In situ vibrational spectroscopy of adsorbed nitrogen in porous carbon materials

This study uses in situ vibrational spectroscopy to probe nitrogen adsorption to porous carbon materials, including single-wall carbon nanotubes and Maxsorb super-activated carbon, demonstrating how the nitrogen Raman stretch mode is perturbed by adsorption. In all porous carbon samples upon N₂ physisorption in the mesopore filling regime, the N₂ Raman mode downshifts by ∼2 cm^-1, a downshift comparable to liquid N₂. The relative intensity of this mode increases as pressure is increased to saturation, and trends in the relative intensity parallel the volumetric gas adsorption isotherm. This mode with ∼2 cm^-1 downshift is thus attributed to perturbations arising due to N₂-N₂ interactions in a condensed film. The mode is also observed for the activated carbon at 298 K, and the relative intensity once again parallels the gas adsorption isotherm. For select samples, a mode with a stronger downshift (>4 cm^-1) is observed, and the stronger downshift is attributed to stronger N₂-carbon surface interactions. Simulations for a N₂ surface film support peak assignments. These results suggest that N₂ vibrational spectroscopy could provide an indication of the presence or absence of porosity for very small quantities of samples.