Low coverages of catalytically active metals deposited onto less active metal surfaces can form single atom alloys (SAAs), which often display unique catalytic properties. Such alloys are particularly attractive for selective hydrogenation reactions. We have used reflection absorption infrared spectroscopy (RAIRS) to monitor the hydrogenation of acetylene and propyne over a Pd-Cu(111) SAA. We have also used RAIRS of adsorbed CO to characterize the structure of the SAAs. At certain values of the surface temperature and/or CO pressure, CO adsorbs only on Pd atoms and not on Cu or Ag sites. For low Pd coverages, only CO adsorbed at atop Pd sites is detected. At higher Pd coverages, CO adsorption at Pd bridge sites is observed indicating that Pd-dimers or other aggregates have formed. For acetylene and propyne hydrogenation, polarization dependent RAIRS is used to monitor gas phase reactants and products while simultaneously detecting surface species. No hydrogenation activity is detected on the clean C(111) surface, but hydrogenation to the alkene is readily detected over the Pd-Cu(111) SAA surface. Full hydrogenation to the alkane is not detected. It is also found that coupling reactions occur to produce a carbonaceous layer on the surface, but that hydrogenation proceeds even in the presence of this layer.

Selective hydrogenation reactions over a Pd-Cu(111) single-atom alloy studied with ambient pressure infrared spectroscopy

ACS Fall 2021

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