Reactivity of shape-controlled crystals and metadynamics simulations locate the weak spots of alumina in water

Abstract

The kinetic stability of any material in water relies on the presence of surface weak spots responsible for chemical weathering by hydrolysis. Being able to identify the atomistic nature of these sites and the first steps of transformation is therefore critical to master the decomposition processes. This is the challenge that we tackle here: combining experimental and modeling studies we investigate the stability of alumina in water. Exploring the reactivity of shape-controlled crystals, we identify experimentally a specific facet as the location of the weak spots. Using biased ab initio molecular dynamics, we recognize this weak spot as a surface exposed tetra-coordinated Al atom and further provide a detailed mechanism of the first steps of hydrolysis. This understanding is of great importance to heterogeneous catalysis where alumina is a major support. Furthermore, it paves the way to atomistic understanding of interfacial reactions, at the crossroad of a variety of fields of research.

Fig. 1

Implication of the (110) facet in the decomposition of γ-Al₂O₃. a General topology of a γ-Al₂O₃ nanoparticle exhibiting three major facets: (110) in dark blue, (100) in light blue and (111) in gray. b, c Structures of two inhibitors: b, sorbitol (red) and c, xylitol (blue). d, e, f Comparisons between the inhibiting coverage (sorbitol in red, xylitol in blue) and the fraction of exposed (110) (d), (111) (e), and (100) (f) surface areas. The experimental data sets are fitted to the zero-intercept linear model derived in Supplementary Note 4. The fitted curves are represented as straight lines. A correlation could only be established in the case of the (110) surface (d) with slopes of 0.367 ± 0.009 and 0.443 ± 0.005 nm⁻² for xylitol and sorbitol, respectively (R² > 0.99). Error bars are calculated from high-performance liquid chromatography (HPLC) analysis standard deviation (see Supplementary Note 3). g, h, i, j Transmission electronic microscopy images of the four differently shaped γ-Al₂O₃ nanoparticles ordered in increasing (110) fractional area: plates (g), fibers (h), rods (i), and commercial γ-Al₂O₃ (j)

Fig. 2

Exploration of the reactivity of tetrahedral Al_β on γ-Al₂O₃ (110) from ab initio metadynamics. a, b Structure of the initial surface with chemisorbed water molecules or fragments (the oxygen atoms of which are shown in blue) adsorbed on tetrahedral Al_α and Al_β and octahedral Al₍₁₎ and Al₍₂₎. c Free energy surface obtained from the metadynamics simulation on Al_β using the coordination numbers to alumina oxygen atoms (CN_a) and to water molecules and fragments (CN_w) as variables. The simulation starts from the point of coordinates (CN_a,CN_w) = (3,1). d, e Structure of the last intermediate obtained from the hydration of tetrahedral Al_β with (CN_a, CN_w) = (1,5). f Projected free energy profile with the structure of each intermediate and the corresponding (CN_a,CN_w). Yellow and white balls represent aluminum and hydrogen atoms, respectively. The color red is used for alumina oxygen atoms and the associated CN_a. The color blue is used for water oxygen atoms and the associated CN_w

Fig. 3

Inhibition of the decomposition of γ-Al₂O₃ in the presence of chemisorbed xylitol. a γ-Al₂O₃(110)/water interface with the average occupation volume of water in gray. b Geometry of adsorbed xylitol at the γ-Al₂O₃(110)/water interface with the iso-surface of the average occupation volume of xylitol in ochre, and the average occupation volume of water in gray. The iso-surfaces were computed from an extra regular ab initio molecular dynamics simulation. More details can be found in Supplementary Note 8. c Free energy surface obtained from the metadynamics simulation on Al_β in presence of xylitol. CN_a is the coordination number of Al_β to alumina oxygen atoms. CN_w+p is the coordination number of Al_β to water and polyols oxygens. Yellow and white balls represent aluminum and hydrogen atoms, respectively. The color red is used for alumina oxygen atoms and the associated CN_a. The color blue is used for water oxygen atoms and the associated CN_w