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In situ time-resolved characterization of Ni-MoO<inf>2</inf> catalysts for the water - Gas shift reaction

Abstract:

Active catalysts for the water-gas shift (WGS, CO + H2O → H2 + CO2) reaction were synthesized from nickel molybdates (β-NiMoO4 and nH2O·NiMoO4) as precursors, and their structural transformations were monitored using in situ time-resolved X-ray diffraction and X-ray absorption near-edge spectroscopy. In general, the nickel molybdates were not stable and underwent partial reduction in the presence of CO or CO/H2O mixtures at high temperatures. The interaction of β-NiMoO4 with the WGS reactants at 500°C led to the formation of a mixture of Ni (∼24 nm particle size) and MoO 2 (∼10 nm particle size). These NÍ-MoO2 systems displayed good catalytic activity at 350, 400, and 500°C. At 350 and 400°C, catalytic tests revealed that the Ni-MoO2 system was much more active than isolated Ni (some activity) or isolated MoO2 (negligible activity). Thus, cooperative interactions between the admetal and oxide support were probably responsible for the high WGS activity of Ni-MoO 2. In a second synthetic approach, the NiMoO4 hydrate was reduced to a mixture of metallic Ni, NiO, and amorphous molybdenum oxide by direct reaction with H2 gas at 350°C. In the first pass of the water-gas shift reaction, MoO2 appeared gradually at 500°C with a concurrent increase of the catalytic activity. For these catalysts, the particle size of Ni (∼4 nm) was much smaller than that of the MoO 2 (∼13 nm). These systems were found to be much more active WGS catalysts than Cu-MoO2, which in turn is superior to commercial low-temperature Cu-ZnO catalysts. © 2008 American Chemical Society.