Activity Of Single-atom Catalysts Research Articles

Selective Hydrogenation on a Highly Active Single-Atom Catalyst of Palladium Dispersed on Ceria Nanorods by Defect Engineering.

Single-atom catalysis represents a new frontier that integrates the merits of homogeneous and heterogeneous catalysis to afford exceptional atom efficiency, activity, and selectivity for a range of catalytic systems. Herein we describe a simple defect engineering strategy to construct an atomically dispersed palladium catalyst (Pdδ+, 0 < δ < 2) by anchoring the palladium atoms on oxygen vacancies created in CeO2 nanorods. This was confirmed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurement. The as-prepared catalyst showed exceptional catalytic performance in the hydrogenation of styrene (99% conversion, TOF of 2410 h-1), cinnamaldehyde (99% conversion, 99% selectivity, TOF of 968 h-1), as well as oxidation of triethoxysilane (99% conversion, 79 selectivity, TOF of 10 000 h-1). This single-atom palladium catalyst can be reused at least five times with negligible activity decay. The palladium atoms retained their dispersion on the support at the atomic level after thermal stability testing in Ar at 773 K. Most importantly, this synthetic method can be scaled up while maintaining catalytic performance. We anticipate that this method will expedite access to single-atom catalysts with high activity and excellent resistance to sintering, significantly impacting the performance of this class of catalysts.

Engineering the Low Coordinated Pt Single Atom to Achieve the Superior Electrocatalytic Performance toward Oxygen Reduction.

Configuring metal single-atom catalysts (SACs) with high electrocatalytic activity and stability is one efficient strategy in achieving the cost-competitive catalyst for fuel cells' applications. Herein, the atomic layer deposition (ALD) strategy for synthesis of Pt SACs on the metal-organic framework (MOF)-derived N-doped carbon (NC) is proposed. Through adjusting the ALD exposure time of the Pt precursor, the size-controlled Pt catalysts, from Pt single atoms to subclusters and nanoparticles, are prepared on MOF-NC support. X-ray absorption fine structure spectra determine the increased electron vacancy in Pt SACs and indicate the Pt-N coordination in the as-prepared Pt SACs. Benefiting from the low-coordination environment and anchoring interaction between Pt atoms and nitrogen-doping sites from MOF-NC support, the Pt SACs deliver an enhanced activity and stability with 6.5 times higher mass activity than that of Pt nanoparticle catalysts in boosting the oxygen reduction reaction (ORR). Density functional theory calculations indicate that Pt single atoms prefer to be anchored by the pyridinic N-doped carbon sites. Importantly, it is revealed that the electronic structure of Pt SAs can be adjusted by adsorption of hydroxyl and oxygen, which greatly lowers free energy change for the rate-determining step and enhances the activity of Pt SACs toward the ORR.

Recent advances in single-atom catalysts for CO oxidation

ABSTRACT Single-atom catalysts (SACs) have received boosting attention due to their high atom utilization and incredible activities in a wide range of catalytic processes. Numerous SACs have been investigated for CO oxidation both experimentally and theoretically, including noble-metal catalysts (Pt, Au, Pd, etc.) and non-noble-metal catalysts (Fe, Co, Ni, etc.), in which the atomically dispersed metal atoms are anchored on supports via strong metal-support interactions. This unique structure of SACs contributes to activating the adsorbed CO and O2 and stabilizing the intermediates. Electron transfer between the metal atom and the support plays an important role in tuning the electronic structure, which can greatly influence the activity, selectivity, and stability of SACs. In this review, the design principles and synthesis methods of SACs for CO oxidation are discussed with emphasis on single-atom active sites and metal-support interactions. Four CO oxidation mechanisms over SACs are evaluated. Moreover, the challenges and future research directions for SAC-catalyzed CO oxidation are outlined.

Hydrodechlorination and deep hydrogenation on single-palladium-atom-based heterogeneous catalysts

Halophenols are widely present in wastewater and groundwater, which threaten global water safety and human health. Importantly, hydrodechlorination (HDC) via heterogeneous catalysis is efficient and environmentally friendly. To further improve the catalytic performance and atomic economy, Pd-based single atom catalysts (SACs) were prepared through electrostatic attraction and used in the catalytic HDC for the first time. By substituting different defect-rich supports, the coordination environments of atomically dispersed Pd-atoms were mediated with different catalytic performance. The results demonstrate that the Pd/CeO2 SACs have the highest HDC activity and remain stable during cycle tests. The moderately intensive metal-support interactions (MIMSI) effect was identified as a critical factor for the higher activity of SACs, unlike the traditional strong metal-support interaction effect. The HDC process on Pd SACs was further investigated by combining kinetic experiments with density functional theory calculations, which facilitated the recognition of HDC intermediates and an understanding of the reaction mechanism.

