TiO2-based photocatalysis : role of defects during photocatalytic hydrogen evolution and oxidation of methanol

Abstract

Climate change caused by anthropogenic greenhouse gas emissions is currently one of the most pressing global issues. One promising mitigation strategy is the replacement of fossil fuels with renewable energy sources, e.g. solar fuels like H2 obtained from photocatalytic water splitting. In the first part of my work, I investigate the deactivation of TiO2 loaded with Pt during photocatalytic H2 evolution reaction (HER) from aqueous methanol solu-tions under UV irradiation. I attribute this deactivation to a shift in the reaction mechanism that favors the formation of a methyl formate intermediate and which is accompanied by an increased CO generation. Photocatalytic experiments and thorough ex situ characterization suggest that oxygen vacancies that are inherent to the metal oxide or photoinduced during the reaction stabilize this mechanism shift, possibly through electron back-donation by surface Ti3+ or higher selectivity for detrimental side reactions. I suggest that the ratio of Pt particles to oxygen vacancies is a critical factor for the stability of the H2 generation rate. Further-more, transmission electron microscopy (TEM) and X-ray photoelectron spectros-copy (XPS) suggest possible strong metal-support interaction(SMSI)-like encapsu-lation of Pt by suboxides invoked by an increasing degree of reduction during HER. This work shows that the complexity and dynamics of heterogeneous pho-tocatalytic reactions are still not fully understood. Understanding individual steps of the HER at the example of the reference system of Pt-loaded TiO2 will help in designing better photocatalysts. A novel liquid phase ATR-FTIR cell has been de-veloped that allows for systematic studies at controlled parameters in order to identify possible different reaction pathways. In the second part of this work, I am performing a broad comparative study of different synthesis routes for the novel photocatalyst black TiO2. Generally, the synthesis of black TiO2 seems to need more specific conditions than often stated in literature rendering comparison across literature tedious to impossible. How-ever, concluding from this work, surface amorphization independent of valence band modifications is the crucial parameter for the synthesis of highly active pho-tocatalysts: H2-treated, largely amorphous TiO2 followed by a rapid cooling step outperforms P25-type TiO2 during photocatalytic H2 production under visible light by a factor of 2. Identification of key features in this work for the outstand-ing photocatalytic performance of black TiO2 can be applied to future material designs for improved photocatalytic performances.