In Situ Time-Resolved Characterization of Ni−MoO₂ Catalysts for the Water−Gas Shift Reaction

Active catalysts for the water−gas shift (WGS, CO + H 2 O → H 2 + CO 2 ) reaction were synthesized from nickel molybdates (β-NiMoO 4 and n H 2 O·NiMoO 4 ) 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 2 O mixtures at high temperatures. The interaction of β-NiMoO 4 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 Ni−MoO 2 systems displayed good catalytic activity at 350, 400, and 500 °C. At 350 and 400 °C, catalytic tests revealed that the Ni−MoO 2 system was much more active than isolated Ni (some activity) or isolated MoO 2 (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 NiMoO 4 hydrate was reduced to a mixture of metallic Ni, NiO, and amorphous molybdenum oxide by direct reaction with H 2 gas at 350 °C. In the first pass of the water−gas shift reaction, MoO 2 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−MoO 2, which in turn is superior to commercial low-temperature Cu−ZnO catalysts.