Decomposition of chlorofluorocarbons over metal phosphate catalysis. II. Origin of the stability of AlPO₄ and the location of CE as a promoter

Aluminum-phosphate catalysts prepared from aqueous solution of Al(NO₃)₃6H₂O and H₃PO₄ in acidic condition are effective for hydrolysis of CCl₂F₂. Active species of the AlPO₄ catalysts were studied and it was found that not Al(PO₃)₃ but pure AlPO₄ was active. AlPO₄ was amorphous when calcined at 1000°C for 5 h. But a slight crystallization of AlPO₄ took place after the reaction at 400°C for 1000 h. Crystallization of AlPO₄ proceeded only in the presence of both CCl₂F₂ and water vapor. Replacement of 5-10 at.% of Al in AlPO₄ by Ce increased the catalytic activity without changing the selectivity. The Ce-promoted catalysts were composed of the mixture of AlPO₄ and CePO₄. The compositions of the catalysts were the same as those of parent liquor in preparation and it was almost uniform in the surface and the bulk. AlPO₄ in the Ce-promoted AlPO₄ catalyst was partially crystallized after the calcination at 1000°C for 5 h. (C) 2000 Elsevier Science B.V.