CORRELATED INFRARED STUDIES OF PLATINUM SINGLE-CRYSTALS AND SUPPORTED CATALYSTS

On platinum surfaces, the vibrational frequency of carbon monoxide molecules adsorbed at defect sites is somewhat lower than that of molecules on terrace sites. Because of this difference, dipolar coupling interactions modify the infrared spectrum by transferring intensity from the "defect" band to the "terrace" band. This can severely distort the spectrum, especially when there exists a high proportion of surface defects, as on small catalyst particles. We have recorded spectra of (CO)-C-12/(CO)-C-13 mixtures adsorbed on alumina-supported platinum and correlated the results with data from single-crystal studies, thereby showing that terrace sites constitute a monority of the adsorption sites on such catalysts, even though they dominate the normal, single-isotope spectrum.