Self-Oscillatory Regimes of Methane Oxidation over Supported Pd Catalysts
Self-oscillations of methane oxidation rate over Pd catalysts are usually observed in methane-rich reaction mixtures. The oscillations are interesting to understand the methane oxidation mechanism, but they are detrimental for industrial catalysis. Self-oscillations and temperature hysteresises during methane oxidation over Pd/Al2O3 catalysts were studied both in a tubular quartz flow reactor and a combined thermogravimetric-mass-spectrometric setup. Temperature ranges of selfoscillation regimes were measured and shown to shorten with a decreasing of Pd particle size. The catalyst weight and gaseous components oscillations were detected and the results allowed to reveal the nature of Pd variations during the oscillation cycle: periodic oxidation-reduction of Pd and carbon accumulation-removal as PdCx. An extremal relationship between the catalytic activity and Pd oxidation degree x of PdOx was found: Pd activity is maximum at x=0; it drops to minimum and gradually increases with x rise from 0 to 0.3.