Nature of Cu Active Centers in Cu-SSZ-13 and Their Responses to SO₂ Exposure

Abstract

The effect(s) of SO 2 on the two types of active sites on Cu-SSZ-13 NH 3 −SCR catalysts, Z2Cu and ZCuOH, were investigated. Two Cu-SSZ-13 catalysts with Si:Al ratios of 6 and 30 were synthesized, and they provide very different distributions of these two active sites. Inductively coupled plasma optical emission spectroscopy (ICP-OES), H 2 temperature-programmed reduction (H 2 -TPR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were utilized to characterize catalyst samples and quantify the amounts of total Cu, Z2Cu and ZCuOH. In situ DRIFTS results show that Z2Cu and ZCuOH responses to low-temperature (<200 °C) SO 2 poisoning were site-dependent. Results of SO 2 and SO 2 + NH 3 temperatureprogrammed desorption (TPD) and DRIFTS experiments, supplemented with DFT calculations, revealed that the different observed responses correspond to different sulfur intermediates that form. On Z2Cu sites, SO 2 only adsorbs when it is cofed with NH 3 via formation of ammonium sulfate, with its fingerprint TPD feature at 380 °C. However, low-temperature interaction between SO 2 and ZCuOH leads to copper bisulfite species formation, which can be further oxidized to form copper bisulfate with increasing temperature. In terms of low-temperature SCR functionality, the activity of both Cu-SSZ-13 samples were found to be significantly inhibited by SO 2 . However, in terms of regeneration (i.e., desulfation) behavior, Cu-SSZ-13 with a Si:Al = 30 (higher ZCuOH compared to Z2Cu) seemed to require higher desulfation temperatures (>550 °C). Therefore, compared with Z2Cu, ZCuOH sites are more susceptible to severe low-temperature SO 2 poisoning because of the formation of more stable bisulfite and ultimately bisulfate species.

Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts

“…An extra benefit for this treatment, is that a mildly aged Cu/SSZ-13 catalyst will be more resistant to sulfur poisoning. Notably, studies by Luo et al [16] and Epling and coworkers [23] provide strong evidence to demonstrate that [Cu(OH)] + -Z is much more vulnerable than Cu 2+ -2Z toward sulfur poisoning. CuO x formed during hydrothermal aging also causes SCR selectivity to decrease because this species does not efficiently catalyze SCR but is instead active in catalyzing the undesirable NH 3 oxidation reaction at temperatures above~300 • C. This problem may be circumvented with low Cu loaded catalysts where the dominance of Cu 2+ -2Z prevents CuOx formation during hydrothermal aging.…”

“…Now after 3 years, new and exciting results have appeared in literature, especially with respect to the developing molecular-level understanding of the nature of the active Cu species under realistic operating (i.e., situ/operando) conditions, as well as the SCR reaction mechanisms [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. As such, it seems timely to review these new fundamental understandings.…”

Site selective adsorption and relocation of SOx in deactivation of Cu–CHA catalysts for NH3-SCR

“…Ultra-low sulfur diesel contains less than 10 wt ppm (Europe) or 15 wt ppm (US) sulfur, and the SO2 concentration in the exhaust gas typically reaches a few ppmv, which is nevertheless sufficient to reduce the low-temperature activity of the Cu-CHA catalysts significantly. [13][14][15][16][17][18][19][20][21][22][23][24] To be able to preserve the good lowtemperature activity, it is important to understand the impact of SO2 on the NH3-SCR activity of Cu-CHA based catalysts. We have recently shown that deactivation of Cu-CHA catalysts depends on the total SO2 exposure, which is the product of the SO2 concentration and the exposure time.…”

“…13,14,17 During the regeneration, SO2 is released from the catalyst, but a certain fraction of the sulfur is retained in a stable form that resemble Cu sulfate. 13,14,[17][18][19]22 The observations that heating to 550 C results both in a partial regeneration of Cu-CHA catalysts and in the formation of a certain amount Cu sulfate, indicates that there are distinct Cu sites with a different reactivity towards SO2. 13,15,17,22 The most obvious possibility for distinct Cu sites in Cu-CHA catalysts is that the Cu can be associated with either one or two Al centers in the zeolite framework.…”

The Role of SO3 Poisoning in CU/SSZ-13 NH3-SCR Catalysts

“…Shih et al [23] further used a theoretical method to support that Cu(OH) + was more prone to SO 2 poisoning than Cu 2+ . Yasser et al [24] studied that SO 2 only adsorbed on Cu 2+ when ammonia was present and formed ammonium sulfate as the main poisoning route on this site, while, SO 2 could react with [Cu(OH)] + to form copper bisulfate. Wei et al [25] studied the impact of hydrothermal aging on SO 2 poisoning over Cu/SSZ-13.…”