Single-Atom Catalysts: Advances & Design

• Single-atom catalysts (SACs) are heterogeneous catalysts featuring atomically dispersed metal atoms anchored to supports, offering unique reactivity and maximal atom-utilization efficiency.
• Precise control of geometric structure, coordination motifs, and thermodynamic stability is essential to tune catalytic activity and prevent aggregation.
• Emerging synthesis techniques, advanced characterization methods, and computational modeling enable scalable production and rational design for energy conversion and chemical transformations.

Single-atom catalysts (SACs) are heterogeneous catalysts in which atomically dispersed individual metal atoms are anchored to the surface of a solid support, yielding unique active sites that maximize atom-utilization efficiency and unlock reactivity modes inaccessible to classical nanoparticles and bulk metals. Distinguished by their absence of metal–metal bonds at active sites, SACs exhibit a spectrum of catalytic behaviors dictated by local coordination geometry, electronic structure, and support selection. Substantial advances in experimental characterization, high-throughput computational modeling, and synthetic strategies have established SACs as a central research platform in energy conversion, environmental remediation, and selective chemical transformations.

1. Geometric Structure and Coordination Motifs

Determination of the local coordination environment at the atomic scale is central to modeling, tuning, and benchmarking SAC activity and stability. On model oxide supports such as α-Fe₂O₃(1̄102), global-optimization evolutionary searches (e.g. CMA-ES with DFT-PBE+U) revealed that deposited Pt atoms can locally reconstruct the substrate, breaking three Fe–O bonds and displacing two lattice oxygen atoms, to form a pseudo-linear O₁–Pt–O₂ motif with Pt–O bond lengths of 1.91 Å and 2.08 Å, and an O₁–Pt–O₂ angle of ≈174° (Rafsanjani-Abbasi et al., 2024). This linear two-fold coordination geometry "digs its own site" and differs fundamentally from bulk-like and four/five-fold motifs prevalent in unreconstructed surfaces (Parkinson, 2017, Kraushofer et al., 2022).

Stabilization of SACs across diverse supports (porous ceramics, carbon nitrides, MOFs, COFs, engineered polymers) involves precise control of primary coordination spheres. Examples include Co–N₃ chelation along terpyridine-functionalized 1D polymers (Kinikar et al., 2024), Ni–N₄ penta-coordination in nitrogen-intercalated nanoporous carbon (Priyanga et al., 2024), and Pt anchored by four iodide ions replacing methylammonium on hybrid perovskite surfaces (Fu et al., 2018). Key geometric and electronic parameters (bond lengths, coordination number, charge state, site asymmetry) are typically retrieved from a combination of STM/nc-AFM, XPS, XAS/EXAFS, and DFT structural optimization.

2. Thermodynamics, Energetics, and Site Stability

Atomic-scale dispersion is thermodynamically favored only under conditions balancing atom–support binding against aggregation and sintering. Adsorption energies of isolated atoms on reconstructed supports can reach E_ads ≈ –3.56 eV (Pt/α-Fe₂O₃) for the pseudo-linear configuration, exceeding the –2.72 eV of unreconstructed surfaces (ΔE = –0.84 eV) (Rafsanjani-Abbasi et al., 2024). The energy gain arises from the formation of strong metal–ligand bonds that compensate for broken substrate bonds.

Kinetic stabilization, as established by non-equilibrium ion-implantation protocols, exploits high-energy ion irradiation to kinetically trap single atoms at defect/vacancy sites generated in situ. The binding energies of SACs at these sites (Pt_sub@MoS₂ ≈ 3.2 eV, Pt_ads on S ≈ 1.4 eV (Li et al., 18 Jan 2026)) vastly exceed thermal energies, yielding negligible atomic mobility at room temperature. Key performance descriptors—overpotential η, Tafel slope b, mass/TOF activity—are tightly linked to the density and accessibility of thermodynamically stable single-atom sites, with Pt/MoS₂ fabricated by ion implantation reaching η₁₀ = 26 mV at 10 mA cm⁻² and mass activity exceeding commercial Pt/C by 2.4× (Li et al., 18 Jan 2026).

3. Electronic Structure, Reactivity, and Selectivity

The catalytic properties of SACs stem from tailored electronic states that differ markedly from bulk metals and clusters. The density-of-states (DOS), d-band center, oxidation state, and ligand field orchestrate reactant adsorption, bond activation, and product desorption. Charge transfer between support and atom can dramatically alter reactivity: for Pt single atoms on Fe₂O₃, Bader charge is +0.1 e for the linear site, supporting a near-metallic but catalytically open character (Rafsanjani-Abbasi et al., 2024). Similar charge tuning is observed for I–Pd on MoS₂, where axial ligands engineer impurity bands near E_F that optimize H* adsorption free energies (ΔG_H = –0.13 eV for I–Pd@MoS₂, comparable to Pt(111), versus +0.84 eV for unligated Pd) (Sun et al., 8 Mar 2025).

Electronic asymmetry and frontier orbital engineering—realized in metal–organic SAPs or N-doped carbons—tune π–d hybridization and reactivity towards small molecules, such as CO and CO₂ (Kinikar et al., 2024, Yang et al., 2024). Computational, spectroscopic, and machine-learning analyses converge on d-band center and hybrid descriptors (e.g., mixing G_adsCO and G_adsOH to predict CO₂RR activity) as critical selectors for catalytic performance (Yang et al., 2024). For main-group metals (Mg, Al, Ga), the s/p band centers can be rendered pseudo-local in tailored N₄ environments, rationalizing high NO reduction activity and selectivity (Wu et al., 2021).

