Hydrogen absorption of Mg-Based composites prepared by mechanical milling: Factors affecting its characteristics

Journal

ELSEVIER

Journal of Alloys and Compounds

of

232 (1996) 218-223

Hydrogen absorption of Mg-Based composites prepared by mechanical milling: Factors affecting its characteristics Hayao Imamura, Department of Advanced

Nozomu Sakasai, Yasuyuki Kajii

Materials Science and Engineering, Received

Faculty of Engineering,

14 April

1995; in final form

Yamaguchi

University, 2557 Tokiwadai, Ube 755, Japan

12 May 1995

Abstract Hydriding and dehydriding properties of Mg-based composites (Mg-G and Mg-Pd-G) which are prepared by mechanical milling of magnesium powder and graphite (G) or graphite supporting 5 wt.% Pd (5 wt.% Pd-G) in the presence of various additives (tetrahydrofuran, benzene or cyclohexane) have been studied. Such composites were effective as hydrogen storage materials even under mild reaction conditions (500 Torr, 453 K). Factors affecting the hydriding characteristics of the composites were extensively investigated in connection with the preparative conditions (additives and their amounts, milling times, Mg:G component ratios, etc.). In particular, the presence of tetrahydrofuran in the milling process strongly affected the hydriding and dehydriding kinetics of the resulting composites. The active composites for hydrogen storage are those in which finely divided magnesium is in intimate contact with graphite. It is expected that their activity results from certain synergetic interactions between Mg and aromatic carbon atoms of graphite containing charge transfer to some extent. Keywords:

Magnesium;

Hydrogen

absorption;

Graphite;

Composite;

1. Introduction There is a considerable potential and intrinsic interest in magnesium and magnesium-based alloys as rechargeable hydrogen storage materials [l]. In earlier

publications [2-61, we have described hydriding aspects of magnesium-containing systems modified chemically with various organic compounds. This provides strong evidence that such chemical modifications constitutes a novel way of improving the sorption properties of the hydrogen storage materials containing magnesium. As one modification method, the treatment of Mg-containing alloys with polycondensed-ring compounds such as anthracene, chrysene or perylene leads to improved hydriding behaviour [2,3]. This is attributable to the formation of electron donor-acceptor complexes by charge transfer between the alloys and aromatic molecules. This type of complex is generally accepted to be active for the catalytic activation of hydrogen [7]. 09258388/96/$15.00 0 1996 Elsevier SSDI 0925-8388(95)01882-4

Science

S.A. All rights

reserved

Mechanical

milling;

Metal

hydride

The studies have recently extended to include the composites (Mg-G) prepared by mechanical milling of magnesium powder (Mg) and graphite (G) [8]. Graphite is characterized as having a polycondensedring structure whose carbon net is considered as an infinite array of aromatic molecules and accumulates vertically in the three-dimensional framework. If complexation between magnesium and aromatic carbon atoms of graphite similarly occurs during the mechanical milling, modification effects are expected, resulting in the occurrence of improved hydriding properties. We have demonstrated that the composites obtained by milling of magnesium and graphite in the presence of tetrahydrofuran (THF) exhibit excellent characteristics for hydrogen storage [8]. In this paper, to provide information as to the factors responsible for the characteristics of the composites, the effects of differences in additives used (THF, benzene and cyclohexane) and Mg:G component ratios on the hydriding and dehydriding properties are investigated.

H. Imamura et al. I Journal of Alloys and Compounds

The synergetic effect induced during the milling of magnesium and graphite is discussed.

2. Experimental

details

2.1. Materials

Magnesium (99.95%; Rare Metallic Co. Ltd.) was obtained in powder form (100 mesh). Anhydrous PdCl, (99.9% ) was commercially obtained from Rare Metallic Co. Ltd. and used without further purification. Graphite was spectroscopic powder (grade, PS-1) from the Union Carbide Co. THF benzene and cyclohexane were of research purity and further purified by distillation in the presence of sodium wire. 2.2. Sample preparation,

