Effects of alcohols and dissolved gases on sonochemical generation of hydrogen in a 300 kHz sonoreactor

The sonochemical generation of hydrogen (H(2)) was investigated using various water/alcohol solutions under argon (Ar) 100 % in a 300 kHz sonoreactor. Five types of alcohols–methanol, ethanol, isopropanol, n-propanol, and n-butanol–were used at various concentrations (0 – 100 % v/v). The H(2) generation rate in water was 0.31 μmol/min in the absence of alcohols. The H(2) generation rate increased, peaked, and then decreased as the alcohol concentration increased. The concentrations used for the peak H(2) generation were 5 %, 1 %, 0.5 %, 0.5 %, and 0.1 % for methanol, ethanol, isopropanol, n-propanol, and n-butanol, respectively. The highest generation rate (5.46 μmol/min) was obtained for methanol 5 % among all conditions in this study, and no H(2) was detected for 100 % alcohol concentrations. The reason for the enhancement of the sonochemical H(2) generation by the addition of alcohols might be due to strong scavenging effect of alcohols for sonochemically generated oxidizing radicals and vigorous reactions of alcohol molecules and their derivatives with H radicals. No significant correlations were found between the H(2) generation rates and physicochemical properties of the alcohols in any of the data in this study. As alcohol concentration increased, the calorimetric power decreased. This indicates that the calorimetric power does not represent the degree of sonochemical reactions in the water/alcohol mixtures. The effect of oxygen (O(2)) content in the dissolved gases on the generation of H(2)O(2) (representing sonochemical oxidation activity) and H(2) (representing sonochemical reduction activity) was investigated using Ar/O(2) mixtures for water, methanol 5 % and n-propanol 0.5 %. In water, the highest H(2)O(2) generation was obtained for Ar/O(2) (50:50), which is similar to previous research results. However, the H(2)O(2) generation increased as the O(2) content increased. In addition, H(2) generation decreased as the O(2) content increased under all liquid conditions (water, methanol, and n-propanol).