Time (min)Time (min)(b)

Inte

nsi

ty (

a.u

.)


Figure 6. 3: HPLC chromophores of OII solutions vs. retention time for (a) MgSrCuO and (b)

CeSrCuO.

C h a p t e r 6 P a g e | 94

250 500 750 250 500 750

RT (min)

(a)

Inte

ns

ity (

a.u

.)


1.1

1.2

RT (min)

(b)

Inte

ns

ity (

a.u

.)


1.1

1.2

Figure 6. 4: UV-vis spectra of eluted species by (a) Mg0.5Sr0.5CuO and (b) Ce0.5Sr0.5CuO at the

retention time (RT) of 1.0 and 1.2 min.

The stability and recyclability of a heterogeneous catalyst is one of the important indexes for practical

application. Therefore, both catalysts were tested for five consecutive recycling runs of OII

degradation and results are shown in Fig. 6.5. Both catalysts suffered from a gradual decrease in the

catalytic activity at each subsequent cycle. However, it should be noted that both retained relatively

high removal efficiency of OII after five cycle runs (43% for MgSrCuO and 37% for CeSrCuO).

These results are much higher than those for the conventional iron oxide Fenton-like catalyst which

tend to be fully catalytic inactive at the 5th cycle [36, 37] due to the passivation of the active sites

Fe2+ to non-active Fe3+. Therefore, the decrease in catalytic activity might be due to the loss of

active sites and/or blockage from adsorbed byproducts on catalyst surface.

1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

1-C

/C0

Cycle NO.

1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

1-C

/C0

Cycle NO.

Figure 6. 5: Cycling performance of MgSrCuO and CeSrCuO for OII degradation. Experimental

conditions: [OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, in the dark and under magnetic stirring.

6.6.2 Structural and morphological analysis

The as-synthsized Mg0.5Sr0.5CuO and Ce0.5Sr0.5CuO samples were first characterized by X-ray

diffraction (XRD). As displayed in Fig. 6.6, the XRD patterns of both samples display strong and

C h a p t e r 6 P a g e | 95

sharp diffraction peaks, indicating a high degree of crystallinity. Mg0.5Sr0.5CuO is indexed to two

phases assigned to SrCu2O3 (PDF 00-039-0250) and MgO (PDF 04-009-5447). XRD patterns of

Ce0.5Sr0.5CuO are assigned to Sr8Cu13O22.6 (PDF 00-045-0049) and CeO2 (PDF 04-013-4361).

Multicomponent samples were prepared instead of a single phase probably owing to the mismatch of

cation sizes.

10 20 30 40 50 60 70 80 90

Mg0.5

Sr0.5

CuO SrCu2O3

MgO

Inte

ns

ity

(a

.u.)

Ce0.5

Sr0.5

CuOSr8Cu13O22.6

CeO2

()

Figure 6. 6: XRD patterns of as-prepared Mg0.5Sr0.5CuO and Ce0.5Sr0.5CuO samples.

SEM images in Fig. 6.7 show the morphologies of Mg0.5Sr0.5CuO and Ce0.5Sr0.5CuO samples. A

representative SEM image of Mg0.5Sr0.5CuO shows a dense surface with randomly distributed

micron-sized particles (1-10 μm) (Fig. 6.7a). Ce0.5Sr0.5CuO is dominated with disperse particles with

the longest dimension ranging from 1 to 8 μm (Fig. 6.7b). Overall, SEM imaging shows non-porous

structures dominated by large inter-particle spacing. This is consistent with N2 adsorption-desorption

isotherms (Fig. 6.7c) showing low adsorbed volume at low relative pressure (i.e., non-porous) and

the increased volume at high relative pressure (i.e., inter-particle space). The surface areas were below

2 m2 g-1, consistent with non-porous particulate materials.

Figure 6. 7: SEM images (a,b) and N2 adsorption-desorption isotherms (c) of as-prepared

Mg0.5Sr0.5CuO3-δ (a) and Ce0.5Sr0.5CuO3-δ (b).

To shed more light on the reduced catalytic performance observed in Fig. 6.5, XPS was carried out

to determine the chemical states of metals and relative amounts of elements on metal oxide surface.

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

Qu

an

tity

Ad

sorb

ed (

cm

3/g

ST

P)

Relative Pressure (P/P0)

MgSrCuO

CeSrCuO

(c)

C h a p t e r 6 P a g e | 96

Fig. 6.8 displays the XPS survey spectra for both pristine (i.e., as synthesized and not tested by OII

degradation) samples. The XPS spectra of pristine/spent (i.e., catalytic tested) Cu of MgSrCuO and

CeSrCuO and pristine/spent Ce in CeSrCuO are also shown in Fig. 6.8. Deconvolution of Cu 2p3/2

XPS spectra demonstrates the coexistence of Cu1+ and Cu2+

for both pristine and spent MgSrCuO and

CeSrCuO (Fig. 6.8a and Fig. 6.8b). The peak at ~934 eV can be identified as Cu2+ species and the

peak at ~ 932.9 eV as Cu1+ species [38, 39]. The appearance of a shakeup satellite peak (denoted as

Sat.) centered at ~942.6 eV confirmed the presence of paramagnetic Cu2+ ions [39]. The high-

resolution XPS spectra of Ce in pristine/spent CeSrCuO are shown in Fig. 6.8c. The two pairs of spin-

orbit doublets ((v0, u0), (v, u)) represent the presence of Ce3+. The other three pairs ((v, u), (v, u),

(v, u)) are characteristic peaks for Ce4+ [40].

1200 1000 800 600 400 200 0

Sr 3d

Mg

KL

L

O 1s

C 1

s

Ce 3d

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

MgSrCuO

CeSrCuO

Cu 2p

950 945 940 935 930 925 950 945 940 935 930 925 920 910 900 890 880

(a)Cu 2p3/2

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Sat.Cu

2+

Cu 2p3/2

Cu1+

pristine

spent

Inte

nsi

ty (

a.u

.)

Cu 2p3/2

Sat. Cu2+

Cu 2p3/2

Cu1+

pristine

spent

(b)

Binding Energy (eV)

spent

pristine

u'u''

Inte

nsi

ty (

a.u

.)

Ce 3d (c)

Binding Energy (eV)

v0

v

v'

v''u

0

v'''uu'''

Ce4+

: v,v'',v'''

u,u'',u'''

Ce3+

: v0,v',u

0,u'

V: 3d5/2

U:3d3/2

Figure 6. 8: XPS survey spectra and high-resolution XPS spectra of Cu for pristine/spent in (a)

MgSrCuO and (b) CeSrCuO (b): and (c) Ce for pristine/spent CeSrCuO.

