Carbonate formation on Al2O3 thin film model catalyst supports

The formation of carbonate species on alumina upon CO and CO2 exposure was studied by PM-IRAS and XPS, utilizing the model system of an ultrathin alumina film grown on NiAl(110). No carbonates were detected under UHV conditions, even after exposures up to 100.000 L of the gasses. In contrast, in a 100 mbar CO2 atmosphere the formation of monodentate carbonates was identified. The surface concentration of the carbonates increased after generating defects on the alumina film by Ar+ ion bombardment. It is suggested that this kind of carbonate species is produced by reaction of coordinatively unsaturated O2− ions of alumina with the C-atom of the CO2 molecule. This is corroborated by the observation that the amount of carbonates further increased when CO2 and oxygen were dosed simultaneously. In agreement with the “water gas shift” mechanism previously proposed for carbonate formation on high surface area alumina powders, no carbonates were detected from CO even upon mbar exposure, consistent with the absence of the required OH-groups on the model alumina support.