Surface structure of hydroxylated and sulfated zirconia. A periodic density-functional study

The surface structure of sulfated zirconia (SZ) is examined by density-functional theory (DFT) with periodic boundary conditions. Adsorption of H2O and SO3 (or H2SO4) on the (101) surface of tetragonal zirconia is studied for different loadings up to H2SO4.3H(2)O and 2H(2)SO(4).2H(2)O per two surface unit cells (four Zr surface sites). The considered surface species include H2O, [H+,OH-], SO3, [H+, HSO4-], [2H(+),SO42-], [H+,HS2O7-], and [2H(+),S2O72-]. Statistical thermodynamics is used to evaluate the relative stability of different surface structures for different temperatures and pressures of H2O and SO3 (or H2SO4). The simulated surface phase diagrams show a strong dependency on the considered sulfur species (H2SO4 or SO3) as well as on pressure and temperature. Monosulfates and pyrosulfates may occur, but higher condensated sulfates are not observed. In agreement with infrared experiments, we predict transformation of water-rich structures, [SO42-, 2H(+),3H(2)O(+)], into pyrosulfate structures, [S2O72-, 2H(+),H2O], during calcination. Further increase of the temperature yields adsorbed SO3 before the clean surface is reached. Water adsorbed on the t-ZrO2(101) surface leaves in three steps upon heating from 250 to 730 K at 0.01 bar pressure: physisorbed water below room temperature, the first chemisorbed water at about 440 K and the last water at about 730 K.