Hydrogenation of acetylene-ethylene mixtures over Pd and Pd-Ag alloys: First-principles-based kinetic Monte Carlo simulations

The kinetics for the selective hydrogenation of acetylene-ethylene mixtures over model Pd(1 1 1) and bimetallic Pd-Ag alloy surfaces were examined using first principles-based kinetic Monte Carlo (KMC) simulations to elucidate the effects of alloying and reaction conditions. The elementary steps that control the selective and unselective pathways, including hydrogenation, dehydrogenation, and C-C bond breaking, were analyzed using first-principle density functional theory (DFT) calculations. The results were used to construct an intrinsic kinetic database that was used in a variable time step kinetic Monte Carlo simulation to follow the kinetics and the molecular transformations in the selective hydrogenation of acetylene-ethylene feeds over Pd and Pd-Ag surfaces. Through-surface and through-space lateral interactions between coadsorbates were estimated using DFT-parameterized bond order conservation and van der Waal interaction models, respectively. The simulations show that the rate of acetylene hydrogenation as well as ethylene selectivity increases with temperature over both the Pd(1 1 1) and the Pd-Ag/ Pd(1 1 1) alloy surfaces. The selective hydrogenation of acetylene to ethylene proceeds via the formation of a surface vinyl intermediate. The unselective formation of ethane is the result of the over-hydrogenation of ethylene as well as over-hydrogenation of vinyl to form ethylidene. Ethylidene further hydrogenates to form ethane and dehydrogenates to form ethylidyne. While ethylidyne is not reactive, it can block adsorption sites and thus limit the availability of hydrogen on the surface which enhances the selectivity. Alloying Ag into the Pd surface decreases the overall rate but increases the ethylene selectivity significantly by promoting the selective hydrogenation of vinyl to ethylene and concomitantly suppressing the unselective path involving the hydrogenation of vinyl to ethylidene and the dehydrogenation of ethylidene to ethylidyne. This is consistent with experimental results which suggest that only the predominant hydrogenation path which involves the sequential addition of hydrogen to form vinyl and ethylene exists over the Pd-Ag alloys. Ag enhances the desorption of ethylene and hydrogen from the surface thus limiting their ability to undergo subsequent reactions. The simulated apparent activation barriers were calculated to be 32-44 kJ/mol on Pd(1 1 1) and 26-31 kJ/mol on Pd-Ag/Pd(1 1 1), respectively. The reaction was found to be essentially first order in hydrogen and -0.4 and -0.21 order in acetylene over Pd(1 1 1) and Pd-Ag/Pd(1 1 1) surfaces, respectively. The results reveal that increases in the hydrogen partial pressure increase the activity but decrease ethylene selectivity over both Pd and Pd-Ag/Pd(1 1 1) surfaces.