Facile Synthesis of Spherical Porous Tricobalt Tetroxide and Metal Cobalt for High Quality Cobalt Production and Recycling

Abstract

Liquid-phase synthesis is suitable for controlling the morphological and structural properties of functional inorganic materials and such control is important because they strongly affect the functionality of the product. Cobalt-based materials is one group of materials that requires control of powder morphology and structure. Cobalt is an important metal element used in various industrial fields such as secondary batteries, superalloys, permanent magnets, hard metals, and catalysts. The ever-growing demand of cobalt highlights that there is an urgent necessity to develop high quality material synthesis methods and to secure cobalt resources for countries that rely on import like Japan. Especially, technologies to recycle cobalt in a form reusable for secondary batteries and catalysts are desired because further increase in demand is anticipated. Meanwhile, spherical porous tricobalt tetroxide secondary particles obtained by calcination of spherical cobalt carbonate precursors have been reported to be promising materials for battery applications. Moreover, porous metal cobalt obtained by hydrogen reduction of tricobalt tetroxide is known to be effective for catalyst applications. However, conventional methods to synthesize spherical cobalt carbonate requires addition of additives in the reaction mixtures or otherwise requires hydrothermal treatment in pressure vessels. These are unfavorable to develop a practical and sustainable process for cobalt-based material production and recycling. In this study, a facile room temperature synthesis method to prepare spherical cobalt carbonate precursors from solutions of cobalt chloride and ammonium hydrogen carbonate that does not require any additives or hydrothermal treatment is developed by optimizing the carbonate solution concentration, carbonate/cobalt salt molar ratio, and stirring time during reaction. Furthermore, we succeeded in obtaining porous tricobalt tetroxide and porous metal cobalt that carry on the exterior spherical morphology of the original cobalt carbonate precursor by calcination and hydrogen reduction. The developed process shall contribute to realize high quality cobalt-based material production and recycling.

1. Introduction

Liquid phase synthesis or wet chemical synthesis is a promising method to synthesize diverse functional inorganic materials. It is generally simple and does not require complicated equipments. Thus, it is considered to be an effective industrial synthesis method of materials and it has already been long applied in the ceramic industry.¹⁾ Liquid phase synthesis basically proceeds under mild conditions compared to high temperature methods (e.g., solid-state reaction and thermal decomposition) and for this reason, it is expected to be a more economical and environmental-friendly process with less energy consumption and less emission of hazardous volatile substances.²⁾ In addition, the availability of a wide range of chemical reactions and physical phenomena of liquid-phase synthesis is suitable for controlling the morphological and structural properties of ceramic powders consisting of nano-, micro-, and macro-sized crystals.^1,3) Such control of morphology and structure is extremely important because it is well known that functionality of inorganic materials is strongly dependent on their chemical composition, particle morphology, size, and so on.⁴⁾ Indeed, many reviews are being published even recently on morphology and structure control using liquid phase synthesis for various materials such as metal oxides,^2,4) layered double hydroxides⁵⁾ and lithium-ion battery cathode materials.⁶⁾

One group of materials that require control of powder morphology and structure is cobalt(Co)-based materials. Co is an important metal element widely used in industry as ingredients for secondary batteries, superalloys, permanent magnets, hard metals, catalysts and so on.^7,8) Recently, the demand for Co is continuously increasing and it has been reported that approximately half of it is used for secondary batteries and around 5% for catalysts in 2017.⁹⁾ These products are essential for realizing the upcoming carbon neutral society and thus the demand is expected to increase even further.^8,10,11) However, the producing countries of Co are unevenly distributed because Co is a rare metal and there is an urgent necessity to secure Co resources for countries that rely on import like Japan.⁸⁾ From these point of view, development of technologies to enable production and recycling of high quality Co-based powders with controlled morphology and structures for secondary batteries and catalysts is of great importance.

