Distorted carbon nitride nanosheets with activated n → π* transition and preferred textural properties for photocatalytic CO2 reduction

Engineering Built-In Electric Field Microenvironment of CQDs/g-C3N4 Heterojunction for Efficient Photocatalytic CO2 Reduction

Graphitic carbon nitride (CN), as a nonmetallic photocatalyst, has gained considerable attention for its cost-effectiveness and environmentally friendly nature in catalyzing solar-driven CO2 conversion into valuable products. However, the photocatalytic efficiency of CO2 reduction with CN remains low, accompanied by challenges in achieving desirable product selectivity. To address these limitations, a two-step hydrothermal-calcination tandem synthesis strategy is presented, introducing carbon quantum dots (CQDs) into CN and forming ultra-thin CQD/CN nanosheets. The integration of CQDs induces a distinct work function with CN, creating a robust interface electric field after the combination. This electric field facilitates the accumulation of photoelectrons in the CQDs region, providing an abundant source of reduced electrons for the photocatalytic process. Remarkably, the CQD/CN nanosheets exhibit an average CO yield of 120 µmol g-1, showcasing an outstanding CO selectivity of 92.8%. The discovery in the work not only presents an innovative pathway for the development of high-performance photocatalysts grounded in non-metallic CN materials employing CQDs but also opens new avenues for versatile application prospects in environmental protection and sustainable cleaning energy.

S-scheme heterojunction of crystalline carbon nitride nanosheets and ultrafine WO3 nanoparticles for photocatalytic CO2 reduction

Highly crystalline carbon nitride polymers have shown great opportunities in overall water photosplitting; however, their mission in light-driven CO2 conversion remains to be explored. In this work, crystalline carbon nitride (CCN) nanosheets of poly triazine imide (PTI) embedded with melon domains are fabricated by KCl/LiCl-mediated polycondensation of dicyandiamide, the surface of which is subsequently deposited with ultrafine WO3 nanoparticles to construct the CCN/WO3 heterostructure with a S-scheme interface. Systematic characterizations have been conducted to reveal the compositions and structures of the S-scheme CCN/WO3 hybrid, featuring strengthened optical capture, enhanced CO2 adsorption and activation, attractive textural properties, as well as spatial separation and directed movement of light-triggered charge carriers. Under mild conditions, the CCN/WO3 catalyst with optimized composition displays a high photocatalytic activity for reducing CO2 to CO in a rate of 23.0 µmol/hr (i.e., 2300 µmol/(hr·g)), which is about 7-fold that of pristine CCN, along with a high CO selectivity of 90.6% against H2 formation. Moreover, it also manifests high stability and fine reusability for the CO2 conversion reaction. The CO2 adsorption and conversion processes on the catalyst are monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), identifying the crucial intermediates of CO2*-, COOH* and CO*, which integrated with the results of performance evaluation proposes the possible CO2 reduction mechanism.

Developing Polymeric Carbon Nitrides for Photocatalytic H2O2 Production

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

Photocatalytic Efficiency for CO2 Reduction of Co and Cluster Co2O2 Supported on g-C3N4: A Density Functional Theory and Machine Learning Study

It is well known from experimental results that a single atom of cobalt supported on g-C3N4 is an efficient photocatalyst for the reduction of CO2 to CO, with a better photocatalytic activity than g-C3N4. In this work, we investigate the performance as catalysts for the CO2 reduction of single atoms of cobalt and Co2O2 clusters supported on graphitic carbon nitride (g-C3N4). Employing density functional theory plus Hubbard (DFT + U) calculations, we investigate in detail the reduction mechanisms to CO and HCOOH for the first time. We find that deposition of cobalt on g-C3N4 decreases the work function of g-C3N4 to 6.6 eV and provides a better candidate for the reduction reaction. In addition, we find that the preferred product of CO2 reduction on Co@g-C3N4 is CO, with a rate-determining barrier of 0.97 eV, while on Co2O2@g-C3N4, CO2 reduces to formate with a rate-determining barrier of 0.44 eV. We determine the creation of CO2 from COOH to only take place on Co2O2@g-C3N4, with a (relatively high) barrier of 2.27 eV. In order to obtain more easily the transition state energies of the reactions mentioned above, we applied machine learning methods to search for high-importance descriptors for these quantities, in the case of single transition metal atoms supported on C3N4. Interestingly, our results show that our quantities of interest are closely related to the adsorption energies of products and normalized valence electrons of the products of the elementary reactions as well as those of the metal atoms. The former of these two sets of features can be straightforwardly obtained via DFT, while the latter energies are extensively tabulated. Our results offer guidance for the design of catalysts and photocatalysts for CO2 reduction on single-metal atoms supported on C3N4.

