Thermodynamics on Ammonia Absorption of Metal Halides and

Jul 28, 2014 - ABSTRACT: Ammonia absorption properties of metal halides and borohydrides were systematically investigated by pressure−composition...
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Thermodynamics on Ammonia Absorption of Metal Halides and Borohydrides Taihei Aoki,† Takayuki Ichikawa,*,‡ Hiroki Miyaoka,§ and Yoshitsugu Kojima‡ †

Graduate School of Advanced Sciences of Matter, ‡Institute for Advanced Materials Research, and §Institute for Sustainable Sciences and Development, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8530, Japan S Supporting Information *

ABSTRACT: Ammonia absorption properties of metal halides and borohydrides were systematically investigated by pressure−composition isothermal measurements. By systematical research, we clarified that the plateau pressure is strongly correlated with the electronegativity of cation or anion in the metal halides or borohydrides. The material with higher electronegativity of the cation showed lower plateau pressure on the ammonia absorption. For the anion, the opposite trend between the electronegativity and plateau pressure was found. With respect to sodium borohydride, the changes of standard enthalpy and entropy corresponding to the ammonia absorption were estimated to be −29 kJ/mol and −98 J/(mol K).

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INTRODUCTION Ammonia (NH3) is a promising hydrogen carrier because it reveals higher gravimetric density of hydrogen, 17.8 wt %, than any other chemical hydrides and it can be transported via worldwide infrastructures.1 However, in order to obtain a high volumetric density at 293 K, NH3 should be liquefied by compression of more than 0.85 MPa. The vapor pressure is high from the viewpoint to safely utilize the liquid NH3.2−4 In addition, some NH3 leakages are of concern for any NH3 application. In the case of polymer electrolyte membrane (PEM) fuel cell, NH3 has to be removed before the feed gas, in which H2 is generated from NH3, enters the fuel cell because NH3 poisons the anode catalyst and the acidic membrane of the fuel cell.5 Some kinds of halides or complex hydrides absorb large amount of NH3 in the solid phase to form ammine complex, and then the volumetric hydrogen densities of ammine complex are almost comparable with that of liquid NH3.6,7 Thus, by use of the above materials, NH3 can be stored, and therefore, its vapor pressure can be controlled effectively and safely. Furthermore, in the case of the H2 utilization to PEM fuel cell, the NH3 absorbing materials could be useful to reduce the NH3 concentration in feed gas as well. Although NH3 desorption properties of ammine complex of metal halides were reviewed in 1943 by Hart,8 these systematical studies concerning the ammine complex were carried out by using a manometer to measure ammonia vapor pressure of ammine complex. A manometer is only able to measure a narrow range of pressure. Ammonia absorption properties of alkaline earth halides into their ammine complexes have been reported with respect to the plateau pressure of ammonia absorption up to 80 kPa.9 Thermodynamic properties of ammine complex of metal © 2014 American Chemical Society

borohydrides have been reported as hydrogen storage materials.10,11 However, there are few reports on ammonia absorption properties of metal borohydrides to form their ammine complexes.12 The reactions to form the ammine complexes after the NH3 absorption into metal halides and metal borohydrides are shown in the following eqs 1 and 2, respectively. MX m + n NH3 ⇆ M(NH3)n X m

(1)

M(BH4)m + n NH3 ⇆ M(NH3)n (BH4)m

(2)

In this work, we systematically investigate the ammonia absorption properties of various kinds of metal halides and borohydrides by volumetric technique. The NH3 pressure from 0.001 to 0.8 MPa is measured by using a pressure sensor. By systematical research for various kinds of ammine complexes, the correlation between plateau pressure of ammine complex and electronegativity of cation or anion of metal halides or borohydrides is discussed to obtain the guidelines for designing NH3 storage materials for safe practical use.

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EXPERIMENTAL SECTION As ammonia absorbing materials, nine kinds of metal halides and five kinds of metal borohydrides were prepared, which are lithium fluoride (LiF, 99.98%, Aldrich), lithium chloride (LiCl, 99.998%, Aldrich), lithium bromide (LiBr, 99.995%, Kojundo Chemical Laboratory), sodium chloride (NaCl, 99.9%, Aldrich), sodium iodide (NaI, 99.999%, Aldrich), calcium Received: May 20, 2014 Revised: July 12, 2014 Published: July 28, 2014 18412

