Studies of bimetallic surfaces using infrared reflection absorption spectroscopy

Abstract

Well-defined bimetallic surfaces have been studied using carbon monoxide adsorption in conjunction with infrared reflection–absorption spectroscopy. The vibrational frequency of CO on low Cu coverages [∼0.1 monolayer (ML)] on Rh(100) and Mo(110) is blue-shifted relative to its gas phase stretching frequency of 2143 cm−1. This blue shift is explained as arising from CO adsorbed on well-dispersed Cu adatoms which are slightly positively charged due to polarization arising from the CO–Cu interaction. As the sample temperature is increased to 150–200 K, CO becomes mobile on these surfaces as evidenced by the migration of CO from submonolayer Pd overlayers to a W(110) substrate. Three-dimensional Cu clusters, well-ordered pseudomorphic two-dimensional Cu islands, and single Cu atoms are distinctively characterized by their CO infrared (IR) peaks. In addition, it is found that IR spectra of adsorbed CO show a remarkable sensitivity to surface structural phase transitions. Both order–order and disorder–order transitions are observed for Ni and Co overlayers on a Mo(110) substrate. It is further shown that localized segregation and ordering of C and O on Mo(110), B on Ru(0001) and Co on S/Mo(110) are observable.