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

Cu-VWT Catalysts for Synergistic Elimination of NOx and Volatile Organic Compounds from Coal-Fired Flue Gas

A dual-function catalyst, designated as Cu5-VWT, has been constructed for the synergistic removal of NO_x and volatile organic compounds under complex coal-fired flue gas conditions. The removal of toluene, propylene, dichloromethane, and naphthalene all exceeded 99% (350 °C), and the catalyst could effectively block the generation of polycyclic aromatic hydrocarbons. Mechanistic studies have shown that Cu sites on the Cu5-VWT catalyst facilitate catalytic oxidation, while V sites facilitate NO_x reduction. Thus, toluene oxidation and NO_x reduction can proceed simultaneously. The removal of total hydrocarbons and nonmethane total hydrocarbons from 1200 m³·h^-1 real coal-fired flue gas by a monolithic catalyst were determined as 92 and 96%, respectively, much higher than those of 54 and 72% over a commercial VWT catalyst, indicating great promise for industrial application.

Single-Atom Ce-Modified α-Fe2O3 for Selective Catalytic Reduction of NO with NH3

A single-atom Ce-modified α-Fe₂O₃ catalyst (Fe_0.93Ce_0.07O_x catalyst with 7% atomic percentage of Ce) was synthesized by a citric acid-assisted sol-gel method, which exhibited excellent performance for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) over a wide operating temperature window. Remarkably, it maintained ∼93% NO conversion efficiency for 168 h in the presence of 200 ppm SO₂ and 5 vol % H₂O at 250 °C. The structural characterizations suggested that the introduction of Ce leads to the generation of local Fe-O-Ce sites in the FeO_x matrix. Furthermore, it is critical to maintain the atomic dispersion of the Ce species to maximize the amounts of Fe-O-Ce sites in the Ce-doped FeO_x catalyst. The formation of CeO₂ nanoparticles due to a high doping amount of Ce species leads to a decline in catalytic performance, indicating a size-dependent catalytic behavior. Density functional theory (DFT) calculation results indicate that the formation of oxygen vacancies in the Fe-O-Ce sites is more favorable than that in the Fe-O-Fe sites in the Ce-free α-Fe₂O₃ catalyst. The Fe-O-Ce sites can promote the oxidation of NO to NO₂ on the Fe_0.93Ce_0.07O_x catalyst and further facilitate the reduction of NO_x by NH₃. In addition, the decomposition of NH₄HSO₄ can occur at lower temperatures on the Fe_0.93Ce_0.07O_x catalyst containing atomically dispersed Ce species than on the α-Fe₂O₃ reference catalyst, resulting in the good SO₂/H₂O resistance ability in the NH₃-SCR reaction.

Low-Temperature Combustion of Toluene over Cu-Doped SmMn2O5 Mullite Catalysts via Creating Highly Active Cu2+-O-Mn4+ Sites

Catalytic combustion of volatile organic compounds (VOCs) at low temperatures is still an urgent issue to be solved. Herein, low-temperature combustion of toluene over Cu-doped SmMn₂O₅ mullite catalysts via creating highly active Cu²⁺-O-Mn⁴⁺ sites has been originally demonstrated. Cu-doped SmMn₂O₅ mullite catalysts exhibited 90% conversion of toluene at 206 °C and displayed robust stability even in the presence of water. It has been demonstrated that Cu doping created Cu²⁺-O-Mn⁴⁺ active composite sites that were more exposed after removing surface Sm species via acid-etching. Benefiting from this, the redox and oxygen activation ability of catalysts was significantly enhanced. The consumption of benzaldehyde and benzoic acid as intermediate species and the CO₂ generation ability were apparently promoted, which were the direct reasons for the enhanced low-temperature combustion of toluene. This work provides novel ideas for the development of high-performance catalysts for low-temperature VOC combustion, which has great industrial application prospects.

In situ deposition of 0D CeO2 quantum dots on Fe2O3-containing solid waste NH3-SCR catalyst: Enhancing redox and NH3 adsorption ability

As NO_x has been turning into a crucial environmental problem, NH₃-SCR technology with relatively simple device, reliable operation and low secondary pollution, has become a widely used commercial and mature de-nitration technology. However, some weaknesses restricted the further application of commercialized V₂O₅-WO₃/TiO₂ NH₃-SCR catalysts, while Fe₂O₃-based catalysts have received much attention due to their high thermal stability, passable N₂ selectivity and low cost. In this study, Fe₂O₃-containing solid waste derived from Zn extraction process of electric arc furnace dust was exploited as the base material for catalyst preparing. Owing to the complementary and synergistic effect of CeO₂ and Fe₂O₃, 0D CeO₂ quantum dots (CeQDs) with fully-exposed active sites, large specific surface area, and rapid charge transfer have been introduced and deposited onto Fe₂O₃-containing solid waste nanorods. The in-situ deposition of CeQDs led to the admirable enhancement in NH₃-SCR catalytic activity, N₂ selectivity and SO₂ tolerance of the extremely low-cost Fe₂O₃ catalyst. Comprehensive characterizations and DFT calculations describing the adsorption of O₂ and NH₃ were applied to analyze the catalyst structure and further investigate the detailed relationship between structural properties and activity as well as reaction mechanism. This work provides new insights for the high-value utilization of iron-containing solid waste and a practical reference for boosting the performance of NH₃-SCR catalysts by introducing quantum dots.

