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High-surface-area corundum by mechanochemically induced phase transformation of boehmite

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Science  25 Oct 2019:
Vol. 366, Issue 6464, pp. 485-489
DOI: 10.1126/science.aaw9377

Milling corundum nanoparticles

High-purity corundum (α-Al2O3) nanoparticles could enable applications such as more stable catalyst supports or precursors for high-strength ceramics. Milling of corundum only produces micrometer-scale particles, and direct synthesis from other aluminum oxides that would be likely starting materials, such as γ-Al2O3, fails because of the high activation barrier for converting the lattice structure of these cubic close-packed oxides. Amrute et al. show that ball milling of boehmite, γ-AlOOH, created ∼13-nanometer-diameter corundum nanoparticles of high purity through a mechanically induced dehydration reaction and by the effect of milling impacts on the surface energy of the particles.

Science, this issue p. 485

Abstract

In its nanoparticulate form, corundum (α-Al2O3) could lead to several applications. However, its production into nanoparticles (NPs) is greatly hampered by the high activation energy barrier for its formation from cubic close-packed oxides and the sporadic nature of its nucleation. We report a simple synthesis of nanometer-sized α-Al2O3 (particle diameter ~13 nm, surface areas ~140 m2 g−1) by the mechanochemical dehydration of boehmite (γ-AlOOH) at room temperature. This transformation is accompanied by severe microstructural rearrangements and might involve the formation of rare mineral phases, diaspore and tohdite, as intermediates. Thermodynamic calculations indicate that this transformation is driven by the shift in stability from boehmite to α-Al2O3 caused by milling impacts on the surface energy. Structural water in boehmite plays a crucial role in generating and stabilizing α-Al2O3 NPs.

Corundum (α-Al2O3) in high-surface-area nanoparticle (NP) form (1, 2) would enable a number of applications. For example, in auto-exhaust catalysis, gamma alumina (γ-Al2O3) is used in wash-coat formulations, but α-Al2O3 has higher mechanical stability and could be used directly as support without wash coating if available as NPs (3, 4). It could also provide improved hydrothermal stability when water is present in the exhaust gas. For ammonia synthesis, the presence of α-Al2O3 in the catalyst formulation was shown to increase catalyst activity by threefold (5). Corundum is widely used in ceramic applications, including dental implants, prostheses, and high-speed cutting tools, where high fracture strength and toughness is desired (6, 7). The use of nanoparticulate, fine-grained corundum precursors would allow the manufacture of ceramics with improved fracture toughness and high density at reduced sintering temperatures, enabling energy savings (6, 7).

A number of studies—mostly theoretical (1, 810) but a few experimental (1, 2)—have addressed the thermodynamics and synthesis of corundum NPs. However, none of the efforts made has thus far resulted in a method for the fabrication of high-purity corundum with surface areas reaching or even exceeding 100 m2 g−1 (1, 2, 11). One reason for this lack of success may be the high activation energy barrier (485 kJ mol−1), requiring temperatures above 1473 K, to facilitate a rearrangement of oxide ions from the cubic close-packed structure found in transition aluminas to the hexagonally close-packed form in corundum (Fig. 1, middle) (9, 12). This transition typically leads to substantial mass transfer during the transformation that is associated with a loss in surface area, generally to values well below 10 m2 g−1.

Fig. 1 Methods for the synthesis of α-Al2O3.

In contrast to the harsh conditions needed in conventional routes (top and middle), ball milling (bottom) enables us to synthesize α-Al2O3 in the nanoparticulate form at room temperature from easily accessible boehmite precursors. Bottom insets show the crystal structures (two-unit cells) of boehmite [crystal system: orthorhombic; space group: Cmcm (30)], diaspore [crystal system: orthorhombic; space group: Pbnm (31)], tohdite [crystal system: hexagonal; space group: P63mc (10)], and α-Al2O3 [crystal system: hexagonal-trigonal; space group: R3¯c no. 167 (32)] along the c-axis. In boehmite, aluminum and oxygen atoms form double layers of octahedra that sandwich hydrogen atoms in a zigzag manner. The diaspore structure consists of the “double rutile strings” formed by the aluminum-centered oxygen octahedra and involves hexagonal close packing of the oxygen ions (AB-AB…). The tohdite structure is composed of oxygen planes stacked in the sequence of A-B-A-C-A… and a unit cell contains 10 Al atoms, eight of which occupy octahedral and two tetrahedral sites. In the α-Al2O3 structure, each Al3+ center is octahedrally coordinated and the oxygen ions form a hexagonal close-packed structure with the aluminum ions filling two-thirds of the octahedral interstices. Atom colors are as follows: Al, blue; O, pink; and H, black.

