Generation of highly reactive OH groups at the surface of TiO2 nanotubes

Titania with nanotubular morphology was synthesized hydrothermally via the reaction of a commercial TiO2 anatase with NaOH solution at 413 K followed by a washing procedure with an HCl solution. The obtained nanotubes were dried at 373 K and then annealed at 573 and 673 K. All samples were characterized using X-ray diffraction, Raman spectroscopy, HRTEM, nitrogen physisorption and FTIR of adsorbed CO. Tubular morphology was preserved up to 673 K although the collapse of nanotube walls took place and formation of anatase microdomains was observed. Low temperature CO adsorption was followed by FTIR on orthorhombic layered titanate and on anatase transformed nanotubes. The samples were annealed in situ and ex situ in order to know the origin of reactivity of surface OH groups. In situ treated samples presented higher adsorption of CO molecules than samples treated ex situ, nevertheless, the latter present higher activity toward CO oxidation into CO2. FTIR results reveal the existence of highly reactive OH groups toward CO oxidation at the surface of nanotubular titania calcined ex situ. These hydroxyl groups might be generated from the dissociation of water molecules adsorbed on the strongly deformed TiO6 octahedra in the curved structure of nanotubes. This fact suggests that enhancement of CO oxidation activity of TiO2 nanotubes can be achieved by their exposure to adequate doses of water vapor, producing CO2 at temperatures as low as 100 K. Total extinction of CO and CO2 vibration bands in FTIR spectra at temperatures above 240 K evidenced the reversibility of the adsorption process.