Metal-Free Photocatalysis: Two-Dimensional Nanomaterial Connection toward Advanced Organic Synthesis

Engineered Polymeric Carbon Nitride for Photocatalytic Diverse Functionalization of Electronic-Rich Alkenes

Engineered polymeric carbon nitride represents a promising class of metal-free semiconductor photocatalysts for organic synthesis. Herein, we utilized engineered polymeric carbon nitride nanosheets, which exhibit an increased specific surface area and band gap due to enhanced quantum confinement from vacancy enrichment. These nanosheets serve as a heterogeneous organic semiconductor photocatalyst to facilitate diverse functionalizations of electron-rich alkenes, including arylsulfonylation, aminodifluoroalkylation, and oxytrifluoromethylation. This catalytic system operates under mild conditions, offering excellent functional group compatibility and high yields. Additionally, the catalyst demonstrates outstanding recyclability and efficiency in flow reactors, highlighting its significant potential for industrial applications.

Sheets Copper-Cobalt Graphitic Carbon Nitride Dual Single-Atom Catalysts for the Epoxidation of Styrene

Dual single-atom catalysts have attracted considerable research interest due to their higher metal atom loading and more flexible active sites compared to single-atom catalysts (SACs). We pioneered the one-step synthesis of sheets copper-cobalt graphitic carbon nitride dual single-atom (S-Cu/Co-g-C3N4) using folding fan-shaped aluminum foil as a template, and used them as catalysts in the epoxidation of styrene respectively. Through XAFS (X-ray Absorption Fine Structure) and other characterizations, it is found that Cu and Co single atoms are stabilized separately on g-C3N4 via coordination with nitrogen (N), hindered the ordered growth of sheets, and formed more pore structures, which not only increased more catalytically active sites, but also effectively prevented the flakes re-aggregate during the catalytic process. And the synergistic effect between Cu and Co changes the energy band structure of the material and facilitates electron transfer during catalysis, hence an excellent catalytic effect of 89 % styrene conversion and 85 % styrene oxide selectivity was achieved when S-Cu/Co-g-C3N4-1 : 1 was applied in the epoxidation of styrene. Furthermore, the mechanisms of the epoxidation of styrene with S-Cu/Co-g-C3N4-1 : 1 was probed by the density functional theory (DFT) based on the slab model.

Ionic liquid-assisted sustainable preparation of photo-catalytically active nanomaterials and their composites with 2D materials

The preparation of nanomaterials employing ionic liquids (ILs) and surface active ionic liquids (SAILs) in a relatively sustainable manner for different applications is reviewed. ILs offer structure directing and templating effects via inherent bi-continuous structures formed by the segregation of polar and non-polar domains. On the other hand, SAILs offer a structure-directing effect governed by their ability to lower the surface tension, self-assembling nature and interaction with precursors via ionic head groups. Binary mixtures of ILs with other relatively greener solvents or utilization of metal-based ILs (MILs), which act as precursors of metal ions, templates and stabilizing agents propose a new way to prepare a variety of nanomaterials. The introduction of SAILs that exfoliate 2D materials under low-energy bath sonication and also aid in photoreduction and stabilization of photocatalytically active nanomaterials at the surface of 2D materials poses a distinctive perspective in sustainable preparation and utilization of nanomaterials in different photocatalytic applications. The present feature article reviews the employment of distinctive properties of ILs in precise morphological control of nanomaterials, and their after-effects on their catalytic efficiencies.

Molybdenum Disulfide-Based Catalysts in Organic Synthesis: State of the Art, Open Issues, and Future Perspectives

In the field of heterogeneous organic catalysis, molybdenum disulfide (MoS2) is gaining increasing attention as a catalytically active material due to its low toxicity, earth abundance, and affordability. Interestingly, the catalytic properties of this metal-based material can be improved by several strategies. In this Perspective, through the analysis of some explicative examples, the main approaches used to prepare highly efficient MoS2-based catalysts in relevant organic reactions are summarized and critically discussed, namely: i) increment of the specific surface area, ii) generation of the metallic 1T phase, iii) introduction of vacancies, iv) preparation of nanostructured hybrids/composites, v) doping with transition metal ions, and vi) partial oxidation of MoS2. Finally, emerging trends in MoS2-based materials catalysis leading to a richer organic synthesis are presented.

