Decorating tungsten single atoms on MnO2 nanorods for enhanced selective catalytic reduction of NO with NH3

Manganese dioxide (MnO2) shows significant potential for selective catalytic reduction with NH3 (NH3-SCR). However, the selectivity and water vapor tolerance of MnO2 are generally unsatisfactory. This study tackles these issues by decorating tungsten single atoms (W SAs) onto MnO2 nanorods. The resulting W/MnO2 catalysts exhibit markedly improved performance, especially the 1.8 wt% W/MnO2 catalyst, which exhibits superior reactivity (over 90% conversion and over 80% N2 selectivity) across an extended operational temperature range of 75-350 °C, along with improved water tolerance. Structural characterizations based on X-ray diffraction (XRD) and aberration-corrected scanning transmission electron microscopy (AC-STEM) reveal that the initial W/MnO2 catalyst is characterized by W SAs that are partially embedded within the MnO2 lattice and partially dispersed on the surface. During the reaction, the catalyst undergoes structural transformations, characterized by the further incorporation of surface-dispersed W SAs into the MnO2 lattice. The incorporation of W SAs enhances both the surface acidity and oxygen vacancy density of the catalyst, thereby improving its catalytic performance. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies suggest that the NH3-SCR reaction proceeds via both the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. This work provides valuable insights into the structure-performance relationships of W/MnO2 catalysts in NH3-SCR, offering important implications for the design and fabrication of efficient SCR catalysts.

Strikingly Facile Cleavage of N-H/N-O Bonds Induced by Surface Frustrated Lewis Pair on CeO2(110) to Boost NO Reduction by NH3

Ceria with surface solid frustrated Lewis pairs (FLPs), formed by regulating oxygen vacancies, demonstrate remarkable ability in activating small molecules. In this work, we extended the application of FLPs on CeO2(110) to the selective catalytic reduction of NO by NH3 (NH3-SCR), finding a notable enhancement in performance compared to ordinary CeO2(110). Additionally, an innovative approach involving H2 treatment was discovered to increase the number of FLPs, thereby further boosting the NH3-SCR efficiency. Typically, NH3-SCR on regular CeO2 follows the Eley-Rideal (E-R) mechanism. However, density functional theory (DFT) calculations revealed a significant reduction in the energy barriers for the activation of N-O and N-H bonds under the Langmuir-Hinshelwood (L-H) mechanism with FLPs present. This transition shifted the reaction mechanism from the E-R pathway on regular R-CeO2 to the L-H pathway on FLP-rich FR-CeO2, as corroborated by the experimental findings. The practical application of FLPs was realized by loading MoO3 onto FLP-rich FR-CeO2, leveraging the synergistic effects of acidic sites and FLPs. This study provides profound insights into how FLPs facilitate N-H/N-O bond activation in small molecules, such as NH3 and NO, offering a new paradigm for catalyst design based on catalytic mechanism research.

Research progress of Mn-based low-temperature SCR denitrification catalysts

Selective catalytic reduction (SCR) is a efficiently nitrogen oxides removal technology from stationary source flue gases. Catalysts are key component in the technology, but currently face problems including poor low-temperature activity, narrow temperature windows, low selectivity, and susceptibility to water passivation and sulphur dioxide poisoning. To develop high-efficiency low-temperature denitrification activity catalyst, manganese-based catalysts have become a focal point of research globally for low-temperature SCR denitrification catalysts. This article investigates the denitrification efficiency of unsupported manganese-based catalysts, exploring the influence of oxidation valence, preparation method, crystallinity, crystal form, and morphology structure. It examines the catalytic performance of binary and multicomponent unsupported manganese-based catalysts, focusing on the use of transition metals and rare earth metals to modify manganese oxide. Furthermore, the synergistic effect of supported manganese-based catalysts is studied, considering metal oxides, molecular sieves, carbon materials, and other materials (composite carriers and inorganic non-metallic minerals) as supports. The reaction mechanism of low-temperature denitrification by manganese-based catalysts and the mechanism of sulphur dioxide/water poisoning are analysed in detail, and the development of practical and efficient manganese-based catalysts is considered.

