Research on Performance and Mechanism of the NH3-SCR Reaction over Fly Ash-derived Mn-Ce/Zeolite X

Abstract

Zeolite X was synthesized from fly ash under optimized conditions, which was supported with Mn, Ce, and Mn-Ce respectively. Fly ash-derived zeolite X catalysts exhibited similar catalytic performance as compared with the commercial zeolite X catalyst. XRD, N2 adsorption, SEM, XPS, and H2-TPR were used to investigate the relationship among the structure, physicochemical properties, and catalytic activities of the catalysts. Finally, the mechanism of NH3-SCR reaction on Mn-Ce/zeolite X was systematically performed by using diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The characterization results showed that the amorphous structure and good dispersion between Mn and Ce on the surface of the zeolite changed the electronic properties of the active components, improved its low-temperature catalytic activity, brought it characteristics of Mn and Ce at the same time, and broaden the reaction temperature range. The NO conversion rate of Mn-Ce-FX catalyst remained above 80% at 200°C–300°C. From DRIFTS we suggest that Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism are simultaneously carried out on the catalyst surface. According to the FTIR results, the Eley-Rideal mechanism has a great influence on the reaction below 250°C, and the Brønsted acid sites adsorb a large amount of NH3, resulting in an excellent low-temperature activity. When above 250°C, Langmuir-Hinshelwood mechanism plays a dominant role. The nitrates on the surface gradually convert to bidentate nitrates, impeding the SCR reaction, could be one of the reasons for reducing the high-temperature activity.