The interaction of CO molecules on Au-Rh bimetallic nanoclusters supported on a thin film of Al₂O₃/NiAl(100)

The interaction of CO molecules adsorbed on Au-Rh bimetallic nanoclusters supported on an ordered thin film of Al₂O₃/NiAl(100) was studied, primarily with infrared reflection absorption spectroscopy and density-functional-theory calculations. The bimetallic clusters, grown by sequential deposition of vapor Au and Rh onto the Al₂O₃/NiAl(100) surface at 300 K, had diameters of 1.2-3.0 nm and heights of 0.4-1.2 nm; they had a fcc phase and grew in the orientation (100). The infrared absorption line for CO adsorbed on Au sites (CO_Au) of the bimetallic clusters at 110 K was narrow (centered about 2100 cm⁻¹) and intense, which results largely from the small adsorption energy and large dipole moment of CO_Au, whereas that on Rh sites (CO_Rh) was broad (1880-2100 cm⁻¹) and weak, which contrasts also with its counterpart on pure Rh clusters. Upon increasing the temperature to remove CO_Au, the absorption line for CO_Rh narrowed and the intensity increased; at 300 K, the line width decreased by 30-40% and the absorption intensity was enhanced by 40-60%. The former arose, after the desorption of CO_Au, from a decreased CO-CO interaction and inhomogeneous broadening; the latter corresponded to an enhanced dipole moment of CO_Rh, attributed to a promoted charge transfer from the CO_Rh-binding Rh to the neighboring Au and consequently increased charge donated from CO_Rh to Rh. The varied IR absorption for adsorbed CO can thus serve as an indicator for the charge transfer between the components in bimetallic clusters.