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The effects of plasma treatment on electrochemical activity of Co–B–P catalyst for hydrogen production by hydrolysis of NaBH4
Journal of the Energy Institute xxx (2016) 1e10
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Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen production by hydrolysis of NaBH4 € Omer S¸ahin a, Duygu Elma Karakas¸ b, Mustafa Kaya b, Cafer Saka c, * a
Faculty of Engineering and Architecture, Siirt University, 56100 Siirt, Turkey Central Research Laboratory, Siirt University, 56100 Siirt, Turkey c School of Healthy, Siirt University, 56100 Siirt, Turkey b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2016 Received in revised form 6 March 2016 Accepted 9 March 2016 Available online xxx
In this paper, the hydrogen generation from the hydrolysis of NaBH4 with plasma treated CoeBeP catalyst are investigated as based on NaBH4 concentration, NaOH concentration, temperature, plasma applying time and plasma gases. The changes in surface chemistry and morphology induced by the plasma treatment are examined using scanning electron microscopy (SEM), N2 adsorptionedesorption, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The hydrogen generation rate from hydrolysis of NaBH4 with CoeBeP prepared in the presence of plasma is completed in 12 min time intervals that exhibited a higher hydrogen generation rate of 60%, while the CoeBeP produced in known method let to slower hydrogen release, and the hydrolysis is completed in 30 min time intervals. The maximum hydrogen generation rates of the plasma treated and plasma untreated CoeBeP catalysts toward NaBH4 hydrolysis are 3976 and 1807 mL/g/min, respectively. The activation energy for zero-order is found to be 49.11 kJ mol1. © 2016 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Plasma Sodium borohydride Cobaltephosphoruseboron catalyst Hydrolysis Hydrogen generation
1. Introduction Hydrogen production is one of the most promising alternative energy technologies. Hydrogen is the best candidate as energy carrier. Hydrogen can be produced from a wide variety of primary energy sources and different production technologies. However, its production and use still require energy-consuming and costly processes, and the need for new infrastructure. Hydrogen storage using materials-based approaches such as ammonia borane, hydrides, amides, composite materials, metal-organic frameworks, organic molecules is being explored extensively. Chemical boron-hydrides like NaBH4, NH3BH3, LiBH4, etc. are the most potential candidates as a source to supply pure hydrogen to fuel cells at room temperature [1,2]. NaBH4 is regarded as the most promising material to release hydrogen because of its high content of Hydrogen (10.8 wt%), nonflammable and non-toxic, the reaction products environmentally benignable, the rate of hydrogen generation easily controllable [3]. Hydrogen is generated from NaBH4 by following hydrolysis reaction [4]: NaBH4 þ 2H2O / NaBO2 þ 4H2[ þ heat
(1)
However, it is required to accelerate this hydrolysis reaction using different catalysts in a controllable manner because only a small percentage of the theoretical amount of hydrogen is liberated by hydrolysis reaction of NaBH4 and H2O [5]. Until now, there are many reports about catalysts for hydrolysis of NaBH4. For instance, Ru supported catalyst [6e8], cobalt-based catalyst [9e14], Ni-based catalyst [15,16], CueCo based catalyst [17,18], NieCoeB [19] etc. have been developed to accelerate the hydrolysis reaction of the NaBH4. All of these catalysts used in hydrolysis of NaBH4 act as heterogeneous catalysts [20]. The catalytic properties of metal catalysts are mainly affected by their surface properties. The limited surface area of the heterogeneous catalysts causes to a lower catalytic activity [21].
* Corresponding author. Tel.: þ90 484 223 12 24; fax: þ90 484 223 66 31. E-mail address: [email protected] (C. Saka). http://dx.doi.org/10.1016/j.joei.2016.03.003 1743-9671/© 2016 Energy Institute. Published by Elsevier Ltd. All rights reserved.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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In recent years, plasma techniques had attracted considerable attention in the field of preparing effective catalysts with higher activity due to the reinforcing effect from plasma modification of surface [22e24]. Plasma produces a variety of active species such as electrons, ions and radicals. Among various plasmas, nonthermal plasma, also known as non-equilibrium plasma, can be effective for assisting to the catalytic hydrolysis of NaBH4 for hydrogen production. Non-thermal plasma has a non-equilibrium characteristic due to a significant difference in temperature between the electrons and heavy particles (ions, atoms, free radicals and excited species). The highly energetic electrons can reach energy of 1e10 eV, while the gas kinetic temperature of the plasma can be as low as room temperature, which enables reactions thermodynamically unfavoured to occur at low temperature [25,26]. The use of plasma in combination with solid catalysts has the potential to enhance the conversion of feed gases, improve the selectivity toward the desirable products and to reduce the operating temperature of the catalyst, which both increases the energy efficiency of the processing and improves the stability of the catalyst by reducing poisoning, coking and sintering [27e29]. The interaction of plasma with catalyst could generate a synergistic effect, which might provide a unique way to separate the activation steps from the selective reactions [30,31]. In recent studies, for example, we have investigated the hydrogen production from hydrolysis of NaBH4 in the presence of the catalytic properties of Ni(0) with a dielectric barrier discharge plasma method. Hydrolysis reaction of NaBH4 with Ni(0) prepared in the presence of dielectric barrier discharge plasma is completed in 45-min time intervals with fast hydrogen generation while the Ni(0) produced in a known method led to a slow hydrogen release and hydrolysis is completed in 70-min time intervals [16]. In addition, we have reported the hydrogen production from the hydrolysis of NaBH4 with Co(0) catalyst with a cold plasma method under nitrogen atmosphere. The hydrolysis reaction completed within around 10-min time intervals with cold plasma, while the hydrolysis reaction in the known method completed within around 20-min time intervals [9]. A plasma treatment of CoeWeB catalyst increases the rate of hydrogen generation from the hydrolysis of NaBH4. The catalytic properties of CoeWeB prepared in the presence of plasma have been investigated. The CoeWeB catalyst prepared with cold plasma effect hydrolysis in only 12 min, where as the CoeWeB catalyst prepared in known method with no plasma treatment in 23 min [32]. In addition, we have presented the hydrogen generation from the hydrolysis of NaBH4 with Ni (0) catalyst prepared in the presence of plasma. The results indicated that Ni catalyst reduced with plasma was completed in only 35 min, while the nickel catalyst produced in known method is completed in 80 min [18]. All of these studies show