Kwang-Wook Kim

Study on the electro-activity and non-stochiometry of a Ru-based mixed oxide electrode

Catalytic oxide electrodes of Ru/Ti, RuSn/Ti, and RuSnTi/Ti effective in destroying refractory organics in aqueous waste were studied in terms of material and electrochemical properties. The portion of non-stochiometry, RuOx (0<x<2) and chloride-bonded Ru within the Ru-based oxide were evaluated by deconvolution of XPS of the oxide with a change of sintering temperature. RuOx mostly existed around 450°C, and disappeared at 600°C. The component of chloride-bonded Ru decreased nearly linearly from 350 to 600°C. The non-stochiometry resulted from the incomplete decomposition of metal chloride in the precursor solution. The existence of non-stochiometry was confirmed to have a close relation to the electrochemical activity of the oxide electrode. The segregation phenomena of Sn and the existence of chloride in RuSnTi/Ti oxide after sintering were confirmed by AES, XPS, and EPMA.

A relation between the non-stoichiometry and hydroxyl radical generated at photocatalytic TiO2 on 4CP decomposition

Abstract In order to know an interrelationship between OH radical generated on the TiO 2 surface under an UV irradiation and the existence of non-stoichiometric titanium oxide of TiO x (0 x 2 , SIMS measurements and peak analyses of XPS on three kinds of TiO 2 samples were carried out, and ESR spectra of DMPO-OH adduct through spin-trapping of OH radical generated on the TiO 2 samples were measured. The generation of OH radical was approximately linear to the degree of non-stoichiometry of the TiO 2 . Removal of TOC of 4CP was affected by the total surface area of the particles in the TiO 2 slurry and the non-stoichiometry within TiO 2 . The particle size in the slurry depended on the mixing condition of the preparation of the slurry and the morphology of agglomeration of TiO 2 particles. The TiO 2 particles in the slurry were dispersed in much smaller size than those of TiO 2 samples on SEM.

Effects of the different conditions of uranyl and hydrogen peroxide solutions on the behavior of the uranium peroxide precipitation.

The dynamic precipitation characteristics of UO(4) in different solution conditions (pH, ionic strength, uranium and H(2)O(2) concentrations) were characterized by measuring changes in the absorbance of the precipitation solution and by monitoring the change of particle size in a circulating particle size analyzer. The precipitation solution conditions affected the precipitation characteristics such as the induction time, precipitation rate, overall precipitation time, and particle size in a complex manner. With increases in both pH and ionic strength, the induction time was prolonged, and the individual particle size decreased, but the individual particles tended to grow by aggregation to form larger precipitates. The uranium concentration and the ionic strength of the solution affected the induction time and precipitation rate to the greatest extent.

A Conceptual Process Study for Recovery of Uranium Alone from Spent Nuclear Fuel by Using High-Alkaline Carbonate Media

Abstract This work studied a conceptual process to recover uranium alone from spent nuclear fuel using high-alkaline carbonate media with hydrogen peroxide for the purposes of reducing the volume of high-level active waste and recycling of uranium from the spent fuel with greatly enhanced proliferation resistance, environmental friendliness, and operational safety. The transuranium (TRU) elements were evaluated to be undissolved and precipitated together with other fission products during the oxidative leaching of uranium from the spent fuel. The leaching ratio of uranium dioxide to TRU dioxide from spent fuel in the carbonate solution with H2O2 was estimated to be more than about 108. Only Cs, Tc, Mo, and Te among the major fission products in the spent fuel were dissolved together in the carbonate solution. In the carbonate solution with H2O2, UO2 was dissolved in the form of uranyl peroxo-carbonato complex ions, which could be recovered in the form of uranium peroxide precipitate with a very low solubility by acidification of the solution in a succeeding step. All the inorganic salts of Na2CO3, NaOH, and HNO3 used in the process suggested could be almost completely recovered and recycled into the process again without any generation of secondary wastes.

Material and Organic Destruction Characteristics of High Temperature-Sintered RuO2 and IrO2 Electrodes

For Ru and Ir oxide electrodes sintered at different temperatures, in this work, surface resistivity, X-ray photoelectron spectroscopy, electrode lifetime, voltammetric charge capacity, and total organic carbon of 4-chlorophenol (4CP) decomposition at the electrodes were measured, and then intermediates during the electrolysis were identified by gas chromatography-mass spectroscopy to predict the destruction path of 4CP at the electrodes. A sintering temperature of around 650°C, rather than 400-550°C suggested in the literature for the fabrication of Ru and Ir oxide electrode, showed the highest organic destruction yield. The sintering temperature strongly affected the electrode lifetime as well. During the high temperature sintering, increase of the sintering time caused the oxidation of the Ti substrate to result in the increase of oxide weight of the electrode and the solid diffusion of the generated TiO 2 to the electrode surface which decreased the electrode activity so that the organic destruction yield went down slowly. The destruction path of 4CP at a high temperature-sintered electrode was suggested to be different from that at a low temperature-sintered one. The Ru oxide electrode sintered at 450°C generated several complicated aliphatic intermediates.

