[PPT] - of Pt- and Pd-based catalysts for benzene hydrogenation Maarten K. PowerPoint Presentation

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Methusalem Advisory Board meeting, Ghent, 17 June 2011

First-principles based design

f Pt- and Pd-based catalysts

for benzene hydrogenation

Maarten K. Sabbe, Gonzalo Canduela, Marie- Françoise Reyniers, Guy B. Marin

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Introduction: benzene hydrogenation on Pt(111)

 Regressions to experimental data suggest other dominant path (Thybaut): DPregressed  Experimental work: no consensus on the rate determining step  Entropy contributions difficult at cluster level: include using periodic calculations Current status of computational models: dominant path proposed based on Pt22 cluster calculations (DPcluster)

Electronic reaction barriers BP86/DZ on Pt22 cluster of Pt(111) Saeys.M J.Phys.Chem.B, 109,2064- 2063 (2005)

Pt22 cluster

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Benzene hydrogenation: applications in hydrotreating, hydrocracking, cyclohexane production

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Aim

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Pt(111)  Evaluate reaction barriers based on periodic calculations  Calculate entropy contibutions and rate coefficients  Perform reactor simulations and compare yields to experiment Pt- and Pd-based catalyst design  evaluate stability and hydrogenation reactivity of Pt3M alloys and surface alloys (M= Ag,Au,Cu,Fe,Co,Ni,Pd)  Pd: start design of Pd-based catalysts by developing a first principles kinetic model on Pd(111)

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Computational approach

Methusalem Advisory Board meeting, Ghent, 17 June 2011

3 x 3 unit cell used to model the Pt(111) surface: 9 atoms/layer
moderate lateral interactions: coverage degree ≈ 30%

Unit cell Top view Unit cell Side views

Vacuum layer 10.6 Å Relax 2 upper layers Fix 2 bottom layers Lattice constant: 4.011Å

Artifical dipole layer

Periodic structure

Surface with unit cell indicated

PW91 functional (GGA)
plane waves; PAW; 400 eV; no spin polarization (for clean Pt)
5 x 5 x 1 k-point Monkhorst-Pack grid
first order Methfessel-Paxton smearing, σ=0.20 eV
TS determination: NEB, followed by DIMER calculation

DFT (VASP)

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Part I: Hydrogenation of benzene on Pt(111): from

molecule to reactor

Reaction network: electronic barriers
Entropy contributions
Rate coefficients
Compare reactor simulations to experiment
Part II: Catalyst-descriptor based design of

hydrogenation catalysts

Outline

Methusalem Advisory Board meeting, Ghent, 17 June 2011

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Based on ΔEel: no clear dominant path

Pt(111) network: electronic reaction barriers

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Electronic energy barriers ΔEel forward reverse

DPcluster,135THB dominant path on Pt22 cluster level MEPperiodical,123THB minimum energy path (periodical calculations)

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Entropy contributions are important for K and k

Methusalem Advisory Board meeting, Ghent, 17 June 2011

j i ij

q q E H

2

) q ( ) q ( Hq q 2 1 2 1

H 3 2 2

E q m

N i i i

Immobile species: Harmonic frequency analysis

vibrational Schrödinger equation Kinetic energy Potential energy requires knowledge of Hessian H Hessian

qi= Δx, Δy, Δz around equilbrium geometry

N i T k h B i

T B k i h B i

e e T k h R S

3 1 HO rovib,

1 ln 1 Vibrational contribution to entropy

Mobile species

free rotation and/or free translation  Replace 2 ‘translational’ and 1 ‘rotational’ frequency A: 10-19 m² for H*; 5 10-19 m² for hydrocarbon species identify mobility of surface species: calculate diffusion barriers

2 2 1 ) , ( ' ln

trans surf transl,

T A q R S 2 1 ) ( ln

, Z rot,

T q R S

Z rot

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Entropy contributions: mobile mode identification

Methusalem Advisory Board meeting, Ghent, 17 June 2011

All species immobile at 450 K except H and cyclohexane (barrier < 9 kJ/mol)

Species + motion ΔE° kJ/mol Hydrogen (top to top) 9.2 Hydrogen (top to hollow) 11.6 Benzene (hollow to bridge-rotation) 21.1 135 THB (translation) 233.0 1235 THB (rotation) 99.8 Cyclohexyl (translation) 98.5 Cyclohexyl (rotation around C-Pt bond) 12.7 Cyclohexane (rotation) 5.9

2 4 6 8 10 E-Etop kJ/mol Translational Coordinate

H* top to top diffusion (NEB) 135-THB translation (diffusion barrier 233 kJ/mol) Determine transition states for diffusion (NEB+dimer) Initial state Final state

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no clear dominant path  Evaluate full reaction network in simulation

Rate coefficients indicate dominant path

Methusalem Advisory Board meeting, Ghent, 17 June 2011

rate coefficients k (s-1) forward reverse

DPcluster dominant path at Pt22 cluster level MEPperiodical minimum energy path (periodical calculations) DPperiodical,k dominant path based on rate coefficients (periodical calculations)

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Experimental data: Berty set-up