Singlet Oxygen Formation Mechanism for the H2O2-Based Fenton-like Reaction Catalyzed by the Carbon Nitride Homojunction

The singlet oxygen (1O2) oxidation process activated by metal-free catalysts has recently attracted considerable attention for organic pollutant degradation; however, the 1O2 formation remains controversial. Simultaneously, the catalytic activity of the metal-free catalyst limits the practical application. In this study, carbon nitride (HCCN) containing an intramolecular homojunction, a kind of metal-free catalyst, exhibits excellent activity compared to g-C3N4 (CN) and crystalline carbon nitride (HCN) for tetracycline hydrochloride degradation through the H2O2-based Fenton-like reaction. The rate constant for HCCN increased about 16.1 and 8.9 times than that of CN and HCN, respectively. The activity of HCCN was enhanced, and the dominant reactive oxygen species (ROS) changed from hydroxyl radicals (•OH) to 1O2 with an increase in pH from 4.5 to 11.5. A novel formation pathway of 1O2 was revealed. This result is different from the normal reference, in which •OH is always the primary ROS in the H2O2-based Fenton-like reaction. This study may provide a possible strategy for the investigation on the nonradical oxidation process in the Fenton-like reaction.

Thin-wall hollow porous cystic-like graphitic carbon nitride with awakened n→π* electronic transitions and exceptional structural features for superior photocatalytic degradation of sulfamethoxazole

Effective photoexcitation and carrier migration are the essential aspects to strengthen semiconductor-engaged redox reaction. Herein, a three-dimensional thin-wall hollow porous cystic-like g-C₃N₄ (HPCN) with curved layer edge was successfully fabricated via a non-template thermal-condensation strategy. The construction of unique distorted structure can evoke the hard-to-activate n→π* electronic transition to some extent, broadening the absorption spectrum to 800 nm. And benefiting from the multiple reflections of incident light, the effective photoactivation can be further achieved. Moreover, the thin-wall porous framework can shorten the diffusion distance and accelerate migration of photogenerated charge, favouring interfacial redox reactions. The optimized HPCN_1.0 demonstrated an excellent photocatalytic degradation of SMX under blue-LED light irradiation, which was dramatically superior to that of pristine g-C₃N₄ (CN, 11.4 times). Ultimately, in consideration of reactions under several influencing factors with four different water samples, we demonstrated that the HPCN photocatalyst could be utilized far more productively for the elimination of SMX under real-world aqueous conditions. This work provides a straightforward approach for the removal of SMX and has immense potential to contribute to global scale environmental remediation.

Heterogenised Molecular Catalysts for Sustainable Electrochemical CO2 Reduction

There has been a rapid rise in interest regarding the advantages of support materials to protect and immobilise molecular catalysts for the carbon dioxide reduction reaction (CO₂ RR) in order to overcome the weaknesses of many well-known catalysts in terms of their stability and selectivity. In this Review, the state of the art of different catalyst-support systems for the CO₂ RR is discussed with the intention of leading towards standard benchmarking for comparison of such systems across the most relevant supports and immobilisation strategies, taking into account these multiple pertinent metrics, and also enabling clearer consideration of the necessary steps for further progress. The most promising support systems are described, along with a final note on the need for developing more advanced experimental and computational techniques to aid the rational design principles that are prerequisite to prospective industrial upscaling.

Carbon Nitride Photocatalysts with Integrated Oxidation and Reduction Atomic Active Centers for Improved CO2 Conversion

Single-atom active-site catalysts have attracted significant attention in the field of photocatalytic CO₂ conversion. However, designing active sites for CO₂ reduction and H₂ O oxidation simultaneously on a photocatalyst and combining the corresponding half-reaction in a photocatalytic system is still difficult. Here, we synthesized a bimetallic single-atom active-site photocatalyst with two compatible active centers of Mn and Co on carbon nitride (Mn₁ Co₁ /CN). Our experimental results and density functional theory calculations showed that the active center of Mn promotes H₂ O oxidation by accumulating photogenerated holes. In addition, the active center of Co promotes CO₂ activation by increasing the bond length and bond angle of CO₂ molecules. Benefiting from the synergistic effect of the atomic active centers, the synthesized Mn₁ Co₁ /CN exhibited a CO production rate of 47 μmol g^-1  h^-1 , which is significantly higher than that of the corresponding single-metal active-site photocatalyst.

