Produktion von Wasserstoffperoxid durch photokatalytische Prozesse

Molecular Level Modulation of Anthraquinone-containing Resorcinol-formaldehyde Resin Photocatalysts for H2 O2 Production with Exceeding 1.2 % Efficiency

Designing polymeric photocatalysts at the molecular level to modulate the photogenerated charge behavior is a promising and challenging strategy for efficient hydrogen peroxide (H₂ O₂ ) photosynthesis. Here, we introduce electron-deficient 1,4-dihydroxyanthraquinone (DHAQ) into the framework of resorcinol-formaldehyde (RF) resin, which modulates the donor/acceptor ratio from the perspective of molecular design for promoting the charge separation. Interestingly, H₂ O₂ can be produced via oxygen reduction and water oxidation pathways, verified by isotopic labeling and in situ characterization techniques. Density functional theory (DFT) calculations elucidate that DHAQ can reduce the energy barrier for H₂ O₂ production. RF-DHAQ exhibits excellent overall photosynthesis of H₂ O₂ with a solar-to-chemical conversion (SCC) efficiency exceeding 1.2 %. This work opens a new avenue to design polymeric photocatalysts at the molecular level for high-efficiency artificial photosynthesis.

Oxidation-Reduction Molecular Junction Covalent Organic Frameworks for Full Reaction Photosynthesis of H2 O2

The full reaction photosynthesis of H₂ O₂ that can combine water-oxidation and oxygen-reduction without sacrificial agents is highly demanded to maximize the light-utilization and overcome the complex reaction-process of anthraquinone-oxidation. Here, a kind of oxidation-reduction molecular junction covalent-organic-framework (TTF-BT-COF) has been synthesized through the covalent-coupling of tetrathiafulvalene (photo-oxidation site) and benzothiazole (photo-reduction site), which presents visible-light-adsorption region, effective electron-hole separation-efficiency and photo-redox sites that enables full reaction generation of H₂ O₂ . Specifically, a record-high yield (TTF-BT-COF, ≈276 000 μM h^-1  g^-1 ) for H₂ O₂ photosynthesis without sacrificial agents has been achieved among porous crystalline photocatalysts. This is the first work that can design oxidation-reduction molecular junction COFs for full reaction photosynthesis of H₂ O₂ , which might extend the scope of COFs in H₂ O₂ production.

Covalent Organic Frameworks Containing Dual O2 Reduction Centers for Overall Photosynthetic Hydrogen Peroxide Production

Covalent organic frameworks (COFs) are highly desirable for achieving high-efficiency overall photosynthesis of hydrogen peroxide (H₂ O₂ ) via molecular design. However, precise construction of COFs toward overall photosynthetic H₂ O₂ remains a great challenge. Herein, we report the crystalline s-heptazine-based COFs (HEP-TAPT-COF and HEP-TAPB-COF) with separated redox centers for efficient H₂ O₂ production from O₂ and pure water. The spatially and orderly separated active sites in HEP-COFs can efficiently promote charge separation and enhance photocatalytic H₂ O₂ production. Compared with HEP-TAPB-COF, HEP-TAPT-COF exhibits higher H₂ O₂ production efficiency for integrating dual O₂ reduction active centers of s-heptazine and triazine moieties. Accordingly, HEP-TAPT-COF bearing dual O₂ reduction centers exhibits a remarkable solar-to-chemical energy efficiency of 0.65 % with a high apparent quantum efficiency of 15.35 % at 420 nm, surpassing previously reported COF-based photocatalysts.

Metal-Free Photocatalysts for Conversion of H2 O into Hydrogen Peroxide

Hydrogen peroxide (H₂ O₂ ) is an important green oxidizing agent for environmental protection and chemical production. In comparison to the traditional anthraquinone method, photosynthesis is a green and energy-saving process for H₂ O₂ production. To improve the stability and practical application value of the H₂ O₂ synthesized by photocatalysis, the H₂ O₂ photosynthesis should be conducted in pure water without involving any sacrificial reagents. In this regard, organic semiconducting catalysts pose as a suitable candidate for photocatalytic H₂ O₂ synthesis owing to their metal-free nature to prevent H₂ O₂ decomposition by the metal ions. In this Perspective, the H₂ O₂ photosynthesis history is firstly introduced, followed by a review of the organic semiconductor photocatalysts reported to date. Finally, the main problems to thwart the advances of current pure H₂ O-to-H₂ O₂ photosynthesis are discussed, followed by proposed solutions to address these issues in order to pave new ways for the development of highly efficient metal-free organic photocatalysts for sustainable pure H₂ O-to-H₂ O₂ conversion.

