Low-temperature selective catalytic reduction of NO_x with NH₃ over metal oxide and zeolite catalysts-A review

The removal of NO_x by catalytic technology at low temperatures (100-300°C) is significant for flue gas of industry and exhaust gas of diesel engine; however, to develop the low-temperature catalyst (LTC) for selective catalytic reduction of NO_x with ammonia (NH₃ -SCR) is still a challenge especially at temperature below 200 ° C. This study reviews two types of LTC, the metal oxide catalyst and metal exchanged zeolite catalyst. The performances of Mn-based metal oxide with and without supports have been attempted to correlate with preparation method, precursor, and various supports. The role of manganese oxides with different phases as the most effective low temperature active component and the limitation of stability in the presence of H₂ O and SO₂ are discussed. Fe, Cu exchanged zeolites as potential real application catalysts in diesel engine have been investigated for NH₃-SCR of NO_x in the past decades, the activity, selectivity and thermal stability related to types of metal, and zeolite, and reaction conditions are reviewed. The research progress in active sites and reaction mechanisms of Mn-based catalyst and Fe-zeolite catalysts are described and compared. Finally, future research directions in the developing LTC for removal of NO_x are proposed.