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Nov 9, 2015 - Taihei Aoki†, Hiroki Miyaoka‡, Hitoshi Inokawa‡, Takayuki Ichikawa§∥, and Yoshitsugu Kojima†∥. †Graduate School of Advanc...
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Activation on Ammonia Absorbing Reaction for Magnesium Chloride Taihei Aoki,† Hiroki Miyaoka,‡ Hitoshi Inokawa,‡ Takayuki Ichikawa,*,§,∥ and Yoshitsugu Kojima†,∥ †

Graduate School of Advanced Sciences of Matter, ‡Institute for Sustainable Sciences and Development, and ∥Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8530 Japan § Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima, 739-8521 Japan S Supporting Information *

ABSTRACT: Hexa-ammine complex (Mg(NH3)6Cl2) is directly formed at room temperature by the reaction between magnesium chloride (MgCl2) and ammonia (NH3) without formation of mono- and diammine complexes (Mg(NH3)Cl2 and Mg(NH3)2Cl2) even though these are more stable phases. The high kinetic barrier exists for the formation of low coordinated ammine complexes. The activation by using heat treatment and ball milling is carried out, and then the NH3 absorption properties are investigated to understand kinetic properties of the ammine complexes formation of MgCl2. At 373 K, the formation of Mg(NH3)2Cl2 is realized. Furthermore, it is found that a higher temperature than 573 K is required to form Mg(NH3)Cl2. Interestingly, the ball milled MgCl2 can transform into low coordinated ammine complexes even at room temperature, indicating that the structural disorder state generated by the ball milling induces the formation of the above phases with low activation energy. Therefore, it is expected that the kinetic barrier to form ammine complexes of MgCl2 is strongly related to the process on the structural change.

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INTRODUCTION Ammonia (NH3) has attracted much interest recently as one of the materials having significantly high hydrogen capacity because its hydrogen capacity is 17.8 wt %.1 However, in order to use NH3 compactly as hydrogen carrier at 293 K, it should be liquefied by compression to more than 0.85 MPa. From the viewpoint of safe utilization, the vapor pressure of liquid NH3 is relatively high.2−5 Moreover, the influence of NH3 poisoning is of concern for NH3 applications such as polymer electrolyte membrane (PEM) fuel cells even though it is a small amount, below a few parts per million. In this case, the NH3 concentration should be kept to less than 0.1 ppm level before introducing hydrogen into the fuel cell because the anode catalyst and the acidic membrane are damaged by NH3.6 Series of halides and complex hydrides absorb several moles of NH3 by forming the solid phase of ammine complex of which volumetric densities are almost comparable with that of liquid NH3.7 Then, a vapor pressure of NH3 can be controlled effectively.8 Particularly, for PEM fuel cells, the NH3 absorbing materials could be useful to reduce the NH3 concentration in feed gas. So far, the NH3 absorption properties of MgCl2 have been reported by several researchers because of its high gravimetric hydrogen density and/or low vapor pressure. Moreover, NH3 desorption properties of hexa-ammine complex of MgCl2 (Mg(NH3)6Cl2) were also studied by thermal analysis and proceeded in the following steps:7,9−11 Mg(NH3)6 Cl 2 → Mg(NH3)2 Cl 2 + 4NH3

(1)

Mg(NH3)2 Cl 2 → Mg(NH3)Cl 2 + NH3

(2)

© 2015 American Chemical Society

Mg(NH3)Cl 2 → MgCl2 + NH3

(3)

A hysteresis behavior between NH3 absorption and desorption of halides was reported by Trudel et al. There are several possible reasons for the hysteresis.12 As one of the reasons, an activation barrier for the formation of ammine complex of halides exists due to large volume expansion and atomic rearrangement. The ammine complex of MgCl2 drastically expands during its formation process, and then the absorption reaction is prevented due to the volume expansion in a limited space.10 Actually, the volume expansion ratios of Mg(NH3)2Cl2 and Mg(NH3)6Cl2 compared with MgCl2 can be estimated from the densities of MgCl2 (2.35 g/cm3), Mg(NH3)2Cl2 (1.70 g/cm3), and Mg(NH3)6Cl2 (1.24 g/cm3) to be 1.9 and 3.9, respectively.13 In fact, after the NH3 desorption from Mg(NH3)6Cl2, the “skeletal aggregate structure” of Mg(NH3)6Cl2 remains with narrow pores,13 suggesting that the apparent volume of the sample is preserved after the NH3 desorption. By this effect, the activation barrier to expand the material volume decreases after Mg(NH3)6Cl2 desorbs NH3. The kinetics of the ammine complex formation in other chloride also have been reported, suggesting that strontium chloride has the surface absorption state before the bulk absorption state.14 As mentioned above, the kinetic modification is important to control the NH3 sorption properties of MgCl2. Received: August 15, 2015 Revised: November 7, 2015 Published: November 9, 2015 26296

