(621bv) 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.

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