Identification of adsorbed species on Cu-ZSM-5 under NH3 SCR conditions

Abstract

The selective catalytic reduction of nitrogen oxides with ammonia as the reducing agent was studied using Fourier transform infrared (FTIR) spectroscopy. The adsorbed species found on a Cu-ZSM-5 powder during exposure to NO, NO 2 or NH 3 was compared to the adsorbed species identified during SCR conditions. A blocking effect caused by ammonia at 175°C was investigated by a stepwise increase of the ammonia concentration, and the spectra indicated that the formation of nitrites or nitrates decreased as surface coverage of ammonia increased. No such effect was observed at 350°C, since the oxidation of ammonia results in very low ammonia coverage. The effect of changes in the NO to NO 2 ratio was also studied at 350°C, and the species identified during SCR reaction indicated that the enhanced activity at equimolecular amounts of NO and NO 2 possibly involves gas phase components as well as adsorbed species.

Impact of sulfation and desulfation on NO reduction using Cu-chabazite SCR catalysts

“…This effect is clearly observed at temperatures above 250 • C. The de-greened sample displays the highest oxidation, with a maximum of 40% at 300 • C. The activity of the sample sulfated at 400 • C and desulfated for 30 min at 600 • C is lower, but is still significant with a maximum of 22% at 300 • C. Additionally, both of these samples show at least 5% NH 3 oxidation at 250 • C. Meanwhile, both sulfated samples and the sample desulfated for only 10 min at 500 • C (sulfated at 250 • C) show limited NH 3 oxidation, with all three displaying less than 5% conversion at 300 • C and essentially zero activity at 250 • C. These results agree with Kumar et al [14], who also observed a decrease in NH 3 oxidation over a Cu-SAPO SCR catalyst after exposure to sulfur. It is generally agreed upon that, as with NO oxidation, NH 3 oxidation in Cu-zeolite SCR catalysts occurs over active Cu sites in the washcoat [22,23]. Thus, based upon the decreased oxidation activity for both NO and NH 3 observed over sulfated catalysts in the current study, it appears that sulfur is poisoning or blocking Cu sites in the chabazite washcoat.…”

Mechanism of propene poisoning on Cu-SSZ-13 catalysts for SCR of NOx with NH3

“…6(b)). The IR bands at 1602 cm −1 and 1573 cm −1 could be attributed to the different nitrates adsorbed on the surface [40,41], while there was no other carbonaceous species observed. The IR bands almost disappeared within 300 • C implying the nitrate desorption or decomposition at high temperatures.…”

A multi-site kinetic model for NH3-SCR over Cu/SSZ-13

“…Ammonia storage and release is a crucial function for the transient operation of an SCR catalyst, especially for zeolite containing materials since they store very large quantities [7,18,20,49]. We Table 1 Reactions and rate expressions.…”

“…Further, the NO oxidation activity for this catalyst was low, and we only receive 20-25 ppm NO 2 at 500-600 • C when exposing the catalyst to 400 ppm NO, 8% O 2 and 5% H 2 O (results not shown here). In a DRIFT study by Sjövall et al [7] using Cu/ZSM-5 it was found that the coverage of nitrites/nitrates decreased due to ammonia inhibition. Thus, it is likely that ammonia also inhibits the NO oxidation, which would result in even lower NO oxidation in SCR conditions.…”

“…Vanadia supported on titania [1][2][3][4][5] and Cu- [6][7][8][9][10][11][12][13][14] and Fe-exchanged [15][16][17] zeolites have been extensively studied for this reaction. A major advantage of Cu/zeolites is that they exhibit a high level of activity at low temperature [18].…”