Polarization-Dependent Infrared Spectroscopy of Adsorbed Carbon Monoxide To Probe the Surface of a Pd/Cu(111) Single-Atom Alloy

Single-atom alloys can be formed by depositing a low coverage of active metal onto the surface of another metal. Few experimental techniques permit verification that the deposited metal exists as single-atom sites on the host metal surface. Here we use polarization-dependent reflection absorption infrared spectroscopy (PD-RAIRS) to characterize the surface of a Pd/Cu(111) single-atom alloy through the use of CO as a probe molecule. In the presence of 1 x 10(-2) Torr of CO at 300 K, significant coverages of CO are only achieved when Pd is present on the surface. The Pd coverage at the surface is determined with RAIRS from the C-O stretch peak areas while the total amount of Pd in the first few atomic layers is measured with Auger electron spectroscopy. Isolated Pd atoms in Cu(111) are revealed with RAIRS by the C-O stretch peak of CO bound on top of a Pd atom. The appearance of C-O stretch peaks due to bridge-site CO occurs at Pd coverages where Pd atoms start to agglomerate. An isosteric heat of adsorption for CO of 32 kJ/mol was measured with PD-RAIRS for CO in dynamic equilibrium with the Pd/Cu(111) surface and provides new information on the bonding properties of isolated Pd atoms in the Cu(111) host.