DOI: 10.1021/acs.jpcc.5b07965 J. Phys. Chem. C 2015, 119, 26296−26302

Article

The Journal of Physical Chemistry C In this work, the NH3 absorption and desorption properties of pristine and activated MgCl2 were investigated in further detail at different temperatures to discuss the kinetic properties of the ammine complex formation of MgCl2.

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EXPERIMENTAL SECTION Commercial magnesium chloride (MgCl2, anhydrous, beads, 10 mesh, purity: 99.9%, Aldrich) was initially grinded for 10 min and then used for the experiments. As activation of MgCl2, treatments of NH3 ab/desorption and mechanical ball milling were performed. For the NH3 treatment, NH3 is first absorbed into MgCl2 at room temperature under 0.8 MPa of NH3 for 2 h to form Mg(NH3)6Cl2, and then NH3 was desorbed from Mg(NH3)6Cl2 to form MgCl2 by keeping vacuum condition at 723 K for 2 h. For the mechanical treatment, the ball milling was carried out by planetary type apparatus (Fritsch, P7) for a total of 10 h with repeating 30 min milling and 15 min rest, in which 20 zirconia balls with 8 mm diameter and 200 mg MgCl2 were put into the specially designed milling vessel made of high-carbon high-chromium steel (SKD-11) with inner volume of 30 cm3 under Ar atmosphere. In addition, the coordinated number of ammine complex for each activated MgCl2 was evaluated by thermal NH3 gas desorption measurement. For an investigation on the kinetic properties of the activated MgCl2, the samples for this evaluation were prepared by absorbing 1 mol NH3 into 1 mol MgCl2 at 293, 373, and 573 K. In order to eliminate the influence of water absorption, all the samples were handled in a glovebox (Miwa, MDB-2BL) filled with purified argon gas. The NH3 absorption properties of the pristine and activated MgCl2 were investigated by volumetric technique, which is pressure−composition (PC) isothermal measurement (BEL JAPAN, Inc., BELSORP-max) at several temperatures. The pressure was monitored every 5 min. An equilibrium pressure is determined when the pressure variation is within 0.3% in each 5 min period. Unless the conditions for the determination are satisfied, the waiting time is extended. To characterize the state of ammine complexes of MgCl2, thermal NH3 gas desorption behavior was examined by thermogravimetry-mass spectroscopy (TG-MS) (TG: Rigaku, TG8120, MS: Anelva, MQA200TS). In this measurement, Ar gas was flowed as a carrier gas, and heating rate was fixed at 5 K/min. Powder X-ray diffraction (XRD) (Rint-2500 V, Rigaku, Cu Kα radiation) measurement was performed at an ambient condition to identify the solid phase of the prepared MgCl2 samples and the products after NH3 absorption where these samples were covered by a film (Du Pont-Toray Co., Ltd., Kapton) to prevent oxidation of the samples during the measurements.

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Figure 1. PC isotherm for NH3 absorption of MgCl2 after NH3 treatment (□) and pristine MgCl2 (○) at 293 K.

