Surface studies of planar model HDN catalysts
Creators: name: Leung, Yin-Ling
Publisher: name: University of British Columbia
Year: 2009

description: 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. Observations of the Al(KLL) Auger electron peak and changes in the corresponding Auger parameter, suggest that the calcination at 450°C yields a new surface bonding. It is hypothesized that direct Mo-O-Al surface linkages are formed at this particular temperature, and that this provides the driving force for the enhanced Mo dispersion observed. 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. The sulfiding of the 450°C-calcined sample seems particularly influenced by the Mo-support interaction, and the Auger parameter changes in a way that suggests the rupture of its Mo-O-Al linkages. High-area catalysts were treated as similarly as possible to the model catalysts for some initial tests on the hydrodenitrogenation (HDN) reaction for pyridine. The sulfided 450°C-calcined sample provides better HDN activity than the other sulfided samples. XPS study of the sulfided 450°C-calcined model sample indicated the presence of a non-stoichiometric sulfide (MoS₂-x) which may be a factor in the reactivity. 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 but the degree of reduction depends on the reaction MoO₃ being reduced temperature. Heating to 350°C indicates some conversion to Mo(+5.) and "O-rich" Mo(+4) components, while heating to 450°C and then to 700°C give respectively a "N-rich" Mo(+4) form and a Mo(+3) oxynitride as the dominant components. Comparisons are made with treating the original MoO₃/Mo sample by cold plasmas formed by H₂ and by N₂. The whole evidence is consistent with the nitridation of MoO₃/Mo by NH₃ involving initial hydrogenation, with the subsequent elimination of water, reduction and the effective replacement of O by N. 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 MoO₃/Mo system. In general, the reduction behavior for MoO₃/AlOx is similar to that of MoO₃/Mo, but the metal in MoO₃/SiO₂ is more easily reduced (Mo(0) is detected after the reaction with NH₃ on the SiO₂ system but not on AlOx). Comparisons of heating rates for the nitridation step from 350 to 450°C were made for the MoO₃/Mo and MoO₃/AlOx samples. Differences between the higher heating rate (100 K/h) and lower heating rate (40 K/h) are incremental but definite. In this work, the lower heating rate appears to help both the O-N replacement and the metal reduction. These observations contrast with conclusions reached previously from x-ray diffraction for the evolution of bulk phases during the nitridation process the different heating rates.