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fluoride (CaF2, 99.99%, Aldrich), calcium chloride (CaCl2, 99.99%, Aldrich), nickel chloride (NiCl2, 99.99%, Aldrich), nickel bromide (NiBr2, 98%, Aldrich), lithium borohydride (LiBH4, 95%, Aldrich), sodium borohydride (NaBH4, 98%, Aldrich), potassium borohydride (KBH4, 99.9%, Aldrich), magnesium brohydride (Mg(BH4)2, 95%, Aldrich), and calcium borohydride (Ca(BH4)2, Aldrich). These samples were handled in a glovebox (Miwa, MDB-2BL) filled with purified argon gas in order to avoid water absorption. The NH3 absorption properties of metal halides and borohydrides were evaluated by pressure−composition (PC) isothermal measurements. Schematic image of a PC isothermal apparatus is shown in Figure 1. This apparatus consists of a

Figure 2. PC isotherms for NH3 absorption of LiF (○), LiCl (□), LiBr (△), NaCl (▽), NaI (●), CaF2 (■), CaCl2 (▲), NiCl2 (▼), and NiBr2 (☆) at 293 K.

Figure 1. Schematic image of PCI measurement system.

pressure gauge (Druck, DPI280), water bath (EYELA, NTB221), NH3 cylinder (NH3 purity, 99.999%), buffer, and sample holder. Samples with 1 mmol were set into the sample holder, and its temperature was kept at 293 K by water bath. NH3 absorption properties were measured in the pressure range from 0.001 to 0.8 MPa. The lower limit of 0.001 MPa is due to the measurement range of the pressure gauge, and the higher limit of 0.8 MPa is due to the liquefied pressure of NH3 at 293 K. When the pressure value did not change in 5 min, the system was judged to be in an equilibrium state. The amount of absorbed NH3 was evaluated by using ammonia density at each temperature and pressure, which is referred from NIST (National Institute of Standards and Technology) database.13 Powder X-ray diffraction (Rint-2500V, Rigaku, Cu Kα radiation) measurement was carried out at room temperature to identify the solid phase before and after the PC isothermal measurements.

Figure 3. PC isotherms for NH3 absorption of LiBH4 (○), NaBH4 (□), KBH4 (△) Mg(BH4)2 (●), and Ca(BH4)2 (■) at 293 K.

conditions. LiCl showed a similar pressure variation to the above materials at low pressure region. However, with increase in the pressure, the plateau area appeared at 0.178 MPa, and NH3 was absorbed up to 4 mol. This plateau feature in the PC isotherm indicates the reaction to form the ammine complex phase of LiCl(NH3)4. After this phase was generated, the NH3 absorption was stopped and its pressure increased immediately to 0.8 MPa. In the case of LiBr, the NH3 pressure was not changed from 0.001 MPa until 2 mol of NH3 was introduced, suggesting that the introduced NH3 is absorbed into LiBr, and the equilibrium pressure between LiBr and the ammine complex phase is below 0.001 MPa (out of detectable range in this experiment). NaI started NH3 absorption from 0.055 MPa, and the absorption amount was gradually increased up to 5 mol at this plateau-like feature. After the plateau-like region, the NH3 pressure was increased with relatively high slope and reached up to 0.325 MPa, and then the NH3 pressure was gradually increased with increasing NH3 amount introduced up to 9 mol in total. Any anomalous phenomenon is not found for all the other halide samples except NaI. CaCl2 exhibited the longest plateau area at 0.030 MPa, which is corresponding to

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RESULTS AND DISCUSSION The amount of absorbed NH3 and the corresponding plateau pressure were determined by the PC isothermal measurements for the various kinds of samples as mentioned in the Experimental Section. The PC isotherms corresponding to the halides and borhydrides measured at 293 K are shown in Figures 2 and 3. In these figures, NH3 pressures are plotted as a function of the amount of absorbed NH3 for 1 mol of halides or borohydrides (mol/mol halides or borohydrides). In the cases of LiF, NaCl, and CaF2, the equilibrium pressure vertically increased and immediately reached 0.8 MPa, indicating that NH3 is not absorbed into these materials under the above 18413

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Table 1. Ammonia Absorption Amount (n) and Equilibrium Pressure at Plateau (Peq) of All Samples in This Work at 293 K reaction

n (moles of NH3/moles of material)

