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Hydrogen evolution reaction (HER) is one of the most important reactions in electrochemistry. This is not only because it is the simplest way to produce high purity hydrogen and the fact that it is the side reaction in many other technologies. HER actually shaped current electrochemistry because it was in focus of active research for so many years (and it still is). The number of catalysts investigated for HER is immense, and it is not possible to overview them all. In fact, it seems that the complexity of the field overcomes the complexity of HER. The aim of this review is to point out some of the latest developments in HER catalysis, current directions and some of the missing links between a single crystal, nanosized supported catalysts and recently emerging, single-atom catalysts for HER.

Chemical Science, 10 (2), ISSN:2041-6520, ISSN:2041-6539

Single-atom heterogeneous catalysts with welldefined 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 substratedependent catalytic responses; structure insensitive for alkynols with g-OH and unfunctionalized alkynes, and sensitive for alkynols with a-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.

Controlling the structure sensitivity of catalyzed reactions over metals is central to developing atom-efficient chemical processes. Approaching the minimum ensemble size, the properties enter a non-scalable regime in which each atom counts. Almost all trends in this ultra-small frontier derive from surface science approaches using model systems, because of both synthetic and analytical challenges. Exploiting the unique coordination chemistry of carbon nitride, we discriminate through experiments and simulations the interplay between the geometry, electronic structure, and reactivity of palladium atoms, dimers, and trimers. Catalytic tests evidence applicationdependent requirements of the active ensemble. In the semihydrogenation of alkynes, the nuclearity primarily impacts activity, whereas the selectivity and stability are affected in Suzuki coupling. This powerful approach will provide practical insights into the design of heterogeneous catalysts comprising well-defined numbers of atoms.

Palladium-catalysed cross-coupling reactions, central tools in fine-chemical synthesis, predominantly employ soluble metal complexes despite recognized challenges with product purification and catalyst reusability1– 3. Attempts to tether these homogeneous catalysts on insoluble carriers have been thwarted by suboptimal stability, which leads to a progressively worsening performance due to metal leaching or clustering4. The alternative application of supported Pd nanoparticles has faced limitations because of insufficient activity under the mild conditions required to avoid thermal degradation of the substrates or products. Single-atom heterogeneous catalysts lie at the frontier5–18. Here, we show that the Pd atoms anchored on exfoliated graphitic carbon nitride (Pd-ECN) capture the advantages of both worlds, as they comprise a solid catalyst that matches the high chemoselectivity and broad functional group tolerance of stateof- the-art homogeneous catalysts for Suzuki couplings, and also demonstrate a robust stability in flow. The adaptive coordination environment within the macroheterocycles of ECN facilitates each catalytic step. The findings illustrate the exciting opportunities presented by nanostructuring single atoms in solid hosts for catalytic processes that remain difficult to heterogenize.