Catalytic performance and reaction mechanisms of NO removal with NH at low and medium temperatures on Mn-W-Sb modified siderite catalysts.

Iron-based catalysts have been explored for selective catalytic reduction (SCR) of NO due to environmentally benign characters and good SCR activity. Mn-W-Sb modified siderite catalysts were prepared by impregnation method based on siderite ore, and SCR performance of the catalysts was investigated. The catalysts were analyzed by X-ray diffraction, H-temperature-programmed reduction, Brunauer-Emmett-Teller, Thermogravimetry-derivative thermogravimetry and in-situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The modified siderite catalysts calcined at 450°C mainly consist of FeO, and added Mn, W and Sb species are amorphous. 3Mn-5W-1.5Sb-siderite catalyst has a wide temperature window of 180-360°C and good N selectivity at low temperatures. In-situ DRIFTS results show NH, coordinated NH, NH, NO species (bidentate), NO species (nitro, nitro-nitrito, monodentate), and adsorbed NO can be discovered on the surface of Mn-W-Sb modified siderite catalysts, and doping of Mn will enhance adsorbed NO formation by synergistic catalysis with Fe. In addition, the addition of Sb can inhibit sulfates formation on the surface of the catalyst in the presence of SO and HO. Time-dependent in-situ DRIFTS studies also indicate that both of Lewis and Brønsted acid sites play a role in SCR of NO by ammonia at low temperatures. The mechanism of NO removal on the 3Mn-5W-1.5Sb-siderite catalyst can be discovered as a combination of Eley-Rideal and Langmuir-Hinshelwood mechanisms with three reaction pathways. The mechanism of NO, oxidized by synergistic catalysis of Fe and Mn to form NO among three pathways, reveals the reason of high NO conversion of the catalyst at medium and low temperatures.