J. Phys. Chem. C 2007, 111, 3795-3798
3795
Al K Edge XANES Measurements in NaAlH4 Doped with TiCl3 by Ball Milling Aline Le´ on,*,† Antonella Balerna,‡ Gianfelice Cinque,‡ Christoph Frommen,† and Maximilian Fichtner† Institut fu¨r Nanotechnologie, Forschungszentrum Karlsruhe GmbH, P.O. Box 36 40, 76021 Karlsruhe, Germany, and INFN-Frascati National Laboratory - Via E. Fermi 40, Frascati 00044, Italy ReceiVed: July 18, 2006; In Final Form: January 8, 2007
Al K edge X-ray absorption spectroscopy has been applied to investigate pure sodium alanate and sodium alanate doped with 5 mol % Ti on the basis of TiCl3 by ball milling. These first experimental data indicate that the chemical state of Al in NaAlH4 does not change upon cycling. It confirms also that the valence state of Al is higher than that of metallic Al in NaAlH4. Moreover, a change in the orbital occupancy of Al atoms or structural disorder in the local Al surroundings is occurring most probably after doping sodium alanate and subsequent cycling under hydrogen. Indeed, an evolution of the X-ray absorption near-edge structure (XANES) features can be observed with increasing number of cycles under hydrogen compared to the pure alanate sample. After several cycles, a new phase containing metallic Al is formed, which corresponds to the mixture of Al and the formation of small Ti-Al clusters already observed in Ti K edge measurements. These preliminary results suggest the presence of molecular-scale inhomogeneities within the average structure of sodium alanate.
Introduction For the development of new, higher-efficiency hydrogen storage materials, the underlying principles controlling hydrogen release and uptake need to be elucidated. Among the hydrogen storage materials, the complex low- and mediumtemperature hydride, namely, sodium alanate, NaAlH4, is the one which has been widely studied. In this case, the decomposition reaction has been made reversible under moderate temperature and pressure conditions by using transitionmetal precursors which were added to the alanate by wet impregnation.1,2 Thermal dissociation of NaAlH4 to NaH, Al, and H2 proceeds in two steps according to the following equations:
3NaAlH4 / Na3AlH6 + 2Al + 3H2 3 Na3AlH6 / 3NaH + Al + H2 2
(3.7 wt %) (1) (1.8 wt %)
(2)
The decomposition of NaH occurring at temperatures higher than 400 °C is not considered for practical purposes. From the first two steps, a reversible hydrogen capacity of 5.6 wt % is expected. This amount of reversible H2, however, decreases with the increasing amount of dopant and increasing number of cycles under hydrogen. In the case of Na alanate, Ti halides3 and Ti13· 6THF4,5 are the most efficient precursors to accelerate the kinetics of the desorption and absorption reaction of hydrogen. Recent studies on the atomic scale6-8 elucidated the state of the dopant in different stages of the dehydrogenation/rehydrogenation reaction. A comparison of the behavior of the Ti-based precursor-doped materials revealed that most of the Ti species do not remain at the surface upon milling and subsequent cycling, irrespective of the Ti-based precursor used to activate * To whom correspondence should be addressed. † Institut fu ¨ r Nanotechnologie. ‡ INFN-Frascati National Laboratory
the reversible decomposition reaction of Na alanate.9 Furthermore, the chemical state of Ti is relevant to the desorption/ absorption reaction rate and the storage capacity. Reduction of Ti species to the metallic state upon ball milling (as occurring for TiCl3)6-8 or during the first absorption reaction (as occurring for Ti colloid precursor)10 promotes the formation of small bimetallic entities between Ti and Al. 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. The formation of this nanoscale Ti-Al alloy is correlated with the decrease of the hydrogen storage capacity and the desorption/absorption reaction rate.9 Considering the structure of NaAlH4, X-ray diffraction (XRD) analysis by Rietveld refinement indicates that the Bragg positions and the lattice parameters of Ti-doped sodium alanate are preserved. However, some diffraction peak intensities of the doped alanate are not properly described by the refinement.10 Some new insights into the reaction have been gained, but the physical and chemical processes involved in hydrogen release and uptake still are not fully understood. There are still open questions concerning the state of the catalysts as well as the relationships between the structure and the characteristic properties observed with this material. Herein, it is attempted to bring some elements of answers by investigating, for the first time to our knowledge, NaAlH4 doped with 5 mol % Ti on the basis of TiCl3 by ball milling using Al K edge X-ray absorption spectroscopy, in particular, the XANES features (X-ray absorption near-edge structure, i.e., the region which ends about 50 eV above the edge). From the presence, intensity, and energy position of distinct XANES features, information about the oxidation state, the coordination geometry, and the orbital occupancy of the absorbing atoms can be derived. The spectra can be interpreted by comparison with reference spectra (“fingerprint”) or theoretical calculations.11,12
10.1021/jp0645418 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/14/2007
3796 J. Phys. Chem. C, Vol. 111, No. 9, 2007
Le´on et al.
