Surface Structure of Hydroxylated and Sulfated Zirconia. A Periodic Density-Functional Study

Abstract

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 H2SO4·3H2O and 2H2SO4·2H2O per two surface unit cells (four Zr surface sites). The considered surface species include H2O, [H+,OH-], SO3, [H+,HSO4 -], [2H+,SO4 2-], [H+,HS2O7 -], and [2H+,S2O7 2-]. 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, [SO4 2-,2H+,3H2O], into pyrosulfate structures, [S2O7 2-,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.

Adsorption–desorption equilibrium investigations of n-butane on nanocrystalline sulfated zirconia thin films

“…Such a model results in the calculated thickness of the sulfate layer being in excess of a monolayer coverage, which is not considered viable given the preparation route of the zirconia thin films and the fact that polymeric sulfates (higher than disulfates) were not observed by theoretical studies. [8] Previously reported IR studies indicate the adsorption of n-butane to occur on hydroxyl groups on sulfated zirconia. [73] Furthermore, XPS investigations on the interaction of sulfated zirconia thin films with n-butane under reactive conditions [74] show the attenuation of the zirconium signal to be greater than sulfur upon carbon deposition, which suggests adsorption on zirconia rather than coverage of the sulfate groups.…”

“…XPS measurements show that the Zr:S atomic ratio (based on the Zr 3d and S 2p peak areas and reported atomic sensitivity factors [40]) of the activated sulfated zirconia thin film is 5.4. Assuming a homogeneous distribution of sulfur and zirconium within the thin film over the XPS measurement depth, a sulfate surface area of 31 Ų [71] and a zirconia surface 2 x 2 unit cell of 6.425 x 7.284 Å, [8] the sulfate groups are shown to cover ~33% of the surface, which is equivalent to a surface site density of ~1.1x10 18 S atoms/m². Alternatively assuming the sulfate groups are situated on the surface of zirconia crystals, a layer model that accounts for attenuation of the zirconium signal can be used to estimate the sulfate surface density.…”

“…It is thus proposed that the strong chemisorption sites arise from the presence of a minority, active, disulfate species. [8,9] Hence the weaker, more dominant chemisorption sites are ascribed to mono-sulfate species. The range of n-butane heats of adsorption for both the disulfate and mono-sulfate species are attributed to there being a distribution of similar adsorption sites.…”

“…[6] Sulfate is a key component, and numerous propositions of its structure on the zirconia surface have been published, the majority of which were developed with the design of strong Brønsted or Lewis acid sites in mind. [7] Recent experimental and theoretical findings [8,9] have shown that active catalysts possess an IR band at ~1404 cm -1 , which is ascribed to the S=O bond stretching vibrations in disulfate or adsorbed SO 3 molecules. These surface species are proposed to initiate alkane conversion by oxidative dehydrogenation.…”

The Balance Between Reactivity and Stability of Modified Oxide Surfaces Illustrated by the Behavior of Sulfated Zirconia Catalysts

“…Sulfate can assume a multitude of structures on the surface of zirconia, depending on the loading and on the degree of hydration [29]. Sulfate can be removed or converted to an inactive sulfur-oxygen compound through dissolution in water [30], thermal decomposition [31], or reduction, for example by pyridine or benzene [32,33] or alkane reactants [34].…”