Coated bifunctional catalysts for NOx SCR with C3H6: Part II. In situ spectroscopic characterization

The effect of finely coating Cu-ZSM-5 with nanoparticles of ceria has been studied in situ by XANES, DRIFTS and TPR. The combination of these techniques reveals a lower temperature of reduction for both metals (Cu and Ce) in the bifunctional catalyst. Copper interacts with ceria nanocrystals at the oxide–zeolite interface and forms a localized phase comparable to doped ceria solid solutions commonly used as oxidation catalysts. As a result the catalyst is able to activate C3H6 and form NO2 at low temperatures. Analysis of the stability of adsorbed CO by DRIFTS suggests that the addition of water promotes the oxidation of Cu1+ on the coated catalyst. In addition, TPD and TGA results, in combination with supplementary DRIFTS experiments, indicate that the ceria coating retains water at the interface during reaction conditions. The combination of these events yields relatively high C3H6-SCR activity at temperatures as low as 250 °C. It is believed that the affinity towards water and extended coverage of the coating also protects the zeolite against degradation by restricting the diffusion of water molecules into the zeolite pores.