Effect of initial support particle size of MnO _{: X} /TiO ₂ catalysts in the selective catalytic reduction of NO with NH ₃

A series of manganese-based catalysts supported by 5-10 nm, 10-25 nm, 40 nm and 60 nm anatase TiO ₂ 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 NH ₃ . All catalysts were characterized by transmission electron microscopy (TEM), N ₂ 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 /TiO ₂ catalysts were similar after the calcination process, although the initial TiO ₂ support particle sizes were different. However, the initial TiO ₂ support particle sizes were found to have a significant influence on the SCR catalytic performance. XPS and NH ₃ -TPD results of the MnO _x /TiO ₂ catalysts illustrated that the surface Mn ⁴⁺ /Mn molar ratio and acid amount could be influenced by the initial TiO ₂ support particle sizes. The order of surface Mn ⁴⁺ /Mn molar ratio and acid amount over the MnO _x /TiO ₂ catalysts was as follows: MnO _x /TiO ₂ (10-25) > MnO _x /TiO ₂ (40) > MnO _x /TiO ₂ (60) > MnO _x /TiO ₂ (5-10), which agreed well with the order of SCR performance. In situ DRIFTS results revealed that the NH ₃ -SCR reactions over MnO _x /TiO ₂ 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 Mn ⁴⁺ showed superior SCR activity among all the adsorbed NO _x species. The variation of the initial TiO ₂ support particle size over MnO _x /TiO ₂ could change the surface Mn ⁴⁺ /Mn molar ratio, which could influence the adsorption of NO _x species, thus bringing about the diversity of the SCR catalytic performance.