Selective Hydrogenation of Acetylene to Ethylene in the Presence of a Carbonaceous Surface Layer on a Pd/Cu(111) Single-Atom Alloy

Reflection absorption infrared spectroscopy (RAIRS) was used to simultaneously monitor gas-phase and surface species in the presence of ambient pressures of acetylene and hydrogen over a single-atom alloy (SAA). The alloy consisted of isolated Pd atoms at surface coverages in the range of 0.0028-0.085 ML in a Cu(111) surface. When C2H2(g) is present, but not H-2(g), the RAIR spectra are similar for Cu(111) with and without Pd, independent of C2H2(g) pressure for temperatures between 180 and 500 K. The addition of H2(g) leads to different RAIR spectra depending on the presence of Pd. With a C2H2:H-2 ratio of 1:100 and a SAA-Pd/Cu(111) surface with less than 196 Pd, C2H2(g) is converted to C2H4(g) at 400 K at total pressures up to 10 Torr. From the rate of change in the gas-phase IR peaks, a range of initial turnover frequencies was estimated, which depend on which sites are assumed to be active for hydrogenation. Postreaction surface analysis with Auger electron spectroscopy (AES) showed a significant carbon coverage, which decreased with increasing Pd coverage. The combined RAIRS and AES results suggest that, by increasing reactivity for ethylene formation, Pd also limits the amount of carbon that is deposited, while also changing the extent of oligomer formation.