CoxB composite: Influence of catalyst properties on hydrogen generation rate

Catalysis Today 245 (2015) 86–92

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Hydrogen storage systems based on solid-state NaBH4 /Cox B composite: Influence of catalyst properties on hydrogen generation rate O.V. Netskina ∗ , A.M. Ozerova, O.V. Komova, G.V. Odegova, V.I. Simagina Laboratory of Hydrides Investigation, Boreskov Institute of Catalysis SB RAS, Pr. Akademika Lavrentieva 5, Novosibirsk 630090, Russia

a r t i c l e

i n f o

Article history: Received 30 November 2013 Received in revised form 10 April 2014 Accepted 20 May 2014 Available online 28 June 2014 Keywords: Solid state NaBH4 pellet Hydrogen storage Hydrolysis Hydrogen generation Catalyst

a b s t r a c t Hydrogen generation from solid-state pellets of sodium borohydride with cobalt boride catalysts has been studied. The nature of the reducing agent (NaBH4 , NH3 BH3 ) and the catalyst precursor (CoCl2 ·6H2 O, Co3 O4 ) was shown to have a substantial influence on the rate of hydrogen evolution. The effect of pH on the activity of the catalysts added to the solid-state pellets of sodium borohydride was estimated. It was found that the cobalt catalysts formed under the action of sodium borohydride undergo strong changes in an alkali medium due to boron leaching from cobalt boride and formation of low active Co0 and Co(OH)2 . © 2014 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen can be used directly in proton exchange membrane fuel cells (PEMFC) to generate electricity and water as a by-product. PEMFCs have a high power density and are able to operate at low temperatures. This makes them suitable energy sources for portable devices. However, for a safe operation they need pure hydrogen. Materials for the production and delivery of pure H2 must have high volumetric and gravimetric hydrogen densities. Chemical hydrides are such materials [1]. Among them, sodium borohydride (NaBH4 ) deserves special attention due to a high content of H2 (10.8 wt.%), a reasonable price, non-toxicity and inflammability [2,3]. Besides, NaBH4 interaction with water leads to a two-fold increase in the hydrogen yield due to reduction of water protons [4–6]: NaBH4 + 4H2 O → NaB(OH)4 + 4H2 ↑

(1)

Aqueous alkali-stabilized solutions of sodium borohydride can be stored without considerable losses of hydrogen and have conventionally been used in such applications [7,8]. Solutions of sodium borohydride are mainly used in flow catalytic reactors.

∗ Corresponding author. Tel.: +7 383 330 74 58; fax: +7 383 330 76 91/+7 383 330 73 36. E-mail address: [email protected] (O.V. Netskina). http://dx.doi.org/10.1016/j.cattod.2014.05.029 0920-5861/© 2014 Elsevier B.V. All rights reserved.

The dissolved hydride passes through a bed of a catalyst, which starts hydrogen generation [9–14]. The rate of hydrogen generation depends on the type of the catalyst [15,16] and can be controlled by changing the feed rate of the solution. However, a safe delivery of solutions to the places of their potential application requires measures for preventing possible leakages of alkaline solutions. Besides, storage of NaBH4 in solutions is unpractical because of low storage capacity and gradual decomposition of the hydride. The problem may be solved by using the solid-state hydride composites that have a higher hydrogen storage capacity [17]. Works are under way to create reactors with cartridges containing a solid-state form of NaBH4 [18,19]. In [20,21] hydrogen generation occurs by adding a solution containing catalytic salts to the hydride in the form of a powder or a gel. The active component of the catalyst forms at the solution/solid-state NaBH4 interface [22]. Since 2010, there have been intensive studies of solid-state NaBH4 composites with added Ni, Co and Ru catalysts [23,24]. These materials combine high activity in hydride hydrolysis with a reasonable price, which is important for practical applications. Hydrogen generation from such composites is started by the mere addition of water. The generation of hydrogen has been shown [24] to depend on the amount of the added water. For the process to proceed to completion, the H2 O/NaBH4 ratio must be equal to 8 mol/mol. Depending on the amounts of the added water, the resulting by-products have different contents of crystalline water. The more water is added, the higher is the water content of the

