Doping of Magnéli Phase-New Direction in Pollutant Degradation and Electrochemistry

Abstract

This review summarizes the recent developments in titanium suboxide (TSO) doping and the application of doped materials in pollutant degradation and electrochemistry. Doping is mainly limited to transition and rare-earth metals, with some exceptions, of similar ionic radii and charge, that can replace Ti ions in TSO without too much disturbance to the lattice. Consequently, doping is limited to below 10 at%, which predominantly induces oxygen vacancy formation. Doping mechanisms are weighted, and their effect on conductivity, stability, and catalytic activity is overviewed. High-temperature H2 reduction of TiO2 is still the dominant preparation method, with carbothermal reduction and Ti reduction gaining ground due to safety and energy concerns. Doping predominantly increases the conductivity 2-5 times, while the stability can be both improved or worsened, depending on the size and charge of the doping ion. Electrochemical oxidation, at positive overpotentials, of per- and polyfluoroalkyl substances (PFAS), antibiotics, and other water pollutants, is the main avenue of application. Doping almost exclusively leads to complete selected pollutant degradation and improvement of the pristine TSO, which is summarized in detail. New niche applications of peroxide, hydrogen, and chlorine production are also viable on doped TSO and are touched upon. Complementing experimental results are theoretical calculations, and we give an overview of density functional theory (DFT) results of transition metal-doped TSOs, identifying active centers, degradation trends, and potential new doping candidates.