A Perspective on the Selective Catalytic Reduction (SCR) of NO with NH3 by Supported V2O5–WO3/TiO2 Catalysts

Theoretical insight into H2O impact on V2O5/TiO2 catalysts for selective catalytic reduction of NOx

H2O in flue gas causes the deactivation of V2O5/TiO2 catalysts for selective catalytic reduction (SCR) of NOx with NH3 at low temperatures. Developing water resistance requires understanding the theoretical mechanism of H2O impact on the catalysts. The aim of this work was to clarify the adsorption process of H2O and the deactivation mechanism induced by H2O through density functional theory (DFT). The process of H2O adsorption was studied based on a modeled V2O5/TiO2 catalyst surface. It was found that H2O had a strong interaction with exposed titanium atoms. Water adsorption on the catalyst surface significantly alters the electronic structure of VOx sites, transforming Lewis acid sites into Brønsted acid sites. Exposed titanium sites contribute to the decrease of Lewis acidity via adsorbed water. Ab initio thermodynamic calculations show that H2O adsorption on V2O5/TiO2 is stable at low coverage but less favorable at high coverage. Adsorption of NH3 is the most critical step for the SCR of NOx, and the adsorption of H2O can hinder this process. The H2O coverage below 15% of adsorption sites could enhance the NH3 adsorption rate and have a limited effect on the acidity, while higher coverage impeded the adsorption ability of VOx sites. This work provided electron-scale insight into the adsorption impact of H2O on the surface of V2O5/TiO2 catalysts, presented thermodynamic analysis of the adsorption of H2O and NH3, paving the way for the exploration of V2O5/TiO2 catalysts with improved water resistance.

Plasma-assisted manipulation of vanadia nanoclusters for efficient selective catalytic reduction of NOx

Supported nanoclusters (SNCs) with distinct geometric and electronic structures have garnered significant attention in the field of heterogeneous catalysis. However, their directed synthesis remains a challenge due to limited efficient approaches. This study presents a plasma-assisted treatment strategy to achieve supported metal oxide nanoclusters from a rapid transformation of monomeric dispersed metal oxides. As a case study, oligomeric vanadia-dominated surface sites were derived from the classic supported V2O5-WO3/TiO2 (VWT) catalyst and showed nearly an order of magnitude increase in turnover frequency (TOF) value via an H2-plasma treatment for selective catalytic reduction of NO with NH3. Such oligomeric surface VOx sites were not only successfully observed and firstly distinguished from WOx and TiO2 by advanced electron microscopy, but also facilitated the generation of surface amide and nitrates intermediates that enable barrier-less steps in the SCR reaction as observed by modulation excitation spectroscopy technologies and predicted DFT calculations.

Low-temperature CeCoMnOx spinel-type catalysts prepared by oxalate co-precipitation for selective catalytic reduction of NO using NH3: A structure-activity relationship study

CeCoMnOx spinel-type catalysts for the selective catalytic reduction of NO using NH3 (NH3-SCR) are usually prepared by alkaline co-precipitation. In this paper, a series of CeCoMnOx spinel-type catalysts with different calcination temperatures were prepared by acidic oxalate co-precipitation. The physicochemical structures and NH3-SCR activities of the CeCoMnOx spinel-type catalysts prepared by oxalate co-precipitation and conventional ammonia co-precipitation were systematically compared. The results show that the CeCoMnOx spinel-type catalysts prepared by the oxalate precipitation method (CeCoMnOx-C) have larger specific surface area, more mesopores and surface active sites, stronger redox properties and adsorption activation properties than those prepared by the traditional ammonia co-precipitation method at 400 °C (CeCoMnOx-N-400), and thus CeCoMnOx-C have better low-temperature NH3-SCR performance. At the same calcination temperature of 400 °C, the NO conversion of CeCoMnOx-C-400 exceeds 89 % and approaches 100 % within the reaction temperature of 100-125 °C, which is 14.8 %-2.5 % higher than that of CeCoMnOx-N-400 at 100-125 °C. In addition, the enhanced redox and acid cycle matching mechanisms on the CeCoMnOx-C surface, as well as the enhanced monoadsorption Eley-Rideal (E-R) and double adsorption Langmuir-Hinshelwood (L-H) reaction mechanisms, are also derived from XPS and in situ DRIFTS characterization.

C≡N and N≡O Bond Cleavages of Acetonitrile and Nitrosyl Ligands at a Dimolybdenum Center to Render Ethylidyne and Acetamidinate Ligands

Extended reduction of [Mo2Cp2(μ-Cl)(μ-PtBu2)(NO)2] (1) with Na(Hg) in acetonitrile (MeCN) at room temperature resulted in an unprecedented full cleavage of the C≡N bond of a coordinated MeCN molecule to yield the vinylidene derivative Na[Mo2Cp2(μ-PtBu2)(μ-CCH2)(NO)2], which upon protonation with (NH4)PF6 gave the ethylidyne complex [Mo2Cp2(μ-PtBu2)(μ-CMe)(NO)2] [Mo1-Mo2 = 2.9218(2) Å] in a selective and reversible way. Controlled reduction of 1 at 273 K yielded instead, after protonation, the 30-electron acetamidinate complex [Mo2Cp2(μ-PtBu2)(μ-κN:κN'-HNCMeNH)(μ-NO)]PF6 [Mo1-Mo2 = 2.603(2) Å], in a process thought to stem from the paramagnetic MeCN-bridged intermediate [Mo2Cp2(μ-PtBu2)(μ-NCMe)(NO)2], followed by a complex sequence of elementary steps including cleavage of the N≡O bond of a nitrosyl ligand.

