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

Abstract

The effect(s) of SO2 on the two types of active sites on Cu-SSZ-13 NH3–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), H2 temperature-programmed reduction (H2-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) SO2 poisoning were site-dependent. Results of SO2 and SO2 + NH3 temperature-programmed 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, SO2 only adsorbs when it is cofed with NH3 via formation of ammonium sulfate, with its fingerprint TPD feature at 380 °C. However, low-temperature interaction between SO2 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 SO2. 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 SO2 poisoning because of the formation of more stable bisulfite and ultimately bisulfate species.