(482c) Molybdenum Carbide-Supported Metal Catalysts: Synthesis, Characterization, and Catalytic Properties

Early transition metal carbides have been shown to be active for a variety of reactions [1] and tolerant to sulfur poisoning for some reactions [2]. Moreover, these materials can be synthesized with high surface areas [3], leading to their use as supports for other metals. For example, a Pt/Mo₂C catalyst produced by depositing Pt directly onto the native Mo₂C surface [4] was demonstrated to be more active than conventional catalysts for reactions such as water gas shift [5] and methanol steam reforming [6]. The basis for this high activity remains ill-defined.

A series of Mo₂C supported Pt, Pd, Cu, Ni, and Fe catalysts was synthesized and characterized using x-ray absorption spectroscopy (XAS) and high resolution transmission electron microscopy (TEM). These catalysts were highly active for the water gas shift and Fischer-Tropsch Synthesis reactions. The XAS results indicated that the PtCl₆^2- complex was reduced to metallic Pt on the surface of Mo₂C during the adsorption process. This result is in agreement with our ability to attain high Pt loadings (>12 wt%) and high dispersions. The Pd (Pd(NH₃)₄(NO₃)₂) and Cu (CuCl₂) complexes also produced high metal dispersions on adsorption to Mo₂C; however, the Ni (NiCl₂) and Fe (FeCl₂) precursors did not. We believe the driving force for reduction of the Pt, Pd, and Cu complexes was a redox reaction between the metal complex in solution and the Mo₂C surface whereby the metal complex was reduced and the Mo₂C surface was oxidized.

The TEM and XAS are consistent with Pt that was in the form of ?raft-like? particles wetting the Mo₂C surface. As the Pt loading increased, the diameter of these rafts increased, but the thickness appeared to remain constant. Work is in progress to exploit the strong interactions between the PtCl₆^2- complex and Mo₂C in the synthesis of core shell particles on Al₂O₃ and carbon supports.

[1] S. T. Oyama, Catalysis Today 15 (1992).

[2] E. J. Markel, J. W. Van Zee, Journal of Catalysis 126 (1990).

[3] J. S. Lee, S. T. Oyama, M. Boudart, Journal of Catalysis 106 (1987).

[4] L. T. Thompson, S. K. Bej, J. J. Patt, C. H. Kim, US Patent 6897178 (2005).

[5] T. E. King, Ph. D. Thesis, University of Michigan, 2007.

[6] W. Setthapun, S. Bej, L. T. Thompson, Top. Catal. 49 (2008).