COF-5/CoAl-LDH Nanocomposite Heterojunction for Enhanced Visible-Light-Driven CO2 Reduction

Photocatalytic conversion of CO₂ into value-added chemical fuels is an attractive route to mitigate global warming and the energy crisis. Reasonable design of optical properties and electronic behavior of the photocatalyst are essential to improve their catalytic activity. Herein, the 1D/2D heterojunction by direct in-situ synthesis of the covalent organic framework (COF)-5 colloid on the surface of CoAl layered double hydroxide (LDH) was used as the prospective photocatalyst for CO₂ reduction. COF-5/CoAl-LDH nanocomposite achieved 265.4 μmol g^-1 of CO with 94.6 % selectivity over CH₄ evolution in 5 h under visible light irradiation, which was 4.8 and 2.3 times higher than those of COF-5 colloid and CoAl-LDH, respectively. The enhanced catalytic activity was derived from the increased visible-light activity and the construction of type II-2 heterojunction, which greatly optimized visible light harvesting and accelerated the efficient separation of the photoinduced holes and electrons. This work paves the way for rational design of heterojunction catalysts in photocatalytic CO₂ reduction.

Atom manufacturing of photocatalyst towards solar CO2reduction

Photocatalytic CO₂reduction reaction (CO₂RR) is believed to be a promising remedy to simultaneously lessen CO₂emission and obtain high value-added products, but suffers from the thwarted activity of photocatalyst and poor selectivity of product. Over the past decade, aided by the significant advances in nanotechnology, the atom manufacturing of photocatalyst, including vacancies, dopants, single-atom catalysts, strains, have emerged as efficient approaches to precisely mediate the reaction intermediates and processes, which push forward in the rapid development of highly efficient and selective photocatalytic CO₂RR. In this review, we summarize the recent developments in highly efficient and/or selective photocatalysts toward CO₂RR with the special focus on various atom manufacturing. The mechanisms of these atom manufacturing from active sites creation, light absorbability, and electronic structure modulation are comprehensively and scientifically discussed. In addition, we attempt to establish the structure-activity relationship between active sites and photocatalytic CO₂RR capability by integrating theoretical simulations and experimental results, which will be helpful for insights into mechanism pathways of CO₂RR over defective photocatalysts. Finally, the remaining challenges and prospects in this field to improve the photocatalytic CO₂RR performances are proposed, which can shed some light on designing more potential photocatalysts through atomic regulations toward CO₂conversion.

MOF nanosheet-derived carbon-layer-coated CoP/g-C3N4 photocatalysts with enhance charge transfer for efficient photocatalytic H2 generation

Catalysts with low cost, high efficiency, and no need for precious metals cocatalysts, are the key to realizing the maximum utilization of solar energy-efficient hydrogen (H2) production. In this study,...

A visible light inducing photoelectrochemical biosensor with high-performance based on a porphyrin-sensitized carbon nitride composite

An outstanding photosensitive material plays a crucial role in building a high-performance and practical photoelectrochemical (PEC) biosensor.

Successful CO2 reduction under visible light photocatalysis using porous NiO nanoparticles, an atypical metal oxide

An efficient wet chemical one-pot synthesis strategy for the preparation of NiO nanoparticles has been reported. The nano-structured NiO has surface bound hexamine and it has emerged as a proficient...

A Short Review on Catalyst, Feedstock, Modernised Process, Current State and Challenges on Biodiesel Production

Biodiesel, comprising mono alkyl fatty acid esters or methyl ethyl esters, is an encouraging option to fossil fuels or diesel produced from petroleum; it has comparable characteristics and its use has the potential to diminish carbon dioxide production and greenhouse gas emissions. Manufactured from recyclable and sustainable feedstocks, e.g., oils originating from vegetation, biodiesel has biodegradable properties and has no toxic impact on ecosystems. The evolution of biodiesel has been precipitated by the continuing environmental damage created by the deployment of fossil fuels. Biodiesel is predominantly synthesised via transesterification and esterification procedures. These involve a number of key constituents, i.e., the feedstock and catalytic agent, the proportion of methanol to oil, the circumstances of the reaction and the product segregation and purification processes. Elements that influence the yield and standard of the obtained biodiesel encompass the form and quantity of the feedstock and reaction catalyst, the proportion of alcohol to feedstock, the temperature of the reaction, and its duration. Contemporary research has evaluated the output of biodiesel reactors in terms of energy production and timely biodiesel manufacture. In order to synthesise biodiesel for industrial use efficaciously, it is essential to acknowledge the technological advances that have significant potential in this sector. The current paper therefore offers a review of contemporary progress, feedstock categorisation, and catalytic agents for the manufacture of biodiesel and production reactors, together with modernised processing techniques. The production reactor, form of catalyst, methods of synthesis, and feedstock standards are additionally subjects of discourse so as to detail a comprehensive setting pertaining to the chemical process. Numerous studies are ongoing in order to develop increasingly efficacious techniques for biodiesel manufacture; these acknowledge the use of solid catalytic agents and non-catalytic supercritical events. This review appraises the contemporary situation with respect to biodiesel production in a range of contexts. The spectrum of techniques for the efficacious manufacture of biodiesel encompasses production catalysed by homogeneous or heterogeneous enzymes or promoted by microwave or ultrasonic technologies. A description of the difficulties to be surmounted going forward in the sector is presented.