TABLE 1: Sample Description labels (Al) NaAlH4 (bm)
sample description Al metallic foil purified sodium alanate, ball-milled for 30 min
NaAlH4 + 5 mol % TiCl3, ball-milled for 30 min at 600 rpm (d1d) sample quenched during the first desorption once 1.9 wt % of hydrogen have been releaseda (a1a) sample after the first absorptionb (a8a) sample after eight absorptionb a
(T ) 150 °C, pH2 ) 0.3 bar)b. b (T ) 100 °C, pH2 ) 100 bar).
Experimental Section All sample preparations were done in an argon-filled glovebox equipped with a recirculation system to keep the water and oxygen concentrations below 1 ppm. Chemical operations were performed on the bench under purified nitrogen using Schlenk tube techniques. Commercially available NaAlH4 (Albemarle, Belgium) was purified by a Soxhlet extraction with THF as described elsewhere.4 According to elemental analysis, the purified NaAlH4 contained 7.6 ( 0.1 wt % of H and 0.2 ( 0.1 wt % of C. The sample doped with the Ti-based catalyst was prepared using TiCl3 (99.999%, Sigma Aldrich). In the Ar atmosphere, the silicon nitride vial containing balls of the same material was filled with 2 g of purified sodium alanate and 285 mg of TiCl3 (5 mol % Ti) and was sealed. The ball-to-powder weight ratio was about 20:1. Milling was carried out in a Fritsch P6 planetary mixer/mill at 600 rpm for 30 min. Absorption and desorption of hydrogen took place in a modified Sieverts apparatus. A more detailed description of the apparatus and the reactor used can be found elsewhere.4,13 The evolution of the local structure around Al was investigated in samples quenched at a defined degree of the dehydrogenationrehydrogenation reaction (cf. Table 1). X-ray absorption spectroscopy was performed on the samples at the Al K edge (around 1560 eV). The measurements were carried out at the DAΦNE soft X-ray beamline (working in the 1-3 keV range), INFN-Laboratori Nazionali di Frascati, Italy. The DAΦNE storage ring operates at an electron beam energy of 0.51 GeV and a circulating current over 1 A. The X-ray monochromator, equipped with a pair of KTP (KTiOPO4, 2d ) 10.95 Å) crystals, in “boomerang” geometry, was used to ensure a fixed exit mode while scanning the whole energy range. Energy resolution is about 1 eV at 1.5 keV. The Al K edge X-ray absorption spectrum of a pure (99.99%, Goodfellow) Al metal foil was measured for energy calibration. Spectra from all the samples were taken in the transmission mode and under vacuum. Nitrogen-filled ionization chambers were used to monitor the incident and transmitted intensity of the beam. Al metal and NaAlH4 were used as reference samples. The sample holder was a specially designed cell, which allowed the samples to be prepared, transferred, and measured without exposing them to air. It consisted of an aluminum frame (19 × 10 mm) with a window (4 × 12 mm) sealed with a mylar film (2.5 µm) on both sides. The sample powder was spread onto one side of the mylar film using silicon grease. The sealed sample holders were fixed on a translation manipulator and were introduced into the evacuated sample chamber at about 10-4 mbar. The spectra were collected at room temperature in the range from 1540.0 to 1606.7 eV with 0.4 eV step width and 3 s acquisition time per step. Data reduction of the experimental absorption spectra was carried out using the WinXAS 2.1 software package.14 XANES spectra for all samples were isolated from XAFS scans by
Figure 1. Normalized Al K edge XANES spectrum of pure sodium alanate, ball-milled for 30 min. For comparison, the spectrum of metallic aluminum is presented. The dashed line at 1560 eV corresponds to the first inflection point in the Al K edge XANES spectrum of the pure metal.
