multi-scale modeling and reaction path analysis of benzene ... advis… · reactor simulation on...

Laboratory for Chemical Technology, Ghent University

http://www.lct.UGent.be

G. Canduela, M.-F. Reyniers, G.B. Marin

1

M2dcR2 advisory board, Gent, 19/06/2012

Multi-scale modeling and reaction path analysis

of benzene hydrogenation over Pd(111)

Annex

� Multi-scale modeling� Ab initio model

� Micro-kinetic model

� Reactor model

� Reaction path analyses� Potential energy surface (PES)

� Rate of production (ROP)

� Degree of rate control (DRC)

� Sensitivity analysis (SA)

� Conclusions� Future work

2


Multi-scale modeling

– Inclusion of chemical details at different scales that influence the performance

– Measure the relation between catalyst properties and reactor performance

– Aiming the integration of quantum mechanical models into reactor models

3


Activity

Catalytic property

Optimal catalytic propertiesReactor simulations

Quantum chemistry methods

Ab initio model: adsorption

4


Atop

Bridge

30° 0°

Hollow-hcp

Hollow-fcc

Benzene (θB= 0.67 ML)Hydrogen (θH=0.11 ML)

top

hcp

bridge

fcc

octasub

hcpfcctop

tetrasub13 octasub tetrasub31

• Bridge30 and hollow0 sites are preferred

� (-115 and -100 kJ mol-1) for Pd(111)

Exp � -125 to -95 kJ mol-1 for Pd(111) (Tysoe et al.)

� (-97 and -60 kJ mol-1) for Pt(111)

Exp � -197 to -63 kJ mol-1 for Pt(111) (Ihm et al.)

• Hollow fcc and hcp sites preferred on Pd(111)

� -61 and -57 kJ mol-1

� Subsurface hydrogen metastable

� Exp � -62 kJ mol-1 (Chou and Vannice)

• Similar stabilities on Pt(111)

� -48 (fcc), -46 (hcp), -41 (bridge) and -40 kJ molH-1 (top)

� No subsurface in Pt(111)

� Exp � -48 kJ mol-1 (Podkolzin et al.)

Chou and Vannice, 1987Journal of Catalysis, Vol 104Podkolzin et al. 2001, J.Physical Chemistry, Vol 105

Tysoe et al., 1993, J.Physical Chemistry , Vol 97Podkolzin et al. 2004, J.Physical Chemistry B, Vol 108

Thermodynamic surface diagram on Pd(111)

5


Study of co-adsorption of benzene hollow (0.67 ML) and different hydrogen coverages.Diagram from the surface (free) energy of each system, embedded in a thermodynamic model

-10

-8

-6

-4

-2

0

2

100 200 300 400 500 600 700

log(

pH

/ p)

Temperature (K)

0.44 ML

0.89 ML

0 ML0 to 0.11 ML

0.11 to 0.33 ML

0.33 to 0.44 ML

0.44 to 0.88 ML0.89 to 1.11 ML

1.11 to 1.22 ML

1.22 to 1.44 ML

1.44 to 1.67 ML

1.67 to 1.78 ML

1.78 to 1.89 ML1.89 ML

Typical range of hydrogenation conditions

What is the state of the catalyst surface at hydrog enation conditions?

Ab initio model: surface reaction

6


Surface reactions of the full reaction network (bri dge and hollow)Regarding the preferred adsorption sites for the reactants, different reactions are evaluatedMedium coverage of benzene, low coverage of hydrogen

Pd(111):∆Eel = 99 for hollow and 118 kJ mol-1 for bridge, 1st hydrogenation step� possible rate determining step (RDS)

Transition state

Reactants Product

Pt(111):∆Eel = 90 kJ mol-1 on hollow network, 3rd hydrogenation step � possible rate determining step (RDS)

Micro-kinetic model

7


Periodic DFT Eel + HO vibrational ν + surface mobility

In combination with statistical thermodynamics

Thermodynamics and KineticsS & H = f (T) and A & Ea = f (T) �� k = A exp(-Ea/RT)

exp(∆S/R)-(∆H/RT) = Keq= kf / kr

θtotal = θ * + θB + θH + θBH +……+ θCHA

rCHA= kd × θCHA – ka × pCHA × θ *

Micro-kinetic model: describes reaction mechanism in terms of elementary reaction stepsFull network Dominant path

Reactor simulation on Pd(111)

8


pH2 = 0.5-1 bar

pB = 0.05-0.1 bar

Ftotal=277.8 µmol/s

PT = 1-30 barT = 350-573 K

Ct = 0.008 molactive sites kgcat-1

W = 0.2 gcat

Aiming to test the modeling tools to be used for catalysts screening in the future

Reaction orders: Benzene, nB ~ 0.8 Hydrogen nH2 ~ 0.45

Reaction orders, T = 500 K:Benzene, nB ~ 0Hydrogen nH2 ~ 1.5

Reaction orders, T = 500 K:Benzene, nB ~ 0.5Hydrogen nH2 ~ 0.5

Adaptations due to the sensitivity of adsorption enthalpies to the coverage

Decreasing the adsorption enthalpy of hydrogen in 40 kJ mol-1 Increasing the adsorption enthalpy of benzene

