Understanding the Water Effect for Selective Catalytic Reduction of NO_x with NH₃ over Cu-SSZ-13 Catalysts

Abstract

The catalytic activity of commercial Cu-SSZ-13 monolith catalysts was measured in the presence and absence of water, which demonstrated an inhibiting role below 300 °C and a promoting role above 300 °C. Furthermore, temperature-programed desorption illustrated that hydrolysis of Z 2 Cu would take place to generate ZCu II OH and [Cu(H 2 O) 6 ] 2+ species, which inhibited ammonia adsorption over Lewis acid sites but promoted nitrate formation and gaseous NO 2 desorption. The temperatureprogramed surface reaction and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were further performed to elucidate the influence of water on reaction mechanisms. Basically, the NH 3 -SCR reaction mainly took place at Cu centers acting as Lewis acid sites of Cu-SSZ-13 catalysts in the low-temperature regime. Water inhibited the reaction by occupying and blocking Lewis acid sites to prevent [Cu(NH 3 ) 2 ] 2+ formation and NO oxidation from occurring, limiting the formation of NH 4 NO 3 intermediates. In the high-temperature regime, gaseous NO 2 could directly interact with NH 4 + molecules adsorbed onto Brønsted acid sites. The addition of water could facilitate the production of gaseous NO 2 decomposed by surface nitrates and further promote the NH 3 -SCR reaction rate at high temperatures.

Investigation of Low-Temperature OHC and RHC in NH3–SCR over Cu-CHA Catalysts: Effects of H2O and SAR

“…In line with our findings on CuCHA10, they observed standard SCR NO x conversion to be promoted by the presence of water in the feed. Conversely, in a recent publication, Ma et al observed the low-T deNO x activity to be inhibited by water over a commercial Cu-SSZ-13 catalyst with a Cu loading ratio of ∼1.4 wt % and SiO 2 /Al 2 O 3 ∼ 9 . Even if apparently in contrast with our results, we have to consider that, in that case, a severe aging pretreatment (at 750 °C for 16 h) was applied by the authors, leading to dealumination and decrease in the catalyst acidity .…”

“…With respect to the recent literature, 17,18,40,43,46,50 which mainly focuses on the effects of operating conditions (e.g., water feed content) or catalyst formulation by directly looking at the global SCR process, once again, the present results demonstrate the importance of analyzing the standard SCR process as two separate half-cycles.…”

“…However, the effect of water on the deNO x performance is still unclear. On a Cu-SSZ-13 catalyst, Ruggeri et al 42 reported a negligible effect of water on the low-T (≤250 °C) standard SCR NO x conversion, while Ma et al 43 observed an inhibiting effect in the same T range. Ottinger et al 40 suggested the effect of water on the deNO x activity to vary depending on the NO x concentration (i.e., promoting and inhibiting effect at high and low NO x concentrations, respectively).…”

Design, Synthesis, and Insights into the Redox Mechanism of Highly Efficient One-Pot Cu-ZSM-5 Catalysts for the Reduction of NOx by NH3-SCR at Low Temperatures

“…In situ DRIFT spectra and corresponding mapping were recorded for further analysis of [Cu­(OH)] + in 10-membered rings and Cu 2+ in 6-membered rings. According to the literature, two distinguished bands around 950 and 900 cm –1 are attributed to T–O–T asymmetric vibrations perturbed by [Cu­(OH)] + and Cu 2+ species for Cu-SSZ-13. − These two bands of zeolite framework vibrations are perturbed by Cu species in the neighborhood of the zeolite lattice, representing the solvation of two different types of Cu species with NH 3 , namely, [Cu­(OH)] + and Cu 2+ species, respectively . Similar phenomena also appear in Cu-SAPO-34 and Cu-SAPO-18, while the FTIR bands for [Cu­(OH)] + and Cu 2+ species are around 900 and 850 cm –1 .…”

In Situ Spectroscopic Studies of NH3 Oxidation of Fe-Oxide/Al2O3

“…When humidity increased from 30 to 60%, NH 3 removal did not decrease further but actually increased in a broad range of temperature, while N 2 selectivity seemed to decrease slightly in the range of 320–400 °C. It can be speculated that H 2 O molecules can compete with NH 3 molecules for adsorption sites, while they can also help conversion of NH 3 to surface residual nitrogen compounds. , The former role of H 2 O can induce reduction of NH 3 removal; however, from a certain level of humidity, the later pathway can become more activated, leading to the improved NH 3 removal and the reduced N 2 selectivity.…”