Photocatalytic CO2 reduction vs. H2 production : the effects of surface carbon-containing impurities on the performance of TiO2-based photocatalysts

Photocatalytic CO2 reduction is a useful way to convert solar energy into fuels, mimicking artificial photosynthesis. The gas phase photocatalytic reduction of CO2 was investigated with a series of TiO2-based photocatalysts, either bare or modified by Cu(II) grafting and/or by Pt nanoparticles deposition and the results obtained in this reaction were compared with those obtained in the photocatalytic production of hydrogen by photosteam reforming of methanol-water mixtures employing the same photocatalysts series. The rates of the two reactions largely depend on the type of photocatalyst and exhibit an almost parallel behaviour, both being higher with photocatalysts containing noble metal nanoparticles. However, the yields in CH4, CO and other products obtained from photocatalytic CO2 reduction still remain very low and often decline during the runs. Furthermore, methane was found to be produced under irradiation when the photocatalysts were contacted with a gas phase containing water vapour and no carbon dioxide. This points to a major role of carbon-containing impurities on the photocatalyst surface, which may act both as hole scavengers and as carbon source. Two step photocatalytic tests, i.e. a cleaning step in the presence of water vapour followed by purging of the gas phase and a second step after CO2 addition, allow one to discern between the carbon-containing gaseous products originated from carbon impurities and those produced in the presence of CO2. In this second step, the presence of copper grafted on the photocatalyst surface favours CO evolution with respect to fully reduced CH4.