desorption treatment has a significant effect as the activation for the NH3 absorption process. As mentioned above, the NH3 treatment improves the kinetics of NH3 absorption due to preserving the apparent volume of Mg(NH3)6Cl2 after the NH3 desorption. It has been reported that NH3 desorption of Mg(NH3)6Cl2 progresses in three steps;7,10 however, the NH3 absorption behavior of MgCl2 at 293 K is not stepwise. It is considered that this plateau region up to 6 mol NH3 indicates the direct formation of Mg(NH3)6Cl2. In order to clarify the reaction process on the NH3 absorption up to 6 mol, the XRD and TG-MS measurements were performed after absorbing 6 and 1 mol of NH3 at 293 K for MgCl2 after the NH3 treatment. Figures 2a and b show the XRD patterns of MgCl2 without and with the NH3 treatment, respectively. The NH3 treated MgCl2 had the diffraction peak broadened slightly, indicating that the NH3 treatments decrease the crystalline size and induce the crystalline strain. Here, by using the Scherrer equation, the crystalline sizes of treated MgCl2 are decreased by 60−70% compared with the pristine sample. The formation of Mg(NH3)6Cl2 by the reaction of pristine MgCl2 with 6 mol of NH3 without any other phases was confirmed as shown in Figure 2c. On the other hand, the diffraction peaks of solid products by the reaction with 1 mol NH3 were totally assigned to MgCl2 and Mg(NH3)6Cl2 as well, as shown in Figure 2d, indicating that Mg(NH3)6Cl2 is directly formed at 293 K not via mono- and diammine complexes even after the activation, although these low coordinated complexes are more stable thermodynamically than Mg(NH3)6Cl2.15,16 Thus, it is expected that the formation of Mg(NH3)Cl2 and Mg(NH3)2Cl2 is kinetically difficult at 293 K. Figure 3a and 3b shows result of TG-MS for solid products by the reactions of MgCl2 with 6 and 1 mol of NH3 at 293 K, which correspond to the samples shown in Figure 2c and d, respectively. Figure 3a shows the NH3 desorption profile of Mg(NH3)6Cl2, indicating that the weight loss in the lower temperature region around 380 K corresponds to 4 mol of NH3 desorption, and the weight loss in higher temperature region around 530 K corresponds to 2 mol of NH3 desorption. Here, these results are almost consistent with previous reports.7,10 The reaction equations of these steps can be expressed by eqs 1, 2, and 3. It is expected that the NH3 desorption around 530 K includes the NH3 desorption from Mg(NH3)2Cl2 and Mg(NH3)Cl2, which are overlapped due to kinetic reasons. In fact, the NH3 desorption from Mg(NH3)6Cl2 is clearly

RESULTS AND DISCUSSION

Figure 1 shows results of NH3 PC isotherm of MgCl2 after the activation treatment by NH3 absorption and desorption at 293 K. In the inset figure, the results of pristine and MgCl2 after the NH3 treatment are compared. The plateau region appeared around 200 Pa and continued until 6 mol in x-axis, which corresponds to the amount of absorbed NH3 per 1 mol MgCl2. Because the PC isotherms were performed under the condition of a certain waiting time for both samples, a pressure of plateau region should strongly be affected by the sample condition. Therefore, the plateau pressure corresponding to the sample after NH3 treatment was clearly lower than that of pristine MgCl2 as shown in the inset, suggesting that the NH3 ab/ 26297

DOI: 10.1021/acs.jpcc.5b07965 J. Phys. Chem. C 2015, 119, 26296−26302

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The Journal of Physical Chemistry C

Mg(NH3)6Cl2. Namely, Mg(NH3)6Cl2 desorbs NH3 with three steps even though it was directly formed in the absorption process. As mentioned above, mono- and diammine complex of MgCl2 is thermodynamically stable compared with hexaammine complex. However, these complexes were not formed by absorbing NH3 of MgCl2 at the above reaction conditions due to its reaction kinetics. Therefore, the NH3 absorption treatments were carried out at various temperature ranges from 293 to 573 K, in which the different thermal activation conditions were produced. Figures 2e and f also show results of XRD measurements for the solid products obtained by the reaction with 1 mol of NH3 at 373 and 573 K, respectively. After the reaction at 293 K, major and minor phases were, respectively, MgCl2 and Mg(NH3)6Cl2 in XRD pattern as shown in Figure 2d. After the reaction at 373 K, the main phase was changed to Mg(NH3)2Cl2 as shown in Figure 2e. Furthermore, unknown peaks were observed in the XRD pattern shown in Figure 2f. Although the XRD pattern of Mg(NH3)Cl2 is not reported so far, it is expected that the unknown peaks originate in Mg(NH3)Cl2 as discussed later. From the XRD patterns, it was found that Mg(NH3)6Cl2 has better symmetry of the structure, which is the same level as MgCl2 itself. On the other hand, the structural symmetry is clearly lowered in the order of Mg(NH3)2Cl2 and unidentified phase, which could correspond to Mg(NH3)Cl2, because of the appearance of many extra peaks. From the structural point of view, MgCl2 is the most isotropic in the three materials because Mg atoms are surrounded by six equivalent chlorine (Cl) atoms in the octahedral shape. On the other hand, ammine complexs have only four equivalenet Cl atoms. To characterize the atomic arrangement of the materials, the distance of the nearest 18 Mg atoms from a fixed Mg atom were examined by geometric analyses using reported crystal structures.17,18 On the basis of the analyses, Mg atoms in MgCl2 are classified as two equivalent Mg species, resulting in high symmetrical structure. In the case of Mg(NH3)6Cl2, Mg atoms are also classified as two kinds of species. However, in the case of Mg(NH3)2Cl2, the classified number of Mg atoms is four, suggesting that structural symmetry is lower than the other two materials. These results are consistent with the results of the XRD measurements. From the results of NH3 absorption properties and structural analyses, it is expected that the formation of Mg(NH3)2Cl2 from MgCl2 has a high kinetic barrier due to the complicated rearrangement of atoms to form low symmetric crystal structure. As a result, the formation of Mg(NH3)6Cl2 is prior to the Mg(NH3)2Cl2 formation around room temperature. Here, for Mg(NH3)Cl2, it is difficult to discuss the kinetic properties in further detail from a structural point of view because the crystal structure is not reported yet. The XRD results indicate that crystal structure of Mg(NH3)Cl2 has the lowest symmetry among all the materials, leading to worse kinetics to form Mg(NH3)Cl2 phase at room temperature like the case of Mg(NH3)2Cl2. At higher temperature, atomic fluctuation by the thermal activation would decrease kinetic barrier, and then the mono- and diammine complexes can be formed before the Mg(NH3)6Cl2 formation. The results obtained in this work indicated that the thermal activation was effective to decrease the kinetic barrier related to structural changes due to the formation of the mono- and diammine complexes. Results of TG-MS for ammine complexes of MgCl2 are also shown in Figures 3b, c, and d, which correspond to the

