Sulfur-Resistant Ceria-Based Low-Temperature SCR Catalysts with the Non-bulk Electronic States of Ceria

Variant Properties of Tungsten Species over CeO2 Induced by Different Lattice Planes for the Selective Catalytic Reduction of Nitric Oxide

Although ceria-based catalysts have been proven to be a promising alternative to conventional vanadyl catalysts for the selective catalytic reduction of NOx with NH3 (NH3-SCR), whether the interaction between tungsten (W) and CeO2 could be well established for enhancing SCR performance is still unclear. Herein, W/CeO2 catalysts with different CeO2 morphologies for SCR are fully investigated. Systematic characterization results confirmed that the crystalline WO3 phase was formed on W-loaded CeO2 nanocubes, nanospindles, and nanospheres. Intriguingly, the W/CeO2 nanorods exhibited exclusively polymeric tungstate species. This divergence originates from the distinct W-CeO2 interaction mediated by preferentially exposed {110} lattice planes, endowing the nanorod catalyst with stronger reducibility, more surface-active oxygen species, and acid sites. Consequently, the NH3-SCR activity followed the order W/CeO2 nanorods > W/CeO2 nanospindles > W/CeO2 nanocubes > W/CeO2 nanospheres. This work reveals the significant role of the CeO2 morphology in the dispersion and interaction of W species, which directly influences the NH3-SCR performance. The insight offers a valuable opportunity for the design of a more efficient SCR catalyst.

Environmental Catalysis for NOx Reduction by Manipulating the Dynamic Coordination Environment of Active Sites

Nowadays, it is challenging to achieve SO2-tolerant environmental catalysis for NOx reduction because of the thermodynamically favorable transformation of reactive sites to inactive sulfate species in the presence of SO2. Herein, we achieve enhanced low-temperature SO2-tolerant NOx reduction by manipulating the dynamic coordination environment of active sites. Engineered by coordination chemistry, SiO2-CeO2 composite oxides with a short-range ordered Ce-O-Si structure were elaborately constructed on a TiO2 support. A dynamic coordination environment of active sites is demonstrated from a Ce-O-Si local structure to a low-coordinated Ce-SO42- species in the presence of SO2. The low-coordinated Ce-SO42- species as new active sites maintain a high NO removal efficiency by preserving the good adsorption and activation capacity of NO and NH3 reactants. This work proposes a new notion to improve the SO2 resistance of catalysts by regulating the coordination environment of sulfated active sites, which is of significance for SO2-tolerant environmental catalysis in practical applications.

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

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

Atom Pairing Enhances Sulfur Resistance in Low-Temperature SCR via Upshifting the Lowest Unoccupied States of Cerium

Environmentally benign cerium-based catalysts are promising alternatives to toxic vanadium-based catalysts for controlling NOx emissions via selective catalytic reduction (SCR), but conventional cerium-based catalysts unavoidably suffer from SO2 poisoning in low-temperature SCR. We develop a strongly sulfur-resistant Ce1+1/TiO2 catalyst by spatially confining Ce atom pairs to different anchoring sites of anatase TiO2(001) surfaces. Experimental results combined with theoretical calculations demonstrate that strong electronic interactions between the paired Ce atoms upshift the lowest unoccupied states to an energy level higher than the highest occupied molecular orbital (HOMO) of SO2 so as to be catalytically inert in SO2 oxidation but slightly lower than HOMO of NH3 so that Ce1+1/TiO2 has desired ability toward NH3 activation required for SCR. Hence, Ce1+1/TiO2 shows higher SCR activity and excellent stability in the presence of SO2 at low temperatures with respect to supported single Ce atoms. This work provides a general strategy to develop sulfur-resistant catalysts by tuning the electronic states of active sites for low-temperature SCR, which has implications for practical applications with energy-saving requirements.

