J. Phys. Chem. B 1999, 103, 7557-7564
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ARTICLES Al K-Edge Near-Edge X-ray Absorption Fine Structure (NEXAFS) Study on the Coordination Structure of Aluminum in Minerals and Y Zeolites J. A. van Bokhoven,† H. Sambe,† D. E. Ramaker,† and D. C. Koningsberger*,‡ Chemistry Department, George Washington UniVersity, Washington, D.C. 20052 and Laboratory of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht UniVersity, 3508 TB Utrecht, The Netherlands ReceiVed: February 9, 1999; In Final Form: June 21, 1999
Curved-wave-multiple-scattering cluster calculations with the FEFF6 code were used to interpret experimental AlK-edge near-edge X-ray absorption fine structure (NEXAFS) spectra of various minerals and Y zeolites for energies ∼15 eV above threshold. Octahedral, tetrahedral, and square planar geometries of Al can be easily distinguished from each other utilizing characteristic features in the NEXAFS data. NEXAFS line shapes are used for determining the geometrical conformations of Al atoms in Y zeolites with one or more conformational geometries. In the H-Y zeolite, separate contributions to the NEXAFS from tetrahedrally and octahedrally coordinated Al atoms are identified. The differences in the octahedrally coordinated Al spectra in the H-Y zeolite compared with spectra for standard octahedrally coordinated Al compounds can be attributed to the presence of very small nonregular clusters of octahedrally coordinated Al dispersed over the zeolite. However, the presence of some pentacoordinated Al cannot be excluded.
Introduction Zeolites are widely used as catalysts and as catalyst supports in industrial processes. Applications can be found in processes such as fluid catalytic cracking, hydro-cracking, paraffin isomerization, aromatic alkylation, ion-exchange resins, molecular sieves, sorbents etc.1-3 Zeolites are open framework aluminosilicates consisting of SiO4-4 and AlO4-5 tetrahedra, interconnected via the oxygen atoms. Charge balance requires the presence of one cation for each Al atom. These cations are not part of the framework and can easily be exchanged.4 Physical properties, like the catalytic activity, are primarily determined by the structure of the zeolite, the presence of extraframework species, etc. Although zeolites have been under investigation for many years, no clear relationship between local and electronic structure and activity has been established.5 For example, the influence of the extraframework Al species on the catalytic activity is still unclear and even the exact structure of the extraframework Al species is not known. Clearly, detailed structural investigations of Al species in zeolites are needed. We present here X-ray absorption spectroscopy (XAFS) studies on the AlK-edge. XAFS spectroscopy is a very useful technique for probing the local atomic environment of different substituents in a material. Application of this technique is widespread, and the material to be studied can be either a gas, a liquid, or a solid, because long-range structure is not required. Extended X-ray absorption fine structure (EXAFS) analysis yields the coordination number, bond distances, type of neighbors, and the local disorder. Near-edge X-ray absorption fine * Corresponding author. Telephone: +31-30-253-74-00. Fax: +31-302511-027. E-mail:
[email protected]. † Chemistry Department. ‡ Laboratory of Inorganic Chemistry and Catalysis.
