J. Phys. Chem. C 2008, 112, 12545–12549
12545
Investigation of the Nature of a Ti-Al Cluster Formed upon Cycling under Hydrogen in Na Alanate Doped with a Ti-Based Precursor Aline Le´on,† Galina Yalovega,‡ Alexander Soldatov,‡ and Maximilian Fichtner*,† Institut fu¨r Nanotechnologie, Forschungszentrum Karlsruhe GmbH, P.O. Box 3640, 76021 Karlsruhe, Germany, and Center for Nanoscale Structure of Matter, Southern Federal UniVersity, Sorge 5, RostoV-na-Donu, 344090 Russia ReceiVed: NoVember 12, 2007; ReVised Manuscript ReceiVed: June 10, 2008
The hydrogen storage material of Ti-doped Na alanate has been studied by using both experimental and theoretical Ti K-edge X-ray absorption near edge structure (XANES) analysis. The results suggest the existence of small TiAlxTiy nanoclusters with not more than 13-35 atoms in NaAlH4 after the intercalation of Ti and hydrogenation-dehydrogenation cycling. The results rule out the possibility of the formation of both TiTi4Al8 nanoparticles and TiAlx nanoparticles having a TiAl3 type of local structure. TABLE 1: Sample Description and Labels
I. Introduction The development of a hydrogen storage material that fulfils all requirements for mobile applications is a challenge. So far, the most advanced systems with respect to thermodynamics, kinetics, and storage capacity have been sodium alanate, NaAlH4, doped with a transition metal-based precursor or, more recently, rare earth metal-based chlorides produced by ball milling.1–6 The hydrogen storage material is usually obtained by ball milling pure NaAlH4 with a precursor concentration of 2 or 5 mol %. So far, the best kinetic properties have been obtained by selecting TiCl3, Ti colloid, or CeCl3 as precursors.4–8 The decomposition of the nanocomposite and reversible reaction occur at medium to low temperature and pressure according to the following two-step mechanism:
3NaAlH4 S Na3AlH6+2Al + 3H2 Na3AlH6S3NaH + Al + 3 ⁄ 2H2
(3.7 wt % H) (1) (1.8 wt % H)
(2)
The third step, which consists of the decomposition of NaH into the elements, is not considered for applications, as it occurs at temperature above 450 °C. As can be seen, a reversible hydrogen storage capacity of 5.5 wt % is available. However, the storage capacity as well as the reaction rate have been observed to decrease with an increasing number of cycles when a Ti-based precursor is used, while they remain stable when CeCl3 is added. The experimental data available for this material provide information on the structure of the majority of phases,9,10 on the energy and kinetics of the transformation process,5,11 on the reaction rate,12,13 on the size of domains,14 and on microstructural changes.15 Moreover, X-ray absorption spectroscopy has been applied to gain more insight into the transformations on the atomic scale. The evolution of the chemical state of Ti and the local structure around Ti in NaAlH4 doped with a Ti-based precursor by ball milling upon cycling under hydrogen has been elucidated. It has been observed that a reduction of Ti species to the metallic state upon ball milling (as occurring for TiCl3)16–18 or during the first absorption reaction (as occurring * Address correspondence to this author. † Institut fu ¨ r Nanotechnologie. ‡ Southern Federal University.
