A Highly Effective Catalyst of Sm-MnOx for the NH3-SCR of NOx at Low Temperature: Promotional Role of Sm and Its Catalytic Performance

Tourmaline-Modified FeMnTiO x Catalysts for Improved Low-Temperature NH3-SCR Performance

Low temperature NH₃ selective catalytic reduction (NH₃-SCR) technology is an efficient and economical strategy for cutting NO _x emissions from power-generating equipment. In this study, a novel and highly efficient NH₃-SCR catalyst, tourmaline-modified FeMnTiO _x is presented, which was synthesized by a simple one-step sol-gel method. We found that the amount of tourmaline has an important impact on the catalytic performance of the modified FeMnTiO _x-based catalysts, and the NO _x conversion exceeded 80% from 160 to 380 °C with the addition of 5 wt % tourmaline. Compared with the pure FeMnTiO _x, the catalytic efficiency at a temperature below 100 °C was increased by nearly 18.9%, and the operation temperature window was broadened significantly. The enhanced catalytic performance of the FeMnTiO _x catalyst was mainly attributed to the small spherical nanoparticles structure around the tourmaline powders, resulting in the increased content of Mn³⁺, Mn⁴⁺, and chemical oxygen on the catalytic surface. These as-developed tourmaline-modified FeMnTiO _x materials have been demonstrated to be promising as a new type highly efficient low temperature NH₃-SCR catalyst.

Fe2O3-CeO2@Al2O3 Nanoarrays on Al-Mesh as SO2-Tolerant Monolith Catalysts for NO x Reduction by NH3

Currently, selective catalytic reduction of NO _x with NH₃ in the presence of SO₂ is still challenging at low temperatures (<300 °C). In this study, enhanced NO _x reduction was achieved over a SO₂-tolerant Fe-based monolith catalyst, which was originally developed through in situ construction of Al₂O₃ nanoarrays (na-Al₂O₃) on the monolithic Al-mesh by a steam oxidation method followed by anchoring Fe₂O₃ and CeO₂ onto the na-Al₂O₃@Al-mesh composite by an impregnation method. The optimum catalyst delivered more than 90% NO conversion and N₂ selectivity above 98% within 250-430 °C as well as excellent SO₂ tolerance at 270 °C. The strong interaction between Fe₂O₃ and CeO₂ enabled favorable electron transfers from Fe₂O₃ to CeO₂ while generating more oxygen vacancies and active oxygen species, consequently accelerating the redox cycle. The improved reactivity of NH₄⁺ with nitrates following the Langmuir-Hinshelwood mechanism and active NH₂ species that directly reacted with gaseous NO following the Eley-Rideal mechanism enhanced the NO _x reduction efficiency at low temperatures. The preferential sulfation of CeO₂ alleviated the sulfation of Fe₂O₃ while maintaining the high reactivities of NH₄⁺ and NH₂ species. Especially, the SCR reaction following the Eley-Rideal mechanism largely improved the SO₂ tolerance because NO does not need to compete with sulfates to adsorb on the catalyst surface as nitrates or nitrites. This work paves a way for the development of high-performance SO₂-tolerant SCR monolith catalysts.

Ru/CeO2 Catalyst with Optimized CeO2 Support Morphology and Surface Facets for Propane Combustion

Tailoring the interfaces between active metal centers and supporting materials is an efficient strategy to obtain a superior catalyst for a certain reaction. Herein, an active interface between Ru and CeO₂ was identified and constructed by adjusting the morphology of CeO₂ support, such as rods (R), cubes (C), and octahedra (O), to optimize both the activity and the stability of Ru/CeO₂ catalyst for propane combustion. We found that the morphology of CeO₂ support does not significantly affect the chemical states of Ru species but controls the interaction between the Ru and CeO₂, leading to the tuning of oxygen vacancy in the CeO₂ surface around the Ru-CeO₂ interface. The Ru/CeO₂ catalyst possesses more oxygen vacancy when CeO₂-R with predominantly exposed CeO₂{110} surface facets is used, providing a higher ability to adsorb and activate oxygen and propane. As a result, the Ru/CeO₂-R catalyst exhibits higher catalytic activity and stability for propane combustion compared with the Ru/CeO₂-C and Ru/CeO₂-O catalysts. This work highlights a new strategy for the design of efficient metal/CeO₂ catalysts by engineering morphology and associated surface facet of CeO₂ support for the elimination of light alkane pollutants and other volatile organic compounds.

