Effect of initial support particle size of MnOx/TiO2 catalysts in the selective catalytic reduction of NO with NH3

Abstract

A series of manganese-based catalysts supported by 5–10 nm, 10–25 nm, 40 nm and 60 nm anatase TiO₂ particles was synthesized via an impregnation method to investigate the effect of the initial support particle size on the selective catalytic reduction (SCR) of NO with NH₃. All catalysts were characterized by transmission electron microscopy (TEM), N₂ physisorption/desorption, X-ray diffraction (XRD), temperature programmed techniques, X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared transform spectroscopy (DRIFTS). TEM results indicated that the particle sizes of the MnO_x/TiO₂ catalysts were similar after the calcination process, although the initial TiO₂ support particle sizes were different. However, the initial TiO₂ support particle sizes were found to have a significant influence on the SCR catalytic performance. XPS and NH₃-TPD results of the MnO_x/TiO₂ catalysts illustrated that the surface Mn⁴⁺/Mn molar ratio and acid amount could be influenced by the initial TiO₂ support particle sizes. The order of surface Mn⁴⁺/Mn molar ratio and acid amount over the MnO_x/TiO₂ catalysts was as follows: MnO_x/TiO₂(10–25) > MnO_x/TiO₂(40) > MnO_x/TiO₂(60) > MnO_x/TiO₂(5–10), which agreed well with the order of SCR performance. In situ DRIFTS results revealed that the NH₃-SCR reactions over MnO_x/TiO₂ at low temperature occurred via a Langmuir–Hinshelwood mechanism. More importantly, it was found that the bridge and bidentate nitrates were the main active substances for the low-temperature SCR reaction, and bridge nitrate adsorbed on Mn⁴⁺ showed superior SCR activity among all the adsorbed NO_x species. The variation of the initial TiO₂ support particle size over MnO_x/TiO₂ could change the surface Mn⁴⁺/Mn molar ratio, which could influence the adsorption of NO_x species, thus bringing about the diversity of the SCR catalytic performance.