Interface sites on vanadia-based catalysts are highly active for NOx removal under realistic conditions

TiO2-supported V2O5 catalysts are commonly used in NOx reduction with ammonia due to their robust catalytic performance. Over these catalysts, it is generally considered that the active species are mainly derived from the vanadia species rather than the intrinsic structure of V-O-Ti entities, namely the interface sites. To reveal the role of V-O-Ti entities in NH3-SCR, herein, we prepared TiO2/V2O5 catalysts and demonstrated that V-O-Ti entities were more active for NOx reduction under wet conditions than the V sites (V=O) working alone. On the V-O-Ti entities, kinetic measurements and first principles calculations revealed that NH3 activation exhibited a much lower energy barrier than that on V=O sites. Under wet conditions, the V-O-Ti interface significantly inhibited the transformation of V=O to V-OH sites thus benefiting NH3 activation. Under wet conditions, meanwhile, the migration of NH4+ from Ti site neighboring the V-O-Ti interface to Ti site of the V-O-Ti interface was exothermic; thus, V-O-Ti entities together with neighboring Ti sites could serve as channels linking NH3 pool and active centers for activation of NH4+. This finding reveals that the V-O-Ti interface sites on V-based catalysts play a crucial role in NOx removal under realistic conditions, providing a new perspective on NH3-SCR mechanism.

Ce doped V-W/Ti as selective catalytic reduction catalysts for cement kiln flue gas denitration

In cement industry, the selection of catalyst temperature window and the inhibition effect of dust composition in flue gas on catalyst are the key issues of flue gas denitrification. In this article, a pilot study with Ce doped V-W/Ti catalyst on the removal of NOx by selective catalytic reduction with ammonia (NH3-SCR) from the cement kiln flue gas was presented. Cement kiln dust loading on catalysts obviously decreased the NO conversion in the absence of SO2 and H2O, while the denitration efficiency restored from 75 to 98% at 280 ℃ after SO2 and H2O introduced into the reaction system, which mainly because the SO2 may enhance the acidic site on the catalyst surface, and prefer to be bonded with the coordinated Ca species, releasing the active sites poisoned by dust. The NH3-temperature programmed desorption (NH3-TPD), X-ray photoelectron spectroscopy (XPS), and H2-temperature programmed reduction (H2-TPR) detections were performed to reveal that the appropriate Ce and W ratios catalyst contributed better denitrification activity. The optimum ratio of Ce doped catalyst was amplified to form the standard honeycomb monomer catalyst, and then, the activity of catalyst was verified on the side line of cement kiln. The effect of temperature and space velocity on denitrification efficiency was investigated, and the denitration efficiency reached to 92.5% at 300℃ and 3000 h-1 space velocity. Moreover, the life of catalyst was verified and predicted by GM (1,1) grey model. The study realized the innovation from the laboratory data rules to the industrial pilot application, providing positive promoting value for the industrial large-scale demonstration application of the catalyst.

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.

Extraction of W, V, and As from spent SCR catalyst by alkali pressure leaching and the pressure leaching mechanism

Spent selective catalytic reduction (SCR) catalysts are environmentally hazardous and resource-enriching. In this work, V, W, and As in a spent SCR catalyst was extracted by alkali pressure leaching. Results showed that the V, W, and As were loaded on the anatase TiO2 crystal grains as amorphous oxides. The optimum pressure leaching conditions were NaOH concentration of 20 wt%, reaction temperature of 180 °C, reaction time of 120 min, L/S of 10 mL/g, and stirring speed of 300 rpm. The leaching efficiency of W, V, and As reached 98.83%, 100%, and 100%, respectively. The experiment revealed the preferential leaching of V and As rather than W, and the leaching mechanisms of V, W, and As were studied through experiment and density functional theory (DFT). The leaching kinetics of W conformed to a variant of the shrinking core model and the leaching process of W is controlled by both chemical reactions and diffusion processes. During the leaching process, Na2Ti2O4(OH)2 product powder layer was generated, which affects the mass transfer of W. The destruction of the TiO2 skeleton in the spent SCR catalyst is essential for adequate W extraction, especially for the extraction of W embedded in the TiO2 lattice. The DFT simulation result indicated that the V and As loaded onto the TiO2 support are easier to absorb hydroxide ions rather than W, and the leaching reaction energy of V and As was lower than W, As, and V has leaching priority over the leaching of W. Furthermore, an anatase TiO2 photocatalyst with the {001} crystal surface exposed was successfully prepared from the alkali pressure leaching residue. This work provides theoretical support for the metal leaching and utilization of spent SCR catalysts via alkali pressure leaching.

Uncovering the Dinuclear Mechanism of NO2-Involved NH3-SCR over Supported V2O5/TiO2 Catalysts

Commercial vanadium oxide catalysts exhibit high efficiency for the selective catalytic reduction (SCR) of NO with NH3, especially in the presence of NO2 (i.e., occurrence of fast NH3-SCR). The high-activity sites and their working principle for the fast NH3-SCR reaction, however, remain elusive. Here, by combining in situ spectroscopy, isotopic labeling experiments, and density functional theory (DFT) calculations, we demonstrate that polymeric vanadyl species act as the main active sites in the fast SCR reaction because the coupling effect of the polymeric structure alters the elementary reaction step and effectively avoids the high energy barrier of the rate-determining step over monomeric vanadyl species. This study unveils the high-activity dinuclear mechanism of the NO2-involved SCR reaction over vanadia-based catalysts and provides a fundamental basis for developing high-efficiency and low V2O5-loading SCR catalysts.

