Z‐Schema‐Photokatalysesysteme für die Kohlendioxidreduktion: Wo stehen wir heute?

Covalent Organic Framework-Semiconductor-Based Heterostructures for Photocatalytic Applications

Photocatalysis is a promising and sustainable technology in the fields of energy conversion/storage and environment purification; however, the utilization of individual component as photocatalyst is generally restricted due to the low catalytic activity deriving from the rapid recombination of photogenerated electrons/holes. Covalent organic framework (COF)-semiconductor-based composite photocatalysts with synergistic effects provide a feasible route to achieve high-performance photocatalytic reactions with more active sites, strong light utilization ability, and high stability. In recent years, significant progress has been made in the rational design and preparation of the COF-semiconductors-based heterostructures for photocatalytic water splitting, carbon dioxide (CO₂ ) reduction, and dye/pollutant degradation. In this Review, the synthetic strategies of COF-semiconductor-based heterostructures are first introduced, which includes the rational design of the morphology, connection modes, and type of heterojunctions. The performance of COF-semiconductor-based heterostructures in different photocatalytic reactions are comprehensively reviewed. The structure-activity relationship and the synergistic effects within the heterostructures are discussed, and the photocatalytic mechanism and the role of COFs during the photocatalytic process are also presented. Finally, an outlook and challenges of realizing COF-semiconductor-based heterostructures with simple synthesis methods, diverse functions, high performance, and well-defined reaction mechanisms are provided.

Engineering a Copper Single-Atom Electron Bridge to Achieve Efficient Photocatalytic CO2 Conversion

Developing highly efficient and stable photocatalysts for the CO₂ reduction reaction (CO₂ RR) remains a great challenge. We designed a Z-Scheme photocatalyst with N-Cu₁ -S single-atom electron bridge (denoted as Cu-SAEB), which was used to mediate the CO₂ RR. The production of CO and O₂ over Cu-SAEB is as high as 236.0 and 120.1 μmol g^-1  h^-1 in the absence of sacrificial agents, respectively, outperforming most previously reported photocatalysts. Notably, the as-designed Cu-SAEB is highly stable throughout 30 reaction cycles, totaling 300 h, owing to the strengthened contact interface of Cu-SAEB, and mediated by the N-Cu₁ -S atomic structure. Experimental and theoretical calculations indicated that the SAEB greatly promoted the Z-scheme interfacial charge-transport process, thus leading to great enhancement of the photocatalytic CO₂ RR of Cu-SAEB. This work represents a promising platform for the development of highly efficient and stable photocatalysts that have potential in CO₂ conversion applications.

Facilitated Photocatalytic CO2 Reduction in Aerobic Environment on a Copper-Porphyrin Metal-Organic Framework

Herein, we fabricated a π-π stacking hybrid photocatalyst by combining two two-dimensional (2D) materials: g-C₃ N₄ and a Cu-porphyrin metal-organic framework (MOF). After an aerobic photocatalytic pretreatment, this hybrid catalyst exhibited an unprecedented ability to photocatalytically reduce CO₂ to CO and CH₄ under the typical level (20 %) of O₂ in the air. Intriguingly, the presence of O₂ did not suppress CO₂ reduction; instead, a fivefold increase compared with that in the absence of O₂ was observed. Structural analysis indicated that during aerobic pretreatment, the Cu node in the 2D-MOF moiety was hydroxylated by the hydroxyl generated from the reduction of O₂ . Then the formed hydroxylated Cu node maintained its structure during aerobic CO₂ reduction, whereas it underwent structural alteration and was reductively devitalized in the absence of O₂ . Theoretical calculations further demonstrated that CO₂ reduction, instead of O₂ reduction, occurred preferentially on the hydroxylated Cu node.

Investigation of Photocatalyst Composites for Pollutant Degradation in a Microslit Reactor Utilizing High Throughput Screening Techniques

The high-throughput screening investigations on TiO2 based photocatalyst composites presented here have been carried out in a 60-fold parallel photoreactor. Additional catalyst testing was performed in a microslit reactor system with immobilized catalysts. For further enhancing the photocatalytic activity of TiO2 (P25), composites of P25 and, for example, Bi2 O3 , CeO2 , g-C3 N4 , WO3 or ZnO were formulated in different nominal molar ratios. The catalysts' performances were assessed by their conversion of 17α-ethinyl estradiol (EE2) in aqueous solutions, determined by LC-MS. Findings show rapid EE2 conversions in short residence times. The extensive testing of catalysts led to the conclusion that the photocatalytic conversion is rather a function of residence time than a function of the materials utilized. This makes adequate process development seem more important than material development. The novelty of this contribution lies in the unique combination of testing a wide range of composite catalysts in a unique microreactor geometry.

