XANES and DRIFTS study of sulfated Sb/V/Ce/TiO2 catalysts for NH3-SCR

Abstract

A study of structural and mechanistic investigation of the Sb/V/Ce/Ti catalysts sulfated at different temperatures was carried out by X-ray absorption near edge spectroscopy (XANES) and diffused reflectance infrared Fourier transformed spectroscopy (DRIFTS). The high temperature sulfated catalyst, especially Sb/V/Ce/Ti-S500 (sulfated at 500°C temperatures), exhibited superior NOx conversion at low temperatures (150–200°C) compared to Sb/V/Ce/Ti-S400 and Sb/V/Ce/Ti-S300 (sulfated at 400 and 300°C) catalysts. The Ce L3 edge XANES spectra of Sb/V/Ce/Ti-S500 catalyst showed the formation of Ce(III) dominant sulfate species, resulting in the enhancement of Lewis and Brønsted acid strength. The formation of Ce(III) sulfate species on Sb/V/Ce/Ti-S500 catalyst was clearly indicated by Ce M4,5 and S K edge XANES spectra peaks at 881.9eV attributed to Ce3+ oxidation state and 2481eV assigned to S6+ oxidation state of sulfate species. Furthermore, the in situ DRIFTS results revealed that the Lewis and Brønsted acid sites of Sb/V/Ce/Ti-S500 catalysts increased significantly, followed by Sb/V/Ce/Ti-S400 and Sb/V/Ce/Ti-S300. At 200°C, the reaction between the pre-adsorbed NH3 species with NO+O2 on sulfated catalysts exhibited the formation of mono-dentate, bi-dentate, bridging nitrates and NO2 species. Meanwhile, the subsequent formation of NO2 via NO oxidation was promoted on Sb/V/Ce/Ti-S400 and Sb/V/Ce/Ti-S500 catalysts, followed by surface interaction with adsorbed NH3 to produce N2 and H2O at low temperatures (<220°C).