In recent years, single-atom catalysts(SACs) have attracted increasing attention in catalysis. However, their stability is considerably challenging. As a result, fine-tuning the interaction of metal single atoms(SA) with different types of supports has emerged as an effective strategy for improving their thermal and chemical stabilities. Owing to its non-toxicity, cost-effectiveness, high abundance, and excellent stability, as well as presence of rich, tunable, and reliable anchor sites for metal SA, TiO₂ has been extensively explored as a superior support for SACs. In this review, recent advances of TiO₂-supported SACs(M₁/TiO₂) are discussed, and synthetic strategies, structure elucidation, and catalytic applications are summarized. First, the recently developed synthetic strategies for M₁/TiO₂ are highlighted and summarized, identifying the major challenges for the precise fabrication of M₁/TiO₂. Subsequently, key characterization techniques for the structure identification of M₁/TiO₂ are discussed. Next, catalytic applications of M₁/TiO₂ are highlighted, viz. photocatalysis, electrocatalysis, and thermocatalysis. In addition, the mechanism via geometric structures and electronic states of metal centers facilitate catalytic reactions is outlined. Finally, opportunities and challenges of M₁/TiO₂ in catalysis are discussed, which may inspire the future development of M₁/TiO₂ for multifunctional catalytic applications.

Although driven by different research interests, single-site catalysts and single-atom catalysts are both believed to be model systems bridging homogeneous and heterogeneous catalysis. The two concepts are similar but different. In this review, we will first explain the difference between single-atom catalysis and single-site catalysis, in terms of their goals, synthetic methods and coordination structures of corresponding catalysts. Then, we will introduce the surface organometallic chemistry method, a method traditionally used for synthesizing single-site catalyst. We will explain why it might benefit the single-atom catalysis community. At last, the choice of support to accommodate the method for synthesizing single-atom catalysts will be discussed.

Single-atom catalysis is the “hot spot” in the field of catalysis due to the special geometries, electronic states, and their unique catalytic performance. Single-atom catalysts(SACs), isolated metal atoms dispersed on the support, show the highest atom efficiency, cutting down the potential cost in the industrial process. Consequently, this “homo-hetero” catalyst could be a promising candidate for the next-generation catalysts. The applications for the SACs are widely reported, like gas-solid reactions, organic reactions, and electro-catalysis. In this mini-review, we will focus on the recent work of SACs on electro-catalysis, including hydrogen evolution reaction(HER), oxygen reduction reaction(ORR), oxygen evolution reaction(OER), CO₂ reduction reaction(CO₂ RR), and nitrogen reduction reaction(NRR).

Single site catalysts(SSCs) are a new type of heterogeneous catalysts formed by isolated metal atoms supported on kinds of substrates. SSCs have shown great potential for energy conversion and storage in recent years, especially for oxygen reduction reactions(ORR). Typically, SSCs are confined on the substrate by strong chemical interactions, such as coordination bonds. Therefore, the surface chemical environment and porous properties of the supports are crucial to the performance of SSCs. In recent years, COFs have become excellent candidates for preparing SSCs as they can precisely assemble monomers into highly ordered crystalline porous materials with a fine structure and definite components. In this review, we not only summarize the characteristics and advantages of COFs based SSCs, but also highlight the applications of COFs constructed from different single active sites for ORR in recent years. Finally, challenges in practical application, feasible strategies and perspectives are proposed for the of COFs based SSCs.

Single atom catalysts(SACs) offer exceptional atom efficiency, activity, and selectivity for many catalytic systems. In recent years, SACs have demonstrated great potential in liquid phase selective hydrogenation. In this review, we discuss the critical challenge of selective hydrogenation reactions. Meanwhile, we highlight recent achievements in the design and construction of SACs, as well as their application in liquid reactions. Finally, the current issues and future opportunities for development in the field of SACs are given.