Mechanism of NH3-SCR over P/CeO2 Catalysts Investigated by Operando Spectroscopies

This study reports a comprehensive investigation into the active sites and reaction mechanism for the selective catalytic reduction of NO by NH3 (NH3-SCR) over phosphate-loaded ceria (P/CeO2). Catalyst characterization and density functional theory calculations reveal that H3PO4 and H2P2O6 species are the dominant phosphate species on the P/CeO2 catalysts under the experimental conditions. The reduction/oxidation half-cycles (RHC/OHC) were investigated using in situ X-ray absorption near-edge structure for Ce L3-edge, ultraviolet-visible, and infrared (IR) spectroscopies together with online analysis of outlet products (operando spectroscopy). The Ce4+(OH-) species, possibly adjacent to the phosphate species, are reduced by NO + NH3 to produce N2, H2O, and Ce3+ species (RHC). The Ce3+ species is reoxidized by aqueous O2 (OHC). The results from IR spectroscopy suggest that the RHC initiates with the reaction between NO and Ce4+(OH-) to yield Ce3+ and gaseous HONO, which then react with NH3 to produce N2 and H2O via NH4NO2 intermediates.

Facile H2O-Contributed O2 Activation Strategy over Mn-Based SCR Catalysts to Counteract SO2 Poisoning

Mn-based catalysts preferred in low-temperature selective catalytic reduction (SCR) are susceptible to SO2 poisoning. The stubborn sulfates make insufficient O2 activation and result in deficient reactive oxygen species (ROS) for activating reaction molecules. H2O has long been regarded as an accomplice to SO2, hastening catalyst deactivation. However, such a negative impression of the SCR reaction was reversed by our recent research. Here, we reported a H2O contribution over Mn-based SCR catalysts to counteract SO2 poisoning through accessible O2 activation, in which O2 was synergistically activated with H2O to generate ROS for less deactivation and more expected regeneration. The resulting ROS benefited from the energetically favorable route supported by water-induced Ea reduction and was actively involved in the NH3 activation and NO oxidation process. Besides, ROS maintained high stability over the SO2 + H2O-deactivated γ-MnO2 catalyst throughout the mild thermal treatment, achieving complete regeneration of its own NO disposal ability. This strategy was proven to be universally applicable to other Mn-based catalysts.

Ultrahighly Alkali-Tolerant NOx Reduction over Self-Adaptive CePO4/FePO4 Catalysts

Catalyst deactivation caused by alkali metal poisoning has long been a key bottleneck in the application of selective catalytic reduction of NOx with NH3 (NH3-SCR), limiting the service life of the catalyst and increasing the cost of environmental protection. Despite great efforts, continuous accumulation of alkali metal deposition makes the resistance capacity of 2 wt % K2O difficult to enhance via merely loading acid sites on the surface, resulting in rapid deactivation and frequent replacement of the NH3-SCR catalyst. To further improve the resistance of alkali metals, encapsulating alkali metals into the bulk phase could be a promising strategy. The bottleneck of 2 wt % K2O tolerance has been solved by virtue of ultrahigh potassium storage capacity in the amorphous FePO4 bulk phase. Amorphous FePO4 as a support of the NH3-SCR catalyst exhibited a self-adaptive alkali-tolerance mechanism, where potassium ions spontaneously migrated into the bulk phase of amorphous FePO4 and were anchored by PO43- with the generation of Fe2O3 at the NH3-SCR reaction temperature. This ingenious potassium storage mechanism could boost the K2O resistance capacity to 6 wt % while maintaining approximately 81% NOx conversion. Besides, amorphous FePO4 also exhibited excellent resistance to individual and coexistence of alkali (K2O and Na2O), alkali earth (CaO), and heavy metals (PbO and CdO), providing long durability for CePO4/FePO4 catalysts in flue gas with multipollutants. The cheap and accessible amorphous FePO4 paves the way for the development and implementation of poisoning-resistant NOx abatement.

Universal Water Disinfection by the Introduction of Fe-N3 Traps between g-C3N4 Layers under Visible Light

Efficient inactivation of bacteria in the sewage via a photocatalytic process represents a promising strategy for the efficient chemical utilization of solar energy. Herein, uniformly dispersed Fe atoms were embedded between layers of g-C3N4 photocatalysts (CNFx), which were facilely prepared by thermal treatment. The optimized photocatalyst (CNF100) first showed excellent photoactivity for killing a variety of bacteria (93.0% for E. coli, 93.9% for Salmonella, and 96.2% for S. aureus) under visible light irradiation. The superior activity can be attributed to the formation of shallow electron traps (Fe-N3) that can capture excitons of excited states, which promote the charge transfer and energy transfer process of activated adsorbed molecular oxygen, respectively, forming reactive oxygen species, improving separation efficiency of photoexcited electrons and holes, and the Fe-N3 traps can also be used as photosensitive sites to broaden the absorption range of visible light. This strategy of constructing shallow electronic traps lays a theoretical foundation for the design of new environmentally friendly and efficient water disinfectants.

