| Abstract
| - Active catalysts for the water−gas shift (WGS, CO + H2O → H2 + CO2) reaction were synthesized fromnickel molybdates (β-NiMoO4 and nH2O·NiMoO4) as precursors, and their structural transformations weremonitored 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/H2Omixtures at high temperatures. The interaction of β-NiMoO4 with the WGS reactants at 500 °C led to theformation of a mixture of Ni (∼24 nm particle size) and MoO2 (∼10 nm particle size). These Ni−MoO2systems displayed good catalytic activity at 350, 400, and 500 °C. At 350 and 400 °C, catalytic tests revealedthat the Ni−MoO2 system was much more active than isolated Ni (some activity) or isolated MoO2 (negligibleactivity). Thus, cooperative interactions between the admetal and oxide support were probably responsiblefor the high WGS activity of Ni−MoO2. In a second synthetic approach, the NiMoO4 hydrate was reducedto 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 concurrentincrease of the catalytic activity. For these catalysts, the particle size of Ni (∼4 nm) was much smaller thanthat of the MoO2 (∼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.
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