From first-principles to catalytic turnovers: Ethylene hydrogenation over palladium

We present a first-principles-based dynamic Monte Carlo method which can be used to model the kinetics of metal catalyzed reaction systems by following the explicit atomic surface structure and individual molecular transformations. The approach uses first-principle density functional quantum chemical calculations to build a comprehensive database of adsorption energies, overall reaction energies, activation barriers, and intermolecular interaction energies. The ab initio calculated lateral interaction energies were subsequently used to develop more approximate but universal interaction models that could be used in-situ in the MC. A radial function model and a bond order conservation model were both developed. The simulation algorithm was used herein to examine ethylene hydrogenation over palladium. The results indicate that it is the repulsive interactions in the adlayer that weaken the metal-carbon and metal-hydrogen bonds thus lowering the barriers for hydrogenation from 15 for ethylene to ethyl and 14.5 for ethyl to ethane to 8.5 and 8.0 kcal/mol for the same steps taken at higher surface coverages. The simulation results provide a very good match against known experiment results. The simulation was subsequently used to examine both the effects of alloying and surface structure. The addition of gold decreased the overall turnover number simply because the number of sites was reduced. On a per palladium basis, however, the activity remains approximately the same. The addition of gold indirectly leads to less hydrogen on the surface since it shuts down H₂ dissociation steps. This, however, is countered by a reduction in the metal-hydrogen bond strength which helps to enhance the activity. These two features tend to balance one another out as the turnover frequencies remain nearly constant. We provide a simple cursory look at the effects of surface structure by examining the changes in the kinetics over Pd(100) and Pd(111) surfaces. The barriers over these two surfaces are 7.1 and 6.4 kcal/mol respectively suggesting that the chemistry is relatively structure insensitive.