Structural features and thermal stability of titania-supported 12-molybdophosphoric heteropoly compounds

The structure and thermal decomposition of unsupported and titania-supported 12-phosphoromolybdic acid (HPMo) and its Co and Ni salts have been studied by different techniques including X-ray diffraction, thermogravimetric analysis, FT-infrared, Raman, and X-ray spectroscopic techniques. The transformation of heteropoly anion of bulk HPMo begins at a temperature lower than for its TiO2-supported counterpart with the formation of a mixture of alpha- and beta-MoO3. In the thermal transformation of TiO2-supported HPMo, two steps have been identified. The intermediate species formed between the hydrated form of the heteropoly compound and the crystallized molybdenum oxides display the same nuclearity and composition as that of the Keggin precursor. The stability of the HPA supported on titania increases due to the anion-support interaction and this is even enhanced in the Co-and Ni-exchanged homologues.