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Bor Dergisi 17 Journal of Boron BORON 2 (1), 28 - 36, 2017 ULUSAL BOR ARAŞTIRMA ENSTİTÜSÜ ISSN: 2149-9020 NATIONAL BORON RESEARCH INSTITUTE BOR DE RGİSİ JOURNAL OF BORON CİLT/VOL SAYI/ISSUE YIL/YEAR 02 01 20 BOR DERGİSİ 17 JOURNAL OF BORON http://dergipark.gov.tr/boron Synthesis of Co2B nanostructures and their catalytic properties for hydrogen generation Tuncay Şimşek1*, Mustafa Barış2 1Hacettepe University, Department of Physics Engineering, 06800 Ankara, Turkey 2Eti Maden Works General Management, 06105 Ankara, Turkey ARTICLE INFO ABSTRACT Article history: Pure cobalt (II) boride nanoparticles/nanocylinders were synthesized in aqueous Received 24 November 2016 media under Argon blanket using cobalt chloride (CoCl2) and sodium borohydride Received in revised form 6 December 2016 (NaBH ) as reactants. CoCl (0.325 g) was dissolved in cold distilled water (DDI) Accepted 6 December 2016 4 2 and the solution was introduced into a sealed glass reactor. Then, NaBH was Available online 16 March 2017 4 dissolved in DDI (90 mL) and the solution was added drop-wise into the reactor Research Article and stirred magnetically at 300 rpm. By the addition of NaBH4 (10 mL, 0.225 g) solution, amorphous black cobalt boride particles were synthesized immediately. Keywords: The presence of crystalline Co2B phase with high purity in the nanocylinders which o Cobalt boride, is obtained by calcination at 500 C was shown by X-ray diffraction spectroscopy. H2 generation, An amorphous Co2B structure was observed with the sample dried under Sodium borohydride, vacuum. In the synthesis runs, Co B nanoparticles with different morphological Hydrolysis, 2 characteristics were achieved by changing the initial CoCl concentration and Solvothermal method 2 the reaction period. A microscopic structure in the form of nanocylinders was observed for the calcined products. The nanocylinder diameter increased from ca. 30 nm to 100 nm by increasing the reaction time from 3 to 120 min. CoCl2 initial concentration was also found another factor increasing the nanocylinder diameter. The nanocylinders with diameters between 80-500 nm were obtained by increasing CoCl2 concentration from 12.6 to 100.1 mM. The lowest and highest saturation magnetization values were obtained 19 and 68.5 emu/g for crystalline sample calcined under air (Co-Co2B mixture) and amorphous Co2B sample obtained by vacuum-drying, respectively. Amorphous and crystalline Co2B samples were used as catalyst for Hydrogen generation by the hydrolysis of NaBH4 in aqueous media. Amorphous Co2B gave significantly higher 2H generation rate with respect to the o catalysts prepared by calcination of amorphous Co2B under air or Ar at 500 C. The maximum H2 generation rate was obtained as 1.1 L/g catalyst.min by using amorphous Co2B with 1 % w/w of initial NaBH4 concentration. 1. Introduction catalysts in aqueous media. Various types of catalysts have been tried for H2 generation by the hydrolysis of Significant attention was paid on H carriers due to 2 NaBH4 in aqueous-alkaline media [1-43]. Generally, the difficulties appeared in the storage and transport transition metal based catalysts are preferred. Cobalt of H . The non-toxic character of H , the flexibility of 2 2 (II) salts were tried as catalyst for generating H2 from source materials used for generation, low-cost produc- sodium borohydride [1]. On the other hand, various tion and taking place of the source reaction at ambi- catalysts based on supported cobalt were also tried ent conditions are known as the basic advantages of [2-11]. the H2 energy. Although various methods have been proposed for H2 generation, the hydrolysis of alkali Metal borides synthesized via mechanochemical, metal hydrides is mostly preferred as a favorable borothermal or carbothermal route, electrolysis in a method. Among these materials, sodium borohydride fused salt, chemical vapor deposition or carbothermal is the most widely used reactant for H2 generation due method are another group of materials widely used for to its high H2 content (10.7 % w/w). All of H2 content H2 generation by the decomposition of NaBH4 [12-14]. of NaBH4 have been recovered by using appropriate On the other hand, the catalysts including the alloys of *Corresponding author: [email protected] 28 Şimşek T. and Barış M. / BORON 2 (1), 28 - 36, 2017 metal borides were recently tried for obtaining higher 2.3. Characterization H2 generation rates in the decomposition of NaBH4 [15-24]. The crystal structure was analysed by X-ray diffraction spectrophotometer (Rigaku, D/MAX-2200) for both There have been numerous studies in the literature on dried and calcined Co2B nanoparticles. The particle the H2 generation by using cobalt boride as catalyst in morphology was determined by Scanning Electron Mi- the decomposition of NaBH4 [25-33]. In most of these croscopy (FEI, Quanta 200F). The magnetic properties studies, the crystalline