Polyoxometalates-derived metal oxides incorporated into graphitic carbon nitride framework for photocatalytic hydrogen peroxide production under visible light

Donor-acceptor type triphenylamine-based porous aromatic frameworks (TPA-PAFs) for photosynthesis of benzimidazoles

The development of efficient and recyclable photocatalysts for organic synthesis is of great interest. This study presents the synthesis of triphenylamine-based porous aromatic frameworks (TPA-PAFs) in an alternating donor-acceptor (D-A) manner. The light absorption range and the optical band gaps of TPA-PAFs are effectively tuned by changing the electron acceptor units, which further determine their photocatalytic properties. As a result, TPA-PAFs exhibit excellent catalytic performance for the photosynthesis of benzimidazoles in high yields (up to 99%), broad substrate scope (18 examples), and good recyclability (up to 10 cycles). This work provides a feasible approach toward the facile design and synthesis of efficient and stable PAF-based photocatalysts, which further broadens the application of PAFs catalytic materials in photocatalytic organic synthesis.

Developing Polymeric Carbon Nitrides for Photocatalytic H2O2 Production

Hydrogen peroxide (H2O2) plays a crucial role in various applications, such as green oxidation processes and the production of high-quality fuels. Currently, H2O2 is primarily manufactured using the indirect anthraquinone method, known for its significant energy consumption and the generation of intensive by-products. Extensive research has been conducted on the photocatalytic production of H2O2 via oxygen reduction reaction (ORR), with polymeric carbon nitride (PCN) emerging as a promising catalyst for this conversion. This review article is organized around two approaches. The first part main consists of the chemical optimization of the PCN structure, while the second focuses on the physical integration of PCN with other functional materials. The objective is to clarify the correlation between the physicochemical properties of PCN photocatalysts and their effectiveness in H2O2 production. Through a thorough review and analysis of the findings, this article seeks to stimulate new insights and achievements, not only in the domain of H2O2 production via ORR but also in other redox reactions.

Photosynthesis of Hydrogen Peroxide Based on g-C3 N4 : The Road of a Cost-Effective Clean Fuel Production

Emerging artificial photosynthesis promises to offer a competitive means for solar energy conversion and further solves the energy crisis facing the world. Hydrogen peroxide (H2 O2 ), which is considered as a benign oxidant and a prospective liquid fuel, has received worldwide attention in the field of artificial photosynthesis on account of the source materials are just oxygen, water, and sunlight. Graphitic carbon nitride (g-C3 N4 )-based photocatalysts for H2 O2 generation have attracted extensive research interest due to the intrinsic properties of g-C3 N4 . In this review, research processes for H2 O2 generation on the basis of g-C3 N4 , including development, fabrication, merits, and disadvantages, and the state-of-the-art methods to enhance the performance are summarized after a brief introduction and the mechanism analysis of an efficient catalytic system. Also, recent applications of g-C3 N4 -based photocatalysts for H2 O2 production are reviewed, and the significance of active sites and synthetic pathways are highlighted from the view of reducing barriers. Finally, this paper ends with some concluding remarks to reveal the issues and opportunities of g-C3 N4 -based photocatalysts for producing H2 O2 in a high yield.

Cu0@CuOx-NC modified Zn2In2S5 for photo-self-Fenton system coupling H2O2 in-situ production and decomposition

Although many concerns have been put into photocatalytic hydrogen peroxide (H₂O₂) production, multifunctional catalysis suitable for continuously in-situ H₂O₂ consumption in the field has rarely been investigated. Herein, Cu⁰@CuO_x@nitrogen-doped graphitic carbon (Cu⁰@CuO_x-NC) decorated Zn₂In₂S₅ was successfully prepared for in-situ production and activation H₂O₂, which could achieve effectively photocatalytic self-Fenton degradation of tetracycline (TC). Under visible light irradiation, 5wt% Cu⁰@CuO_x-NC/Zn₂In₂S₅ (CuZS-5) efficiently generated a high yield of H₂O₂ (0.13 mmol L^-1), and Cu⁰@CuO_x-NC could in-situ consume H₂O₂ to generate hydroxyl radicals (•OH), accelerating the oxidation of TC. As a result, the 5 wt% Cu⁰@CuO_x-NC/Zn₂In₂S₅ degraded about 89.3% of TC within 60 min, and the cycle experiments also exhibited sufficient stability. This study achieves a delicate combination of in-situ production and activation of H₂O₂, which is regarded as a promising strategy to eco-friendly promote pollutant degradation in wastewater.

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.

