Surface Hydroxyls as Proton Transit Stations Enable Hydroxyl‐Density‐Mediated Switching via Proton‐Coupled Electron Transfer for High‐Valent Cobalt‐Oxo Species Evolution

ABSTRACT The selective generation of high‐valent cobalt‐oxo species (Co(IV) = O) in peroxymonosulfate (PMS)‐based advanced oxidation processes (AOPs) is a long‐sought goal for targeted degradation of electron‐rich pollutants such as monoethanolamine (MEA). However, it is kinetically crippled by the high energy barrier of O─H bond deprotonation in PMS. Here, it is demonstrated that surface hydroxyl groups on Co‐based catalysts act as proton acceptors to overcome this bottleneck, unlocking a proton‐coupled electron transfer (PCET) pathway. By systematically varying the cobalt loading to control the aggregation state, three catalysts are constructed, namely single atoms (CoSA), atomic clusters (CoAC), and nanoparticles (CoNP), which exhibit a progressively decreasing surface hydroxyl density. A combination of structural, in situ spectroscopic, and 18 O isotope labeling analyses reveals that the high hydroxyl density on CoSA switches the pathway of Co(IV) = O formation from the conventional two‐electron oxygen transfer pathway to a highly efficient single‐electron process. As a result, CoSA completely removes MEA and other electron‐rich pollutants within 20 min with exceptional selectivity, while the hydroxyl‐deficient CoNP falls back to a non‐selective radical pathway. The causal link from cobalt aggregation state to surface hydroxyl density and catalytic pathway offers a programmable design strategy for AOP catalysts, enabling precise control over reaction selectivity.