doreen nabaho cchange&catsa9 nov
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Presention at the Catalysis Society of South Africa (CATSA) 2010 in Bloemfontein South AfricaTRANSCRIPT
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Hydrogen Spillover Effect in the Fischer-Tropsch Synthesis
The role of platinum as a promoter on cobalt-based catalysts
Doreen Nabaho, Eric van Steen, Michael Claeys
Centre for Catalysis Research, Department of Chemical Engineering
University of Cape Town, Private Bag X3, Rondebosch, 7701, RSA
E-mail: [email protected]
Dry M.E. Hydrocarbon Processing 61 (1982) 121 - 124.
Fischer-Tropsch in slurry reactor: novel but expensive cobalt catalyst
•Crucial to maximize catalyst performance with respect to catalyst life and product selectivity•Prior to optimisation of catalyst, rigorous understanding of fundamental behaviour needed
INTRODUCTIONBACKGROUNDEXPERIMENTAL METHODOLOGYRESULTSCONCLUSIONS
Low Temperature Fischer-Tropsch:Cobalt-based catalyst
Goal: to elucidate the behaviour of the cobalt-based Fischer-Tropsch catalyst
Goal: to elucidate the behaviour of the cobalt-based Fischer-Tropsch catalyst
Composition of patented cobalt-based catalysts
INTRODUCTION
BACKGROUNDEXPERIMENTAL METHODOLOGYRESULTSCONCLUSIONS
Oxide Promoter1-10wt%
Noble metal promoter<0.1wt%
Cobalt 15-30wt%
Catalyst support
Oukaci R., A.H. Singleton and J.G. Goodwin jr. Applied Catalysis A: General 186 (1999) 129-144
• Need reduction promoters (e.g Platinum) to enhance Co3O4 reduction!
• Cobalt catalysts contain many components → complex + difficult to study1
• Reduction promotion + other effects of Platinum suggested to ↑ rate ALTHOUGH exact mechanisms poorly understood
Cobalt oxide (Co3O4) crystallites
10-20nm
Metallic cobalt crystallites(Co0)
Reduction before Fischer-Tropsch
synthesis
(H2 )
Al2O3 support particle~150μm
Cobalt-based Fischer-Tropsch catalyst
INTRODUCTION
BACKGROUNDEXPERIMENTAL METHODOLOGYRESULTSCONCLUSIONS
1Morales, F., and B.M. Weckhuysen: Catalysis, Vol. 19. Royal Society of Chemistry, Cambridge.
Effects of Promoters1
Alumina
CoPt
How does platinum increase Fischer-Tropsch rate?
INTRODUCTION
BACKGROUNDEXPERIMENTAL METHODOLOGYRESULTSCONCLUSIONS
Electronic Promotion•Increases intrinsic cobalt activity
Requires physical contact between cobalt + promoter
•Changes in electronic environment of cobalt
Structural Promotion •No effect on intrinsic cobalt activityDoesn’t require physical contact
between cobalt + promoter?•Increase surface area of Co0 , thus increasing rate
?↑ number of ‘active sites’↑ number of ‘active sites’
↑ activity of each ‘active site’
↑ activity of each ‘active site’
Platinum increases Co0 after reduction of Co3O4 → more active sites
Platinum increases Co0 after reduction of Co3O4 → more active sites
1Morales, F., and B.M. Weckhuysen: Catalysis, Vol. 19. Royal Society of Chemistry, Cambridge.
Co3O4 + 4H2 → 3Co + 4H2O
Cobalt oxide (Co3O4)
Metallic cobalt crystallites(Co0)
Reduction in H2 at 300-450oC
Alumina support
Platinum
higher rate increases vs. unpromoted catalyst due
to increase in Co0
higher rate increases vs. unpromoted catalyst due
to increase in Co0
Limited proof of hydrogen spillover on platinum promoted cobalt-based catalysts available in
reviewed literature
Limited proof of hydrogen spillover on platinum promoted cobalt-based catalysts available in
reviewed literature
Q: BUT how exactly does platinum increase metallic cobalt? A: Often postulated to be due to a HYDROGEN SPILLOVER MECHANISM
Mechanism of structural promotion with platinum
INTRODUCTION
BACKGROUNDEXPERIMENTAL METHODOLOGYRESULTSCONCLUSIONS
Promotion with platinum: Hydrogen spillover mechanism
Another surface could be metal and/or support
Possible routes of spillover hydrogen include: • Metal → Metal• Metal → Support• Metal → Support → Metal• Metal → Support - Support → Metal
INTRODUCTION
BACKGROUNDEXPERIMENTAL METHODOLOGYRESULTSCONCLUSIONS
Definition:The dissociative chemisorption of hydrogen molecules on metal surfaces to form adsorbed hydrogen species (H*) followed by their migration to another surface.1
1Roland U., Braunschweig T. and Roessner F. Journal of Molecular Catalysis A: Chemical 127 (1997), 61-84
1Claeys M. and van Steen E.: “Basic studies in Fischer-Tropsch Technology” (A.P. Steynberg, M.E. Dry, Eds.), Studies in Surface Science and Catalysis 152 (2004), 601-680.