Visible light-driven H2O2 synthesis over Au/C3N4: medium-sized Au nanoparticles exhibiting suitable built-in electric fields and inhibiting reverse H2O2 decomposition

Visible light-driven H₂O₂ production presents the unique merits of sustainability and environmental friendliness. The size of noble metal nanoparticles (NPs) determines their dispersion and electronic structure and greatly affects their photocatalytic activity. In this work, a series of sized Au NPs over C₃N₄ were modulated for H₂O₂ production. The results show that there is a volcanic trend in H₂O₂ with the decrease of Au particle size, and the highest H₂O₂ production rate of 1052 μmol g^-1 h^-1 is obtained from medium-sized Au particles (∼8.7 nm). The relationship between structure and catalytic performance is supported by experimental and theoretical methods. (1) First, medium-sized Au NPs promote photon absorption, and have a suitable built-in electric field at the heterojunction, which can be successfully tuned to achieve a more efficient h⁺-e^- spatial separation. (2) Second, medium-sized Au NPs enhance O₂ adsorption, and create selective 2e^- O₂ reduction reaction sites. (3) Particularly, medium-sized Au NPs promote the desorption of produced H₂O₂ and inhibit H₂O₂ decomposition, finally leading to the highest H₂O₂ selectivity. Excellent catalytic performance will be obtained by finely optimizing the particle size in a certain range. This work provides a new idea for preparing high efficiently photocatalysts for H₂O₂ production.

Glycerol Valorization Towards Glycolic Acid Production Over Cu-Based Biochar Catalyst

Glycerol valorization towards high-value chemicals is of particular importance to increase the value chain of biodiesel production. In this study, the catalytic activity of a series of cheap Cu-based catalysts for glycerol conversion is investigated. Cu supported on activated carbon (AC, obtained through carbonization of coconut shell) exhibits outstanding catalytic activity for the selective conversion of glycerol into glycolic acid (GcA) in O₂ atmosphere, affording up to 68.3 % GcA yield. The combination of experimental results with theoretical calculations reveals that glyceraldehyde is the key reaction intermediate. The high specific surface area and surface oxygenated groups of AC enable the formation of CuO nanoparticles with small size and uniform dispersion. In addition, the surface oxygen vacancy on Cu/AC might help to activate reaction intermediates, and the electron transfer from Cu to AC facilitates the oxidation of glycerol to GcA. Cu loaded onto AC also significantly inhibits C-C breakage to generate formic acid as a byproduct. This work might aid the development of approaches for glycerol application and afford profitable possibilities for sustainable biodiesel.

Self-assembly of bimetallic polyoxometalates and dicyandiamide to form Co/WC@NC for efficient electrochemical hydrogen generation

Graphene carbon-coated and N-doped WC and cobalt (Co) nanoparticles (Co/WC@NC) were synthesized via a one-step straightforward high-temperature calcination. The resultant Co/WC@NC manifests excellent hydrogen evolution activity.

Nanointerface engineering Z-scheme CuBiOS@CuBi2O4 heterojunction with OS interpenetration for enhancing photocatalytic hydrogen peroxide generation and accelerating chromium(VI) reduction

Designing a core-shell nanointerface is beneficial for enhancing the photocatalytic performance of hydrogen peroxide (H₂O₂) production. Hence, a direct Z-scheme one-dimensional (1 D) CuBiOS@CuBi₂O₄ nanorods with a core (oxide)-shell (sulfide) nanostructure and OS interpenetrated nanointerface was controllably synthesized through in-situ anion exchange. The formation of OS interpenetration at the heterogeneous interface with surface oxygen vacancies could effectively boost light absorption, reduce the interface contact resistance, facilitate band bending, and thus enhance charge separation and transfer as a "bridge". The as-prepared catalyst with tunable OS nanointerface greatly improved the photocatalytic performances in the H₂O₂ production with a yield of 201.9 μmol·L^-1 and the in-situ generated H₂O₂ effectively accelerated the reduction of chromium(VI) (Cr(VI), 95.4% within 15 min). The excellent performances were due to the OS interpenetration with rich oxygen vacancies and unique shell-core structure with intimate contact inter-doping nanointerface. Moreover, the photocatalytic mechanism was discussed in detail. This work might provide a guideline in the design and construction of high-performance catalysts with well-defined nanointerface for various applications.

