Abstract
Using the ^14C-radiotracer technique, the adsorptions of carbon monoxide, ethene, and ethyne have been studied in a static system at 293±2 K. CO adsorption showed the Langmuir-type behavior, while the adsorption of both ^14C-ethene and ^14C-ethyne showed typical primary and secondary adsorption regions. A fraction of approximately 30% of the adsorbed hydrocarbon remained on the surface as strongly bound species and could not be removed by evacuation, dihydrogen, or ethyne hydrogenation treatments.
Ethyne hydrogenation on Europt-3 proceeds in two distinct stages. During the first stage, the reaction order is approximately 1.73±0.06 in dihydrogen and approximately -0.95±0.01 for ethyne. The reaction rate constant is approximately 2.4±0.1 x 10² min⁻¹ with an activation energy of approximately 40.7±0.3 kJ/mol. The initial selectivity for ethene formation is approximately 0.68.
Direct surface monitoring during the adsorption or hydrogenation reaction shows that ethene does not adsorb on the ethyne adsorption site, while ethyne adsorbs on the ethene adsorption site. Evidence has been obtained to show that during the hydrogenation reaction, surface coverage by hydrocarbon progressively increases at the commencement of the reaction and decreased linearly as the reaction proceeded.
The hydrogenation of ethyne in the presence of ^14C-ethene shows that the yield of ethane from the hydrogenation of ethyne constitutes a very small fraction; the major route to ethane formation is by direct hydrogenation of ethyne. Three distinct types of surface sites are proposed as being responsible for the hydrogenation: (i) ethyne to ethene; (ii) ethyne to ethane; and (iii) ethene to ethane.
It is proposed that under the influence of hydrocarbon adsorption and hydrogenation reaction, the surface undergoes some reconstruction in which distinctive sites for the adsorption of ethyne and ethene are created.