phases containing either –vari- of Co2B nanoparticles were determined by Vibrating ous- cobalt borides or metallic cobalt in the form of Sample Magnetometer (PPMS, P525). The specific irregular nanoparticles were tested as catalyst for H2 surface area (SSA) was measured by a surface area production. In our study, both the synthesis of cobalt and pore-size analyzer (Quantaochrome, Nova 220E) boride and the drying and calcination of cobalt boride using nitrogen adsorption-desorption method together product were carried out under Ar atmosphere. Hence with Brunauer-Emmett-Teller (BET) equation. both the synthesis and calcination conditions were carefully controlled. Then amorphous Co2B or crys- 2.4. H generation using Co B nanoparticles as talline pure Co B phases were obtained in the form 2 2 2 catalyst of nanocylinders by a modified form of solvothermal method based on the chemical reaction between co- H2 generation runs by the hydrolysis of NaBH4 were balt chloride and sodium borohydride. Here, we wish conducted in a pyrex glass reactor equipped with an to report their properties and catalytic performance of apparatus capable of measuring the H2 evolved ac- pure amorphous/crystalline Co2B in H2 generation by cording to the water-displacement method (Figure 1). the decomposition of NaBH4. Two separate sets of H2 generation runs were per- formed by using the calcined Co B samples obtained 2. Experimental procedure 2 with different CoCl2 initial concentrations and differ- 2.1. Materials ent reaction periods, respectively. A third set of runs for H2 generation were also performed by using the Cobalt chloride (CoCl2), sodium borohydride (NaBH4) amorphous Co2B obtained by vacuum-driying of reac- were supplied from Aldrich Chem. Corp., and used o tion product at 70 C, the crystalline Co2B obtained by as received. The synthesis reactions were conducted calcining the reaction product under Ar at 500 oC and under high purity of Ar atmosphere (Habaş, Turkey). the crystalline Co2B-Co mixture obtained by calcining Distilled-deionized water was used in all experiments. the reaction product under air at 500 oC as catalysts. 2.2. Synthesis of Co2B particles The synthesis of cobalt boride (Co2B) nanoparticles was performed in an aqueous medium. In the syn- thesis runs, CoCl2 concentration in the initial reaction medium and the reaction period were changed. Typi- cally, CoCl2 (0.325 g) was dissolved in cold DDI water (90 mL) and the solution was introduced into a sealed, three-necked glass reactor stirred magnetically at 300 rpm, placed in an ice-bath kept at +4 oC. Then, the flow of Ar into the reactor was initiated. The reactor was stirred for 10 min for thermal equilibrium. NaBH4 was then dissolved in DDI water (10 mL) and the solu- tion was added drop-wise into the reactor within 3 min. By the addition of NaBH4 solution (10 mL, 0.225 g), the reaction medium turned to black. The reaction was continued under Ar atmosphere to prevent oxidation. Figure 1. H generation measuring apparatus The black cobalt boride particles were synthesized im- 2 mediately. Following to the completion of reaction-pe- riod selected, the reaction medium was filtrated under Typically, the solution (10 mL) DDI and NaBH4 (1 % Ar-protection. The black cake was extensively washed w/w) were prepared. Then, CoB2 catalyst (0.1 g) was with DDI water under Ar atmosphere. The nanopar- added into the solution under magnetic stirring at 300 ticles isolated were dried in vacuo at 70 oC for 24 h. rpm. The valve on the line passing to the displacement o Dry cobalt boride powder was calcined at 500 C for tube was opened. The volume of H2 generated was 2 hours, under Ar atmosphere with a heating rate of measured by water-displacement method with the 2 oC/min. progressing hydrolysis. 29 Şimşek T. and Barış M. / BORON 2 (1), 28 - 36, 2017 3. Results and discussion to 300 nm. The calcined powder under air conditions o at 500 C was found to consist of Co-Co2B fingertype 3.1. Characterization of Co2B nanostructures structured particles ranging from 50-300 nm. Calcin- ing the powders under the Ar atmosphere at the same Cobalt boride particles were obtained by the reaction temperature resulted in fingertype pure Co2B of 50-100 of CoCl2 with NaBH4 in the aqueous media kept at +4 nm size. The specific surface areas (SSA) were deter- oC, under Ar atmosphere according to the following mined as 10.0, 6.0 and 3.0 m2/g for the amorphous equation [44]. Co2B, Co2B calcined under Ar and Co2B calcined un- der air, respectively. Wu et al. have calcined the pow- 2CoCl2 + 4NaBH4 + 9H2O Co2B + 4NaCl + ders they synthesized in a similar study under vacuum 12.5H2 + 3B(OH)3 (1) at 90-300-500-700 °C. They reported that the structure at 90 °C was amorphous while the structures at 300 Equation 1 shows the reduction reaction of CoCl2 with and 500 °C consisted of Co-Co2B and the structure at effective reducing agent NaBH4.
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