Heterogeneity of the vanadia phase dispersed on titania. Co-existence of distinct mono-oxo VOx sites

The structural and configurational characteristics of the species comprising the (VO_x)_n phase dispersed on TiO₂(P25) are studied under oxidative dehydration conditions by in situ molecular vibrational spectroscopy (Raman, FTIR) complemented by in situ Raman/¹⁸O isotope exchange and Raman spectroscopy under static equilibrium at temperatures of 175-430 °C and coverages in the 0.40-5.5 V nm^-2 range. It is found that the dispersed (VO_x)_n phase consists of distinct species with different configurations. At low coverages of 0.40 and 0.74 V nm^-2, isolated (monomeric) species prevail. Two distinct mono-oxo species are found: (i) a majority Species-I, presumably of distorted tetrahedral OV(-O-)₃ configuration with VO mode at 1022-1024 cm^-1 and (ii) a minority Species-II, presumably of distorted octahedral-like OV(-O-)₄ configuration with VO mode at 1013-1014 cm^-1. Cycling the catalysts in the 430 → 250 → 175 → 430 °C sequence results in temperature-dependent structural transformations. With decreasing temperature, a Species-II → Species-I transformation with concomitant surface hydroxylation takes place by means of a hydrolysis mechanism mediated by water molecules retained by the surface. A third species (Species-III, presumably of di-oxo configuration with ν_s/ν_as at ∼995/985 cm^-1) occurs in minority and its presence is increased when further lowering the temperature according to a Species-I → Species-III hydrolysis step. Species-II (OV(-O-)₄) shows the highest reactivity to water. For coverages above 1 V nm^-2, an association of VO_x units takes place leading to gradually larger polymeric domains when the coverage is increased in the 1.1-5.5 V nm^-2 range. Polymeric (VO_x)_n domains comprise building units that maintain the structural characteristics (termination configuration and V coordination number) of Species-I, Species-II, and Species-III. The terminal VO stretching modes are blue-shifted with increasing (VO_x)_n domain size. A lower extent of hydroxylation is evidenced under static equilibrium forced dehydrated conditions, thereby limiting the temperature dependent structural transformations and excluding the possibility of incoming water vapors as the cause for the temperature dependent effects observed in the in situ Raman/FTIR spectra. The results address open issues and offer new insight in the structural studies of VO_x/TiO₂ catalysts.

Improved N2 Selectivity of MnOx Catalysts for NOx Reduction by Engineering Bridged Mn3+ Sites

Mn-based catalysts are promising for selective catalytic reduction (SCR) of NO_x with NH₃ at low temperatures due to their excellent redox capacity. However, the N₂ selectivity of Mn-based catalysts is an urgent problem for practical application owing to excessive oxidizability. To solve this issue, we report a Mn-based catalyst using amorphous ZrTiO_x as the support (Mn/ZrTi-A) with both excellent low-temperature NO_x conversion and N₂ selectivity. It is found that the amorphous structure of ZrTiO_x modulates the metal-support interaction for anchoring the highly dispersed active MnO_x species and constructs a uniquely bridged Mn³⁺ bonded with the support through oxygen linked to Ti⁴⁺ and Zr⁴⁺, respectively, which regulates the optimal oxidizability of the MnO_x species. As a result, Mn/ZrTi-A is not conducive to the formation of ammonium nitrate that readily decomposes to N₂O, thus further increasing N₂ selectivity. This work investigates the role of an amorphous support in promoting the N₂ selectivity of a manganese-based catalyst and sheds light on the design of efficient low-temperature deNO_x catalysts.

NO Selective Catalytic Reduction over Atom-Pair Active Sites Accelerated via In Situ NO Oxidation

Selective catalytic reduction (SCR) of NO_x with NH₃ is the most efficient technology for NO_x emissions control, but the activity of catalysts decreases exponentially with the decrease in reaction temperature, hindering the application of the technology in low-temperature SCR to treat industrial stack gases. Here, we present an industrially practicable technology to significantly enhance the SCR activity at low temperatures (<250 °C). By introducing an appropriate amount of O₃ into the simulated stack gas, we find that O₃ can stoichiometrically oxidize NO to generate NO₂, which enables NO reduction to follow the fast SCR mechanism so as to accelerate SCR at low temperatures, and, in particular, an increase in SCR rate by more than four times is observed over atom-pair V₁-W₁ active sites supported on TiO₂(001) at 200 °C. Using operando SCR tests and in situ diffuse reflectance infrared Fourier transform spectra, we reveal that the introduction of O₃ allows SCR to proceed along a NH₄NO₃-mediated Langmuir-Hinshelwood model, in which the adsorbed nitrate species speed up the re-oxidation of the catalytic sites that is the rate-limiting step of SCR, thus leading to the enhancement of activity at low temperatures. This technology could be applicable in the real stack gas conditions because O₃ exclusively oxidizes NO even in the co-presence of SO₂ and H₂O, which provides a general strategy to improve low-temperature SCR efficacy from another perspective beyond designing catalysts.