labels
sample description
Ti Ti metallic foil TiAl3 Ti/Al alloy as received (99.5%, Alfa Aesar) Exp TiCl3 NaAlH4 + 5 mol % of TiCl3, ball milled 30 min and stopped after eight absorptions (T ) 100 °C, pH2 ) 100 bar) Exp Ti13 NaAlH4 + 5 mol % Ti on the basis of Ti13 · 6THF, ball milled 30 min and stopped after eight absorptions (T ) 100 °C, pH2 ) 100 bar)
for the Ti colloid precursor)19 promotes the formation of small bimetallic entities between Ti and Al. The local environment resulting after several cycles under hydrogen consists of small clusters of Ti, surrounded by about 10 Al atoms at 2.80 ( 0.02 Å and a small Ti contribution (around 1 atom) at 3.88 ( 0.02 Å, irrespective of the nature of the Ti precursor used. The formation of this nanoscale Ti-Al alloy is correlated with the decrease of the hydrogen storage capacity and the desorption/ absorption reaction rate.19 CeCl3-doped NaAlH4 is still being investigated. However, a full understanding of where Ti or Ce resides and how it helps to lower the hydrogen desorption temperature is still lacking. So far, density functional theory calculations have been carried out to understand the role of titanium in hydrogen desorption in crystalline sodium alanate and to determine the site of Ti. Several theoretical hypotheses have been studied in combination with experimental data to identify the mechanism of the chemical processes involved.20–24 The present work will focus on the theoretical calculation of the Ti K-edge absorption spectra of Ti-based precursor-doped NaAlH4 to evaluate the properties of the Ti-Al cluster formed upon cycling under hydrogen. For this purpose, the theoretical spectra will be compared with experimental data. The real-space multiple scattering (RSMS) approach will be applied to investigate the Ti K-edge XAS spectra.25 It represents an ab initio method for general calculations of XAS over an extended energy range. The theoretical method used will be first validated by calculating the expected absorption spectrum of Ti foil and TiAl3. Then, the method will be applied to NaAlH4 doped with a Ti-based precursor by ball milling (see a detailed description of the samples in Table 1). Finally, the nature and the size of
10.1021/jp710796a CCC: $40.75 2008 American Chemical Society Published on Web 07/22/2008
12546 J. Phys. Chem. C, Vol. 112, No. 32, 2008 TABLE 2: The Structure of TiAlxTi Clusters Used in the Simulations atoms
coordinates
Ti Al Al Al Al Al Al Al Al Al Al Al Al Ti
0.00000 1.98000 -1.98000 1.98000 -1.98000 1.98000 -1.98000 0.00000 0.00000 1.98000 -1.98000 0.00000 0.00000 3.8800
TiAl12Ti 0.00000 1.98000 1.98000 -1.98000 -1.98000 0.00000 0.00000 1.98000 -1.98000 0.00000 0.00000 1.98000 -1.98000 0.00000
0.00000 0.00000 0.00000 0.00000 0.00000 1.98000 1.98000 1.98000 1.98000 -1.98000 -1.98000 -1.98000 -1.98000 0.00000
TiAl8Ti Ti Al Al Al Al Al Al Al Al Ti
0.00000 1.94464 -1.94464 1.94464 -1.94464 1.94464 -1.94464 1.94464 -1.94464 3.8800
0.00000 1.94464 1.94464 -1.94464 -1.94464 0.00000 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 0.00000 0.00000 1.94464 1.94464 -1.94464 -1.94464 0.00000
the Ti-Al cluster formed after several cycles under hydrogen will be discussed.
Le´on et al. oxygen concentrations below 1 ppm. Chemical operations are performed on the bench under purified nitrogen, using Schlenk tube techniques. A detailed description of the sample preparation can be found elsewhere.5,12 X-ray absorption spectroscopy is performed at the Ti K-edge (4966 eV) of the samples. The measurements are carried out at the ANKA-INE beamline, Forschungszentrum Karlsruhe, Germany. The ANKA storage ring is operated with an electron beam of 2.5 GeV and a mean electron current of 100 mA. The X-ray monochromator, equipped with a pair of Si {111} crystals (2d ) 6.271 Å), is operated in the fixed-exit mode. The Ti K-edge X-ray absorption spectrum of a Ti metal foil is measured for energy calibration in the transmission mode. Ti metal is also used as a reference sample. Due to the low concentration of Ti in the doped sodium alanate samples, these spectra are taken in the fluorescence mode at room temperature. Ti KR radiation (∼4510 eV) is collected by a solid-state detector (5-element Canberra LEGe). An air-filled ionization chamber at 0.4 bar gas pressure is used to monitor the incident intensity of the beam. Up to 9 scans are averaged to improve the signal-tonoise ratio. The sample holder is a specially designed cell that allows the samples to be prepared, transferred, and measured without exposing them to air.19 XANES spectra (X-ray Absorption Near-Edge Structure, i.e., the region that ends roughly 50 eV above the edge) of all samples are isolated from XAFS scans by subtraction of the pre-edge background absorption (approximated by a polynomial function) and normalization of the edge jump to unity to allow for a quantitative comparison. A detailed description of the background removal, normalization procedure, and the Fourier transformation can be found elsewhere.19
