Toward Rational Design of Single-Atom Catalysts

Carbon-based single-atom catalysts derived from biomass: Fabrication and application

Single-atom catalysts (SACs) with active metals dispersed atomically have shown great potential in heterogeneous catalysis due to the high atomic utilization and superior selectivity/stability. Synthesis of SACs using carbon-neutral biomass and its components as the feedstocks provides a promising strategy to realize the sustainable and cost-effective SACs preparation as well as the valorization of underused biomass resources. Herein, we begin by describing the general background and status quo of carbon-based SACs derived from biomass. A detailed enumeration of the common biomass feedstocks (e.g., lignin, cellulose, chitosan, etc.) for the SACs preparation is then offered. The interactions between metal atoms and biomass-derived carbon carriers are summarized to give general rules on how to stabilize the atomic metal centers and rationalize porous carbon structures. Furthermore, the widespread adoption of catalysts in diverse domains (e.g., chemocatalysis, electrocatalysis and photocatalysis, etc.) is comprehensively introduced. The structure-property relationships and the underlying catalytic mechanisms are also addressed, including the influences of metal sites on the activity and stability, and the impact of the unique structure of single-atom centers modulated by metal/biomass feedstocks interactions on catalytic activity and selectivity. Finally, we end this review with a look into the remaining challenges and future perspectives of biomass-based SACs. We expect to shed some light on the forthcoming research of carbon-based SACs derived from biomass, manifestly stimulating the development in this emerging research area.

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.

Targeted Spin-State Regulation to Boost Oxygen Reduction Reaction

Catalytic reactions are known to be significantly affected by spin states and their variations during reaction processes, yet the mechanisms behind them remain not fully understood, thus preventing the rational optimization of catalysis. Here, we explore the relationship between the spin states of active sites and their catalytic performance, taking the oxygen reduction reaction as an example. We demonstrate that the catalytic performance is spin-state-dependent and can be improved by adjusting spin states during the catalytic process. To this end, we further investigate the possibility of altering the spin states of transition metals through the application of external fields, such as adsorbed species. By studying the influence of the strength of adsorbed ligands on spin states and its impact on catalytic performance, our results show that optimal catalytic performance is achieved when the strength of the external field is neither too strong nor too weak, forming a volcano-like relationship between the catalytic performance and the external field strength. Our findings can have far-reaching implications for the rational design of high-performance catalysis.

Catalytic Properties of Biochar as Support Material Potential for Direct Methanol Fuel Cell: A Review

With the evolution and emergence of compounding environmental problems and issues, renewable energy promises to be a sustainable future technology. One technology considered is the fuel cell, which thrives on the primary function of electrocatalytic activities. Thus this review article envisages and presents a comprehensive summary of the applications of activated carbonaceous material as supports for electrocatalysts in fuel cells. The different techniques utilized to produce these carbon materials are discussed in detail. The overview architecture and the principle of the operation of fuel cells are also addressed. Additionally, electrocatalysts and the importance of support materials, their characteristics, and the role they play in the performance of the electrocatalyst will be reviewed. Unfortunately, the carbon-support-based electrocatalyst suffers long-term instability due to corrosion. Previously, carbon black has been used as a carbon support in various fuel cells. In recent years, there has been progress in the incorporation of nanostructured carbon supports in electrocatalysts in various fuel cells; however, there is still a great deal of distance to cover for nanostructured carbon-supported electrocatalysts in fuel cells to realize full commercialization and large-scale industrial purposes due to shortcomings in electrocatalysts, which are low-cost and highly efficient. This review therefore discusses the progress of incorporation of biochar extracted from sugar cane bagasse as carbon support in electrocatalysts for direct methanol fuel cells with the intention to provide insight into the quest of producing highly efficient and low cost fuel cells.