Unveiling Secondary-Ion-Promoted Catalytic Properties of Cu-SSZ-13 Zeolites for Selective Catalytic Reduction of NOx

The incorporation of secondary metal ions into Cu-exchanged SSZ-13 zeolites could improve their catalytic properties in selective catalytic reduction of NO_x with ammonia (NH₃-SCR), but their essential roles remain unclear at the molecular level. Herein, a series of Cu-Sm-SSZ-13 zeolites have been prepared by ion-exchanging Sm ions followed by Cu ions, which exhibit superior NH₃-SCR performance. The NO conversion of Cu-Sm-SSZ-13 is nearly 10% higher than that of conventional Cu-SSZ-13 (175-250 °C) after hydrothermal ageing, showing an enhanced low-temperature activity. The Sm ions are found to occupy the six-membered rings (6MRs) of SSZ-13 by X-ray diffraction Rietveld refinement and aberration-corrected scanning transmission electron microscopy. The Sm ions at 6MRs can facilitate the formation of more active [ZCu²⁺(OH)]⁺ ions at 8MRs, as revealed by temperature-programmed reduction of hydrogen. X-ray photoelectron spectroscopy and density functional theory (DFT) calculations indicate that there exists electron transfer from Sm³⁺ to [ZCu²⁺(OH)]⁺ ions, which promotes the activity of [ZCu²⁺(OH)]⁺ ions by decreasing the activation energy of the formation of intermediates (NH₄NO₂ and H₂NNO). Meanwhile, the electrostatic interaction between Sm³⁺ and [ZCu²⁺(OH)]⁺ results in a high-reaction energy barrier for transforming [ZCu²⁺(OH)]⁺ ions into inactive CuO_x species, thus enhancing the stability of [ZCu²⁺(OH)]⁺ ions. The influence of the ion-exchanging sequence of Sm and Cu ions into SSZ-13 is further investigated by combining both experiments and theoretical calculations. This work provides a mechanistic insight of secondary ions in regulating the distribution, activity, and stability of Cu active sites, which is helpful for the design of high-performance Cu-SSZ-13 catalysts for the NH₃-SCR reaction.

Tungsten Oxide Modified V2O5-Sb2O3/TiO2 Monolithic Catalyst: NH3-SCR Activity and Sulfur Resistance

In this study, a V2O5-Sb2O3/TiO2 monolithic catalyst was modified by introducing WO3. The WO3-modified catalyst exhibited enhanced catalytic activity in the measuring temperature range of 175–320 °C. The changes in dispersion of vanadia species were investigated by ultraviolet-visible (UV-Vis) spectroscopy and H2 temperature-programmed reduction (H2-TPR). A durability test was conducted in a wet SO2-containing atmosphere at 220 °C for 25 h. The sulfate deposition was estimated by temperature-programmed decomposition (TPDC) of sulfates, thermo-gravimetric (TG) analysis, and temperature-programmed desorption (TPD) of NH3. Isothermal SO2 oxidation and temperature-programmed surface reaction (TPSR) of NH4HSO4 with NO were performed. Based on these characterizations, effects of WO3 modification on the sulfate tolerance of the catalyst were explored.

Structure-Directing Role of Support on Hg0 Oxidation over V2O5/TiO2 Catalyst Revealed for NOx and Hg0 Simultaneous Control in an SCR Reactor

The crystal structure of TiO₂ strongly influences the physiochemical properties of supported active sites and thus the catalytic performance of the as-synthesized catalyst. Herein, we synthesized TiO₂ with different crystal forms (R = rutile, A = anatase, and B = brookite), which were used as supports to prepare vanadium-based catalysts for Hg⁰ oxidation. The Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B was the best, followed by V₂O₅/TiO₂-A and V₂O₅/TiO₂-R. Further experimental and theoretical results indicate that gaseous Hg⁰ reacts with surface-active chlorine species produced by the adsorbed HCl and the reaction orders of Hg⁰ oxidation over V₂O₅/TiO₂ catalyst with respect to HCl and Hg⁰ concentration were approximately 0 and 1, respectively. The excellent Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B can be attributed to lower redox temperature, larger HCl adsorption capacity, and more oxygen vacancies. This work suggests that to achieve the best simultaneous removal of NO_x and Hg⁰ on state-of-the-art V₂O₅/TiO₂ catalyst, a combination of anatase and brookite TiO₂-supported vanadyl tandem catalysts is supposed to be employed in the SCR reactor, and the brookite-type catalyst should be on the downstream of the anatase-based catalyst due to the inhibition of NH₃ on Hg⁰ oxidation.

Hydrothermal Aging Treatment Activates V2O5/TiO2 Catalysts for NOx Abatement

Thermal stability is crucial for the practical application of deNO_x catalysts. Vanadia-based catalysts are widely applied for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). Generally, hydrothermal aging at high temperatures induces the deactivation of deNO_x catalysts. However, in this work, a remarkable increase in low- and medium-temperature NH₃-SCR activity was observed for a V₂O₅/TiO₂ catalyst after hydrothermal aging treatment, especially at 750 °C for 16 h. After the vanadia-based catalyst was hydrothermally treated at 750 °C, the specific surface area decreased and the surface VO_x density and surface V ratio increased significantly. Therefore, the aged catalyst presented more abundant polymeric vanadyl species than the fresh one. Furthermore, the redox capability was improved markedly after hydrothermal treatment due to the strong interaction of vanadia and titania, contributing to the NH₃-SCR reaction. 750 °C is the optimal temperature to activate the V₂O₅/TiO₂ catalyst, improving the SCR performance significantly. This study provides an in-depth understanding of vanadia-based catalysts for practical applications.