Asymmetric Coordination of Single-Atom Ru Sites Achieves Efficient N(sp3)-H Dehydrogenation Catalysis for Ammonia Oxidation

Ruthenium single-atom catalysts have great potential in ammonia-selective catalytic oxidation (NH3-SCO); however, the stable sp3 hybrid orbital of NH3 molecules makes N(sp3)-H dissociation a challenge for conventional symmetrical metallic oxide catalysts. Herein, we propose a heterogeneous interface reverse atom capture strategy to construct Ru with unique asymmetric Ru1N2O1 coordination. Ru1N2O1/CeO2 exhibits intrinsic low-temperature conversion (T100 at 160 °C) compared to symmetric coordinated Ru-based (280 °C), Ir-based (220 °C), and Pt-based (200 °C) catalysts, and the TOF is 65.4 times that of Ag-based catalysts. The experimental and theoretical studies show that there is a strong d-p orbital interaction between Ru and N atoms, which not only enhances the adsorption of ammonia at the Ru1N2O1 position but also optimizes the electronic configuration of Ru. Furthermore, the affinity of Ru1N2O1/CeO2 to water is significantly weaker than that of conventional catalysts (the binding energy of the Pd3Au1 catalyst is -1.19 eV, but it is -0.39 eV for our material), so it has excellent water resistance. Finally, the N(sp3)-H activation of NH3 requires the assistance of surface reactive oxygen species, but we found that asymmetric Ru1N2O1 can directly activate the N(sp3)-H bond without the involvement of surface reactive oxygen species. This study provides a novel principle for the rational design of the proximal coordination of active sites to achieve its optimal catalytic activity in single-atom catalysis.

Unlocking Mixed-Metal Oxides Active Centers via Acidity Regulation for K&SO2 Poisoning Resistance: Self-Detoxification Mechanism of Zeolite-Confined deNOx Catalysts

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) is an efficient NOx reduction strategy, while the denitrification (deNOx) catalysts suffer from serious deactivation due to the coexistence of multiple poisoning substances, such as alkali metal (e.g., K), SO2, etc., in industrial flue gases. It is essential to understand the interaction among various poisons and their effects on the deNOx process. Herein, the ZSM-5 zeolite-confined MnSmOx mixed (MnSmOx@ZSM-5) catalyst exhibited better deNOx performance after the poisoning of K, SO2, and/or K&SO2 than the MnSmOx and MnSmOx/ZSM-5 catalysts, the deNOx activity of which at high temperature (H-T) increased significantly (>90% NOx conversion in the range of 220-480 °C). It has been demonstrated that K would occupy both redox and acidic sites, which severely reduced the reactivity of MnSmOx/ZSM-5 catalysts. The most important, K element is preferentially deposited at -OH on the surface of ZSM-5 carrier due to the electrostatic attraction (-O-K). As for the K&SO2 poisoning catalyst, SO2 preferred to be combined with the surface-deposited K (-O-K-SO2ads) according to XPS and density functional theory (DFT) results, the poisoned active sites by K would be released. The K migration behavior was induced by SO2 over K-poisoned MnSmOx@ZSM-5 catalysts, and the balance of surface redox and acidic site was regulated, like a synergistic promoter, which led to K-poisoning buffering and activity recovery. This work contributes to the understanding of the self-detoxification interaction between alkali metals (e.g., K) and SO2 on deNOx catalysts and provides a novel strategy for the adaptive use of one poisoning substance to counter another for practical NOx reduction.

Role of the In-Situ-Formed Surface (Pt-S-O)-Ti Active Structure in SO2-Promoted C3H8 Combustion over a Pt/TiO2 Catalyst

Typically, SO2 unavoidably deactivates catalysts in most heterogeneous catalytic oxidations. However, for Pt-based catalysts, SO2 exhibits an extraordinary boosting effect in propane catalytic oxidation, but the promotive mechanism remains contentious. In this study, an in situ-formed tactful (Pt-S-O)-Ti structure was concluded to be a key factor for Pt/TiO2 catalysts with a substantial SO2 tolerance ability. The experiments and theoretical calculations confirm that the high degree of hybridization and orbital coupling between Pt 5d and S 3p orbitals enable more charge transfer from Pt to S species, thus forming the (Pt-S-O)-Ti structure with the oxygen atom dissociated from the chemisorbed O2 adsorbed on oxygen vacancies. The active oxygen atom in the (Pt-S-O)-Ti active structure is a robust site for C3H8 adsorption, leading to a better C3H8 combustion performance. This work can provide insights into the rational design of chemical bonds for high SO2 tolerance catalysts, thereby improving economic and environmental benefits.

