Dynamic Evolution of Atomically Dispersed Cu Species for CO2 Photoreduction to Solar Fuels

Copper Tantalate by a Sodium-Driven Flux-Mediated Synthesis for Photoelectrochemical CO2 Reduction

Copper-tantalate, Cu2Ta4O11 (CTO), shows significant promise as an efficient photocathode for multi-carbon compounds (C2+) production through photoelectrochemical (PEC) CO2 reduction, owing to its suitable energy bands and catalytic surface. However, synthesizing CTO poses a significant challenge due to its metastable nature and thermal instability. In this study, this challenge is addressed by employing a flux-mediated synthesis technique using a sodium-based flux to create sodium-doped CTO (Na-CTO) thin films, providing enhanced nucleation and stabilization for the CTO phase. To evaluate the PEC performance and catalytic properties of the films, copper(II) oxide (CuO) at the Na-CTO surface is selectively etched. The etched Na-CTO shows a lower dark current, with decreased contribution from photocorrosion, unlike the non-etched Na-CTO which has remaining CuO on the surface. Furthermore, Na-CTO exhibits 7.3-fold ethylene selectivity over hydrogen, thus highlighting its promising potential as a photocathode for C2+ production through PEC CO2 reduction.

Solar-driven production of renewable chemicals via biomass hydrogenation with green methanol

Solar-driven, selective biomass hydrogenation is recognized as a promising route to renewable chemicals production, but remains challenging. Here, we report a TiO2 supported Cu single-atom catalyst with a four-coordinated Cu1-O4 structure, which can be universally applied for solar-driven production of various renewable chemicals from lignocellulosic biomass-derived platform molecules with good yields using green methanol as a hydrogen donor, to address this challenge. It is significant that the biomass upgrading driven by natural sunlight on a gram scale demonstrates the great practical potential. By combining in situ soft X-ray absorption spectroscopy with theoretical calculations, we successfully identify the dynamic evolution of Cu sites along with the biomass hydrogenation and methanol oxidation, where the tandem process is enabled by the photogenerated electrons and holes to complete a chemical cycle. The concept of solar-driven biomass hydrogenation proposed here provides an efficient and sustainable methodology for the sustainable production of renewable chemicals.

Influence of Copper Valence in CuOx/TiO2 Catalysts on the Selectivity of Carbon Dioxide Photocatalytic Reduction Products

The Cu cocatalyst supported on the surface of TiO2 photocatalysts has demonstrated unique activity and selectivity in photocatalytic CO2 reduction. The valence state of copper significantly influences the catalytic process; however, due to the inherent instability of copper's valence states, the precise role of different valence states in CO2 reduction remains inadequately understood. In this study, CuOx/TiO2 catalysts were synthesized using an in situ growth reduction method, and we investigated the impact of various valence copper species on CO2 photocatalytic reduction. Our results indicate that Cu+ and Cu0 serve as primary active sites, with the selectivity for CH4 and CO products during CO2 photoreduction being closely related to their respective ratios on the catalyst surface. The adsorption and activation mechanisms of CO on both Cu+ and Cu0 surfaces are identified as critical factors determining product selectivity in photocatalytic processes. Furthermore, it is confirmed that Cu+ primarily facilitates CH4 production while Cu0 is responsible for generating CO. This study provides valuable insights into developing highly selective photocatalysts.

Dynamic evolution processes in electrocatalysis: structure evolution, characterization and regulation

Reactions on electrocatalytic interfaces often involve multiple processes, including the diffusion, adsorption, and conversion of reaction species and the interaction between reactants and electrocatalysts. Generally, these processes are constantly changing rather than being in a steady state. Recently, dynamic evolution processes on electrocatalytic interfaces have attracted increasing attention owing to their significant roles in catalytic reaction kinetics. In this review, we aim to provide insights into the dynamic evolution processes in electrocatalysis to emphasize the importance of unsteady-state processes in electrocatalysis. Specifically, the dynamic structure evolution of electrocatalysts, methods for the characterization of the dynamic evolution and the strategies for the regulation of the dynamic evolution for improving electrocatalytic performance are summarized. Finally, the conclusion and outlook on the research on dynamic evolution processes in electrocatalysis are presented. It is hoped that this review will provide a deeper understanding of dynamic evolution in electrocatalysis, and studies of electrocatalytic reaction processes and kinetics on the unsteady-state microscopic spatial and temporal scales will be given more attention.

Sulfate residuals on Ru catalysts switch CO2 reduction from methanation to reverse water-gas shift reaction

Efficient heterogeneous catalyst design primarily focuses on engineering the active sites or supports, often neglecting the impact of trace impurities on catalytic performance. Herein, we demonstrate that even trace amounts of sulfate (SO42-) residuals on Ru/TiO2 can totally change the CO2 reduction from methanation to reverse-water gas shift (RWGS) reaction under atmospheric pressure. We reveal that air annealing causes the trace amount of SO42- to migrate from TiO2 to Ru/TiO2 interface, leading to the significant changes in product selectivity from CH4 to CO. Detailed characterizations and DFT calculations show that the sulfate at Ru/TiO2 interface notably enhances the H transfer from Ru particles to the TiO2 support, weakening the CO intermediate activation on Ru particles and inhibiting the further hydrogenation of CO to CH4. This discovery highlights the vital role of trace impurities in CO2 hydrogenation reaction, and also provides broad implications for the design and development of more efficient and selective heterogeneous catalysts.

Recent progress in solar-driven CO2 reduction to multicarbon products

Currently, most catalysts used for photoconverting carbon dioxide (CO2) typically produce C1 products. Achieving multicarbon (C2+) products, which are highly desirable due to their greater energy density and economic potential, still remains a significant challenge. This difficulty is primarily due to the kinetic hurdles associated with the C-C coupling step in the process. Given this, devising diverse strategies to accelerate C-C coupling for generating multicarbon products is requisite. Herein, we first give a classification of catalysts involved in the photoconversion of CO2 to C2+ fuels. We summarize metallic oxides, metallic sulfides, MXenes, and metal-organic frameworks as catalysts for CO2 photoreduction to C2+ products, attributing their efficacy to the inherent dual active sites facilitating C-C coupling. In addition, we survey covalent organic frameworks, carbon nitrides, metal phosphides, and graphene as cocatalysts for CO2 photoreduction to C2+ products, owing to the incorporated dual active sites that induce C-C coupling. In the end, we provide a brief conclusion and an outlook on designing new photocatalysts, understanding the catalytic mechanisms, and considering the practical application requirements for photoconverting CO2 into multicarbon products.