The relative concentrations of Cu1+ and Cu2+ species present on the outer surface (5-10 nm) of pristine and

spent materials are displayed in Table 1 and were calculated based on integrated areas via Eqs. 6.1-6.2 [37]:

[𝐶𝑢1+] =𝐴1

𝐴+𝐵 Eq. (6.1)

[𝐶𝑢2+] =𝐵+𝐴2

𝐴+𝐵 Eq. (6.2)

where A is the total integrated area of the main peak; B is the integrated area of the whole shakeup

satellite peak; A1 is the integrated peak area contributed by Cu1+ species and A2 is the integrated peak

area contributed by Cu2+ species; A1+A2=A. The results in Table 6.1 clearly indicate that the relative

C h a p t e r 6 P a g e | 97

concentration of Cu1+ for spent materials increased after the reaction compared their counterparts in

pristine materials. This suggests Cu2+ underwent irreversible reduction reaction during OII

degradation. The relative concentration of Ce3+ before (15.33%) and after (10.84%) reaction was

calculated using methods reported elsewhere [41]. These results also confirm changes of CeO3,

though in the opposite direction of CuO reduction, as Ce3+ oxidized by losing electron to form Ce4+.

Table 6. 1: Relative concentrations of Cu1+ to (Cu2++ Cu1+) in pristine and spent samples.

Materials MgSrCuO CeSrCuO

Cu1+/(Cu2++ Cu1+) Pristine 6.04% 6.86%

Spent 14.64% 12.34%

6.6.3 Degradation mechanism

The catalytic degradation mechanism of OII using MgSrCuO and CeSrCuO was investigated by

exploring the effects of active species potentially involved in the catalytic process using radical

scavengers. Scavengers were added to OII solutions before the addition of catalysts. EDTA-2Na (10

mM) was adopted as a chelating agent to rule out the formation of reactive species by chelating with

transition metal ions (e.g., Cu) on catalyst surface [38]. As can be seen from Fig. 6.9, the degradation

efficiency of OII is close to ~100% within the first 30 min for both MgSrCuO and CeSrCuO without

scavengers. In the presence of EDTA-2Na, the reaction is significantly suppressed as OII degradation

was less than 5%. This result shows that OII could not access the surface of the catalyst due to the

chelating action of EDTA-2Na. In other words, this result strongly suggests that the breakdown of

azo bonds took place via OII molecules contacting with the surface of the catalyst. The addition of

hydroxyl radical (•OH) quencher KI (10 mM) and electron (e-1) quencher AgNO3 (2.5% w/v)

inhibited OII degradation remarkably as well, indicating •OH and e-1 played an important role in OII

degradation.

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0(a)

No quencher

EDTA-2Na

KI

AgNO3

C/C

0

Time (min)

No quencher

EDTA-2Na

KI

AgNO3

(b)

C/C

0

Time (min)

Figure 6. 9: Radical quenching effect on OII degradation using (a) MgSrCuO and (b) CeSrCuO.

Experiment conditions: [OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, in the dark and under constant

stirring.

C h a p t e r 6 P a g e | 98

Based on the analysis of all characterization and experiment results, potential degradation mechanism

of OII by MgSrCuO and CeSrCuO is proposed as schematically shown in Fig. 6.10. Under dark

conditions, OII molecules contacted the surface of MgSrCuO and CeSrCuO as per Eq. 6.3. As the

surface of these metal oxides are catalytically active, the first step in the reaction is the breakdown of

the azo bonds (Eq. 6.4) which is supported by the results in Fig. 6.1. This reaction causes the release

of electrons which tend to react with O2 (Eq. 6.5), leading to the generation of a series of reactive

species (Eqs. 6.6-6.8). The formation of electrons and hydroxyl radicals has been ascertained by

radical quenching experiment (Fig. 6.9). Hydroxyl radicals resulted in the degradation of OII/OII+

(Eq. 6.9), the generation of by-products as confirmed by the HPLC results (Figs. 6.3-6.4) and partial

mineralization based on TOC analysis (Fig. 6.2). This formation of hydroxyl radicals was also

reported when using SrCeFeO [13] and CaSrCuO [24] were used as catalysts for OII degradation in

the dark.

OII + MeSrCuO → CSC(OII) Eq. (6.3)

MeSrCuO(OII) → MeSrCuO + OII+ + e− Eq. (6.4)

e− + O2 → O2− Eq. (6.5)

O2− + H+ → HO2

Eq. (6.6)

2HO2 → O2 + H2O2 Eq. (6.7)

H2O2 + e− →∙ OH + OH− Eq. (6.8)

∙ OH + OII OII+⁄ → ⋯ → byproducts + CO2 Eq. (6.9)

MeSrCuO(Cu2+) + e− →MeSrCuO(Cu1+) Eq. (6.10)

A parallel mechanism is the reduction of Cu2+ to Cu1+

(Eq. 6.10) which was confirmed by the XPS

results (Fig. 6.8 and Table 6.1). This proves that Cu2+ in MeSrCuO is the active site as supported by

the decline in catalytic activity after each subsequent cycling test (Fig. 6.5). Hence, this parallel

mechanism provides a second non-catalytic degradation pathway for OII. In addition, as electrons are

consumed by MeSrCuO during the degradation of OII, this reduces the ability of the reaction to

generate hydroxyl radicals. This explains that the mineralization was partial only as ascertained by

TOC analysis (Fig. 6.2).

C h a p t e r 6 P a g e | 99

Figure 6. 10: Schematic presentation of potential reaction mechanisms involved in the

degradation of OII by MgSrCuO and CeSrCuO.

6.7 Conclusions

Metal oxides Me0.25Sr0.25Cu0.5O (Me=Mg and Ce) degraded OII in the dark at room temperature

without additional reagents and energy/light irradiation. The materials proved to have excellent

catalytic activity and fast kinetics with more than 95% OII degradation occurring within the first 5

min. The surface of MeSrCuO was catalytically active for the breakdown of azo bonds of OII dye,

leading to the generation of electrons. Subsequently, the electrons reacted with O2 from the aqueous

solution, resulting in the formation of reactive species (e.g., •OH) which are responsible for OII

degradation. This reaction pathway resulted in the formation of clear solutions, a confirmation that

the azo bonds were destructed, though generating by-products as confirmed by HPLC analyses.

Further, mineralisation was only partial, indicating that not all electrons could lead to the formation

of hydroxyl radicals. XPS analyses confirmed that the reduction of Cu2+ from the pristine sample to

Cu1+ in the spent sample, thus consuming electrons and providing a non-catalytic pathway. Therefore,

Cu2+ in MeSrCuO proved to be the active phase for OII degradation.

