Comparative DFT study of the oxy(hydr)oxides of iron and aluminum – structural, electronic and surface properties.

Abstract The Goethite (α-FeOOH, Goe), hematite (α-Fe2O3, Hem) and their Al-isomorphs diaspore (α-AlOOH, Dia) and corundum (α-Al2O3, Cor) are an important class of materials. Hem and Cor have been intensely investigated. However, the Goe and Dia have not received much attention commensurate with their importance for many geochemical and technological processes. We have reviewed the literature and revisited by DFT/Plane Waves calculations the structural and electronic properties of their bulk and their predominant surfaces. The calculated values have been compared and discussed in the light of the available experimental and previously published theoretical data. Insights of relevant and important properties of the less investigated Goe and Dia are provided, fulfilling the lack of information about these systems. Furthermore, the topological analysis of the electron density has been performed for all investigated minerals. The differences in the structural, electronic, and adsorptive properties of their surfaces have been discussed in detail. The smaller structural and energetic relaxation of the Fe-isomorph surfaces compared to their Al-isomorphs is explained in terms of larger covalent character cushioning the changes of the metal electronic structure. The water adsorption on the predominant surfaces has been investigated and the dissociative mechanism is at least 10 kJ mol−1 more favored than the molecular mechanism. The estimated adsorption energies for the Goe and Dia oxyhydroxides are 70 and 39 kJ mol−1, respectively. For the Hem and Cor oxides, the water adsorption energies are estimated to be 105 and 148 kJ mol−1, respectively. These values reinforce the observations that the Goe and Dia present different surface reactivities which must be observed in geochemical processes.