The interaction of CO and Pt(100). I. Mechanism of adsorption and Pt phase transition

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Abstract
It is known that clean Pt(100) normally exists in a reconstructed (‘‘hex’’) phase, that a metastable clean unreconstructed phase [the (1×1)] can be prepared, and that during adsorption of CO the hex→(1×1) transition occurs. In this and the following paper, we describe an investigation of the CO/Pt(100) system which clarifies the mechanism of this adsorbate‐induced Pt phase transition. The experimental techniques included fast video‐LEED techniques, thermal desorption spectroscopy, electron energy loss spectroscopy, and work function measurements. Adsorption of CO at low temperature (T≲400 K) is discussed in this paper. CO on (1×1)‐Pt forms a c(2×2) overlayer near and at the ideal coverage of θ=0.5, in which the CO molecules occupy on‐top adsorption sites. Repulsive CO–CO interactions cause this structure to form. CO adsorption on hex‐Pt proceeds via formation of small areas with the same local structure, a c(2×2) layer of CO on (1×1)‐Pt, even when the total coverage of CO is low (0.05<θc(2×2). This apparent contradiction is resolved by considering the energy contribution of the Pt phase transition which accompanies the island formation, the whole process being well described as a ‘‘nucleation and trapping’’ mechanism. The nucleation process strongly limits the long‐range order of the CO: At θ=0.5, the c(2×2) domains which result from CO adsorption on (1×1) are about five times larger than for hex‐Pt, even though in the latter case, the substrate has been completely converted to (1×1)‐Pt at θ=0.5. Differences in the long‐range order of CO at θ≥0.5 are also observed in the surface work functions and in the distributions of occupied adsorption sites.

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