Hydrogen trapping: Synergetic effects of inorganic additives with cobalt sulfide absorbers and reactivity of cobalt polysulfide

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Hydrogen trapping: Synergetic effects of inorganic additives with cobalt sulfide absorbers and reactivity of cobalt polysulfide David Chartier a,*, Christophe Joussot-Dubien a, Catherine Pighini b, Elisabeth Sciora b, Frederic Bouyer b a b

Commissariat a` l’E´nergie Atomique, CEA, DEN, BP 17171, F-30207 Bagnols-sur-Ce`ze Cedex, France Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 5209 CNRS-Universite´ de Bourgogne, BP 47870, F-21078 Dijon Cedex, France

article info

abstract

Article history:

The biphasic product CoS2 þ Co(OH)2 obtained by oxidation of cobalt sulfide is known to

Received 26 April 2012

trap hydrogen at room temperature and low pressure according to a balanced reduction

Received in revised form

equation. Adding various inorganic compounds to this original absorber induces their

13 June 2012

reduction by hydrogen in the same conditions at a significant rate: (i) excess cobalt

Accepted 1 July 2012

hydroxide is reduced to metallic cobalt; (ii) nitrate ions are reduced to ammonia; (iii) sulfur

Available online 24 July 2012

and sodium thiosulfate are reduced to H2S or NaHS and Na2S, respectively. Without

Keywords:

synergetic effects involving H2 and the hydrogen absorber. Amorphous cobalt polysulfide,

Hydrogen

CoS5, is also reduced by hydrogen at room temperature and releases H2S gas. In the

a hydrogen absorber these inorganic compounds are not reduced by H2, suggesting

Cobalt sulfide

presence of a base to neutralize H2S gas, the reaction rate is initially slower than with the

Cobalt polysulfide

CoS2 þ Co(OH)2 mixture due to the higher stability of polysulfide chains but the H2 trapping

Hydrogen sulfide

yield is improved, making CoS5 a good candidate for H2 trapping.

Hydrogen trapping

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Several recent articles have addressed the mechanism of hydrogen trapping at room temperature and low pressure by cobalt sulfide prepared by aqueous precipitation using Na2S and a Co(II) salt [1e4]. According to our previous work [1], the hydrogen absorber is obtained after partial oxidation of the primary CoS precipitate to give an equimolar mixture of Co(OH)2 and CoS2. This binary system is also described as cobalt oxysulfide, CoSOH, in the literature [2e5]. H2 trapping is a two-step mechanism involving (i) the reduction of this solid mixture at room temperature and low pressure to form amorphous cobalt sulfide CoS and the oxidation of H2 to H2O,

and (ii) the reduction of CoS to crystallized Co9S8 and the oxidation of H2 to H2S. The total amount of hydrogen trapped by this absorber is slightly more than half the total amount of cobalt. One of the possible applications of this solid mixture is to trap hydrogen arising from radiolysis of organic matter in nuclear wastes, especially in the bitumen matrix used to immobilize aqueous effluent decontamination residues [6]. This novel hydrogen absorber was patented in 2005 [7]. Various kinds of impurities such as sodium nitrate and sodium thiosulfate are present in the decontamination residues and may influence the efficiency of the hydrogen absorber. Urban [8] showed that hydrogenation of thiosulfate

* Corresponding author. E-mail address: [email protected] (D. Chartier). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.07.003

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to sulfide was catalyzed at high pressure and temperature in the presence of cobalt sulfide combined with porous carrier material. We therefore modified the composition of the hydrogen absorber by adding a large amount of NaNO3 or Na2S2O3$5H2O to better understand their role in H2 trapping. In addition, we studied the addition of elemental sulfur or Co(OH)2, which are also expected to trap hydrogen. To investigate the influence of Co/S on hydrogen trapping, a cobalt polysulfide, CoSn, was also synthesized by precipitation of a cobalt salt and NaSn.

2.

Material and methods

Analytical grade cobalt sulfate (CoSO4$7H2O), sodium sulfide (Na2S$7-9 H2O), sodium nitrate (NaNO3), sodium thiosulfate (Na2S2O3$5H2O), and elemental orthorhombic sulfur (S) were purchased from VWR. All chemicals were used as received. Cobalt hydroxide (Co(OH)2) was prepared by adding excess sodium hydroxide (OH/Co2þ > 2) to an aqueous solution of cobalt sulfate heptahydrate (CoSO4$7H2O) prepared from analytical grade (Merck). The preparation of the hydrogen absorber and the hydrogenation reactions are described in our previous article [1]. Briefly, cobalt sulfide (CoS) is initially precipitated by mixing cobalt sulfate and sodium sulfide at a 1:1 M ratio in water. CoS is then oxidized in air to form a mixture of CoS2 þ Co(OH)2. To study the influence of the mineral additives on H2 trapping, the additives are mixed with the hydrogen absorber in wet conditions to obtain slurries. Hydrogen trapping experiments are carried out at 20.5  0.5  C in stainless steel reactors equipped with a gas valve and a pressure transmitter (Fig. 1). The reactor free volume is measured for each experiment. Prior to the hydrogenation reaction, the absorber (alone or mixed with inorganic additives) is dried in situ under vacuum at room temperature.

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The product is considered dry if the pressure does not increase when the vacuum is stopped. Then pure helium is introduced at PHe ¼ 1200 hPa. The pressure must remain constant for at least 20 days (deviation less than 0.2 hPa/day) to consider the system free of gas leakage. If the leak test is conclusive, hydrogen Alphagaz 1 (Air Liquide) is introduced in the reactor at an initial pressure of 1200 hPa and the total pressure is recorded as a function of time. H2 refilling is performed whenever the pressure drops below 500 hPa. The total pressure drop observed during H2 trapping is converted to a gas quantity using the ideal gas law, the reactor free volume (V) and the temperature (T ): Dn ¼ DPV/RT. As DP is negative in our experiments, Dn represents the amount of H2 trapped in most cases, provided the release of other gases is negligible. A gas analysis is systematically performed at the end of the experiment to compute the amount of H2 trapped and the amounts of other gases released. Significant release of other gases is detected when sodium nitrate and sodium thiosulfate are added to the absorber (releasing NH4 and H2S, respectively). On the contrary, no significant amount of H2S can be detected in the experiments conducted with NaOH traps in the reactor (experiment in which elemental sulfur is added to the absorber and experiment with cobalt polysulfide). In these experiments LDn represents the amount of H2 trapped considering that no volatile elements other than H2S are conceivable in such cases. In the experiment where extra cobalt hydroxide is added to the absorber (with no NaOH trap), the presence of cobalt hydroxide does not allow H2S gas accumulation in the reactor and LDn also represents the amount of H2 trapped. This is cheeked by a gas analysis during the experiment (Section 3.1.2). Gas analyses are performed with a MAT 271 mass spectrometer (Thermo Electron Corp.) on samples taken from the reactors. Gases are identified according to the mass-to-charge ratio of ions and their fragments, and quantified by ion intensity after calibration with standard mixes.

Fig. 1 e Experimental device to measure H2 trapping by cobalt-based absorbers.

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X-ray diffraction (XRD) measurements are obtained on a Siemens D5000 diffractometer (Cu Kb radiation ˚ ) and a graphite monochromator operating at l ¼ 1.39222 A 40 kV and 40 mA. Diffractograms are analyzed using ICDD files. The crystallite size is determined from the XRD diffractograms using TOPAS software. Chemical analyses are performed by ICP-AES using an IRIS Intrepid ER/S spectrometer. Prior to analysis, cobalt sulfide is dissolved in acidic media. Basic traps consisting of NaOH granules are dissolved in water and treated with excess H2O2 to oxidize sulfide ions to sulfate ions prior to elemental analysis of sulfur.

3.

