Attenuated total reflection infrared spectroscopy for studying adsorbates on planar model catalysts: CO adsorption on silica supported Rh nanoparticles

Abstract

A sensitive method is presented for studying adsorption of gaseous species on metal surfaces in vacuum by attenuated total internal reflection Fourier transform infrared spectroscopy (ATR). The method is illustrated by CO adsorption experiments on silica supported Rh nanoparticles. An experimental setup and a procedure are described in detail to obtain a sensitivity of reflectance change of ∼5×10−5 absorbance units. Here, a silicon ATR crystal with a 50nm layer of hydroxylated silica acts as the support for the Rh nanoparticles. These particles are easily prepared by spincoat impregnation from a RhCl3 solution followed by H2 reduction. X-ray photoelectron spectroscopy before and after reduction shows that rhodium is reduced to Rh0 and that all chlorine is removed. Atomic force microscope images the distribution of the particles, which are 3–4nm in height. When the crystal is exposed to pressures up to 1mbar of CO, a gas which is inert to the silica support, the stretch vibration of linearly adsorbed CO on the Rh nanoparticles is detected at 2023cm−1, while no bridged CO or geminal dicarbonyl species can be distinguished. The minimum detectable coverage is estimated ∼0.005COpernm2 substrate area or ∼5×10−4ML.