The electrochemical reduction of hydrogen peroxide on uranium dioxide under intermediate pH to acidic conditions

摘要

The cathodic reduction of H{sub}2O{sub}2 has been studied at rotating SIMFUEL disc electrodes in sodium chloride solutions over the pH range 1-9. The surfaces were analyzed by X-ray photoelectron spectroscopy. The mechanism and rate of reduction did not change over the pH range 4-9. Within this pH range, H{sub}2O{sub}2 chemically oxidizes the UO{sub}2 surface to U{sup}v which is subsequently electrochemically reduced. The U{sup}v state is incorporated into a surface layer of UO{sub}(2+x). For pH values below 3, reduction is catalyzed by the chemical creation of an adsorbed Uv state. For solutions from pH 3 to 9, currents rise to the diffusion-limited value at sufficiently negative potentials since the rate of formation of U{sup}v (in UO{sub}(2+x)) is sufficiently rapid, and the Uv state formed is sufficiently stable, that reduction is never limited by the availability of this state. However, the U{sup}v state formed in acidic solutions (pH 1,2) is less stable and can be further oxidized to the insulating U{sup}VI state prior to its dissolution as (UO{sub}2){sup}(2+), or cathodically reduced at less negative potentials than required in neutral solutions. This instability limits the availability of the catalytic state and the currents are always well below the diffusion-limited value.