Kinetics of the selective low-temperature oxidation of CO in H2-rich gas over Au/α-Fe2O(3

The selective CO oxidation (also referred to as PROX) on a Au/alpha-Fe2O3 catalyst in simulated reformer gas (low concentrations of CO and O-2, 75 kPa H-2, balance N-2) at atmospheric pressure was investigated over almost two orders of magnitude in CO partial pressure (0.025-1.5 kPa) and over a large range of p(O2)/p(CO) ratios (0.25-10). Quantitative evaluation of CO oxidation rates as a function of CO and O-2 partial pressure at 80 degrees C yields reaction orders with respect to CO and O-2 of 0.55 and 0.27, respectively. The apparent activation energy for this reaction evaluated in the temperature range of 40-100 degrees C is 31 kJ/mol. At 80 degrees C, the selectivity, defined as the ratio of oxygen consumption for CO oxidation to the total oxygen consumption, reaches 75% at large CO partial pressures (1.5 kPa), but decreases significantly with diminishing p(CO). This is related to the fact that the H-2 oxidation rate is independent of the CO partial pressure, consistent with a reaction mechanism where oxygen adsorbed at the metal/metal oxide interface reacts with H and CO adsorbed at low coverages on the supported Au nanoclusters. The selectivity increases with decreasing temperature, reflecting a higher apparent activation energy for H-2 oxidation than for CO oxidation. A comparison with Pt/gamma-Al2O3, a commonly used PROX catalyst with an optimum operating temperature of ca. 200 degrees C, demonstrates that Au/alpha-Fe2O3 already offers comparable activity and selectivity at 80 degrees C. (C) 1999 Academic Press.