O.V. Netskina et al. / Catalysis Today 245 (2015) 86–92

solid by-products [23]. To reduce the content of the crystalline water, a modification of the solid-state NaBH4 composites has been proposed through the addition of small amounts of an organic polymer—carboxymethyl cellulose. This removes the crystalline water from NaB(OH)4 and the generation process can be efficiently accomplished with the stoichiometric amount of the added water [25]. For a uniform distribution of the catalyst within the solid-state NaBH4 composite, it has been proposed to include a stage of a mechanical activation with subsequent pressing of pellets [26]. However, in this case, hydrogen evolution occurs instantaneously upon contact with water. To ensure a uniform gas generation during a given period of time, silicon rubber was added to the solid-state NaBH4 composite with the Co2+ /IRA catalyst [27]. Due to surface hydrophobicity, silicon rubber serves as a stabilizer to reduce water diffusion into the composite. It has been shown experimentally that the use of such solid-state reacting system of NaBH4 ensures the production of hydrogen for 2 W PEMFC during 2 h. This time is sufficient to charge a cell phone. Nevertheless, although the above mentioned studies are of great practical importance, the effect of the composition and physicochemical properties of the catalyst on the efficiency of hydrogen generation from solid-state NaBH4 composites has not been given sufficient attention. In this work, the rate of hydrogen generation from NaBH4 pellets has been studied depending on the nature of the active component precursor of cobalt boride catalysts, and the effect of alkali on the state of the catalyst has been estimated.

2. Experimental 2.1. Material All commercial chemical reagents were used as received, including sodium borohydride, NaBH4 (Acros Organics, 98 wt.%); ammonia borane, NH3 BH3 (JSC “Aviabor”, 98 wt.%); cobalt(II) chloride hexahydrate, CoCl2 ·6H2 O (GOST 4525-77); cobalt(II, III) oxide, Co3 O4 (GOST 4467-79); cobalt nanopowder, Co0 (No. 697745, Aldrich); sodium hydroxide, NaOH (GOST 4328-77); acetone (GOST 2603-79).

2.2. Cobalt boride preparation Three types of cobalt boride catalysts were used in the pellets: (I) Cox B-(Cl)-NaBH4 prepared by reduction of cobalt chloride in an aqueous solution of sodium borohydride; (II) Cox B-(O)-NaBH4 prepared by reduction of Co3 O4 in an aqueous solution of sodium borohydride; and (III) Cox B-(Cl)-NH3 BH3 prepared by reduction of cobalt chloride in an aqueous solution of ammonia borane. Cobalt borides were synthesized from CoCl2 ·6H2 O and Co3 O4 . The cobalt salt or oxide was added to a 0.12 M solution of NaBH4 or NH3 BH3 at 40 ◦ C under continuous stirring. After the end of gas evolution, the resulting black precipitate was separated from the mother liquor using a magnet, thoroughly washed with acetone and vacuum dried at 70 ◦ C for 3 h. The product was stored in an Ar atmosphere.

2.3. Pellet preparation Appropriate weighted mixtures of NaBH4 and Co3 O4 or cobalt boride catalysts were ground with a mortar grinder (PULVERISETTE 2, Germany). Then the mixtures were made into tablets using a manual tablet press machine (TDP-0, China).

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2.4. Sample characterization Bulk concentrations of Co and B in the studied samples were determined by inductively coupled plasma atomic emission spectrometry using an Optima 4300 DV instrument. TEM images were obtained on a JEM-2010 microscope (accelerating voltage of 200 kV, resolution of 0.14 nm). The images were analyzed and filtered using the DigitalMicrograph program (GATAN). Elemental analysis of the samples was carried out by energy dispersive X-ray microanalysis (EDX) on an EDAX Phoenix spectrometer equipped with a Si(Li) detector with an energy resolution of 130 eV or higher. The samples to be analyzed were applied to a holey carbon film mounted on a standard copper grid. To construct the particle size distribution diagrams, the diameters of at least 300 particles have been determined. The mean Pd particle sizes are quoted from [28] as a number average diameter (d¯n ), d¯n =

 nd i i i , n i i

(2)

