Article pubs.acs.org/JPCC
Tomography and High-Resolution Electron Microscopy Study of Surfaces and Porosity in a Plate-like γ‑Al2O3 Libor Kovarik,*,† Arda Genc,‡ Chongmin Wang,† Annie Qiu,† Charles H. F. Peden,§ János Szanyi,§ and Ja Hun Kwak*,§ †
Environmental Molecular Sciences Laboratory and §Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ‡ FEI Company, 5350 Northease Dawson Creek Drive, Hillsboro, Oregon 97124, United States ABSTRACT: Morphological and surface characteristics of γAl2O3 are topics of high relevance in the field of catalysis. Using tomography and high-resolution transmission electron microscopy (TEM) imaging, we have studied the surface characteristics of a model γ-Al2O3 synthesized in the shape of platelets and macroscopically defined by (110)Al2O3 and (111)Al2O3 surface facets. We show that the dominant (110)Al2O3 surface of the synthesized γ-Al2O3 is not atomically flat but undergoes a significant reconstruction, forming nanoscale (111)Al2O3 facets. In addition to high-resolution imaging, tomographic analysis was carried out, enabling an examination of the pores/voids, which were found to be mostly enclosed within the bulk and inaccessible to gases or solvents carrying precursors for metal particles. Tomographic analysis shows that the surfaces of the pores are defined exclusively by (100)Al2O3 and (111)Al2O3 facets. The importance of these findings is discussed in the context of relative surface energies of low index surfaces and ethanol desorption characteristics.
1. INTRODUCTION γ-Al2O3 represents one of the most prominent support materials for noble metals and oxide catalysts that are widely employed for reduction, oxidation, and reforming reactions in automotive exhaust control and petroleum refining processes.1,2 As a catalytic support, one of the most attractive properties of γAl2O3 is that it can be prepared with and can maintain a relatively large surface area in excess of 200 m2/g. In the role as catalyst support, it is also very attractive that γ-Al2O3 has highly defective surfaces over a range of several length scales (from atomic level to a macroscopic level). The surface defects seem to play important roles as anchoring sites for catalytic particles/ clusters.3,4 The morphological and size characteristics of γ-Al2O3 and the way these characteristics change as a function of the preparation techniques have been extensively studied in the past.5 As a result, a considerable variety of aqueous solution preparation methods have been developed, many of which provide very precise control over size, surface, and morphological characteristics and are commonly used for catalytic applications.6−9 Because of structural complexity and small crystallite sizes, the crystallography of γ-Al2O3 is poorly understood and is still actively debated in the literature. Traditionally, γ-Al2O3 has been understood as a defective spinel structure with vacancies occupying Al cation sites to maintain the correct stoichiometry and charge balance. However, this traditional view of the γAl2O3 structure has been repeatedly questioned on the basis of © 2012 American Chemical Society
X-ray diffraction (XRD) and density functional theory (DFT) calculations, and models with Al cations not limited to spinel sites have been proposed.10−14 Presently, there is a relatively large number of structural models that are used for interpretation and modeling of γ-Al2O3.15−17 Analogously to the crystal structure, the structural characteristics and properties of low-index surfaces remain actively debated. Several structural models have been proposed on the basis of theoretical approaches using DFT modeling,11,16−18 but relatively large differences exist among these models, in terms of ranking of both surface energies and corresponding properties. For example, the surface models developed by Digne et al.11 suggest that a fully dehydrated (100) surface represents the lowest energy surface, while (110) has approximately 1.5-times higher energy and (111) has approximately 2-times higher energy than (100). This is in strong contrast to the work of Pinto et al.,16 who have shown that both (111) and (100) are energetically comparable and represent the thermodynamically preferred surfaces. Interestingly, the DFT work of Pinto et al.16 shows that (110) is the least stable surface, which reconstructs to (111) facets during molecular dynamics DFT calculations. It has also been shown in the work of Pinto that the (111) facets of the macroscopiReceived: July 9, 2012 Revised: December 7, 2012 Published: December 10, 2012 179
dx.doi.org/10.1021/jp306800h | J. Phys. Chem. C 2013, 117, 179−186
The Journal of Physical Chemistry C
Article
single-tilt tomography holder, which provides tilts from +70° to −70°. The tomography tilt series were acquired manually with 2° increments, generally at a tilt range that is just a few degrees short of the maximum capabilities. STEM tomographic series were acquired at an accelerating voltage of 200 kV and with a substantially reduced half convergence angle of 10 mrad to improve the depth of focus. Tomographic reconstructions were performed with an FEI Inspect 3D software package using the Simultaneous Iterative Reconstruction Technique (SIRT), and employing 20 iterations. Visualization and analysis of the tomograms were performed with the Amira 5.2 software package. TEM sample preparation involved either direct dispersion of the dry heterogeneous catalyst powders onto lacey carboncoated Cu grids, or dispersion of the heterogeneous catalysts in ethanol and sonication for 5 min to improve the dispersion of the particles and then subsequent application of small drops of the ethanol slurry onto a lacey carbon-coated Cu grid.
