Electrodes Based on Zeolites Modified with Cobalt and/or Molybdenum for Pesticide Degradation. Part I: Physicochemical Characterization and Efficiency of the Electrodes for O₂ Reduction and H₂O₂ Production

With the purpose of obtaining inexpensive electrodes for the degradation of organic pesticides by the electro-Fenton reaction, the required H₂O₂ being obtained by the 2-electron reduction of dissolved O₂, we have prepared glassy carbon electrodes coated with a mixture of graphite with Mo- and/or Co-modified zeolites. Three zeolites were used, Linde type A (ZA), Faujasite (ZY), and MFI (ZSM5), whose maximum possible cation exchange, directly given by the Al/Si ratio, and their hydrophilicity increases in the order ZSM5 < ZY < ZA. The zeolites were modified with Mo and/or Co by the wet impregnation method and characterized by different techniques. The outer surfaces of the three Mo-modified zeolites showed Mo-containing grains (in ZA) or needles (in ZY and ZSM5), which could be largely washed away with hot water. Electrodes were made by depositing on a disc of glassy carbon (GC) a mixture of graphite, zeolite, and a binder. Quite unexpectedly, the cyclic voltammograms (CVs) of the three Mo-modified zeolites showed at least five pairs of anodic–cathodic peaks, which we assume are due to the presence of the Mo₇O₂₄ ⁶⁻ isopolyoxomolybdate anion, proceeding from the impregnating solution, and anchored on the zeolites’ surface. With a rotating ring-disc electrode, the highest efficiency for H₂O₂ production at − 0.2 V_RHE, namely, 12.7%, was obtained with the GC/graphite-(CoMo-exchanged ZA) electrode, but this efficiency decreased with time. On the contrary, the three zeolites modified only with Mo were stable in 4-h electrolyses at − 0.2 V_RHE and yielded the highest H₂O₂ concentrations, which we attribute to the Mo₇O₂₄ ⁶⁻ isopolyoxomolybdate anchored on the zeolites. The H₂O₂ yield was the same for the three Mo-modified zeolites, irrespective of their exchange capacity and hydrophobic/hydrophilic character. [Figure not available: see fulltext.].