Selective Catalytic Reduction of NO_<i>x</i> with NH₃ by Using Novel Catalysts: State of the Art and Future Prospects

Selective catalytic reduction with NH₃ (NH₃-SCR) is the most efficient technology to reduce the emission of nitrogen oxides (NO_<i>x</i>) from coal-fired industries, diesel engines, etc. Although V₂O₅-WO₃(MoO₃)/TiO₂ and CHA structured zeolite catalysts have been utilized in commercial applications, the increasing requirements for broad working temperature window, strong SO₂/alkali/heavy metal-resistance, and high hydrothermal stability have stimulated the development of new-type NH₃-SCR catalysts. This review summarizes the latest SCR reaction mechanisms and emerging poison-resistant mechanisms in the beginning and subsequently gives a comprehensive overview of newly developed SCR catalysts, including metal oxide catalysts ranging from VO_<i>x</i>, MnO_<i>x</i>, CeO₂, and Fe₂O₃ to CuO based catalysts; acidic compound catalysts containing vanadate, phosphate and sulfate catalysts; ion exchanged zeolite catalysts such as Fe, Cu, Mn, etc. exchanged zeolite catalysts; monolith catalysts including extruded, washcoated, and metal-mesh/foam-based monolith catalysts. The challenges and opportunities for each type of catalysts are proposed while the effective strategies are summarized for enhancing the acidity/redox circle and poison-resistance through modification, creating novel nanostructures, exposing specific crystalline planes, constructing protective/sacrificial sites, etc. Some suggestions are given about future research directions that efforts should be made in. Hopefully, this review can bridge the gap between newly developed catalysts and practical requirements to realize their commercial applications in the near future.