268929 Water-Gas Shift Catalysis Over Transition Metals Supported On Molybdenum Carbide

Tuesday, October 30, 2012: 3:15 PM
321 (Convention Center )
Kaiwalya D. Sabnis1, Mayank Shekhar1, Wen-Sheng Lee1, Jeffrey T. Miller2, W. Nicholas Delgass1 and Fabio H. Ribeiro1, (1)School of Chemical Engineering, Purdue University, West Lafayette, IN, (2)CSE, Argonne National Laboratory, Argonne, IL

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 H2. 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.

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