Single-atom catalysis: Bridging the homo-and heterogeneous catalysis

Single-atom catalysis,the catalysis by single-atom catalysts(SACs),has attracted considerable attention in recent years as a new frontier in the heterogeneous catalysis field.SACs have the advantages of both homogeneous catalysts(isolated active sites)and heterogeneous catalysts(stable and easy to separate),and are thus predicted to be able to bridge the homo-and heterogeneous catalysis.This prediction was first experimentally demonstrated in 2016.In this mini-review,we summarize the few homogeneous catalysis progresses reported recently where SACs have exhibited promising application:a)Rh/ZnO and Rh/CoO SAC have been used successfully in hydroformylation of olefin of which the activity are comparable to the homogeneous Wilkinson’s catalyst;b)a Pt/Al2O3 SAC has shown excellent performance in hydrosilylation reaction;and c)M-N-C SACs(M=Fe,Co etc.)have been applied in the activation of C–H bonds.All of these examples suggest that fabrication of suitable SACs could provide a new avenue for the heterogenization of homogeneous catalysts.These pioneering works shed new light on the recognition of single-atom catalysis in bridging the homo-and heterogeneous catalysis.

First-Principles Study of Pd Single-Atom Catalysis to Hydrogen Desorption Reactions on MgH2（110） Surface

MgH2 is a promising and popular hydrogen storage material.In this work,the hydrogen desorption reactions of a single Pd atom adsorbed MgH2(110)surface are investigated by using first-principles density functional theory calculations.We find that a single Pd atom adsorbed on the MgH2(110)surface can significantly lower the energy barrier of the hydrogen desorption reactions from 1.802 eV for pure MgH2(110)surface to 1.154 eV for Pd adsorbed MgH2(110)surface,indicating a strong Pd single-atom catalytic effect on the hydrogen desorption reactions.Furthermore,the Pd single-atom catalysis significantly reduces the hydrogen desorption temperature from 573K to 367K,which makes the hydrogen desorption reactions occur more easily and quickly on the MgH2(110)surface.We also discuss the microscopic process of the hydrogen desorption reactions through the reverse process of hydrogen spillover mechanism on the MgH2(110)surface.This study shows that Pd/MgH2 thin films can be used as good hydrogen storage materials in future experiments.

Single-atom catalysis for carbon neutrality

Currently,more than 86%of global energy consumption is still mainly dependent on traditional fossil fuels,which causes resource scarcity and even emission of high amounts of carbon dioxide(CO_(2)),resulting in a severe“Greenhouse effect.”Considering this situation,the concept of“carbon neutrality”has been put forward by 125 countries one after another.To achieve the goals of“carbon neutrality,”two main strategies to reduce CO_(2) emissions and develop sustainable clean energy can be adopted.Notably,these are crucial for the synthesis of advanced single-atom catalysts(SACs)for energyrelated applications.In this review,we highlight unique SACs for conversion of CO_(2) into high-efficiency carbon energy,for example,through photocatalytic,electrocatalytic,and thermal catalytic hydrogenation technologies,to convert CO_(2) into hydrocarbon fuels(CO,CH_(4),HCOOH,CH_(3)OH,and multicarbon[C_(2+)]products).In addition,we introduce advanced energy conversion technologies and devices to replace traditional polluting fossil fuels,such as photocatalytic and electrocatalytic water splitting to produce hydrogen energy and a high-efficiency oxygen reduction reaction(ORR)for fuel cells.Impressively,several representative examples of SACs(including d-,ds-,p-,and f-blocks)for CO_(2) conversion,water splitting to H2,and ORR are discussed to describe synthesis methods,characterization,and corresponding catalytic activity.Finally,this review concludes with a description of the challenges and outlooks for future applications of SACs in contributing toward carbon neutrality.

Highlights of the major progress in single-atom catalysis in 2015 and 2016

The idea that single metal atoms dispersed on a solid support can act as an efficient heterogeneous catalyst was raised in2011when single Pt atoms on an FeOx surface were reported to be active for CO oxidation and preferential oxidation of CO in H2.The last six years have witnessed tremendous progress in the field of single‐atom catalysis.Here we introduce the major achievements on this topic in2015and2016.Some particular aspects of single‐atom catalysis are discussed in depth,including new approaches in single‐atom catalyst(SAC)synthesis,stable gold SACs for various reactions,the high selectivity of Pt and Pd SACs in hydrogenation,and the superior performance of non‐noble metal SACs in electrochemistry.These accomplishments will encourage more efforts by researchers to achieve the controllable fabrication of SACs and explore their potential applications.

