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 urgent need for the treatment of flue gas from non-electric industries has stimulated great interest in developing NH3-SCR (selective catalytic reduction of NO with NH3) catalysts that can work stably at low temperatures (

Conquering ammonium bisulfate poison over low-temperature NH3-SCR catalysts: A critical review

SO2 poisoning of NH3-SCR catalysts at low temperature (< 300 °C) is still an austere challenge. In this work, γFe2O3 was taken as a model catalyst and the effect of reaction temperature on the catalytic activity in the presence of SO2 were fully revealed. SO2 introduction has no negative effect on the activity at 300 °C, which gradually improves with the extension of time. While for 225−275 °C, the activity decreases firstly and then increases slowly. The formatted sulfate species inhibits the adsorption of NOx, cuts off L-H reaction pathway and leads to the initial decline. While the deposited ammonium bisulfate (ABS) can be consumed continuously by NO + O2, implying the formation and consumption of ABS have reached a dynamic equilibrium. Moreover, the formation of ferric sulfate species results in the enhancement of surface acidity, which leads to the promotion of the E-R reaction pathway and further facilitates the increase of activity.

Insight into the SO2 resistance mechanism on γ-Fe2O3 catalyst in NH3-SCR reaction: A collaborated experimental and DFT study

Extensive research has been conducted on controlling carbon dioxide (CO2), sulfur oxides (SOx), and nitrogen oxides (NOx) emissions in recent years with increasing environmental awareness. The primary contributor to climate change is the exponentially growing anthropogenic CO2, NOx, and SOx in the air from various energy and industrial sectors. Hence, several pre-combustion and post-combustion processes to mitigate these gases have been introduced and implemented. Despite the benefits of CO2, SOx, and NOx abatement technologies, researchers still have major challenges to deal with. While the removal efficiencies, operational costs, and resulting byproducts of conventional removal technologies are being improved, new technologies are also emerging. As it is evident from the current abundant reserves of coal that it will continue to be in a significant position among the principal sources of energy for more than a decade, the research for the reduction of CO2, NOx, and SOx is inevitable to save future generations from the harmful impacts of these pollutants. This review paper summarizes the progress of different emission control technologies at the plant level and introduces emerging technologies that are environment friendly.

Review on the progress in emission control technologies for the abatement of CO2, SOx and NOx from fuel combustion

A series of supported TiO2/CeO2 catalysts were synthesized to investigate the morphology and crystal-plane effects of CeO2 on these TiO2/CeO2 catalysts for selective catalytic reduction by ammonia (NH3-SCR). The experiment results show that TiO2/CeO2-NC, TiO2/CeO2-NP, and TiO2/CeO2-NR catalysts present nanocubes (mainly exposed {100} facet), nanopolyhedrons (mainly exposed {111} and {100} facets), and nanorods (mainly exposed {110} and {100} facets), respectively. Furthermore, TiO2/CeO2-NR catalyst exhibits the best dispersion of TiO2 species, the most excellent reduction behavior and surface acidity, and the largest amount of Ce3+ ions and chemisorbed oxygen, which is due to the largest specific surface area and pore volume of CeO2-NR support, as well as the strongest interaction between the surface dispersed TiO2 species and the {110} facet of CeO2-NR support. All of them result in the best denitration performance of TiO2/CeO2-NR catalyst during NH3-SCR reaction.

Morphology and Crystal-Plane Effects of CeO2 on TiO2/CeO2 Catalysts during NH3-SCR Reaction

The deactivation of NH3-selective catalytic reduction (SCR) catalysts due to NH4HSO4 deposition at low temperatures (

Understanding the deposition and reaction mechanism of ammonium bisulfate on a vanadia SCR catalyst: A combined DFT and experimental study

Formation of NH4HSO4 and its deposition on catalyst surface are the main obstacle for the application of NH3-SCR technology at low temperature. In this work, the deposited NH4HSO4 was found to be highly active towards NOx in the NO/NO2 contained atmosphere on the V2O5-WO3/TiO2 catalyst. The reaction profiles indicate that the NO/NO2/O2 mixture can rapidly react with NH4+ of NH4HSO4 above 100 °C, while NO/O2 mixture can only strongly react with NH4HSO4 above 300 °C. Regenerations of NH4HSO4-deactivated catalyst were performed in both NO/O2 and NO/NO2/O2 atmospheres and the performances were compared. Results from FTIR, XPS and EDS characterizations indicate the S-containing groups of deposited NH4HSO4 will be stabilized as metal sulfate after NH4+ is consumed by NOx. The BET and BJH analysis shows that the NO/NO2/O2 mixture can recover the pore structure of NH4HSO4-deposited catalyst more effectively than NO/O2 mixture does. The SCR activity tests reveal that the regeneration by NO/NO2/O2 mixture can remarkably restore the activity of NH4HSO4-deactivated catalyst.

Promotion of NH4HSO4 decomposition in NO/NO2 contained atmosphere at low temperature over V2O5-WO3/TiO2 catalyst for NO reduction

Oxidation of SO2 to SO3 is one of the major drawbacks of the commonly used vanadia-based selective catalytic reduction catalyst. Density functional theory calculations were applied to study the int...

Oxidation of Sulfur Dioxide over V2O5/TiO2 Catalyst with Low Vanadium Loading: A Theoretical Study

Supported metal sulfates on Ce–TiOx as catalysts for NH3–SCR of NO: High resistances to SO2 and potassium

The deposition of NH4HSO4 on the catalyst surface is the main obstacle to the stable removal of NOx in SO2-contained flue gas at a low temperature. In this work, the promotional effect of NO2 on the decomposition of NH4HSO4 is presented for the first time. The NH3-selective catalytic reduction (SCR) experiments show that the V2O5–WO3/TiO2 catalyst exhibits higher SCR activity and excellent resistance to SO2 and H2O when NO2 is present in the flue gas at 250 °C. Results from Brunauer–Emmett–Teller, scanning electron microscopy, and Fourier transform infrared spectroscopy characterizations indicate that NH4HSO4 will not excessively deposit on the surface of the catalyst because the ammonia ion of NH4HSO4 will be rapidly consumed by the NO2/NO mixture. Our study reveals that it is promising to stably and efficiently remove NOx from SO2-contained flue gas at a low temperature by properly adjusting the NO2/NO ratio.

Simultaneous Fast Decomposition of NH4HSO4 and Efficient NOx Removal by NO2 Addition: An Option for NOx Removal in H2O/SO2-Contained Flue Gas at a Low Temperature

Xiangmin Wang

Papers

Understanding the deposition and reaction mechanism of ammonium bisulfate on a vanadia SCR catalyst: A combined DFT and experimental study

Promotion of NH4HSO4 decomposition in NO/NO2 contained atmosphere at low temperature over V2O5-WO3/TiO2 catalyst for NO reduction

Oxidation of Sulfur Dioxide over V2O5/TiO2 Catalyst with Low Vanadium Loading: A Theoretical Study

Supported metal sulfates on Ce–TiOx as catalysts for NH3–SCR of NO: High resistances to SO2 and potassium

Simultaneous Fast Decomposition of NH4HSO4 and Efficient NOx Removal by NO2 Addition: An Option for NOx Removal in H2O/SO2-Contained Flue Gas at a Low Temperature