ISOTHERMAL KINETIC MEASUREMENTS FOR THE HYDROGENATION OF ETHYLENE ON PT(111) UNDER VACUUM - SIGNIFICANCE OF WEAKLY-BOUND SPECIES IN THE REACTION-MECHANISM

The kinetics of the hydrogenation of ethylene on Pt(111) was studied i sothermally and under vacuum by using a variation of the dynamic molec ular beam method originally devised by King and Wells. At surface temp eratures above 240 K ethylidyne formation competes with both ethylene hydrogenation and ethylene desorption. At temperatures below 240 K, on the other hand, the decomposition of ethylene is slow, and the adsorp tion and hydrogenation kinetics for ethylene on both clean and hydroge n-covered surfaces could be investigated independently. Ethylene adsor ption was found to be precursor-mediated at low coverages and Langmuir ian near saturation. A certain population of weakly-adsorbed species c an also be maintained at coverages near saturation by exposure of the surface to a constant flux of ethylene molecules. The presence of coad sorbed hydrogen reduces the total ethylene uptake but increases the am ount of weakly-adsorbed ethylene as compared to that on the clean Pt(1 11). The main conclusion from this work is the fact that this weakly-a dsorbed species appears to be essential for the hydrogenation of ethyl ene: the kinetic orders of the reaction were determined to be 1.2 +/- 0.3 and 0.8 +/- 0.2 with respect to the weakly-adsorbed ethylene and h ydrogen surface coverages, respectively. An activation energy of 6 +/- 1 kcal/mol was measured for the hydrogenation of ethylene to ethane u nder the conditions of these experiments. Finally, the presence of eth ylidyne on the surface was found to not influence the hydrogenation re action in any other way than by blocking surface sites.