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

Abstract

A series of transient kinetic, semi-steady-state kinetic andin situFourier transform infrared spectroscopy (FTIR) experiments were conducted using NH3-presorbed H-form pentasil zeolites to reduce NOxto N2. Semi-steady-state kinetic experiments were performed at temperatures below 210°C to determine reaction rate parameters for the reduction of NO over NH3-presorbed H-mordenite. Transient test results suggest that pairs of NH3molecules adsorbed onto neighboring acid sites are necessary for NO reduction to N2. Results ofin situFTIR experiments were used to identify the structures of adsorbed NH3and intermediate NOxspecies. These FTIR results suggest that adsorbed NH3is most reactive when it is bonded to the Brønsted acid sites in zeolites through three hydrogen bonds.In situFTIR spectra also indicate that an NO2-type intermediate is formed on the zeolite surface during selective catalytic reduction (SCR). Formation of this NO2-type species appears to be a necessary step in the SCR reaction mechanism as the concentration of adsorbed NH3does not decrease (i.e., react) until a band corresponding to this NO2-type species appears. Combined analyses of transient andin situFTIR results were used to develop a reaction mechanism which describes NO or NO2reduction by NH3over H-form zeolites.