First-principles-based kinetic Monte Carlo simulation of the selective hydrogenation of acetylene over Pd(111)

Abstract

The kinetics for the selective hydrogenation of acetylene over Pd(111) was investigated by using first-principles-based kinetic Monte Carlo simulations. Density functional theory (DFT) calculations were carried out to obtain intrinsic kinetic data for a Horiuti–Polanyi-type reaction mechanism involving the sequential hydrogenation of acetylene. The results were subsequently used to develop a detailed intrinsic kinetics database that includes the adsorption energies of the reactants, intermediates, and products, the reaction energies and activation barriers of the elementary steps in the proposed reaction mechanism. The DFT-calculated energies and activation barriers were initially performed at lower surface coverages, to probe the intrinsic surface chemistry. Subsequent calculations were carried out at higher coverages, to capture the influence of the local environment on the reaction kinetics, and used to parameterize coarse-grained models that describe adsorbate interactions. A van der Waals force field model and a modified bond order conservation model were subsequently used within the simulation to calculate the local through-space interactions and the lateral through-surface interactions occurring between coadsorbates, respectively, and to assess the influence of the local reaction environment. The intrinsic DFT-derived kinetic data and the coarse-grained reaction environment models were used together in a variable time step kinetic Monte Carlo simulation to track the molecular transformations involved in acetylene hydrogenation over the (111) surface of Pd. The kinetic Monte Carlo simulation method [E.W. Hansen, M. Neurock, Chem. Eng. Sci. 54 (1999) 3411; E.W. Hansen, M. Neurock, J. Catal. 196 (2000) 241; E.W. Hansen, M. Neurock, Surf. Sci. 464 (2000) 91; E.W. Hansen, M. Neurock, J. Phys. Chem. B 105 (2001) 9218] used herein explicitly treats the atomic surface structure, the effects of the local reaction environment, and the reaction conditions on the surface kinetics. The simulated apparent activation energy for acetylene hydrogenation was calculated as 8.0 ± 0.6 kcal / mol at P H 2 = 100 Torr and P C 2 H 2 = 100 Torr over the temperature range of 300–500 K, in very good agreement with the value of 9.6 kcal/mol reported from experimental studies over well-defined Pd(111) surfaces [H. Molero, B.F. Bartlett, W.T. Tysoe, J. Catal. 181 (1999) 49]. The reaction orders were calculated as − 0.52 ± 0.03 for acetylene and 1.16 ± 0.03 for hydrogen, which agree very well with the experimental reaction orders by Molero et al. [H. Molero, B.F. Bartlett, W.T. Tysoe, J. Catal. 181 (1999) 49] of −0.66 and 1.04, respectively. A comparison of the simulations carried out assuming non-interacting adsorbates (hard sphere) and those that include lateral interactions between adsorbates showed that although the overall apparent activation energy was weakly sensitive to the presence of lateral interactions, the surface coverages and intrinsic rates changed considerably due to the presence of lateral interactions. The addition of lateral interactions between coadsorbates was found to be essential in simulating the correct overall selectivity behavior and appropriately predicting the apparent reaction orders with respect to hydrogen and acetylene.