N₂O Formation Pathways over Zeolite-Supported Cu and Fe Catalysts in NH₃-SCR

Abstract

N2O is a common byproduct in the NH3-SCR reaction. We analyzed the N2O formation pathways in NH3-SCR over various catalysts (Cu-ZSM-5, Fe-ZSM-5, Cu-SAPO-34, Fe-SAPO-34, Cu-SSZ-13, and Fe-SSZ-13), aided by catalyst characterization using XRD, XPS, EDS mapping, and NH3-TPD. The results showed that the NH3 nonselective catalytic reduction was the major N2O formation pathway for most of the Cu catalysts. The N2O formation at lower temperatures (<300 °C) originated mainly from decomposition of NH4NO3. In addition, NH3 nonselective oxidation was another reaction that formed N2O especially at higher temperatures. The N2O resulting from the Eley–Rideal mechanism was also favored at higher temperatures. The decomposition of NO to N2O and O2 also led to N2O formation, although its contribution was minimal. The absence of N2O yield over most Fe catalysts could be attributed to active N2O decomposition and N2O-SCR reactions. Moreover, varying O2 and H2O concentrations in the feed exerted strong influence on both N2O formation and SCR activity. Decrease in O2 level from 14% to 3% led to continual decline in N2O formation but had no effect on SCR activity until reaching a threshold concentration of 2%. H2O in lower concentrations (2–3%) facilitated N2O formation and NO conversion due to increase in Brønsted acidity, while H2O in higher concentrations (>5%) led to suppression of these reactions due to the coverage of active sites.

Catalytic Performances of Cu/MCM-22 Zeolites with Different Cu Loadings in NH3-SCR

“…In contrast, the density of strong acid sites dramatically increased from 361 µmol/g to 1034 µmol/g. According to the literature [22,[47][48][49][50], the results in Table 2 demonstrate that some of the weak and medium acid sites were covered by Cu species, but the introducing of Cu species also led to the increasing of strong acid sites. As shown in Figure 3, the shift of TPD peaks towards higher temperatures also confirms that the global acid strength of xCu/MCM-22 zeolites gradually increases with Cu loadings, which is in agreement with the quantitative results in Table 2.…”

The dynamic nature of Cu sites in Cu-SSZ-13 and the origin of the seagull NOx conversion profile during NH3-SCR

“…S3b †). Such a behavior could be explained based on recent studies on N 2 O formation over Cu-SSZ-13 catalysts, 45,46 which claim two different paths: via NH 4 NO 3 at low temperatures and via NH 3 oxidation at high temperatures. Also the participation of different active sites for the SCR reaction and N 2 O formation cannot be ruled out.…”

Integrating pore interconnectivity and adaptability in a single crystal hierarchical zeolite for liquid alkylation

“…The chemical state of these species was characterized by XPS, and the results were displayed in Figure S1. For Fe‐containing MEL zeolites, two peaks were observed at 711.8 and 725.4 eV corresponding to Fe 2p3/2 and Fe 2p1/2 in Fe 3+ , respectively 25,26 . Moreover, additional satellite peaks at ~716.3 and 729.9 eV were also observed in P‐MEL@Fe (Figure S1a) and H‐MEL@Fe‐ x (Figure S1b–S1d), which represented the binding energies of Fe 2p3/2 and Fe 2p1/2 in Fe(III) of Fe 2 O 3 , respectively 27 .…”

“…For Fe-containing MEL zeolites, two peaks were observed at 711.8 and 725.4 eV corresponding to Fe 2p3/2 and Fe 2p1/2 in Fe 3+ , respectively. 25,26 Moreover, additional satellite peaks at~716.3 and 729.9 eV were also observed in P-MEL@Fe (Figure S1a) and H-MEL@Fe-x (Figure S1b-S1d), which represented the binding energies of Fe 2p3/2 and Fe 2p1/2 in Fe(III) of Fe 2 O 3 , respectively. 27 The UV-Vis spectroscopy data for the resultant zeolite samples was given in Figure S2, and the corresponding percentages for the different iron species of zeolites were listed in Table S1.…”