Étude des interactions chimiques gaz-solide par la technique du faisceau moléculaire: V. réactions de l'oxygène et de l'oxyde de carbone avec des rubans polycristallins de tantale

We use the reflection of a noble gas (helium) molecular beam to study the superficial reactions of oxygen and carbon monoxide with polycrystalline tantalum. The fraction of the incident beam which is specularly reflected gives direct information on the formation of an oxygen chemisorbed layer and we observed only a single apparent binding state for oxygen chemisorbed on tantalum. The initial value of the sticking coefficient of oxygen on clean polycrystalline tantalum is 0.86, decreasing rapidly during the formation of a chemisorbed layer. The specularly reflected fraction of the incident beam is also modified by the chemisorption of carbon monoxide and this modification would confirm the dissociative character of the CO chemisorption on tantalum. The influence of the partial pressure of CO on the temperature at which the surface is completely covered by the products of the dissociative chemisorption of CO shows that the coverage becomes equal to unity at temperatures at which the solubility limit of CO is attained and tantalum carbide is formed.