Thermodynamic calculations coupled to adsorption microcalorimetry and thermogravimetric analyses by Navrotsky et al. suggest that at room temperature, γ-Al2O3 could actually be the thermodynamically most stable phase for NPs, corresponding to surface areas of 100 to 200 m2 g−1 (1, 13). At 800 K, γ-Al2O3 might become stable even at lower surface areas. Thus, a high-temperature process, which tends to lead to the formation of the most stable phase, should never lead to high-surface-area α-Al2O3. However, density functional theory simulations by Nørskov et al. showed that the surface energy, and thus the stability, of an alumina polymorph depends strongly on the size of the crystallite and degree of hydroxylation (9).

Accordingly, some hydroxylated surfaces of alumina polymorphs, especially θ-Al2O3, but also α-Al2O3, should have negative surface energies, and the nanocrystalline form of alumina should become thermodynamically most stable. In fact, a later study by Navrotsky et al. also showed lower surface energy for the hydroxylated form of α-Al2O3 than for a similarly hydroxylated γ-Al2O3 irrespective of the surface area (13). Hence, α-Al2O3 with a high surface area >100 m2 g−1 could be accessible and synthesizable, but numerous studies to date have proven unsuccessful at producing it through the thermal route at 1500 K.

The topotactic transformation of diaspore (α-AlOOH) to α-Al2O3 produces corundum at moderate temperatures of 800 to 873 K (Fig. 1, top) (1, 14), but the synthesis of stable diaspore requires very harsh conditions (723 K, 1200 bar, 35 days) (15, 16). Synthesis of a mixture of α-Al2O3 and α-Fe2O3 with 1:5 mole ratio by co-precipitation followed by calcination is also shown to reduce the formation temperature of α-Al2O3 to 973 K as α-Fe2O3 acts as seeds and thereby prevented the excessive agglomeration and growth of α-Al2O3 NPs. However, a separation of a huge excess of Fe matrix needs a series of harsh HCl treatments (17). Mechanochemical reactions such as that induced by ball milling, which finds increasing attention as a simple, economical, and scalable method for the preparation of nanomaterials and for carrying out chemical reactions (1821), have also been reported as a method to obtain α-Al2O3. Synthesis of α-Al2O3 with particle sizes below 10 nm is reported by a mechanochemical reaction between Fe2O3 and Al powders (1:1 Fe:Al mole ratio) and by a top-down route involving ball milling of micron-sized preformed α-Al2O3 (22, 23). Nonetheless, these approaches also require a separation step to remove huge amounts of Fe using harsh HCl treatments. Besides, a fractional-coagulation using HCl as a coagulating agent is also used in these methods to achieve a narrow size distribution.

Ball milling of γ-Al2O3 has also been reported to form α-Al2O3 (24, 25). Nonequilibrium processes involved in breaking and making chemical bonds by mechanical energy input enable this phase transformation to take place at room temperature. However, γ-Al2O3 undergoes continuous agglomeration until it fully transforms to α-Al2O3, which severely reduces the specific surface area (by a factor of >2.2) (24).

The observed effect might be related to the so-called “milling equilibrium” between cold welding, leading to the formation of larger agglomerates, and fragmentation, producing smaller particles (26). The dominance of one process over the other determines the eventual size of the produced particles. As γ-Al2O3 is softer (bulk modulus, BVR = 196.9 GPa, and shear modulus, GVR = 97.45 GPa; see table S1), it could undergo excessive cold welding, which would cease with the formation of harder α-Al2O3 (BVR = 239.6 GPa and GVR = 138.6 GPa), as harder materials are difficult to cold weld and also difficult to further fragmentize without external aids.