where ni is the number of particles with diameter di . IR spectra were recorded with CsI pellets in air at room temperature on an MB-102 (Bomem) Fourier transform spectrometer. The phase analysis of cobalt borides was performed using a URD63 (Seifert-FPM, Germany) diffractometer with a CuK␣ radiation. 2.5. Hydrogen generation experiments Hydrogen generation experiments were carried out at 40 ◦ C in a temperature-controlled glass reactor. 2.5.1. NaBH4 solution A weighed amount of NaBH4 (0.0465 g) was placed into the reactor and dissolved in 10 mL of distilled water. A weighed amount of the catalyst (0.0117 g) was added in the NaBH4 solution. 2.5.2. Solid-state NaBH4 composite A solid-state NaBH4 pellet (0.0465 g of NaBH4 with 0.0117 g of the catalyst) was placed into the reactor with subsequent addition of 10 mL of distilled water. The volume of the produced hydrogen gas was measured using a 100 mL gas burette with a resolution of 0.2 mL. The data obtained were corrected to N.T.P. based on three repeated experiments under the same conditions; the experimental uncertainty was less than 5%. 3. Results and discussion 3.1. Ways for carrying out the process of hydrogen generation from sodium borohydride Hydrogen generation was performed from (1) pellets consisting of sodium borohydride and Co3 O4 , (2) a mechanical borohydride/catalyst mixture, and (3) a solution of the borohydride in water in the presence of Co3 O4 (Fig. 1). In the case of the aqueous solution and mechanical mixture, the kinetic curves of hydrogen generation were shown to be quite similar. There was an initial period when the active phase of the catalyst was formed within 2 min as a result of Co3 O4 reduction to cobalt boride [29]. During this time, the sodium borohydride of the mechanical mixture was completely dissolved in the water. The rate of hydrolysis increased as a more active phase was accumulated, and within 5 min reached its maximum and then remained virtually unchanged to the end of the reaction. A different regularity was observed for the pellets (Fig. 1). Hydrolysis started immediately upon contact with water and the

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Fig. 1. Hydrogen generation from an aqueous solution of sodium borohydride, a mechanical hydride/Co3 O4 mixture and solid-state pellet at 25 ◦ C. The volume of water 10 mL. The mass ratio of «Catalyst:NaBH4 = 1:4».

reaction rate was virtually constant. It appears that during the preparation of the pressed pellets there takes place an interaction of sodium borohydride with Co3 O4 to produce cobalt boride, as can be concluded from the 10% decrease in the hydrogen yield. Therefore, the use of solid-state pellets containing sodium borohydride and cobalt oxide not only leads to an increased hydrogen storage capacity of the system but also to a shorter time of the cobalt boride phase formation and a higher rate of hydrogen generation. 3.2. Effect of way of catalyst preparation on the rate of hydrogen generation Three types of cobalt boride catalysts were used in the pellets: (I) Cox B-(Cl)-NaBH4 prepared by reduction of cobalt chloride in an aqueous solution of sodium borohydride; (II) Cox B-(O)-NaBH4 prepared by reduction of Co3 O4 in an aqueous solution of sodium borohydride; and (III) Cox B-(Cl)-NH3 BH3 prepared by reduction of cobalt chloride in an aqueous solution of ammonia borane. Hydrogen generation from pellets containing the above cobalt borides after their contact with water is shown in Fig. 2. The highest rate of hydrogen generation was observed in the presence of cobalt boride Cox B-(Cl)-NaBH4 . Cobalt boride Cox B-(O)-NaBH4 was less active. However, this catalyst showed nearly a twofold higher rate of hydrolysis during the first 5 min of the reaction when a 50% conversion was reached. Cobalt boride Cox B-(Cl)-NH3 BH3 was the least active. In contrast to the cobalt borides prepared using sodium borohydride Cox B-(Cl)-NaBH4 and Cox B-(O)-NaBH4 , pellets with this catalyst ensure a constant rate of hydrogen generation throughout the entire process including the initial period. From the results in Fig. 2 it can be concluded that it is the type of the reducing agent and of the active component precursor used that determines the rate of NaBH4 hydrolysis from the pellets. This

Fig. 2. The rate of hydrogen generation depending on the way of cobalt boride preparation used in the pellets. 25 ◦ C. The volume of water 10 mL. The mass ratio of «Catalyst:NaBH4 = 1:4».