2 distinct incidences in which the cobalt catalyst is exposed to H2
1. During reduction: Co3O4 + 4H2 → 3Co + 4H2O
Alumina
metallic
cobalt
gaseous H2
molecules
• High mobility of chemisorbed hydrogen1
• Chemisorbed hydrogen can move between crystallites• Hydrogen adsorption and consumption does not need to occur at the same site
The Fischer-Tropsch Synthesis: Hydrogen spillover
INTRODUCTION
BACKGROUNDEXPERIMENTAL METHODOLOGYRESULTSCONCLUSIONS
Pt
Could dissociated hydrogen FROM PLATINUM spillover TO COBALT during the
Fischer-Tropsch Synthesis?
Could dissociated hydrogen FROM PLATINUM spillover TO COBALT during the
Fischer-Tropsch Synthesis?
Cobalt oxide
Platinum promoter may therefore be expected to improve catalyst activity as follows:1.During reduction: ↑ metallic cobalt and thus number of active sites2.During Fischer-Tropsch synthesis: Spilling hydrogen over to cobalt and thus increasing reaction rate
2. During Fischer-Tropsch synthesis: nCO + 2nH2 → n(CH2)n + nH2O
‘Hybrid’ catalysts were used to eliminate Pt-Co physical contact
cobalt oxide (Co3O4)
alumina support
platinum promoterplatinum promoted
cobalt catalyst
platinumcatalyst
cobaltcatalyst
Experimental methodology: Hybrid catalysts
INTRODUCTIONBACKGROUNDEXP’TAL METHODOLOGYRESULTSCONCLUSIONS
Experimental Methodology: Catalysts
INTRODUCTIONBACKGROUNDEXP’TAL METHODOLOGYRESULTSCONCLUSIONS
Co/γ-Al2O3
‘Hybrid’Pt-Co
‘Hybrid’Pt-Co
Pt/γ-Al2O3
Prepared by slurry impregnationPrecursors: Co(NO3)2 Pt[(NH3)4]Cl2 Pt:Co
mass ratio = 1:40
‘co-impregnated’Pt-Co/γ-Al2O3
Pt/Al2O3
50nm100nm
Pt-Co/Al2O3
200nm
Co/Al2O3
Catalyst characterisation INTRODUCTIONBACKGROUNDEXPERIMENTAL METHODOLOGY
RESULTSCONCLUSIONS
Co3O4 clustersdcluster = 150-200 nmdcrystallite ,TEM= 7-9 nm
dcrystallite ,XRD= 9.3 nm (Co/Al2O3) & 9.6nm (Pt-Co/Al2O3)
Pt crystallitesdcrystallite,TEM = 1-2 nmdcrystallite,COchem = 1.9 nm
Prepared loading (wt%) Co/Al2O3 Pt-Co/Al2O3
Cobalt 19.4 19.4Platinum - 0.5
Pt/Al2O3
-0.5
Low Temperature Fischer-Tropsch Synthesis
220oC, 20 bar, H2:CO = 2•Turn Over Number•Product distribution
Low Temperature Fischer-Tropsch Synthesis
220oC, 20 bar, H2:CO = 2•Turn Over Number•Product distribution
Investigating H2 Spillover in the Fischer-Tropsch Synthesis
During Co3O4 reduction During Fischer-Tropsch synthesis
Temperature Programmed Reduction
• Obtain Co3O4 reduction peak positions
• Degree of reduction 12 hrs, 350oC, 1 bar H2
Temperature Programmed Reduction
• Obtain Co3O4 reduction peak positions
• Degree of reduction 12 hrs, 350oC, 1 bar H2
Experimental Methodology:Summary of experiments
INTRODUCTIONBACKGROUNDEXP’TAL METHODOLOGYRESULTSCONCLUSIONS
H2 Spillover during reduction: Temperature Programmed Reduction (TPR)
Co3O4 reduction studied with TPR
INTRODUCTIONBACKGROUNDEXP’TAL METHODOLOGY
RESULTSCONCLUSIONS
MILL
Co3O4 + 4H2 → 3Co + 4H2O
Catalyst Characterisation: XRD
INTRODUCTIONBACKGROUNDEXPERIMENTAL METHODOLOGY
RESULTSCONCLUSIONS
Co/Al2O3
(milled)
Hybrid Pt-Co(milled)
Pt-Co/Al2O3
Radiation source: Co K-α
Co/Al2O3
milled
Hybrid Pt-Comilled
Pt-Co/Al2O3
INTRODUCTIONBACKGROUNDEXPERIMENTAL METHODOLOGY
RESULTSCONCLUSIONS
NB: Prior to Fischer-Tropsch• Degree of reduction (% metallic cobalt) determined after 12h reduction in H2 @
350oC and 1 bar• Degree of reduction determined with milled catalysts (Co/Al2O3 and Pt-Co hybrid)• Milled catalysts used for Fischer-Tropsch testing
H2 Spillover during reduction: Temperature Programmed Reduction (TPR)
Degree of Reduction (%Co0)
Co/Al2O3 22.2
Pt-Co hybrid
