CHARACTERIZATION OF DIFFUSE REFLECTANCE FT-IR SPECTROMETRY FOR HETEROGENEOUS CATALYST STUDIES

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.