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

Abstract

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.