Altering the CO2 Electroreduction Pathways Towards C1 or C2+ Products via Engineering the Strength of Interfacial Cu-O Bond

Copper (Cu)-based catalysts have established their unique capability for yielding wide value-added products from CO2. Herein, we demonstrate that the pathways of the electrocatalytic CO2 reduction reaction (CO2RR) can be rationally altered toward C1 or C2+ products by simply optimizing the coordination of Cu with O-containing organic species (squaric acid (H2C4O4) and cyclohexanehexaone (C6O6)). It is revealed that the strength of Cu-O bonds can significantly affect the morphologies and electronic structures of derived Cu catalysts, resulting in the distinct behaviors during CO2RR. Specifically, the C6O6-Cu catalysts made up from organized nanodomains shows a dominant C1 pathway with a total Faradaic efficiency (FE) of 63.7 % at -0.6 V (versus reversible hydrogen electrode, RHE). In comparison, the C4O4-Cu with an about perfect crystalline structure results in uniformly dispersed Cu-atoms, showing a notable FE of 65.8 % for C2+ products with enhanced capability of C-C coupling. The latter system also shows stable operation over at least 10 h with a high current density of 205.1 mA cm-2 at -1.0 VRHE, i.e., is already at the boarder of practical relevance. This study sheds light on the rational design of Cu-based catalysts for directing the CO2RR reaction pathway.

M/BiOCl-(M = Pt, Pd, and Au) Boosted Selective Photocatalytic CO2 Reduction to C2 Hydrocarbons via *CHO Intermediate Manipulation

Selective CO2 photoreduction to C2 hydrocarbons is significant but limited by the inadequate adsorption strength of the reaction intermediates and low efficiency of proton transfer. Herein, an ameliorative *CO adsorption and H2O activation strategy is realized via decorating bismuth oxychloride (BiOCl) nanostructures with different metal (Pt, Pd, and Au) species. Experimental and theoretical calculation results reveal that distinct *CO binding energies and *H acquisition abilities of the metal cocatalysts mediate the CO2 reduction activity and hydrocarbon selectivity. The relatively moderate *CO adsorption and *H supply over Pd/BiOCl endows it with the lowest free energy to generate *CHO, leading to its highest activity of hydrocarbon production. Specifically, the Pt cocatalyst can efficiently participate in H2O dissociation to deliver more *H for facilitating the protonation of the *CHO and *CHOH, thereby favoring CH4 production with 76.51% selectivity. A lower *H supply over Pd/BiOCl and Au/BiOCl results in a large energy barrier for *CHO or *CHOH protonation and thus a more thermodynamically favored OC─CHO coupling pathway, which endows them with vastly increased C2 hydrocarbon selectivity of 81.21% and 92.81%, respectively. The understanding of efficient C2 hydrocarbon production in this study sheds light on how materials can be engineered for photocatalytic CO2 reduction.

Shaping and Doping Metal-Organic Framework-Derived TiO2 to Steer the Selectivity of Photocatalytic CO2 Reduction toward CH4

Steering selectivity in photocatalytic conversion of CO2, especially toward deep reduction products, is vital to energy and environmental goals yet remains a great challenge. In this work, we demonstrate a facet-dependent photocatalytic selective reduction of CO2 to CH4 in Cu-doped TiO2 catalysts exposed with different facets synthesized by a topological transformation from MIL-125 (Ti) precursors. The optimized round cake-like Cu/TiO2 photocatalyst mainly exposed with the (001) facet exhibited a high photocatalytic CO2 reduction performance with a CH4 yield of 40.36 μmol g-1 h-1 with a selectivity of 94.1%, which are significantly higher than those of TiO2 (001) (4.70 μmol g-1 h-1 and 52.6%, respectively), Cu/TiO2 (001 + 101) (18.95 μmol g-1 h-1 and 69.6%, respectively), and Cu/TiO2 (101) (14.73 μmol g-1 h-1 and 78.9%, respectively). The results of experimental and theoretical calculations demonstrate that the Cu doping dominating the promoted separation and migration efficiencies of photogenerated charges and the preferential adsorption on (001) facets synergistically contribute to the selective reduction of CO2 to CH4. This work highlights the significance of synergy between facet engineering and ion doping in the design of high-performance photocatalysts with respect to selective reduction of CO2 to multielectron products.

Oxidizing Role of Cu Cocatalysts in Unassisted Photocatalytic CO2 Reduction Using p-GaN/Al2O3/Au/Cu Heterostructures

Photocatalytic CO2 reduction to CO under unassisted (unbiased) conditions was recently demonstrated using heterostructure catalysts that combine p-type GaN with plasmonic Au nanoparticles and Cu nanoparticles as cocatalysts (p-GaN/Al2O3/Au/Cu). Here, we investigate the mechanistic role of Cu in p-GaN/Al2O3/Au/Cu under unassisted photocatalytic operating conditions using Cu K-edge X-ray absorption spectroscopy and first-principles calculations. Upon exposure to gas-phase CO2 and H2O vapor reaction conditions, the composition of the Cu nanoparticles is identified as a mixture of CuI and CuII oxide, hydroxide, and carbonate compounds without metallic Cu. These composition changes, indicating oxidative conditions, are rationalized by bulk Pourbaix thermodynamics. Under photocatalytic operating conditions with visible light excitation of the plasmonic Au nanoparticles, further oxidation of CuI to CuII is observed, indicating light-driven hole transfer from Au-to-Cu. This observation is supported by the calculated band alignments of the oxidized Cu compositions with plasmonic Au particles, where light-driven hole transfer from Au-to-Cu is found to be thermodynamically favored. These findings demonstrate that under unassisted (unbiased) gas-phase reaction conditions, Cu is found in carbonate-rich oxidized compositions rather than metallic Cu. These species then act as the active cocatalyst and play an oxidative rather than a reductive role in catalysis when coupled with plasmonic Au particles for light absorption, possibly opening an additional channel for water oxidation in this system.