4. Synthetic Strategies and Atomic Dispersion Control

Manufacturing SACs at scale requires precise strategies to achieve atomic dispersion without undesired clustering. Wet-chemistry impregnation, co-precipitation, photochemical deposition, atomic-layer deposition (ALD), and MOF/COF-derived pyrolysis are central protocols, each associated with characteristic atomic loadings, anchoring mechanisms, and aggregation thresholds (Rai, 2021). Non-equilibrium ion implantation, recently advanced, achieves wafer-scale, high-throughput SACs with kinetic trapping of isolated atoms across multiple supports (MoS₂, graphene, GDY, CNT), constructing libraries of ≥36 SAC compositions (Li et al., 18 Jan 2026).

On-surface synthesis with controlled polymerization and functional group engineering yields atomically precise metal-organic SAPs with tunable site isolation and electronic asymmetry (Kinikar et al., 2024). Defect engineering, vacancy creation, and substitutional doping (N-doped carbons, graphene single vacancies) further enable robust atomic anchoring and enhanced stability under harsh catalytic or electrochemical conditions (Jovanović et al., 2021).

5. Experimental Characterization and Theoretical Modeling

Accurate determination of SAC structure and electronic state requires synergy between advanced experimental probes and high-fidelity computation. STM and nc-AFM reveal atom position, coordination symmetry, and dynamic ligation; XPS (Pt 4f, N 1s) and EXAFS/XANES yield oxidation state, coordination number, and local disorder (Rafsanjani-Abbasi et al., 2024, Li et al., 7 Nov 2025). In situ XAS under photocatalytic or thermal operation allow real-time tracking of coordination shell evolution and charge redistribution (Li et al., 7 Nov 2025).

Surface science model systems (UHV-prepared single crystals with controlled facets, steps, and vacancies) enable direct benchmarking of DFT-computed adsorption energies, reaction barriers, and oxidation states to real atomic-scale sites (Kraushofer et al., 2022, Parkinson, 8 Jan 2025). Global-optimization algorithms (CMA-ES, machine-learning potentials) operate on PES landscapes to recover the true energetic minima of active sites and reject intuition-biased bulk truncation models (Rafsanjani-Abbasi et al., 2024, Parkinson, 8 Jan 2025). Microkinetic modeling, scaling relations, volcano plot construction, and hybrid-descriptor CNNs systematically screen structure–activity relationships to inform rational design (Yang et al., 2024).

6. Applications and Mechanistic Pathways

SACs are deployed in key energy and environmental transformations, typically enabled by unique reaction pathways absent in conventional catalysts. In CO oxidation, pseudo-linear O–Pt–O or twofold-coordinated Pt on Fe-oxides catalyze Langmuir–Hinshelwood-type cycles, with activation barriers of 0.49–0.58 eV and net reaction energies ΔE ≈ –1.8 to –6.4 eV (Rafsanjani-Abbasi et al., 2024, Fu et al., 2018). Water–gas shift (WGS), hydrogen evolution reaction (HER), nitrogen reduction, and selective CO₂ reduction are accessed via site-specific mechanisms dependent on charge state, adsorption free energies, and modular coordination (Rai, 2021, Ping et al., 2022, Sun et al., 8 Mar 2025, Zhang et al., 2023). Complex bond formation (C–N, C–C) through migratory insertion is realized on Ni–N₄/NPC sites with barriers <1.2 eV (Priyanga et al., 2024).

Faradaic efficiency, turnover frequency, mass activity, and selectivity in electrochemical reactions are maximized by optimizing interface composition and site structure: e.g., M–N₄–graphene for ORR/NERR, Ti–N–C for dichromate binding in Cr(VI) reduction, P-block metals on C₃N₄ for CO₂RR (Dobrota et al., 2021, Ping et al., 2022, Yang et al., 2024). Dual-metal and bimetallic sites, site asymmetry, and ligand-based modulation extend the design space for multielectron and tandem catalysis (Li et al., 7 Nov 2025).

7. Rational Design Rules and Future Directions

Integrative design of SACs combines high-throughput screening, global-optimization search, machine-learning prediction, and operando measurement, guided by a portfolio of physical metrics (E_ads, ΔG_H, d-band center, s/p band hybridization, site asymmetry). Linear coordination motifs, two/three-fold active sites on reducible oxides, multi-vacancy or N-doping in carbons, and precisely tuned ligands confer optimal stability, reactivity, and selectivity (Rafsanjani-Abbasi et al., 2024, Parkinson, 8 Jan 2025, Kinikar et al., 2024, Sun et al., 8 Mar 2025).

SAC stability under real operating conditions—aggregation, sintering, ligand loss—is managed by kinetic trapping (ion implantation), orbital-level charge tuning, and defect engineering (Li et al., 18 Jan 2026). Benchmarking DFT models against rigorous surface science and in situ characterization ensures reliable transfer to scalable powder catalysts (Parkinson, 8 Jan 2025, Kraushofer et al., 2022).

Emerging directions include wafer-scale manufacturing, on-chip integration, co-doped and multi-metal SACs for tandem catalysis, quantitative operando monitoring, and extension to main-group metals using tailored s/p band states (Li et al., 18 Jan 2026, Wu et al., 2021). The unification of atomic-precision synthesis, electronic-structure engineering, robust characterization, and machine-learning-driven screening is set to deliver next-generation SACs with maximal atom economy, tunable activity, and application breadth across catalysis science.