Structure sensitivity of methane oxidation over platinum and palladium

A series of supported platinum and palladium catalysts were tested for methane oxidation at 260 to 370°C, 50 Torr methane, 110 Torr oxygen, 900 Torr helium, and conversions below 2%. The intrinsic rate varies by more than 5000 from the least active to the most active catalyst, indicating that the reaction is structure sensitive. The catalytic activity of platinum depends on the distribution of the metal between a dispersed and a crystalline phase. These two phases are distinguished by absorbances at 2068 and 2080 cm−1, respectively, in the infrared spectrum of adsorbed carbon monoxide. The catalytic activity of palladium depends on the metal particle size. For the different classes of catalysts, the mean steady state turnover frequency (TOF) at 335°C and the mean apparent activation energy (Ea) are as follows: dispersed phase of platinum, TOF = 0.005 s−1 and Ea = 36 kcal/mole; crystalline phase of platinum, TOF = 0.08 s−1 and Ea = 28 kcal/mole; small particles of palladium, TOF = 0.02 s−1 and Ea = 27 kcal/mole; and large particles of palladium, TOF = 1.3 s−1 and Ea = 29 kcal/mole. The structure sensitivity may be explained by differences in the reactivity of the adsorbed oxygen on these metal surfaces.