Kinetic Monte Carlo Simulation of PdZn Alloying and Density Functional Study of PdZn Surface Reactivity towards Water Dissociation

Two-monolayer-Zn covered Pd(1 1 1) annealed at (low) 500 K exhibits nice CO2 selectivity for CH3OH+H2O to CO2+H2, whereas CO is yielded exclusively when annealed at (high) 650 K. To unravel the reason behind the phenomenon, kinetic Monte Carlo (KMC) simulations were used to study the alloying process. It shows the low temperature annealing produces a multilayer 1:1 PdZn alloy, whereas the high temperature operation results in a Zn-lean multilayer alloy, not the previously assumed monolayer PdZn alloy on Pd. The geometry and electronic structures of the models derived from KMC simulation agrees with the relevant experiments. The low temperature sample is more active than the high temperature one for H2O dissociation, in line with the assumption that H2O dissociation controls CO2 selectivity. It is revealed that triple Zn ensembles which form three-fold hollow sites account for the activity for H2O dissociation.