Periodic Transformations of the Catalyst During Self-Oscillating Methane Oxidation on Bulk Nickel

Periodic changes in the composition and morphology of the surface of Ni foil (2 × 8 and 6 × 6 mm) and nickel foam during self-oscillations of the methane oxidation reaction rate were studied using mass spectrometry, thermogravimetry, XRD, SEM, and EMF. Self-oscillations of the reaction rate were observed in the flow of a mixture of 42.3% CH₄ − 7.7% O₂ − 3.8% Ar − 46.2% in the temperature range of 500−700°C. It is shown that the period of one oscillation consists of two phases with different catalytic activity. In the first phase, partial oxidation of methane occurs, the rate of which decreases with time. In the second phase, there is a total oxidation of methane, the rate of which increases towards the end of this phase. At various points in the oscillation cycle, the reaction was interrupted and samples were obtained to study the composition and morphology of the surface. It has been established that oxygen in the form of NiO and carbon in the form of graphite-like structures periodically accumulate on the Ni surface under conditions of self-oscillation. The dynamics of accumulation/conversion of NiO and surface carbon during various phases of the oscillatory cycle, which correlate with the above-mentioned changes in catalytic activity, has been quantitatively studied.

methane oxidation, self-oscillations, nickel