(361f) NH3-SCR Over Cu/SAPO-34 - Zeolite Acidity and Active Site Changes As a Function of Cu Loading
AIChE Annual Meeting
Tuesday, November 5, 2013 - 4:55pm to 5:15pm
NH3 Selective Catalytic Reduction (NH3-SCR) is a widely applied technology to efficiently treat NOx 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.