LiF + nNH3 ⇆ LiF(NH3)n LiCl + 4NH3 ⇆ LiCl(NH3)4 LiBr + 2NH3 ⇆ LiBr(NH3)2 CaF2 + nNH3 ⇆ CaF2(NH3)n CaCl2 + 8NH3 ⇆ CaCl2(NH3)8 NaI + 5NH3 ⇆ NaI(NH3)5 NiCl2 + 6NH3 ⇆ NiCl2(NH3)6 NiBr2 + 6NH3 ⇆ NiBr2(NH3)6 NaCl + nNH3 ⇆ NaCl(NH3)n KBr + nNH3 ⇆ KBr(NH3) n MgF2 + nNH3 ⇆ MgF2(NH3)n LiBH4 + 3NH3 ⇆ LiBH4(NH3)3 NaBH4 + 2NH3 ⇆ NaBH4(NH3)2 KBH4 + nNH3 ⇆ KBH4(NH3)n Ca(BH4)2 + 5NH3 ⇆ Ca(BH4)2(NH3)2 MgBH4 + 5NH3 ⇆ MgBH4(NH3)5 a

4 2 8 5 6 6

3 2 5 5

Peq (MPa)

xp differencea

>0.800 0.178 0.800 0.030 0.055 0.800 >0.800 0.800 K, 0.82, Mg, 1.31 > Ca, 1.00 > Ba, 0.89, and F, 3.98 > Cl, 3.16 > Br, 2.96 > I, 2.66. For a series of lithium halides that are compounds of Li+ and different halogen anions the plateau pressures of LiF, LiCl, and LiBr are more than 0.8 (not absorbed), 0.178, and less than 0.001 MPa, respectively. Thus, the plateau pressure increases with increasing χp value of the corresponding anion. Furthermore, this tendency is also found in the series of Na and Ca halides as follows, Peq(LiF) > Peq(LiCl) > Peq(LiBr), Peq(CaF2) > Peq(CaCl2), and Peq(NaCl) > Peq(NaI). In a series of metal borohydrides that are composed of different cations and BH4− anion the plateau pressure of NaBH4 was 0.09 MPa, being higher than that of LiBH4, Mg(BH4)2, and Ca(BH4)2. In addition, KBH4 showed no NH3 absorption, suggesting that the plateau pressure is thought to be higher than 0.8 MPa. Therefore, the tendency of Peq is expressed as follows, Peq(KBH4) > Peq(NaBH4) > Peq(LiBH4), Peq(Mg(BH4)2), and Peq(Ca(BH4)2), indicating that the plateau pressure increases with the decreasing χp value of according cation. And the results for ammonia PC isotherms of all materials are summarized in Table 1. In this table, the amount of absorbed NH3 (n), and the value of plateau pressure (Peq) at 293 K are shown. The plateau pressures of NH3 absorption for LiCl, LiBr, CaCl2, and NaI are plotted as a function of the value of χp difference between cation and anion in Figure 4. Here, the numerical data are shown in Table 2. The plateau area of LiBr appeared at 0.03 MPa in the measurement at 323 K, although the plateau pressure was out of measurement range at 293 K. Plateau pressure corresponding to BaBr2 was estimated from a database8 and added in the figure. Closed symbols and open symbol show the results at 293 and 323 K, respectively. A dotted line located at 0.001 MPa shows the lowest limit of experimental pressure range in this work. The highest pressure limit, above 0.8 MPa, is shown by a broken line. It is found that the plotted plateau pressure of halides has a linear relation to the increase in the χp difference at both temperatures. However, NaI did not follow the trend, indicating that the plateau pressure would be related to material properties as well as χp or kinetic properties might strongly affect the PC isothermal properties in this experimental condition. In fact, only NaI was liquefied above 0.325 MPa among the examined halides,

