The role of hydrogen-deuterium exchange reactions in the conversion of ethylene to ethylidyne on Pt(111)

Reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to study H/D scrambling reactions during the conversion of ethylene to ethylidyne in coadsorbed layers of C2H4 and C2D4, C2H4 and D-2, and C2D4 and H-2 on Pt(111). The infrared spectra obtained after coadsorbing C2H4 and C2D4 at 330 K reveal the formation of partially deuterated ethylidyne. Extensive isotope scrambling was also observed when coadsorbing C2D4 with H-2, indicating that the exchange most likely involves surface hydrogen. Additional evidence points to the idea that the observed H/D exchange is not necessarily connected with the formation of ethylidyne, but due to other reactions occurring simultaneously on the surface. In particular, TPD experiments with C2H4 coadsorbed with either D-2 or C2D4 display significant desorption signals for partially substituted ethylenes; it is that exchanged ethylene the most likely source of the partially deuterated ethylidyne observed in the RAIRS experiments. Arguments are presented here in favor of a mechanism for the ethylene H/D exchange involving the formation of an ethyl intermediate. H/D exchange between ethylidyne and surface hydrogen or deuterium does occur too, but at a much slower rate than ethylidyne formation.