Manganese oxide-based catalysts for low-temperature selective catalytic reduction of NO_x with NH₃: A review

Selective catalytic reduction (SCR) technology has been widely used for the removal of NO_x from flue gas. However, it is still a challenge to develop novel low-temperature catalysts for SCR of NO_x, especially at temperatures below 200 °C. This paper reviewed the recent progress on the Mn-based catalysts for low-temperature SCR de-NO_x with NH₃. Catalysts were divided into four categories, single MnO_x, Mn-based multi-metal oxide, Mn-based multi-metal oxide with support, and Mn-based monolith catalyst. In the section of single MnO_x, the effects of several factors, such as Mn oxidation state, crystallization state, specific surface area and morphology on catalytic activity were systematically reviewed. In the section of multi-metal oxide catalysts, the various roles played by the components of catalysts were intentionally summarized from four aspects, improving de-NO_x efficiency, enhancing N₂ selectivity, improving resistance to SO₂ and H₂O, extending operation temperature window, respectively. Moreover, the newly emerging morphology-dependent nanocatalysts were highlighted at the end of this section. In the introduction of supported metal oxide catalysts, the effects of supports were systematically analyzed according to their types, such as Al₂O₃, TiO₂, carbon materials, etc. Considering the actual operation, Mn-based monolith catalysts were also introduced with regard to monolith supports, such as ceramics, metal wire mesh, etc. Subsequently, NH₃-SCR mechanisms at low temperature, including E-R and L-H mechanisms, were discussed. At last, the perspective and the future direction of low-temperature SCR of NO_x were proposed.