The catalytic behaviour of H5PV, Mo, , 0, , heteropoly acid in the oxidation of n-butane to maleic anhydride is analysed in relation to that of its partially salified Cs salt and their FTlR spectra, UV-VIS diffuse reflectance spectra, thermogravimetry and reducibility tests as well as solid state 51V NMR and EPR characterizations are also compared. At temperatures around 573 K the acid sample dehydrates with partial reduction and the formation of lacunar 0x0-anions but with preservation of the Keggin unit structure. The creation of inter-Keggin V-0-V bonds during dehydration is suggested. The presence of the alkali metal in the secondary structure inhibits these effects. The superior catalytic performance of the acid sample in butane transformation is discussed in terms of the role of the presence of V5+ sites associated with a n inter-Keggin interaction.

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EPR and XPS have been used to study the redox reaction on CuTh oxide catalysts with different Cu/Th atomic ratios. Apart from the substitutional (S) sites occupied by Cu2+ ions both in the bulk and on the surface of thoria, other Cu2+ ions, occupying different sites on the surface were susceptible to H, and/or 0, treatment at different temperatures. In particular, copper(ii) ion pairs which can be obtained at high copper contents (Cu/Th > 0.01) are formed from one copper(i1) ion, reducible by hydrogen, and another, corresponding to substitutional surface sites (SJ, which remains stable towards hydrogen. The migration of some Cu2+ ions, from sites very susceptible to the cyclic redox treatment, is responsible for the growth of CuO crystallites on the surface of CuTh oxide catalysts.