FTIR Spectroscopy Combined with Isotope Labeling and Quantum Chemical Calculations to Investigate Adsorbed Bicarbonate Formation Following Reaction of Carbon Dioxide with Surface Hydroxyl Groups on Fe₂O₃ and Al₂O₃

FTIR spectroscopy combined with isotope labeling experiments and quantum chemical calculations is used to investigate the adsorption of carbon dioxide on hydroxylated metal oxide surfaces. In particular, transmission FTIR spectra following CO₂ adsorption on hydroxylated nanoparticulate Fe₂O₃, α-Al₂O₃, and γ-Al₂O₃ particles at 296 K are reported. As expected, reaction of CO₂ with these surfaces results in the formation of adsorbed bicarbonate and carbonate. In this study, the vibrational spectrum of the bicarbonate product is analyzed in detail through the use of isotope labeling experiments and quantum chemical calculations. The experimental and calculated vibrational frequencies of adsorbed HC¹⁶O₃^-, DC¹⁶O₃^-, HC¹⁸O₃^-, HC¹⁶O¹⁸O₂^-, and HC¹⁸O¹⁶O₂^- indicate that bicarbonate bonds to the surface in a bridged structure. There is some evidence from the mixed isotope experiments that following initial nucleophilic attack of OH, the formation of the final bicarbonate structure involves a proton transfer. On the basis of energetic considerations, the proton transfer mechanism most likely occurs through an intermolecular process involving either coadsorbed hydroxyl groups and/or carbonate.