Editorial for the special issue“Single-Atom Catalysis”

Single-atom catalysts(SACs),featuring a structure consisting of iso-lated metal atoms dispersed on solid supports,can provide the maximum atom utilization efficiency and great potential for bridging the gap be-tween heterogeneous and homogeneous catalysis.Since Zhang and co-workers reported the single Pt atom on iron oxide for catalyzing CO oxidation in 2011,1 SACs have become a new frontier in catalysis sci-ences and have attracted numerous attention in various aspects,such as photocatalysis,electrocatalysis and thermal catalysis.2,3。

Single-atom catalysis for advanced oxidation and reduction systems in water decontamination

Water scarcity is an escalating global crisis,posing a severe threat to populations worldwide.Consequently,exploring various materials to remove emerging contaminants from freshwater sources has garnered significant attention.In this regard,single-atom catalysis(SACs) has emerged as a catalyst of scientific progress in water purification and treatment methodologies during recent decades.SACs exhibit exceptional catalytic activity,selectivity and stability,due to their near-perfect atom utilization,highly unsaturated coordination environment and uniform reaction centers.However,a comprehensive and critical review encompassing the successful integration of SACs into water purification processes needs to be completed.This review aims to accentuate recent trends by presenting the synthesis,structure,and environment and energy application-relevant properties of SACs.The results show that a comprehensive and multi-perspective summary of the advantages of SACs in environmental remediation can have significant benefits,such as fast kinetics,costeffectiveness,selectivity.The oxidation and reduction processes of SACs and functional SACs materials in water purification were emphasized.Furthermore,the last section is devoted to the current research gaps and further perspectives on the application of SACs in water treatment,which are summarized and analyzed.

Theoretical and experimental investigations on single-atom catalysis:Pt_(1)/FeO_(x) for water-gas shift reaction

Oxide-supported metal single-atom catalysts(SACs)have exhibited excellent catalytic performance for water-gas shift(WGS)reaction.Here,we report the single-atom catalyst Pt1/FeOx exhibits excellent medium temperature catalytic performance for WGS reactions by the density functional theory(DFT)calculations and experimental results.The calculations indicate that H_(2)O molecules are easily dissociated at oxygen vacancies,and the formed*OH and*O are adsorbed on Pt1 single atoms and the adjacent O atoms,respectively.After studying four possible reaction mechanisms,it is found that the optimal WGS reaction pathway is proceeded along the carboxyl mechanism(pathway III),in which the formation of*COOH intermediates can promote the stability of Pt_(1)/FeO_(x) SAC and the easier occurrence of WGS reaction.The energy barrier of the rate-determining step during the entire reaction cycle is only 1.16 eV,showing the high activity for the medium temperature WGS reaction on Pt_(1)/FeO_(x) SAC,which was verified by experimental results.Moreover,the calculated turnover frequencies(TOFs)of CO_(2)and H_(2)formation on Pt1/FeOx at 610 K(337℃)can reach up to 1.14×10^(-3)s^(-1)·site^(-1)through carboxyl mechanism.In this work,we further expand the application potential of Pt1/FeOx SAC in WGS reaction.

