Kinetics of catalytic oxidation of methane over palladium oxide by wire microcalorimetry

The kinetics of catalytic oxidation of methane (1-3% in air) over a palladium oxide (PdO) surface was investigated by wire microcalorimetry at atmospheric pressure and over the temperature range from 560 to 800 K. Wire surface structures and compositions were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and atom force microscopy. It was found that a porous PdO layer with a constant thickness of 1-2 μm was formed on the Pd wire after it was heat treated in nitrogen followed by air at elevated temperatures. Under the condition of the experiment, the reaction was found to be in the pseudo-first-order regime with respect to the methane concentration. The apparent rate constant of methane oxidation on PdO was determined to be k_app(cm/s) = (3.2 ± 0.8) × 10⁴e ^{-(62.8±1.6)(kJ/mol)/RT} for 600 < T < 740 K. Experimental data were analyzed using a gas-surface reaction model proposed previously. Analysis shows that the overall catalytic oxidation rate is governed by equilibrium adsorption/desorption of molecular oxygen, which determines the density of surface palladium sites and dissociative adsorption of methane on these sites. The equilibrium constant of O₂ adsorption and desorption was estimated from literature values of desorption energy and molecular parameters of adsorbed oxygen atoms. The rate coefficient of methane dissociative adsorption was estimated to be k₁₆(cm/s) = (7.7 ± 1.6) × 10⁴e^{-(59.9±1.2)(kJ/mol)/RT}, derived from the equilibrium constant of oxygen adsorption over the same temperature range.