Computational Lattice-Gas Modeling of the Electrosorption of Small Molecules and Ions

TL;DR

This paper develops lattice-gas models for electrochemical adsorption of small molecules and ions on metal surfaces, accurately fitting experimental data and revealing phase transitions in adsorbate layers.

Contribution

The study introduces specific lattice-gas models for urea on Pt(100) and (bi)sulfate on Rh(111), validated against experimental data and used to analyze phase behavior.

Findings

Ordered submonolayer phases form due to geometric fit.

Phase transitions are observed between ordered phases and hydrogen adsorption.

Models agree with experimental cyclic voltammetry and surface measurements.

Abstract

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 involve the specific adsorption of small molecules or ions on well-characterized single-crystal electrodes, and they provide a particularly good fit between the adsorbate geometry and the substrate structure. The close geometric fit facilitates the formation of ordered submonolayer adsorbate phases 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 phase transition, signified in cyclic voltammograms by a sharp current peak. Based on data from {\it in situ\/} radiochemical surface concentration measurements, cyclic voltammetry, and scanning tunneling micro- scopy, and {\it ex situ\/} Auger electron spectroscopy and low-energy electron diffraction, we have developed specific lattice-gas models for the two systems. These models were studied by group-theoretical ground-state calcu- lations and numerical Monte Carlo simulations, and effective lattice-gas inter- action parameters were determined so as to provide agreement with experiments.