Effect of Cu loading content on the catalytic performance of Cu-USY catalysts for selective catalytic reduction of NO with NH3

Series of Cu-USY zeolite catalyst with different Cu loading content were synthesized through simple impregnation method. The obtained catalysts were subjected to selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) performance evaluation, structural/chemical characterizations such as X-ray diffraction (XRD), N₂ adsorption/desorption, H₂ temperature-programmed reduction (H₂-TPR), NH₃ temperature-programmed desorption (NH₃-TPD) as well as detailed in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments including CO adsorption, NH₃ adsorption and NO+O₂ in situ reactions. Results show that Cu-USY with proper Cu loading (in this work 5Cu-USY with 5 wt.% Cu) could be promising candidates with highly efficient NH₃-SCR catalytic performance, relatively low byproduct formation and excellent hydrothermal stability, although its SO₂ poisoning tolerability needs alleviation. Further characterizations reveal that such catalytic advantages can be attributed to both active cu species and surface acid centers evolution modulated by Cu loading. On one hand, Cu species in the super cages of zeolites increases with higher Cu content and being more conducive for NH₃-SCR reactivity. On the other hand, higher Cu loading leads to depletion of Brønsted acid centers and simultaneous formation of abundant Lewis acid centers, which facilitates NH₄NO₃ reduction via NH₃ adsorbed on Lewis acid centers, thus improving SCR reactivity. However, Cu over-introduction leads to formation of surface highly dispersed CuO_x, causing unfavorable NH₃ oxidation and inferior N₂ selectivity.

Revealing the crystal facet effect on N2O formation during the NH3-SCR over α-MnO2 catalysts

The detailed atomic-level mechanism of the effect induced by engineering the crystal facet of α-MnO₂ catalysts on N₂O formation during ammonia-selective catalytic reduction (NH₃-SCR) was ascertained by combining density functional theory (DFT) calculations and thermodynamics/kinetic analysis. The surface energies of α-MnO₂ with specific (100), (110), and (310) exposed planes were calculated, and the adsorptions of NH₃, NO, and O₂ on three surfaces were analyzed. The adsorption energies showed that NH₃ and NO molecules could be strongly adsorbed on the surface of the α-MnO₂ catalyst, while the adsorption of O₂ was weak. Moreover, the key steps in the oxidative dehydrogenation of NH₃ and the formation of NH₂NO as well as dissociation of NH₂ were studied to evaluate the catalytic ability of NH₃-SCR reaction and N₂ selectivity. The results revealed that the α-MnO₂ catalyst exposed with the (310) plane exhibited the best NH₃-SCR catalytic performance and highest N₂ selectivity, mainly due to its low energy barriers in NH₃ dehydrogenation and NH₂NO generation, and difficulty in NH₂ dissociation. This study deepens the comprehension of the facet-engineering of α-MnO₂ on inhibiting N₂O formation during the NH₃-SCR, and points out a strategy to improve their catalytic ability and N₂ selectivity for the low-temperature NH₃-SCR process.

A comprehensive review of the heavy metal issues regarding commercial vanadium‑titanium-based SCR catalyst

Facing the increasing demand of atmosphere pollutant control, selective catalytic reduction (SCR) technology has been widely applied in various industries for NO_x abatement. However, in the condition of complicated flue gas components, the heavy metal issue is a great challenge to the catalyst deactivation and atmospheric pollution control. In this review, with the comprehensive consideration of SCR catalysts in heavy metal-rich flue gas scenarios, the distribution character of heavy metals in SCR system is firstly summarized, then the detailed interaction mechanism between heavy metals and the vanadium‑titanium-based catalyst is discussed. Focusing on the mercury oxidation as well as against arsenic/lead poisoning, certain modification strategies are also concluded to develop novel SCR catalysts with multiple functions. Furthermore, the state-of-the-art technologies regarding the regeneration, the valuable metal recovery, and the harmless treatment of the spent SCR catalyst are also reported. This paper provides theoretical guidance for the manufacture of novel SCR catalysts under multiple scenarios, as well as the synergistic control of NO_x and heavy metals.

Hydrothermal Aging Mechanism and Modeling for SCR Catalysts

Based on the activity evaluation and characterization test, we explored the hydrothermal aging mechanism of a vanadium-based SCR catalyst and constructed a dual-site hydrothermal aging kinetic model in this study. The vanadium-based catalyst contains Brønsted acidic sites and Lewis acidic sites, which show different sensitivities to hydrothermal aging, and the reduction of active sites is the main reason for the NOx conversion efficiency reduction after hydrothermal aging. The ammonia storage capacities of both sites have a high correlation coefficient with the NOx conversion efficiency. Based on the method of NH₃-TPD curve peak resolution, we quantified the transformations of the two active sites and established the relationship between the site density, the aging temperature, and the duration to determine the aging factor. Then, a hydrothermal aging kinetic model was constructed, and the parameter identification and verification of the model were carried out through flow reactor experiments. The results show that the model constructed in this study can accurately reflect the catalyst activity after hydrothermal aging.