Synergistic effect of atomically dispersed Cu species and Ti-defects for boosting photocatalytic CO2 reduction over hierarchical TiO2

The photocatalytic water-mediated CO2 reduction reaction, which holds great promise for the conversion of CO2 into valuable chemicals, is often hindered by inefficient separation of photogenerated charges and a lack of suitable catalytic sites. Herein, we have developed a glycerol coordination assembly approach to precisely control the distribution of atomically dispersed Cu species by occupying Ti-defects and adjusting the ratio between Cu species and Ti-defects in a hierarchical TiO2. The optimal sample demonstrates a ∼4-fold improvement in CO2-to-CO conversion compared to normal TiO2 nanoparticles. The high activity could be attributed to the Ti defects, which enhance the photogenerated charge separation and simultaneously facilitate the adsorption of water molecules, thereby promoting the water oxidation reaction. Moreover, by means of in situ EPR and FTIR spectra, we have demonstrated that Cu species can effectively capture photogenerated electrons and facilitate the adsorption of CO2, so as to catalyze the reduction of CO2. This work provides a strategy for the construction of atomic-level synergistic catalytic sites and the utilization of in situ techniques to reveal the underlying mechanism.

Engineering Spatially Adjacent Redox Sites with Synergistic Spin Polarization Effect to Boost Photocatalytic CO2 Methanation

The integration of oxidation and reduction half-reactions to amplify their synergy presents a considerable challenge in CO2 photoconversion. Addressing this challenge requires the construction of spatially adjacent redox sites while suppressing charge recombination at these sites. This study introduces an innovative approach that utilizes spatial synergy to enable synergistic redox reactions within atomic proximity and employs spin polarization to inhibit charge recombination. We incorporate Mn into Co3O4 as a catalyst, in which Mn sites tend to enrich holes as water activation sites, while adjacent Co sites preferentially capture electrons to activate CO2, forming a spatial synergy. The direct H transfer from H2O at Mn sites facilitates the formation of *COOH on adjacent Co sites with remarkably favorable thermodynamic energy. Notably, the incorporation of Mn induces spin polarization in the system, significantly suppressing the recombination of photogenerated charges at redox sites. This effect is further enhanced by applying an external magnetic field. By synergizing spatial synergy and spin polarization, Mn/Co3O4 exhibits a CH4 production rate of 23.4 μmol g-1 h-1 from CO2 photoreduction, showcasing a 28.8 times enhancement over Co3O4. This study first introduces spin polarization to address charge recombination issues at spatially adjacent redox sites, offering novel insights for synergistic redox photocatalytic systems.

Synchrotron Radiation Based X-ray Absorption Spectroscopy: Fundamentals and Applications in Photocatalysis

Photocatalysis is one of the most promising green technologies to utilize solar energy for clean energy achievement and environmental governance. There is a knotty problem to rational designing high-performance photocatalyst, which largely depends on an in-depth insight into their structure-activity relationships and complex photocatalytic reaction mechanisms. Synchrotron radiation based X-ray absorption spectroscopy (XAS) is an important characterization method for photocatlayst to offer the element-specific key geometric and electronic structural information at the atomic level, on this basis, time-resolved XAS technique has a huge impact on mechanistic understanding of photochemical reaction owing to their powerful ability to probe, in real-time, the electronic and geometric structures evolution within photocatalysis reactions. This review will focus on the fundamentals of XAS and their applications in photocatalysis. The detailed applications obtained from XAS is described through the following aspects: 1) identifying local structure of photocatalyst; 2) uncovering in situ structure and chemical state evolution during photocatalysis; 3) revealing the photoexcited process. We will provide an in depth understanding on how the XAS method can guide the rational design of highly efficient photocatalyst. Finally, a systematic summary of XAS and related significance is made and the research perspectives are suggested.

Understanding the Dynamic Aggregation in Single-Atom Catalysis

The dynamic response of single-atom catalysts to a reactive environment is an increasingly significant topic for understanding the reaction mechanism at the molecular level. In particular, single atoms may experience dynamic aggregation into clusters or nanoparticles driven by thermodynamic or kinetic factors. Herein, the inherent mechanistic nuances that determine the dynamic profile during the reaction will be uncovered, including the intrinsic stability and site-migration barrier of single atoms, external stimuli (temperature, voltage, and adsorbates), and the influence of catalyst support. Such dynamic aggregation can be beneficial or deleterious on the catalytic performance depending on the optimal initial state. Those examples will be highlighted where in situ formed clusters, rather than single atoms, serve as catalytically active sites for improved catalytic performance. This is followed by the introduction of operando techniques to understand the structural evolution. Finally, the emerging strategies via confinement and defect-engineering to regulate dynamic aggregation will be briefly discussed.

Catalyzing Artificial Photosynthesis with TiO2 Heterostructures and Hybrids: Emerging Trends in a Classical yet Contemporary Photocatalyst

Titanium dioxide (TiO2) stands out as a versatile transition-metal oxide with applications ranging from energy conversion/storage and environmental remediation to sensors and optoelectronics. While extensively researched for these emerging applications, TiO2 has also achieved commercial success in various fields including paints, inks, pharmaceuticals, food additives, and advanced medicine. Thanks to the tunability of their structural, morphological, optical, and electronic characteristics, TiO2 nanomaterials are among the most researched engineering materials. Besides these inherent advantages, the low cost, low toxicity, and biocompatibility of TiO2 nanomaterials position them as a sustainable choice of functional materials for energy conversion. Although TiO2 is a classical photocatalyst well-known for its structural stability and high surface activity, TiO2-based photocatalysis is still an active area of research particularly in the context of catalyzing artificial photosynthesis. This review provides a comprehensive overview of the latest developments and emerging trends in TiO2 heterostructures and hybrids for artificial photosynthesis. It begins by discussing the common synthesis methods for TiO2 nanomaterials, including hydrothermal synthesis and sol-gel synthesis. It then delves into TiO2 nanomaterials and their photocatalytic mechanisms, highlighting the key advancements that have been made in recent years. The strategies to enhance the photocatalytic efficiency of TiO2, including surface modification, doping modulation, heterojunction construction, and synergy of composite materials, with a specific emphasis on their applications in artificial photosynthesis, are discussed. TiO2-based heterostructures and hybrids present exciting opportunities for catalyzing solar fuel production, organic degradation, and CO2 reduction via artificial photosynthesis. This review offers an overview of the latest trends and advancements, while also highlighting the ongoing challenges and prospects for future developments in this classical yet rapidly evolving field.