We first approached this problem by performing ball milling of γ-Al2O3 in the presence of process control agents (PCAs), which are known to limit cold welding and enhance fragmentation (26). Stearic acid, alcohols, and heavier alkanes are commonly used as PCAs (26). However, we opted for water as an environmentally benign and cost-effective PCA. The ball-milling experiments were performed in a vibration mill. Typically, 1 g of precursor powder is charged in the tungsten carbide (WC) vessel with three WC balls (see section S2 of the supplementary materials for more details). The effect of abrasion of milling balls, the vessel, or both caused contamination of the samples with WC after milling. WC was selectively removed by an oxidative leaching step for all the samples obtained using the WC milling jar and media (supplementary text and fig. S1). Ball milling of γ-Al2O3 (denoted as γ-Al2O3-1, SBET = 109 m2 g−1; see fig. S2A for morphology) in the presence of 1 to 5 weight % (wt %) H2O for 120 min fully transformed the material into α-Al2O3, but the surface area dropped to 70 m2 g−1 (fig. S2B). This decrease in surface area is similar to its ball milling without water (fig. S2C). A further increase in H2O to 15 wt % resulted in no phase transformation of γ-Al2O3, suggesting that an excess amount of water in the system is detrimental because it might reduce the energy transfer during the milling impact.

These results suggest that high-surface-area γ-Al2O3 is a suitable precursor for the production of α-Al2O3 with moderately high surface area, but that alternative precursors might be better. Ball milling was also reported to convert boehmite to α-Al2O3 (27), but there is no information on the surface area, intermediates, or agglomeration phenomena, and thus it remains unclear whether boehmite could lead to high-surface-area corundum. Boehmite attracted our interest also because it formally has one water molecule in its structure per alumina (i.e., 15 wt % water with respect to alumina) and can be economically produced, for example, by a modified Bayer process of bauxite refining (28). With this material, an effective milling impact can still be achieved, and the structural water of boehmite, which will be released upon its dehydration to alumina, could act as a PCA to limit cold welding or to facilitate the fragmentation of agglomerates.

On the basis of these considerations, we ball milled a boehmite sample (denoted as γ-AlOOH-1; SBET = 89 m2 g−1; see fig. S3 for morphology) for different times using a WC jar and media (Fig. 2A). The boehmite phase remained mostly unaltered upon ball milling for up to 30 min, but x-ray diffraction (XRD) reflections corresponding to α-Al2O3 and α-AlOOH appeared after 60 min of milling. Further continuation of milling for 180 min fully converted boehmite to a mixture of corundum (major, ~70 to 80%) and diaspore (minor, ~20 to 30%). Similar results were obtained upon ball milling of boehmite with a steel milling jar and media (fig. S4A), except instead of diaspore, another mineral phase, tohdite (5Al2O3·H2O, which is structurally similar to α-Al2O3, see Fig. 1, bottom insets, and rearranges to the latter upon dehydration), was detected.

Fig. 2 Evolution of the α-Al2O3 phase and changes in the surface area upon ball milling of boehmite.

(A) Diffractograms of γ-AlOOH-1 before and after ball milling at room temperature in the vibration mill. Milling conditions: WC jar (25 cm3), 3 × 12 mm WC balls, 1 g of powder charge, fMill = 25 Hz, t = 10 to 720 min. Identified crystalline phases: ◇, γ-AlOOH; ●, α-Al2O3; and △, α-AlOOH. (B) Surface area and hydrodynamic diameter of particles versus milling time for γ-AlOOH (circles) and γ-Al2O3 (triangles) precursors. (C and D) TEM images of 60-min ball-milled γ-AlOOH-1 (C) and 180-min ball-milled γ-AlOOH-1 (D). The ball milling of boehmite leads to the formation of α-Al2O3 with diaspore and/or tohdite (fig. S4A) intermediates. The formation of the latter relies on the milling conditions. The phase transformation to α-Al2O3 proceeds through severe microstructural rearrangements.

The observation of these rare phases by ball milling of boehmite was surprising because their formation normally requires extremely harsh conditions (16, 29). However, in view of the thermodynamic analysis (see below), the formation of such phases is possible for the high-surface-area systems, which was confirmed by performing experiments under variable conditions (fig. S5). In the ball milling of γ-Al2O3, no intermediate phase was observed (24, 25), which we confirmed by milling γ-Al2O3-1 for different times (fig. S2C).

Ball milling of boehmite for 720 min led to the formation of pure α-Al2O3 (Fig. 2A and fig. S4A), showing that these intermediates can also be transformed to the desired corundum phase (alternatively, the remaining diaspore after 180 min of milling can be topotactically converted to α-Al2O3 by a mild calcination at 823 K; Fig. 2A, green diffractogram). This result indicates that the conversion of these phases is slower compared with that of boehmite, likely because they are mechanically harder (table S1). Increased milling frequency (30 Hz) led to a faster transformation of boehmite and intermediates to α-Al2O3 (fig. S4B and table S2).