Figure 2. XRD patterns of (a) pristine MgCl2, (b) MgCl2 after NH3 treatment, (c) a product by the reaction of MgCl2 after NH3 treatment with NH3 (mole ratio is 1:6) at 293 K, (d), (e), and (f) a product by the reaction of MgCl2 after NH3 treatment with NH3 (mole ratio is 1:1) at 293, 373, and 573 K, respectively.

Figure 3. NH3 desorption profiles of solid products by the reactions of (a) MgCl2 with 6 equiv of NH3 at 293 K, and MgCl2 after NH3 treatment with 1 mol equiv of NH3 at (b) 293, (c) 373, and (d) 573 K.

separated to three steps under NH3 pressure (Figure S1), suggesting that the NH3 desorption is faster under inert gas flow condition and the third NH3 desorption is more sensitive to the partial pressure of NH3 gas than the second one. Figure 3b shows the NH3 desorption profile of the product by the reaction with 1 mol NH3, which is similar to that of 26298

DOI: 10.1021/acs.jpcc.5b07965 J. Phys. Chem. C 2015, 119, 26296−26302

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The Journal of Physical Chemistry C

isothermal condition. The higher measurement temperature led to the higher plateau pressure, and then the plateau pressures at 293, 308, and 323 K were about 0.2, 0.8, and 3.4 kPa, respectively. These plateaus were flatly continued beyond 1 mol of NH3 absorption, indicating that the NH3 absorption behavior in the temperature range would originate in the direct formation of Mg(NH3)6Cl2. Figure 4b shows the PC isotherms measured at 501, 549, and 572 K, which are chosen to evaluate the thermodynamic properties of the Mg(NH3)Cl2 formation. Here, the solid line in Figure 4b is the expected plateau pressures of the Mg(NH3)6Cl2 formation estimated from the results shown in Figure 4a. The plateau pressures at 501, 549, and 572 K obtained in Figure 4b were around 2.5, 8.7, and 16.9 kPa, respectively, which are clearly lower than those of the Mg(NH3)6Cl2 formation. It was noted that the pressure was vertically increased after 1 mol of NH3 absorption. Therefore, these results indicate that Mg(NH3)Cl2 can be generated by the heat activation around 500−572 K. In addition, the PC isothermal measurement at 373 K was performed to observe the plateau of Mg(NH3)2Cl2 formation (Figure S2); however, the clear plateau was not obtained. If the behaviors would be caused by poor kinetics, the optimization of reaction conditions and improvement of kinetics would be necessary to evaluate the thermodynamic properties of the Mg(NH3)2Cl2 formation. From the observed plateau pressure (Peq) and the experimental temperature (T), the standard enthalpy change (ΔH0) and entropy change (ΔS0) for the corresponding reaction can be evaluated by following van’t Hoff equation:

