Catalyst Characterization Using Quantitative FTIR: CO on Supported Rh

Abstract

Abstract Qualitative FTIR has been and continues to be one of the most utilized tools in the characterization of supported metal catalysts. Quantitative FTIR has the potential to allow catalysis researchers to determine the surface concentrations of active intermediates. However, its successful application depends upon an understanding of the factors affecting integrated absorption intensities (coefficients relating IR absorbance to surface concentration). This work addresses the effect of metal particle size and temperature on the absorption intensities for CO chemisorbed on Rh/SiO 2 . Absorption intensities for both linear and bridged CO surface species ( A 1 and A b ) were determined by combining peak area data from IR spectra with uptake measurements obtained in gravimetric experiments. This resulted in an A 1 value of 13 (±2) and an A b value of 42 (±6) cm/μmol. No statistically significant particle size effect has been observed for average spherical particle diameters ranging from 13 to 58 angstroms (100 to 22% dispersion). Also, integrated absorption intensities for linear and bridged CO were shown to vary little over the temperature range of 323 to 473 K. The discovery that absorption intensities determined for one temperature and metal dispersion may be used for other temperatures and dispersions is a welcome result which may broaden the application of quantitative FTIR. Rh dispersions were determined for Rh/SiO 2 samples of five different weight loadings using the absorption intensities determined in this study. The variation of Rh dispersion with Rh loading was practically identical to that observed in hydrogen chemisorption experiments conducted on another series of Rh/SiO 2 catalysts. Also, it was observed that the ratio of linear to bridged CO surface concentrations increased from 2 to 5 as Rh dispersion increased from 22 to 100%. These observations demonstrate the usefulness of a more fully developed quantitative FTIR technique.