(361f) NH3-SCR Over Cu/SAPO-34 - Zeolite Acidity and Active Site Changes As a Function of Cu Loading

NH₃ Selective Catalytic Reduction (NH₃-SCR) is a widely applied technology to efficiently treat NO_x emissions from heavy duty diesel vehicles. Copper-exchanged chabazite (CHA) catalysts exhibit both superior activity and hydrothermal stability for this reaction in comparison with Cu-BEA and Cu-ZSM-5, and have therefore been recently adapted for on-vehicle application. In this study, a series of SAPO-34 catalyst with various Cu loadings (ranging 0.7-4.0 wt%) were prepared by the solid state ion exchange method. The acid properties were characterized by XRD, NH₃-TPD, in-situ DRIFTS, H₂-TPR and UV-vis techniques and active sites for NH₃-SCR were also probed. The data show that both Bronsted acid sites and Lewis acid sites were present in the as-prepared samples. The Lewis acid sites were created by Cu ions and increased monotonically with increasing the Cu loading. Furthermore, four different Cu species were discovered in the samples: isolated Cu²⁺ ions, Cu⁺ ions, large CuO particles and nano-sized CuO. However, the proportion of these species on each sample was found to be different, resulting in various SCR behaviors. For example, the amount of isolated Cu²⁺ ions was proved to increase monotonically with increasing Cu loading. However, the total number of Cu⁺ ions increased as the Cu loading increased up to 2.5 wt% then decreased with higher Cu loading. Isolated Cu²⁺ species were found to be the active sites over Cu-SAPO-34 catalysts at low temperature (130-300°C). However, the NH₃-SCR activity at high temperature (300-550°C) could also be attributed to the nano-sized CuO species that are probably located in the zeolite pores. In addition, the particle size of the CuO species has various impacts on NH₃ oxidation activity and selectivity. The data show that large CuO particles could promote the competitive NH₃ oxidation reaction and therefore decrease the NO conversion of standard SCR at high temperature. On the other hand, the nano-sized CuO particles actually increased the selectivity of NH₃ oxidation to NO. Moreover, Cu⁺ ions likely do not affect the SCR activity.