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

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 X 10(-2) Torr H-2 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)/H-2(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-sigma-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 it-ethylene was also present during the annealing experiment at 350 K but disappeared at 370 K, indicating that it is also an intermediate in the reaction. In a separate experiment to compare acetylene hydrogenation with ethylene hydrogenation at 370 K, spectra were acquired with a C2H4(g)/H2(g) ratio of 1:10. Ethylene hydrogenation proceeds approximately three times faster when starting with ethylene as compared with hydrogenation of the ethylene produced by acetylene hydrogenation. This indicates that the surface is covered with different intermediates when it is first exposed to acetylene. The results presented here demonstrate a simple way to use polarization-dependent RAIRS to distinguish surface species from gas phase reactants and products. The method should be applicable to a wide range of catalytic reactions over metal surfaces and offers new opportunities for operando studies in catalysis.