Acknowledgment





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C h a p t e r 7 P a g e | 104

7 Surface and catalytic properties of stable Me(Ba, Ca and Mg)SrCoO for the degradation of

Orange II dye under dark conditions

7.1 Introduction

As reviewed in Section 2.3.1, A-site cation substitution potentially contributes to influence the

structural framework and further to affect the catalytic activity of perovskite catalysts. Diverse A-site

cations might result in catalysts with differing catalytic behavior. As Mg, Ca and Ba are in the same

group with Sr in the Periodic Table with the same oxidation states, they should be compatible well

with each other when doping into A0.5Sr0.5CoO. Therefore, A0.5Sr0.5CoO (A=Mg, Ca, and Ba) is

designed to investigate the effect of A-site cation substitution on the physicochemical properties of

mother compounds.

7.2 Contributions

Chapter 7 has been submitted to Applied Surface Science. This chapter is wholly my own work with

the exception of the contribution by Prof. João C. Diniz da Costa, Dr. Julius Motuzas and Dr. Wayde

Martens.

C h a p t e r 7 P a g e | 105

Surface and catalytic properties of stable Me(Ba, Ca and Mg)SrCoO for the degradation of Orange

II dye under dark conditions





Australia.


7.3 Abstract

This work investigates the surface and catalytic properties of Co containing metal oxides MeSrCoO

by partially substitution of Sr with another alkaline metal Me (Ba, Ca and Mg). The catalysts were

used for the degradation of a textile dye orange II (OII) under dark conditions and without the addition

of any chemical additive, light irradiation or energy input. Although all catalysts were prepared under

the same conditions, BaSrCoO formed a pure perovskite phase, while MgSrCoO and CaSrCoO

resulted in a mixture of perovskite and metal oxide phases. All these catalysts were characterised by

a non-porous materials with low surface areas (< ~1 m2 g-1). A total value of ~80% OII degradation

percentage was reached in 4 hours and ~90% in 8 hours, though their surface properties resulted in

different reaction kinetics. For instance, BaSrCoO and MgSrCoO reaction kinetics were faster and

fitted a second order reaction whilst CaSrCoO was slower and fitted a first order reaction. OII

degradation was mainly attributed to the catalytic surface properties of these metal oxides, as sorption

was not significant except for CaSrCoO which explains the lower reaction kinetics. All catalysts

demonstrated good stability over 7 cycle testing (56 h), which was also confirmed by XRD and XPS

analysis of pristine and spent samples. Interestingly, non-substituted SrCoO resulted in a similar OII

degradation rate, but decayed cycling stability. Hence, the partial substitution of Sr with alkaline

metals (Ba, Ca and Mg) conferred increased surface stability. Due to the catalytic surface property of

MeSrCoO, the primary reaction mechanism was the contact of OII with the catalyst, leading to the

generation of electrons. Subsequently, the electrons reacted with dissolved O2 in the solution, and in

a series of reactions, formed hydroxyl radicals and singlet oxygen, leading to OII further degradation.

Keywords: Metal oxides; heterogeneous catalysis; stable catalytic surfaces; dark ambient conditions;

OII degradation.


C h a p t e r 7 P a g e | 106

7.4 Introduction

Advanced oxidation processes (AOPs) that produce highly reactive oxygen species in situ have

attracted increasing attention for the degradation of organic pollutants in water from both research

and industry [1-3]. Photocatalysis [4], Fenton-like reaction [5-7], ozonation [8], electrochemical

oxidation [9] and sulphate radical based AOPs [10, 11] are widely reported for water remediation.

All these technologies intrinsically require external inputs such as light illumination, pH adjustment,

chemical additives (persulfate/peroxymonosulfate, ozone or peroxide) and electricity. A more

promising technical solution to degrade organic pollutants in wastewater is via a catalytic process

without any external input [12]. Within this quest in mind, heterogeneous catalysis carried out under

dark condition and room temperature offers many advantages to simplify engineering design of plants

whilst cutting operating costs at the same time.

Recently, metal oxides such as perovskites (ABO3) or containing a perovskite phase have become an

important class of heterogeneous catalysts for the degradation of textile dyes. Metal oxides are very

versatile materials and there is a large number of cations that can be used to prepare different catalysts

[13-16]. As a result, the surface of these catalysts have different physicochemical properties, which

in turn affect their overall degradation performance [17-19]. The surface properties of the A-site

composition in perovskites is particularly important for catalytic degradation of dyes. For instance,

BaFeO3 [20] was reported to have a very low degradation rate (< 5days) of methyl orange. However,

the substitution of Ba with Sr in the A-site, thus forming SrFeO3 [21], resulted in fast degradation

rates (~60 min) of acid orange 8. Perovskites also allow partial substitution of the cation A with

another cation A’, thus forming a compound with the general formula AA’BO3. Tummino and co-

workers [22] showed that the partial substitution of Sr with Ce, forming a perovsksite CeSrFeO3,was

effective in the degradation of orange II (OII). All these examples are catalysis under dark condition.

On a similar basis, the surface and physical properties of perovskites with different B-site cations also

play an important role in the performance of catalyst for degradation of dyes [23, 24]. For instance,

LaCuO3 [25] degraded phenol by using peroxide in a Fenton-like reaction. Pursuing new strategies

to improve the surface properties of metal oxides for catalyses under dark conditions, Chen et al. [26]

also used Cu in the B-site and formed a compound CaSrCuO3 which was effective in the degradation

of OII dye. Very recently, Chen et al. [27] replaced Cu with Ni, forming a metal oxide compound

CaSrNiO. This catalyst resulted in very fast OII degradation kinetics of 97% within 5 min. However,

the Ni2+ active phase of CaSrNiO catalyst was oxidized to a non-active Ni3+ phase, resulting in loss

of activity. To address this problem, perovskites also allow the partial substitution of the cation B

with other B’ cations, thus forming a compound by the general formula AA’BB’O3. In subsequent

work, Ni was partially substituted with Cu that proved to be a good strategy as CaSrNiCuO3 which

C h a p t e r 7 P a g e | 107

was effective and stable for the degradation of azo dyes in the dark due to the Cu2+ active sites. In

many of these studies, a pure perovskite phase is not formed, so the final catalyst may contain

perovskites and metal oxide phases.

Another important cation that is extensively used in the B-site of perovskites is Co. Indeed, Co

containing perovskites have been report for the degradation of organic compounds in wastewaters.

