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

Abstract

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.