Isotopic labeling studies of the selective non-catalytic reduction of NO with NH3

Abstract

The reaction of NO and NH3 in the presence of excess O2 has been investigated using 15N-isotopic labeling techniques under selective noncatalytic reduction (SNCR) conditions. Experiments were conducted with and without added H2. The results show that 14N15N is always the predominant nitrogen-containing product. Conversion of NH3 to 14NO is a prerequisite for the formation of both 14N2 and 14N2O. Experiments conducted in the presence of added H2 result in higher selectivities to N2O: at temperatures above the maximum NO conversion, a large proportion (43%) of the nitrous oxide is 14N2O. In general, the experimentally determined isotopic product distributions were in good agreement with model predictions based on accepted detailed kinetic schemes for the SNCR process. However, under some conditions, the model predicted significant concentrations of 15N2 and 15N2O, but these doubly labeled species were not observed experimentally. Measured concentrations of 15N2O never exceeded the detection limit (0.05 ppm). Sensitivity and rate-of-progress analyses show that these doubly labeled 15N species are produced via channels in which the first steps are reactions of the NNH species with H radicals. An NNH lifetime of less than 10−8 s decreased the predicted amounts of these species. However, this lifetime would imply that H atoms should be produced in kinetic studies of the NH2+NO reaction, but this has not been observed. Alternately, the kinetics of the reaction involving the N2Hi species should be reviewed. The source of HCN produced from the propane-promoted SNCR reaction has also been investigated using labeling techniques. Both HC14N and HC15N, in similar concentrations, are produced from an equimolar mixture of 15NO and 14NH3, under conditions in which ammonia oxidation to NO is insignificant. The results therefore imply that both NO-propane and NH3-propane interactions are important in HCN formation under competitive conditions.