Assignment of Vibrational Bands of Critical Surface Species Containing Nitrogen in the Selective Catalytic Reduction of NO by NH3

The selective catalytic reduction (SCR) of NO by NH₃ on metal oxides plays a key role in minimizing NO_x emissions. Electronic structure calculations at the density functional theory level have been performed to predict the vibrational modes of NH₃/NH₄⁺ bound to validated cluster models of vanadium oxide bound to a TiO₂ surface. Excellent agreement of the scaled calculated values with the observed bands attributed to surface-bound species is found. The presence of NH₃ bound to Lewis acid sites and NH₄⁺ bound to Brønsted acid sites when VOH groups are present is supported by our predictions. NH₄⁺ is expected to dominate the spectra even at low concentrations, with predicted intensities 5 to 30 times greater than those predicted for surface-bound NH₃. This is particularly evident in the lowest-energy N-H stretches of surface NH₄⁺ due to partial proton transfer interactions with the vanadium oxide surface model. The current work is consistent with experimental vibrational spectroscopy results and does not support the presence of a significant amount of NH₂ on the catalyst surface for the SCR reaction on VO_x/TiO₂. The combined experimental and computational results support the presence of both NH₃- and NH₄⁺-type species bound to the surface.

Extraordinary deactivation offset effect of zinc and arsenic on V2O5 -WO3/TiO2 catalysts: Like cures like

The commercial V₂O₅ -WO₃/TiO₂ (VWTi) catalysts often suffer from a serious joint deactivation by multiple heavy metals in the flue gas for NO_x removal by NH₃-SCR. Herein, we report an extraordinary deactivation offset effect between Zn and As on VWTi with alleviation of the toxic effects of the heavy metals by "like cures like". With the As&Zn content of 4 wt%, VWTi-As&Zn exhibited over 97% NO conversion under a GHSV of 100,000 h^-1 and good SO₂/H₂O tolerance (> 93% NO conversion). It's presented 85% of fresh VWTi, exceeding those of VWTi-Zn (15%) by 5.6-fold and VWTi-As (70%) by 1.2-fold. Structure analysis showed that, unlike VWTi-As and VWTi-Zn, the VO vibration and dispersion state of VO_x sites over VWTi-As&Zn were hardly affected. Moreover, VWTi-As&Zn possessed both the Lewis and Brønsted acid sites while VWTi-Zn and VWTi-As had only one type of them. The operando infrared/Raman/UV-vis spectroscopy and DFT calculations verified that the less affected VO_x sites mainly reflected in three aspects: 1) the electron interaction between As and Zn; 2) the active VO Lewis acid sites; 3) lower energy barrier for N - H bond breaking. The "like cures like" phenomenon may open up an innovative pathway for the control of hazardous heavy metals.

Advances in emission control of diesel vehicles in China

Diesel vehicles have caused serious environmental problems in China. Hence, the Chinese government has launched serious actions against air pollution and imposed more stringent regulations on diesel vehicle emissions in the latest China VI standard. To fulfill this stringent legislation, two major technical routes, including the exhaust gas recirculation (EGR) and high-efficiency selective catalytic reduction (SCR) routes, have been developed for diesel engines. Moreover, complicated aftertreatment technologies have also been developed, including use of a diesel oxidation catalyst (DOC) for controlling carbon monoxide (CO) and hydrocarbon (HC) emissions, diesel particulate filter (DPF) for particle mass (PM) emission control, SCR for the control of NOx emission, and an ammonia slip catalyst (ASC) for the control of unreacted NH₃. Due to the stringent requirements of the China VI standard, the aftertreatment system needs to be more deeply integrated with the engine system. In the future, aftertreatment technologies will need further upgrades to fulfill the requirements of the near-zero emission target for diesel vehicles.

An Atom-Pair Design Strategy for Optimizing the Synergistic Electron Effects of Catalytic Sites in NO Selective Reduction

Effective adsorption and speedy surface reactions are vital requirements for efficient active sites in catalysis, but it remains challenging to maximize these two functions simultaneously. We present a solution to this issue by designing a series of atom-pair catalytic sites with tunable electronic interactions. As a case study, NO selective reduction occurring on V₁ -W₁ /TiO₂ is chosen. Experimental and theoretical results reveal that the synergistic electron effect present between the paired atoms enriches high-energy spin charge around the Fermi level, simultaneously rendering reactant (NH₃ or O₂ ) adsorption more effective and subsequent surface reactions speedier as compared with single V or W atom alone, and hence higher reaction rates. This strategy enables us to rationally design a high-performance V₁ -Mo₁ /TiO₂ catalyst with optimized vanadium(IV)-molybdenum(V) electronic interactions, which has exceptional activity significantly higher than the commercial or reported catalysts.

Nitrogen atom coordination tuned transition metal catalysts for NO oxidation and reduction

Developing an efficient catalyst for NO oxidation and reduction at ambient temperature is a significant challenge. Recent studies have suggested that the N-coordinated transition metal (TM) single atom catalysts (SACs) have high catalytic activity and stability. Herein, we report the activation potential of a series of 3d TM atoms supported on N coordination-tuned graphene (GR) for NO oxidation and reduction. The results show that the N coordination pattern can greatly alter the catalytic reactivity of TM on the catalysts, and the TM atom on the catalysts with three-coordinated pyridinic nitrogen TM-N₃@GR exhibit the strongest chemical activity. Among the TM-N₃@GR catalysts, Ti-N₃@GR is the most promising candidate. The rate constants and equilibrium constants were calculated to evaluate the kinetic and thermodynamic feasibility of the catalytic reaction, respectively. Our results demonstrate that the reduction of NO to N₂ on Ti-N₃@GR can occur at ambient temperature with a large exotherm of 6.99 eV, and the oxidation of NO to NO₂ on Ti-N₃@GR can easily proceed when the temperature reaches 360 K with a large equilibrium constant. Our studies are of great significance for understanding the performance of N coordination-tuned catalysts and designing Ti-based catalysts for NO oxidation and reduction.

