Mechanistic Study of Electrocatalytic Perchlorate Reduction using an Oxorhenium Complex Supported on a Ti₄O₇ Support

Developing a stable and active catalyst for ClO4- reduction at nonacidic pH has presented a significant challenge to the catalysis field. Previous research has demonstrated that by depositing an organometallic Re catalyst onto a Ti4O7 support (Re/Ti4O7), it was possible to stabilize the catalyst and obtain active electrocatalytic ClO4- reduction at circumneutral pH. Thus, the focus of this work was on elucidating the mechanisms of electrocatalytic ClO4- reduction in water with the Re/Ti4O7 system. Density functional theory (DFT) simulations indicated that the adsorption of the Re catalyst was exothermic on Ti4O7, and X-ray photoelectron spectroscopy (XPS) characterization indicated that Re adsorption caused a net reduction of the Ti oxidation state on the Ti4O7 surface. After ClO4- reduction experiments, XPS results indicated the presence of Ti(0)/Ti(II) surface sites. Cyclic voltammetry experiments in an acetonitrile solvent provided supporting evidence that these surface sites were electroactive and likely participated in the ClO4- reduction reaction. Analysis of batch reduction experiments in acetonitrile via kinetic modeling estimated a catalyst turnover number of 332 +/- 23, which provided further evidence that the reduced Ti sites could regenerate the Re catalyst. However, these reduced Ti sites were finite in number and required the production of adsorbed hydrogen via water reduction to facilitate continuous ClO4- reduction. DFT results indicated that the reduction of ClO4- to Cl- was exothermic and that reduced Ti sites participated in the reduction reaction. The experimental and DFT results allowed a preliminary mechanism for ClO4- reduction on Re/Ti4O7 to be proposed.