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

Reversibly and irreversibly adsorbed water on AlPO4 and CePO4-AlPO4 (Ce/Al = 1/9) were determined by a TPD method and an electro-balance. The amounts of reversible and irreversible adsorption over AlPO4 were larger than that Of CePO4-AlPO4. However, the surface concentration of surface hydroxyls on AlPO4 was smaller than that on CePO4-AlPO4. Catalytic activity for CCl2F2 decomposition of AlPO4 is smaller than that Of CePO4-AlPO4. To explain these results, a surface intermediate, O-surface-CF2-O-surface, is proposed. The stabilization energy was calculated for the surface species; CCl2F2 interacting with surface hydroxyls and the O-CX2-O (X = Cl or F) type surface species. The calculations suggest that P-(OsurfaceHCl)-Cl-...-CF2-(ClHOsurface)-H-...-P species and Al-O-surface-CF2-O-surface-Al species are the most stable. To clarify the reaction mechanism, trace amounts of by-products in the decomposition of CH2FCF3 which is hydrofluorocarbon with two carbon atoms were analyzed and found the formation of HO-CH2CF2-OH which may be derived from the surface intermediate, O-surface-CH2CF2-O-surface. This strongly supports the formation of bidentate surface intermediates. (C) 2004 Elsevier B.V. All rights reserved.