Selective catalytic reduction (SCR) is now an established NOx removal technology for industrial flue gas as well as for diesel engine exhaust gas. However, it is still a big challenge to develop a novel low-temperature catalyst for NH3-SCR of NOx, especially at a temperature below 200°C. In the past few years, many studies have demonstrated the potential of iron (Fe)-based catalysts as low-temperature catalysts for NH3-SCR of NOx. Herein, we summarize the recent progress and performance of Fe-based catalysts for low-temperature NH3-SCR of NOx. Catalysts are divided into three categories: single FexOy, Fe-based multimetal oxide, and Fe-based multimetal oxide with support catalysts. The catalytic activity and selectivity of Fe-based catalysts are systematically analyzed and summarized in light of some key factors such as activation energy, specific surface area, morphology, crystallinity, preparation method and precursor, acid sites, calcination temperature, other metal dopant/substitute, and redox property of catalysts. In addition, H2O/SO2 tolerance and the NH3-SCR reaction mechanism over Fe-based catalysts, including Eley-Rideal and Langmuir-Hinshelwood mechanism, are emphasized. Lastly, the perspectives and future research directions of low-temperature NH3-SCR of NOx are also proposed.

The selective catalytic reduction of NO with NH3 at low temperatures has been investigated with natural iron ore catalysts. Four iron ore raw materials from different locations were taken and processed to be used as catalysts. The methods of X-ray diffraction (XRD), X-ray fluorescence (XRF), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), ammonia temperature-programmed desorption (NH3-TPD), scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) were used to characterize the materials. The results showed that the sample A (comprised mainly of α-Fe2O3 and γ-Fe2O3), calcined at 250 °C, achieved excellent selective catalytic reduction (SCR) activity (above 80% at 170–350 °C) and N2 selectivity (above 90% up to 250 °C) at low temperatures. Suitable calcination temperature, large surface area, high concentration of surface-adsorbed oxygen, good reducibility, lots of acid sites and adsorption of the reactants were responsible for the excellent SCR performance of the iron ore. However, the addition of H2O and SO2 in the feed gas showed some adverse effects on the SCR activity. The FT-IR analysis indicated the formation of sulfate salts on the surface of the catalyst during the SCR reaction in the presence of SO2, which could cause pore plugging and result in the suppression of the catalytic activity.

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The time evolution of the methane reburn process at high temperature was investigated by the simulation method. The simulated results show that the time evolutions of each component are different with the stoichiometric ratio (SR) of the reburn zone and the reaction time. In the simulation conditions, CH4, C2H2, and H appear as SR is from 0.66 to 0.96; CH3 appears as SR is from 0.66 to 0.76; HCN, NH3, and HNCO appear as SR is from 0.66 to 0.86; OH appears as SR is from 0.82 to 0.96. In the case with SR of 0.76, C2H2 and CH3 only appear in the initial 0.2 s, NH3 and HNCO appear in the whole 2.0 s; In the case with SR of 0.82, C2H2 also appears in the initial 0.2 s, NH3 and HNCO only appear in initial 0.5 s, CH3 does not appear in the whole reaction time of 2.0 s. The reaction between CH3 and NO mainly occurs in the strong reducing atmosphere, and contributes significantly to remove NO to HCN, NH3, and HNCO. The interactions of NO with H and OH radicals mainly occur in the weak reducing atmosphere, and contribute to remove a small part of NO to N2. Through comparing the time evolutions of total fixed nitrogen (TFN) with different SR, it is suggested that the optimum SR for reducing TFN in the methane reburn process is about 0.82. © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.