Encapsulating Co and Pd Nanoparticles as Spatially Separated Dual Active Sites for Heterogeneous Persulfate Activation: Synergistic Catalysis and Switching of the Primary Reaction Pathway

This study demonstrates that the carbon encapsulation of Pd and Co as spatially isolated redox-active sites can synergistically enhance the activation of peroxymonosulfate (PMS) and peroxydisulfate (PDS) and enable persulfate precursor-sensitive degradation routes. The superiority of bimetal-carbon composites (i.e., Pd/Co@NC) was confirmed based on a higher efficiency of Pd/Co@NC with varying Pd/Co ratios for persulfate activation than the sum of efficiencies of single metal-component catalysts applied at corresponding dosages. Treatment performances of Pd/Co@NC with different metal compositions aligned with the dependence of electrical conductivity and binding affinity of Pd/Co@NC on the relative metal content. Reflecting differential reactivity of monometallic components toward persulfate, the primary degradation pathway was switched, depending on the persulfate type. Pd/Co@NC caused radical-induced oxidation upon PMS addition while initiating nonradical PDS activation through electron-transfer mediation, based on retarding effects of radical scavengers, reactivity toward multiple organics, Koutecký-Levich plots, electron paramagnetic spectral features, and product distribution. The fabrication strategy to enable the separate carbon encapsulation of two metallic sites with different catalytic reactivity created metal-carbon composites that retained the advantages of radical and nonradical persulfate activation under realistic treatment conditions; i.e., treatability of a wide spectrum of organics and minimal interference of background compounds in complex water matrices.