Results and discussion

3.1. Synergetic effects of inorganic additives with cobalt sulfide absorbers 3.1.1.

Addition of sodium nitrate to the hydrogen absorber

As previously mentioned, many impurities such nitrate derivatives are present in the decontamination residues. Nitrogen exhibits several redox states from þV (NO 3 ) to eIII (NH3), and redox reactions between NO 3 and H2 may be expected. Hydrogenation of nitrate ions in aqueous media has received more attention in recent decades as an alternative process to biological treatments for wastewater purification. This denitration process always requires metal-based catalysts [9e14]. However although nitrate ions are well known oxidizing species, we did not observe any reactivity of sodium nitrate alone with H2 at room temperature and low pressure (results not shown). In another experiment, sodium nitrate was mixed with the hydrogen absorber in equimolar quantities (8 mmol Co/ 8 mmol NaNO3). Fig. 2 reports the variation in the amount of gas, Dn, in the reactor over time (Fig. 2a) and for the trapping experiment with the absorber alone (Fig. 2b). Fig. 2b clearly shows that the H2 trapping reaction is almost complete after 20 days with the formation of CoS species as follows [1]:

CoS2 þ H2 /CoS þ H2 S

(1a)

CoðOHÞ2 þ H2 S/CoS þ 2H2 O

(1b)

The overall reaction is thus: CoS2 þ CoðOHÞ2 þ H2 /2CoS þ 2H2 O

(2)

CoS is unstable and tends to form crystallized Co9S8: 9CoS þ H2 /Co9 S8 þ H2 S

(3)

In the presence of nitrate ions (Fig. 2a), Dn is much larger than with the hydrogen absorber alone, due to a synergetic effect of nitrates with the hydrogen absorber. This phenomenon was evidenced and patented in 2006 [15], but the products of this reaction have not been identified until now. The gas analysis performed at 120 days in the reactor revealed the presence of a significant amount of ammonia (11.6 vol%) and a high N2/O2 ratio (Table 1). Hydrogenation of NO 3 in the presence of catalysts involves successive reactions leading to the formation of N2 and NH3 as described in the literature [12,13,16]. Our results support the formation of NH3 and N2 described by the following redox equations:  NO 3 þ 4H2 /NH3 þ 2H2 O þ OH

(4)

 NO 3 þ 5=2H2 /1=2N2 þ 2H2 O þ OH

(5)

According to the gas analysis (Table 1), Eq. (4) is the main reaction occurring during reduction of nitrates. We can thus expect to trap 4 mmol of H2 per mmol of NO 3 ; this corresponds to Dn ¼ 3 mmol (because of NH3 release) per mmol of NO 3 and Dntot,theo ¼ 24 mmol. In fact, Fig. 2a shows that Dntot,exp is only about 4 mmol after 120 days. This low yield means that the reduction of sodium nitrate is a slow process and requires a longer time to complete. To support this hypothesis, the reactor was filled with H2 after 57 days and a discontinuity in Dn was observed. This indicates that the kinetics are sensitive to the hydrogen pressure under these conditions and that much longer reaction times or experiments in dynamic conditions would be necessary to obtain complete reduction of nitrate.

3.1.2.

Addition of cobalt hydroxide to the hydrogen absorber

Co(OH)2 is the main compound with CoS2 in the hydrogen absorber formulation and is used to neutralize H2S release (Eq. (1a)). In the literature, Co(OH)2 has also been shown to be reduced by H2 to form metallic cobalt powder in the presence of PdCl2 catalyst at moderate temperature (about 450 K) and high pressure (a few MPa) [17]. But to the best of our knowledge, no hydrogen reactivity with cobalt hydroxide alone has been reported at room temperature and low pressure. We previously showed that the hydrogen absorber induced

Table 1 e Gas analysis after 120 days of reaction between sodium nitrate (8 mmol) and the hydrogen absorber (8 mmol) with pure hydrogen in a closed reactor. Fig. 2 e Influence of sodium nitrate on hydrogen trapping. (a) 8 mmol of hydrogen absorber mixed with 8 mmol of NaNO3; (b) 8 mmol of hydrogen absorber alone.

