Artificial Photosynthesis over Crystalline TiO2-Based Catalysts: Fact or Fiction?

The mechanism of photocatalytic conversion of CO2 and H2O over copper oxide promoted titania, Cu(I)/TiO2, was investigated by means of in situ DRIFT spectroscopy in combination with isotopically labeled 13CO2. In addition to small amounts of 13CO, 12CO was demonstrated to be the primary product of the reaction by the 2115 cm−1 Cu(I)−CO signature, indicating that carbon residues on the catalyst surface are involved in reactions with predominantly photocatalytically activated surface adsorbed water. This was confirmed by prolonged exposure of the catalyst to light and water vapor, which significantly reduced the amount of CO formed in a subsequent experiment in the DRIFT cell. In addition, formation of carboxylates and (bi)carbonates was observed by exposure of the Cu(I)/TiO2 surface to CO2 in the dark. These carboxylates and (bi)carbonates decompose upon light irradiation, yielding predominantly CO2. At the same time a novel carbonate species is produced (having a main absorption at ∼1395 cm−1) by adsorption of photocatalytically produced CO on the Cu(I)/TiO2 surface, most likely through a reverse Boudouard reaction of photocatalytically activated CO2 with carbon residues. The finding that carbon residues are involved in photocatalytic water activation and CO2 reduction might have important implications for the rates of artificial photosynthesis reported in many studies in the literature, in particular those using photoactive materials synthesized with carbon containing precursors