Effects of Passivation on Synthesis, Structure and Composition of Molybdenum Carbide Supported Platinum Water-Gas Shift Catalysts

High surface area molybdenum carbide (Mo₂C) is an attractive catalyst and support for a number of metals. A practical challenge associated with using carbides and nitrides is their pyrophoric nature. These materials are typically passivated prior to use as catalysts or supports.  Research described in this paper compared the synthesis, structural and compositional properties, and water gas shift activities of catalysts produced by depositing platinum (Pt) onto unpassivated and passivated Mo₂C. Passivation had a profound effect on the character of interactions between aqueous solutions of H₂PtCl₆ and the Mo₂C surface.  Contacting the unpassivated Mo₂C with the H₂PtCl₆ solution produced nanoscale Pt metal particles (Figure 1a). Conversely, contact with the passivated Mo₂C resulted in very large Pt particles (Figure 1b).  In situ X-ray absorption spectroscopy revealed that H₂PtCl₆ was reduced on contact with the unpassivated Mo₂C, resulting in the commensurate oxidation of Mo. The mechanism for the deposition of Pt onto the passivated material (p-Mo₂C) was very different and resulted in much lower Pt loadings than were achieved for the unpassivated Mo₂C.  Water gas shift (WGS) activities for the Pt/Mo₂C catalyst were significantly higher than those for the Pt/p-Mo₂C catalyst, as well as, for a Cu-Zn-Al catalyst. The results are consistent with previous reports that active sites for WGS on the Pt/Mo₂C catalyst exist at the interface between the Pt and Mo₂C surface [1]. Overall, results in this paper indicate that passivation of the Mo₂C prior to the deposition of Pt had deleterious effects on the Pt loadings and structures, and on the water gas shift rates.   

[1] N.M. Schweitzer, J.A. Schaidle, O.K. Ezekoye, X. Pan, S. Linic, L.T. Thompson, J. Am. Chem. Soc., 133 (2011) 2378-2381.