that hydrogen production rate considerably increased by the applications of plasma. Earlier, Patel et al. [33] reported the hydrogen generation by hydrolysis of NaBH4 with CoePeB catalyst with the maximum hydrogen production rate of 2200 mL/min/g. However, our objective in the study is to study the influence of the using of plasma in the preparation of CoeBeP catalyst. In this paper, plasma technique was firstly applied to treat CoeBeP catalyst. It is expected to improve the activity of CoeBeP catalyst by plasma treatment for the hydrogen generation from hydrolysis of NaBH4. The effect on the catalytic properties of CoeBeP catalyst after plasma treatment was investigated for the hydrogen production from hydrolysis of NaBH4 in comparison with the CoeBeP without plasma treatment. The changes in surface chemistry and morphology induced by the plasma treatment are examined using SEM, N2 adsorptionedesorption, XRD, FT-IR and XPS. 2. Experimental All reagents used in this research were of analytical grade. NaBH4 (molecular weight: 37.83 g mol1, assay 98%, Aldrich Chemical Co.) was used for the catalytic properties of CoeBeP. The solubility of NaBH4 in water at 25 C is 55 g/100 g water, but the solubility of sodium metaborate is 28 g/100 g water. In this study, the hydrogen generation from NaBH4 hydrolysis with plasma treated CoeBeP catalyst was tested depending on N2, CO2 and Ar plasma gases, plasma applying time (5e20 min), NaBH4 (1.5e5%) and NaOH (1e10%) solutions and temperature (30e50 C). 2.1. Catalyst characterization XRD patterns of the samples were acquired in a Bruker D8 Advance X-ray diffractometer with Cu Ka sources. The Ka radiation was selected with a diffracted beam monochromator. An angular range 2q from 5 to 80 was recorded using step scanning. SEM (Zeiss EVO 50 Model) was carried out to show the surface morphologies of the untreated and plasma treated CoeBeP catalysts. The SEM images were taken in secondary electrons; the acceleration voltage was equal to 20 kV, and the emission current was 20 pA. N2 adsorption studies were used to examine the porous properties of each sample. The measurements were carried out on Micromeritics ASAP 2000 adsorptive and desorptive apparatus, and all the samples were pretreated in vacuum at 350 C for 20 h before the measurements. The BET surface area is calculated from the isotherms using the BrunauereEmmetteTeller (BET) equation. FT-IR spectroscopy analyses were performed using a spectrometer Perkin Elmer 283. The samples for FT-IR analyses were prepared by mixing them with KBr powder and pressing the mixture into pellets. The FT-IR spectra were recorded between 4000 and 400 cm1. XPS was used to study the chemical composition and the oxidation state of the elements on the catalyst surface. The XPS instrument, a VG Escalab 200R spectrometer with an Al Ka X-ray source, was equipped with a pre-treatment chamber with controlled atmosphere and temperature in which the catalyst samples could be treated under various conditions. 2.2. Catalyst preparation The CoeBeP catalyst in known method was synthesized by chemical reduction method, according to the method described at literature [33]. € S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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2.2.1. Plasma treatment The plasma treatment of the CoeBeP catalyst was carried out by using plasmochemical reactor (Femto, Diener electronic, Germany) with a chamber of 100 mm diameter and 270 mm length, pressure of 2.5 Pa, and power input of 80 W. Catalyst powder (about 0.5 g) was loaded in a quartz boat and put into the discharge cell. The dried precipitate sample was put into the reactor and treated by CO2 plasma. The duration of the plasma treatment was 5 min for each sample. The obtained catalyst was stored in airtight plastic container for further use. 2.3. Activity test The volume of hydrogen generated in the presence of catalysts was measured by using a water-displacement method. In a typical measurement, the reaction solution containing NaBH4 and NaOH was thermostated in a sealed flask fitted with an outlet for collection of evolved hydrogen gas, and then the CoeBeP catalyst was dropped into the designated temperature solution to initiate hydrolysis reaction. The solution temperature was maintained at 30 C. As the reaction proceeded, the water displaced from a graduated cylinder connected to the reaction flask was continually monitored. A measured volume of released gas was subsequently converted into yield of produced hydrogen after the total amount of gas had been collected. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. XPS measurements XPS measurements were carried out to examine the changes in surface chemical states due to plasma treatment (Fig. 1). The Co2p spectrum show a doublet containing a low energy band (Co2p3/2) and a high-energy band (Co2p1/2) at 779 and 792 eV. After the plasma treatment, the shift in the Co2p peaks to higher binding energies at 781 and 797 eV indicates that Co becomes more oxidized during this process. XPS peak with binding energy of 187 eV assigned to boron are also observed. After plasma treatment, the peak of B shifted to higher binding energy. The lower BE is attributed to the metallic B while the higher BE is assigned to the oxidized B. Comparing the binding energy values of B species in plasma treated CoeBeP (190 eV) and plasma untreated CoeBeP (187 eV), positive shifts were observed. In the X-ray photoelectron spectra, P peak corresponding to the P 2p level with binding energy of 133 eV was observed. Comparing the binding energy values of P species in plasma treated CoeBeP and plasma untreated CoeBeP with that of the pure P, negative shifts as plasma treated B species (130 eV) were observed, showing that partial electrons transferred from Co or B to the vacant orbital of P. Patel et al. [33] reported that the higher electron density on active metal site is an important aspect for the enhancement of the catalytic activity of catalyst powder for hydrolysis of NaBH4. This implies that the synergistic effect between CoeB and P was enhanced which is due to more electrons transferring from Co or B to P. The plasma treatment improved the dispersion of Co, B and P on the surface of CoeBeP catalyst, in agreement with the results of XRD. 3.1.2. X-ray diffraction measurements Fig. 2 showed XRD patterns of the plasma treated and untreated CoeBeP catalyst samples, respectively. From Fig. 1, very differences of the surface morphology between the untreated and plasma-modified samples are observed. The figure shows that 5-min plasma treatment is quite effective to crystallize the catalyst. Apparently, the diffraction peaks in the range of 50e80 gradually were increased peak intensity
Fig. 1. The comparison of XPS spectra for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C with CoeBeP catalyst before (a) and after (b) plasma treatment with CO2 plasma gas.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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Fig. 2. The comparison of XRD spectra of CoeBeP catalyst before (a) and after (b) plasma treatment with CO2 plasma gas for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C.
