Computational Modeling of the Structure and Properties of Zr(OH)₄

Abstract

Ab initio molecular dynamics (AIMD), based on density functional theory, has been used to develop and test a model for amorphous Zr­(OH)4, which is of interest as an agent for the adsorption of toxic gases. Beginning with an idealized and highly ordered structure based on two-dimensional layers of polymeric Zr­(OH)4, AIMD at 300 K and above yields an amorphous structure. Disordering is seen to occur concomitantly with a reaction between acidic bridging OH sites and basic terminal OH to produce H2O and Zr–O–Zr bridges. The modeling also shows that Zr­(OH)4 can be maintained in an artificial ordered state either by a regular array of hydrogen bonds within layers or by interaction between layers. Radial distribution functions and distributions of hydrogen-bond parameters are calculated for both the ordered and the amorphous structures, and a comparison between the two sets of results provides insight regarding the nature of the amorphous state. Infrared and proton nuclear magnetic resonance spectra are computed for the amorphous model, and chemical properties are evaluated by examining the acidity of different types of OH groups and also the reactions with SO2 and CO2. In particular, various mechanisms leading to sulfite formation during exposure to SO2 are examined in some detail. In all cases, good agreement with experiment is found, which indicates that the model is a suitable basis for analyzing the adsorption properties of amorphous Zr­(OH)4.

Surface Chemistry of Sulfur Dioxide on Zr(OH)4 Powder: The Role of Water

“…Given the high binding energy, we expect that once SO 2 forms a surface-coordinated SO 3 , it can no longer be easily removed, although it is possible that further reactions can occur. Furthermore, previous calculations 22 show that the adsorption of SO 2 on a bilayer of Zr(OH) 4 with no defects gives a ΔE ads = −0.91 eV, with the SO 2 forming a hydrogen bond with both bridging and terminal OH sites. Iordanov et al 22 show SO 2 can bond to either bridging OH or to both terminal and bridging OH sites.…”

“…The positive charge on the Zr (eq 4) is representative of the surface providing the counterbalancing positive charge and not the oxidation state of zirconium. Additionally from previous theory, 22 the surface likely contains bare oxygen (O(ads)), which is depicted in eq 6. A summary of the fingerprint IR bands and their assignments is shown in Table 1.…”

“…The other new IR band at 1180 cm –1 is assigned to sulfate, SO 4 , ,, consistent with 1172 cm –1 on TiO 2 . , As such, possible reaction mechanisms for bisulfite (solvated (HSO 3 – )) and adsorbed (HSO 3 (ads)) and sulfate could be The positive charge on the Zr (eq ) is representative of the surface providing the counterbalancing positive charge and not the oxidation state of zirconium. Additionally from previous theory, the surface likely contains bare oxygen (O­(ads)), which is depicted in eq . A summary of the fingerprint IR bands and their assignments is shown in Table .…”

“…A full explanation of the Zr(OH) 4 surface model used and our previous work on the reactions of sulfur species, specifically near surface defects, can be found in the literature. 22 Previous molecular dynamics calculations 22 followed by geometry optimizations show that molecular SO 2 initially binds on the Zr(OH) 4 surface through dipole−dipole interactions with surface hydroxyls. Due to the amorphous nature of Zr(OH) 4 , it is impossible to assign a single binding energy (BE) for SO 2 adsorbed on Zr(OH) 4 , because the BE depends on the local surface site.…”

“…All PAW calculations included a Grimme D2 semiempirical dispersion correction . A variable cell optimization for an amorphous, 2-layer, two-dimensionally periodic slab with mixed terminal and bridging OH groups gave similar lattice constants, 14.34 and 14.63 Å, with only a difference of 1.8% with that seen in the a and b lattice constants of monoclinic ZrO 2 . Due to the large size of the unit cell, all calculations were for the Γ-point only.…”