Recent trends in vanadium-based SCR catalysts for NOx reduction in industrial applications: stationary sources

Vanadium-based catalysts have been used for several decades in ammonia-based selective catalytic reduction (NH₃-SCR) processes for reducing NO_x emissions from various stationary sources (power plants, chemical plants, incinerators, steel mills, etc.) and mobile sources (large ships, automobiles, etc.). Vanadium-based catalysts containing various vanadium species have a high NO_x reduction efficiency at temperatures of 350-400 °C, even if the vanadium species are added in small amounts. However, the strengthening of NO_x emission regulations has necessitated the development of catalysts with higher NO_x reduction efficiencies. Furthermore, there are several different requirements for the catalysts depending on the target industry and application. In general, the composition of SCR catalyst is determined by the components of the fuel and flue gas for a particular application. It is necessary to optimize the catalyst with regard to the reaction temperature, thermal and chemical durability, shape, and other relevant factors. This review comprehensively analyzes the properties that are required for SCR catalysts in different industries and the development strategies of high-performance and low-temperature vanadium-based catalysts. To analyze the recent research trends, the catalysts employed in power plants, incinerators, as well as cement and steel industries, that emit the highest amount of nitrogen oxides, are presented in detail along with their limitations. The recent developments in catalyst composition, structure, dispersion, and side reaction suppression technology to develop a high-efficiency catalyst are also summarized. As the composition of the vanadium-based catalyst depends mostly on the usage in stationary sources, various promoters and supports that improve the catalyst activity and suppress side reactions, along with the studies on the oxidation state of vanadium, are presented. Furthermore, the research trends related to the nano-dispersion of catalytically active materials using various supports, and controlling the side reactions using the structure of shaped catalysts are summarized. The review concludes with a discussion of the development direction and future prospects for high-efficiency SCR catalysts in different industrial fields.

Monolith or Powder: Improper Sample Pretreatment May Mislead the Understanding of Industrial V2O5-WO3/TiO2 Catalysts Operated in Stationary Resources

Although various characterizations are widely applied to commercial V₂O₅-WO₃/TiO₂ catalysts, the influence of the catalyst physical structure, i.e., monolith or powder, on the characterization results has not been investigated. Several important catalytic behaviors and phenomena were observed in this study using V₂O₅-WO₃/TiO₂ monolithic catalysts employed for over 5000 h in various stationary flue gases, and many of the results were only observable on monolithic catalysts, such as depth-dependent distribution of external elements, penetration of As₂O₃, and the formation of Tl₂O-TiO₂ p-n junctions. If the monolith is ground into powder states, it will alter or destroy the catalyst surface and remove important clues closely related to catalytic performance under working conditions. The redox and acidity properties of V₂O₅-WO₃/TiO₂ obtained from powder samples may be significantly different from their true state under working conditions, resulting in a misperception of catalyst performance. Therefore, a cautious pretreatment should be taken into careful consideration when analyzing commercial honeycomb V₂O₅-WO₃/TiO₂ catalysts.

Polyol-Mediated Synthesis of V2O5-WO3/TiO2 Catalysts for Low-Temperature Selective Catalytic Reduction with Ammonia

We demonstrated highly efficient selective catalytic reduction catalysts by adopting the polyol process, and the prepared catalysts exhibited a high nitrogen oxide (NO_X) removal efficiency of 96% at 250 °C. The V₂O₅ and WO₃ catalyst nanoparticles prepared using the polyol process were smaller (~10 nm) than those prepared using the impregnation method (~20 nm), and the small catalyst size enabled an increase in surface area and catalytic acid sites. The NO_X removal efficiencies at temperatures between 200 and 250 °C were enhanced by approximately 30% compared to those of the catalysts prepared using the conventional impregnation method. The NH₃-temperature-programmed desorption and H₂-temperature-programmed reduction results confirmed that the polyol process produced more surface acid sites at low temperatures and enhanced the redox ability. The in situ Fourier-transform infrared spectra further elucidated the fast absorption of NH₃ and its reduction with NO and O₂ on the prepared catalyst surfaces. This study provides an effective approach to synthesizing efficient low-temperature SCR catalysts and may contribute to further studies related to other catalytic systems.

Insights of Selective Catalytic Reduction Technology for Nitrogen Oxides Control in Marine Engine Applications

The international shipping industry is facing increasingly stringent limitations on nitrogen oxide (NOx) emissions. New solutions for reducing NOx emitted by marine engines need to be investigated to find the best technology. Selective Catalytic Reduction (SCR) is an advanced active emissions control technology successfully used in automotive diesel engines; it could be applied to marine engines with ad-hoc solutions to integrate it in the exhaust of large engines. In this study, a commercial SCR was tested at the exhaust of a diesel engine in inlet gas conditions typical of a marine engine. The SCR system consisted of a custom monolith (provided by Hug-Engineering AG) that enabled seamless integration for a broad range of engine sizes; the active phases were V2O5 (3 wt%)-WO3 (7 wt%)-TiO2 (75 wt%). The monolith was studied at the laboratory scale for its in-depth chemical/physical characterization and by means of an intermediate-scale engine, reproducing the exhaust gas conditions of a full-scale marine engine. The system’s effectiveness in terms of NOx removal for the selected engine operating conditions was evaluated in a wide range of temperature and NOx emissions values and for different quantities of the reduction agent (AdBlue or ammonia) added to exhaust gases. The investigated technological solution resulted in efficient NOx emission control from a marine engine.