subtraction of the preedge background absorption (approximated by a polynomial function) and normalization of the edge jump to unity to allow for comparison. Results and Discussion Figure 1 displays the normalized Al K edge XANES spectra of pure sodium alanate. The spectrum is compared to the spectrum of metallic aluminum, which is taken as reference. From the first derivatives of the spectra (not shown), the position of the first inflection point in the spectra of NaAlH4 (bm) and (Al) is at 1565.5 and 1560 eV, respectively. This confirms that the valence state of Al in sodium alanate is in a higher oxidation state compared to metallic aluminum, which is expected, for example, on the basis of theoretical calculations.15 The spectrum of the pure NaAlH4 (bm) sample shows no preedge structure but does show a shoulder of the broad peak with the highest intensity in the near-edge region, followed by a small plateau which separates the first and the second resonance. Moreover, the Al K edge XANES peak at 1565.5 eV in NaAlH4 confirms the 4-fold coordinated Al in this material. Indeed, Al K edge XANES studies in a wide variety of compounds indicate that the main characteristic of XANES for tetrahedrally coordinated Al is the presence of an intense line located between 1565.4 and 1566.8 eV, whereas the main XANES features for 6-fold coordinated Al occur at an energy that is higher by about 2 eV.16,17 Figure 2 represents the normalized Al K edge XANES spectra of pure sodium alanate NaAlH4 (bm) and of sodium alanate doped with 5 mol % Ti on the basis of TiCl3 during the first desorption (d1d), after first absorption (a1a), and after eight absorption cycles (a8a). For more clarity, Figure 2 is separated in (a) with the spectra of NaAlH4 (bm) and (d1d) and in (b) with the spectra of NaAlH4 (bm), (a1a), and (a8a). The sample (d1d) was obtained by quenching the decomposition reaction once 1.9 wt % of H2 had been released. The phases present at this stage of the reaction are 50 wt % of NaAlH4, 31 wt % of Na3AlH6, and 17 wt % of Al. The main phase in samples (a1a) and (a8a) is NaAlH4 with an increasing concentration of Al from the first to the eight cycles under hydrogen. In the latter stage together with metallic Al, there is also presence of bimetallic entities between Al and Ti as observed at the Ti K edge.9
Al K Edge XANES Measurements by Ball Milling
J. Phys. Chem. C, Vol. 111, No. 9, 2007 3797 evolution occurring around Al atoms in NaAlH4 after one and several cycles under hydrogen. The shape of the resonances in a XANES spectrum actually depends on several structural aspects, including point symmetry of the atom, distribution of bond lengths in the coordination polyhedra, and number of sites.11 Therefore, the decrease of the intensity observed in the features (C) of the XANES spectra of pure sodium alanate and Ti-based precursor-doped NaAlH4 indicates a change in the orbital occupancy of Al atoms or structural disorder in the local Al surroundings after doping with Ti by ball milling and subsequent cycling under hydrogen. Furthermore, in the spectrum of samples (a8a), a low-energy feature (A) preceding the alanate signal appears at 1560 eV. This energy position indicates the presence in this sample of a phase containing significant amounts (at least a few percent as it is detectable in the XANES) of metallic Al. After eight cycles under hydrogen, the presence of metallic Al was observed by XRD,10 and the formation of a very small entity between Ti and Al was revealed by Ti K-extended X-ray absorption fine structure (EXAFS) measurements.9 Therefore, this low-energy feature can be related to the mixture of these two compounds but the contribution of each of them cannot be separated at this stage. A low-energy feature was also expected in the spectrum of samples (d1d) and (a1a) as these samples contain metallic Al. However, the contribution of the metallic Al to the intensity of the signal between 1560 and 1565 eV is comparably small, although differing from the intensity of pure NaAlH4 in this region. More Al K-XANES references have to be investigated using different compositions of Al with NaAlH4, Na3AlH6, or Ti-Al bimetallic entity to distinguish the contribution of these four phases. Summary
Figure 2. (a) Normalized Al K edge XANES spectra of pure sodium alanate ball-milled for 30 min and sodium alanate doped with 5 mol % Ti on the basis of TiCl3 by ball milling during the first desorption (d1d). The dashed line at 1560 eV corresponds to the first inflection point in the Al K edge XANES spectrum of the pure metal. (b) Normalized Al K edge XANES spectra of pure sodium alanate ballmilled for 30 min and sodium alanate doped with 5 mol % Ti on the basis of TiCl3 by ball milling after the first absorption (a1a) and after the eight absorption (a8a). The dashed line at 1560 eV corresponds to the first inflection point in the Al K edge XANES spectrum of the pure metal.
