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要旨:
Self-sustained reaction rate oscillations in the oxidation of propane and in the propane steam reforming with oxygen over nickel foil have been studied in situ by near-ambient pressure X-ray photoelectron spectroscopy and mass-spectrometry. It was found that regular relaxation-type oscillations in both reactions proceed under similar conditions. In the former case, the peaks of CO, CO2, H2, and H2O were detected by mass-spectrometry as gas-phase products. In contrast, in the latter case, after addition of water to the reaction feed, the mass-spectrometric signal of water decreased simultaneously with the signals of O2 and C3H8, whereas the signals of CO, CO2, and H2 increased. It means that in the presence of water in the reaction feed, the propane steam reforming proceeds with a significant rate. In both cases, the oscillations arise due to spontaneous oxidation and reduction of the catalyst. According to the Ni2p and O1s core-level spectra measured in situ, the high-active catalyst surface is represented by nickel in the metallic state, and the transition to the low-active state is accompanied by the growth of a NiO film on the catalyst surface. The oscillations in the gas phase are accompanied by oscillations in the catalyst temperature, which reflects proceeding endothermic and exothermic processes. An oscillatory mechanism, which can be common for oxidative catalytic reactions over transitional metals, is discussed.
