Nature and role of surface carbonates and bicarbonates in CO oxidation over RuO2

Base metal oxides have long been of interest as catalysts for oxidation of small molecules such as CO or NO, but practical applications are limited by surface poisoning processes. With growing interest in the oxidation activity of metal oxides, it is important to understand and ultimately to learn to bypass surface poisoning. RuO2, as a model metal oxide oxidation catalyst, is active for CO oxidation under UHV conditions but is deactivated by some surface poisoning processes at ambient pressures. In this work, we use plane-wave, supercell DFT calculations to characterize the structures of carbonate and bicarbonate on the RuO2(110) surface and determine their thermodynamic stability by constructing phase diagrams. We find that while a surface carbonate (CO32-) is stable at low O-2 pressures and high CO2 pressures, it is not stable under practical catalytic conditions. A surface bicarbonate (HCO3-) is more stable and deactivates the RuO2 surface over a wide range of CO2 and oxygen pressures in the presence of trace amounts of water. Therefore, bicarbonate is likely the species responsible for experimentally observed surface poisons that deactivates RuO2 during CO oxidation. OH* might also be a candidate responsible for surface poisoning when CO2 pressure is very low. This study demonstrates that surface poisoning is sensitive to reaction environments such as water and CO2 pressures.