Recent Developments on the Single Atom Supported at 2D Materials Beyond Graphene as Catalysts

Two dimensional (2D) layered materials have proven to be crucial platforms for anchored individual and isolated metal atoms acting as active single atom catalysts (SACs). Therefore, an accurate loc...

Cover Feature: Controlled One‐pot Synthesis of Nickel Single Atoms Embedded in Carbon Nanotube and Graphene Supports with High Loading (ChemNanoMat 7/2020)

The cover feature shows the controllable synthesis of nickel single atoms embedded in nitrogen-doped carbon nanotubes (NiSA−N-CNT) and graphene (NiSA−N-G). Addition of citric acid introduces amorphous carbon on the layered g-C3N4-Ni and, after annealing under the same conditions, NiSA−N-G was obtained instead of NiSA−N-CNT. NiSA−N-G with Ni atomic loading of ∼6 wt% catalysts shows better activity and stability for the CO2 reduction reaction than NiSA−N-CNT with Ni loading of ∼15 wt% due to the more open and exposed active sites in NiSA−N-G. The exposed active sites are most important for the electrocatalytic activity of single atom catalysts. More information can be found in the Full Paper by Chang Liu, Qianfan Zhang, San Ping Jiang et al.

Metal-Organic Framework-Based Catalysts with Single Metal Sites.

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Descriptor-Based Design Principle for Two-Dimensional Single-Atom Catalysts: Carbon Dioxide Electroreduction.

Single-atom catalysis has recently emerged as a promising approach for catalyzing the carbon dioxide reduction reaction (CO2RR). In this study, we present a principle for designing active single-atom catalysts (SACs) for CO2RR. We systematically examine totally 24 transition metals supported by a graphitic carbon nitride (g-CN) monolayer and find that their catalytic activities are highly correlated with the adsorption free energies of two intermediate species (OH and OCH). We then identify two important intrinsic descriptors, namely, the number of electrons in the outmost d-shell and the enthalpy of vaporization of the transition metal. Test calculations on transition metals supported by a C2N monolayer indicate that both descriptors are quite universal for SACs of CO2RR. Based on these results, we show that Ni@g-CN, Cu@g-CN, and Co@C2N are promising SACs for CO2RR. This study offers an effective principle for designing highly active SACs for CO2RR on the basis of intrinsic properties of transition metals.

Molecular Design of Single‐Atom Catalysts for Oxygen Reduction Reaction

AbstractFuel cells are highly attractive for direct chemical‐to‐electrical energy conversion and represent the ultimate mobile power supply solution. However, presently, fuel cells are limited by the sluggish kinetics of the cathodic oxygen reduction reaction (ORR), which requires the use of Pt as a catalyst, thus significantly increasing the overall cost of the cells. Recently, nonprecious metal single‐atom catalysts (SACs) with high ORR activity under both acidic and alkaline conditions have been recognized as promising cost‐effective alternatives to replace Pt in fuel cells. Considerable efforts have been devoted to further improving the ORR activity of SACs, including tailoring the coordination structure of the metal centers, enriching the concentration of the metal centers, and engineering the electronic structure and porosity of the substrate. Herein, a brief introduction to fuel cells and fundamentals of the ORR parameters of SACs and the origin of their high activity is provided, followed by a detailed review of the recently developed strategies used to optimize the ORR activity of SACs in both rotating disk electrode and membrane electrode assembly tests. Remarks and perspectives on the remaining challenges and future directions of SACs for the development of commercial fuel cells are also presented.

A perspective on oxide-supported single-atom catalysts.

