Site surgery in single atom catalysis: machine learning assisted precise regulation of coordination environment and decoding of structure-activity relationships

ConspectusSingle atom catalysis has garnered widespread attention in the past decade. In general, single atom catalysis refers to a catalytic process involving the participation of single atom sites. A single atom site refers to an active site with only one metal atom playing the main role in the catalysis process. This metal atom is usually coordinated by other atoms in the active site and anchored on the support. Heterogeneous catalysts in which all active sites are single atom sites are referred to as single atom site catalysts (SASCs). Owing to their distinctive active site architecture, single atom catalysis has shown ultrahigh atomic utilization efficiency and unique catalytic activity and selectivity in many systems such as thermal catalysis, electrocatalysis, and environmental catalysis.However, the preparation of SASCs poses significant challenges as metal species tend to sinter and agglomerate during high-temperature treatment. In the past decade, we have challenged the controllable preparation of SASCs based on more than ten years of experience in nanomaterial synthesis. Several representative SASC preparation strategies have been proposed by our group, such as pyrolysis of MOFs with metal ions doped in the skeleton, M@ZIF-8 with a metal precursor fixed by the host-guest interaction, and M/N-rich support precursors. Additionally, "top-down" strategies starting from metal nanoparticles have been established. Based on these synthetic strategies, a systematic SASCs toolbox has been successfully built.In addition to the SASCs, we have expanded the toolbox to include the dual-atom-site catalysts (DASCs) (including active sites with dual active metal atoms or including dual kinds of single atom sites) and nanosingle-atom-site catalysts (NSASCs) (catalysts with both single atom sites and nanoparticle/cluster sites). With the help of a variety of "tools" in the toolbox, single atom catalysis has shown its application value in many important processes such as bulk chemical catalysis, fine chemical catalysis, energy catalysis, and environmental catalysis. Based on laboratory-scale research, we further explored the feasibility of single atom catalysis in industrial catalysis and successfully applied single atom catalysis to industrial-level automobile exhaust purification, which will soon achieve commercialization.This Account provides a concise overview of the evolution of single atom catalysis, summarizes the preparation strategies for SASCs pioneered by our group over the past decade, and highlights the SASCs toolbox developed through these strategies. We also showcase the practical applications of the SASCs toolbox in key catalytic fields, highlight our progress in advancing the industrialization of single atom catalysis (particularly for automobile exhaust purification), and discuss the future prospects of single atom catalysis in industrial applications.

Exploring the energetics of Suzuki cross-coupling reaction: A computational study of palladium and nickel catalysts

Single Atom Catalysis (SAC) is an expanding field of heterogeneous catalysis in which single metallic atoms embedded in different materials catalyze a chemical reaction, but these new catalytic materials still lack fundamental understanding when used in electrochemical environments. Recent characterizations of non-noble metals like Fe deposited on N-doped graphitic materials have evidenced two types of Fe-N4 fourfold coordination, either of pyridine type or of porphyrin type. Here, we study these defects embedded in a graphene sheet and immersed in an explicit aqueous medium at the quantum level. While the Fe-pyridine SAC model is clear cut and widely studied, it is not the case for the Fe-porphyrin SAC that remains ill-defined, because of the necessary embedding of odd-membered rings in graphene. We first propose an atomistic model for the Fe-porphyrin SAC. Using spin-polarized ab initio molecular dynamics, we show that both Fe SACs spontaneously adsorb two interfacial water molecules from the solvent on opposite sides. Interestingly, we unveil a different catalytic reactivity of the two hydrated SAC motives: while the Fe-porphyrin defect eventually dissociates an adsorbed water molecule under a moderate external electric field, the Fe-pyridine defect does not convey water dissociation.

Single-atom catalysis (SACs) has attracted considerable attention because of its distinctive structural characteristics and strong potential for catalytic innovation. The performance of atomically dispersed catalysts depends on the local microenvironment surrounding the single atoms and neighboring active species. Moreover, the local microenvironment constrains the electronic structure and geometry of the catalyst, thereby determining the efficiency of the energy-conversion devices. However, significant challenges persist in accurately designing the electronic coordination environments and geometric configurations of catalysts at the sub-nanometer scale, which limits effective regulation of the catalytic microenvironment and improvement of catalytic activity. This review provides a comprehensive overview of the cutting-edge progress in enhancing energy conversion efficiency via micro-environmental regulation of single-atom catalysts. Typical techniques for regulating local coordination microenvironments are discussed, including heterogeneous atom anchoring, atomic molecular bridging, defect engineering, spatial confinement, and construction of local microinterfaces. Characterization techniques for probing microenvironments, such as X-ray absorption fine structure spectroscopy, are also summarized. The optimization of single-atom efficiency via local microenvironment regulation has been demonstrated for the HER, OER, ORR, CO2RR, and NRR. The discussion concludes with an assessment of application prospects and remaining challenges associated with engineering-based microenvironment control, aiming to guide future developments in single-atom precision catalysis and energy conversion devices.