A Prospective Life Cycle Assessment of Electrochemical CO2 Reduction to Selective Formic Acid and Ethylene

Converting CO₂ into valuable C₁ -C₂ chemicals through electrochemical CO₂ reduction (ECR) has potential to remedy the ever-increasing climate problems owing to the intensification of industrial activity. In this work, cradle-to-gate life cycle assessment (LCA) was performed to quantify the environmental impacts of formic acid (FA) and ethylene production through ECR benchmarked with the conventional processes. At the midpoint level, global warming potential (GWP) effects of FA and ethylene production through ECR recorded 5.6 and 1.6-times that of the conventional process, respectively. Although ECR currently has limited environmental benefits, the incorporation of hydropower has vast potential after evaluating four sustainable electricity sources, namely hydropower, wind, solar, and biomass. Notably, ECR to FA recorded a 24 % reduction in petrochemical usage. For ethylene production, human health damage, ecosystem damage, and petrochemical use were reduced by 67, 94, and 110 %, respectively. Sensitivity analysis indicated that a sustainable energy supply chain for ECR will accelerate the development of a circular economy.

Recent Advances in TiO2-based Photoanodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting has attracted a great attention in the past several decades which holds great promise to address global energy and environmental issues by converting solar energy into hydrogen. However, its low solar-to-hydrogen (STH) conversion efficiency remains a bottleneck for practical application. Developing efficient photoelectrocatalysts with high stability and high STH conversion efficiency is one of the key challenges. As a typical n-type semiconductor, titanium dioxide (TiO 2 ) exhibits high PEC water splitting performance, especially high chemical and photo stability. But, TiO 2 has also disadvantages such as wide band gap and fast electron-hole recombination rate, which seriously hinder its PEC performance. This review focuses on recent development in TiO 2 -based photoanodes as well as some key fundamentals. The corresponding mechanisms and key factors for high STH, and controllable synthesis and modification strategies are highlighted in this review. We conclude finally with an outlook providing a critical perspective on future trends on TiO 2 -based photoanodes for PEC water splitting.

Reductive Carbon-Carbon Coupling on Metal Sites Regulates Photocatalytic CO2 Reduction in Water Using ZnSe Quantum Dots

Colloidal quantum dots (QDs) consisting of precious-metal-free elements show attractive potentials towards solar-driven CO₂ reduction. However, the inhibition of hydrogen (H₂ ) production in aqueous solution remains a challenge. Here, we describe the first example of a carbon-carbon (C-C) coupling reaction to block the competing H₂ evolution in photocatalytic CO₂ reduction in water. In a specific system taking ZnSe QDs as photocatalysts, the introduction of furfural can significantly suppress H₂ evolution leading to CO evolution with a rate of ≈5.3 mmol g^-1  h^-1 and a turnover number (TON) of >7500 under 24 h visible light. Meanwhile, furfural is upgraded to the self-coupling product with a yield of 99.8 % based on the consumption of furfural. Mechanistic insights show that the reductive furfural coupling reaction occurs on surface Zn-sites to consume electrons and protons originally used for H₂ production, while the CO formation pathway at surface anion vacancies from CO₂ remains.

Electro- and Photocatalytic Oxidative Upgrading of Bio-based 5-Hydroxymethylfurfural

Conversion of biomass into biofuels and high value-added chemicals is a promising strategy to solve the increasingly deteriorating environmental problems caused by fossil energy consumption. The development of efficient technologies and methods is the premise and guarantee to realize the high-value conversion of biomass. 5-Hydroxymethylfurfural (HMF), as a versatile biomass platform compound, is generated via dehydration of hexoses (e. g., fructose and glucose) derived from cellulosic biomass. This Review gives an overview of the advances and challenges of electro- and photocatalytic oxidation of biomass-derived HMF to high-value chemicals such as 2,5-formylfuran (DFF) and 2,5-furandicarboxylic acid (FDCA). These strategies and methods for the preparation of high-value chemicals by electro- and photocatalytic oxidation of HMF, coupled with, for example, hydrogen evolution reaction, organic substrate reduction, CO₂ reduction reaction, or N₂ reduction reaction, were summarized and discussed. Moreover, the catalytic efficiency and mechanism of different types of catalysts were also introduced in these conversion systems.

