Molecules (Apr 2012)
Kinetic Study of Oxygen Adsorption over Nanosized Au/γ-Al<sub>2</sub>O<sub>3</sub> Supported Catalysts under Selective CO Oxidation Conditions
Abstract
O<sub>2</sub> 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 O<sub>2</sub> adsorption, desorption and surface bonding, as well as the respective activation energies, over a nanosized Au/γ-Al<sub>2</sub>O<sub>3</sub> 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 H<sub>2</sub>, resulted in mechanistic insights and kinetic as well as energetic comparisons, on the sorption processes of SCO reactants. In the absence of H<sub>2</sub>, the rate of O<sub>2</sub> binding, over Au/γ-Al<sub>2</sub>O<sub>3</sub>, drastically changes with rising temperature, indicating possible O<sub>2</sub> dissociation at elevated temperatures. H<sub>2</sub> facilitates stronger O<sub>2</sub> bonding at higher temperatures, while low temperature binding remains practically unaffected. The lower energy barriers observed, under H<sub>2</sub> rich conditions, can be correlated to O<sub>2</sub> dissociation after hydrogenation. Although, H<sub>2</sub> enhances both selective CO reactant’s desorption, O<sub>2</sub> 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 O<sub>2</sub> 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.
