Efficient SO2 Removal and Highly Synergistic H2O2 Production Based on a Novel Dual-Function Photoelectrocatalytic System

Solar-powered enzymatic-photoelectrocatalytic system constructed with three-dimensional dual photoelectrodes for highly efficient purification of coking wastewater

The coupling of photoelectrocatalysis (PEC) and enzymatic catalysis was explored for efficient treatment of practical phenol-containing coking wastewater outdoors. The developed enzymatic-photoelectrocatalytic (EPEC) system consisted of three-dimensional dual photoelectrodes fabricated with carbon fiber cloth (CFC), namely a CFC-supported WO3 photoanode with oxygen vacancies and a CFC-supported Au-BiOBr photocathode with immobilized horseradish peroxidase (HRP). The experimental results and computational fluid dynamics study revealed that dual CFC-based photoelectrodes with three-dimensional network structures could optimize the fluid diffusion on the electrode surface and enhance the efficiency of pollutant degradation. Further investigation indicated that the introduction of oxygen vacancies promoted the PEC oxidation activity of the WO3 photoanode while the Au nanoparticles modification was favorable to improving the immobilization of HRP and PEC generation of H2O2 on BiOBr photocathode. Moreover, photovoltaic (PV) cells were integrated in the EPEC system to provide applied potential and promote the utilization of solar irradiation. The mineralization rate of coking wastewater reached 88.5% using the PV-integrated EPEC system driven by natural solar energy, which demonstrated potential for treating actual industrial wastewater.

Integration of Ag3PO4/WO3 photoanode with bifunctional CuO cathode exhibiting photocatalytic and peroxidase-mimetic properties for nanozyme-coupled photoelectrocatalytic degradation of persistent pollutant

The nanozyme-coupled photoelectrocatalytic (PEC) technology, which integrates the semiconductor photovoltaic effects with the enzyme-mimetic properties of nanomaterials, offers a promising and sustainable approach for removing organic contaminants from wastewater. In this study, a dual-photoelectrode PEC system comprising an Ag3PO4/WO3 anode and a bifunctional CuO cathode was investigated for the efficient degradation of 2,5-dichlorophenol (2,5-DCP). Under simulated solar irradiation, the Ag3PO4/WO3 photoanode demonstrated superior PEC activity for 2,5-DCP degradation and H2O2 production. The bifunctional CuO cathode significantly enhanced the degradation efficiency of 2,5-DCP, which can be attributed to its inherent photocatalytic activity and peroxidase-mimicking capability in the presence of in-situ generated H2O2. Consequently, the degradation efficiency of 2,5-DCP reached 91.2 % after 90 min of nanozyme-coupled PEC treatment. The intermediate products generated during the degradation process were identified using liquid chromatography-mass spectrometry, and a potential degradation pathway for 2,5-DCP was also proposed. This study highlights the potential of a bifunctional cathode with both photocatalytic and peroxidase-mimicking properties in the development of nanozyme-coupled PEC systems for the degradation of persistent pollutants.

Indium Oxide Layer Dual Functional Modified Bismuth Vanadate Photoanode Promotes Photoelectrochemical Oxidation of Water to Hydrogen Peroxide

The photoelectrochemical (PEC) dual-electron pathway for water oxidation to produce hydrogen peroxide (H2O2) shows promising prospects. However, the dominance of the four-electron pathway leading to O2 evolution competes with this reaction, severely limiting the efficiency of H2O2 production. Here, we report a In2O3 passivator-coated BiVO4 (BVO) photoanode, which effectively enhances the selectivity and yield of H2O2 production via PEC water oxidation. Based on XPS spectra and DFT calculations, a heterojunction is formed between In2O3 and BVO, promoting the effective separation of interface and surface charges. More importantly, Mott-Schottky analysis and open-circuit potential measurements demonstrate that the In2O3 passivation layer on the BVO photoanode shifts the hole quasi-Fermi level towards the anodic direction, enhancing the oxidation level of holes. Additionally, the widening of the depletion layer and the flattening of the band bending on the In2O3-coated BVO photoanode favor the generation of H2O2 while suppressing the competitive O2 evolution reaction. In addition, the coating of In2O3 can also inhibit the decomposition of H2O2 and improve the stability of the photoanode. This work provides new perspectives on regulating PEC two/four-electron transfer for selective H2O2 production via water oxidation.