Tricobalt tetroxide (Co₃O₄) is known to be one of the major ingredients to prepare cathode materials for secondary batteries.¹²⁾ Indeed, the morphology and purity of the Co₃O₄ powder is reported to significantly affect the performance of the final products. Because the morphology and purity of Co₃O₄ strongly rely on the quality of the precursor material, selection of a suitable precursor material is an important factor in preparing Co₃O₄ for the previously introduced applications.^13–16) So far, various types of precursor materials have been investigated and porous secondary particles of Co₃O₄ that carry on the exterior morphology of the original precursor materials including capsules,¹⁷⁾ cubes,¹⁸⁾ hexagonal nano-platelets¹⁹⁾ and spheres^20–26) have been developed. Among them, spherical cobalt carbonate (CoCO₃) precursors which can produce spherical porous secondary particles of Co₃O₄ has received increasing interest in recent years owing to their advantageous features. Previous studies have clarified that calcination of such spherical CoCO₃ precursors provides spherical Co₃O₄ secondary particles with high porosity and high tap density, which is suitable for secondary battery application.^20,21) Furthermore, such porous Co₃O₄²⁷⁾ and Co₃O₄ reduced porous metal Co²⁸⁾ have been reported to be effective catalysts. Thus, a facile process to synthesize spherical CoCO₃ precursors is desired.

Conventionally, studies on the synthesis of spherical CoCO₃ has been conducted by mixing and reacting solutions of Co salts, such as cobalt chloride (CoCl₂), cobalt nitrate, cobalt sulfate, and cobalt acetate, and carbonate salts like ammonium carbonate or ammonium hydrogen carbonate (NH₄HCO₃) in the presence of structure directing agents or additives and/or by subjecting the Co salt and carbonate salt solution mixtures to hydrothermal treatment in pressure vessels.^22–26) For instance, Miyata et al.²⁴⁾ reported that addition of a non-ionic surfactant benzene solvent in the Co salt/carbonate mixture solution results in formation of a spherical CoCO₃ powder. Nassar²⁵⁾ found that hydrothermal treatment of Co salt/carbonate mixture solution at 160°C for 24 h can produce spherical CoCO₃ powder. Li et al.²⁶⁾ reported the combination of adding polyvinyl-pyrrolidone and urea with hydrothermal treatment at 200°C. However, to realize a practical and sustainable production and recycling process, a more facile synthesis process that does not require additives or hydrothermal treatment is desired.

In this study, we aimed to develop a facile process to obtain spherical CoCO₃ powder from CoCl₂ and NH₄HCO₃ solutions at room temperature, that does not use any additives or hydrothermal techniques, by optimizing the condition of simply supplying a concentrated NH₄HCO₃ solution into a CoCl₂ solution. The effects of carbonate solution concentration, NH₄HCO₃/CoCl₂ molar ratio, and stirring time during reaction were investigated. In addition, the effects of calcination temperature to transform CoCO₃ into Co₃O₄ and the hydrogen reduction of Co₃O₄ to metal Co against the initial CoCO₃ powder structure is discussed.

2. Experimental Procedure

2.1 Materials

CoCl₂ hexahydrate (CoCl₂·6H₂O, Guaranteed reagent, Purity ≧ 99.0%, Nacalai Tesque Inc., Japan) and NH₄HCO₃ (Guaranteed reagent, Purity ≧ 96.0%, Nacalai Tesque Inc., Japan) were used as the raw materials for Co and carbonate sources, respectively. Distilled water was prepared using a distillation apparatus (RFD240NC, Toyo Seisakusho Kaisha, Ltd., Japan) installed in our laboratory.

