CHEMISORPTION ON STEPPED METAL-SURFACES - CO VICINAL NI(100)/

The chemisorption properties of carbon monoxide on two vicinal Ni(100) surfaces have been studied with surface infrared reflection-absorptio n spectroscopy and low energy electron diffraction. For coverages less -than-or-equal-to 0.50 monolayer, equilibrium adlayers are formed in w hich CO populates atop sites on the low-index (100) terrace, as well a s twofold bridging sites along both the highly-kinked and close-packed step edges of the Ni[(100)-1.4-degrees(010BAR)] and Ni[(100)-9-degree s(011BAR)] surfaces investigated. Low energy electron diffraction (LEE D) measurements confirm that all three long-range structures establish ed on the (100) surface(2 x 2) at 0.50 ML, hexagonal at 0.61 ML, and c ompressed-hexagonal at 0.69 ML-are also formed on the Ni[(100)-l.4-deg rees(010BAR)] surface. On the Ni [(100)-9-degrees(011BAR)] surface, ho wever, only the ordered c (2 X 2) structure appears. A simple Arrheniu s analysis of the relative population of step and terrace sites estima tes a small binding energy preference for populating step sites. This weak preference is of comparable magnitude to the CO-CO interactions t hat produce long range structures. To evaluate quantitatively the bind ing energy difference between adsorption at step and terrace sites, st ep adsorption isotherms are measured as a function of total coverage a t select temperatures over the 90-300 K window. The isotherms are mode led with simple Monte Carlo simulations of adsorption on stepped surfa ces, which include a 1.0 kcal/mol binding energy preference for step s ites. The data and simulations indicate that the primary role played b y the steps in the chemisorption of CO is to serve as nucleation cente rs for island growth.