Formation of selected organic compounds from aqueous sodium carbonate solution via hetero-coupling of carbon and hydrogen by in-liquid plasma

Abstract

This study explores an in-liquid plasma process for converting carbon dioxide into value-added organic compounds using aqueous sodium carbonate solutions derived from CO₂ captured by sodium hydroxide. The method operates without hydrogen gas or catalysts, aligning with carbon-neutral strategies. During plasma treatment, the measured emission spectroscopy (ES) confirmed the formation of reactive species, including CH (431, 387 nm), CO (336, 295 nm), OH (308, 283, 282 nm), O (777, 845 nm), and H radicals (656 nm [H_α], 486 nm [H_β]). Electron temperature estimated by [H_β]/[H_α] of ES is in the range between 4500 and 5500 K for this plasma condition. These species are generated via vibronic coupling: interactions between molecular vibrations and orbital electronic states in CO₂ and H₂O at these plasma temperatures. CH radicals, formed by hetero-coupling of C and H, play a key role in subsequent synthesis. Gas chromatography-mass spectrometry (GC-MS) at quenching process detected acetone (retention time: 1.56 min) and ethanol (2.06 min). Ab initio calculations reveal the reaction pathways: 2 CH + 4 H + CO → CH₃COCH₃ ，2 CH + 3 H + OH → C₂H₅OH. The yield of acetone (8 mg/L) is lower compared with ethanol (14.3 mg/L) under the cooling conditions. The process proceeds through repeated plasma–quenching cycles, approaching an apparent chemical equilibrium within 60 min. This plasma method demonstrates an efficient and sustainable route for CO₂ utilization of carbonate and water, offering a promising approach for carbon-neutral fuel production and Carbon Capture, Utilization, and Storage (CCUS) process.