structure (NEXAFS) can provide additional information on the coordination geometry, the symmetry of the unoccupied states, and the effective atomic charge on the absorbing atom.6 Previous analysis of the EXAFS data for the Y zeolite with different cations (H+, NH4+, and Na+) indicated that these three materials have slightly different AlsO bond lengths and possess primarily 4-fold (probably tetrahedrally) coordinated Al sites.7 The previously published NEXAFS data are analyzed in detail in this work. In this paper we demonstrate the usefulness of NEXAFS data for the determination of the local Al environment in minerals and Y zeolites. The Al coordinations we consider are the tetrahedral (undistorted and distorted), the square planar, and the octahedral. We perform curved-wave multiple-scattering calculations with the FEFF6 code on various clusters representing the mineral or zeolite. The calculated results are compared with the experimental AlK-edge NEXAFS data from a wide array of aluminum oxide minerals. Some of these data are taken from the literature. From these comparisons, we recognize characteristic features or “fingerprints” in the spectra arising from the different Al coordination geometries. After establishing the characteristic features that allow distinctions between the Al coordinations, we analyze the NEXAFS range of the Y zeolite absorption spectra just mentioned. A technique to separate the different contributions from different Al coordinations in a NEXAFS spectrum is described. Although some part of this work has been briefly reported,8 we provide here the full analysis and results. Experimental and Theoretical Methods Experimental spectra are compared on an energy scale relative to the maximum or shoulder of the first intense peak (whiteline)
10.1021/jp990478t CCC: $18.00 © 1999 American Chemical Society Published on Web 08/20/1999
7558 J. Phys. Chem. B, Vol. 103, No. 36, 1999 in the spectra. This comparison meant a shift of, on average, 1568 eV for tetrahedrally coordinated Al, and 1570 and 1572 eV for distorted and regular octahedral, respectively, in Perovskite. These AlK-edge XAFS spectra of the Y zeolites were taken at the soft X-ray XAFS station 3.4 of the Synchrotron Radiation Source at Daresbury (UK). Experimental details have been published elsewhere.7 Standard procedures were used to analyze the data.9 Briefly, the data were normalized to 1 at 50 eV above the edge, and the postedge background was subtracted using cubic spline routines.10 All experimental spectra presented in this study were obtained using total electron yield detection (TEY). Curved-wave-multiple-scattering (CW-MS) cluster calculations utilizing the FEFF6 code developed by Rehr and Albers11,12 were performed in this work. We have previously13,14 performed FEFF6 calculations on alkali halides and condensed rare gases to test the validity of this code and found that the code reproduces almost all of the experimentally observed peaks except the highly localized excitonic peaks very near or below the edge (particularly for Ne). For the first row transition metals and lighter elements, the absolute energy must be shifted by 0-5 eV for optimum agreement with experimentally observed results. Here, the FEFF6 results were optimally aligned with the experimental data. The theoretical spectra were also normalized at 50 eV above the edge. The input parameters for the theoretical calculations include the atomic number of each unique atom in the cluster, the coordinates of each atom in the cluster, the choice of exchange potential (Hedin-Lundquest, Dirac-Hara, or ground state), the maximum path length, criteria for the path filter that determines the amount of multiple scattering, the Debye-Waller factor (σ2), and the charge on the ions. We employed the ground-state exchange-correlated potential, which does not allow for inelastic energy losses. We used a zero imaginary part in the potential, thus ignoring any lifetime and experimental broadening effects. The maximum path length was set to 8-10 Å for the 1 and 2 shell calculations, and to 15 Å for the 7 shell calculations. The path filter was chosen to be 1% for the plane waves and 2% for the curved waves (i.e., 1/2 of the default values) to include all-important multiple scattering paths. The Debye-Waller factor was set to 0.007 Å2.
van Bokhoven et al.
Figure 1. Experimental NEXAFS data for two different perovskites [YAlO3 (1) and NbAlO3 (2)15] with octahedral Al sites. Also shown are results from FEFF6 calculations on clusters representing nondistorted (perovskite) and distorted (corundum) Al sites.
Results and Discussion Octahedral Coordination. Experimental AlK-edge NEXAFS data for aluminum oxide compounds with an octahedral coordination are given in Figures 1 and 2. Figure 1 gives results over an extended range, 0-50, and Figure 2 emphasizes the whiteline region, 0-15 eV. Basically, the features can be divided into three regions: the whiteline region (0-15 eV), the intermediate region (15-40 eV), and the high-energy region (40-55 eV). In Figure 1, experimental data are given for the compounds that exhibit the Perovskite structure (1 ) YAlO3, 2 ) NdAlO3) with a nondistorted octahedral Al coordination.15 FEFF6 multiple-scattering calculated results for a single-shell AlO6 cluster with the symmetric Perovskite structure with an Al-O bond length of 1.92 Å are also given. Finally, FEFF6 multiple-scattering results for an Al15O18-10 cluster with the crystal structure of corundum, a distorted octahedral, are shown. All spectra, the single- and multi-shell FEFF6 results, regardless of local symmetry, and the experimental data, reveal essentially the same feature in the 40-55 eV region. These results very strongly suggest that this feature arises from Al-O single EXAFS scattering, which is confirmed by analyzing the scattering paths that indicate that this feature has its origin in
Figure 2. Experimental NEXAFS spectra for several materials possessing nondistorted octahedral or distorted octahedral Al sites; perovskite (YAlO3), topaz,17 kayanite,17 jadeite (dotted,17 solid19), and corundum20 are shown.
