Metal-organic framework-derived multifunctional photocatalysts

Catalytic applications of organic-inorganic hybrid porous materials

Organic-inorganic hybrid porous materials (OIHMs) inherit the unique properties from both organic and inorganic components, and the flexibility in the incorporation of functional groups renders the OIHMs an ideal platform for the construction of catalytic materials with multiple active sites. The preparation of OIHMs with precise locations of organic-inorganic components and tunable structures is one of the important topics for the catalytic application of OIHMs, but it is still very challenging. In this feature article, we describe our work related to the preparation of OIHMs via confining active sites in the nanostructure and a layer-by-layer assembly method and their applications in acid-base catalysis, catalytic hydrogenation and photocatalysis with a focus on the elucidation of the synergistic effects of different active sites and the unique properties of OIHMs in catalysis.

Zirconium metal-organic framework photocatalysis with TEMPO for blue light-powered aerobic sulfoxidation

A zirconium metal-organic framework (MOF), NU-1003, along with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), is constructed as a system of cooperative photocatalysis for blue light-powered aerobic sulfoxidation. The incorporation of 4 mol% of TEMPO enhances the hole transfer and thereby aerobic sulfoxidation by NU-1003 photocatalysis, demonstrating the pivotal role of a redox mediator in transforming an established MOF.

Polyoxometalate-encapsulated metal-organic frameworks for photocatalytic uranium isolation

Recycling uranium (U) via adsorption and controlled conversion is crucial for the sustainable development of nuclear energy, in which photocatalytic reduction of U(vi) from aqueous solutions is considered one of the most effective strategies. The primary challenge in the photocatalytic elimination of U(vi) resides in the demand for photocatalysts with exceptional properties for effective U(vi) adsorption and charge separation. Herein, we developed the hybrids of polyoxometalate@Cu-metal-organic frameworks (POM@Cu-MOFs) through a self-assembly strategy and demonstrated the efficient removal of U(vi) via synergistic adsorption and photocatalysis. The abundant oxygen-rich groups in POM served as the adsorption sites, endowing POM@Cu-MOFs with a remarkable removal capacity (1987.4 mg g-1 under light irradiation) to remove 99.4% of UO2 2+. The attraction of electrons from Cu atoms within Cu-MOFs effectively accelerated the carrier dynamics due to their pronounced electronegativity. A mechanism associated with the synergetic effects of adsorption and photocatalytic reduction of U(vi) was proposed. This work provides a feasible approach for efficiently eliminating U(vi) from aqueous solutions in environmental pollution cleanup using the POM@Cu-MOF photocatalyst.

Boosting Photocatalytic Hydrogen Production by MOF-Derived Metal Oxide Heterojunctions with a 10.0 % Apparent Quantum Yield

Photocatalytic hydrogen production offers an alternative pathway to establish a sustainable energy economy, utilizing the Earth's natural sunlight and water resources to address environmental concerns associated with fossil fuel combustion. While numerous photoactive materials exhibit high potential for generating hydrogen from water, the synergy achieved by combining two different materials with complementary properties in the form of heterojunctions can significantly enhance the rate of hydrogen production and quantum efficiency. Our study describes the design and generation of the metal-organic framework-derived (MOF) metal oxide heterojunction herein referred to as RTTA, composed of RuO2/N,S-TiO2. The RuO2/N,S-TiO2 is generated through the pyrolysis of MOFs, Ru-HKUST-1, and the amino-functionalized MIL-125-NH2 in the presence of thiourea. Among the various RTTA materials tested, RTTA-1, characterized by the lowest RuO2 content, exhibited the highest hydrogen evolution rate, producing 10,761 μmol ⋅ hr-1 ⋅ g-1 of hydrogen with an apparent quantum yield of 10.0 % in pure water containing glycerol. In addition to RTTA-1, we generated two other MOF-derived metal oxide heterojunctions, namely ZTTA-1 (ZnO/N,S-TiO2) and ITTA-1 (In2O3/N,S-TiO2). These heterojunctions were tested for their photocatalytic activity, leading to apparent quantum yields of 0.7 % and 0.3 %, respectively. The remarkable photocatalytic activity observed in RTTA-1 is thought to be attributed to the synergistic effects arising from the combination of metallic properties inherent in the metal oxides, complemented by the presence of suitable band alignment, porosity, and surface properties inherited from the parent MOFs. These properties enhance electron transfer and restrict hole movement. The photocatalytic efficiency of RTTA-1 was further demonstrated in actual water samples, producing hydrogen with a rate of 8,190 μmol ⋅ hr-1 ⋅ g-1 in tap water, and 6,390 μmol ⋅ hr-1 ⋅ g-1 in river water.