S-Scheme Interface Between K-C3N4 and FePS3 Fosters Photocatalytic H2 Evolution

In photocatalysis, photogenerated charge separation is pivotal and can be achieved through various mechanisms. Building heterojunctions is a promising method to enhance charge separation, where effective contact and charge exchange between heterojunction components remains challenging. Mostly used synthesis processes for making heterostructures require high temperatures, difficult processes, or expensive materials. Herein, a heterojunction of potassium intercalated graphitic carbon nitride (K-CN) and nanoflakes of iron phosphor trisulfide (FPS) is designed via a simple mechanical grinding process to boost the hydrogen evolution by a factor of more than 25 compared to pure K-CN. This significant improvement is rarely reached by other combinations of two semiconductors without cocatalysts, such as platinum. It can be attributed to the band alignment and band bending of an S-scheme that is validated via optical and X-ray photoelectron spectroscopy. As a consequence, strong quenching of the photoluminescence and significant H2 evolution occur for this unique heterojunction. Furthermore, the excellent durability of the designed photocatalytic heterostructure is confirmed by monitoring the catalysts' H2-evolution rate and crystal structure after 72 h under light illumination. This study opens up promising and simple pathways for constructing efficient S-scheme heterojunctions for photocatalytic water-splitting.

Binaphthyl Mediated Low Temperature Synthesis of Carbon Nitride Photocatalyst for Photocatalytic Hydrogen Evolution

Metal-free graphitic carbon nitrides are on the rise as polymer photocatalysts under visible light illumination, taking shares in a range of promising photocatalytic reactions, including water splitting. Their simple synthesis and facile structural modification afford them exceptional tunability, enabling the creation of photocatalysts with distinct properties. While their metal-free nature marks a significant step towards environmental sustainability, the high energy consumption required to produce carbon nitride photocatalysts remains a substantial barrier to their widespread adoption. Furthermore, the process of condensation at approximately 550 °C typically results in solid yields of less than 15 %, significantly challenging their economic viability. Here, we report on lowering manufacturing conditions of carbon nitride photocatalysts whilst enhancing photocatalytic activity by introducing binaphthyl diamine as a structural mediator. At 450 °C in 2 hours, carbon nitride photocatalyst shows a lower bandgap and enables visible light induced hydrogen evolution (194 μmol h-1) comparable to benchmark carbon nitride photocatalysts.

A novel dual S-scheme Co9S8/MnCdS/Co3O4 heterojunction for photocatalytic hydrogen evolution under visible light irradiation

Rational design and synthesis of a unique heterojunction photocatalyst structure is an important strategy to enhance its performance and structural stability. Herein, Co9S8/MnCdS/Co3O4 photocatalysts with double S-scheme heterojunctions were successfully prepared by coupling Co9S8 and Co3O4 sheet structures with n-type MnCdS nanoparticles through a simple solvothermal and mechanical mixing method. The construction of the dual S-scheme heterostructure offers the possibility to expand the light absorption range, extend the carrier lifetime and maximise the redox capacity. In addition, the mechanism of charge transfer and the reason for the improvement of photocatalytic activity were explored through photoelectrochemical characterization. The lamellar structures of Co9S8 and Co3O4 not only provide excellent dispersion and slow down the agglomeration of MnCdS nanoparticles, but also promote charge transfer, which improves the photocatalytic hydrogen production effect. Under simulated solar irradiation, the evolution rate of H2 after 5 h was as high as 46.44 μmol, which was 3.49 and 1.49 times higher than those of pristine MnCdS and MnCdS/Co3O4, respectively. Meanwhile, it has good stability under 20 h irradiation. This work demonstrates a novel idea for the rational design of double S-scheme photocatalysts with efficient space separation.

GFP Chromophore Integrated Conjugated Microporous Polymers toward Bioinspired Photocatalytic CO2 Reduction to CO

The development of highly active, durable, and low-cost metal-free catalysts for the photocatalytic CO2 reduction reaction (CO2RR) is an efficient and environmentally friendly solution to address significant problems like global warming and high energy demand. In the present study, we have demonstrated the design and synthesis of a donor-acceptor based conjugated microporous polymer (CMP), TPA-GFP, by integrating an electron donor, tris(4-ethynylphenyl)amine (TPA), with a green fluorescent protein chromophore analogue (Z)-4-(2-hydroxy-3,5-diiodobenzylidene)-1-(4-iodophenyl)-2-methyl-1H-imidazol-5(4H)-one (o-HBDI-I3) (GFP). In comparison to nondonor 1,3,5-triethynylbenzene (TEB) based TEB-GFP CMP, photocatalytic CO2 reduction using donor-acceptor based TPA-GFP CMP displays a 3-fold increment of CO production yield with a maximum CO yield of 1666 μmol g-1 at 12 h. Further, the CO selectivity increases significantly from a mere 54% in TEB-GFP to an impressive 95% in TPA-GFP. The impressive CO2 reduction efficiency and selectivity for TPA-GFP can be attributed to the efficient light-harvesting capability and facile charge separation and migration through donor-acceptor building units of the CMP. The mechanistic aspect of the photocatalytic CO2 reduction process is explored using in situ DRIFTS and DFT calculation, and a plausible photocatalytic mechanism is proposed.

