Effects of alloying Pd and Au on the hydrogenation of ethylene: An ab initio-based dynamic Monte Carlo study

An ab initio-based dynamic Monte Carlo simulation was developed and used to examine the kinetics of ethylene hydrogenation over Pd and PdAu alloys. The rate of ethylene hydrogenation was controlled by the strength of the metal-adsorbate bonds and the surface coverage. Alloying palladium with gold weakens the metal-carbon and metal-hydrogen bond strength which increases the intrinsic rate of ethylene hydrogenation. However, Au addition also shut down sites for H₂ activation. This reduced the surface coverage of hydrogen that decreases the rate of hydrogenation. Au addition led to both electronic and geometric effects. The electronic effects were very minor compared with the geometric effects and are primarily attributed to rehybridization. There was a negligible charge transfer between palladium and gold. Alloying palladium with gold did not affect the activity but had a significant effect on the initial selectivity. At higher temperatures and lower partial pressures of hydrogen, the pure Pd(111) surface readily promoted ethylene dehydrogenation to ethylidyne.