Plasma-Driven Catalytic Conversion of Biogas to Methanol and Acetic Acid and the Role of Water in Tailoring Products

The conversion of biogas, a widely available renewable source, to the desired liquid products under mild conditions still remains challenging. Integrating nonthermal plasma with catalysts makes it possible to produce acetic acid and methanol directly from biogas consisting mainly of CO₂ and CH₄. Among various catalysts examined, plasma-reduced Cu/Al₂O₃ with a flower-like morphology substantially enhances methanol formation. Adding a small amount of water vapor into biogas significantly enhances methanol production under cold plasma. In situ optical emission spectroscopy, probe tests with isotope-labeled D₂O and H₂¹⁸O, and plasma desorption analysis provide clear evidence that water not only participates in and promotes the methanol-forming reaction but also facilitates methanol desorption from the catalyst surface under plasma. Increasing the plasma discharge power, however, can dramatically switch the liquid products from methanol-dominated to acetic acid-dominated oxygenates. These novel findings provide a new strategy for the direct conversion of biogas to chemicals and fuels.