Water-Driven Surface Lattice Oxygen Activation in MnO2 for Promoted Low-Temperature NH3-SCR

Water is ubiquitous in various heterogeneous catalytic reactions, where it can be easily adsorbed, chemically dissociated, and diffused on catalyst surfaces, inevitably influencing the catalytic process. However, the specific role of water in these reactions remains unclear. In this study, we innovatively propose that H2O-driven surface lattice oxygen activation in γ-MnO2 significantly enhances low-temperature NH3-SCR. The proton from water dissociation activates the surface lattice oxygen in γ-MnO2, giving rise to a doubling of catalytic activity (achieving 90% NO conversion at 100 °C) and remarkable stability. Comprehensive in situ characterizations and calculations reveal that spontaneous proton diffusion to the surface lattice oxygen reduces the orbital overlap between the protonated oxygen atom and its neighboring Mn atom. Consequently, the Mn-O bond is weakened and the surface lattice oxygen is effectively activated to provide excess oxygen vacancies available for converting O2 into O2-. Therefore, the redox property of Mn-H is improved, leading to enhanced NH3 oxidation-dehydrogenation and NO oxidation processes, which are crucial for low-temperature NH3-SCR. This work provides a deeper understanding and fresh perspectives on the water promotion mechanism in low-temperature NOx elimination.

A novel high activity MnXFe3-XO4 spinel catalyst for selective catalytic reduction of NO using NH3 prepared by a short process from natural minerals for low-temperature sintering flue gas: Effect of X value on catalytic mechanism

The process of smelting and purifying the catalyst precursor salt from minerals is extremely complex, which directly leads to high catalyst costs and serious secondary pollution. In order to achieve energy saving and emission reduction in the catalyst preparation process, in-situ synthesis of catalyst materials from natural minerals is a new research direction. In this study, we firstly explored the optimal X value of MnXFe3-XO4 for the NH3 selective catalytic reduction of NO (NH3-SCR) reaction, i.e., the Mn, Fe ratio, and then prepared a novel highly active mineral-based pure phase MnXFe3-XO4 spinel NH3-SCR catalyst by natural ferromanganese ore fines with iron-red fines (Fe2O3) allotment through in situ solid-phase synthesis and magnetic separation methods according to this ratio. The results show that the X value of 1.5 (Mn1.5Fe1.5O4) is the best for NH3-SCR reaction. Mn1.5Fe1.5O4 nano-particles (201 nm) has nearly 100 % NO conversion (with 5 % H2O(g)) at 125-300 °C. The combination of characterizations and density functional theory (DFT) calculation shows that the catalytic process of Eley-Rideal (E-R) dehydrogenation is enhanced at both the active site Mn site and Fe site, which is a key factor in the acceleration of the NH3-SCR reaction with increasing X value at the MnXFe3-XO4 surface.

The Resistance of SO2 and H2O of Mn-Based Catalysts for NO x Selective Catalytic Reduction with Ammonia: Recent Advances and Perspectives

The treatment of NO _x has become an urgent issue due to it being difficult to degrade in air and its tremendous adverse impact on public health. Among numerous NO _x emission control technologies, the technology of selective catalytic reduction (SCR) using ammonia (NH₃) as the reducing agent (NH₃-SCR) is regarded as the most effective and promising technique. However, the development and application of high-efficiency catalysts is severely limited due to the poisoning and deactivation effect by SO₂ and H₂O vapor in the low-temperature NH₃-SCR technology. In this review, recent advances in the catalytic effects from increasing the rate of the activity in low-temperature NH₃-SCR by manganese-based catalysts and the stability of resistance to H₂O and SO₂ during catalytic denitration are reviewed. In addition, the denitration reaction mechanism, metal modification, preparation methods, and structures of the catalyst are highlighted, and the challenges and potential solutions for the design of a catalytic system for degenerating NO _x over Mn-based catalysts with high resistance of SO₂ and H₂O are discussed in detail.

Purification Technologies for NOx Removal from Flue Gas: A Review

Nitrogen oxide (NOx) is a major gaseous pollutant in flue gases from power plants, industrial processes, and waste incineration that can have adverse impacts on the environment and human health. Many denitrification (de-NOx) technologies have been developed to reduce NOx emissions in the past several decades. This paper provides a review of the recent literature on NOx post-combustion purification methods with different reagents. From the perspective of changes in the valence of nitrogen (N), purification technologies against NOx in flue gas are classified into three approaches: oxidation, reduction, and adsorption/absorption. The removal processes, mechanisms, and influencing factors of each method are systematically reviewed. In addition, the main challenges and potential breakthroughs of each method are discussed in detail and possible directions for future research activities are proposed. This review provides a fundamental and systematic understanding of the mechanisms of denitrification from flue gas and can help researchers select high-performance and cost-effective methods.

