Effect of titania surface modification of mesoporous silica SBA-15 supported Au catalysts: Activity and stability in the CO oxidation reaction

As part of an ongoing effort to understand the deactivation and improve the stability of metal oxide-supported Au catalysts in the low-temperature CO oxidation reaction while maintaining their high activity, we have investigated the influence of a mesoporous silica SBA-15 substrate on the activity and stability of Au/TiO₂ catalysts, which consist of a SBA-15 support surface modified by a monolayer of TiO_x with Au nanoparticles on top. The extent of the TiO_x surface modification was systematically increased, while the Au loading and the Au particle sizes were largely kept constant. Employing kinetic measurements at three different temperatures (30 °C, 80 °C, 180 °C) and a number of ex situ methods as well as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for catalyst characterization, we found that the activity of these catalysts increases significantly with the Ti concentration and with reaction temperature. The tendency for deactivation remains essentially unchanged. Detailed in situ DRIFTS measurements reveal that the Au nanoparticles are largely formed on the TiO_x surface-modified areas of the SBA-15 support and that the tendency for surface carbonate formation is very low. The observed deactivation may at least partly be related to the accumulation of molecularly adsorbed H₂O species, in particular at low temperatures (30 °C). These are likely to be formed from surface hydroxyl groups, they may affect the reaction either by blocking of active sites or by blocking the adsorption of reactants on the substrate. Other effects, such as reaction induced changes in the titania layer, must however, play a role as well, both at 80 °C and in particular at 180 °C, where accumulation of adsorbed species is negligible. The mechanistic ideas are supported by reactivation tests subsequent to calcination at 400 °C, which were found to fully restore the initial activity.