Fabrication of wide temperature FexCe1-xVO4 modified TiO2-graphene catalyst with excellent NH3-SCR performance and strong SO2/H2O tolerance

Selective catalytic reduction of NO with NH3 (NH3-SCR) is one of the most common technique for elimination of NOx. The promotional effect of Fe additive on the NH3-SCR activity of the CeVO4/TiO2-graphene (GE) is systematically studied. The results exhibited that the low-temperature NOx conversion could be enhanced dramatically via the addition of Fe and Fe0.5Ce0.5VO4/TiO2-GE displayed the highest conversion of NOx in the wide temperature window (200-400 °C). It is because that Fe3+ + Ce3+ ↔ Fe2+ + Ce4+ facilitated the oxidization of NO to NO2 at low temperature and led to the "Fast SCR," thereby raising the SCR performance. What is more, the introduction of Fe enhanced redox ability, the surface relative percentage of Ce3+, V5+ and the chemical adsorbed oxygen. Furthermore, the high surface concentration of Ce3+ species can produce more active oxygen and leads to the "Fast SCR" reaction. In addition, the Fe0.5Ce0.5VO4/TiO2-GE catalyst showed excellent H2O/SO2 tolerance, which may be due to the decomposition of ammonium bisulphite under high temperature and the hydrophobicity of graphene. What is more, it displayed outstanding the stability. This work would provide theoretical reference for the practical application of NOx abatement via NH3-SCR.

Physicochemical Features and NH3-SCR Catalytic Performance of Natural Zeolite Modified with Iron—The Effect of Fe Loading

In modern dual-pressure nitric acid plants, the tail gas temperature usually exceeds 300 °C. The NH3-SCR catalyst used in this temperature range must be resistant to thermal deactivation, so commercial vanadium-based systems, such as V2O5-WO3 (MoO3)-TiO2, are most commonly used. However, selectivity of this material significantly decreases above 350 °C due to the increase in the rate of side reactions, such as oxidation of ammonia to NO and formation of N2O. Moreover, vanadium compounds are toxic for the environment. Thus, management of the used catalyst is complicated. One of the alternatives to commercial V2O5-TiO2 catalysts are natural zeolites. These materials are abundant in the environment and are thus relatively cheap and easily accessible. Therefore, the aim of the study was to design a novel iron-modified zeolite catalyst for the reduction of NOx emission from dual-pressure nitric acid plants via NH3-SCR. The aim of the study was to determine the influence of iron loading in the natural zeolite-supported catalyst on its catalytic performance in NOx conversion. The investigated support was firstly formed into pellets and then impregnated with various contents of Fe precursor. Physicochemical characteristics of the catalyst were determined by XRF, XRD, low-temperature N2 sorption, FT-IR, and UV–Vis. The catalytic performance of the catalyst formed into pellets was tested on a laboratory scale within the range of 250–450 °C using tail gases from a pilot nitric acid plant. The results of this study indicated that the presence of various iron species, including natural isolated Fe3+ and the introduced FexOy oligomers, contributed to efficient NOx reduction, especially in the high-temperature range, where the NOx conversion rate exceeded 90%.

Single-atom Ir1 supported on rutile TiO2 for excellent selective catalytic oxidation of ammonia

Gaseous ammonia (NH₃) in the atmosphere is potentially harmful to both human health and the environment. The selective catalytic oxidation of NH₃ (termed as NH₃-SCO) into N₂ and H₂O is a promising method for decreasing NH₃ emissions. A highly efficient catalyst is required for controlling NH₃ emissions by this method in practice. In this study, we prepared Ir/TiO₂ catalysts using different crystal structures of TiO₂ (rutile, P25 or anatase) as supports by a simple impregnation method and evaluated their performance in the NH₃-SCO. We found that the Ir/TiO₂-R (rutile) catalyst performed better than the Ir/TiO₂-P25 (mixed-phase) and Ir/TiO₂-A (anatase) catalyst. High-angle annular dark-field images of the aberration-corrected scanning transmission electron microscopy revealed that the Ir species were mainly atomically dispersed on the TiO₂ support in Ir/TiO₂-R with 1 wt% Ir loading, whereas the Ir species agglomerated to form clusters or nanoparticles in Ir/TiO₂-P25 and Ir/TiO₂-A. The combined results of X-ray absorption fine structure, H₂-temperature-programmed reduction, and in situ diffuse reflectance for infrared Fourier Transform spectroscopy studies suggested that atomically dispersed Ir species had stronger electronic metal-support interaction with rutile TiO₂, which resulted in easier to adsorb and activate O₂ at the interface and thus, better low-temperature activity of the Ir/TiO₂-R catalyst.

