Stagnant liquid layer as “Microreaction System” in submerged plasma Micro-Jet for formation of carbon quantum dots

This study aims to intensify reactivity via bespoke transient hydrodynamics in a plasma-activated three-phase catalyst system. Likewise, three-phase plasma systems in literature are not well designed, understood and commercially available. The synthesised N-doped carbon quantum dots, with application as fertilisers and wastewater treatment was evaluated as a model reaction. A plasma microjet was produced using a commercial plasma system, creating an almost flat, large interface covering a thin stagnant liquid layer with a catalyst bed underneath. In this hydrodynamic regime, plasma can penetrate the catalyst bed via the stagnant thin liquid film and polarise the plasma-liquid interface. The new process regime's effectiveness is proven by the enhanced reaction rate achieved by raising liquid diffusivity toward the catalyst bed through solvent viscosity reduction. Results indicate that the reaction rate depends on the small surface area interacted with the plasma jet amid an excess of excited species, not the total gas–liquid interface area. The plasma process competes energetically with the best dielectric barrier discharge processes and uses less energy than microfluidic processing in chemical microreactors. High momentum transfer from the plasma jet to the liquid partly evaporates non-water additives, impacting process design and requiring reduction.