A metal-free photocatalyst for highly efficient hydrogen peroxide photoproduction in real seawater

Artificial photosynthesis of H2O2 from H2O and O2, as a spotless method, has aroused widespread interest. Up to date, most photocatalysts still suffer from serious salt-deactivated effects with huge consumption of photogenerated charges, which severely limit their wide application. Herein, by using a phenolic condensation approach, carbon dots, organic dye molecule procyanidins and 4-methoxybenzaldehyde are composed into a metal-free photocatalyst for the photosynthetic production of H2O2 in seawater. This catalyst exhibits high photocatalytic ability to produce H2O2 with the yield of 1776 μmol g-1h-1 (λ ≥ 420 nm; 34.8 mW cm-2) in real seawater, about 4.8 times higher than the pure polymer. Combining with in-situ photoelectrochemical and transient photovoltage analysis, the active site and the catalytic mechanism of this composite catalyst in seawater are also clearly clarified. This work opens up an avenue for a highly efficient and practical, available catalyst for H2O2 photoproduction in real seawater.

Enhanced Photocatalytic Hydrogen Production of the Polyoxoniobate Modified with RGO and PPy

The development of high-efficiency, recyclable, and inexpensive photocatalysts for water splitting for hydrogen production is of great significance to the application of solar energy. Herein, a series of graphene-decorated polyoxoniobate photocatalysts Nb₆/PPy-RGO (Nb₆ = K₇HNb₆O₁₉, RGO = reduced graphene oxide, PPy = polypyrrole), with the bridging effect of polypyrrole were prepared through a simple one-step solvothermal method, which is the first example of polyoxoniobate-graphene-based nanocomposites. The as-fabricated photocatalyst showed a photocatalytic H₂ evolution activity without any co-catalyst. The rate of 1038 µmol g⁻¹ in 5 h under optimal condition is almost 43 times higher than that of pure K₇HNb₆O₁₉·13H₂O. The influencing factors for photocatalysts in photocatalytic hydrogen production under simulated sunlight were studied in detail and the feasible mechanism is presented in this paper. These results demonstrate that Nb₆O₁₉ acts as the main catalyst and electron donor, RGO provides active sites, and PPy acted as an electronic bridge to extend the lifetime of photo-generated carriers, which are crucial factors for photocatalytic H₂ production.

Nitrogen-deficient g-C3Nx/POMs porous nanosheets with P-N heterojunctions capable of the efficient photocatalytic degradation of ciprofloxacin

The direct shedding of piperazine rings is critical for the degradation of antibiotic persistent organic pollutants. In this work, nitrogen-deficient g-C₃N₄ loaded with polyoxometalates porous photocatalysts with P-N heterojunctions were carried out through the formation of chemical bonds between the nitrogen-deficient C⁺ in g-C₃N_x and the bridging oxygen in polyoxometalates (POMs), including phosphomolybdic acid (PMA), phosphotungstic acid (PTA) and silicotungstic acid (STA). The adsorption and photocatalysis experiments confirm the ability of the g-C₃N_x/POMs nanosheets to efficiently remove ciprofloxacin via the synergistic effects of adsorption and photo-catalysis. Approximately, g-C₃N_x/POMs-30 exhibits the optimal degradation ability, and the degradation rates of g-C₃N_x/PMA-30, g-C₃N_x/PTA-30 and g-C₃N_x/STA-30 could respectively reach 93.1%, 97.4% and 95.6% within only 5 min under visible light. The free radical scavenging experiment and ESR free radical capture experiments confirm that ·OH and ·O₂^- are free radicals that effectively degrade CIP. According to the results of the LC-MS analysis, the intermediates produced after CIP degradation and the efficient degradation pathway are proposed. The direct shedding of piperazine rings in the decarboxylation and defluorination process leads to the most efficient degradation of CIP into the small molecules.

Recent Progress, Challenges, and Prospects in Two-Dimensional Photo-Catalyst Materials and Environmental Remediation

The successful photo-catalyst library gives significant information on feature that affects photo-catalytic performance and proposes new materials. Competency is considerably significant to form multi-functional photo-catalysts with flexible characteristics. Since recently, two-dimensional materials (2DMs) gained much attention from researchers, due to their unique thickness-dependent uses, mainly for photo-catalytic, outstanding chemical and physical properties. Photo-catalytic water splitting and hydrogen (H2) evolution by plentiful compounds as electron (e-) donors is estimated to participate in constructing clean method for solar H2-formation. Heterogeneous photo-catalysis received much research attention caused by their applications to tackle numerous energy and environmental issues. This broad review explains progress regarding 2DMs, significance in structure, and catalytic results. We will discuss in detail current progresses of approaches for adjusting 2DMs-based photo-catalysts to assess their photo-activity including doping, hetero-structure scheme, and functional formation assembly. Suggested plans, e.g., doping and sensitization of semiconducting 2DMs, increasing electrical conductance, improving catalytic active sites, strengthening interface coupling in semiconductors (SCs) 2DMs, forming nano-structures, building multi-junction nano-composites, increasing photo-stability of SCs, and using combined results of adapted approaches, are summed up. Hence, to further improve 2DMs photo-catalyst properties, hetero-structure design-based 2DMs' photo-catalyst basic mechanism is also reviewed.