Well-defined diatomic catalysis for photosynthesis of C2H4 from CO2

Owing to the specific electronic-redistribution and spatial proximity, diatomic catalysts (DACs) have been identified as principal interest for efficient photoconversion of CO2 into C2H4. However, the predominant bottom-up strategy for DACs synthesis has critically constrained the development of highly ordered DACs due to the random distribution of heteronuclear atoms, which hinders the optimization of catalytic performance and the exploration of actual reaction mechanism. Here, an up-bottom ion-cutting architecture is proposed to fabricate the well-defined DACs, and the superior spatial proximity of CuAu diatomics (DAs) decorated TiO2 (CuAu-DAs-TiO2) is successfully constructed due to the compact heteroatomic spacing (2-3 Å). Owing to the profoundly low C-C coupling energy barrier of CuAu-DAs-TiO2, a considerable C2H4 production with superior sustainability is achieved. Our discovery inspires a novel up-bottom strategy for the fabrication of well-defined DACs to motivate optimization of catalytic performance and distinct deduction of heteroatom synergistically catalytic mechanism.

Copper nanoparticles encapsulated in zeolitic imidazolate framework-8 as a stable and selective CO2 hydrogenation catalyst

Metal-organic frameworks have drawn attention as potential catalysts owing to their unique tunable surface chemistry and accessibility. However, their application in thermal catalysis has been limited because of their instability under harsh temperatures and pressures, such as the hydrogenation of CO2 to methanol. Herein, we use a controlled two-step method to synthesize finely dispersed Cu on a zeolitic imidazolate framework-8 (ZIF-8). This catalyst suffers a series of transformations during the CO2 hydrogenation to methanol, leading to ~14 nm Cu nanoparticles encapsulated on the Zn-based MOF that are highly active (2-fold higher methanol productivity than the commercial Cu-Zn-Al catalyst), very selective (>90%), and remarkably stable for over 150 h. In situ spectroscopy, density functional theory calculations, and kinetic results reveal the preferential adsorption sites, the preferential reaction pathways, and the reverse water gas shift reaction suppression over this catalyst. The developed material is robust, easy to synthesize, and active for CO2 utilization.

Accelerated Photocatalytic Carbon Dioxide Reduction and Water Oxidation under Spatial Synergy

Photocatalytic conversion of CO2 and H2 O into fuels and oxygen is a highly promising solution for carbon-neutral recycling. Traditionally, researchers have studied CO2 reduction and H2 O oxidation separately, overlooking potential synergistic interplay between these processes. This study introduces an innovative approach, spatial synergy, which encourages synergistic progress by bringing the two half-reactions into atomic proximity. To facilitate this, we developed a defective ZnIn2 S4 -supported single-atom Cu catalyst (Cu-SA/D-ZIS), which demonstrates remarkable catalytic performance with CO2 reduction rates of 112.5 μmol g-1  h-1 and water oxidation rates of 52.3 μmol g-1  h-1 , exhibiting a six-fold enhancement over D-ZIS. The structural characterization results indicated that the trapping effect of vacancy associates on single-atom copper led to the formation of an unsaturated coordination structure, Cu-S3 , consequently giving rise to the CuZn 'VS ⋅⋅VZn " defect complexes. FT-IR studies coupled with theoretical calculations reveal the spatially synergistic CO2 reduction and water oxidation on CuZn 'VS ⋅⋅VZn ", where the breakage of O-H in water oxidation is synchronized with the formation of *COOH, significantly lowering the energy barrier. Notably, this study introduces and, for the first time, substantiates the spatial synergy effect in CO2 reduction and H2 O oxidation through a combination of experimental and theoretical analyses, providing a fresh insight in optimizing photocatalytic system.

Halogen-Modulated 2D Coordination Polymers for Efficient Hydrogen Peroxide Photosynthesis under Air and Pure Water Conditions

H2 O2 is a widely used eco-friendly oxidant and a potential energy carrier. Photocatalytic H2 O2 production from water and O2 is an ideal approach with the potential to address the current energy crisis and environmental issues. Three zig-zag two-dimensional coordination polymers (2D CPs), named CuX-dptz, were synthesized by a rapid and facile method at room temperature, showing preeminent H2 O2 photoproduction performance under pure water and open air without any additives. CuBr-dptz exhibits a H2 O2 production rate high up to 1874 μmol g-1  h-1 , exceeding most reported photocatalysts under this condition, even comparable to those supported by sacrificial agents and O2 . The coordination environment of Cu can be modulated by halogen atoms (X=Cl, Br, I), which in turn affects the electron transfer process and finally determines the reaction activity. This is the first time that 2D CPs have been used for photocatalytic H2 O2 production in such challenging conditions, which provides a new pathway for the development of portable in situ H2 O2 photosynthesis devices.

Nanoscale Metal Particle Modified Single-Atom Catalyst: Synthesis, Characterization, and Application

Single-atom catalysts (SACs) have attracted considerable attention in heterogeneous catalysis because of their well-defined active sites, maximum atomic utilization efficiency, and unique unsaturated coordinated structures. However, their effectiveness is limited to reactions requiring active sites containing multiple metal atoms. Furthermore, the loading amounts of single-atom sites must be restricted to prevent aggregation, which can adversely affect the catalytic performance despite the high activity of the individual atoms. The introduction of nanoscale metal particles (NMPs) into SACs (NMP-SACs) has proven to be an efficient approach for improving their catalytic performance. A comprehensive review is urgently needed to systematically introduce the synthesis, characterization, and application of NMP-SACs and the mechanisms behind their superior catalytic performance. This review first presents and classifies the different mechanisms through which NMPs enhance the performance of SACs. It then summarizes the currently reported synthetic strategies and state-of-the-art characterization techniques of NMP-SACs. Moreover, their application in electro/thermo/photocatalysis, and the reasons for their superior performance are discussed. Finally, the challenges and perspectives of NMP-SACs for the future design of advanced catalysts are addressed.