with the plasma treatment. After being plasma treated, the sharper diffraction peaks appeared in XRD patterns of CoeBeP, indicating that the evolution of the structure to a crystalline phase. 3.1.3. Effects of plasma upon surface topography Fig. 3aed present the SEM images of the catalysts treated and untreated by plasma. Visible changes in surface topography can be observed after plasma treatment, as evident from SEM images. Agglomerations with irregular shapes are easily detectable in the plasma untreated CoeBeP catalyst micrograph. In contrast, the micrograph of the plasma treated CoeBeP catalyst shows a uniform distribution and much smaller particles. The structure of small pores is unnoticeable and becomes looser after plasma treatment as shown in Fig. 3, which may be helpful to the improvement of catalytic activity. It was concluded that some changes would take place on the surface if the support were loaded with active species according to the above catalysts SEM images. 3.1.4. FTIR analysis Fig. 4 shows the FT-IR spectra of the plasma untreated and plasma treated CoeBeP catalysts. The spectrum of the plasma treated CoeBeP catalyst was similar to that of the original one. The FTIR spectra consist of four peaks at 1000, 1400, 1650 and 3450 cm1 in the plasma untreated and plasma treated CoeBeP catalyst samples. The stretching vibration for structural eOH and adsorbed water at wave numbers about 3450 and 1649 cm1 for both samples were observed. Absorption peaks below 1000 cm1 are related to metal oxides arising from inter-atomic vibrations. The metal oxide absorption peaks are strong and broad for both catalysts that show high crystallinity of metal oxides in these samples [34]. This observation is in good accordance with XRD results. 3.1.5. Textural properties and reducibility of the catalysts The surface areas of the plasma untreated and plasma treated CoeBeP catalysts were 25.58 and 38.29 m2/g, respectively. The surface area was enhanced for the new catalyst prepared by plasma treatment, which was 49% more than that of plasma untreated CoeBeP catalyst. In the study, total pore volume for the plasma treated catalyst was 0.343 cc/g for pores smaller than 1229.7 Å (Radius) at P/Po ¼ 0.99215. In addition, average pore radius for the plasma treated catalyst was 179.26 Å. The larger surface area benefited a higher dispersion of active components further improved improve the catalytic performances. This result was attributed to the surface modification effect of plasma. Rahemi et al. [34] reported that plasma treatment after impregnation increases the surface area through two different mechanisms. In the first mechanism, as a result of uniform dispersion in plasma environment, the metal particles could not agglomerate and, hence no pore plugging can be occurred. 3.2. Catalytic performances The tests of catalytic activity for the hydrogen generation rate from hydrolysis of NaBH4 with the plasma untreated and plasma treated CoeBeP catalysts were performed. The hydrolysis is shown in Fig. 5. The plasma treatment had a significant effect on catalytic performance € S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
Fig. 3. The comparison of SEM for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C with CoeBeP catalyst before (a, b) and after (c, d) plasma treatment with CO2 plasma gas.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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Fig. 4. The comparison of FTIR spectra for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C with CoeBeP catalyst before (a) and after (b) plasma treatment with CO2 plasma gas.
of CoeBeP catalyst. It can be seen that the hydrogen generation rate from hydrolysis of NaBH4 with CoeBeP prepared in the presence of plasma is completed in 12 min time intervals that exhibited a higher hydrogen generation rate of 60%, while the CoeBeP produced in known method let to slower hydrogen release, and the hydrolysis is completed in 30 min time intervals. The surface of CoeBeP catalyst was undertaken the actions of high-energy atoms, electrons and ions during the plasma treatment. It resulted in a new kind of surface modification technique, could change effectively the interaction between metal species, obtaining new active materials with higher reaction performances [22,35,36]. The maximum hydrogen generation rates of the plasma treated and plasma untreated CoeBeP catalysts toward NaBH4 hydrolysis are 3976 and 1807 mL/g/min, respectively. The maximum generation rate obtained by using the plasma treated catalyst is about two times higher than that by using the plasma untreated catalyst. These results suggest that the interactions between Co, B and P species could change the properties of the catalysts, and consequently enhance the performance of the plasma-catalytic the hydrolysis of NaBH4 for the hydrogen generation. 3.3. Effects of plasma gases To examine the effects of plasma gases, the hydrogen generation yield was measured by hydrolysis of 2.5 wt% NaBH4 þ 2.5% NaOH solution with 7 min plasma applying time and 80 W plasma applying power at 30 C by using three different plasma gases, namely, argon (Ar), carbon dioxide (CO2) and nitrogen (N2) (Fig. 6). As shown in Fig. 6, plasma treated CoeBeP catalyst with CO2 was exhibited much greater hydrogen generation rate compared with the plasma treated CoeBeP catalyst with N2 and Ar gases. This result shows that Ar, CO2 and N2 gases can be used to accelerate the hydrogen generation rate. We were used CO2 plasma gas for the hydrogen generation with CoeBeP in this study. We therefore conclude that changes in hydrogen generation performance are related to plasma treated catalyst surface properties. 3.4. Effect plasma treatment time To examine the effects of plasma application time, the hydrogen generation yield was measured by hydrolysis of 2.5 wt% NaBH4 þ 2.5% NaOH solution with CO2 plasma and 80 W plasma applying power at 30 C by using four different plasma application time (3, 5, 10, 20). As shown in Fig. 7, the hydrogen generation rate is dependent on the duration of cold plasma treatment. As observed in Fig. 7, the hydrogen generation rate is increased with increasing of the plasma treatment time from 3 to 5 min and it arrives at a maximum value at 5 min. 10 and 20 min compared to plasma application time of 5 min. However, the hydrogen generation rate with rising plasma treatment from 5 to
Fig. 5. The tests of catalytic activity for the hydrogen generation rate from hydrolysis of NaBH4 with the plasma untreated and plasma treated CoeBeP catalysts.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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Fig. 6. Effect of plasma gases on the hydrogen production from NaBH4 with plasma treated and untreated CoeBeP catalyst for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C.
20 min is decreased. It appears that is decreased the hydrogen generation rate from NaBH4 hydrolysis. In plasma treatment time in 5e20 min range, the decrease in the hydrogen generation rate was most likely to be due to the destruction of the catalyst surface of longer plasma treatment. Therefore, the optimum plasma treatment time was selected as 5 min. 3.5. Effect of NaOH concentration The concentration of NaOH important influences the hydrogen production from hydrolysis of NaBH4. Fig. 8 illustrates the hydrogen generation rate with different NaOH concentration, i.e. 1 wt%, 3 wt%, 5 wt%, and 10 wt%, in 2.5 wt% NaBH4 solution with 25 mg of CoeBeP catalyst at 30 C. When the NaOH concentration increased from 1 wt% to 3 wt%, the hydrogen production rate increased. Then, when the NaOH concentration increased from 3 wt% to 10 wt%, the hydrogen production rate decreased. The probable reason is that hydroxyl ion is involved in the hydrolysis of NaBH4. This occurs primarily because the hydroxyl ions strongly complex water, thus decreasing the available free water needed for NaBH4 hydrolysis [6]. 3.6. Effect of NaBH4 concentration The rate of hydrogen generated versus the concentration of NaBH4 in wt % is given in Fig. 9. Effect of NaBH4 concentration on the hydrogen generation rate was measured using x wt% NaBH4 (x ¼ 1.5, 2.5, 5), 5 wt% NaOH solutions at 30 C using 25 mg of CoeBeP catalyst. As the NaBH4 concentration increases from 1 wt% to 2.5 wt%, the hydrogen generation rate rises monotonically. Then, as the NaBH4 concentration increases from 2.5 wt% to 5 wt%, the rate of hydrogen generation decreases with increase in NaBH4 concentration. It reaches a maximum value around a concentration of 2.5 wt% of NaBH4 and subsequently decreases with further increase in NaBH4 concentration. A great yield of the hydrogen production at lower weight percentage of NaBH4 solution is possibly explained by the reduction of solution viscosity as explained in the hydrolysis of NaBH4 [12].
