Transition Metal-doped Rare-earth Oxysulfide Catalysts for High Temperature Dry Reforming of Methane

Carbon dioxide reforming of methane is a catalytic reaction utilizing two kinds of greenhouse gases and converting them into a useful industrial gas stream, “syngas”. However, sulfur poisoning and coke formation are two major challenges for this reaction. In this study, we have synthesized and examined several Ce-La and Ce-Zr oxides, with different transition metal additives. A rapid screening technique was developed to measure reforming and coking rates at low partial pressures. It is a good indicator of catalyst behavior at higher conversions and partial pressures. Following the rapid screening, select catalysts were examined at longer times on stream. Those containing Ni and Co together were the most stable. Catalysts containing Ce-La oxides lacked practicality, partly due to more reverse water-gas shift. Catalysts containing Ce-Zr oxides fared better, with Ce/Zr = 3 (molar) showing the best stability for Ni-based catalysts. Reaction and deactivation results for Ni- and Ni/Co-containing catalysts could be explained partly in terms of DFT calculations, and partly in terms of spent catalyst characterizations. The Ni interacts strongly with the mixed oxides, even when it is in a mostly reduced state, as in Ce-Zr. Ni-based catalysts were also examined for sulfur tolerance based on long-term reactor tests. Catalysts prepared on both Ce/Zr and Ce/La oxide supports, some without and some with additional Co metal, were tested under low and high conversion conditions. Long-term reaction runs were conducted both with and without sulfur added to the feed. The catalysts were also characterized by TEM, XPS, XAFS, and CO chemisorption. Only catalysts where Co is also present, and with the metals combined with a Ce-Zr oxide, are capable of extended sulfur tolerance at \u3e20 ppm sulfur. This tolerance, and also a greatly reduced coking rate, is linked to Co in contact with the Ni and existing in small aggregates where CH4 activation takes place, anchored and influenced electronically by the oxide support. Larger metal aggregates formed during reaction appear to be spectators. Measured activation energies for the dry reforming reaction support the hypothesis that CO2 activation takes place at the oxide interface and is a kinetically significant step