The interaction of CO and Pt(100). II. Energetic and kinetic parameters

Abstract

We have investigated the mechanism and driving force of the CO-induced phase transition of the Pt(100) surface. In a preceding paper, we have concentrated on the mechanism by which CO removes the surface reconstruction. As discussed, the clean reconstructed (hex) surface of Pt(100) is more stable than the unreconstructed (1×1) phase, but if the CO coverage on hex exceeds a small critical value (θ≳0.05) nucleation of (1×1) patches occurs and proceeds until, at θ=0.5, the entire surface has been converted to the (1×1) phase. In this paper we present data which quantitatively describe the energetics of this system. Values for the heats of adsorption of CO on both the reconstructed and unreconstructed phases, as well as values for the preexponential factors for the desorption rates, are determined from quasiequilibrium LEED measurements at different coverages. The difference in the low coverage heats of adsorption of CO on the hex and (1×1) phases (27.5 vs 37.5 kcal/mol) is the driving force in the Pt phase transformation during adsorption. On the other hand, during desorption, the hex phase does not return until the coverage has decreased below 0.3. This measurement allows an estimation of the difference in stability between the clean (1×1) and hex phases: about 9–13 kcal per mole of Pt atoms. The transition between the two phases which are in equilibrium [COgas+hex-Pt(100) vs COads+(1×1)-Pt(100)] shows a hysteresis due to kinetic limitations. These kinetic effects are characteristic of a nucleation process involving a critical coverage.