Metal Single Atom Strategy Greatly Boosts Photocatalytic Methyl Activation and C–C Coupling for the Coproduction of High-Value-Added Multicarbon Compounds and Hydrogen

Single Atom Cocatalysts in Photocatalysis

Single-atom (SA) cocatalysts (SACs) have garnered significant attention in photocatalysis due to their unique electronic properties and high atom utilization efficiency. This review provides an overview of the concept and principles of SA cocatalyst in photocatalysis, emphasizing the intrinsic differences to SAs used in classic chemical catalysis. Key factors that influence the efficiency of SAs in photocatalytic reactions, particularly in photocatalytic hydrogen (H2) production, are highlighted. This review further covers synthesis methods, stabilization strategies, and characterization techniques for common SAs used in photocatalysis. Notably, "reactive deposition" method, which often shows a self-homing effect and thus achieves a maximum utilization efficiency of SA cocatalysts, is emphasized. Furthermore, the applications of SA cocatalysts in various photocatalytic processes, including H2 evolution, carbon dioxide reduction, nitrogen fixation, and organic synthesis, are comprehensively reviewed, along with insights into common artifacts in these applications. This review concludes by addressing the challenges faced by SACs in photocatalysis and offering perspectives on future developments, with the aim of informing and advancing research on SAs for photocatalytic energy conversion.

Single-Atom-Induced Hybridization States Promote the Direct Trapping of Hot Carriers by Reactants for Photocatalysis

Single-atom manipulation has emerged as an effective strategy for enhancing the photocatalytic efficiency. However, the mechanism of photogenerated carrier dynamics under single-atom modulation remains unclear. Combining first-principles calculations and non-adiabatic molecular dynamics simulations, we systematically studied carrier transfer and recombination in the oxygen reduction reaction of single-atom-doped C3N4 systems. Unlike the conventional two-step process, where single atoms trap photogenerated carriers before transferring them to reactants, our findings reveal a direct one-step electron transfer process, where single-atom-induced hybridization states facilitate the direct trapping of hot carriers by reactants from photocatalysts. Specifically, photogenerated electron transfer time through the one-step process is 237 and 325 fs for Sb and Cu single-atom-doped systems, respectively, considerably faster than the two-step process (hundreds of picoseconds). Moreover, these systems exhibit a nanosecond-level photogenerated carrier lifetime, driving a high photocatalytic efficiency. This study elucidates the carrier dynamics in single-atom photocatalysts, facilitating the screening of high-performance photocatalysts.

Photocatalytic Reforming of Ethanol in the Liquid Phase Using a Ternary Composite of Rh/TiO2/g-C3N4 as a Catalyst

Photocatalytic reforming of ethanol provides an effective way to produce hydrogen energy using natural and nontoxic ethanol as raw material. Developing highly efficient catalysts is central to this field. Although traditional semiconductor/metal heterostructures (e.g., Rh/TiO2) can result in relatively high catalyst performance by promoting the separation of photoinduced hot carriers, it will still be highly promising to further improve the catalytic performance via a cost-effective and convenient method. In this study, we developed a highly efficient photocatalyst for ethanol reformation by preparing a ternary composite structure of Rh/TiO2/g-C3N4. Hydrogen is the main product, and the reaction rate could reach up to 27.5 mmol g-1 h-1, which is ∼1.41-fold higher than that of Rh/TiO2. The catalytic performance here is highly dependent on the wavelength of the light illumination. Moreover, the photocatalytic reforming of ethanol and production of hydrogen were also dependent on the Rh loading and g-C3N4:TiO2 ratio in Rh/TiO2/g-C3N4 composites as well as the ethanol content in the reaction system. The mechanism of the enhanced hydrogen production in Rh/TiO2/g-C3N4 is determined as the improvement in the separation of photoinduced hot carriers. This work provides an effective photocatalyst for ethanol reforming, largely expanding its application in the field of renewable energy and interface science.

Photorefinery of Biomass and Plastics to Renewable Chemicals using Heterogeneous Catalysts

The photocatalytic conversion of biomass and plastic waste provides opportunities for sustainable fuel and chemical production. Heterogeneous photocatalysts, typically composed of semiconductors with distinctive redox properties in their conduction band (CB) and valence band (VB), facilitate both the oxidative and reductive valorization of organic feedstocks. This article provides a comprehensive overview of recent advancements in the photorefinery of biomass and plastics from the perspective of the redox properties of photocatalysts. We explore the roles of the VB and CB in enhancing the value-added conversion of biomass and plastics via various pathways. Our aim is to bridge the gap between photocatalytic mechanisms and renewable carbon feedstock valorization, inspiring further development in photocatalytic refinery of biomass and plastics.

