CHEMISORPTION OF CO, H-2, AND O-2 ON ORDERED SN/PT(111) SURFACE ALLOYS

The chemisorption of CO, H2, and O2 on ordered Pt-Sn surface alloys has been investigated by using thermal desorption mass spectroscopy (TDMS), high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), Auger electron (AES), X-ray photoelectron (XPS) and ultraviolet photoelectron spectroscopy (UPS), and low-energy ion scattering spectroscopy (LEISS). Alloying with Sn causes only slight decreases in the CO desorption peak temperature compared to Pt(111). A (2√3×2√3)R30° LEED pattern is observed for the saturation CO coverage on the p(2×2) ordered alloy surface. No ordered CO overlayers were observed by LEED from the (√3× √3)R30° alloy surface. HREELS indicates the atop-bonded CO is the most strongly adsorbed species on both ordered alloy surfaces, as on Pt(111), and that both atop and bridge sites are populated for saturation CO coverage on both ordered alloy surfaces. No dissociative H2 adsorption is observed on either the p(2×2) or (√3×√3)R30° Sn/Pt ordered alloy surfaces. However, H atoms will adsorb on both surfaces. H2 desorption from the H adlayers occurs with a slightly higher peak temperature than on Pt(111). In addition to this shifted desorption peak, another low-temperature H2 desorption state is observed for the (√3×√3)R30° Sn/Pt alloy surface. No molecular or dissociative O2 adsorption is observed on either of the two ordered alloy surfaces over the range 100-600 K. This suppression of chemisorption for the alloy surfaces is in contrast to increased oxygen uptake by unalloyed Sn overlayers on Pt(111) and the difference is due to changes in the electronic structure of the surface that accompany alloy formation. These changes have been examined by using UPS for the ordered Sn/Pt(111) alloys. Models are proposed to explain the adsorption and desorption behavior for the three gases that were studied. © 1990 American Chemical Society.