Transient reaction analysis and steady-state kinetic study of selective catalytic reduction of NO and NO + NO 2 by NH 3 over Fe/ZSM-5

Abstract

The reaction kinetics of selective catalytic reduction (SCR) by NH 3 on NO (standard SCR) and on NO + NO 2 (fast SCR) over Fe/ZSM-5 were investigated using transient and steady-state analyses. In the standard SCR, the N 2 production rate was transiently promoted in the absence of gaseous NH 3; this enhancement can be attributed to the negative reaction order of NH 3 (between −0.21 and −0.11). The steady-state data for the standard SCR could be fit to a Langmuir–Hinshelwood-type reaction between NO ad and O ad to form NO 2. In the fast SCR, however, the promotion behavior in the absence of gaseous NH 3 was not observed and the apparent NH 3 order changed from positive to negative with NH 3 concentration. The steady-state rate analysis combined with elementary reaction modeling suggested that competitive adsorption between NO 2 and NH 3 was occurring due to strong NO 2 adsorption; this must be the main reason for the absence of the promotion effect.