Hierarchical nickel carbonate hydroxide nanostructures for photocatalytic hydrogen evolution from water splitting

Metal carbonate hydroxides have emerged as novel and promising candidates for water splitting due to their good electrochemical properties and eco-friendly features. In this study, the hierarchical mesoporous structure of nickel carbonate hydroxide hydrate (Ni2(CO3)(OH)2·4H2O) was synthesized by a one-pot facile hydrothermal method. It demonstrated photocatalytic properties for the first time, exhibiting a hydrogen evolution reaction yield of 10 μmol g-1 h-1 under white light irradiation with a nominal power of 0.495 W. This facile synthesis strategy and the good photocatalytic properties indicate that nickel carbonate hydroxide is a promising material for application in photocatalytic hydrogen evolution.

Progress of disinfection catalysts in advanced oxidation processes, mechanisms and synergistic antibiotic degradation

Human diseases caused by pathogenic microorganisms make people pay more attention to disinfection. Meanwhile, antibiotics can cause microbial resistance and increase the difficulty of disease treatment, resulting in risk of triggering a vicious circle. Advanced oxidation process (AOPs) has been widely studied in the field of synergistic treatment of the two contaminates. This paper reviews the application of catalytic materials and their modification strategies in the context of AOPs for disinfection and antibiotic degradation. It also delves into the mechanisms of disinfection such as the pathways for microbial inactivation and the related influencing factors, which are essential for understanding the pivotal role of catalytic materials in disinfection principles by AOPs. More importantly, the exploratory research on the combined use of AOPs for disinfection and antibiotic degradation is discussed, and the potential and prospects in this field is highlighted. Finally, the limitations and challenges associated with the application of AOPs in disinfection and antibiotic degradation are summarized. It aims to provide a starting point for future research efforts to facilitate the widespread use of advanced oxidation processes in the field of public health.

Fluorinated photoreduced graphene oxide with semi-ionic C-F bonds: An effective carbon based photocatalyst for the removal of volatile organic compounds

There is much interest in developing metal-free halogenated graphene such as fluorinated graphene for various catalytic applications. In this work, a fluorine-doped graphene oxide photocatalyst was investigated for photocatalytic oxidation (PCO) of a volatile organic compound (VOC), namely gaseous methanol. The fluorination process of graphene oxide (GO) was carried out via a novel and facile solution-based photoirradiation method. The fluorine atoms were doped on the surface of the GO in a semi-ionic C-F bond configuration. This presence of the semi-ionic C-F bonds induced a dramatic 7-fold increment of the hole charge carrier density of the photocatalyst. The fluorinated GO photocatalyst exhibited excellent photodegradation up to 93.5% or 0.493 h-1 according pseudo-first order kinetics for methanol. In addition, 91.7% of methanol was mineralized into harmless carbon dioxide (CO2) under UV-A irradiation. Furthermore, the photocatalyst demonstrated good stability in five cycles of methanol PCO. Besides methanol, other VOCs such as acetone and formaldehyde were also photodegraded. This work reveals the potential of fluorination in producing effective graphene-based photocatalyst for VOC removal.

Electrochemical impedance spectroscopy, another arrow in the arsenal to study the biodegradability of two-dimensional materials

Carbon nitride (C3N4) is an innovative material with a high potential in many applications including energy storage, catalysis, composites, and biomedicine. C3N4 appears remarkably interesting not only for its properties but also because its simple preparation routes involve low-cost starting materials and reagents. However, there is still a lack of information on its degradability. For this reason, in this study, we evaluate the environmental persistence of C3N4 and its oxidized form by applying the photo-Fenton reaction. The morphological and structural changes of both materials were monitored by transmission electron microscopy and Raman spectroscopy respectively. In addition, electrochemical impedance spectroscopy has been used as an original technique to validate the degradation process of C3N4.

Activating two-dimensional semiconductors for photocatalysis: a cross-dimensional strategy

The emerging two-dimensional (2D) semiconductors substantially extend materials bases for versatile applications such as semiconductor photocatalysis demanding semiconductive matrices and large surface areas. The dimensionality, while endowing 2D semiconductors the unique properties to host photocatalytic functionality of pollutant removal and hydrogen evolution, hurdles the activation paths to form heterogenous photocatalysts where the photochemical processes are normally superior over these on the mono-compositional counterparts. In this perspective, we present a cross-dimensional strategy to employ thenD (n= 0-2) clusters or nanomaterials as activation partners to boost the photocatalytic activities of the 2D semiconductors. The formation principles of heterogenous photocatalysts are illustrated specifically for the 2D matrices, followed by selection criteria of them among the vast 2D database. The computer investigations are illustrated in the density functional theory route and machine learning benefitted from the vast samples in the 2D library. Synthetic realizations and characterizations of the 2D heterogenous systems are introduced with an emphasis on chemical methods and advanced techniques to understand materials and mechanistic studies. The perspective outlooks cross-dimensional activation strategies of the 2D materials for other applications such as CO2removal, and materials matrices in other dimensions which may inspire incoming research within these fields.

