Bulk Defect Dependence of Low-Temperature Partial Oxidation of Methanol and High-Temperature Hydrocarbon Formation on Rutile TiO2 (110)

Titanium dioxide (TiO2) is among the most studied model (photo)catalyst materials. The influence of surface point defects, like oxygen vacancies and particularly bulk defects such as Ti3+ interstitials, is usually underestimated or even ignored. We present a systematic study under well-defined UHV conditions illustrating the importance of such defects for the thermal reaction of methanol at the rutile TiO2 (110) single crystal surface by using temperature-programmed reaction spectroscopy (TPRS) and Fourier-transform infrared reflection-absorption spectroscopy (FT-IRRAS). It will be shown that the population of different reaction pathways, namely, the partial oxidation of methanol to formaldehyde and the deoxygenation forming hydrocarbons, especially methane, depends on the bulk defect density and the presence or absence of oxygen adsorbates. While at elevated temperatures molecular desorption is pronounced for the less defective substrates, for higher reduction grades the high-temperature deoxygenation channel via methoxy intermediates is favored. In addition, preadsorption of oxygen enables low- and high-temperature partial oxidation forming formaldehyde, likely from a dioxomethylene-like adsorbate.