The adsorption of CO on a 5% Pt/SiO2 catalyst has been studied using infrared spectroscopy. Spectra obtained under different conditions showed the presence of two distinct adsorbed CO species. The high-frequency infrared band centered at 2070 cm- (Θ296k = 0.35) was assigned to the stretching frequency of CO linearly adsorbed on Pt. These adsorptive CO molecules were characterized as having a nonordered structure with large dipole-dipole interactions. The low-frequency infrared band centered at 2047 cm- (Θ = 0.35) was assigned to the stretching frequency of desorptive CO with ordered structure adsorbed on Pt surface sites having a low surface coordination. Dipole-dipole interaction for CO molecules adsorbed on these surface sites was small. Adsorption-desorption hysteresis curves suggested that the CO coverage-dependent shifts were due to an adsorbed disordered-ordered transition when a migration occurred from low Miller index planes to high Miller index planes. By carefully controlling the adsorption-desorption conditions, it was possible to obtain a CO doublet at 2070 and 2049 cm- indicating that both ordered and disordered CO could coexist on silica-supported Pt surface. The relative intensities of the adsorbed CO species suggested that the ordered structure had a higher extinction coefficient than the disordered CO structure.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry