Enhanced electrochemical oxidation of phenol over manganese oxides under mild wet air oxidation conditions

Low-cost manganese oxide, MnOx-based electrocatalysts, containing a-MnO2 and mixed a-Mn2O3/a-
MnO2 phases, were synthesized by scalable anodic and cathodic electrodeposition methods, respectively.
Their morphological and chemical composition were characterized by means of Field Emission Scanning
Electronic Microscopy (FESEM), X-Ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS).
These electrodes were tested for the electro-oxidation of a recalcitrant molecule (i.e. phenol) in a labscale
high temperature and high pressure (HTHP) batch electrocatalytic reactor. Their electrocatalytic
activity was compared with that of state-of-the-art anodes for phenol electro-oxidation: antimonydoped
tin oxide (SnO2eSb5þ) and ruthenium oxide (RuO2): first, under standard ambient conditions, and
then, under the conditions of a Polymeric Electrolyte Membrane (PEM) electrolyzer (i.e. 85 C and 30 bar)
and of mild Catalytic Wet Air Oxidation (CWAO, i.e. 150 C and 30 bar). Both reaction time and current
density were varied to investigate their effect in the performances of the system as well as on the reaction
mechanism. Both MnOx electrodes reported enhanced conversion efficiencies, up to ~75%, at the
highest pressure and temperature, and at the lowest applied current density, which influenced the
process by improving dissolution of the O2 evolved, the reaction kinetics and thermodynamics, and by
minimizing irreversibilities, respectively. The here reported MnOx films achieved conversion and
mineralization efficiencies comparable to Sb-SnO2 (that is the more toxic) and RuO2 (that is more
expensive) materials, operating under mild CWAO operation conditions, which demonstrate the potential
of the electrocatalytic HTHP process as a sustainable advanced oxidation technology for wastewater
treatment or electrosynthesis applications.