The surface structure of sulfated zirconia:: Periodic ab initio study of sulfuric acid adsorbed on ZrO2(101) and ZrO2(001)

Periodic plane wave pseudopotential calculations based on density functional theory are performed to reveal the structure of sulfur species on the surface of tetragonal zirconia. The most stable configurations found are a tridentate sulfate anion on the (101) surface and an SO3 complex on the (001) surface which is also S-fold coordinated but unlike the sulfate anion is bonded to the surface via two oxygen atoms and the sulfur atom. The adsorption energies of these tridentate complexes are -322 kJ/mol for the former and -467 kJ/mol for the latter structure. On the (001) surface we also identified a bidentate sulfate complex as a stable structure with an adsorption energy of -408 kJ/mol. However, as MD simulations at a temperature of 800 K show, this bidentate configuration is transformed into a 5-fold coordinated structure accompanied by a reconstruction in the oxygen top layer. The observed LR spectra can be explained by the presence of sulfate anions on both crystallographic planes studied in this work. The calculated vibrational frequencies of the two tridentate surface complexes exhibit a gap of about 360 cm(-1) between the v(S=O) and v(S-O) stretching bands, which agrees well with experimental IR spectra of sulfated zirconia samples calcined at about 900 K. For the bidentate sulfate complex as well as for a less stable hydrogen sulfate anion we calculate v(S-O) stretching frequencies in the range 1250-900 cm(-1) which qualitatively explain the observed IR spectra of sulfated zirconia samples calcined at 800 K. On the basis of the calculated deprotonation energies, which are in the range 1350-1550 kJ/mol, we conclude that the hydroxyl groups on the two surfaces studied are less acidic than bridged hydroxyls in zeolites, regardless of the presence or absence of sulfate anions. The -1170 kJ/mol proton affinity of oxygen atoms on the (001) surface indicates that the zirconia surface is a strong base. This result and our finding of a strong electrostatic interaction with the surface explain why adsorbed sulfuric acid is completely deprotonated.