Universal Phenomena of CO Adsorption on Gold Surfaces with Low-Coordinated Sites

Since Au turned out to be an active catalyst for CO oxidation at low temperatures, CO adsorption on various Au surfaces has been in the scope of numerous surface science studies. Interestingly, supported particles as well as stepped and rough single-crystal surfaces exhibit very similar adsorption behavior. To elucidate the origin of these similarities, we have performed temperature-programmed desorption and infrared absorption spectroscopy for a whole range of Au surfaces from nanoparticles grown on HOPG to Au(111) surfaces roughened by argon ion bombardment. In line with previous results, we have observed two desorption states at ∼130−145 and ∼170−185 K, respectively, and one infrared peak at around 2120 cm -1 in all cases. In addition to the experiments, we have carried out theoretical studies of CO adsorption on Au(332). The calculations show that CO desorption states above 100 K may be located at step-edges but not on terrace sites. Reducing the coordination of Au atoms further leads to successively higher binding energies with an unchanged anharmonic frequency. Therefore, we conclude that both desorption peaks belong to CO on low-coordinated Au atoms at steps and kinks. For the sputtered Au(111) surface, scanning tunneling microscopy reveals a rough pit-and-mound morphology with a large number of such sites. In annealing experiments we observe that the loss of these sites coincides with the loss of CO adsorption capacity, corroborating our conclusions.