Computer Simulation of Dissociative Adsorption of Water on the Surfaces of Spinel MgAl₂O₄

Abstract

Atomistic simulation techniques have been used to model the dissociative adsorption of water onto the low-index {100}, {110}, and {111} surfaces of spinel MgAl 2 O 4 . The Born model of solids and the shell model for oxygen polarization have been used. The resulting structures and chemical bonding on the clean and hydrated surfaces are described. The calculations show that the dissociative adsorption of water on the low-index surfaces is generally energetically favorable. For the {110} and {111} orientations, the surfaces cleaved between oxygen layers show high absorption and stability. The calculations also show that, for the {111} orientation, the surfaces may absorb chemically water molecules up to ϳ90% coverage and have the highest stability. It is suggested that, during fracture, only partial hydration occurs, leading to cleavage preferentially along the {100} orientation.

Noncontact atomic force microscopy study of the spinel MgAl2O4(111) surface

“…According to a theoretical study by Harding et al, and other theoretical studies [15–17], the oxygen-terminated MgAl 2 O 4 (111) is evaluated to be in its lowest energy state with 42% of the oxygen atoms removed from the oxygen crystal plane to fulfill the stabilization requirements. Furthermore, calculations show that dissociative adsorption of water up to a 90% coverage is very favorable and leads to high stability [18–19]. The NC-AFM data presented in this work reveal the MgAl 2 O 4 (111) surface to have a characteristic surface morphology consisting of triangular patches, the orientation and coverage of which are in agreement with the theoretical predictions for an oxygen-terminated surface with a certain percentage of the surface-layer atoms removed.…”

Characterization of Nano‐Particles During MG‐AL‐Spinel‐Formation Calculated by MD‐ Simulations

“…The surface energies γ as a function of the temperature T was fitted by the equation: γ = mT + b, where the slopes for the surface energy was determined as m=2.5 * 10 -4 Jm -2 K -1 for the layered geometry in all three materials. These values correspond very well to previous calculations [4,10], and the experimental values [9] for all three materials (For Al 2 O 3 also other data with a steeper slope 7.8 * 10 -4 Jm -2 K -1 are published [9]). The slopes for the nano-particles, however, are much smaller (MgO 1 * 10 -4 , Al 2 O 3 0.2 * 10 -4 , MgAl 2 O 4 1.3 * 10 -4 Jm -2 K -1 , respectively).…”

“…3). In the graphs the round symbols refer to the particles, the square symbols to the layered geometry, and the asterisk to literature values [4,[9][10][11]. These values depend on the crystallographic orientation and in the case of Al 2 O 3 the Al-terminating surface has been found to have a larger energy than the O-terminating plane.…”

“…Molecular dynamics (MD) is a suitable method for analyzing structural nano-size effects in ceramics and is also applied for ceramic surfaces in aqueous media [4]. The software "Moldy" [5] allows the use of the Bucking potential with a Morse part including the covalent bonding term and the bond stretch correction term.…”

Non-contact atomic force microscopy study of hydroxyl groups on the spinel MgAl2O4(100) surface

“…Magnesium aluminate (MgAl 2 O 4 ) belongs to this class of materials [8][9][10], and until recently mainly theoretical studies have been applied to study its surface structure. The few experimental studies in the literature have provided little conclusive atomic-scale insight on the possible surface terminations [11][12][13][14][15]. However, more recently atom-resolved non-contact atomic force microscopy (NC-AFM) measurements [16,17], combined with density functional theory (DFT) calculations and NC-AFM simulations [18] were successfully applied in [19] to determine that the surface structure of MgAl 2 O 4 (100) adopts the so-called O 4 -Al 4 -O 4 termination.…”