2.2 Synthesis of CoCO₃ powder

Since Co is generally leached using hydrochloric acid or nitric acid in the current recycling process and since a purification process that can produce extremely pure CoCl₂ solutions from the leached crude CoCl₂ solution has been developed,²⁹⁾ CoCl₂ solution was selected as the initial Co source considering the practical process. 12.0 g of CoCl₂·6H₂O was dissolved in distilled water and the total volume was adjusted to 100 ml to obtain a 0.5 M CoCl₂ solution while 200 ml solutions containing certain amounts of NH₄HCO₃ were prepared as the carbonate source. The carbonate solution was fed into the beaker containing the 0.5 M CoCl₂ solution using a pump (MASTERFLEX 7524-10, Cole-Parmer Instrument Company, LLC., United States) at a constant speed of 10 ml/min. The mixture was stirred at 300 rpm using stirring blade while the carbonate solution was supplied. After completely introducing the NH₄HCO₃ solution into the CoCl₂ solution, the mixture was continued to stir for a particular time and then statically aged for 24 h at room temperature in a desktop fume hood. The obtained solid was collected by suction filtration using a hydrophilic polytetrafluoroethylene filter (Omnipore membrane filter: pore size 0.2 µm, Merck KGaA, Germany). The obtained solid was rinsed with distilled water, dried at 60°C for more than 24 h and then ground using an alumina mortar. The Co concentration in filtrate solution was analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES, S-II, Seiko Instrument, Japan) to calculate the recovery rate of Co, R_Co, using the following eq. (1),

  

\begin{equation} R_{\text{Co}} = (1 - \text{C}/\text{C}_{0})\times 100 \end{equation} (1)

where C and C₀ are the Co concentration in the filtrate and the initial reaction mixture, respectively.

The effect of concentration of carbonate solution was investigated by supplying 0.5, 1.0, and 1.5 M NH₄HCO₃ solutions while maintaining the NH₄HCO₃/CoCl₂ molar ratio to 1, which is the stoichiometric ratio of CoCO₃, by adjusting their volumes to 100, 50, and 33 ml, respectively. The stirring time of 1 h after adding the NH₄HCO₃ solution completely was employed. When considering the effect of NH₄HCO₃/CoCl₂ molar ratio, a 1.5 M NH₄HCO₃ solution was supplied and their volumes were adjusted to meet NH₄HCO₃/CoCl₂ molar ratio of 1, 2, 3, and 6. The effect of stirring time after completely adding the NH₄HCO₃ solution was considered by stirring for 1 or 24 h after completely adding the 1.5 M NH₄HCO₃ solution to the 0.5 M CoCl₂ solution. The obtained samples were characterized using X-ray diffraction (XRD, Ultima IV, Rigaku Corporation, Japan) and field emission scanning electron microscope (FE-SEM; JSM-6330F, JEOL, Japan) to determine the crystalline phase and morphology of the solid product, respectively.

2.3 Verification on formation behavior of spherical CoCO₃ precursor

Using the experimental condition in section 2.2 that resulted in spherical CoCO₃, the formation behavior of the spherical morphology of the CoCO₃ was investigated. The condition that 200 ml of 1.5 M NH₄HCO₃ solution was pumped into a 100 ml of 0.5 M CoCl₂ solution and stirring for 1 h was adopted. The formation behavior was observed by a sampling test. Specifically, a small amount of solid containing solution was collected at 1, 5, 10, 20, 35, 50, 80, 560, 1040, and 1520 min after starting the addition of NH₄HCO₃ solution. 1, 5, 10, 20 min is during the NH₄HCO₃ solution addition, 35, 50, 80 min is after NH₄HCO₃ solution was finished adding but still stirred, and 560, 1040, 1520 min is the period the mixed solution was statically aged at room temperature in a desktop fume hood. The pH was measured throughout the experiment using a pH meter (AS800, AS ONE Corporation, Japan) to anticipate the reaction during the formation of CoCO₃. All collected solids were rinsed with distilled water, dried at 60°C for more than 24 h and then ground using an alumina mortar. The collected sample was analyzed using XRD and FE-SEM.