Reactivity of the Intramolecular Vicinal Group-13/P- and B/Group-15-Based Frustrate Lewis Pairs with Sulfur Dioxide: Mechanistic Insight from DFT

The emission of SO2 gas by industrialized societies contributes to the occurrence of acid rain in natural environments. In this study, we put forward a theoretical investigation into the capture reactions of SO2. Our analysis centers on the energy profiles of intramolecular 1,2-cyclohexylene-bridged FLP-associated molecules. We will particularly examine the reactions involving G13/P-based (with G13 denoting Group 13 element) and B/G15-based (with G15 representing Group 15 element) FLP-associated molecules. Except for Tl/P-FLP, B/N-FLP, and B/Bi-FLP, our theoretical examinations indicate that the remaining six FLP-associated molecules, namely G13'/P-FLP (G13' = B, Al, Ga, and In) and B/G15 ' -FLP (G15' = P, As, and Sb), can easily undergo SO2 capture reactions due to their energetic feasibility. Particularly, our theoretical findings suggested that 1,2-cyclohexylene-bridged Al/P-FLP, Ga/P-FLP, B/As-FLP, and B/Sb-FLP are capable of undergoing a reversible reaction and returning to the initial reactant state. Our theoretical evidence indicates that the G13-G15 bond length in the 1,2-cyclohexylene-linked G13/G15-FLP can serve as a basis for evaluating the free activation barrier associated with its reaction with SO2. Two theoretical methods, namely, the frontier molecular orbital theory and the energy decomposition analysis-natural orbitals of chemical valence approach, are utilized to investigate the electronic structure and bonding nature of the reactions under consideration. Moreover, the analyses based on the activation strain model revealed that it is the geometrical deformation energies of G13/G15-FLP, which is the key factor that greatly influences the activation barriers of such SO2 capture reactions. Further, our theoretical computations indicate that such capturing reactions of SO2 by intramolecular 1,2-cyclohexylene-linked G13/G15-based FLP-type molecules obey the Hammond postulate.

Mechanistic and thermodynamic insights into the SO2 oxidation on MnO2 catalysts: A combined theoretical and experimental study

Manganese oxide (especially manganese dioxide [MnO₂]) is an excellent catalytic material for SO₂ removal in flue gas desulfurization. In this study, the effect of crystalline structure of MnO₂ (α-MnO₂, β-MnO₂, γ-MnO₂ and δ-MnO₂) on their activity for SO₂ oxidation was studied based on density functional theory with Hubbard U corrections (DFT + U). The calculated results showed that α-MnO₂ has mild energy barriers of 0.69 eV and 0.46 eV, and β-MnO₂ has poor redox performance on SO₂ molecules, which has the highest energy barrier of 2.17 eV and the largest oxygen formation energy of 1.74 eV, making it difficult for the oxygen atom to remove from the surface lattice to form reactive sites. Thermodynamic calculations showed that α-MnO₂ is suitable for SO₂ oxidation for its low energy barriers, reaction energy close to zero in the first half, and relatively high spontaneity in the whole reaction. Experimental tests showed that α-MnO₂ had the best catalytic oxidation effect, with the highest sulfur capacity (304.11 mg/g), but β-MnO₂ had poor catalytic oxidation performance, with a sulfur capacity of 41.59 mg/g. This work studies the catalytic performance and mechanism of SO₂ removal and proposes a strategy to improve the catalytic activity by phase structure.

Boosting Photosynthetic H2O2 of Polymeric Carbon Nitride by Layer Configuration Regulation and Fluoride-Potassium Double-Site Modification

Photocatalytic hydrogen peroxide (H₂O₂) production will become a burgeoning strategy for solar energy utilization by selective oxygen reduction reaction (ORR). Polymeric carbon nitride (PCN) shows relatively high two-electron ORR selectivity for H₂O₂ production but still limited low H₂O₂ production efficiency due to slow exciton dissociation. Herein, we constructed a heptazine/triazine layer stacked carbon nitride heterojunction with fluorine/potassium (F/K) dual sites (FKHTCN). The introduction of F/K not only can regulate layer components to enhance the charge separation efficiency but, more importantly, also optimize the adsorption of surface oxygen molecules and intermediate *OOH during H₂O₂ production. Consequently, FKHTCN efficiently improves the photocatalytic H₂O₂ production rate up to 3380.9 μmol h^-1 g^-1, nearly 15 times higher than that of traditional PCN. Moreover, a production-utilization cascade system was designed to explore their practical application in environmental remediation. This work lays out the importance of engineering a layer-stacked configuration and active sites for enhancing photocatalysis.

Schematic Design of Metal-Free NHC-Mediated Sequestering and Complete Conversion of SO2 to Thiocarbonyl S-Oxide Derivatives at Room Temperature

The sequestering and complete conversion of SO₂ to valuable chemicals in a metal-free pathway is highly demanded. The recent success of SO₂ fixation by N-heterocyclic carbenes instigated further studies in this regard. Previous reports were confined within the carbene-SO₂ reaction mechanism and the stability of oxathiirane S-oxide derivatives. The complete conversion of captured SO₂ to precious chemicals was not studied. The present inquisition has accomplished the scarcity of the earlier studies. It is observed that in the presence of an excess amount of carbene, the registered SO₂ is converted to the ketone derivative and thiocarbonyl S-oxide derivative. An electronic level investigation of these reactions is carried out. From the change of the molecular orbitals along the reaction path, it is concluded that the reaction between the oxathiirane S-oxide derivative and carbene follows a frog's hunting mechanism.

