Preparation And Properties Of High Performance Modified Metal Oxides Electrodes For The Degradation Of Nitrophenols In Aqueous Solutions

Electrocatalytic oxidation as one of the Advanced Oxidation Processes (AOPs) was effectively to treat some intractable organic pollutants, such as phenols, the aromatic amines, the aromatic hydrocarbons and the pesticides. Both the development of electrodes with high stability and enhancement of efficiency of electrochemical system are hot topics in the field of electrochemistry. Nitrophenols (NPs) represent a class of widely synthesized chemicals particularly involving in the manufactures of pesticides, dyes and pharmaceuticals, which are anthropogenic, toxic, inhibitory and bio-refractory organic compounds and are considered as hazardous substances and priority toxic pollutants. It is of significant importance to develop new treatment technologies for the destruction and mineralization of NPs in wastewater. In the present work, the following investigation have been carried out for the purpose of development of metal oxides electrodes with high stability, degradation of nitrophenols and enhancement of efficiency of entire electrochemical system.A set of modified PbO₂ anodes doped with the oxides of Bismuth and Cobalt were prepared by the means of electrodeposition in nitrate solutions. Scanning electronic microscopy (SEM), Atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to characterize the morphology and crystal structure of modified PbO₂ anodes. The electrochemical properties of these modified PbO₂ anodes were studied by means of linear sweep, cyclic voltammetry and electrochemical impedance spectroscopy (EIS), respectively. The service lives of modified anodes were obtained in terms with accelerated life tests. Oxidants such as hydroxyl radical, hydrogen peroxide and hypochlorite ion were determined. Electrocatalytic oxidation of o-nitrophenol (o-NP) was conducted by using these electrodes as anode and stainless steel sheet as cathode. The results indicated that Bismuth doped PbO₂ electrode (Ti/Bi-PbO₂) was characterized of smaller crystal size, compact structure and rough surface. This anode displayed a better electrocatalytic activity and higher stability than those of Ti/β-PbO₂ anode, as well as other modified anodes. In addition, the current efficiency of Ti/Bi-PbO₂ was the highest of all PbO₂ anodes. Ti/Bi-PbO₂ anode had the highest electrocatalytic activities and the lowest energy consumption. The Ti/Bi-PbO₂ anode was a promising anode for the treatment of organic pollutants.The electrocatalytic oxidation of o-nitrophenol (o-NP), m-nitrophenol (m-NP) and p-nitrophenol (p-NP), 2, 4-dinitrophenol (2, 4-DNP), 2, 5-dinitrophenol (2, 5-DNP), 2, 6-dinitrophenol (2, 6-DNP) and 2, 4, 6-trinitrophenol (2, 4, 6-TNP) has been studied on Bi-doped lead dioxide anodes in acid medium by cyclic voltammetry and bulk electrolysis. The mineralization and kinetics of organic compounds were studied in the course of electrolysis. The intermediates accumulated during the electrolysis were analyzed qualitatively and quantitatively. Furthermore, possible degradation pathways of nitrophenols were proposed. The results of voltammetric studies indicated that these nitrophenols were indirectly oxidized by·OH radical in the solutions. Within the present experimental conditions used, almost complete elimination of nitrophenols and more than 90% mineralization were achieved. The electrocatalytic oxidation of NPs lay in the order: o-NP >m-NP >p-NP >2, 6-DNP >2, 5-DNP >2, 4-DNP >2, 4, 6-TNP. The intermediates generated during the electrolysis of nitrophenols were determined and analyzed. Nitrate ion is identified as the major nitrogen final reaction product during the NPs oxidation, while a minor amount of ammonia is left at the end of electrolysis, which indicated that almost all the nitro groups detached from aromatic rings. The results of LC / MS and HPLC suggest that three kinds of intermediates are generated, i.e. polyhydroxylated intermediates, reduction products of NPs and carboxylic acids. In the long run, polyhydroxylated intermediates and reduction products of NPs were eventually oxidized to micromolecular carboxylic acids, such as maleic acid, oxalic acid, acetic acid and formic acid, etc. The possible degradation pathways of NPs were proposed, including three major steps: 1) the denitration and substitution by hydroxyl radicals on aromatic rings seem to be the first stage; 2) aromatic ring-opening reactions took place to generate carboxylic acids; 3) carboxylic acids were further oxidized into CO₂ and H2O.Cerium doped lead dioxide anode, i.e. Ti/Ce-PbO₂, was prepared by electrodeposition. After the optimization of preparation technique, a Ti/Ce-PbO₂ anode with lower charge transfer resistance and higher electrocatalytic activity was obtained. SEM, AFM, XRD and X-ray photoelectron spectrometry (XPS) were used to characterize the morphology, crystal structure and elements states of modified anode. The results of SEM, AFM and XRD showed that the crystal size of Ti/Ce-PbO₂ anode was smaller than undoped PbO₂ (Ti/β-PbO₂) and no diffraction peaks corresponding to CeO₂ formed. The result of accelerated life test implied that Ti/Ce-PbO₂ anode had favorable electrochemical stability. The electrochemical oxidation of o-NP on Ti/Ce-PbO₂ anode displayed a faster degradation rate and higher mineralization efficiency than Ti/β-PbO₂ anode (the degradation rate of former was 2.29 times higher than that of latter). In addition, Ti/Ce-PbO₂ anode had higher current efficiency and lower energy consumption (the energy consumption of former decreased by 18% compared with latter). Comparing the properties of Ti/Ce-PbO₂ anode before and after use, this modified lead dioxide electrode displayed benign stability. The results indicated that the incorporation of Cerium fascinated to improve the stability and electrocatalytic activity of lead dioxide anode. The investigations on the effect of co-exsting substances on electrochemical oxidation of o-NP came into the following conclusions. The degradation of o-NP was nearly not impacted in the presence of metal ions, alcohols, organic acids and surfactant. The present electrochemical system displayed a reliable stability and resistance to impact of additional load.The preparation parameters for the Cerium doped ternary SnO₂ based oxides anode were optimized. When the molar percentages of Cerium and Ruthenium were 1% and 5%, the prepared anode had high electrocatalytic activity and stability. The results of SEM and XRD revealed that the incorporation of Cerium could decrease the cracks of anode surface, enhance the specific surface area and diminish the crystal size of modified SnO₂ anode, as well as cause a better dispersion of oxides. The results of accelerated life test indicated that the service life of Ce-Ru-SnO₂ anode was longer than that of traditional SnO₂-Sb2O5 anode. Comparing the properties of Ce-Ru-SnO₂ anode before and after use, almost no evident difference was observed, which demonstrated that this anode had benign stability. Nitrophenols could be effectively eliminated on Ce-Ru-SnO₂ anode. The degradation of NPs lies in the order: o-NP>p-NP>m-NP>2, 5-DNP>2, 4-DNP>2, 6-DNP>2, 4, 6-TNP. The comparison of Ce-Ru-SnO₂ anode with Ti/Bi-PbO₂ anode indicated that the electrocatalytic activity of former was superior to that of latter. Nevertheless, the service life of latter was longer.