Infrared reflection absorption spectral study for CO adsorption on Pd/Pt(1 1 1) bimetallic surfaces

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on 0.15 nm-thick–0.6 nm-thick Pd-deposited Pt(1 1 1) bimetallic surfaces: Pdx/Pt(1 1 1) (where x is the Pd thickness in nanometers) fabricated using molecular beam epitaxial method at substrate temperatures of 343 K, 473 K, and 673 K. Reflection high-energy electron diffraction (RHEED) measurements for Pd0.15–0.6 nm/Pt(1 1 1) surfaces fabricated at 343 K showed that Pd grows epitaxially on a clean Pt(1 1 1), having an almost identical lattice constant of Pt(1 1 1). The 1.0 L CO exposure to the clean Pt(1 1 1) at room temperature yielded linearly bonded and bridge-bonded CO–Pt bands at 2093 and 1855 cm−1. The CO–Pt band intensities for the CO-exposed Pdx/Pt(1 1 1) surfaces decreased with increasing Pd thickness. For Pd0.3 nm/Pt(1 1 1) deposited at 343 K, the 1933 cm−1 band caused by bridge-bonded CO–Pd enhanced the spectral intensity. The linear-bonded CO–Pt band (2090 cm−1) almost disappeared and the bridge-bonded CO–Pd band dominated the spectra for Pd0.6 nm/Pt(1 1 1). With increasing substrate temperature during the Pd depositions, the relative band intensities of the CO–Pt/CO–Pd increased. For the Pd0.3 nm/Pt(1 1 1) deposited at 673 K, the linear-bonded CO–Pt and bridge-bonded CO–Pd bands are located respectively at 2071 and 1928 cm−1. The temperature-programmed desorption (TPD) spectrum for the 673 K-deposited Pd0.3 nm/Pt(1 1 1) showed that a desorption signal for the adsorbed CO on the Pt sites decreased in intensity and shifted ca. 20 K to a lower temperature than those for the clean Pt(1 1 1). We discuss the CO adsorption behavior on well-defined Pd-deposited Pt(1 1 1) bimetallic surfaces.