Amidation crosslinking of polymeric carbon nitride for boosting photocatalytic hydrogen peroxide production

Site engineering of linear conjugated polymers to regulate oxygen adsorption affinity for boosting photocatalytic production of hydrogen peroxide without sacrificial agent

Artificial photosynthesis of hydrogen peroxide (H2O2) is a hopeful alternative to the industrial anthraquinone process. However, rational fabrication of the photocatalysts for the production of H2O2 without any sacrificial agents is still a formidable challenge. Herein, two kinds of linear conjugated polymers (LCPs) including pyridinic N functionalized polymer (DEB-N2) and pyridinic N non-contained polymer (DEB-N0) were successfully synthesized. DEB-N2 displays enhanced light capturing ability and good dispersion in water, leading to a substantial initial H2O2 generation rate of 3492μmol g-1h-1 as well as remarkable photocatalytic stability in pure water. Furthermore, the temperature programmed desorption (TPD) and density functional theory (DFT) analysis reveal that highly electronegative pyridine-N atoms in DEB-N2 boost the adsorption affinity of oxygen molecules, which facilitates the occurrence of the oxygen reduction reaction, therefore enhancing the performance of photocatalytic H2O2 production. This study unveils that the presence of pyridinic N in DEB-N2 has a significant impact on photocatalytic H2O2 production, suggesting the precise manipulation of the chemical structure of polymer photocatalysts is essential to achieve efficient solar-to-chemical energy conversion.

Pyridine-Based Covalent Organic Frameworks with Pyridyl-Imine Structures for Boosting Photocatalytic H2O2 Production via One-Step 2e- Oxygen Reduction

Bipyridine-based covalent organic frameworks (COFs) have emerged as promising contenders for the photocatalytic generation of hydrogen peroxide (H2O2). However, the presence of imine nitrogen alters the mode of H2O2 generation from an efficient one-step two-electron (2e-) route to a two-step 2e- oxygen reduction pathway. In this work, we introduce 3,3'-bipyridine units into imine-based COF skeletons, creating a pyridyl-imine structure with two adjacent nitrogen atoms between the pyridine ring and imine linkage. This unique bipyridine-like architecture can effectively suppress the two-step 2e- ORR process at the single imine-nitrogen site, facilitating a more efficient one-step 2e- pathway. Consequently, the optimized pyridyl-imine COF (PyIm-COF) exhibits a remarkable H2O2 production rate of up to 5850 μmol h-1 g-1, nearly double that of pristine bipyridine COFs. This work provides valuable insight into the rational design of functionalized COFs for enhanced H2O2 production in photocatalysis.

Recent developments in photocatalytic production of hydrogen peroxide

Hydrogen peroxide (H2O2), an environmentally friendly strong oxidant and energy carrier, has attracted widespread attention in photocatalysis. Artificial photosynthesis of H2O2 using water and oxygen as raw materials, solar energy as an energy source, and semiconductor materials as catalysts is considered a promising technology. In the past few decades, encouraging progress has been made in the photocatalytic production of H2O2. Therefore, we summarize the research achievements in this field in recent years. This review first briefly introduces the reaction pathway, detection techniques and evaluation metrics. Then, the recent advances in photocatalysts are highlighted. Furthermore, the existing challenges and possible solutions in this field are presented. At last, we look forward to the future development direction of this field. This review provides valuable insights and guidance for efficient photocatalytic H2O2 production.

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.

Unraveling the Synergistic Mechanism of Boosted Photocatalytic H2O2 Production over Cyano-g-C3N4/In2S3/Ppy Heterostructure and Enhanced Photocatalysis-Self-Fenton Degradation Performance