may be due to the different state of the active phase of the cobalt catalysts used in the pellets. To confirm this assumption, samples of the prepared cobalt borides have been studied by physicochemical methods. 3.3. Study of the prepared cobalt borides by physicochemical methods The prepared Cox B-(Cl)-NaBH4 , Cox B-(O)-NaBH4 and Cox B-(Cl)NH3 BH3 borides are X-ray amorphous black powders. According to the chemical analysis (Table 1), the overall content of cobalt and boron in Cox B-(Cl)-NaBH4 and Cox B-(O)-NaBH4 is less than 100% due to the presence of oxygen [30]. As can be seen from Table 1, the molar ratio of the elements depends on the nature of the active component precursor of the catalyst and the hydride used to prepare cobalt borides. The state of the active component of the cobalt catalysts added to the pellets has been studied by high-resolution transmission spectroscopy, X-ray structural analysis and IR spectroscopy. 3.3.1. Cox B-(Cl)-NH3 BH3 A TEM study of the Cox B-(Cl)-NH3 BH3 catalyst (Fig. 3A) has shown that the boride particles formed by reduction of cobalt chloride in the aqueous solution of ammonia borane have a globular shape and an average size of about 250 nm. An EDX spectrum (Fig. 3B) of the particles indicates that, along with cobalt and boron, they also contain oxygen. An EDX analysis of the near-surface regions of the catalyst particles (Fig. 3C and D) allowed us to conclude that oxygen is mainly localized in the shell surrounding the catalyst particle. The IR spectroscopy data are also indicative of the presence of oxygen-containing compounds on the surface of the particles (Fig. 4). Thus, in the IR spectrum of the Cox B-(Cl)-NH3 BH3 catalyst in the region of 2000–400 cm−1 , against a structureless absorption, there are the absorption bands of small intensity at

Table 1 Results of the chemical analysis of cobalt catalysts. Catalysts

Initial compound

Cox B-(Cl)-NaBH4 Cox B-(Cl)-NaBH4 -NaOH Cox B-(Cl)-NaBH4 -H2 O Cox B-(O)-NaBH4 Cox B-(Cl)-NH3 BH3

CoCl2 ·6H2 O CoCl2 ·6H2 O CoCl2 ·6H2 O Co3 O4 CoCl2 ·6H2 O

Content (wt.%)

Molar ratio Co:B:O

Co

B

81.7 86.1 66.3 77 94.3

7.4 0.48 2.7 1.95 5.7

2:1:1 1.7:0.05:1 0.58:0.12:1 1:0.14:1 3:1:0

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Fig. 3. A TEM micrographs of the Cox B-(Cl)-NH3 BH3 catalyst: (A) morphology of the sample and (B) EDX analysis of a catalyst particle; (C) near-surface region of the catalyst particles and (D) EDX analysis of near-surface region.

1392 and 1066 cm−1 corresponding to the B O vibrations [31]. The thickness of this oxygen-containing shell is insignificant relative to the particle size; so, it was not possible to estimate the amount of oxygen in Cox B-(Cl)-NH3 BH3 by chemical analysis. Thus, in the medium of ammonia borane, the reduction of cobalt chloride leads to the formation of rather large particles of cobalt boride surrounded by a very thin shell with the oxygen-containing compounds of boron.

Fig. 4. IR spectra: (A) Cox B-(Cl)-NaBH4 , (B) Cox B-(Cl)-NH3 BH3 , (C) Co2 B2 O5 —cobalt borate, (D) the initial Cо3 O4 and (F) Cox B-(O)-NaBH4 .

3.3.2. Cox B-(Cl)-NaBH4 The reduction of cobalt chloride in an aqueous solution of sodium borohydride produces cobalt boride with a Cо:В molar ratio of 2.0:1. A TEM study showed that Cox B-(Cl)-NaBH4 is represented by spherical particles of an average size of about 30 nm (Fig. 5A). An EDX analysis revealed that, similar to the case of Cox B(Cl)-NH3 BH3 , the particles of this catalyst are also covered by an oxygen-containing shell (Fig. 5C and D) with an average thickness of 3 nm. From this it follows that the oxygen-containing shell constitutes 1/3 of the particle volume. This oxygen-containing shell is however not uniform in structure. Its amorphous surface (Fig. 5B) has small crystalline particles of about 3 nm in size with an ordered structure. Their interplanar spacing of 0.47 nm allows them to be classified as Co3 O4 . IR spectroscopy data indicate the presence of oxygen-containing compounds of boron on the surface of cobalt boride particles (Fig. 4). This follows from the observation, against a structureless absorption, of the absorption bands at 1392, 1066, 857 and 704 cm−1 corresponding to the B O vibrations [31]. It should be noted that their positions are identical to those of the absorption bands of amorphous cobalt borate prepared under conditions most closely approaching those of the catalytic hydrolysis of NaBH4 in the presence of CoCl2 . The formation, under certain experimental conditions, of an amorphous cobalt borate phase, as a by-product,