Single-atom catalysis enables long-life, high-energy lithium-sulfur batteries

With high energy density and low material cost,lithium sulfur batteries(LSBs)emerge quite expeditiously as a fascinating energy storage system over the past decade.Broad applications of LSBs ranging from electric vehicles to stationary grid storage seem rather bright in recent literatures.However,there still exist many pressing challenges to be addressed because we do not yet fully understand and control the electrode-electrolyte interface chemistries during battery operation,such as polysulide shuttling and poor utilization of active sulfur.Single-atom catalysts(SACs)pave new possibilities of tackling the tough issues due to their decent applicability in the atomic-level identification of structure-activity relationships and reaction mechanism,as well as their structural tunability with atomic precision.This review comprehensively summarizes the very recent advances in utilization of highly active SACs for LSBs by stating and discussing the related publications,which involves catalyst synthesis routes,battery pertormance,catalytic mechanisms,optimization strategies,and promises to achieve long-lite,high-energy LSBs.We see that endeavors to employ SACs to modify sulfur cathode have allowed efficient polysulfide conversion and confinement,leading to the minimization of shuttle effect.Parallel efforts are being devoted to extending the scope of SACs to cell separator and lithium metal anode in order to unlock the full potential of LSBs.We also obtain mechanistic insights into battery chemistries and nature of SACs in their strong interactions with polysulfides through advanced in situ characterizations documented.Overall,acceleration in the development of LSBs by introducing SACs is noticeable,and this cutting edge needs more attentions to further promoting the design of better LSBs.

Single-atom catalysis for organic reactions

Metal-based catalysis,including homogeneous and heterogeneous catalysis,plays a significant role in the modern chemical industry.Heterogeneous catalysis is widely used due to the high efficiency,easy catalyst separation and recycling.However,the metal-utilization efficiency for conventional heterogeneous catalysts needs further improvement compared to homogeneous catalyst.To tackle this,the pursing of heterogenizing homogeneous catalysts has always been attractive but challenging.As a recently emerging class of catalytic material,single-atom catalysts(SACs)are expected to bridge homogeneous and heterogeneous catalytic process in organic reactions and have arguably become the most active new frontier in catalysis field.In this review,a brief introduction and development history of single-atom catalysis and SACs involved organic reactions are documented.In addition,recent advances in SACs and their practical applications in organic reactions such as oxidation,reduction,addition,coupling reaction,and other organic reactions are thoroughly reviewed.To understand structure-property relationships of single-atom catalysis in organic reactions,active sites or coordination structure,metal atom-utilization efficiency(e.g.,turnover frequency,TOF calculated based on active metal)and catalytic performance(e.g.,conversion and selectivity)of SACs are comprehensively summarized.Furthermore,the application limitations,development trends,future challenges and perspective of SAC for organic reaction are discussed.

Ensemble of single-atom catalysis and defect engineering in Cu_(1)/CeO_(2) nanozymes for tumor therapy

As a class of nanomaterials with natural enzyme-like characteristics, nanozymes have shown their great potential in various applications. Reducible metal oxides featured with defect structures, and single-atom catalysts with isolated metal sites are regarded as two of the most promising nanozymes. However, the strategies to construct highly performed nanozymes by combining these advantages are rarely reported. Herein, we report the coordination-unsaturated single-atomic Cu species supported on sintered CeO_(2), which combines the advantages of defect engineering and single-atom catalysis, exhibiting a largely enhanced peroxidase(POD)-like activity. The high-temperature calcination induces the transformation of inert Cu_(1)O_(4) species into coordination-unsaturated Cu_(1)O_(3) sites. This novel Cu_(1)O_(3) active sites with an unsaturated coordination work as a new type of defect sites to greatly activate the isolated Cu atoms and accelerate the dissociation of H_(2)O_(2) to form hydroxyl radicals(·OH). The obtained nanozyme with a high POD-like activity possesses low cytotoxicity, showing potential applications for the tumor inhibition in vitro and in vivo.

Recent advance in single-atom catalysis

Single-atom catalysts(SACS) have obtained a great deal of attention in many catalytic fields due to the high atom utilization efficiency and high catalytic activity.Recently,great achievements on S ACs have been made for thermocatalysis,electrocatalysis,and photocatalysis which play an important role in obtaining value-added products.However,it remains a great challenge to fabricate S ACs with high metal loading and investigate their reaction mechanisms.Therefore,it is necessary to highlight the recent development of S ACs in these fields to guide future research.In this review,we overviewed the thermocatalysis applications of SACs in CO oxidation,preferential oxidation of CO,water-gas shift reaction,methane conversion,methanol steam reforming,aqueous-phase reforming of methanol,hydrogenation of alkynes and dienes,hydrogenation of CO,and hydrogenation of substituted nitroarenes.Moreover,the oxygen reduction reaction(ORR),hydrogen evolution reaction(HER),oxygen evolution reaction(OER),CO2 reduction reaction(CO2 RR),and N2 reduction reaction(N2 RR) for photocatalytic and electrocatalytic fields were also overviewed.Lastly,the opportunities and challenges of SACs were pointed out.