products measured by XRD in Figures 2d, e, and f, respectively. The ammine complex formed at 293 K desorbed NH3 stepwise as shown in Figure 3b. On the other hand, the ammine complexes formed at 373 and 573 K did not desorb NH3 in the low temperature region below 400 K and desorbed NH3 in only the high temperature region above 450 K, which included the NH3 desorption from Mg(NH3)2Cl2 and Mg(NH3)Cl2 as mentioned above. On the basis of thermal analyses shown in Figure 3 (see also Figure S1), the NH3 desorption from the ammine complex of MgCl2 is roughly classified by the temperature region. The reactions 1, 2, and 3 seem to proceed at 293−400, 400−500, and more than 500 K, respectively, although reactions 2 and 3 are difficult to be separated due to the thermodynamic reason. The ammine complex formed at 373 K desorbed NH3 from 400 K, and the MS profile revealed a small shoulder around 520 K. Thus, the main NH3 desorption in the temperature range from 400 to 500 K would be caused by reaction 2. Reaction 3 continuously proceeds at higher temperature and forms the shoulder. The NH3 desorption of the ammine complex formed by 573 K was located at more than 500 K, indicating that Mg(NH3)Cl2 was generated by the NH3 absorption at 573 K. Therefore, the heat activation is effective for MgCl2 to form mono- or diammine complexes. In order to evaluate thermodynamic parameters, the PC isotherms were measured at various temperatures, and the results are shown in Figure 4. In the temperature range from 293 to 323 K, it is expected on the basis of the above results that the hexa-ammine complex is directly formed in our PC

ln(Peq /P 0) = ΔH 0/RT − ΔS 0/R

(5) 0

Figure 5 shows van’t Hoff plot, ln(Peq/P ) vs 1000/RT. The slope and intercept on the vertical axis give the values of ΔH0

Figure 5. Van’t Hoff plot of plateau pressure for NH3 absorption of MgCl2.

and ΔS0, respectively. As a result, ΔH0 and ΔS0 corresponding to the direct formation of Mg(NH3)6Cl2 and Mg(NH3)Cl2 from MgCl2 were evaluated to be ΔH0 = −58 ± 6 kJ/mol, ΔS0 = −150 ± 20 J/mol K, and ΔH0 = −64 ± 1kJ/mol, ΔS0 = −97 ± 2 J/mol K, respectively. Actually, monoammine complex Mg(NH3)Cl2 forms low symmetric crystal structure due to high configuration number of NH3 occupation. On the other hand, hexa-ammine complex Mg(NH3)6Cl2 forms high symmetry in the crystal structure, suggesting that the motion of NH3 molecules is limited by the occupation of the others. Therefore, smaller entropy change corresponding to monoammine complex should be quite reasonable. For Mg(ND3)6Cl2, Sørby et al. reported that the ND3 molecules in ammine complex phase has highly correlated rotational and translational motions.19 Thus, it is expected that the degree-of-freedom of

Figure 4. (a) PC isotherm for NH3 absorption of MgCl2 after NH3 treatment at 293, 308, and 323 K; (b) PC isotherm for NH3 absorption of MgCl2 after NH3 treatment at 501, 549, and 572 K. The limitation of pressure for measurements by this apparatus is shown by the dotted line. The solid line is the expected plateau pressure of the reaction of Mg(NH3)6Cl2 formation estimated by the result shown in (a). 26299

