Mesoporous carbon nitride synthesized by nanocasting with urea/formaldehyde and metal-free catalytic oxidation of cyclic olefins

Hydrogen Evolution Activity of Nitrogen-Rich g-C3-xN4+x Synthesized by Solid-Gas Interface Method

Synthesis of a metal-free carbon nitride (g-C3N4) photocatalyst in the form of nitrogen-rich g-C3-xN4+x derivatives is desirable for efficient solar to hydrogen conversion and remains a challenging task to achieve. Herein we report the development of homogeneous sheets of nitrogen-rich graphitic carbon nitride samples from melamine by a solid-gas interface approach. Using this method, pure g-C3N4 (CN), g-C3-xN4+x under ammonia flow (CN-NH3) and g-C3-xN4+x under nitrogen flow (CN-N2) are prepared. The g-C3-xN4+x (CN-NH3) sample shows better surface conductivity, wide optical absorbance in the visible region, reduced recombination and high electron donor density, and higher performance toward photoelectrochemical hydrogen evolution (HER). The g-C3-xN4+x (CN-NH3) generates a photocurrent of 2.06 μA cm-2, which is 2.5 times higher than that of the pure g-C3N4 (CN) sample (0.85 μA cm-2). It also shows higher photocatalytic water splitting ability compared to the CN and CN-N2 samples, generating 634 μmol g-1 hydrogen without cocatalyst and 1163 μmol g-1 of hydrogen with Pt cocatalyst. Density functional calculations suggest that the progressive band gap reduction with the increase in the N-dopant percentage can be attributed to the gradual increase in the partial π-occupations, which can lead to a significant stabilization of the conduction band minima. The theoretical modeling, however, indicates a saturation in the band gap effect after 75% of N-dopant. The onset potential of g-C3-xN4+x for HER appears at η = 0.43 V in dark and η = 0.34 V vs Ag/AgCl under solar light illumination of 1 sun.

Efficient CO2 methanation using nickel nanoparticles supported mesoporous carbon nitride catalysts

The CO₂ methanation technique not only gives a solution for mitigating CO₂ emissions but can also be used to store and convey low-grade energy. The basic character and large surface area of mesoporous carbon nitride, (MCN), are considered promising properties for the methanation of CO₂. So, a series (5-20 wt.%) of Ni-doped mesoporous carbon nitride catalysts were synthesized by using the impregnation method for CO₂ methanation. the prepared catalysts were characterized by several physicochemical techniques including XRD, BET, FT-IR, Raman spectroscopy, TEM, TGA analysis, Atomic Absorption, H₂-TPR, and CO₂-TPD. The catalytic performance was investigated at ambient pressure and temperature range (200-500 °C) using online Gas chromatography system. The prepared catalysts showed good performance where 15%Ni/MCN exhibited the best catalytic conversion and methane yield with 100% methane selectivity at 450 °C for investigated reaction conditions.

Advancing charge carriers separation and transformation by nitrogen self-doped hollow nanotubes g-C3N4 for enhancing photocatalytic degradation of organic pollutants

The rapid recombination of photogenerated electrons and holes, low utilization of visible light and weak oxidation capacity significantly limit the photocatalytic activity for the degradation of organic pollutants. Doping is used as a conventional strategy for regulating the electronic structure of photocatalysts to obtain a wider light absorption, but also suffers from the problems of reduced charge mobility and oxidation capacity, which is not conducive to photocatalytic degradation of pollutants. To address this issue, a nitrogen self-doped hollow nanotubes g-C₃N₄ (N-PCN) was synthesized by synergistic self-doping and quantum confinement effects. The N-PCN exhibits excellent efficiency in photocatalytic degradation of TC compared to the pristine g-C₃N₄. The synthesized N-PCN has a more positive conduction band minimum and can generate more photogenerated electrons to reduce oxygen to superoxide radicals. In addition, experimental and theoretical evidence shows that N-self-doping not only suppresses the recombination of photogenerated charge carriers but also facilitates the adsorption of oxygen molecules. Consequently, more superoxide radicals and singlet oxygen are generated through oxygen activation process.

High-efficiency photocatalytic water splitting by a N-doped porous g-C3N4 nanosheet polymer photocatalyst derived from urea and N,N-dimethylformamide

N-Doped porous g-C₃N₄ nanosheets for high-efficiency photocatalytic water splitting were prepared by mixing the precursor of N,N-dimethylformamide and urea through a simple one-step thermal copolymerization method.

