Catalytic Combustion of Ethane over Palladium Foil in the 300–450°C Range: Kinetics and Surface Composition Studies

Abstract

Catalytic oxidation of ethane over palladium foils was studied and the kinetic parameters were determined for the reaction. The studies were carried out at 800 Torr total pressure over a wide range of reaction conditions in the temperature range of 300–425°C. The catalyst surface composition was characterized before and after reaction by Auger electron spectroscopy (AES). While we detected only metallic palladium before reaction, the postreaction analysis showed significant formation of PdOx, a surface oxide, at all temperatures and under all ethane/oxygen ratios. The catalyst that exhibits optimum combustion activity is the one covered with 0.3 to 0.5 monolayer of oxygen. Carbon monoxide temperature-programmed desorption (CO-TPD) was used to evaluate the surface area of the metallic and oxidized palladium in order to normalize the measured catalytic reaction rates. The turnover rate for ethane combustion is in the range of 1–260 s−1(molecule(s) of CO2produced per surface palladium atom per second) in the studied temperature range. Under stoichiometric reaction conditions at 325°C the rate for ethylene combustion is 100 times faster than that of ethane and the turnover rate for ethylene formation is calculated to be 0.6 s−1(molecule(s) of C2H4produced per surface palladium atom per second). At 325°C the kinetic orders are 1, 0, and −1 for ethane, carbon dioxide, and water, respectively. The order in oxygen is −1/2 under stoichiometric and fuel-rich conditions and 0 under oxidizing conditions. Apparent activation energies of 104, 116, and 189 kJ/mol under fuel lean, stoichiometric, and fuel-rich conditions, respectively, were observed. While CO2is the main carbonaceous product, unoxidized and partially oxidized species, methane and ethylene, are produced in small amounts. The product distribution is typically 98.6 mol% CO2, 0.9 mol% C2H4, and 0.5 mol% CH4at 10% conversion during a reaction at 350°C under stoichiometric conditions.