Selective catalytic reduction with NH3 (NH3-SCR) is the most efficient technology to reduce the emission of nitrogen oxides (NOx) 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 VOx, MnOx, 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.

Selective Catalytic Reduction of NOx with NH3 by Using Novel Catalysts: State of the Art and Future Prospects.

The Sm doped CeO2-TiO2 mixed oxide catalyst, which exhibited excellent activity and tolerance to H2O and SO2 in the NH3-SCR reaction, was synthesized. The reasons for the high activity and SO2 resistance of the catalyst were investigated by a series of characterization. The H2-TPR and O2-TPD results suggested that the reducibility and oxygen storage capacity (OSC) of CeTi catalyst were promoted by the addition of Sm species, which was beneficial for improving the activity of catalyst. The in situ DRIFTS results revealed that the adsorptive ability of NOx species and activation ability of NH3 were enhanced by Sm doping, which was also propitious to enhance the activity. XPS combined with DFT calculated results confirmed that the transfer of electron by Sm2++Ce4+⇌Sm3++Ce3+ circles occurred in the SmCeTi catalyst. The redox circles may be the reason of the good SO2 tolerance of the SmCeTi catalyst, for which suppressed the electron transferring from adsorbed SO2 to Ce4+. Through in situ DRIFTS and TG-DSC results, it can be concluded that the sulphation of catalyst was lowered by samarium doping into CeTi catalyst. Consequently, the SmCeTi catalyst exhibited significant SO2 tolerance ability.

Improved activity and significant SO2 tolerance of samarium modified CeO2-TiO2 catalyst for NO selective catalytic reduction with NH3

Cerium and titanium oxides are considered as promising alternative catalysts for selective catalytic reduction with NH3 (NH3-SCR) to remove NOx. However, the poor SO2 or H2O tolerance and stability limit their practical applications. Herein, CeTiOx with nanotube structure (CeTiOx-T) was prepared by hydrothermal method and used for NH3-SCR reaction. CeTiOx-T shows the excellent catalytic activity, SO2 and H2O tolerance and stability, in which more than 98 % NO conversion can be achieved in the range of 180−390 °C with 100 % N2 selectivity. The characterizations verify that CeTiOx-T exhibits amorphous structure due to the strong interaction between Ce and Ti to form short-range ordered Ce-O-Ti species. As results, CeTiOx-T displays the larger BET surface area, more surface Bronsted acid amounts and chemisorbed oxygen, leading to its higher NH3-SCR performance. In situ DRIFTS results suggest the SCR reaction mainly follow L-H and E-R mechanisms at low and high temperature for over CeTiOx-T, respectively.

Two steps synthesis of CeTiOx oxides nanotube catalyst: Enhanced activity, resistance of SO2 and H2O for low temperature NH3-SCR of NOx

Low temperature high activity of M (M = Ce, Fe, Co, Ni) doped M-Mn/TiO2 catalysts for NH3-SCR and in situ DRIFTS for investigating the reaction mechanism

Xu Feng

Papers

Ni-Ce-Ti as a superior catalyst for the selective catalytic reduction of NOx with NH3