Kinetic study of selective Co oxidation in H_2-rich gas on a Ru/gamma-Al_2O_3 catalyst

摘要

The perferential oxidation (PROX) of CO on Ru/gamma-Al_2O_3 in simulatead reformer gas (1.0 kPa CO, 75 kPa H_2, rest N_2) was investigated over a wide range of CO partial pressures (0.02-1.5 kPa) and O_2 excess (p_O_2: P_CO = 0.5-5.0). Integral flow measurements in a microreactor at lambda = 2 show that approx 150 deg C is the optimum temperature for the PROX of CO, combining sufficiently high rates with a high seletivity (S approx = 50%). Ru/gamma-Al_2O_3 shows a significantly higher activity and selectivity under these conditions than the conventinally used Pt/gamma-Al_2O_3 catalyst. Differentail flow experiments allow the quantitative determination of CO and O_2 oxidation rates at 150 deg C, with reaction orders of alpha = - 0.48 for CO and alpha = 0.85 for O_2. In the low temperature regime, 135-200 deg C, the apparent activation energy, E_a~*, is 95 (+-) 5 kJ mol~(-1). Our findings are consistent with a Langmuir-Hinshelwood reaction mechanism in the low-rate branch, where both CO oxidation and H_2 oxidation are limited by the presence of CO adlayer. The saturation of the reaction rate at high oxygen excess (lambd > =6) as well as previous data support a model where the reaction proceeds on oxygen loaded/surface oxide covered Ru particles. The minimum amount of catalyst and oxygen excess for complete removal (< 50 ppm) of CO from methanol reformate in the second stage of a double stage reactor (p_CO < = 0.2 kPa) was calculated in reaction simulations, which were based on a plug-flow reactor model and use the kinetic parameters determined in this study. Including contributions from CO and CO_2 methanation and from the reverse water gas shift reaction (RWGS) the Ru/gamma-Al_2O_3 catalyst shows a much better performance than Pt/gamma-Al_2O_3, both because of its much lower minimum noble metal mass, lower by a fator of 20, and the much better behavior under dynamic load conditions, where only the Ru catalyst remains below the critical CO level of 50 ppm for laod variations to 10% and 1%, respectively.