Single Atomic Site Catalysts Research Articles

Electronic structure regulations of single-atom site catalysts and their effects on the electrocatalytic performances

In recent years, single-atom site catalysts (SACs) have achieved great advancements in heterogeneous and electrochemical catalysis due to the merits of maximal atom utilization, unique electronic and geometric structures, low costs, and high catalytic performances. The electronic structures of the isolated metal centers can greatly influence the adsorption energies of reactants and intermediates, thus determining the catalytic activities of SACs. Especially, the electronic structure regulation of the metal sites has been advanced to the atomic level and proven a powerful strategy to enhance their electrocatalytic performances, which provides great opportunities for the further development of SACs. Based on the reported synthesis and structural modulation methods, in this review, the experimental and computational advances in the electronic structure regulation strategies for SACs, including coordination adjustment, electronic metal-support interaction, oxidation state modulation, and strain engineering, will be summarized. Then, the effects of electronic structures on the adsorption behaviors and, thus, the electrocatalytic activities of water splitting and O2/CO2 reduction reactions will be emphatically exemplified and discussed. In the end, a brief conclusion of this paper and the existing challenges and future opportunities in this research direction will be proposed. This review aims to highlight the understanding of electronic structure in association to the electrocatalytic activity for SACs and provide guidance for their further development in electrochemical applications.

Atom-level interfacial synergy of single-atom site catalysts for electrocatalysis

Single-atom site catalysts (SACs) have made great achievements due to their nearly 100% atomic utilization and uniform active sites. Regulating the surrounding environment of active sites, including electron structure and coordination environment via atom-level interface regulation, to design and construct an advanced SACs is of great significance for boosting electrocatalytic reactions. In this review, we systemically summarized the fundamental understandings and intrinsic mechanisms of SACs for electrocatalytic applications based on the interface site regulations. We elaborated the several different regulation strategies of SACs to demonstrate their ascendancy in electrocatalytic applications. Firstly, the interfacial electronic interaction was presented to reveal the electron transfer behavior of active sites. Secondly, the different coordination structures of metal active center coordinated with two or three non-metal elements were also summarized. In addition, other atom-level interfaces of SACs, including metal atom–atom interface, metal atom-X-atom interface (X: non-metal element), metal atom-particle interface, were highlighted and the corresponding promoting effect towards electrocatalysis was disclosed. Finally, we outlooked the limitations, perspectives and challenges of SACs based on atomic interface regulation.

Synthesis of Platinum Based Single-Atom Catalysts over Intermetallic Compounds for Hydrogen Evolution Reaction By Electrochemically Assisted Dissolution-Deposition

