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

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.