Experimental evidence of the mechanism behind NH3 overconsumption during SCR over Fe-zeolites

Isotope labeled 15NO was used to investigate the mechanism of the unusual overconsumption of NH3 during standard SCR over Fe-zeolite, under conditions during which NH3 oxidation with O2 alone (without NO) was unfavorable. When the Fe-BEA catalyst was exposed to the 15NO + 14NH3 + O2 gas mixture at 250 and 300 °C, the resulting products included 14NO, a product of 14NH3 oxidation under SCR conditions. However, 14NO was not detected when Fe-BEA was exposed to 14NH3 + O2. Since the only source of 14N derives from 14NH3, with the labeled gas mixture used, the 14NO during SCR must have originated from oxidation of 14NH3. Furthermore, twice as much 14N14N was observed at 300 °C under SCR conditions in comparison with NH3 oxidation using O2 alone. Under SCR conditions, the 14NO formed through the unusual oxidation route further reacted with 14NH3 to produce 14N2. Thus, for the first time, we have experimental evidence for the unusual overconsumption of ammonia during SCR over Fe-zeolites.

The stoichiometry of NH3 to NO in standard SCR is larger than one over Fe zeolites. Isotopic experiments with 15NO, 14NH3, and O2 show evidence for parasitic ammonia oxidation at low temperature in the presence of NO, which explains the overconsumption of NH3.Figure optionsDownload high-quality image (99 K)Download as PowerPoint slideHighlights
► The stoichiometry of NH3 to NO in standard SCR is larger than one over Fe zeolites.
► Isotopic experiments with 15NO, 14NH3, and O2 were conducted over Fe-BEA.
► 14NH3 is oxidized to 14NO, only in the presence of NO (do not occur with O2 only).
► The selectivity for 14NO production is increasing with increasing NH3 concentration.
► 14NO is than further reacted with 14NH3 to produce 14N.