The promoting/inhibiting effect of water vapor on the selective catalytic reduction of NOx

In the field of nitrogen oxides (NO_x) abatement, developing selective catalytic reduction (SCR) catalysts that can operate stably in the practical conditions remains a big challenge because of the complexity and uncertainty of actual flue gas emissions. As water vapor is unavoidable in the actual flue gas, it is indispensable to explore its effect on the performance of SCR catalysts. Many studies have proved that the effects of H₂O on de-NO_x activity of SCR catalysts were indeed observed during SCR reactions operated under wet conditions. Whether the effect is promotive or inhibitory depends on the reaction conditions, catalyst types and reducing agents used in SCR reaction. This review focuses on the effect of H₂O on SCR catalysts and SCR reaction, including promoting effect, inhibiting effect, as well as the effecting mechanism. Besides, various strategies for developing a water-resistant SCR catalyst are also included. We hope that this work can give a more comprehensive insight into the effects of H₂O on SCR catalysts and help with the rational design of water-resistant SCR catalysts for further practical application in NO_x abatement field.

Brønsted acid enhanced hexagonal cerium phosphate for the selective catalytic reduction of NO with NH3: In situ DRIFTS and DFT investigation

The possible effect of optimized acid sites on NH₃-SCR performance and the fundamental mechanism are barely illustrated. In this work, we report two model catalysts of hexagonal (h-CPO) and monoclinic (m-CPO) cerium phosphate with disparate acidity that show different NH₃-SCR activities under the same reaction conditions. Brønsted acid sites were found to be crucial for NH₃-SCR performance at both low and high temperature. The electron localization discrepancy of h-CPO was more pronounced as compared with m-CPO, leading to the enrichment of P-OH (Brønsted acid site) which could strongly absorb NH₃ and then generate NH₄⁺ to participate in fast SCR via Langmuir-Hinshelwood mechanism, resulting in good activity at low temperature. The zeolitic water stored in the open channels of h-CPO could be released as supplement for P-OH sites which prevent the depletion and non-selective oxidation of NH₃ thus maintaining its high activity at high temperature via the Eley-Rideal mechanism. Meanwhile, as DFT calculation revealed, cerium is the electron deficient center which can easily fix NO and NO₂ from the intake, generating active NO_2(ad) or nitrites and facilitating fast SCR by reacting with NH₄⁺ species. Hence, the superior protonation ability to form P-OH and low energy barrier to generate active nitrites of h-CPO led its T₈₀ NO_x conversion to a broaden temperature of 150-450 ^oC under high GHSV of 177,000 h^-1. Furthermore, experimental and DFT calculation also demonstrated that the enriched Brønsted acid sites over h-CPO have largely suppressed SO₂ adsorption_, thus significantly reducing the formation of metal sulfates and achieving great SO₂ resistance. The ammonium sulfate deposits can be storage of NH₃, supplying additional reductant to promote high temperature activity and selectivity.

Recent progress of low-temperature selective catalytic reduction of NOx with NH3 over manganese oxide-based catalysts

Selective catalytic reduction with NH3 (NH3−SCR) was the most efficient approach to mitigate the emission of nitrogen oxides (NOx). Although the conventional manganese oxide-based catalyst had gradually become a kind...

Understand the role of redox property and NO adsorption over MnFeOx for NH3-SCR

Widening the operation temperature window of selective catalytic reduction NO by NH3 (NH3-SCR) is a challenge to meet the increasingly stringent emission control regulations of NOx. Hence, MnFeOx with different...

Comparative study of HSOA-/SOA2- versus H3-BPO4B- functionalities anchored on TiO2-supported antimony oxide-vanadium oxide-cerium oxide composites for low-temperature NOX activation

TiO₂-supported antimony oxide-vanadium oxide-cerium oxide (SVC) imparts Lewis acidic (L)/Brönsted acidic (B) sites, labile (O_α)/mobile oxygens (O_M), and oxygen vacancies (O_V) for selective catalytic NO_X reduction (SCR). However, these species are harmonious occasionally, readily poisoned by H₂O/sulfur/phosphorus/carbon, thus limiting SCR performance of SVC. Herein, a synthetic means is reported for immobilizing HSO_A^-/SO_A^2- (A= 3-4) or H_3-BPO₄^B- (B= 1-3) on the L sites of SVC to form SVC-S and SVC-P. HSO_A^-/SO_A^2-/H_3-BPO₄^B- acted as additional B sites with distinct characteristics, altered the properties of O_α/O_M/O_V species, thereby affecting the SCR activities and performance of SVC-S and SVC-P. SVC-P activated Langmuir-Hinshelwood-typed SCR better than SVC-S, as demonstrated by a greater O_α-directed pre-factor and smaller binding energy between O_α and NO. Meanwhile, SVC-S provided a larger B-directed pre-factor, thereby outperforming SVC-P in activating Eley-Rideal-typed SCR that dictated the overall SCR activities. Compared with SVC-S, SVC-P contained fewer O_V species, yet, had higher O_M mobility, thus enhancing the overall redox cycling feature, while providing greater Brönsted acidity. Consequently, the resistance of SVC-P to H₂O or soot were greater than or similar to that of SVC-S. Conversely, SVC-S revealed greater tolerance to hydro-thermal aging and SO₂ than SVC-P. This study highlights the pros and cons of HSO_A^-/SO_A^2-/H_3-BPO₄^B- functionalities in tailoring the properties of metal oxides in use as SCR catalysts.

