Adsorption of CO on Rh(100) studied by infrared reflection–absorption spectroscopy

Abstract

The interaction of CO with a Rh(100) surface at 90 and 300 K has been investigated with infrared reflection–absorption spectroscopy (IRAS). Absorption bands due to the C–O stretch are found in both the linear and bridging regions at all coverages for both adsorption temperatures. For adsorption at 300 K, an ordered CO layer is formed at ∼0.4 monolayers (ML) as evidenced by a sharp, highly symmetrical, linear-CO band. At θCO>0.45 ML, the reduction in intermolecular separation and increasing intermolecular repulsive forces in the CO adlayer are evidenced by the increased broadness of both the linear- and the bridged-CO bands. CO adsorption at 90 K yields a nonuniform adlayer dominated by island formation at θCO<0.5 as indicated by peak splitting of the linear-CO band and the presence of a broad bridged-CO band. At θCO∼0.5, a highly ordered CO adlayer is suggested by the appearance of a highly symmetrical linear-CO band as well as the formation of a sharp c(2×2) LEED pattern. The presence of multiple components in the linear-CO band along with the drastic variations in the relative absorption intensities for both CO bands at coverages between 0.5 and 0.7 ML imply that the CO adlayer undergoes a phase transition within this coverage regime. These IRAS data are in agreement with the observed transition of a sharp c(2×2) pattern at θCO=0.5 to an ordered LEED pattern with a p(4×4)R45° coincidence structure at θCO=0.7. From a series of temperature-dependent IRAS measurement, it has been determined that the difference in binding energy of linear and bridged sites at θCO=0.2–0.5 ranges from ∼0.1 to ∼0.4 kcal/mol with the bridged-bound CO being the more stable species.