Low temperature selective catalytic reduction of NOx with NH₃ over Mn-based catalyst: A review

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.