Platinum metal is not inert as it undergoes irreversible surface oxidation1. Platinum oxide (PtO) formation and corresponding reduction brings irreversibility on the surface due to famous place-exchange phenomena. Rise of this irreversibility during PtO reduction results in Pt dissolution2. Pt dissolution has been investigated upto an appreciable extent, in rigorous fuel cell conditions, which gives immense opportunity to restrain it by controlling accessible electrochemical operational parameters3. Advantageousness of modulated electrochemical dissolution of Pt, deployed as sacrificial electrode (S.E), renders ultra-low amount of simultaneous Pt electrodeposition through electrolyte on an anchoring substrate electrode (A.S.E). Thereby isolated atoms and subnanometric clusters becomes achievable over suitable A.S.E, utilizing altered rates of dissolution and deposition, by a four-electrode assembly (FEA). Presently, field of electrochemical energy conversion is evolving with highly active and stable electrocatalysts for Hydrogen Evolution Reaction (HER). One such pinnacle group of electrocatalysts emerged as metal-based single-atom catalysts (SACs) are quite efficient as well as stable for HER4. Current research work is focussed on using Pt dissolution phenomena to prepare SACs over certain stable substrates by using four-electrode assembly (FEA).Electrochemically assisted Dissolution-Deposition (EADD) by using FEA is utilized in present work to prepare Pt SACs supported by Intermetallic compounds. EADD being a novel, controlled and efficient synthesis technique has helped to evaluate the insitu Pt dissolution-deposition mechanism by Alternating Current Voltammetry. Authors also demonstrates that typical three-electrode assembly (TEA) used for Pt dissolution from Pt counter electrode is an uncontrolled method for Pt deposition over working electrode. In TEA, Open Circuit Potential (OCP) measurements of counter electrode conducted during cyclic voltammetry (CV) has helped to evaluate the scan rates experienced by Pt counter electrode. Finally, scan rates has given the insights about Pt dissolution and redeposition kinetics demonstrating the problems associated with TEA in context of dissolution-deposition synthesis. While using FEA the challenges experienced in TEA has also overcome. Synthesized Pt SACs tested for HER showed onset potential (η0) of 0mV and over potential at 10mA/cm2 (η10) of 50mV with a quite stable performance for 20h. Ultimately, EADD is summarized as a general, unique and effective surface synthesis technique in the domain of single atom catalysis availing immense opportunities for various other catalytic applications too.(1) H. ANGERSTEIN-KOZLOWSKA; B. E. CONWAY and W. B. A. SHARP. Faraday Trans. I, 1973. Langmuir 1973, 43, 9–36.(2) Gómez-Marín, A. M.; Clavilier, J.; Feliu, J. M. Sequential Pt(1 1 1) Oxide Formation in Perchloric Acid: An Electrochemical Study of Surface Species Inter-Conversion. J. Electroanal. Chem. 2013, 688, 360–370. https://doi.org/10.1016/j.jelechem.2012.07.016.(3) Topalov, A. A.; Cherevko, S.; Zeradjanin, A. R.; Meier, J. C.; Katsounaros, I.; Mayrhofer, K. J. J. Towards a Comprehensive Understanding of Platinum Dissolution in Acidic Media. Chem. Sci. 2014, 5 (2), 631–638. https://doi.org/10.1039/c3sc52411f.(4) Ji, S.; Chen, Y.; Wang, X.; Zhang, Z.; Wang, D.; Li, Y. Chemical Synthesis of Single Atomic Site Catalysts. Chem. Rev. 2020. https://doi.org/10.1021/acs.chemrev.9b00818. Figure 1

Single-Atomic Site Catalyst with Heme Enzymes-Like Active Sites for Electrochemical Sensing of Hydrogen Peroxide.

Heme enzymes, with the pentacoordinate heme iron active sites, possess high catalytic activity and selectivity in biosensing applications. However, they are still subject to limited catalytic stability in the complex environment and high cost for broad applications in electrochemical sensing. It is meaningful to develop a novel substitute that has a similar structure to some heme enzymes and mimics their enzyme activities. One emerging strategy is to design the Fe-N-C based single-atomic site catalysts (SASCs). The obtained atomically dispersed Fe-Nx active sites can mimic the active sites of heme enzymes effectively. In this work, a SASC (Fe-SASC/NW) is synthesized by doping single iron atoms in polypyrrole (PPy) derived carbon nanowire via a zinc-atom-assisted method. The proposed Fe-SASC/NW shows high heme enzyme-like catalytic performance for hydrogen peroxide (H2 O2 ) with a specific activity of 42.8Umg-1 . An electrochemical sensor based on Fe-SASC/NW is developed for the detection of H2 O2 . This sensor exhibits a wide detection concentration range from 5.0× 10-10 m to 0.5 m and an excellent limit of detection (LOD) of 46.35× 10-9 m. Such excellent catalytic activity and electrochemical sensing sensitivity are attributed to the isolated Fe-Nx active sites and their structural similarity with natural metalloproteases.

A Supported Pd2Dual‐Atom Site Catalyst for Efficient Electrochemical CO2Reduction

AbstractDual‐atom site catalysts (DACs) have emerged as a new frontier in heterogeneous catalysis because the synergistic effect between adjacent metal atoms can promote their catalytic activity while maintaining the advantages of single‐atom site catalysts (SACs), like 100 % atomic utilization efficiency and excellent selectivity. Herein, a supported Pd2DAC was synthesized and used for electrochemical CO2reduction reaction (CO2RR) for the first time. The as‐obtained Pd2DAC exhibited superior CO2RR catalytic performance with 98.2 % CO faradic efficiency at −0.85 V vs. RHE, far exceeding that of Pd1SAC, and coupled with long‐term stability. The density functional theory (DFT) calculations revealed that the intrinsic reason for the superior activity of Pd2DAC toward CO2RR was the electron transfer between Pd atoms at the dimeric Pd sites. Thus, Pd2DAC possessed moderate adsorption strength of CO*, which was beneficial for CO production in CO2RR.

