Oxidation stability and activity of bulk, supported and promoted molybdenum carbide catalysts for methane reforming

The objective of the research presented here is to understand and to improve the oxidation stability of molybdenum carbide (Mo2C) as a catalyst for oxidative methane reforming. The first of three studies was aimed at identifying the behavior of low surface area, bulk Mo2C in the presence of reforming gases. Reforming products, such as CO and H 2, were found to inhibit Mo2C oxidation. However, the carburization of MoO2 by CH4 at these temperatures was found to be insignificant. A "stability ratio", was then formulated to correlate the partial pressure of reforming gases to the stability of Mo2C. In the second study, the oxidation stability of supported and promoted Mo 2C was studied. A ceria-promoted Mo2C/gamma-Al2O 3 was the most stable catalyst, though stability was heavily influenced on the synthesis procedure. The interaction between ceria and gamma-Al 2O3 is hypothesized to preserve the ceria particles from agglomeration allowing the ceria to actively undergo redox reaction on the surface of the catalyst during reforming. The effect of reforming products, specifically CO, on the stability of both the unpromoted and ceria-promoted Mo2C/gamma-Al2O3 catalysts was also investigated. Whereas high CO partial pressure was beneficial for the stability of Mo 2C/gamma-Al2O3, it significantly lowered the stability of the ceria-promoted Mo2C/gamma-Al2O 3 by reducing most of the ceria to Ce2O3, which primarily diminished the redox ability of the ceria, and also increased the tendency for CO disproportionation to form inactive carbon. The third study was a successful determination of the kinetics of reforming over the ceria-promoted Mo2C/gamma-Al2O3 catalyst. The high activation energy of the ceria-promoted Mo2C (45.5 kcal/mol) was similar to that previously measured for a bulk Mo2C catalyst, although the activity of the ceria-promoted catalyst was higher. The ceria greatly enhanced the activation of CO2, which supported our previous observation of the high stability of this catalyst during reforming. A reaction mechanism, taking into account the activation of CH4 on the surface of Mo 2C and the activation of CO2 on the surface of ceria and Mo2C, was consistent with the kinetic model. The rate-determining step was hypothesized to be the extraction of carbidic carbon by oxygen adsorbed on the ceria.