Recent Strategies for Hydrogen Peroxide Production by Metal-Free Carbon Nitride Photocatalysts

Regulation of Photogenerated Redox Species through High Crystallinity Carbon Nitride for Improved C-S Coupling Reactions

A novel and efficient approach for the synthesis of α, β-unsaturated sulfones through heterogeneous photocatalyzed C-S coupling reactions have been developed. The use of molten-salt method derived carbon nitride (MCN), a transition metal-free polymeric photocatalyst, combined with enhanced crystallinity and potassium iodide as an additive, effectively modulates photogenerated reactive redox species, markedly increasing the overall reaction selectivity. This method achieves the shortest reaction time (2 h) with high yield (up to 95 %) among the reported heterogeneous catalytic C-S bond formation reactions, matching the efficiency of the homogeneous photocatalysts. Furthermore, the application to challenging alkyne substrates has been demonstrated, underscoring the potential for a broad range of applications in pharmaceutical research and synthetic chemistry.

Robust Light Driven Enzymatic Oxyfunctionalization via Immobilization of Unspecific Peroxygenase

Unspecific peroxygenases have attracted interest in synthetic chemistry, especially for the oxidative activation of C-H bonds, as they only require hydrogen peroxide (H2 O2 ) instead of a cofactor. Due to their instability in even small amounts of H2 O2 , different strategies like enzyme immobilization or in situ H2 O2 production have been developed to improve the stability of these enzymes. While most strategies have been studied separately, a combination of photocatalysis with immobilized enzymes was only recently reported. To show the advantages and limiting factors of immobilized enzyme in a photobiocatalytic reaction, a comparison is made between free and immobilized enzymes. Adjustment of critical parameters such as (i) enzyme and substrate concentration, (ii) illumination wavelength and (iii) light intensity results in significantly increased enzyme stabilities of the immobilized variant. Moreover, under optimized conditions a turnover number of 334,500 was reached.

Expression and biochemical characterization of a Bacillus subtilis catalase in Pichia pastoris X-33

Catalase, which catalyzes the decomposition of H₂O₂ to H₂O and O₂, is widely used to reduce H₂O₂ in industrial applications, such as in food processing, textile dyeing and wastewater treatment. In this study, the catalase (KatA) from Bacillus subtilis was cloned and expressed in the yeast Pichia pastoris X-33. The effect of the promoter in the expression plasmid on the activity level of the secreted KatA protein was also studied. First, the gene encoding KatA was cloned and inserted into a plasmid containing an inducible alcohol oxidase 1 promoter (pAOX1) or a constitutive glyceraldehyde-3-phosphate dehydrogenase promoter (pGAP). The recombinant plasmids were validated by colony PCR and sequencing and then linearized and transformed into the yeast P. pastoris X-33 for expression. With the promoter pAOX1, the maximum yield of KatA in the culture medium reached 338.8 ± 9.6 U/mL in 2 days of shake flask cultivation, which was approximately 2.1-fold greater than the maximum yield obtained with the promoter pGAP. The expressed KatA was then purified from the culture medium by anion exchange chromatography, and its specific activity was determined to be 14826.58 U/mg. Finally, the purified KatA exhibited optimum activity at 25 °C and pH 11.0. Its K_m for hydrogen peroxide was 10.9 ± 0.5 mM, and its k_cat/K_m was 5788.1 ± 25.6 s^-1 mM^-1. Through the work presented in this article, we have therefore demonstrated efficient expression and purification of KatA in P. pastoris, which might be advantageous for scaling up the production of KatA for use in a variety of biotechnological applications.

Dual molecules engineered carbon nitride for achieving outstanding photocatalytic H2O2 production