In situ decorated MOF-derived Mn–Fe oxides on Fe mesh as novel monolithic catalysts for NOx reduction

In situ decorated MOF-derived Mn–Fe oxides on Fe mesh were developed as novel monolithic catalysts for NO_x reduction.

Promotional effects of modified TiO2- and carbon-supported V2O5- and MnOx-based catalysts for the selective catalytic reduction of NOx: a review

This review presents the promotional effects of transition metal modification over TiO₂- and carbon-supported V₂O₅- and MnO_x-based SCR catalysts.

A γ-Fe2O3-modified nanoflower-MnO2/attapulgite catalyst for low temperature SCR of NOx with NH3

A novel mesoporous γ-Fe₂O₃-modified nanoflower-MnO₂/attapulgite catalyst has been fabricated through a facile hydrothermal method and used for low temperature SCR of NO_x with NH₃.

Investigation of the selective catalytic reduction of NO with NH3 over the WO3/Ce0.68Zr0.32O2 catalyst: the role of H2O in SO2 inhibition

The effect of H₂O and SO₂ on the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) over WO₃/Ce_0.68Zr_0.32O₂ at 250 °C was systematically investigated using various characterization techniques.

Performance of selective catalytic reduction of NO with NH3 over natural manganese ore catalysts at low temperature

Natural manganese ore catalysts for selective catalytic reduction (SCR) of NO with NH₃ at low temperature in the presence and absence of SO₂ and H₂O were systematically investigated. The physical and chemical properties of catalysts were characterized by X-ray diffraction, Brunauer-Emmett-Teller (BET) specific surface area, NH₃ temperature-programmed desorption (NH₃-TPD) and NO-TPD methods. The results showed that natural manganese ore from Qingyang of Anhui Province had a good low-temperature activity and N₂ selectivity, and it could be a novel catalyst in terms of stability, good efficiency, good reusability and lower cost. The NO conversion exceeded 85% between 150°C and 300°C when the initial NO concentration was 1000 ppm. The activity was suppressed by adding H₂O (10%) or SO₂ (100 or 200 ppm), respectively, and its activity could recover while the SO₂ supply is cut off. The simultaneous addition of H₂O and SO₂ led to the increase of about 100% in SCR activity than bare addition of SO₂. The formation of the amorphous MnO_x, high concentration of lattice oxygen and surface-adsorbed oxygen groups and a lot of reducible species as well as adsorption of the reactants brought about excellent SCR performance and exhibited good SO₂ and H₂O resistance.

Iron oxide-based catalysts for low-temperature selective catalytic reduction of NO x with NH3

Selective catalytic reduction (SCR) is now an established NO x removal technology for industrial flue gas as well as for diesel engine exhaust gas. However, it is still a big challenge to develop a novel low-temperature catalyst for NH3-SCR of NO x , especially at a temperature below 200°C. In the past few years, many studies have demonstrated the potential of iron (Fe)-based catalysts as low-temperature catalysts for NH3-SCR of NO x . Herein, we summarize the recent progress and performance of Fe-based catalysts for low-temperature NH3-SCR of NO x . Catalysts are divided into three categories: single Fe x O y , Fe-based multimetal oxide, and Fe-based multimetal oxide with support catalysts. The catalytic activity and selectivity of Fe-based catalysts are systematically analyzed and summarized in light of some key factors such as activation energy, specific surface area, morphology, crystallinity, preparation method and precursor, acid sites, calcination temperature, other metal dopant/substitute, and redox property of catalysts. In addition, H2O/SO2 tolerance and the NH3-SCR reaction mechanism over Fe-based catalysts, including Eley-Rideal and Langmuir-Hinshelwood mechanism, are emphasized. Lastly, the perspectives and future research directions of low-temperature NH3-SCR of NO x are also proposed.

