Deactivation Effects of Potassium on a CeMoTiOx Catalyst for the Selective Catalytic Reduction of NOx with NH3

Strategy and Technical Progress of Recycling of Spent Vanadium-Titanium-Based Selective Catalytic Reduction Catalysts

Selective catalytic reduction denitration technology, abbreviated as SCR, is essential for the removal of nitrogen oxide from the flue gas of coal-fired power stations and has been widely used. Due to the strong demand for energy and the requirements for environmental protection, a large amount of SCR catalyst waste is produced. The spent SCR catalyst contains high-grade valuable metals, and proper disposal or treatment of the SCR catalyst can protect the environment and realize resource recycling. This review focuses on the two main routes of regeneration and recycling of spent vanadium-titanium SCR catalysts that are currently most widely commercially used and summarizes in detail the technologies of recycling, high-efficiency recycling, and recycling of valuable components of spent vanadium-titanium SCR catalysts. This review also discusses in depth the future development direction of recycling spent vanadium-titanium SCR catalysts. It provides a reference for promoting recycling, which is crucial for resource recovery and green and low-carbon development.

Progress of selective catalytic reduction denitrification catalysts at wide temperature in carbon neutralization

With the looming goal of carbon neutrality and increasingly stringent environmental protection policies, gas purification in coal-fired power plants is becoming more and more intense. To achieve the NOx emission standard when coal-fired power plants are operating at full load, wide-temperature denitrification catalysts that can operate for a long time in the range of 260–420°C are worthy of study. This review focuses on the research progress and deactivation mechanism of selective catalytic reduction (SCR) denitration catalysts applied to a wide temperature range. With the increasing application of SCR catalysts, it also means that a large amount of spent catalysts is generated every year due to deactivation. Therefore, it is necessary to recycle the wide temperature SCR denitration catalyst. The challenges faced by wide-temperature SCR denitration catalysts are summarized by comparing their regeneration processes. Finally, its future development is prospected.

A novel regeneration method for deactivated commercial NH3-SCR catalysts with promoted low-temperature activities

A novel route is developed for regeneration of deactivated commercial NH₃-SCR catalysts, which includes an initial in situ construction of anatase TiO₂ porous film, followed by loading of MnO_x, CeO_x, and Mn-Ce mixed oxides as active components. The regenerated catalysts present largely improved low-temperature denitrification performance due to the synergetic effect of MnO_x and CeO_x. The denitrification efficiency could reach a high value of 97% at 200 °C and 100% at 250 °C when the Ce-Mn mixed oxides are loaded at the optimized molar quantity ratio of 10:9 (Ce:Mn). Properties and reaction mechanisms of the regenerated catalysts are investigated with characterizations of X-ray photoelectron spectroscopy (XPS), NH₃ temperature-programmed desorption (NH₃-TPD), H₂ temperature-programmed reduction (H₂-TPR), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Our results demonstrate that the adsorption and oxidation of NO plays a crucial role for these three catalysts even though a difference exists on the reaction pathways. Graphical abstract.

VxMn(4-x)Mo3Ce3/Ti catalysts for selective catalytic reduction of NO by NH3

A series of vanadium based catalysts (VxMn(4-x)Mo3Ce3/Ti) with different vanadium (x wt.%) and manganese ((4-x) wt.%) contents have been prepared by the wet impregnation method and investigated for selective catalytic reduction (SCR) of NO_x by NH₃ in the presence of 8 vol.% H₂O and 500 ppmV SO₂. The physicochemical characteristics of the catalysts were thoroughly characterized. The SCR of NO_x by NH₃ (NH₃-SCR) activity, especially the low-temperature activity, significantly increased with increasing V₂O₅ content in the catalyst until the V₂O₅ content reached 1.5 wt.%, which corresponds well with the redox properties of the catalyst. All of the metal oxides were well dispersed and strongly interacted with each other on the catalyst surface. V mainly exists in the V⁵⁺ state in the catalysts. The strong synergistic effect between the vanadium and cerium species led to formation of more Ce³⁺ species, and that between the vanadium and manganese species contributed to formation of more manganese species with low valences. All of the catalysts exhibited strong acidity, while the redox properties determined the NH₃-SCR activity, especially the low-temperature activity. H₂O and SO₂ had severe inhibiting effects on the activity of V1.5Mn2.5Mo3Ce3/Ti. However, good H₂O and SO₂ resistance and high NO_x conversion by V1.5Mn2.5Mo3Ce3/Ti could be achieved in the presence of SO₂ and almost no decline was observed in a long-term test at 275°C for 168 hr in the presence of SO₂ and H₂O, which can be attributed to the sulfate species formed on the catalyst surface.

Dual Promotional Effects of TiO2-Decorated Acid-Treated MnO x Octahedral Molecular Sieve Catalysts for Alkali-Resistant Reduction of NO x

Alkali metals generated during waste incineration in power stations are not conducive to the control of nitrogen oxide (NO _x) emission. Hence, improved selective catalytic reduction of NO _x with ammonia (NH₃-SCR) in the presence of alkali metals is a major issue for practical NO _x removal. In this work, we developed a novel TiO₂-decorated acid-treated MnO _x octahedral molecular sieve (OMS-5(H)@TiO₂) catalyst with improved alkali-resistant NO _x reduction at low temperature, and the dual promotional effects of OMS-5(H)@TiO₂ catalysts were clarified. It was found that the special structure of the acid-treated MnO _x octahedral molecular sieve (OMS-5(H)) was responsible for the trapping of alkali metals and high deNO _x activity at low temperature. Subsequently, the decoration by TiO₂ further improved the redox properties by accelerating the high ratio of Mn⁴⁺ and O_α on the surface of the highly active (OMS-5(H)@TiO₂) catalyst. Moreover, a thorough mechanism study via in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs) demonstrated that the acid treatment led to remarkable increment of acid sites, which enabled the catalyst to resist alkali metals in the form of ion exchange. Meanwhile, the decoration of TiO₂ further increased the strength of the Lewis acid sites, assisting more active intermediate species to effectively take part in the deNO _x reaction. Besides, a "fast SCR" process was observed to certify that the decoration of TiO₂ promoted the improvement of low-temperature activity in the presence of alkali metals. The dual effects combining OMS-5(H) with TiO₂ decoration in terms of alkali metal resistance and high catalytic activity at low temperature proved that the high-performance deNO _x catalyst was successfully developed in this work. The work paves a way for the development of superior low-temperature SCR catalysts with improved NO _x reduction efficiency in the presence of alkali metals.

Enhanced low-temperature NH3-SCR performance of Ce/TiO2 modified by Ho catalyst

Holmium was used as a dopant to boost the low-temperature NH3-selective catalytic reduction (SCR) performance of Ce/TiO2 catalyst. It was ascertained that certain amount of Ho-doping species could exceedingly improve the low-temperature SCR activity under 60 000 h-1 of Ce/TiO2, accompanied with the improvement of tolerance to H2O and SO2 at 200°C. Characterization results manifested that Ho modification could not only result in inhibiting the growth of TiO2 crystals and the enlargement of specific surface area but also lead to the enhanced redox ability and the increased amount of surface-adsorbed substances, all of which could promote the low-temperature NH3-SCR performance of Ce/TiO2.

Development of cerium-based catalysts for selective catalytic reduction of nitrogen oxides: a review

Nitrogen oxides (NO_X) are major pollutants of the atmosphere, and selective catalytic reduction of nitrogen oxides using ammonia as a reductant (NH₃-SCR) is an effective method to remove nitrogen oxides.