Rational Design of Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Production

Photocatalytic production of hydrogen peroxide (H2O2) represents a significant approach to achieving sustainable energy generation through solar energy, addressing both energy shortages and environmental pollution. Among various photocatalytic materials, covalent organic frameworks (COFs) have gained widespread attention and in-depth research due to their unique advantages, including high porosity, predesignability, and atomic-level tunability. In recent years, significant progress has been made in the development, performance enhancement, and mechanistic understanding of COF-based photocatalysts. This review focuses on the latest advancements in photocatalytic H2O2 production using COFs, particularly emphasizing the rational design of COF structures to regulate catalytic performance and exploring the fundamental processes involved in photocatalysis. Based on current research achievements in this field, this paper also discusses existing challenges and future opportunities, aiming to provide a reference for the application of COFs in photocatalytic H2O2 production.

Bringing Porous Framework Materials toward Photocatalytic H2O2 Production

Photocatalytic H2O2 production driven by renewable solar energy is a promising and sustainable approach, with porous framework materials such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and hydrogen-bonded organic frameworks (HOFs) emerging as highly efficient catalysts. This Review first presents the current research state of porous framework materials in H2O2 photosynthesis, focusing on the progress in H2O2 production across different porous frameworks and mechanism insights gained through advanced techniques. Furthermore, a systematic categorization of material modifications aimed at enhancing the photocatalytic efficiency is provided, linking structural modifications to improved H2O2 production performance. Key factors such as charge carrier separation and transfer, reaction pathways, and material stability are comprehensively analyzed. Finally, the challenges related to stability, scalability, and cost-effectiveness, are discussed alongside opportunities for future advancements. This Review aims to provide insights into understanding and optimizing porous framework materials for efficient and scalable H2O2 photosynthesis.

Designing 1-nm-Thick MOF Nanosheets with Donor-Acceptor Complexes for Photosynthesis of H2O2 Using Water and Dioxygen Only

Artificial photosynthesis of hydrogen peroxide (H2O2) presents a promising environmentally friendly alternative to the industrial anthraquinone process. This work designed ultrathin metal-organic framework (MOF) nanosheets on which porphyrin ligand as an electron donor (D) and anthraquinone (AQ) as an electron acceptor (A) are integrated as the D-A complexes. The porphyrin component allows the MOF nanosheets to absorb full-spectrum solar light while the acceptor AQ motif promotes central aluminum ion coordination, hindering layer stacking to achieve a thickness of 1.0 nm. The ultrathin D-A design facilitates the separation of electrons from the MOF skeleton to the AQ motif, which induces the direct two-electron oxygen reduction reaction (ORR) mediated by the reversible redox couple of AQ-AQH2 and multielectron water oxidation reaction (WOR) driven by holes remaining on the porphyrin part. In O2-saturated water, the ultrathin MOF nanosheets outperformed the AQ-free bulk and multilayered counterparts by 2.9 and 2.6 times in H2O2 production, respectively, achieving the apparent quantum yield of 4.8% at 420 nm. It also surpasses other benchmark photocatalysts, including the typical MOF photocatalyst, MIL-125-NH2, and organic polymeric photocatalysts. The ultrathin D-A MOF photocatalyst generated H2O2 via both two-electron ORR as a major path and two-electron WOR as a minor path. This approach presents a promising strategy for the rational design of efficient nanostructured photocatalysts for solar fuels and chemicals.

Water quality constraints H2O2 production in a dual-fiber photocatalytic reactor

In-situ hydrogen peroxide (H2O2) finds applications in disinfection and oxidation processes. Photoproduction of H2O2 from water and oxygen, avoids reliance upon organic chemicals, and potentially enables smaller-sized or lower-cost reactors than electrochemical methods. In ultrapure water, we previously demonstrated a novel dual-fiber system coupling a light emitting diode (LED) with a metal-organic framework (MOF) catalyst-coated optical fiber (POF-MIL-101(Fe)) and O2-based hollow-membrane fibers and achieved a remarkable H2O2 yield, 308 ± 1.4 mM h-1 catalyst-g-1. To enable H2O2 production anywhere we sought to understand the impacts of common water quality parameters. The production of H2O2 was not affected by added sodium, potassium, hydroxide, sulfate or nitrate ions. There was consistent performance over a wide pH range (4-10), maintaining a high production rate of 232 ± 3.5 mM h-1 catalyst-g-1 even at pH 10, a condition typically unfavorable for H2O2 photoproduction. Chloride ions produced hypochlorous acid, consuming in-situ produced H2O2. Phosphate adsorption on the iron-based MOF catalysts blocked H2O2 production. Inorganic carbon species inhibited H2O2 production due to in-situ formic acid. Encouraging results were obtained using atmospheric water (i.e., condensate), with rates reaching 288 ± 6.1 mM h-1 catalyst-g-1, comparable to ultrapure water. This underscores atmospheric water as a variable alternative, available in nearly all building air conditioning systems or could overcome geographical constraints, particularly in regions where obtaining pure water resources is challenging, offering a cost-effective solution. The dual-fiber reactor using atmospheric water enables high-efficiency H2O2 production anytime and anywhere.