Visible Light-Driven Selective Reduction of CO2 by Acetylene-Bridged Cobalt Porphyrin Conjugated Polymers

Photocatalytic conversion of CO₂ into renewable fuels with high efficiency and selectivity is desirable for solar energy utilization, but remains a great challenge. Herein, cobalt(II)-porphyrin functionalized conjugated polymers with acetylene bridging units, assembled through the Sonogashira cross coupling reaction, as heterogeneous catalysts for CO₂ photoreduction were presented. Experimental investigations and density functional theory calculations demonstrated the crucial roles of Co centers in porphyrin units for CO₂ activation and conversion, while excessive acetylene group prompted the competing hydrogen evolution reaction and reduced the selectivity. Thus, the CoPor-DBBP afforded superior activity for the CO generation with a rate of 286.7 μmol g^-1  h^-1 and high selectivity of up to 90.4 %. This work presents a new insight for rationally designing of porphyrin-based conjugated polymers as energetic photocatalyst in CO₂ reduction.

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.

A cation exchange strategy to construct Rod-shell CdS/Cu2S nanostructures for broad spectrum photocatalytic hydrogen production

Herein, Cu₂S as the outer shell is grown on CdS nanorods (NRs) to construct rod-shell nanostructures (CdS/Cu₂S) by a rapid, scalable and facile cation exchange reaction. The CdS NRs are firstly synthesized by a hydrothermal route, in which thiourea as the precursor of sulfur and ethylenediamine (EDA) as the solvent. And then, the outer shells of CdS NRs are successfully exchanged by Cu₂S via a cation exchange reaction. The obtained CdS/Cu₂S rod-shell NRs exhibit much enhanced activity of hydrogen production (640.95 μmol h^-1 g^-1) in comparison with pure CdS NRs (74.1 μmol h^-1 g^-1) and pure Cu₂S NRs (0 μmol h^-1 g^-1). The enhanced photocatalytic activity of CdS/Cu₂S rod-shell NRs owns to the following points: i) the photogenerated electrons generated by CdS quickly migrate to Cu₂S without any barrier due to rod-shell structure by the in-situ cation exchange reaction, a decreased carrier recombination is achieved; ii) Cu₂S as outer shells broaden the light absorption range of CdS/Cu₂S rod-shell NRs into visible or even NIR light, which can produce more electrons and holes. This work inspires people to further study the rod-shell structured photocatalyst through the cation exchange strategy to further solar energy conversion.

Highly Durable and Fully Dispersed Cobalt Diatomic Site Catalysts for CO2 Photoreduction to CH4

Dual-atom-site catalysts (DACs) have emerged as a new frontier in heterogeneous catalysis because the synergistic effect between adjacent metal atoms can promote their catalytic activity while maintaining the advantages of single-atom-site catalysts, such as almost 100 % atomic efficiency and excellent hydrocarbon selectivity. In this study, cobalt-based atom site catalysts with a Co₂ -N coordination structure were synthesized and used for photodriven CO₂ reduction. The resulting CoDAC containing 3.5 % Co atoms demonstrated a superior atom ratio for CO₂ reduction catalytic performance, with 65.0 % CH₄ selectivity, which far exceeds that of cobalt-based single-atom-site catalysts (CoSACs). The intrinsic reason for the superior activity of CoDACs is the excellent adsorption strength of CO₂ and CO* intermediates at dimeric Co active sites.

Energy Platform for Directed Charge Transfer in the Cascade Z-Scheme Heterojunction: CO2 Photoreduction without a Cocatalyst

A universal strategy is developed to construct a cascade Z‐Scheme system, in which an effective energy platform is the core to direct charge transfer and separation, blocking the unexpected type‐II charge transfer pathway. The dimension‐matched (001)TiO₂‐g‐C₃N₄/BiVO₄ nanosheet heterojunction (T‐CN/BVNS) is the first such model. The optimized cascade Z‐Scheme exhibits ≈19‐fold photoactivity improvement for CO₂ reduction to CO in the absence of cocatalysts and costly sacrificial agents under visible‐light irradiation, compared with BVNS, which is also superior to other reported Z‐Scheme systems even with noble metals as mediators. The experimental results and DFT calculations based on van der Waals structural models on the ultrafast timescale reveal that the introduced T as the platform prolongs the lifetimes of spatially separated electrons and holes and does not compromise their reduction and oxidation potentials.