Gas

H2

N2

O2

H2O

NH3

H2S

Residual gas

Vol%

86.37

0.8

0.02

0.35

11.6

0

0.86

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hydrogen trapping by nitrate ions in these pressure and temperature conditions, suggesting similar behavior in the presence of an excess of Co(OH)2. Fig. 3 compares Dn over time for equimolar (Co absorber/ extra Co(OH)2 ¼ 1) amounts of hydrogen absorber and extra cobalt hydroxide (Fig. 3a) and for the absorber alone (Fig. 3b). Fig. 3a exhibits two plateaus characteristic of a two-step mechanism. The first quasi-plateau is observed after 35 days and the variation in the total number of moles of gas, Dn, during that time period was about 4 mmol. A gas analysis performed at 78 days (still on the plateau) showed that only H2 (99.2%) and traces of N2 (0.45%), O2 (0.08%) and other gases (Ar, CO2, H2O) were detected. This implies that Dn is equal to the H2 trapped at that time. Furthermore the curve of reduction in the first few days is very similar to the curve obtained with the absorber alone (Fig. 3b). This suggests that the reaction in the first few days is the same with or without the Co(OH)2 additive. The reduction of extra cobalt hydroxide then occurs after the reduction of the hydrogen absorber itself and induces excess trapping of about 8 mmol. This difference of absorption capacity is almost equal to the amount of added Co(OH)2. The two absorbers were characterized by XRD (Fig. 4a and b) after total reaction with H2. Without addition of Co(OH)2 (Fig. 4b), the diffraction peaks evidence the presence of the Co9S8 pentlandite phase, which is in agreement with Eqs (2) and (3). Co9S8 is also observed in the presence of added Co(OH)2 (Fig. 4a) but new peaks characteristic of the cubic and hexagonal cobalt phases are present. All the crystallized phases (Co9S8, metallic cobalt) are in the nanometric range as shown in Tables 2 and 3. The metallic cobalt phases may result from the reduction of added Co(OH)2 according to the following reaction: CoðOHÞ2 þ H2 /Co þ 2H2 O

(6)

in which H2 is trapped at a 1:1 ratio with Co(OH)2 as experimentally observed.

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Fig. 4 e X-ray diffractograms of the absorber with extra Co(OH)2 (a) or alone (b) and after hydrogen trapping. *Co9S8 (ICDD n 019-0364), #cubic Co metal (ICDD n 015-0806). $hexagonal Co metal (ICCD n 001-1277).

As no reactivity is observed between H2 and cobalt hydroxide alone at room temperature, this implies that the first metallic cobalt crystallites are formed at the end of the first reduction reaction. The dissociation of molecular hydrogen on cobalt sulfide dispersed in unreacted Co(OH)2 enables the beginning of the reduction of Co(OH)2 to metallic cobalt. This phenomenon is very slow and explains the almost stable period observed between the two reactions at about 50 days. The second part of the curve is consistent with the fact that metallic Co is well known as a hydrogenation catalyst [18e20] and allows the reduction of Co(OH)2. This process induces the growth of metallic Co nuclei, increasing the total surface area of metallic Co and enhancing H2 trapping. At 110 days, the trapping rate reaches a maximum and then decreases as the area of the Co(OH)2/Co interface decreases (2nd wave of reduction). To validate reaction (6), the amount of water released after reaction was determined and compared with the expected value. After reaction with hydrogen, the wet product was vacuum dried in situ (in the reactor) and a weight loss of 425 mg was measured. Based on Eqs. (2) and (6) and the experimental conditions, 24 mmol of H2O should be formed (16 mmol due to reaction (6) and 8 mmol due to reaction (2)), for a total weight loss of 432 mg. The two values are almost identical and support the initial hypothesis. The formation of metallic cobalt and the increased H2 consumption are therefore consistent with the reduction mechanism of added Co(OH)2 according to synergetic effects.

3.1.3. Addition of elemental sulfur S8 to the hydrogen absorber Fig. 3 e Influence of cobalt hydroxide on hydrogen trapping. (a) 8 mmol of hydrogen absorber mixed with 8 mmol of cobalt hydroxide; (b) 8 mmol of hydrogen absorber alone.

