Hydrogen on rhodium (311): Commensurate adsorption phases, reconstruction, and subsurface binding states

Abstract

Four superstructure phases are being observed in a combined low-energy electron-diffraction, thermal-desorption mass-spectroscopy, and work-function-change study of hydrogen adsorption on rhodium (311). This relatively open surface offers sites of threefold and fourfold coordination for the hydrogen atoms. Three low-coverage phases, 1×3-H, 1×2-H, and 1×3-2H, observed in the temperature range of 90–250 K, are governed by the lateral interaction of the adsorbate and seem to have little influence on the surface structure of the substrate. The high-coverage 1×2 phase, however, is an example of adsorbate-induced reconstruction. In contrast to the previous phases, each of which corresponds to a well-defined coverage Θ≤2/3, the reconstructed phase appears close to Θ=1 and cannot be saturated. Its well-ordered surface structure remains stable even under extended exposure to hydrogen. The reconstruction apparently opens diffusion channels to subsurface sites. Structural models based on a linear arrangement of hydrogen along the densely packed Rh rows are proposed for all four adsorption phases, taking into account the observed reaction kinetics of desorbing hydrogen.