cally defined (110) surface can contain penta-coordinated Al sites, which have been previously exclusively associated with dehydrated (100) surfaces. The penta-coordinated sites have been known to play an important role in influencing the chemistry and activity of the surfaces; for example, the pentacoordinated sites have been determined to be critical in the anchoring of Pt atoms.3 In this contribution, we focus on a detailed characterization of surfaces (both internal and external) for a model γ-Al2O3 synthesized in the shape of platelets. This is a model system that is not fully representative of high-surface-area γ-Al2O3, but because of its well-defined morphology, it is ideally suited for fundamental work on characterization of surface and structural properties otherwise not possible with commercial γ-Al2O3. The current atomic level imaging enables us to experimentally confirm the theoretically predicted decomposition of (110) surfaces to (111) facets, assess the relative surface energies of low index planes, and relate the crystallographic information with the characteristics of ethanol temperature programmed desorption.
3. RESULTS 3.1. Structural (XRD) and Surface (TPD) Characteristics. The presently studied γ-Al2O3 represents a model system with a medium BET surface area of ∼70 m2/g, which is lower than that (∼200 m2/g) commonly found for many commercial γ-Al2O3 products. The lower surface area for the asprepared sample is related to larger primary particle sizes when compared with commercial aluminas. To confirm that the presently studied system is structurally representative of γ-Al2O3, rather than being partially transformed to more stable forms of transition Al2O3, such as δAl2O3 or θ-Al2O3, the crystallographic characteristics were examined by powder XRD. Figure 1 shows an XRD pattern
2. EXPERIMENTAL SECTION Gamma alumina used in the present work was synthesized from aluminum isopropoxide by a hydrolysis method.6 Approximately 10 g of aluminum isopropoxide was added to ∼50 mL of water with vigorous stirring at 80 °C for 1 h. The mixture was subsequently transferred to the 125 mL Teflon liner of a Parr reactor and placed into an oven and kept at 200 °C for 24 h. After cooling to room temperature, the powder was collected by filtration, washed extensively with distilled water, and dried at 100 °C. The as-obtained powder of Boehmite was then converted to γ-Al2O3 by calcination at 800 °C for 2 h under ambient air conditions. To verify that the synthesis protocol leads to the formation of γ-Al2O3, X-ray diffraction (XRD) analysis was carried out on the prepared powder. The XRD analysis was performed on a Philips PW3040/00 X’Pert powder X-ray diffractometer using the Cu K(α) radiation (λ = 1.5406 Å) in step mode between 2θ values of 10° and 75° with a step size of 0.02/s. Verification that the surface characteristics are consistent with that of commercial γ-Al2O3 was achieved by performing ethanol temperature-programmed desorption (TPD) analysis and comparing it with a commercial product (Condea SBA-200). The experiments were carried out using the same experimental procedures as described in our previous report.19,20 A portion of the as-prepared powder of γ-Al2O3 was loaded with 5 wt % of Pt by wet impregnation (incipient wetness) using an aqueous solution of Pt(NH3)4(NO3)2. To form Pt nanoparticles, further calcination−reduction treatments were done at 300 °C for 2 h under O2/He and H2 flowing conditions, respectively. The main purpose of the Pt loading was to provide external markers that are critical for tomographic reconstruction of the studied γ-Al2O3. However, this also enables examination of additional aspects of the Pt nanoparticles' interaction with γ-Al2O3 and Pt distribution on the available surfaces. Microscopy observations reported in this work were performed with a spherical aberration-corrected FEI Titan 80300 operated at 200 and 300 kV using conventional and highresolution transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) equipped with high angle annular dark field (HAADF) detector. The tomography experiments were performed with a Fischione
Figure 1. XRD patterns of synthesized γ−Al2O3 (after calcination at 800 °C for 2 h) and commercial γ-Al2O3 (Condea SBA-200). The indexing of the peaks is done in terms of a spinel cubic lattice.
