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

Abstract

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, OsurfaceCF2Osurface, 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OsurfaceH⋯ClCF2Cl⋯HOsurfaceP species and AlOsurfaceCF2Osurface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, OsurfaceCH2CF2Osurface. This strongly supports the formation of bidentate surface intermediates.