Molecular engineering of carbon nitride (CN) was considered as a suitable and compelling strategy to overcome the intrinsic imperfections and enhance photocatalytic H₂O₂ production. However, the photocatalytic H₂O₂ production of conventional single molecular engineering is still unsatisfactory, and the comprehension of photogenerated carrier migration and separation is still indistinct. Herein, dual molecules were engineered on CN molecular skeleton for achieving an outstanding photocatalytic rate of H₂O₂ production. The photocatalytic H₂O₂ production rate of the dual molecules engineered CN was up to 3320 μmol g^-1 h^-1, which was approximately 25 times than that of the pristine CN. After the dual-molecular engineering, pyrimidine and cyano group were co-grafted. Synchronously, K ion and Na ion were co-embedded near the interlamination of CN layers. The synergistic effect of the dual molecules in CN not only restrained photogenerated carrier recombination and broadened visible light response by modulating the intrinsic energy band structure, but also enhanced the capture of the photogenerated electrons and accelerated the migration of proton. Hence, the photocatalytic 2e^- oxygen reduction reaction, the rate-determining step, was significantly strengthened. Additionally, caused by the positive valence band potential, the H₂O oxidation reaction became an indispensable role in photocatalytic H₂O₂ production. This work provided a viable route to modulate the molecular skeleton of organic semiconductors and presented a promising strategy to obtain high-efficient photocatalytic H₂O₂ production.

Photocatalytic and Electrocatalytic Generation of Hydrogen Peroxide: Principles, Catalyst Design and Performance

Hydrogen peroxide (H2O2) is a high-demand organic chemical reagent and has been widely used in various modern industrial applications. Currently, the prominent method for the preparation of H2O2 is the anthraquinone oxidation. Unfortunately, it is not conducive to economic and sustainable development since it is a complex process and involves unfriendly environment and potential hazards. In this context, numerous approaches have been developed to synthesize H2O2. Among them, photo/electro-catalytic ones are considered as two of the most promising manners for on-site synthesis of H2O2. These alternatives are sustainable in that only water or O2 is required. Namely, water oxidation (WOR) or oxygen reduction (ORR) reactions can be further coupled with clean and sustainable energy. For photo/electro-catalytic reactions for H2O2 generation, the design of the catalysts is extremely important and has been extensively conducted with an aim to obtain ultimate catalytic performance. This article overviews the basic principles of WOR and ORR, followed by the summary of recent progresses and achievements on the design and performance of various photo/electro-catalysts for H2O2 generation. The related mechanisms for these approaches are highlighted from theoretical and experimental aspects. Scientific challenges and opportunities of engineering photo/electro-catalysts for H2O2 generation are also outlined and discussed.

BiVO4-Deposited MIL-101-NH2 for Efficient Photocatalytic Elimination of Cr(VI)

In this study, a flower-like BiVO₄/MIL-101-NH₂ composite is synthesized by a facile and surfactant-free process. The -COO^--Bi³⁺ ionic bond construction was conductive to enhance the interface affinity between BiVO₄ and MIL-101-NH₂. Due to the highly efficient light capture and sufficient electron traps induced by oxygen vacancies and the formation of a heterostructure, the improved separation and transportation rates of charge carriers are realized. In addition, the MIL-101-NH₂/BiVO₄ composite is favorable for Cr(VI) photocatalytic removal (91.2%). Moreover, FNBV-3 (Fe/Bi = 0.25) also exhibited an excellent reusability after five cycles.

Enhanced H2O2 Production via Photocatalytic O2 Reduction over Structurally-Modified Poly(heptazine imide)

Solar H₂O₂ produced by O₂ reduction provides a green, efficient, and ecological alternative to the industrial anthraquinone process and H₂/O₂ direct-synthesis. We report efficient photocatalytic H₂O₂ production at a rate of 73.4 mM h^-1 in the presence of a sacrificial donor on a structurally engineered catalyst, alkali metal-halide modulated poly(heptazine imide) (MX → PHI). The reported H₂O₂ production is nearly 150 and >4250 times higher than triazine structured pristine carbon nitride under UV-visible and visible light (≥400 nm) irradiation, respectively. Furthermore, the solar H₂O₂ production rate on MX → PHI is higher than most of the previously reported carbon nitride (triazine, tri-s-triazine), metal oxides, metal sulfides, and other metal-organic photocatalysts. A record high AQY of 96% at 365 nm and 21% at 450 nm was observed. We find that structural modulation by alkali metal-halides results in a highly photoactive MX → PHI catalyst which has a broader light absorption range, enhanced light absorption ability, tailored bandgap, and a tunable band edge position. Moreover, this material has a different polymeric structure, high O₂ trapping ability, interlayer intercalation, as well as surface decoration of alkali metals. The specific C≡N groups and surface defects, generated by intercalated MX, were also considered as potential contributors to the separation of photoinduced electron-hole pairs, leading to enhanced photocatalytic activity. A synergy of all these factors contributes to a higher H₂O₂ production rate. Spectroscopic data help us to rationalize the exceptional photochemical performance and structural characteristics of MX → PHI.

