Chemisorption of CO on the Pt(111) surface

Abstract

The system CO/Pt(111) was studied by means of LEED, thermal desorption and work function measurements. At 170 K a √ 3 × √3/ R30° structure at θ = 1 3 is continuously transformed with increasing coverage into a c4 × 2 structure at θ = 1 2 and finally into a hexagonal close-packed layer with saturation at about θ = 0.68. Due to a decrease of the adsorption energy by about 4 kcal mole at θ = 0.5 ( =7.5 × 10 14 molecules cm 2 ) adsorption is completed at this coverage at room temperature. The initial adsorption energy is about 33 kcal/rnole. A strong tendency for disordering far below room temperature manifests itself with the LEED patterns and with the Δφ data. The work function at first decreases, exhibits a (temperature-dependent) minimum at θ = 1 3 , attains nearly the value of the clean surface at θ = 1 2 and again exhibits a second (shallow) minimum around θ = 0.6. A detailed discussion reveals that the observed effects may be explained by assuming the occupation of two different adsorption sites at θ⩽ 0.5 with different dipole moments (presumably bridge (A) and threefold coordinated (B)) whose adsorption energy differs by only 0.5 kcal/mole. At low temperatures at θ = 1 2 sites A and at θ = 1 3 sites B are preferentially occupied whereas their small energy difference favours disordering at increasing temperature.