Dynamics of low temperature N2O formation under SCR reaction conditions over a Cu-SSZ-13 catalyst

Mechanism of NH3–Selective Catalytic Reduction (SCR) of NO/NO2 (Fast SCR) over Cu-CHA Zeolites Studied by In Situ/Operando Infrared Spectroscopy and Density Functional Theory

“…Under fast SCR conditions, NO 2 may disproportionate to NO + and nitrate (NO 3 – ) ions. Previous IR studies proposed that NO + and NO 3 – species on Cu-CHA catalysts are possible intermediates or spectators in the standard and fast SCR. ,,,,,,,− However, to our knowledge, very limited IR results were obtained for quantitative comparison of these NO x species on Cu-CHA and H-CHA catalysts in NO 2 and NO/NO 2 mixtures. Here, we carried out a comprehensive in situ IR study of the adsorbed NO x species under NO 2 or equimolar amounts of the NO 2 and NO mixture over Cu-CHA and H-CHA catalysts as a function of temperature.…”

“…Unlike these conventional SCR catalysts, NO 2 addition in the reaction mixture does not significantly improve steady-state NO x conversions over Cu-CHA, and the promotional effect of NO 2 on SCR by Cu-CHA is observed under limited conditions. − Below 150 °C, NO 2 formed in the DOC upstream of the SCR converter can react with NH 3 , leading to the accumulation of ammonium nitrate (NH 4 NO 3 ) in the zeolite pores and hence decreasing the NO x conversion . Additionally, NH 4 NO 3 may act as a precursor of the undesirable byproduct N 2 O. − Based on this background, fundamental understanding of the elementary reaction steps involving NO, NO 2 , and NH 3 in Cu-CHA below 150 °C is indispensable for designing a next generation SCR system workable at low temperatures. Although a large number of papers reported the mechanism of NH 3 –SCR by Cu-CHA, most of them focused on the standard SCR − ,,− and rather few studies proposed elementary steps of fast SCR. − ,,− In situ X-ray absorption study by McEwen et al demonstrated that Cu 2+ is the only evident Cu oxidation state in the fast SCR reaction on Cu-SSZ-13.…”

“…A large number of experimental [ in situ IR and temperature-programmed surface reaction (TPSR)] ,− and theoretical , studies of NO x and NH 3 species adsorbed on Cu-CHA and other zeolites − have been reported by many research groups to identify possible intermediates in SCR, but most of the previous works have focused on the mechanism of standard SCR. On the basis of spectroscopic or indirect evidence, nitrates (NO 3 – ), nitrites (NO 2 – ), and nitrosonium cations (NO + ) adsorbed on Cu-CHA and other zeolites have been proposed to play important roles in SCR. ,− However, intermediacy of these adsorbed NO x species is still under debate due to the lack of comprehensive approach, and limited numbers of reports have utilized combined study of operando infrared (IR) spectroscopy, kinetics, TPSR, and density functional theory (DFT). Furthermore, mechanistic insights into fast SCR for Cu-CHA are less than those for vanadium oxides, Fe-zeolites, ,, and BaNa-zeolites. , Our previous study on Cu-free H-CHA zeolites demonstrated that the Brønsted acid sites (BASs) play important catalytic roles in the elementary reactions of fast SCR, and herein, we further performed a comprehensive study on the fast SCR mechanism over Cu-CHA zeolites.…”

N2O Formation during NH3-SCR over Different Zeolite Frameworks: Effect of Framework Structure, Copper Species, and Water

“…Moreover, the considerable N 2 O formation for H/SSZ-13 suggests that another N 2 O formation pathway exits in contrast to N 2 O formation over Cu–dimer complexes in standard SCR. Significantly lower NOx conversion is generally observed when the NO 2 fraction exceeds 50% of the total NOx at low temperature (below 200 °C) . This indicates that surface nitrate or nitrate species formation is dominant, leading to higher AN formation at low temperature.…”

The Study of C3H6 Impact on Selective Catalytic Reduction by Ammonia (NH3-SCR) Performance over Cu-SAPO-34 Catalysts

“…Especially based on the paper published by Xue et al, it was shown that four copper species in Cu/SAPO-34 samples were identified, including external surface copper oxide clusters, isolated Cu 2+ , nanosized copper oxide crystallites, and Cu + species. It was indicated that the isolated Cu 2+ species were responsible for the active sites for the NH 3 -SCR reaction over the Cu/SAPO-34 catalyst (100-200 • C) [19,20], acidity [21], reaction intermediates [22], reaction mechanism [23][24][25][26], formation and decomposition of the by-products N 2 O [27][28][29][30][31], hydrothermal stability [32,33], and SO x poisoning over Cu-SAPO-34 [34,35].…”