Simultaneous Monitoring of Surface and Gas Phase Species during Hydrogenation of Acetylene over Pt(111) by Polarization-Dependent Infrared Spectroscopy

Abstract

Acetylene hydrogenation was monitored at ambient pressure with polarization-dependent reflection absorption infrared spectroscopy (RAIRS), which permitted gas phase and surface species to be simultaneously monitored as C2H2(g) was converted first to C2H4(g) and then to C2H6(g). Experiments in which an acetylene-covered surface was hydrogenated with 1.0 × 10–2 Torr H2 between 120 and 300 K indicated that vinyl is the intermediate species to ethylene formation and that the addition of one H to acetylene is the rate-limiting step of the reaction. At a C2H2(g)/H2(g) ratio of 1:100, the reaction was monitored from 300 to 370 K and separately in a constant pressure and constant temperature reaction at 370 K. Ethylidyne and di-σ-ethylene were observed on the surface in both reactions and were found to be spectator species in the hydrogenation of ethylene to ethane. A minor hydrogenation pathway involves a third species, which is best assigned to an ethylidene intermediate. A small coverage of π-ethylene was also...