Insights into the formation of methane and methanol via photocatalytic water splitting in in the presence of methyl iodide on a TiO₂(100) surface

The detection of intermediate species during surface photoinduced and photocatalytic reactions and the correlation of their dynamics with the properties of a surface is crucial to fully understand and control heterogeneous reactions. In this study, a novel technique that combines time-of-flight mass spectrometry with laser spectroscopy and fast surface preparation with molecules is employed to investigate the mechanism of photoinduced reactions through the direct detection of intermediate species and final products. As a model system, the photoinduced reaction of D₂O in the presence of CH₃I is investigated on an n-type (Nb) doped TiO₂(100) and surface. The reaction is induced by a femtosecond (fs) laser pulses with the central wavelength at 266 nm that triggers the photocatalytic water splitting and simultaneously the photodissociation of the CH₃I molecule via A-band excitation. A subsequent fs laser pulse in the UV domain is used to ionize the reaction intermediates and final products, which are immediately analyzed by the TOF-MS. Intermediates such as D, OD, and DO₂ radicals are detected when only water is dosed on TiO₂. When both D₂O and CH₃I are dosed on the surface, besides the intermediate species mention above, CH₃ and I radicals as well as methane (CH₃D) and methanol (CH₃OD) are observed. This indicates that D and OD radicals obtained via water splitting can attach to CH₃ radicals to form methane and methanol. Details about the surface properties and how this affects the surface chemical reactions will be provided.