Multifunctional carbon nitride nanoarchitectures for catalysis

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Graphitic Carbon Nitride as Photocatalyst for the Direct Formylation of Anilines

The use of graphitic carbon nitride (g-CN) for the photocatalytic radical formylation of anilines, which represents a more sustainable and attractive alternative to the currently used approaches, is reported herein. Our operationally simple method occurs under mild conditions, employing air as an oxidant. In particular, the chemistry is driven by the ability of g-CN to reach an electronically excited state upon visible-light absorption, which has a suitable potential energy to trigger the formation of reactive α-amino radical species from anilines. Mechanistic investigations also proved the key role of the g-CN to form reactive superoxide radicals from O2 via single electron transfer. Importantly, this photocatalytic transformation provides a variety of functionalized formamides (15 examples, up to 89 % yield).

Single-Atom Catalysis in Organic Synthesis

Single-atom catalysts hold the potential to significantly impact the chemical sector, pushing the boundaries of catalysis in new, uncharted directions. These materials, featuring isolated metal species ligated on solid supports, can exist in many coordination environments, all of which have shown important functions in specific transformations. Their emergence has also provided exciting opportunities for mimicking metalloenzymes and bridging the gap between homogeneous and heterogeneous catalysis. This Review outlines the impressive progress made in recent years regarding the use of single-atom catalysts in organic synthesis. We also illustrate potential knowledge gaps in the search for more sustainable, earth-abundant single-atom catalysts for synthetic applications.

Photocatalytic Activity of the Oxidation Stabilized Ti3 C2 Tx MXene in Decomposing Methylene Blue, Bromocresol Green and Commercial Textile Dye

Two-dimensional MXenes are excellent photocatalysts. However, their low oxidation stability makes controlling photocatalytic processes challenging. For the first time, this work elucidates the influence of the oxidation stabilization of model 2D Ti3 C2 Tx MXene on its optical and photocatalytic properties. The delaminated MXene is synthesized via two well-established approaches: hydrofluoric acid/tetramethylammonium hydroxide (TMAOH-MXene) and minimum intensive layer delamination with hydrochloric acid/lithium fluoride (MILD-MXene) and then stabilized by L-ascorbic acid. Both MXenes at a minimal concentration of 32 mg L-1 show almost 100% effectiveness in the 180-min photocatalytic decomposition of 25 mg L-1 model methylene blue and bromocresol green dyes. Industrial viability is achieved by decomposing a commercial textile dye having 100 times higher concentration than that of model dyes. In such conditions, MILD-MXene is the most efficient due to less wide optical band gap than TMAOH-MXene. The MILD-MXene required only few seconds of UV light, simulated white light, or 500 nm (cyan) light irradiation to fully decompose the dye. The photocatalytic mechanism of action is associated with the interplay between surface dye adsorption and the reactive oxygen species generated by MXene under light irradiation. Importantly, both MXenes are successfully reused and retained approximately 70% of their activity.

Organic Covalent Interaction-based Frameworks as Emerging Catalysts for Environment and Energy Applications: Current Scenario and Opportunities

The term "covalent organic framework" (COF) refers to a class of porous organic polymeric materials made from organic building blocks that have been covalently bonded. The preplanned and predetermined bonding of the monomer linkers allow them to demonstrate directional flexibility in two- or three-dimensional spaces. COFs are modern materials, and the discovery of new synthesis and linking techniques has made it possible to prepare them with a variety of favorable features and use them in a range of applications. Additionally, they can be post-synthetically altered or transformed into other materials of particular interest to produce compounds with enhanced chemical and physical properties. Because of its tunability in different chemical and physical states, post-synthetic modifications, high stability, functionality, high porosity and ordered geometry, COFs are regarded as one of the most promising materials for catalysis and environmental applications. This study highlights the basic advancements in establishing the stable COFs structures and various post-synthetic modification approaches. Further, the photocatalytic applications, such as organic transformations, degradation of emerging pollutants and removal of heavy metals, production of hydrogen and Conversion of carbon dioxide (CO₂ ) to useful products have also been presented. Finally, the future research directions and probable outcomes have also been summarized, by focusing their promises for specialists in a variety of research fields.

Macrocycle-Strutted Coordination Microparticles for Fluorescence-Monitored Photosensitization and Substrate-Selective Photocatalytic Degradation

The prosperous advancement of supramolecular chemistry has motivated us to construct supramolecular hybrid materials with integrated functionalities. Herein, we report an innovative type of macrocycle-strutted coordination microparticle (MSCM) using pillararenes as the struts and "pockets", which performs unique activities of fluorescence-monitored photosensitization and substrate-selective photocatalytic degradation. Prepared via a convenient one-step solvothermal method, MSCM showcases the incorporation of supramolecular hybridization and macrocycles, endowed with well-ordered spherical architectures, superior photophysical properties, and photosensitizing capacity, where a self-reporting fluorescence response is exhibited upon photoinduced generation of multiple reactive oxygen species. Importantly, photocatalytic behaviors of MSCM show marked divergence toward three different substrates and reveal pronounced substrate-selective catalytic mechanisms, attributing to the variety in the affinity of substrates toward MSCM surfaces and pillararene cavities. This study brings new insight into the design of supramolecular hybrid systems with integrated properties and further exploration of functional macrocycle-based materials.

