20724
J. Phys. Chem. B 2006, 110, 20724-20726
Reply to “Comments on ‘Examination of Spinel and Nonspinel Structural Models for γ-Al2O3 by DFT and Rietveld Refinement Simulations’”
TABLE 1: d-Spacings for Transitional Aluminas and the SXPD Pattern γ-Al2O3 a
Alan E. Nelson,*,† Mingyong Sun,† and John Adjaye‡ Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G6, and Edmonton Research Centre, Syncrude Canada Ltd., Edmonton, Alberta, Canada T6N1H4 ReceiVed: March 17, 2006; In Final Form: May 23, 2006 In our original manuscript,1 we examined several spinel and one nonspinel2 structural models for γ-Al2O3 using DFT and Rietveld refinement simulations of synchrotron X-ray powder diffraction (SXPD) data. Our conclusion was that although the nonspinel model previously proposed is consistent with several structural features of γ-Al2O3, the anhydrous defect spinel structure is in better agreement with the experimental SXPD pattern. Thus, the spinel-related structure model is better than the nonspinel model for the description of the bulk structure of γ-Al2O3. In the immediately preceding correspondence, our conclusions are criticized as being based on an incorrect SXPD pattern of δ/θ-Al2O3 instead of γ-Al2O3. Raybaud and coworkers present several perceived discrepancies with the SXPD data and state that our conclusions are circumspect due to experimental inconsistencies and inadequate sample information. However, we believe substantive proof supporting these criticisms is lacking. The major technical argument the authors present is that the sample we used in our study of the structure of alumina (Davicat AL 2700 γ-Al2O3) is δ/θ-Al2O3 instead of γ-Al2O3. Before commenting further, it is worthwhile to present a brief discussion regarding the transitional forms of alumina. γ-Al2O3 is typically formed from an amorphous or boehmite precursor, and occurs at temperatures between 350 and 780 °C3,4 and has been reported to remain present as high as 1200 °C when synthesized from amorphous alumina precursors.5 The thermal transformation sequence is as follows:
boehmite/amorphous Al2O3 f γ-Al2O3 f δ-Al2O3 f θ-Al2O3 f R-Al2O3 As summarized by Trueba and Trasatti (ref 6 and references therein), γ-Al2O3 derived from amorphous precursors has a cubic lattice, while boehmite-derived γ-Al2O3 has been reported as having both a cubic lattice and tetragonal distortion. However, regardless of the degree of tetragonal distortion, γ-Al2O3 has generally been described using the Fd3[bar]m space group.6,7 The diffraction spectra of the transitional forms of alumina are commonly used to distinguish between phases, as noted by Raybaud and co-workers, and a composite spectrum of the seven “synthetic” forms of alumina has been previously published in ref 4 (Figure 2). The d-spacings for γ-Al2O3, δ-Al2O3, and θ-Al2O3, and a comparison to our SXPD data, are presented in Table 1. * Corresponding author. Tel.: +1-780-492-7380. Fax: +1-780-4922881. E-mail address:
[email protected]. † University of Alberta. ‡ Syncrude Canada Ltd.
c
δ-Al2O3 b
θ-Al2O3 c
SXPD data
d (Å)
I/Io
d (Å)
I/Io
d (Å)
I/Io
d (Å)
I/Io
4.560 2.800 2.390 2.280 1.997 1.520 1.395 1.140 1.027 0.989 0.884 0.806
40 20 80 50 100 30 100 20 10 10 10 20
7.600 6.400 5.530 5.100 4.570 4.070 3.610 3.230 3.050 2.881 2.728 2.601 2.460 2.402 2.315 2.279 2.160 1.986 1.953 1.914 1.827 1.810 1.628 1.604 1.538 1.517 1.456 1.407 1.396
4 4 4 8 12 12 4 4 4 8 30 25 60 16 8 40 4 75 40 12 4 8 8 4 8 16 8 50 100
5.700 5.450 4.540 2.837 2.730 2.566 2.444 2.315 2.257 2.019 1.954 1.909 1.800 1.777 1.738 1.681 1.622 1.572 1.543 1.512 1.488 1.453 1.426 1.388
2 10 18 80 65 14 60 45 35 45 8 30 14 6 4 2 6 2 25 6 25 25 10 100
4.550 2.851 2.733 2.437 2.285 2.000 1.949 1.798 1.537 1.490 1.393 1.285 1.261 1.232
9 21 42 33 28 53 28 4 14 11 100 3 3 2
a ICDD 29-1486, 29-0063, 10-0425. b ICDD 16-0394, 04-0877. ICDD 35-0121, 11-0517, 23-1009.