A Supported Pd2Dual‐Atom Site Catalyst for Efficient Electrochemical CO2Reduction

AbstractDual‐atom site catalysts (DACs) have emerged as a new frontier in heterogeneous catalysis because the synergistic effect between adjacent metal atoms can promote their catalytic activity while maintaining the advantages of single‐atom site catalysts (SACs), like 100 % atomic utilization efficiency and excellent selectivity. Herein, a supported Pd2DAC was synthesized and used for electrochemical CO2reduction reaction (CO2RR) for the first time. The as‐obtained Pd2DAC exhibited superior CO2RR catalytic performance with 98.2 % CO faradic efficiency at −0.85 V vs. RHE, far exceeding that of Pd1SAC, and coupled with long‐term stability. The density functional theory (DFT) calculations revealed that the intrinsic reason for the superior activity of Pd2DAC toward CO2RR was the electron transfer between Pd atoms at the dimeric Pd sites. Thus, Pd2DAC possessed moderate adsorption strength of CO*, which was beneficial for CO production in CO2RR.

Fe3C-Assisted Single Atomic Fe Sites for Sensitive Electrochemical Biosensing.

The rational construction of advanced sensing platforms to sensitively detect H2O2 produced by living cells is one of the challenges in both physiological and pathological fields. Owing to the extraordinary catalytic performances and similar metal coordination to natural metalloenzymes, single atomic site catalysts (SASCs) with intrinsic peroxidase (POD)-like activity have shown great promise for H2O2 detection. However, there still exists an obvious gap between them and natural enzymes because of the great challenge in rationally modulating the electronic and geometrical structures of central atoms. Note that the deliberate modulation of the metal-support interaction may give rise to the promising catalytic activity. In this work, an extremely sensitive electrochemical H2O2 biosensor based on single atomic Fe sites coupled with carbon-encapsulated Fe3C crystals (Fe3C@C/Fe-N-C) is proposed. Compared with the conventional Fe SASCs (Fe-N-C), Fe3C@C/Fe-N-C exhibits superior POD-like activity and electrochemical H2O2 sensing performance with a high sensitivity of 1225 μA/mM·cm2, fast response within 2 s, and a low detection limit of 0.26 μM. Significantly, sensitive monitoring of H2O2 released from living cells is also achieved. Moreover, the density functional theory calculations reveal that the incorporated Fe3C nanocrystals donate electrons to single atomic Fe sites, endowing them with improved activation ability of H2O2 and further enhancing the overall activity. This work provides a new design of synergistically enhanced single atomic sites for electrochemical sensing applications.

Single-atomic Pt sites anchored on defective TiO2 nanosheets as a superior photocatalyst for hydrogen evolution

Single-atomic site catalysts have drawn considerable attention because of their maximum atom-utilization efficiency and excellent catalytic activity. In this work, a highly active single-atomic Pt site photocatalyst was synthesized through employing defective TiO2 nanosheets as solid support for photocatalytic water splitting. It indicated that the surface oxygen vacancies on defective TiO2 nanosheets could effectively stabilize the single-atomic Pt sites through constructing a three-center Ti–Pt–Ti structure. The Ti–Pt–Ti structure can hold the stability of isolated single-atomic Pt sites and facilitate the separation and transfer of photoinduced charge carriers, thereby greatly improving the photocatalytic H2 evolution. Notably, our synthesized photocatalyst exhibited a remarkably enhanced H2 evolution performance, and the H2 production rate is up to 13460.7 μmol·h−1·g−1, which is up to around 29.0 and 4.7 times higher than those of TiO2 nanosheets and Pt nanoparticles-TiO2. In addition, a plausible enhanced reaction mechanism was also proposed combining with photo-electrochemical characterizations and density functional theory (DFT) calculation results. Ultimately, it is believed that this work highlights the benefits of a single-site catalyst and paves the way to rationally design the highly active and stable single-atomic site photocatalysts on metal oxide support.