MOF-Derived Defective Co3O4 Nanosheets in Carbon Nitride Nanocomposites for CO2 Photoreduction and H2 Production

In photocatalysis, especially in CO₂ reduction and H₂ production, the development of multicomponent nanomaterials provides great opportunities to tune many critical parameters toward increased activity. This work reports the development of tunable organic/inorganic heterojunctions comprised of cobalt oxides (Co₃O₄) of varying morphology and modified carbon nitride (CN), targeting on optimizing their response under UV-visible irradiation. MOF structures were used as precursors for the synthesis of Co₃O₄. A facile solvothermal approach allowed the development of ultrathin two-dimensional (2D) Co₃O₄ nanosheets (Co₃O₄-NS). The optimized CN and Co₃O₄ structures were coupled forming heterojunctions, and the content of each part was optimized. Activity was significantly improved in the nanocomposites bearing Co₃O₄-NS compared with the corresponding bulk Co₃O₄/CN composites. Transient absorption spectroscopy revealed a 100-fold increase in charge carrier lifetime on Co₃O₄-NS sites in the composite compared with the bare Co₃O₄-NS. The improved photocatalytic activity in H₂ production and CO₂ reduction is linked with (a) the larger interface imposed from the matching 2D structure of Co₃O₄-NS and the planar surface of CN, (b) improvements in charge carrier lifetime, and (c) the enhanced CO₂ adsorption. The study highlights the importance of MOF structures used as precursors in forming advanced materials and the stepwise functionalization of the individual parts in nanocomposites for the development of materials with superior activity.

Modulation of ZnO Nanostructure for Efficient Photocatalytic Performance

Structure has been considered to play an important role in photocatalytic performance of the semiconductors, but the intrinsic factors were rarely revealed. Herein, ZnO nanomaterials in the structures of thin film, nanowire array and nanosheet array were synthesized, and their structural characteristics, optical properties, photocurrent response and photocatalytic efficiency were compared with each other for illustrating the issue. The photoluminescence intensity decreased in the order of nanosheets, thin film and nanowires for improved lifetime of the photoexcited charges. The absorption of the nanosheets and nanowires improved obviously in the visible range with a redshift of the absorption edge than that of the thin film. The nanowires possessed the highest response current of 82.65 μA at a response time of 2.0 ms in a sensitivity of 87.93 at the light frequency of 1 Hz, and gained the largest catalytic efficiency of 2.45 μg/cm² h for the methylene blue degradation in UV light. Nevertheless, the improvement of catalytic efficiency of the nanosheets (up to 42.4%) was much larger than that of nanowires (5.7%) and thin film (2.6%) for the Au coating. The analysis revealed that the photocatalytic efficiency of the ZnO nanomaterials was modulated by the structure as it contained different surface area, roughness, defect and doping states, vacancies, polar and non-polar crystalline faces, which would provide structural design of semiconductor nanomaterials for the photoelectric and photocatalytic applications.

Morphology and Light-Dependent Spatial Distribution of Spin Defects in Carbon Nitride

Carbon nitride (CN) is a heterogeneous photocatalyst that combines good structural properties and a broad scope. The photocatalytic efficiency of CN is associated with the presence of defective and radical species. An accurate description of defective states-both at a local and extended level-is key to develop a thorough mechanistic understanding of the photophysics of CN. In turn, this will maximise the generation and usage of photogenerated charge carriers and minimise wasteful charge recombination. Here the influence of morphology and light-excitation on the number and chemical nature of radical defects is assessed. By exploiting the magnetic dipole-dipole coupling, the spatial distribution of native radicals in CN is derived with high precision. From the analysis an average distance in the range 1.99-2.34 nm is determined, which corresponds to pairs of radicals located approximately four tri-s-triazine units apart.

Insight into the mechanism of deep NO photo-oxidation by bismuth tantalate with oxygen vacancies

Deeply photocatalytic oxidation of nitrogen oxides is still difficult to achieve, mainly limited by few intrinsic active sites and inefficient carrier separation of photocatalysts. Accordingly, we develop a simple room temperature tactic to introduce oxygen vacancies (OVs) into Bi₃TaO₇ (BTO). Based on solid experimental and DFT theoretical supports, we explore the mechanism of NO removal over OVs decorated BTO (OVs-BTO). OVs can not only alter the distribution of local electrons to result in the formation of a fast charge transfer channel between OVs and the adjacent Ta atoms, which improves the transport rate of photogenerated carriers; but also function as active sites to adsorb small molecules (NO, O₂ and H₂O), which being activated and positively drive the NO oxidation reaction. In order to investigate a possible reaction path, a combination of in-situ DRIFTS and simulated Gibbs free energy reveals that the intermediate products of OVs-BTO are helpful to promote the deep oxidation of NO to NO₃^-, while pristine BTO is more likely to produce NO₂ intermediate toxic by-products, which greatly hinders the deep photocatalytic oxidation of NO. This work provides insights into the role of OVs in photocatalysts, and also points out a guideline for the mechanism of semiconductor photocatalysts in eliminating gaseous pollutants.

