The effect of SMSI (strong metal-support interaction) behavior on CO adsorption and hydrogenation on Pd catalysts: I. IR spectra of adsorbed CO prior to and during reaction conditions

Abstract

An infrared cell was designed and operated as a differential, plug-flow reactor so that ir spectra and kinetic data could be obtained simultaneously under well-defined, steady-state conditions. In agreement with results from other reactor systems, turnover frequencies varied over two orders of magnitude from the most active catalysts, TiO 2-supported Pd in the SMSI state, to the least active, Pd SiO 2 . Infrared spectra of CO adsorbed on Pd TiO 2 and Pd SiO 2-Al 2O 3 were recorded for the first time, and spectra for Pd Al 2O 3 and Pd SiO 2 were also obtained. The effects of temperature and hydrogen on the spectra were studied, and spectra were obtained under reaction conditions for five different catalysts. In the absence of H 2, a high-frequency band above 2080 cm −1 and a broader, dominant low-frequency band below 2000 cm −1 were present on all catalysts. Hydrogen had little influence on the CO spectra on the typical Pd catalysts, but it markedly reduced the CO band intensities on the Pd TiO 2 catalysts, even at 300 K. The effect was most pronounced for the Pd TiO 2 (SMSI) catalyst, and under reaction conditions no CO bands were detected whereas all other Pd catalysts had observable ir bands. Primarily for this reason, no obvious correlation was found between observed ir-active CO species and specific activity. However, these results strongly indicate that only a small fraction of the Pd surface atoms constitute active sites, and the large surface concentrations of CO detected by ir are not directly involved in the rate-determining step for methanation.