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

Abstract

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 °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 °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.