Morphological and Coordination Modulations in Iridium Electrocatalyst for Robust and Stable Acidic OER Catalysis

Proton exchange membrane water splitting (PEMWS) technology has high-level current density, high operating pressure, small electrolyzer-size, integrity, flexibility, and has good adaptability to the volatility of wind power and photovoltaics, but the development of both active and high stability of the anode electrocatalyst in acidic environment is still a huge challenge, which seriously hinders the promotion and application of PEMWS. In recent years, researchers have made tremendous attempts in the development of high-quality active anode electrocatalyst, and we summarize some of the research progress made by our group in the design and synthesis of PEMWS anode electrocatalysts with different nanostructures, and makes full use of electrocatalytic activity points to increase the inherent activity of Iridium (Ir) sites, and provides optimization strategies for the long-term non-decay of catalysts under high anode potential in acidic environments. At this stage, these research advances are expected to facilitate the research and technological progress of PEMWS, and providing some research ideas and references for future research on efficient and inexpensive PEMWS anode electrocatalysts.

Performance Regulation of Single-Atom Catalyst by Modulating the Microenvironment of Metal Sites

Metal-based catalysts, encompassing both homogeneous and heterogeneous types, play a vital role in the modern chemical industry. Heterogeneous metal-based catalysts usually possess more varied catalytically active centers than homogeneous catalysts, making it challenging to regulate their catalytic performance. In contrast, homogeneous catalysts have defined active-site structures, and their performance can be easily adjusted by modifying the ligand. These characteristics lead to remarkable conceptual and technical differences between homogeneous and heterogeneous catalysts. As a recently emerging class of catalytic material, single-atom catalysts (SACs) have become one of the most active new frontiers in the catalysis field and show great potential to bridge homogeneous and heterogeneous catalytic processes. This review documents a brief introduction to SACs and their role in a range of reactions involving single-atom catalysis. To fully understand process-structure-property relationships of single-atom catalysis in chemical reactions, active sites or coordination structure and performance regulation strategies (e.g., tuning chemical and physical environment of single atoms) of SACs are comprehensively summarized. Furthermore, we discuss the application limitations, development trends and future challenges of single-atom catalysis and present a perspective on further constructing a highly efficient (e.g., activity, selectivity and stability), single-atom catalytic system for a broader scope of reactions.

Dynamic evolution of the active center driven by hemilabile coordination in Cu/CeO2 single-atom catalyst

Hemilability is an important concept in homogeneous catalysis where both the reactant activation and the product formation can occur simultaneously through a reversible opening and closing of the metal-ligand coordination sphere. However, this effect has rarely been discussed in heterogeneous catalysis. Here, by employing a theoretical study on CO oxidation over substituted Cu₁/CeO₂ single atom catalysts, we show that dynamic evolution of metal-support coordination can significantly change the electronic structure of the active center. The evolution of the active center is shown to either strengthen or weaken the metal-adsorbate bonding as the reaction proceeds from reactants, through intermediates, to products. As a result, the activity of the catalyst can be increased. We explain our observations by extending hemilability effects to single atom heterogenous catalysts and anticipate that introducing this concept can offer a new insight into the important role active site dynamics have in catalysis toward the rational design of more sophisticated single atom catalyst materials.

Recent progress of theoretical studies on electro- and photo-chemical conversion of CO2 with single-atom catalysts

The CO₂ reduction reaction (CO₂RR) into chemical products is a promising and efficient way to combat the global warming issue and greenhouse effect. The viability of the CO₂RR critically rests with finding highly active and selective catalysts that can accomplish the desired chemical transformation. Single-atom catalysts (SACs) are ideal in fulfilling this goal due to the well-defined active sites and support-tunable electronic structure, and exhibit enhanced activity and high selectivity for the CO₂RR. In this review, we present the recent progress of quantum-theoretical studies on electro- and photo-chemical conversion of CO₂ with SACs and frameworks. Various calculated products of CO₂RR with SACs have been discussed, including CO, acids, alcohols, hydrocarbons and other organics. Meanwhile, the critical challenges and the pathway towards improving the efficiency of the CO₂RR have also been discussed.

