Article pubs.acs.org/JPCC
Mobile Species in NaAlH4 Eric G. Sorte,*,† Robert C. Bowman, Jr.,‡ Eric H. Majzoub,§ Margriet H. W. Verkuijlen,∥ Terrence J. Udovic,⊥ and Mark S. Conradi*,† †
Department of Physics, Washington University, One Brookings Drive, Saint Louis, Missouri 63130, United States RCB Hydrides, LLC, 117 Miami Avenue, Franklin, Ohio 45005, United States § Center for Nanoscience and Department of Physics and Astronomy, University of Missouri-St. Louis, One University Boulevard, St. Louis, Missouri 63121, United States ∥ Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands ⊥ National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ‡
S Supporting Information *
ABSTRACT: Sodium alanate (NaAlH4) is the archetypical complex (ioniccovalent) hydride compound for hydrogen storage applications. However, the details of the reactions of this compound upon hydrogen cycling remain unclear. Recently, a new mobile aluminum-containing species termed S105 was discovered in the hydrogen chemistry of sodium alanate using in situ NMR with S105 a likely facilitator of Al-atom transport in NaAlH4. Here, we find that hydroxide impurities play a crucial role in the formation of the mobile species. Specifically, in bare NaAlH4, S105 is present after melting (under sufficient hydrogen pressure to block the dehydriding reaction) only in samples either exposed to H2O vapor or mixed with metal hydroxides. We find that the 27Al line position of S105 is close to that of NaAlH4 (after correcting for second-order quadrupole effects), indicating that S105 involves very mobile AlH4− tetrahedra (rotationally and translationally). We propose that hydroxide impurities promote fast diffusion of nearby AlH4− units, similar to enhanced motions seen in NaH; the hydroxides also react with NaAlH4 to form NaH and subsequently produce Na3AlH6, which is always found to accompany S105. Our measurements reveal that the only chemical components of S105-containing alanate apart from hydroxides are NaAlH4 and Na3AlH6. Presence of the S105 species in NaAlH4 samples also leads to faster dehydriding in hot, undoped NaAlH4 solid, pointing to an enhancement of the hydrogen reaction kinetics by S105.
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INTRODUCTION Hydrogen is an attractive energy carrier due to its high abundance, clean combustion, and good energy density by mass. H2 can also be used in fuel cells for better efficiency in converting chemical energy to work. However, a relatively low gas−liquid critical temperature at around 33 K poses difficulties for its on-board storage as a liquid or highly compressed gas in automotive applications. Consequently, much work has focused on safe and energy-efficient ways to store hydrogen reversibly in solid form, including chemical storage in complex (ioniccovalent) metal hydrides. Sodium alanate (NaAlH4) is a well-studied complex hydride useful in hydrogen storage as it releases 5.6% hydrogen by mass and exhibits reasonable reversibility when doped (traditionally with titanium compounds).1−3 Decomposition is known to occur in two steps,3,4 each liberating Al metal and H2 gas 3NaAlH4 ↔ Na3AlH6 + 2Al + 3H 2 Na3AlH6 ↔ 3NaH + Al +
3 H2 2
decomposition is typically kinetically limited. The thermodynamics (i.e., the reaction equilibrium H2 pressure) have been measured by Bogdanovic.6 A third step involving the decomposition of NaH proceeds only at much higher temperatures and so is generally disregarded. As implied by eqs 1 and 2, the dehydriding reaction not only liberates hydrogen gas but also separates the metal constituents into phase-segregated, nonmobile crystallites (as evidenced by sharp, distinct X-ray diffraction (XRD) reflections of the different reaction products). Rehydriding sodium alanate must then involve recombining the segregated metallic elements (from NaH and Al) into intimate stoichiometric ionic and covalent arrangements. While catalysis with titanium has been shown to make these reactions reversible at reasonable temperatures and pressures, the mechanism whereby this is accomplished remains unclear. Furthermore, the metal-atom transport remains the most likely rate-limiting step in the reaction scheme.7−11
(1)
(2)
Received: January 31, 2013 Revised: March 24, 2013 Published: March 25, 2013
These reactions typically occur above 185 °C for undoped material or at lower temperatures in doped NaAlH4;5 © 2013 American Chemical Society
8105
dx.doi.org/10.1021/jp401134t | J. Phys. Chem. C 2013, 117, 8105−8113
The Journal of Physical Chemistry C
Article
Figure 1. 27Al NMR (static, at ambient conditions) of NaAlH4 after heating to 220 °C for 30 min under 210 bar H2 pressure. (a) Sample maintained in an inert N2 atmosphere prior to P−T treatment. NaAlH4 and Al metal are evident, along with a very small amount of Na3AlH6 (at −3.3 kHz = −42.5 ppm). (b) Sample exposed to ambient air for 5 min prior to P−T treatment. NaAlH4, Na3AlH6, and the sharp S105 appear here.
