Due to its outstanding properties, titanium dioxide is a promising material for heterogenous (photo-) catalysis. The row-like structure of the rutile (110) surface is intrinsically bifunctional offering a regular ordered array of Lewis acidic and basic sites, which is an important factor for adsorption motifs and chemical reactivity. Moreover, rutile TiO2 can be partially reduced under the formation of point defects such as oxygen vacancies and Ti3+ interstitials. Both are carriers of electronic charge, leading to a well controllable self-n-doping. This provokes a drastically increased thermal and photochemical reactivity. By deposition of tungsten oxide clusters on rutile TiO2 (110), we mimic a bifunctional oxide-oxide catalysts. First, the electron transfer in such materials was investigated in an x-ray photoelectron spectroscopy (XPS) based study. In brief, the deposition of (WO3)n clusters enables the accumulation and pinning of Ti3+ near the surface for highly reduced TiO2 and additionally allows an increase of the Lewis acidity as well as basicity of surface sites. Moreover, we suggest the formation of partially anionic [(WO3)n]z- clusters by electron transfer from Ti3+ towards the clusters. We aim to obtain a systematic understanding for the role of such defects from temperature-programmed reaction spectroscopy (TPRS) studies under well-defined UHV conditions. In detail, we present the defect dependent population of different reaction pathways in the conversion of methanol. Particularly the deoxygenation reaction forming methane as well as the partial oxidation under production of formaldehyde turned out to be highly sensitive towards the presence of reduced titanium sites, (WO3)n clusters and different species of oxygen. In addition, the condensation route forming dimethyl ether as well as thermal and photostimulated C-C coupling reactions also appear as Ti3+ mediated processes.

Point defects at rutile TiO2: From minority to gamechangers in bifunctional oxide (photo-)catalysis

ACS Spring 2022

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