We showed in a previous paper that H2 trapping by cobalt sulfide implied breaking SeS bonds [1]. The effect of added elemental sulfur on the hydrogen trapping capacity was investigated. In the literature, the reduction of elemental

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Table 2 e Crystallite size and lattice parameters of the hydrogen absorber mixture with cobalt hydroxide after reaction with hydrogen. Cubic metal cobalt

Hexagonal metal cobalt 001-1277 P63/mmc a ¼ 2.5414 c ¼ 4.105 33.5  1.6 a ¼ 2.51  0.01 c ¼ 4.08  0.01

ICDD file data

ICDD file Space group ˚) Lattice parameters (A

015-0806 Fm-3m a ¼ 3.5447

TOPAS computed data

Crystallite size (nm) ˚) Lattice parameters (A

10.1  1.1 a ¼ 3.49  0.01

sulfur by hydrogen at room temperature and atmospheric pressure has not been observed to date. Eight mmol of the absorber (8 mmol of Co) were mixed with 240 mmol of elemental sulfur. Since H2S is the expected product of the hydrogenation reaction, a large amount of sodium hydroxide was introduced in a separate compartment of the reactor to trap this gas. The results of this trapping experiment are presented in Fig. 5a together with the experiment carried out with the absorber alone (8 mmol of absorber, Fig. 5b). Fig. 5a shows several discontinuities in H2 trapping due to repeated H2 refilling during the experiment (9, 18, 25 days). In the first days the experimental curves are similar, suggesting that sulfur and the hydrogen absorber trap H2 simultaneously. Then the trapping rate is much higher in the presence of elemental sulfur. After 80 days the total amount of H2 trapped, Dn, is 20 mmol. Furthermore the elemental analysis in the NaOH compartment indicates the presence of 16 mmol of sulfur, a small fraction of the initial amount of elemental sulfur (240 mmol). These two results are consistent knowing that 4 mmol of H2 should be trapped by the 8 mmol of hydrogen absorber after 80 days if we consider that in the mixture the hydrogen absorber has the same H2 trapping capacity. According to the following reaction, 16 mmol of H2S should be formed: 1=8S8 þ H2 /H2 S

Co9S8 019-0364 Fm-3m a ¼ 9.932 13.9  0.4 a ¼ 9.89  0.01

the curve is not affected after the last refilling (25 days) indicating that the total pressure no longer affects the reduction mechanism. Finally, in the final days the trapping quantity reaches a plateau. The two hydrogenation reactions occur simultaneously and the presence of the plateau is due to the consumption of the total amount of cobalt disulfide.

3.1.4. Addition of sodium thiosulfate to the hydrogen absorber Trapping experiments were also carried out with a sulfurbased additive: sodium thiosulfate (Na2S2O3$5H2O). Preliminary hydrogenation experiments carried out on sodium thiosulfate alone did not show any reactivity (results not shown). Four mmol of Na2S2O3$5H2O were then mixed with 8 mmol of hydrogen absorber (8 mmol of Co). Fig. 6 shows the trapping results obtained (Fig. 6a) together with the results of the trapping experiment without the mineral additive (Fig. 6b). The presence of sodium thiosulfate significantly increases the amount of trapped hydrogen. After 180 days of reaction Dn gas is about 12 mmol, which may correspond to 4 mmol of H2 trapped by the hydrogen absorber and 8 mmol of H2 trapped by sodium thiosulfate. But at the beginning of the trapping

(7)

Elemental sulfur alone cannot be reduced under these pressure and temperature conditions, implying the presence of the hydrogen absorber throughout the reaction (80 days). This is confirmed by the shape of curve 4a. After each H2 refilling the quantity trapped over 7 days decreases regularly (Dn ¼ 6.1, 4.9, 3.2, and 1.3 mmol). Furthermore the shape of

Table 3 e Crystallite size and lattice parameters of the hydrogen absorber after reaction with hydrogen. Co9S8 ICDD files data

TOPAS computed data

ICDD file Space group Lattice parameters Crystallite size Lattice parameters

˚) (A ˚) (A

019-0364 Fm-3m a ¼ 9.932 17.3  0.3 a ¼ 9.89  0.01

Fig. 5 e Effect on hydrogen absorption of adding elemental sulfur to the hydrogen absorber. (a) 8 mmol of hydrogen absorber mixed with 240 mmol of elemental sulfur. 625 mmol of NaOH are placed in a separate compartment to trap H2S gas. (b) 8 mmol of hydrogen absorber alone.

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first H2 refill (after 32 days) the reaction rate increases again meaning that the reaction depends on the H2 pressure. Conversely, the second H2 refill (after 120 days) has no influence on trapping, which reaches a plateau (Dn ¼ 12 mmol). This means that the hydrogen absorber totally reacted with H2 and cannot produce atomic hydrogen to reduce thiosulfate. The plateau corresponds to the consumption of the hydrogen absorber and about 50% of thiosulfate.