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Berty-reactor: Gas phase CSTR (intrinsic kinetics) Input variables (43 experiments) Benzene Feed (mol s-1) 17 10-6 -57 10-6 T (K) 425-500 P(atm) 10-30 pH2/pB 5-11 Wcat(g) 1.29 -1.8 W/Fbenzene (kgcat s-1mol-1) 22-74 Catalyst: Pt/ZSM-22 (0.5 wt% Pt) Conversion: 9-85%

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Estimated parameters

H2 adsorption enthalpy: strongly coverage

dependent  Estimation of this parameter required

General reduction of activation energy:
calculated Ea larger than experiment
temperature dependence too strong

without reduction of Ea

Reactor simulation approach

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Simulations

CSTR model
Levenberg-Marquardt for parameter

estimation

Goal function=Σ(simulated product yield-

exp.observed)2

K(T) and k(T) with mobile H* and

cyclohexane*, other species are considered immobile

catalyst model: 0.008 active sites/kgcat
PSSA (reaching steady state using

transient solver)

W R F F dt dF

i i i i * * i i

R dt dC

* *

R dt dC

Transient continuity equations: Gas phase species: Surface species: Free sites:

Podkolzin et al., JPCB, 105:8550 (2001)

Ea,i = Ea,i,AbInitio + ΔEa,parameter

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Full network: reactor simulation results

Methusalem Advisory Board meeting, Ghent, 17 June 2011

K(T) and k(T) for mobile H* and cyclohexane* (other immobile)
surface coverage ≈ 1 => take ΔHads(benzene)= -66.1 kJ mol-1 (calculated value)

Estimating only ΔHH2: yields still too low

temperature dependence too strong

without reduction of Ea

Estimate Eareduction

10 20 30 40 50 10 20 30 40 50 Simulated product yield (10-6 mol/s) Experimental product yield (10-6 mol/s)

ΔHads,H2

46.1 ± 2.2 kJ/mol

ΔEa

14.6 ± 2.7 kJ/mol

F 428

Simulation Estimate ΔHH2 and ΔEa

Ea,i = Ea,i,AbInitio + ΔEa,parameter Cyclohexane yield parity plot

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Full network: reaction path analysis

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Electronic energy barriers ΔEel forward reverse 20 bar, 225 °C, 1.8 gcat, 0.13 mol/h benzene, (H2/B)in=5 W/FB=48.4 kgcat s/mol

Clear pathway for step 4, 5 and 6
In step 2 and 3 equilibration between intermediates

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Conclusions and prospects

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Conclusions

No clear dominant path based on electronic energies for full network
Activation energies need to be reduced in order to obtain quantitative

agreement to experimental values

With 2 parameters, a reasonable agreement to experimental yields is
btained

Future work

Multiscale modeling: development of first-principles based kinetic

Monte Carlo simulation tools to assess the validity of the mean field approximation under industrially relevant operating conditions

Introduce method for clean Pt catalysis
If results differ significantly from mean-field results, apply on

bimetallic catalysts as well

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Part I: Hydrogenation of benzene on Pt(111): from

molecule to reactor

Part II: Catalyst-descriptor based design of

hydrogenation catalysts

Pd catalysts
Pt3M catalysts
Conclusions & prospects

Outline

Methusalem Advisory Board meeting, Ghent, 17 June 2011

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→ similar MEP as for Pt(111) Future work: entropy contributions, rate coefficients and multiscale modeling of the reactor

Pd-catalyzed hydrogenation

Methusalem Advisory Board meeting, Ghent, 17 June 2011

First step in design of Pd-based catalysts: develop kinetic model on Pd(111) analogous to Pt(111)

PW91 PAW 400 eV benzene at hollow site 3x3 unit cell Electronic energy barriers ΔEel forward reverse

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Pt3M catalysts: surface segregation

Au, Ag Pd stays in place Fe, Co, Ni, Cu No segregation Antisegregation Segregation Au/Pt Ag/Pt Most stable alloys studied Pt/Pt3M/Pt surface alloys Pt/PtM/Pt3M bulk alloys M=Fe, Ni, Co and Cu Pt3Ag/Pt Pt3Au/Pt Pt3Pd/Pt Pt3Pd bulk alloy ∆Eseg large

Methusalem Advisory Board meeting, Ghent, 17 June 2011

∆Eantiseglarge surface alloy

Pt3M alloys (4x4 supercells) (M= Ag, Au, Cu, Fe, Co, Ni, Pd) →evaluate stability & reactivity

Pt3M Bulk alloy Pt3M/Pt Surface alloy ∆Eseg= Eslab,seg–Eslab,non-seg ∆Eantiseg= Eslab,antiseg–Eslab,non-seg

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Pt2-bri30 Pt2M-fcc0 Pt3-fcc0 PtM-bri30 Pt2M-hcp0 Pt3-hcp0 bri-PtM30 fcc-Pt2M0 fcc-Pt3 bri-Pt2

30

hcp-M0 hcp-Pt0

Adsorption sites

Non-segregated Antisegregated

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Top-Pt Pt3-fcc Top-M Pt2M-hcp Top-Pt1 Pt3-fcc Top-Pt2 Pt3-hcp