For instance, Orge et al. [25] showed that the perovskite LaCoO3 was considered as the best catalyst

for oxalic acid degradation under ozonation. It was interesting to note that the single metal oxide CoO

was effective in the degradation of CI acid blue 113 also in the presence of ozone [28]. Recently,

LaCoO3 showed high catalytic activity toward Rhodamine B in the presence of peroxymonosulfate

[29] and methylene blue degradation under visible light irradiation [30]. Other example include

SrFe0.5Co0.5O3 [31] perovskites that were used in the photo-Fenton reaction systems for acid red dye

and methyl orange dye, respectively. In all these reports, Co containing perovskites required chemical

additives whilst the use this type of compound warrants research under dark conditions.

Therefore, this work investigates the catalytic surface effect under dark condition of ternary metal

oxide compounds containing Sr in the A-site and Co in the B-site of perovskites. As previous studies

of metal oxides showed the formation of both metal oxide and perovskite phases, the general metal

oxide formula Me0.25Sr0.25Co0.5O where Me (Ba, Ca and Mg) together with Sr are alkaline earth group

metals, which are conventional cations incorporated in the A-site of perovskites. OII is a widely

employed as an azo dye in the textile industry and was selected to evaluate the catalytic activity of

Me0.25Sr0.25Co0.5O materials. The OII degradation reactions were carried out in the dark at room

temperature under atmospheric pressure and without the addition of any chemical additive, light

irradiation or energy input. The property of the formed materials was analysed using a series of

characterisation tools such N2 sorption to determine surface areas, XPS and XRD to determine

changes of the catalyst prior and after reaction. The catalysts were also exposed to 7 cycles of reaction

to determine their long-term stability, and their reaction performance is correlated to their surface

properties. A mechanism of reaction for the degradation of OII dye is proposed.

7.5 Materials and method

7.5.1 Materials, synthesis and characterization

Strontium nitrate, barium nitrate and cobalt (II) nitrate hexahydrate were from Alfa Aesar. Calcium

nitrate tetrahydrate, magnesium nitrate hexahydrate, cerium (III) nitrate hexahydrate, citric acid

monohydrate, and ammonia solution were purchased from Chem-Supply Pty Ltd. Ethylenediamine

tetra-acetic acid (EDTA) and potassium iodide (KI) came from Ajar FineChem Pty. Silver nitrate,

C h a p t e r 7 P a g e | 108

1,4-diazabicyclo[2.2.2]octane (DABCO), and Orange II were supplied by Sigma-Aldrich. Aqueous

solutions used were prepared with distilled water. All chemicals were of at least analytical grade.

The synthesis Me0.25Sr0.25Co0.25O materials was carried out by a sol-gel chemistry method. Briefly,

nitrate salts, ammonium hydroxide, EDTA and citric acid with fixed molar ratio of 1: 10: 1.1: 2 were

dissolved in distilled water to obtain clear mixed solution, followed by mild heating under continuous

stirring to to form a viscous sol-gel [32]. Subsequently, the sol-gel was then pre-calcined at 450 °C

and further sintered up to 1000 °C in atmospheric air in a furnace (heating and cooling rates of 5 °C

min-1). Hereinafter, the compounds are denominated as MeSrCoO, and as BaSrCoO, CaSrCoO and

MgSrCoO. The phase compositions were identified by X-ray diffraction (XRD, D8 Advance, Bruker,

USA) with graphite monochromatized Cu Ka radiation in the range of 10-80 in 0.02°. The

microstructure and morphology was examined by field-emission scanning electron microscopy

(FESEM; JSM-7001F, JEOL, Japan). The surface composition and chemical valence analyses were

investigated by X-ray photoelectron spectroscopy (XPS; Kratos Axis ULTRA, Japan) with

monochromatic Al K (hv=1486.6 eV) radiation. All the binding energies in the XPS analysis were

corrected for specimen charging by referencing them to the C 1s peak set at 284.8 eV. Textural studies

were carried out to calculate specific surface areas using N2 sorption (Micromeritics TriStar 3000,

USA).

7.5.2 Catalysis

OII was used as the model organic pollutant to evaluate the catalytic activity of MeSrCoO in the dark

at room temperature under atmospheric pressure without the addition of additional oxidants.

Typically, 0.2 g of catalysts was added into 200 mL OII solution with a concentration of 20 ppm (mg

L-1) in a 250 mL beaker covered with alumina foil in order to keep the dark condition. The solution

was kept under vigorously magnetic stirring throughout the reaction. Aliquot suspension was

periodically collected and filtered to separate the solid catalyst. Degradation efficiency of OII with

time was determined by high-performance liquid chromatography (HPLC; UltiMate 3000, UHPLC+

focused, Thermmo Scientific, USA) method using a C18 column (4.6 mm 150 mm 5 m) and

70:30 aqueous solution of ammonium acetate: acetonitrile as an isocratic mobile phase at 1 mL min-

1 at room temperature. Mineralization degree of OII after reaction was obtained using a total organic

carbon analyzer (TOC; TOC-VCSH, Shimadzu, Japan). Classical quenching tests were employed

using scavengers to probe the reactive species. Thermal analysis was conducted for pristine and spent

materials to investigate the effect of surface sorption properties on OII degradation using a

Thermogravimetric Analyzer (Mettler Toledo).

C h a p t e r 7 P a g e | 109


7.6.1 Characterization of catalysts

Fig. 7.1a-c shows representative SEM images of Me0.5Sr0.5CoO3 catalysts revealing similar

morphology. For instance, BaSrCoO resulted in micro-sized particles (2-10 µm) joined together by

necks caused by the coalescence of particles during sintering with nano particles randomly distributed.

Similar trends are observed for the CaSrCoO and MgSrCoO samples, though the size of the joined

micro-particles are smaller (1-5 µm). N2 sorption results (Fig. 7.1d) display isotherms type III, a

characteristic of non-porous solid materials with a hysteresis at high relative pressure associated with

inter-particle space. This confirms that visual observation in the SEM images of solid micro particles

with inter-particle spacing. The BET surface area for all samples were low and below ~1 m2 g-1,

clearly indicating that the sintering process resulted in the formation of solid particles.

Figure 7. 1: SEM images (a,b,c) and N2 sorption isotherms of (a) BaSrCoO, (b) CaSrCoO, and

(c) MgSrCoO.