Recent Strategies for Hydrogen Peroxide Production by Metal-Free Carbon Nitride Photocatalysts

Hydrogen peroxide (H2O2) is a chemical which has gained wide importance in several industrial and research fields. Its mass production is mostly performed by the anthraquinone (AQ) oxidation reaction, leading to high energy consumption and significant generation of wastes. Other methods of synthesis found in the literature include the direct synthesis from oxygen and hydrogen. However, this H2O2 production process is prone to explosion hazard or undesirable by‑product generation. With the growing demand of H2O2, the development of cleaner and economically viable processes has been under intense investigation. Heterogeneous photocatalysis for H2O2 production has appeared as a promising alternative since it requires only an optical semiconductor, water, oxygen, and ideally solar light irradiation. Moreover, employing a metal-free semiconductor minimizes possible toxicity consequences and reinforces the sustainability of the process. The most studied metal‑free catalyst employed for H2O2 production is polymeric carbon nitride (CN). Several chemical and physical modifications over CN have been investigated together with the assessment of different sacrificial agents and light sources. This review shows the recent developments on CN materials design for enhancing the synthesis of H2O2, along with the proposed mechanisms of H2O2 production. Finally, the direct in situ generation of H2O2, when dealing with the photocatalytic synthesis of added-value organic compounds and water treatment, is discussed.

A Metal-Free Carbon-Based Catalyst: An Overview and Directions for Future Research

Metal-free carbon porous materials (CPMs) have gained the intensive attention of scientists and technologists because of their potential applications, ranging from catalysis to energy storage. Various simple and facile strategies are proposed for the preparation of CPMs with well-controlled sizes, shapes, and modifications on the surface. The extraordinary tenability of the pore structure, the environmental acceptability, the unique surface and the corrosion resistance properties allow them to be suitable materials for a large panel of catalysis applications. This review briefly outlines the different signs of progresses made towards synthesizing CPMs, and their properties, including catalytic efficiency, stability, and recyclability. Finally, we make a comparison of their catalytic performances with other nanocomposites, and we provide an outlook on the expected developments in the relevant research works.

The effects of bismuth (III) doping and ultrathin nanosheets construction on the photocatalytic performance of graphitic carbon nitride for antibiotic degradation

To further enhance the photocatalytic performance of graphitic carbon nitride (g-C₃N₄), we rationally combined two strategies (foreign metal doping and ultrathin nanosheet construction) to synthesize bismuth (III) (Bi³⁺) doped ultrathin g-C₃N₄ nanosheets (Bi-CNNS) via one-step thermal polymerization method using melamine as the raw material, bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) as the dopant source, and nitric acid (HNO₃) and acetic acid (AC) as soft templates for the ultrathin nanosheets construction. The Bi-CNNS catalysts exhibited an excellent photocatalytic performance in tetracycline (TC) degradation. The TC removal efficiency reached to be 94.1% in 30 min under visible-light irradiation over 0.03Bi-CNNS, which is 6.03 times higher than that of pure g-C₃N₄ (CN). The higher specific surface area, narrower bandgap, the improved photoexcited electron-hole pair transfer and separation efficiency, and prolonged carrier lifetimes in the Bi³⁺-doped ultrathin g-C₃N₄ nanosheets led to a significantly enhanced photocatalytic performance. The main radical species responsible for the degradation of tetracycline over 0.03Bi-CNNS were O₂^- and OH. Moreover, the possible photodegradation intermediate products of TC were detected by gas chromatography-mass spectroscopy (GC-MS), and a possible pathway was proposed.