Promoting the efficiency and selectivity of NO3--to-NH3 reduction on Cu-O-Ti active sites via preferential glycol oxidation with holes

The combined reductive and oxidative reaction is the essence of a solar-driven photoredox system. Unfortunately, most of these efforts focus on the specific half-reactions, and the key roles of complete photoredox reactions have been overlooked. Taking the nitrate reduction reaction (NO3-RR) as a typical multiple-electrons involved process, the selective reduction of the NO3- into ammonia (NH3) synthesis with high efficiency is still a grand challenge. Herein, a rational oxidative half-reaction is tailored to achieve the selective conversion of NO3- to NH3 on Cu-O-Ti active sites. Through the coupled NO3-RR with glycol oxidation reaction system, a superior NH3 photosynthesis rate of 16.04 ± 0.40 mmol gcat-1 h-1 with NO3- conversion ratio of 100% and almost 100% of NH3 selectivity is reached on Cu-O-Ti bimetallic oxide cluster-anchored TiO2 nanosheets (CuOx@TNS) catalyst. A combination of comprehensive in situ characterizations and theoretical calculations reveals the molecular mechanism of the synergistic interaction between NO3-RR and glycol oxidation pair on CuOx@TNS. The introduction of glycol accelerates the h+ consumption for the formation of alkoxy (•R) radicals to avoid the production of •OH radicals. The construction of Cu-O-Ti sites facilitates the preferential oxidation of glycol with h+ and enhances the production of e- to participate in NO3-RR. The efficiency and selectivity of NO3--to-NH3 synthesis are thus highly promoted on Cu-O-Ti active sites with the accelerated glycol oxidative half-reaction. This work upgrades the conventional half photocatalysis into a complete photoredox system, demonstrating the tremendous potential for the precise regulation of reaction pathway and product selectivity.

A porous polyacrylonitrile (PAN)/covalent organic framework (COF) fibrous membrane photocatalyst for highly efficient and ultra-stable hydrogen evolution

Photocatalytic water splitting has been regarded as one of the most promising technologies to generate hydrogen as an ideal energy carrier in the future. However, most of the experience for such process are derived from the researches based on the suspension powder photocatalysts under a stirring condition and a practical scaling application is urgently calling for the high-efficient panel reactors based on the membrane photocatalysts. Herein, we develop a new series of flexible and ultrastable membrane photocatalysts through a controllable growth of covalent organic framework (COF) photocatalysts on the polyacrylonitrile (PAN) electrospun fiber membrane. Multiple characterization techniques verify the successful anchoring of the COF-photocatalysts on the PAN fibers, forming a three-dimensional porous PAN/COF membrane photocatalyst with excellent light absorption ability, high specific surface area, and good hydrophily. As a result, the optimized PAN/COF membrane photocatalyst exhibits excellent hydrogen evolution rate up to 1.25 mmol g-1h-1 under visible-light irradiation without stirring, which is even higher than that of the corresponding suspension COF-powder photocatalyst with stirring. In particular, the PAN/COF membrane photocatalyst demonstrates a much more superior hydrogen evolution stability and also a much better recyclability. This study gives some experience for the practical scaling application of solar-driven water splitting.

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

Single-molecule fluorescence microscopy enables the direct observation of individual reaction events at the surface of a catalyst. It has become a powerful tool to image in real time both intra- and interparticle heterogeneity among different nanoscale catalyst particles. Single-molecule fluorescence microscopy of heterogeneous catalysts relies on the detection of chemically activated fluorogenic probes that are converted from a nonfluorescent state into a highly fluorescent state through a reaction mediated at the catalyst surface. This review article describes challenges and opportunities in using such fluorogenic probes as proxies to develop structure-activity relationships in nanoscale electrocatalysts and photocatalysts. We compare single-molecule fluorescence microscopy to other microscopies for imaging catalysis in situ to highlight the distinct advantages and limitations of this technique. We describe correlative imaging between super-resolution activity maps obtained from multiple fluorogenic probes to understand the chemical origins behind spatial variations in activity that are frequently observed for nanoscale catalysts. Fluorogenic probes, originally developed for biological imaging, are introduced that can detect products such as carbon monoxide, nitrite, and ammonia, which are generated by electro- and photocatalysts for fuel production and environmental remediation. We conclude by describing how single-molecule imaging can provide mechanistic insights for a broader scope of catalytic systems, such as single-atom catalysts.

Amorphization-Induced Cation Exchange in Indium Oxide Nanosheets for CO2-to-Ethanol Conversion

Cation exchange (CE) in metal oxides under mild conditions remains an imperative yet challenging goal to tailor their composition and enable practical applications. Herein, we first develop an amorphization-induced strategy to achieve room-temperature CE for universally synthesizing single-atom doped In2O3 nanosheets (NSs). Density functional theory (DFT) calculations elucidate that the abundant coordination-unsaturated sites present in a-In2O3 NSs are instrumental in surmounting the energy barriers of CE reactions. Empirically, a-In2O3 NSs as the host materials successfully undergo exchange with unary cations (Cu2+, Co2+, Mn2+, Ni2+), binary cations (Co2+Mn2+, Co2+Ni2+, Mn2+Ni2+), and ternary cations (Co2+Mn2+Ni2+). Impressively, high-loading single-atom doped (over 10 atom %) In2O3 NSs were obtained. Additionally, Cu/a-In2O3 NSs exhibit an excellent ethanol yield (798.7 μmol g-1 h-1) with a high selectivity of 99.5% for the CO2 photoreduction. This work offers a new approach to induce CE reactions in metal oxides under mild conditions and constructs scalable single-atom doped catalysts for critical applications.

In Situ Loading of Cu Nanocrystals on CsCuCl3 for Selective Photoreduction of CO2 to CH4

Rational design and facile synthesis of efficient environmentally friendly all-inorganic lead-free halide perovskite catalysts are of great significance in photocatalytic CO2 reduction. Aiming at photogenerated charge carrier separation and CO2 reaction dynamics, in this paper, a CsCuCl3 /Cu nanocrystals (NCs) heterojunction catalyst is designed and synthesized via a simple acid-etching solution process by using Cu2 O as the sacrificed template. Due to the disproportionation reaction of Cu2 O induced by concentrated hydrochloric acid, Cu NCs can be deposited onto the surface of CsCuCl3 microcrystals directly and tightly. As revealed by photoelectrochemical analysis, in situ Fourier transform infrared spectra, etc., the Cu NCs contribute a lot to extracting photoelectrons of CsCuCl3 to improve the charge separation efficiency, regulating the CO2 adsorption and activation, and also stabilizing the reaction intermediates. Therefore, CsCuCl3 /Cu heterojunction exhibits a total electron consumption rate of 58.77 µmol g-1 h-1 , which is 2.9-fold of that of single CsCuCl3 . Moreover, high CH4 selectivity of up to 92.7% is achieved, which is much higher than that of CsCuCl3 (50.4%) and most lead-free halide perovskite-based catalysts. This work provides an ingenious but simple strategy to rationally design cocatalysts in situ decorated perovskite catalysts for manipulating both the catalytic activity and the product selectivity.

