A Critical Spin Moment Governs the Spin-Adsorption Relationship in Magnetic Single-Atom Alloys

Spin engineering has emerged as a promising strategy for tuning catalytic activity by manipulating the spin states of active sites. This strategy generally assumes that the local spin moment serves as a universal descriptor for adsorption strength, yet its validity remains largely unexplored. Here, we develop a reverse doping strategy that enables broad modulation of local spin moments in magnetic single-atom alloy catalysts, allowing the spin–adsorption relationship to be systematically revisited. We find that tuning the local spin moment does not uniformly translate into changes in adsorption energy, with particularly weak responses in the low-spin regime. Isolating the contribution of spin to adsorption energy reveals a nonlinear dependence on spin moment; the spin effect becomes pronounced only above a critical spin moment, close to one-half of the ground-state value of the corresponding isolated magnetic atom. Below the critical spin moment, adsorption-induced d-state reconstruction compensates for the spin effect, whereas this compensation breaks down above it. Importantly, this nonlinear behavior is observed across diverse adsorbates, suggesting its generality. These findings establish a critical spin-moment criterion for spin-regulated adsorption and redefine the applicability of spin engineering in heterogeneous catalysis.