Recent surface science studies show autocatalytic regimes in CO oxidation over transition metal oxide surfaces that are hypothesized to be due to the presence of oxygen vacancies. Despite this recent work, the role at a elementary mechanistic level of oxygen vacancies on observed increases in CO oxidation rates has not been elucidated. In our study, we investigated the thermal reduction and CO oxidation of PdO(101) to determine the effect of oxygen vacancies on overall CO oxidation rates. First we employ density functional theory to map out the kinetics of elementary surface processes related to the isothermal reduction and the CO oxidation of PdO(101). In particular, we have determined the kinetics of elementary processes (e.g. oxygen vacancy formation, CO oxidation to form CO2, desorption and adsorption of reactants and products) in the presence and absence of O vacancies. Using the rate constants from DFT results, we have develop a lattice-based kinetic Monte Carlo (kMC) framework[AA1] that takes into account the prescence of adjacent O vacancies. For the thermal reduction of PdO(101), our kMC simulations capture the experimentally observed autocatalytic behavior in which the reduction rate increases as the surface reduces. The simulation results suggest that oxygen vacancies trigger the autocatalytic behavior by facilitating the formation of oxygen vacancies and the formation of O2 at adjacent positions. Therefore, as more oxygen vacancies are generated the surface becomes more active toward further reduction. Similar behavior is also captured from kMC simulations for CO oxidation over PdO(101). The simulation results suggest that the oxygen vacancies also promote CO oxidation over the PdO(101) surface. Initially, the adsorbed CO consumes surface oxygens thereby generating oxygen vacancies. Subsequently, the adsorbed CO and surface oxygens migrate to the reduced oxide phase to form CO2 via a more facile pathway. The CO oxidation rate associated with the above alternate mechanism dramatically increases as the surface reduces, which results in the observed rapid rise in CO oxidation rate as the surface is reduced. We will also present ongoing work in examining steady state CO oxidation at different reaction conditions to evaluate if such O vacancy mediated processes play an important role under reaction conditions

First-principles kinetic Monte Carlo simulation of CO oxidation on PdO(101)

254th National Meeting (2017)

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