Electrocatalysis and Novel Functions of Nano-Structured IrO2-Ta2O5 /Ti Anodes
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In the present work, the main purpose was the development of the IrO2-Ta2O5/Ti anodes to suppress the unwanted side reactions in industrial electrolytic processes. The mixed effects of Ir mole ratio and thermal decomposition temperature on the crystallographic structure, especially amorphization of the IrO2-Ta2O5 coating, were investigated, and those effects on the suppression of anodic oxidation of Pb(II) to PbO2 as a typical example of unwanted side reactions and on the electrocatalysis for oxygen evolution were studied. The relationship between nano-scale surface morphology of the oxide coating and the suppression of PbO2 deposition and electrocatalysis for oxygen evolution was further investigated. Chapter 4 is about IrO2-Ta2O5/Ti anodes preparation at different Ir mole ratios and thermal decomposition temperatures. The IrO2-Ta2O5 coating became amorphous with lowering Ir mole ratio and decreasing thermal decomposition temperature. The amorphization of the oxide coating made the onset potential of PbO2 deposition more positive, and oxygen evolution on the amorphous oxide coating was accelerated at higher Ir mole ratio. However, the reason for the suppression of PbO2 deposition was significantly related to the increase in nucleation overpotential of PbO2 on the amorphous IrO2-Ta2O5 coatings, not simply due to the negative shift of the onset potential of oxygen evolution. In Chapter 5, the surface morphology of IrO2-Ta2O5 coatings was investigated by SEM with low accelerated incident electron beam. The IrO2-Ta2O5 coatings consisting of crystalline IrO2 and amorphous Ta2O5 and of amorphous IrO2 and Ta2O5 were examined, and a significant difference in surface morphology between them was found. The coating comprising crystalline IrO2 showed a mud-cracked morphology with large segregated IrO2 particles, and the flat area was porous with nano holes and cracks in addition to non-uniformly dispersed IrO2 flakes of 20-60 nm. A smooth cracked surface with no segregated IrO2 was observed for the amorphous IrO2-Ta2O5 coating, and the existence of ordered nano IrO2 particles of 5-10 nm were further revealed. In Chapter 6, the catalytic oxide coatings comprising a mixture of IrO2 and Ta2O5 were prepared at 50 mol% Ir to 80 mol% Ir by thermal decomposition at 470 oC, and the crystallographic structures and surface morphologies were investigated. XRD analysis and high-resolution SEM observation using low accelerated incident beam revealed that decreasing Ir ratio made a phase transition of IrO2 from crystalline to partly amorphous, and nano IrO2 particles of 10 nm or less and nano chains made of them, which were uniformly dispersed on the flat area of the coating, were generated at 50-60 mol% Ir. In Chapter 7, the oxygen evolution behavior and the suppression of anodic PbO2 deposition on IrO2-Ta2O5/Ti anodes, which were prepared by thermal decomposition at different temperatures and metal compositions were investigated in acidic aqueous solutions. Ordered nano IrO2 particles were confirmed in amorphous IrO2-Ta2O5 coatings which were obtained at 50 mol% Ir and calcined at 380 °C by SEM observation with low accelerated electron beam. The presence of such ordered nano IrO2 has never been reported before for amorphous IrO2-Ta2O5 coatings. The amorphization was important to suppress anodic PbO2 deposition, because the active sites for anodic PbO2 deposition were aggregated IrO2 crystallites, while oxygen evolution was accelerated with nano IrO2 particles.