How do dissolved gases affect the sonochemical process of hydrogen production? An overview of thermodynamic and mechanistic effects - On the "hot spot theory"

Kaouther Kerboua; Slimane Merouani; Oualid Hamdaoui; Abdulaziz Alghyamah; Md H Islam; Henrik E Hansen; Bruno G Pollet

doi:10.1016/j.ultsonch.2020.105422

How do dissolved gases affect the sonochemical process of hydrogen production? An overview of thermodynamic and mechanistic effects - On the "hot spot theory"

Ultrason Sonochem. 2021 Apr:72:105422. doi: 10.1016/j.ultsonch.2020.105422. Epub 2020 Dec 24.

Authors

Kaouther Kerboua¹, Slimane Merouani², Oualid Hamdaoui³, Abdulaziz Alghyamah³, Md H Islam⁴, Henrik E Hansen⁵, Bruno G Pollet⁴

Affiliations

¹ Higher School of Industrial Technologies, Department of Second Cycle, P.O. Box 218, 23000 Annaba, Algeria. Electronic address: k.kerboua@esti-annaba.dz.
² Laboratory of Environmental Process Engineering, Faculty of Process Engineering, University Salah Boubnider - Constantine 3, 25000 Constantine, Algeria.
³ Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, 11421 Riyadh, Saudi Arabia.
⁴ Hydrogen Energy and Sonochemistry Research Group, Department of Energy and Process Engineering, Faculty of Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway.
⁵ Hydrogen Energy and Sonochemistry Research Group, Department of Energy and Process Engineering, Faculty of Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway; Electrochemistry Research Group, Department of Materials Science and Engineering, Faculty of Natural Sciences, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway.

Abstract

Although most of researchers agree on the elementary reactions behind the sonolytic formation of molecular hydrogen (H₂) from water, namely the radical attack of H₂O and H₂O₂ and the free radicals recombination, several recent papers ignore the intervention of the dissolved gas molecules in the kinetic pathways of free radicals, and hence may wrongly assess the effect of dissolved gases on the sonochemical production of hydrogen. One may fairly ask to which extent is it acceptable to ignore the role of the dissolved gas and its eventual decomposition inside the acoustic cavitation bubble? The present opinion paper discusses numerically the ways in which the nature of dissolved gas, i.e., N₂, O₂, Ar and air, may influence the kinetics of sonochemical hydrogen formation. The model evaluates the extent of direct physical effects, i.e., dynamics of bubble oscillation and collapse events if any, against indirect chemical effects, i.e., the chemical reactions of free radicals formation and consequently hydrogen emergence, it demonstrates the improvement in the sonochemical hydrogen production under argon and sheds light on several misinterpretations reported in earlier works, due to wrong assumptions mainly related to initial conditions. The paper also highlights the role of dissolved gases in the nature of created cavitation and hence the eventual bubble population phenomena that may prevent the achievement of the sonochemical activity. This is particularly demonstrated experimentally using a 20 kHz Sinaptec transducer and a Photron SA 5 high speed camera, in the case of CO₂-saturated water where degassing bubbles are formed instead of transient cavitation.

Keywords: Dissolved gases; Hydrogen production; Mechanistic aspects; Sonochemistry; Thermodynamics.