Low-Temperature FTIR Study of CO Adsorption on Titania Nanotubes

Titania nanotubes were obtained by an alkaline hydrothermal treatment of anatase at 373 K followed by washing with an HCl aqueous solution. Samples were dried at 373 K and calcined at 573 and 673 K. The solids were characterized by X-ray diffraction, Raman spectroscopy, HRTEM, nitrogen physisorption, and FTIR of adsorbed CO. After calcination at 673 K, the nanotubular morphology was well-preserved, although initial transformation to anatase was observed. The interlayer space between the walls of the nanotubes collapsed, resulting in domains of an anatase phase within the walls. The nanostructure obtained after calcination at 673 K was able to adsorb CO at a low temperature (100 K). The CO molecules were adsorbed on the remaining OH groups of the nanotubes and then converted into CO 2 and CO 3 2- by increasing the desorption temperature. FTIR bands related to H 2 O and CO 3 2- appeared concomitantly with CO adsorption and intensified at higher evacuation temperatures. Then, adsorbed CO was transformed into CO 2 at temperatures around 158 K. After evacuation at 233 K, the adsorbed CO 2 was transformed into CO 3 2- species. These results suggest that CO molecules are adsorbed on titania nanotubes via reaction with surface OH groups. Finally, the CO 3 2- species were eliminated, restoring the surface OH groups after evacuation at 473 K.