Theoretical study on NO oxidation using single metal atom catalysis embedded graphene with N4 vacancy

Recently, the missing link between homogeneous and heterogeneous catalysis has been found and it was named single-atom catalysis (SAC). However, the SAC field still faces important challenges, one of which is controlling the bonding/coordination between the single atoms and the support in order to compensate for the increase in surface energy when the particle size is reduced due to atomic dispersion. Excellent candidates to meet this requirement are carbon nitride (CN)-based materials. Metal atoms can be firmly trapped in nitrogen-rich coordination sites in CN materials, which makes them a unique class of hosts for preparing single-atom catalysts (SACs). As one of the most promising two-dimensional supports to stabilize isolated metal atoms, CN materials have been increasingly employed for preparing SACs. Herein, we will cover the most recent advances in single-atoms supported by CN materials. In this review, the most important characterization techniques and the challenges faced in this topic will be discussed, and the commonly employed synthetic methods will be delineated for different CN materials. Finally, the catalytic performance of SACs based on carbon nitrides will be reviewed with a special focus on their photocatalytic applications. In particular, we will prove CN as a non-innocent support. The relationship between single-atoms and carbon nitride supports is two-way, where the single-atoms can change the electronic properties of the CN support, while the electronic features of the CN matrix can tune the catalytic activity of the single sites in photocatalytic reactions. Finally, we highlight the frontiers in the field, including analytical method development, truly controlled synthetic methods, allowing the fine control of loading and multi-element synthesis, and how understanding the two-way exchange behind single-atoms and CN supports can push this topic to the next level.

Single atom catalysis (SAC) is a recent discipline of heterogeneous catalysis for which a single atom on a surface is able to carry out various catalytic reactions. A kind of revolution in heterogeneous catalysis by metals for which it was assumed that specific sites or defects of a nanoparticle were necessary to activate substrates in catalytic reactions. In another extreme of the spectrum, surface organometallic chemistry (SOMC), and, by extension, surface organometallic catalysis (SOMCat), have demonstrated that single atoms on a surface, but this time with specific ligands, could lead to a more predictive approach in heterogeneous catalysis. The predictive character of SOMCat was just the result of intuitive mechanisms derived from the elementary steps of molecular chemistry. This review article will compare the aspects of single atom catalysis and surface organometallic catalysis by considering several specific catalytic reactions, some of which exist for both fields, whereas others might see mutual overlap in the future. After a definition of both domains, a detailed approach of the methods, mostly modeling and spectroscopy, will be followed by a detailed analysis of catalytic reactions: hydrogenation, dehydrogenation, hydrogenolysis, oxidative dehydrogenation, alkane and cycloalkane metathesis, methane activation, metathetic oxidation, CO2 activation to cyclic carbonates, imine metathesis, and selective catalytic reduction (SCR) reactions. A prospective resulting from present knowledge is showing the emergence of a new discipline from the overlap between the two areas.

Coordinating single-atom catalysts on two-dimensional nanomaterials: A paradigm towards bolstered photocatalytic energy conversion

Catalytic performance of La@Mg12O12 as a single atom catalyst toward nitrogen reduction reaction for ammonia generation.

In the past decade, isolated single atoms have been successfully dispersed on various substrates, with their potential applications being intensively investigated in different reactions. While the essential target of research in single-atom catalysis is the precise synthesis of stable single-atom catalysts (SACs) with clear configurations and impressive catalytic performance, theoretical investigations have also played important roles in identifying active sites, revealing catalytic mechanisms, and establishing structure–activity relationships. Nevertheless, special attention should still be paid in theoretical works to the particularity of SACs. In this Perspective, we will summarize the theoretical progress made on the understanding of the rich phenomena in single-atom catalysis. We focus on the determination of local structures of SACs via comparison between experiments and simulations, the discovery of distinctive catalytic mechanisms induced by multiadsorption, synergetic effects, and dynamic evolutions, to name a few, the proposal of criteria for theoretically designing SACs, and the extension of original concepts of single-atom catalysis. We hope that this Perspective will inspire more in-depth thinking on future theoretical studies of SACs.