5.00E-05

1.50E-04

2.50E-04

3.50E-04

4.50E-04

5.50E-04

6.50E-04

7.50E-04

0 0.02 0.04 0.06 0.08 0.1 0.12

Yie

ld, μ

mo

l/s

partial pressure benzene, bar

6.00E-04

7.00E-04

8.00E-04

9.00E-04

1.00E-03

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

Yie

ld, µ

mol

/s

partial pressure hydrogen, bar

Reaction path analysis on Pd(111)

– Catalytic heterogeneous reactions are normally complex process

– Willing to know the most important reaction steps and states to simplify the model

– Benzene catalytic hydrogenation to cyclohexane

9


Adsorption

BenzeneHydrogen

Desorption

Cyclohexane

Surface reactions

Reaction path analysis on Pd(111)• Based on the potential energy surface (PES) and rate coefficients, k :

– Minimum energy path (MEP) at 0 K: the one running from benzene to monohydrobenzene, passing over cyclo-1,3-hexadiene, trihydro-1,2,3-benzene, cyclohexene, cyclohexyl to cyclohexane.

– First reaction as potential rate determining step (RDS)– Same results based on k for wide range of T

10


Hollow networkBridge network

RDS

Rate of production analysis on Pd(111)

– Establish which reactions steps control the overall rate at each step– Measure the net reaction rate of the different elementary steps

11


for T=(400-550) K

95%

90% 97%

100%

Hollow network

Bridge network

103%

103% 73%

73%30%

27% 27% 27%3%

Degree of rate control on Pd(111)

– Developed by Charles T. Campbell – Reactions that control the overall reaction rate– Influence of infinitesimal change in k on the global reaction r (by changing Ea � stability of TS )

12


for T=450 K

Nearly RDS

RDS

XRC-TS Hollow network XRC-TS Bridge network

0.86

0.090.92

0.06

0.02

(C. T. Campbell, Topics on Catalysis. 1, 353 (1994))

XRC more or less constant for T=400-550 K

iijiij KkiKki

iiRC k

r

k

r

r

kX

,,

, ln

ln

≠≠

∂∂=

∂∂⋅

=

Thermodynamic degree of rate control on Pd(111)

– Defined by Stegelmann et al. – Stable surface intermediates that control the overall reaction rate

13


-500

-450

-400

-350

-300

-250

-200

-150

-100

-50

0

En

tha

lpy,

kJ

mo

l-1

B*+ 6H*

BH*+ 5H*

BH2*+ 4H*BH3*+ 3H*

BH4+ 2H*

BH5*+ H* CHA*

CHAgas

Adsorbed reactants�Thermodynamic rate control, XTRC

(Stegelmann et al., J.Catalysis. 204, 520 (2001))

Sensitivity analysis on Pd(111)– Reactions that control the overall reaction rate– Measure of stability of species by changes in the pre-exponential factor of the reaction, A.

14


0.85

0.09 0.85

0.1

for T=450 K

XSA-TS Hollow network XSA-TS Bridge network

iijiij KkiKki

iiSA k

r

k

r

r

kX

,,

, ln

ln

≠≠

∂∂=

∂∂⋅

=

Conclusions

15


Multi-scale modeling

� Good agreement with experimental and theoretical studied for benzene and hydrogen adsorption

� Thermodynamic diagram of surface indicate high hydrogen coverage at reaction conditions

� Approximations (low hydrogen coverage and only (111) surface plane) lead to low activities

� Tuning adsorption enthalpies lead to better comparison to experimental results

� In any case, the modeling tools are available to be used as guideline with different catalysts

Reaction path analysis

� Based on PES and k: Same minimum energy path (MEP) for bridge and hollow network:

� Benzene � monohydrobenzene � cyclo-1,3-hexadiene� trihydro-1,2,3-benzene � CHE � Cyclohexyl � CHA

� First reaction as potential rate determining step

� Based on Rate of production: Same dominant path than MEP for both networks

� Based on Degree of rate control : 3rd transition state (hollow) and 2nd (bridge) � nearly RDS

� Based on Thermodynamic DRC: adsorbed benzene and hydrogen stability controlling the overall rate

� Based on Sensitivity analysis: Same results as DRC

Future work

16


� Study the transition state periodic trends for adso rption of the reactants

� Perform calculation for a selected number of Pd-bas ed bimetallic catalysts

� Evaluate the influence of high coverage on the kine tics and thermodynamics

� Include high coverage results and different surface plane in reactor simulations

� Study non-competitive adsorption of benzene and hyd rogen in the reactor simulations

� Include desorption of the cyclohexadiene and cycloh exene species

17

Thank you for your attention

Glossary

18


Catalyst descriptor: Characteristic for a given catalyst that can

be correlated with kinetic and thermodynamic properties

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)

PES: Potential energy surface

PW91: Perdew-Wang type of DFT functional

RDS: Rate determining step

VASP: Vienna Ab initio Simulation Package

ZPE: Zero point energy

multi-scale modeling and reaction path analysis of benzene ... advis… · reactor simulation on...

Documents