activation of catalysts

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Activation of catalysts

29th

April 2013Daniel Casas Orozco

Preparation of Catalysts

Environmental Catalysis Group

Engineering FacultyUniversidad de Antioquia


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Outline

1. Objectives

2. Introduction

3. Active-phase/support interactions

a) Types of solid reactionsb) Types of interactions

4. Activation by calcination

5. Activation by reduction

6. Reduction-sulfidation7. Conclusions

8. References

Activation of catalysts 2


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Objectives

Describe the most used methods of catalystactivation

Identify the differences and the aplicability ofthe available methods

Ilustrate the effect of activation procedure onthe activity and morphological characteristicsof the final catalyst



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Introduction

Activation1*


Transformation of a

solid precursor to

the material

immediately active

for the desiredreaction

defined as the

With

impact on Activity

Selectivity

Resistance todeactivation

4

1. Ertl, G., Knozinger, H., Schuth, F., & Weitkamp, J. (2008). Handbook of Heterogeneous Catalysis (2nd Edition., p. 4270).

Wiley-VCH

* If not specified, all the information taken from this reference


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Typ

icalexamples

Transformation ofhydroxides to oxides

Reduction ofmetal oxidesto dispersed metal

particles

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Running-in period:

Supported phase has to catalyze the desiredreaction for a certain time in order to reachstable activity and selectivity

In situ transformation for reactions such as:

Vanadium phospate catalysts:Oxidation of benzeneor butane to maleic acid

Hydrodesulfurization (HDS) catalysts:

Supported oxides sulfides (caused by H2/H2Sfeed)



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Active-phase/support interactions


FACTORSAFFECTING

ACTIVATIONPROCESS

Dispersion state ofthe precursor

Solid-statereactions

Interaction withthe active phase

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Change in reactivity is expected if the supported

phase reacts with the carrier

Formation of new, less reactive compounds

Hindering of active species



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Weak forces

Van der Waals hydrogenbonds interactions

Graphite and silica exhibitweak forces with somesupported materials

Electronic

interaction Electronic junction (not

chemical bonds involved)

Electron density canenhance selectivity


Transition layer (I)

Formation of crystallites

Formation of patches or

monolayers (Mo) Formation of a bi-layer

(Co-Mo, Ni-Mo oxide onalumina)

Transition layer(II)

Solid solutions of

supported elements Compounds with ill-

definedsthoichiometry


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MoO3 surface interaction (evaluating support)

Lowinteraction

-Sb2O4

Mediuminteraction

Co3O4

SiO2 TiO2

Highinteraction

-alumina



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Low interaction

Low load (0-8%wt)MoO3 crystallites

Medium interaction

Medium load (8-13%wt)

Polymolibdates (PMA)on the surface as

patched monolayers

High interaction

High loading (13-20%wt)

Silicomolybdic acid

(SMA)

MoO3 surface interaction (on SiO2)


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Activation by calcination

High-temperature treatment in air is often2

The last step in producing oxide catalysts

The next to the last step in producing metal or metalsulfide catalysts

Used to decompose and volatilize the various catalystprecursors formed in preparation

Hydroxides Nitrates

carbonates


2. Farrauto, Robert; Barholomew, Calvin. (1997). Fundamentals of Industrial Catalytic Processes. Blackie Academic &

Professional. New Jersey. 754 p.


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Typically conducted in air at

2

300 500 C for inorganic carriers


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400 500 C

2+ 23

>500 C

Bulk nickel aluminates

Reductionconditions

Very high temperatures

Sintering of support or metal species

Catalystproperties

Loss of porosity Loss of surface area

Higher thermal stability



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Preparationmethod

Co-deposition of Pt and Fe salts oncarbon carrier

Activationtreatment

400 C air oxidation

Finalcharacteristcs

Poorly dispersed, separate Pt and Fe

metallic phases (segregation)



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Knowledge about interaction between active phaseand support allows the control the calcination step,

e.g.:

Favoring the spreading of precursors

Use of additives

Inhibiting promoting the formation of solidsolutions doping the support

Choosing temperature ramps or modifiedcalcination atmospheres Desired oxidation state

Oxide structures



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Activation by reduction2

Final step of unsupported and supported catalystpreparation

Converts oxides and/or catalyst precursor salts tothe corresponding metal

H2, CO, syn gas and hidrazine environments

Sometimes, reduction from oxychloridecomplexes takes place (platinum and some noblemetals)

Direct reduction without intermediate oxide canlead to a higher dispersion





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Special concerns in reduction procedures2

