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The reduction of titanium dioxide (TiO2) by the hydrogen plasma treatment creates a new absorption band in the visible light region and is expected to create photocatalytic activity in this region. Anatase TiO2 powder, which has photocatalytic activity in the ultraviolet (UV) region, was treated using microwave and radio-frequency (RF) plasmas. The TiO2 was reduced thus having an oxygen deficiency. The RF-plasma-treated TiO2 absorbed visible light between 400 and 600 nm and showed a high activity in this region, as measured by the oxidative removal of nitric oxide from the gas phase. The plasma-treated TiO2, which can be easily produced on a large scale, is expected to have a higher efficiency in utilizing solar energy than the raw material.

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.