II. Theoretical Methods Recently, it was demonstrated that relatively large errors may occur in the determination of the local structure parameters from the XANES spectrum when the muffin-tin approximation is used for the crystal potential in the multiple-scattering approach.26 In this approximation the atomic potentials used are spherical, while the outer and the interstitial regions are represented by a constant potential. The materials which are of interest in this study are rather condensed with a very small contribution from covalent bonding. Consequently, hardly any contribution can be expected from the nonmuffin-tin potential. For this reason, the Ti K-edge XANES in metallic Ti, TiAl3 alloy, and Ti-doped alanates has been analyzed theoretically by means of a selfconsistent full multiple scattering method, using the FEFF8.4 code.25 The spectra have been simulated by using several types of exchange potentials: Nonlocal potential, Dirac-Fock potential,27 Hedin-Lundqvist potential,28 and Dirac-Hara potential.29 Moreover, the influence of a relaxation of electrons in the presence of a core hole has been studied. The best agreement with experiment has been achieved when calculating the spectra with the Hedin-Lundqvist potential in the presence of a core hole. For the Ti K-edge calculation of metallic Ti, a hcp crystal lattice is used with the lattice parameters of a ) b ) 2.9500 Å and c ) 4.686 Å.30 For the Ti K-edge calculation of TiAl3 alloy, the crystal structure used is the space group I4/mmm with a ) b ) 3.848 Å and c ) 8.596 Å.31 For the alanates, Table 2 displays the coordinates used for the calculation of Ti K-edge in the model clusters. III. Experimental Methods All sample preparations are done in an argon-filled glovebox equipped with a recirculation system to keep the water and
IV. Discussion The simulation of standards, namely, hcp Ti foil and TiAl3 sample, is performed first in order to validate the theoretical method used for the Ti K-edge XANES analysis. Figure 1 displays the simulated spectra of the Ti foil, Figure 1a, and TiAl3 alloy, Figure 1b. For comparison, the experimental Ti K-edge XANES spectra of Ti foil and TiAl3 are plotted. At first, the size of the cluster is optimized to reproduce all the main features of the XANES region. For Ti foil and for TiAl3 alloy, Figure 1 panels a and b accordingly, the XANES has been calculated for clusters containing 510 atoms (radius about 12.86 Å and 12.3 Å). For TiAl3 alloy a cluster containing 123 atoms (7 Å) is found to be large enough. The predicted and the experimental XANES spectra of metallic Ti and TiAl3 alloy agree well. This indicates that the self-consistent full multiple scattering approach used can reproduce Ti K-edge XANES spectra of Ti foil and TiAl3. As found by the experimental study, the local structure around Ti in Ti-doped Na alanate cycled under hydrogen 8 times does not exactly correspond to the TiAl3 bulk phase. Indeed, the local environment resulting after several cycles under hydrogen consists of small clusters composed of Ti, surrounded by about 10 Al atoms at 2.80 ( 0.02 Å and a small Ti contribution (around 1 atom) at 3.88 ( 0.02 Å, irrespective of the nature of the Ti precursor used.19 Attempts have been made to estimate the size of this Ti-Al cluster in the alanate system from the XANES point of view by means of the present theoretical approach. Figure 2 displays the evolution of the Ti K-edge XANES with the size of a cluster constructed from a TiAl3 alloy crystal. For comparison, the experimental Ti K-edge XAS spectra of TiAl3 (spectrum at the bottom) and of Ti-doped Na alanate after eight absorption cycles (spectrum at the top) are
Investigation of the Nature of a Ti-Al Cluster
J. Phys. Chem. C, Vol. 112, No. 32, 2008 12547
Figure 3. (a) Transformation of the Ti K-edge XANES of a TiAl12Ti cluster with symmetry changes. (b) Model of a Ti-Al12-Ti cluster.
Figure 1. Comparison of the experimental and theoretical Ti K-edge XANES of a Ti foil (a) and a TiAl3 alloy (b).
Figure 2. Transformation of the Ti K-edge XANES with the size of a cluster around the X-ray-absorbing Ti atom.
presented. As can be seen by decreasing the size of the cluster from 123 atoms (corresponding to bulk material) to 13 atoms, the shape of the XANES spectra calculated approaches that of the experimental data. The shape of the calculated spectra is in good agreement with the experimental one in the energy range below 5000 eV. The experimental spectrum of Ti-doped alanate is closer to the 13 atomic cluster of the TiAl3 crystal than the large cluster of the TiAl3 material meaning that one can expect
the Ti surrounding to be made up of a small nanocluster of Al atoms. However, the energy position of the high-energy peaks B and C differs from that of the measured spectra. Moreover, the discrepancy in the energy position of the peaks increases with increasing energy. Such a discrepancy generally is a signature of smaller interatomic distances in measured samples compared to the model cluster used for the simulation. This is also true here, as the theoretical model cluster used for the simulations has an average Ti-Al distance on the order of 2.83 Å, while the Tix nanocluster inside Ti-based precursor-doped Na alanates seems to have a smaller interatomic distance of about 2.80 Å, as will be shown below. The result of the theoretical calculation indicates that a very small entity composed of about 13 atoms is formed upon cycling Ti-doped Na alanate under hydrogen. As agreement is still not good we propose further modifications of the TiAl12 nanocluster changing its symmetry from corresponding to that of the TiAl3 crystal (where there are two sets of Ti-Al distances) to that of the more “spherical” one where all Ti-Al distances are equal. Figure 3 displays the transformation of the Ti K-edge XANES of a small TiAl12Ti cluster with the symmetry changes, Figure 3a, and the view of the model, Figure 3b. The spectrum of the TiAl3 crystal with tetragonal symmetry (4 Al atoms at about 2.72 Å, eight Al atoms at about 2.88 Å from the central Ti ion) is presented by the dashed line. The spectrum calculated by changing this starting symmetry to equal Ti-Al distances at about 2.8 Å is presented by the solid line. As is obvious, the changes in the symmetry of the clusters result in a much better agreement of the theoretical XANES spectrum with the experimental spectrum of the Ti alanate. It may therefore be concluded that a small Ti-Alx nanocluster in the alanate is not simply a nanoparticle of a TiAl3 alloy.