In this work, cyano contained g-C3N4 comodified by In2S3 and polypyrrole (C≡N─CN/IS/Ppy) materials are synthesized for the photocatalytic production of H2O2 and photocatalysis-self-Fenton reaction for highly efficient degradation of metronidazole. The results from UV-vis spectrophotometry, surface photovoltage, and Kelvin probe measurements reveal the promoted transport and separation efficiency of photoinduced charges after the introduction of In2S3 and Ppy in the heterojunction. The existence of a built-in electric field accelerates the photoinduced charge separation and preserves the stronger oxidation ability of holes at the valence band of C≡N─CN. Linear sweep voltammetry measurements, zeta potential analyzations, nitroblue tetrazolium determination, and other measurements show that Ppy improves the conversion ratio of •O2 - to H2O2 and the utilization ratio of •O2 -, as well as suppresses decomposition of H2O2. Accordingly, the H2O2 evolution rate produced via a two-step single-electron reduction reaction reaches almost 895 µmol L-1 h-1, a value 80% and 7.2-fold higher than those obtained with C≡N─CN/IS and C≡N─CN, respectively. The metronidazole removal rate obtained via photocatalysis-self-Fenton reaction attains 83.7% within 120 minutes, a value much higher than that recorded by the traditional Fenton method. Overall, the proposed synthesis materials and route look promising for the H2O2 production and organic pollutants degradation.

P-doped Mn0.5Cd0.5S coupled with cobalt porphyrin as co-catalyst for the photocatalytic water splitting without using sacrificial agents

Photocatalytic water splitting over semiconductors is an important approach to solve the energy demand of human beings. Most photocatalytic H2 generation reactions are conducted in the presence of sacrificial agent. However, the use of sacrificial reagents increases the cost of hydrogen generation. Realizing photocatalytic water splitting for hydrogen production without the addition of sacrificial agents is a major challenge for photocatalysts. The porphyrin MTCPPOMe and P doped MnxCd1-xS make a significant contribution in facilitating the MnxCd1-xS photocatalytic pure water splitting to H2 reaction. Herein, a novel MTCPPOMe/P-MnxCd1-xS (M = 2H, Fe, Co, Ni) composite catalyst which can efficiently split pure water without using sacrificial agents is developed. As a result, the H2 generation rate of CoTCPPOMe/P-Mn0.5Cd0.5S is as high as 2.10 μmol h-1, which is 9.1 and 4.2 times higher than that of Mn0.5Cd0.5S (MCS) and P-Mn0.5Cd0.5S (P-MCS), respectively. P doped MnxCd1-xS inhibits the recombination of photogenerated carriers, and introduction of MTCPPOMe as co-catalyst enhances the reduction capacity. In summary, an efficient and economical photocatalystis prepared for pure water splitting to prepare hydrogen.

Ultratrace Aromatic Anhydride Dopant as Intermediate Island to Promote Charge Transfer of Graphitic Carbon Nitride for Enhancing the Photocatalytic Degradation of Rhodamine B

In this work, 0.75 wt ‰ 2,3-pyridinedicarboxylic anhydride (PDA) as a novel dopant was utilized to obtain modified graphitic carbon nitride with ultratrace doping (3MCN-PDA3) by facile thermal polymerization. Characterization of the microstructure, surface state, and porosity properties of the samples indicated that 3MCN-PDA3 has a thinner sheet-like, larger-scale, and tighter lamellar stacking structure than that of pristine graphitic carbon nitride (3MCN). Based on photo/electrochemical analysis, the PDA dopant formed an extended coplanar conjugated system by anhydride-amine thermal condensation with heptazine rings, and the channels of amide covalent bonds and superconjugation of the solitary pair of electrons of the nitrogen atoms of PDA synergistically promoted the charge transport performance of 3MCN-PDA3. Under visible light, the photodegradation efficiency of Rhodamine B (RhB) over 3MCN-PDA3 reached 92.4% in 60 min and realized almost entire removal in 200 min (99.2%), 1.43 times that of 3MCN. Furthermore, the experimental results and generalized density theory calculations confirmed that PDA acts as an intermediate molecular island and constructs an efficient carrier transfer pathway between different heptazine units. The results indicate that PDA is a promising candidate to enhance the charge transfer performance through ultratrace doping in the large-scale preparation and application of the graphitic carbon nitride photocatalyst.

