Pyridine electrooxidation on polycrystalline platinum: a combined computational and electrochemical mechanistic study

Pyridine is a resistant N-containing heterocyclic pollutant widely found in industrial wastewater, which cannot be effectively removed by traditional treatment techniques. A computational and electrochemical study of the electrooxidation mechanism on polycrystalline platinum is presented. A favorable vertical N-down adsorption geometry was suggested by molecular docking analysis, which was supported by the presence of nitrogen lone pair. Density functional theory (DFT) calculations concluded the HOMO and LUMO energies were -5.670 and -1.697 eV, respectively, indicating good kinetic stability and resistance to oxidation. Based on molecular electrostatic potential (MESP), Fukui function, and global conceptual descriptors, the nitrogen atom and ortho-carbon atoms are shown to be the main reactive sites. Cyclic voltammetry (CV) showed that the oxidation was completely suppressed at pH 2, which was believed to be due to protonation of pyridine. At pH 7 and 12, in contrast, irreversible anodic oxidation was observed which had no cathodic peak. The scan rate studies have confirmed, with validation, that the kinetics are first order with respect to scan rate, which in turn supports the electrochemical chemical (EC) oxidation pathway and confirms that the oxidation is initiated by direct electron transfer from adsorbed pyridine.

The adsorption geometry on a cluster of size Pt₁₃ was carefully modeled using AutoDock Vina v1.1.2. Electronic structures and reactivity descriptors were calculated at the DFT level of calculations (B3LYP/6-31G(d)/ water polarizable continuum model (PCM) using ORCA software. The HOMO-LUMO energies and molecular orbital distributions were directly extracted from the output files of the DFT program ORCA, and subsequently visualized in the Avogadro v2 software. Only CV was used for experimental electrochemical kinetics and only in phosphate buffers on a platinum electrode.