As-synthesized pristine catalysts (before reaction) and the spent counterparts (after reaction) were

characterized by powder XRD measurements as depicted in Fig. 7.2. The sharp and intense XRD

patterns for BaSrCoO demonstrated the formation of a pure perovskite phase as Ba0.5Sr0.5CoO3

(JCPDS: 04-016-5545) without any other impurity peaks. However, the same cannot be said about

the other samples that resulted in the formation of mixed metal oxide and perovskite phases. For

instance, CaSrCoO sample formed a perovskite (Ca0.2Sr0.8)5Co4O12 (JCPDS: 00-060-0753) and single

metal oxide CoO (JCPDS: 04-007-0517), whilst MgSrCoO resulted in the formation of a perovskite

Sr6Co5O15 (JCPDS: 04-014-3926) and a single metal oxide as MgO (JCPDS: 04-017-5048) and CoO

(JCPDS: 04-007-0517). Although the basic SrCoO compound was used in mixtures, the partial

substitution of Sr with Ba, Ca and Mg clearly resulted in the formation of different phases. The XRD

patterns of spent materials showed no obvious changes in comparison with that of pristine

counterparts, indicating the potential stability of these catalysts.

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

10

Qu

an

tity

Ad

sorb

ed (

10

-6m

3/g

ST

P)

Relative Pressure (P/P0)

BaSrCoO

CaSrCoO

MgSrCoO

(a)

10μm 10μm

(b)

2μm

(c) (d)

C h a p t e r 7 P a g e | 110

Ba0.5Sr0.5CoO3

Inte

nsi

ty (

a.u

.)

●

●

♥●♥

●

●●●

●♥ ●

CoO♥(Ca0.2Sr0.8)5Co4O12●

2θ (degrees)

(a)

(b)

(c)

■

■

■

■

■ Sr6Co5O15 MgO

■ ■ ■ ■ ■

■

♥ CoO

♥

♥

♥

Figure 7. 2: XRD patterns of nominal pristine (black line) and spent (red line) (a) BaSrCoO, (b)

CaSrCoO and (c) MgSrCoO.

The surface properties of the samples were analysed by XPS and results for both pristine and spent

MeSrCoO are displayed in Fig. 7.3. The wide scan XPS spectra and high-resolution XPS spectra of

pristine and spent MeSrCoO materials are displayed in Fig. 7.3. It is observed that all elements

remained practically unchanged after reaction, demonstrating the highly stability of as-synthesized

catalysts, which is consistent with the XRD analysis (Fig. 7.2). Specifically, there is negligible

variation in the high-resolution XPS spectra of doped cations (Ba 3d, Ca 2p and Mg 2p) between

pristine and spent materials. The two spin-orbit doublets and three shakeup satellites (denoted as

“Sat.”) that are characteristic of Co2+ and Co3+ cations [33, 34] are observed for all pristine and spent

catalysts and were not subject to change after reaction. This confirms cobalt exists in mixed chemical

valences (Co2+/Co3+) on the surface of the as-prepared catalysts. The XPS narrow spectrum for the

Sr 3d with peaks at ~133.3 and ~135.1 eV corresponds to Sr2+ [33, 34]. It should be noted that the

XPS spectra of Ba 3d and Co 2p overlapped with each other and remained almost unchanged before

and after reaction (Figs. 7.3a and 7.3c).

C h a p t e r 7 P a g e | 111

1200 1000 800 600 400 200 0

Co 2p+Ba 3d

O 1s

C 1

s

Sr

3d

Ba

4d

Co

3p

Inte

nsi

ty (

a.u

.) Co 2p O 1s

Ca 2p C 1

s

Sr

3d

Co

3p

Binding Energy (eV)

BaSrCoO

CaSrCoO

MgSrCoO

Co 2p O 1s

Mg KLL

C 1

s

Sr

3d

Mg

2s

804 798 792 786 780 774 140 136 132 128 804 798 792 786 780 774

354 351 348 345 342 140 136 132 128 804 798 792 786 780 774

54 51 48 45 42 140 136 132 128 804 798 792 786 780 774

Ba 3d(a)

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Sr 3d(b)

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Sat.Sat.Sat.

2p1/2

2p3/2

(c)

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Co 2p

Ca 2p(d)

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Sr 3d(e)

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

(f)

Sat.2p

1/2

Sat. Sat.

2p3/2 Co 2p

Mg 2p(g)

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Sr 3d(h)

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

(i)

Sat.

2p1/2

Sat.Sat.

2p3/2

Co 2p

Figure 7. 3: Survey XPS spectra of pristine and spent catalysts and high-resolution XPS spectra

of pristine (black line) and spent (red line) catalyst for (a,b,c) BaSrCoO, (d,e,f) CaSrCoO and

(g,h,i) MgSrCoO.

C h a p t e r 7 P a g e | 112

7.6.2 Catalyst activity

Fig. 7.4a shows the activity of MeSrCoO catalysts for the degradation of OII as the model organic

pollutant based on HPLC results. The MgSrCoO and BaSrCoO gave similar trends in OII degradation,

characterised by fast degradation rates up to 78% in 2 h reaction and reaching 89% at 8 h. Contrary

to this, the CaSrCoO reaction kinetics was slow resulting in 54% OII degradation in 2h, though this

catalyst still maintained a good activity up to 8h delivering the best 93% OII degradation. The UV-

vis spectra of the initial OII and perovskite treated OII solutions are shown in Fig. 7.4b. It is clear

that the UV-vis spectra of treated OII are different from that of OII solution in terms of peak intensity

and peak positions, indicating that OII was degraded after reaction. Specifically, the characteristic

peak of OII at 485 nm assigned to the hydrozone band [35] decreased significantly. This is attributed

to the breakdown of azo bonds suggests, which can be further evidenced (see Appendix Fig. 7.A3)

by the change in colour from dark yellowish for the OII solutions prior to reaction to the almost pale

solutions after treatment. In addition, it is also observed that the intensity of the band at 230 nm

assigned to benzene ring in OII [36] also decreased. These results suggest that the MeSrCoO catalysts

were effective in breaking down OII molecules, though there are differences between the performance

of each catalyst. For instance, the intensity of the band at 430 nm was higher for CaSrCoO than that

of BaSrCoO and MgSrCoO. Further, all treated OII solutions adsorption bands remained in the visible

region (Fig. 7.4b), indicating that OII degradation produced intermediate by-products. This is also

supported by the visible pale colour of the treated solutions (Fig. 7.4c).

Figure 7. 4: (a) Normalized OII concentrations based on HPLC, (b) UV-vis spectra of initial

and treated OII solutions, and colour intensity variation of OII solution at 8h treated by

MeSrCoO catalyst. Experimental conditions: [OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, in the

dark and under stirring.

Fig. 7.5 shows that the chromatograms of HPLC results at the retention time of ~1.8 min attributed

to OII decreased gradually with time. This is indicative of the progressive catalytic degradation of

OII. Meanwhile, two new downward chromatograms appeared at the retention time of ~1.1 and ~1.4

min for BaSrCoO and MgSrCoO catalysts, which are attributed to the formation of by-products.

