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.