Abstract
Chemisorptions of CH{sub 4} on well-degassed MgO were examined by means of the temperature-programmed desorption and ab initio molecular orbital methods. Three types of chemisorptions (M{sub 1}, M{sub 2}, and IM) were found experimentally, all of which are very similar in nature and give heterolytically dissociated species, CH{sub 3}{sup {minus}} and H{sup +}. The active site M{sub 1} consists of the nearest pair of low-coordinated ions, Mg{sub LC}{sup 2+} and O{sub LC}{sup 2{minus}}, while the M{sub 2} and the IM sites consist of an isolated pair of them. Therefore, M{sub 1} adsorption could proceed even below room temperature but M{sub 2} IM adsorptions were observable only under severe adsorption conditions. The molecular orbital calculations strongly supported the above experimental results, especially the presence of the M{sub 1} adsorption. Among four models examined, only heterolytic adsorption which led to Mg-CH{sub 3} and O-H bond formations gave a stable product with an activation energy of 10.7 kcal mol{sup {minus}1} at the MP2 level (or 7.2 kcal mol{sup {minus}1} at the HF level).
Published Version
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