Modifications of a Diffuse Reflectance Cell to Allow the Characterization of Carbon-Supported Metals by DRIFTS

Abstract

Carbon-supported metal catalysts have been investigated with the use of Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) for the first time. In order to successfully pretreat such catalysts, obtain IR spectra, and characterize these highly opaque materials, one must make extensive modifications of DRIFTS cells, commercial or otherwise. These modifications include the arrangements for capability and ease of sample height adjustment from outside the spectrometer chamber, the improvement of the thermal performance of the cell to provide high temperatures in flowing H₂, the accurate measurement of sample temperature, and the guarantee of adequate gas flows through the catalyst bed. These capabilities are essential for proper in situ investigations of catalysts. This improved DRIFTS cell was used to follow the thermal decomposition of carbon-supported Os₃(CO)₁₂ for the first time. The cluster transformed rapidly in flowing H₂ to a more stable H₄Os₄(CO)₁₂ terra-nuclear cluster, similar to the reaction in solution, which then further decomposed at higher temperatures. The spectrum of CO adsorbed on carbon-supported Os crystallites after complete decomposition was also obtained, and it showed the presence of only reduced metallic Os. These findings provide the first IR evidence for the relative inertness of oxygen-free carbon, compared to oxide supports, towards the formation of stable, positive-valent Os species. In order to facilitate the effective use of DRIFTS for characterization of opaque materials in general, our modifications to a commercial DRIFTS cell are described and representative spectra are shown.