Development of manganese oxide films for the electro-oxidation of phenol at high temperature and pressure

This thesis is divided into three main parts. In the first part, the concept of a MJ-PEM reactor will be introduced, and its design and calculations will be explained. A MJ-PEM reactor is the result of the coupling of a Multi-Junction Solar Cell (MJSC) and a Polymer Electrolyte Membrane (PEM) electrolyzer, able to work at high temperatures and pressures (up to 150°C and 30 bar). Two scenarios for the application of this system were investigated: in the first one, the anodic chamber is used for the oxidation of recalcitrant organics contained in wastewater, while the cathodic compartment is used for the evolution of H2, for storage or direct use on site; in the second one, the H2 produced at the cathode is sent to an anaerobic digestion process, to boost the biomethanation step, whereas at the anode O2 is evolved and it is exploited for the digestate stabilization and disinfection. Both the scenarios proved to be feasible and effective, due to a high degree of integration between stoichiometric and thermal requirements of different systems, allowing to carry out both waste or wastewater treatment on one side, and hydrogen or natural gas production on the other side. The second part of this work concerns the synthesis and the characterization of electrodes based on manganese oxides, for the electro-oxidation of recalcitrant organics. Phenol was chosen as target molecule, due to its high refractoriness and stability, and its wide presence in industrial plants. Manganese oxides are extensively used in electrochemistry, and they were chosen because of their low cost, high abundance, and low toxicity. Different types of manganese oxides (MnOx) were synthesized by electrodeposition on two substrates, namely metallic titanium and titania nanotubes (TiO2-NTs). X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) were used to analyze the oxidation states of manganese, whereas Field Emission Scanning Electronic Microscopy (FESEM) was employed to investigate the morphology of the samples and the penetration of manganese oxides inside the NTs. The electrochemical properties of the electrodes have been investigated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV), showing that both calcination and electrodeposition over TiO2-NTs gave more stable electrodes, exhibiting a marked increase in the current density. The activity of the proposed nanostructured samples towards phenol degradation has been investigated. Tetravalent manganese (α-MnO2) resulted to be the most active phase, with a phenol conversion of 42.7%. Trivalent manganese (α-Mn2O3), instead, reported the highest stability, with an average working potential of 2.9 V vs. RHE, and the highest tendency for oxygen evolution reaction, reaching 0.4 mA/cm2 at 2.5 V vs. RHE. TiO2-NTs interlayer contributed in all cases to the decrease in the final potential reached after the reaction time of about 1 – 1.5 V, due to the improved contact with the catalyst film and the prevention of passivation of the titanium substrate. In the third part, the most performing electrodes were selected from the ones synthesized in the second part. They were tested in High Temperature, High Pressure (HTHP) reactor, designed in Politecnico di Torino for kinetic studies on electro-degradation of refractory organics in wastewaters, under Catalytic Wet Air Oxidation (CWAO) conditions, i.e. 150°C and 30 bar. The most stable (α-Mn2O3) and the most active (α-MnO2) manganese oxides were compared, both at ambient and CWAO operative conditions, with some of the most effective electrodes used in this field: Sb-doped SnO2 and RuO2. Results showed that manganese oxides, especially α-Mn2O3, is more than tripled at 150°C and 30 bar, reaching values of phenol oxidation close to the ones of Sb-SnO2 and RuO2. This phenomenon can be attributed to the higher tendency of manganese in its Mn3+ form to oxidize water to O2, that is wasted at ambient conditions, while is better employed at high temperatures (high kinetics, low overpotentials) and high pressures (improved O2 solubility).