Photocatalytic CO Oxidation over Nanoparticulate Au-Modified TiO2 Aerogels: The Importance of Size and Intimacy

We determine the activity of catalytic carbon monoxide (CO) oxidation as a function of the size of gold nanoparticles (Au NP), their weight loading, and their interfacial intimacy with similarly sized TiO2 NPs covalently bonded in an aerogel support. We use this Au+TiO2 series to correlate mechanisms that influence catalytic activity including hot electron injection and production, electron-hole pair formation, oxygen vacancy content, and electronic metal-support interaction (EMSI) stemming from photoinduced charge transfer from TiO2 to Au. The Au weight loading varies from 1 and 8 wt %, and Au NPs are either entrained within the oxide network of the aerogel during sol-gel synthesis (Au-IN-TiO2) or deposited on as-formed TiO2 aerogel (Au-ON/TiO2). Electron tomography reveals that Au-IN-TiO2 exhibits greater metal parallel to metal-oxide intimacy than Au-ON/TiO2 at the same weight loading, while increasing Au content (especially with Au-ON/TiO2) yields broader size distributions, larger Au NPs, and lower Au parallel to TiO2 intimacy. Comparing broadband photoactivity to dark activity reveals that high Au parallel to TiO2 intimacy and monodisperse small (similar to 5 nm) Au NPs diminishes catalytic activity under broadband illumination, while Au+TiO2 composites with high weight loading, some large (>50 nm) Au NPs, and decreased Au parallel to TiO2 intimacy enhance activity under broadband illumination.