Chemistry of a Nitrosyl Ligand κ:η-Bridging a Ditungsten Center: Rearrangement and N–O Bond Cleavage Reactions

The novel nitrosyl-bridged complex [W₂Cp₂(μ-P^tBu₂)(μ-κ:η-NO)(CO)(NO)](BAr₄) [Ar = 3,5-C₆H₃(CF₃)₂] was prepared in a multistep procedure starting from the hydride [W₂Cp₂(μ-H)(μ-P^tBu₂)(CO)₄] and involving the new complexes [W₂Cp₂(μ-P^tBu₂)(CO)₄](BF₄), [W₂Cp₂(μ-P^tBu₂)(CO)₂(NO)₂](BAr₄), and [W₂(μ-κ:η⁵-C₅H₄)Cp(μ-P^tBu₂)(CO)(NO)₂] as intermediates, which follow from reactions with HBF₄·OEt₂, NO, and Me₃NO·2H₂O, respectively. The nitrosyl-bridged cation easily added chloride upon reaction with [N(PPh₃)₂]Cl, with concomitant NO rearrangement into the terminal coordination mode, to give [W₂ClCp₂(μ-P^tBu₂)(CO)(NO)₂], and underwent N–O and W–W bond cleavages upon the addition of CN^tBu to give the mononuclear phosphinoimido complex [WCp(NP^tBu₂)(CN^tBu)₂](BAr₄). Another N–O bond cleavage was induced upon photochemical decarbonylation at 243 K, which gave the oxo- and phosphinito-bridged nitrido complex [W₂Cp₂(N)(μ-O)(μ-OP^tBu₂)(NO)](BAr₄), likely resulting from a N–O bond cleavage step following decarbonylation.

The π binding of the NO ligand in the cation [W₂Cp₂(μ-P^tBu₂)(μ-κ:η-NO)(CO)(NO)]⁺ facilitates the addition of ligands with concomitant rearrangement of the bridging nitrosyl into the terminal coordination mode and also facilitates cleavage of the N−O bond of that ligand at low temperature possibly in two different ways: either through the oxidative addition of this ligand to the dimetal center or through deoxygenation by another ligand.

Formic Acid-Mediated Regeneration Strategy for As-Poisoned V2O5-WO3/TiO2 Catalysts with Lossless Catalytic Activity and Simultaneous As Recycling

Regeneration of spent V₂O₅-WO₃/TiO₂ catalysts is highly desirable, especially for those containing hypertoxic As, which is categorized as hazardous waste. However, common solution-leaching methods suffer from the trade-off between As removal and V₂O₅ retention, and it would be necessary to introduce extra proceedings like ingredients reimplantation and As-bearing waste treatment after regeneration. Herein, a formic acid-mediated regeneration strategy has been developed to achieve superior catalytic activity, short timescale regeneration, and nontoxic metallic As recycling with controllable and safe conduction. The specific activity of the optimal regenerated catalyst reaches 98.3% of the fresh catalyst with 99.1% As removal and less than 1.8% V loss within 15 min. Structure characterizations reveal that the distorted VO_x molecular structure, surface acidity, and redox property recover to the fresh level after regeneration. In situ investigation of the regeneration process indicates that As-OH removal together with V-OH generation occurs at the first regeneration stage, followed by the active center V═O sites over-reduction at the second stage. The retained V═O species by suitable regeneration temperature and time are essential for NH₃-selective catalytic reduction (SCR) since As existence and VO_x over-reduction will separately cause unstable and excessive NH₃ adsorption to further suppress the reaction cycle. The developed strategy and improved understanding of active site protection would exert benefits on the development of efficient and time-saving regeneration methods for spent catalysts.

Selective Catalytic Removal of High Concentrations of NOx at Low Temperature

Three vanadium-based catalysts were used to remove high concentrations of nitrogen oxides, and the catalysts’ performance of de-NOx and anti-H2O under the high concentrations of NOx were investigated. The V-Mo-W/TiO2 catalysts were tested under 1500 mL/min gas flow (GHSV = 500 h−1, 2.4% NO2, 4.78% NH3, 13% O2, 4% H2O, 5% CO2) and characterized by BET, SEM, EDS, XRD, XPS, H2-TPR, and NH3-TPD; then, their physical and chemical properties were analyzed. The results showed that under the influence of H2O, the NOx conversion of the V-Mo-W/TiO2 catalysts remained above 97% at 200–280 °C indicating that the catalysts had high catalytic activity and strong water resistance. The analysis of the characterization results showed that the larger specific surface area of the catalyst, the higher acid content, stronger redox ability, and higher V4+ and V3+ content were the reasons for the high NOx conversion. The surface area decreased and the microstructure become smoother after the reaction, which may be caused by thermal sintering, but the overall morphology did not change. Comparing the H2-TPR and NH3-TPD of V1.6Mo1.7W1.8/TiO2 before and after NH3-SCR reaction, it was found that the reduction peak and the intensity of the acid sites of the sample had not changed, which indicated that the catalyst had good anti-sintering performance and a long lifetime. This is significant for followup long-term engineering application experiments.

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.

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.