Fourier-transform infrared, attenuated total reflection spectroscopy: a complementary tool for the investigation of the adsorption of CO on thin metal layers

Abstract

The application of the attenuated total reflection (ATR) technique to the study of the adsorption of CO on thin films of Pt or Pd from dry and wet gases and from water was investigated. Metal layers, thinner than the depth of penetration of IR light, were deposited on a germanium reflection element by sputtering or by evaporation. These films were characterized by different techniques such as STM, electron microprobe analysis and SIMS. Adsorption of CO was carried out in a flow-through cell. The effect of increasing coverage was studied.

On Pt the adsorbed CO was mainly linearly bonded (> 2000 cm^–1). The asymmetric shape of the bands indicated that different CO bond strengths coexist on the rough surface. Both linear and bridged-bonded CO adsorb at a lower frequency in the wet than in the dry state. The band position shifts to higher frequencies with increasing coverage. The platinum layer was not stable in contact with the liquid.

On Pd the adsorbed CO was both linearly bonded (ca. 2060 cm^–1) and bridged bonded (ca. 1880 cm^–1). The relative intensity of the two bands varied considerably from one experiment to another. The asymmetry of the band for the bridged CO was greater than that for the linear CO. The relative intensityl_linear/l_bridged increased from dry to wet gas and decreased from wet gas to liquid. The frequency of the bridged CO was also increased from dry to wet gas and decreased from wet gas to liquid. Band positions within one experiment are constant, but vary with the Pd layer.