| In this paper, the preparations, analytical methods, physical characteristics of ferrates (Ⅵ) and the application of ferrates (Ⅵ) in the fields of synthesis of organic compounds, treatment of waste water and the usage as cathode material in alkaline batteries are summarized. The electrochemical preparation of sodium ferrate (Ⅵ) is described and the relationships between the main parameters in the process are deeply investigated. One method of in-situ electrosynthesizing potassium ferrate(Ⅵ) of high purity with both high efficiency and low energy consumption is invented. The cyclic voltammogram and feature characterization of iron anode in alkaline solutions is studied. One preparation method of ex-situ electrosynthesizing tripotassium sodium ferrate(Ⅵ) of high purity is created. Many kinds of modern techniques are used to characterize the obtained K3Na(FeO4)2 sample.The preparation of ferrate (Ⅵ) by the anodic dissolution of an iron wire gauze at an average current density of 4.8 mA·cm-2 is described in the third chapter of this paper. The cyclic voltammogram and SEM are used to characterize the iron electrode polarization. And the relationships between the main parameters in the process are deeply investigated. By using this method, an anolyte of 0.35～0.49 mol·L-1 Na2FeO4 can be produced during 3～6 h electrolysis in initial the 16 mol·L-1 NaOH solution at 35℃. Analysis in both experiment and theory indicate that the electrogeneration rate of ferrate(Ⅵ) is close related to the factors such as current density, alkaline concentration and the ratio of effective surface area to anolyte volume, and the quantitative relationship between them is given. The apparent electrogeneration rate of ferrate(Ⅵ) is determined by three factors such as the electrogeneration and decomposition of ferrate(Ⅵ) and passivation film formed on the iron anode surface. And the relationships of the above parameters are described. The experiments show that it is hard for the anode passivation to occur during a continuous electrolysis in the NaOH solution with a concentration larger than the critical value between 14 and 15 mol·L-1 at 35℃.In the fourth chapter, an electrosynthesis from an iron wire gauze in KOH electrolyte is first presented for the in situ direct synthesis of the solid K2FeO4 with high purity by one step, and the techniques of FTIR, XRD, SEM, EDX are used to characterize the obtained sample. This method of preparing K2FeO4 can reach a highest efficiency of 73.2%, purities of 95.3%～98.1% and a highest yield of 49 g·l-1 K2FeO4 and its energy consumption is only 2.1 kWh·(kg K2FeO4)-1. The optimal parameters are as below: electrolyte temperature 65～75℃, 14.5 mol·L-1 KOH, current density 1.0～1.7 mA·cm-2. Comparison experiment shows that electro -synthesis of solid K2FeO4 in KOH solution is far better than that in NaOH as a whole. It is found that the higher temperature is very favorable for the in-situ electrosynthesis of K2FeO4 and it is stable very much under this condition. Comparison analysis for the K2FeO4 samples shows that the in-situ and ex-situ electrochemically synthesized K2FeO4 powders exhibit similar IR absorption spectra and XRD patterns to the chemical synthesized K2FeO4, but different contents of sodium and crystal morphologies. As the cathode materials, their discharging specific capacity at different discharging rates are some different. The electrochemical experiments show that the higher content of electric agent is helpful to increase the discharge efficiency of ferrates(Ⅵ). But K2FeO4 is unstable and decomposed in the wet state during preservation in seal.In the fifth chapter, the CV and EIS are used to study the polarizations of iron electrode in the systems of NaOH and KOH, respectively. The results show that the current peak of Fe(Ⅵ) will appear on the CV curves of both the systems of NaOH and KOH under the suitable conditions of temperature and alkaline concentration when the electrode potential scanning rate is slow enough. The evolution potential difference between Fe(Ⅵ) and oxygen in KOH system is 30～50mV, larger than that of NaOH one. It displays that the former is more suitable to electrosynthesize ferrate(Ⅵ) and easier to gain higher current efficiency.The polarization of pure iron electrode in NaOH is investigated in this chapter. The result shows that the higher the temperature, the lower the alkaline solution concentration, the greater the current density, the easier to occur the passivation of iron anode. The passivation of iron may become too weak to see under a suitable condition, and the condition is of: temperature 35℃, current＜4.7 mA·cm-2, alkaline concentration＞14.5 mol·L-1. At meantime, the three-step characterization during the polarization of iron electrode in NaOH solution is described.In the sixth chapter, an electrosynthesis in concentrated NaOH electrolyte is first presented for the synthesis of 0.83 mol·L-1 Na2FeO4 solution, and then for the synthesis of solid K3Na(FeO4)2 with high purity and yield. The techniques of FTIR, XRD, AAS, SEM, EDX, TG/DSC are used to characterize the obtained sample. The conditions have been explored. The synthesis experiment shows that it is easy to obtain a solution of 0.8～0.83 mol·L-1Na2FeO4 by using the two-step method. The optimal parameters are as below: temperature 31±1℃, 20 mol·L-1 NaOH, current density 4.3 mA·cm-2, and the corresponding cumulative efficiency 43%～50%. The key operation in this method is that it is nesissary to decrease the catalysis decomposition effects from the great amount of Fe(OH)3 produced during the electrolysis. Crystal experiment shows that adding less KOH to the above solution of Na2FeO4-NaOH can lead to the solid K3Na(FeO4)2, but adding more KOH to it can lead to the solid K2FeO4. And the dissolution-precipitation curve of K3Na(FeO4)2 in the mixed NaOH-KOH solutions is determined. The measured solubility of K3Na(FeO4)2 in concentrated KOH electrolytes is almost the same as that of K2FeO4. Different from K2FeO4 crystals, the synthesized K3Na(FeO4)2 powders exhibit three characteristic IR absorption peaks (787, 801～802 and 858～862 cm-1) and a hexagonal unit cell with the space group P3 ml (164). The results of TG/DSC experiments show that the K3Na(FeO4)2 powders will not decompose until 197℃in the atmosphere of Ar, but less stable than K2FeO4.
|
PDF Full Text Request