2.4 Preparation of Co₃O₄ by calcination of CoCO₃

The CoCO₃ precursors obtained from the experiment in section 2.2 with different morphologies were used in this experiment. The condition that 200 ml of 1.5 M NH₄HCO₃ solution was pumped into a 100 ml of 0.5 M CoCl₂ solution and thereafter mixed for 1 h and 24 h was adopted. To consider the effect of calcination temperature, CoCO₃ powder with the 2 different morphologies were calcined in air atmosphere. The heating rate was fixed at 10°C/min and was held for 1 h at 500, 700 or 900°C followed by furnace cooling. The obtained samples were characterized using XRD and FE-SEM.

2.5 Hydrogen reduction of spherical porous Co₃O₄

The hydrogen reduction effect on structural morphology is investigated using the spherical porous Co₃O₄ powder obtained by calcination at 500°C in section 2.4. The spherical porous Co₃O₄ secondary particle was placed in a alumina boat and set in a hydrogen reduction furnace (FT-101VAC, Full-Tech Co., LTD., Japan) under continuous flow of hydrogen. The reduction was carried out using a heating rate of 10°C/min and was held at 400°C for 2 h followed by furnace cooling. The obtained samples were characterized again using XRD and FE-SEM.

3. Results and Discussions

3.1 Effect of experimental parameters on crystal phase and morphology

3.1.1 Effect of concentration of NH₄HCO₃ solution

The possibility to form spherical CoCO₃ by changing the concentration of the carbonate source was considered first. To supply enough carbonate ions (CO₃²⁻) for Co ions (Co²⁺) to form CoCO₃, NH₄HCO₃/CoCl₂ molar ratio of 1, the stoichiometric ratio of CoCO₃, was employed in this experiment. 100, 50, and 33 ml of 0.5, 1.0, and 1.5 M NH₄HCO₃ solutions were supplied to the 0.5 M CoCl₂ solution followed by stirring for 1 h and static aging for 24 h. The XRD patterns and FE-SEM images of the three samples are shown in Fig. 1 and Fig. 2. The results of XRD indicated that pure CoCO₃ could not be obtained in all three conditions. The obtained crystal phase was mainly hydrated or dehydrate form of cobalt carbonate basic with a CO₃²⁻/Co²⁺ ratio of 0.5, which is half of the target material, namely 1 for CoCO₃. This indicates that supplying equimolar of NH₄HCO₃ against CoCl₂ was insufficient for the formation of pure CoCO₃. Meanwhile, FE-SEM images show that the morphology of the obtained powder changed from irregular shape to flake-like crystals. This implies that the concentration of the carbonate source itself was not the primary factor for forming CoCO₃ but had effect on the crystallinity of the formed product. This is also supported from the fact that peak intensity in XRD of Co(CO₃)_0.5(OH)·0.11H₂O increased by increasing the concentration of NH₄HCO₃ from 1.0 to 1.5 M.

Fig. 1

XRD patterns of solid products obtained when using (a) 0.5 M NH₄HCO₃, (b) 1.0 M NH₄HCO₃, and (c) 1.5 M NH₄HCO₃ solutions.

Fig. 2

FE-SEM images of solid products obtained when using (a) 0.5 M NH₄HCO₃, (b) 1.0 M NH₄HCO₃, and (c) 1.5 M NH₄HCO₃ solutions.