From the first derivatives of the spectra (not shown), the position of the first inflection point in the spectra (a1a) and (a8a) is obtained to be located at 1565.0 and 1565.5 eV, respectively. This indicates that the chemical state of Al in NaAlH4 does not change with cycling under hydrogen. As can be seen, the XANES features are subject to an evolution when the sodium alanate is doped with a Ti-based precursor and with an increasing number of cycles under hydrogen compared to the pure alanate sample. A significant difference appears in the resonance amplitude (C) of sample NaAlH4 (bm) and resonance of the (d1d), (a1a), and (a8a) samples. Indeed, the edge height observed for the pure NaAlH4 is strongly dampened in the Ti-doped sodium alanate in different stages of the reaction. There are similarities in the main resonances of samples (d1d), (a1a), and (a8a), but there are pronounced differences in the shoulder (B) of the rising edge. The intensity of the features (B) at around 1566 eV decreases with an increasing number of cycles. A comparison of the NaAlH4 (bm), (a1a), and (a8a) spectra clearly indicates the
According to the Al K edge XANES analysis, the doping of sodium alanate by a Ti-based precursor induces local modifications around Al atoms within the alanate structure. Moreover, an evolution around Al atoms in the alanate phase takes place upon cycling under hydrogen. After several cycles, a new phase containing metallic Al is formed, which corresponds to a mixture of metallic Al and the formation of small Ti-Al clusters already observed in Ti K edge measurements. Moreover, the evolution of the absorption feature around 1570 eV suggests the presence of molecular-scale inhomogeneities in the average structure of sodium alanate. As confirmed by the first results at the Al K edge, XAS analyses of these materials are feasible. Al K edge XANES seems to be a sensitive probe for investigating further the local structure around Al atoms in pure sodium alanate or in Ti-doped sodium alanate. Acquisition of the fingerprint of samples with different composition of the phases involved in Ti doped sodium alanate and improvements of the experimental setup (sample thickness optimization for a better signal-to-noise ratio and improvement of the counting statistics to achieve also EXAFS spectra) are envisaged in a next experimental step to distinguish the four different coordination geometries that are present around Al in this material. This will also allow to better quantify the local environment around Al atoms in Ti-based precursor-doped sodium alanate. Acknowledgment. Financial support of the work by EU-IP “StorHy” (contract # 502667) and the Helmholtz Initiative “FuncHy” is gratefully acknowledged. The authors would like to cordially thank Prof. A. Soldatov and Prof. C. Marcelli for their suggestion to perform these investigations using the
3798 J. Phys. Chem. C, Vol. 111, No. 9, 2007 DAΦNE soft X-ray beamline and for valuable technical discussion. Thanks are also due to Dr. J. Rothe for fruitful discussions. References and Notes (1) Bogdanovic´, B.; Schwickardi, M. J. Alloys Compd. 1997, 1, 253254. (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, 400, 356-357. (4) Fichtner, M.; Fuhr, O.; Kircher, O.; Rothe, J. Nanotechnology 2003, 14, 778. (5) Bogdanovic´, B.; Felderhoff, M.; Kaskel, S.; Pommerin, A.; Schlichte, K.; Schu¨th, F. AdV. Mater. 2003, 15, 1012. (6) Graetz, J.; Reilly, J. J.; Johnson, J.; Ignatov, A. Yu.; Tyson, T. A. Appl. Phys. Lett. 2004, 85, 500. (7) Le´on, A.; Kircher, O.; Rothe, J.; Fichtner, M. J. Phys. Chem. B 2004, 108, 16372.
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