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.

Chemical Synthesis of Single Atomic Site Catalysts.

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Single-Atom Catalysts across the Periodic Table.

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 catalyti...

Surface Coordination Chemistry of Atomically Dispersed Metal Catalysts.

Layered 2D materials are a vital class of electrocatalys for the hydrogen evolution reaction (HER), due to their large area, excellent activity, and facile fabrication. Theoretical caculations domenstrate, however, that only the edges of the 2D nanosheets act as active sites, while the much larger basal plane exhibits passive activity. Here, from a distinguishing perspective, RhSe2 is reported as a "3D" electrocatalyst for HER with top-class activity, synthesized by a facile solid-state method. Superior to 2D materials, multiple crystal facets of RhSe2 exhibit near-zero free energy change of hydrogen adsorption (ΔGH ), which guarantees high performance in most common morphologies. Density functional theory calculations reveal that the low-coordinated Rh atoms act as the active sites in acid, which enables the modified Kubas-mediated pathway, while the Se atoms act as the active sites in an alkaline medium. The overpotentials of HER activity of RhSe2 are measured to be 49.9 and 81.6 mV at 10 mA cm-2 in acid and alkaline solutions, respectively. This work paves the way to new transition metal chalcogenide catalysts.

RhSe 2 : A Superior 3D Electrocatalyst with Multiple Active Facets for Hydrogen Evolution Reaction in Both Acid and Alkaline Solutions

Structural transformation and evolution of active metal sites can occur in metal-catalyzed reactions in both homogeneous and heterogeneous systems. Such structural changes have an important impact on the catalytic behavior, including activity, selectivity, and stability. Aiming to establish a link between homogeneous and heterogeneous catalytic systems, this review begins with a discussion on dynamic structural transformations of metal catalysts in homogeneous reactions and the corresponding implications. We then discuss the evolution of isolated metal atoms and clusters in heterogeneous catalysts during catalyst activation and under reaction conditions. Finally, strategies for stabilizing subnanometric metal species on solid supports are presented for potential industrial applications.

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Evolution of Isolated Atoms and Clusters in Catalysis

Single-atom Rh based bipyridine framework porous organic polymer: A high active and superb stable catalyst for heterogeneous methanol carbonylation

Supported noble metal nanoclusters and single-metal-site catalysts are inclined to aggregate into particles, driven by the high surface-to-volume ratio. Herein, we report a general method to atomically disperse noble metal nanoparticles. The activated carbon supported nanoparticles of Ru, Rh, Pd, Ag, Ir and Pt metals with loading up to 5 wt. % are completely dispersed by reacting with CH3I and CO mixture. The dispersive process of the Rh nanoparticle is investigated in depth as an example. The in-situ detected I• radicals and CO molecules are identified to promote the breakage of Rh-Rh bonds and the formation of mononuclear complexes. The isolated Rh mononuclear complexes are immobilized by the oxygen-containing functional groups based on the effective atomic number rule. The method also provides a general strategy for the development of single-metal-site catalysts for other applications. Supported noble metal nanoclusters and single-metal-sites catalysts are inclined to aggregate into particles. Here, the authors report a general method with CO and CH3I to disperse the metal nanoparticles of Ru, Rh, Pd, Ag, Ir and Pt completely into single atoms with loading up to 5 wt%.

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In situ formation of mononuclear complexes by reaction-induced atomic dispersion of supported noble metal nanoparticles

A highly efficient single site Rh-POL-PPh3 catalyst for heterogeneous methanol carbonylation

Although homogeneous methanol carbonylation is industrialized to produce acetic acid, it still suffers from the problems of difficult separation and metal leaching. Here, we report a heterogeneous ...

Feng Siquan

Papers

Single-atom Rh based bipyridine framework porous organic polymer: A high active and superb stable catalyst for heterogeneous methanol carbonylation

In situ formation of mononuclear complexes by reaction-induced atomic dispersion of supported noble metal nanoparticles

A highly efficient single site Rh-POL-PPh3 catalyst for heterogeneous methanol carbonylation

La-Stabilized, Single-Atom Ir/AC Catalyst for Heterogeneous Methanol Carbonylation to Methyl Acetate

The role of H2 on the stability of the single-metal-site Ir1/AC catalyst for heterogeneous methanol carbonylation