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Abstract
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.
Highlights
The discovery of the catalytic properties of gold nano-particles was one of the most exciting recent scientific findings, which led to a true renaissance of gold chemistry, and catalysis by gold has rapidly become a hot topic in chemistry
O2 adsorption is a key process for further understanding the mechanism of selective CO oxidation on gold catalysts
The performance of the Au/γ-Al2O3 catalyst both under normal preferential oxidation (PROX)-type operation as well as for normal CO oxidation has been studied in detail in previous publications [48,53,54,55]
Summary
The discovery of the catalytic properties of gold nano-particles was one of the most exciting recent scientific findings, which led to a true renaissance of gold chemistry, and catalysis by gold has rapidly become a hot topic in chemistry. An efficient selective CO oxidation (SCO) catalyst must be highly active in CO oxidation, at temperatures compatible with the operation of PEM fuel cells, and very selective towards. Nanometer-sized gold particles supported on γ-Al2O3 have been investigated as efficient catalysts for the low temperature CO oxidation as well as the selective catalytic oxidation of CO, under conditions compatible with the operation of PEM fuel cells [48]. For successful operation as a selective CO oxidation catalyst in a reformer-PEMFC system, the catalyst must operate in a complex feed comprising CO, O2, H2, CO2, H2O, and N2, and be capable of converting almost 100% of the CO Taken together, this represents a demanding target, in which coadsorption effects play a vital role in the catalytic action, since they have a large effect on the kinetics and dynamics of surface reactions. The present study was carried-out in a wide temperature range (50–300 C), both in excess as well as in the absence of H2, permitting mechanistic insights and kinetic as well as energetic comparisons, of the sorption processes of SCO reactants over Au/γ-Al2O3
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