Purity of reducing agents: removal of oxygen,

sulfur, water and hydrocarbons. Removal of oxygen: high-surface area Pt catalyst

Drying and hydrocarbon removal: molecuar sieves

De-sulfuring: ZnO catalysts





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Temperature:

Optimization for high metal dispersion,

surface area and extent of reduction (given a

metal loading and support)

Typical ranges are 250- 350 C: noble metals (2-6 h)

350-500 C: base metals (350 500 C)

Calcination temperature prior to reduction



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Precursor loading

Supported base metals :High metal loading (15 25 %): more easily reduced catalysts than lowloading (


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Heating rate and hydrogen space velocity:

Lower heating rates: 1-5 C/min

Space velocity: 2000-3000 h-1

Allow water withdrawal (inhibit reduction and

facilitate metal species transport



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Reduction-sulfidation

Catalyst preparation method for hydrotreating

reactions

Hydrodesulfurization

Hydrogenation

Hydrodeoxygenation

Hydrodemetillation

Hydrocracking



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Main active species Group VI metals: molybdenum, tungsten

Nickel and/or cobalt, iron

Environmental concerns make mandatory the use of

noble metals for complete hydrodesulfurization andde-aromatization of gasolines

Formation of active sulfided species necessitatesa reduction and a sulfidation of oxide precursor



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Typically, involves exposing the catalyst to and

H2S/H2 mixture at high temperature

Used conditions: previously calcined catalysts

subjected to

350-400 C in 10 % H2S/H2 mixture (1 atm):laboratory applications

2-3 % H2S/H2 mixture (higher pressure) for

industrial catalysts

Industrial practice: hydrogen and sulfur-containing

feed (spiked petroleum fraction)



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Role of procedure parameters

Activation temperature and sequence steps



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Catalyst preparation: deposition-precipitation

method with urea

Metal precursor: HAuCl4.3H2O (Gold

Trichloride Trihydrate)

Catalyst activation

U reactor

2 C/min, 1mL/min/mgprecursor (hydrogen or air)

Catalysts tested on CO oxidation reaction

100 mL/min gas (1% CO, 1% O2, 98% N2)


Titanium butoxide


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Stirring for 1

h

Stirring for

30 min

Heating to 90 Cand reflux for 15

h

Water

Nitric acid

Titanium butoxide

Stirringfor 1 h

HAuCl4.3H2 sltn

Urea sltn

Water

Heating to 80 C

mantained for

15 h

Centrifugation,washing and

drying

Titania (rutile)

Centrifugation,

washing and

drying

FINALCATALYST

Activation

Air or H2

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Results

Average particle size does not increase inhydrogen treated materials: reduction ofneigboring support sites (oxygen and titanium as

pinning centers)

In air-treated materials, poor interaction betweengold particles and support leads to growing gold

particles. Conversion is diminished in thesematerials



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Catalytic activity in CO

oxidation reaction anddeactivation of catalysts

Caused by carbonate-

poisoning Aging in atmospheric

conditions

Typical carbonate bandsfollowed by infraredspectroscopy (1710 cm-1peaks)


Samples activated in H2


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Hydrogen-treated Air-treated


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Conclusions

Interactions between active phase and support are a keyconcept in choosing an appropriate method of activation

Several parameters must be adjusted in activationprocedure in order to increase final catalyst activity. Designof experiments can be an important tool to systematicallyapporach the study of activation conditions

A compromise between catalyst stability and activity is

found in the reviewed methods, factor which must be takeninto account in the final design of the material



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Calcination method is usually not as a uniqueactivation procedure but in conjunction with

other activation steps

Reduction-sulfurization methods are one of

the most applied activation methods in

petrochemical and large industrial catalytic

processes



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References

1. Ertl, G., Knozinger, H., Schuth, F., & Weitkamp, J. (2008). Handbookof Heterogeneous Catalysis (2nd Edition., p. 4270). Wiley-VCH.

2. Farrauto, Robert; Barholomew, Calvin. (1997). Fundamentals ofIndustrial Catalytic Processes. Blackie Academic & Professional.New Jersey. 754 p.

3. Bokhimi, X., Zanella, R., Morales, A., Maturano, V., & Carlos, A.(2011). Au / Rutile Catalysts: Effect of the Activation Atmosphereon the Gold- Support Interaction. Journal of Physical Chemistry C,115, 58565862.

4. Bokhimi, X., & Zanella, R. (2007). Crystallite Size and Morphology

of the Phases in Au/TiO2 and Au/Ce-TiO 2 Catalysts. Journal ofPhysical Chemistry C, 111, 25252532.

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activation of catalysts

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