These results show that the formation of intermediates depends on the milling conditions and that high impact energy favors the nucleation of the α-Al2O3 phase. This finding was substantiated by ball milling of boehmite in the planetary mill using steel, sintered corundum, or zirconia milling jars and media. The higher impact and friction forces involved in planetary milling led to the direct formation of α-Al2O3 (see fig. S6). Notably, milling equipment made of corundum can also be used to exclude the presence of any foreign material by abrasion.

We obtained deeper insight into the phase transformation discussed above from the thermodynamic analysis of the involved phases (see section S4 of the supplementary materials). At standard state, γ-AlOOH should be the most stable phase irrespective of the surface area (Fig. 3A and table S3). For the dense bulk phases, the thermodynamic stability decreases in the order γ-AlOOH > α-Al2O3 > γ-Al2O3 and above a surface area of 190 m2 g−1, γ-Al2O3 becomes more stable than corundum. This trend agrees with those reported for these alumina polymorphs (13). Assuming that water vacancies are the primary defects among others (see section S4 in the supplementary materials), the modification of γ-AlOOH by ball milling effectively increases its surface energy and reverses the mutual stability orders as follows: α-Al2O3 > γ-AlOOH > γ-Al2O3 (Fig. 3, B and C, and fig. S7). At a surface area >108 m2 g−1 for the anhydrous state and 52 m2 g−1 for the hydroxylated state, α-Al2O3 becomes the thermodynamically most stable phase, which provides the driving force for the boehmite-to-corundum transformation.

Fig. 3 Free energy of γ-Al2O3 and γ-AlOOH with respect to α-Al2O3 as a function of surface area at standard state (T = 298.15 K and P = 1 bar).

Data in (A) correspond to the conditions discussed in (1), where at high surface area, γ-Al2O3 is thermodynamically more stable than α-Al2O3. (B) Modification of boehmite surface in the milling process effectively increases the surface energy of this compound (above 4 J m−2; fig. S7), which drives the conversion to α-Al2O3 above 108 m2 g−1. (C) Alumina surface hydroxylation changes the mutual stability of the phases, where α-Al2O3 becomes the most stable phase above 52 m2 g−1. For the corresponding data for diaspore and tohdite, see figs. S8 and S9.

Mechanochemical conversion is a nonequilibrium process in which intermediates can be observed. Comparison of the surface energies of diaspore and tohdite with boehmite and corundum suggests that boehmite has the largest thermodynamic driving force for its conversion under our milling conditions (figs. S8C and S9). Thus, intermediate phases such as diaspore or tohdite could also form, together with the most stable α-Al2O3. Diaspore or tohdite would also ultimately convert to α-Al2O3, but the driving force for their conversion is lower and requires longer times or higher milling intensity.

To determine whether these phase changes involve agglomeration, fragmentation, or both phenomena, we further characterized the ball-milled boehmite and γ-Al2O3 samples. The surface area gradually decreased with milling time for γ-Al2O3 and reached a plateau after 120 min (full conversion to α-Al2O3; Fig. 2B). This result is in agreement with previous work (24). Likewise, boehmite also displayed a loss in surface area during the first 60 min of milling, which suggests that agglomeration was also prominent for boehmite in the initial phase of milling. A faster decrease in surface area for boehmite compared with γ-Al2O3 likely reflects their different mechanical properties (table S1); boehmite is softer and prone to cold weld faster than γ-Al2O3.

However, in contrast to γ-Al2O3, the surface area was recovered after 60 min for the boehmite precursor (Fig. 2B; see table S2 for steel milling jar and media). To understand this difference, the samples were characterized by dynamic light scattering (Fig. 2B). An increase in the hydrodynamic diameter during the first hour of milling and a subsequent decrease upon further milling indicated that the sample was agglomerated during the first hour of milling, and then these agglomerates were fragmented into smaller NPs. This process was visualized by transmission electron microscopy (TEM) analysis (Fig. 2, C and D). Agglomeration of the sample milled for 60 min was seen, and finely rounded NPs were detected after 180 min of milling. Thus, the structural water in boehmite seems to play a crucial role in generating and stabilizing the corundum NPs. Water released during the dehydration of boehmite to α-Al2O3 could create fissures in the agglomerates that facilitate their disintegration during milling, and the formed NPs of α-Al2O3 would be stabilized through hydroxylation of the surface by the released water. Hydroxylation has been demonstrated to lower the surface energy of α-Al2O3 (13). Further evidence highlighting the effect of structural water was derived by ball milling of boehmite at elevated temperatures (373 to 473 K) (fig. S10).

