In situ XPS study of self-sustained oscillations in catalytic oxidation of propane over nickel

Self-sustained oscillations in the propane oxidation over a nickel foil were studied in situ with the use of ambient-pressure X-ray photoelectron spectroscopy (XPS) coupled with on-line mass-spectrometry and gas chromatography (GC). Regular oscillations of a relaxation type were observed at 0.5 mbar in the temperature range of 600–750 °C in oxygen-deficient conditions. CO, CO2, H2, H2O, and propylene were detected as products. CO selectivity in active half-periods achieved 98% decreasing to 40–60% in inactive half-periods. It has been found that the chemical state of the catalyst drastically changes together with the oscillations of the catalytic activity. According to the Ni2p and O1s core-level spectra measured in situ, the active catalyst surface is represented by metallic nickel, whereas it is covered with a layer of NiO with a thickness of at least 3 nm during the inactive half-periods. It means that the oscillations in the propane oxidation over nickel are originated from the reversible bulk oxidation of Ni to NiO. We suggest that the propane oxidation over the metallic Ni surface occurs via the Langmuir–Hinshelwood mechanism, whereas the Mars–van Krevelen mechanism prevails when the reaction proceeds over NiO. The switching between the metallic surface and the oxide shows a significant change in the catalytic activity. According to GC measurements the activity of metallic nickel is approximately 40-fold higher than that of NiO.