3.1.2 Effect of NH₄HCO₃/CoCl₂ molar ratio

The results of 3.1.1 indicated that adding NH₄HCO₃ of a stoichiometric amount was insufficient for the formation of pure CoCO₃. Therefore, we considered to increase the NH₄HCO₃/CoCl₂ molar ratio. However, because 1.5 M was nearly the saturated concentration for NH₄HCO₃, NH₄HCO₃/CoCl₂ molar ratio was increased from 1 to 2, 3, and 6 by increasing the volume of the supplied 1.5 M NH₄HCO₃ solution from 33 ml to 67, 100, 200 ml. The XRD patterns and FE-SEM images of the four samples are shown in Fig. 3 and Fig. 4. The results of XRD indicated that the portion of CoCO₃ increased along with increasing the NH₄HCO₃/CoCl₂ molar ratio and pure CoCO₃ could be obtained when the NH₄HCO₃/CoCl₂ molar ratio reached 6. This indicates that addition of an excess amount of carbonate source is critical to obtain pure CoCO₃. The FE-SEM image of the obtained samples turned into a clear spherical morphology with smaller particle size as the NH₄HCO₃/CoCl₂ molar ratio was increased, even though no additives or hydrothermal treatment was applied. Indeed, the solution temperature monitored during the experiment revealed that the solution temperature was kept below room temperature throughout the experiment. The reason why excess amount of NH₄HCO₃ solution was necessary maybe explained by the ionic state of carbonates in the solution. At the pH of the initial solution of CoCl₂ solution, pH around 5.0, most of the carbonate exists mainly as hydrogen carbonate ion (HCO₃⁻) or carbon dioxide (CO₂).³⁰⁾ The lack in carbonate ions in the solution must be suppressing the CoCO₃ formation reaction and also weakens the pH buffering ability of NH₄HCO₃. The increase in the amount of supplied 1.5 M NH₄HCO₃ solution must have compensated the shortfall of carbonate ions in the solution. The recovery rate of Co, R_Co, for the four conditions are shown in Fig. 5. The recovery rate of Co increases as the NH₄HCO₃/CoCl₂ molar ratio was increased. R_Co reached >95% when 1.5 M NH₄HCO₃ solution was supplied until a NH₄HCO₃/CoCl₂ molar ratio of above 3. The high recovery rate of Co is essential for developing a practical Co recycling process.

Fig. 3

XRD patterns of solid products obtained at NH₄HCO₃/CoCl₂ molar ratio of (a) 1, (b) 2, (c) 3, and (d) 6.

Fig. 4

FE-SEM images of solid products obtained at NH₄HCO₃/CoCl₂ molar ratio of (a) 1, (b) 2, (c) 3, and (d) 6.

Fig. 5

Relationship between the NH₄HCO₃/CoCl₂ molar ratio and recovery rate of Co from the initial solution.

3.1.3 Effect of stirring time during reaction

The effect of stirring time during reaction was considered utilizing the condition that produced spherical pure CoCO₃ in section 3.1.2., namely the condition with NH₄HCO₃/CoCl₂ molar ratio of 6. The stirring time after completely adding the NH₄HCO₃ solution was extended from 1 to 24 h followed by static aging for 24 h and the obtained product was compared. The XRD patterns and FE-SEM images of the two samples are shown in Fig. 6 and Fig. 7. The results of XRD indicated that pure CoCO₃ was obtained regardless of stirring time. This also supports the anticipation that high NH₄HCO₃/CoCl₂ molar ratio was the crucial factor in forming CoCO₃. Although both conditions resulted in pure CoCO₃, FE-SEM images of the samples were completely changed. The CoCO₃ particles obtained with stirring for 24 h showed irregular and inhomogeneous morphology with smaller particle size while the sample stirred only for 1 h showed homogeneous spherical structure with similar particle size. The longer stirring time was anticipated to cause shear stress on the particles starting to transforming into CoCO₃, resulting in the shattered small particles with irregular morphology. Furthermore, the R_Co was found to decrease by 5% (from 95.6 to 90.6%) when elongating the stirring time. This maybe because the finely ground particle of weakly crystallized CoCO₃ redissolved into the solution. It seems that the stirring time has significant effect on particle morphology and control of stirring conditions is essential to obtain CoCO₃ particles with spherical morphology.

Fig. 6

XRD patterns of solid products obtained at stirring times of (a) 1 h and (b) 24 h.

Fig. 7

FE-SEM images of solid products obtained at stirring times of (a) 1 h and (b) 24 h.