Ordered Mesoporous MnAlOx Oxides Dominated by Calcination Temperature for the Selective Catalytic Reduction of NOx with NH3 at Low Temperature

Manganese alumina composited oxides (MnAlOx) catalysts with ordered mesoporous structure prepared by evaporation-induced self-assembly (EISA) method was designed for the selective catalytic reduction (SCR) of NOx with NH3 at low temperature. The effect of calcination temperature of MnAlOx catalysts was investigated systematically, and it was correlated with SCR activity. Results showed that with an increase in calcination temperature, the SCR activity of MnAlOx catalysts increased. When the calcination temperature was raised up to 800 °C, the NOx conversion was more than 90% in the operation temperature range of 150~240 °C. Through various characterization analysis, it was found that MnAlOx-800 °C catalysts possessed enhanced redox capacities as the higher content of Mn4+/(Mn3+ + Mn4+). Moreover, the improved redox properties could contribute to a higher NOx adsorption and activation ability, which lead to higher SCR performance of MnAlOx-800 °C catalysts. In situ DRIFTs revealed that the adsorbed NO2 and bidentate nitrate are the reactive intermediate species, and NH3 species bonded to Lewis acid sites taken part in SCR progress. The SCR progress predominantly followed E–R mechanism, while L–H mechanism also takes effect to a certain degree.

Unraveling the structure and role of Mn and Ce for NOx reduction in application-relevant catalysts

Mn-based oxides are promising for the selective catalytic reduction (SCR) of NOx with NH₃ at temperatures below 200 °C. There is a general agreement that combining Mn with another metal oxide, such as CeOx improves catalytic activity. However, to date, there is an unsettling debate on the effect of Ce. To solve this, here we have systematically investigated a large number of catalysts. Our results show that, at low-temperature, the intrinsic SCR activity of the Mn active sites is not positively affected by Ce species in intimate contact. To confirm our findings, activities reported in literature were surface-area normalized and the analysis do not support an increase in activity by Ce addition. Therefore, we can unequivocally conclude that the beneficial effect of Ce is textural. Besides, addition of Ce suppresses second-step oxidation reactions and thus N₂O formation by structurally diluting MnOx. Therefore, Ce is still an interesting catalyst additive.

Effect of MnOx/α-Fe2O3 Prepared from Goethite on Selective Catalytic Reduction of NO with NH3

A low-cost goethite and manganese acetate were used to prepare a MnOx/α-Fe2O3 composite catalyst by simple impregnation method for novel high-efficiency selective catalytic reduction (SCR) of NO with NH3, and its denitration performance of composite catalyst under different conditions was investigated in a thermal fixed-bed catalytic reaction system. The results showed that MnOx/α-Fe2O3 with Mn/Fe molar ratio of 0.1 and calcination temperature of 400 °C had the best low-temperature catalytic activity and wider reaction temperature window compared with α-Fe2O3. It achieved over 90% NO conversion efficiency with a space velocity of 72,000 h−1 at 200~350 °C and possessed a good resistance of H2O and SO2. Characterization by XRD, BET, H2-TPR, and NH3-TPD revealed that the main reason for the high catalytic activity of MnOx(0.1)/α-Fe2O3(400) was that the addition of Mn changed phase types of catalyst and valence composition of Fe, resulting in a larger specific surface area, more acidic sites, and higher redox performance.

NOx Reduction over Smart Catalysts with Self-Created Targeted Antipoisoning Sites

Selective catalytic reduction of NO_x in the presence of alkali (earth) metals and heavy metals is still a challenge due to the easy deactivation of catalysts. Herein, NO_x reduction over smart catalysts with self-created targeted antipoisoning sites is originally demonstrated. The smart catalyst consisted of TiO₂ pillared montmorillonite with abundant cation exchange sites to anchor poisoning substances and active components to catalyze NO_x into N₂. It was not deactivated during the NO_x reduction process in the presence of alkali (earth) metals and heavy metals. The enhanced surface acidity, reducible active species, and active chemisorbed oxygen species of the smart catalyst accounted for the remarkable NO_x reduction efficiency. More importantly, the self-created targeted antipoisoning sites expressed specific anchoring effects on poisoning substances and protected the active components from poisoning. It was demonstrated that the tetrahedrally coordinated aluminum species of the smart catalyst mainly acted as self-created targeted antipoisoning sites to stabilize the poisoning substances into the interlayers of montmorillonite. This work paves a new way for efficient reduction of NO_x from the complex flue gas in practical applications.

Abatement of Nitrogen Oxides via Selective Catalytic Reduction over Ce1-W1 Atom-Pair Sites

Environmentally benign CeO₂-WO₃/TiO₂ catalysts are promising alternatives to commercial toxic V₂O₅-WO₃/TiO₂ for controlling NO_x emission via selective catalytic reduction (SCR), but the insufficient catalytic activity of CeO₂-WO₃/TiO₂ catalysts is one of the obstacles in their applications because of a lack of an in-depth understanding of the CeO₂-WO₃ interactions. Herein, we design a Ce₁-W₁/TiO₂ model catalyst by anchoring Ce₁-W₁ atom pairs on anatase TiO₂(001) to investigate the synergy between Ce and W in SCR. A series of characterizations combined with density functional theory calculations and in situ diffuse-reflectance infrared Fourier-transform experiments reveal that there exists a strong electronic interaction within Ce₁-W₁ atom pairs, leading to a much better SCR performance of Ce₁-W₁/TiO₂ compared with that of Ce₁/TiO₂ and W₁/TiO₂. The Ce₁-W₁ synergy not only shifts down the lowest unoccupied states of Ce₁ near the Fermi level, thus enhancing the abilities in adsorbing and oxidizing NH₃ but also makes the frontier orbital electrons of W₁ delocalized, thus accelerating the activation of O₂. The deep insight of the Ce-W synergy may assist in the design and development of efficient catalysts with an SCR activity as high as or even higher than V₂O₅-WO₃/TiO₂.

