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 functiona l theory are performed to reveal the structure of sulfur species on the sur face of tetragonal zirconia. The most stable configurations found are a tri dentate 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 bo nded to the surface via two oxygen atoms and the sulfur atom. The adsorptio n 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 identif ied a bidentate sulfate complex as a stable structure with an adsorption en ergy of -408 kJ/mol. However, as MD simulations at a temperature of 800 K s how, this bidentate configuration is transformed into a 5-fold coordinated structure accompanied by a reconstruction in the oxygen top layer. The obse rved LR spectra can be explained by the presence of sulfate anions on both crystallographic planes studied in this work. The calculated vibrational fr equencies of the two tridentate surface complexes exhibit a gap of about 36 0 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 hydr ogen sulfate anion we calculate v(S-O) stretching frequencies in the range 1250-900 cm(-1) which qualitatively explain the observed IR spectra of sulf ated zirconia samples calcined at 800 K. On the basis of the calculated dep rotonation energies, which are in the range 1350-1550 kJ/mol, we conclude t hat the hydroxyl groups on the two surfaces studied are less acidic than br idged hydroxyls in zeolites, regardless of the presence or absence of sulfa te anions. The -1170 kJ/mol proton affinity of oxygen atoms on the (001) su rface 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.