Article pubs.acs.org/cm
Unraveling the Origin of Structural Disorder in High Temperature Transition Al2O3: Structure of θ‑Al2O3 Libor Kovarik,*,† Mark Bowden,† Dachuan Shi,‡ Nancy M. Washton,† Amity Andersen,† Jian Zhi Hu,‡ Jaekyoung Lee,§ János Szanyi,‡ Ja-Hun Kwak,§ and Charles H. F. Peden‡ †
Chem. Mater. 2015.27:7042-7049. Downloaded from pubs.acs.org by UNIV OF TOLEDO on 09/28/18. For personal use only.
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ‡ Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States § Department of Chemical Engineering, UNIST, Ulsan, Korea S Supporting Information *
ABSTRACT: The crystallography of transition Al2O3 has been extensively studied in the past, because of the advantageous properties of the oxide in catalytic and a range of other technological applications. However, existing crystallographic models are insufficient to describe the structure of many important Al2O3 polymorphs, because of their highly disordered nature. In this work, we investigate structure and disorder in high-temperature-treated transition Al2O3 and provide a structural description for θ-Al2O3 by using a suite of complementary imaging, spectroscopy, and quantum calculation techniques. Contrary to current understanding, our high-resolution imaging shows that θ-Al2O3 is a disordered composite phase of at least two different end-members. By correlating imaging and spectroscopy results with density functional theory (DFT) calculations, we propose a model that describes θ-Al2O3 as a disordered intergrowth of two crystallographic variants at the unit-cell level. One variant is based on β-Ga2O3, and the other on a monoclinic phase that is closely related to δAl2O3. The overall findings and interpretations afford new insight into the origin of poor crystallinity in transition Al2O3, and we also provide new perspectives on structural complexity that can emerge from intergrowth of closely related structural polymorphs.
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INTRODUCTION Transition Al2O3 derived from dehydration of boehmite is a group of oxides with a high degree of inherent structural disorder. The way in which this structural disorder manifests as a function of various heat-treatment conditions is a highly relevant topic in many technological fields, with perhaps the highest relevance in the field of catalysis, where transition Al2O3 is heavily used as catalysts and catalytic support materials,1−3 and where detailed understanding of structure and structural disorder could provide a much needed foundation for rationalization of unique surface chemistry of these materials.4−8 In the case of transition Al2O3 heat-treated at relatively high temperatures (>900 °C), the two phases that form are δ-Al2O3 and θ-Al2O3.9−11 To understand high-temperature transition Al2O3, it is important to understand the defective nature of the individual phases and, perhaps more importantly, the way the individual crystallographic phases coexist in common microstructures.9,12,13 The crystallography of what is now known as δ-Al2O3 is rather intriguing, as it is becoming recognized that the interpretation of structure in terms of a unique crystallographic © 2015 American Chemical Society
structure is not appropriate. Instead, it is necessary to describe the structure as a complex intergrowth of closely related polymorphs belonging to the δ-Al2O3 family.13 While this interpretation is relatively new, it should be pointed out that numerous studies have previously reported several crystallographic symmetries and lattice parameters for δ-Al2O3,9−11 and did not provide descriptions in terms of a unique structural polymorph. A clarification regarding the actual nature of δAl2O3 has been obtained and discussed in Kovarik et al.,13 based on quantitative high-angle annular dark field (HAADF) observations. It was shown that there are two main closely related polymorphs that coexist in a common intergrowth that macroscopically appears as a single phase. The two closely related polymorphs were denoted as δ1-Al2O3 and δ2-Al2O3,13 and were found to intergrow along the [100] direction. In addition to these two polymorphs, there are other crystal symmetries and lattice parameters that have been previously proposed for δ-Al2O3,9−11 but verification of these structures Received: July 1, 2015 Revised: September 22, 2015 Published: September 22, 2015 7042
DOI: 10.1021/acs.chemmater.5b02523 Chem. Mater. 2015, 27, 7042−7049
Article
Chemistry of Materials
Figure 1. (a) Section of the XRD pattern for alumina sample (Sasol) heat-treated at 1050 °C. (b) XRD pattern of laboratory-synthesized alumina sample heat-treated at 1100 °C for 3 h with θ-Al2O3 as the main component of the microstructure.