A theoretical descriptor for screening efficient NO reduction electrocatalysts from transition-metal atoms on N-doped BP monolayer

An electrochemical nitric oxide (NO) reduction reaction (NORR) is proposed as an attractive method for simultaneous realization of NO removal and ammonia (NH₃) synthesis. Here, the potentials of 29 transition-metal atoms anchored on the nitrogen-doped BP monolayer (MN₃/BP) as efficient NORR catalysts are systematically examined using first-principles calculations. Combining the adsorption Gibbs free energies of the N and OH species, a simple descriptor is constructed and a volcano plot of the NORR limiting potentials on the single atom catalysts (SACs) is established. Consequently, the MoN₃/BP and IrN₃/BP SACs are picked out as promising NORR electrocatalysts for NH₃ synthesis with the limiting potentials of -0.10 V and -0.06 V, respectively. Their corresponding rate constants are significantly larger than or close to that of the excellent Pt(111) surface. The electronic analysis shows that the Mo-4d or Ir-5d orbitals can be well hybridized with the NO-2p orbitals, sufficiently activating the adsorbed NO species. Particularly, the MoN₃/BP and IrN₃/BP SACs possess high thermal stabilities and can be easily synthesized by using MoCl₃ and IrCl₃ as precursors, respectively. This work not only offers a simple descriptor to efficiently design NORR electrocatalysts but also provides a comprehensive atomic understanding on the mechanism of NO-to-NH₃ conversion.

Enhancing Electrocatalytic NO Reduction to NH3 by the CoS Nanosheet with Sulfur Vacancies

Electrochemical reduction of NO to NH₃ is of great significance for mitigating the accumulation of nitrogen oxides and producing valuable NH₃. Here, we demonstrate that the CoS nanosheet with sulfur vacancies (CoS_1-x) behaves as an efficient catalyst toward electrochemical NO-to-NH₃ conversion. In 0.2 M Na₂SO₄ electrolyte, such CoS_1-x displays a large NH₃ yield rate (44.67 μmol cm^-2 h^-1) and a high Faradaic efficiency (53.62%) at -0.4 V versus the reversible hydrogen electrode, outperforming the CoS counterpart (27.02 μmol cm^-2 h^-1; 36.68%). Moreover, the Zn-NO battery with CoS_1-x shows excellent performance with a power density of 2.06 mW cm^-2 and a large NH₃ yield rate of 1492.41 μg h^-1 mg_cat.^-1. Density functional theory was performed to obtain mechanistic insights into the NO reduction over CoS_1-x.

SO2-Tolerant Catalytic Reduction of NOx via Tailoring Electron Transfer between Surface Iron Sulfate and Subsurface Ceria

Currently, SO₂-induced catalyst deactivation from the sulfation of active sites turns to be an intractable issue for selective catalytic reduction (SCR) of NO_x with NH₃ at low temperatures. Herein, SO₂-tolerant NO_x reduction has been originally demonstrated via tailoring the electron transfer between surface iron sulfate and subsurface ceria. Engineered from the atomic layer deposition followed by the pre-sulfation method, the structure of surface iron sulfate and subsurface ceria was successfully constructed on CeO₂/TiO₂ catalysts, which delivered improved SO₂ resistance for NO_x reduction at 250 °C. It was demonstrated that the surface iron sulfate inhibited the sulfation of subsurface Ce species, while the electron transfer from the surface Fe species to the subsurface Ce species was well retained. Such an innovative structure of surface iron sulfate and subsurface ceria notably improved the reactivity of NH_x species, thus endowing the catalysts with a high NO_x reaction efficiency in the presence of SO₂. This work unraveled the specific structure effect of surface iron sulfate and subsurface ceria on SO₂-toleant NO_x reduction and supplied a new point to design SO₂-tolerant catalysts by modulating the unique electron transfer between surface sulfate species and subsurface oxides.

Application and Development of Selective Catalytic Reduction Technology for Marine Low-Speed Diesel Engine: Trade-Off among High Sulfur Fuel, High Thermal Efficiency, and Low Pollution Emission

In recent years, the International Maritime Organization (IMO), Europe, and the United States and other countries have set up different emission control areas (ECA) for ship exhaust pollutants to enforce more stringent pollutant emission regulations. In order to meet the current IMO Tier III emission regulations, an after-treatment device must be installed in the exhaust system of the ship power plant to reduce the ship NOx emissions. At present, selective catalytic reduction technology (SCR) is one of the main technical routes to resolve excess NOx emissions of marine diesel engines, and is the only NOx emission reduction technology recognized by the IMO that can be used for various ship engines. Compared with the conventional low-pressure SCR system, the high-pressure SCR system can be applied to low-speed marine diesel engines that burn inferior fuels, but its working conditions are relatively harsh, and it can be susceptible to operational problems such as sulfuric acid corrosion, salt blockage, and switching delay during the actual ship tests and ship applications. Therefore, it is necessary to improve the design method and matching strategy of the high-pressure SCR system to achieve a more efficient and reliable operation. This article summarizes the technical characteristics and application problems of marine diesel engine SCR systems in detail, tracks the development trend of the catalytic reaction mechanism, engine tuning, and control strategy under high sulfur exhaust gas conditions. Results showed that low temperature is an important reason for the formation of ammonium nitrate, ammonium sulfate, and other deposits. Additionally, the formed deposits will directly affect the working performance of the SCR systems. The development of SCR technology for marine low-speed engines should be the compromise solution under the requirements of high sulfur fuel, high thermal efficiency, and low pollution emissions. Under the dual restrictions of high sulfur fuel and low exhaust temperature, the low-speed diesel engine SCR systems will inevitably sacrifice part of the engine economy to obtain higher denitrification efficiency and operational reliability.

