Kinetic study of oxygen adsorption over nanosized Au/γ-Al₂O₃ supported catalysts under selective CO oxidation conditions.

O₂ adsorption is a key process for further understanding the mechanism of selective CO oxidation (SCO) on gold catalysts. Rate constants related to the elementary steps of O₂ adsorption, desorption and surface bonding, as well as the respective activation energies, over a nanosized Au/γ-Al₂O₃ catalyst, were determined by Reversed-Flow Inverse Gas Chromatography (RF-IGC). The present study, carried-out in a wide temperature range (50-300 °C), both in excess as well as in the absence of H₂, resulted in mechanistic insights and kinetic as well as energetic comparisons, on the sorption processes of SCO reactants. In the absence of H₂, the rate of O₂ binding, over Au/γ-Al₂O₃, drastically changes with rising temperature, indicating possible O₂ dissociation at elevated temperatures. H₂ facilitates stronger O₂ bonding at higher temperatures, while low temperature binding remains practically unaffected. The lower energy barriers observed, under H₂ rich conditions, can be correlated to O₂ dissociation after hydrogenation. Although, H₂ enhances both selective CO reactant's desorption, O₂ desorption is more favored than that of CO, in agreement with the well-known mild bonding of SCO reactant's at lower temperatures. The experimentally observed drastic change in the strength of CO and O₂ binding is consistent both with well-known high activity of SCO at ambient temperatures, as well as with the loss of selectivity at higher temperatures.