DOI: 10.1021/acs.jpcc.5b07965 J. Phys. Chem. C 2015, 119, 26296−26302

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The Journal of Physical Chemistry C NH3 molecule in Mg(NH3)Cl2 is extremely high. Form the ΔS0 obtained in this work, the entropy of solid at standard conditions of Mg(NH3)6Cl2 and Mg(NH3)Cl2 were estimated to be S0 = 352 and 185 J/mol K, respectively (Table S1),20 where the S0 per NH3 of Mg(NH3)6Cl2 and Mg(NH3)Cl2 was described as 59 and 185 J/mol K, respectivaly. It is noteworthy that the S0 value of Mg(NH3)Cl2 is close to that of gaseous NH3, indicating that the motion of NH3 molecules is quite active even in solid phase. The results of experiments and their analyses are very consistent with the above speculation. Moreover, thermodynamics properties of the reaction between various chlorides including MgCl2 and NH3 were frequently referred to the literature written by P. Neveu et al.,21 which show the values of ΔH0 and ΔS0 corresponding to the Mg(NH3)6Cl2 and Mg(NH3)Cl2, as the estimataed values by the equilibrium pressure and under certain temperature to be ΔH0 = −56 kJ/mol, ΔS0= −231 J/mol K, and ΔH0 = −87 kJ/ mol, ΔS0 = −231 J/mol K. Compared with them, the absolute value of ΔS0 of both mono- and hexa-ammine complex determined in this study is very low. With respect to ΔH0, the values of hexa-ammine complexes obtained in this work and shown in Neveu’s work are close each other. On the other hand, those of monoammine complexes are largely different. The difference of ΔH0 would originate in kinetic reason, which should strongly affect the measurement for monoammine complex because it was demonstrated that the heat activation is necessary to form Mg(NH3)Cl2. When the material has kinetic problems, the apparent hysteresis between the absorption and desorption is often shown as mentioned in Figure 1. However, the thermodynamic parameters determined form the absorption processes are well matched to the results estimated by the desorption process under 0.1 MPa of NH3 by Sorensen et al., indicating that the kinetics barrier was decreased enough by the NH3 treatment in this work. Figure 6 separately shows the NH3 PC isotherms of MgCl2 activated by the NH3 treatment and the mechanical treatment at 293 K (upper), and the NH3 PC isotherms of MgCl2 after the mechanical treatment at several temperatures (lower). At 293 K, MgCl2 after the mechanical treatment gradually absorbed NH3 from lower pressure than that of the NH3 treated one, indicating that the ammine complexes with lower equilibrium pressure were possibly formed. However, the profile of MgCl2 after the mechanical treatment shaped into the slope, and the plateau region was not clearly observed. It was obvious that the milling process was able to activate the NH3 absorption of MgCl2 as well, but it had different effect from the activation process using the NH3 treatment. With respect to the measurement of MgCl2 after mechanical treatment at several temperatures, the sloped profiles were observed at the measurement temperatures 297 and 373 K. In contrast, the clear plateau region appeared, and ammonia pressure increased vertically from 1.0 mol/mol (x-axis) in the profile resulting from the measurement at 573 K, indicating the formation of Mg(NH3)Cl2. The flat plateau pressure and PC isothermal curve observed at 573 K were almost the same as that observed in the PC isotherms of MgCl2 after the NH3 treatment at 572 K shown in Figure 4b, suggesting that the activated state of MgCl2 by the NH3 treatment and the mechanical treatment is similar at 573 K. Figure 7 shows results of XRD measurements for MgCl2 after the mechanical treatment and a solid product obtained by the reaction with 1 mol of NH3 at 293, 373, and 573 K for MgCl2 after the mechanical treatment. The diffraction peaks

Figure 6. PC isotherm for NH3 absorption of MgCl2 after NH3 treatment (open symbol) and after mechanical treatment (closed symbol) at 293 K (upper), and for MgCl2 after mechanical treatment at 293, 373, and 573 K (lower).

Figure 7. XRD patterns of (a) MgCl2 after mechanical treatment, (b) a product by the reaction of MgCl2 after mechanical treatment with NH3 (mole ratio is 1:1) at 293 K, (c) a product by the same reaction of (b) at 373 K, and (d) a product by the same reaction of (b) and (c) at 573 K.

corresponding to MgCl2 were clearly broadened and weakened compared with the pristine MgCl2 as shown in Figure 2a. In Figure 7b, it was difficult to identify the observed phase because of the unclear diffraction peaks due to the milling and the similarity of XRD patterns for MgCl2 and Mg(NH3)6Cl2. It is 26300

DOI: 10.1021/acs.jpcc.5b07965 J. Phys. Chem. C 2015, 119, 26296−26302

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The Journal of Physical Chemistry C found that the Mg(NH3)2Cl2 was formed by the reaction at 373 K from the XRD pattern shown in Figure 7c. After the reaction at 573 K, unknown diffraction peaks were observed in the XRD profile of the product obtained at 573 K as shown in Figure 7d, where the observed pattern was similar to Figure 2f and would be assigned to Mg(NH3)Cl2. Here, the peak shape of the only product by the reaction at 573 K is sharpened compared with those at lower temperature, and it is almost the same as the peaks observed in the case of MgCl2 after the NH3 treatment. These results are consistent with the similarity of PC isothermal curves. Figure 8 shows results of TG-MS for MgCl2 after the mechanical treatment and the reaction with NH3, which

