Deactivation of Cu-SSZ-13 in the presence of SO2 during hydrothermal aging

Cu2+-exchanged zeolite catalysts with the chabazite (CHA) structure have been thought to be optimal candidates for selective catalytic reduction of NOx with NH3. In real applications, however, SCR catalysts readily undergo hydrothermal aging and sulfur poisoning. In this work, the co-effect of SO2 and hydrothermal aging at high temperature was investigated. Different from the reversible inhibition of SO2 poisoning that occurs at low temperatures, the sulfur poisoning at high temperature is permanent due to the destruction of the zeolite structure, and no deposit of sulfate is observed. Cu-SSZ-13 catalysts were characterized through solid state Al-27 nuclear magnetic resonance (Al-27-NMR), X-ray diffraction (XRD), temperature-programmed desorption of NH3 and NO (NH3/NO-TPD), electron paramagnetic resonance (EPR), temperature-programmed reduction by H-2(H2TPR), in situ DRIFTS, and thermogravimetric analysis with mass spectrometric detection (TG-Mass) to develop an understanding of the degradation mechanisms during hydrothermal aging and sulfurization at high temperature. The results indicated that SO2 dislodged the extra-framework Al atoms that resulted from the dealumination process that occurs during hydrothermal aging. More Cu2+ species were accumulated as CuOx for CuSSZ-13 after sulfurization at high temperature compared to that treated by hydrothermal aging only. The dealumination and accumulation of Cu2+ species caused a loss of acid and active sites for the Cu-SSZ-13 catalyst, and resulted in degradation of NH3-SCR performance.