Photocatalytic oxidation of methanol using porous Au/WO3 and visible light

Abstract

Porous tungsten oxide (WO₃) visible light photocatalysts were prepared via hard templating using mesoporous silica (SiO₂). Nitrogen physisorption and powder XRD confirmed the greater surface area and smaller nanocrystallites of porous WO₃ compared to bulk WO₃ ((B)-WO₃). Sonochemical deposition of gold nanoparticles (Au NPs) was used to further modify the porous WO₃ catalyst. The photocatalytic oxidation of methanol (MeOH) under continuous flow conditions was used to evaluate the activity of the porous photocatalysts. Porous WO₃ exhibited significantly higher MeOH oxidation activity compared to (B)-WO₃ due to its increased surface area. In separate experiments, WO₃ and SiO₂-supported Au NPs were highly selective MeOH photooxidation catalysts for the production of formaldehyde (HCHO, 60%) and methyl formate (MF), respectively. This indicated that two different photocatalytic mechanisms, namely band gap excitation (WO₃) and surface plasmon resonance (SPR, Au NPs), were responsible for the two different oxidation products. When combined with Au NPs, WO₃ showed higher rates of MeOH oxidation compared to pure WO₃ due to more efficient electron and hole separation. In addition, the reduction of O₂ to H₂O by photoexcited electrons was more efficient in the presence of Au NPs compared to pure WO₃. At higher Au NP loadings, however, selective oxidation of MeOH decreased at the expense of greater CO₂ formation due to increased oxidation activity of the combined WO₃–Au catalyst.