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

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.

Unexpected Redox Role of WO3 in V2O5-WO3/TiO2 Catalysts for Selective Reduction of NO by Forming V-W Dinuclear Sites

Promoters can greatly improve the activity of catalysts; however, the mechanism by which they interact with the primary catalyst to enhance catalytic activity is not always clear. It has been widely demonstrated that WO3 in V2O5-WO3/TiO2 catalysts acts as a promoter to enhance the surface acidity and regulate the dispersion of active V2O5 species, while it is believed that the redox process takes place entirely at the V sites in the selective catalytic reduction of NOx with NH3 (NH3-SCR). Here, by combining in situ spectroscopic measurements and density functional theory (DFT) calculations, we validate that the WO3 directly participates in the oxidative activation of NH3, and hence explicitly exerts a redox effect in NH3-SCR by forming V-W dinuclear sites with V2O5. This study sheds new light on a long-standing puzzle regarding the role of WO3 as a promoter and hence advances the understanding of the working principle of V2O5-WO3/TiO2 catalysts.

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.

Recent Progress on Low-Temperature Selective Catalytic Reduction of NOx with Ammonia

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) has been implemented in response to the regulation of NOx emissions from stationary and mobile sources above 300 °C. However, the development of NH3-SCR catalysts active at low temperatures below 200 °C is still needed to improve the energy efficiency and to cope with various fuels. In this review article, recent reports on low-temperature NH3-SCR catalysts are systematically summarized. The redox property as well as the surface acidity are two main factors that affect the catalytic activity. The strong redox property is beneficial for the low-temperature NH3-SCR activity but is responsible for N2O formation. The multiple electron transfer system is more plausible for controlling redox properties. H2O and SOx, which are often found with NOx in flue gas, have a detrimental effect on NH3-SCR activity, especially at low temperatures. The competitive adsorption of H2O can be minimized by enhancing the hydrophobic property of the catalyst. Various strategies to improve the resistance to SOx poisoning are also discussed.

New insights and reaction mechanisms in the design of catalysts for the synergistic removal of NOx and VOCs from coke oven flue gas: Dual regulation of oxidative properties and acidic sites

The acid and redox sites of the MnCo catalysts are simultaneously fine-tuned by the addition of V. A dual-function catalyst, designated as V0.5Mn5Co5, has been constructed for the synergistic removal of NOx and volatile organic compounds under coke-oven flue gas conditions, which exhibits > 95 % NOx conversion and > 80 % N2 selectivity at 180-300 °C. Meanwhile, it removes 70 % of ethylene at 240 °C. Besides it has excellent sulfur and water resistance. The characterization results indicate that this acid-redox dual sites modulation strategy appropriately weakens the oxidation capacity of the catalysts while increasing the surface acidity of the catalysts. The catalyst mainly performs SCR reaction through the E-R mechanism, and N2O is generated through the transition dehydrogenation of NH3 and NSCR reaction. Ethylene is first adsorbed on the catalyst surface then oxidized to form carbonate species, and finally decomposed to CO2. Ethylene oxidation follows the MvK mechanism. There is a competitive adsorption between NH3 and C2H4, and a mutual inhibition between the SCR reaction and the ethylene oxidation reaction. V0.5Mn5Co5 exhibits excellent synergistic removal of NOx and VOCs in coke oven flue gas compared with commercial VWTi catalysts, which indicates great promise for industrial application.

Challenges and Perspectives of Environmental Catalysis for NO x Reduction

Environmental catalysis has attracted great interest in air and water purification. Selective catalytic reduction with ammonia (NH3-SCR) as a representative technology of environmental catalysis is of significance to the elimination of nitrogen oxides (NO x ) emitting from stationary and mobile sources. However, the evolving energy landscape in the nonelectric sector and the changing nature of fuel in motor vehicles present new challenges for NO x catalytic purification over the traditional NH3-SCR catalysts. These challenges primarily revolve around the application limitations of conventional industrial NH3-SCR catalysts, such as V2O5-WO3(MoO3)/TiO2 and chabazite (CHA) structured zeolites, in meeting both the severe requirements of high activity at ultralow temperatures and robust resistance to the wide array of poisons (SO2, HCl, phosphorus, alkali metals, and heavy metals, etc.) existing in more complex operating conditions of new application scenarios. Additionally, volatile organic compounds (VOCs) coexisting with NO x in exhaust gas has emerged as a critical factor further impeding the highly efficient reduction of NO x . Therefore, confronting the challenges inherent in current NH3-SCR technology and drawing from the established NH3-SCR reaction mechanisms, we discern that the strategic manipulation of the properties of surface acidity and redox over NH3-SCR catalysts constitutes an important pathway for increasing the catalytic efficiency at low temperatures. Concurrently, the establishment of protective sites and confined structures combined with the strategies for triggering antagonistic effects emerge as imperative items for strengthening the antipoisoning potentials of NH3-SCR catalysts. Finally, we contemplate the essential status of selective synergistic catalytic elimination technology for abating NO x and VOCs. By virtue of these discussions, we aim to offer a series of innovative guiding perspectives for the further advancement of environmental catalysis technology for the highly efficient NO x catalytic purification from nonelectric industries and motor vehicles.

