C. Su, J. Yeh, C. Chen
Aug 15, 2000
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0
Influential Citations
24
Citations
Journal
Journal of Catalysis
Abstract
Abstract The adsorption, thermal reactions, and photochemistry of methyliodide were studied on powdered TiO 2 by transmission infrared spectroscopy, and on a TiO 2 (110) single-crystal surface by temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS). CH 3 I is either adsorbed molecularly or adsorbed dissociatively, forming methoxy groups on powdered TiO 2 by thermal activation as observed by IR spectroscopy. The dissociation of CH 3 I is enhanced by the presence of surface hydroxyl groups. CH 4(g) evolves thermally as a major product when CH 3 I reacts with powdered TiO 2 in the absence of oxygen. However, only oxygen-containing reaction intermediates and products are found when oxygen is present, including CH 2 O (g) , (CH 3 ) 2 O (g) , CO (g) , CO 2(g) , and H 2 O (g) . (CH 3 ) 2 O (g) is formed via a coupling reaction of two adsorbed methoxy groups. For the thermal reactions on TiO 2 (110), desorption of (CH 3 ) 2 O (g) is observed at multilayer coverages by TPD study. The formation of (CH 3 ) 2 O (g) on TiO 2 (110) occurs at much lower temperatures. It starts to evolve at ∼200 K and reaches its maximum rate at 250–330 K depending on surface heterogeneity in the TPD study. The Williamson synthesis process is proposed for the generation of (CH 3 ) 2 O (g) in this case. In addition, surface iodine formed by X-ray irradiation on CH 3 I-covered TiO 2 is not stable and is desorbed by 570 K. In the photochemistry of CH 3 I over powdered TiO 2 , when O 2 is absent, negligible reactions occur. In the presence of O 2 , some CH 3 I is photooxidized to surface formate groups and CO 2(g) . This reaction is most likely initiated by superoxide, O 2 − , formed upon irradiation.