Composition and catalytic properties of products from the reduction of NiMoO 4

Abstract

Reduction of α-NiMoO 4 by hydrogen was studied in the temperature range 350–700°C using X-ray diffraction, temperature-programmed reduction and magnetisation measurements. Water vapour was found to have a strong influence on the composition of the reduction products. In the presence of water, pure ferromagnetic nickel and crystalline MoO 2 were found to be reaction products, whereas without water the products were non-magnetic Ni-Mo alloy and poorly crystalline MoO 2. A reduction mechanism is proposed, including initial formation of nickel and MoO 2, followed by their interaction due to spillover of hydrogen. The effect of water was found to be kinetic in nature. Hydrogen temperature-programmed desorption (TPD) spectra of reduction products were studied vs. reduction temperature and time. Maxima on TPD curves at 125,340 and 450° C were tentatively assigned to various three-fold adsorption sites on the surface of Ni-Mo alloy. Catalytic activity in toluene hydrogenation and hydrogenolysis was investigated. Ni-Mo alloying was found to suppress both catalytic reactions, but especially the hydrogenation activity. Trends of activity in both reactions were related to low-temperature features of the TPD spectra. Results of catalytic tests were explained using the ensemble model: only nickel atoms participate in hydrogenation ensembles (Ni 3), whereas hydrogenolysis can proceed on Ni-Mo mixed species.