Identification and exclusion of intermediates of photocatalytic CO2 reduction on TiO2 under conditions of highest purity

Abstract

Using a high-purity gas phase photoreactor and highly sensitive trace gas analysis, new insights into the mechanism of photocatalytic CO₂ reduction on TiO₂ P25 have been obtained. The reactor design and sample pretreatment excludes product formation from intermediates. Apart from CO₂, the only other reactant offered to the catalyst is water. The main products found on this prominent photocatalyst are methane and carbon monoxide. To distinguish between the three possible mechanisms reported in previous studies, likely intermediates of the reaction were added to the TiO₂ photocatalyst and their reactivity was followed by gas chromatographic analysis. Based on the results, we can clearly rule out CO as intermediate of any photocatalytic reaction pathway on TiO₂, because CO was not converted at all within a course of six hours. An improvement of carbonate formation on TiO₂ brought about by surface-doping with sodium decreased product yields, so carbonates are unlikely intermediates as well. Methanol, formaldehyde and formic acid were exclusively oxidized back to CO₂. We thus support a mechanism running over C₂-intermediates, and we tested our hypothesis by reacting glyoxal, glyoxylic acid, acetic acid and acetaldehyde on TiO₂. The reactions of acetaldehyde and acetic acid led to product distributions very similar to those obtained from CO₂ under the standard reaction conditions, strongly supporting the C₂ mechanism. This mechanism can also explain the small amounts of ethane usually found in the product mixture.