0 2 4 6 80.0

0.2

0.4

0.6

0.8

1.0

C/C

0

Reaction time (h)

BaSrCoO

CaSrCoO

MgSrCoO

200 300 400 500 600 7000.0

0.5

1.0

1.5

2.0

Wavelength (nm)

Ab

sorb

an

ce (

a.u

.)

OII

BaSrCoO

CaSrCoO

MgSrCoO

(a) (b)(c)

C h a p t e r 7 P a g e | 113

Contrary to these results, CaSrCoO did not show a peak ~1.4 min, though the intensity of the peak at

~1.1 min was higher than that of the other catalysts. These results clearly indicate that the surface

properties of CaSrCoO differ from those of BaSrCoO and MSrCoO in terms of catalytic degradation

of OII, which is in line with UV-vis spectra results (Fig. 7.4).

0 1 2 3 0 1 2 3 0 1 2 3

(a)

8 h

6 h

4 h

2 h

1 h

In

ten

sity

(a.u

.)


0 h

(b)

Inte

nsi

ty (

a.u

.)


8 h

6 h4 h

2 h

1 h

0 h

(c)

Inte

nsi

ty (

a.u

.)Retention time (min)

8 h

6 h

4 h

2 h

1 h

0 h

Figure 7. 5: HPLC chromophores of OII solutions vs. retention time for (a) BaSrCoO; (b)

CaSrCoO; and (c) MgSrCoO.

Fig. 7.6 shows the UV-vis spectra of eluted by-products generated by the catalysts against the spectra

of OII. The two observed UV-vis spectra of eluted species at ~1.1 and ~1.4 min are distinct from each

other in terms of the numbers and positions of absorption peaks. The UV-vis spectra of eluted species

at the retention time of 1.1 min have strong absorbance above the wavelength of 400 nm. This

explains the remaining pale colour (see Appendix Fig. 7.A3) after treatment and confirms the

formation of by-products. However, it should be noted that only one chromatogram was observed for

the treated sample by CaSrCoO. This suggests that the degradation pathway by CaSrCoO differs from

that of BaSrCoO and MgSrCoO catalysts.

Fig. 7.6 shows the UV-vis spectra of eluted by-products generated by the catalysts against the spectra

of OII. The two observed UV-vis spectra of eluted species at ~1.1 and ~1.4 min show they are distinct

from each other in terms of the numbers and positions of absorption peaks. The UV-vis spectra of

eluted species at the retention time of 1.1 min have strong absorbance above the wavelength of 400

nm. This explains the remaining pale colour (see Appendix Fig. 7.A3) after treatment and confirms

the formation of by-products. This suggests that the degradation pathway by CaSrCoO differs from

that of BaSrCoO and MgSrCoO catalysts.

C h a p t e r 7 P a g e | 114

300 450 600 300 450 600 300 450 600

1.1 min

1.37 min

OII

Inte

nsi

ty (

a.u

.)

Inte

nsi

ty (

a.u

.)

Retention time (min) Retention time (min) Retention time (min)In

ten

sity

(a.u

.)

1.1 min

OII

1.1 min

1.37 min

OII

(c)(a) (b)

Figure 7. 6: UV-vis spectra of OII and eluted species by (a) BaSrCoO, (b) CaSrCoO and (c)

MgSrCoO.

7.6.3 Surface properties and reaction mechanism

Surface properties of blank (exposed to water) and spent (after catalysis) catalysts were analyzed by

TGA to determine if OII degradation results in Figs. 7.4-6 were due to catalytic reaction of the

MeSrCoO samples or due to surface sorption. Fig. 7.7 shows the mass of the samples and compares

the mass against the degradation of OII in air as a function of temperature. In principle, all results

show that mass differences between the samples were not significant. A closer inspection of the insert

in Fig. 7.7 clearly indicates that the MgSrCoO results remained the same for both blank and spent

samples, whilst minor variations in mass were observed for the BaSrCoO samples reaching 0.03% at

550 C, equivalent to 0.3 ppm adsorption out of 20 ppm. The CaSrCoO sample show a mass

difference of 0.68% at 550 C, thus a higher adsorption of 6.8 ppm. For an initial solution containing

20 ppm, these results suggest that 34% of OII or its intermediate by-products sorbed on the surface

of CaSrCoO.

C h a p t e r 7 P a g e | 115

100 200 300 400 5000

20

40

60

80

100

200 40098

99

100

Ma

ss (

%)

Temperature (C)

OII

BaSrCoO Spent Blank

CaSrCoO Spent Blank

MgSrCoO Spent Blank

Figure 7. 7: TGA results of blank and spent samples and OII in flowing air.

The sorption properties of CaSrCoO explains the slow kinetics of OII degradation observed in Fig.

7.4a. For instance, OII degradation reached 54% at 2 h for CaSrCoO, though at a much faster rate for

both BaSrCoO (74.3%) and MgSrCoO (78.3%). However, this slow degradation kinetics did not deter

the reaction from continuing, as from 4 to 8 h reaction, evidenced by the OII removal by CaSrCoO

being higher than the other two catalysts. The degradation kinetics in Fig. 7.8 also show that CaSrCoO

has different surface catalytic properties. For instance, the CaSrCoO apparent rate constant (k = 0.368

h-1) fitted a first order reaction, whilst a second order reaction for both BaSrCoO (k = 0.0414 ppm-1

h-1) and MgSrCoO (k = 0.0519 ppm-1 h-1). All the k values fitted well the first and second order

reactions with R2 > 0.95.

0 2 4 6 80.0

0.1

0.2

0.3

0.4

0.5

0.6

Reaction time (h)

1/C

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

MgSrCoO

CaSrCoO Ln

(C/C

0)

BaSrCoO

Figure 7. 8: OII degradation kinetics by (a) BaSrCoO, (B) CaSrCoO and (c) MgSrCoO.

C h a p t e r 7 P a g e | 116

Fig. 7.9a shows the cycling results up to seven cycles (56 h) under identical conditions. All samples

demonstrated good cycling stability, thus confirming the XRD (Fig. 7.2) and XPS (Fig. 7.3) results

that no significant changes occurred between the pristine and spent catalysts. However, there are

differences in long-term performance observed in Fig. 7.9. Within the experimental variation (±5%),

the CaSrCoO sample proved to be the most stable as the average degradation of OII over 7 cycles

only slightly reduced. The other catalysts BaSrCoO and MgSrCoO average degradation decayed from

initially values to ~80 and 70%, respectively, though the OII degradation stabilised around the third

cycle. This high activity is beneficial, contrary to conventional iron oxide Fenton catalysts for which

degradation activity decreased from 70% to 10% in 4 cycles only [37], or Ni containing metal oxide

catalysts that suffered similar reduction of reaction activity in 4 cycles for the degradation of OII dyes

under dark conditions [27]. A SrCoO sample was also tested and cycling results (Fig. 7.9b) show that

the activity of the catalyst constantly decayed. Hence, the partial substitution of Sr with alkaline

metals improved the stability of the catalyst, particularly Ca that delivered the most stable catalyst.

