Dispersed NbOx catalytic phases in silica matrixes : influence of Nb-concentration and preparative route

Niobium oxide catalytic phase was dispersed in/over silica host structures by using different Nb sources (ammonium niobium oxalate complex and niobium pentaethoxide) and methodologies (coprecipitation, sol-gel, and impregnation). Three series of completely amorphous silica-niobia catalysts were obtained with 5, 15, 30, 45, and 60 mass % Nb2O5 by coprecipitation (aqueous route, from ammonium niobium oxalate) and sol-gel (organic route, from niobium pentaethoxide) and with 5, 10, and 20 mass % Nb2O5 by impregnation on a finite silica, from ammonium niobium oxalate. The surface and bulk catalyst properties of all the series of samples were studied by several physicochemical techniques [N2 adsorption/desorption, thermal gravimetric analysis (TGA), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflection spectra (UV-vis DRS), temperature-programmed reduction (TPR)], and the acid properties of the surfaces were investigated by base (2-phenylethylamine, PEA) titration in liquid (cyclohexane). Evidence from both XPS (3d Nb bands and O Is band) and UV-vis DRS (ligand-to-metal charge-transfer transitions (LMCT) from O2- to Nb5+ and edge energies) measurements showed that the Nb dispersion increased with Nb dilution in the host silica structures. The number of coordinated oxygens around the Nb center was 6, for the highest diluted samples, and converged to 2.5, indicating the progressive formation of Nb 2O5 nanodomains. As the Nb2O5 concentration decreased in the catalyst composition, the surface area values expanded and Nb dispersion increased, owing to Nb-O-Si linkage formation. The synthesis by sol-gel permitted accommodating a higher surface concentration of niobia than by the coprecipitation route.