Photocatalytic Methanol Dehydrogenation Promoted Synergistically by Atomically Dispersed Pd and Clustered Pd

Supported metal in the form of single atoms, clusters, and particles can individually or jointly affect the activity of supported heterogeneous catalysts. While the individual contribution of the supported metal to the overall activity of supported photocatalysts has been identified, the joint activity of mixed metal species is overlooked because of their different photoelectric properties. Here, atomically dispersed Pd (Pd1) and Pd clusters are loaded onto CdS, serving as oxidation and reduction sites for methanol dehydrogenation. The Pd1 substitutes Cd2+, forming hole-trapping states for methanol oxidation and assisting the dispersion of photodeposited Pd clusters. Therefore, methanol dehydrogenation on CdS with supported Pd1 and Pd clusters exhibits the highest turnover frequency of 1.14 s-1 based on the Pd content and affords H2 and HCHO with a similar apparent quantum yield of 87 ± 1% at 452 nm under optimized reaction conditions. This work highlights the synergistic catalysis of supported metal for improved photocatalytic activity.

Photocatalytic Transformation of Organics to Valuable Chemicals - Quo Vadis?

Recent development in photocatalysis is increasingly focused on transforming organic compounds toward producing fine chemicals. Simple, non-selective oxidation reactions (degradation of pollutants) and very demanding solar-to-chemical energy conversion processes (production of solar fuels) face severe economic limitations influenced by still low efficiency and insufficient stability of the systems. Synthesis of fine chemicals, including reductive and oxidative selective transformations, as well as C-C and C-N coupling reactions, can utilise the power of photocatalysis. Herein, we present the recent progress in photocatalytic systems designed to synthesise fine chemicals. In particular, we discuss the factors influencing the efficiency and selectivity of the organic transformations, dividing them into intrinsic (related to individual properties of photocatalysts) and extrinsic (originating from the reaction environment). A rational design of the photocatalytic systems, based on a deep understanding of these factors, opens new perspectives for applied photocatalysis.

Enhanced Photoinduced Carrier Separation in Fe-MOF-525/CdS for Photocatalytic Hydrogen Evolution: Improved Catalytic Dynamics with Specific Active Sites

Single-atom metal-anchored porphyrin-based metal-organic frameworks (MOFs) have shown excellent light absorption, catalytic sites, and high stability during photocatalytic reactions, while there are still challenges for facile assembly with quantum dots to enhance catalytic dynamics. Herein, a kind of Fe single atom-doped MOF material (Fe-MOF-525) was ball milled with CdS in a proper ratio through Fe-N4 and Fe-N-C bonding, which showed the enhanced photoinduced carrier separation ability. As a result, extended light absorption ranges of CdS/Fe-MOF-5252.3 induced the promotion of the photocatalytic hydrogen (H2) value (3638.6 μmol g-1 h-1), which was 7.2 and 2.3 times higher than those of Fe-MOF-525 and CdS. In this work, the facile synthetic technique, specific active sites, and enhanced catalytic dynamics in the composite highlight the future research on MOF-based heterojunctions and their potential photocatalysis applications..

Enhancing Photocatalysis: Understanding the Mechanistic Diversity in Photocatalysts Modified with Single-Atom Catalytic Sites

Surface modification of heterogeneous photocatalysts with single-atom catalysts (SACs) is an attractive approach for achieving enhanced photocatalytic performance. However, there is limited knowledge of the mechanism of photocatalytic enhancement in SAC-modified photocatalysts, which makes the rational design of high-performance SAC-based photocatalysts challenging. Herein, a series of photocatalysts for the aerobic degradation of pollutants based on anatase TiO2 modified with various low-cost, non-noble SACs (vanadate, Cu, and Fe ions) is reported. The most active SAC-modified photocatalysts outperform TiO2 modified with the corresponding metal oxide nanoparticles and state-of-the-art benchmark photocatalysts such as platinized TiO2 and commercial P25 powders. A combination of in situ electron paramagnetic resonance spectroscopy and theoretical calculations reveal that the best-performing photocatalysts modified with Cu(II) and vanadate SACs exhibit significant differences in the mechanism of activity enhancement, particularly with respect to the rate of oxygen reduction. The superior performance of vanadate SAC-modified TiO2 is found to be related to the shallow character of the SAC-induced intragap states, which allows for both the effective extraction of photogenerated electrons and fast catalytic turnover in the reduction of dioxygen, which translates directly into diminished recombination. These results provide essential guidelines for developing efficient SAC-based photocatalysts.