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Fig. 5. TEM micrographs and EDX spectra of the Cox B-(Cl)-NaBH4 catalyst: (A) the morphology of the particles; (B) a high-resolution TEM micrograph of a near-surface region of a catalyst particle; (C) and (D) EDX analysis of a near-surface region and a core part of a catalyst particle.

when preparing cobalt borides in solution has been described in the literature [32–34]. 3.3.3. Cox B-(O)-NaBH4 A detailed study of cobalt oxide reduction by sodium borohydride has been described in our earlier paper [29]. It was shown that the formation of the cobalt boride phase occurs gradually and the hydrolysis rate increases with its accumulation. In this work, Co3 O4 pre-reduced by sodium borohydride was added to the hydrogen-generating pellets. According to the chemical analysis, the Cox B-(O)-NaBH4 sample contains 77 wt.% Cо and 1.95 wt.% В, the rest being the oxygen. According to the IR spectroscopy (Fig. 4), Co3 O4 is not reduced completely. The absorption bands at 663 and 573 cm−1 , corresponding to the vibrations of the octahedral МO6 group in the Co2+ (Co3+ )2 O4 2− spinel, are still observed in the spectrum. It can be suggested that upon contact of the pellet with water there takes place an additional reduction of Co3 O4 to form cobalt borides. This increase in the amount of the active phase explains the increased rate of hydrogen generation during hydrolysis in the presence of Cox B-(O)-NaBH4 (Fig. 2). A TEM study of Cox B-(O)-NaBH4 (Fig. 6) has shown that the unreduced cobalt oxide also serves as a place of attachment for the forming cobalt boride particles. Thus, in the dark field image (Fig. 6C) one can clearly see light particles, which strongly differ from the less contrasting grey particles of cobalt oxide. An EDX-spectrum (Fig. 6B) of one of such particles indicates that they predominantly consist of cobalt. A stronger magnification reveals that the particle is built up by agglomerates with an average size of about 30 nm fixed within the oxide. Their irregular shape suggests them to be a cluster of nanosized particles of cobalt boride formed in the reduction of Co3 O4 by sodium borohydride. Analysis of the data from physicochemical studies of the boride particles forming from cobalt chloride under the action of sodium borohydride and ammonia borane allows us to conclude that they

have the same structure consisting of a cobalt boride core surrounded by an oxygen-containing shell of amorphous cobalt borate and Co3 O4 crystallites. However, in the case of sodium borohydride, the forming particles are smaller than in ammonia borane, which makes them more active in the production of hydrogen from NaBH4 based pellets. Thus, the 30 nm particles of the Cox B-(Cl)-NaBH4 catalyst ensure a nearly tenfold higher rate of hydrogen generation than the big particles of the Cox B-(Cl)-NH3 BH3 (250 nm). It should be noted that the particles forming in the reduction by sodium borohydride have close sizes both for cobalt oxide and cobalt chloride. The lower activity of the Cox B-(O)-NaBH4 catalyst is explained by an incomplete reduction of the starting Co3 O4 during catalyst preparation. The use of such a catalyst in a NaBH4 based pellet increases the amount of the active phase in the course of hydrolysis through additional reduction of the cobalt oxide left in the catalyst. The forming particles of cobalt boride attached to the surface of unreduced Co3 O4 ensure a high rate of hydrogen generation. 3.4. Effect of alkali on the rate of hydrogen generation from the pellets Sodium tetrahydroxyborate, NaB(OH)4 , forms as a by-product of sodium borohydride hydrolysis [35] and dissociates in the aqueous medium to liberate a hydroxide ion: [B(OH)4 ]− + H2 O ↔ B (H2 O) (OH)3 + OH−

pKb = 4.76

(3)

The increase in рН of the reaction medium in the course of the reaction may affect the state of the nanosized particles of the active cobalt boride component. To estimate changes in the cobalt borides caused by the presence of the hydroxide ions, the Cox B-(Cl)NaBH4 catalyst was stirred at 700 rpm in a 5% solution of NaOH at 40◦ C for 5 min. For a correct comparison, the same procedure was performed in water. Pellets on the basis of sodium borohydride

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Fig. 6. TEM micrographs and an EDX spectrum of Cox B-(O)-NaBH4 : (A) the morphology of the sample; (B) EDX analysis of the central part of the catalyst particle; (C) a dark field image of the oxide phase and the metal in the sample (A); (D) the morphology of the nanosized particle.