Extended Conjugation Tuning Carbon Nitride for Non-sacrificial H2 O2 Photosynthesis and Hypoxic Tumor Therapy

Artificial photocatalysis offers a clean approach for producing H₂ O₂ . However, the poor selectivity and activity of H₂ O₂ production hamper traditional industrial applications and emerging photodynamic therapy (PDT)/chemodynamic therapy (CDT). Herein, we report a C₅ N₂ photocatalyst with a conjugated C=N linkage for selective and efficient non-sacrificial H₂ O₂ production in both normoxic and hypoxic systems. The strengthened delocalization of π-electrons by linkers in C₅ N₂ downshifted the band position, thermodynamically eliminating side H₂ evolution reaction and kinetically promoting water oxidation. As a result, C₅ N₂ had a competitive solar-to-chemical conversion efficiency of 0.55 % in overall H₂ O₂ production and exhibited by far the highest activity under hypoxic conditions (698 μM h^-1 ). C₅ N₂ was further applied to hypoxic PDT/CDT with outstanding performance in apparent cancer cell death and synchronous bioimaging. The study sheds light on the photosynthesis of H₂ O₂ by carbon nitrides for health applications.

Sulfone-Decorated Conjugated Organic Polymers Activate Oxygen for Photocatalytic Methane Conversion

Oxidizing CH₄ into liquid products with O₂ under mild conditions still mainly relies on metal catalysis. We prepared a series of sulfone-modified conjugated organic polymers and found that the catalyst with proper S^VI content (0.10) could drive O₂ →H₂ O₂ →⋅OH to oxidize CH₄ into CH₃ OH and HCOOH directly and efficiently at room temperature under light irradiation. Experimental results showed that after 4 h reaction, decomposition rate and residual amounts of H₂ O₂ were 81.21 % and 4.83 mmol g_cat ^-1 respectively, and CH₄ conversion rate was 22.81 %. Mechanism studies revealed that illumination could induce the homolytic dissociation of S=O bonds on catalyst to produce oxygen and sulfur radicals, where the ⋅O could adsorb and activate CH₄ , and the ⋅S could supply electrons for ¹ O₂ to generate H₂ O₂ and then for decomposing the H₂ O₂ into ⋅OH timely to oxidize CH₄ . This research provided a novel organic catalysis approach for oxygen activation and utilization.

Efficient Generation of Hydrogen Peroxide and Formate by an Organic Polymer Dots Photocatalyst in Alkaline Conditions

A photocatalyst comprising binary organic polymer dots (Pdots) was prepared. The Pdots were constructed from poly(9,9-dioctylfluorene-alt-benzothiadiazole), as an electron donor, and 1-[3-(methoxycarbonyl)propyl]-1-phenyl-[6.6]C₆₁ , as an electron acceptor. The photocatalyst produces H₂ O₂ in alkaline conditions (1 M KOH) with a production rate of up to 188 mmol h^-1  g^-1 . The external quantum efficiencies were 30 % (5 min) and 14 % (75 min) at 450 nm. Furthermore, photo-oxidation of methanol by Pdots, followed by a disproportionation reaction and an oxidation reaction, produced the high-value chemical formate. On the basis of various spectroscopic and electrochemical measurements, the photophysical processes of the system were studied in detail and a reaction mechanism was proposed.

Noble Metal-Free Inorganic Photocatalyst Consisting of Antimony-Doped Tin Oxide Nanorod and Titanium oxide for Two-Electron Oxygen Reduction Reaction

This study reports a noble metal-free robust inorganic photocatalyst for H 2 O 2 synthesis via two-electron oxygen reduction reaction (ORR). Antimony-doped tin oxide nanorods were heteroepitaxially grown from rutile TiO 2 seed crystals with an orientation of (001)ATO//(001)TiO 2 (ATO-NR//TiO 2 , // denotes heteroepitaxial junction) by a hydrothermal method. UV-light irradiation of ATO-NR//TiO 2 particles stably and continuously produces H 2 O 2 from aerated aqueous solution of ethanol. Electrochemical measurements using rotating electrodes show that Sb-doping into SnO 2 greatly enhances the electrocatalytic activity for two-electron ORR. The striking photocatalytic activity of ATO-NR//TiO 2 stems from the effective charge separation, electrocatalytic activity for two-electron ORR, and low catalytic activity for H 2 O 2 decomposition.

Molecularly Engineered Covalent Organic Frameworks for Hydrogen Peroxide Photosynthesis

Synthesizing H₂ O₂ from water and air via a photocatalytic approach is ideal for efficient production of this chemical at small-scale. However, the poor activity and selectivity of the 2 e^- water oxidation reaction (WOR) greatly restricts the efficiency of photocatalytic H₂ O₂ production. Herein we prepare a bipyridine-based covalent organic framework photocatalyst (denoted as COF-TfpBpy) for H₂ O₂ production from water and air. The solar-to-chemical conversion (SCC) efficiency at 298 K and 333 K is 0.57 % and 1.08 %, respectively, which are higher than the current reported highest value. The resulting H₂ O₂ solution is capable of degrading pollutants. A mechanistic study revealed that the excellent photocatalytic activity of COF-TfpBpy is due to the protonation of bipyridine monomer, which promotes the rate-determining reaction (2 e^- WOR) and then enhances Yeager-type oxygen adsorption to accelerate 2 e^- one-step oxygen reduction. This work demonstrates, for the first time, the COF-catalyzed photosynthesis of H₂ O₂ from water and air; and paves the way for wastewater treatment using photocatalytic H₂ O₂ solution.