A DRIFTS Study of Cu–ZSM-5 Prior to and during Its Use for N2O Decomposition

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

“…Hence, the reduced band at 1572 cm )1 , which indicates a decreased nitrite or nitrate formation in presence of high ammonia coverage, can be one explanation to the blocking effect observed in previous study [14]. At 350°C (Figure 4) no evidence of adsorbed ammonia can be found and the only observed surface components are nitrites or nitrates (1500-1650 cm )1 ), water and other possible OH groups that may contribute to the broad feature between 2200 and 3800 cm )1 [40][41][42]. The stepwise increase of gas phase ammonia from 300 to 700 ppm results in enhanced water formation and nitrite or nitrate consumption, which suggests that these species are active intermediates in ammonia SCR over Cu-ZSM-5 catalysts.…”

“…It is probable that some water was present during the adsorption of ammonia and the band at 1620 cm )1 may thus be a combination of bands due to both ammonia and water. The broad feature in the 2200-3500 cm )1 region has been assigned to water [40][41][42]. However, it is a combination of several peaks and a contribution from bands assigned to NH 4 + and weakly bound NH 3 cannot be ruled out.…”

“…These peaks have both been assigned to ammonium ions formed at Brønsted acid sites but the result shows that the two bands may be induced by additional adsorption sites with similar adsorption properties. During the copper ion exchange, complexes may be formed that generates additional hydroxyl groups [40], and it is possible that ammonia may be adsorbed to a different extent on the various sites at 175 and 350°C.…”

Mechanistic Study of Lean NO2 Reduction by Propane Over HZSM-5 in the Presence of Water

“…However, this reaction route involves formation of N 2 O 3 on the sample surface and even though we find a peak in the N=O region (1930-1880 cm )1 ) [10] the symmetric and asymmetric stretching vibrations of NO 2 , at 1590-1550 and 1305-1290 cm )1 , respectively [10], are not detected. Of course, disturbances from the zeolite framework are significant in this area (below 2000 cm )1 ) and may influence the peak observations [19][20][21], thus it is possible that these peaks exists. On the other hand, Hadjiivanov et al [22] state that the NO + species is the only stable NO x surface species on HZSM-5 and Svedberg et al [42] have investigated the NO x storage ability on several materials and conclude that the amounts of stored NO x on acidic samples is negligible above 300°C.…”

“…The FTIR measurements are performed in the region above 2000 cm )1 since below 2000 cm )1 the background noise level is significant, probably related to perturbed vibration frequencies of the zeolite framework [19][20][21], which makes peak observation and assignment difficult in this area.…”