Oscillatory behaviour of Pd/Al2O3, Pd-Pt/Al2O3 and Pd/Al2O3-CeO2 in the oxidation of methane

The reaction of methane (2%) with oxygen (2.8-4.6%) in nitrogen at atmospheric pressure over Pd/Al2O3, Pd-Pt/Al2O3 and Pd/Al2O3-CeO2 at temperatures in the range 250-600 degrees C has been studied using catalytic microcalorimeters (beads) with simultaneous analysis of gas mixtures by FTIR. With close-to-stoichiometric gas mixtures, these catalysts showed oscillatory behaviour at 350-550 degrees C. Oscillations were associated with changes in catalytic activity due to oxidation or reduction of the catalyst. Corresponding changes in morphology were also implicated by the enhanced thermal emissivity of the more active partially oxidised catalyst. Oscillations were weakened over Pd/Al2O3-CeO2 but were considerably enhanced over Pd-Pt/Al2O3. Using two beads of Pd/Al2O3, interactions between oscillating systems were investigated. Heat-transfer effects were appreciable at separations of less than 2 mm but gas-phase communication, via changes in reactant concentrations, were observed at higher separations. Oscillations were eliminated at very close separations.