12548 J. Phys. Chem. C, Vol. 112, No. 32, 2008
Figure 4. Transformation of the Ti K-edge XANES of a TiAl12 cluster with and without a Ti atom in the second shell.
Figure 5. Transformation of the Ti K-edge XANES of a TiAl8 cluster with and without a Ti atom in the second shell.
The possibility of finding a Ti atom in the second shell of TiAlx nanoclusters by an EXAFS data analysis19 shall be evaluated below. Figures 4 and 5 display the transformation of the Ti K-edge XANES of TiAl12 and TiAl8 cluster with and without a Ti atom in the second shell. It is evident that the two theoretical spectra have mutually complementary features (regions A′ and C′). Figure 6 displays both the experimental and theoretical Ti K-edge XANES for the samples with two different precursors, namely, TiCl3 and Ti13 clusters (more details can be found elsewhere19). Theoretical spectra are obtained by averaging the spectra of TiAlx and TiAlxTi clusterssi.e., assuming that half of the TiAlx clusters have a Ti atom in the second shell, while the other half do not have a Ti atom. As can be seen, the relative changes in the shape of the theoretical spectra follow the changes observed in the experimental data. Hence, these results indicate that TiAlx and TiAlxTi clusters coexist in the Ti-based precursordoped Na alanate cycled under hydrogen 8 times and quenched in the absorbed state. In a previous work based on EXAFS analysis,18 the existence of TiTi4Al8 clusters was proposed with a certain probability (at least, the differences in the goodness of the fits, R factor, for this type of cluster and TiAl12 nanocluster were rather small). To select between these two types of clusters and to identify the nature of the Ti-Al cluster, a simulation is performed with
Le´on et al.
Figure 6. Transformation of the experimental and theoretical Ti K-edge XANES for two different precursors.
Figure 7. Comparison of the Ti K-edge XANES of TiAl12 and TiTi4Al8 clusters.
Figure 8. The Ti K-edge XANES of a Ti ion in a metal Al matrix with the size of the Al cluster around the Ti ion.
both models. Figure 7 displays the calculated Ti K-edge XANES for TiTi4Al8 and TiAl12 clusters. For comparison, the experimental data are presented. It is found that XANES analysis definitely rejects the TiTi4Alx model. Therefore, it can be concluded that the formation of TiAl12 is more plausible.
Investigation of the Nature of a Ti-Al Cluster It was proposed previously32 that the Ti K-edge XANES spectra of an Al matrix doped even with a low level of Ti atoms (2 mol %) looked very close to the spectrum of TiAl3. To verify the conclusion of Ti atoms in this case transforming the Al matrix into TiAl3, the Ti K-edge XANES of a single Ti atom dissolved in a rigid Al matrix is simulated. Figure 8 displays the result of the simulation for different sizes of the cluster. Obviously, the spectra of the Ti ion in a metal Al matrix for all sizes of Al clusters around the Ti ion do agree with neither TiAl3 nor Ti alanate spectra. Thus, the hypothesis that Ti atoms transform the Al matrix into TiAl3 does not seem to be valid. V. Conclusions Detailed analysis of the Ti K-edge XANES obtained from advanced theoretical simulations support the hypothesis of small TiAlxTiy nanoclusters existing in Ti alanates after the intercalation of Ti and hydrogenation-dehydrogenation cycling. The present results rule out the possibility of both TiTi4Al8 nanoparticles and TiAlx nanoparticles having a TiAl3 type of local structure being formed. References and Notes (1) Bogdanovic´, B.; Schwickardi, M. J. Alloys Compd. 1997, 253254, 1. (2) Bogdanovic´, B.; Brand, R.; Marjanovic´, A.; Schwickardi, M.; To¨lle, J. J. Alloys Compd. 2000, 302, 36. (3) Anton, D. L. J. Alloys Compd. 2003, 356-357, 400. (4) Bogdanovic´, B.; Sandrock, G. MRS Bull. 2002, 712. (5) Fichtner, M.; Fuhr, O.; Kircher, O.; Rothe, J. Nanotechnology 2003, 14, 778. (6) Bogdanovic´, B.; Felderhoff, M.; Kaskel, S.; Pommerin, A.; Schlichte, K.; Schu¨th, F. AdV. Mater. 2003, 15, 1012. (7) Bogdanovic´, B.; Felderhoff, M.; Pommerin, A.; Schu¨th, F.; Spielkamp, N. AdV. Mater. 2006, 1198. (8) Majzoub, E. H.; Herberg, J. L.; Stumpf, R.; Spangler, S.; Maxwell, R. S. J. Alloys Compd. 2005, 394, 265. (9) Gross, K. J.; Sandrock, G.; Thomas, G. J. Alloys Compd. 2002, 330, 691.