Recent Progress of Single-Atom Photocatalysts Applied in Energy Conversion and Environmental Protection

Photocatalysis driven by solar energy is a feasible strategy to alleviate energy crises and environmental problems. In recent years, significant progress has been made in developing advanced photocatalysts for efficient solar-to-chemical energy conversion. Single-atom catalysts have the advantages of highly dispersed active sites, maximum atomic utilization, unique coordination environment, and electronic structure, which have become a research hotspot in heterogeneous photocatalysis. This paper introduces the potential supports, preparation, and characterization methods of single-atom photocatalysts in detail. Subsequently, the fascinating effects of single-atom photocatalysts on three critical steps of photocatalysis (the absorption of incident light to produce electron-hole pairs, carrier separation and migration, and interface reactions) are analyzed. At the same time, the applications of single-atom photocatalysts in energy conversion and environmental protection (CO₂ reduction, water splitting, N₂ fixation, organic macromolecule reforming, air pollutant removal, and water pollutant degradation) are systematically summarized. Finally, the opportunities and challenges of single-atom catalysts in heterogeneous photocatalysis are discussed. It is hoped that this work can provide insights into the design, synthesis, and application of single-atom photocatalysts and promote the development of high-performance photocatalytic systems.

Optimizing the semiconductor–metal-single-atom interaction for photocatalytic reactivity

Metal single-atom (MSA) catalysts with 100% metal atom utilization and unique electronic properties are attractive cocatalysts for efficient photocatalysis when coupled with semiconductors. Owing to the absence of a metal-metal bond, MSA sites are exclusively coordinated with the semiconductor photocatalyst, featuring a chemical-bond-driven tunable interaction between the semiconductor and the metal single atom. This semiconductor-MSA interaction is a platform that can facilitate the separation/transfer of photogenerated charge carriers and promote the subsequent catalytic reactions. In this Review, we first introduce the fundamental physicochemistry related to the semiconductor-MSA interaction. We highlight the ligand effect on the electronic structures, catalytic properties and functional mechanisms of the MSA cocatalyst through the semiconductor-MSA interaction. Then, we categorize the state-of-the-art experimental and theoretical strategies for the construction of the efficient semiconductor-MSA interaction at the atomic scale for a wide range of photocatalytic reactions. The examples described include photocatalytic water splitting, CO₂ reduction and organic synthesis. We end by outlining strategies on how to further advance the semiconductor-MSA interaction for complex photocatalytic reactions involving multiple elementary steps. We provide atomic and electronic-scale insights into the working mechanisms of the semiconductor-MSA interaction and guidance for the design of high-performance semiconductor-MSA interface photocatalytic systems.

Photothermal Synergistic Effect of Pt1/CuO-CeO2 Single-Atom Catalysts Significantly Improving Toluene Removal

Photothermal synergistic catalytic oxidation of toluene over single-atom Pt catalysts was investigated. Compared with the conventional thermocatalytic oxidation in the dark, toluene conversion and CO₂ yield over 0.39Pt₁/CuO-CeO₂ under simulated solar irradiation (λ = 320-2500 nm, optical power density = 200 mW cm^-2) at 180 °C could be increased about 48%. An amount of CuO was added to CeO₂ to disperse single-atom Pt with a maximal Pt loading of 0.83 wt %. The synergistic effect between photo- and thermocatalysis is very important for the development of new pollutant treatment technology with high efficiency and low energy consumption. Both light and heat played an important role in the present photothermal synergistic catalytic oxidation. 0.39Pt₁/CuO-CeO₂ showed good redox performance and excellent optical properties and utilized the full-spectrum solar energy. Light illumination induced the generation of reactive oxygen species (^•OH and ^•O₂^-), which accelerated the transformation of intermediates, promoted the release of active sites on the catalyst surface, and improved the oxidation reaction.

Considering single-atom catalysts as photocatalysts from synthesis to application

With the ever-increased greenhouse effect and energy crisis, developing novel photocatalysts to realize high-efficient solar-driven chemicals/fuel production is of great scientific and practical significance. Recently, single-atom photocatalysts (SAPs) are promising catalysts with maximized metal dispersion and tuneable coordination environments. SAPs exhibit boosted photocatalytic performance by enhancing optical response, facilitating charge carrier transfer behaviors or directly manipulating surface reaction processes. In this regard, this article systematically reviews the state-of-the-art progress in the development and application of SAPs, especially the mechanism and performance of SAPs on various reaction processes. Some future challenges and potential research directions over SAPs are outlined at the final stage.

