Atomically precise heterogeneous catalysts offer unprecedented opportunities for establishing clear structure-activity relationships in catalysis. However, the precise control over the number of metal atoms in such systems remains a significant challenge, primarily due to difficulties in maintaining atomic-level dispersion, preventing metal migration and aggregation. This study develops a tandem synthesis strategy combining ligand protection and precursor engineering. First, atomic-level precision precursors Pt₁(C₄₄H₃₂P₂)₂(CO)₄, Pt₂(C₁₈H₁₂Cl₃P)₃Br₂(CO)₁ and Pt₃(C₁₈H₂₁P)₅(CO)₂ are synthesized using mixed ligands. Subsequently, by integrating Joule induction rapid thermal annealing technology, structurally well-defined Pt₁/OKB, Pt₂/OKB, and Pt₃/OKB catalysts are successfully constructed on oxidized Ketjenblack (OKB) support. In mild conditions (60 °C, 0.5 MPa H₂) for nitrobenzene selective hydrogenation, the Pt₃/OKB catalyst achieves>99% conversion and>99% aniline selectivity, significantly outperforming Pt single-atom, Pt dual-atom, and Pt nanoparticle catalysts. Density functional theory (DFT) calculations demonstrate that the unique coordination environment in the triatomic structure optimizes adsorption strength, suppressing excessive benzene ring hydrogenation while facilitating H₂ activation and intermediate stabilization, thereby achieving synergistic regulation of activity and selectivity at the atomic scale. This study provides an atomically precise synthetic method for heterogeneous catalysts and elucidates the dependence of catalytic hydrogenation performance on the number of metal atoms.

Pt-based catalysts are used commercially for the hydrogen evolution reaction (HER), even though the low earth abundance and high cost of platinum hinder scale-up applications. Ru metal is a promising alternative catalyst for HER owing to its lower cost but similar metal–hydrogen bond strength to Pt. However, designing an efficient and robust Ru-based electrocatalyst for pH-universal HER is challenging. Herein, we successfully synthesized N-doped carbon (NC) supported ruthenium catalysts with different Ru sizes (single-atoms, nanoclusters and nanoparticles), and then systematically evaluated their performance for HER. Among these catalysts, the Ru nanocluster catalyst (Ru NCs/NC) displayed optimal catalytic performance with overpotentials of only 14, 30, and 32 mV (at 10 mA·cm⁻²) in 1 M KOH, 1 M phosphate buffer saline (PBS), and 0.5 M H₂SO₄, respectively. The corresponding mass activities were 32.2, 12.1 and 8.1 times higher than those of 20 wt.% Pt/C, and also much better than those of the Ru single-atoms (Ru SAs/NC) and Ru nanoparticle (Ru NPs/NC) catalysts, at an overpotential of 100 mV under alkaline, neutral and acidic conditions, respectively. Density functional theory (DFT) calculations revealed that the outstanding HER performance of the Ru NCs/NC catalyst resulted from a strong interaction between the Ru nanoclusters and the N-doped carbon support, which downshifted the d-band center and thus weakened the *H adsorption ability of Ru sites.