Adsorption of chlorine on the Pt(100) face: structural aspects and desorption kinetics

Abstract

The interaction of chlorine with the Pt(100) single crystal face has been studied using LEED and thermal desorption mass spectrometry. Cl 2 molecules adsorb readily on the initially clean reconstructed Pt(100)-hex surface to form a chemisorbed adlayer. Adsorption of chlorine effects the removal of surface reconstruction which is completed at about 4 × 10 14 Cl atoms/cm 2. At higher concentrations a c(2 × 2) structure is formed. Beyond saturation of the c(2 × 2) structure the rate of further chlorine adsorption is largely diminuished. Eventually a (2 × 3) structure develops after heating to 500 K., corresponding to the limiting observed chlorine to platinum surface atom ratio of 2 3 . In this stage the adlayer is interpreted to consist of a distorted hexagonal arrangement of adsorbed chlorine atoms, which are in part out of registry with the substrate. Temperature programmed thermal desorption is observable from 580 to 1100 K and yields exclusively Cl atoms and Cl 2 molecules. The desorption kinetics is influenced by the coverage dependent desorption energy and by the substrate reconstruction taking place during desorption. A kinetic model encompassing surface diffusion of Cl atoms, substrate reconstruction and atomic and molecular desorption was proven by calculating desorption traces on the basis of a lattice gas model with lateral repulsive interactions using Monte Carlo simulations as well as an analytical treatment.