Transient studies on carbon monoxide oxidation over supported gold catalysts: support effects

摘要

The origin of support effects on the activity of supported gold catalysts was examined in details via the Temporal Analysis of Products (TAP) approach. Supported catalysts derived from interaction of an Au–phosphine complex Au(PPh3)(NO3) with as-precipitated titanium hydroxide Ti(OH)4* afforded remarkably high catalytic activity for low-temperature carbon monoxide oxidation compared to catalysts obtained by supporting Au(PPh3)(NO3) to conventional titanium oxide TiO2. At 300 K, for a carbon monoxide to oxygen 1:1 ratio, carbon monoxide conversion was 50% on Au/Ti(OH)4* catalyst, but 1% only on Au/TiO2. Single-pulse TAP experiments evidenced that, among the other factors, morphology and specific surface area of catalyst supports have an influence on intraparticle diffusion and adsorption features, which in turn have an influence on the reaction rate. While for the mesoporous Au/Ti(OH)4* catalyst, its internal surface area is efficiently utilized equally to the external surface for carbon monoxide oxidation, for the micro-porous Au/TiO2 catalyst, carbon monoxide oxidation reaction occurs pre-eminently on the external surface, because the internal diffusion was extremely slow. Based on pulse-response results, a new method to determine the intraparticle Knudsen diffusivity, Dp, was proposed. For argon on Au/TiO2, 1.4 × 10−7 cm2 s−1 Dp calculated value was in good agreement with values reported in literature.