Water activation by single Pt atoms supported on a Cu2O thin film

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

Simply yet powerfully, single-atom catalysts (SACs) with atomically dispersed metal active centers on supports have received a growing interest in a wide range of catalytic reactions. As a specific example, SACs have exhibited distinctive performances in CO2 chemical conversions. The unique structures of SACs are appealing for adsorptive activation of CO2 molecules, transfer of intermediates from support to active metal sites, and production of desirable products in CO2 conversion. In this Account, we have exemplified our recent endeavors in the development of SACs toward CO2 conversions in thermal catalysis and electrocatalysis. In terms of the support not only stabilizing but also working collaboratively with the single active sites, the proper choice of support is of great importance for its stability, activity, and selectivity in single-atom catalysis. Three distinctive strategies for SAC architectures-lattice-matched oxide supported, heteroatom-doped carbon anchored, and mimetic ligand chelated-are intensively discussed from the perspective of support design for SACs in different reaction environments. To achieve a high-temperature thermal reduction of CO2 to CO, TiO2 (rutile), lattice-matched to the IrO2 active site, was chosen as a support to realize the thermal stability of Ir1/TiO2 SAC, and it shows great capability toward CO2 conversion and excellent selectivity to CO due to the effective block of the over-reduction of CO2 to methane over single Ir active sites. In the electrochemical reduction of CO2 at low temperature, sulfur co-doped N-graphene was developed to achieve unique d9-Ni single atoms on the conductive graphene support, by which not only were the atomic Ni active sites trapped into the matrix of graphene for its stabilization, but also the modulation of electronic configuration of mononuclear Ni centers promoted the CO2 activation through facile electron transfer with an improved electroreduction activity. Inspired by the Ir mononuclear homogeneous catalysts in CO2 hydrogenation to formate, porous organic polymers (POPs) functionalized with a reticular aminopyridine group were purposely fabricated to mimic the homogeneous ligand environment for chelating the Ir single-atom active center, and this quasi-homogeneous Ir1/POP catalyst manifests high efficiency for hydrogenation of CO2 to formate under mild conditions in the liquid phase. Such SACs are of paramount importance for the transformation of CO2, with their coordination environment helping in the activation of CO2. Since the energy barrier for the dissociation of the second C-O bond of CO2 on single-atom sites is very high, these catalysts can give high selectivities toward CO or formate products. Thanks to SACs, the conversion of CO2 has become much easier in various chemical environments.

Summary“Single‐Atom” Catalysis (SAC) is a rapidly emerging field aimed at minimizing the amount of precious metals required to perform important catalytic reactions. Modern heterogeneous catalysts already utilize nanoparticles containing 100s to 1000s of atoms on an inexpensive support, but the dream of SAC is to do the same chemistry with single atoms. The concept is firmly entrenched, and SAC systems have demonstrated activity for a variety of reaction, metal, and support combinations. Nevertheless, the topic remains controversial because it is extremely difficult to characterize a catalyst based on single atom active sites, and even harder to figure out how they work. In our group in Vienna, we study model SAC systems in a highly controlled ultrahigh vacuum environment using a variety of state‐of‐the‐art surface‐science techniques to discover what makes a stable single atom catalyst, the mechanisms underlying their catalytic activity, and the processes leading to their deactivation.

Single atom catalysts (SACs) have recently attracted great attention in heterogeneous catalysis and have been regarded as ideal models for investigating the strong interaction between metal and support. Despite the huge progress over the past decade, the deep understanding on the structure-performance correlation of SACs at a single atom level still remains to be a great challenge. In this study, we demonstrate that the variation in the coordination number of the Pt single atom can significantly promote the propylene selectivity during propyne semihydrogenation (PSH) for the first time. Specifically, the propylene selectivity greatly increases from 65.4% to 94.1% as the coordination number of Pt-O increases from ∼3.4 to ∼5, whereas the variation in the coordination number of Pt-O slightly influences the turnover frequency values of SACs. We anticipate that the present work may deepen the understanding on the structure-performance of SACs and also promote the fundamental research in single atom catalysis.