Clean energy innovation has triggered the development of single-atom catalysts(SACs) due to their excellent catalytic activity, high tunability and low cost. The success of SACs for many catalytic reactions has opened a new field where the fundamentals of catalytic property-structure relationship at atomic level await exploration, and thus raises challenges for structural characterization. Among the characterization techniques for SACs, aberration-corrected transmission electron microscopy(TEM) has become an essential tool for direct visualization of single atoms. In this review, we briefly summarize recent studies on SACs using advanced TEM. We first introduce TEM methods, which are particularly important for SACs characterization, and then discuss the applications of advanced TEM for SAC characterization, where not only atomic dispersion of single atoms can be studied, but also the distribution of elements and the valence state with local coordination can be resolved. We further extend our review towards in-situ TEM, which has increasing importance for the fundamental understanding of catalytic mechanism. Perspectives of TEM for SACs are finally discussed.

Single-atom catalysts(SACs) have attracted much attention for their superior catalytic performance in various fields. It has been widely accepted that the selection of appropriate substrates is crucial to the fabrication and application of SACs. Layered double hydroxides(LDHs) have been developed as one of the promising substrates for single-atoms due to their unique adjustable supramolecular structures. In this review, we comprehensively sort out the research of SACs based on LDHs. By analyzing the characteristics of LDHs and the single-atoms, respectively, the preparation strategies of SACs by using LDHs are summarized. Their applications as efficient catalysts in electrocatalysis, photocatalysis and thermal catalysis are then discussed. Finally, we summarize the opportunities and challenges for the rational design and application expansion of SACs based on LDHs in the future.

Photocatalytic CO₂ reduction to C₁ fuels is considered to be an important way for alleviating increasingly serious energy crisis and environmental pollution. Due to the environment-friendly, simple preparation, easy formation of highly-stable metal-nitrogen(M-N _x) coordination bonds, and suitable band structure, polymeric carbon nitride-based single-atom catalysts(C₃N₄-based SACs) are expected to become a potential for CO₂ reduction under visible-light irradiation. In this review, we summarize the recent advancement on C₃N₄-based SACs for photocatalytic CO₂ reduction to C₁ products, including the reaction mechanism for photocatalytic CO₂ reduction to C₁ products, the structure and synthesis methods of C₃N₄-based SACs and their applications toward photocatalytic CO₂ reduction reaction(CO₂RR) for C₁ production. The current challenges and future opportunities of C₃N₄-based SACs for photoreduction of CO₂ are also discussed.

Carrier dynamics and surface reaction are two critical processes for determining the performance of photocatalytic reaction. Highly designable polymer-based photocatalysts have shown promising protectives in energetic and environmental applications. In this prospective, we first distinguished the differences of physiochemical properties between polymer-based semiconductors and traditional inorganic semiconductors. Then, the effects of single-atom sites on the charge dynamics and reaction kinetics of polymer-based photocatalysts are further elaborated. Time(excitation)-space(wavefunction) population analysis, which can provide relevant information to clarify the structure-excitation relationships after introducing the single atom sites was also reviewed. In the future, with the further development of artificial intelligence, the establishment of an energy function with a regression accuracy close to or reaching the level of density functional theory is highly desired to infer the energetic diagram of the photocatalytic systems at the excited states. Furthermore, coordination structures, interaction with inorganic semiconductors, photocatalytic stability and solvent effects should also be carefully considered in the future studies of polymer-based photocatalyst.

Developing high-performance catalysts for oxygen reduction reaction to replace costly platinum-based materials is of great importance but still confronted with challenges. Herein, a kind of supported palladium liquid metal catalyst, which is prepared by galvanic replacement, surpasses commercial Pt/C and Pd/C in oxygen reduction catalysis with a higher half-wave potential of 0.92 V, mass activity of 1.85 A/mg_Pd at 0.90 V, and superior durability. The liquid metal support can both optimize the electronic structures of Pd sites and guarantee the dispersion of Pd atoms, which explains the enhanced activity and durability, respectively. This work opens an avenue for rational design of catalysts.