Boosted C-C coupling with Cu-Ag alloy sub-nanoclusters for CO2-to-C2H4 photosynthesis

The selective photocatalytic conversion of CO2 and H2O to high value-added C2H4 remains a great challenge, mainly attributed to the difficulties in C-C coupling of reaction intermediates and desorption of C2H4* intermediates from the catalyst surface. These two key issues can be simultaneously overcome by alloying Ag with Cu which gives enhanced activity to both reactions. Herein, we developed a facile stepwise photodeposition strategy to load Cu-Ag alloy sub-nanoclusters (ASNCs) on TiO2 for CO2 photoreduction to produce C2H4. The optimized catalyst exhibits a record-high C2H4 formation rate (1110.6 ± 82.5 μmol g-1 h-1) with selectivity of 49.1 ± 1.9%, which is an order-of-magnitude enhancement relative to current work for C2H4 photosynthesis. The in situ FT-IR spectra combined with DFT calculations reveal the synergistic effect of Cu and Ag in Cu-Ag ASNCs, which enable an excellent C-C coupling capability like Ag and promoted C2H4* desorption property like Cu, thus advancing the selective and efficient production of C2H4. The present work provides a deeper understanding on cluster chemistry and C-C coupling mechanism for CO2 reduction on ASNCs and develops a feasible strategy for photoreduction CO2 to C2 fuels or industrial feedstocks.

Efficient Photocatalytic CO2 Reduction with High Selectivity for Ethanol by Synergistically Coupled MXene-Ceria and the Charge Carrier Dynamics

The primary factors that govern the selectivity and efficacy of CO2 photoreduction are the degree of activation of CO2 on the active surface sites of photocatalysts and charge separation/transfer kinetics. In this context, the rational synthesis of heterostructured MXene-coupled CeO2-based photocatalysts with different loading concentrations of Ti3C2MXene via a one-step hydrothermal approach has been undertaken. These photocatalysts exhibit a shift in X-ray diffraction peaks to higher 2θ values and changes in stretching vibrations of 5 wt % Ti3C2MXene/CeO2(5-TC/Ce) that indicate interaction between Ti3C2MXene and CeO2. Moreover, XPS analysis confirms the presence of the Ce3+/Ce4+ states. A sharp band at 2335 cm-1 observed during the CO2 photoreduction process corresponds to bidentate b-CO32-, which facilitates the adsorption of CO2 at the surface of the catalyst as revealed by the TPD analysis. Furthermore, the Schryvers test and NMR analysis were undertaken to confirm the formaldehyde intermediate formation during CO2 photoreduction to C2H5OH. The decrease in emission intensity, reduced lifetimes (2.68 ns), and lower interfacial resistance, as revealed by PL, TR-PL, and EIS analysis, imply an efficient charge separation and charge transfer in the case of the Ti3C2MXene/CeO2 heterojunction. The decrease in the intensity of peaks in the EPR spectrum in the case of 5-TC/Ce further confirms efficient charge transfer kinetics across the interface. The optimized 5-TC/Ce shows CO2 reduction with a drastically enhanced yield of ethanol on the order of 6127 μmol g-1 at 5 h with 98% selectivity and 7.54% apparent quantum efficiency, which is 6-fold higher than that of ethanol produced by bare CeO2. Herein, CeO2 that acts as a redox couple (Ce3+/Ce4+) when coupled with MXene having a metallic nature that reduces the electron transfer resistance is in unison, enabling an enhanced mobilization of electrons. Thereby, the synergistic coupling of Ti3C2MXene with CeO2 leads to an efficient photoreduction of CO2 under visible light illumination.

Construction of Zr-Metal-Organic Frameworks-Based Composite Materials toward Anhydrous Proton Conduction and Photocatalytic CO2 Reduction

Metal-organic frameworks constructed from Zr usually possess excellent chemical and physical stability. Therefore, they have become attractive platforms in various fields. In this work, two families of hybrid materials based on ZrSQU have been designed and synthesized, named Im@ZrSQU and Cu@ZrSQU, respectively. Im@ZrSQU was prepared through the impregnation method and employed for proton conduction. Im@ZrSQU exhibited terrific proton conduction performance in an anhydrous environment, with the highest proton conduction value of 3.6 × 10-2 S cm-1 at 110 °C. In addition, Cu@ZrSQU was synthesized via the photoinduction method for the photoreduction of CO2, which successfully promoted the conversion of CO2 into CO and achieved the CO generation rate of up to 12.4 μmol g-1 h-1. The photocatalytic performance of Cu@ZrSQU is derived from the synergistic effect of Cu NPs and ZrSQU. Based on an in-depth study and discussion toward ZrSQU, we provide a versatile platform with applications in the field of proton conduction and photocatalysis, which will guide researchers in their further studies.

Cu-doped mesoporous SnO2 nanoparticles with rich grain boundaries and oxygen vacancies for photocatalytic CO2-to-CO conversion

Photocatalytic conversion of carbon dioxide into fuels provides an effective approach to realize carbon resource utilization. However, the photocatalytic efficiency is still relatively low due to the recombination of photogenerated charges. Herein, we have designed Cu-doped SnO2 nanoparticles (Cu-SnO2) using a glucose-involved hydrothermal crystallization method for the photocatalytic reduction of CO2. The rich oxygen vacancies facilitated the separation and transfer of photogenerated charges, and the confined effect of the typical mesoporous structure promoted the adsorption of CO2, especially a high density of grain boundaries (GBs) and the doping of atomic Cu would introduce new active sites to activate CO2 molecules. This elaborately designed catalyst exhibited super and stable photocatalytic conversion activity of CO2-into-CO, with a CO optimal yield of 107 µmol g-1 in 4 h, which was 2.75 times that over pure SnO2. In situ Raman results indicated that the CO2 reduction reaction followed a *COOH pathway on Cu-SnO2. This work provides implications for the construction of a catalyst with rich defects in the field of energy and environmental catalysis.