Role of heteroatom-doping in enhancing catalytic activities and the stability of single-atom catalysts for oxygen reduction and oxygen evolution reactions

Single-atom catalysts (SACs) are promising as efficient electrocatalysts for clean energy technologies such as fuel cells, water splitting, and metal-air batteries. Still, the unsatisfactory loading density and stability of the catalytic active centers limit their applications. Herein, a doping strategy is explored to achieve highly efficient and stable SACs for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The stability, electronic structures, and ORR/OER overpotentials of S-doped transition metal-nitrogen-carbon SAC structures were investigated using first-principles calculation methods. An intrinsic descriptor linking the intrinsic properties of catalysts and the catalytic activity was established for screening the best SACs. The theoretical predictions are well consistent with the experimental results, which provide a theoretical basis for understanding the catalytic mechanism and an approach for the rational design of SACs for clean energy conversion and storage.

Applications of Single Atom Catalysts for Environmental Management

With the rapid development of industrialization, human beings have caused many negative effects on the environment that have endangered the survival and development of human beings, such as the greenhouse effect, water pollution, energy depletion, etc [...].

Effect of the Charge State on the Catalytic Activity of a Fullerene-Based Molecular Electrocatalyst: A Theoretical Study

The charge state of a catalyst is significant for its catalytic activity. In this work, taking molecular electrocatalysts of fullerene C₆₀ with a doped transition metal (TM-C₆₀, where TM = Fe, Co, or Ni) as an example, we conducted first-principles calculations to study the effect of the charge state on the cathodic nitrogen reduction reaction (NRR) and anodic oxygen evolution reaction (OER). Our calculated results suggest that the maximal free energy barrier of the NRR with a dissociative mechanism is a nearly linear function of the number of negatively charged electrons (0-3). Nevertheless, the NRR activity with an associative mechanism is insensitive to the charge state effect. The OER activity of TM-C₆₀ with a 0-3 e⁺ charge state exhibits a volcano-shaped trend, which indicates that it is important to tailor a particular charge state toward effective catalytic activity. This study provides new insight into the effect of the charge state on catalytic activity, which could help us improve our understanding of the catalytic mechanism and tailor a new efficient catalyst.

Elucidation of Metal Local Environments in Single-Atom Catalysts Based on Carbon Nitrides

The ability to tailor the properties of metal centers in single-atom heterogeneous catalysts depends on the availability of advanced approaches for characterization of their structure. Except for specific host materials with well-defined metal adsorption sites, determining the local atomic environment remains a crucial challenge, often relying heavily on simulations. This article reports an advanced analysis of platinum atoms stabilized on poly(triazine imide), a nanocrystalline form of carbon nitride. The approach discriminates the distribution of surface coordination sites in the host, the evolution of metal coordination at different stages during the synthesis of the material, and the potential locations of metal atoms within the lattice. Consistent with density functional theory predictions, simultaneous high-resolution imaging in high-angle annular dark field and bright field modes experimentally confirms the preferred localization of platinum in-plane in the corners of the triangular cavities. X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and dynamic nuclear polarization enhanced ¹⁵ N nuclear magnetic resonance (DNP-NMR) spectroscopies coupled with density functional theory (DFT) simulations reveal that the predominant metal species comprise Pt(II) bound to three nitrogen atoms and one chlorine atom inside the coordination sites. The findings, which narrow the gap between experimental and theoretical elucidation, contribute to the improved structural understanding and provide a benchmark for exploring the speciation of single-atom catalysts based on carbon nitrides.