Fig. 7. Effect of plasma treatment time on the hydrogen production from NaBH4 with plasma treated CoeBeP catalyst for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C with CO2 plasma gas.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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Fig. 8. Effect of NaOH concentration on the hydrogen generation rate measured using x wt% NaOH (x ¼ 1, 3, 5, 10) þ 2.5 wt% NaBH4 solutions 30 C, using 25 mg with plasma treated CoeBeP catalyst.
3.7. Effect of temperature Influence of temperature on hydrogen generation rate in solutions containing 2.5 wt% NaBH4 and 5 wt% NaOH was investigated at temperatures ranging from 30 C to 50 C (Fig. 10). As expected, hydrogen generation rate increases with the temperature. Furthermore, the linear relationship between amount of hydrogen generated and reaction time at each temperature studied was observed. Moreover, the linear dependence between hydrogen generated and reaction time at each temperature was observed, which means to a zero-order reaction. In order to find Arrhenius constant (activation energy, E) for zero-order reaction model, the plot of ln(k) versus 1/T for the temperatures of 30, 40 and 50 C was obtained. The activation energy of zero-order can be obtained from the slope and intercept of the regression line. Arrhenius plot of the hydrogen production rate using CoeBeP catalyst gives the activation energy of about 49.11 kJ mol1. This value is lower than many the activation energy found in literature. In addition, the maximum hydrogen production rate of about 3976 mL/min/g is obtained which is better than many other catalysts (Table 1). 4. Conclusions In this study, the hydrogen production from hydrolysis of NaBH4 in the presence of the catalytic properties of plasma treated CoeBeP catalyst was investigated based on NaBH4 concentration, NaOH concentration, plasma treatment time, plasma gases and temperature. The catalytic activity of CoeBeP catalyst samples was enhanced greatly by plasma treatment, as compared to pure CoeBeP prepared with untreated plasma. It has been demonstrated that the plasma has a beneficial effect on textural properties and catalytic activity. The hydrogen generation rate from hydrolysis of NaBH4 with CoeBeP prepared in the presence of plasma is completed in 12 min time intervals, while the CoeBeP produced in known method let to slower hydrogen release, and the hydrolysis is completed in 30 min time intervals. The maximum hydrogen generation rates of the plasma treated and plasma untreated CoeBeP catalysts toward NaBH4 hydrolysis are 3976 and 1807 mL/g/
Fig. 9. Effect of NaBH4 concentration on the hydrogen generation rate measured using x wt% NaBH4(x ¼ 2.5, 5, 7.5) þ 5 wt% NaOH solutions 30 C, with plasma treated 25 mg CoeBeP catalyst.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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Fig. 10. Effect of temperature on the hydrogen generation rate measured using 2.5% NaBH4 þ 5 wt% NaOH solutions using 25 mg plasma treated CoeBeP catalyst.
Table 1 Comparison of the catalytic effectiveness for NaBH4 hydrolysis with previous results. Catalysts
Activation energy Ea (kJ mol1)
Hydrogen production rate (mL/min/g)
Reference
Raney Co CoeCo2B CoeB powder Co CoeB CoeMneB CoeCueB Plasma treated CoeBeP Plasma untreated CoeBeP
53.7 35.24 45 75 64.87 52.1 49.6 49.11 e
267.5 350 681 e 880 1440 212 3976 1807
[10] [37] [38] [39] [11] [40] [41] This study This study
min, respectively. The maximum generation rate obtained by using the plasma treated catalyst is about two times higher than that by using the plasma untreated catalyst. Characteristics of this plasma treated CoeBeP/based catalyst were carried out by using BET, FTIR, XPS, SEM and XRD. Hydrolysis kinetics of NaBH4 was investigated at a temperature range of 30e50 C and zero-order kinetic was applied to the obtained data. The activation energy was found to be 49.11 kJ mol1 for zero-order. References [1] L. Schlapbach, A. Zuttel, Hydrogen-storage materials for mobile applications, Nature 414 (2001) 353e357. [2] E. Fakioglu, Y. Yuruma, T.N. Veziroglu, A review of hydrogen storage systems based on boron and its compounds, Int. J. Hydrogen Energy 29 (2004) 1371e1376. [3] J.Y. Liang, Y.L. Li, Y.Q. Huang, J.Y. Yang, H.L. Tang, Z.D. Wei, et al., Sodium borohydride hydrolysis on highly efficient Co-B/Pd catalysts, Int. J. Hydrogen Energy 33 (2008) 4048e4054. [4] H.I. Schlesigner, H.C. Brown, A.E. Finholt, I.R. Gilbreath, H.R. Hoekstra, E.K. Hyde, Sodium borohydride, its hydrolysis and its use as a reducing agent and in the generation of hydrogen, J. Am. Chem. Soc. 75 (1953) 215e219. [5] N. Patel, R. Fernandes, A. Miotello, Promoting effect of transition metal-doped CoeB alloy catalysts for hydrogen production by hydrolysis of alkaline NaBH4 solution, J. Catal. 271 (2010) 315e324. [6] S.C. Amendola, S.L. Sharp-Goldman, M.S. Janjua, M.T. Kelly, P.J. Petillo, M. Binder, An ultrasafe hydrogen generator: aqueous, alkaline borohydride solutions and Ru catalyst, J. Power Sources 85 (2000) 186e195. [7] J.S. Zhang, W.N. Delgass, T.S. Fisher, J.P. Gore, Kinetics of Ru-catalyzed sodium borohydride hydrolysis, J. Power Sources 164 (2007) 772e781. [8] C.L. Hsueh, C.Y. Chen, J.R. Ku, S.F. Tsai, Y.Y. Hsu, F. Tsau, et al., Simple and fast fabrication of polymer template-Ru composite as a catalyst for hydrogen generation from alkaline NaBH4 solution, J. Power Sources 177 (2008) 485e492. € S¸ahin, F. Hansu, C. Saka, O. Baytar, Hydrogen generation from NaBH4 solution with the high-performance Co (0) catalyst using a cold plasma method, Energy Sources [9] O. Part A 36 (2014) 1578e1587. [10] B.H. Liu, Z.P. Li, S. Suda, Nickel- and cobalt-based catalysts for hydrogen generation by hydrolysis of borohydride, J. Alloys Compd. 