Reduction of CO2 with H2O was carried out by microwave plasma for the formation of methanol. The results of steam chromatography and mass spectrometry showed that the plasma products contained methanol. The methanol formation was also found in H2O-plasma-cleaning process, in which materials which had been deposited in the reaction between CO2 and H2O were removed. The most adequate plasma energy density for the formation of methanol was found to be 0.26 GJ kg−1 of W/FM. The methanol yield at the system pressure of 400 Pa was 3.5 times higher than at 240 Pa for both the CO2–H2O synthetic process and the H2O-cleaning process.

Five kinds of synthetic polymers, poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), poly(methacrylic acid) (PMAA), poly(methyl acrylate) (PMA), and high-density polyethylene (HDPE), were subjected to microwave HO plasma. In the plasma, the polymers were decomposed, and the formation of methanol in the process was confirmed. The decomposition rate of the polymers in HO plasma decreased in the following order: PMMA > PEMA > PMAA > PMA > HDPE. Methanol showed the greatest yield among the organic compounds in the products for the each polymer. The conversion into methanol for respective polymers was in the following ratio: PMMA PEMA : PMAA : PMA : HDPE = 2 :1:1:1:0. This result shows good agreement with the total number of methyl group and methoxycarbonyl group (methyl ester) per repeating unit in respective polymers. This result implied that both methyl and methoxycarbonyl groups in the polymers contributed to the decomposition and formation of methanol in H2O plasma. Other products obtained were acetaldehyde, ethanol, acetone, 2-propanol, and acetic acid.