Abstract
The removals of NOx by catalytic technology at low temperatures (100–300 °C) for industrial flue gas treatment have received increasing attention. However, the development of low temperature catalysts for selective catalytic reduction (SCR) of NOx with ammonia is still a challenge especially in the presence of SO2. The current status of using Mn-based catalysts for low temperature SCR of NOx with ammonia (NH3-SCR) is reviewed. Reaction mechanisms and effects of operating factors on low temperature NH3-SCR are addressed, and the SCR efficiencies of Mn-based metal oxides with and without SO2 poisoning have also been discussed with different supports and co-metals. The key factors for enhancing low temperature NH3-SCR efficiency and SO2 resistance with Mn-based catalysts are identified to be (1) high specific surface area; (2) high surface acidity; (3) oxidation states of manganese; (4) well dispersion of manganese oxide metals; (5) more surface adsorbed oxygen; (6) more absorbed NO3− on the catalyst surface; (7) easier decomposition of ammonium sulfates. Moreover, the regenerative methods such as water washing, acid and/or alkali washing and heat treatment to the poisoned catalysts could help to recover the low temperature SCR efficiency to its initial level.
Highlights
The combustion of fossil fuels in power plants, vehicles and factories are major sources of air pollution, and the exhaust air pollutants containing sulfur oxides, carbon monoxide, particulate matter and nitrogen oxides (NOx), etc
Reaction mechanisms and effects of operating factors were addressed, and the selective catalytic reduction (SCR) efficiencies of Mn-based metal oxides with and without SO2 poisoning have been discussed under different supports and co-doping metals
From the review of reaction mechanisms for low-temperature SCR, it is concluded that the key factors for enhancing SCR efficiency and SO2 resistance include (1) high specific surface area; (2) high surface acidity; (3) oxidation states of manganese; (4) well dispersion of manganese oxide metals; (5) more surface adsorbed oxygen. (6) more absorbed NO3− on the catalyst surface; (7) easier decomposition of ammonium sulfates
Summary
The combustion of fossil fuels in power plants, vehicles and factories are major sources of air pollution, and the exhaust air pollutants containing sulfur oxides, carbon monoxide, particulate matter and nitrogen oxides (NOx), etc. One of the well-known post-combustion control techniques is the selective catalytic reduction (SCR) process, where ammonia based reagent or hydrocarbons (HCs) are used as the reducing agents. The SCR process that employs HCs as the reducing agent is termed as HC-SCR, which is being investigated for the vehicle NOx emission controls. The low temperature SCR process for stationary source controls using Mn-based catalysts is reviewed. The V2O5–WO3/ TiO2-based catalyst has a narrow and high working temperature window of 300–400 °C [3,4,5,6,7,8,9]. Singoredjo et al [11] prepared manganese oxide based catalysts in 1992, which showed both high activity and good selectivity at T < 250 °C. Studies on low-temperature SCR catalyst increase rapidly.
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