Sonochemical hydrogen production in a 300 kHz sonoreactor: Comparative effects of short‑chain carboxylic acids, pH,...

Sonochemical hydrogen production via ultrasonic cavitation offers a promising but still inefficient route for sustainable hydrogen generation. This study investigates how short‑chain carboxylic acids affect hydrogen (H₂) production in a 300 kHz sonoreactor under well‑defined pH and gas conditions. Formic (FA), acetic (AA), propionic (PA), and butyric acid (BA) were examined at 0.1-5.0% v/v and pH 3, 6, and 9 under 100% Ar and 100% N₂. At pH 3, 5% BA achieved a maximum H₂ production rate of about 3.3 µmol/min, approximately ten times higher than pure water, whereas FA showed little enhancement. In parallel, H₂O₂ formation followed the inverse order FA > AA > PA > BA, consistent with the notion that carboxylic acids can act as hydroxyl radical scavengers that redistribute radicals between oxidative (H₂O₂ production) and reductive (H₂ production) pathways, as inferred from measured H₂ and H₂O₂ rates. The dependence of H₂ yield on pH and acid identity suggests that volatility, hydrophobicity, and pH‑dependent speciation are important factors influencing interfacial accumulation and cavitation behavior, although interfacial properties and radicals were not directly measured. Substituting Ar with N₂ reduced H₂ production in pure water, but a 5% (0.57 M) BA solution under 100% N₂ still produced more H₂ than Ar‑saturated pure water, providing a proof‑of‑concept that suitable additive can partially offset the thermodynamic limitations of N₂. Overall, this work offers a comparative framework for understanding how carboxylic acids modulate sonochemical H₂ production and highlights opportunities and limitations for using homogeneous additives to enhance hydrogen yields in high‑frequency sonoreactors.