Insight into the mechanisms of activity promotion and SO2 resistance over Fe-doped Ce-W oxide catalyst for NOx reduction

Ceria-based catalysts for the selective catalytic reduction of NOx with NH3 (NH3-SCR) are always subject to deactivation by sulfur poisoning. In this study, Fe-doped Ce-W mixed oxides, which were synthesized by the co-precipitation method, improved the SCR activity and SO2 durability at low temperatures of undoped Ce-W oxides. The improved low-temperature activity was mainly due to the enhancement of redox properties at low temperatures and more active oxygen species, together with the adsorption and activation of more abundant NOx species, facilitating the "fast SCR" reaction. In the presence of SO2, doping with Fe species effectively prevented sulfate deposition on the CeW catalyst, due to the interaction between Fe, Ce, and W species inducing electron transfer among different metal sites and altering the electron distribution. The competitive adsorption behavior between NO and SO2 was changed by Fe doping, in which the adsorption and oxidation of SO2 were restrained. Besides, the elevated NO oxidation accelerated the decomposition of ammonium bisulfate, causing the SCR reaction to not be greatly suppressed. Hence, Fe-doped Ce-W oxides catalysts showed excellent sulfur resistance. This study provides an in-depth understanding of efficient Ce-based catalysts for SO2-tolerance strategies.

Unique Compensation Effects of Heavy Metals and Phosphorus Copoisoning over NOx Reduction Catalysts

Selective catalytic reduction (SCR) of NO_x from the flue gas is still a grand challenge due to the easy deactivation of catalysts. The copoisoning mechanisms and multipoisoning-resistant strategies for SCR catalysts in the coexistence of heavy metals and phosphorus are barely explored. Herein, we unexpectedly found unique compensation effects of heavy metals and phosphorus copoisoning over NO_x reduction catalysts and the introduction of heavy metals results in a dramatic recovery of NO_x reduction activity for the P-poisoned CeO₂/TiO₂ catalysts. P preferentially combines with Ce as a phosphate species to reduce the redox capacity and inhibit NO adsorption. Heavy metals preferentially reduced the Brønsted acid sites of the catalyst and inhibited NH₃ adsorption. It has been demonstrated that heavy metal phosphate species generated over the copoisoned catalyst, which boosted the activation of NH₃ and NO, subsequently bringing about more active nitrate species to relieve the severe impact by phosphorus and maintain the NO_x reduction over CeO₂/TiO₂ catalysts. The heavy metals and P copoisoned catalysts also possessed more acidic sites, redox sites, and surface adsorbed oxygen species, which thus contributed to the highly efficient NO_x reduction. This work elaborates the unique compensation effects of heavy metals and phosphorus copoisoning over CeO₂/TiO₂ catalysts for NO_x reduction and provides a perspective for further designing multipoisoning-resistant CeO₂-based catalysts to efficiently control NO_x emissions in stationary sources.

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.

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.

Simultaneous catalytic removal of NO, mercury and chlorobenzene over WCeMnOx/TiO2-ZrO2: Performance study of microscopic morphology and phase composition

Nitrogen oxides, mercury and chlorobenzene are important air pollutants emitted by waste incineration and other industries. Coordinated control of multiple pollutants has become an important technology for air pollution control. Through solid-phase structure control, the catalytic performance of the WCeMnO_x/TiO₂-ZrO₂ catalyst for simultaneous catalytic removal of NO, mercury and simultaneous removal of NO and chlorobenzene were improved. MnWO₄ improved the solid acidity of the catalyst and improved the catalytic activity at high temperature. The formation of Ce_0·75Zr_0·25O₂, Ce₂WO₆, Ce₂Zr₂O₇ and Ce₂Ti₂O₇ improved the catalytic activity at low temperature. The presence of TiOSO₄ would affect the valence of metal ions and the reduction of chemisorbed oxygen, thereby reducing the catalytic activity at low temperature. Within the same size range of nanoparticles, cyclic nanoparticles exposed more active sites due to their hollow structure, and their catalytic performance was better than spherical nanoparticles. The thickness of the circular nanoparticles of WCM/TZ-14 catalyst was about 14 nm, and the diameter was about 40 nm Ce_0.75Zr_0.25O₂ and MnWO₄ were also present in the phase composition. Therefore, it exhibited the best performance for simultaneous catalytic removal of NO, mercury and simultaneous removal of NO and chlorobenzene. The coincidence temperature window was 347-516 °C. Finally, WCM/TZ-14 catalyst followed both E-R and L-H mechanisms in the NH₃-SCR reaction.

