KINETIC AND MECHANISTIC STUDY OF NOX REDUCTION BY NH3 OVER H-FORM ZEOLITES .2. SEMI-STEADY-STATE AND IN-SITU FTIR STUDIES

A series of transient kinetic, semi-steady-state kinetic and in situ F ourier transform infrared spectroscopy (FTIR) experiments were conduct ed using NH3-presorbed H-form pentasil zeolites to reduce NOx to N-2. Semi-steady-state kinetic experiments were performed at temperatures b elow 210 degrees C to determine reaction rate parameters for the reduc tion of NO over NH3-presorbed H-mordenite. Transient test results sugg est that pairs of NH3 molecules adsorbed onto neighboring acid sites a re necessary for NO reduction to N-2. Results of in situ FTIR experime nts were used to identify the structures of adsorbed NH3 and intermedi ate NOx species. These FTIR results suggest that adsorbed NH3 is most reactive when it is bonded to the Bronsted acid sites in zeolites thro ugh three hydrogen bonds. lit situ FTIR spectra also indicate that an NO2-type intermediate is formed on the zeolite surface during selectiv e catalytic reduction (SCR). Formation of this NO2-type species appear s to be a necessary step in the SCR reaction mechanism as the concentr ation of adsorbed NH3 does not decrease (i.e., react) until a band cor responding to this NO2-type species appears. Combined analyses of tran sient and in situ FTIR results were used to develop a reaction mechani sm which describes NO or NO2 reduction by NH3 over H-form zeolites. (C ) 1997 Academic Press.