| Model catalysts based on oxidized Mo, and formed on planar supports, have been studied by x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES) and secondary electron microscopy (SEM). The reason for using planar catalysts is to reduce the effects of uncertainties that may occur, as a result of differential charging, when direct measurements are made on real catalysts. The model catalyst approach is designed to provide new information about the chemistry which takes place during the preparation stages of real catalysts. It is hoped that the observations made here may help to understand behaviors for high-area supported Mo catalysts.;Alumina supported molybdena model catalysts, prepared by wet deposition of ammonium molybdate on oxide-coated planar Al, were studied as a function of calcination temperature (200, 350 and 450°C). Characterizations by XPS, SIMS and SEM indicate that the Mo disperses more uniformly on the support when the heating was done at 450°C as opposed to the lower temperatures.;Comparative sulfidabilities of these model catalysts (i.e. the uncalcined form and those calcined at 200, 350 and 450°C) were assessed by XPS. The Mo dispersion affects the sulfiding of the samples calcined at lower temperatures, and in particular the amount of Mo(+4) sulfide formed through the treatment with H2S is greater when the Mo is better dispersed on the initial sample.;Reaction pathways associated with the nitridation by NH3 of MoO3 thin films, formed on a Mo substrate, were studied by XPS. Core level and valence spectra are consistent with the MoO3 being reduced, but the degree of reduction depends on the reaction temperature.;The nitridation of MoO3 samples formed on alumina and silica supports have also been characterized by XPS, and comparisons made with behavior for the NH3 reaction with MoO3/Mo. The samples on the oxide supports appear to show easier O-N replacement compared with the MoO 3/Mo system. In general, the reduction behavior for MoO3/AlO x is similar to that of MoO3/Mo, but the metal in MoO 3/SiO2 is more easily reduced (Mo(0) is detected after the reaction with NH3 on the SiO2 system but not on AlOx). (Abstract shortened by UMI.). |
