Investigation of Plasma Sulfonation Mechanism with Dilute Sulfuric Acid

Carbon sulfonation is a process to modify carbon materials with sulfonic group (SO₃H), carboxyl group (COOH), and hydroxyl group (OH) so that sulfonated carbon materials exhibit excellent catalytic performance for biomass transformation. A conventional method for carbon sulfonation requires concentrated sulfuric acid at elevated temperatures. On the contrary, we use a gas-liquid interfacial plasma (GLIP) process for carbon sulfonation which just requires dilute (1 mol/L) sulfuric acid. Moreover, after the plasma process, the catalytic performance and stability of carbon materials are much better than catalysts produced by the conventional method. However, the plasma sulfonation mechanism is not clearly understood. Consequently, we set optical emission spectroscopy (OES) and Fourier-transform infrared (FT-IR) spectrometry for finding gas-phase elements in the plasma sulfonation process. These experiments were conducted without carbon materials. In addition to the H₂SO₄ solution, Na₂SO₄ solution was used as a liquid electrode to generate plasma, which cannot sulfonate carbon materials. We did not observe any SO_X by OES measurements with the H₂SO₄ and Na₂SO₄ solutions. On the other hand, it was confirmed by FT-IR measurements that SO₂ gas can be generated by plasma discharge with H₂SO₄ solution whereas could not be generated with Na₂SO₄ solution. An increased discharge power could enhance SO₂ generation. In addition to OES and FT-IR measurements, titration experiments were conducted to measure the amounts of functional groups in the carbon materials and to confirm the importance of the SO₂ gas in the plasma sulfonation process. In the future, we will compare the catalytic performance of sulfonated carbons with different experimental conditions to investigate the optimum condition for carbon sulfonation by the plasma process.