Electrocatalytic nitrogen reduction reaction(NRR) is a promising way to produce ammonia(NH₃) at ambient temperature and pressure. Herein, we have constructed single Fe dimer catalysts on a molybdenum disulfide monolayer for potential nitrogen activation. By employing ab initio molecular dynamics simulations, it is suggested that a dual iron-single atom site can be dynamically formed, which exhibits the similar Fe-S-Fe structure as the nitrogenase. We further identify an iron dimer with a sulfur vacancy as the active center for realistic nitrogen activation by the free energy calculations since the bridged sulfur is easy to be released in the form of H₂S during the reduction process. It is shown that N₂ mainly adsorbs on the Fe₂ dimer at the sulfur vacancies in the pattern of side-on configuration, and the nitrogen reduction reaction is proceeded by an enzymatic mechanism. Charge analyses further show that the Fe₂ dimer mainly works as an electron reservoir while MoS₂ substrate with one sulfur vacancy acts as an inert carrier to stabilize the Fe₂ dimer. Overall, our work provides important insights into how N₂ molecules were adsorbed and activated on Fe₂-doped MoS₂, and provides new ideas for the transformation of actual reaction sites during electrochemical reactions.

Lithium-sulfur(Li-S) batteries have been puzzled by the “shuttle effect”. In the recent years, catalytic materials present a huge potential for solving this problem. However, the exploitation for catalytic activity was still challenging in Li-S batteries. In this article, we put forward a single atom catalyst (SAC) of FeN₄ coupled with Fe₃C on the N-doped carbon (FeN₄/Fe₃C@NC) by one-step pyrolysis method. The FeN₄ and Fe₃C synergistically catalyze the polysulfides conversion when the N-doped carbon provides the high conductive three-dimensional skeleton in Li-S batteries. As a result, the FeN₄/Fe₃C@NC shows a specific capacity of 1100 mA·h/g at 0.2 C(1 C=1675 mA/g). In addition, the FeN₄/Fe₃C@NC maintains 99.01% of the pristine specific capacity after 100 cycles at 0.5 C, indicating the improved electrochemical performance in Li-S batteries. This work sheds new lights on the design of engineering catalysts for developing high-performance Li-S batteries.

Wacker oxidation is an industry-adopted process to transform olefins into value-added epoxides and carbonyls. However, traditional Wacker oxidation involves the use of homogeneous palladium and copper catalysts for the olefin addition and reductive elimination. Here, we demonstrated an ultrahigh loading Cu single atom catalyst(14% Cu, mass fraction) for the palladium-free Wacker oxidation of 4-vinylanisole into the corresponding ketone with N-methylhydroxylamine hydrochloride as an additive under mild conditions. Mechanistic studies by ¹⁸O and deuterium isotope labelling revealed a hydrogen shift mechanism in this palladium-free process using N-methylhydroxylamine hydrochloride as the oxygen source. The reaction scope can be further extended to Kucherov oxidation. Our study paves the way to replace noble metal catalysts in the traditional homogeneous processes with single atom catalysts.

Polymeric carbon nitride(PCN) has emerged as a promising candidate for photocatalytic hydrogen evolution, but its dependence on scarce and high-cost noble metal co-catalysts severely limits its extensive application. It will be of great promise to develop non-noble metal single-atom co-catalysts with low-cost and high atom utilization to improve the photocatalytic performance over PCN. Herein, single Ni atoms are successfully anchored onto carbon-vacant PCN nanosheets(CCN-SANi) via a two-step ammonia thermal treatment and photo-deposition process. Theoretical calculations and experimental results demonstrate that the optical absorption property and the charge transfer ability of CCN-SANi have been significantly improved with the introduction of single Ni atoms to form Ni−N₃ sites. In comparison to carbon-vacant PCN(CCN) loaded with Ni clusters, the obtained CCN-SANi exhibits 11.4 times increased photocatalytic performance, with the highest hydrogen evolution rate reaching 511 µmol/(g·h), which is even 1.7 times higher than that of CCN loaded with Pt clusters. This research proposes an inspiring and reliable strategy to design novel single-atom semiconducting polymers with electronic structures manipulated for efficient photocatalysis.

In this work, the effect of Nafion ionomer content on the structure and catalytic performance of direct CO polymer electrolyte membrane fuel cell(CO-PEMFC) by using Rh-N-C single-atom catalyst as the anode catalyst layers was studied. The ionic plaque and roughness of the anode catalyst layers increase with the increase of Nafion ionomer content. Furthermore, the contact angle measurement results show that the hydrophilicity of the anode catalyst layers also increases with the increase of Nafion ionomer content. However, when the Nafion ionomer content is too low, the binding between microporous layers, catalyst layers and membrane cannot meet the requirement for either electric conductivity or mass transfer. While Nafion ionomer content increased above 30%, the content of water in anode is difficult to control. Therefore, it was found that AN 30(30% Nafion ionomer content of anode) is the best level to effectively extend the three-phase boundary and improve CO-PEMFCs performance.