Novel photoelectrochemical 3D-system for water disinfection by deposition of modified carbon nitride on vitreous carbon foam

Graphitic carbon nitride (GCN) is an optical semiconductor with excellent photoactivity under visible light irradiation. It has been widely applied for organic micropollutant removal from contaminated water, and less investigated for microorganisms' inactivation. The photocatalytic degradation mechanism using GCN is attributed to a series of reactions with reactive oxygen species and photogenerated holes that can be boosted by modifying its physical-chemical structure. This work reports a successful improvement of the overall photocatalytic and electrocatalytic activities of the pristine material by thermal and chemical modification by a copolymerisation synthesis method. The copolymerisation of dicyandiamide as a precursor with barbituric acid strongly reduced photoluminescence due to the enhanced charge separation thus improving the catalyst efficiency under visible light irradiation. The material with 1.6 wt% of barbituric acid showed the best photocatalytic performance and electrochemical properties. This photocatalyst was selected for immobilisation on a conductive carbon foam, which promotes a higher electrochemical active surface area and enhanced mass transfer. This three-dimensional metal-free electrode was employed for the photoelectrochemical inactivation of two different microorganisms, Escherichia coli, and Enterococcus faecalis, obtaining removals below the detection limit after 30 min in simulated faecal-contaminated waters. This photoelectrochemical reactor was also applied to treat polluted river and urban waste waters, and the faecal contamination indicators were vastly reduced to values below the detection limit in 60 min in both cases, showing the wide applicability of this innovative photoelectrode for different types of polluted aqueous matrices.

Microtubular cellulose-derived kapok fibre as a solid electron donor for boosting photocatalytic H2O2 production over C-doped g-C3N4 hybrid complexation

Cellulose continues to play an important and emerging role in photocatalysis, and its favourable properties, such as electron-rich hydroxyl groups, could enhance the performance of photocatalytic reactions. For the first time, this study exploited the kapok fibre with microtubular structure (t-KF) as a solid electron donor to enhance the photocatalytic activity of C-doped g-C₃N₄ (CCN) via ligand-to-metal-charge-transfer (LMCT) to improve hydrogen peroxide (H₂O₂) production performance. As confirmed by various characterisation techniques, the hybrid complex consisting of CCN grafted on t-KF was successfully developed in the presence of succinic acid (SA) as a cross-linker via a simple hydrothermal approach. The complexation formation between CCN and t-KF results in the CCN-SA/t-KF sample displaying a higher photocatalytic activity than pristine g-C₃N₄ to produce H₂O₂ under visible light irradiation. The enhanced physicochemical and optoelectronic properties of CCN-SA/t-KF imply that the LMCT mechanism is crucial in improving photocatalytic activity. This study promotes utilising the unique t-KF material's properties to develop a low-cost and high-performance cellulose-based LMCT photocatalyst.

Sulfone-Modified Covalent Organic Frameworks Enabling Efficient Photocatalytic Hydrogen Peroxide Generation via One-Step Two-Electron O2 Reduction

Photocatalytic oxygen reduction reaction (ORR) offers a promising hydrogen peroxide (H2 O2 ) synthetic strategy, especially the one-step two-electron (2e- ) ORR route holds great potential in achieving highly efficient and selectivity. However, efficient one-step 2e- ORR is rarely harvested and the underlying mechanism for regulating the ORR pathways remains greatly obscure. Here, by loading sulfone units into covalent organic frameworks (FS-COFs), we present an efficient photocatalyst for H2 O2 generation via one-step 2e- ORR from pure water and air. Under visible light irradiation, FS-COFs exert a superb H2 O2 yield of 3904.2 μmol h-1  g-1 , outperforming most reported metal-free catalysts under similar conditions. Experimental and theoretical investigation reveals that the sulfone units accelerate the separation of photoinduced electron-hole (e- -h+ ) pairs, enhance the protonation of COFs, and promote O2 adsorption in the Yeager-type, which jointly alters the reaction process from two-step 2e- ORR to the one-step one, thereby achieving efficient H2 O2 generation with high selectivity.