A MOLECULAR-BEAM STUDY OF THE CO-INDUCED SURFACE PHASE-TRANSITION ON PT(100)

The CO-induced hex --> (1 X 1) surface phase transition on Pt{100} is driven by the higher adsorption heat of CQ on the (1 X 1) phase than on the hex phase and occurs by the sequential steps of CO adsorption on the hex phase and growth of islands of CO trapped on the (1 X 1) phase. We have investigated the dynamics of CO chemisorption on both the rotated hex (hex-R) and (1 X 1) surfaces by measuring the sticking probability as a function of surface temperature, beam energy, and beam incidence angle using the King and Wells beam reflectivity method. For both clean surfaces, the adsorption is nonactivated and there is no evidence for adsorption via an intrinsic precursor. However, at finite CO coverages, there is an additional adsorption pathway involving an extrinsic physisorbed precursor. At surface temperatures above 350 K, the net sticking probability on the initial hex-R surface at finite CO coverages is less than below 350 K due to CO desorption, primarily from the hex-R phase, competing with island growth. We have developed a technique to measure the local CO coverage on the hex-R phase during the CO-induced hex-R --> (1 X 1) phase transformation. The island growth rate has a power law dependence on the local CO coverage on the hex-R phase with an apparent reaction order of 4.5 +/- 0.4. These kinetics manifest themselves as a strongly flux-dependent net sticking probability. The absence of a decrease in the local CO coverage on the hex-R phase after the onset of island growth is unexpected considering the nucleation-and-growth process, and two possible microscopic mechanisms for the surface phase transition are proposed.