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.

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.

Insight into the Promotion Effect of Trace Pd Doping on the Catalytic Performance of Ag/Al2O3 for C3H6-SCR of NOx

The mechanistic cause of the enhancement of the C3H6-SCR activity of Ag/Al2O3 by trace Pd doping and the corresponding structure-property relationship were investigated. Pd doping enhanced the water resistance of Ag/Al2O3 for C3H6-SCR by changing the reaction pathway. Under wet conditions, a series of in situ DRIFT studies indicated that the production of an active acetate intermediate on Ag/Al2O3 was suppressed during the partial oxidation of C3H6, while trace Pd doping promoted the formation of another active intermediate, an enolic species. Furthermore, a pathway for the formation of enolic species by the reaction of acrylate with hydroxyl species was proposed. DFT calculations revealed that the surface of Ag clusters was easily covered by hydroxyl in the presence of water vapor, which could inhibit the formation of acetates. Doping with Pd facilitated the activation of acrylate which might further react with hydroxyl species to form enolic species. These findings can be helpful for the future design of efficient HC-SCR 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.

Single-Atom Catalysts in Environmental Engineering: Progress, Outlook and Challenges

Recently, single-atom catalysts (SACs) have attracted wide attention in the field of environmental engineering. Compared with their nanoparticle counterparts, SACs possess high atomic efficiency, unique catalytic activity, and selectivity. This review summarizes recent studies on the environmental remediation applications of SACs in (1) gaseous: volatile organic compounds (VOCs) treatment, NO_x reduction, CO₂ reduction, and CO oxidation; (2) aqueous: Fenton-like advanced oxidation processes (AOPs), hydrodehalogenation, and nitrate/nitrite reduction. We present the treatment activities and reaction mechanisms of various SACs and propose challenges and future opportunities. We believe that this review will provide constructive inspiration and direction for future SAC research in environmental engineering.

Enhanced H2O2 utilization efficiency in Fenton-like system for degradation of emerging contaminants: Oxygen vacancy-mediated activation of O2

Hydrogen peroxide (H₂O₂) is the most commonly used oxidant in advanced oxidation processes for emerging organic contaminant degradation. However, the activation of H₂O₂ to generate reactive oxygen species is always accompanied by O₂ generation resulting in H₂O₂ waste. Here, we prepare a Ti doped Mn₃O₄/Fe₃O₄ ternary catalyst (Ti-Mn₃O₄/Fe₃O₄) to create abundant oxygen vacancies (OVs), which yields electron delocalization impacts on enhancing the electrical conductivity, accelerating the activation of O₂ to produce H₂O₂. In Ti-Mn₃O₄/Fe₃O₄/H₂O₂ system, OVs-mediated O₂/O₂^•-/H₂O₂ redox cycles trigger the activation of locally generated O₂, boost the regeneration of O₂^•- and on site produce H₂O₂ for replenishment. This leads to a 100% removal of tiamulin in 30 min at an unprecedented H₂O₂ utilization efficiency of 96.0%, which is 24 folds higher than that with Fe₃O₄/H₂O₂. Importantly, further integration of Ti-Mn₃O₄/Fe₃O₄ catalysts into membrane filtration achieved high rejections of tiamulin (> 83.9%) from real surface water during a continuous 12-h operation, demonstrating broad pH adaptability, excellent catalytic stability and leaching resistance. This work demonstrates a feasible strategy for developing OVs-rich catalysts for improving H₂O₂ utilization efficiency via activation of locally generated oxygen during the Haber-Weiss reaction.