Solar Light-Driven Molecular Oxygen Activation by BiOCl Nanosheets: Synergy of Coexposed {001}, {110} Facets and Oxygen Vacancies

Single-crystalline BiOCl nanosheets with coexposed {001} and {110} facets, as well as oxygen vacancies, were synthesized using a simple method. These nanosheets have the ability to activate molecular oxygen, producing reactive superoxide radicals (77.8%) and singlet oxygen (22.2%) when exposed to solar light. The BiOCl demonstrated excellent photocatalytic efficiency in producing H2O2 under simulated solar light and in oxidatively hydroxylating phenylboronic acid under blue LED light. Our research highlights the significance of constructing coexposed {001} and {110} facets, as well as oxygen vacancies, in enhancing photocatalytic performance. The BiOCl nanosheets have the capability to produce H2O2 with a solar-to-chemical energy conversion efficiency of 0.11%.

S-scheme NiO/C3N5 heterojunctions with enhanced interfacial electric field for boosting photothermal-assisted photocatalytic H2 and H2O2 production

Heterostructured visible-light-responsive photocatalysts represent a prospective approach to achieve efficient solar-to-chemical energy conversion. Herein, we propose a facile self-assembly technique to synthesize NiO nanoparticles/C3N5 nanosheets (NOCN) heterojunctions for hydrogen (H2) evolution catalysis and hydrogen peroxide (H2O2) production under visible light. In this regard, the black NiO nanoparticles (NPs) were tightly anchored on the surface of C3N5 nanosheets (CNNS) to construct S-scheme NOCN heterojunction, enabling efficient charge separation and high redox capability. Obtained results elucidated that the incorporated NiO NPs significantly promote light-harvesting efficiency and photo-to-thermal capacity over the NOCN composites. The enhanced photothermal effect facilitates the charge carrier transfer rate across the heterojunction and boosts the surface reaction kinetics. Accordingly, the photocatalytic performances of CNNS for H2 release and H2O2 production can be manipulated by introducing NiO NPs. The modified photocatalytic properties of NOCN composites are ascribed to the synergistic effects of all integrated components and the S-scheme heterojunction formation. Impressively, the high H2 evolution photocatalysis efficiency of NOCN nano-catalysts in seawater certifies their potential environmental applicability. Among all, the 12-NOCN nano-catalyst exhibits a higher photocatalytic efficiency for H2 release (112.2 μmol∙g-1∙h-1) and H2O2 production (91.2 μmol∙L-1∙h-1). Besides, the 12-NOCN nano-catalyst reveals excellent recyclability and structural stability. Additionally, the possible mechanism for photothermal-assisted photocatalysis is proposed. This work affords a feasible pathway to design photothermal-assisted S-scheme heterojunctions for diverse photocatalytic applications.

Structure determines performance: isomeric Ti-MOFs for photocatalytic synthesis of hydrogen peroxide

NH2-UiO-66(Ti) and NH2-MIL-125(Ti) were successfully prepared by using post-synthesis exchange (PSE) and hydrothermal methods, and then these frameworks were tested for photocatalytic hydrogen peroxide production in pure water under visible light. NH2-MIL-125(Ti) exhibits superior activity compared to NH2-UiO-66(Ti) due to its shorter Ti-O bond. In addition, NH2-MIL-125-D (defective) demonstrates a high photocatalytic yield of hydrogen peroxide owing to the presence of defect-rich titanium rich sites.

