Chemisorption on stepped metal surfaces: CO/vicinal Ni(100)

Abstract

The chemisorption properties of carbon monoxide on two vicinal Ni(100) surfaces have been studied with surface infrared reflection–absorption spectroscopy and low energy electron diffraction. For coverages ≤0.50 monolayer, equilibrium adlayers are formed in which CO populates atop sites on the low-index (100) terrace, as well as twofold bridging sites along both the highly-kinked and close-packed step edges of the Ni[(100)-1.4°(01̄0)] and Ni[(100)-9°(01̄1̄)] surfaces investigated. Low energy electron diffraction (LEED) measurements confirm that all three long-range structures established on the (100) surface—c(2×2) at 0.50 ML, hexagonal at 0.61 ML, and compressed-hexagonal at 0.69 ML—are also formed on the Ni [(100)-1.4°(01̄0)] surface. On the Ni [(100)-9°(01̄1̄)] surface, however, only the ordered c(2×2) structure appears. A simple Arrhenius analysis of the relative population of step and terrace sites estimates a small binding energy preference for populating step sites. This weak preference is of comparable magnitude to the CO–CO interactions that produce long range structures. To evaluate quantitatively the binding energy difference between adsorption at step and terrace sites, step adsorption isotherms are measured as a function of total coverage at select temperatures over the 90–300 K window. The isotherms are modeled with simple Monte Carlo simulations of adsorption on stepped surfaces, which include a 1.0 kcal/mol binding energy preference for step sites. The data and simulations indicate that the primary role played by the steps in the chemisorption of CO is to serve as nucleation centers for island growth.