J. Phys. Chem. C 2009, 113, 6845–6851
6845
Hydrogen Absorption/Desorption Mechanism in Potassium Alanate (KAlH4) and Enhancement by TiCl3 Doping Jose R. Ares,*,† Kondo-Francois Aguey-Zinsou,‡ Fabrice Leardini,† Isabel Jı´menez Ferrer,† Jose-Francisco Fernandez,† Zheng-Xiao Guo,‡ and Carlos Sa´nchez† Laboratorio de Materiales de Intere´s Ene´rgetico, Departamento de Fı´sica de Materiales C-IV, Facultad de Ciencias, UniVersidad Auto´noma de Madrid, Espan˜a, and Department of Chemistry, UniVersity College London, WC1H 0AJ, London, U.K. ReceiVed: August 12, 2008; ReVised Manuscript ReceiVed: February 17, 2009
Thermodynamic and kinetic properties of potassium alanate (KAlH4) are investigated. Its pressure-compositionisotherm measurement exhibits two plateaus for hydrogen absorption/desorption in KAlH4, with gravimetric hydrogen densities of 1.2 ( 0.1 and 2.6 ( 0.2 mass% and reaction enthalpies of 81 and 70 kJ · mol-1 H2, respectively. However, the nonisothermal decomposition of KAlH4 occurs through three endothermic events at temperatures of 294, 311, and 347 °C with the release of hydrogen. Whereas the high temperature event is clearly attributed to K3AlH6 decomposition, the low temperature events occur by two reactions, denoting the existence of an intermediate phase during KAlH4 decomposition. FTIR measurements suggest that this intermediate phase is a KyAlHx compound (y g 1, x g 4) with a high coordination about the aluminum. TiCl3-doped KAlH4 also exhibits three decomposition events, but with significant reduction of desorption temperatures (∼50 °C) as well as activation energies that is attributed to particle size reduction and creation of charged vacancies. 1. Introduction The past decade has witnessed strong efforts in the development of materials to store hydrogen for vehicular applications.1 The main problem encountered for the development of such materials is the difficulty of meeting all the practical requirements of high volumetric and gravimetric storage capacities, moderate pressures and temperatures for hydrogen absorption/ desorption, and fast hydrogen absorption/desorption kinetics. Among the wide range of potential materials for hydrogen storage, i.e., light metal hydrides,2,3 imides,4 etc., compounds based on AlH4- are very attractive because of their high hydrogen content and low decomposition temperatures.5 In particular, sodium alanate (NaAlH4) has been at the center of extensive studies since its reversibility at moderate temperatures and pressures can be achieved through the addition of titanium compounds.6 To date, investigations on alanates have mainly focused on understanding the role of the additives and how reversibility was achieved,7-9 with the aim of obtaining similar improvements with other alanates of higher hydrogen capacities such as LiAlH4.10 However, despite strong research efforts, reversibility in other alanates of a high hydrogen capacity has not been achieved so far. This is partly due to the fact that some mechanistic aspects of the reactions of alanates with hydrogen and the role of the additives remain unclear. Whereas some studies suggest that reversibility is achieved by the destabilization of alanates (formation of vacancies in the crystal lattice due to the additive11 or by the formation of intermediate compounds12) other investigations claim that it is just an influence of the hydrogen kinetics of alanates.13 Recently, A. Borschulte et al.14 have noted the importance of an intermediate species and its vacancy-mediated diffusion by H/D exchange experiments. In this context, investigation of the mechanism * Corresponding author. Tel: 0034 (0) 914974777; fax: 0034 (0) 914978579; e-mail:
[email protected]. † Universidad Autonoma de Madrid. ‡ University College London.
behind the reversibility in potassium aluminum hydride (KAlH4) should contribute to the further understanding of the overall reversibility issues in alanates of high hydrogen capacities. The main advantage of KAlH4 over sodium and lithium alanates are its reversible hydrogen sorption at low pressures (350 °C) compared to other alkaline alanates could prevent the additive influence on the desorption mechanism by the removal of defects. Finally, because no values of Ea for doped KAlH4 have been previously reported in the literature, it is interesting to compare the values obtained for KAlH4 with those reported for a well investigated alanate such as NaAlH4.39 First, as shown in Table 2, activation energies are into the same range, suggesting a similar rate-limiting step in all alanates. However, whereas doping does not affect to activation energies of the decomposition reactions of LiAlH4 compared to those of nondoped samples, NaAlH4 and KAlH4 exhibit a decrease of Ea of ∼40 kJ · mol-1 and ∼30-70 kJ · mol-1, respectively, regarding the first decomposition step. For the second decomposition step, a decrease of ∼20 kJ · mol-1 and ∼30 kJ · mol-1, respectively, is observed. Despite the need for further analysis, this trend can be related to the expected strong influence of vacancies on species diffusion and, subsequently, on Ea. That influence should be more drastic in KAlH4 or NaAlH4 than in LiAlH4 because of greater cation size.
