Measurement of Radicals Generated by Plasma in Contact with Dilute Sulfuric Acid by Using Electron Spin Resonance (ESR) Method*

Catalyst for hydrolysis of cellulose to glucose is very important to highly efficient biofuel process. Our group has achieved to generate a high-performance hydrolysis catalyst, which was formed by sulfonating carbon material, by using plasmas in contact with dilute sulfuric acid. However, the mechanism of the sulfonation process is not fully understood. In this study, to better understand the sulfonation mechanism, we investigated radicals generated by plasma in contact with the dilute sulfuric acid by using an electron spin resonance (ESR) method and a spin trapping reagent of CYPMPO. When the plasma was generated by negative pulsed voltage and irradiated to the 500 mM dilute sulfuric acid for 300 s, we observed an ESR spectrum derived from CYPMPO-OH, but any radicals formed from the sulfuric acid, such as sulfate ion radicals (SO₄⁻), were not detected. On the other hand, in the case of the positive polarity, unknown peaks in addition to those derived from the CYPMPO-OH were observed in the 500 mM dilute sulfuric acid after 300 s plasma irradiation.