Plasma-Tuned nitrogen vacancy graphitic carbon nitride sphere for efficient photocatalytic H2O2 production

Graphitic carbon nitride (CN) is a promising photocatalyst for sustainable energy conversion. Meanwhile, N vacancies are useful for H₂O₂ generation; however, they are hard to control. In this study, the N vacancy CN sphere (NVCNS) is synthesized by H₂ plasma treatment to tune the NV. The as-synthesized NVCNS exhibits an efficient and stable photocatalytic H₂O₂ yield of 4413.1 μmol g_cat^-1h^-1, which is 2.5 and 4.6 times higher than that of CNS (1766.4 μmol g_cat^-1h^-1) and bulk CN (956.6 μmol g_cat^-1h^-1), respectively, using a Xe lamp with an intensity of 100 mWcm^-2. In particular, the charges recombination rate is remarkably reduced by introducing N defect state, promoting electron accumulation and O₂ adsorption, through theoretical calculation and experiments. Furthermore, the NV creates abundant unsaturated sites and induces strong interlayer interactions, leading to effective electronic excitation and the promotion of charge transport.

Global evolution of research on green energy and environmental technologies:A bibliometric study

To better grasp developments and trends in research on green energy and environmental technologies, the published literatures in the Web of Science Core Collection database from 1998 to 2020 were utilized to reveal critical information and guidance on what has been investigated, and what are the changes in research interests using a bibliometric method. Herein, yearly quantitative distribution of literatures, author contribution and collaboration, productive and influential institutions and countries/territories, co-citation analysis, keywords co-occurrence analysis, and research frontier identification are analyzed via information visualization technology. The results show that the publications on green energy and environmental technologies have grown exponentially, that China, the USA, and Italy are the most active countries, but the global cooperation is not close at present. The research frontier identification results reveal that the categories of energy, wastewater, and performance remain stable, while the trending up and emerging categories of catalyst and CO₂ emission show clear shift over the last decade, indicating that catalytic production of clean energy and value-added chemicals, strategies to reduce greenhouse gas emissions, and other related studies to solve the global energy crisis and environmental problems are the research frontiers. This bibliometric study provides unique insights and offers research guidance on green energy and environmental technologies.

Oxidation process of lead sulfide nanoparticle in the atmosphere or natural water and influence on toxicity toward Chlorella vulgaris

Lead sulfide nanoparticle (nano-PbS) released into environment can cause hazards to human or ecosystem. Nano-PbS potentially undergoes oxidation in the environment, but oxidation mechanism is not understood yet. Herein, oxidation kinetics and products of nano-PbS by ozone (O₃), hydrogen peroxide (H₂O₂) and hydroxyl radical (HO·) in the atmosphere or natural water were investigated. Results show that oxidation process of nano-PbS can be divided into three stages, producing sulfate, ions and oxides of lead in sequence. O₃ or HO·leads to faster release of ionic lead from nano-PbS in the initial stage than H₂O₂, but causes significant decrease of ionic lead by transforming divalent lead to tetravalent lead oxides in the second or third stage. Toxicity determined taking Chlorella Vulgaris as an example follows an order of PbO₂ < Pb₃O₄ < nano-PbS < PbO < PbSO₄. Toxicity of lead particles is mainly determined by sizes influencing cellular uptake and solubility product constant (K_sp) related with dissolution of lead in cells. The results indicate that the toxicity of nano-PbS increases in an initial oxidation stage and decreases in further oxidation stages. This study provides new insights into environmental behavior of nano-PbS and mechanism understandings for assessing ecological risks of nano-PbS.