Catalytic performance and reaction mechanisms of NO removal with NH3 at low and medium temperatures on Mn-W-Sb modified siderite catalysts

Iron-based catalysts have been explored for selective catalytic reduction (SCR) of NO due to environmentally benign characters and good SCR activity. Mn-W-Sb modified siderite catalysts were prepared by impregnation method based on siderite ore, and SCR performance of the catalysts was investigated. The catalysts were analyzed by X-ray diffraction, H₂-temperature-programmed reduction, Brunauer-Emmett-Teller, Thermogravimetry-derivative thermogravimetry and in-situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The modified siderite catalysts calcined at 450°C mainly consist of Fe₂O₃, and added Mn, W and Sb species are amorphous. 3Mn-5W-1.5Sb-siderite catalyst has a wide temperature window of 180-360°C and good N₂ selectivity at low temperatures. In-situ DRIFTS results show NH₄⁺, coordinated NH₃, NH₂, NO₃^- species (bidentate), NO₂^- species (nitro, nitro-nitrito, monodentate), and adsorbed NO₂ can be discovered on the surface of Mn-W-Sb modified siderite catalysts, and doping of Mn will enhance adsorbed NO₂ formation by synergistic catalysis with Fe³⁺. In addition, the addition of Sb can inhibit sulfates formation on the surface of the catalyst in the presence of SO₂ and H₂O. Time-dependent in-situ DRIFTS studies also indicate that both of Lewis and Brønsted acid sites play a role in SCR of NO by ammonia at low temperatures. The mechanism of NO removal on the 3Mn-5W-1.5Sb-siderite catalyst can be discovered as a combination of Eley-Rideal and Langmuir-Hinshelwood mechanisms with three reaction pathways. The mechanism of NO, oxidized by synergistic catalysis of Fe³⁺ and Mn^4+/3+ to form NO₂ among three pathways, reveals the reason of high NO_x conversion of the catalyst at medium and low temperatures.

Amorphous Boron Carbide on Titanium Dioxide Nanobelt Arrays for High-Efficiency Electrocatalytic NO Reduction to NH3

Electrocatalytic NO reduction is regarded as an attractive strategy to degrade the NO contaminant into useful NH₃ , but the lack of efficient and stable electrocatalysts to facilitate such multiple proton-coupled electron-transfer processes impedes its applications. Here, we report on developing amorphous B_2.6 C supported on a TiO₂ nanoarray on a Ti plate (a-B_2.6 C@TiO₂ /Ti) as an NH₃ -producing nanocatalyst with appreciable activity and durability toward the NO electroreduction. It shows a yield of 3678.6 μg h^-1  cm^-2 and a FE of 87.6 %, superior to TiO₂ /Ti (563.5 μg h^-1  cm^-2 , 42.6 %) and a-B_2.6 C/Ti (2499.2 μg h^-1  cm^-2 , 85.6 %). An a-B_2.6 C@TiO₂ /Ti-based Zn-NO battery achieves a power density of 1.7 mW cm^-2 with an NH₃ yield of 1125 μg h^-1  cm^-2 . An in-depth understanding of catalytic mechanisms is gained by theoretical calculations.

Low-Temperature NH3-SCR on Cex-Mn-Tiy Mixed Oxide Catalysts: Improved Performance by the Mutual Effect between Ce and Ti

A series of Cex-Mn-Tiy catalysts were synthesized using the coprecipitation method, and sodium carbonate solution was used as a precipitant. The various catalysts were assessed by selective catalytic reduction of NOx with NH3, and characterized by X-ray diffraction, Raman spectroscopy, H2 temperature-programmed reduction, NH3 temperature-programmed desorption, and X-ray photoelectron spectroscopy to investigate the physicochemical properties, surface acidity, and redox abilities of the Cex-Mn-Tiy catalysts. The Ce0.1-Mn-Ti0.1 catalyst exhibited the best catalytic performance (more than 90% NOx conversion in the range of 75 to 225 °C), as a result of proper redox ability, abundant acid sites, high content of Mn4+ and Ce3+, and surface-adsorbed oxygen (OS). The results of in situ DRIFT spectroscopy showed that the NH3-SCR reaction followed both the E-R and L-H paths over the Ce0.1-Mn-Ti0.1 catalyst, and it occurred faster and more sharply when it mainly abided by the E-R mechanism.