Several discrepancies exist regarding the identification and presence of the intermediate δ-Al2O3 phase. Trueba and Trasatti6 noted that previous studies have reported that there is no distinct difference between δ-Al2O3 and γ-Al2O3,8,9 while others have not observed the δ-Al2O3 phase.10,11 The unique difference between δ-Al2O3 and γ-Al2O3 is suggested to be the arrangement of the Al atoms in the framework, where γ-Al2O3 contains a distribution of tetrahedral and octahedral locations, and δ-Al2O3 only contains octahedral sites.6 It is clear from this brief discussion, however, that the exact identification of the metastable phases of alumina is a significant research challenge. Raybaud and co-workers suggest that the SXPD spectrum used in our previous study corresponds to δ/θ-Al2O3 instead of γ-Al2O3 on the basis of (1) the rise of the (220) reflection, (2) the shoulder of the (400) reflection, (3) two peaks at about 27°, and (4) the slight shift of the (440) reflection toward a larger angle. We will address each of these criticisms separately in the following discussion. First, the shoulder on the (400) reflection is not evidence of δ/θ-Al2O3, as this shoulder is clearly evident in spectra of γ-Al2O3 [ref 4, Figure 2c; ref 7, Figure 6]. Tsuchida et al.12 also concluded that boehmite with greater crystallinity leads to higher crystallinity γ-Al2O3 and tetragonal distortion and peak splitting of the (400) reflection. As noted by Paglia et al.,7 this tetragonal distortion has been attributed to several factors, including the distribution of residual hydroxyl groups13 and vacancy ordering of tetrahedral positions.8,14 Second, the increase in the intensity of the (220) reflection can be attributed to an increase in crystallinity, and this is evident in our sample. Third, the two peaks at d-spacings at 1.537 and 1.490 Å (27°) do not correspond to reflections that can be
10.1021/jp0616720 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/27/2006
Comments ascribed to δ-Al2O3, and are shifted in comparison to the d-spacings for θ-Al2O3. These peaks are also evident as a single unresolved feature in γ-Al2O3, which becomes more pronounced with greater crystallinity and more resolved with SXPD compared to Cu KR X-ray diffraction (XRD). Finally, the (440) reflection of our γ-Al2O3 (1.393 ( 0.003 Å) is not significantly shifted to a higher reflection angle (lower d-spacing) compared to International Centre of Diffraction data (ICDD) (1.395 Å), as suggested by Raybaud and co-workers, and this shift would not normally be observed in a Cu KR diffraction pattern. Therefore, we believe the SXPD spectrum contains the characteristic features of γ-Al2O3 suitable for Rietveld analysis of model γ-Al2O3 structures. Raybaud and co-workers also note that the tetrahedral aluminum content determined by NMR for γ-Al2O3 is typically 20-30%, as compared to θ-Al2O3 which is typically 40-50%, and suggest that, in the absence of a 27Al NMR characterization of our sample, they “...strongly suspect that the commercial sample is not a γ-Al2O3 sample.” Recently, we have performed 27Al MAS NMR on our γ-Al O sample using a Bruker Avance 2 3 500 mHz NMR (11.75 T) at a spinning rate of 14 kHz. The spectrum was baseline corrected, and the peaks were integrated to determine the distribution of Al in the bulk structure. The analysis indicated only the presence of tetrahedral and octahedral aluminum with a tetrahedral aluminum concentration of 28% (( 1%), which is in excellent agreement with the range stated by Raybaud and co-workers and other studies referenced in our original manuscript.15-17 Therefore, this lends further support to our previous discussion that the sample used in our study is consistent with γ-Al2O3 based on the SXPD spectral features and tetrahedral aluminum content. Raybaud and co-workers state that the inability of the nonspinel model to accurately reproduce the primary reflection features of the SXPD data “...must be seen as a proof of the validity of the nonspinel model for γ-Al2O3.” Clearly, the nonspinel model would be expected to reproduce the key reflections and be consistent with the primary features of γ-Al2O3, which it does not at present. To further illustrate this shortcoming of the nonspinel model, we have calculated the reflections for the defect spinel model, the fully hydrogenated spinel model, and the nonspinel model and compared the reflections with the XRD data published by Raybaud and coworkers.2 The unrefined spectra and direct comparison to their XRD data is presented in Figure 1. It can be clearly seen that the (220) reflection of the nonspinel model is completely out of phase with their published experimental XRD data. This shift of the (220) reflection is also evident in their previous work [ref 2, Figure 7], where the (220) reflection for the nonspinel model (S0,25) is completely out of phase with their experimental XRD pattern of γ-Al2O3 between 30° and 40° (Cu KR). This inability to accurately reproduce one of the key reflections would indicate the nonspinel model requires further investigation, and is not validated as they suggest. Additionally, the calculated lattice parameters for the nonspinel model [ref 2, Table 2, S0,25] have greater deviation (a ) 7.86 Å, c ) 8.00 Å) than the spinel-based model (a ) 8.01 Å, c ) 7.81 Å) presented in our study (ref 1, Table 4) when compared to experimental data (ref 14; a ) 7.96 Å, c ) 7.81 Å). Also, as noted in our study, the refined spinel structure accurately reproduces the lattice angles and tetrahedral aluminum content when modeled with a reasonable particle size. Thus, we conclude that the spinel model is a better representation than the nonspinel model for γ-Al2O3 based on the ability to model all key structural features.