commercially. The laboratory-synthesized samples, prepared from alkoxides, were in the form of morphologically welldefined platelets defined by rhombus facets. The size of particles was ∼10−20 nm in thickness, and ∼50 nm across the short rhombus diagonal. Detailed information about the synthesis protocol and morphology of the particles, as obtained from transmission electron microscopy (TEM) tomographic measurement, can be found in our previous work.19 In addition to the laboratory-synthesized samples, we also studied morphologically well-defined transition alumina obtained by calcination of a commercial boehmite material supplied by Sasol. The microstructural investigation was performed with an aberration-corrected electron microscope (FEI, Model Titan 80-300) in scanning transmission electron microscopy (STEM) mode. The probe convergence angle was set to ∼18 mrad and the inner detection angle on HAADF detector was ∼3 times higher than the probe convergence angle. The TEM sample preparation involved dispersion of the alumina materials in ethanol and sonication for ∼3−5 min in order to improve the dispersion of the particles, and then subsequent application of a small drop of the ethanol slurry onto a TEM grid. A selected set of samples were heat-treated inside a TEM system with an Aduro Protochips heating holder, and imaged after the in situ heat-treatment conditions. Simulation of HAADF images was performed with a computer code developed by E. Kirkland.20 The calculations were performed with microscope parameters that closely correspond to the experimental conditions (E = 300 kV, cs = 5 μm, convergence angle = 18 mrad, inner collection angle = 70 mrad, and outer collection angle = 240 mrad). The simulated images were convoluted with a Gaussian full width at half maximum (fwhm) of 0.08 nm to account for spatial incoherence of the imaging system identical to previous simulations13 27 Al direct polarization nuclear magnetic resonance (NMR) experiments were conducted on a Varian/Agilent VNMRs system utilizing a 1.6 mm HXY probe operating in DR mode tuned to 221.421829 MHz. A calibrated π/20 pulse of 0.25 μs, a recycle delay of 2s, a spinning speed of 38 kHz, and a constant 20 °C temperature were used to collect 1024 transients. Timedomain-free induction decays were apodized with 25 Hz
and determination of their crystallographic nature is yet to be performed. At higher temperatures, δ-Al2O3 gradually transforms to θAl2O3, which is commonly accepted to possess a monoclinic symmetry and a structure that is isomorphous to β-Ga2O3. The models available in the literature have minor differences, which are mostly due to partial occupancies and lattice parameters. The model of Zhou and Snyder12 suggests that the structure has partial occupancies at selected atomic sites while the models proposed by Yamaguchi et al. and Repelin et al.14,15 consider full occupancy at Al sites. Numerous studies have shown that one of the factors leading to poor crystallinity in θ-Al2O3 can be ascribed to twinning, which is often found at relatively high densities.9,13,16 Other polymorphs or structures aside from δ-Al2O3 and θAl2O3 have also been reported to evolve during hightemperature aging of boehmite/γ-Al2O3 precursors.17,18 Perhaps the most intriguing in this regard is a disordered structure of Al2O3 that has been reported to coexist with δ-Al2O3 and θAl2O3. As shown by Kovarik et al.,13 the disordered structure is unique as it has only one-dimensional (1D) crystallographic periodicity, which makes it clearly distinguishable from the existing models of δ-Al2O3 and θ-Al2O3. The current study is motivated by the paucity of information detailing structural characteristics of high-temperature-treated transition Al2O3. Addressing the structural complexity in these materials, we find that θ-Al2O3 is much more complex than that previously described in the literature. In particular, we show that even after extensive high-temperature treatment, θ-Al2O3 is a disordered phase that deviates from its commonly accepted structure of β-Ga2O3. The disorder is accomplished by structural intergrowth of a crystal type of β-Ga2O3 and crystal that is closely related to δ-Al2O3. The reported analysis on θAl2O3 provides new crystallographic insight into the origin of poor crystallinity in transition aluminas, and also affords a new perspective on the structural complexity that can arise from the intergrowth of closely related structural polymorphs.
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METHODS Two sources of boehmite were used to prepare transition Al2O3 in this work: one laboratory-synthesized and the other obtained 7043
DOI: 10.1021/acs.chemmater.5b02523 Chem. Mater. 2015, 27, 7042−7049
Article
Chemistry of Materials
Figure 2. Detailed atomic level view of θ-Al2O3 microstructure (3 h @ 1100 °C, laboratory-synthesized sample) as investigated with HAADF imaging: (a,b) HAADF images from the [010] zone; (c) [010] HAADF image simulation of Al2O3 with β-Ga2O3 structure; (d,e) HAADF images from the [001] zone (the microstructure is found to be aperiodic); and (f) corresponding [001] HAADF image simulation of the β-Ga2O3 structure.
exclusively to θ-Al2O3 were not synthesized under any heattreatment conditions. At the higher temperatures of 1100 °C, the transformation of δ-Al2O3 to θ-Al2O3 is competing with the formation α-Al2O3, which then can quickly become a dominant phase. An example of XRD pattern from the laboratorysynthesized sample at 1100 °C and with some of the highest proportions of θ-Al2O3 is shown in Figure 1b. Under these relatively severe conditions, a small volume fraction of α-Al2O3 (