Figure 7. 9: Cycling performance of MeSrCoO (a) and SrCoO (b) perovskite catalysts for OII

degradation. Experiment conditions: [OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, each run is

8h, in the dark and under constant stirring.

As the samples contained distinct phases of metal oxides and perovskites as per XRD results (Fig.

7.2), it is important to ascertain which compound is most active. In the case of BaSrCoO, it formed a

pure perovskite phase, so there is no need for further assessment. However, CaSrCoO and MgSrCoO

formed mixed metal oxide and perovskite phases. Hence, a number of compounds were synthesized

and assessed separately for OII degradation and results are given in Fig. 7.10. According to the XRD

results, MgSrCoO was composed by SrCoO, MgO and CoO phases. Indeed, the non-partially

substituted SrCoO reached similar OII degradation value of ~86% as MgSrCoO, whilst MgO gave

an OII degradation of 35%. Nevertheless, the partial substitution of Sr with Mg was positive and

increased the cycling stability performance (Figs. 7.9 and Fig. 7.A4). CaSrCoO formed a perovskite

(a)

1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

1-C

/C0

Cycle No.

SrCoO

1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

1-C

/C0

Cycle No.

BaSrCoO

CaSrCoO

MgSrCoO

(b)

C h a p t e r 7 P a g e | 117

while not all Co was able to be incorporate into the perovskite phase, thus forming CoO, which had

a very low catalytic degradation of ~2%. Hence, OII degradation was attributed to the CaSrCoO

perovskite phase.

Figure 7. 10: OII degradation of several SrCoO, CoO and MgO. Experiment conditions:

[OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, each run is 8h, in the dark and under constant

stirring.

Figure 7. 11: Radical quenching effect on OII degradation using (a) BaSrCoO, (b) CaSrCoO

and (c) MgSrCoO. Experiment conditions: [OII]0=20 ppm, [catalyst]=1 g L-1, T=20 °C, in the

dark and under constant stirring.

Scavenging experiments were conducted to distinguish the potential involved reactive species for OII

degradation. The degradation profiles of OII versus time with and without quenchers are shown in

Fig. 7.11a-c for the MeSrCoO catalysts. Air was bubbled into the reaction suspension to increase the

dissolved O2 concentration to probe the role of O2 in OII degradation. It is found that air increased

the catalytic activity of all catalysts, thus confirming that O2 plays a role in the reaction. KI is used as

a hydroxyl radical scavenger and results clearly indicate that the OII degradation was suppressed in

the presence of KI, indicating hydroxyl radicals are responsible for OII degradation. DABCO is an

efficient singlet oxygen (1O2) scavenger [38], which proved to suppress OII degradation in this study,

C h a p t e r 7 P a g e | 118

confirming 1O2 was generated during OII degradation. TOC tests were also carried out though results

showed that OII mineralisation did not take place up to 8 h.

The breaking down of OII molecules under dark conditions was attributed to the catalytic surface

properties of MeSrCoO based on the surface analysis and catalytic experiment results. A potential

degradation mechanism is proposed as displayed in Fig. 7.12. Under dark conditions, OII molecules

contacted the surface of MeSrCoO as per Eq. 7.1. Due to the catalytically active surface of MeSrCoO,

the OII degradation occurs primarily at the azo bonds of OII, resulting in the colour change from dark

yellowish to pale colour from before to after reaction (see Appendix Fig. 7.A3), respectively. This

reaction causes the release of electrons (Eq. 7.2) which react with available dispersed O2 (Eq. 7.3) in

the OII solution to form superoxide anion radicals, where the role of O2 in the reaction was ascertained

in Fig. 7.11. Subsequently, there is a number of reactions resulting in the formation of reactive species

(Eqs. 7.4-7.6). Of particular note is the generation of hydroxyl radicals (Eq. 7.6), which was identified

by the by radical quenching experiment (Fig. 7.11). The generation of hydroxyl radicals was also

reported for other type of metal oxides catalysts such as SrCeFeO [22] and CaSrCuO [26] for OII

degradation in the dark. Notably, singlet oxygen 1O2 can be also generated via Eqs. 7.7-7.9 as

ascertained by DABCO trapping experiment (Fig. 7.11), which are also reported in Fenton-like

reactions [39, 40]. Hydroxyl radicals and singlet oxygen are responsible for the degradation of

OII/OII+ (Eq. 7.10), as confirmed by the presence of by-products based on HPLC results (Fig. 7.4).

OII + MeSrCuO → CSC(OII) Eq. (7.1)

MeSrCuO(OII) → MeSrCuO + OII+ + e− Eq. (7.2)

e− + O2 → O2− Eq. (7.3)

O2− + H+ → HO2

Eq. (7.4)

2HO2 → O2 + H2O2 Eq. (7.5)

H2O2 + e− →∙ OH + OH− Eq. (7.6)

O2− + HO ∙→ 1𝑂2 + OH

− Eq. (7.7)

HO2 + O2

− → 1O2 + HOO− Eq. (7.8)

2HO2 → 1O2 + H2O2 Eq. (7.9)

∙ OH/1O2 + OII OII+⁄ → ⋯ → byproducts Eq. (7.10)

C h a p t e r 7 P a g e | 119

Figure 7. 12: Illustration of OII degradation by MeSrCoO catalysts at room temperature under

dark conditions.

However, the mineralization of OII using the MeSrCoO catalysts did not occur. These results suggest

that electron generation by the breakdown of the azo bonds was not enough to generate a large number

of hydroxyl radicals which are the active reactive species for the mineralization reaction to generate

CO2 (Eq. 7.10). Similar performance was also observed for a copper containing MgSrCuO [41],

though this type of catalyst was not as stable as the cobalt containing MgSrCoO catalyst for OII

degradation under identical conditions. In some cases, such as nickel containing CaSrNiO, the OII

degradation kinetics reached 97% in 5 min [27], though this catalyst was very unstable due to the

oxidation of the active phase Ni2+ to the non-active Ni3+. MeSrCoO catalysts do not undergo oxidation

reactions, thus further confirming the stability of these catalysts. Due to the stability of cobalt

containing MeSrCoO, these catalysts have potential application in degrading OII dyes from textile

industrial sector.

