Beyond d-Band Models: Unoccupied Valence Band Ratio Governs CO Oxidation on BiCuSeO-Supported Single-Atom Catalysts

Developing reliable descriptors that correlate catalyst electronic structure with performance is crucial for designing efficient single-atom catalysts (SACs). Here, we report a systematic first-principles study of CO oxidation on single transition-metal atoms (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt) anchored on 2D BiCuSeO. These catalysts primarily follow the Eley-Rideal mechanism, exhibiting remarkably low rate-limiting barriers ranging from 0.19 to 0.39 eV. We propose a comprehensive, orbital-resolved descriptor, the unoccupied valence band ratio, which integrates both s and d state contributions and serves as a superior descriptor for catalytic activity (R2 = 0.90), outperforming conventional d-band center and unoccupied d-band ratio models. Electronic structure analyses clarify that metal s orbitals play a critical, often-overlooked role in synergistic electron donation/back-donation during adsorbate activation. These findings position BiCuSeO as a promising SAC support for CO oxidation, underscoring the importance of leveraging non-d valence states for rational SAC design.