prepared from these two treatments were studied in hydrogen generation upon contact with water. According to the results in Fig. 7, the activity of the cobalt boride decreases in the presence of alkali since analogous treatment in water leads to less dramatic consequences. The study of the elemental composition of samples of the Cox B-(Cl)-NaBH4 , Cox B-(Cl)-NaBH4 -NaOH and Cox B-(Cl)-NaBH4 -H2 O catalysts showed that after a prolonged treatment of the catalysts in water and in an alkaline solution there was a decrease in their content of boron (Table 1) and a change in the state of the active component of the cobalt catalyst. According to the X-ray structural analysis (Fig. 8), a portion of the cobalt boride oxidizes in water to form Co3 O4 . In the alkaline medium, along with the oxidation of the

Fig. 7. The effect of the treatments of cobalt borides on hydrogen generation from pellets containing sodium borohydride and cobalt catalyst. 25 ◦ C. The volume of water 10 mL. The mass ratio of «Catalyst:NaBH4 = 1:4».

Fig. 8. X-ray analysis of Cox B-(Cl)-NaBH4 , Cox B-(Cl)-NaBH4 -NaOH, and Cox B-(Cl)NaBH4 -H2 O catalysts.

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provide for a nearly tenfold higher rate of hydrogen generation than particles of the Cox B-(Cl)-NH3 BH3 catalyst with the average size of 250 nm. The effect of рН on the activity of cobalt boride catalyst has been estimated. It was found that in the alkali sodium borohydride medium the active cobalt boride phase undergoes strong changes such as boron leaching and formation of metallic cobalt Cо0 and Co(OH)2 which have a low activity. Acknowledgements This work was supported by the Russian Academy of Sciences (Project No 24.54). The authors are grateful to A.V. Ischenko for the HR TEM study and O.A. Bulavchenko for the XRD study. References Fig. 9. The effect of the nature of precursor of cobalt boride catalysts forming in situ in the reaction medium of NaBH4 from Co3 O4 and Co(OH)2 at 40 ◦ C on their activity and a comparison with the catalytic activity of metallic cobalt Cо0 . The volume of water 10 mL. The mass ratio of «Catalyst:NaBH4 = 1:4».

active component and the formation of Co(OH)2 , the boron is leached to leave metallic cobalt (Fig. 8). It was found, in separate experiments, that in the medium of sodium borohydride the forming compounds of cobalt can be reduced to cobalt borides to catalyze the process of NaBH4 hydrolysis (Fig. 9) but at a different rate. Thus, in the presence of Cо0 and Co(OH)2 the hydrolysis is very slow and Cо3 O4 quickly reduces in the reaction medium to the cobalt boride phase [36], which leads to a high rate of hydrogen generation (Fig. 2). Thus, the continuous increase in pH in the hydrogen generation by hydrolysis of sodium borohydride may lead to a loss in activity of the cobalt catalysts due to the decreasing content of boron leading to the formation of low active Cо0 and Cо(OН)2 phases. 4. Conclusion Generation of hydrogen from solid-state pellets of sodium borohydride with cobalt catalysts has been studied. It was shown that the nature of the reducing agent (NaBH4 , NH3 BH3 ) and the catalyst precursor (CoCl2 ·6H2 O, Co3 O4 ) has a substantial influence on the activity of cobalt boride used for preparation of the pellets. The state of the starting cobalt borides has been studied by physicochemical methods (elemental analysis, high-resolution TEM, IR spectroscopy). The active particles forming upon CoCl2 ·6H2 O and Co3 O4 reduction in the medium of sodium borohydride have an average size of 30 nm, while in the case of ammonia borane the active particles have an average size of 250 nm. All samples have a cobaltboride core surrounded by an oxygen-containing shell. A distinctive feature of Co3 O4 is its incomplete reduction, which leads to stabilization of the active particles on unreduced oxide surface. The use of such a catalyst in the pellet on the basis of NaBH4 leads to additional reduction of cobalt oxide in the hydride medium with the formation of additional amount of active cobalt boride phase and this is the reason for the increase in hydrogen generation rate. In the presence of cobalt boride formed in the reduction of cobalt chloride, the rate of hydrogen generation remains virtually the same until the conversion of sodium borohydride reaches 50% and is determined by the size of boride particles. Thus, particles of the Cox B-(Cl)-NaBH4 catalyst with an average size of 30 nm

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