Recent advances in synergistically enhanced single-atomic site catalysts for boosted oxygen reduction reaction

The development of electrocatalysts for the oxygen reduction reaction (ORR) with high activity and cost-effectiveness is indispensable for sustainable energy storage and conversion technologies. Featured with maximum atomic efficiency and well-defined structure characteristics, carbon-based single-atomic site (SAS) catalysts are regarded as the most potential next-generation ORR catalysts. Despite the substantial efforts on the study of SAS catalysts, there still exists a large gap between them and platinum group metal (PGM) catalysts in the performance. Particularly, the unique metal-support interactions are believed to alter the charge state of the SAS by electron transfer between SAS and the support, resulting in the activated reactants and enhanced catalytic process of SAS catalysts. Inspired by this, the addition of some synergistic components is desirable to accurately tune the interactions and endow the SAS with the optional local environment, leading to improved performance. In this review, we provide a comprehensive overview of recent advances in synergistically enhanced SAS for boosted ORR. First, ORR mechanism and description of SAS catalysts towards ORR are provided. Then, a variety of novel synthetic strategies and advanced characterization techniques for SAS catalysts are briefly introduced. Emphatically, several synergistic components for SAS were systematically summarized. Finally, the current challenges and perspectives are proposed for the future development of SAS catalysts towards ORR. A comprehensive overview of recent advances in SAS catalysts for boosted ORR is provided, with a strong emphasis on the role of synergistic components for metal single sites. • The ORR mechanisms, novel synthesis strategies of single-atomic site (SAS) catalysts are introduced. • The interactions between SAS and synergistic components are emphasized. • Future trends and challenges for the development of synergistically enhanced SAS catalysts are highlighted.

Transforming cobalt hydroxide nanowires into single atom site catalysts

Abstract The stability of the catalyst has always received considerable attention in materials science and catalysis. Single atom site catalysts (SACs) with high surface energy and low coordination number are generally believed not as stable as nanocatalysts. Herein, we discovered that nanocrystals, such as nanowires (NWs), which are unstable at high temperature, can be directly converted into SACs without undergoing particles aggregation and growth. In-situ environmental transmission electron microscopy, X-ray absorption spectroscopy, and density functional calculation studies demonstrated that the high temperature overcomes the energy barrier of metal atomization while N-doped carbon materials regulate the atomic adsorption energy, both of which jointly cause the transformation of NWs into SACs. Notably, the obtained SACs exhibited excellent catalytic performance for the hydrogenation of nitroarenes to the corresponding amines due to the considerable decrease in energy barriers of the rate-determining step.

Cobalt single atom site catalysts with ultrahigh metal loading for enhanced aerobic oxidation of ethylbenzene

The oxidation of hydrocarbons to produce high value-added compounds (ketones or alcohols) using oxygen in air as the only oxidant is an efficient synthetic strategy from both environmental and economic views. Herein, we successfully synthesized cobalt single atom site catalysts (Co SACs) with high metal loading of 23.58 wt.% supported on carbon nitride (CN), which showed excellent catalytic properties for oxidation of ethylbenzene in air. Moreover, Co SACs show a much higher turn-over frequency (19.6 h−1) than other reported non-noble catalysts under the same condition. Comparatively, the as-obtained nanosized or homogenous Co catalysts are inert to this reaction. Co SACs also exhibit high selectivity (97%) and stability (unchanged after five runs) in this reaction. DFT calculations reveal that Co SACs show a low energy barrier in the first elementary step and a high resistance to water, which result in the robust catalytic performance for this reaction.

A gel-limiting strategy for large-scale fabrication of Fe–N–C single-atom ORR catalysts

Although transition metal single atom site catalysts (SASCs) show great potential for electrocatalysis, their large-scale controllable and flexible preparation remains a great challenge.

The atomic-level regulation of single-atom site catalysts for the electrochemical CO2 reduction reaction.