415 (2006) 288. [11] S.U. Jeong, R.K. Kim, E.A. Cho, H.J. Kim, S.W. Nam, I.H. Oh, S.A. Hong, S.H. Kim, A study on hydrogen generation from NaBH4 solution using the high-performance Co-B catalyst, J. Power Sources 144 (2005) 129e134. [12] K.W. Cho, H.S. Kwon, Effects of electrodeposited Co and CoeP catalysts on the hydrogen generation properties from hydrolysis of alkaline sodium borohydride solution, Catal. Today 120 (2007) 298e304. [13] D. Kilinç, C. Saka, O. Sahin, Hydrogen generation from catalytic hydrolysis of sodium borohydride by a novel Co (II)eCu (II) based complex catalyst, J. Power Sources 217 (2012) 256e261. _ [14] O. Sahin, M. Kaya, S. Izgi, C. Saka, The effect of microwave irradiation on a Co-B-based catalyst for hydrogen generation by hydrolysis of NaBH4 solution, Energy Sources Part A 37 (2014) 462e467. [15] H. Dong, H.X. Yang, X.P. Ai, C.S. Cha, Kinetics of sodium borohydride hydrolysis reaction for hydrogen generation, Int. J. Hydrogen Energy 28 (2003) 1095e1100. [16] O. Sahin, O. Baytar, F. Hansu, C. Saka, Hydrogen generation from hydrolysis of sodium borohydride with Ni (0) catalyst in dielectric barrier discharge method, Energy Sources Part A 36 (2014) 1886e1894. [17] C. Saka, O. Sahin, H. Demir, A. Karabulut, A. Sarıkaya, Hydrogen generation from sodium borohydride hydrolysis with a CueCo-based catalyst: a kinetic study, Energy Sources Part A 37 (2015) 956e964. € S¸ahin, C. Saka, O. Baytar, F. Hansu, Influence of plasma treatment on electrochemical activity of Ni (0)-based catalyst for hydrogen production by hydrolysis of NaBH4, [18] O. J. Power Sources 40 (2013) 729e735. [19] J.C. Ingersoll, N. Mani, J.C. Thenmozhiyal, A. Muthaiah, Catalytic hydrolysis of sodium borohydride by a novel nickelecobalteboride catalyst, J. Power Sources 173 (2007) 450e457. [20] I.I. Korobov, N.G. Mozgina, L.N. Blinova, Kinet. Catal. 48 (1995) 380e384. [21] J.K. Zeng, J.G. Wang, L.Y. Bi, R.S. Li, Q.B. Kan, Catalytic Action Basis, third ed., Science Press, Beijing, 2005.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen production by hydrolysis of NaBH4 € Omer S¸ahin a, Duygu Elma Karakas¸ b, Mustafa Kaya b, Cafer Saka c, * a
Faculty of Engineering and Architecture, Siirt University, 56100 Siirt, Turkey Central Research Laboratory, Siirt University, 56100 Siirt, Turkey c School of Healthy, Siirt University, 56100 Siirt, Turkey b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2016 Received in revised form 6 March 2016 Accepted 9 March 2016 Available online xxx
In this paper, the hydrogen generation from the hydrolysis of NaBH4 with plasma treated CoeBeP catalyst are investigated as based on NaBH4 concentration, NaOH concentration, temperature, plasma applying time and plasma gases. The changes in surface chemistry and morphology induced by the plasma treatment are examined using scanning electron microscopy (SEM), N2 adsorptionedesorption, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The hydrogen generation rate from hydrolysis of NaBH4 with CoeBeP prepared in the presence of plasma is completed in 12 min time intervals that exhibited a higher hydrogen generation rate of 60%, while the CoeBeP produced in known method let to slower hydrogen release, and the hydrolysis is completed in 30 min time intervals. The maximum hydrogen generation rates of the plasma treated and plasma untreated CoeBeP catalysts toward NaBH4 hydrolysis are 3976 and 1807 mL/g/min, respectively. The activation energy for zero-order is found to be 49.11 kJ mol1. © 2016 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Plasma Sodium borohydride Cobaltephosphoruseboron catalyst Hydrolysis Hydrogen generation
1. Introduction Hydrogen production is one of the most promising alternative energy technologies. Hydrogen is the best candidate as energy carrier. Hydrogen can be produced from a wide variety of primary energy sources and different production technologies. However, its production and use still require energy-consuming and costly processes, and the need for new infrastructure. Hydrogen storage using materials-based approaches such as ammonia borane, hydrides, amides, composite materials, metal-organic frameworks, organic molecules is being explored extensively. Chemical boron-hydrides like NaBH4, NH3BH3, LiBH4, etc. are the most potential candidates as a source to supply pure hydrogen to fuel cells at room temperature [1,2]. NaBH4 is regarded as the most promising material to release hydrogen because of its high content of Hydrogen (10.8 wt%), nonflammable and non-toxic, the reaction products environmentally benignable, the rate of hydrogen generation easily controllable [3]. Hydrogen is generated from NaBH4 by following hydrolysis reaction [4]: NaBH4 þ 2H2O / NaBO2 þ 4H2[ þ heat
(1)
However, it is required to accelerate this hydrolysis reaction using different catalysts in a controllable manner because only a small percentage of the theoretical amount of hydrogen is liberated by hydrolysis reaction of NaBH4 and H2O [5]. Until now, there are many reports about catalysts for hydrolysis of NaBH4. For instance, Ru supported catalyst [6e8], cobalt-based catalyst [9e14], Ni-based catalyst [15,16], CueCo based catalyst [17,18], NieCoeB [19] etc. have been developed to accelerate the hydrolysis reaction of the NaBH4. All of these catalysts used in hydrolysis of NaBH4 act as heterogeneous catalysts [20]. The catalytic properties of metal catalysts are mainly affected by their surface properties. The limited surface area of the heterogeneous catalysts causes to a lower catalytic activity [21].