SO2-Induced Alkali Resistance of FeVO4/TiO2 Catalysts for NOx Reduction

Selective catalytic reduction of nitrogen oxides with ammonia (NH₃-SCR) is an efficient NO_x abatement strategy, but deNO_x catalysts suffer from serious deactivation due to the coexistence of multiple poisoning substances such as K, SO₂, etc. in the flue gas. It is essential to understand the interaction among various poisons and their effects on NO_x abatement. Here, we unexpectedly identified the K migration behavior induced by SO₂ over K-poisoned FeVO₄/TiO₂ catalysts, which led to alkali-poisoning buffering and activity recovery. It has been demonstrated that the K would occupy both redox and acidic sites, which severely reduced the reactivity of FeVO₄/TiO₂ catalysts. After the sulfuration of the K-poisoned catalyst, SO₂ preferred to be combined with the surface K₂O, lengthened the K-O_Fe and K-O_V, and thus released the active sites poisoned by K₂O, thereby preserving an increase in the activity. As a result, for the K-poisoned catalyst, the conversion of NO_x increased from 21 to 97% at 270 °C after the sulfuration process. This work contributes to the understanding of the specific interaction between alkali metals and SO₂ on deNO_x catalysts and provides a novel strategy for the adaptive use of one poisoning substance to counter another for practical NO_x reduction.

Synergistic promotion of transition metal ion-exchange in TiO2 nanoarray-based monolithic catalysts for the selective catalytic reduction of NOx with NH3

The improved performance of the multi-component Cu–Ce–Mn/TNA catalysts over the mono-metallic catalysts demonstrated the synergistic promotion of multi-transition-metal-doped nanoarray catalysts for efficient NO abatement.

Ceria–tungsten–tin oxide catalysts with superior regeneration capacity after sulfur poisoning for NH3-SCR process

A combined study on the anti-sintering ability, SO2-poisoning mechanism and thermal regeneration property of CeWSnOx catalysts for NH3-SCR reaction.

Alkali-Resistant Catalytic Reduction of NOx by Using Ce-O-B Alkali-Capture Sites

Reducing the poisoning effect arising from alkali metals over catalysts for selective catalytic reduction (SCR) of NO_x by NH₃ is still an urgent issue to be solved. Herein, alkali-resistant NO_x reduction over B-doped CeO₂/TiO₂ catalysts (Ce-B/TiO₂) with Ce-O-B alkali-capture sites was originally demonstrated. It was noted that boron was confirmed to be doped into the lattice of CeO₂ to form the Ce-O-B structure. In this way, more active Ce(III) species and oxygen vacancies were generated from B-doped CeO₂, thus accelerating the redox cycle and enhancing the adsorption/activation of NO. Gratifyingly, the created Ce-O-B sites as alkali-capture sites could be effectively combined with K and release the poisoned Ce active sites, which maintained efficient NH₃ and NO adsorption/activation over K poisoned Ce-B/TiO₂. This work paves a way for designing highly efficient and alkali-resistant SCR catalysts in both academic and industrial fields.

Status and development for detection and control of ammonium bisulfate as a by-product of SCR denitrification

When denitrification technology using NH₃ or urea as the reducing agent is applied to remove NOx from the flue gas, ammonium bisulfate (ABS) by-product will also be generated in the flue gas. ABS has an impact on catalyst life span, denitrification efficiency etc., air preheater and its downstream thermal equipment also have a significant negative impact due to its plugging and corrosion. The requirement for NOx removal efficiency is improved by ultra-low emissions in China. However, wide-load denitrification makes the flue gas composition and temperature changing more complicated. Increasing ammonia injection can improve the NOx removal effect, but too much ammonia injection will lead to the formation of ABS and the increase of deposition risk, the contradiction between these two aspects is amplified by ultra-low emissions and wide-load denitrification in many plants. Coordinating NOx control and reducing the impact of ABS on equipment are issues that the industry needs to solve urgently. In recent years, extensive research on ABS had been carried out deeply, consequently, there has been a relatively in-deepth knowledge foundation for ABS formation, formation temperature, deposition temperature, dew point temperature, decomposition behavior, etc., but the existing researches are insufficient to support the problem of ABS under full load denitrification completely resolved. Therefore, some analysis and detection methods related to ABS are reviewed in this paper, and the impact of ABS on SCR, air preheater and other equipment and the existing research results on reducing the impact of ABS are summarized also. It is hoped that this review will provide a reference for the industry to solve the problems of ABS that hinder wide-load denitrification and affect ultra-low emissions.

Selective catalytic reduction of NO over W–Zr-Ox/TiO2: performance study of hierarchical pore structure

A series of W–Zr-Ox/TiO2 catalysts with hierarchical pore structure were prepared and used for selective catalytic reduction of NO by NH3.