HYDROGEN ADSORPTION ON A PT50NI50(111) SINGLE-CRYSTAL ALLOY STUDIED BY NRA, TDS, AND DELTA-PHI

Nuclear reaction analysis (NRA), thermal desorption spectroscopy (TDS), and work function changes (Delta phi) were used to study hydrogen adsorption on Pt50Ni50(111) at 115 K. Low-energy ion spectroscopy (LEIS) allowed us to measure the surface concentration of Ni atom: C-Ni (%) = 11 +/- 2, in agreement with a strong segregation of platinum on the topmost layer. The work function decreases during the hydrogen adsorption on the alloy as on Pt(111). The activation energy for desorption and the initial sticking coefficient are lowered on the alloy compared to Pt(111). The saturation coverage at 115 K is only 0.2 monolayer for P(H-2) > 10(-5) Torr. The TD spectra show several peaks which have been related to the local order on this alloy surface, with different types of triangular sites. It is proposed that only the sites involving at least one Ni atom are active for hydrogen adsorption.