First-principles based design of Pt- and Pd-based ... M... · 1 1 Methusalem Advisory Board...

1 1

Methusalem Advisory Board meeting, Ghent, 17 June 2011

First-principles based design of Pt- and Pd-based catalysts for benzene hydrogenation

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

2

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 barriersBP86/DZ on Pt22 cluster of Pt(111)Saeys.M J.Phys.Chem.B, 109,2064-2063 (2005)

Pt22 cluster


Benzene hydrogenation:

applications in hydrotreating, hydrocracking, cyclohexane production

3

Aim


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)

4

Computational approach


• 3 x 3 unit cell used to model the Pt(111) surface: 9 atoms/layer

• moderate lateral interactions: coverage degree ≈ 30%

Unit cellTop view

Unit cellSide views

Vacuum layer10.6 Å

Relax 2 upperlayersFix 2 bottomlayers

Lattice constant: 4.011Å

Artifical dipole layer

Periodicstructure

Surface withunit 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)

5

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


6

Based on ΔEel: no clear dominant path

Pt(111) network: electronic reaction barriers


Electronic energy barriers ΔEel

forward

reverse

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

7

Entropy contributions are important for K and k


ji

ijqq

EH

2

)q()q(Hqq2

1

2

1 H3

2

2

Eqm

N

i ii

Immobile species: Harmonic frequency analysisvibrational Schrödinger equation

Kineticenergy

Potential energy requiresknowledge of Hessian H

Hessian qi= Δx, Δy, Δz aroundequilbrium geometry

N

iTk

h

B

iTBk

ih

B

i

e

eTk

hRS

3

1

HOrovib, 1ln

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

21),('ln transsurf transl, TAqRS

2

1)(ln ,Zrot, TqRS Zrot

8

Entropy contributions: mobile mode identification


All species immobile at 450 K except H and cyclohexane

(barrier < 9 kJ/mol)

Species + motionΔE°

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

0

2

4

6

8

10

E-Et

op

kJ/

mo

l

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

9

no clear dominant path Evaluate full reaction network in simulation

Rate coefficients indicate dominant path


rate coefficients k (s-1)

forwardreverse

DPcluster dominant path at Pt22 cluster level

MEPperiodical minimum energy path (periodical calculations)

DPperiodical,k dominant path based on rate coefficients (periodical calculations)

10

Experimental data: Berty set-up


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%

11

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


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 usingtransient solver)

0

0 WRFFdt

dFiii

i

**

ii 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

12

Full network: reactor simulation results


• 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 Ea reduction

0

10

20

30

40

50

0 10 20 30 40 50

Sim

ula

ted

pro

du

ct y

ield

(1

0-6

mo

l/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

SimulationEstimate ΔHH2 and ΔEa

Ea,i = Ea,i,AbInitio + ΔEa,parameter

Cyclohexane yield parity plot

13

Full network: reaction path analysis



forwardreverse

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

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

14

Conclusions and prospects


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

obtained

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

15

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


16

→ similar MEP as for Pt(111)

Future work: entropy contributions, rate coefficients and multiscale

modeling of the reactor

Pd-catalyzed hydrogenation


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

PW91 PAW 400 eVbenzene at hollow site3x3 unit cell


forward

reverse

1717

Pt3M catalysts: surface segregation

0segE

0segE

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


∆Eantiseg large

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

1818

Pt2-bri30Pt2M-fcc0

Pt3-fcc0

PtM-bri30

Pt2M-hcp0Pt3-hcp0

bri-PtM30fcc-Pt2M

0

fcc-Pt30

bri-Pt230

hcp-M0 hcp-Pt0

Adsorption sites

Non-segregatedAntisegregated


Top-Pt

Pt3-fcc

Top-M

Pt2M-hcp

Top-Pt1

Pt3-fcc

Top-Pt2

Pt3-hcp

Non-segregated

Anti-segregated

Hydrogen Benzene

1919

-140

-120

-100

-80

-60

-40

-20

0

20

40

Au Ag Fe Co Ni Cu Pd

Pt3M: Benzene adsorption energy

Adsorption Energy (kJ mol-1)

Bridge

Adsorption of benzene

Segregation No segregationAntisegregation

Bridge

Hollow hcp

Hollow hcp

Pt3M/PtSurface alloys

Pt3MBulk alloys

Pt(111) (bridge)-119 kJ/mol4x4 unit cell

60 to 90 kJ/mol weaker than Pt(111) bridge


up to 50 kJ/mol weaker

2020

-80

-60

-40

-20

0

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 hydrogen0.5 H2 + * → H*

Top

Segregation No segregationAntisegregation

Pt3M/PtSurface alloys

Pt3MBulk alloys




Top

Hollow fcc

Hollow fcc

Pt(111) fcc site-47kJ/mol

2x2 unit cell

2121

Pt3M: activation energies first step

SegregationNo segregationAntisegregation

Electronic barrier Eel = ETS + EPt - EBads - EHads

Pt+B+H TS BH

0

20

40

60

80

100

120

140

160

Co Ni Fe Cu Pd Au Ag


try to add correlation with Eads

Step 1

Pt3MBulk alloys

Electronic Barrier (kJ/mol)

Pt3M/PtSurface alloys 92 kJ/mol Pt(111)

Activation energies are lower on Pt3Co, Pt3Ni,

Pt3Fe, Pt3Cu and Pt3Fe/Pt than on pure Pt(111)

2222

Activation energies correlate well with Eads

0

30

60

90

120

150

-150 -100 -50 0

0

30

60

90

120

150

-60 -40 -20 0

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)


Eads hydrogen (kJ/mol)

Can activation energy however be directly linked to electronic

catalyst properties?

2323

d-band descriptors as catalyst descriptordensity 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


Efermi

DOS at Fermid-band center

Density of states (eV-1)

Energy (E-Ef)

-140-120-100

-80-60-40-20

020

-2.80 -2.60 -2.40 -2.20

Ead

s (k

J/m

ol)

εd - Ef

Pt3Au/PtPt3Ag/Pt

40

60

80

100

120

140

-2.80 -2.60 -2.40 -2.20

Ea (

kJ/m

ol)

εd - Ef

Best correlation with occupied d-band center

Pt3Ag/Pt

Pt3Au/Pt

: bulk alloys: surface alloys

Pt

Pt

24

Conclusions & prospects


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

25

Acknowledgements


Lucía Laín AmadorJoris Thybaut

Fund for scientific research - FlandersLong Term Structural Methusalem Funding bythe Flemish Government – grant number BOF09/01M00409

Questions?

26

Glossary


DFT: Density Functional TheoryDimer method: force-based transition state search algorithmGGA: generalized gradient approximation (within DFT theory)MEP: Minimum Energy PathNEB: Nudged Elastic Band method for the calculation of MEPsPAW: Plane Augmented Waves (periodic calculation technique)PW91: Perdew-Wang type of DFT functionalVASP: Vienna Ab initio Simulation Package

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