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A single-atom catalyst is a heterogeneous catalyst in which the active metal is dispersed as isolated single atoms on a support, so that the active sites are atomically uniform and the metal is used with maximum efficiency ^[1]. The concept was established for CO oxidation over Pt1/FeOx, where singly dispersed platinum atoms on iron oxide were reported to be active while platinum nanoparticles and clusters were not ^[2]. It now covers a broad family of systems based on oxides, carbon materials, and metal–organic frameworks ^[3]. Synthesis generally relies on anchoring individual metal atoms at defect sites, nitrogen or sulfur coordination environments, or vacancy-rich supports, with typical metal loadings of 0.1–5 wt%, and higher loadings are frequently accompanied by the formation of dimers and clusters ^[4]. The electronic structure of an isolated metal atom differs from that of the bulk metal, and coordination number, charge state, and metal–support interaction are the properties most often invoked to explain the altered activity and selectivity ^[3][5]. Beyond oxide supports, the same atomically dispersed motif has been examined on carbon materials and metal–organic frameworks, and dedicated reviews now cover single-atom systems for biological and for biofuel-related catalysis ^[4,5]. Because single atoms tend to migrate and aggregate under reaction conditions, and because the structure of the working site is difficult to establish during catalysis, long-term stability and the identity of the true active species remain debated ^[3][4].