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

The removal of NOx 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 NOx 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 H2O and SO2 are discussed. Fe, Cu exchanged zeolites as potential real application catalysts in diesel engine have been investigated for NH3-SCR of NOx 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 NOx are proposed.

Manganese oxides are the most active components for NH3-SCR of NO at low temperature. The crystallinity, specific surface area, and oxidation state of MnOx are all correlated to the performance to some extent. The crystallinity is dependent on the preparation methods and calcination temperature. However, its low N2 selectivity required further improvement. Compared to the Fe/zeolite, Cu–zeolite has better activity at low temperature.Figure optionsDownload high-quality image (242 K)Download as PowerPoint slideHighlights
► Manganese oxides are the most active components for low temperature NH3-SCR.
► The activity is correlated with the valence of manganese and phase structure of MnOx.
► Cu/zeolite has better activity at low temperature than Fe/zeolite.
► N2 selectivity and resistance of H2O and SO2 are challenges for low-temperature SCR.