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

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 1 1

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Introduction: benzene hydrogenation on Pt(111) Benzene hydrogenation: applications in hydrotreating, hydrocracking, cyclohexane production Current status of computational models: dominant path proposed based on Pt 22 cluster calculations (DP cluster ) Pt 22 cluster Electronic reaction barriers BP86/DZ on Pt 22 cluster of Pt(111) Saeys.M J.Phys.Chem.B , 109,2064- 2063 (2005)  Regressions to experimental data suggest other dominant path (Thybaut): DP regressed  Experimental work: no consensus on the rate determining step  Entropy contributions difficult at cluster level: include using periodic calculations 2

Methusalem Advisory Board meeting, Ghent, 17 June 2011 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 Pt 3 M 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) 3

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Computational approach Periodic • 3 x 3 unit cell used to model the Pt(111) surface: 9 atoms/layer structure • moderate lateral interactions: coverage degree ≈ 30% Vacuum layer Artifical dipole layer 10.6 Å Relax 2 upper layers Fix 2 bottom layers Lattice constant: 4.011Å Surface with Unit cell Unit cell unit cell indicated Top view Side views • PW91 functional (GGA) DFT (VASP) • 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 4

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Outline  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 5

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Pt(111) network: electronic reaction barriers Based on Δ E el : no clear dominant path DP cluster,135THB dominant path on Pt 22 cluster level MEP periodical,123THB minimum energy path (periodical calculations) Electronic energy barriers Δ E el forward reverse 6

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Entropy contributions are important for K and k Immobile species: Harmonic frequency analysis  vibrational Schrödinger equation Vibrational contribution to entropy 2 3 N 1 1 H q Hq ( q ) E ( q ) 2 2 m q 2 i i i h i 3 N h k T B Kinetic Potential energy requires i S R ln 1 e rovib, HO h i energy knowledge of Hessian H i 1 k T k T 1 e B B 2 E Hessian H q i = Δ x, Δ y, Δ z around ij q q equilbrium geometry i j Mobile species  free rotation and/or free translation  Replace 2 ‘ translational ’ and 1 ‘ rotational ’ frequency 2 1 S R ln q ' ( A , T ) 1 S R ln q ( T ) transl, surf trans rot, Z rot , Z 2 2 A: 10 -19 m² for H*; 5 10 -19 m² for hydrocarbon species  identify mobility of surface species: calculate diffusion barriers 7

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Entropy contributions: mobile mode identification H* top to top diffusion (NEB) Determine transition states for diffusion 10 (NEB+dimer) 8 E-Etop kJ/mol Δ E ° 6 Species + motion kJ/mol 4 Hydrogen (top to top) 9.2 2 Hydrogen (top to hollow) 11.6 0 Translational Coordinate Benzene (hollow to bridge-rotation) 21.1 135 THB (translation) 233.0 1235 THB (rotation) 99.8 135-THB translation (diffusion barrier 233 kJ/mol) Cyclohexyl (translation) 98.5 Cyclohexyl (rotation around C-Pt bond) 12.7 Cyclohexane (rotation) 5.9 All species immobile at 450 K except H and cyclohexane (barrier < 9 kJ/mol) Initial state Final state 8

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Rate coefficients indicate dominant path no clear dominant path  Evaluate full reaction network in simulation DP cluster dominant path at Pt 22 cluster level MEP periodical minimum energy path (periodical calculations) DP periodical, k dominant path based on rate coefficients (periodical calculations) rate coefficients k (s -1 ) forward reverse 9

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Experimental data: Berty set-up Catalyst: Pt/ZSM-22 (0.5 wt% Pt) Conversion: 9-85% Input variables (43 experiments) Benzene Feed (mol s -1 ) 17 10 -6 -57 10 -6 T (K) 425-500 P(atm) 10-30 p H2 /p B 5-11 W cat (g) 1.29 -1.8 W/F benzene (kg cat s -1 mol -1 ) 22-74 Berty-reactor: Gas phase CSTR (intrinsic kinetics) 10