Non-segregated Anti-segregated Hydrogen Benzene

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140
120
100
80
60
40
20

20 40

Au Ag Fe Co Ni Cu Pd

Pt3M: Benzene adsorption energy

Adsorption Energy (kJ mol-1)

Bridge

Adsorption of benzene Segregation No segregation Antisegregation

Bridge Hollow hcp Hollow hcp

Pt3M/Pt Surface alloys Pt3M Bulk alloys

Pt(111) (bridge)

119 kJ/mol

4x4 unit cell 60 to 90 kJ/mol weaker than Pt(111) bridge

Methusalem Advisory Board meeting, Ghent, 17 June 2011

up to 50 kJ/mol weaker

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20 20

80
60
40
20

20 40 60

Ag Au Cu Co Ni Fe Pd

Pt/PtM/Pt3M Pt3M/Pt Pt/PtM/Pt3M Pt/Pt3M/Pt

Pt3M: Hydrogen adsorption energy

Adsorption Energy (kJ/mol)

Adsorption of hydrogen 0.5 H2 + * → H*

Top

Segregation No segregation Antisegregation

Pt3M/Pt Surface alloys Pt3M Bulk alloys

up to 15 kJ/mol weaker up to 30 kJ/mol weaker

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Top Hollow fcc Hollow fcc

Pt(111) fcc site

47kJ/mol

2x2 unit cell

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Pt3M: activation energies first step

Segregation No segregation Antisegregation Electronic barrier Eel = ETS + EPt - EBads - EHads Pt+B+H TS BH

20 40 60 80 100 120 140 160

Co Ni Fe Cu Pd Au Ag

Methusalem Advisory Board meeting, Ghent, 17 June 2011

try to add correlation with Eads Step 1

Pt3M Bulk alloys

Electronic Barrier (kJ/mol)

Pt3M/Pt Surface alloys

92 kJ/mol Pt(111) Activation energies are lower on Pt3Co, Pt3Ni, Pt3Fe, Pt3Cu and Pt3Fe/Pt than on pure Pt(111)

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Activation energies correlate well with Eads

30 60 90 120 150

150
100
50

30 60 90 120 150

60
40
20

Ea (kJ/mol) Eads benzene (kJ/mol) Ea (kJ/mol)

Electronic barriers are well correlated to the adsorption energies of the reactants Eads as descriptor of reactivity

Bulk Pt3M alloys Surface Pt3M/Pt alloys Pt (111)

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Eads hydrogen (kJ/mol)

Can activation energy however be directly linked to electronic catalyst properties?

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d-band descriptors as catalyst descriptor

density of states projected on d-band of surface atoms of clean slab DOS-based descriptors Efermi center of occupied d-band DOS at Fermi Work function Ф=Ef–Evacuum DOS-based descriptors Work function

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Efermi DOS at Fermi d-band center

Density of states (eV-1) Energy (E-Ef)

140
120
100
80
60
40
20

20

2.80
2.60
2.40
2.20

Eads (kJ/mol) εd - Ef

Pt3Au/Pt Pt3Ag/Pt

40 60 80 100 120 140

2.80
2.60
2.40
2.20

Ea (kJ/mol) εd - Ef

Best correlation with occupied d-band center

Pt3Ag/Pt Pt3Au/Pt

: bulk alloys : surface alloys

Pt Pt

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Conclusions & prospects

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Benzene hydrogenation on Pt(111):

Succesful reaction simulation using only 2 optimized parameters

Benzene hydrogenation on Pt3M bimetallic alloys

Adsorption energies of benzene and hydrogen of the Pt3M alloys are, compared to pure

Pt(111), weaker when alloying with Au, Ag, Fe, Co, Ni and Cu

On the bulk alloys Pt3Co, Pt3Ni, Pt3Fe, Pt3Cu and the Pt3Fe/Pt surface alloy the activation

energies are lower than on pure Pt(111)

the d-band center correlates well with benzene adsorption energies and hydrogenation

barriers for the studied alloys. Prospects

Development of first-principles based kinetic Monte Carlo simulation tools to assess the

validity of the mean field approximation under industrially relevant operating conditions

Further evaluate the d-band center as useful catalyst descriptors relating the variation in

activity and selectivity in going from Pt(111) to other metal catalysts, and screen the d-band center of other promising alloys

Definition of optimal catalyst properties: simultaneous optimization of catalyst

properties, industrial process conditions and reactor configuration

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Acknowledgements

Methusalem Advisory Board meeting, Ghent, 17 June 2011

Lucía Laín Amador Joris Thybaut Fund for scientific research - Flanders Long Term Structural Methusalem Funding by the Flemish Government – grant number BOF09/01M00409 Questions?

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Glossary

Methusalem Advisory Board meeting, Ghent, 17 June 2011

DFT: Density Functional Theory Dimer method: force-based transition state search algorithm GGA: generalized gradient approximation (within DFT theory) MEP: Minimum Energy Path NEB: Nudged Elastic Band method for the calculation of MEPs PAW: Plane Augmented Waves (periodic calculation technique) PW91: Perdew-Wang type of DFT functional VASP: Vienna Ab initio Simulation Package