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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
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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*
Activation of catalysts
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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Activation of catalysts
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
Activation of catalysts
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
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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
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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
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>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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2. Farrauto, Robert; Barholomew, Calvin. (1997). Fundamentals of Industrial Catalytic Processes. Blackie Academic &
Professional. New Jersey. 754 p.
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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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2. Farrauto, Robert; Barholomew, Calvin. (1997). Fundamentals of Industrial Catalytic Processes. Blackie Academic &
Professional. New Jersey. 754 p.
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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)
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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)
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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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