Environmental electrochemistry of organic compounds at metal oxide electrodes

[摘要] ENGLISH ABSTRACT: This study investigates the electrochemical oxidation of phenol. Phenol is a major toxinand water pollutant. In addition, during water treatment it reacts with chlorine to producecarcinogenic chlorophenols. lts treatment down to trace levels is therefore of increasingconcern.For this purpose, dynamically stable anodes for the breakdown of phenols to carbondioxide or other less harmful substances were developed and characterized. The anodeswere prepared from mixed oxides of tin (Sn) and the precious metals ruthenium (Ru),tantalum (Ta) and iridium (Ir), which in tum were prepared using sol-gel techniques. Thisinvolved dip-coating the aqueous salts of the respective metals onto titanium substratesand heating to temperatures of several hundreds of degree Celsius.The properties of these mixed oxide thin films were investigated and characterized usingthermal gravimetric analysis (TGA), scanning electron microscopy (SEM), atomic forcemicroscopy (AFM), elemental dispersive energy X-ray analysis (EDX), X-ray diffraction(XRD), Rutherford backscattering spectrometry (RBS), particle induced X-ray emission(PIXE) and electrochemical measurements.A variety of different electrode materials including Til Sn02-Ru02-Ir02, Ti/Ta20s-Ir02and Ti/RhOx-Ir02 were developed and tested for their potential as oxidation catalysts fororganic pollutants in wastewaters. Depending on the anode type, phenol was found to be electrochemically degraded, to different extents, on these surfaces during electrolysis. Itwas however found that the oxidation rate not only depended on the chemicalcomposition but also on the oxide morphology revealed, resulting from the preparationprocedure. The Ti/SnOz-Ru02-Ir02 film was found to be the most efficient surface for theelectrolytic breakdown of phenol. This film oxidized phenol at a potential of 200 mV vsAg/AgC!.The activity of the catalytic systems was evaluated both on the basis of phenol removalefficiency as well as the kinetics of these reactions. Phenol removal efficiency was morethan 90% for all the film surfaces prepared and the rate of the reaction followed firstorder kinetics. A pathway for the electrochemical degradation of phenol was derivedusing techniques such as HPLC to identify the breakdown products. These pathwayproducts included the formation of benzoquinone and the further oxidation ofbenzoquinone to the carboxylic acids malic, malonic and oxalic.

[发布机构] Stellenbosch University