The Nickel Age in Synthetic Dual Photocatalysis: A Bright Trip Toward Materials Science

Recently, the field of dual photocatalysis has grown rapidly, to become one of the most powerful tools for the functionalization of organic molecules under mild conditions. In particular, the merging of Earth-abundant nickel-based catalytic systems with visible-light-activated photoredox catalysts has allowed the development of a number of unique green synthetic approaches. This goes in the direction of ensuring an effective and sustainable chemical production, while safeguarding human health and environment. Importantly, this relatively new branch of catalysis has inspired an interdisciplinary stream of research that spans from inorganic and organic chemistry to materials science, thus establishing itself as one dominant trend in modern organic synthesis. This Review aims at illustrating the milestones on the timeline evolution of the photocatalytic systems used, with a critical analysis toward novel applications based on the use of photoactive two-dimensional carbon-based nanostructures. Lastly, forward-looking opportunities within this intriguing research field are discussed.

Treated activated carbon as a metal-free catalyst for effectively catalytic reduction of toxic hexavalent chromium

In this work, activated carbon treated in N₂ atmosphere, as a non-metallic catalyst, exhibits excellent catalytic performance in reduction of Cr (VI) to Cr (III) using HCOOH as the reducing agent at room temperature. A series of characterizations and control experiments were carried out to deduce the possible reaction mechanism. The results showed that the improved catalytic performance can be attributed to the enhanced graphitization degree and basic sites such as pyrone-like, which favor electron transferring and activation of reactant. The reaction rate constant observed herein for the C-800 was 22 and 6 times more than that for C-0 and Pd/C catalyst, respectively. In addition, C-800 showed good recycle performance, and no loss of activity was observed after 5 cycles. This study broadens the application of nonmetallic catalyst and provides an easy-available and cost-effective catalytic material for removing toxic Cr (VI).

Progress in Development of Photocatalytic Processes for Synthesis of Fuels and Organic Compounds under Outdoor Solar Light

With photovoltaics becoming a mature, commercially feasible technology, society is willing to allocate resources for developing and deploying new technologies based on using solar light. Analysis of projects supported by the European Commission in the past decade indicates exponential growth of funding to photocatalytic (PC) and photoelectrocatalytic (PEC) technologies that aim either at technology readiness levels (TRLs) TRL 1-3 or TRL > 3, with more than 75 Mio€ allocated from the year 2019 onward. This review provides a summary of PC and PEC processes for the synthesis of bulk commodities such as solvents and fuels, as well as chemicals for niche applications. An overview of photoreactors for photocatalysis on a larger scale is provided. The review rounds off with the summary of reactions performed at lab scale under natural outdoor solar light to illustrate conceptual opportunities offered by solar-driven chemistry beyond the reduction of CO₂ and water splitting. The authors offer their vision of the impact of this area of research on society and the economy.

Carbon Nanomaterials (CNMs) and Enzymes: From Nanozymes to CNM-Enzyme Conjugates and Biodegradation

Carbon nanomaterials (CNMs) and enzymes differ significantly in terms of their physico-chemical properties-their handling and characterization require very different specialized skills. Therefore, their combination is not trivial. Numerous studies exist at the interface between these two components-especially in the area of sensing-but also involving biofuel cells, biocatalysis, and even biomedical applications including innovative therapeutic approaches and theranostics. Finally, enzymes that are capable of biodegrading CNMs have been identified, and they may play an important role in controlling the environmental fate of these structures after their use. CNMs' widespread use has created more and more opportunities for their entry into the environment, and thus it becomes increasingly important to understand how to biodegrade them. In this concise review, we will cover the progress made in the last five years on this exciting topic, focusing on the applications, and concluding with future perspectives on research combining carbon nanomaterials and enzymes.

Two-dimensional N-doped Pd/carbon for highly efficient heterogeneous catalysis

A novel two-dimensional ZIF-derived Pd@CN material prepared via one-step calcination exhibits outstanding catalytic activity in heterogeneous hydrogenation. Its well-developed porous structure, low dimensions and low density make active sites more accessible. This facile and effective strategy can guide the synthesis of highly active and durable Pd@CN catalysts with specific morphologies.

Penta-BCN monolayer: a metal-free photocatalyst with a high carrier mobility for water splitting

The penta-BCN monolayer is semiconducting with a considerable band gap and shows appropriate band edge positions for photocatalytic water splitting.

