Supported nickel catalysts: Preparation and characterisation of alumina-, molybdena-, and silica-supported nickel, and the identification of reactive oxygen on these catalysts by exchange with isotopically labelled carbon dioxide

Abstract

Nickel catalysts, supported on alumina, silica, and molybdena, have been prepared by impregnation and co-crystallization. In the precursor state the catalysts were characterised by UV–visible spectroscopy, thermogravimetric analysis/differential thermal analysis (TGA/DTA), and X-ray photoelectron spectroscopy (XPS). The nickel was principally in the 2+ oxidation state with an octahedral coordination. However, the ligand sphere surrounding the nickel ion was sensitive to the support, indicating that the species on the different supports were not identical thus suggesting a metal complex–support interaction. Reduction was followed by temperature programmed reduction (TPR) and TGA, the results of which indicated that reduction and decomposition of nickel nitrate occurred simultaneously. X-ray diffraction (XRD) analysis revealed that, with the Ni/MoO₃ sample, no hydrogen bronze was formed on reduction. The reduced catalysts were characterised by carbon monoxide chemisorption, carbon dioxide chemisorption, and by reaction of buta-1,3-diene with dihydrogen. In the absence of a dihydrogen stream it was found that the catalysts adsorbed no carbon monoxide due the presence of sub-monolayer quantities of surface oxygen. The extent of the oxygen was quantified by exchange with isotopically labelled carbon dioxide. Differences in the electronic nature of the nickel between the Ni/MoO₃ sample and the other catalysts wererevealedbytheirbehaviourtowardsbuta-1,3- diene hydrogenation.