Effective stabilization of atomic hydrogen by Pd nanoparticles for rapid hexavalent chromium reduction and synchronous bisphenol A oxidation during the photoelectrocatalytic process

Atomic hydrogen (H*) plays a vital role in the synchronous redox of metallic ions and organic molecules. However, H* is extremely unstable as it is easily converted to hydrogen. Herein, we designed a novel strategy for the effective stabilization of H* to enhance its utility. The synthesized Pd nanoparticles grown on the defective MoS₂ (DMS) of TiO₂ nanowire arrays (TNA) (TNA/DMS/Pd) photocathode exhibited rapid Cr(VI) reduction (~95% in 10 min) and bisphenol A (BPA) oxidation (~97% in 30 min), with the kinetic constants almost 24- and 6-fold higher than those of the TNA photocathode, respectively. This superior performances could be attributed to: (i) the generated interface heterojunctions between TNA and DMS boosted the separation efficiencies of photogenerated electrons, thereby supplying abundant valance electrons to lower the overpotential to create a suitable microenvironment for H* generation; (ii) the stabilization of H* by Pd nanoparticles resulted in a significant increase in the yield of hydroxyl radical (•OH). This research provides a new strategy for the effective utilization of H* toward rapid reduction of heavy metals and synchronous oxidation of the refractory organics.

Preparation of TiO2/porous glass-H with the coupling of photocatalysis oxidation-adsorption system in the initial position and its desulfurization performance on model fuel

TiO₂/porous glass-H as composite catalysts were synthesized hydrothermally in the presence of H₂O₂ using porous glass microspheres as carriers. The photocatalytic-adsorptive desulfurization of model fuel by composite catalysts was investigated under UV irradiation. The structure and morphology of the composite catalysts were characterized via scanning electron microscopy (SEM), N₂ adsorption, X-ray diffraction (XRD) and ultraviolet-visible spectroscopy (UV-vis). The results showed that TiO₂/porous glass-H exhibited a significantly enhanced photocatalytic-adsorption desulfurization performance due to its enhanced surface area, highly enhanced light absorption, and reduced recombination of photogenerated electron pairs compared with TiO₂/porous glass synthesized in the absence of H₂O₂. The optimized TiO₂ loading was 20% and the reaction temperature was 303.15 K, which could achieve almost 100% sulfur removal when 0.1 g catalyst was applied to a sulfide concentration of 300 mg L⁻¹. Based on the kinetic fitting of the obtained data, it was found that the rate-controlling step of sulfide adsorption on the catalyst was a molecular diffusion process and the adsorption intensity and adsorption capacity of the composite catalyst were significantly improved compared with the porous glass-H in the adsorption thermodynamic curve, and ΔS, ΔH and ΔG of the adsorption process were calculated. In addition, TiO₂/porous glass-H could be regenerated via simple heat treatment, exhibiting similar efficiency as the original TiO₂/porous glass-H after three regeneration cycles.

TiO₂/porous glass-H as composite catalysts were synthesized hydrothermally in the presence of H₂O₂ using porous glass microspheres as carriers.

Recent advances in hybrid wet scrubbing techniques for NOx and SO2 removal: State of the art and future research

Recently, the discharge of flue gas has become a global issue due to the rapid development in industrial and anthropogenic activities. Various dry and wet treatment approaches including conventional and hybrid hybrid wet scrubbing have been employing to combat against these toxic exhaust emissions. However, certain issues i.e., large energy consumption, generation of secondary pollutants, low regeneration of scrubbing liquid and high efficieny are hindering their practical applications on industrial level. Despite this, the hybrid wet scrubbing technique (advanced oxidation, ionic-liquids and solid engineered interface hybrid materials based techniques) is gaining great attention because of its low installation costs, simultaneous removal of multi-air pollutants and low energy requirements. However, the lack of understanding about the basic principles and fundamental requirements are great hurdles for its commercial scale application, which is aim of this review article. This review article highlights the recent developments, minimization of GHG, sustainable improvements for the regeneration of used catalyst via green and electron rich donors. It explains, various hybrid wet scrubbing techniques can perform well under mild condition with possible improvements such as development of stable, heterogeneous catalysts, fast and in-situ regeneration for large scale applications. Finally, it discussed recovery of resources i.e., N₂O, NH₃ and N₂, the key challenges about several competitive side products and loss of catalytic activity over time to treat toxic gases via feasible solutions by hybrid wet scrubbing techniques.

A Review of Photoelectrocatalytic Reactors for Water and Wastewater Treatment

The photoexcitation of suitable semiconducting materials in aqueous environments can lead to the production of reactive oxygen species (ROS). ROS can inactivate microorganisms and degrade a range of chemical compounds. In the case of heterogeneous photocatalysis, semiconducting materials may suffer from fast recombination of electron–hole pairs and require post-treatment to separate the photocatalyst when a suspension system is used. To reduce recombination and improve the rate of degradation, an externally applied electrical bias can be used where the semiconducting material is immobilised onto an electrically conducive support and connected to a counter electrode. These electrochemically assisted photocatalytic systems have been termed “photoelectrocatalytic” (PEC). This review will explain the fundamental mechanism of PECs, photoelectrodes, the different types of PEC reactors reported in the literature, the (photo)electrodes used, the contaminants degraded, the key findings and prospects in the research area.