The electrochemical CO2 reduction reaction (CO2RR) is viewed as a promising way to remove the greenhouse gas CO2 from the atmosphere and convert it into useful industrial products such as methane, methanol, formate, ethanol, and so forth. Single-atom site catalysts (SACs) featuring maximum theoretical atom utilization and a unique electronic structure and coordination environment have emerged as promising candidates for use in the CO2RR. The electronic properties and atomic structures of the central metal sites in SACs will be changed significantly once the types or coordination environments of the central metal sites are altered, which appears to provide new routes for engineering SACs for CO2 electrocatalysis. Therefore, it is of great importance to discuss the structural regulation of SACs at the atomic level and their influence on CO2RR activity and selectivity. Despite substantial efforts being made to fabricate various SACs, the principles of regulating the intrinsic electrocatalytic performances of the single-atom sites still needs to be sufficiently emphasized. In this perspective article, we present the latest progress relating to the synthesis and catalytic performance of SACs for the electrochemical CO2RR. We summarize the atomic-level regulation of SACs for the electrochemical CO2RR from five aspects: the regulation of the central metal atoms, the coordination environments, the interface of single metal complex sites, multi-atom active sites, and other ingenious strategies to improve the performance of SACs. We highlight synthesis strategies and structural design approaches for SACs with unique geometric structures and discuss how the structure affects the catalytic properties.

Non-carbon-supported single-atom site catalysts for electrocatalysis

Non-carbon supported SACs were classified into eight categories based on the nature of their substrates for the first time, and their corresponding anchoring and stabilization mechanisms have been systematically summarized and discussed.

Single-atom site catalysts for environmental catalysis

In recent decades, the environmental protection and long-term sustainability have become the focus of attention due to the increasing pollution generated by the intense industrialization. To overcome these issues, environmental catalysis has increasingly been used to solve the negative impact of pollutants emission on the global environment and human health. Supported platinum-metal-group (PGM) materials are commonly utilized as the state-of-the-art catalysts to eliminate gaseous pollutants but large quantities of PGMs are required. By comparison, single-atom site catalysts (SACs) have attracted much attention in catalysis owing to their 100% atom efficiency and unique catalytic performances towards various reactions. Over the past decade, we have witnessed burgeoning interests of SACs in heterogeneous catalysis. However, to the best of our knowledge, the systematic summary and analysis of SACs in catalytic elimination of environmental pollutants has not yet been reported. In this paper, we summarize and discuss the environmental catalysis applications of SACs. Particular focus was paid to automotive and stationary emission control, including model reaction (CO oxidation, NO reduction and hydrocarbon oxidation), overall reaction (three-way catalytic and diesel oxidation reaction), elimination of volatile organic compounds (formaldehyde, benzene, and toluene), and removal/decomposition of other pollutants (Hg0 and SO3). Perspectives related to further challenges, directions and design strategies of single-atom site catalysts in environmental catalysis were also provided.

Gram‐Scale Synthesis of High‐Loading Single‐Atomic‐Site Fe Catalysts for Effective Epoxidation of Styrene

Single-atomic-site (SAS) catalysts, a new frontier of catalysts, always show extremely high atom efficiency and unexpected catalytic properties. Herein, a pyrolyzing coordinated polymer (PCP) strategy is developed, which is facile and widely applicable in the synthesis of a series of SAS catalysts including SAS-Fe, SAS-Ni, SAS-Cu, SAS-Zn, SAS-Ru, SAS-Rh, SAS-Pd, SAS-Pt, and SAS-Ir. The as-obtained SAS catalysts can be easily synthesized at gram scale and the metal loading of SAS-Fe catalysts achieves a record value of 30 wt%, which meets the requirement of practical applications. Moreover, it is discovered that SAS-Fe catalysts show unprecedented catalytic performance for epoxidation of styrene using O2 as the only oxidant (yield: 64%; selectivity: 89%), while Fe nanoparticles and ironporphyrin are inactive. This discovery is believed to pave the way for exploiting the unparalleled properties of SAS catalysts and promoting their industrial applications.

Cation Exchange Strategy to Single-Atom Noble-Metal Doped CuO Nanowire Arrays with Ultralow Overpotential for H2O Splitting.

