(328a) Water-Gas Shift Catalysis Over Transition Metals Supported On Molybdenum Carbide

We have measured the water-gas shift (WGS) reaction kinetics over molybdenum carbide (Mo₂C) and Au, Pt, Pd, Ag, Ni and Cu supported on Mo₂C. The Mo₂C support was prepared by temperature programmed carburization of ammonium paramolybdate in presence of 15% CH₄ in H_2. The reaction orders presented here are first to be reported for these systems. Mo₂C has a WGS reaction rate per gram of catalyst (3×10^-7 mol H₂ (g cat)^-1 (s)^-1) comparable to the commercial Cu/Zn/Al₂O₃ catalyst at 120 °C under 7% CO, 8.5% CO₂, 22% H₂O, 37% H₂ and balance Ar. This rate is further promoted by a factor of 3-6 by the addition of metals such as Pt, Pd, Au and Ni and is unaltered by the addition of metals such as Cu and Ag on Mo₂C. The increase in the WGS reaction rate upon addition of Pt, Pd, Au and Ni on Mo₂C is accompanied by an increase in H₂O order and a decrease in CO order and apparent activation energy. Higher H₂O order and lower CO order compared to the Mo₂C itself, i.e. lower relative coverage of hydroxyl species and higher relative coverage of CO, are the characteristics of the admetals which lead to a promotion in WGS reaction rate on Mo₂C.

In addition, we report an alternative route for the synthesis of Pt/Mo₂C wherein, molybdenum is deposited on multi-walled carbon nanotubes, followed by a reduction in H₂ at 600 °C,and deposition of Pt by incipient wetness method.  Pt/Mo₂C and Pt/Mo/MWCNT catalysts have similar reaction kinetics and comparable WGS reaction rates per total mole of Pt at 120 °C. This route obviates the use of methane for carburization of Mo precursor to produce bulk Mo₂C.

In situ EXAFS over fresh Au/Mo₂C indicates that the carburization pretreatment at 600 °C leads to irreversible re-dispersion of Au nanoparticles. Platinum nanoparticles on the Pt/Mo₂C catalyst that was reduced at 450 °C in H₂ during the preparation were already well-dispersed, and no change was observed in the Pt-Pt and Pt-Mo coordination during carburization and subsequent WGS. Thus, Au and Pt seem to have a strong interaction with the Mo₂C support. However, the WGS reaction rates for these catalysts decreased by two orders of magnitude when the catalysts were exposed to air, although the metal dispersion did not change. This suggests that both small metal nanoparticles (~2-3 nm) and the carburized Mo₂C surface are the prerequisites for the high WGS reaction rates on a metal/Mo₂C system.