Radical defects modulate the photocatalytic response in 2D-graphitic carbon nitride

Graphitic carbon nitride (gCN) is an important heterogeneous metal-free catalytic material. Thermally induced post-synthetic modifications, such as amorphization and/or reduction, were recently used to enhance the photocatalytic response of these materials for certain classes of organic transformations, with structural defects possibly playing an important role. The knowledge of how these surface modifications modulate the photocatalytic response of gCN is therefore not only interesting from a fundamental point of view, but also necessary for the development and/or tuning of metal-free gCN systems with superior photo-catalytic properties. Herein, employing density functional theory calculations and combining both the periodic and molecular approaches, in conjunction with experimental EPR measurements, we demonstrate that different structural defects on the gCN surface generate distinctive radical defect states localized within the electronic bandgap, with only those correlated with amorphous and reduced gCN structures being photo-active. To this end, we (i) model defective gCN surfaces containing radical defect states; (ii) assess the interactions of these defects with the radical precursors involved in the photo-driven alkylation of electron-rich aromatic compounds (namely perfluoroalkyl iodides); and (iii) describe the photo-chemical processes triggering the initial step of that reaction at the gCN surface. We provide a coherent structure/photo-catalytic property relationship on defective gCN surfaces, elaborating how only specific defect types act as binding sites for the perfluoroalkyl iodide reagent and can favor a photo-induced charge transfer from the gCN surface to the molecule, thus triggering the perfluoroalkylation reaction.

The nature of radical defects governs the photocatalytic activity of graphitic carbon nitride.

Hydrazone-linked 2D porphyrinic covalent organic framework photocatalysis for visible light-driven aerobic oxidation of amines to imines

Covalent organic frameworks (COFs) have recently gained rising consideration for visible light photocatalysis. Their property could be accurately established with specific reactions in which the most investigated one turns out to be the aerobic oxidation of amines. In this contribution, a hydrazone-linked 2D (two-dimensional) porphyrinic COF, Por-DETH-COF, was assembled from 5,10,15,20-tetrakis(4-benzaldehyde)porphyrin (p-Por-CHO) and 2,5-diethoxyterephthalohydrazide (DETH) and its photocatalytic activity was duly appraised with the aerobic oxidation of amines. Thereby, the red light-driven selective oxidation of benzyl amines to imines was obtained in very high conversions and selectivities with ambient air as the oxidant. Importantly, the photocatalytic system exhibited remarkable compatibility of functional groups and extensive scope of benzyl amines. Notably, the Por-DETH-COF photocatalyst displayed outstanding recyclability after five successive cycles. This work suggests that 2D COFs could contribute a unique juncture for selective organic transformations by photocatalysis.

Blue light-powered hydroxynaphthoic acid-titanium dioxide photocatalysis for the selective aerobic oxidation of amines

Solar photocatalysis is the key to resolve many environmental challenges but is usually hard to achieve over a metal oxide semiconductor. Therefore, assembling π-conjugated molecules onto semiconductors becomes an efficient approach to solar conversion via ligand-to-metal charge transfer. Here, a rational design of ligands for titanium dioxide (TiO₂) is presented to produce robust visible light photocatalysts. Three hydroxynaphthoic acids (HNAs) were selected as ligands by extending an extra benzene ring of salicylic acid (SA) at 3,4 or 4,5 or 5,6 positions. These ligands could regulate the performance of TiO₂ in which 2-hydroxy-1-naphthoic acid (2H1NA) endows the best outcome. In detail, blue light-powered cooperative photocatalysis of 2H1NA-TiO₂ with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 5 mol%) inaugurates the expeditious formation of imines by oxidation of amines with atmospheric oxygen (O₂). Interestingly, the increase of the O₂ pressure from 1 atm to 0.4 MPa promoted the selective oxidation of benzylamine but thereafter declined with a further boost to 0.6 MPa. Notably, an electron transfer between the oxidatively quenched 2H1NA-TiO₂ and TEMPO is established, offering a new pathway for environmental applications. This work presents a strategy in designing cutting-edge visible light photocatalysts via altering semiconductors with surface ligands.

General, Metal-free Synthesis of Carbon Nanofiber Assemblies from Plant Oils

We designed a metal-free synthesis of carbon nanofiber based on ketene chemistry using phosphorus pentoxide (P2 O5 ) and vegetable oil. Based on the characterization of intermediates, P2 O5 -oil reaction yielded most possibly alkylketenes, which polymerized into poly(ketene) with abundant enol groups. The enol groups further reacted with P2 O5 , forcing the poly(ketene) to assemble into a nano-sized preassembly structure. Moderate heating transforms these structures into carbonaceaus nanofibers. This approach is applicable to other chemicals with similar structure to vegetable oil. The carbon nanofibers with P-O-C functionalization show relatively high graphitization degree and promising textural properties. The C-O-P environment accounts for 66 at % of the total P and creates a superior thermal stability. As a model application, a CDI system built of a carbon-nanofiber-based electrode countered by an activated carbon-based electrode exhibited exceptional performance.