the NH3 absorption of 8 mol. NiCl2 and NiBr2 absorbed 6 mol of NH3 below 0.001 MPa. Furthermore, for the other metal halides, KBr and MgF2, the NH3 absorption phenomena were not observed, where these results are omitted because the profiles are the same as those of LiF, NaCl, and CaF2. Ammonia absorption profiles at 293 K of metal borohydrides, LiBH4, NaBH4, KBH4, Mg(BH4)2, and Ca(BH4)2, are shown in Figure 3. LiBH4 absorbed about 3 mol of NH3 below 0.001 MPa, and then the NH3 pressure suddenly increased with a little increase of NH3 amount. Moreover, the second plateaulike feature appeared at 0.62 MPa. Thus, LiBH4 ammine complex has a characteristic behavior indicating continuous change of the state. It has been reported that LiBH4 itself, LiBH4(NH3), and LiBH4(NH3)3 are solid phases, but the intermediate LiBH4(NH3)2 phase is in liquid phase.10 Therefore, the complicated reaction process with NH3 absorption was observed in this pressure range. Actually, the plateau regions below 0.001 and at 0.62 MPa correspond to solid phases, and the slope phenomenon should be due to the liquid phase. In addition, the plateau pressure of NaBH4 was observed around 0.090 MPa, and the corresponding amount of absorbed NH3 was 2 mol. After that, the NH3 pressure was linearly increased with increasing NH3 amount like NaI and LiBH4. In this pressure region, it was confirmed that solid NaBH4 ammine complex could be changed to liquid phase like liquid ammonia solution of NaBH4 by using a glass vessel (Hyper glass cylinder, TAIATSU TECHNO). This phenomenon would be similar to deliquescence of salt and water systems such as well-known NaOH−H2O, MgCl2−H2O, and so on. One of these kinds of systems, the NaCl−H2O system, is also known. In that system, water uptake occurs on the NaCl surface, and then deliquescence continuously proceeds.13 In other words, this liquefaction phenomenon can be understood as a phenomenon of boiling point elevation when a solvent dissolves a solute. It was expected that the characteristic PC isotherms of LiBH4 and NaI were also originated in such liquefied phenomenon. On the other hand, KBH4 showed no NH3 absorption. Mg(BH4)2 and Ca(BH4)2 absorbed 5 mol of NH3 by similar profile to each other, and the plateau pressure was below 0.001 MPa. From the viewpoint of materials chemistry, the thermodynamic properties of ammine complex formation are discussed as follows. It is well-known that the Pauling electronegativities (χp) of alkali, 18414

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Figure 4. Value of equilibrium pressure at plateau of metal halides as a function of the value of Pauling electronegativity difference. Ammonia absorption properties of LiCl (●, ○), CaCl2 (■, □), BaBr2a) (◆, ◇), LiBr (△), and NaI (▼, ▽) were measured at 293 K (closed symbol) and 323 K (open symbol). Estimations were from a database.8

Table 2. Equilibrium Pressure at Plateau (Peq) of the Reaction at 293 or 323 K reaction

T (K)

Peq (MPa)

xp differencea

LiCl + 4NH3 ⇆ LiCl(NH3)4 LiBr + 2NH3 ⇆ LiBr(NH3)2 NaI + 5NH3 ⇆ NaI(NH3)5 CaCl2 + 8NH3 ⇆ CaCl2(NH3)8 BaBr2 + 8NH3 ⇆ BaBr2(NH3)8 b BaBr2 + 8NH3 ⇆ BaBr2(NH3)8 b

323 323 323 323 293 323

0.640 0.030 0.252 0.164 0.057 0.057

2.18 1.98 1.73 1.25 2.07 2.07

Figure 5. (a) PC isotherms for NH3 absorption of NaBH4 measured at 323 K (○), 293 K (●), 273 K (□), and 263 K (■). (b) van’t Hoff plot of plateau pressure for NH3 absorption of NaBH4.

a The value of electronegativity (xp) difference between cation and anion. bEstimated from database.8

⎛ P ⎞ ΔH 0 ΔS 0 ln⎜ 0 ⎟ = − ⎝P ⎠ RT R

Figure 5b shows the van’t Hoff plot (ln(Peq/P0) vs T−1). The slope and intercept on vertical axis give the values of ΔH0 and ΔS0, respectively. As a result, ΔH0 and ΔS0 were evaluated to be −29 kJ/mol and −98 J/(mol K) for the NH3 absorption process. The values of ΔH0 and ΔS0 of NaBH4 are almost the same as these of liquefaction of NH3 (ΔH0 = −23 kJ/mol, ΔS0 = −97 J/(mol K)). These similar values indicate that NaBH4(NH3)2 and liquid NH3 have close vapor pressures. It was noteworthy that the value of entropy change ΔS0 was almost the same as that of liquefaction of NH3 itself, indicating that a degree of freedom corresponding to the NH3 molecule in the ammine complex NaBH4(NH3)2 is similar to that of liquefied NH3 although NaBH4 (NH 3) 2 is solid phase. Considering the continuous liquefaction with further NH3 absorption as mentioned above, the S0 value with high degree of freedom corresponding to NH3 would be expected even in the solid phase. The results are consistent with the above discussion about NH3 absorption properties of NaI. In addition, XRD profiles of NaBH4 before and after PC isothermal measurement at 293 K reveal that both samples were totally assigned to NaBH4 (Figure S1 in Supporting Information). After the PCI measurement, the sample holder is evacuated to remove the NH3 gas and analyze the product. Thus, this result indicated that the NH3 absorption and desorption processes do not cause generation of any byproducts, suggesting that the NH3 absorption into NaBH4 can be controlled at 293 K by only the pressure.