Directing the Selectivity of Oxygen Reduction to Water by Confining a Cu Catalyst in a Metal Organic Framework

Electrocatalysis is to play a key role in the transition towards a sustainable chemical and energy industry and active, stable and selective redox catalysts are much needed. Porous structures such as metal organic frameworks (MOFs) are interesting materials as these may influence selectivity of chemical reactions through confinement effects. In this work, the oxygen reduction catalyst Cu-tmpa was incorporated into the NU1000 MOF. Confinement of the catalyst within NU1000 steers the selectivity of the oxygen reduction reaction (ORR) towards water rather than peroxide. This is attributed to retention of the obligatory H2 O2 intermediate in close proximity to the catalytic center. Moreover, the resulting NU1000|Cu-tmpa MOF shows an excellent activity and stability in prolonged electrochemical studies, illustrating the potential of this approach.

Metal-organic frameworks as photocatalysts in energetic and environmental applications

Metal-organic frameworks (MOFs) are an exciting new class of porous materials with great potential for photocatalytic applications in the environmental and energy sectors. MOFs provide significant advantages over more traditional materials when used as photocatalysts due to their high surface area, adaptable topologies, and functional ability. In this article, we summarize current developments in the use of MOFs as photocatalysts for a variety of applications, such as CO2 reduction, water splitting, pollutant degradation, and hydrogen production. We discuss the fundamental properties of MOFs that make them ideal for photocatalytic applications, as well as strategies for improving their performance. The opportunities and challenges presented by this rapidly expanding field are also highlighted.

Oxygen Vacancy-Mediated Z-Scheme Charge Transfer in a 2D/1D B-Doped g-C3N4/rGO/TiO2 Heterojunction Visible Light-Driven Photocatalyst for Simultaneous/Efficient Oxygen Reduction Reaction and Alcohol Oxidation

Hydrogen peroxide (H₂O₂) is a powerful oxidant that directly or indirectly oxidizes many organic and inorganic contaminants. The photocatalytic generation of H₂O₂ is achieved by using a semiconductor photocatalyst in the presence of alcohol as a proton source. Herein, we have synthesized oxygen vacancy (O_v)-mediated TiO₂/B-doped g-C₃N₄/rGO (TBCN@rGO) ternary heterostructures by a simple hydrothermal technique. Several characterization techniques were employed to explore the existence of oxygen vacancies in the crystal structure and investigate their impact on the optoelectronic properties of the catalyst. Oxygen vacancies offered additional sites for adsorbing molecular oxygen, activating alcohols, and facilitating electron migration from TBCN@rGO to the surface-adsorbed O₂. The defect creation (oxygen vacancy) and Z-scheme mechanistic pathways create a suitable platform for generating H₂O₂ by two-electron reduction processes. The optimized catalyst showed the highest photocatalytic H₂O₂ evolution rate of 172 μmol/h, which is 1.9 and 2.5 times greater than that of TBCN and BCN, respectively. The photocatalytic oxidation of various lignocellulose-derived alcohols (such as furfural alcohol and vanillyl alcohol) and benzyl alcohol was also achieved. Photocatalytic activity data, physicochemical and optoelectronic features, and trapping experiments were conducted to elucidate the structure-activity relationships. The TBCN@rGO acts as a multifunctional Z-scheme photocatalyst having an oxygen vacancy, modulates surface acidity-basicity required for the adsorption and activation of the reactant molecules, and displays excellent photocatalytic performance due to the formation of a large number of active surface sites, increased electrical conductivity, improved charge transfer properties, outstanding photostability, and reusability. The present study establishes a unique strategy for improving H₂O₂ generation and alcohol oxidation activity and also provides insights into the significance of a surface vacancy in the semiconductor photocatalyst.

Lignin-Supported Heterogeneous Photocatalyst for the Direct Generation of H2O2 from Seawater