Unveiling the potential of MOF-based single-atom photocatalysts for the production of clean fuel and valuable chemical

Harnessing solar energy through photocatalysis has excellent potential for powering sustainable chemical production, supporting the United Nations' environmental goals. Single-atoms (SAs) dispersed on catalyst surfaces are gaining attention for their highly active and durable nature. Metal-organic frameworks (MOFs) can provide enough reactive sites to sustain selectivity and durability over time because of their tunable channels and functional groups. Owing to their organized structures, MOFs are ideal platforms for securing individual atoms and promoting solar-driven reactions. Few reviews have, however, reflected the possibility of combining MOFs and SAs to produce potent photocatalysts that may produce clean fuels and valuable chemicals. This review provides a general overview of methods for combining MOFs and SAs to generate photocatalysts. The challenges associated with these MOF-based single-atom systems are also critically examined. Their future development is discussed as continued refinement helps to more fully leverage their advantages for boosting photocatalytic performances - turning sunlight into chemicals in a manner that supports sustainable development. Insights gained here could illuminate pathways toward realizing the profound potential of MOF-based single-atom photocatalysts to empower production driven by renewable solar energy.

Synergistic Effects of MOFs and Noble Metals in Photocatalytic Reactions: Mechanisms and Applications

Photocatalytic technology can efficiently convert solar energy to chemical energy and this process is considered as one of the green and sustainable technology for practical implementation. In recent years, metal-organic frameworks (MOFs) have attracted widespread attention due to their unique advantages and have been widely applied in the field of photocatalysis. Among them, noble metals have contributed significant advances to the field as effective catalysts in photocatalytic reactions. Importantly, noble metals can also form a synergistic catalytic effect with MOFs to further improve the efficiency of photocatalytic reactions. However, how to precisely control the synergistic effect between MOFs and noble metals to improve the photocatalytic performance of materials still needs to be further studied. In this review, the synergistic effects of MOFs and noble metal catalysts in photocatalytic reactions are firstly summarized in terms of noble metal nanoparticles, noble metal monoatoms, noble metal compounds, and noble metal complexes, and focus on the mechanisms and advantages of these synergistic effects, so as to provide useful guidance for the further research and application of MOFs and contribute to the development of the field of photocatalysis.

Photothermal-Enhanced S-Scheme Heterojunction of Hollow Core-Shell FeNi2S4@ZnIn2S4 toward Photocatalytic Hydrogen Evolution

Herein, guided by the results of density functional theory prediction, the study rationally designs a hollow core-shell FeNi2S4@ZnIn2S4 (FNS@ZIS) Step-scheme (S-scheme) heterojunction for photocatalytic H2 evolution with photothermal-assisted. The hollow FNS spheres offered substrate for coating the ZIS nanosheets, which can inhibit ZIS nanosheets from agglomerating into pellet, enrich the active site, increase specific surfaces, and raise the light absorption. Notably, due to its excellent photothermal properties, FNS core generated heat unceasingly inside under visible-light irradiation and effectively prevent the heat loss of the reaction system, which increased the local temperature of photocatalysts and thus accelerated the charge migration. In addition, the S-scheme heterojunction construction via in situ growth has a tight interface, which can facilitate the separation and transfer of carriers and achieve high redox potential. Owning to the distinctive construction, the hollow core-shell FNS@ZIS S-scheme heterojunction show extraordinary stability and photocatalytic H2 evolution rate with 7.7 mmol h-1 g-1, which is ≈15.2-fold than pristine ZIS. Based on the double evidence of theoretical predictions and experimental confirmations, the photothermal effect and electron transfer mechanism of this innovative material are investigated in depth by the following infrared thermography technology and deep DFT calculations.

Efficient metal-organic framework-based dual co-catalysts system assist CdS for hydrogen production from photolysis of water

Metal-organic framework materials (MOFs) and their derivatives have been widely used in the field of photocatalytic water decomposition for hydrogen production. In this study, NiS/CdS was initially acquired and subsequently combined with DUT-67 via ultrasound to create a unique ternary photocatalyst NiS/CdS@DUT-67. The rate of hydrogen production for NiS/CdS@DUT-67 is 9618 μmol·g NiS/CdS-1·h-1 for NiS/CdS@DUT-67, which is 32 times and 2.5 times higher than that for CdS and NiS/CdS, respectively. Of particular interest is the fact that even after 50 h of photocatalysis, the hydrogen production rate did not show a significant decrease, demonstrating its excellent stability compared to CdS and NiS/CdS. In this ternary system, NiS and DUT-67 function as dual co-catalysts for CdS, collaborating to enhance charge separation during the photocatalysis. This study presents a clear demonstration of the advantages of utilizing metal-organic framework derivatives (MOF-derivatives) cophotocatalysts and their synergistic effect, resulting in improved photocatalytic activity and stability of semiconductors. This innovative approach provides a new perspective on constructing photocatalytic materials with exceptional performance.

