Spectroscopic Signatures of Pressurized Carbon Dioxide in Diffuse Reflectance Infrared Spectroscopy of Heterogeneous Catalysts

Using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy to study surface species of reaction intermediates on heterogeneous catalysts requires particular precautions, when assigning IR bands to intermediates apparently resulting from the reaction of H-2, CO, and CO2. In accordance with earlier work, it is shown that in the investigation of the heterogeneous methanol synthesis, several gas phase bands of CO2 were misassigned in previous studies as adsorbates on the catalyst surface. Thus, several combination bands and overtones of CO2 in the 2200-750 cm(-1) range - notably those at 2130, 2112, 2107, 2094, 2077, similar to 2056, 1933, similar to 1920, similar to 1079, similar to 1050, similar to 973, and similar to 948 cm(-1) - were already misinterpreted as adsorbates/intermediates. Some of these bands exhibit similar (low) intensities as surface species and are in the range of typical adsorbed CO or methoxy/methanol vibrations. Higher pressures and temperatures, which are necessary to study industrial catalysts by in situ IR spectroscopy, even amplify this effect. In addition, due to a Fermi resonance at a CO2 partial pressure above similar to 10 bar, two further bands appear at 1388 and 1285 cm(-1). This is also within the range typically associated with surface adsorbates. In order to avoid misassignments of IR bands for in situ or operando DRIFT spectroscopy, those occurring at CO2 pressures up to 30 bar in the widely used Praying Manti(TM) High Temperature Reaction Chamber are presented here and assigned to their origin as combination bands and overtones of gaseous CO2.