The structure and catalytic activity of anatase and rutile titania supported manganese oxide catalysts for selective catalytic reduction of NO by NH3

Abstract

The structure and catalytic properties of anatase and rutile supported manganese oxide catalysts prepared by impregnation method have been studied by using X-ray diffraction (XRD), laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (H₂-TPR) and BET surface area measurements combined with activity testing of selective catalytic reduction (SCR) of NO by NH₃. It has been shown that the manganese oxide loadings on the two TiO₂ supports exert great influences on the SCR activity. For the rutile supported manganese oxide catalysts, increasing manganese oxide loading leads to the increase of reducibility of dispersed manganese oxide species and the rate constant k, which reaches a maximum around 9.6 × 10⁻⁶ mol g_Mn⁻¹ s⁻¹ at 0.5 mmol Mn per 100 m²TiO₂. When the manganese oxide loading is beyond this value, the existence of amorphous MnO_x multiple layers will certainly reduce the ratio of manganese oxide species exposed on the surface and the reducibility of dispersed manganese oxide species, resulting in the rapid decrease of rate constant k. The LRS and XPS results have revealed that for the anatase supported manganese oxide catalysts manganese oxide species exist in Mn⁺⁴ as a major species with Mn⁺³ species and partially undecomposed Mn-nitrate as the minor species. Under the SCR reaction conditions, Mn⁺³ species on anatase are oxidized to Mn⁺⁴ species, inserting in the surface of anatase and promoting the anatase-to-rutile transformation in the surface layers of the anatase support. Since these Mn⁴⁺ cations are actually dispersed on the support with a rutile shell–anatase core structure and its concentration is very near to that of MnO_x/TiO₂ (R) catalyst, the relation between the rate constant k and the MnO_x loading on the anatase support is similar to that on the rutile support, and that the rate constant k values for anatase and rutile supported manganese oxide catalysts are very close at the same MnO_x loading.