Formic acid production using submerged microwave plasma jet in aqueous glycerol mixtures

• Revealed the role of submerged microwave plasma jet in formic acid production via water dissociation into OH radicals. • Demonstrated the significance of liquid phase reactions evidenced by the optimal glycerol concentration in aqueous glycerol mixtures. • Identified the critical impact of plasma-liquid interaction, driven by argon flow, in enhancing formic acid production. The conversion of glycerol into formic acid for energy and chemical applications represents a promising value-added strategy. In-liquid nanosecond repetitively pulsed discharge (NRPD) has demonstrated the potential of plasma-assisted glycerol conversion, highlighting the importance of water content in aqueous glycerol mixtures for generating OH radicals and introducing them into the liquid phase . Here we present formic acid formation results using a submerged microwave plasma jet (MWPJ) of argon in aqueous glycerol mixtures, motivated by the contrasting plasma characteristics of MWPJ (gas temperature ∼ O(10 3 ) K, electron temperature ∼ O(10 –1 ) eV) compared to NRPD (O(10 2 ) K and O(10) eV, respectively). Similar to NRPD, the primary conversion reactions occurred in the liquid phase, driven by OH radicals formed through gas phase reactions involving excited metastable states of Ar. Depending on the combination of MW power and plasma processing time, an optimum energy efficiency was achieved, attributed to substantial water evaporation caused by high gas temperature and prolonged processing time. Notably, even with constant specific input energy, increased Ar flow rate significantly enhanced formic acid formation, emphasizing the crucial role of plasma-liquid interactions. Estimated gas temperatures and measured OH radical distributions further supported the observed formic acid formation. The performance of a submerged MWPJ system can be improved by augmenting plasma-liquid interaction, suggesting a direction for future studies.