Quasi-compounds of WO2 on a W(0 0 1) surface: cluster-model calculations

We analyzed the mechanism of oxygen-atom-pair chemisorption on a tungsten (0 0 1) surface by performing theoretical cluster-model calculations based on the density functional theory. In the analysis, we supposed that p(4×1) structures of oxygen-covered tungsten (0 0 1) surface above coverage of a fourth monolayer come from chains of quasi-compounds of WO2. Calculated electronic structures and vibrational modes normal to the surface of the quasi-compounds of WO2 agrees well with the experimental results. The model clusters for the quasi-compound are however more unstable than ones for single-site chemisorption of oxygen at fourfold hollow and bridge site from comparing their adsorption energies. In order to explain the driving force behind the quasi-compound reaction, we analyzed electrovalent chemisorption energies and electronic dipole moments of the model clusters for oxygen chemisorption on the tungsten (0 0 1) surface and then we propose that the driving force is due to the dipole repulsion between surface polarization induced by oxygen chemisorption and the polarization that occurs between the incoming oxygen molecule and the surface.