Insight into the synergism between MnO2 and acid sites over Mn-SiO2@TiO2 nano-cups for low-temperature selective catalytic reduction of NO with NH3

The rational synthesis of low-temperature catalysts with high catalytic activity and stability is highly desirable for the selective catalytic reduction of NO with NH3. Here we synthesized a Mn-SiO2/TiO2 nano-cup catalyst via the coating of the mesoporous TiO2 layers on SiO2 spheres and subsequent inlay of MnO2 nanoparticles in the narrow annulus. This catalyst exhibited superior catalytic SCR activities and stability for low-temperature selective catalytic reduction of NO with NH3, with NO conversion of ∼100%, N2 selectivity above 90% at a temperature ∼140 °C. The characterization results, such as BET, XRD, H2-TPR, O2/NH3-TPD and XPS, indicated that this nano-cup structure catalyst possesses high concentration and dispersion of Mn4+ active species, strong chemisorbed O- or O2 2- species and highly stable MnO X active components over the annular structures of the TiO2 shell and SiO2 sphere, and thus enhanced the low-temperature SCR performance.

Rational selection of Fe2V4O13 over FeVO4 as a preferred active site on Sb-promoted TiO2 for catalytic NOX reduction with NH3

Fe₂V₄O₁₃ outperforms FeVO₄ as an active site for NH₃-SCR and resists SO₂/ABS/Na poisons with the inclusion of an Sb promoter.

Dechlorination of chlorobenzene on vanadium-based catalysts for low-temperature SCR

Chlorobenzene (CB) inhibits SCR activity of V-based catalysts at low temperature, though the adsorption amount is small.

Rationally Designed Porous MnOx-FeOx Nanoneedles for Low-Temperature Selective Catalytic Reduction of NOx by NH3

In this work, a novel porous nanoneedlelike MnO_x-FeO_x catalyst (MnO_x-FeO_x nanoneedles) was developed for the first time by rationally heat-treating metal-organic frameworks including MnFe precursor synthesized by hydrothermal method. A counterpart catalyst (MnO_x-FeO_x nanoparticles) without porous nanoneedle structure was also prepared by a similar procedure for comparison. The two catalysts were systematically characterized by scanning and transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, ammonia temperature-programmed desorption, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFT), and their catalytic activities were evaluated by selective catalytic reduction (SCR) of NO_x by NH₃. The results showed that the rationally designed MnO_x-FeO_x nanoneedles presented outstanding low-temperature NH₃-SCR activity (100% NO_x conversion in a wide temperature window from 120 to 240 °C), high selectivity for N₂ (nearly 100% N₂ selectivity from 60 to 240 °C), and excellent water resistance and stability in comparison with the counterpart MnO_x-FeO_x nanoparticles. The reasons can be attributed not only to the unique porous nanoneedle structure but also to the uniform distribution of MnO_x and FeO_x. More importantly, the desired Mn⁴⁺/Mnⁿ⁺ and O_α/(O_α + O_β) ratios, as well as rich redox sites and abundant strong acid sites on the surface of the porous MnO_x-FeO_x nanoneedles, also contribute to these excellent performances. In situ DRIFT suggested that the NH₃-SCR of NO over MnO_x-FeO_x nanoneedles follows both Eley-Rideal and Langmuir-Hinshelwood mechanisms.

Controllable synthesis of Ce-doped α-MnO2for low-temperature selective catalytic reduction of NO

On account of the highly active exposed Mn atom on α-MnO₂with a mesoporous channel, the catalyst exhibits high performance for NH₃-based selective catalytic reduction of NO.

A review of Mn-containing oxide catalysts for low temperature selective catalytic reduction of NOx with NH3: reaction mechanism and catalyst deactivation

The reactions over Mn-containing selective catalytic reduction (SCR) catalysts.

Construction of hybrid multi-shell hollow structured CeO2–MnOx materials for selective catalytic reduction of NO with NH3

Multi-shell CeO₂–MnO_x hollow spheres with superior catalytic performance were prepared via modification of the calcination rate.

Effect of CeO2 for a high-efficiency CeO2/WO3–TiO2 catalyst on N2O formation in NH3-SCR: a kinetic study

The effect of CeO₂ for a high-efficiency CeO₂/WO₃–TiO₂ catalyst on N₂O formation in NH₃-SCR reaction was investigated using a kinetic method.

Catalytic performance of Co–Fe mixed oxide for NH3-SCR reaction and the promotional role of cobalt

Co-modified iron oxide (Co-FeO_x) catalysts were prepared by a citric acid method for the low temperature NH₃-SCR of NO in the presence of O₂.

Promotion mechanism of CeO2addition on the low temperature SCR reaction over MnOx/TiO2: a new insight from the kinetic study

A new insight into CeO₂addition on the SCR reaction over MnO_x/TiO₂was drawn according to steady-state kinetic analysis.