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.

Pérez-Ramírez, J. (2020). Nanostructuring unlocks high performance of platinum singleatom catalysts for stable vinyl chloride production. Nature Catalysis, 3(4), 376-385.

Distinct gold nanostructures on activated and nitrogen-doped carbons are evaluated in acetylene hydrochlorination.

Nitrogen-doped carbons are promising materials for a broad range of applications. However, their rational design is greatly hampered by the lack of efficient methods to control the nitrogen speciation, which not only causes controversy about the roles of specific nitrogen functionalities, but also hinders investigations into other physicochemical characteristics of these materials. We herein present a cutting-edge strategy that allows a systematic tuning of the electrical conductivity of polyaniline-derived N-doped carbons at a defined nitrogen speciation and content, and similar porous properties. By applying these model systems in acetylene hydrochlorination, a major industrial technology for the production of polyvinylchloride, we provide insights into the active sites and the reaction mechanism and disclose two key catalytic descriptors for N-doped carbons in this reaction: (i) a high content of pyrrolic-N functionalities, promoting the adsorption of the reactants, and (ii) good electrical conductivity, likely influencing the surface diffusion of adsorbed species. The adverse correlation of the two descriptors to calcination temperature causes a trade-off that determines the catalytic activity. This understanding enabled the design of the first N-doped carbon catalyst that rivals the unprecedented activity of benchmark gold-based systems in acetylene hydro-chlorination.

Single‐atom heterogeneous catalysts with well‐defined architectures are promising for deriving structure–performance relationships, but the challenge lies in finely tuning the structural and electronic properties of the metal. To tackle this point, a new approach based on the surface diffusion of gold atoms on different cavities of N‐doped carbon is presented. By controlling the activation temperature, the coordination neighbors (Cl, O, N) and the oxidation state of the metal can be tailored. Semi‐hydrogenation of various alkynes on the single‐atom gold catalysts displays substrate‐dependent catalytic responses; structure insensitive for alkynols with γ‐OH and unfunctionalized alkynes, and sensitive for alkynols with α‐OH. Density functional theory links the sensitivity for alkynols to the strong interaction between the substrate and specific gold‐cavity ensembles, mimicking a molecular recognition pattern that allows to identify the cavity site and to enhance the catalytic activity.

The potential implementation of ruthenium‐based catalysts in polyvinyl chloride production via acetylene hydrochlorination is hindered by their inferior activity and stability compared to gold‐based systems, despite their 4‐fold lower price. Combining in‐depth characterization and kinetic analysis we reveal the superior activity of ruthenium nanoparticles with an optimal size of 1.5 nm hosted on nitrogen‐doped carbon (NC) and identify their deactivation modes: 1) nanoparticle redispersion into inactive single atoms and 2) coke formation at the metal sites. Tuning the density of the NC carrier enables a catalytic encapsulation of the ruthenium nanoparticles into single layer graphene shells at 1073 K that prevent the undesired metal redispersion. Finally, we show that feeding O2 during acetylene hydrochlorination limits coke formation over the nanodesigned ruthenium catalyst, while the graphene layer is preserved, resulting in a stability increase of 20 times, thus rivalling the performance of gold‐based systems.

Controlling the precise atomic architectures of supported metals is central to unlock their potential as heterogeneous catalysts, as recently exemplified for nanostructured platinum and ruthenium systems in acetylene hydrochlorination, a key industrial technology for vinyl chloride production. This opens the possibility to build on historically established activity correlations. Here, we derive quantitative activity, selectivity, and stability descriptors, accounting for the metal-dependent speciation and host effects. To achieve this, we generate a platform of Au, Pt, Ru, Ir, Rh, and Pd single atoms and nanoparticles supported on functionalized carbons and assess their evolution during synthesis and under reaction conditions. Combining kinetic, transient and chemisorption analyses with modelling, we identify the acetylene adsorption energy as a speciation-sensitive activity descriptor, further determining the catalyst's selectivity with respect to coke formation.The stability of the different metal nanostructures is governed by the interplay between single atomsupport interaction and the chlorine affinity, promoting redispersion or agglomeration, respectively.

For decades, carbons have been the support of choice in acetylene hydrochlorination, a key industrial process for polyvinyl chloride manufacture. However, no unequivocal design criteria could be established to date, due to the complex interplay between the carbon host and the metal nanostructure. Herein, we disentangle the roles of carbon in determining activity and stability of platinum-, ruthenium-, and gold-based hydrochlorination catalysts and derive descriptors for optimal host design, by systematically varying the porous properties and surface functionalization of carbon, while preserving the active metal sites. The acetylene adsorption capacity is identified as central activity descriptor, while the density of acidic oxygen sites determines the coking tendency and thus catalyst stability. With this understanding, a platinum single-atom catalyst is developed with stable catalytic performance under two-fold accelerated deactivation conditions compared to the state-of-the-art system, marking a step ahead towards sustainable PVC production.