Oxygen and carbon monoxide isotope exchange reactions on rhenium

The catalytic efficiency, E, of rhenium at high temperatures for the equilibration of a mixture of carbon monoxide isotopes (¹²C¹⁸O + ¹³C¹⁶O) is reduced by pre-adsorbed oxygen; E at 1300 K declines linearly to zero at an oxygen uptake of about 5 × 10¹⁴ atoms cm^?2. The replacement of one pre-adsorbed carbon monoxide isotope by another can be correlated with the characteristic desorption temperatures of the two main states (α and β) of CO on Re. The observation that a considerable fraction of CO is non-replaceable at filament temperatures below 700 K suggests a high activation energy for migration of some adsorbed CO. The probability of exchange of ¹⁶O between an oxygenated rhenium filament and gaseous ¹²C¹⁸O for oxygen coverages ?4 × 10¹⁴ O atoms cm^?2 is 0.012 per 10¹⁴ O atoms cm^?2 per collision with the filament at 900 K. The surface reaction $\text{Re-}{}^{16}\text{O +}{}^{12}\text{C}{}^{18}\text{O}{}_{\mathrm{(g)}}\text{= Re-}{}^{18}\text{O +}{}^{12}\text{C}{}^{16}\text{O}{}_{\mathrm{(g)}}$ is completely reversible. However, in the presence of nitrous oxide no reaction is observed until the filament temperature exceeds 1600 K, when continuous decomposition of N₂O is appreciable. Possible transition states for isotope exchange are discussed.