* Corresponding author. Tel.: þ90 484 223 12 24; fax: þ90 484 223 66 31. E-mail address: [email protected] (C. Saka). http://dx.doi.org/10.1016/j.joei.2016.03.003 1743-9671/© 2016 Energy Institute. Published by Elsevier Ltd. All rights reserved.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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In recent years, plasma techniques had attracted considerable attention in the field of preparing effective catalysts with higher activity due to the reinforcing effect from plasma modification of surface [22e24]. Plasma produces a variety of active species such as electrons, ions and radicals. Among various plasmas, nonthermal plasma, also known as non-equilibrium plasma, can be effective for assisting to the catalytic hydrolysis of NaBH4 for hydrogen production. Non-thermal plasma has a non-equilibrium characteristic due to a significant difference in temperature between the electrons and heavy particles (ions, atoms, free radicals and excited species). The highly energetic electrons can reach energy of 1e10 eV, while the gas kinetic temperature of the plasma can be as low as room temperature, which enables reactions thermodynamically unfavoured to occur at low temperature [25,26]. The use of plasma in combination with solid catalysts has the potential to enhance the conversion of feed gases, improve the selectivity toward the desirable products and to reduce the operating temperature of the catalyst, which both increases the energy efficiency of the processing and improves the stability of the catalyst by reducing poisoning, coking and sintering [27e29]. The interaction of plasma with catalyst could generate a synergistic effect, which might provide a unique way to separate the activation steps from the selective reactions [30,31]. In recent studies, for example, we have investigated the hydrogen production from hydrolysis of NaBH4 in the presence of the catalytic properties of Ni(0) with a dielectric barrier discharge plasma method. Hydrolysis reaction of NaBH4 with Ni(0) prepared in the presence of dielectric barrier discharge plasma is completed in 45-min time intervals with fast hydrogen generation while the Ni(0) produced in a known method led to a slow hydrogen release and hydrolysis is completed in 70-min time intervals [16]. In addition, we have reported the hydrogen production from the hydrolysis of NaBH4 with Co(0) catalyst with a cold plasma method under nitrogen atmosphere. The hydrolysis reaction completed within around 10-min time intervals with cold plasma, while the hydrolysis reaction in the known method completed within around 20-min time intervals [9]. A plasma treatment of CoeWeB catalyst increases the rate of hydrogen generation from the hydrolysis of NaBH4. The catalytic properties of CoeWeB prepared in the presence of plasma have been investigated. The CoeWeB catalyst prepared with cold plasma effect hydrolysis in only 12 min, where as the CoeWeB catalyst prepared in known method with no plasma treatment in 23 min [32]. In addition, we have presented the hydrogen generation from the hydrolysis of NaBH4 with Ni (0) catalyst prepared in the presence of plasma. The results indicated that Ni catalyst reduced with plasma was completed in only 35 min, while the nickel catalyst produced in known method is completed in 80 min [18]. All of these studies show that hydrogen production rate considerably increased by the applications of plasma. Earlier, Patel et al. [33] reported the hydrogen generation by hydrolysis of NaBH4 with CoePeB catalyst with the maximum hydrogen production rate of 2200 mL/min/g. However, our objective in the study is to study the influence of the using of plasma in the preparation of CoeBeP catalyst. In this paper, plasma technique was firstly applied to treat CoeBeP catalyst. It is expected to improve the activity of CoeBeP catalyst by plasma treatment for the hydrogen generation from hydrolysis of NaBH4. The effect on the catalytic properties of CoeBeP catalyst after plasma treatment was investigated for the hydrogen production from hydrolysis of NaBH4 in comparison with the CoeBeP without plasma treatment. The changes in surface chemistry and morphology induced by the plasma treatment are examined using SEM, N2 adsorptionedesorption, XRD, FT-IR and XPS. 2. Experimental All reagents used in this research were of analytical grade. NaBH4 (molecular weight: 37.83 g mol1, assay 98%, Aldrich Chemical Co.) was used for the catalytic properties of CoeBeP. The solubility of NaBH4 in water at 25 C is 55 g/100 g water, but the solubility of sodium metaborate is 28 g/100 g water. In this study, the hydrogen generation from NaBH4 hydrolysis with plasma treated CoeBeP catalyst was tested depending on N2, CO2 and Ar plasma gases, plasma applying time (5e20 min), NaBH4 (1.5e5%) and NaOH (1e10%) solutions and temperature (30e50 C). 2.1. Catalyst characterization XRD patterns of the samples were acquired in a Bruker D8 Advance X-ray diffractometer with Cu Ka sources. The Ka radiation was selected with a diffracted beam monochromator. An angular range 2q from 5 to 80 was recorded using step scanning. SEM (Zeiss EVO 50 Model) was carried out to show the surface morphologies of the untreated and plasma treated CoeBeP catalysts. The SEM images were taken in secondary electrons; the acceleration voltage was equal to 20 kV, and the emission current was 20 pA. N2 adsorption studies were used to examine the porous properties of each sample. The measurements were carried out on Micromeritics ASAP 2000 adsorptive and desorptive apparatus, and all the samples were pretreated in vacuum at 350 C for 20 h before the measurements. The BET surface area is calculated from the isotherms using the BrunauereEmmetteTeller (BET) equation. FT-IR spectroscopy analyses were performed using a spectrometer Perkin Elmer 283. The samples for FT-IR analyses were prepared by mixing them with KBr powder and pressing the mixture into pellets. The FT-IR spectra were recorded between 4000 and 400 cm1. XPS was used to study the chemical composition and the oxidation state of the elements on the catalyst surface. The XPS instrument, a VG Escalab 200R spectrometer with an Al Ka X-ray source, was equipped with a pre-treatment chamber with controlled atmosphere and temperature in which the catalyst samples could be treated under various conditions. 2.2. Catalyst preparation The CoeBeP catalyst in known method was synthesized by chemical reduction method, according to the method described at literature [33]. € S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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2.2.1. Plasma treatment The plasma treatment of the CoeBeP catalyst was carried out by using plasmochemical reactor (Femto, Diener electronic, Germany) with a chamber of 100 mm diameter and 270 mm length, pressure of 2.5 Pa, and power input of 80 W. Catalyst powder (about 0.5 g) was loaded in a quartz boat and put into the discharge cell. The dried precipitate sample was put into the reactor and treated by CO2 plasma. The duration of the plasma treatment was 5 min for each sample. The obtained catalyst was stored in airtight plastic container for further use. 2.3. Activity test The volume of hydrogen generated in the presence of catalysts was measured by using a water-displacement method. In a typical measurement, the reaction solution containing NaBH4 and NaOH was thermostated in a sealed flask fitted with an outlet for collection of evolved hydrogen gas, and then the CoeBeP catalyst was dropped into the designated temperature solution to initiate hydrolysis reaction. The solution temperature was maintained at 30 C. As the reaction proceeded, the water displaced from a graduated cylinder connected to the reaction flask was continually monitored. A measured volume of released gas was subsequently converted into yield of produced hydrogen after the total amount of gas had been collected. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. XPS measurements XPS measurements were carried out to examine the changes in surface chemical states due to plasma treatment (Fig. 1). The Co2p spectrum show a doublet containing a low energy band (Co2p3/2) and a high-energy band (Co2p1/2) at 779 and 792 eV. After the plasma treatment, the shift in the Co2p peaks to higher binding energies at 781 and 797 eV indicates that Co becomes more oxidized during this process. XPS peak with binding energy of 187 eV assigned to boron are also observed. After plasma treatment, the peak of B shifted to higher binding energy. The lower BE is attributed to the metallic B while the higher BE is assigned to the oxidized B. Comparing the binding energy values of B species in plasma treated CoeBeP (190 eV) and plasma untreated CoeBeP (187 eV), positive shifts were observed. In the X-ray photoelectron spectra, P peak corresponding to the P 2p level with binding energy of 133 eV was observed. Comparing the binding energy values of P species in plasma treated CoeBeP and plasma untreated CoeBeP with that of the pure P, negative shifts as plasma treated B species (130 eV) were observed, showing that partial electrons transferred from Co or B to the vacant orbital of P. Patel et al. [33] reported that the higher electron density on active metal site is an important aspect for the enhancement of the catalytic activity of catalyst powder for hydrolysis of NaBH4. This implies that the synergistic effect between CoeB and P was enhanced which is due to more electrons transferring from Co or B to P. The plasma treatment improved the dispersion of Co, B and P on the surface of CoeBeP catalyst, in agreement with the results of XRD. 3.1.2. X-ray diffraction measurements Fig. 2 showed XRD patterns of the plasma treated and untreated CoeBeP catalyst samples, respectively. From Fig. 1, very differences of the surface morphology between the untreated and plasma-modified samples are observed. The figure shows that 5-min plasma treatment is quite effective to crystallize the catalyst. Apparently, the diffraction peaks in the range of 50e80 gradually were increased peak intensity
Fig. 1. The comparison of XPS spectra for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C with CoeBeP catalyst before (a) and after (b) plasma treatment with CO2 plasma gas.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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Fig. 2. The comparison of XRD spectra of CoeBeP catalyst before (a) and after (b) plasma treatment with CO2 plasma gas for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C.