3.2 Formation behavior of spherical CoCO₃

To understand the formation behavior of the spherical CoCO₃ particles, a sampling test was conducted using the optimized condition: NH₄HCO₃/CoCl₂ molar ratio of 6 and 1 h stirring after NH₄HCO₃ solution addition followed by static aging for 24 h. The sampling was carried out 4 times while supplying the NH₄HCO₃ solution (1, 5, 10, 20 min), 3 times during stirring (35, 50, 80 min), and 3 times during static aging (560, 1040, 1520 min). The XRD patterns and FE-SEM images of the ten collected samples are shown in Fig. 8 and Fig. 9. It was found from XRD results that at the initial stage of synthesis, the solid product was not CoCO₃ but hydrated cobalt carbonate basic (Co(CO₃)_0.5(OH)·0.11H₂O) even though NH₄HCO₃ was excess (NH₄HCO₃/CoCl₂ molar ratio of 6). CoCO₃ was detected after 80 min, which corresponds to the point that the stirring was stopped. The Co(CO₃)_0.5(OH)·0.11H₂O gradually transformed into pure CoCO₃ during the statical aging step. The results of FE-SEM observation well corresponded with that of XRD. The morphology of the solid product started to change after 80 min from small irregular shape to bulky isotropic particles and finally changed into spherical shape at 1040 min, which is the same time that pure CoCO₃ was confirmed by XRD. Considering that cobalt carbonate basic is detected at lower NH₄HCO₃/CoCl₂ molar ratios, it is anticipated that CoCO₃ becomes more stable than cobalt carbonate basic at higher NH₄HCO₃ concentrations and thus triggers the transformation to CoCO₃. In addition, because extension of stirring from 1 to 24 h was found to result in irregular shaped CoCO₃ particles, the slow transformation of cobalt carbonate basic into CoCO₃ in the statically aged solution must have been important to allow transformation into spherical morphology with the smallest free energy.

Fig. 8

XRD patterns of solid products collected at different periods of spherical CoCO₃ formation experiment: (a) 1 min; (b) 5 min; (c) 10 min; (d) 20 min; (e) 35 min; (f) 50 min; (g) 80 min; (h) 560 min; (i) 1040 min; (j) 1520 min.

Fig. 9

FE-SEM images of solid products collected at different periods of spherical CoCO₃ formation experiment: (a) 1 min; (b) 5 min; (c) 10 min; (d) 20 min; (e) 35 min; (f) 50 min; (g) 80 min; (h) 560 min; (i) 1040 min; (j) 1520 min.