Alkali and Heavy Metal Copoisoning Resistant Catalytic Reduction of NOx via Liberating Lewis Acid Sites

The catalyst deactivation caused by the coexistence of alkali and heavy metals remains an obstacle for selective catalytic reduction of NO_x with NH₃. Moreover, the copoisoning mechanism of alkali and heavy metals is still unclear. Herein, the copoisoning mechanism of K and Cd was revealed from the adsorption and variation of reaction intermediates at a molecular level through time-resolved in situ spectroscopy combined with theoretical calculations. The alkali metal K mainly decreased the adsorption of NH₃ on Lewis acid sites and altered the reaction more depending on the formation of the NH₄NO₃ intermediate, which is highly related to NO_x adsorption and activation. However, Cd further inhibited the generation of active nitrate intermediates and thus decreased the NO_x abatement about 60% on potassium-poisoned CeTiO_x catalysts. Physically mixing with acid additives for CeTiO_x catalysts could significantly liberate the active Lewis acid sites from the occupation of alkali metals and relieve the high dependence on NO_x adsorption and activation, thus recovering the NO_x removal rate to the initial state. This work revealed the copoisoning mechanism of K and Cd on Ce-based de-NO_x catalysts and developed a facile anti-poisoning strategy, which paves a way for the development of durable catalysts among alkali and heavy metal copoisoning resistant catalytic reduction of NO_x.

SO2- and H2O-Tolerant Catalytic Reduction of NOx at a Low Temperature via Engineering Polymeric VOx Species by CeO2

Selective catalytic reduction (SCR) of NO_x over V₂O₅-based oxide catalysts has been widely used, but it is still a challenge to efficiently reduce NO_x at low temperatures under SO₂ and H₂O co-existence. Herein, SO₂- and H₂O-tolerant catalytic reduction of NO_x at a low temperature has been originally demonstrated via engineering polymeric VO_x species by CeO₂. The polymeric VO_x species were tactfully engineered on Ce-V₂O₅ composite active sites via the surface occupation effect of Ce, and the obtained catalysts exhibited remarkable low-temperature activity and strong SO₂ and H₂O tolerance at 250 °C. The strong interaction between Ce and V species induced the electron transfer from V to Ce and tuned the SCR reaction via the E-R pathway between the NH₄⁺/NH₃ species and gaseous NO. In the presence of SO₂ and H₂O, the polymeric VO_x species had not been hardly influenced, while the formation of sulfate species on Ce sites not only promoted the adsorption of NH₄⁺ species and the reaction between gaseous NO and NH₄⁺ but also facilitated the decomposition of ammonium bisulfate through weakening the strong bond between HSO₄^- and NH₄⁺. This work provided a new strategy for SO₂- and H₂O-tolerant catalytic reduction of NO_x at a low temperature.

Phosphate on ceria with controlled active sites distribution for wide temperature NH3-SCR

Practical catalysts that work well at a wide operation window for selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) are essential for the purification of non-isothermal emission such as vehicle exhaust. However, NH₃-SCR catalyst with high low-temperature performance has excellent NO activation and oxidation ability, leading inevitably to NH₃-intermediates over-oxidation and N₂ selectivity deterioration at high operation temperatures. By far the best performance ceria-based catalyst with a super-wide temperature window of 175-400 ^oC for 90% NO_x conversion in ideal environment and 225-475 ^oC for 90% NO_x conversion by addition of 50 ppm SO₂ and 5% H₂O is obtained via distributing phosphate over the outer of ceria. NH₃ protection strategy is the key for keeping high-temperature activity. Brønsted acidity surged as the formation of P-OH network via a charge compensatory mechanism of phosphate. NH₃ was prone to be captured by the surface P-OH network, forming NH₄⁺ species, avoiding being oxidized and contributing to both low and high temperature activity. NO can also be readily absorbed and oxidized to the absorbed NO₂(ad) species over phosphate as reflected by in situ DRIFTS and DFT calculation, providing a facile pathway for 'fast SCR' by reacting with NH₄⁺ species to form N₂ and H₂O. The reaction followed the L-H mechanism and contributed to catalytic activity under 300 ^oC. This directional structure fabricate strategy helps to increases the NO_x conversion and N₂ selectivity under a broaden temperature window. The enriched Brønsted acid sites over phosphate treated ceria were also demonstrated to have largely suppressed SO₂ adsorption, which significantly slowed down the catalyst poisoning. A dynamic equilibrium between the poisoning and regeneration process can be achieved according to the shrinking-core model for each nanosphere, leading to the excellent resistance.

Self-Defense Effects of Ti-Modified Attapulgite for Alkali-Resistant NOx Catalytic Reduction

Nowadays, the serious deactivation of deNO_x catalysts caused by alkali metal poisoning was still a huge bottleneck in the practical application of selective catalytic reduction of NO_x with NH₃. Herein, alkali-resistant NO_x catalytic reduction over metal oxide catalysts using Ti-modified attapulgite (ATP) as supports has been originally demonstrated. The self-defense effects of Ti-modified ATP for alkali-resistant NO_x catalytic reduction have been clarified. Ti-modified ATP with self-defense ability was obtained by removing alkaline metal cation impurities in the natural ATP materials without destroying its initial layered-chain structure through the ion-exchange procedure, accompanied with an obvious enrichment of Brønsted acid and Lewis acid sites. The self-defense effects embodied that both ion-exchanged Ti octahedral centers and abundant Si-OH sites in the Ti-ion-exchange-modified ATP could effectively anchor alkali metals via coordinate bonding or ion-exchange process, which induced alkali metals to be immobilized by the Ti-ion-exchange-modified ATP carrier rather than impair active species. Under this special protection of self-defense effects, Ti-ion-exchange-modified ATP supported catalysts still retained plentiful acidic sites and superior redox ability even after alkali metal poisoning, giving rise to the maintenance of sufficient NH_x and NO_x adsorption and the subsequent efficient reaction, which in turn resulted in high NO_x catalytic reduction capacity of the catalyst. The strategy provided new inspiration for the development of novel and efficient selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) catalysts with high alkali resistance.

