Study of adsorption and reactions of methyl iodide on TiO₂

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.