Adsorption of water vapor on the AlPO4-based catalysts and reaction mechanism for CFCs decomposition

2004 
Abstract Reversibly and irreversibly adsorbed water on AlPO 4 and CePO 4 AlPO 4 (Ce/Al = 1/9) were determined by a TPD method and an electro-balance. The amounts of reversible and irreversible adsorption over AlPO 4 were larger than that of CePO 4 AlPO 4 . However, the surface concentration of surface hydroxyls on AlPO 4 was smaller than that on CePO 4 AlPO 4 . Catalytic activity for CCl 2 F 2 decomposition of AlPO 4 is smaller than that of CePO 4 AlPO 4 . To explain these results, a surface intermediate, O surface CF 2 O surface , is proposed. The stabilization energy was calculated for the surface species; CCl 2 F 2 interacting with surface hydroxyls and the OCX 2 O (X = Cl or F) type surface species. The calculations suggest that PO surface H⋯ClCF 2 Cl⋯HO surface P species and AlO surface CF 2 O surface Al species are the most stable. To clarify the reaction mechanism, trace amounts of by-products in the decomposition of CH 2 FCF 3 which is hydrofluorocarbon with two carbon atoms were analyzed and found the formation of HOCH 2 CF 2 OH which may be derived from the surface intermediate, O surface CH 2 CF 2 O surface . This strongly supports the formation of bidentate surface intermediates.
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