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of a heterogeneous catalyst. Any atom not at the catalyst surface is not directly used to drive chemical conversions, which is a critical consideration for the use of precious metal catalysts. To that end, the use of smaller particles with high surface/ volume ratios has been the preferred choice to maximize metal utilization. Recent advancements in the synthesis of nanomaterials with targeted shapes and sizes have resulted in the production of catalysts with designed catalytic properties. Single atom catalysts (SACs) have furthered the field of catalysis due to the potential for carefully controlled active site properties and distinct capabilities compared to bulk and nanoparticle catalysts. The reduction in size of the metal modifies the behavior through several mechanisms including: quantum size effect, [1][2][3][4] strong metal-support interactions (SMSI), [5][6][7] surface effects, [8][9][10][11] and varied oxidation states. [12] As a result of these influences, the coordination environment of single atoms on supports control their reactivity.Visualizing SACs is important to understand the formation, coordination environment, and stability of the catalyst. Oftentimes, bulk and nanoparticle catalysts release atoms during a catalytic process, which may contribute to the measured catalytic activity and changes in activity as a function of time. Recent work visualizing the release of Au single atoms, from nanoporous Au surfaces during methane pyrolysis, demonstrates the importance of visualizing catalysts at the atomic scaleThe drive for atom efficient catalysts with carefully controlled properties has motivated the development of single atom catalysts (SACs), aided by a variety of synthetic methods, characterization techniques, and computational modeling. The distinct capabilities of SACs for oxidation, hydrogenation, and electrocatalytic reactions have led to the optimization of activity and selectivity through composition variation. However, characterization methods such as infrared and X-ray spectroscopy are incapable of direct observations at atomic scale. Advances in transmission electron microscopy (TEM) including aberration correction, monochromators, environmental TEM, and microelectro-mechanical system based in situ holders have improved catalysis study, allowing researchers to peer into regimes previously unavailable, observing critical structural and chemical information at atomic scale. This review presents recent development and applications of TEM techniques to garner information about the location, bonding characteristics, homogeneity, and stability of SACs. Aberration corrected TEM imaging routinely achieves sub-Ångstrom resolution to reveal the atomic structure of materials. TEM spectroscopy provides complementary information about local composition, chemical bonding, electronic properties, and atomic/molecular vibration with superior spatial resolution. In situ/operando TEM directly observe the evolution of SACs under reaction conditions. This review concludes with remarks on the challenges an...

Aimed toward the pursuit of manufacturing ammonia in a carbon-neutral and decentralized manner, the electrocatalytic nitrate reduction reaction (NO 3 RR) not only promises an effective route for carbon-neutral ammonia synthesis but also offers potential advantages to wastewater remediation. Here, we describe the efficacy of bioinspired, atomically dispersed catalysts for the NO 3 RR in aqueous media via a catalytic cascade. Compared to nanoparticles with extended catalytic surfaces, atomically dispersed catalysts are largely underexplored in this field, despite their intrinsic selectivity toward mono-nitrogen species over their dinitrogen counterparts. Herein, we specifically report on a series of nitrogen-coordinated mono-and bimetallic, atomically dispersed, iron-and molybdenum-based electrocatalysts for ammonia synthesis via the NO 3 RR. The key role of the *NO 2 /NO 2 − intermediates was identified both computationally and experimentally, wherein the Fe−N 4 sites and Mo−N 4 /*O− Mo−N 4 sites carried distinct associative and dissociative adsorption of NO 3 − molecules, respectively. By integrating individual Fe and Mo sites on a single bimetallic catalyst, the unique reaction pathways were synergized, achieving a Faradaic efficiency of 94% toward ammonia. Furthermore, the robustness of the bimetallic FeMo−N−C catalyst was highlighted by five consecutive 12 h electrolysis cycles with the Faradaic efficiency being maintained above 90% over the entire 60 h. The utilization of catalytic cascades, synergizing distinct reaction pathways on heterogeneous single-atom sites, is largely unconstrained by linear scaling relations of reaction intermediates and sheds light on designing electrocatalysts for highly selective, efficient, and durable ammonia synthesis.

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