(578b) First-Principles Kinetic Monte Carlo Simulation of the Thermal Reduction of PdO(101)

The impacts of oxygen vacancies on CO or alkane oxidation over transition metal oxide surfaces have been recently reported in catalysis and surface science studies. Oxygen vacancies may enhance CO and methane oxidation rates under reaction conditions, including potential autocatalytic behavior during CO oxidation over the transition metal oxide surfaces. Despite this rising attention, oxygen vacancy effects have not been closely studied. As a first step in our study, we investigate the thermal reduction of PdO(101) to elucidate oxygen vacancy effects in the absence of any reactants. First we employ density functional theory (DFT) to map out the kinetics of elementary surface processes related to the thermal reduction of PdO(101). Using the DFT calculations and harmonic transition state theory, we perform kinetic Monte Carlo (kMC) to examine the evolution of the PdO(101) surface as it begins to reduce at high temperatures. Our kMC simulations capture the experimentally observed autocatalytic behavior in which the decomposition rate increases as the surface decomposes. The simulation results show that an oxygen vacancy triggers the autocatalytic behavior by facilitating the formation of oxygen vacancies at adjacent positions. The presence of oxygen vacancies also promotes formation of O₂, which is the rate-determining step in the removal of oxygen from the surface. Moreover, we also elucidate the role of healing surface oxygen vacancies through diffusion of subsurface oxygens by conducting kMC simulations with a sublayer. The role of subsurface oxygen is found to delay and moderate the autocatalytic behavior. Future work will examine the role of surface processes identified in the thermal reduction (e.g. autocatalytic formation of oxygen vacancies, subsurface oxygen healing) on CO oxidation on PdO(101) surfaces at lower reaction temperatures.