In 2010, Ivancic et al.12 discovered a mobile species (S105) in NaAlH4 using in situ 27Al NMR measurements at high temperatures under pressures sufficient to block both reactions 1 and 2 from evolving left to right (dehydriding).6 They reported a sharp, motionally narrowed resonance in both the 27 Al and 1H static NMR spectra of sodium alanate, leading them to identify the resonance as an AlxHy species that was undergoing rapid motion. The narrow resonance at approximately 105 ppm comprises about 10% of the 27Al spectral weight (area) and was proposed to be a defect-ridden form of NaAlH4. Calculations by Gunaydin and Ozolins indicate that neutral AlH3 vacancies in the NaAlH4 bulk are a likely vacancy species.11−13 Rapid migration of AlH3 vacancies would result in diffusion of Al and H in NaAlH4, explaining the line-narrowing of S105. Furthermore, this study prescribed conditions under which the new resonance (termed S105 for the approximately 105 ppm chemical shift of its 27Al resonance) could be retained at room temperature and pressure for study and characterization. Unfortunately, initial attempts by other groups to produce the S105 resonance under similar conditions were unsuccessful.14 Here, we report the results of a systematic study of the mobile species S105, including how to reliably produce and maintain the species in sodium alanate. In addition to extensive static NMR measurements under a wide variety of conditions to examine the creation of the mobile species, we performed an array of characterization measurements examining S105containing samples including X-ray diffraction, Raman and neutron vibrational spectroscopies, and MAS NMR. Finally, we show that the mobile S105 may indeed be important for enhancing the normally sluggish dehyriding and rehydriding chemistry of undoped sodium alanate.
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Missouri-St. Louis, and M. Felderhoff at MPI-Muelheim synthesized the NaAlD4. The Ti- and Sc-doped NaAlH4 samples were prepared in a SPEX 8000 mechanical mill for 30 min using a WC milling pod and six WC balls. All materials were handled in an argon glovebox with H2O and O2 below 10 ppm. Pressure and temperature (P−T) treatment of samples (typically at 220 °C and 210 bar H2) was performed in an 8 cc stainless-steel high-pressure reactor with an external resistive heater driven by a temperature controller. Hydrogen overpressures were delivered to the sample by metering from a 400 bar high-pressure hydrogen cylinder. Static NMR and in situ static NMR experiments were performed in a field of 7.04 T (27Al Larmor frequency 78.085 MHz) using effective 90° pulses (central transition only) of 3.5 μs (corresponding to a rotating frame rf field strength of 24 kHz), coadding spectra with a 1 s (typical) repetition time. In situ static NMR was performed in a home-built brass highpressure NMR probe with an internal radio frequency (rf) coil and an rf feedthrough. The probe was heated by a forced-air oven, with the samples contained in short glass vessels, all under 210 bar H2 pressure. All static NMR signals were acquired on a home-built spectrometer and processed as described elsewhere.16,17 Solid-state 27Al MAS NMR was performed at 20 T (221.4 MHz 27Al and 850 MHz 1H frequencies) on a Varian VNMRS spectrometer at Nijmegen using a 1.6 mm HXY MAS probe spinning at 35 kHz. 27Al single pulse excitation spectra were obtained using short rf pulses of 0.2 microseconds at an effective rf field strength of 100 kHz, giving a spin nutation angle of 7.2°. Single-phase (gated continuous wave) 1H decoupling at an rf field strength of 10 kHz (optimized for maximum decoupling effect) was applied. Typical experiments consisted of 1000 spectra added with a 10 s repetition time. Some 27Al MAS NMR measurements were obtained at 7.04 T (78.085 MHz 27Al and 299.7 MHz 1H frequencies) with hydrogen decoupling and occasionally at 13.85 T (153.7 MHz 27 Al) without decoupling. All 27Al spectra are referenced to 1 M Al(NO3)3 at 0 ppm. XRD reflections were measured using a Rigaku Ultima IV diffractometer with Cu-Kα radiation at 40 kV and 44 mA. The XRD pattern was obtained at room temperature using a 1 s
EXPERIMENTAL SECTION
Samples were prepared and maintained in 7 mm diameter tubes and stored in a nitrogen flow-through glovebag before and after measurements. NaAlH4 samples were obtained as “storage grade” NaAlH4 powder from Sigma Aldrich15 with a stated purity of 99.3% and “technical grade” with a stated purity of 95%. NaAlH4 melt-infiltrated in graphitic carbon was prepared by J. Gao of University of Utrecht; samples of Ti-doped and Scdoped NaAlH4 were prepared by E. Majzoub at University of 8106
dx.doi.org/10.1021/jp401134t | J. Phys. Chem. C 2013, 117, 8105−8113
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Article
Figure 2. 27Al NMR (static) of samples at 22 °C. (a) NaAlH4 sample mixed with Al2O3 and then P−T treated (220 °C and 210 bar H2 for 30 min). (b) NaAlH4 sample mixed with Al(OH)3 and then P−T treated.