Defect Engineering of Photocatalysts towards Elevated CO2 Reduction Performance

Photocatalytic CO₂ reduction provides a promising solution to address the crises of massive CO₂ emissions and fossil energy shortages. As one of the most effective strategies to promote CO₂ photoconversion, defect engineering shows great potential in modulating the electronic structure and light absorption properties of photocatalysts while increasing surface active sites for CO₂ activation and conversion. This Review summarizes the recent progress in defect engineering of photocatalysts to promote CO₂ reduction performances from the following four aspects: 1) Approaches to defect (mainly vacancy and dopant) generation in photocatalysts; 2) defect structure characterization techniques; 3) physical and chemical properties of defect-engineered photocatalysts; 4) CO₂ reduction performance enhancements in activity, selectivity, and stability of photocatalysts by defect engineering. This Review is expected to present readers with a comprehensive view of progress in the field of photocatalytic CO₂ reduction through defect engineering for elevated CO₂ -to-fuels conversion efficiency.

Prospects of Z-Scheme Photocatalytic Systems Based on Metal Halide Perovskites

Considering the attractive optoelectronic properties of metal halide perovskites (MHPs), their introduction to the field of photocatalysis was only a matter of time. Thus far, MHPs have been explored for the photocatalytic generation of hydrogen, carbon dioxide reduction, organic synthesis, and pollutant degradation applications. Of growing research interest and possible applied significance are the currently emerging developments of MHP-based Z-scheme heterostructures, which can potentially enable efficient photocatalysis of highly energy-demanding redox processes. In this Perspective, we discuss the advantages and limitations of MHPs compared to traditional semiconductor materials for applications as photocatalysts and describe emerging examples in the construction of MHP-based Z-scheme systems. We discuss the principles and material properties that are required for the development of such Z-scheme heterostructure photocatalysts and consider the ongoing challenges and opportunities in this emerging field.

Coupling Strategy for CO2 Valorization Integrated with Organic Synthesis by Heterogeneous Photocatalysis

Photocatalytic reduction of CO₂ to solar fuels and/or fine chemicals is a promising way to increase the energy supply and reduce greenhouse gas emissions. However, the conventional reaction system for CO₂ photoreduction with pure H₂ O or sacrificial agents usually suffers from low catalytic efficiency, poor stability, or cost-ineffective atom economy. A recent surge of developments, in which photocatalytic CO₂ valorization is integrated with selective organic synthesis into one reaction system, indicates an efficient modus operandi that enables sufficient utilization of photogenerated electrons and holes to achieve the goals for sustainable economic and social development. In this Review we discuss current advances in cooperative photoredox reaction systems that integrate CO₂ valorization with organics upgrading based on heterogeneous photocatalysis. The applications and virtues of this strategy and the underlying reaction mechanisms are discussed. The ongoing challenges and prospects in this area are critically discussed.

The synthesis of interface-modulated ultrathin Ni(ii) MOF/g-C3N4 heterojunctions as efficient photocatalysts for CO2 reduction

It is highly desirable to improve charge separation and to provide catalytic functions for the efficient photocatalytic CO2 reduction reaction (CO2RR) on g-C3N4 (CN). Here, dimension-matched ultrathin NiMOF/CN heterojunctions have been successfully constructed by the in situ growth of NiMOF nanosheets on hydroxylated and 1,4-aminobenzoic acid (AA) functionalized CN nanosheets, respectively, with ultrasonic assistance. The resultant NiMOF/CN heterojunctions exhibited excellent photocatalytic activities for the CO2RR to produce CO and CH4, especially NiMOF/CN-AA, which had photoactivity 18 times higher than that of bare CN. Based on the surface photovoltage responses, wavelength-dependent photocurrent action spectra, electrochemical impedance spectra, and CO2 electrochemical reduction data, it is clearly confirmed that the exceptional photoactivity mainly resulted from the favorable charge transport properties of ultrathin CN and coupled NiMOF, and from the greatly enhanced charge separation via excited high-level electron transfer from CN to NiMOF in the resultant intimately contacted heterojunction caused by the induction effect of AA, and also from the provided catalytic functionality of the central Ni(ii) for CO2 activation. This work provides a feasible synthetic protocol to fabricate MOF-containing dimension-matched heterojunctions with good charge separation for efficient photocatalysis.

Industrial carbon dioxide capture and utilization: state of the art and future challenges

This review covers the sustainable development of advanced improvements in CO₂ capture and utilization.