F. Zaera et al., REFLECTION-ABSORPTION INFRARED-SPECTROSCOPY AND KINETIC-STUDIES OF THE REACTIVITY OF ETHYLENE ON PT(111) SURFACES, Surface science, 368, 1996, pp. 371-376
The chemistry of ethylene on Pt(111) single-crystal surfaces has prove
n quite complex because it involves the simultaneous occurrence of sev
eral reactions, namely molecular desorption, dehydrogenation to ethyli
dyne, H-D exchange within the adsorbed molecules, and hydrogenation to
ethane. Reflection absorption infrared spectroscopy (RAIRS) has been
used here in conjunction with isothermal kinetic measurements to ident
ify the possible intermediates involved in each of those reactions, an
d to follow their thermal chemistry on the platinum surface. All vinyl
, ethyl and ethylidene moieties were prepared by thermal decomposition
of their corresponding iodides and characterized by RAIRS. The experi
mental data available to date favors the formation of ethylidene as an
intermediate in the conversion of ethylene to ethylidyne, but the com
plexity of the kinetics of that reaction, which changes significantly
with changing surface coverages, makes the final proof of this mechani
sm quite difficult. In addition, a side ethylene-ethyl equilibrium whi
ch starts at temperatures below those required for the formation of et
hylidyne is responsible for H-D exchange in ethylene. Finally, the hyd
rogenation of ethylene to ethane also involves an ethyl intermediate,
but only occurs at the ethylene high coverages needed for the transiti
on of the di-sigma strongly bonded species to a weak pi configuration.
The relevance of the reactions seen under vacuum to the high-pressure
catalytic hydrogenation of ethylene is briefly discussed.