At these air-exposure times, the alanate gains about 5% by mass from atmospheric absorption. Much longer exposure times eventually result in thorough degradation of the sample. Comparing the behavior of newly purchased NaAlH4 and that used in the earlier study,12 it appears that Ivancic’s reagent bottle had been air-exposed. Ivancic et al.12 reported the S105 resonance at variously 100−105 ppm chemical shift. The present careful study of the line position at 7.04 T finds this sharp S105 resonance at 101.6 ± 0.3 ppm. Exposure to the laboratory atmosphere subjects the alanate to a gas mixture that is approximately 80% N2 and 20% O2 with some water vapor (about 10 Torr, which is ∼1.3% H2O by volume). In order to understand the genesis of the mobile S105 species, samples of NaAlH4 were selectively exposed to individual atmospheric gases, either dry or after saturating the gas with water vapor close to 100% relative humidity. The samples were maintained in a closed gas-delivery system while the selected gas was flowed over the powdered, dispersed sample for 5 min. For water vapor saturation, the gas first bubbled through a fine glass frit submerged in deoxygenated water before delivery to the alanate. The sample was then returned to the dry nitrogen glovebag and prepared for P−T treatment. Each sample was heated to 220 °C under 210 bar of H2 pressure for 30 min, then cooled under pressure, and finally depressurized for room temperature NMR experiments. At 220 °C, the equilibrium hydrogen pressure of NaAlH4 (reaction 1) is about 170 bar;6 greater hydrogen pressures will thermodynamically block the dehydriding of sodium alanate. For all the gases tested (nitrogen, oxygen, helium, and carbon dioxide), the dry gas exposures did not result in the narrow 27Al S105 NMR signal in the alanate after the P−T treatment (hence, all their spectra are similar to Figure 1a), whereas water-saturated gases did result in the appearance of S105 after P−T treatment (i.e., similar to Figure 1b). Clearly, water vapor plays a key role here. The individual spectra for each wet/dry gas exposure experiment appear in Figure S4 of the Supporting Information. Role of Added Hydroxides. Water reacts with NaAlH4 to form hydroxides,21 suggesting that hydroxide species, and not water itself, may be involved in the generation of S105. To test this possibility, we prepared powder mixtures of pristine NaAlH4 with various hydroxides (Al(OH)3, Mg(OH)2, and NaOH) and oxides (Al2O3 and MgO). Each sample of storage grade NaAlH4 powder was mixed with one of the above powder hydroxide or oxide compounds in the mass ratio of 10 parts to
dwell time and a step size of 0.02 degrees (2θ). Samples were loaded onto XRD slides, covered with a mylar film, and transported under an argon atmosphere. Matched reference patterns were generated using the known I41/a and P21/c structures for NaAlH4, and Na3AlH6, respectively. NIST corundum Al2O3 was used as a lattice parameter standard. Hydrogen evolution (dehydriding) experiments were performed in a type 316 stainless steel vessel with a 42 cc volume. The resistively heated reactor was equipped with an MKS Baratron hydrogen pressure transducer to measure the pressure of evolved hydrogen gas.
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RESULTS AND DISCUSSION S105 was reported12 to be produced in the molten state (Tmelt = 183 °C) in undoped sodium alanate and at lower temperatures in doped (i.e., catalyzed with titanium or scandium compounds) solid sodium alanate. It was found to persist at room temperature and pressure in NaAlH4 that had been melted under sufficient hydrogen pressure (208 bar) to thermodynamically block the forward reactions 1 and 2.6 However, initial attempts by us and other groups14 to produce S105 in uncatalyzed (undoped, bare) sodium alanate samples (both Aldrich technical grade and storage grade) proved unsuccessful. In situ NMR measurements during heat treatment under H2 pressure (210 bar) going up to 220 °C showed the undoped sodium alanate melting around 183 °C as expected, and then returning to solid NaAlH4 as the temperature was lowered with no appearance of the expected S105 species (Figure 1a). The peak of the broad 27Al (spin 5/2) NMR resonance of the sodium alanate central transition appears in Figure 1a at 7.4 kHz (95 ppm) along with aluminum metal at 128 kHz, a 1640 ppm Knight shift; these are consistent with earlier NMR studies of these phases.18−20 The small shoulder at −3.3 kHz (−43.5 ppm) is a small amount of Na3AlH6. Formation of S105. Role of Water Vapor. Further investigation revealed that S105 can be reliably produced in undoped NaAlH4 that has been briefly exposed to ambient air prior to heating above the melting point under H2 pressure (210 bar; see Figure 1b). Short air-exposure times (