Through screening Ce precursors and pyrolysis temperatures [Ce(acac)₃ as Ce precursors and pyrolysis at 900 °C], zeolitic imidazolate framework-8(ZIF-8) derived nitrogen-doped carbon supported cerium single atom catalyst(Ce₁/NC) is successfully prepared by ball milling method. The Ce₁/NC catalyst exhibits exceptional catalytic performance in the selective oxidation of saturated C-H bonds in aromatic compounds, e.g., 91% conversion and 99% selelctivity can be achieved in the oxidation of ethylbenzene to acetophenone under mild reaction conditions.

For a practical high-loading single-atom catalyst, it is prone to forming diverse metal species owing to either the synthesis inhomogeneity or the reaction induced aggregation. The diversity of this metal species challenges the discerning about the contributions of specific metal species to the catalytic performance, and thus hampers the rational catalyst design. In this paper, a distinct solution of dispersion analysis based on transmission electron microscopy imaging specialized for metal-supported catalysts has been proposed in the capability of full-metal-species quantification(FMSQ) from single atoms to nanoparticles, including dispersion densities, shape geometry, and crystallographic surface exposure. This solution integrates two image-recognition algorithms including the electron microscopy-based atom recognition statistics (EMARS) for single atoms and U-Net type deep learning network for nanoparticles in different shapes. When applied to the C₃N₄- and nitrogen-doped carbon-supported catalysts, the FMSQ method successfully identifies the specific activity contributions of Au single atoms and particles in butadiene hydrogenation, which presents remarkable variation with the metal species constitution. This work demonstrates a promising value of our FMSQ strategy for identifying the activity origin of heterogeneous catalysis.

The oxygen reduction reaction in direct glycol fuel cells heavily relies on noble metal-based electrocatalysts. In this work, novel Pt group metal-free catalysts based on porous Fe-N-C materials are successfully synthesized as catalysts with high activity and durability for the cathode oxygen reduction reaction (ORR). Through the encapsulation of NH₄SCN salt, the surface elements and pore structure of the catalyst are effectively changed, and the active sites of Fe effectively are increased. The half-wave potential of the best Fe-N-C catalyst was −0.02 V vs. Hg/HgO in an alkaline environment. The porous Fe-N-C catalyst possesses a large specific surface area(1158 m²/g) and shows good activity and tolerance to glycol. The direct glycol fuel cell with the Fe-N-C cathode achieved a maximum power density of 62.2 mW/cm² with 4 mol/L KOH and 4 mol/L glycol solution at 25 °C and maintained discharge for more than 250 h at a 50 A/cm² current density.

This work chooses Cu/Fe single-atom catalysts(SACs) with weak/strong oxygen affinity to clarify the effect of dual-atom configuration on oxygen reduction reaction(ORR) performance based on density functional theory(DFT) calculations. The stability and ORR activity of single or dual Cu/Fe atomic sites anchored on nitrogen-doped graphene sheets(Cu-N₄-C, Cu₂-N₆-C, Fe-N₄-C, and Fe₂-N₆-C) are investigated, and the results indicate the dual-atom catalysts(Cu₂-N₆-C and Fe₂-N₆-C) are thermodynamically stable enough to avoid sintering and aggregation. Compared with single-atom active sites of Cu-N₄-C, which show weak oxygen affinity and poor ORR performance with a limiting potential of 0.58 V, the dual-Cu active sites of Cu₂-N₆-C exhibit enhanced ORR activity with a limiting potential up to 0.87 V due to strengthened oxygen affinity. Interestingly, for Fe SACs with strong oxygen affinity, the DFT results show that the dual-Fe sites stabilize the two OH^* ligands structure[Fe₂(OH)₂-N₆-C], which act as the active sites during ORR process, resulting in greatly improved ORR performance with a limiting potential of 0.90 V. This study suggests that the dual-atom design is a potential strategy to improve the ORR performance of SACs, in which the activity of the single atom active sites is limited with weak or strong oxygen affinity.

