Gold-assisted E′ centres formation on the silica surface of Au/SBA-15 catalysts for low temperature CO oxidation

Abstract

SH-functionalized SBA-15 was synthesized and gold nanoclusters were grafted inside its channels. Different portions of such gold-containing material were subjected to a variety of thermal treatments, including calcination in the 300-560 °C temperature range and further treatments under a hydrogen atmosphere at diverse temperatures (up to 600 °C). The resulting materials were thoroughly characterized by CHS elemental analysis, X-ray diffraction, transmission electron microscopy, N2 physisorption and H2 chemisorption, as well as by Raman, NMR, FTIR and EPR spectroscopies, in order to investigate the influence of the temperature and atmosphere of the pretreatment on the gold particle size and oxidation state, the formation of defective E' centres on the silica support and their ability to induce the formation of reactive O2(-) surface species. Catalytic testing for the CO oxidation reaction was carried out in the 40-150 °C temperature range. Significant differences in catalytic behaviour were observed among the catalysts, which are discussed in the light of the characterization results. A mechanism for the "activation" of the non-reducible ("inert") SBA-15 support involving Au(0)→ Au(+) transformation is proposed, where gold metal plays an essential twofold role: it induces the formation of atomic hydrogen by dissociatively adsorbing H2 and triggers the formation of E' centres by transferring an electron to positively charged silicon species.