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

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<θ<0.5). This comparison between adsorption on the two phases indicates that the mechanism of the hex→(1×1) transition involves, effectively, ‘‘island’’ formation of adsorbed CO in spite of the repulsive character of the CO–CO interaction in the c(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.