Adsorption and Oxidation of Carbon Monoxide on (100) Nickel

Exposure of a clean (100) nickel surface to CO results in the formation of a structure designated as (100)Ni c2X2-CO whose unit mesh is twice as large as that of the clean surface. This is interpreted to mean that CO is adsorbed with one molecule to every two atoms of the (100) nickel surface, in contrast to the (110) surface on which one molecule adsorbs to each atomic site. The work function increase due to adsorbed CO was less than for (110) nickel, and a much higher temperature was required to remove the CO from the (100) surface. These results suggest that CO may be bound in a bridged structure on the (100) face rather than a linear bond as on the (110). Heating at temperatures below that required to remove the CO and then cooling to room temperature produced a large increase in the diffraction intensity, suggesting a possible reconstruction of the nickel substrate. Exposure of the CO covered surface to O-2 extinguished the diffraction pattern and caused the work function to decrease. If the crystal was then heated, a pattern having the same unit mesh as the initial CO structure appeared, but was shown to be that of the (100)Ni c2X2-O structure, the preadsorbed CO having reacted to form CO2 which was desorbed by heating. CO did not react with a (100) nickel surface which had received prior exposure to O-2.