Single-atom catalysts (SACs) can not only maximize the metal atom utilization efficiency, but also show drastically improved catalytic performance for various important catalytic processes. Insights into the working principles of SACs provide rational guidance to design and prepare advanced catalysts. Many factors have been claimed to affect the performance of SACs, which makes it very challenging to clarify the correlation between the catalytic performance and physicochemical characteristics of SACs. Oxide-supported SACs are one of the most extensively explored systems. In this minireview, some latest developments on the determining factors of the stability, activity and selectivity of SACs on oxide supports are overviewed. Discussed also are the reaction mechanisms for different systems and methods that are employed to correlate the properties with the catalyst structures at the atomic level. In particular, a recently proposed surface free energy approach is introduced to fabricate well-defined modelled SACs that may help address some key issues in the development of SACs in the future.

Geometric structures, electronic characteristics, stabilities, catalytic activities, and descriptors of graphene-based single-atom catalysts

Abstract Single-atom catalysts (SACs) have been a research hotspot due to their high catalytic activity, selectivity, and atomic utilization rates. However, the theoretical research of SACs is relatively fragmented, which restricts further understanding of SAC stability and activity. To address this issue, we report our analysis of the geometric structures, electronic characteristics, stabilities, catalytic activities, and descriptors of 132 graphene-based single-atom catalysts (M/GS) obtained from density functional theory calculations. Based on the calculated formation and binding energies, a stability map of M/GS was established to guide catalyst synthesis. The effects of metal atoms and support on the charge of metal atoms are discussed. The catalytic activities of M/GS in both nitrogen and oxygen reduction reactions are predicted based on the calculated magnetic moment and the adsorption energy. Combined with the electronegativity and d-band center, a two-dimensional descriptor is proposed to predict the O adsorption energy on M/GS. More importantly, this theoretical study provides predictive guidance for the preparation and rational design of highly stable and active single-atom catalysts using nitrogen doping on graphene.

Enhanced Activity of C2N-Supported Single Co Atom Catalyst by Single Atom Promoter.

The remarkable chemical activity of metal single-atom catalysts (SACs) lies in their unique electronic states associated with the low-coordination nature of single-atom sites. Yet, electronic state manipulation normally requires direct contact with other atoms, which inevitably changes the low-coordination environment. Herein, we found by first-principle calculations that the activity of a Co SAC for HCOOH dehydrogenation is appreciably enhanced via electronic state manipulation by a noncontact single atom promoter. A Co atom and a Sn/Ge/Pb atom are anchored in the same cavity of a graphitic C2N monolayer. Surprisingly, the nonbonded promoter makes two far splitting spin states of Co almost degenerate via charge redistribution of C2N support. Further, the high-spin Co gives a remarkably low reaction barrier comparable to Pt or Pd catalysts. Our results demonstrate that the activity of a SAC can be tuned via a noncontact promoter, casting new insights into electronic state modulation of SACs on graphene-like support.

Coordination tailoring towards efficient single-atom catalysts for N2 fixation: A case study of iron-nitrogen-carbon (Fe@N-C) systems

Abstract N2 fixation using electrochemical methods offers a promising strategy for NH3 production. However, it remains a grand challenge to efficiently produce NH3 in a large scale due to the low activity and poor selectivity of the current electrocatalysts. Herein, by means of density functional theory (DFT) computations, we systematically explored the great potential of two-dimensional (2D) single-atom catalysts (SACs), namely carbon-based iron-nitrogen systems (Fe@Nx, x = 0–4), for N2 electroreduction. Significantly, the catalytic activity and selectivity of Fe@Nx systems are influenced by the coordination environment of single Fe atom. Among all the proposed configurations, Fe@N2-opp, in which two N atoms are located at the opposite coordination sites of single Fe atom, exhibits the highest activity and selectivity for N2 electroreduction, with a limiting potential of −0.63 V. By examining the scaling relations for the single vacancy and double vacancy-based SACs, we constructed a volcano plot to describe the catalytic activity of carbon-based SACs and proposed an optimal nitrogen adsorption energy for N2 electroreduction. This work calls for more attention to the coordination effect to the single-atom active site, and helps provide guidance for further developing more effective carbon-based SACs for N2 fixation.

Building Up a Picture of the Electrocatalytic Nitrogen Reduction Activity of Transition Metal Single-Atom Catalysts.

