Orbital Engineering in Single-Atom Catalysts for Benzene to Phenol Oxidation

The rational design of high-performance single-atom catalysts (SACs) for the selective oxidation of benzene to phenol under mild conditions requires a deep understanding of orbital-level interactions governing catalytic activity. Herein, we report a systematic investigation of transition metal SACs (M/NC, M = Mn, Fe, Co, Ni, Cu, Zn) embedded in N-doped carbon nanosheets for benzene oxidation to phenol. Our study reveals that the catalytic performances exhibit a volcano-shaped trend with respect to the metal atomic number, with Cu/NC showing the highest activity at 81.4% benzene conversion, and Zn/NC and Mn/NC exhibiting nearly negligible activity. The selective oxidation of benzene to phenol over M/NC catalysts proceeds in two consecutive steps: H2O2 is initially activated at the metal site, followed by benzene oxidation at the adsorbed oxygen atom. Given this two-step mechanism, and based on density functional theory (DFT) studies, we propose a dynamic active-sites theory: initial H2O2 activation depends on the metal d-band center, whereas subsequent benzene oxidation is facilitated by the p-band center of the adsorbed oxygen intermediate. These orbital descriptors exhibit opposing electronic requirements, creating a dual-constraint design principle for optimal catalysis. Cu/NC simultaneously satisfies the dual requirements, possessing a moderate d-band center that facilitates H2O2 dissociation and an elevated oxygen p-band center that promotes C–H bond activation, thereby minimizing the overall energy barrier. This work establishes a quantitative orbital-level framework for SACs and provides a general design strategy for dynamic active sites in selective oxidation reactions.