The development of smart and sustainable photocatalysts is in high priority for the synthesis of H₂O₂ because the global demand for H₂O₂ is sharply rising. Currently, the global market share for H₂O₂ is around 4 billion US$ and is expected to grow by about 5.2 billion US$ by 2026. Traditional synthesis of H₂O₂ via the anthraquinone method is associated with the generation of substantial chemical waste as well as the requirement of a high energy input. In this respect, the oxidative transformation of pure water is a sustainable solution to meet the global demand. In fact, several photocatalysts have been developed to achieve this chemistry. However, 97% of the water on our planet is seawater, and it contains 3.0-5.0% of salts. The presence of salts in water deactivates the existing photocatalysts, and therefore, the existing photocatalysts have rarely shown reactivity toward seawater. Considering this, a sustainable heterogeneous photocatalyst, derived from hydrolysis lignin, has been developed, showing an excellent reactivity toward generating H₂O₂ directly from seawater under air. In fact, in the presence of this catalyst, we have been able to achieve 4085 μM of H₂O₂. Expediently, the catalyst has shown longer durability and can be recycled more than five times to generate H₂O₂ from seawater. Finally, full characterizations of this smart photocatalyst and a detailed mechanism have been proposed on the basis of the experimental evidence and multiscale/level calculations.

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.

A covalent modification strategy for di-alkyne tagged metal–organic frameworks to access efficient heterogeneous catalysts toward C–C bond formation

A di-alkyne tagged MOF was proposed to introduce diverse metals, and UiO-66-(alkyne-Co)2 displayed an efficient catalytic performance for the Knoevenagel reaction.

A metal free catalyst for efficient and stable one-step photocatalytic production of pure hydrogen peroxide

Hydrogen peroxide (H2O2) is widely used as a green and clean energy. The pure H2O2 solution has attracted much attention for its special applications in many fields. Photocatalytic water splitting...

Lewis acid-triggered photocatalytic hydrogen peroxide production in an aluminum-based metal–organic framework

Al-based MIL-101-NH2, which was previously regarded as having silent photo-features, exhibits photocatalytic H2O2 production via O2 reduction accompanied by efficient suppression of undesired H2O2 decomposition.

Photocatalytic production of H2O2 over facet-dependent Ti-MOF

The Z-scheme structure between Ti-MOF (001) and (111) facets for photocatalytic H2O2 production.

Effect of Ligand Functionalization on the Rate of Charge Carrier Recombination in Metal-Organic Frameworks: A Case Study of MIL-125

Ligand functionalization is a powerful approach for modifying the electronic structure of metal-organic frameworks when targeting the optimal electronic properties for photocatalysis and photovoltaics. However, its effect on the charge carrier lifetimes and recombination pathways remains unexplored. In this work, first-principles simulations, including nonadiabatic molecular dynamics, are performed for the representative TiO₂-based metal-organic framework systems MIL-125-X to unravel the impact of ligand functionalization on the nonradiative electron-hole recombination process, decoherence rates, and phonon modes giving the largest contribution to the nonradiative decay. Nonradiative recombination rates, simulated using the PBE0 density functional, are in excellent agreement with experiment. The ligand functionalization in MIL-125-X influences the recombination rates, unraveling the trend opposite to the evolution of the band gap and affecting the nonadiabatic coupling coefficients. Ligand modification impacts the phonon modes, which contribute most to the recombination process, altering the distribution between soft phonon modes and vibrational modes associated with specific structural motifs.

Efficient capturing of hydrogen peroxide in dilute aqueous solution by co-crystallization with amino acids

X-ray structure analyses of co-crystals of H2O2 and l-Phe, dl-Phe, or dl-Asp prepared in a dilute aqueous solution (30 wt%) indicated that multi-layer motifs including water molecule is important for highly efficient H2O2 capture in dilute solutions.

Recent Advances in Photocatalytic CO2 Utilisation Over Multifunctional Metal–Organic Frameworks

The efficient conversion of carbon dioxide (CO2) to high-value chemicals using renewable solar energy is a highly attractive but very challenging process that is used to address ever-growing energy demands and environmental issues. In recent years, metal–organic frameworks (MOFs) have received significant research attention owing to their tuneability in terms of their composition, structure, and multifunctional characteristics. The functionalisation of MOFs by metal nanoparticles (NPs) is a promising approach used to enhance their light absorption and photocatalytic activity. The efficient charge separation and strong CO2 binding affinity of hybrid MOF-based photocatalysts facilitate the CO2 conversion process. This review summarises the latest advancements involving noble metal, non-noble-metal, and miscellaneous species functionalised MOF-based hybrid photocatalysts for the reduction of CO2 to carbon monoxide (CO) and other value-added chemicals. The novel synthetic strategies and their corresponding structure–property relationships have also been discussed for solar-to-chemical energy conversion. Furthermore, the current challenges and prospects in practical applications are also highlighted for sustainable energy production.