Ft nicolet 5700 spectrometer

Infrared spectra were collected on a Thermo Electron Corporation FT Nicolet 5700 Spectrometer (resolution: 4 cm⁻¹). Self‐standing pellets were placed into an IR cell with KBr windows permanently attached to a vacuum line (residual pressure: 10⁻⁴ mbar), allowing all treatments and adsorption–desorption experiments to be carried out in situ. Before the gas adsorption tests, the silica samples were outgassed at 135 °C (heating ramp: 5 °C min⁻¹, holding: 3 h) by using an oil‐free apparatus and a grease‐free vacuum line. Afterwards the samples were cooled at room temperature for the collection of IR spectra upon CO₂ adsorption. All the spectra were normalized by dividing for the density of the self‐supported pellets. Furthermore, the proper normalization of the IR spectra was verified taking as reference the intensity of the overtones and combination modes of the silica framework (bands in the 2200–1600 cm⁻¹ range). In this way, differences in the band intensities among different samples related to intrinsic oscillators of the materials (e. g. hydroxyl groups) can be associated with actual differences in the amount of such species in the samples. As a consequence of the normalization, the absorbance values are reported as the arbitrary unit (a.u.).