Kinetic study of selective CO oxidation in H2-rich gas on a Ru/γ-Al2O3 catalyst

Abstract

The preferential oxidation (PROX) of CO on Ru/γ-Al2O3 in simulated reformer gas (1.0 kPa CO, 75 kPa H2, rest N2) was investigated over a wide range of CO partial pressures (0.02–1.5 kPa) and O2 excess (pO2∶pCO = 0.5–5.0). Integral flow measurements in a microreactor at λ = 2 show that ∼150 °C is the optimum temperature for the PROX of CO, combining sufficiently high rates with a high selectivity (S ≈ 50%). Ru/γ-Al2O3 shows a significantly higher activity and selectivity under these conditions than the conventionally used Pt/γ-Al2O3 catalyst. Differential flow experiments allow the quantitative determination of CO and O2 oxidation rates at 150 °C, with reaction orders of α = −0.48 for CO and α = 0.85 for O2. In the low temperature regime, 135–200 °C, the apparent activation energy, Ea*, 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 H2 oxidation are limited by the presence of a CO adlayer. The saturation of the reaction rate at high oxygen excess (λ 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 (pCO ⩽ 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 CO2 methanation and from the reverse water gas shift reaction (RWGS) the Ru/γ-Al2O3 catalyst shows a much better performance than Pt/γ-Al2O3, both because of its much lower minimum noble metal mass, lower by a factor 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 load variations to 10% and 1%, respectively.