Chemical states of metal-loaded titania in the photoreduction of CO2

Various sol–gel procedures and post-treatments were applied to modify the distribution of Cu on the surface of Cu/TiO2 catalysts in order to increase the production of methanol in the photoreduction of CO2. The chemical states of Cu in 2 wt.% Cu/TiO2 were characterized in detail as a follow-up the high Cu dispersion found in previous studies. XRD, XPS and XAS analysis reveal that the active Cu state for the photoreaction of CO2 is suggested to be highly dispersed Cu(I). The photoactivity decreases when Cu(I) changes to Cu(0) or aggregates after reduction with H2. An optimal distribution of Cu exists between the surface and bulk of TiO2 particles. The photocatalytic activity attains maximum when near 25% of the total Cu loading is located on the outermost surface of a TiO2 particle. Cu/TiO2 is a more active catalyst than Ag/TiO2 because Cu particles act as electron trapping sites while still maintain the mobility of photoelectrons.