Facile Synthesis of Single Iron Atoms over MoS2 Nanosheets via Spontaneous Reduction for Highly Efficient Selective Oxidation of Alcohols

The facile creation of high-performance single-atom catalysts (SACs) is intriguing in heterogeneous catalysis, especially on 2D transition-metal dichalcogenides. An efficient spontaneous reduction approach to access atomically dispersed iron atoms supported over defect-containing MoS₂ nanosheets is herein reported. Advanced characterization methods demonstrate that the isolated iron atoms situate atop of molybdenum atoms and coordinate with three neighboring sulfur atoms. This Fe SAC delivers exceptional catalytic efficiency (1 atm O₂ @ 120 °C) in the selective oxidation of benzyl alcohol to benzaldehyde, with 99% selectivity under almost 100% conversion. The turnover frequency is calculated to be as high as 2105 h^-1 . Moreover, it shows admirable recyclability, storage stability, and substrate tolerance. Density functional theory calculations reveal that the high catalytic activity stems from the optimized electronic structure of single iron atoms over the MoS₂ support.

Atomically Defined Undercoordinated Copper Active Sites over Nitrogen-Doped Carbon for Aerobic Oxidation of Alcohols

Selective aerobic oxidation of alcohols offers an attractive means to address challenges in the modern chemical industry, but the development of non-noble metal catalysts with superior efficacy for this reaction remains a grand challenge. Here, this study reports on such a catalyst based on atomically defined undercoordinated copper atoms over nitrogen-doped carbon support as an efficient, durable, and scalable heterogeneous catalyst for selective aerobic oxidation of alcohols. This catalyst exhibits extremely high intrinsic catalytic activity (TOF of 7692 h^-1 ) in the oxidation of cinnamyl alcohol to afford cinnamaldehyde, along with exceptional recyclability (at least eight cycles), scalability, and broad substrate scope. DFT calculations suggest that the high activity derives from the low oxidation state and the unique coordination environment of the copper sites in the catalyst. These findings pave the way for the design of highly active and stable single atom catalysts to potentially address challenges in synthetic chemistry.

Mo3(C6O6)2 monolayer as a promising electrocatalyst for the CO2 reduction reaction: a first-principles study

Mo3(C6O6)2 monolayers are potential electrocatalysts for CO2 reduction reaction (CRR). The electrochemical performances can be further improved by coordinating with hydroxyl groups, which show improved performance for the production of methane.

Active site engineering of single-atom carbonaceous electrocatalysts for the oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is the vital process at the cathode of next-generation electrochemical storage and conversion technologies, such as metal-air batteries and fuel cells. Single-metal-atom and nitrogen co-doped carbonaceous electrocatalysts (M-N-C) have emerged as attractive alternatives to noble-metal platinum for catalyzing the kinetically sluggish ORR due to their high electrical conductivity, large surface area, and structural tunability at the atomic level, however, their application is limited by the low intrinsic activity of the metal-nitrogen coordination sites (M-N x ) and inferior site density. In this Perspective, we summarize the recent progress and milestones relating to the active site engineering of single atom carbonous electrocatalysts for enhancing the ORR activity. Particular emphasis is placed on the emerging strategies for regulating the electronic structure of the single metal site and populating the site density. In addition, challenges and perspectives are provided regarding the future development of single atom carbonous electrocatalysts for the ORR and their utilization in practical use.

Dilute Element Compounds: A Route to Enriching Inorganic Functional Materials

The development of functional materials calls for ever-enriching the inorganic material database. Doping is an effective way of achieving this purpose. Herein, we propose the concept of dilute element compounds (DECs), which contain a small amount of a dopant element distributed in a host crystal structure in an ordered manner. Different from dilute alloys or solid solutions, the DECs could be more resistant to segregation and are ideal for dispersing functional elements for applications such as single-atom catalysts. It is also expected that the DECs will serve as a route to discovering new inorganic functional materials by controlling phase transitions and tuning intrinsic properties of the host materials with applications including, but not limited to, thermoelectrics and solid-state electrolytes for secondary batteries. As an initial work, we quantify the diluteness of DECs and find the limits of diluteness in existing DECs. We further provide a classification scheme for the DECs to guide future discoveries.