suggesting that NaI has characteristic thermodynamic properties different from those of other halides. In fact, the standard entropy change of NaI ammine complex (161 J/mol) is about 30 J/(mol K) higher than that of other monovalent metal halides, LiCl and LiBr, ammine complex (133 and 136 J/(mol K)), where the standard entropy of ammine complex of NaI and LiBr was estimated from the database.8 The high standard entropy might be related to the liquefaction behavior. Here, it is expected that ammine complex state of LiBH4 and NaBH4, which also showed continuous liquefaction, might have high standard entropy. From the above results, it is clarified that the plateau pressure of halides with simple NH3 absorption phenomena would be expected by χp difference, although the basic material properties as solid phase such as crystal structure and internal atomic mobility should be considered to totally understand the essential mechanism of the NH3 absorption phenomena for halides and hydrides. As discussed above, NaBH4 has moderate properties as NH3 absorption material because the plateau pressure is close to ambient conditions. Thus, the thermodynamic properties of NaBH4 were investigated as a representative system. In Figure 5 a, the PC isotherms of NaBH4 at different temperatures are shown. Plateau pressures were located on 0.267, 0.090, 0.044, and 0.022 MPa at 323, 293, 273, and 263 K, respectively. From the plateau pressure (Peq) and the experimental temperature (T), the enthalpy change (ΔH0) and the entropy change (ΔS0) for the corresponding reaction can be evaluated by following the van’t Hoff equation, 18415

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(8) Hart, A. B.; Partington, J. R. Dissociation pressures of compounds of ammonia and trideuterammonia with some metallic salts. J. Chem. Soc. 1943, 104−118. (9) Liu, C. Y.; Aika, K. Ammonia absorption into alkaline earth metal halide mixtures as an ammonia storage material. Ind. Eng. Chem. Res. 2004, 43, 7484−7491. (10) Guo, Y.; Xia, G.; Zhu, Y.; Gao, L.; Yu, X. Hydrogen release from amminelithium borohydride, LiBH4·NH3. Chem. Commun. 2010, 46, 2599−2601. (11) Guo, Y.; Jiang, Y.; Xia, G.; Yu, X. Ammine aluminium borohydrides: an appealing system releasing over 12 wt% pure H2 under moderate temperature. Chem. Commun. 2012, 48, 4408−4410. (12) Sullivan, E. A.; Johnson, S. The lithium borohydride−ammonia system: pressure−composition−temperature relationships and densities. J. Phys. Chem. 1959, 63, 233−238. (13) Ewing, G. E. H2O on NaCl: From single molecule, to clusters, to monolayer, to thin film, to deliquescence. Struct. Bonding 2005, 116, 1−25.

CONCLUSION In this work, the NH3 absorption properties of various kinds of metal hydrides or borohydrides were systematically investigated by PC isotherm measurements in the pressure range from 0.001 to 0.8 MPa at 293 K. The correlation between the plateau pressure and the electronegativity of cation or anion in the metal halides or borohydrides was clarified. The materials with higher electronegativity for the cation and anion showed, respectively, lower and higher equilibrium pressure on the NH3 absorption. Therefore, the material with smaller difference of electronegativity between cation and anion exhibited a much lower equilibrium pressure. By use of this correlation, the ammine complex with suitable vapor pressure in accordance with requirements of applications can be chosen. From the PCIs obtained at different temperatures, thermodynamic parameters for the NH3 absorption of NaBH4 were evaluated. Results were that ΔH0 was −29 kJ/ mol and ΔS0 was −98 J/(mol K), which are close to the values of NH3 liquefaction, indicating that NaBH4(NH3)2 possesses high S0, although NaBH4(NH3)2 is still in the solid phase.

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ASSOCIATED CONTENT

* Supporting Information S

Figure S1 of XRD profiles of NaBH4 before and after PC isothermal measurement at 293 K. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-82-424-5744. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work is partially supported by Energy Carrier Project (Ammonia Utilization Team) in Advanced Low Carbon Technology Research and Development Program (ALCA) of Japan Science and Technology Agency (JST).

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