Constructing atomically dispersed active sites with densely exposed and dispersed double metal-S _x catalytic sites for favorable OER catalytic activity remains rare and challenging. Herein, we design and construct a Fe₁S _x@Co₃S₄ electrocatalyst with Fe single atoms epitaxially confined in Co₃S₄ nanosheets for catalyzing the sluggish alkaline oxygen evolution reaction(OER). Consequently, in ultralow concentration alkaline solutions(0.1 mol/L KOH), such a catalyst is highly active and robust for OER with low overpotentials of 300 and 333 mV at current densities of 10 and 30 mA/cm², respectively, accompanying long-term stability without significant degradation even for 350 h. In addition, Fe₁S _x@Co₃S₄ shows a turnover frequency(TOF) value of 0.18 s^{− 1}, nearly three times that of Co₃S₄(0.07 s^{− 1}), suggesting the higher atomic utilization of Fe single atoms. Mössbauer and in-situ Raman spectra confirm that the OER activity of Fe₁S _x@Co₃S₄ origins from a thin catalytic layer of Co(Fe)OOH that interacts with trace-level Fe species in the electrolyte, creating dynamically stable active sites. Combined with experimental characterizations, it suggests that the most active S-coordinated dual-metal site configurations are 2S-bridged (Fe-Co)S₄, in which Co-S and Fe-S moieties are shared with two S atoms, which can strongly regulate the adsorption energy of reaction intermediates, accelerating the OER reaction kinetics.

Electrochemical reduction of carbon dioxide into value-added products is a promising way to recycle the greenhouse gas, thus solving the crisis of global warming. Pressing challenges remain in regulating the catalytic selectivity. In this work, we demonstrated a metal-organic frameworks-assisted approach to synthesizing In species loaded on the surface of N doped carbon matrix. By controlling the particle sizes, the catalytic selectivity can be easily altered. The obtained In_c/NC possesses the outstanding capability for converting CO₂ into CO. And 80.09% Faraday efficiency (FE) of CO can be achieved at 0.8 V vs. RHE. While the In₂O₃/C exhibits different catalytic behaviors, the main product is formic acid and the FE is more than 50% at 0.8 V vs. RHE. The selectivity reversal can be attributed to the strong interactions between In clusters and N atoms of carbon supports, which efficiently inhibits the formation of the by-product, formic acid. Our research has paved a new way to modulate catalytic selectivity by manipulating the fine structures of the catalysts.

In this study, we have provided a facile solution to synthesize well-aligned titanium dioxide nanorods by using hydrothermal reaction. By calcining the materials under different atmospheres and temperatures, a batch of titanium dioxides with excellent oxygen evolution reaction(OER) catalytic efficiency were obtained. This new structured TiO₂ photoanode material yields a high photocurrent density of 5.69 mA/cm² at 1.23 V vs. reversible hydrogen electrode(RHE) under simulated solar light(100 mW/cm²). Surface photovoltage techniques and other measurements were carried out to confirm that the enhanced photoelectrochemical performances were attributed to the synergistic effect of the phase junction and a certain content of surface states, which accelerate the separation and transmission of the photogenerated charges. This material with phase junction and surface states promises a potential application in the field of photoelectric catalysis under solar light.

Regulating Lewis acid sites with well-defined electronic state and steric environment is still challenging for achieving high catalytic efficiency. Here we show coordinating zirconium nodes in the typical metal-organic framework known as MOF-545 with the monocarboxylate modulators including trifluoroacetic acid(TFA) or benzoic acid(BA) over meso-tetra(4-carboxyphenyl)-porphine(H₂TCPP), denoted as MOF-545-TFA or MOF-545-BA. Impressively, MOF-545-TFA shows the significantly enhanced performance for the catalytic ring-opening reaction of various epoxides with alcohols and good recyclability at 40 °C in respect with MOF-545-BA and ZrO₂. This mainly originates from the stronger Lewis acidity and more active zirconium sites induced by the electron-withdrawing TFA, resulting in the increased ability for activation of epoxides. This modulation approach is promising for enlarging the toolbox to extend the MOFs-based Lewis acid catalysis.