The adsorption, thermal reactions, and photochemistry of methyl iodide (CH3I) were studied on powdered TiO2 by transmission IR spectroscopy, and on a TiO2(110) single-crystal surface by TPD and XPS. CH3I dissociated on the powdered surface to form CH3O. The presence of isolated hydroxyl groups enhanced its dissociation rate. In the thermal chemistry, CH4(g) was the major product in the absence of O2. However, only oxygen-containing intermediates and products were found when oxygen was present, including CH2O(g), (CH3)2O(g), CO(g), CO2(g), and H2O(g) was formed via coupling reaction of two adsorbed methoxy groups. O2 was crucial for CH3I oxidation in the photochemistry. On the TiO2 single crystal, the TPD experiment was performed with a multilayer of CH3I, making a link to the solid-liquid interaction. (CH3)2O(g) was also the desorption product in the TPD study. The formation of (CH3)2O(g) on TiO2(110) occurred at much lower temperatures. It started to evolve at ~ 200 K and reached its maximum rate at 250-330 K depending on surface heterogeneity in the TPD study. The Williamson synthesis process was proposed for the generation of (CH3)2O(g) in this case. Surface iodine formed by X-ray irradiation on CH3I-covered TiO2 was not stable and was desorbed by 570 K.
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