The reaction kinetics of selective catalytic reduction (SCR) by NH3 on NO (standard SCR) and on NO + NO2. (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 NH3: this enhancement can be attributed to the negative reaction order of NH3 (between -0.21 and -0.11). The steady-state data for the standard SCR could be fit to a Langmuir-Hinshelwood-type reaction between NOad and O-ad to form NO2. In the fast SCR, however, the promotion behavior in the absence of gaseous NH3 was not observed and the apparent NH3 order changed from positive to negative with NH3 concentration. The steady-state rate analysis combined with elementary reaction modeling suggested that competitive adsorption between NO2 and NH3 was occurring due to strong NO2 adsorption; this must be the main reason for the absence of the promotion effect. (C) 2009 Elsevier B.V. All rights reserved.