Single-atom site catalysts (SACs) have aroused enormous attention and brought about new opportunities for many applications. Herein, we report a versatile strategy to rhodium (Rh) SAC by a facile cation exchange reaction. Remarkably, the Rh SAC modified CuO nanowire arrays on copper foam (Rh SAC-CuO NAs/CF) show unprecedented alkaline oxygen evolution reaction (OER) activity with a high current density of 84.5 mA cm-2@1.5 V vs reversible hydrogen electrode (RHE), 9.7 times that of Ir/C/CF. More strikingly, when used as an anode and a cathode for overall water splitting, the Rh SAC-CuO NAs/CF can achieve 10 mA cm-2 at only 1.51 V. Density functional theory calculations reveal that the high OER and HER intrinsic catalytic activities result from moderate adsorption energy of intermediates on Rh SAC. Finally, we demonstrate the general synthesis of different single-atom noble-metal catalysts on CuO NAs (M SAC-CuO NAs/CF, where M = Ru, Ir, Os, and Au).

Room-Temperature Synthesis of Single Iron Site by Electrofiltration for Photoreduction of CO2 into Tunable Syngas

Developing a convenient and effective method to prepare single-atom catalysts at mild synthetic conditions remains a challenging task. Herein, a voltage-gauged electrofiltration method was demonstrated to synthesize single-atom site catalysts at room temperature. Under regulation of the graphene oxide membrane, a bulk Fe plate was directly converted into Fe single atoms, and the diffusion rate of Fe ions was greatly reduced, resulting in an ultralow concentration of Fe2+ around the working electrode, which successfully prevented the growing of nuclei and aggregating of metal atoms. Monatomic Fe atoms are homogeneously anchored on the as-prepared nitrogen-doped carbon. Owing to the fast photoelectron injection from photosensitizers to atomically dispersed Fe sites through the highly conductive supported N-C, the Fe-SAs/N-C exhibits an outstanding photocatalytic activity toward CO2 aqueous reduction into syngas with a tunable CO/H2 ratio under visible light irradiation. The gas evolution rates for CO and H2 are 4500 and 4950 μmol g-1 h-1, respectively, and the tunable CO/H2 ratio is from 0.3 to 8.8. This article presents an efficient strategy to develop the single-atom site catalysts and bridges the gap between heterogeneous and homogeneous catalysts toward photocatalytic CO2 aqueous reduction into syngas.

Chemical Synthesis of Single Atomic Site Catalysts.

Manipulating metal atoms in a controllable way for the synthesis of materials with the desired structure and properties is the holy grail of chemical synthesis. The recent emergence of single atomic site catalysts (SASC) demonstrates that we are moving toward this goal. Owing to the maximum efficiency of atom-utilization and unique structures and properties, SASC have attracted extensive research attention and interest. The prerequisite for the scientific research and practical applications of SASC is to fabricate highly reactive and stable metal single atoms on appropriate supports. In this review, various synthetic strategies for the synthesis of SASC are summarized with concrete examples highlighting the key issues of the synthesis methods to stabilize single metal atoms on supports and to suppress their migration and agglomeration. Next, we discuss how synthesis conditions affect the structure and catalytic properties of SASC before ending this review by highlighting the prospects and challenges for the synthesis as well as further scientific researches and practical applications of SASC.

Electrocatalyst engineering and structure-activity relationship in hydrogen evolution reaction: From nanostructures to single atoms

With the ever-pressing issues of global energy demand and environmental pollution, molecular hydrogen has been receiving increasing attention as a clean alternative energy carrier. For hydrogen production, the design and development of high-performance catalysts remains rather challenging. As the compositions and structures of catalyst interfaces have paramount influences on the catalytic performances, the central topic here has always been to design and engineer the interface structures via rational routes so as to boost the activities and stabilities of electrocatalysts on hydrogen evolution reaction (HER). Here in this review, we focus on the design and preparation of multi-scale catalysts specifically catering to HER applications. We start from the design and structure-activity relationship of catalytic nanostructures, summarize the research progresses related to HER nanocatalysts, and interpret their high activities from the atomistic perspective; then, we review the studies regarding the design, preparation, HER applications and structure-activity relationship of single-atom site catalysts (SASCs), and thereupon discuss the future directions in designing HER-oriented SASCs. At the end of this review, we present an outlook on the development trends and faced challenges of catalysts for electrochemical HER.