with the plasma treatment. After being plasma treated, the sharper diffraction peaks appeared in XRD patterns of CoeBeP, indicating that the evolution of the structure to a crystalline phase. 3.1.3. Effects of plasma upon surface topography Fig. 3aed present the SEM images of the catalysts treated and untreated by plasma. Visible changes in surface topography can be observed after plasma treatment, as evident from SEM images. Agglomerations with irregular shapes are easily detectable in the plasma untreated CoeBeP catalyst micrograph. In contrast, the micrograph of the plasma treated CoeBeP catalyst shows a uniform distribution and much smaller particles. The structure of small pores is unnoticeable and becomes looser after plasma treatment as shown in Fig. 3, which may be helpful to the improvement of catalytic activity. It was concluded that some changes would take place on the surface if the support were loaded with active species according to the above catalysts SEM images. 3.1.4. FTIR analysis Fig. 4 shows the FT-IR spectra of the plasma untreated and plasma treated CoeBeP catalysts. The spectrum of the plasma treated CoeBeP catalyst was similar to that of the original one. The FTIR spectra consist of four peaks at 1000, 1400, 1650 and 3450 cm1 in the plasma untreated and plasma treated CoeBeP catalyst samples. The stretching vibration for structural eOH and adsorbed water at wave numbers about 3450 and 1649 cm1 for both samples were observed. Absorption peaks below 1000 cm1 are related to metal oxides arising from inter-atomic vibrations. The metal oxide absorption peaks are strong and broad for both catalysts that show high crystallinity of metal oxides in these samples [34]. This observation is in good accordance with XRD results. 3.1.5. Textural properties and reducibility of the catalysts The surface areas of the plasma untreated and plasma treated CoeBeP catalysts were 25.58 and 38.29 m2/g, respectively. The surface area was enhanced for the new catalyst prepared by plasma treatment, which was 49% more than that of plasma untreated CoeBeP catalyst. In the study, total pore volume for the plasma treated catalyst was 0.343 cc/g for pores smaller than 1229.7 Å (Radius) at P/Po ¼ 0.99215. In addition, average pore radius for the plasma treated catalyst was 179.26 Å. The larger surface area benefited a higher dispersion of active components further improved improve the catalytic performances. This result was attributed to the surface modification effect of plasma. Rahemi et al. [34] reported that plasma treatment after impregnation increases the surface area through two different mechanisms. In the first mechanism, as a result of uniform dispersion in plasma environment, the metal particles could not agglomerate and, hence no pore plugging can be occurred. 3.2. Catalytic performances The tests of catalytic activity for the hydrogen generation rate from hydrolysis of NaBH4 with the plasma untreated and plasma treated CoeBeP catalysts were performed. The hydrolysis is shown in Fig. 5. The plasma treatment had a significant effect on catalytic performance € S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
Fig. 3. The comparison of SEM for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C with CoeBeP catalyst before (a, b) and after (c, d) plasma treatment with CO2 plasma gas.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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Fig. 4. The comparison of FTIR spectra for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C with CoeBeP catalyst before (a) and after (b) plasma treatment with CO2 plasma gas.
of CoeBeP catalyst. It can be seen that the hydrogen generation rate from hydrolysis of NaBH4 with CoeBeP prepared in the presence of plasma is completed in 12 min time intervals that exhibited a higher hydrogen generation rate of 60%, while the CoeBeP produced in known method let to slower hydrogen release, and the hydrolysis is completed in 30 min time intervals. The surface of CoeBeP catalyst was undertaken the actions of high-energy atoms, electrons and ions during the plasma treatment. It resulted in a new kind of surface modification technique, could change effectively the interaction between metal species, obtaining new active materials with higher reaction performances [22,35,36]. The maximum hydrogen generation rates of the plasma treated and plasma untreated CoeBeP catalysts toward NaBH4 hydrolysis are 3976 and 1807 mL/g/min, respectively. The maximum generation rate obtained by using the plasma treated catalyst is about two times higher than that by using the plasma untreated catalyst. These results suggest that the interactions between Co, B and P species could change the properties of the catalysts, and consequently enhance the performance of the plasma-catalytic the hydrolysis of NaBH4 for the hydrogen generation. 3.3. Effects of plasma gases To examine the effects of plasma gases, the hydrogen generation yield was measured by hydrolysis of 2.5 wt% NaBH4 þ 2.5% NaOH solution with 7 min plasma applying time and 80 W plasma applying power at 30 C by using three different plasma gases, namely, argon (Ar), carbon dioxide (CO2) and nitrogen (N2) (Fig. 6). As shown in Fig. 6, plasma treated CoeBeP catalyst with CO2 was exhibited much greater hydrogen generation rate compared with the plasma treated CoeBeP catalyst with N2 and Ar gases. This result shows that Ar, CO2 and N2 gases can be used to accelerate the hydrogen generation rate. We were used CO2 plasma gas for the hydrogen generation with CoeBeP in this study. We therefore conclude that changes in hydrogen generation performance are related to plasma treated catalyst surface properties. 3.4. Effect plasma treatment time To examine the effects of plasma application time, the hydrogen generation yield was measured by hydrolysis of 2.5 wt% NaBH4 þ 2.5% NaOH solution with CO2 plasma and 80 W plasma applying power at 30 C by using four different plasma application time (3, 5, 10, 20). As shown in Fig. 7, the hydrogen generation rate is dependent on the duration of cold plasma treatment. As observed in Fig. 7, the hydrogen generation rate is increased with increasing of the plasma treatment time from 3 to 5 min and it arrives at a maximum value at 5 min. 10 and 20 min compared to plasma application time of 5 min. However, the hydrogen generation rate with rising plasma treatment from 5 to
Fig. 5. The tests of catalytic activity for the hydrogen generation rate from hydrolysis of NaBH4 with the plasma untreated and plasma treated CoeBeP catalysts.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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Fig. 6. Effect of plasma gases on the hydrogen production from NaBH4 with plasma treated and untreated CoeBeP catalyst for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C.
