Quantification of photocatalytically-generated hydrogen peroxide in the presence of organic electron donors: Interference and reliability considerations

Polyvinyl alcohol as solid proton donor to modify g-C3N4via hydrogen bonding enabling efficient photocatalytic H2O2 production from H2O and O2

Polyvinyl alcohol (PVA) was used as a solid proton donor to improve the photocatalytic performance of graphitic carbon nitride (CN) for hydrogen peroxide (H2O2) production. The modified CN (CN/PVA) was prepared by mixing CN and PVA at room temperature. The H2O2 production efficiency of CN/PVA was 5.65 times higher than that of CN in pure water. Photocurrent measurement, electrochemical impedance spectroscopy (EIS), and photoluminescence (PL) analysis proved that PVA increased charge separation of CN. X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared (FTIR) analyses further suggested that PVA acted as the proton donor during H2O2 production by interacting with CN via hydrogen bonds. The combination of the charge separation enhancer and proton donor from PVA promoted the sequential two-step single-electron reduction of O2 for H2O2 production. This study paves the way for the modification of g-C3N4 with hydroxyl-containing materials as solid proton donors for photocatalytic H2O2 production.

Keto-anthraquinone covalent organic framework for H2O2 photosynthesis with oxygen and alkaline water

Hydrogen peroxide photosynthesis suffers from insufficient catalytic activity due to the high energy barrier of hydrogen extraction from H2O. Herein, we report that mechanochemically synthesized keto-form anthraquinone covalent organic framework which is able to directly synthesize H2O2 (4784 μmol h-1 g-1 at λ > 400 nm) from oxygen and alkaline water (pH = 13) in the absence of any sacrificial reagents. The strong alkalinity resulted in the formation of OH-(H2O)n clusters in water, which were adsorbed on keto moieties within the framework and then dissociated into O2 and active hydrogen, because the energy barrier of hydrogen extraction was largely lowered. The produced hydrogen reacted with anthraquinone to generate anthrahydroquinone, which was subsequently oxidized by O2 to produce H2O2. This study ultimately sheds light on the importance of hydrogen extraction from H2O for H2O2 photosynthesis and demonstrates that H2O2 synthesis is achievable under alkaline conditions.

Biomimetic leaves with immobilized catalase for machine learning-enabled validating fresh produce sanitation processes

Washing and sanitation are vital steps during the postharvest processing of fresh produce to reduce the microbial load on the produce surface. Although current process control and validation tools effectively predict sanitizer concentrations in wash water, they have significant limitations in assessing sanitizer effectiveness for reducing microbial counts on produce surfaces. These challenges highlight the urgent need to improve the validation of sanitation processes, especially considering the presence of dynamic organic contaminants and complex surface topographies. This study aims to provide the fresh produce industry with a novel, reliable, and highly accurate method for validating the sanitation efficacy on the produce surface. Our results demonstrate the feasibility of using a food-grade, catalase (CAT)-immobilized biomimetic leaf in combination with vibrational spectroscopy and machine learning to predict microbial inactivation on microgreen surfaces. This was tested using two sanitizers: sodium hypochlorite (NaClO) and hydrogen peroxide (H2O2). The developed CAT-immobilized leaf-replicated PDMS (CAT@L-PDMS) effectively mimics the microscale topographies and bacterial distribution on the leaf surface. Alterations in the FTIR spectra of CAT@L-PDMS, following simulated sanitation processes, indicate chemical changes due to CAT oxidation induced by NaClO or H2O2 treatments, facilitating the subsequent machine learning modeling. Among the five algorithms tested, the competitive adaptive reweighted sampling partial least squares discriminant analysis (CARS-PLSDA) algorithm was the most effective for classifying the inactivation efficacy of E. coli on microgreen leaf surfaces. It predicted bacterial reduction on microgreen surfaces with 100% accuracy in both training and prediction sets for NaClO, and 95% in the training set and 86% in the prediction set for H2O2. This approach can improve the validation of fresh produce sanitation processes and pave the way for future research.

Pt-Black-Modified (Hemi)spherical AFM Sensors: In Situ Imaging of Light-Driven Hydrogen Peroxide Evolution

In this work, we present (hemi)spherical atomic force microscopy (AFM) sensors for the detection of hydrogen peroxide. Platinum-black (Pt-B) was electrodeposited onto conductive colloidal AFM probes or directly at recessed microelectrodes located at the end of a tipless cantilever, resulting in electrocatalytically active cantilever-based sensors that have a small geometric area but, due to the porosity of the films, exhibit a large electroactive surface area. Focused ion beam-scanning electron microscopy tomography revealed the porous 3D structure of the deposited Pt-B. Given the accurate positioning capability of AFM, these probes are suitable for local in situ sensing of hydrogen peroxide and at the same time can be used for (electrochemical) force spectroscopy measurements. Detection limits for hydrogen peroxide in the nanomolar range (LOD = 68 ± 7 nM) were obtained. Stability test and first in situ proof-of-principle experiments to achieve the electrochemical imaging of hydrogen peroxide generated at a microelectrode and at photocatalytically active structured poly(heptazine imide) films are demonstrated. Force spectroscopic data of the photocatalyst films were recorded in ambient conditions, in solution, and by applying a potential, which demonstrates the versatility of these novel Pt-B-modified spherical AFM probes.

