9. 超音波化学反応による貴金属の析出 : アルコール・有機酸添加効果

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Palladium metal was deposited sonochemically from the aqueous solution of palladium(II) chloride by the sonication using the ultrasonic homogenizer(20 kHz, 60W). The effect of some organic additives such as various kinds of alcohols and organic acids, which have high reactivity with OH radicals, on the metal deposition yield was examined. Addition of alcohols enhanced the metal deposition, the efficiency being in the order of 1,1-dimethylethanol(tert-butyl alcohol), 2-propanol, 1-propanol, ethanol and methanol, while formic acid and acetic acid gave little effect. It was found that the efficiency depended on the hydrophobicity ratio of the solute(ratio of number of hydrophobic to hydrophylic groups in a molecule), which was pointed out by Henglein. The same dependence was found for the sonochemical formation of hydrogen gas from the aqueous solutions of these additives without containing palladium ions, the higher hydrophobicity of the solute, the higher yield of hydrogen. These results were interpreted in terms of an accumulation of solute molecules at the liquid-gas interface of the cabitation bubbles, where the solute molecules scavenged the OH radicals and suppressed the recombination of OH radicals with H radicals. These radicals were generated from the sonolysis of water in the cabities. The H radicals that would otherwise recombine dimerise to hydrogen and react with palladium ions if in the presence of palladium ions in the solution. The increase of the palladium deposition and the compatible decrease of hydrogen formation with the increase of the initial concentration of palladium chloride were found in the sonolysis of aqueous 2-propanol solutions (52 mmol dm^<-3>). These results indicate the occurrence of the competitive reaction of H radical with its dimerization and the palladium ion reduction. The similar experiments were performed for hexachloroplatinic acid(IV) solutions, however, different results from those of palladium solutions were obtained.
日本ソノケミストリー学会の論文

9. 超音波化学反応による貴金属の析出 : アルコール・有機酸添加効果

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