In-situ formation of carboxylate species on TiO2 nanosheets for enhanced visible-light photocatalytic performance

It still remains challenge for expanding the photo-response range of TiO₂ with dominant {0 0 1} facets due to the hardly achieving modification of the electronic structure without destroying the formation of TiO₂ high energy facets. Herein, we report the construction of carboxylate species modified TiO₂ nanosheets with dominant {0 0 1} facets by employing ethanol as a carbon source through a low-temperature (300 °C) carbonization method. The as-obtained samples were investigated in detail by using various characterization techniques. The results indicate that the carboxylate species derived from the oxidation and carbonization of ethanol are coordinated to the {0 0 1} facets in a bidentate bridging mode. The electron-withdrawing carboxylate species induce TiO₂ to form a lower valence band edge and a narrower bandgap, which enhances the oxidation ability of photogenerated holes and expands the photo-response range. The partially carbonized carboxylate species can also act as a photosensitizer to induce visible-light photocatalytic activity of TiO₂ nanosheets. In addition, the carboxylate species can further promote the separation of photogenerated charge carriers. The findings of this work may provide a new perspective for tuning the band structure of TiO₂ with dominant {0 0 1} facets and improving its photocatalytic performance.

Preparation of a g-C3N4/Co3O4/Ag2O ternary heterojunction nanocomposite and its photocatalytic activity and mechanism

A g-C₃N₄/Co₃O₄/Ag₂O nanocomposite shows good photocatalytic activities towards the degradation of RhB and H₂O₂ production via the two-electron reduction of oxygen.

Facile synthesis of multi-type carbon doped and modified nano-TiO2 for enhanced visible-light photocatalysis

Nano-TiO₂ is a type of environment-friendly and inexpensive substance that could be used for photocatalytic degradation processes. In this study, the multi-type carbon species doped and modified anatase nano-TiO₂ was innovatively synthesized and developed to overcome the deficiency of common nano-TiO₂ photocatalysts. The multi-type carbon species were derived from tetrabutyl titanate and ethanol as the internal and external carbon sources, respectively. Meanwhile, diverse characterization methods were applied to investigate the morphology and surface properties of the photocatalyst. Finally, the visible-light photocatalytic degradation activity of the collected samples was evaluated by using methyl orange as a model pollutant. The promotion mechanism of multi-type carbon species in the photocatalytic process was also discussed and reported. The results in this work show that the doping and modification of multi-type carbon species successfully narrows the bandgap of nano-TiO₂ to expand the light absorption range, reduces the valence band position to improve the oxidation ability of photogenerated holes, and promotes the separation of photogenerated charge carriers to improve quantum efficiency. In addition, the further modification of the external carbon source can promote the surface adsorption of MO and stabilize the multi-type carbon species on the surface of nano-TiO₂.

The synergistic modification of nano-TiO₂ by multi-type carbon species results in excellent and stable visible-light photocatalytic degradation activity.

A comparative perspective of electrochemical and photochemical approaches for catalytic H2O2 production

Recent advances in the design, preparation, and applications of different catalysts for electrochemical and photochemical H₂O₂ production are summarized, and some invigorating perspectives for future developments are also provided.

Visible-light-driven photocatalytic degradation of sulfamethazine by surface engineering of carbon nitride：Properties, degradation pathway and mechanisms

Polymeric carbon nitride semiconductor has been explored as emerging metal-free photocatalyst for solving the energy shortage and environmental issues. However, the efficiency of carbon nitride is still not satisfying. Herein, a facile copolymerization between L-cysteine and dicyandiamide has been applied to forming the modified carbon nitride photocatalysts. The photocatalytic performance was evaluated through degrading sulfamethazine under visible light illumination. The ameliorative structure and tuned energy band result in visible-light adsorption enhancement. In addition, nitrogen vacancies offer more sites to adsorbing molecular oxygen, thereby facilitating the transfer of electrons from carbon nitride to the surface adsorbed oxygen. As a result, the degradation rate of optimized modified carbon nitride sample for sulfamethazine was 0.1062 min^-1, which was almost 12 times than that of carbon nitride (0.0086 min^-1). Superoxide radicals and holes were mainly responsible for the sulfamethazine photodegradation by modified carbon nitride. Two reaction intermediates/products were observed and identified by high performance liquid chromatography-mass spectrometer, and a possible reaction pathway was proposed. This study provides new insights into the design of highly efficient photocatalyst for other organic pollutants degradation.