procedures

and analyses

Graphite-supported palladium (denoted by Pd-G) was prepared by impregnation of graphite with a palladium chloride aqueous solution to obtain a palladium loading of 5 wt.%. The impregnating slurry was vigorously stirred, dried, calcined at 623 K and finally reduced at 623 K with flowing hydrogen at a standard pressure. The preparative method of composites using a planetary-type ball mill was described in a previous study [8]. The magnesium and palladium content in the sample was determined by atomic absorption spectrophotometry (Shimazu AA 610) after dissolution of the composites in aqueous nitric acid solutions, and the fraction of graphite carbon was calculated from a mass balance. Hydrogen absorption measurements were made using glass volumetric equipment provided with a high vacuum system (about lo-" Torr). Prior to the measurements the sample was subjected to evacuation treatment at 453-623 K for 2 h, set at the reaction temperature (453 K) and then the absorption of hydrogen was initiated by admitting hydrogen gas at 500Torr. The changes in pressures of hydrogen on absorption were monitored with a baratron (MKS Instruments Inc, 127AA-OlOOOB). Carbon monoxide chemisorption was carried out with an ordinary volumetric technique at room temperature, the details of which have been described elsewhere [8]. Surface areas were obtained by the Brunauer-Emmett-Teller (BET) method using nitrogen adsorption at 77 K. Temperature-programmed desorption (TPD) experiments were carried out with a constant heating rate of 3 K min-’ to increase the mass spectrometer signal. The composite was brought into contact with hydrogen gas to absorb hydrogen at 453 K, followed by evacuating the gas phase to ca. lo-’ Torr at 273 K.

232 (1996) 218-223

219

The sample was heated in a programmed manner and the hydrogen desorbed was continuously monitored with a quadrupole mass spectrometer. UV spectra were recorded on a Shimazu UV 2200 spectrometer. The preparation of the sample was carried out as follows. Mg powder and anthracene were placed in a Schlenk tube flushed thoroughly with dry nitrogen, to which anhydrous THF (about 10 cm”) was added. The mixture was vigorously stirred at room temperature. X-ray diffraction (XRD) spectra were recorded on a Shimazu diffractometer (VG-107R) using Cu Ka radiation.

3. Results and discussion 3.1. General features of Mg-G composites

and Mg-Pd-G

XRD spectra obtained when magnesium (4.2 g) and graphite (1.8 g) were milled mechanically for O120 min are shown in Fig. 1. A representative diffraction peak of graphite corresponding to (002) planes

ZU

40

60

80

28 / degree Fig. 1. XRD spectra of Mg-G: (a) Mg as a starting material; (b) graphite; Mg-G after milling for (c) 10 min, (d) 20 min, (e) 60min and (f) 120min.

H. Imamura et al. I Journal of Alloys and Compounds

220

decreased in relative height with milling of Mg-G. This indicates that the graphite layer structure of a regular hexagonal net of carbon atoms is gradually destroyed with milling and almost decomposed after 1 h of mechanical milling. On milling, metallic magnesium existed in a finely divided state and, moreover, there were no XRD changes in lattice parameters of its hexagonal structure. The Mg diffraction peaks were broadened considerably with milling (Fig. 1); thus a high state of dispersion of magnesium in the Mg-G composites can be achieved by this technique. XRD measurements yielded crystallite size estimates for the magnesium in the composite after 1 h of milling at about 17-18nm, in comparison with 34nm of magnesium as a starting material. The mode of decomposition of the graphite structure was similar for the Mg-Pd-G composites obtained when graphite supporting 5 wt.% Pd (5 wt.% Pd-G) was used instead of graphite. The effects of THF in the milling process on the resulting composites, designated hereafter as (Mgwere quite significant [8]. G) THF and (Mg-Pd-G),,,, However, there were no great differences in XRD spectra irrespective of milling in the presence or absence of THF; thus the destruction of the graphite layer structure and the grinding effect of the magnesium particles occurred similarly (Fig. 3 of Ref. [S]). On the whole, THF tended to depress pulverization of magnesium in the composite. Mg-Pd-G prepared without THF yielded Mg particle sizes of 17-18 nm, while the addition of THF (6.0 cm’) resulted in a value of 27 nm. As shown in Table 1, the period of milling for (Mg-Pd-G),,, strongly affected the specific surface areas and CO chemisorption. Milling in the absence of THF did not influence the increase in the surface areas and CO chemisorption of Mg-G and Mg-Pd-G very much, but these values obtained for increased markedly with increased (Mg-Pd-G),,, period of milling (O-240 min) (Table 1). On milling in the presence of THF (6.0 cm”) for 4 h, the surface area of (Mg-Pd-G),,, (S-10) increased from initial values of 3.6 m2 g-’ to as much as 195.8 m2 g-‘. The CO