absorption reaction. The formation of both ammine complexes proceeds at 373 K in the slope region of the NH3 PC isotherm shown in Figure 6. The product by the reaction of MgCl2 after the mechanical treatment and reaction with NH3 at 573 K revealed a similar NH3 desorption profile with a peak around 550 K to that generated by the reaction of MgCl2 after the NH3 treatment, resulting in Mg(NH3)Cl2 formation. Although two peaks around 550 K were found in the profile, we concluded that two peaks should originate in a kinetic effect because any NH3 desorption was confirmed below 500 K. Therefore, the mechanical treatment improves the kinetics of the low coordinated ammine complexes formation and is more effective than the NH3 treatment. The NH3 desorption region determined by the TG-MS measurements and corresponding ammine complex formed in each NH3 absorption condition are listed in Table 1. Generally, the mechanical energy applied by Table 1. List of the NH3 Absorption Conditions and the NH3 Desorption Profiles Shown in Figures 3 and 8a NH3 absorption condition activation method

temp (K)

NH3 desorption region

corresponding ammine complex

NH3 treatment

293 373 573 293

A and B B C A, B, and C

373 573

B and C C

Mg(NH3)6Cl2 Mg(NH3)2Cl2 Mg(NH3)Cl2 Mg(NH3)6Cl2, Mg(NH3)nCl2 (n ≤ 2) Mg(NH3)2Cl2, Mg(NH3)Cl2 Mg(NH3)Cl2

mechanical treatment

a

NH3 desorption profile

A, below 400 K; B, 400−500 K; and C, above 500 K.

the ball milling affects the structural properties of solid material such as crystalline size and strain; namely, the lattice disorder is introduced into the crystal structure. As described above, the activation barrier for the ammine complex formation of MgCl2 is strongly related to the structural changes. Therefore, the obtained experimental results suggest that the structural disorder state induced by the ball milling decreases the activation barrier and accelerates the reaction speed. At lower temperature than 373 K, the disordered state generated by the ball milling is effective to form mono- and diammine complexes, where the formation processes of these ammine complexes for the higher crystalline samples would have high activation energy due to the atomic rearrangement as discussed above. With respect to the similarity between the NH3 absorption properties of the NH3 and mechanical treated MgCl2 at 573 K, it is expected that the crystallinity is recovered by the annealing effect at 573 K, and the milling effects would be weakened or lost. As a result, the effect of only thermal activation remains, and then clear plateau region appears.

Figure 8. NH3 desorption profiles of solid products by the reactions of MgCl2 after mechanical treatment with a molar equivalent of NH3 at 293, 373, and 573 K.

correspond to the products measured by XRD in Figure 7b, c, and d. The product obtained at 293 K desorbed NH3 by three steps and revealed the low temperature desorption around 360 K, suggesting that the hexa-ammine complex was formed at 293 K. Here, if only Mg(NH3)6Cl2 was selectively generated like the reaction as discussed above, the weight losses below and above 400 K, which correspond to reactions 1, and 2, and 3, respectively, should be in a 2:1 ratio as shown in Figure 3. In the case of the product at 293 K, however, the weight losses ratio of the lower and higher temperature desorption was 1:2, suggesting that the mono- and/or diammine as well as hexaammine complexes were simultaneously generated in the reaction at 293 K. This result is consistent with the result of the PC isotherm of MgCl2 after the mechanical treatment shown in Figure 6. Namely, in the slope region of the PC isotherm, it is expected that the ammine complexes with different composition were independently formed. The MS profile of the product at 373 K showed the NH3 desorption with two peaks. The NH3 desorption around 450 and 550 K would originate in reactions 2 and 3 because the observed temperature ranges are well matched. The weight loss at higher temperature was slightly larger than that at lower temperature, indicating that Mg(NH3)Cl2 was also formed with the formation of Mg(NH3)2Cl2 as a main phase during the NH3

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CONCLUSION The activation on the reaction of ammine complex formation of MgCl2 was investigated. It was found that the formation of Mg(NH3)6Cl2 was kinetically prior to the mono- and diammine complexes at room temperature in spite of the thermodynamically unstable phase. The direct formation of Mg(NH3)6Cl2 would be caused by the high kinetic barrier for the formation of Mg(NH3)2Cl2 and Mg(NH3)Cl2. In fact, the thermal activation realized the formation of these ammine complexes. By using the NH3 treated MgCl2, thermodynamic parameters can be evaluated to be ΔH0 = −58 ± 6 kJ/mol, ΔS0= −150 ± 20 J/ 26301

