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.

Au@TiO2 Core–Shell Composites for the Photocatalytic Reduction of CO2

“…Considering that the formation of all products observed in the present study occurs by processes consuming only electrons, this decreasing production rate of H 2 for all tested samples (Figure ) can be explained by the lack of an efficient hole‐consuming reaction, that is, the lack of formation of gaseous dioxygen. As has been described previously for bare TiO 2 , instead of the formation of O 2 in the gas phase, oxygen‐derived species remain on or in the photocatalyst. The presence of those species eventually favors the back reaction, that is, oxidation of the formed hydrocarbons and hydrogen to CO 2 and H 2 O.…”

“…No mechanistic studies were conducted here, but the roughly equal amounts of CO and CH 4 formed photocatalytically may be indicative for the decomposition of a common intermediate in the mechanism. One option may be acetaldehyde, suggested previously as intermediate on bare TiO 2 . It must be noted that the mechanism of CH 4 formation on TiO 2 is still under debate, particularly because some of the proposed elementary steps are thermodynamically not possible on bare TiO 2 , such as the formation of the CO 2 anion radical located at −1.9 eV on the electrochemical energy scale.…”

“…One option may be acetaldehyde, suggested previouslya si ntermediate on bare TiO 2 . [32,33] It must be noted that the mechanism of CH 4 formation on TiO 2 is still under debate, particularlyb ecause some of the proposed elementarys teps are thermodynamically not possible on bare TiO 2 ,s uch as the formation of the CO 2 anion radical locateda t À1.9 eV on the electrochemical energys cale. In thermal catalysis with aA u/TiO 2 system, it was found that acetaldehyde strongly interacts with the perimeter site of Au nanoparticles on TiO 2 .…”

“…In brief, each catalyst sample was cleaned photocatalytically in an atmosphere containing only gaseous water and helium. This removed hydrocarbon impurities and minimized product formation from carbon sources other than CO 2 . For testing the activity in photocatalytic CO 2 reduction, each catalyst was brought in contact with a reactant gas mixture of 1.5 % of CO 2 and 0.6 % of H 2 O balanced by He.…”

“…Details concerning the photoca- talytic cleaning procedure and the actual photocatalytic CO 2 reduction can be found in ref. [33].I nb rief, each catalyst sample was cleaned photocatalytically in an atmosphere containing only gaseous water and helium. This removed hydrocarbon impurities and minimized product formation from carbon sources other than CO 2 .…”