Influence of the Degree of Carburization on the Density of Sites and Hydrogenating Activity of Molybdenum Carbides

Abstract

Temperature-programmed carburization of molybdic acid with CH4/H2 mixtures led to hexagonal Mo2C with specific surface areas ranging from 40 to 91 m2 g−1. The carburization was found to go through Mo4O11, MoO2, and Mo metal. Whatever was the parameter of carburization, no significant change of the lattice parameters of Mo2C was detected by XRD. The primary particles of the resulting carbides were found to be single crystals. Elemental analysis evidenced different bulk carbon contents of carbides depending on the conditions of synthesis. Nevertheless, this kind of analysis only gives global data to monitor the completion of carburization. Without free carbon contamination, all accessible Mo atoms were able to chemisorb oxygen. So oxygen chemisorption was proposed as a molecular probe to control the free carbon deposition. Counting of noble metal-like sites was possible by selective CO chemisorption, which was found to selectively deactivate active sites in benzene hydrogenation. CO titrated from 3 to 58% of a monolayer of surface Mo atoms. This evolution of the density of sites titrated by CO was interpreted in terms of the “degree of carburization” of materials linked to both surface carbidic carbon and residual oxygen contents as evidenced by TPR measurements of Mo2C prepared in situ. The evaluation of the quality of each Mo site titrated by CO was done by benzene hydrogenation at room temperature. The carbides were found to be as active as Ru/Al2O3. A significant increase of the activity per site was observed on Mo2C, when the density of sites titrated by CO was increased. It means that the degree of carburization affects not only the number of active sites, but also the quality of these sites. These results are in good agreement with the general concept speculated on the noble metal-like behavior of Mo2C: the higher the carbidic carbon content and the lower the residual oxygen content in Mo lattice, the higher will be the noble metal-like behavior.