Understanding the dual-acting of iron and sulfur dioxide over Mn-Fe/AC catalysts for low-temperature SCR of NO

Poor N₂ selectivity and low SO₂-tolerant are dominated weaknesses for low-temperature selective catalytic reduction of NO with NH₃ (NH₃-SCR) catalysts, which must be overcome for practical applications. Herein, a Mn-Fe mixed catalyst supported on active carbon (Mn-Fe/AC) for low-temperature NH₃-SCR of NO was prepared, and the influence of SO₂ on this process was also studied. Both iron and sulfur dioxide exhibited a dual-acting over Mn-Fe/AC catalyst for NH₃-SCR of NO. Sulfur dioxide had two effects when SO₂ was introduced in the feed gas under SCR conditions, including inhibiting the low temperature activity of the catalysts and improving N₂ selectivity which exceeded 90% during the whole testing temperature. The introduction of SO₂ could promote the formation of NH₂NO which was the main intermediate and then decomposed into N₂. Fe species in Mn-Fe/AC catalyst could improve N₂ selectivity. In the presence of SO_2, it showed considerable SO₂ resistance compared to Mn/AC catalyst. On the one hand, iron species improved the selectivity through forming new active sites promoting the formation of NH₂NO and suppressing the free NO₃^- generation. On the other hand, iron existing in Mn-Fe/AC catalyst could decrease the amount of sulfur species adsorbent and SO₂ oxidation, thereby exhibiting better SO₂ resistance.