Evolution of sample optical pathlength during diffuse reflectance FT-IR analyses

In situ and operando diffuse reflectance FT-IR (DRIFTS) studies are often carried out over samples that may undergo changes in optical properties due to stronger absorptivity (darkening), for instance when coking or being reduced. The variation of IR optical pathlength of two Fe and Co-based Fischer-Tropsch catalysts treated at different temperatures was investigated through the use of an internal standard CaCO3 that was physically mixed with the catalyst. The CaCO3 overtone band at 1795 cm-1 was used, exhibiting an integral molar absorption coefficient & epsilon; = 2.68 x 105 cm mol-1. The IR penetration depth was negligible over the iron oxide sample once reduced above 400 degrees C, due to the formation of a strongly absorbing material. A reduction of the cobalt sample up to 350 degrees C revealed large changes of optical pathlength (dropping four-fold from 66 to 16 & mu;m), likely related to the onset of cobalt oxide deep reduction. These data indicate that changes in optical pathlength should be considered when attempting quantitative DRIFTS analyses and that mixing a small concentration of CaCO3, e.g. 2 wt%, is a suitable way of doing so.

Automated summary

In situ and operando diffuse reflectance FT-IR (DRIFTS) studies often require consideration of changes in optical properties. This study investigated the variation of IR optical pathlength of Fe and Co-based Fischer-Tropsch catalysts by using an internal standard CaCO3 mixed with the catalyst. The results showed the importance of considering changes in optical pathlength for quantitative DRIFTS analysis.