MUSIC Speciation of γ-Al₂O₃ at the Solid Liquid Interface: How DFT Calculations Can Help with Amorphous and Poorly Crystalline Materials

Abstract

Transition aluminum oxides, such as γ-Al 2 O 3 or alumina, are widely used in many different technical applications that rely on the surface reactivity of this material at the solid liquid interface. The speciation of surface sites of this material confronts several obstacles. On the one hand, alumina is a poorly crystalline oxide, thus allowing for a limited amount of empirical structural information for an important number of surface sites with different trends in reactivity and, on the other hand, it is a metastable material. In this work, we show several ways in which the Multi Site Complexation Model (MUSIC), combined with atomistic information from Density Functional Theory and Ab Initio Molecular Dynamics, can manage to perform speciation calculations of γ-Al 2 O 3 surface sites at the solid liquid interface. Although the results are in good qualitative agreement with experimental titration curves, and they can serve as a guide for the interpretation of the reactivity of this material at the initial stages of an impregnation experiment and chemical weathering phenomena, this work highlights the need of more complex AIMD simulations to accurately model these phenomena in γ-Al 2 O 3 surface/liquid interfaces.

Phosphate Adsorption on γ-Alumina: A Surface Complex Model Based on Surface Characterization and Zeta Potential Measurements

“…The description of the γ-alumina surface is based on the model built by Corral Valero et al from the simulation of the acid–base titration curve of the surface. They used DFT information , (density and charge of each site) combined to a MUSIC model , in order to determine the p K a of each hydroxyl sites (see Supporting Information, Table S1 and S2).…”

“…PHREEQC2 speciation software was used to model the variation of the samples IEP with increasing P loading. The acid−base equilibria at the alumina surface determined by Corral Valero et al 25 have been used. The electrostatic model was the basic Stern model, and the capacitance C 1 selected is 1.39 F/m 2 , as used by Mayordormo et al 27 to model the bidentate adsorption of selenium ion on alumina surface.…”

“…Other authors using mainly spectroscopic information proposed different adsorption modes and phosphate complex geometry: Zheng et al using ATR-IR in situ and Li et al by NMR have concluded on the presence of bidentate binuclear complex without ruling out the presence of monodentate species with different degrees of protonation. Recently, considerable progress has been made with DFT calculations , to describe the surface as well as DFT-based ab initio molecular dynamics (AIMD) simulation , and the MUSIC model to perform speciation calculations of surface sites at the solid/solution interface.…”

Surface Complexation Modeling Approach for Aluminum-Substituted Ferrihydrites

“…The MUSIC model was utilized 46,47 since it has been successfully used to describe protonation on iron oxides 39,46,48 and aluminum oxides. 2,49 Protonation log K constant values for all reactive Fe surface groups were taken from previous SCMs for Fh. 37,39 The protonation constants for singly and triply coordinated surface oxygens for Fe sites in Fh were not changed from previously published values in those studies for our Fh-Al surfaces.…”

“…The basic parameters involved in our SCM are site densities, protonation constants, binding constants for background electrolyte ions, and a Stern layer capacitance value. The MUSIC model was utilized , since it has been successfully used to describe protonation on iron oxides ,, and aluminum oxides. , …”