Cell-free biocatalysis coupled with photo-catalysis and electro-catalysis: Efficient CO_(2)-to-chemical conversion

The increasing atmospheric carbon dioxide (CO_(2)) concentration has exposed a series of crises in the earth's ecological environment.How to effectively fix and convert carbon dioxide into products with added value has attracted the attention of many researchers.Cell-free enzyme catalytic system coupled with electrical and light have been a promising attempt in the field of biological carbon fixation in recent years.In this review,the research progresses of photoenzyme catalysis,electroenzyme catalysis and photo-electroenzyme catalysis for converting carbon dioxide into chemical products in cell-free systems are systematically summarized.We focus on reviewing and comparing various coupling methods and principles of photoenzyme catalysis and electroenzyme catalysis in cell-free systems,especially the materials used in the construction of the coupling system,and analyze and point out the characteristics and possible problems of different coupling methods.Finally,we discuss the major challenges and prospects of coupling physical signals and cell-free enzymatic catalytic systems in the field of CO_(2) fixation,suggesting possible strategies to improve the carbon sequestration capacity of such systems.

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.

Non-Gaussian quantum states generated via quantum catalysis and their statistical properties

A new kind of non-Gaussian quantum catalyzed state is proposed via multiphoton measurements and two-mode squeezing as an input of thermal state.The characteristics of the generated multiphoton catalysis output state depends on the thermal parameter,catalyzed photon number and squeezing parameter.We then analyze the nonclassical properties by examining the photon number distribution,photocount distribution and partial negativity of the Wigner function.Our findings indicate that nonclassicality can be achieved through the implementation of multiphoton catalysis operations and modulated by the thermal parameter,catalyzed photon number and squeezing parameter.

Identification of origin of insulating polymer maneuvered photoredox catalysis

Solid non-conjugated polymers have long been regarded as insulators due to deficiency of delocalizedπelectrons along the molecular chain framework.Up to date,origin of insulating polymer regulated charge transfer has not yet been uncovered.In this work,we unleash the root origin of charge transport capability of insulating polymer in photocatalysis.We ascertain that insulating polymer plays crucial roles in fine tuning of electronic structure of transition metal chalcogenides(TMCs),which mainly include altering surface electron density of TMCs for accelerating charge transport kinetics,triggering the generation of defect over TMCs for prolonging carrier lifetime,and acting as hole-trapping mediator for retarding charge recombination.These synergistic roles contribute to the charge transfer of insulating polymer.Our work opens a new vista of utilizing solid insulating polymers for maneuvering charge transfer toward solar energy conversion.

Ordered mesoporous materials for water pollution treatment:Adsorption and catalysis

To meet the growing emission of water contaminants,the development of new materials that enhance the efficiency of the water treatment system is urgent.Ordered mesoporous materials provide opportunities in environmental processing applications due to their exceptionally high surface areas,large pore sizes,and enough pore volumes.These properties might enhance the performance of materials concerning adsorption/catalysis capability,durability,and stability.In this review,we enumerate the ordered mesoporous materials as adsorbents/catalysts and their modifications in water pollution treatment from the past decade,including heavy metals(Hg^(2+),Pb^(2+),Cd^(2+),Cr^(6+),etc.),toxic anions(nitrate,phosphate,fluoride,etc.),and organic contaminants(organic dyes,antibiotics,etc.).These contributions demonstrate a deep understanding of the synergistic effect between the incorporated framework and homogeneous active centers.Besides,the challenges and perspectives of the future developments of ordered mesoporous materials in wastewater treatment are proposed.This work provides a theoretical basis and complete summary for the application of ordered mesoporous materials in the removal of contaminants from aqueous solutions.