Iron oxide-promoted photochemical oxygen reduction to hydrogen peroxide (H2O2)

Hydrogen peroxide (H2O2) is a valuable green oxidant with a wide range of applications. Furthermore, it is recognized as a possible future energy carrier achieving safe operation, storage and transportation. The photochemical production of H2O2 serves as a promising alternative to the waste- and energy-intensive anthraquinone process. Following the 12 principles of Green Chemistry, we demonstrate a facile and general approach to sustainable catalyst development utilizing earth-abundant iron and biobased sources only. We developed several iron oxide (FeOx) nanoparticles (NPs) for successful photochemical oxygen reduction to H2O2 under visible light illumination (445 nm). Achieving a selectivity for H2O2 of >99%, the catalyst material could be recycled for up to four consecutive rounds. An apparent quantum yield (AQY) of 0.11% was achieved for the photochemical oxygen reduction to H2O2 with visible light (445 nm) at ambient temperatures and pressures (9.4-14.8 mmol g-1 L-1). Reaching productivities of H2O2 of at least 1.7 ± 0.3 mmol g-1 L-1 h-1, production of H2O2 was further possible via sunlight irradiation and in seawater. Finally, a detailed mechanism has been proposed on the basis of experimental investigation of the catalyst's properties and computational results.

Photocatalytic Synthesis of Hydrogen Peroxide from Molecular Oxygen and Water

Hydrogen peroxide is a powerful and green oxidant that allows for the oxidation of a wide span of organic and inorganic substrates in liquid media under mild reaction conditions, and forms only molecular water and oxygen as end products. Hydrogen peroxide is therefore used in a wide range of applications, for which the well-documented and established anthraquinone autoxidation process is by far the dominating production method at the industrial scale. As this method is highly energy consuming and environmentally costly, the search for more sustainable synthesis methods is of high interest. To this end, the article reviews the basis and the recent development of the photocatalytic synthesis of hydrogen peroxide. Different oxygen reduction and water oxidation mechanisms are discussed, as well as several kinetic models, and the influence of the main key reaction parameters is itemized. A large range of photocatalytic materials is reviewed, with emphasis on titania-based photocatalysts and on high-prospect graphitic carbon nitride-based systems that take advantage of advanced bulk and surface synthetic approaches. Strategies for enhancing the performances of solar-driven photocatalysts are reported, and the search for new, alternative, photocatalytic materials is detailed. Finally, the promise of in situ photocatalytic synthesis of hydrogen peroxide for water treatment and organic synthesis is described, as well as its coupling with enzymes and the direct in situ synthesis of other technical peroxides.

Robust route to H2O2 and H2 via intermediate water splitting enabled by capitalizing on minimum vanadium-doped piezocatalysts

H₂O₂ is an environmentally friendly chemical for a wide range of water treatments. The industrial production of H₂O₂ is an anthraquinone oxidation process, which, however, consumes extensive energy and produces pollution. Here we report a green and sustainable piezocatalytic intermediate water splitting process to simultaneously obtain H₂O₂ and H₂ using single crystal vanadium (V)-doped NaNbO₃ (V-NaNbO₃) nanocubes as catalysts. The introduction of V improves the specific surface area and active sites of NaNbO₃. Notably, V-NaNbO₃ piezocatalysts of 10 mg exhibit 3.1-fold higher piezocatalytic efficiency than the same catalysts of 50 mg, as more piezocatalysts lead to higher probability of aggregation. The aggregation causes reducing active sites and decreased built-in electric field due to the neutralization between different nano-catalysts. Remarkably, piezocatalytic H₂O₂ and H₂ production rates of V-NaNbO₃ (10 mol%) nanocubes (102.6 and 346.2 µmol·g^-1·h^-1, respectively) are increased by 2.2 and 4.6 times compared to the as-prepared pristine NaNbO₃ counterparts, respectively. This improved catalytic efficiency is attributed to the promoted piezo-response and more active sites of NaNbO₃ catalysts after V doping, as uncovered by piezo-response force microscopy (PFM) and density functional theory (DFT) simulation. More importantly, our DFT results illustrate that inducing V could reduce the dynamic barrier of water dissociation over NaNbO₃, thus enhancing the yield of H₂O₂ and H₂. This facile yet robust piezocatalytic route using minimal amounts of catalysts to obtain H₂O₂ and H₂ may stand out as a promising candidate for environmental applications and water splitting.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (typical Raman spectra of NaNbO₃ and V-NaNbO₃ with various doping concentrations (Fig. S1). XPS spectra of Na 1s (Fig. S2). PL spectra of solution obtained from the piezocatalytic system using NaNbO₃ and V-NaNbO₃ (10 mol%) as the catalysts after 1 h (Fig. S3). The length of NaNbO₃ and V-NaNbO₃ nanocubes calculated from XRD data of their (101) planes (Table S1)) is available in the online version of this article at 10.1007/s12274-022-4506-0.

