Probing adsorption sites of silica-supported platinum with 13C16O+12C16O and 13C18O+12Cl16O mixtures:: A comparative Fourier transform infrared investigation

A comparative investigation of the adsorption of (CO)-C-13-O-18 + (CO)-C-12-O-16 and (CO)-C-13-O-16 + (CO)-C-12-O-16 mixtures on silica-supported Pt has been conducted. It is advantageous to use (CO)-C-13-O-18 + (CO)-C-12-O-16 mixtures rather than (CO)-C-13-O-16 + (CO)-C-11-O-16 to probe the adsorption sites and electronic state of supported Group VIII metals because the vibrational bands of the adsorbed (CO)-C-13-O-18 and (CO)-C-12-O-16 isotopic molecules do not overlap. In addition, while an intensity redistribution suppresses the lower-frequency band with adsorbed (CO)-C-11-O-16 and (CO)-C-12-O-16 With vibrational frequencies differing by 50 cm(-1), the intensity redistribution is less pronounced with the adsorbed (CO)-C-13-O-18 and (CO)-C-12-O-16 in which the frequency difference is 100 cm(-1). Moreover, the small intensity redistribution that does occur between the bands of adsorbed (CO)-C-13-O-18 and (CO)-C-12-O-16 still allows the detection of the vibrational band of adsorbed (CO)-C-13-O-18 at (CO)-C-13-O-18 gas-phase concentrations as low as 3%. At such low concentrations, the dipole-dipole interaction between adsorbed (CO)-C-13-O-18 molecules is negligible, and, hence, both the singleton frequency and the dipole-dipole shift for adsorbed CO may be obtained in a single experiment. Two types of strongly bound and one type of weakly bound linear CO-Pt adsorption complexes have been identified and characterized by their singleton frequencies and dipole-dipole coupling shifts. The origin of these CO adsorption modes is discussed.