A Novel N-Doped Nanoporous Bio-Graphene Synthesized from Pistacia lentiscus Gum and Its Nanocomposite with WO3 Nanoparticles: Visible-Light-Driven Photocatalytic Activity

This paper reports the synthesis of a new nitrogen-doped porous bio-graphene (NPBG) with a specific biomorphic structure, using Pistacia lentiscus as a natural carbon source containing nitrogen that also acts as a bio-template. The obtained NPBG demonstrated the unique feature of doped nitrogen with a 3D nanoporous structure. Next, a WO₃/N-doped porous bio-graphene nanocomposite (WO₃/NPBG-NC) was synthesized, and the products were characterized using XPS, SEM, TEM, FT-IR, EDX, XRD, and Raman analyses. The presence of nitrogen doped in the structure of the bio-graphene (BG) was confirmed to be pyridinic-N and pyrrolic-N with N1 peaks at 398.3 eV and 400.5 eV, respectively. The photocatalytic degradation of the anionic azo dyes and drugs was investigated, and the results indicated that the obtained NPBG with a high surface area (151.98 m²/g), unique electronic properties, and modified surface improved the adsorption and photocatalytic properties in combination with WO₃ nanoparticles (WO₃-NPs) as an effective visible-light-driven photocatalyst. The synthesized WO₃/NPBG-NC with a surface area of 226.92 m²/g displayed lower bandgap and higher electron transfer compared with blank WO₃-NPs, leading to an increase in the photocatalytic performance through the enhancement of the separation of charge and a reduction in the recombination rate. At the optimum conditions of 0.015 g of the nanocomposite, a contact time of 15 min, and 100 mg/L of dyes, the removal percentages were 100%, 99.8%, and 98% for methyl red (MR), Congo red (CR), and methyl orange (MO), respectively. In the case of the drugs, 99% and 87% of tetracycline and acetaminophen, respectively, at a concentration of 10 mg/L, were removed after 20 min.

Strain-Induced Ferroelectric Heterostructure Catalysts of Hydrogen Production through Piezophototronic and Piezoelectrocatalytic System

In this work, we discover a piezoelectrocatalytic system composed of a ferroelectric heterostructure of BaTiO₃ (BTO)@MoSe₂ nanosheets, which exhibit piezoelectric potential (piezopotential) coupling with electrocatalyzed effects by a strain-induced piezopotential to provide an internal bias to the catalysts' surface; subsequently, the catalytic properties are substantially altered to enable the formation of activity states. The H₂ production rate of BTO@MoSe₂ for the piezoelectrocatalytic H₂ generation is 4533 μmol h^-1 g^-1, which is 206% that of TiO₂@MoSe₂ for piezophototronic (referred to as piezophotocatalytic process) H₂ generation (∼2195.6 μmol h^-1 g^-1). BTO@MoSe₂ presents a long-term H₂ production rate of 21.2 mmol g^-1 within 8 h, which is the highest recorded value under piezocatalytic conditions. The theoretical and experimental results indicate that the ferroelectric BTO acts as a strain-induced electric field generator while the few-layered MoSe₂ is facilitating piezocatalytic redox reactions on its active sites. This is a promising method for environmental remediation and clean energy development.

Metal-Free Catalysis: A Redox-Active Donor-Acceptor Conjugated Microporous Polymer for Selective Visible-Light-Driven CO2 Reduction to CH4

Achieving more than a two-electron photochemical CO₂ reduction process using a metal-free system is quite exciting and challenging, as it needs proper channeling of electrons. In the present study, we report the rational design and synthesis of a redox-active conjugated microporous polymer (CMP), TPA-PQ, by assimilating an electron donor, tris(4-ethynylphenyl)amine (TPA), with an acceptor, phenanthraquinone (PQ). The TPA-PQ shows intramolecular charge-transfer (ICT)-assisted catalytic activity for visible-light-driven photoreduction of CO₂ to CH₄ (yield = 32.2 mmol g^-1) with an impressive rate (2.15 mmol h^-1 g^-1) and high selectivity (>97%). Mechanistic analysis based on experimental results, in situ DRIFTS, and computational studies reveals that the potential of TPA-PQ for catalyzing photoreduction of CO₂ to CH₄ was energetically driven by photoactivated ICT upon surface adsorption of CO₂, wherein adjacent keto groups of PQ unit play a pivotal role. The critical role of ICT for stimulating photocatalysis is further illustrated by synthesizing another redox-active CMP (TEB-PQ), bearing triethynylbenzene (TEB) and PQ, that shows 8-fold lesser activity for photoreduction toward CO₂ to CH₄ (yield = 4.4 mmol g^-1) as compared to TPA-PQ. The results demonstrate a novel concept for CO₂ photoreduction to CH₄ using an efficient, sustainable, and recyclable metal-free robust organic photocatalyst.