COMPUTATIONAL LATTICE-GAS MODELING OF THE ELECTROSORPTION OF SMALL MOLECULES AND IONS

We present two recent applications of lattice-gas modeling techniques to electrochemical adsorption on catalytically active metal substrates : urea on Pt(100) and (bi)sulfate on Rh(111). Both systems involve the specific adsorption of small molecules or ions on single-crystal elec trodes, and they are characterized by a particularly good fit between the adsorbate geometry and the substrate structure. The close geometri c fit facilitates the formation of ordered submonolayer adsorbate phas es in a range of electrode potential positive of the range in which an adsorbed monolayer of hydrogen is stable. In both systems the ordered -phase region is separated from the adsorbed-hydrogen region by a phas e transition, signalled in cyclic voltammograms by a sharp current pea k. Based on data from in situ radiochemical surface concentration meas urements, cyclic voltammetry, and scanning tunneling microscopy, and e x situ Auger electron spectroscopy and low-energy electron diffraction , we have developed specific lattice-gas models for the two systems. T hese models were studied by group-theoretical ground-state calculation s and numerical Monte Carlo simulations, and effective lattice-gas int eraction parameters were determined so as to provide agreement with th e experimental results.