Chemisorption and displacement reactions on tungsten, molybdenum and tantalum films

Abstract

The desorption of chemisorbed hydrogen by N₂(g) and CO(g) has been investigated with tungsten, tantalum and molybdenum films. Carbon monoxide is particularly effective, about 40 % of the hydrogen chemisorbed on tungsten and molybdenum at 10^–3 mm and 0°C being desorbed at the same temperature and a CO pressure of ∼10^–5 mm; the replacement reaction conforms to a stoichiometric relation in that two molecules of CO replace two hydrogen adatoms. On the other hand, less than 0.1 % of the hydrogen chemisorbed on tungsten is desorbed by N₂(g) under the same conditions. Moreover, there is both a high and low coverage (θ≃ 0.8 and 0.94) limit to the hydrogen that can be desorbed by N₂(g) at 0°C. More hydrogen (0.5 % of a monolayer) can be desorbed from a molybdenum surface which suggests a greater preponderance of low energy sites than with tungsten. No desorption was observed with either N₂(g) or CO(g) from the α-hydride of tantalum although both were chemisorbed extensively. A tungsten surface which has chemisorbed a monolayer of CO at 0° can retain hydrogen atoms impinging from the gas phase up to [CO]/[H] ratio of unity.