Metal-organic framework-based S-scheme heterojunction photocatalysts

Photocatalysis is a promising technology to resolve energy and environmental issues, where the design of high-efficiency photocatalysts is the central task. As an emerging family of photocatalysts, semiconducting metal-organic frameworks (MOFs) with remarkable features have demonstrated great potential in various photocatalytic fields. Compared to MOF-based photocatalysts with a single component, construction of S-scheme heterojunctions can render MOFs with enhanced charge separation, redox capacity and solar energy utilization, and thus improved photocatalytic performance. Herein, an overview of the recent advances in the design of MOF-based S-scheme heterojunctions for photocatalytic applications is provided. The basic principle of S-scheme heterojunctions is introduced. Then, three types of MOF-based S-scheme heterojunctions with different compositions are systematically summarized including MOF/non-MOF, MOF-on-MOF and MOF-derived heterojunctions. Afterwards, the enhanced performances of MOF-based S-scheme heterojunctions in hydrogen production, CO2 reduction, C-H functionalization, H2O2 production and wastewater treatment are highlighted. Lastly, the current challenges and future prospects regarding the design and applications of MOF-based S-scheme heterojunctions are discussed to inspire the further development of this emerging field.

Metal-organic frameworks (MOFs) for photoelectrocatalytic (PEC) reducing carbon dioxide (CO2) to hydrocarbon fuels

MOF-based photoelectrocatalysis (PEC) using CO2 as an electron donor offers a green, clean, and extensible way to make hydrocarbon fuels under more tolerant conditions. Herein, basic principles of PEC reduction of CO2 and the preparation methods and characterization techniques of MOF-based materials are summarized. Furthermore, three applications of MOFs for improving the photoelectrocatalytic performance of CO2 reduction are described: (i) as photoelectrode alone; (ii) as a co-catalyst of semiconductor photoelectrode or as a substrate for loading dyes, quantum dots, and other co-catalysts; (iii) as one of the components of heterojunction structure. Challenges and future wave surrounding the development of robust PEC CO2 systems based on MOF materials are also discussed briefly.

Rational Construction of MOF-Derived Porous ZnTiO3/TiO2 Heterostructured Photocatalysts with Remarkable Photocatalytic Performance

TiO2 has been widely used in photodegradation of pollutants, but it suffers from inferior photocatalytic performance under solar light illumination. Thus, novel porous ZnTiO3/TiO2 heterostructured photocatalysts are constructed by hydrothermal and carbonization techniques using ZIF-8 as a sacrificial template. After coating with TiO2, ZIF-8 nanocubes are selectively etched and subsequently coprecipitated with Ti ions during the hydrothermal process. Thereafter, the pores generated from carbonized ZIF-8 provide a large specific surface area and abundant active reaction sites for photocatalysis after annealing, producing stable ZnTiO3/TiO2 nanocomposites. Thus, porous ZnTiO3/TiO2 heterostructured photocatalysts exhibit excellent photocatalytic performance under solar light irradiation due to the boosted electron-hole separation/transfer. The kinetic constant of ZnTiO3/TiO2 nanocomposites (4.66 × 10-1 min-1) is almost 100 and 3.7 times higher than that of self-degradation (4.69 × 10-3 min-1) and TiO2 (1.27 × 10-1 min-1), respectively. This facile strategy provides a deep insight into synthesizing heterostructured photocatalysts with high efficiency in the field of environmental remediation.

Visible-light-induced photocatalytic CO2 reduction over zirconium metal organic frameworks modified with different functional groups

The reduction of CO₂ into high value-added chemicals and fuels by a photocatalytic technology can relieve energy shortages and the environmental problems caused by greenhouse effects. In the current work, an amino-functionalized zirconium metal organic framework (Zr-MOF) was covalently modified with different functional groups via the condensation of Zr-MOF with 2-pyridinecarboxaldehyde (PA), salicylaldehyde (SA), benzaldehyde (BA), and trifluoroacetic acid (TA), named Zr-MOF-X (X = PA, SA, BA, and TA), respectively, through the post-synthesis modification. Compared with Zr-MOF and Zr-MOF-TA, the introduction of PA, SA, or BA into the framework of Zr-MOF can not only enhance the visible-light harvesting and CO₂ capture, but also accelerate the photogenerated charge separation and transfer, thereby improving the photocatalytic ability of Zr-MOF for CO₂ reduction. These results indicate that the modification of Zr-MOF with electron-donating groups can promote the photocatalytic CO₂ reduction. Therefore, the current work provides an instructive approach to improve the photocatalytic efficiency of CO₂ reduction through the covalent modification of MOFs.

