Studies On Surported Molybdenum Carbide Catalysts: Preparation, Characterization And Selective Hydrogenation

Transition metal carbides have shown catalytic properties resembling groupVIII metals in a number of hydrogen-involved reactions, including hydrogenation,hydrodenitrogenation, hydrodesulfurization and isomerization, etc. However,there are few researches dealing with the catalytic active sites and reactionmechanisms on fresh transition metal carbides. In this thesis, the adsorption andreaction behaviors of some simple molecules were studied on fresh Mo₂C/γ-Al₂O₃catalysts mainly by in situ FT-IR spectroscopy, aiming at a better understanding ofthe surface properties of fresh carbide and providing the explanations for reactionmechanisms.For the fresh Mo₂C/Al₂O₃ catalyst, Mo^δ+(0 < δ< 2) is dominant on thesurface as probed by adsorbed CO giving an IR band at 2054 cm^-1. Unlikeadsorbed CO on reduced passivated Mo₂C/Al₂O₃, the adsorption of CO on freshMo₂C/Al₂O₃ catalyst is very strong and stable. The characteristics of COadsorption on fresh Mo₂C/Al₂O₃ catalyst indicate that fresh carbide resembles theproperties of noble metals in terms of the IR band position and adsorptionbehavior of CO. This is the direct IR evidence demonstrating the similarity insurface electronic propertybetween molybdenum carbide and noble metals. TPRand IR results on the fresh catalyst treated by H2 at different temperatures indicatethat temperatures above 450 K can decrease the amount of the carbon speciesdeposited on the surface of carbide and also partially remove the carbon atomsfrom molybdenum carbide. Flushing the fresh sample with H₂ at a hightemperature (> 873 K) can extensively remove both the carbon atoms at surfaceregion of carbide and deposited carbon species on carbide and dramatically alterthe surface structure of molybdenum carbide. Freshly prepared carbides are violently oxidized when they are exposed to air,so they have to be passivated using a 1% O2/He mixture to avoid the violentoxidation. Oxidation with oxygen is too fast to control the oxidation state of thesurface so that a mild oxidant is desired for the passivation of the freshmolybdenum carbide. We have used three different methods to passivateMo2C/Al2O3. Unlike fresh Mo2C/Al2O3 catalyst, IR spectra of adsorbed CO on aMo2C/Al2O3 catalyst passivated by O2 show that the surface oxide layer cannot beremoved by a H2-reduction at temperatures below 873 K. A H2-treatment attemperature higher than 873 K could eliminate oxygen of the surface oxide layer,but some carbon atoms of the carbide is also removed and a metallic Mo layerinstead of Mo2C is formed. It appears that oxygen, as a oxidant, is too active,while H2O or CO2 are mild enough to form a thin layer of protective oxycarbide atthe surface of molybdenum carbide. When the catalyst passivated with H2O orCO2 is reduced by H2 at 673 K, the active sites can be regenerated, while thecatalyst passivated withO2 is difficult to reduce. In order to get insight into the sulfur effect on carbide catalysts, thiopheneand H2S were chosen as the model sulfur-containing compounds to study theirinfluences on the nature of the surface sites of fresh Mo2C/Al2O3 catalysts.Thiophene can be easily hydrodesulfurized in the presence of H2 on Mo2C/Al2O3catalyst at temperatures as low as 473 K, at the expense of the sulfidation of thecarbide catalyst. The influence of H2S on Mo2C/Al2O3 catalyst is more evidentthan that of thiophene, and H2S can even sulfide the surface at temperatures closeto RT. The sulfided surface of Mo2C/Al2O3 catalyst can be partially regeneratedby a recarburization with a CH4/H2 at 1033 K, but it can be completelyregenerated by a first oxidation and a subsequent recarburization. Benzene can be easily hydrogenated on Mo2C/Al2O3 catalyst in the presenceof H2 even at RT and the only product detected is cyclohexane. CH3CN can bealso hydrogenated on Mo2C/Al2O3 catalyst at temperatures as low as 423 Kresulting in high selectivity of imines. 1,3-butadiene can be hydrogenated onMo2C/Al2O3 catalyst but withlow selectivityto 1-butene.