Dilla Martin, Jakubowski Alina, Ristig Simon, Strunk Jennifer, Schlögl Robert
Max Planck Institute for Chemical Energy Conversion, 45470 Mülheim an der Ruhr, Germany.
Phys Chem Chem Phys. 2019 Jul 24;21(29):15949-15957. doi: 10.1039/c8cp07765g.
Although the photocatalytic reduction of CO2 to CH4 by using H2O as the oxidant presupposes the formation of O2, it is often not included in the product analysis of most of the studies dealing with photocatalytic CO2 reduction or it is reported to be not formed at all. The present study aims to clarify the absence of O2 in the photocatalytic gas phase CO2 reduction on TiO2. By modifying P25-TiO2 with IrOx co-catalysts it was possible to observe photocatalytic water splitting, i.e. the formation of gaseous O2 and H2 in almost stoichiometric amounts, without the use of sacrificial agents, while bare P25-TiO2 showed no activity in H2 and O2 formation under similar reaction conditions. Investigating the effect of improved H2O oxidation properties on the photocatalytic CO2 reduction revealed that the CH4 formation on P25 from CO2 was completely inhibited as long as the H2O splitting reaction proceeded. Furthermore, we found that a certain amount of O2 is consumed under conditions of photocatalytic water oxidation. A quantification showed it to be in the same order of magnitude as the oxygen which is missing as a byproduct from photocatalytic CO2 conversion. A detailed interpretation of the results in the context of the general understanding of the photocatalytic CO2 reduction with H2O on TiO2 allows the hypothesis that P25-TiO2 undergoes a stoichiometric reaction, meaning that the CH4 formation is not based on a true catalytic cycle and runs only as long as TiO2 can consume oxygen.
尽管以水作为氧化剂光催化还原二氧化碳生成甲烷预先假定了氧气的生成，但在大多数涉及光催化二氧化碳还原的研究的产物分析中，氧气往往未被纳入，或者据报道根本没有生成。本研究旨在阐明在二氧化钛上进行光催化气相二氧化碳还原时氧气缺失的情况。通过用氧化铱助催化剂对P25 - 二氧化钛进行改性，能够观察到光催化水分解，即在不使用牺牲剂的情况下，几乎以化学计量比生成气态氧气和氢气，而在类似反应条件下，裸露的P25 - 二氧化钛在氢气和氧气生成方面没有活性。研究改善的水氧化性能对光催化二氧化碳还原的影响表明，只要水分解反应进行，P25上由二氧化碳生成甲烷的过程就会被完全抑制。此外，我们发现，在光催化水氧化条件下会消耗一定量的氧气。定量分析表明，其数量级与光催化二氧化碳转化过程中作为副产物缺失的氧气相同。结合对二氧化钛上用水进行光催化二氧化碳还原的一般理解对结果进行详细解释，可以得出这样的假设：P25 - 二氧化钛发生的是化学计量反应，这意味着甲烷的生成并非基于真正的催化循环，并且仅在二氧化钛能够消耗氧气时才会进行。
