FT-IR Spectroscopic Studies Of The Adsorption And Reaction Of Some Small Molecules On Supported Molybdenum Nitride Catalysts

Transition metal nitrides have shown catalytic properties resembling group VIII metals in a number of hydrogen-involved reactions, including hydrogenation, hydrodenitrogenation, hydrodesulfurization and isomerization, etc. However, there are few researches dealing with the reaction mechanisms. In this thesis, the adsorption and reaction behaviors of some simple molecules were studied on both nitrided and reduced passivated Mo2N/y-Al2O3 catalysts mainly by in situ FT-IR spectroscopy, aiming at a better understanding of the surface properties of fresh nitrides and providing initial explanations of reaction mechanisms.Microcalorimetric studies of CO adsorption on molybdenum nitrides show that the distribution of surface sites on nitrided Mo is more heterogeneous than that on reduced passivated one. There are both Mo (in low valence state) and N sites on nitrided Mo2N while only Mo sites (in high valence state) on reduced passivated one.The adsorption mode of thiophene on Mo2N/y-Al2O3 catalysts is determined to be V(S). Nitrided Mo2N/y-Al2O3 is more reactive to thiophene/H2 compared with reduced passivated one, which is attributed to both the high density of surface sites and special electronic property of nitrided Mo2N/y-Al2O3. The surface reaction of thiophene leads to the sulfidation of molybdenum nitride catalysts and the formation surface a-bonded C4 species, which are considered as the reasons for the deactivation of nitride catalysts in HDS process. H2S is found more aggressive than thiophene to the surface sites of nitride. Two kinds of sulfur species are formed on the sulfided Mo2N/y-Al2O3 catalyst. A complete regeneration of the sulfided nitrides needs a first oxidation and a subsequent renitridation.By a comparative study of the adsorbed species from 1,3-butadiene and 1-butene on nitrided Mo2N/y-Al2O3, it is proposed that the high selectivity in 1 -butene for the hydrogenation of 1,3-butadiene on y-Mo2N can be explained in term of competitive adsorption, namely, the Tid-adsorbed butadiene strongly inhibits the adsorption of 1 -butene and blocks its further hydrogenation and isomerization.A study of low-temperature adsorption of 1-butene shows that the double-bond migration of 1-butene is mainly proceeded via an alkyl species on the surface cus Mo sites of nitrided Mo2N/y-Al2O3 catalyst. This resembles the behaviors of Group VIII metals in the isomerization of butenes.