Synergetic metal-semiconductor interaction: Single-atomic Pt decorated CdS nano-photocatalyst for highly water-to-hydrogen conversion

Solar driven water-to-hydrogen conversion is a promising technology for the typical sustainable production mode, so increasing efforts are being devoted to exploit high-performance photocatalytic materials. Cadmium sulfide (CdS) is widely used to prepare highly active photocatalysts owing to its merits of broadband-light harvesting and feasible band structure. However, the slow photo-carriers' migration in CdS body structure generally results in high-frequency carriers recombination, which leads to unsatisfied photoactivity. Metallic single-atom surface decoration is an effective method to build the strong metal-support interaction for promotion of photo-carriers' migration. Herein, a simple light-induced reduction procedure was proposed to decorate single-atomic Pt on the surface of CdS nanoparticles for highly photocatalytic HER activity. Research showed that the synergetic metal (Pt)-semiconductor (CdS) interaction significantly promoted the body-to-surface (BTS) photo-carriers' migration of CdS, thereby the high light-to-fuel conversion efficiency (AQY_{500 nm} = 25.70%) and 13.5-fold greater simulated sunlight driven HER rate of bare CdS was achieved by this CdS-Pt nano-photocatalyst. Based on the photo-electrochemical analysis and density functional theory calculations, the remarkably improved HER photoactivity can be attributed to the enhanced light-harvesting, promoted BTS electron migration and reduced reaction energy barriers. This study provides a facile procedure to obtain CdS based photocatalyst with metallic single-atom sites for high-performance HER photocatalysis.

Photocatalytic Anaerobic Oxidation of Aromatic Alcohols Coupled With H2 Production Over CsPbBr3/GO-Pt Catalysts

Metal halide perovskites (MHPs) have been widely investigated for various photocatalytic applications. However, the dual-functional reaction system integrated selective organic oxidation with H₂ production over MHPs is rarely reported. Here, we demonstrate for the first time the selective oxidation of aromatic alcohols to aldehydes integrated with hydrogen (H₂) evolution over Pt-decorated CsPbBr₃. Especially, the functionalization of CsPbBr₃ with graphene oxide (GO) further improves the photoactivity of the perovskite catalyst. The optimal amount of CsPbBr₃/GO-Pt exhibits an H₂ evolution rate of 1,060 μmol g⁻¹ h⁻¹ along with high selectivity (>99%) for benzyl aldehyde generation (1,050 μmol g⁻¹ h⁻¹) under visible light (λ > 400 nm), which is about five times higher than the CsPbBr₃-Pt sample. The enhanced activity has been ascribed to two effects induced by the introduction of GO: 1) GO displays a structure-directing role, decreasing the particle size of CsPbBr₃ and 2) GO and Pt act as electron reservoirs, extracting the photogenerated electrons and prohibiting the recombination of the electron–hole pairs. This study opens new avenues to utilize metal halide perovskites as dual-functional photocatalysts to perform selective organic transformations and solar fuel production.

Al3+ Dopants Induced Mg2+ Vacancies Stabilizing Single-Atom Cu Catalyst for Efficient Free-Radical Hydrophosphinylation of Alkenes

Utilizing heterogeneous catalysts to overcome obstacles for homogeneous reactions is fascinating but very challenging owing to the difficult fabrication of such catalysts based on the character of target reactions. Herein, we report a Al³⁺ doping strategy to construct single-atom Cu on MgO nanosheets (Cu₁/MgO(Al)) for boosting the free-radical hydrophosphinylation of alkenes. Al³⁺ dopants in MgO bring about abundant Mg²⁺ vacancies for stabilizing dense independent Cu atoms (6.3 wt %), while aggregated Cu nanoparticles are formed without Al³⁺ dopants (Cu/MgO). Cu₁/MgO(Al) exhibits preeminent activity and durability in the hydrophosphinylation of various alkenes with great anti-Markovnikov selectivity (99%). The turnover frequency (TOF) value reaches up to 1272 h^-1, exceeding those of Cu/MgO by ∼6-fold and of traditional homogeneous catalysts drastically. Further experimental and theoretical studies disclose that the prominent performance of Cu₁/MgO(Al) derives from the accelerated initiating step of phosphinoyl radical triggered by individual Cu atoms.

