Kinetic reaction models for the selective reduction of NO by methane over multifunctional zeolite-based redox catalysts

Kinetic measurements of the selective catalytic reduction (SCR) of NO by methane were performed over CeO2/H-ZSM-5, In-ZSM-5, and CeO2/ln-ZSM-5 catalysts. The parameter space covered NO, CH4, and O-2 concentrations varying from 250 to 1000 ppm, from 500 to 2000 ppm, and from 0.5 to 10 vol.-%, respectively, space velocities between 5000 and 90000 h(-1) and temperatures between 573 and 873 K depending on the catalyst activities. With CeO2/In-ZSM-5 an additional series of measurements was performed with moistened feed gas (0.5-10 vol.-% H2O). On the basis of a pseudo-homogeneous, one-dimensional fixed-bed reactor model, the data were fitted to a kinetic model that includes power rate laws for the reduction of NO and for the unselective total oxidation of methane. From analyses of isothermal data sets, almost all reaction orders were found to vary significantly with changing temperature, which indicates that the simple kinetic model cannot reflect the complex reaction mechanism correctly. Nevertheless, the data measured with In-ZSM-5 could be modeled with good accuracy over a wide range of reaction temperatures (150 K) while the accuracy was less satisfactory with the remaining data sets, in particular for data with the moist feed over CeO2/In-ZSM-5. With the latter catalyst it was not possible to represent the data measured in dry and in moist feed in a single model even upon confinement to fixed reaction temperatures. A comparison of the separate models established showed strong changes in the reaction orders in the presence of water, which occur apparently already at a very low water content (less than or equal to0.5 vol.-%). The kinetic parameters found are in agreement with earlier conclusions about the reaction mechanisms. With In-ZSM-5, both reaction orders and the activation energy show a rate-limiting influence of NO oxidation on the NO reduction path which is removed by the presence of the CeO2 promoter. A difference in the reaction mechanism over CeO2/In-ZSM-5 and CeO2/H-ZSM-5 is reflected in different kinetic parameters. The differences of the kinetic parameters between dry-feed and moist-feed models for CeO2/In-ZSM-5 reflect adsorption competition between the reactants and water.