Constructing and controlling of highly dispersed metallic sites for catalysis

Electrochemical conversion of CO2 into added-value chemicals is an important approach to recycle CO2. Heterogeneous catalysis is widely used in industrial applications because of the possibility of facile separation, which reduces the operating costs, although heterogeneous catalysts often have limited selectivity. In contrast, homogeneous catalysts are very selective although they have limited industrial applications due to their cost, the use of precious metals, and the difficulty in separating and recovering the catalysts. Currently, the research community is trying to combine the properties of homogeneous and heterogeneous catalysts. From the heterogeneous catalyst perspective, research has been focused on creating smaller and dispersed catalyst particles. Single-atom catalysts (SACs), which comprise atoms of metal species dispersed on a solid support, are expected to bridge the homogeneous and heterogeneous catalyst properties.The work described herein explores, by means of density functional simulations, the electrocatalytic CO2 reduction reaction (CO2RR) using several single transition metal atoms anchored in 2D graphitic carbon nitride (g-C3N4),1 focusing on the group XI transition metals since they include Cu, the only transition metal capable of reducing CO2 to hydrocarbons and alcohols with acceptable faradaic efficiencies. Moreover, the Cu1/g-C3N4 system has been experimentally evaluated as CO2RR electrocatalysts. 2D g-C3N4 has been demonstrated to be a competitive candidate for electrocatalytic CO2 reduction since it can act as an active support for single metal-atom catalysts, mainly Cu, Pd, and Pt, and the deposition of Au single atom was experimentally characterized.The computational hydrogen electrode model has been used to explore the suitability of several transition metals atoms anchored to C3N4, showing that single atoms enhance the catalytic activity of the system as the first proton–electron transfer is thermodynamically favored in comparison to bare carbon nitride support. Our theoretical interpretations are consistent with the experimental results using Cu1/g-C3N4,2 revealing that the competitive H2 generation is favored due to the strong CO binding energies. This fact reinforced the capability of our computational models to predict the behavior of several single metal atom electrocatalysts to reduce CO2, for instance, predicting that Au can promote the methane formation after eight electron-proton transfer processes. Our computational study paves the road to finding suitable metals that catalyze the first proton–electron transfer in the carbon dioxide reduction reaction. Posada-Pérez, A. Vidal-López, M. Solà, and A. Poater, 2023, Phys. Chem. Chem. Phys, 25, 8574.Cometto, A. Ugolotti, E. Grazietti, A. Moretto, G. Bottaro, L. Armelao, C. Di Valentin, L. Calvillo and G. A. Granozzi, npj 2D Mater. Appl., 2021, 5, 63. Figure 1

ConspectusSupported metal catalyst, has been one of the most important systems in the field of heterogeneous catalysis. The great complexity of both the compositions and structures of such supported metal catalysts provides a great degree of freedom for tuning their catalytic properties, which has essentially triggered the explosive growth in research on design and control active metals' surface structures for decades. An ideal metal catalyst theoretically features maximum active sites and optimal intrinsic reactivity to facilitate a desired chemical reaction. Inspired by the catalytic concepts brought by natural enzymes and homogeneous catalysis, the fabrication of heterogeneous catalysts with atomically dispersed metal atoms has attracted much attention and been extensively explored in recent years.Atomically dispersed metal catalysts (ADMCs) including single-atom catalyst (SACs) and fully exposed cluster catalyst (FECCs), as shining stars in heterogeneous catalysts have recently drawn much attention. The advantages of ADMCs mainly include the following three aspects: (1) the fully exposed active metal atoms can realize the utmost atomic utilization efficiency and reduce the cost of catalysts; (2) the geometric and electronic structure can be effectively regulated by altering the coordination environments of metal atoms and then further tuning the catalytic performance in terms of activity, selectivity, and stability; (3) the precisely designed structures provide a promising platform for digging the structure–performance relationships of active sites with the assistance of theoretical calculations. Owing to these advantages, ADMCs have been used in thermal-catalysis, electrocatalysis, photocatalysis, etc. until now.In this Account, a summary of recent progress regarding ADMCs for heterogeneous thermal catalysis in our group will be presented from the following aspects. First, an overview of great opportunities brought by nanodiamond and its derivatives as substrates for anchoring atomically dispersed metals (ADMs) and tailoring their structures. Next, our recent progress in achieving desirable catalytic performance, including activity, selectivity, and stability over nanodiamond–graphene (ND@G) supported ADMCs will be introduced in detail. Finally, a brief outlook regarding the development directions for ADMCs by discussing current challenges and opportunities will be proposed. It is hoped that this Account can inspire the development of the rational design and various application of ADMCs.

