SMSI effects on CO adsorption and hydrogenation on Pt catalysts: I. Infrared spectra of adsorbed CO prior to and during reaction conditions

Abstract

Infrared spectroscopy has been employed to characterize adsorbed CO species under steadystate reaction conditions in the presence of H 2 and also under equilibrium adsorption conditions in the presence of He. During reaction, kinetic data were obtained simultaneously with ir spectra using a special ir reactor cell. Turnover frequencies on Pt were found to vary more than 100-fold as the support material varied, and the activity ranking of these catalysts was: Pt TiO 2 (SMSI) > Pt TiO 2 (LT) ≥ Pt η- Al 2 O 3 − Pt SiO 2- Al 2 O 3 > Pt SiO 2 . Two principal ir bands were observed on Pt—a high-frequency band between 2050 and 2080 cm −1 and a low-frequency band between 1780 and 1860 cm −1. Except for TiO 2-supported Pt, very similar behavior was observed, i.e., intense ir bands occurred which were nearly insensitive to temperature and the presence of H 2, and these catalysts exhibited strong bands under reaction conditions. In contrast, the Pt TiO 2 (SMSI) catalyst showed almost no ir-detectable CO during reaction and, even more surprising, CO band intensities were also markedly reduced at 300 K by the presence of H 2. No obvious correlation was found between detectable CO species and specific activity; however, this study strongly infers that only a very small fraction of surface Pt atoms constitute active sites, and the higher activity occurring on the SMSI Pt TiO 2 catalyst is attributed to a weakened CO-metal bond resulting in more competitive H 2 chemisorption and higher surface concentrations of hydrogen under reaction conditions.