Interaction of water with titania and zirconia surfaces

Abstract

Isotopic exchange of water on the surface of monoclinic zirconia and the anatase form of titania was studied experimentally using an ordinary gas chromatograph/mass spectrometer (GC/MS) as a pulse microreactor. Interaction of water with these surfaces was modeled by first principles calculations for periodic structures within density functional theory. Experimental data support computational results showing dissociation of water on monoclinic zirconia's most stable surfaces ( 1 ¯ 1 1 ), ( 1 ¯ 0 1 ) and (1 1 1). Water is adsorbed molecularly on the anatase (1 0 1) surface and dissociatively on the anatase (1 0 0) surface. Interaction of water with the anatase (0 0 1) surface proceeds through insertion into a Ti O bond. By monitoring the concentration of the mixed water isotope, DHO, during the isotopic exchange of surface water with D 2O, it was found that exchange of water on anatase surfaces at 200–400 °C proceeds by whole molecules from 30% to 70% depending on the temperature and the surface coverage. In contrast, water exchanges on zirconia by single hydrogen atoms with a complete scrambling of hydrogens, at least 99%, under the same conditions. The proposed mechanism of water exchange on zirconia surface includes interaction through hydrogen bonding between hydroxyl groups adsorbed on neighboring sites. The observed anomaly on anatase is in agreement with the preferential molecular adsorption on a (1 0 1) surface and with the insertion of water into Ti O bonds on a (0 0 1) surface. This study contributes to the understanding of the atomic structure of acid–base catalytic sites on anatase titania and monoclinic zirconia surfaces.