Liquid fuel synthesis via CO2 hydrogenation by coupling homogeneous and heterogeneous catalysis

Single (metal) atoms (SAs) have attracted great attention in virtue of their intriguing physical and chemical properties, representing a fertile playground for fundamental studies of electronic, magnetic and chemical phenomena. Placing SAs on clean, crystalline and atomically flat substrates allows the study of their properties and functions via the whole plethora of surface science tools, offering direct access to their features with ultimate resolution. Moreover, the isolated metal atoms exhibit peculiar properties and tend to behave similarly to reactive centres in solution or gas phase, bridging homogeneous and heterogeneous catalysis. In these regards, atomically dispersed metal catalysts (ADCs) have recently received great attention [1, 2]. However, they suffer some intrinsic bottlenecks such as a poor tunability and limited maximum density.Here we show the successful achievement of single atom platforms (SAPs), where SAs are coordinated to atomically precise organic templates, confined to a two dimensional surface. The templates are obtained via the on-surface synthesis (OSS) of carefully designed molecular precursors, and consist of carbon-based, covalent polymers equipped with side moieties that act as coordination sites. These groups capture metal atoms that are deposited on the substrate in a post-synthetic step, and together form active site with rich potential applications. Conceptually, the obtained SAP offers rich tunability prospects by varying not only the substrate and the SA, but also the organic ligand. Its shape and composition (e.g. heteroatoms, functional groups, strain, etc.) will influence the properties of the SA and enable their fine tuning. Moreover, the versatility of the OSS strategy offers a wide plethora of possible architectures, allowing also the precise modification of the SA density.We have characterized each stage of the SAP preparation via high-resolution scanning tunnelling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) with functionalized tips [3]. The trapping and conversion ability of these well-defined active sites towards CO and CO2 was then studied. Our investigation allows the direct visualization of reactants and binding motifs with unprecedented resolution, and opens new avenues for the study of chemical reactions localized at the active sites of our SAP.[1] Wang, A.; Li, J.; Zhang, T. Heterogeneous Single-Atom Catalysis. Nature Reviews Chemistry 2018, 2 (6), 65–81.[2] Hannagan, R. T.; Giannakakis, G.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Single-Atom Alloy Catalysis. Chem. Rev. 2020, 120 (21), 12044–12088.[3] Gross, L.; Mohn, F.; Moll, N.; Liljeroth, P.; Meyer, G. The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy. Science 2009, 325 (5944), 1110–1114. Figure 1. Single atom platform concept. Sketch of surface-adsorbed single atoms (orange dots) and of a single atom platform. Here, the organic part is represented by 1D polymers adsorbed on a surface, and the single atoms decorate the coordination sites (U-shaped motifs). Figure 1

Application of heterogeneous catalysts in olefin hydroformylation

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Chapter 3 - Applications of heteropoly acids as heterogeneous catalysts

Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

Organometallic cluster compounds with metal frameworks containing more than three metal atoms are akin to metal particles and stepped metal surfaces having neibouring multimetallic centers. These metal centers are chemically accomodated by appropriate ligand molecules such as carbonyls, phosphines, aryls and olefins. The similarity between very small metal particles and molecular clusters was firsly proposed by Muetterties(1) and Chini(2) on the basis of the chemical behavior of lignads such as CO coordinated to a cluster framework and chemisorption onto the metal surface. The adjacent metal sites in polynuclear metal clusters make available multi-metallic coordination environments(e.g., face, edge and kink) that can not be realized at a single metal atom or ionic site typical of most homogeneous metal complex catalysts. Transition metal cluster complexes are ideal objects for the study of collective behavior in stoichiometric and catalytic reactions. In this sense, metal clusters occupy an intermediate position between molecular metal complexes(homogeneous catalysts) and bulky metals such as films and crystals (classic heterogeneous catalysts) in catalysis. Heterogeneous catalysts which involve colloidal, crystalline, and supported metal particles have the advantage of being readily separable from the products, which gives a practical advantage over homogeneous metal complex catalysts even with a high selectivity and uniformity. Catalytically active metal/alloy particles of several nanometer(nm) diameters are usually dispersed on supporting materials such as metal oxides, sulfides or carbon having large surface area to prevent further metal agglomeration (“sintering”). Several examples of supported multi-metal catalysts used in industrial proccesses are Fe/K-Al2O3 for ammonia synthesis, Cu/ZnO for methanol synthesis, Ag/Re-Al2O3 for selective epoxidation of ethylene, and Pt/Re(or Ru/Cu)Al2O3 which is used for naphtha reforming. The activity and selectivity of supported metal catalysts generally depend on the state of metal dispersion(ensemble sizes), structure(shape and morphology), metal composition(geometric distribution of multi metals), and metal-support(metal-oxide/sulfide) interactions.

Metal Cluster Compounds as Molecular Precursors for Tailored Metal Catalysts

In heterogeneous single-metal-site catalysts (HSMSCs) the active metal centres are located individually on a support and are stabilized by neighbouring surface atoms such as nitrogen, oxygen or sulfur. Modern characterization techniques allow the identification of these individual metal atoms on a given support, and the resulting materials are often referred as single-atom catalysts. Their electronic properties and catalytic activity are tuned by the interaction between the central metal and the neighbouring surface atoms, and their atomically dispersed nature allows for metal utilization of up to 100%. In this way, HSMSCs provide new opportunities for catalysis, and with respect to structure build a bridge between homogeneous and heterogeneous catalysis. Herein, selected publications from 2010 in this area are reviewed and their perspectives for the near future are highlighted. Where appropriate, comparisons between HSMSCs and homogeneous/heterogeneous counterparts are presented. Single-atom catalysts have drawn increasing attention as methods for their preparation and characterization improve. Here, Beller and co-workers discuss the latest developments in the field of single-metal-site catalysts, discussing how this catalyst class bridges heterogeneous and homogeneous catalysis, and providing a perspective on how the field might continue to develop.