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The Journal of Physical Chemistry C mol K, and ΔH0 = −64 ± 1 kJ/mol, ΔS0 = −97 ± 2 J/mol K, for Mg(NH3)6Cl2 and Mg(NH3)Cl2 formation, respectively. Furthermore, it was noteworthy that the mechanical treatment by ball milling enhanced the thermal activation effects, resulting in the formation of low coordinated ammine complexes even at 293 K. Therefore, it is expected that the high activation barrier is related to the structural properties of the ammine complexes because the ball milling induces the formation of disordered state such as lattice distortion.

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(10) Bevers, E. R. T.; Oonk, H. A. J.; Haije, W. G.; van Ekeren, P. J. Investigation of Thermodynamic Properties of Magnesium Chloride Amines by HPDSC and TG. J. Therm. Anal. Calorim. 2007, 90, 923− 929. (11) Jones, M. O.; Royse, D. M.; Edwards, P. P.; David, W. I. F. The Structure and Desorption Properties of the Ammines of the Group Π Halides. Chem. Phys. 2013, 427, 38−43. (12) Trudel, J.; Hosatte, S.; Ternan, M. Solid−Gas Equilibrium in Chemical Heat Pumps: The NH3−COCl2 System. Appl. Therm. Eng. 1999, 19, 495−511. (13) Jacobsen, H. S.; Hansen, H. A.; Andreasen, J. W.; Shi, Q.; Andreasen, A.; Feidenhans’l, R.; Nielsen, M. M.; Stahl, K.; Vegge, T. Nanoscale Structural Characterization of Mg(NH3)6Cl2 during NH3 Desorption: An in Situ Small Angle X-ray Scattering Study. Chem. Phys. Lett. 2007, 441, 255−260. (14) Ammitzboll, A. L.; Lysgaard, S.; Klukowska, A.; Vegge, T.; Quaade, U. J. Surface Adsorption in Strontium Chloride Ammines. J. Chem. Phys. 2013, 138, 164701−1−6. (15) Hart, A. B.; Partington, J. R. Dissociation Pressures of Compounds of Ammonia and Trideuterammonia with Some Metallic Salts. J. Chem. Soc. 1943, 104−118. (16) Liu, C. Y.; Aika, K. Ammonia Absorption on Alkaline Earth Halides as Ammonia Separation and Storage Procedure. Bull. Chem. Soc. Jpn. 2004, 77, 123−131. (17) Partin, D. E.; O’Keeffe, M. The Structures and Crystal Chemistry of Magnesium Chloride and Cadmium Chloride. J. Solid State Chem. 1991, 95, 176−183. (18) Tekin, A.; Hummelshøj, J. S.; Jacobsen, H. S.; Sveinbjornsson, D.; Blanchard, D.; Nørskov, J. K.; Vegge, T. Ammonia Dynamics in Magnesium Ammine from DFT and Neutron Scattering. Energy Environ. Sci. 2010, 3, 448−456. (19) Sørby, M. H.; Løvvik, O. M.; Tsubota, M.; Ichikawa, T.; Kojima, Y.; Hauback, B. C. Crystal Structure and Dynamics of Mg(ND3)6Cl2. Phys. Chem. Chem. Phys. 2011, 13, 7644−7648. (20) Chase, M. W., Jr. NIST−JANAF Themochemical Tables. J. Phys. Chem. Ref. Data 1998, 4, 1−1951. (21) Neveu, P.; Castaing, J. Solid-Gas Chemical Heat-Pumps - Field of Application and Performance of the Internal Heat of Reaction Recovery Process. Heat Recovery Syst. CHP 1993, 13, 233−251.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07965. Table of thermodynamics properties of ammine complex and figures of NH3 desorption profiles of Mg(NH3)6Cl2 under NH3 pressure, PC isotherm for NH3 absorption of two kinds designed NH3, and PC isotherms for NH3 absorption of MgCl2 after mechanical treatment at 293, 373, and 573 K (PDF)

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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 This work was partially supported by Council for Science, Technology and Innovation(CSTI), Cross-Ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST).

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b07965 J. Phys. Chem. C 2015, 119, 26296−26302