3.2.

Fig. 6 e Influence of sodium thiosulfate on hydrogen trapping. (a) 8 mmol of hydrogen absorber mixed with 4 mmol of Na2S2O3$5H2O; (b) 8 mmol of hydrogen absorber alone.

experiment, the shape of Fig. 6a is different from the previous curves, suggesting a complex mechanism. With nitrate ions or elemental sulfur as additives, the reactivity of the system in the first few days is similar to that of the absorber alone. The trapping rate is slower in the presence of sodium thiosulfate, suggesting that the reactivity of the hydrogen absorber is also lower. Urban previously showed that thiosulfate ions can be reduced by hydrogen for sulfide production at high pressure and temperature (P > 104 hPa and T ¼ 473 K) in the presence of various sulfide catalysts (CoS, NiS, FeS, etc.) [8]. In that experiment the molar ratio of hydrogen to ammonium thiosulfate was 40. The following hydrogenation reaction was proposed: ðNH4 Þ2 S2 O3 þ 4H2 /2NH4 HS þ 3H2 O

(8)

Based on Urban’s study, we could expect in our experiment that the reduction of CoS2 induces the formation of atomic hydrogen, as explained above, which reacts with sodium thiosulfate according to the balance equation: Na2 S2 O3 þ 4H2 /2NaHS þ 3H2 O

(9)

However, the gas analysis at the end of the reaction indicates the presence of a large amount of H2S (4.33 vol% i.e. 0.7 mmol). This result cannot be explained by Eq. (9) and the following reactions may occur: Na2 S2 O3 þ 4H2 /Na2 S þ H2 S þ 3H2 O

(10a)

Na2 S þ H2 S/2NaHS

(10b)

Fig. 6a clearly shows that the reaction rate with thiosulfate is initially slower. The H2 consumption is not only related to the reduction of CoS2 and we may expect that reactions involving thiosulfate occur from the beginning of the reaction. In other words, these reactions occur simultaneously, which partially inhibits H2 trapping. Fig. 6a indicates that after the

Hydrogen trapping based on cobalt polysulfide

Our previous work evidenced that CoS was not an efficient candidate for trapping H2 compared to CoS2 [1]. Furthermore, the addition of sulfur-based additives to the hydrogen absorber (CoS2 þ Co(OH)2) improves H2 trapping. In all cases, the mechanisms are similar and H2S was formed as an intermediate species. It thus appears that the S/Co balance is an important parameter for H2 trapping. Another strategy to investigate the influence of S/Co ratio on H2 trapping would be to use a cobalt sulfide with a S/Co ratio higher than 2. Cobalt polysulfide (CoSn where n > 2) can be obtained by coprecipitation of sodium polysulfide and a cobalt(II) salt in aqueous solution [5,21]. First, sodium polysulfide Na2Sn was synthesized by adding elemental (orthorhombic) sulfur to a solution of sodium sulfide at moderate temperature (333 K) until saturation. The following reaction occurred: Na2 SðsolubleÞ þ ðn  1ÞSðinsolubleÞ/Na2 Sn ðsolubleÞ

(11)

The excess elemental sulfur, which is insoluble, was removed by filtration. From this synthesis, a mixture of sodium polysulfides is usually obtained. The existence of these polysulfides is based only on arithmetic analysis of spectroscopic or mass data [22e24]. However, under these conditions of synthesis, the value of n is about 5 [25]. Second, a solution of cobalt sulfate was added to the sodium polysulfide solution. An excess of cobalt sulfate was used (CoSO4/Na2Sn ¼ 2) so that all polysulfide could be precipitated according to: CoSO4 þ Na2 Sn /CoSn þ Na2 SO4

(12)

Under these conditions, a black precipitate was obtained spontaneously. The precipitate was washed abundantly with deionized water to remove Na2SO4 and the excess CoSO4. Elemental analyses of the precipitate after dissolution in a hot and highly concentrated acidic solution (10 mL HNO3 65% þ 20 mL HClO4 71%) indicated that the average composition of the cobalt polysulfide is CoS4.9 (see Table 4). A hydrogen trapping experiment was carried out with the cobalt polysulfide. Typically, 3.3 mmol of CoSn and 125 mmol