Stepwise dechlorination of chlorinated alkenes on an Fe-Ni/rGO/Ni foam cathode: Product control by one-electron-transfer reactions

Research on the stepwise hydrogenation dechlorination of chlorinated alkenes forms an important basis for eliminating toxic intermediate incomplete dechlorination products. The low-cost Fe-Ni/rGO/Ni foam cathode both supplied electrons and exhibited hydrogen conversion activity, and it was an excellent tool for the study of stepwise dechlorination. Electrochemical reduction experiments were carried out on homologous chlorinated alkenes. The conditions affecting the dechlorination efficiency and the repeatability of the catalytic electrode were analyzed. The trichloroethylene (TCE) removal rates were all above 78.0% over 8 cycles. The maximum EHDC efficiency was as high as 86.1%, and the faradaic efficiency was over 78.8%. Electrochemical methods combined with the calculation of the electron transfer number are proposed to verify the good hydrogenation ability of the electrode and the stepwise reduction ability at proper voltages. The stepwise dechlorination electroreduction characteristics of chlorinated alkenes were explained. The C-Cl bond dissociation enthalpies of chlorinated alkenes were calculated by density functional theory (DFT), and the 4-Cl and 5-Cl of TCE were expected to be removed first. The stepwise cleavage of chlorinated alkenes on Fe-Ni/rGO/Ni foam during dichlorination provided a reference for controlling the reduction products of chlorinated alkenes and preventing the pollution caused by toxic intermediate products formed during incomplete dechlorination.

Enhanced Photocatalytic hydrolysis performance of titanium disulfide nanosheets by chiral molecules loaded

The photoelectrochemical (PEC) water decomposition is a promising method to produce hydrogen from water. In order to improve the water decomposition efficiency of the PEC process, it is necessary to inhibit the generation of H 2 O 2  byproducts and reduce the overpotential required by cheap catalysts and a high current density. Studies have shown that coating the electrode with chiral molecules or chiral films can increase the hydrogen production and reduce the generation of H 2 O 2  byproducts. This is interpreted as the result of a chiral induced spin selectivity (CISS) effect, which induces a spin correlation between the electrons that are transferred to the anode. Here, we report the adsorption of chiral molecules onto titanium disulfide nanosheets. Firstly, titanium disulfide nanosheets were synthesized via thermal injection and then dispersed through ultrasonic crushing. This strategy combines the CISS effect with the plasma effect caused by the narrow bandgap of two-dimensional sulfur compounds to promote the PEC water decomposition with a high current density.

Biodegradable Protein-Stabilized Inorganic Nanoassemblies for Photothermal Radiotherapy of Hepatoma Cells

Inorganic nanomaterials require optimal engineering to retain their functionality yet can also biodegrade within physiological conditions to avoid chronic accumulation in their native form. In this work, we have developed gelatin-stabilized iron oxide nanoclusters having a primary crystallite size of ∼10 nm and surface-functionalized with indocyanine green (ICG)-bound albumin-stabilized gold nanoclusters (Prot-IONs). The Prot-IONs are designed to undergo disintegration in an acidic microenvironment of tumor in the presence of proteolytic enzymes within 72 h. These nanoassemblies demonstrate bio- and hemocompatibility and show significant photothermal efficiency due to strong near infrared absorption contributed by ICG. The surface gold nanoclusters could efficiently sensitize hepatoma cells to γ-irradiation with substantial cytoskeletal and nuclear damage. Sequential irradiation of Prot-ION-treated cancer cells with near infrared (NIR) laser (λ = 750 nm) and γ-irradiation could cause ∼90% cell death compared to single treatment groups at a lower dose of nanoparticles. The superparamagnetic nature of Prot-IONs imparted significant relaxivity (∼225 mM^-1 s^-1) for T₂-weighted magnetic resonance imaging. Additionally, they could also be engaged as photoacoustic and NIR imaging contrast agents. This work demonstrates bioeliminable inorganic nanoassemblies with significant theranostic potential.