Mesoporous carbon nitrides: synthesis, functionalization, and applications

Mesoporous carbon nitrides (MCNs) with large surface areas and uniform pore diameters are unique semiconducting materials and exhibit highly versatile structural and excellent physicochemical properties, which promote their application in diverse fields such as metal free catalysis, photocatalytic water splitting, energy storage and conversion, gas adsorption, separation, and even sensing. These fascinating MCN materials can be obtained through the polymerization of different aromatic and/or aliphatic carbons and high nitrogen containing molecular precursors via hard and/or soft templating approaches. One of the unique characteristics of these materials is that they exhibit both semiconducting and basic properties, which make them excellent platforms for the photoelectrochemical conversion and sensing of molecules such as CO₂, and the selective sensing of toxic organic acids. The semiconducting features of these materials are finely controlled by varying the nitrogen content or local electronic structure of the MCNs. The incorporation of different functionalities including metal nanoparticles or organic molecules is further achieved in various ways to develop new electronic, semiconducting, catalytic, and energy harvesting materials. Dual functionalities including acidic and basic groups are also introduced in the wall structure of MCNs through simple UV-light irradiation, which offers enzyme-like properties in a single MCN system. In this review article, we summarize and highlight the existing literature covering every aspect of MCNs including their templating synthesis, modification and functionalization, and potential applications of these MCN materials with an overview of the key and relevant results. A special emphasis is given on the catalytic applications of MCNs including hydrogenation, oxidation, photocatalysis, and CO₂ activation.

Melamine-derived graphitic carbon nitride as a new effective metal-free catalyst for Knoevenagel condensation of benzaldehyde with ethylcyanoacetate

Melamine-derived C₃N₄ was tested in Knoevenagel condensation of benzaldehyde with ethylcyanoacetate.

Recent advances in functional mesoporous graphitic carbon nitride (mpg-C3N4) polymers

Mesoporous micro-/nanostructures acting as supports for catalysts or used directly in catalysis reactions generally show fascinating performances that could lead to great potential for application. In the past few decades, extensive efforts have been devoted to the exploration and enrichment of graphitic carbon nitride (g-C₃N₄) based research. Especially, mesoporous g-C₃N₄ (mpg-C₃N₄) with controllable porosity and electronic/atomic structure can bring to bear unique physicochemical properties and has been widely applied in the fields of photocatalysis, adsorbents, sensors and chemical templates. However, a comprehensive summary on mpg-C₃N₄ micro/nanostructures is less reported and there is an urgent need to further promote the development of function-oriented mpg-C₃N₄-based materials. Herein, we will overview the significant advances in functional mpg-C₃N₄ polymers, including general synthesis strategies and growth mechanisms, modifications of electronic/atomic structures and interfacial properties (such as exfoliation, doping and hybridizing), as well as their current applications. Finally, several emerging issues and perspectives are also proposed.

Mesoporous carbon nitride grafted with n-bromobutane: a high-performance heterogeneous catalyst for the solvent-free cycloaddition of CO2 to propylene carbonate

Mesoporous carbon nitride grafted n-bromobutane shows high catalytic activity in the solvent-free cycloaddition of CO₂ with propylene oxide to propylene carbonate, affording a maximum yield of 88% with a TOF up to 10.7 h⁻¹.

A Comparative Study on the Role of Precursors of Graphitic Carbon Nitrides for the Photocatalytic Degradation of Direct Red 81

The graphitic carbon nitride (g-C3N4) materials have been synthesized from nitrogen rich precursors such as urea and thiourea by directly heating at 520 °C for 2 h. The as-synthesized carbon nitride samples were characterized by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV-vis) absorption spectroscopy, photoluminescence (PL) and particle size analysis. The photoelectrochemical measurements were performed using several on-off cycles under visible-light irradiation. The x-ray diffraction peak is broader which indicates the fine powder nature of the synthesized materials. The estimated crystallite size of carbon nitrides synthesized from urea (U-CN) and thiourea (T-CN) are 4.0 and 4.4 nm respectively. The particle size of U-CN and T-CN were analysed by particle size analyser and were found to be 57.3 and 273.3 nm respectively. The photocatalytic activity for the degradation of the textile dye namely, direct red-81 (DR81) using these carbon nitrides were carried out under visible light irradiation. In the present investigation, a comparison study on the carbon nitrides synthesized from cheap precursors such as urea and thiourea for the degradation of DR81 has been carried out. The results inferred that U-CN exhibited higher photocatalytic activity than T-CN. The photoelectrochemical studies confirmed that the (e--h+) charge carrier separation is more efficient in U-CN than that of T-CN and therefore showed high photocatalytic degradation. Further, the smaller particle size of U-CN is also responsible for the observed degradation trend.

Post-functionalization of graphitic carbon nitrides by grafting organic molecules: toward C–H bond oxidation using atmospheric oxygen

We present a feasible approach in the post-modification of carbon nitrides for the selective oxidation of 3,5,5-trimethylcyclohex-3-en-1-one.