CO adsorption on cobalt: Prediction of stable surface phases

Adsorption is often described by a Langmuir isotherm, sometimes accounting for a gradual decrease in the adsorption energy with coverage. Using density functional theory, we show that CO adsorption on cobalt does not follow this typical behavior. Instead, adsorption on Co(0001) is dominated by two surface phases. At low pressures, the (root 3 x root 3)R30 degrees-CO structure is the stable phase, and CO forms (root 3 x root 3)R30 degrees-CO islands for coverages below 1/3 ML because of attractive CO-CO interactions. Increasing the pressure does not gradually increase the coverage beyond 1/3 ML Instead, a transition to a high coverage (2 root 3 x 2 root 3)R30 degrees-7CO surface structure occurs at 0.1 mbar at room temperature and at 21 bar at 500 K. These two phases are also the dominant structures that have been characterized experimentally on Co(0001), and the conditions where each phase was observed match the first principle surface phase diagram. (C) 2015 Elsevier B.V. All rights reserved.