20 min is decreased. It appears that is decreased the hydrogen generation rate from NaBH4 hydrolysis. In plasma treatment time in 5e20 min range, the decrease in the hydrogen generation rate was most likely to be due to the destruction of the catalyst surface of longer plasma treatment. Therefore, the optimum plasma treatment time was selected as 5 min. 3.5. Effect of NaOH concentration The concentration of NaOH important influences the hydrogen production from hydrolysis of NaBH4. Fig. 8 illustrates the hydrogen generation rate with different NaOH concentration, i.e. 1 wt%, 3 wt%, 5 wt%, and 10 wt%, in 2.5 wt% NaBH4 solution with 25 mg of CoeBeP catalyst at 30 C. When the NaOH concentration increased from 1 wt% to 3 wt%, the hydrogen production rate increased. Then, when the NaOH concentration increased from 3 wt% to 10 wt%, the hydrogen production rate decreased. The probable reason is that hydroxyl ion is involved in the hydrolysis of NaBH4. This occurs primarily because the hydroxyl ions strongly complex water, thus decreasing the available free water needed for NaBH4 hydrolysis [6]. 3.6. Effect of NaBH4 concentration The rate of hydrogen generated versus the concentration of NaBH4 in wt % is given in Fig. 9. Effect of NaBH4 concentration on the hydrogen generation rate was measured using x wt% NaBH4 (x ¼ 1.5, 2.5, 5), 5 wt% NaOH solutions at 30 C using 25 mg of CoeBeP catalyst. As the NaBH4 concentration increases from 1 wt% to 2.5 wt%, the hydrogen generation rate rises monotonically. Then, as the NaBH4 concentration increases from 2.5 wt% to 5 wt%, the rate of hydrogen generation decreases with increase in NaBH4 concentration. It reaches a maximum value around a concentration of 2.5 wt% of NaBH4 and subsequently decreases with further increase in NaBH4 concentration. A great yield of the hydrogen production at lower weight percentage of NaBH4 solution is possibly explained by the reduction of solution viscosity as explained in the hydrolysis of NaBH4 [12].
Fig. 7. Effect of plasma treatment time on the hydrogen production from NaBH4 with plasma treated CoeBeP catalyst for the hydrogen production from 2.5 wt% NaBH4 hydrolysis in 5 wt% NaOH at 30 C with CO2 plasma gas.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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Fig. 8. Effect of NaOH concentration on the hydrogen generation rate measured using x wt% NaOH (x ¼ 1, 3, 5, 10) þ 2.5 wt% NaBH4 solutions 30 C, using 25 mg with plasma treated CoeBeP catalyst.
3.7. Effect of temperature Influence of temperature on hydrogen generation rate in solutions containing 2.5 wt% NaBH4 and 5 wt% NaOH was investigated at temperatures ranging from 30 C to 50 C (Fig. 10). As expected, hydrogen generation rate increases with the temperature. Furthermore, the linear relationship between amount of hydrogen generated and reaction time at each temperature studied was observed. Moreover, the linear dependence between hydrogen generated and reaction time at each temperature was observed, which means to a zero-order reaction. In order to find Arrhenius constant (activation energy, E) for zero-order reaction model, the plot of ln(k) versus 1/T for the temperatures of 30, 40 and 50 C was obtained. The activation energy of zero-order can be obtained from the slope and intercept of the regression line. Arrhenius plot of the hydrogen production rate using CoeBeP catalyst gives the activation energy of about 49.11 kJ mol1. This value is lower than many the activation energy found in literature. In addition, the maximum hydrogen production rate of about 3976 mL/min/g is obtained which is better than many other catalysts (Table 1). 4. Conclusions In this study, the hydrogen production from hydrolysis of NaBH4 in the presence of the catalytic properties of plasma treated CoeBeP catalyst was investigated based on NaBH4 concentration, NaOH concentration, plasma treatment time, plasma gases and temperature. The catalytic activity of CoeBeP catalyst samples was enhanced greatly by plasma treatment, as compared to pure CoeBeP prepared with untreated plasma. It has been demonstrated that the plasma has a beneficial effect on textural properties and catalytic activity. The hydrogen generation rate from hydrolysis of NaBH4 with CoeBeP prepared in the presence of plasma is completed in 12 min time intervals, while the CoeBeP produced in known method let to slower hydrogen release, and the hydrolysis is completed in 30 min time intervals. The maximum hydrogen generation rates of the plasma treated and plasma untreated CoeBeP catalysts toward NaBH4 hydrolysis are 3976 and 1807 mL/g/
Fig. 9. Effect of NaBH4 concentration on the hydrogen generation rate measured using x wt% NaBH4(x ¼ 2.5, 5, 7.5) þ 5 wt% NaOH solutions 30 C, with plasma treated 25 mg CoeBeP catalyst.
€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003
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Fig. 10. Effect of temperature on the hydrogen generation rate measured using 2.5% NaBH4 þ 5 wt% NaOH solutions using 25 mg plasma treated CoeBeP catalyst.
Table 1 Comparison of the catalytic effectiveness for NaBH4 hydrolysis with previous results. Catalysts
Activation energy Ea (kJ mol1)
Hydrogen production rate (mL/min/g)
Reference
Raney Co CoeCo2B CoeB powder Co CoeB CoeMneB CoeCueB Plasma treated CoeBeP Plasma untreated CoeBeP
53.7 35.24 45 75 64.87 52.1 49.6 49.11 e
267.5 350 681 e 880 1440 212 3976 1807
[10] [37] [38] [39] [11] [40] [41] This study This study
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€ S¸ahin, et al., The effects of plasma treatment on electrochemical activity of CoeBeP catalyst for hydrogen Please cite this article in press as: O. production by hydrolysis of NaBH4, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.03.003