The α-Al2O3 NPs derived by ball milling using WC jar and balls, followed by mild calcination, were further characterized in depth (Fig. 4). TEM images showed that the NPs ranged in size from 4 to 32 nm, with an arithmetic average particle size (dParticle) of 13 nm (standard deviation = 6 nm) (Fig. 4, A and D). The domain size determined by the application of the Scherrer equation to the corresponding XRD pattern was 18 nm (Table 1). High-resolution TEM showed that the NPs display a crystalline rather than an amorphous surface (Fig. 4B). Determination of the d spacing at different spots provided an average value of 2.079 Å, which agrees well with the 2.085 Å d spacing for the (113) plane of α-Al2O3.

Fig. 4 Properties of nanoparticulate α-Al2O3 derived by ball milling of boehmite.

(A) TEM and (B and C) high-resolution TEM of nanoparticulate α-Al2O3 obtained by 180-min ball milling of γ-AlOOH-1, followed by calcination at 823 K. (D) Particle size (dParticle) distribution from TEM images of nanoparticulate α-Al2O3. (E) Pore size distribution determined by the application of density functional theory model to the N2 isotherm. Inset in (E) shows N2 adsorption-desorption isotherms. Ball-milling–derived α-Al2O3 from boehmite is composed of rounded NPs (dParticle = 13 nm) with surface roughness and defects and pores in the range of 3 to 10 nm.

Table 1 Surface area, phase composition, and domain size of boehmite before and after ball milling using a WC jar and media in the vibration mill and after calcination of the ball-milled sample.

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The TEM images also showed some irregularity, surface roughness, or defects as well as some pores on the NPs (Fig. 4C). Pore size determined from N2 sorption analysis using the density functional theory model indicated that pores in the size range from 3 to 10 nm were present (Fig. 4E), which might include a contribution from interparticle pores. The surfaces were also highly dehydrated (fig. S11). Thus, the observed surface area can be attributed to nanometer-sized particles and a combination of surface roughness, defects, and pores. Similar results were obtained for other boehmite precursors of a different nature (i.e., different particle size, surface area, and water contents; see Table 1, figs. S12 to S14, and corresponding captions). In all cases, full boehmite conversion was achieved after 180 min of milling, with surface areas for α-Al2O3 exceeding 130 m2 g−1.

Boehmite and related compounds were found to be highly effective and economically accessible precursors to produce phase-pure nanocrystalline α-Al2O3 with surface areas exceeding 120 m2 g−1 through mechanochemistry. These results may have high practical relevance, for instance, for the optimization of alumina processing in ceramics and for the design of stable industrial catalysts (e.g., as supports for automotive or Fischer–Tropsch synthesis catalysts).

Supplementary Materials

science.sciencemag.org/content/366/6464/485/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S14

Tables S1 to S3

References (3343)

References and Notes

Acknowledgments: The electron microscopy department at the Max-Planck-Institut für Kohlenforschung is gratefully acknowledged for the electron microscopy and the EDX analyses. We thank G. Prieto for providing boehmite samples (γ-AlOOH-1 and γ-AlOOH-4). PL-Grid is acknowledged for CPU allocation. Funding: Max-Planck-Institut für Kohlenforschung is acknowledged for financial support. Author contributions: F.S. conceived the idea of investigating ball milling to produce high-surface-area α-Al2O3. A.P.A. and F.S. designed a strategy to study boehmite compounds. A.P.A. synthesized the starting materials for ball milling, designed and executed ball-milling experiments, and performed characterizations. Z.Ł. formulated the thermodynamic model, performed the calculations of elastic and thermodynamic properties, and wrote the corresponding part of the text. H.S. first detected the transformation of γ-Al2O3 to α-Al2O3 in experiments to synthesize supported catalysts in the ball mill and performed some of the experiments involving ball milling of γ-Al2O3. C.W. assisted in the measurement of selected samples on a PANalytical X'Pert Pro diffractometer and performed Rietveld refinement of selected samples. A.P.A. and F.S. wrote the manuscript. All authors commented on the manuscript. Competing interests: The authors have filed a patent application on the synthesis process to obtain high-surface-area alpha-alumina. Data and materials availability: All data are available in the main text or the supplementary materials.

Correction (12 May 2020): Three additional references on ball-milling synthesis of alpha-alumina were unintentionally omitted and are now cited as references 17, 22, and 23. Text has been added where the references were inserted.

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