Single-atom catalysts (SACs), with their structurally simplified active sites, provide a unique platform for probing catalytic mechanisms at the atomic level. The performance of SACs—both activity and stability—is fundamentally governed by synergistic frontier orbital interactions at the metal–adsorbate and metal–support interfaces. Yet, a quantitative atomic-scale understanding of how these interactions determine catalytic performance remains elusive. Here, we show that the lowest unoccupied molecular orbital (LUMO) positions of pristine semiconducting supports, measured via Mott–Schottky analysis across 14 distinct materials, scale approximately linearly with the hydrogenation activity of supported palladium (Pd₁) atoms. Strikingly, downsizing the support to a few nanometers elevates its LUMO position, leading to record-high activity and outstanding stability in acetylene semihydrogenation. Theoretical calculations reveal that this LUMO elevation reduces the energy gap with the highest occupied molecular orbital (HOMO) of the Pd₁ atoms, which reinforces Pd₁–support orbital hybridization for enhanced stability; in turn, it sequentially tunes the LUMO of the anchored Pd₁ to amplify Pd₁–adsorbate interactions for improved activity. These findings are consistent with frontier molecular orbital (FMO) theory and establish the LUMO position of semiconducting supports as a general descriptor for the rational design of metal–support pairs with predictable hydrogenation performance.

We introduce experimental investigations on the high T_c superconductivity at 80 K in the bilayer nickelate La₃Ni₂O₇ under a 14 GPa pressure. The superconductivity emerges coincidently with a structural transition from Amam to I4/mmm. Both zero resistance and diamagnetic response, which are essential for superconductivity, were observed. A multislice electron ptychography technique was employed to visualize the oxygen. The results confirm the superconducting phase is the bilayer phase of La₃Ni₂O₇ with low oxygen vacancies.