Hydrogen atom reactions in the sonolysis of aqueous solutions

Abstract

Aqueous solutions of the spin trap 5,5-dimethylpyrroline TV-oxide (DMPO) and of Br2,12, Mn04~, AuCl4", and Ag+ were irradiated with 1-MHz ultrasound under argon-hydrogen mixtures of various compositions. DMPO was found to trap only very small amounts of hydrogen atoms while substantially greater amounts could be scavenged by the other solutes. The yield of H atoms as measured by the ESR signal of the DMPO adduct decreased with increasing H2 concentration in the gas mixture. The yields of the reduction products of the other solutes passed through a maximum at about 20 vol % H2. The efficiency of scavenging did not correlate to the rate constant of reaction with H in homogeneous solution. It seems that the ability of a solute to be accumulated at the liquid-gas interface of cavitation bubbles strongly influences the scavenging efficiency. Thallium ions are reduced in solutions of high pH. This reduction is attributed to hydrated electrons which are formed in the reaction of the H atoms with OH" anions. It is concluded that the local concentration of H atoms in the interfacial region of the cavitation spots is about 10"1 23 M and that hydrated electrons are not primary products of sonolysis.

Sonochemical Processes and Formation of Gold Nanoparticles within Pores of Mesoporous Silica

“…It was observed that the presence of isopropanol has a crucial influence on the sonochemical reduction rate of AuCl − 4 ions. Without the additive, H ᭹ and ᭹ OH radicals would readily recombine to give a variety of products as shown in reaction [1] (27). Some other additives used in our investigation were also found to act as radical-scavenger and accelerator of the sonochemical reduction reaction.…”

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…Mason suggested a fourth site originating from a two site model that considers a liquid phase surrounding the hot bubble, that can either be a heated shell of liquid around the collapsing bubble or liquid droplets injected into the bubble by instabilities at the surface [100]. Of these sites the interfacial region is the greatest area of •OH population and recombination [20,30,[105][106][107][108][109][110]. Merouani et al through numerical simulation determined that of the numerous products, H 2 occurred most abundantly and more readily in the gas phase via reaction (8) [111].…”

Ultrasound in Heterogeneous Systems and Applications in Food Processing

“…,H þ OH À /H 2 O þ e aq (11.2) Gutiérrez et al were able to reduce Tl þ in aqueous solutions using the hydrated electron to form colloidal thallium nanoparticles [7]. Metal ion/complex reduction reactions making use of the sonolytically produced hydrated electrons have only received limited attention [8].…”