Pivotal Role of Holes in Photocatalytic CO 2 Reduction on TiO 2

Evidence is provided that in a gas‐solid photocatalytic reaction the removal of photogenerated holes from a titania (TiO 2 ) photocatalyst is always detrimental for photocatalytic CO 2 reduction. The coupling of the reaction to a sacrificial oxidation reaction hinders or entirely prohibits the formation of CH 4 as a reduction product. This agrees with earlier work in which the detrimental effect of oxygen‐evolving cocatalysts was demonstrated. Photocatalytic alcohol oxidation or even overall water splitting proceeds in these reaction systems, but carbon‐containing products from CO 2 reduction are no longer observed. H 2 addition is also detrimental, either because it scavenges holes or because it is not an efficient proton donor on TiO 2 . The results are discussed in light of previously suggested reaction mechanisms for photocatalytic CO 2 reduction. The formation of CH 4 from CO 2 is likely not a linear sequence of reduction steps but includes oxidative elementary steps. Furthermore, new hypotheses on the origin of the required protons are suggested.