A diffuse reflectance infrared fourier-transform spectra and density functional theory study of CO adsorption on Rhγ-Al2O 3

Abstract

The vibrational frequencies and bond dissociation energies of carbon monoxide adsorbed to various rhodium clusters are computed, and the diffuse reflectance infrared Fourier-transform (DRIFT) spectrum of carbon monoxide adsorbed onto Rh/γ-Al2O3 is reported. The computations reveal that the spectral features that have previously been assigned to symmetric and antisymmetric vibrational modes of isolated dicarbonyl species on Rh can also be assigned to vibrational coupling of adjacent linearly bound carbonyl groups (atop M-CO). We explore, in small rhodium clusters, the effects of binding configuration, surface coverage, metal-support charge transfer, and cluster dimension on the vibrational frequency of the C-O bond and the adsorption energy of CO to rhodium. In particular, we find a linear correlation between the vibrational frequency of the carbon-oxygen bond with respect to the metal-carbonyl bond adsorption energy as well as the carbon-oxygen bond length for small rhodium clusters. A semiquantitative relationship is derived that may be used to calculate bond dissociation enthalpies of rhodium-carbonyl bonds with the use of crystallographic or IR data associated to the rhodium-carbonyl bond. © 2007 American Chemical Society.