Kinetics of the Selective CO Oxidation in H2-Rich Gas on Pt/Al2O3

Abstract

The selective CO oxidation reaction on Pt/γ-Al2O3insimulated reformer gas (75% H2; the rest is N2) was investigated over a wide range of CO concentrations (0.02–1.5%)at low stoichiometric O2excess (pO2/pCO=0.5–1.5). Integral flow measurements in a microreactor showed that the optimumtemperature for the PROX process (preferential oxidation) was∼200°C; rates of CO methanation in this range wereinsignificant. At higher temperatures a significant loss inselectivity was observed. The quantitative determination of COoxidation rates and selectivities as a function of CO and O2concentration between 150 and 250°C by differential flow measurements led to reaction orders of −0.4 forpCOand +0.8 forpO2at an apparentactivation energy of 71 kJ/mol. These kinetics are consistentwith the selective CO oxidation reaction occurring in thelow-rate branch, on a surface predominantly covered withadsorbed CO. Its ramifications on the observed dependence of theselectivity on both CO concentration and temperature arediscussed. A H2-induced increase in the CO oxidation rate by afactor of ∼2 was observed. Comparison of the kinetic rateexpression for the selective CO oxidation with the performanceof a plug-flow reactor (variable flow rates) led to quantitativeagreement.