Keggin and Wells-Dawson heteropolyanions, PW12O403- and P2W18O626-, respectively, are nanosized transition metal-oxygen clusters belonging to the heteropolyacids (HPAs) family. They are widely used as catalysts, due to their strong Brönsted acidity, and their dispersion on solid supports favours the accessibility to their acid sites increasing their catalytic activity. Notably, the UV-irradiation significantly increases their catalytic performance .
A series of binary materials composed of Keggin or Wells-Dawson HPAs and SiO2, TiO2, ZrO2 or C3N4 has been prepared by impregnation and/or solvothermal methods. Remarkable differences in the (photo)catalytic activities of unsupported HPAs and binary materials are related to the structural changes of the supported HPAs due to the cluster-support interaction, which modifies their acidic strength and redox potentials. The nature of the support, particularly its acidic or basic character, defines the state of the deposited HPA. The application of XAS allowed to study the modes of interaction between the metal-oxide cluster and the support surface. Fitting of W L3-edge EXAFS spectrum of the HPA/SiO2 prepared by impregnation revealed only a slight increment of the coordination number of the tungsten atom both in W=O and W-O-W bonds, suggesting that the supported HPA remained almost intact. The solvothermal impregnation, although gave almost the same result for silica support, drastically affected the W arrangement in HPA/TiO2 and HPA/ZrO2. The increase of the coordination number of W=O and the decrease of that of the W-O-W implies the occurrence of strong HPA interactions with the TiO2 and ZrO2 surfaces. For the solvothermal preparation of the HPA-oxide composites the increase of the W=O coordination number followed the sequence SiO2<TiO2<ZrO2 having the values of 1.25, 3.80, 4.40, respectively. This dramatic change could be explained by the partial destruction of the HPA cluster forming WO3 species. These changes, in addition to the bulk and surface features of the supported HPAs, influenced the reaction performance of the (photo)catalytic propylene hydration  and 2-propanol dehydration [4,5] to a significant extent.References: