Propyne Hydrogenation over a Pd/Cu(111) Single-Atom Alloy Studied using Ambient Pressure Infrared Spectroscopy

The hydrogenation of propyne (C3H4) to propene (C3H6) using a Pd/Cu(111) single-atom alloy (SAA) has been studied using polarization-dependent reflection absorption infrared spectroscopy. This method allows for simultaneous monitoring of reactants and products in the gas phase and species adsorbed on the surface during the reaction. The results were compared with the hydrogenation of propyne using Pd-free Cu(111) as well as with previous studies on Pd/Cu SAA catalysts supported on alumina. Propene production occurs at temperatures of 383 K and above as indicated by the appearance of an infrared peak at 912 cm(-1), which is a unique characteristic feature of gas phase propene. Propyne was found to adsorb on the surface at 300 K in the presence of gas phase propyne to form a di-s/di-p structure, as the spectrum was identical to that reported in the literature for propyne adsorbed on Cu(111) at 150 K in ultrahigh vacuum. The presence of a carbonaceous layer on the surface is indicated by a dramatic increase in the intensity of a peak at 2968 cm(-1) at temperatures above 400 K. The progression of gas phase peaks at 912 and 3322 cm-1 was used to calculate the rate of production of propene and the rate of consumption of propyne, respectively. This reaction rate was used to determine a turnover frequency of 25.4 s(-1) at 383 K for the reaction on the Pd/Cu(111) SAA surface. The reaction was not impeded by the presence of the carbonaceous layer, even for a layer so thick that only carbon was detectable on the surface with Auger electron spectroscopy.