87.9
Pt-Co/Al2O3 97.8
Hyd
roge
n co
nsum
ption
Increasingly easier to reducewith Pt
closer to Co
Increasing trend in agreement with
hydrogen spillover theory
IllustrationTurn Over Number
@80 h (mmolCO/gCo
.s)
Co/Al2O3 3.35 X 10-2
Pt/Al2O3
n/a
Hybrid Pt-Co (Pt/Al2O3 + Co/Al2O3)
3.53 X 10-2
Pt-Co/Al2O3 3.15 x 10-2
Hydrogen Spillover during FT:Turn over numbers & product analysis
INTRODUCTIONBACKGROUNDEXPERIMENTAL METHODOLOGY
RESULTSCONCLUSIONS
Reaction conditions: 220oC, 20 bar, H2:CO = 2:1
C1+C2
Reference gas: cyclohexane
Platinum is hardly active under these Fischer Tropsch conditions!
Could surface be covered with carbon monoxide and thus hydrogen adsorption inhibited?1
FID Chromatogram of Pt/Al2O3
1McKee D.W. Journal of Catalysis 8 (1967), 240-249
Hydrogen Spillover during FT: Olefin content in product
INTRODUCTIONBACKGROUNDEXPERIMENTAL METHODOLOGY
RESULTSCONCLUSIONS
Co/Al2O3 Pt-Co hybrid Pt-Co/Al2O3
olefin content in linear hydrocarbons and 1-olefin content in linear olefins (@ XCO=20%) similar and thus unaffected by platinum
• Platinum is known as a hydrogenation catalyst• Addition of platinum expected to increase secondary hydrogenation and thus lower
olefin content
INTRODUCTIONBACKGROUNDEXPERIMENTAL METHODOLOGY
RESULTSCONCLUSIONS
Hydrogen Spillover during FT: Anderson-Schulz-Flory distribution
Determined from C4-C10
Hydrogen Spillover in the Fischer-Tropsch Synthesis: A summary
Fischer-Tropsch so far: Platinum is hardly active under these Fischer-Tropsch conditions & also doesn’t
affect product selectivity in promoted or unpromoted catalysts
Platinum inactivity likely due to strong carbon monoxide adsorption
→platinum surface not accessible to hydrogen making spillover unlikely
Further work: in-situ XRD to obtain better Co0 estimates for
Turn Over Number, cobalt oxide reduction peak identification,
characterisation of spent catalysts.
Contribution to Catalysis Research:- understanding spillover effects during catalyst reduction and Fischer-Tropsch synthesis- Add to limited understanding of promoters in Cobalt-based catalysts to enable better catalyst design
INTRODUCTIONBACKGROUNDEXPERIMENTAL METHODOLOGYRESULTS
CONCLUSIONS
Platinum increases the Fischer-Tropsch rate by increasing degree of reduction
of cobalt catalysts.
Mechanism suspected to be via hydrogen spillover and TPR results
obtained so far point to this
ACKNOWLEDGEMENTS
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Department of Science and Technology
Questions &
Comments?
Is H2 involved in the Rate Determining Step of the FT Reaction?
Kinetic expression: rate depends on PH2 & thus surface coverage of H2 on Co
Van Helden (2008) : DFT to study CO & H2 co-adsorption on Fe
• Sharp increase in dissociation barrier of H2: CO coverage increased from θ=0 to 0.5, EaH2-diss from 30 to 130 kJ/mol
• Fewer sites for H2 dissociation with CO: on precovered surfaces as seen under FT, even A would be lowered as fewer active sites & thus fewer
dissociation pathways would be available to H2
H2 may not be freely available under FT: van Santen and Niemantsverdriet (1995) stated that small A may make dissociative processes the RDS in catalytic cycles
OnHCHnnHnCO n 222 )(2 2)1(
2
CO
HCOFT bP
PkPr
)2(
2RTdissEaH
dissH eAk
?
H2 probably in RDS
Gold Promoters for Cobalt-based Catalysts
INTRODUCTION
BACKGROUNDHYPOTHESES EXPERIMENTAL METHODOLOGYCURRENT RESULTSFUTURE WORK
In order for spillover H2 to increase FT activity, H2 must be involved in rate determining step