Table 4 e Elemental ICP-AES analyses of the black precipitate in hot acidic solution (10 mL HNO3 65% D 20 mL HClO4 71%). Co (mmol/L) 30.5  2

S (mmol/L)

S/Co

148  8

4.9  0.4

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of NaOH were placed in a reactor with two compartments, as illustrated in Fig. 7a. The presence of NaOH was previously used to trap the expected release of H2S in the case of CoS2 [1] and is reported in Fig. 7b. After trapping, the S/Co ratio in the CoS5 compartment is about 1 (3.32 mmol of Co and 3.38 mmol of S). The NaOH compartment thus contains 13.8 mmol of sulfur. This value is close to the amount of H2 trapped after 100 days (Dn ¼ 13.5 mmol), corresponding to a yield of about 100%. These results indicate that the reaction involves the formation of H2S gas as an intermediate species according to the following reactions: CoSn þ ðn  1ÞH2 /CoSðn  1ÞH2 S

(13)

ðn  1ÞH2 S þ 2ðn  1ÞNaOH/ðn  1ÞNa2 S þ ðn  1Þ2H2 O

(14)

Considering these equations and the experimental data after trapping, n ¼ Dn/nCo þ 1 ¼ 5, where nCo is the number of moles of cobalt in the absorber compartment. This suggests the formula CoS5 for the polysulfide and agrees with the ICP analysis of the polysulfide (CoS4.9). Fig. 7a shows an induction time, contrary to Fig. 7b; this suggests moderate reactivity of CoS5 towards hydrogen compared to CoS2. To confirm this hypothesis, a new H2 trapping experiment with CoSn alone was carried out in a closed reactor. No significant pressure drop was recorded (DP < 0.1 hPa/day). Gas analysis after 6 days indicated the presence of 99.07 vol% of H2 and 0.38 vol% of H2S. By comparison, the same experiment with CoS2 resulted in the release of 40 vol% of H2S after 3 days [1]. The lower reactivity of CoS5 compared to CoS2 comes from the higher stability of the polysulfide chains. Indeed the structures of CoS2 and CoS5 are different since the polysulfide ion is a chain composed of about 5 sulfur atoms [26,27]. Breaking SeS bonds in S2 5 chains thus requires more energy and delays the H2 trapping. Once the SeS bonds than for S2 2

Fig. 7 e Influence of cobalt polysulfide on hydrogen trapping (a) 3.3 mmol of cobalt polysulfide CoSnz5; 125 mmol of NaOH are placed in a separate compartment to trap H2S gas (b) 4 mmol of CoS2; 26 mmol of NaOH in a separate compartment.

are broken, H2 trapping occurs in the same way as for CoS2 since CoS is obtained as a final product. With CoS5 the H2/Co ratio is about 4, whereas it is only 1 with the CoS2 hydrogen absorber (or 0.5 if we consider the mixture of CoS2 þ Co(OH)2). This could be a significant asset in terms of cost for industrial use.

4.

Conclusions

The addition of inorganic compounds (sodium nitrate, sodium thiosulfate, cobalt hydroxide, and elemental sulfur) to a cobalt sulfide hydrogen absorber (CoS2 þ Co(OH)2 mixture) significantly increases the amount of trapped hydrogen. The reaction of the hydrogen absorber with H2 occurs spontaneously and produces water and another cobalt sulfide, CoS (or Co9S8). This absorber reaction is required to allow further reduction of the inorganic additives by H2. Whatever the additive, the amount of trapped H2 increases but the reaction rate is much slower. In the case of Co(OH)2, the trapping reaction is a twostep mechanism that is quantitative and involves the formation of metallic cobalt. In the presence of sodium nitrate, sodium thiosulfate, or elemental sulfur, the reduction of the absorber and the additives occurs simultaneously after a few days and induces gas formation (NH3, N2 or H2S) that must be trapped in the case of industrial applications. A cobalt polysulfide, CoS5, was synthesized. Its hydrogen trapping capacity is much higher than that of CoS2 but the reaction rate is much smaller in the first few days due to the greater stability of the polysulfide chains.

references

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