Decrease in DRIFTS Optical Path Length Induced by Sample Reduction Led to Cu–Ce Phase Being IR-Invisible

The impact of thermal treatments on the DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) optical path length of catalysts routinely used for CO2 hydrogenation was investigated. Cu-Ce binary oxides and In2O3/ZrO2 were examined, using CaCO3 as the DRIFTS internal standard. The CaCO3 bands at 2510 and 1795 cm-1 were employed for estimating the global optical path length of the solid mixture. The results show that reduction significantly affected the optical path length, particularly in high metal-loaded samples. These changes were greater than those induced by temperature variations or sorbate adsorption/desorption. Notably, Cu80Ce20 (51.8 wt % Cu) exhibited such a strong IR absorption after reduction that it was impossible to obtain any further signal relating to this phase (bulk and surface) and only signals relating to the internal standard were detected. These findings emphasize the importance of considering optical path length variations during DRIFTS studies, especially when catalysts undergo significant structural changes under reaction conditions. This work also underlines the necessity of using internal standards for reliable quantitative DRIFTS analyses.