Characterization of Diffuse Reflectance FT-IR Spectrometry for Heterogeneous Catalyst Studies

Abstract

The characteristics of diffuse reflectance (DR) infrared spectrometry for the study of adsorbed species were investigated. DR spectra of adsorbed CO on supported catalysts with a high surface area may be obtained at high signal-to-noise ratio with detection limits approaching 10−6 monolayer coverage on the metal surface. Band intensities of adsorbed species as they appear in the Kubelka-Munk (KM) spectra are linear with surface coverage over a low coverage range but show a negative deviation from linearity at high coverage. This effect is due either to a nonlinear response of the KM function with analyte concentration or possibly to a change in the absorptivities of adsorbates with increasing coverage. Several factors affect band intensity reproducibility including: (1) gas equilibration in the sample, (2) baseline variations due to changes in the scattering coefficient, and (3) signal loss at elevated sample temperatures when a mercury-cadmium-telluride detector is used, because unmodulated radiation leads to conduction band or preamplifier saturation. Reproducibility between different catalyst samples is affected by sample packing and treatment history.