Understanding the synergetic effect from foreign metals in bimetallic oxides for PMS activation: A common strategy to increase the stoichiometric efficiency of oxidants

Synergistic effect was commonly observed in bimetallic oxide catalysts in sulfate radical-based advanced oxidation processes by forming spinel structures. In this paper, we found the incorporation of Cu demonstrated the most remarkable promotional effect to Co oxides comparing with Fe and Mn in PMS activation, and no spinel structure of mixed metals was found. Moreover, the synergism between oxides of Cu and Co was observed on various supports including MgAl layered double hydroxides, mesoporous MnO2 and ZnO. Under conditions of 0.2 g L-1 catalysts, 2 mM PMS, and 40 ppm 4-chlorophenol (4-CP), CuCo@ZnO/PMS system demonstrated excellent performance that 96.0% 4-CP could be removed, while the removal efficiency of 4-CP in Cu@ZnO/PMS and Co@ZnO/PMS systems was only 12.1% and 45.4%, respectively. The simultaneous incorporation of Co and Cu bimetallic oxides in CuCo@ZnO led to a higher surface hydroxyl density, and this surface hydroxyl density showed a linear relationship to their first order kinetic constant in 4-CP degradation. Moreover, the results of peroxymonosulfate (PMS) decomposition, quantification of sulfate radical (SO4 center dot-), EPR, XPS, H2-TPR, and CV indicated that the synergism of Cu/Co also arose from the electron transfer between Cu(I) and Co(III), which altered the redox circle of Co(II)/Co(III) and increased the steady concentration of oxidative radicals. Due to this synergism, the degradation percentage of organic contaminants, stability of heterogeneous catalysts and the stoichiometric efficiency of PMS were all improved. These findings were highly suggestive to the further development of bimetallic heterogeneous catalysts in persulfate based advanced oxidation processes.