Table 1 Effects of the period

232 (1996) 218-223

chemisorption, by the amounts of which the number of active sites formed on the composites can be evaluated simultaneously, increased about 1000 times. The present composites exhibited relatively high thermostability, despite the ease of sintering for such small magnesium particles. XRD revealed negligible changes in the diffraction patterns of the composites even after thermal treatments at elevated temperatures (473-673 K). Thus the presence of graphitic carbon obviously results in an enhanced thermostability of the parent magnesium in the composites compared with the case of magnesium alone. We have demonstrated that magnesium is also used efficiently when deposited on an activated charcoal support [5,6]. Nor&s et al. [9] report that in Mg,PdC, prepared using synthetic techniques the residual carbon has a stabilizing effect on the structure. 3.2. Hydriding and dehydriding As shown in Fig. 2, the effects of additives in the milling process on the hydriding properties were also quite significant; milling without the additives led to negligible or very low hydriding activity. THF was the most effective of the additives so far studied and the activity tended to increase markedly with an increase in the amounts of THF added (O-6.0 cm3), the reaching a factor of about 140 (Fig. 3) [8]. Benzene and cyclohexane were better than nothing, but they were inferior to THF in hydriding kinetics. This is consistent with the characteristics of the Mg-Pd-G composites summarized in Table 2. The composites prepared in the presence of additives, in particular THF, exhibited increased surface areas (34.4-65.2 m2 g-l). Since the surface area of the composites (S-2) with the smallest Mg particle sizes (18 nm) was not as large as expected,

cyclohexane of milling

on the characteristics

of (Mg-Pd-

G),,,” Sample number

Milling (min)

S,,, (m’g-‘)

Mg crystallite (nm)

s-o s-1 S-8 S-6 s-10

0 10 20 60 240

3.6 10.1 18.5 36.4 195.8

34 30 28 27 23

size

CO chemisorption (*mol

g-‘)

1.5 1 4 544 1098

a The composites were prepared by mechanical milling of Mg (4.2-4.3 g) and 5 wt.% Pd-G (1.8-2.0 g) in the presence of THF (6.0 cm’).

4

6

8

10

time / hr Fig. 2. Effects of various additives on hydriding properties of MgPd-G: 0, S-2; A, S-05; 0, S-15; 0, S-5. The samples were evacuated at 453 K for 2 h, followed by admission of hydrogen at 500Torr at 453 K.

H. Imamura et al. I Journal of Alloys and Compounds

I

0_8i-

0

5

10

time/h

Fig. 3. Effects of amount of THF on hydrogen absorption by Mg-Pd-G: 0, 0 cm” (S-2); Q, 2.0 cm3 (S-4): 0, 4.5 cm’ (S-5); 0, 6.0 cm3 (S-6).

this increase in surface areas is considered to be largely due to significant destruction of the graphite layer or cleavage of each carbon layer of graphite rather than fine grinding of magnesium particles during the milling. However, as described above, the graphite structure was completely destroyed for S-2 and the composites (S-4, S-5 and S-6) prepared using THF in Table 2. Therefore, an interesting feature is that only carbon atoms of graphite decomposed in the presence of THF exhibited a significant increase in surface areas. The role of THF is not clear during the milling process, but it is inferred that the presence of THF strongly affects the mode of destruction of graphite. Only when THF was used as an additive did CO chemisorption also exhibit a marked increase. From the Mg particle sizes estimated by X-ray line broadening, the effect of grinding on the composite (S-2) without use of additives was rather pronounced (Table 2) which did not reflect the hydriding kinetics as shown in Fig. 3. It has been reported elsewhere that ball milling techniques applied to magnesium systems increased the hydrogen absorption rates [lo-121. Therefore, the mechanical milling in the presence of

Table 2 Effects of additives

on the characteristics

of Mg-Pd-G

Sample number

Additive

s-2 s-05 s-15 s-5 s-4 S-6

None Benzene (4.5) Cyclohexane (4.5) Tetrahydrofuran (4.5) Tetrahydrofuran (2.0) Tetrahydrofuran (6.0)

a The composites additives.