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Reactor simulation approach Estimated parameters Simulations  H2 adsorption enthalpy: strongly coverage  CSTR model  Levenberg-Marquardt for parameter dependent  Estimation of this parameter required estimation  Goal function= Σ (simulated product yield- Podkolzin et al., JPCB, exp.observed) 2 105:8550 (2001)  K( T ) and k( T ) with mobile H* and cyclohexane*, other species are considered immobile  catalyst model: 0.008 active sites/kg cat  PSSA (reaching steady state using transient solver) Transient continuity equations:  General reduction of activation energy: dF • calculated E a larger than experiment 0 i Gas phase species: F F R W i i i 0 • temperature dependence too strong dt without reduction of E a dC i * Surface species: R E a,i = E a,i, AbInitio + Δ E a ,parameter i * dt dC * Free sites: R * dt 11

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Full network: reactor simulation results • K( T ) and k( T ) for mobile H* and cyclohexane* (other immobile) • surface coverage ≈ 1 => take Δ H ads (benzene)= -66.1 kJ mol -1 (calculated value) Simulation Estimate Δ H H2 and Δ E a Cyclohexane yield parity plot E a,i = E a,i, AbInitio + Δ E a ,parameter 50 Simulated product yield (10-6 mol/s) Δ H ads,H2 -46.1 ± 2.2 kJ/mol 40 Δ E a -14.6 ± 2.7 kJ/mol 30 F 428 20 10 Estimating only Δ H H2 : yields still too low 0 • temperature dependence too strong 0 10 20 30 40 50 without reduction of E a Experimental product yield (10-6 mol/s) • Estimate E a reduction 12

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Full network: reaction path analysis 20 bar, 225 °C, 1.8 g cat , 0.13 mol/h benzene, Electronic energy barriers Δ E el (H 2 /B) in =5 forward W/F B =48.4 kg cat s/mol reverse • Clear pathway for step 4, 5 and 6 • In step 2 and 3 equilibration between intermediates 13

Methusalem Advisory Board meeting, Ghent, 17 June 2011 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 14

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Outline  Part I: Hydrogenation of benzene on Pt(111): from molecule to reactor  Part II: Catalyst-descriptor based design of hydrogenation catalysts • Pd catalysts • Pt 3 M catalysts • Conclusions & prospects 15

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Pd-catalyzed hydrogenation First step in design of Pd-based catalysts: develop kinetic model on Pd(111) analogous to Pt(111) → similar MEP as for Pt(111) PW91 PAW 400 eV Electronic energy barriers Δ E el benzene at hollow site forward 3x3 unit cell reverse Future work : entropy contributions, rate coefficients and multiscale modeling of the reactor 16

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Pt 3 M catalysts: surface segregation Pt 3 M alloys (4x4 supercells) Pt 3 M/Pt Pt 3 M (M= Ag, Au, Cu, Fe, Co, Ni, Pd) Surface alloy Bulk alloy →evaluate stability & reactivity Segregation Most stable alloys studied Au, Ag Au/Pt ∆E seg large Ag/Pt ∆ E seg = E slab,seg – E slab,non-seg No segregation Pt 3 Ag/Pt surface alloy Pt 3 Au/Pt Pd stays in place Pt 3 Pd/Pt Pt 3 Pd bulk alloy Antisegregation Pt/Pt 3 M/Pt surface alloys Fe, Co, Ni, Cu Pt/PtM/Pt 3 M bulk alloys ∆ E antiseg large M=Fe, Ni, Co and Cu ∆ E antiseg = E slab,antiseg – E slab,non-seg 17 17

Methusalem Advisory Board meeting, Ghent, 17 June 2011 Adsorption sites Hydrogen Benzene Non-segregated Top-Pt Pt 3 -fcc Pt 2 M-hcp fcc-Pt 2 M 0 Pt 2 M-fcc 0 bri-PtM 30 Top-M Pt 2 -bri 30 0 Pt 3 -fcc 0 fcc-Pt 3 bri-Pt 2 PtM-bri 30 30 Anti-segregated Top-Pt 1 Pt 2 M-hcp 0 hcp-Pt 0 Pt 3 -hcp 0 hcp-M 0 Pt 3 -fcc Pt 3 -hcp Non-segregated Antisegregated Top-Pt 2 18 18