The influence of Si/Al ratio on the catalytic property and hydrothermal stability of Cu-SSZ-13 catalysts for NH3-SCR

Abstract

Cu-SSZ-13 catalysts with three Si/Al ratios of 6, 15 and 30 were synthesized by a one-pot method. The catalytic property and hydrothermal stability of as-prepared catalysts were studied to understand the influence of Si/Al ratio on the NH3-SCR performance over Cu-SSZ-13 catalysts. Although the three catalysts contain equivalent Cu loading, they perform different catalytic activities for NO reduction. The EPR and H2-TPR results clearly show that the Si/Al ratio not only has influence on the nature of Cu species but also on the Cu species distribution. With increasing the Si/Al ratio, the amount of CuO increases while the number of highly stable Cu+ ions decreases. Such a phenomenon could be attributed to the less Al content in high-Si/Al-ratio catalysts. Besides, the acidity is also found to be affected by the Si/Al ratio. The NH3-TPD results demonstrate that the total number of acid sites decreases with increasing the Si/Al ratio. Lacking sufficient acidity is also responsible for the inferior NO reduction activity of Cu-SSZ-13 with Si/Al = 30. Meanwhile, kinetic analysis over fresh catalysts was conducted and the conclusion correlated well with the NH3-SCR performance. Additionally, three catalysts exhibit distinct resistances to hydrothermal treatment. The XRD, N2 physisorption and NH3-SCR performance results consistently indicate the Cu-SSZ-13 with Si/Al = 15 is much more resistant to hydrothermal aging than another two catalysts probably due to its considerably stability of zeolite structure and Cu ions.