A Review of Low Temperature NH3-SCR for Removal of NOx

The importance of the low-temperature selective catalytic reduction (LT-SCR) of NOx by NH3 is increasing due to the recent severe pollution regulations being imposed around the world. Supported and mixed transition metal oxides have been widely investigated for LT-SCR technology. However, these catalytic materials have some drawbacks, especially in terms of catalyst poisoning by H2O or/and SO2. Hence, the development of catalysts for the LT-SCR process is still under active investigation throughout seeking better performance. Extensive research efforts have been made to develop new advanced materials for this technology. This article critically reviews the recent research progress on supported transition and mixed transition metal oxide catalysts for the LT-SCR reaction. The review covered the description of the influence of operating conditions and promoters on the LT-SCR performance. The reaction mechanism, reaction intermediates, and active sites are also discussed in detail using isotopic labelling and in situ FT-IR studies.

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.

Metal organic frameworks-assisted fabrication of CuO/Cu2O for enhanced selective catalytic reduction of NOx by NH3 at low temperatures

Porous CuO/Cu₂O heterostructure was successfully synthesized through a metal organic frameworks (MOFs)-assisted template method. Tunable production of pure phase CuO and Cu₂O could be achieved by regulating the coordination environment of copper. The copper oxides inherited the polyhedral morphology from the Cu MOFs and possessed higher surface area and larger pore volume. Compared with pure CuO and Cu₂O, heterostructured CuO/Cu₂O displayed remarkably enhanced NH₃-SCR de-NO_x activity and N₂ selectivity in a low temperature range of 170-220 °C. Systematical in situ DRIFT characterization revealed that the NH₃-SCR of NO_x over CuO/Cu₂O heterostructure followed Eley-Rideal (E-R) mechanism, which was greatly improved by the abundant Lewis acid sites, improved O₂ adsorption and the synergistic effect between Cu⁺ and Cu²⁺. In addition, CuO/Cu₂O heterostructure exhibited excellent H₂O, SO₂, alkali metals, and hydrocarbon durability, indicating its potential use in industrial NH₃-SCR of NO_x.

Effect of ceria loading on Zr-pillared clay catalysts for selective catalytic reduction of NO with NH3

The effect of ceria loading on Zr-pillared clay catalysts was investigated and its reaction mechanism was explored.

Effect of the TiO2 crystal structure on the activity of TiO2-supported platinum catalysts for ammonia synthesis via the NO–CO–H2O reaction

The catalytic activity of Pt/TiO₂ for ammonia synthesis via the NO–CO–H₂O reaction is affected by the TiO₂ crystal structure below 250 °C.

Doping effect of Sm on the TiO2/CeSmOx catalyst in the NH3-SCR reaction: structure–activity relationship, reaction mechanism and SO2 tolerance

A good balance between the redox properties and surface acidity induces the high activity of the Sm doped TiO₂/CeO₂ catalyst.

Sulfate promotion of selective catalytic reduction of nitric oxide by ammonia on ceria

Selective catalytic reduction of nitric oxide by ammonia (NH₃-SCR) is a promising technology for NO_x emission control.

The poisoning effect of PbO and PbCl2 on CeO2-TiO2 catalyst for selective catalytic reduction of NO with NH3

The poisoning effect of PbO and PbCl₂ on CeO₂-TiO₂ catalyst for selective catalytic reduction of NO with NH₃ was investigated and compared. Both Pb species could deactivate the CeO₂-TiO₂ catalyst and PbO had a stronger poisoning effect than PbCl₂. From the characterization results of BET, XRD, XPS, NH₃-TPD and H₂-TPR, it was concluded that the more serious deactivation by PbO could be ascribed to smaller BET surface area, fewer surface Ce³⁺ and chemisorbed oxygen, stronger interaction between PbO and CeO₂-TiO₂ catalyst, lower redox properties and surface acidity. The in situ DRIFT study results revealed that the NH₃-SCR reaction over CeO₂-TiO₂ catalyst was governed by both E-R and L-H mechanisms, which wasn't changed over the Pb-poisoned samples. The greater loss of Brønsted acid sites attributed to fewer surface Ce³⁺ and more serious inhibition of NO oxidation to NO₂ due to fewer surface chemisorbed oxygen were two key factors responsible for more serious deactivation by PbO. Furthermore, the presence of Pb species inhibited the NH₃ adsorption on the Lewis acid sites, aggravating the deactivation of CeO₂-TiO₂ catalyst.

