Preparation of Small Samples for 14C Accelerator Targets by Catalytic Reduction of CO

Abstract

Graphite in various forms has become the standard target for accelerator ¹⁴C dating. Graphite has been made by catalytic graphitization of charcoals (Lowe, 1984). Thin films of graphite have also been produced by thermal cracking (Beukens & Lee, 1981), electric discharge (Andrée et al, 1984; Wand, Gillespie & Hedges, 1984). Vogel et al (1984) pointed out the ease of graphite formation on iron from CO₂ and H₂ mixtures at ca 600°C. The deposition reactions of carbon from the CO, H₂, and CO₂ equilibria are well known (Wagman et al, 1945) and well studied. Formation of graphite from CO₂ was discussed extensively by Boudouard (1902) and Schenck and Zimmerman (1903), and was known to chemists in France in 1851. We have used a related method, where graphite forms away from the iron, by using a higher temperature, and reduction of CO₂ to CO over Zn in the presence of H₂ (Jull et al, 1986) as an alternative to the use of Fe alone. The object of this paper is to point out an even simpler graphite preparation system, which eliminates hydrogen. The decomposition reaction of CO (Boudouard, 1902) takes place according to reaction (1).