A Method of Decomposing Carbon Dioxide for Fixation of Carbon-14

Abstract

To consecutively decompose ¹⁴CO₂ into carbon (¹⁴C) through its reaction with Hz, an apparatus using microwave discharge and its conditioning were investigated. The reaction produces CO as an intermediate, and proceeds in the two steps of (1) “CO₂+H₂ → CO+H₂O” and (2) “CO+H₂+C_n → C_n+1+H₂O”, where C_n denotes the carbon already deposited on the wall of the discharge tube. Preliminary dispersion of carbon to the wall of the discharge tube by sputtering of a graphite particle was effective to promote the reaction. Two silica discharge tubes (6 mm O.D., 4 mm I.D., and 150 mm length each) were connected in series to proceed the former reaction in the first discharge tube and the latter one in the second one. When a 1:3 mixture of CO₂ and H₂ (total pressure 0.67kPa) was passed through the discharge tubes at a linear gas velocity of approximately 30mm/s and discharged for 60 h under microwave of 30–40 W supplied from two 2,450MHz power generators (200 W each), more than 90% of CO₂ was converted into CO in the 1st tube and about 23% of the CO was then decomposed into carbon in the 2nd tube. However, about 50% of the CO escaped from the tube without being decomposed, and about 0.5% and 1% of the carbon fed were hydrogenated into CH₄ and C₂H₂, respectively. The rest about 25% which was not confirmed was probably evacuated from the 2nd tube as microparticles of carbon. To completely decompose CO₂ into carbon, additional discharge tubes are necessary downstream of the 2nd tube.

KEYWORDS:

• carbon 14 fixation
• decomposition
• carbon dioxide
• microwave discharge
• hydrogen
• carbon monoxide
• graphite
• methane
• acetylene
• material balance