Session: General Catalysis:

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.

Mihai Vaida

University of Central Florida

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