Furthermore, the pH was monitored throughout the entire experiment to consider the phenomena on formation of Co(CO₃)_0.5(OH)·0.11H₂O and its transformation into pure CoCO₃. The variation of pH during the experiment is shown in Fig. 10. The pH was found to change in 3 steps. First, the pH quickly increased from ∼5 to 7.3 ± 0.1 while supplying the NH₄HCO₃ solution into CoCl₂ solution (0 to 20 min). Next, the pH gradually increased to 7.7 ± 0.1 until the stirring finished (20 to 80 min). And finally, the pH very slowly increased to 7.8 ± 0.1 during the static aging period (80 to 1520 min). Taking into account the calculated state of carbonate and ammonia in aqueous solutions at the observed pH region as shown in Fig. 11, the following mechanism is proposed. As the NH₄HCO₃ solution was supplied, the pH quickly increases owing to the pH buffering ability of NH₄HCO₃ while the supplied carbonate is partially lost as CO₂ gas, leaving some free ammonium ions (NH₄⁺) in the solution. In fact, intense bubble generation was observed immediately after the NH₄HCO₃ solution was introduced into CoCl₂ solution. This also explain one of the reasons why excess NH₄HCO₃ was necessary: i.e., to cover the shortage of carbonate ions lost as CO₂ gas. As the pH increases, the fraction of HCO₃⁻ in the solution drastically increases and portion of CO₂ gas decreases (as shown in Fig. 11(a)), thus allowing the precipitation of Co(CO₃)_0.5(OH)·0.11H₂O. The precipitation of Co(CO₃)_0.5(OH)·0.11H₂O consumes HCO₃⁻, leaving more free NH₄⁺ in the solution. This precipitation reaction occurs from the middle of NH₄HCO₃ solution supplying period along with pH increase, changing the transparent solution into an opaque suspension, and continuously occurs until the stirring is stopped as can be anticipated from the pH increase until ∼7.7. Simultaneously, some of the NH₄⁺ is converted into ammonia (NH₃) at this stage reaching approximately 3% at pH7.7 according to Fig. 11(b). Therefore, the suspension consist of a solid phase Co(CO₃)_0.5(OH)·0.11H₂O and an aqueous solution containing NH₄⁺, HCO₃⁻ and small amount of NH₃ and CO₂ when the static aging starts. According to previous literature, cobalt carbonate basic is reported to be soluble in ammonium carbonate solutions while CoCO₃ is insoluble in ammonia solution.³¹⁾ Therefore, the following may be occurring during the aging period. In the initial stage, sedimentation of Co(CO₃)_0.5(OH)·0.11H₂O proceeds and forms a sedimentary layer. Meanwhile, gentle but continuous release of CO₂ gas bubbles occur, resulting in larger amount of NH₄⁺ compared to carbonate ions and very slight increase in pH during the aging period. This slight increase in pH also increases the portion of NH₃, possibly making CoCO₃ stabler in the solution in line with the increase in NH₃ concentration. On the other hand, Co(CO₃)_0.5(OH)·0.11H₂O layer shall partially redissolves in the ammonium carbonate solution, providing an supersaturated environment in the sedimentation layer. As the solution environment becomes more stable for CoCO₃, dissolution-recrystallization process from Co(CO₃)_0.5(OH)·0.11H₂O to CoCO₃ will progress and finally form pure CoCO₃. This dissolution-recrystallization process may be similar to the one reported for the slow transformation of vaterite into calcite, which are polymorphs of calcium carbonates.³²⁾ The hypothesis also seems to not contradict with the results obtained from the XRD analysis and FE-SEM observations.

Fig. 10

Variation of pH during the spherical CoCO₃ formation experiment.

Fig. 11

State of (a) carbonate and (b) ammonia in aqueous solutions at various pH.

3.3 Calcination of CoCO₃ to obtain Co₃O₄

Transformation of CoCO₃ into Co₃O₄ by calcination was investigated using the CoCO₃ with spherical and irregular morphologies. The effect of calcination temperature above 500°C was selected according to the reports of Du et al.¹⁷⁾ saying that the thermal decomposition of CoCO₃ to Co₃O₄ begins at 384°C. The XRD patterns and FE-SEM images of the six samples are shown in Fig. 12 and 13. The XRD results indicated that regardless of the morphology of the CoCO₃ particles, pure Co₃O₄ can be obtained by calcination above 500°C. Meanwhile, only the Co₃O₄ obtained from spherical CoCO₃ precursor calcined at 500°C preserved the exterior morphology of the original precursor material, resulting in a secondary particle with spherical porous morphology. It is anticipated that gaps formed by the release of CO₂ from the following reaction (2) combined during the grain growth of primary Co₃O₄ particles, resulting in the porous structure.

