Catalytic Effect of Silver Cathodes on 3,4,5,6-Tetrachloropicolinic Acid Dechlorination in Aqueous Solutions

Dechlorination of 3,4,5,6-tetrachloropicolinic acid (TeCP) on roughened silver (Ag(r)) cathodes provides an unexpected example showing extraordinary catalytic effect in aqueous solution, which is counter to what has been reported in electrochemical reduction of organic halides using aprotic media. To fully recognize this extraordinary catalytic effect of silver cathodes on electrochemical reduction of 3,4,5,6-tetrachloropicolinic acid in aqueous solutions, we conduct a comprehensive study from the aspect of surface characterization, in situ electrochemical study, and theoretical calculation. Transmission electron microscopy (TEM) images and X-ray photoelectron spectroscopy (XPS) spectra are presented to observe the surface structure and chemical state of Ag(r). Experimental results show that Ag nanoparticle can be formed in the oxidation-reduction cyclic (ORC) process, which leads to an increase in the degree of surface disorder. Density functional theory (DFT) calculations of the first electron transfer (ET) process, integrated with an in situ electrochemical surface-enhanced Raman spectroscopy (SERS) study and a cyclic voltammetry (CV) experiment with the aid of H+, were performed to characterize various surface species in different electrode potential regions. Experimental evidence shows that the first ET process is catalyzed by silver for the radical derivate (TeCP center dot-) formed by the ET process is adsorbed more strongly than TeCP. TeCPads center dot- ads is the key intermediate of the dechlorination process, implying that the dechlorination mechanism could drastically differ from the outer-sphere reduction at the glass carbon (GC) electrode.