Fast photocatalytic degradation of rhodamine B using indium-porphyrin based cationic MOF under visible light irradiation

A broad light-harvesting range and efficient charge separation are two main ways to enhance the visible photocatalytic performance of semiconductors. Herein, an ionic porphyrin MOF [In(TPyP)]·(NO3) (1) (TPyP = 5,10,15,20-tetrakis(4-pyridyl)-21H,23H-porphyrin) was synthesized via in situ metalation. The orderly arranged porphyrin photosensitizer and the internal electric field between the MOF host and NO3- guests enable effective visible light response and electron-hole separation. Consequently, the as-synthesized MOF shows efficient photocatalytic degradation of rhodamine B (RhB), methyl orange (MO) and methylene blue (MB) organic pollutants. It can degrade 99.07% of RhB within only 20 minutes under visible light irradiation (λ > 420 nm) with a high chemical reaction rate constant of 0.2400 min-1. The photocatalytic activity of the title MOF is more efficient than those of other reported MOFs, COFs and even inorganic semiconductors. The reusability, energy level, band gap, charge distribution and main degradation mechanisms of the photocatalyst were well studied.

UiO-66 MOF-Derived Ru@ZrO2 Catalysts for Photo-Thermal CO2 Hydrogenation

The use of metal–organic frameworks (MOFs) as templates or precursors in the manufacture of heterogeneous catalysts is highly attractive due to the transfer of MOFs’ inherent porosity and homogeneous metallic distribution to the derived structure. Herein, we report on the preparation of MOF-derived Ru@ZrO2 catalysts by controlled thermal treatment of zirconium-based MOF UiO-66 with ruthenium moieties. Ru3+ (3 or 10 mol%) precursor was added to UiO-66 synthesis and, subsequently, the as-synthesized hybrid structure was calcined in flowing air at different temperatures (400–600 °C) to obtain ZrO2-derived oxides doped with highly dispersed Ru metallic clusters. The materials were tested for the catalytic photo-thermal conversion of CO2 to CH4. Methanation experiments were conducted in a continuous flow (feed flow rate of 5 sccm and 1:4 CO2 to H2 molar ratio) reactor at temperatures from 80 to 300 °C. Ru0.10@ZrO2 catalyst calcined at 600 °C was able to hydrogenate CO2 to CH4 with production rates up to 65 mmolCH4·gcat.–1·h–1, CH4 yield of 80% and nearly 100% selectivity at 300 °C. The effect of the illumination was investigated with this catalyst using a high-power visible LED. A CO2 conversion enhancement from 18% to 38% was measured when 24 sun of visible LED radiation was applied, mainly due to the increase in the temperature as a result of the efficient absorption of the radiation received. MOF-derived Ru@ZrO2 catalysts have resulted to be noticeably active materials for the photo-thermal hydrogenation of CO2 for the purpose of the production of carbon-neutral methane. A remarkable effect of the ZrO2 crystalline phase on the CH4 selectivity has been found, with monoclinic zirconia being much more selective to CH4 than its cubic allotrope.

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO2 reduction reaction (CRR) is an effective strategy because it affords the conversion of CO2 into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst-promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.

Collaborative hydrothermal and calcination fabrication of ZnOS heterostructures for visible-light-driven H2 production

Photocatalytic water splitting is forecasted as a promising strategy for H₂ production. In this work, novel zinc oxide/zinc sulfide (ZnOS-x) (x = 1, 2, 3 and 4) heterostructures were fabricated by a collaborative hydrothermal and calcination method with different amounts of trithiocyanuric acid. The formation of ZnOS-x heterostructures was confirmed by PXRD, XPS, and HRTEM. Moreover, ZnOS-3 nanoparticles exhibited homogeneous and smooth surface morphology structure. ZnOS-3 displayed efficient charge separation and transfer efficiency upon photoinduction. ZnOS-3 showed the highest average H₂ evolution reaction rate (78.87 μmol h^-1) under visible-light irradiation, which increased with increase in the ratio of trithiocyanuric acid in the ZnOS-x series. This work provides a new insight to prepare uniformly integrated heterostructures of metal oxides/sulfides for visible-light-driven H₂ generation.