In Operando Photoswitching of Cu Oxidation States in Cu-Based Plasmonic Heterogeneous Photocatalysis for Efficient H2 Evolution

Metal nanoparticles (NP) supported on TiO₂ are known to be efficient photocatalysts for solar-to-chemical energy conversion. While TiO₂ decorated with copper NPs has the potential to become an attractive system, the poor oxidative stability of Cu severely limits its applicability. In this work, we demonstrate that, when Cu NPs supported on TiO₂ nanobelts (NBs) are engaged in the photocatalytic generation of H₂ from water under light illumination, Cu is not only oxidized in CuO but also dissolved under the form of Cu⁺/Cu²⁺ ions, leading to a continuous reconstruction of nanoparticles via Ostwald ripening. By nanoencapsulating the CuO_x (Cu/CuO/Cu₂O) NPs by a few layers of carbon supported on TiO₂ (TC@C), Ostwald ripening can be suppressed. Simultaneously, the resulting CuO_x@C NPs are photoreduced under light illumination to generate Cu@C NPs. This photoswitching strategy allows the preparation of a Cu plasmonic photocatalyst with enhanced activity for H₂ production. Remarkably, the photocatalyst is even active when illuminated with visible light, indicating a clear plasmonic enhancement of photocatalytic activity from the surface plasmonic resonance (SPR) effect of Cu NPs. Three-dimensional electromagnetic wave-frequency domain (3D-EWFD) simulations were conducted to confirm the SPR enhancement. This advance bodes for the development of scalable multifunctional Cu-based plasmonic photocatalysts for solar energy transfer.

Deciphering the Superior Electronic Transmission Induced by the Li-N Ligand Pairs Boosted Photocatalytic Hydrogen Evolution

Atomic level decoration route is designated as one of the attractive methods to regulate both the charge density and band structure of photocatalysts. Moreover, to enable more efficient separation and transport of photocarriers, the construction of novel active sites can enhance both the reactivity and electrical conductivity of the crystal. Herein, an Li-N ligand is constructed via co-doping lithium and nitrogen atoms into ZnIn₂ S₄ lattice, which achieves a promoted photocatalytic H₂ evolution at 9737 µmol g^-1 h^-1 . The existence of Li-N ligand pairs and the behaviors of photocarriers on L₄₀ N₅ ZIS are determined systematically, which also provides a unique insight into the mechanism of the improved photocarrier migration rate. With the introduction of Li-N dual sites, the vacancy form of ZnIn₂ S₄ has changed and the photocatalytic stability is significantly improved. Interestingly, the change of charge density around Li-N ligand in ZnIn₂ S₄ is determined by theoretical simulations, as well as the regulated energy barrier of photocatalytic water splitting caused by Li-N dual sites, which act as both adsorption site for H₂ O and stronger reactive sites. This work helps to extend the understanding of ZnIn₂ S₄ and offers a fresh perspective for the creation of a Li-N co-doped photocatalyst.

Recent Advances and Perspectives of Core-Shell Nanostructured Materials for Photocatalytic CO2 Reduction

Photocatalytic CO₂ conversion into solar fuels is a promising technology to alleviate CO₂ emissions and energy crises. The development of core-shell structured photocatalysts brings many benefits to the photocatalytic CO₂ reduction process, such as high conversion efficiency, sufficient product selectivity, and endurable catalyst stability. Core-shell nanostructured materials with excellent physicochemical features take an irreplaceable position in the field of photocatalytic CO₂ reduction. In this review, the recent development of core-shell materials applied for photocatalytic reduction of CO_{2 is introduced} . First, the basic principle of photocatalytic CO₂ reduction is introduced. In detail, the classification and synthesis techniques of core-shell catalysts are discussed. Furthermore, it is also emphasized that the excellent properties of the core-shell structure can greatly improve the activity, selectivity, and stability in the process of photocatalytic CO₂ reduction. Hopefully, this paper can provide a favorable reference for the preparation of efficient photocatalysts for CO₂ reduction.

Chitosan Sponge/Cu-WO3-x Composite for Photodynamic Therapy of Wound Infection

One of the potential treatments for bacterial wound infections is photodynamic therapy. WO_3-x semiconductor materials can generate reactive oxygen species when exposed to light, which can inactivate bacteria. In this work, we improved their photocatalytic performance by doping WO_3-x with Cu. The wound dressing was prepared by loading Cu-WO_3-x into a highly biocompatible chitosan sponge. The composite sponge dressing showed significant inactivation of Escherichia coli and Staphylococcus aureus, and in vitro toxicity assays on L929 cells demonstrated the biosafety of the dressing. Through in vivo wound healing trials, composite sponge dressings have been shown to accelerate wound healing, and this composite chitosan sponge can be possibly used for photodynamic therapy of bacterial wound infections.

A p-n Junction by Coupling Amine-Enriched Brookite-TiO2 Nanorods with CuxS Nanoparticles for Improved Photocatalytic CO2 Reduction

Photocatalytic CO₂ reduction is a promising technology for reaching the aim of "carbon peaking and carbon neutrality", and it is crucial to design efficient photocatalysts with a rational surface and interface tailoring. Considering that amine modification on the surface of the photocatalyst could offer a favorable impact on the adsorption and activation of CO₂, in this work, amine-modified brookite TiO₂ nanorods (NH₂-B-TiO₂) coupled with Cu_xS (NH₂-B-TiO₂-Cu_xS) were effectively fabricated via a facile refluxing method. The formation of a p-n junction at the interface between the NH₂-B-TiO₂ and the Cu_xS could facilitate the separation and transfer of photogenerated carriers. Consequently, under light irradiation for 4 h, when the Cu_xS content is 16%, the maximum performance for conversion of CO₂ to CH₄ reaches at a rate of 3.34 μmol g^-1 h^-1 in the NH₂-B-TiO₂-Cu_xS composite, which is approximately 4 times greater than that of pure NH₂-B-TiO₂. It is hoped that this work could deliver an approach to construct an amine-enriched p-n junction for efficient CO₂ photoreduction.