State-of-the-art and current challenges for TiO2/UV-LED photocatalytic degradation of emerging organic micropollutants

The development of ultraviolet light-emitting diodes (UV-LED) opens new possibilities for water treatment and photoreactor design. TiO₂ photocatalysis, a technology that has been continuously drawing attention, can potentially benefit from LEDs to become a sustainable alternative for the abatement of organic micropollutants (OMPs). Recently reported data on photocatalytic degradation of OMPs and their parameters of influence are here critically evaluated. The literature on OMP degradation in real water matrices, and at environmentally relevant concentrations, is largely missing, as well as the investigations of the impact of photoreactor design in pollutant degradation kinetics. The key factors for reducing UV-LED treatment technology costs are pointed out, like the increase in external quantum and wall-plug efficiencies of UV-LEDs compared to other technologies, as well as the need for an appropriate design optimizing light homogeneity in the reactor. Controlled periodic illumination, wavelength coupling and H₂O₂ addition are presented as efficiency enhancement options. Although electrical energy per order (E_EO) values for UV-LED photocatalysis have decreased to the range of traditional mercury lamps, values are still not low enough for practical employment. Moreover, due to the adoption of high initial OMP concentration in most experiments, it is likely that most literature E_EO values are overestimated. Given the process characteristics, which are favoured by translucent matrices and small diameters for more homogenous light distribution and better transportation of radicals, innovative reactor designs should explore the potential of point-of-use applications to increase photocatalysis applicability at large scale.

Graphitic carbon nitride photocatalysis: the hydroperoxyl radical role revealed by kinetic modelling

The behaviour of graphitic carbon nitride photocatalysis for phenol removal and H2O2 evolution was fully analysed by kinetic modelling, rediscovering the contribution of oxygen, reactive oxygen species, photogenerated holes and intermediate products.

KNO3-Assisted incorporation of K dopants and N defects into g-C3N4 with enhanced visible light driven photocatalytic H2O2 production

Facile KNO3-assisted synthesis of g-C3N4 incorporated with K dopants and N defects for efficient visible light driven photocatalytic H2O2 production.

Photooxidation of water with heptazine-based molecular photocatalysts: Insights from spectroscopy and computational chemistry

We present a conspectus of recent joint spectroscopic and computational studies that provided novel insight into the photochemistry of hydrogen-bonded complexes of the heptazine (Hz) chromophore with hydroxylic substrate molecules (water and phenol). It was found that a functionalized derivative of Hz, tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a homogeneous photochemical reaction. This allows the exploration of the basic mechanisms of the proton-coupled electron-transfer (PCET) process involved in the water photooxidation reaction in well-defined complexes of chemically tunable molecular chromophores with chemically tunable substrate molecules. The unique properties of the excited electronic states of the Hz molecule and derivatives thereof are highlighted. The potential energy landscape relevant for the PCET reaction has been characterized by judicious computational studies. These data provided the basis for the demonstration of rational laser control of PCET reactions in TAHz-phenol complexes by pump-push-probe spectroscopy, which sheds light on the branching mechanisms occurring by the interaction of nonreactive locally excited states of the chromophore with reactive intermolecular charge-transfer states. Extrapolating from these results, we propose a general scenario that unravels the complex photoinduced water-splitting reaction into simple sequential light-driven one-electron redox reactions followed by simple dark radical-radical recombination reactions.

Fabrication of Fe3O4@PVA-Cu Nanocomposite and Its Application for Facile and Selective Oxidation of Alcohols

Fe₃O₄@PVA-Cu nanocomposite was introduced as an affordable catalyst for selective oxidation of alcohols into various aldehydes and ketones. The synthesized nanocomposite was characterized by applying essential analyses. The peaks that are appeared in FT-IR spectroscopy confirmed the production of the Fe₃O₄@PVA-Cu nanocomposite. In addition, EDX analysis proved the presence of oxygen, carbon, iron, and copper elements in the catalyst. Further, TGA analysis showed high thermal stability of the nanocomposite. VSM technique was applied to examine the magnetic property of the nanocomposite. The results demonstrated a high magnetic property in the catalyst, which enables easy separation of it from the reaction solution. TEM and SEM imaging showed the nanoscale size of the particles (~20 nm) in the catalyst. Additionally, XRD data was compatible with that of Fe₃O₄ nanoparticles. The application of the nanocomposite has been studied in the selective oxidation of alcohols in the presence of acetonitrile as solvent, and hydrogen peroxide as a supplementary oxidizing agent. This technique is remarkably facile and inexpensive. Further, the products showed high yields. In addition, the calculated TON and TOF values indicated the phenomenal efficiency of the nanocomposite in preparation of targeted products.

Synthesis and characterization of vanadium-doped Mo(O,S)2 oxysulfide for efficient photocatalytic degradation of organic dyes

We developed simple and low cost synthesis methods at low temperature to synthesize V-doped Mo(O,S)₂ for the photocatalytic degradation of dyes.