Revealing the key oxidative species generated by Pt-loaded metal oxides under dark and light conditions

The role of key oxidative species in formic acid oxidation using neat and platinum-loaded TiO2, CeO2 and SiO2 was investigated. The catalytic reactions under three different illumination conditions; dark catalysis (i.e. no illumination), dark catalysis with UV light pre-treatment (denoted as pre-30) and photocatalysis (i.e. continuous UV illumination), were studied. The activities of neat TiO2, CeO2 and SiO2 were low under dark and pre-30 conditions while under illumination (photocatalysis) the semiconductor metal oxides, TiO2 and CeO2 showed much higher oxidation rates. For the Pt-loaded catalysts, the key active species were surface active oxygen (PtOads and O-ads(-)) under the dark catalytic condition with and without light pre-treatment while under photo catalytic condition, photogenerated holes and electrons are believed to form hydroxyl radicals (center dot OH) and superoxide radicals (center dot O-2(-)), respectively, for CeO2, TiO2, Pt/TiO2 and Pt/CeO2. Unexpectedly, Pt/SiO2 showed the highest activity under the dark catalytic condition. The cuboctahedral shape of the Pt deposits on the SiO2 surface, which promote a greater number of sharp edges/comers at the interface, is believed to have been advantageous for dissociating and activating adsorbed oxygen species compared to the Pt deposits on TiO2 and CeO2. The Pt deposit shape is thought to be responsible for the observed high activity of Pt/SiO2 for the dark catalytic oxidation of formic acid.