Atomically dispersed metal catalysts (ADCs), as an emerging class of heterogeneous catalysts, have been widely investigated during the past two decades. The atomic dispersion nature of the catalytic metal centers makes them an ideal system for bridging homogeneous and heterogeneous metal catalysts. The recent rapid development of new synthetic strategies has led to the explosive growth of ADCs with a wide spectrum of metal atoms dispersed on supports of different chemical compositions and natures. The availability of diverse ADCs creates a powerful materials platform for investigating mechanisms of complicated heterogeneous catalysis at the atomic levels. Considering most dispersed metal atoms on ADCs are coordinated by the donors from supports, this review will demonstrate how the surface coordination chemistry plays an important role in determining the catalytic performance of ADCs. This review will start from the link between coordination chemistry and heterogeneous catalysis. After the brief description on the advantages and limitations of common structure characterization methods in determining the coordination structure of ADCs, the surface coordination chemistry of ADCs on different types of supports will be discussed. We will mainly illustrate how the local and vicinal coordination species on different support systems act together with the dispersed catalytic metal center to determine the catalytic activity, selectivity, and stability of ADCs. The dynamic coordination structure change of ADCs in catalysis will be highlighted. At the end of the review, personal perspectives on the further development of the field of ADCs will be provided.

The adjacent chemical microenvironment of single metal atoms in heterogeneous catalysis is crucial to their chemical activity for various catalytic processes. Here, based on first-principles calculations, 25 single transition metal atom catalysts coordinated to sulfur species embedded in graphene (TM-S4-G-SACs) are reported for nitrogen reduction under ambient condition. It shows that nine TM-S4-G-SACs (TM = Mo, Sc, Cr, V, W, Ti, Nb, Mn, and Re) are promising nitrogen reduction catalysts with an optimal potential of -0.425 V. Meanwhile, 18 TM-S4-G-SACs have better catalytic activity than those with nitrogen coordination. Particularly, the catalytic activity of TM-S4-G-SACs and the adsorption energy of intermediate NH2* conform to a volcano-type correlation, which can be described by a universal electronic descriptor φ, defined by the electronegativity of the metal, adjacent coordinated atoms, and the valence electron occupancy. The above findings suggest the potential of sulfur-coordinated single metal atoms as electrocatalytic nitrogen reduction catalysts and an applicable descriptor to achieve optimal performance.

Lactic acid (2-hydroxypropionic acid, CH3CHOHCOOH) is one of the platform chemicals derived from biomass. It is used in the food industry and in the manufacture of biodegradable plastics and useful chemicals. Recently, various examinations were carried out not only by fermentation but also by the chemical methods using heterogeneous and homogenous catalysts. This chapter focuses on the chemical processes with heterogeneous catalysts in lactic acid and lactate ester productions from sugars. Bronsted basic catalysts and Lewis acid catalysts gave lactic acid in high yields. In the lactic acid productions from triose, lactic acid ester is obtained with high yields of nearly 100 % in alcohols around 100 °C using Sn-β zeolite, Sn–carbon–silica, and H-USY catalysts. In the lactic acid production from hexose, lactic acid ester or a lactate salts was obtained from glucose, fructose, and sucrose with the comparatively high selectivity of about 50 % by several catalytic processes, that were in water around 50 °C using heterogeneous basic catalysts, such as activated hydrotalcite catalyst, in hydrothermal water around 300 °C using homogeneous basic catalysts, such as NaOH and ZnSO4, and in alcohols around 160 °C using heterogeneous Lewis acid catalysts, such as Sn-β zeolite.

This mini-review contrasts the characteristics of traditional heterogeneous (solid) catalysts with those of homogeneous ones: the nature of the active sites in each case is very different, a fact well illustrated in ammonia synthesis. It is recalled that certain chemical transformations can be effected only with heterogeneous catalysts. It is also demonstrated that the scope for introducing multifunctional sites is greater with open-structured inorganic heterogeneous catalysts than with homogeneous ones: for example, TiIV ions distributed in a spatially isolated and accessible manner at the large areas of a nanoporous support smoothly convert cyclohexene to adipic acid (with H2O2) in a cascade of six consecutive reactions. A sharp distinction is drawn between nanocluster and nanoparticle “metal” catalysts, both electronic and geometric arguments being utilized to explain this difference. In the extreme case, a few (or single) metal atoms (supported on oxides) have been shown (see refs. Fu et al. Science 301:935, 2003 and Rim et al J Phys Chem C 113:10198, 2009) to be more important determinants of catalytic activity than nanoparticle metals such as Au and Pd. Recent advances in high-resolution electron microscopy is a key technique in this facet of catalysis. The merits of immobilizing single-site homogeneous catalysts and of creating atomically well-defined single-site heterogeneous ones on high-area solids are illustrated both from a practical viewpoint and also as a strategy for the design of new catalysts.