Probing adsorption sites of silica-supported platinum with (13)C(16)O + (12)C(16)O and (13)C(18)O + (12)C(16)O mixtures: a comparative fourier transform infrared investigation

A comparative investigation of the adsorption of (13)C(18)O + (12)C(16)O and (13)C(16)O + (12)C(16)O mixtures on silica-supported Pt has been conducted. It is advantageous to use (13)C(18)O + (12)C(16)O mixtures rather than (13)C(16)O + (12)C(16)O to probe the adsorption sites and electronic state of supported Group VIII metals because the vibrational bands of the adsorbed (13)C(18)O and (12)C(16)O isotopic molecules do not overlap. In addition, while an intensity redistribution suppresses the lower-frequency band with adsorbed (13)C(16)O and (12)C(16)O with vibrational frequencies differing by 50 cm(-1), the intensity redistribution is less pronounced with the adsorbed (13)C(18)O and (12)C(16)O in which the frequency difference is 100 cm(-1). Moreover, the small intensity redistribution that does occur between the bands of adsorbed (13)C(18)O and (12)C(16)O still allows the detection of the vibrational band of adsorbed (13)C(18)O at (13)C(18)O gas-phase concentrations as low as 3%. At such low concentrations, the dipole-dipole interaction between adsorbed (13)C(18)O 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.