Spin engineering of single-site metal catalysts.

Single-site metal atoms (SMAs) on supports are attracting extensive interest as new catalytic systems because of maximized atom utilization and superior performance. However, rational design of configuration-optimized SMAs with high activity from the perspectives of fundamental electron spin is highly challenging. Herein, N-coordinated Fe single atoms are successfully distributed over axial carbon micropores to form dangling-FeN(4) centers (d-FeN(4)). This unique d-FeN(4) demonstrates much higher intrinsic activity toward oxygen reduction reaction (ORR) in HClO(4) than FeN(4) without micropore underneath and commercial Pt/C. Both theoretical calculation and electronic structure characterization imply that d-FeN(4) endows central Fe with medium spin (t(2g) (4) e(g) (1)), which provides a spin channel for electron transition compared with FeN(4) with low spin. This leads to the facile formation of the singlet state of oxygen-containing species from triplet oxygen during the ORR, thus showing faster kinetics than FeN(4). This work provides an in-depth understanding of spin tuning on SMAs for advanced energy catalysis.