Total Oxidation of Propane over a Ru/CeO2 Catalyst at Low Temperature

Ruthenium (Ru) nanoparticles (∼3 nm) with mass loading ranging from 1.5 to 3.2 wt % are supported on a reducible substrate, cerium dioxide (CeO₂, the resultant sample is called Ru/CeO₂), for application in the catalytic combustion of propane. Because of the unique electronic configuration of CeO₂, a strong metal-support interaction is generated between the Ru nanoparticles and CeO₂ to stabilize Ru nanoparticles for oxidation reactions well. In addition, the CeO₂ host with high oxygen storage capacity can provide an abundance of active oxygen for redox reactions and thus greatly increases the rates of oxidation reactions or even modifies the redox steps. As a result of such advantages, a remarkably high performance in the total oxidation of propane at low temperature is achieved on Ru/CeO₂. This work exemplifies a promising strategy for developing robust supported catalysts for short-chain volatile organic compound removal.

Active Site of O2 and Its Improvement Mechanism over Ce-Ti Catalyst for NH3-SCR Reaction

The current study on Ce-Ti catalyst was mainly focused on the function of NH3 and NO adsorption sites. In our study, by comparing Ce-Ti (doped catalyst) to Ce/Ti (supported catalyst), the active site of O2 and its improvement mechanism over Ce-Ti catalyst for NH3-Selective catalytic reduction (SCR) reactions were investigated. For Ce-Ti catalyst, a cerium atom was confirmed entering a TiO2 crystal lattice by X-ray diffraction (XRD) and Raman; the structure of Ce-□-Ti (□ represents oxygen vacancy) in Ce-Ti catalyst was confirmed by X-ray photoelectron spectroscopy (XPS) and Photoluminescence spectra (PL spectra). The nature of this structure was characterized by electron paramagnetic resonance (EPR), Ammonia temperature-programmed desorption (NH3-TPD), hydrogen temperature-programmed reduction (H2-TPR), Nitric oxide temperature-programmed desorption (NO-TPD) and In situ DRIFT. The results indicated that oxygen vacancies had a promotive effect on the adsorption and activation of oxygen, and oxygen was converted to superoxide ions in large quantities. Also, because of adsorption and activation of NO and NH3, electrons were transferred to adsorbed oxygen via oxygen vacancies, which also promoted the formation of superoxide ions. We expected that our study could promote understanding of the active site of O2 and its improvement mechanism for doped catalyst.

Effects of Nd-modification on the activity and SO2 resistance of MnOx/TiO2 catalysts for low-temperature NH3-SCR

MnO_x/TiO₂ (MnTi) and Nd-modified MnO_x/TiO₂ (MnNdTi) were prepared via a coprecipitation method.

Mesoporous Mn–Ti amorphous oxides: a robust low-temperature NH3-SCR catalyst

Mn–Ti amorphous oxides prepared by the combinedin situdeposition and freeze-drying strategy exhibited excellent activities and stability in low-temperature NH₃-SCR.

Theoretical and Experimental Evidence for the Carbon-Oxygen Group Enhancement of NO Reduction

The relation between a catalytic center and the surrounding carbon-oxygen groups influences the catalytic activity in various reactions. However, the impact of this relation on catalysis is usually discussed separately. For the first time, we proved that carbon-oxygen groups increased the reducibility of Fe-C bonds toward NO reduction. Experimentally, we compared the reductive activities of materials with either one or both factors, i.e., carbon-oxygen groups and Fe-C bonds. As a result, graphene oxide-supported Fe (with both factors) showed the best activity, duration of activity, and selectivity. This material reduced 100% of NO to N₂ at 300 °C. Moreover, theoretical calculations revealed that the adsorption energy of graphene for NO increased from -13.51 (physical adsorption) to -327.88 kJ/mol (chemical adsorption) after modification with Fe-C. When the graphene-supported Fe was further modified with carboxylic acid groups, the ability to transfer charge increased dramatically from 0.109 to 0.180 |e^-|. Therefore, the carbon-oxygen groups increased the reducibility of Fe-C. The main results will contribute to the understanding of NO reduction and the design of effective catalysts.