Abstract
Aiming at an improved mechanistic understanding of the preferential oxidation of CO on supported Au catalysts, we have investigated the competition between CO and H2 for stable, active oxygen (Oact) species on a Au/TiO2 catalyst during the simultaneous exposure to CO and H2 with various CO/H2 ratios at 80 °C and 400 °C by quantitative temporal analysis of products (TAP) reactor measurements. It is demonstrated that, at both higher and lower temperature, the maximum amount of active oxygen removal is (i) independent of the CO/H2 ratio and (ii) identical to the amount of active oxygen removal by CO or H2 alone. Hence, under preferential CO oxidation (PROX) reaction conditions, in the simultaneous presence of CO and H2, CO and H2 compete for the same active oxygen species. In addition, also the dependency of the selectivity towards CO oxidation on the CO/H2 ratio was evaluated from these measurements. Consequences of these findings on the mechanistic understanding of the PROX reaction on Au/TiO2 will be discussed.
Highlights
Gold nanoparticles supported on a variety of metal oxides are well known for their exceptional high activity and selectivity for various oxidation and reduction reactions already at very low temperatures [1,2,3,4]
Based on similar type temporal analysis of products (TAP) reactor measurements, from the identical amount of active oxygen species for the CO oxidation and the H2 oxidation, and from their almost identical dependency on the Au particle size, we recently proposed that hydrogen oxidation at 80 ̋ C and higher proceeds via a Au-assisted Mars-van Krevelen mechanism, identical to CO
CO and hydrogen are oxidized by the same stable active oxygen species under present reaction conditions, in the simultaneous presence of CO and H2 in the reaction atmosphere, as evidenced by the similar amounts of active oxygen removal from a O2 /Ar pulse oxidized
Summary
Gold nanoparticles supported on a variety of metal oxides are well known for their exceptional high activity and selectivity for various oxidation and reduction reactions already at very low temperatures [1,2,3,4]. CO oxidation (or preferential CO oxidation—PROX) reaction requires catalysts that are highly active for the CO oxidation (Equation (1)). The catalysts should have a very low activity for the continuous oxidation of H2 to water (Equation (2)), H2 reaches concentrations up to 75% (depending on the process of H2 generation) and is, present in large excess compared to CO in the reaction gas [6,9,10,11]. That an oxidation of H2 to adsorbed hydroperoxy-like species, which have previously been proposed to represent reaction intermediates in the preferential CO oxidation in the presence of H2 , may even be beneficial for the PROX activity of supported Au catalysts [10,12]
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