The increase in global energy demand has brought to prominence the investigation of surface photocatalytic reactions that lead to the production of fuels from water and carbon-containing molecules. In this study, a novel technique that combines time-of-flight mass spectrometry (TOF-MS) 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 doped TiO2(110) 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 deuterium (D), hydroxyl (OD), atomic oxygen (O), and hydroperoxyl (DO2) radicals are detected when only water is dosed on TiO2(110). They indicate that the water splitting reaction leads primarily to the formation of D and OD radicals. The detection of DO2 radical is attributed to the formation of a vibrationally excited hydrogen peroxide D2O2 that loses a D atom through collisions with the surface and adjacent molecules, a mechanism that has been predicted theoretically. When both D2O and CH3I are dosed on the surface, besides the intermediate species mention above, methyl (CH3) and iodine (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. This investigation demonstrates that our experimental approach can be used to obtain new insights into the mechanism of surface photocatalytic reactions.

Mechanistic understanding of methane and methanol formation via photocatalytic water splitting in the presence of methyl iodide on a TiO2(110) surface

ACS Spring Meeting 2021

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