from the synthesized γ-Al2O3. From the shape and the relative intensities of the peaks, it can, indeed, be confirmed that the structure is characteristics of γ-Al2O3. This fact can be further confirmed by a comparison of the XRD pattern with a commercial product (Condea SBA-200), as shown in Figure 1. 180
dx.doi.org/10.1021/jp306800h | J. Phys. Chem. C 2013, 117, 179−186
The Journal of Physical Chemistry C
Article
The surface characteristics of the synthesized γ-Al2O3 were also compared with those of commercial γ-Al2O3 by performing ethanol TPD experiments. The ethanol TPD results for the model platelet-like and commercial γ-Al2O3 samples are shown in Figure 2. Both synthesized and commercial γ-Al2O3 have the
Figure 3. (a) Conventional TEM images reveals that the synthesized γ-Al2O3 has a plate-like morphology with rhombus facets. Lighter γAl2O3 particles are in a near-normal orientation to the electron beam and the darker particles are in a near-edge-on orientation. (b) Detailed view of a γ-Al2O3 particle in near-normal orientation revealing the rhombus morphology. (c) Detailed view of the edge-on orientation of two γ-Al2O3 particles. (d) Schematic depiction of the γ-Al2O3 particle morphology with the indexing of the crystallographic planes and directions in terms of a cubic crystal lattice.
Figure 2. Ethanol TPD profiles of commercial (Condea SBA-200) and synthesized model (red) γ-Al2O3. Ethanol was adsorbed at room temperature after activation of the samples at 500 °C for 2 h.
same features in the desorption profiles, with a hightemperature ethanol TPD peak maximum at 231 °C. The only difference is that the total ethanol desorption amount for the synthesized γ-Al2O3 is significantly smaller than that for the commercial γ-Al2O3 as a result of the smaller surface area of the model sample.19 The fact that the ethanol TPD profiles are fully comparable suggests that the surfaces of the synthesized alumina are sustained even when calcined at 800 °C during preparation. In our prior study on a commercial γ-Al2O3 calcined at progressively higher temperatures, we have clearly shown that the ethanol TPD profile was extremely sensitive to the surface structure of the alumina particles. For example, we reported that γ-Al2O3 particles calcined at 800 °C for 2 h developed surface characteristics of θ-Al2O3, even though the bulk structure did not change.19,20 The current TPD results suggest that such transformations do not take place for the larger crystallites of synthesized γ-Al2O3, and instead, the results demonstrate that these larger crystallites are more thermally stable with respect to the gamma-to-theta phase transformation. 3.2. Conventional and High-Resolution TEM Analysis. The general morphological and size characteristics of the studied γ-Al2O3 are shown in Figure 3. The presented TEM images show the γ-Al2O3 particles in several orientations and reveal that the majority of particles are thin platelets that have a roughly rhombus shape. On average, the particle thickness is between 10 and 20 nm, and the short diagonal distance in the plane view is ∼50−70 nm. As confirmed from the electron diffraction and high-resolution imaging from two orthogonal directions presented in Figures 4 and 5, the macroscopic shape of the synthesized γ-Al2O3 is defined by (110) and (111) type surfaces. The (110) surface is the main “broad” surface of the plate-like particles, and the side surfaces of the rhombus are
(111)-oriented. A schematic illustration describing the crystallography of the surfaces, as observed for the majority of particles, is shown in Figure 3d. It should be noted that in some instances, a few particles were also found to have truncated tops of the rhombus, exposing additional facets of the (100) type. The observed morphological characteristics are fully consistent with previous reports on γ-Al2O3 synthesized from Boehmite under normal pH conditions.7 This is a protocol that leads to (110) planes as the main surface and strongly favors the (111) surfaces over (100).7 Both (110) and (111) surfaces of the plate-like γ-Al2O3 particles were examined in detail with high-resolution imaging in several crystallographic orientations. To study the broad (110) surface, high-resolution imaging was performed on several zone axes for the edge on oriented particles. The [11̅0] zone axis, as schematically shown in Figure 3d, was found to be the key orientation to study the broad surface in the edge-on orientation. In particular, the imaging on [11̅0] shows that the (110) surfaces are not atomically flat but, instead, consist of a series of periodically repeating structural facets. As shown in Figure 4, the facets are very short, mostly