  

\begin{equation} \text{3 CoCO$_{3}$} + \text{1/2 O$_{2}$} \to \text{Co$_{3}$O$_{4}$} + \text{3CO$_{2}{}\uparrow$} \end{equation} (2)

Increase in calcination temperature to above 700°C deformed the spherical morphology due to excess grain growth of primary Co₃O₄ particles into an irregular one and caused aggregation. Meanwhile, the Co₃O₄ obtained from CoCO₃ precursor with irregular morphology was found to intensively form aggregates in all tested temperatures. The degree of aggregation was found to show the same trend as in the spherical case, i.e. increase along with the increase in calcination temperature as indicated by the circles in Fig. 13. The size of primary particles of Co₃O₄ also increased as the calcination temperature increased. Calcination of spherical CoCO₃ precursors at 500°C was found to produce spherical porous Co₃O₄ secondary particles told to be promising for applications such as batteries and catalysts.

Fig. 12

XRD patterns of calcined samples of spherical CoCO₃ at (a) 500°C, (b) 700°, (c) 900°C and of irregular shape CoCO₃ at (d) 500°C, (e) 700°, (f) 900°C.

Fig. 13

FE-SEM images of calcined samples of spherical CoCO₃ at (a) 500°C, (b) 700°, (c) 900°C and of irregular shape CoCO₃ at (d) 500°C, (e) 700°, (f) 900°C observed at (-1) high, (-2) intermediate, and (-3) low magnifications.

3.4 Hydrogen reduction of spherical porous Co₃O₄ into metal Co

The non-aggregated spherical porous Co₃O₄ secondary particles obtained by calcination of spherical CoCO₃ precursors at 500°C in section 3.3 was further subjected to hydrogen reduction. According to Tucakovic et al.,³³⁾ Co₃O₄ can be reduce to metal Co at temperatures between 250 to 350°C in hydrogen atmosphere. Therefore, 400°C was selected as the reduction temperature in this study to be sure that all Co₃O₄ is reduced to metal Co. The XRD patterns and FE-SEM images of the reduced sample are shown together in Fig. 14. The XRD clearly indicates that pure metal Co with the hexagonal crystal structure was obtained by the reduction at 400°C in hydrogen atmosphere. The primary particle size of metal Co increase from that of the Co₃O₄ by the hydrogen reduction; however, it was confirmed that the exterior spherical morphology of the secondary particle was preserved. The primary particles of metal Co were around several hundred nanometers and were forming spherical secondary particles of 1 to several micrometers. The obtained spherical porous metal Co secondary particle shall be an interesting material for catalyst due to its porous structure with high surface area and ease in handling.

Fig. 14

(a) XRD pattern and (b) FE-SEM image of spherical porous Co₃O₄ particles after hydrogen reduction.

4. Conclusion

In this study, we aimed to develop and facile process to synthesize spherical porous CoCO₃, a promising precursor material for producing high quality Co₃O₄ and metal Co, at room temperature using CoCl₂ solution and NH₄HCO₃ solution as the Co and carbonate source without using any additives or hydrothermal treatments. The effects NH₄HCO₃ concentration, NH₄HCO₃/CoCl₂ molar ratio, and stirring time during the reaction was investigated. It was found that the NH₄HCO₃/CoCl₂ molar ratio was the crucial factor to form CoCO₃ and pure CoCO₃ could be obtained when a NH₄HCO₃/CoCl₂ molar ratio of 6 was applied. Meanwhile, the stirring time and the following static aging step was found to be critical in obtaining CoCO₃ with spherical morphology. Excess stirring was found to cause destruction of the spherical morphology leading to irregular shape particles and the static aging allowed the slow transformation of the initially formed Co(CO₃)_0.5(OH)·0.11H₂O into the spherical CoCO₃. In addition, the effects of calcination temperature and hydrogen reduction on the structure of Co₃O₄ and metal Co powder was considered. It was found that low temperature calcination of 500°C was necessary to preserve the exterior spherical morphology of the CoCO₃ precursor during thermal decomposition to Co₃O₄ and this exterior spherical morphology could be carried on to metal Co by hydrogen reduction at 400°C. This indicates that porous Co₃O₄ and metal Co that carry on the exterior spherical morphology of the original precursor materials was successfully obtained. The develop process shall contribute to realize high quality cobalt production and recycling.

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