A RAIRS and HREELS study of acetone on Pt(111)

Abstract

Abstract The use of RAIRS (reflection-absorption infrared spectroscopy) has provided the first IR spectra of acetone adsorbed on an unsupported metal surface. These results were complemented by HREELS. Both sets of spectra gave peak positions for multilayer acetone that were very similar to those associated with liquid-phase acetone, whereas the η1 (“end-on” coordination) monolayer state had a CO stretch frequency red-shifted to 1638 cm−1, thus making it easy to identify. Direct evidence for an η2 species, which has been proposed to exist in a “side-on” configuration with its CO bond parallel to the surface, was not easy to obtain on this essentially defect-free Pt(111) surface because of the absence of higher-energy binding states that have been previously reported for Pt(111), Pd(111) and Ru(001) surfaces. However, at 1 K s a second desorption peak at 199 K could be resolved from the larger η1 peak at 184 K. These thermal desorption spectra also indicated that no acetone decomposition occurred during desorption. This more strongly bound acetone species is associated with an η2 species, but the small binding energy difference ( 1 kcal mol ) appears to be less on this surface compared to the others and increases the difficulty of isolating this η2 state. Additional evidence for a monolayer species other than the η1 phase was provided by RAIRS and HREELS. First, the η1 species that initially forms gradually disappears as the multilayer phase develops. Second, after flashing to temperatures below 200 K the 1638 cm−1 band is removed, yet peaks at 1086, 1360, and 1428 cm−1, associated with the methyl groups, not only are retained but also grow in intensity, and new bands appear between 1500 and 1610 cm−1. Finally, these latter bands are also formed after the dissociative adsorption of isopropyl alcohol to give adsorbed hydrogen and acetone.