Structure design and electrochemical properties of carbon-based single atom catalysts in energy catalysis:A review

Single atom catalysts(SACs) possessing regulated electronic structure, high atom utilization, and superior catalytic efficiency have been studied in almost all fields in recent years. Carbon-based supporting SACs are becoming popular materials because of their low cost, high electron conductivity, and controllable surface property. At the stage of catalysts preparation, the rational design of active sites is necessary for the substantial improvement of activity of catalysts. To date, the reported design strategies are mainly about synthesis mechanism and synthetic method. The level of understanding of design strategies of carbon-based single atom catalysts is requiring deep to be paved. The design strategies about manufacturing defects and coordination modulation of catalysts are presented. The design strategies are easy to carry out in the process of drawing up preparation routes. The components of carbon-based SACs can be divided into two parts: active site and carbon skeleton. In this review, the manufacture of defects and coordination modulation of two parts are introduced, respectively. The structure features and design strategies from the active sites and carbon skeletons to the overall catalysts are deeply discussed.Then, the structural design of different nano-carbon SACs is introduced systematically. The characterization of active site and carbon skeleton and the detailed mechanism of reaction process are summarized and analyzed. Next, the applications in the field of electrocatalysis for oxygen conversion and hydrogen conversion are illustrated. The relationships between the superior performance and the structure of active sites or carbon skeletons are discussed. Finally, the conclusion of this review and prospects on the abundant space for further promotion in broader fields are depicted. This review highlights the design and preparation thoughts from the parts to the whole. The detailed and systematic discussion will provide useful guidance for design of SACs for readers.

Why the abnormal phenomena of D-band center theory exist?A new BASED theory for surface catalysis and chemistry

Since the D-band center theory was proposed,it has been widely used in the fields of surface chemistry by almost all researchers,due to its easy understanding,convenient operation and relative accuracy.However,with the continuous development of material systems and modification strategies,researchers have gradually found that D-band center theory is usually effective for large metal particle systems,but for small metal particle systems or semiconductors,such as single atom systems,the opposite conclusion to the D-band center theory is often obtained.To solve the issue above,here we propose a bonding and anti-bonding orbitals stable electron intensity difference(BASED)theory for surface chemistry.The newly-proposed BASED theory can not only successfully explain the abnormal phenomena of D-band center theory,but also exhibits a higher accuracy for prediction of adsorption energy and bond length of intermediates on active sites.Importantly,a new phenomenon of the spin transition state in the adsorption process is observed based on the BASED theory,where the active center atom usually yields an unstable high spin transition state to enhance its adsorption capability in the adsorption process of intermediates when their distance is about 2.5Å.In short,the BASED theory can be considered as a general principle to understand catalytic mechanism of intermediates on surfaces.

Cobalt phthalocyanine promoted copper catalysts toward enhanced electro reduction of CO_(2)to C_(2):Synergistic catalysis or tandem catalysis?

The activity and selectivity of electrocatalytic CO_(2)reduction reaction(CO_(2)RR)to C_(2)products on metal catalysts can be regulated by molecular surfactants.However,the mechanism behind it remains elusive and debatable.Herein,copper nanowires(Cu NWs)were fabricated and decorated with cobalt phthalocyanine(CoPc).The electronic interaction between the Cu NWs,CoPc,CO_(2) and CO_(2)RR intermediates were explored by density functional theory(DFT)calculations.It was found that the selectivity and activity of CO_(2)RR towards C_(2)products on Cu NWs were considerably enhanced from 35.2%to 69.9%by surface decoration of CoPc.DFT calculations revealed that CO_(2)RR can proceed in the interphase between Cu substrate and CoPc,and the CO_(2)RR intermediates could synergistically bond with both Cu and Co metal centre in CuNWs-CoPc,which favours the adsorption of CO_(2),CO and CO_(2)RR intermediates,thus reducing the free energy for CO-COcoupling towards C_(2)products.The synergistic interaction was further extended to phthalocyanine(Pc)and other metal phthalocyanine derivatives(MPc),where a relatively weaker synergistic interaction of COintermediates with MPc and Cu substrate and only a slight enhancement of CO_(2)RR towards C_(2) products were observed.This study demonstrates a synergistic catalysis pathway for CO_(2)RR,a novel perspective in interpreting the role of CoPc in enhancing the activity and selectivity of CO_(2)RR on Cu NWs,in contrast to the conventional tandem catalysis mechanism.

Surface Organometallic Chemistry for Single-site Catalysis and Single-atom Catalysis

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.