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In situ time-resolved characterization of Ni-MoO2 catalysts for the water - Gas shift reaction

Abstract:

Active catalysts for the water-gas shift (WGS, CO + H<inf>2</inf>O → H<inf>2</inf> + CO<inf>2</inf>) reaction were synthesized from nickel molybdates (β-NiMoO<inf>4</inf> and nH<inf>2</inf>O·NiMoO<inf>4</inf>) 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/H<inf>2</inf>O mixtures at high temperatures. The interaction of β-NiMoO<inf>4</inf> with the WGS reactants at 500°C led to the formation of a mixture of Ni (∼24 nm particle size) and MoO <inf>2</inf> (∼10 nm particle size). These NÍ-MoO<inf>2</inf> systems displayed good catalytic activity at 350, 400, and 500°C. At 350 and 400°C, catalytic tests revealed that the Ni-MoO<inf>2</inf> system was much more active than isolated Ni (some activity) or isolated MoO<inf>2</inf> (negligible activity). Thus, cooperative interactions between the admetal and oxide support were probably responsible for the high WGS activity of Ni-MoO <inf>2</inf>. In a second synthetic approach, the NiMoO<inf>4</inf> hydrate was reduced to a mixture of metallic Ni, NiO, and amorphous molybdenum oxide by direct reaction with H<inf>2</inf> gas at 350°C. In the first pass of the water-gas shift reaction, MoO<inf>2</inf> 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 <inf>2</inf> (∼13 nm). These systems were found to be much more active WGS catalysts than Cu-MoO<inf>2</inf>, which in turn is superior to commercial low-temperature Cu-ZnO catalysts. © 2008 American Chemical Society.