Acid catalytic properties of reduced tungsten and niobium-tungsten oxides

•Structure of (Nb)WO3 modified by varying the calcination temperature.•NH3 and NH4+ stabilize hexagonal WO3.•Intermediate calcination temperature and Pd reduction: (NH4)0.33−x(H3O)yWO3−z.•(NH4)0.33−x(H3O)yWO3−z strong acid component in reduced Pd/(Nb)WOx.•Pd-loaded oxides are highly acidic and active hydroisomerization catalysts. Tungsten oxide and mixed niobium–tungsten oxides were prepared by calcination of re-precipitated tungsten and niobium precursors. These materials were characterized by N2 physisorption, XRD, XPS, UV–vis and UV Raman spectroscopy and H2-TPR. With increasing calcination temperature the oxides were seen to transform from a mixture of h-WO3 and HATB (T