single scattering. As such, this feature should be highly dependent on the Al-O bond length. A small energy shift does exist between the two FEFF6 spectra, which arises from the different bond lengths utilized in the calculations. The corundum structure has two different Al-O bond lengths (1.86 and 1.97 Å, respectively16), but only one peak is visible in the calculated spectra in this region. The set of small features (appearing almost as 4 features) around 15-40 eV in the experimental spectra, most clearly visible in Perovskite 2, are reproduced in the calculated spectrum by two broader peaks. Adding countercations (Nd) and including more shells (up to 7 shells) in the calculation does not result in
Al K-Edge NEXAFS Study of Aluminum the splitting of these two peaks. Experimental Al NEXAFS spectra for KAlF4 and K2NaAlF6, which exhibit structures that are associated with the Perovskite structure, do show only two peaks around 15-40 eV.15 Thus, the splitting of the two peaks in the experimental spectra for NdAlO3 might be a result of small irregularities in these crystal structures, such as variations in bond length. These peaks between 15 and 40 eV can be assigned to Al-O σ* antibonding orbitals because they appear already in the FEFF results for a multiple-scattering calculation on a single-shell cluster, but they do not appear in a singlescattering calculation. These peaks are dependent on the local geometry because Al-O-O multiple scattering is also involved. From a molecular orbital point of view, these σ* antibonding orbitals should also depend on the local geometry. Now, we consider the whiteline region at 0-15 eV. Figure 2 shows experimental Al NEXAFS data for materials that possess distorted octahedrally coordinated Al sites; topaz,17,18 kyanite,17,18 jadeite,17,19 and corundum.18-20 The spectrum of topaz from ref 15 was shifted about one extra electronvolt to lower energy for optimal agreement in whiteline position with the other spectra. In addition, an experimental AlK-edge NEXAFS spectrum for an undistorted octahedral Perovskite structure (YAlO3) is given. The spectrum of the Perovskite structure shows a single whiteline peak, whereas the materials with distorted octahedral Al sites all show a splitting of the whiteline by ∼4 eV. It has been established in the literature17-20 that materials with distorted octahedral Al sites have edge features at 1568 and 1572 eV, whereas the Perovskite structure, with undistorted octahedral Al sites, has a single whiteline at 1570 eV15 (the average of these two energies). Although the magnitude of the whiteline splitting in all the distorted structures remains constant around 4 eV, the relative intensities of the two peaks vary. In addition to the split whiteline, a weak preedge feature (at ca. -2 eV) can be seen in all of the spectra of the distorted octahedral compounds. The strong dependence of the whiteline region on the nondistorted versus distorted octahedral symmetry, suggests that the whiteline depends primarily on the local geometry; however, strong evidence exists that it also depends on the long-range structure. For example, the experimental spectrum for KAl(SO4)2‚12H2O, in which the aluminum has a nondistorted octahedral coordination, still shows a split whiteline.21 This result illustrates the influence of the long-range structure on the whiteline of octahedral Al. The FEFF6 results do not reproduce the whiteline region of corundum despite the full multiple-scattering calculations on several shells. Recently, Cabaret et al.22 calculated the AlKedge spectrum for corundum using very large clusters (up to 15.3 Å), full multiple scattering, and the “extended continuum” code developed by Natoli et al.23 They obtained excellent agreement with the experimental whiteline. However, for cluster diameters