Ares et al. 4. Conclusions The thermolysis of KAlH4 involves two endothermic events at low temperatures (290-330 °C) accompanied with the release of 2.7 mass% of hydrogen and a third endothermic event at higher temperatures (330-350 °C) releasing 1.1 mass% of hydrogen. The two endothermic events are related to the decomposition of KAlH4 into K3AlH6 and the formation of an unknown intermediate phase, which further decomposes into KH. According to in situ FTIR results, the unknown phase is an intermediate compound KyAlHx with an increased coordination about the aluminum atom. Further analysis of the thermodynamic properties of KAlH4 shows that the isothermal decomposition/formation of KAlH4 involves only two equilibrium plateaus pressures of 0.09 and 0.8 MPa at 355 °C, with enthalpies of 81 and 70 kJ · mol-1 H2, respectively. The presence of two equilibrium plateaus, and not three, suggests that the unknown phase is not stable within the range of pressures and temperatures used. Moreover, the activation energies of the events occurring during the decomposition of KAlH4 are higher than those related to NaAlH4 and LiAlH4, supporting an influence of metal cation size on the hydrogen desorption kinetics. Addition of TiCl3 to KAlH4 favors a drastic reduction of the decomposition temperatures of KAlH4 which benefits its utilization as a H-storage material at moderate temperatures in the near future. However, TiCl3 does not affect the decomposition temperature of K3AlH6. This result contrasts with previous observations for alanates such as NaAlH4 for which TiCl3 doping enhances the thermolysis of Na3AlH6. This dissimilarity may be attributed to the high temperatures involved in the thermolysis of K3AlH6 which precludes the formation of defects. Acknowledgment. The authors acknowledge the Ramo´n y Cajal Spanish Program (RYC-2005-001054), MEyC (MAT200506738-C02-01) and the UK EPSRC (EP/E040071/1 and EP/ E046193/1) for financial support. Supporting Information Available: Details of the influence of temperature on the vibration peak of AlH6-3 (Figure S1) and DTA-TG analysis at different heating rates to be used in the Kissinger equation (Figure S2). SEM and BSE micrographs of the as-prepared KAlH4 and the milled KAlH4 (Figure S3). Rietveld analysis of the X-ray pattern of the doped KAlH4 (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353. (2) Schu¨th, F.; Bogdanovic, B.; Felderhoff, M. Chem. Commun. 2004, 2249. (3) Aguey-Zinsou, K.-F.; Yao, J.; Guo, Z. X. J. Phys. Chem. B 2007, 111 (43), 12531. (4) Ping, C.; Xiong, Z.; Luo, Z.; Lin, J.; Tan, K. L. Nature 2002, 420, 302. (5) Orimo, S. I.; Nakamori, Y.; Eliseo, J. R.; Zu¨ttel, A.; Jensen, C. M. Chem. ReV. 2007, 107 (10), 4111. (6) Bogdanovic, B.; Schwickardi, M. J. Alloys Compd. 1997, 253254, 1. (7) Anton, D. L. J. Alloys Compd. 2003, 356-357, 400. (8) Brinks, H. W.; Hauback, B. C.; Srinivasan, S. S.; Jensen, C. M. J. Phys. Chem. B 2005, 109, 15780. (9) Ping, W.; Xiang, D. K.; Cheng, H. M. ChemPhysChem 2005, 6, 2488. (10) Ares, J. R.; Aguey- Zinsou, F.-K.; Porcu, M.; Dornheim, M.; Sykes, J.; Klassen, T.; Bormann, R. Mater. Res. Bull. 2008, 43 (5), 1263. (11) Sun, D.; Kiyobayashi, T.; Takeshita, H. T.; Kuriyama, N.; Jensen, C. M. J. Alloys Compd. 2002, 337, L8. (12) Bellosta, J. M.; Schmidt, W.; Felderhoff, M.; Bogdanovic, B.; Schu¨th, F. Angew. Chem. 2006, 45, 3663.
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