The lack of chemical understanding and efficient catalysts impedes the development of electrocatalytic nitrogen reduction reaction (eNRR) for ammonia production. In this work, we employed density functional theory calculations to build up a picture (activity trends, electronic origins, and design strategies) of single-atom catalysts (SACs) supported on nitrogen-doped carbons as eNRR electrocatalysts. To construct such a picture, this work presents systematic studies of the eNRR activity of SACs covering 20 different transition metal (TM) centers coordinated by nitrogen atoms contained in three types of nitrogen-doped carbon substrates, which gives 60 SACs. Our study shows that the intrinsic activity trends could be established on the basis of the nitrogen adatom adsorption energy (Δ EN*). Furthermore, the influence of metal and support (ligands) on Δ EN* proved to be related to the bonding/antibonding orbital population and regulating the scaling relations for adsorption of intermediates, respectively. Accordingly, a two-step strategy is proposed for improving the eNNR activity of TM-SACs, which involves the following: (i) selection of the most promising family of SACs (g-C3N4 supported SACs as predicted in this work) and (ii) further improvement of the activity of the best candidate in the aforementioned family via tuning the adsorption strength of the key intermediates. Also, the stability of N-doped carbon supports and their selectivity in comparison to the competing hydrogen evolution need to be taken into consideration for screening the durable and efficient candidates. Finally, an effective strategy for designing active, stable, and selective SACs based on the mechanistic insights is elaborated to guide future eNRR studies.

Nanowrinkled Carbon Aerogels Embedded with FeN x Sites as Effective Oxygen Electrodes for Rechargeable Zinc-Air Battery.

Rational design of single-metal atom sites in carbon substrates by a flexible strategy is highly desired for the preparation of high-performance catalysts for metal-air batteries. In this study, biomass hydrogel reactors are utilized as structural templates to prepare carbon aerogels embedded with single iron atoms by controlled pyrolysis. The tortuous and interlaced hydrogel chains lead to the formation of abundant nanowrinkles in the porous carbon aerogels, and single iron atoms are dispersed and stabilized within the defective carbon skeletons. X-ray absorption spectroscopy measurements indicate that the iron centers are mostly involved in the coordination structure of FeN4, with a minor fraction (ca. 1/5) in the form of FeN3C. First-principles calculations show that the FeNx sites in the Stone-Wales configurations induced by the nanowrinkles of the hierarchically porous carbon aerogels show a much lower free energy than the normal counterparts. The resulting iron and nitrogen-codoped carbon aerogels exhibit excellent and reversible oxygen electrocatalytic activity, and can be used as bifunctional cathode catalysts in rechargeable Zn-air batteries, with a performance even better than that based on commercial Pt/C and RuO2 catalysts. Results from this study highlight the significance of structural distortions of the metal sites in carbon matrices in the design and engineering of highly active single-atom catalysts.

Single-atom catalyst: a rising star for green synthesis of fine chemicals

Abstract The green synthesis of fine chemicals calls for a new generation of efficient and robust catalysts. Single-atom catalysts (SACs), in which all metal species are atomically dispersed on a solid support, and which often consist of well-defined mononuclear active sites, are expected to bridge homogeneous and heterogeneous catalysts for liquid-phase organic transformations. This review summarizes major advances in the SAC-catalysed green synthesis of fine chemicals in the past several years, with a focus on the catalytic activity, selectivity and reusability of SACs in various organic reactions. The relationship between catalytic performance and the active site structure is discussed in terms of the valence state, coordination environment and anchoring chemistry of single atoms to the support, in an effort to guide the rational design of SACs in this special area, which has traditionally been dominated by homogeneous catalysis. Finally, the challenges remaining in this research area are discussed and possible future research directions are proposed.

Single‐Atom Catalysts for the Hydrogen Evolution Reaction

AbstractElectrocatalytic hydrogen evolution reaction (HER) is recognized as a promising way to generate clean hydrogen energy. However, its large‐scale application requires the development of cheap and efficient catalysts. An increasing number of reports have been published lately on the HER over single atom catalysts (SACs) owing to the high catalytic activity, stability and maximum atom utilization of SACs, of which the final point endows noble‐metal catalysts with low costs. In this Review, we highlight the electrochemical applications of all reported SACs in the HER, with a particular focus on the discussion of their catalytic properties and mechanisms at the atomic level. The main advantages and challenges of SACs are also summarized. This Review aims to provide a comprehensive understanding of current and future applications of SACs in the HER.