were

and amount

prepared

221

218-223

THF leads to speculations on the occurrence of certain synergetic effects between magnesium and graphite, containing charge transfer as shown in the following section. This is probably involved in the formation of active sites, which results in enhanced activity. It was found that the addition of THF in the milling process was effective for dehydriding as well as hydriding. To evaluate the dehydriding properties, TPD studies of hydrogen for (Mg-G& and (Mg-Pd=nF with various levels of THF added were carried G) out (Fig. 4). The temperature at which the desorption of hydrogen began was evidently shifted to lower sides with an increase in the amounts of THF in the composites and the desorption peak of hydrogen in the TPD spectra was found to appear at much lower temperatures as a function of THF addition. It seems that these results approximately correspond to the characteristics of the composites shown in Table 2. These results imply that surface reactions of a sequence of events involved in the sorption process are important in determining the hydriding and dehydriding kinetics of the present composites. It was found by comparison of TPD of (Mg-G)THF and that the surface reactions were more (Mg-Pd-G),,, accelerated by the presence of Pd dispersed on graphite carbon when 5 wt.% Pd-G was used instead of graphite. It is expected that the palladium metal dispersed on graphitic carbon catalytically dissociates the hydrogen bond and efficiently spills atomic hydrogen over to the magnesium phase as hydrogen acceptors to form the hydride. In activated-charcoal-supported palladium, the phenomena associated with hydrogen spillover and their catalytic consequences are known to be effectively operative [13]. The catalytic active phase for spillover effects is that in which highly dispersed Pd-G in intimate contact with finely divided magnesium is formed by the mechanical milling. 3.3. Active sites of Mg-G

composites

To obtain information as to the nature of active sites formed, the gravimetric ratios of magnesium and

composites”

(cm’)

by mechanical

232 (19%)

milling

s BET

Mg crystallite

(m’ 8-l)

(nm)

(umof

8.3 14.1 15.9 65.2 34.4 36.4

18 26 26 25 24 27

6 7 12 340 101 544

(1 h) of Mg (4.2-4.3

g) and 5 wt.%

Pd-G

size

(1.8-2.0

CO chemisorption

g) in the presence

g-l)

of various

222

H. Imamura et al. I Journal of Alloys and Compounds 232 (19%) 218-223

r

__--

3 473

513

553

473

593

temperature/K

Fig. 4. TPD of hydrogen for (a) (Mg-G),,, 6.0cm3 (----).

513

553

teaperature/

and (b) (Mg-Pd-G),,,

graphite in the composites were changed. Magnesium alone, as well as graphite, exhibited no activity toward the hydrogen absorption under mild conditions (5OOTorr, 458 K). As shown in Fig. 5, when magnesium and graphite were milled in the presence of THF (6.0cm3), the hydriding activity increased with Mg:G ratio and passed through a maximum at 7:3 with enhancement by over a factor of about 60. This obviously indicates that there is an optimum value for the Mg:G ratio in the formation of composites. Moreover, that CO chemisorption also reached a maximum around similar Mg:G ratios (Table 3) strongly suggests the generation of active sites in a certain component ratio between magnesium and graphitic carbon. We further examined this point using anthracene which has a polycondensed-ring structure similar to

time/hr

Fig. 5. Influences of Mg:G ratio on hydrogen absorption of (MgThe sample was evacuated at 623 K for 2 h, followed by G),,,. admission of hydrogen at 500 Torr at 453 K.

for THF additions of 0 cm3 (--),

Table 3 Characteristics ratios Mg:G ratio 0:lO 3:7 5:5 I:3 9:l lo:o

of Mg-G SBET

593 K

2.0cm’ (-),

composites”

4.5 cm? (--)

with different

and

component

(m” g-‘)

Mg crystallite size (nm)

CO chemisorption (pm01 g-‘)

2.2 51.0 69.1 36.4 22.3 0.1

32 29 26 26 34

_ 17 640 460 14

a The composites were prepared by mechanical milling (1 h) of Mg and graphite in the presence of THF (6.0cm3).

graphite. Magnesium and anthracene readily gave yellowish complexes in a high yield when mixed in a solution of THF at room temperature. As shown in Fig. 6, UV spectra of the product solution in THF exhibited a strong band based on charge transfer absorption around 405 nm. Experimental findings are described which various workers have cited as evidence to show that this type of charge transfer complex has a high ability for catalytic activation of hydrogen [7]. Therefore, the milling in the presence of THF might cause the occurrence of similar charge transfer between magnesium and aromatic carbon atoms of graphite, probably leading to enhanced hydriding activity. This implies that aromatic carbon atoms derived from graphite during the mechanical milling in the presence of THF chemically modify finely divided magnesium in THF medium, which is an important factor determining the hydriding ability of the composites. That benzene and cyclohexane were not effective is probably because the dielectric constant of these organic solvents is too low to form such charge transfer complexes compared with that of THF.

H. Imamura et al. I Journal of Alloys and Compounds

232 (19%)

218-223

223

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0.6 -

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oW~~nthr~~$~~s~~ 500

600

1111B. Tanguy, J.L. Soubeyroux,

wavelengthinm

Fig. -,

UV spectra of Mg-anthracene Mg/anthracene.

6.

in THF: ---,

anthracene;

Less-Common

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