Co-embedding oxygen vacancy and copper particles into titanium-based oxides (TiO2, BaTiO3, and SrTiO3) nanoassembly for enhanced CO2 photoreduction through surface/interface synergy

Photocatalytic CO₂ reduction into valuable fuel and chemical production has been regarded as a prospective strategy for tackling with the issues of the increasing of greenhouse gases and shortage of sustainable energy. A composite photocatalysis system employing a semiconductor enriched with oxygen vacancy and coupled with metallic cocatalyst can facilitate charge separation and transfer electrons. In this work, mesoporous TiO₂ and titanium-based perovskite oxides (BaTiO₃ and SrTiO₃) nanoparticle assembly incorporated with abundant oxygen vacancy and copper particles have been successfully synthesized for CO₂ photoreduction. As an example, the TiO₂ decorated with different amounts of Cu particles has an impact on photocatalytic CO₂ reduction into CH₄ and CO. Particularly, the optimal TiO₂/Cu-0.1 exhibits nearly 13.5 times higher CH₄ yield (22.27 μmol g^-1 h^-1) than that of pristine TiO₂ (1.65 μmol g^-1 h^-1). The as-obtained BaTiO₃/Cu-0.1 and SrTiO₃/Cu-0.1 also show enhanced CH₄ yields towards photocatalytic CO₂ reduction compared with pristine ones. Based on the temperature programmed desorption (TPD) and photo/electrochemical measurements, the co-embedding of Cu particles and abundant oxygen vacancy into the titanium-based oxides could promote CO₂ adsorption capacity as well as separation and transfer of photoinduced electron-hole pairs, and finally result in efficient CO₂ photoreduction upon the TiO₂/Cu, SrTiO₃/Cu, and BaTiO₃/Cu composites.

Cu-functionalised porous boron nitride derived from a metal-organic framework

Porous boron nitride (BN) displays promising properties for interfacial and bulk processes, e.g. molecular separation and storage, or (photo)catalysis. To maximise porous BN's potential in such applications, tuning and controlling its chemical and structural features is key. Functionalisation of porous BN with metal nanoparticle represents one possible route, albeit a hardly explored one. Metal-organic frameworks (MOFs) have been widely used as precursors to synthesise metal functionalised porous carbon-based materials, yet MOF-derived metal functionalised inorganic porous materials remain unexplored. Here, we hypothesise that MOFs could also serve as a platform to produce metal-functionalised porous BN. We have used a Cu-containing MOF, i.e. Cu/ZIF-8, as a precursor and successfully obtained porous BN functionalised with Cu nanoparticles (i.e. Cu/BN). While we have shown control of the Cu content, we have not yet demonstrated it for the nanoparticle size. The functionalisation has led to improved light harvesting and enhanced electron-hole separation, which have had a direct positive impact on the CO2 photoreduction activity (production formation rate 1.5 times higher than pristine BN and 12.5 times higher than g-C3N4). In addition, we have found that the metal in the MOF precursor impacts porous BN's purity. Unlike Cu/ZIF-8, a Co-containing ZIF-8 precursor led to porous C-BN (i.e. BN with a large amount of C in the structure). Overall, given the diversity of metals in MOFs, one could envision our approach as a method to produce a library of different metal functionalised porous BN samples.

Bioinspired copper single-atom nanozyme as a superoxide dismutase-like antioxidant for sepsis treatment

Sepsis is a systemic inflammatory response syndrome with high morbidity and mortality mediated by infection-caused oxidative stress. Early antioxidant intervention by removing excessively produced reactive oxygen and nitrogen species (RONS) is beneficial to the prevention and treatment of sepsis. However, traditional antioxidants have failed to improve patient outcomes due to insufficient activity and sustainability. Herein, by mimicking the electronic and structural characteristics of natural Cu-only superoxide dismutase (SOD5), a single-atom nanozyme (SAzyme) featuring coordinately unsaturated and atomically dispersed Cu-N4 site was synthesized for effective sepsis treatment. The de novo-designed Cu-SAzyme exhibits a superior SOD-like activity to efficiently eliminate O2 •-, which is the source of multiple RONS, thus blocking the free radical chain reaction and subsequent inflammatory response in the early stage of sepsis. Moreover, the Cu-SAzyme effectively harnessed systemic inflammation and multi-organ injuries in sepsis animal models. These findings indicate that the developed Cu-SAzyme possesses great potential as therapeutic nanomedicines for the treatment of sepsis.

Partial Metalation of Porphyrin Moieties in Hydrogen-Bonded Organic Frameworks Provides Enhanced CO2 Photoreduction Activity

In natural photosynthesis, the architecture of multiproteins integrates more chromophores than redox centers and simultaneously creates a well-controlled environment around the active site. Herein, we demonstrate that these features can be emulated in a prototype hydrogen-bonded organic framework (HOF) through simply varying the proportion of metalated porphyrin in the structure. Further studies demonstrate that changing the metalloporphyrin content not only realizes a fine tuning of the photosensitizer/catalyst ratio, but also alters the microenvironment surrounding the active site and the charge separation efficiency. As a result, the obtained material achieves the challenging overall CO₂ reduction with a high HCOOH production rate (29.8 μmol g^-1  h^-1 , scavenger free), standing out from existing competitors. This work unveils that the degree of metalation is vital to the catalytic activity of the porphryinic framework, presenting as a new strategy to optimize the performance of heterogeneous catalysts.

Single-Atom Catalysts (SACs) for Photocatalytic CO2 Reduction with H2 O: Activity, Product Selectivity, Stability, and Surface Chemistry

In recent years, single-atom catalysts (SACs) have attracted the interest of researchers owing to their suitability for various catalytic applications. For instance, their optoelectronic features, site-specific activity, and cost-effectiveness make SACs ideal for photocatalytic CO₂ reduction. The activity, product selectivity, and photostability of SACs depend on various factors such as the nature of the metal/support material, the interaction between the metal atoms and support, light-harvesting ability, charge separation behavior, CO₂ adsorption ability, active sites, and defects. Consequently, it is necessary to investigate these factors in depth to elucidate the working principle(s) of SACs for catalytic applications. Herein, the recent progress in the development of SACs for photocatalytic CO₂ reduction with H₂ O is reviewed. First, a brief overview of CO₂ photoreduction and SACs for CO₂ conversion is provided. Several synthesis strategies and useful techniques for characterizing SACs employed in heterogeneous catalysis are then described. Next, the challenges of SACs for photocatalytic CO₂ reduction and related optimization strategies, in terms of activity, product selectivity, and stability, are explored. The progress in the development of noble metal- and transition metal-based SACs and dual-SACs for photocatalytic CO₂ reduction is discussed. Finally, the prospects of SACs for CO₂ reduction are considered.