Niobia-supported nickel molybdenum catalysts: Characterisation of the oxide form

Nickel, molybdenum and nickel molybdenum catalysts supported on niobia and on alumina were prepared by wet-point impregnation and calcination at 723 K. The catalysts were characterised in their calcined form using nitrogen adsorption (BET method), X-ray diffraction, X-ray fluorescence spectroscopy, diffuse reflectance spectroscopy in the UV visible region, Raman spectroscopy, X-ray photoelectron spectroscopy and temperature-programmed reduction. Pronounced support effects were observed on the nature of the supported species. Thus, on alumina, the molybdenum was highly dispersed and predominantly present as polymolybdate clusters while, on niobia, there was evidence for a strong molybdena-support interaction, possibly with formation of a mixed niobium molybdenum oxide. The alumina support strongly stabilised supported oxidic nickel species towards reduction, in contrast with the niobia support. This was explained by a smaller polarisation of the Ni-O bond in the niobia than in the alumina environment. On alumina, the nickel and molybdenum oxidic species, when simultaneously present in the catalysts, interacted preferentially with each other rather than with the support, whereas the opposite occurred on the niobia support.