The effect of specific surface area on the activity of nano-scale ceria catalysts for methanol decomposition with and without steam at SOFC operating temperatures

Nano-particulate high surface area CeO2 was found to have a useful methanol decomposition activity producing H2, CO, CO2, and a small amount of CH4 without the presence of steam being required under solid oxide fuel cell temperatures, 700-1000 °C. The catalyst provides high resistance toward carbon deposition even when no steam is present in the feed. It was observed that the conversion of methanol was close to 100% at 850 °C, and no carbon deposition was detected from the temperature programmed oxidation measurement. The reactivity toward methanol decomposition for CeO2 is due to the redox property of this material. During the decomposition process, the gas-solid reactions between the gaseous components, which are homogeneously generated from the methanol decomposition (i.e., CH4, CO2, CO, H2O, and H2), and the lattice oxygen (O^xo) on ceria surface take place. The reactions of adsorbed surface hydrocarbons with the lattice oxygen (CnHm+ O^xo (arraw over)nCO+m/2(H2)+Vo+2e` can produce synthesis gas (CO and H2) and also prevent the formation of carbon species from hydrocarbons decomposition reaction (CnHm (left right double arrow) nC+m/2H2). VO...· denotes an oxygen vacancy with an effective charge 2+. Moreover, the formation of carbon via Boudouard reaction (2CO CO2+C) is also reduced by the gas-solid reaction of carbon monoxide with the lattice oxygen Co+ O^xo (CnHm (left right double arrow) CO2+VO..+2e`. At steady state, the rate of methanol decomposition over high surface area CeO2 was considerably higher than that over low surface area CeO2 due to the significantly higher oxygen storage capacity of high surface area CeO2, which also results in the high resistance toward carbon deposition for this material. In particular, it was observed that the methanol decomposition rate is proportional to the methanol partial pressure but independent of the steam partial pressure at 700-800 °C. The addition of hydrogen to the inlet stream was found to have a significant inhibitory effect on the rate of methanol decomposition.