J. Phys. Chem. B, Vol. 110, No. 41, 2006 20725
Figure 1. Unrefined simulated XRD patterns of spinel and nonspinel models of γ-Al2O3, in comparison with the experimental XRD pattern reprinted from ref 2.
It is clear from the previous discussion, and we fully agree with Raybaud and co-workers, that significant progress has been made resolving the detailed structure of γ-Al2O3. However, we do require additional research due to the metastable nature of the transition phase and inherent lack of crystallinity, which are key experimental challenges requiring further investigation to accurately describe the structure of γ-Al2O3. In summary, it is our belief that the research community will undoubtedly close the material and experimental gaps to further resolve the structure of γ-Al2O3. References and Notes (1) Sun, M.; Nelson, A. E.; Adjaye, J. J. Phys. Chem. B 2006, 110, 2310-2317. (2) Krokidis, X.; Raybaud, P.; Gobichon, A. E.; Rebours, B.; Euzen, P.; Toulhoat, H. J. Phys. Chem. B 2001, 105, 5121-5130. (3) Wefers, K.; Misra, C. Oxides and Hydroxides of Aluminum; ALCOA Laboratories: Alcoa Center, PA, 1987; p 20. (4) Souza Santos, P.; Souza Santos, H.; Toledo, S. P. Mater. Res. 2000, 3, 104-114. (5) Chou, T. C.; Nieh, T. G. J. Am. Ceram. Soc. 1991, 74, 22702279. (6) Trueba, M.; Trasatti, S. P. Eur. J. Inorg. Chem. 2005, 17, 33933403. (7) Paglia, G.; Buckley, C. E.; Rohl, A. L.; Hart, R. D.; Winter, K.; Studer, A. J.; Hunter, B. A.; Hanna, J. V. Chem. Mater. 2004, 16, 220236. (8) Wilson, S. J.; McConnell, J. D. C. J. Solid State Chem. 1980, 34, 315-322. (9) French, R. H.; Mullejans, H.; Jones, D. J. J. Am. Ceram. Soc. 1998, 81, 2549-2557.
20726 J. Phys. Chem. B, Vol. 110, No. 41, 2006 (10) Latella, B. A.; O’Connor, B. H. J. Am. Ceram. Soc. 1997, 80, 29412944. (11) Wang, J. A.; Bokhimi, X.; Morales, A.; Novaro, O.; Lopez, T.; Gomez, R. J. Phys. Chem. B 1999, 103, 299-303. (12) Tsuchida, T.; Furuichi, R.; Ishii, T. Thermochim. Acta 1980, 39, 103-115. (13) Yanagida, H.; Yamaguchi, G. Bull. Chem. Soc. Jpn. 1964, 37, 1229-1231.
Comments (14) Wilson, S. J. J. Solid State Chem. 1979, 30, 247-255. (15) Lee, M.-H.; Cheng C.-F.; Heine, V.; Klinowski, J. Chem. Phys. Lett. 1997, 265, 673-676. (16) Pecharroman, C.; Sobrados, I.; Iglesias, J. E.; Gonzalez-Carreno, T.; Sanz, J. J. Phys. Chem. B 1999, 103, 6160-6170. (17) John, C. S.; Alma, N. C. M.; Hays, G. R. Appl. Catal. 1983, 6, 341-346.