Interaction Mechanism for Simultaneous Elimination of Nitrogen Oxides and Toluene over the Bifunctional CeO2-TiO2 Mixed Oxide Catalyst

Simultaneous catalytic elimination of nitrogen oxides (NO_x, x = 1 and 2) and volatile organic compounds (VOCs) is of great importance for environmental preservation in China. In this work, the interactions of simultaneous removal of NO_x and methylbenzene (PhCH₃) were investigated on a CeO₂-TiO₂ mixed oxide catalyst, which demonstrated excellent bifunctional removal efficiencies for the two pollutants. The results indicated that NO_x positively promotes PhCH₃ oxidation, while NH₃ negatively inhibits through competitive adsorption with PhCH₃. The underlying mechanism is that a pseudo PhCH₃-SCR reaction happened in this process is parallel to NH₃-SCR. Combined with in situ diffuse reflectance infrared Fourier transform spectroscopy and quasi in situ X-ray photoelectron spectroscopy, the interaction mechanism between NO_x and PhCH₃ is proposed. Specifically, NO_x is adsorbed on the catalyst surface to produce nitrate species, which reacts with the carboxylate generated during PhCH₃ oxidation to form organic nitrogen intermediates that create N₂ and CO₂ in the following reactions. In the reaction process, the superoxide (O₂^-) generated by O₂ activation on the catalyst surface is an important species for the propelling of oxidation reaction. This work could provide guidelines for the design of state-of-the-art catalysts for simultaneous catalytic removal of NO_x and VOCs.

Dynamic Change of Active Sites of Supported Vanadia Catalysts for Selective Catalytic Reduction of Nitrogen Oxides

Selective catalytic reduction of NOx by ammonia (NH₃-SCR) on V₂O₅/TiO₂ catalysts is a widely used commercial technology in power plants and diesel vehicles due to its high elimination efficiency for NOx removal. However, the mechanistic aspects of the NH₃-SCR reaction, especially the active sites on the V₂O₅/TiO₂ catalysts, are still a puzzle. Herein, using combined operando spectroscopy and density functional theory calculations, we found that the reactivity of the Lewis acid site was significantly overestimated due to its conversion to the Brønsted acid site. Such interconversion makes it challenging to measure the intrinsic reactivity of different acid sites accurately. In contrast, the abundant V-OH Brønsted acid sites govern the overall NOx reduction rate in realistic exhaust containing water vapor. Moreover, the vanadia species cycle between V⁵⁺═O and V⁴⁺-OH during NOx reduction, and the re-oxidation of V⁴⁺ species to form V⁵⁺ is the rate-determining step.

Efficient Electron Transfer by Plasmonic Silver in SrTiO3 for Low-Concentration Photocatalytic NO Oxidation

Photocatalysis presents a feasible option to control low-concentration NO emissions from industrial burning facilities, and increasing excitons in quantity and improving surface activity are the crucial issues to be solved. Plasmonic silver with the orientation of the (111) plane is uniformly distributed on the Ti-O termination of the SrTiO₃ (STO) (100) plane (major). The NO conversion rate has a sixfold increment compared to pristine STO. Meanwhile, the toxic NO₂ had a significant decline in the absence of water. This high performance could be attributed to the unique property of the localized surface plasmonic resonance of silver particles, which increases the optical response range of the catalyst. Meanwhile, the formation of a Schottky junction could promote the charge separation and enhance the lifetime of excitons via the electron transfer from silver particles to STO. More importantly, the Ag-O bond of the heterojunction increases the charge density of adjacent Ti, preferring to bond with the antibonding orbital electron of adsorbed molecules, which offers a favorable channel for the NO adsorption and activation of reactive oxidation species.

Like Cures like: Detoxification Effect between Alkali Metals and Sulfur over the V2O5/TiO2 deNOx Catalyst

The V₂O₅/TiO₂ (VTi) catalyst has been widely employed for the NH₃ selective catalytic reduction (NH₃-SCR) reaction, and sulfur (S) and alkali metals (K) were usually considered as poisons during this reaction. In this work, the synergistic effect of S and K over the VTi catalyst for the NH₃-SCR reaction was analyzed and discussed. It is surprisingly observed that the synergistic effects of S and K exhibited a detoxification effect on the NH₃-SCR reaction. That is, although the VTi catalyst exhibited moderate resistance to S poisoning and unsatisfactory resistance to K deactivation, the SCR activity was restored to close to fresh VTi when K and S coexisted. This detoxification effect also could occur between other alkali metals (e.g., Ca and Na) and sulfur. X-ray photoelectron spectroscopy and charge density difference studies both indicate that the introduction of K could significantly affect the electronic structure of V, but this toxic effect was recovered by the further addition of S because of the strong interaction between S and K. Therefore, this detoxification effect can occur in the practical reaction atmosphere, which alleviates the alkali metal poisoning of commercial catalysts.