A C-14 RADIOTRACER STUDY OF THE HYDROGENATION OF ETHYNE OVER EUROPT-1

The adsorption and hydrogenation of ethyne over EUROPT-1 has been studied using C-14-ethyne and C-14-ethene as tracers at 295 K. Adsorption of both ethyne and ethene show typical primary and secondary regions. Only a fraction of the initially adsorbed radioactivity can be removed evacuation, treatment in hydrogen or under ethyne hydrogenation conditions at the adsorption temperature. Ethene cannot be adsorbed on an ethyne precovered surface, although ethyne will displace only 68% preadsorbed ethene. In the hydrogenation of ethyne, evidence has been obtained to show that the surface coverage by hydrocarbon progressively increases during the course of the reaction up to the acceleration point in the pressure-time curves. The initial selectivity for ethene formation is ca. 0.25. Hydrogenation of C-12-ethyne/C-14-ethene mixtures shows that the further hydrogenation of ethene to ethane is of only minor importance in determining the selectivity. A reaction scheme is proposed in which three types of surface site are identified as being responsible for the hydrogenation of (a) ethyne to ethene; (b) ethyne to ethane and (c) ethene to ethane.