Efficient and Stable Catalytic Hydrogen Evolution of ZrO2/CdSe-DETA Nanocomposites under Visible Light

Composite photocatalysts are crucial for photocatalytic hydrogen evolution. In this work, ZrO2/CdSe-diethylenetriamine (ZrO2/CdSe-DETA) heterojunction nanocomposites are synthesized, and efficiently and stably catalyzed hydrogen evolution under visible light. X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscope (HRTEM) confirm the formation of heterojunctions between ZrO2 (ZO) and CdSe-DETA (CS). Ultraviolet–visible spectroscopy diffuse reflectance spectra (UV-vis DRS), Mott–Schottky, and theoretical calculations confirm that the mechanism at the heterojunction of the ZrO2/CdSe-DETA (ZO/CS) nanocomposites is Type-I. Among the ZO/CS nanocomposites (ZO/CS-0.4, ZO/CS-0.6, and ZO/CS-0.8; in the nanocomposites, the mass ratio of ZO to CS is 0.1:0.0765, 0.1:0.1148, and 0.1:0.1531, respectively). ZO/CS-0.6 nanocomposite has the best photocatalytic hydrogen evolution activity (4.27 mmol g−1 h−1), which is significantly higher than ZO (trace) and CS (1.75 mmol g−1 h−1). Within four cycles, the ZO/CS-0.6 nanocomposite maintains an efficient catalytic hydrogen evolution rate. Due to the existence of the heterojunction of the composites, the photogenerated electron-hole pairs can be effectively separated, which accelerates the photocatalytic hydrogen evolution reaction and reduces the progress of photocorrosion. This work reveals the feasibility of ZO/CS nanocomposite photocatalysts for hydrogen evolution.

Designing a novel heterostructure AgInS2@MIL-101(Cr) photocatalyst from PET plastic waste for tetracycline degradation

Semiconductor-containing porous materials with a well-defined structure could be unique scaffolds for carrying out selective organic transformations driven by visible light. We herein introduce for the first time a heterostructure of silver indium sulfide (AgInS₂) ternary chalcogenide and a highly porous MIL-101(Cr) metal-organic framework (MOF) synthesised from polyethylene terephthalate plastic waste. Our results demonstrate that AgInS₂ nanoparticles were uniformly attached to each lattice plane of the octahedral MIL-101(Cr) structure, resulting in a nanocomposite with a high distribution of semiconductors in a porous media. We also demonstrate that the nanocomposite with up to 40% of AgInS₂ doping exhibited excellent catalytic activity for tetracycline degradation under visible light irradiation (∼99% tetracycline degraded after 4 h) and predominantly maintained its performance after five cycles. These results could promote a new material circularity pathway to develop new semiconductors that can be used to protect water from further pollution.

Recent Progress of Metal-Organic Frameworks and Metal-Organic Frameworks-Based Heterostructures as Photocatalysts

In the field of photocatalysis, metal-organic frameworks (MOFs) have drawn a lot of attention. MOFs have a number of advantages over conventional semiconductors, including high specific surface area, large number of active sites, and an easily tunable porous structure. In this perspective review, different synthesis methods used to prepare MOFs and MOFs-based heterostructures have been discussed. Apart from this, the application of MOFs and MOFs-based heterostructures as photocatalysts for photocatalytic degradation of different types of pollutants have been compiled. This paper also highlights the different strategies that have been developed to modify and regulate pristine MOFs for improved photocatalytic performance. The MOFs modifications may result in better visible light absorption, effective photo-generated charge carriers (e-/h+), separation and transfer as well as improved recyclability. Despite that, there are still many obstacles and challenges that need to be addressed. In order to meet the requirements of using MOFs and MOFs-based heterostructures in photocatalysis for low-cost practical applications, future development and prospects have also been discussed.

Synthesis of a Zr4(embonate)6-cobalt based superstructure for photocatalytic hydrogen production

Herein, we report an efficient method to construct cage-based MOF materials with exposed metal active sites for catalysis. By employing Zr₄L₆ (L = embonate) cages as precursors for assembly with N-containing ligands and Co²⁺ ions, a new Zr₄L₆-Co based chain structure (PTC-318) has been generated through two-step reactions. Interestingly, in the absence of a photosensitizer, PTC-318 exhibits notable photocatalytic activity for H₂ evolution under visible-light irradiation.