HRTEM and STEM-HAADF characterisation of Au–TiO2 and Au–Al2O3catalysts for a better understanding of the parameters influencing their properties in CO oxidation

Abstract

Gold catalysts supported on titania (Au–TiO₂) and alumina (Au–Al₂O₃) were prepared by deposition–precipitation with urea and then activated before characterisation and reaction in CO oxidation, either by calcination in air at 500 °C or reduction under H₂ at 300 °C. Gold nanoparticles with average size in the range 2–4 nm were obtained, with calcination leading to larger gold nanoparticles than reduction. For Au–TiO₂, high activity was observed in CO oxidation at room temperature, independent of the activation treatment. This high activity could not be correlated to the presence of sub-nanometer gold clusters as reported in the literature, since they could not be detected by atomic-resolution high-angle annular dark-field scanning-transmission electron microscopy (HAADF-STEM). In the case of Au–Al₂O₃, the performance in CO oxidation was found to strongly depend on the water content in the reaction gas feed and on the activation conditions, with calcination resulting in a poorly active catalyst whereas reduction gave activity of the same order as Au–TiO₂. A comparative study of Au–TiO₂ and Au–Al₂O₃ by electron microscopy did not reveal distinct differences in the shapes of the Au nanoparticles, which are mostly flattened through interaction with the substrate in both samples, with side profile shapes varying from rounded hemispherical to well faceted truncated cubo-octahedra. More faceting is found for the samples calcined at 500 °C than reduced at 300 °C. Various possible parameters affecting the catalytic properties of gold in CO oxidation are discussed in the context of the relevant literature.