7.7 Conclusions

MeSrCoO (Me = Ba, Ca and Mg) were tested as heterogeneous catalysts for the degradation of OII

dyes under dark ambient conditions (e.g., without chemical additives and energy input). All the

catalysts degraded OII efficiently, reaching ~80% degradation in 4 h, though CaSrCoO degradation

kinetics was lower due to minor surface adsorption. Interestingly, the degradation rate of a non-

substitute catalyst SrCoO was similar to those of MeSrCoO. However, cycling experiments showed

that MeSrCoO samples reached good stability whilst SrCoO continued to decay. Hence, the partial

substitution of Sr with Me (Ba, Ca and Mg) conferred stability to the catalyst also confirmed by XRD

and XPS results. The OII degradation occurred mainly at the azo bonds via releasing electrons and

generating intermediate by-products. The primary reaction mechanism was attributed to catalytic

surface properties of MeSrCoO. The generation of electrons resulted in the formation of reactive

radical species, which reacted further with OII or by-products, and formed more intermediate by-

products. Mineralisation reactions did not occur, suggesting the amount of electrons generated from

breaking down the azo groups leading to the formation of hydroxyl radicals and singlet oxygen were

used mainly to generate intermediate by-products instead of CO2.

Acknowledgment

C h a p t e r 7 P a g e | 120





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8 Conclusions and recommendations

8.1 Conclusions

This thesis focused on the development of heterogeneous metal oxide catalysts for dye degradation

in dark ambient conditions without external irradiation sources or chemical input. Particularly, this

thesis endeavored to investigate the A-/B- site metal substitution effect on the catalytic efficiency,

stability and recyclability of AA’BB’O based metal oxide catalysts. With this goal in mind, various

metal cations were systematically investigated to probe their role on the catalytic behavior of the

resultant catalysts. To ascertain the specific catalytic role of as-obtained materials, the single/mixed

component(s) of metal oxide catalysts were tested under identical conditions.

The first contribution of this thesis was the finding that Cu (CaxSr1-xCuO3) and Co (Me0.5Sr0.5CoO3,

Me=Ba, Ca, Mg, and Sr) based perovskites were very effective for discoloring OII by breaking -N=N-

bonds in dark ambient conditions without external energy or chemical input. The fast discoloration

occurred via OII contacting with catalyst surface, inducing the generation of hydroxyl radicals or

single oxygen that accounted for OII further degradation. Both CaxSr1-xCuO3 and Me0.5Sr0.5CoO3

endured long-term stability after at least seven continuous cycles with catalytic activity loss less than

10%. It was found CaxSr1-xCuO3 with higher Ca content was slightly more effective due to the

compounding effect of composite oxides and pH. Co-based perovskites demonstrated the equimolar

substitution of Sr with alkaline metals (Ba, Ca and Mg) conferred surface stability properties

compared with unsubstituted SrCoO3, leading to different degradation kinetics for BaSrCoO,

MgSrCoO, and CaSrCoO toward OII degradation in dark ambient conditions.

The second contribution of this thesis was the identification of catalytically active role of nickel phase

in Ca0.5Sr0.5NiO (CSN) oxides for OII discoloration. The Ni2+ phase was found to account for the fast

discoloration of OII with 97% OII decolorized within 5 min. The fast degradation kinetics was

associated with dual electron-generating pathways (e.g., the breakdown of azo groups and the

oxidation of Ni2+ to Ni3+) that endowed the facile generation of powerful HO•. Moreover, the

irreversible oxidation of Ni2+ into Ni3+ after reaction resulted in the deactivation of CSN in the cycling

experiment.

The third contribution of this thesis was that the synergy effect of B-site cations (Ni and Cu) in

nominal Ca0.5Sr0.5NixCu1-xO (CSNC) enhanced the catalytic performance in terms of both

mineralization ability and recyclability. CSNC catalysts exhibited high recyclability over 15 runs with

catalytic efficiency of ~90%. TOC removal of OII by CSNC after 120 min was higher (54%) than

that by CSC (46%) and CSN (10%). The optimized degradation efficiency was obtained when the

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ratio of Ni to Cu was 1:1. The contact of OII on CSNC surface was a prerequisite for the subsequent

generation of reactive species that accounted for OII degradation.

Besides the abovementioned major contributions, there was further important finding in this thesis.

Nominal Me0.5Sr0.5CuO (Me= Mg and Ce) and SrCuO were prepared via EDTA-citric acid

complexation method and evaluated for OII degradation in dark ambient conditions. The resultant

materials were a mixture of MgO/CeO2 and strontium copper oxides. It was found the composite of

metal oxides demonstrated superior catalytic performance than any single component. Specifically,

the presence of Mg oxide and Ce oxide enhanced the catalytic efficiency of SrCuO from 90.5% to

97% in the first 30 min. This suggests the benign synergy existed in the metal oxide composite for

OII degradation. The reduced catalytic activity accompanied by the irreversible reduction of Cu2+ to

Cu1+ confirmed the catalytic active role of Cu2+ for OII degradation.

This thesis clearly reports a number of metal oxide compounds with the desirable properties to

degrade organic dyes under dark conditions. In addition, these catalysts did not require any chemical

additives and energy/irradiation input as it happens to the majority of the APO processes. Therefore,

the catalysts developed in this thesis work are desirable in practical application as it can greatly

simplify the engineering design of plants whilst cutting operating costs at the same time. As the

reported catalysts in this project demonstrated both high catalytic activity and excellent recyclability

meanwhile dispensing the requirement of additional chemicals and energy input, they can be

promising candidate materials in cost-effective AOPs for the remediation of organics-contaminated

waterbodies.

8.2 Recommendations

Although this thesis shed further insights into the knowledge of substitution effect on the catalytic

performance of heterogeneous metal oxide catalysts that work in dark ambient conditions without

external irradiation sources or chemical input, the following recommendations are made for future

work.

1. Further investigation should be conducted to testify the applicability of synthesized

heterogeneous catalysts for the degradation of other organic pollutants (e.g., Methyl Orange,

Methylene Blue, Rhodamine B, and phenol) under identical.

2. The fast degradation kinetics of CSN makes Ni promising element in the fast discoloration of

dye-containing wastewaters. Nevertheless, the recover ability of the active phase (Ni2+) is

worthy further investigation probably by adding additional sacrificial reagents (i.e. electron

donors).

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3. It is vital to improve the TOC removal further probably by increasing temperature or by

employing light/ultrasound irradiation. It is also interesting to probe whether the addition of

H2O2/O3/PS/PMS can result in higher TOC removal.

4. Non-metal doping (e.g., N and S) to replace O partially or completely in ABO3 might

introduce some unexpected properties related to catalytic performance.

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