Doreen Nabaho Cchange&Catsa9 Nov

Enter title of presentation here

Enter author’s names here

If University Logo is required, please adjust size to fit below the line, e.g.

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


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


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


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


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


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


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


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


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


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


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


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

Insert University Collaboration Logo’s Here, e.g.

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