Evaluation of the Behavior of Uranium Peroxocarbonate Complexes in Na–U(VI)–CO3–OH–H2O2 Solutions by Raman Spectroscopy

In this work, the formation of uranium species and their stabilities in Na-U(VI)-CO(3)-OH-H(2)O(2) solutions at different pHs are studied by Raman spectroscopy. In this solution, the UO(2)(O(2))(CO(3))(2)(4-) species was formed together with three other uranium species of UO(2)(O(2))(2)(2-), UO(2)(CO(3))(3)(4-), and a speculated uranium species of the uranyl carbonate hydroxide complex, UO(2)(CO(3))(x)(OH)(y)(2-2x-y), which showed remarkable Raman peaks at approximately 769, 848, 811, and 727 cm(-1), respectively. The UO(2)(O(2))(CO(3))(2)(4-) species disappeared at pH conditions where bicarbonate dominated, and its Raman peak could be clearly observed in only a narrow pH range from approximately 9 to 12. When the pH of the solution increased further, the UO(2)(O(2))(CO(3))(2)(4-) species changed to UO(2)(CO(3))(3)(4-) and the UO(2)(CO(3))(x)(OH)(y)(2-2x-y) species. Moreover, the UO(2)(O(2))(CO(3))(2)(4-) species continuously decomposed into uranyl tricarbonate in the carbonate solution at an elevated temperature because of the instability of the peroxide ion, O(2)(2-), in alkaline conditions.

Evaluation of the stability of uranyl peroxo-carbonato complex ions in carbonate media at different temperatures

This work studied the stability of peroxide in uranyl peroxo carbonato complex ions in a carbonate solution with hydrogen peroxide using absorption and Raman spectroscopies, and evaluated the temperature dependence of the decomposition characteristics of uranyl peroxo carbonato complex ions in the solution. The uranyl peroxo carbonato complex ions self-decomposed more rapidly into uranyl tris-carbonato complex ions in higher temperature carbonate solutions. The concentration of peroxide in the solution without free hydrogen peroxide represents the concentration of uranyl peroxo carbonato complex ions in a mixture of uranyl peroxo carbonato complex and uranyl tris-carbonato complex ions. The self-decomposition of the uranyl peroxo carbonato complex ions was a first order reaction, and its activation energy was evaluated to be 7.144×10(3) J mol(-1). The precipitation of sodium uranium oxide hydroxide occurred when the amount of uranyl tris-carbonato complex ions generated from the decomposition of the uranyl peroxo carbonato complex ions exceeded the solubility of uranyl tris-carbonato ions in the solution at the solution temperature.

Effect of an Etching Ti Substrate on a Catalytic Oxide Electrode

This work has investigated the effects of the etching method of a Ti substrate for a catalytic oxide electrode on the electrochemical characteristics of the electrode. The HCl etching develops a finer and more homogeneous roughness on the Ti substrate than an oxalic acid etching. The etching results affect the physical adhesion of the oxide on the substrate, the morphology of the oxide surface after sintering, and the loading amount of oxide loaded after painting precursor solutions simultaneously. Such changes subsequently bring out the changes in the electrochemical properties of the oxide electrodes such as electrochemical activity and lifetime.

Electrolysis of Nitric Acid by Using a Glassy Carbon Fiber Column Electrode System

The electrochemical redox behavior of nitric acid was studied using a glassy carbon fiber column electrode system, and its reaction mechanism was suggested and confirmed in several ways. Electrochemical reactions in less than 2.0M nitric acid was not observed. However, in more than 2.0M nitric acid, the reduction of nitric acid to nitrous acid occurred and the reduction rate was slow so that the nitric acid solution had to be in contact with an electrode for a period of time long enough for an apparent reduction current of nitric acid to nitrous acid to be observed. The nitrous acid generated in more than 2.0M nitric acid was rapidly and easily reduced to nitric oxide by an autocatalytic reaction. Sulfamic acid was confirmed to be effective to destroy the nitrous acid. At least 0.05M sulfamic acid was necessary to scavenge the nitrous acid generated in 3.5M nitric acid.

A study on characteristics of an electrolytic–photocatalytic reactor using an anode coated with TiO2

A photocatalytic-electrolysis reactor using an anode coated with a TiO2 thin film of an anatase structure, a low surface resistivity, and a large surface area has shown an enhancement of TiO2 photocatalytic reaction efficiency by a reduction of the recombination of photogenerated electron–hole pairs. At the photocatalytic anode under UV irradiation and with a potential to generate oxygen evolution being applied, the photocatalytic enhancement was about 90% because of suppression of the recombination of holes and electrons by taking out forcedly the generated electrons through an external bias into a cathode and by the oxygen generated by the electrolytic reaction acting as an acceptor to the electrons. The photocatalytic enhancement effect occurred only when the cell voltage applied to the photocatalytic-electrolysis reactor was over a certain value. The photocatalytic reaction observed on the catalytic oxide electrodes of RuO2 and IrO2 was because of the existence of TiO2 on the electrode surface, which resulted from the oxidation of Ti substrate itself during sintering for the fabrication of the electrodes.