Nickel sulfide nanorods decorated on graphene as advanced hydrogen evolution electrocatalysts in acidic and alkaline media

Nowadays, the fabrication of robust and earth-abundant hydrogen evolution electrocatalysts with noble-metal-like catalytic activities is still facing great challenges. In this report, nanorod (NR)-shaped nickel sulfide (NiS) is successfully decorated on graphene (Gr) by utilizing carbon cloth (CC) as a substrate (NiS-Gr-CC). Benefiting from the NR morphology and strong interfacial synergetic effect between NiS and Gr, the NiS-Gr-CC electrocatalyst shows good catalytic activity for hydrogen evolution reaction (HER). Specifically, the low Tafel slopes of 46 and 56 mV dec^-1 along with the small overpotentials of 66 and 71 mV at 10 mA cm^-2 are obtained in the acidic and alkaline electrolytes, respectively. Density functional theory results indicate that the combination of NiS and Gr can optimize the adsorption energy of H* during the HER process. The long-term durability measurement result reveals that our NiS-Gr-CC heterostructure has good electrocatalytic cycling stability (∼80 h) in both acidic and alkaline electrolytes. These results confirm that the NiS-Gr-CC heterostructure is a promising candidate for hydrogen evolution electrocatalyst with high catalytic activity.

Emerging Dual-Atomic-Site Catalysts for Efficient Energy Catalysis

Atomically dispersed metal catalysts with well-defined structures have been the research hotspot in heterogeneous catalysis because of their high atomic utilization efficiency, outstanding activity, and selectivity. Dual-atomic-site catalysts (DASCs), as an extension of single-atom catalysts (SACs), have recently drawn surging attention. The DASCs possess higher metal loading, more sophisticated and flexible active sites, offering more chance for achieving better catalytic performance, compared with SACs. In this review, recent advances on how to design new DASCs for enhancing energy catalysis will be highlighted. It will start with the classification of marriage of two kinds of single-atom active sites, homonuclear DASCs and heteronuclear DASCs according to the configuration of active sites. Then, the state-of-the-art characterization techniques for DASCs will be discussed. Different synthetic methods and catalytic applications of the DASCs in various reactions, including oxygen reduction reaction, carbon dioxide reduction reaction, carbon monoxide oxidation reaction, and others will be followed. Finally, the major challenges and perspectives of DASCs will be provided.

Boosting CO2 Electroreduction over a Cadmium Single-Atom Catalyst by Tuning of the Axial Coordination Structure

Guided by first-principles calculations, it was found that Cd single-atom catalysts (SACs) have excellent performance in activating CO₂ , and the introduction of axial coordination structure to Cd SACs cannot only further decrease the free energy barrier of CO₂ reduction, but also suppress the hydrogen evolution reaction (HER). Based on the above discovery, we designed and synthesized a novel Cd SAC that comprises an optimized CdN₄ S₁ moiety incorporated in a carbon matrix. It was shown that the catalyst exhibited outstanding performance in CO₂ electroreduction to CO. The faradaic efficiency (FE) of CO could reach up to 99.7 % with a current density of 182.2 mA cm^-2 in a H-type electrolysis cell, and the turnover frequency (TOF) value could achieve 73000 h^-1 , which was much higher than that reported to date. This work shows a successful example of how to design highly efficient catalysts guided by theoretical calculations.

Single-Atom Photocatalysts for Emerging Reactions

Single-atom photocatalysts have demonstrated an enormous potential in producing value-added chemicals and/or fuels using sustainable and clean solar light to replace fossil fuels causing global energy and environmental issues. These photocatalysts not only exhibit outstanding activities, selectivity, and stabilities due to their distinct electronic structures and unsaturated coordination centers but also tremendously reduce the consumption of catalytic metals owing to the atomic dispersion of catalytic species. Besides, the single-atom active sites facilitate the elucidation of reaction mechanisms and understanding of the structure-performance relationships. Presently, apart from the well-known reactions (H2 production, N2 fixation, and CO2 conversion), various novel reactions are successfully catalyzed by single-atom photocatalysts possessing high efficiency, selectivity, and stability. In this contribution, we summarize and discuss the design and fabrication of single-atom photocatalysts for three different kinds of emerging reactions (i.e., reduction reactions, oxidation reactions, as well as redox reactions) to generate desirable chemicals and/or fuels. The relationships between the composition/structure of single-atom photocatalysts and their activity/selectivity/stability are explained in detail. Additionally, the insightful reaction mechanisms of single-atom photocatalysts are also introduced. Finally, we propose the possible opportunities in this area for the design and fabrication of brand-new high-performance single-atom photocatalysts.