An experimental and chemical kinetic modeling study of the role of potassium in the moist oxidation of CO

The effect of KCl on moist CO oxidation in a laminar flow quartz reactor was investigated. Experiments were conducted in the absence of O2 (gasification), as well as under reducing and fuel-lean conditions, in the tem-perature range 873-1573 K, and the results were interpreted in terms of a chemical kinetic model. The impact of reactions on the quartz surface of the reactor was carefully examined. Under the conditions of interest, KCl reacts with SiO2 to form potassium silicates, releasing HCl to the gas phase. This reaction alters the condition of the quartz surface, making it more active in catalyzing radical recombination, even in the presence of water vapor. Under gasification and reducing conditions, loss of hydrogen atoms on the wall, enhanced by the exposure to KCl, strongly inhibits CO oxidation, with gas-phase inhibition playing a minor role. Under fuel-lean conditions, the state of the surface does not affect the CO oxidation and the observed inhibition can be attributed to gas-phase reactions. The most important reaction for the inhibition is the chain terminating step KO2 + OH ⇆ KOH + O2. Assuming that this reaction proceeds without a barrier, the rate constant is controlled by a long-range dipo-le-dipole interaction. We calculate a capture rate constant of 2.5E15 T-0.163 cm3 mol-1 s- 1. This value, which is significantly higher than earlier estimates used in modeling, allows a satisfactory prediction of the inhibiting effect of KCl on CO oxidation under oxidizing conditions.