J. Phys. Chem. C, Vol. 112, No. 32, 2008 12549 (10) Brinks, H.; Sulic, M.; Jensen, C. M.; Hauback, B. C. J. Phys. Chem. B 2006, 10, 2740. (11) Sandrock, G.; Gross, K. J.; Thomas, G. J. Alloys Compd. 2002, 339, 299. (12) Kircher, O.; Fichtner, M. J. Appl. Phys. 2004, 95, 7748. (13) Luo, W.; Gross, K. J. J. Alloys Compd. 2004, 385, 224. (14) Gomes, S.; Renaudin, G.; Hagemann, H.; Yvon, K.; Sulic, M. P.; Jensen, C. M. J. Alloys Compd. 2005, 390, 305. (15) Le´on, A.; Kircher, O.; Ro¨sner, H.; De´camps, B.; Leroy, E.; Fichtner, M.; Percheron-Gue´gan, A. J. Alloys Compd. 2006, 414, 190. (16) Graetz, J.; Reilly, J. J.; Johnson, J.; Ignatov, A.Yu.; Tyson, T. A. Appl. Phys. Lett. 2004, 85, 500. (17) Le´on, A.; Kircher, O.; Rothe, J.; Fichtner, M. J. Phys. Chem. B 2004, 108, 16372. (18) Felderhoff, M.; Klementiev, K.; Gru¨nert, W.; Spliethoff, B.; Tesche, B.; Bellosta von Colbe, J. M.; Bogdanovic´, B.; Ha¨rtel, M.; Pommerin, A.; Schu¨th, F.; Weidenthaler, C. Phys. Chem. Chem. Phys. 2004, 6, 4369. (19) Le´on, A.; Kircher, O.; Fichtner, M.; Rothe, J.; Schild, D. J. Phys. Chem. B 2006, 110, 1192. (20) Løvvik, O. M.; Opalka, S. M. Phys. ReV. B 2005, 71, 054103. (21) Chaudhuri, S.; Muckerman, J. T. J. Phys. Chem. Lett. 2005, 109, 6952. (22) Moyse´s Arau´jo, C.; Li, S.; Ahuja, R.; Jena, P. Phys. ReV. B 2005, 72, 165101. (23) Chaudhuri, S.; Graetz, J.; Ignatov, A.; Reilly, J. J.; Muckerman, J. T. J. Am. Chem. Soc. 2006, 128, 11404. (24) Moyse´s Arau´jo, C.; Ahuja, R.; Guille´n, J. M. O.; Jena, P Appl. Phys. Lett. 2005, 86, 251913. (25) Rehr, J. J.; Ankudinov, A. L. Coord. Chem. ReV. 2005, 249, 131. (26) Smolentsev, G.; Soldatov, A. V.; Feiters, M. C. Phys. ReV. B 2007, 75, 144106. (27) Hedin, L.; Lundqvist, S. Solid State Phys. 1969, 23, 1. (28) Dirac, P. A. M. Proc. Cambridge Philos. Soc. 1930, 26, 376. (29) Labzowsky, L. N.; Goidenko, I. RelatiVistic Electronic Structure Theory. Part I. Fundamentals; Elsevier: Amsterdam, The Netherlands, 2002; pp 401-467. (30) Wyckoff, R. W. G. Crystal Structures; Interscience: New York, 1965. (31) Sikora, T.; Hug, G.; Jaouen, M.; Rehr, J. J. Phys. ReV. B 2000, 62, 1723. (32) Graetz, J.; Ignatov, A. Yu.; Tyson, T. A.; Reilly, J. J.; Johnson, J. Mater. Res. Soc. Symp. Proc. 2005, 837, N2.4.1.
JP710796A
