Oscillatory Behavior in Oxidation of Propane Over Nickel Foil and Films

The oxidation of propane was studied over Ni foil, a thick Ni/Si film, and a thin Ni/Si film in a flow reactor under oxygen-deficient conditions at a total pressure near 1 mbar. The thickness of the foil was 125 µm, while the thickness of the films was 20 and 1.5 µm, respectively. It was shown that regular self-sustained reaction-rate oscillations arise only over the Ni foil and the thick Ni/Si film. The relaxation-type oscillations appear at 700 °C after an induction period of approximately 50 min, which is accompanied by the formation of a rough and porous structure over their surfaces. According to krypton adsorption measurements, the effective surface area of the treated catalysts increases significantly. CO, CO2, H2, and H2O were detected as products in the gas phase. The results of operando X-ray absorption spectroscopy study indicate that the oscillations originate due to periodical oxidation and reduction of nickel. The highly active state is Ni in the metallic state, while the transition to the poorly active state is accompanied by the growth of NiO layer on the catalyst surface. The upper layer of the catalysts with a thickness of at least 15 nm is involved in the oxidation/reduction process. In contrast, no oscillations appeared in the oxidation of propane over the thin Ni/Si film. In this case, the thickness of the Ni layer is not sufficient for the formation of a porous structure over the thin Ni film. As a result, the thin Ni film cannot provide high activity in the catalytic oxidation of propane that is sufficient for the self-sustained oscillations to occur. The reasons that lead to oscillatory behavior in the oxidation of propane over nickel are discussed.