Catalyzed H2O2 decomposition over iron oxides and oxyhydroxides: Insights from oxygen production and organic degradation

Iron minerals, such as iron oxides and iron oxyhydroxides, are the main influential soil components in catalyzed hydrogen peroxide propagation (CHP). Due to their dual effects on H₂O₂ activation to produce reactive oxygen species (ROS) and invalid consumption to produce oxygen, the intrinsic reactivity of iron minerals toward H₂O₂ decomposition requires comprehensive investigations. Herein, six iron minerals (hematite, magnetite, maghemite, goethite, feroxyhyte, and ferrihydrite) for H₂O₂ decomposition were investigated by a combination of normalized kinetic rate constants of H₂O₂ decomposition (Nk_H2O2), O₂ production (Nk_O2), benzoic acid degradation (Nk_BA), and hexachloroethane degradation (Nk_HCA) over the surface area of each mineral. The results indicate H₂O₂ decomposition over iron minerals is a surface-related heterogeneous process. Hematite and goethite are the most promising minerals for environmental cleanup in terms of ROS production, because their H₂O₂ utilization efficiency for benzoic acid (BA) degradation (0.138 and 0.024 mol BA/mol H₂O₂ for hematite and goethite, respectively) are highest among the six iron minerals. Magnetite and maghemite are highly active for both H₂O₂ decomposition and O₂ production at neutral and basic pHs. The presence of organic compounds suppresses O₂ production by more than 60%, which favors H₂O₂ utilization. Ferrihydrite and feroxyhyte are considered as the problematic mineral for CHP due to that the two minerals acquire a high O₂ production and negligible ROS generation at all pHs. The results of this study provide new insights to increase the understandings of H₂O₂-iron mineral systems and guide the application of iron minerals in chemical oxidation technologies.

Inorganic Metal-Oxide Photocatalyst for H2 O2 Production

Hydrogen peroxide (H₂ O₂ ) is a mild but versatile oxidizing agent with extensive applications in bleaching, wastewater purification, medical treatment, and chemical synthesis. The state-of-art H₂ O₂ production via anthraquinone oxidation is hardly considered a cost-efficient and environment-friendly process because it requires high energy input and generates hazardous organic wastes. Photocatalytic H₂ O₂ production is a green, sustainable, and inexpensive process which only needs water and gaseous dioxygen as the raw materials and sunlight as the power source. Inorganic metal oxide semiconductors are good candidates for photocatalytic H₂ O₂ production due to their abundance in nature, biocompatibility, exceptional stability, and low cost. Progress has been made to enhance the photocatalytic activity toward H₂ O₂ production, however, H₂ O₂ photosynthesis is still in the laboratory research phase since the productivity is far from satisfaction. To inspire innovative ideas for boosting the H₂ O₂ yield in photocatalysis, the most well-studied metal oxide photocatalysts are selected and the modification strategies to improve their activity are listed. The mechanisms for H₂ O₂ production over modified photocatalysts are discussed to highlight the facilitating role of the modification methods. Besides, methods for the quantification of H₂ O₂ and associated radical intermediates are provided to guide future studies in this field.

Persistent free radicals in biochar enhance superoxide-mediated Fe(III)/Fe(II) cycling and the efficacy of CaO2 Fenton-like treatment

Superoxide radicals (O₂^•-) produced by the reaction of Fe(III) with H₂O₂ can regenerate Fe(II) in Fenton-like reactions, and conditions that facilitate this function enhance Fenton treatment. Here, we developed an efficient Fenton-like system by using calcium peroxide/biochar (CaO₂/BC) composites as oxidants and tartaric acid-chelated Fe(III) as catalysts, and tested it for enhanced O₂^•--based Fe(II) regeneration and faster sulfamethoxazole (SMX) degradation. SMX degradation rates and peroxide utilization efficiencies were significantly higher with CaO₂/BC than those with CaO₂ or H₂O₂ lacking BC. The CaO₂/BC system showed superior activity to reduce Fe(III), while kinetic analyses using chloroform as a O₂^•- probe inferred that the O₂^•- generation rate by CaO₂/BC was one-half of that by CaO₂. Apparently, O₂^•- is utilized more efficiently in this system to regenerate Fe(II) and enhance SMX degradation. Additionally, a positive correlation between SMX degradation rate constants and EPR signal intensities of biochar-derived persistent free radicals (PFRs) in CaO₂/BC was obtained. We postulate that PFRs enhanced Fe(III) reduction by shuttling electrons donated by O₂^•-. This represents a new strategy to augment the ability of superoxide to accelerate Fe(III)/Fe(II) cycling for increased hydroxyl radical production and organic pollutant removal in Fenton-like reactions.