Kinetic and Mechanistic Study of NOxReduction by NH3over H-Form Zeolites

Abstract

A study of the kinetics and mechanism of the selective catalytic reduction (SCR) of NO by NH3over H-ZSM-5 was undertaken. Steady-state kinetic experiments performed at temperatures above 330°C with and without the presence of H2O indicate that H2O does not affect SCR by NH3at these temperatures. Analysis of the kinetic data indicates that SCR is positive order in NO and O2and inhibited by NH3. A series of transient tests were also performed to determine the roles of NO, O2, and NH3in the mechanism of NO SCR. Transient test results indicate that O2reacts with NO to form an active intermediate species (possibly NO2or NO+) which reacts with adsorbed NH3. Transient test results also suggest that although adsorbed NH3is necessary for NO reduction to proceed, excess gaseous NH3inhibits SCR. Comparisons of SCR activity exhibited by samples of H-ZSM-5 and H-mordenite of different Si/Al ratios suggest that a limiting Al content is necessary for H-form zeolites to be active. This result suggests that pairs of neighboring Brønsted acid sites are necessary for adsorption of NH3, such that the adsorbed NH3molecules are close enough together that they can both bond with other reactant molecules.