Recent advances in the precise control of isolated single-site catalysts by chemical methods

Abstract The search for constructing high-performance catalysts is an unfailing topic in chemical fields. Recently, we have witnessed many breakthroughs in the synthesis of single-atom catalysts (SACs) and their applications in catalytic systems. They have shown excellent activity, selectivity, stability, efficient atom utilization and can serve as an efficient bridge between homogeneous and heterogenous catalysis. Currently, most SACs are synthesized via a bottom-up strategy; however, drawbacks such as the difficulty in accessing high mass activity and controlling homogeneous coordination environments are inevitably encountered, restricting their potential use in the industrial area. In this regard, a novel top-down strategy has been recently developed to fabricate SACs to address these practical issues. The metal loading can be increased to 5% and the coordination environments can also be precisely controlled. This review highlights approaches to the chemical synthesis of SACs towards diverse chemical reactions, especially the recent advances in improving the mass activity and well-defined local structures of SACs. Also, challenges and opportunities for the SACs will be discussed in the later part.

RETRACTED ARTICLE: A universal principle for a rational design of single-atom electrocatalysts

Developing highly active single-atom catalysts for electrochemical reactions is a key to future renewable energy technology. Here we present a universal design principle to evaluate the activity of graphene-based single-atom catalysts towards the oxygen reduction, oxygen evolution and hydrogen evolution reactions. Our results indicate that the catalytic activity of single-atom catalysts is highly correlated with the local environment of the metal centre, namely its coordination number and electronegativity and the electronegativity of the nearest neighbour atoms, validated by available experimental data. More importantly, we reveal that this design principle can be extended to metal–macrocycle complexes. The principle not only offers a strategy to design highly active nonprecious metal single-atom catalysts with specific active centres, for example, Fe-pyridine/pyrrole-N4 for the oxygen reduction reaction; Co-pyrrole-N4 for the oxygen evolution reaction; and Mn-pyrrole-N4 for the hydrogen evolution reaction to replace precious Pt/Ir/Ru-based catalysts, but also suggests that macrocyclic metal complexes could be used as an alternative to graphene-based single-atom catalysts.

Understanding the Activation and Stabilization of Electrocatalytic Single Atom Catalysts

The discovery of single atom catalysts (SACs) paths new frontier in heterogeneous catalysis. Recently, increasing interests have been focused on the construction of atomically-dispersed transition metal centers in nitrogen doped carbon through supports-assisted pyrolytic synthesis for various applications, such as electrocatalysis, CO oxidation, and organic synthesis. However, the evolution, activation and stabilization of SAC in synthesis and catalysis, which is critical for its applications, still remain unexplored yet. Here, we examined the critical importance of supports and anions for the single-atom transition-metal-sites(Co, Fe, and Ni) construction and modification, combing synchrotron X-ray absorption studies, and the electron microscopic characterizations, and electrochemical measurements. The supports and anions are essential parts which are involved in the pyrolytic synthesis. However, both of them are often excluded from construction of catalytic sites in SACs. The formation of atomic metal sites is often accompanies with the evolution of metallic metal/metal carbide nanoparticles encased by graphite layers. Here, we demonstrate a versatile anions-regulated selective generation of metal sites in carbon and demonstrate the metal-sites dependence catalytic performances for oxygen reduction reaction (ORR) and hydrogen evolution reaction. Besides achieving the controlled evolution of atomically-dispersed metal sites, we also demonstrate the roles of supports on the structural reconstruction and activation of SAC and the functional relationship between the electrocatalytic activity and electrochemical stability in SAC. Different from the previous models of integration of fragments on carbon, we initially reveal the supports-assisted structural reconstruction on the evolution of electrocatalytic SAC. More importantly, the enhanced catalytic activity and selectivity are found to result from more unoccupied 3d orbitals at low energy positions for single-atom-dispersed metal sites within non-planar coordination. This is mainly due to the facilitated interactions between metal sites and adsorbates. More importantly, such activity descriptors can further modified through controlling the atomic coordination geometry or structures. Interestingly, decreasing the atomic coordination numbers enhances the ORR activity, however, also damages the electrochemical stability of atomically dispersed metal sites under potentials, demonstrating the coordination-structure sensitivity of its electrochemical stability. Therefore, we present a functional relationship between the catalytic activity and electrochemical stability of SAC for electrocatalysis. Our studies offer molecular insights on the evolution of SAC, pathing new directions to SAC engineering with improved activity and stability for various reactions, such as ORR, water splitting, and CO2 reduction. Figure 1