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The effects of plasma treatment on electrochemical activity of Co–W–B catalyst for hydrogen production by hydrolysis of NaBH4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 5 2 9 5 e1 5 3 0 1
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
The effects of plasma treatment on electrochemical activity of CoeWeB catalyst for hydrogen production by hydrolysis of NaBH4 Arzu Ekinci a, O¨mer S‚ahin b, Cafer Saka c,*, Tu¨lin Avci b a
Faculty of Science and Letters, Siirt University, 56100 Siirt, Turkey Faculty of Engineering and Architecture, Siirt University, 56100 Siirt, Turkey c School of Healthy, Siirt University, 56100 Siirt, Turkey b
article info
abstract
Article history:
A plasma treatment of CoeWeB catalyst increases the rate of hydrogen generation from
Received 17 July 2013
the hydrolysis of NaBH4. The catalytic properties of CoeWeB prepared in the presence of
Received in revised form
plasma have been investigated as a function of NaBH4 concentration, NaOH concentration,
10 September 2013
temperature, plasma applying time, catalyst amount and plasma gases. The CoeWeB
Accepted 16 September 2013
catalyst prepared with cold plasma effect hydrolysis in only 12 min, where as the CoeWeB
Available online 9 October 2013
catalyst prepared in known method with no plasma treatment in 23 min. The activation energy for first-order reaction is found to be 29.12 kJ mol1.
Keywords:
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Plasma Sodium borohydride CobalteWolframeBoron catalyst Hydrolysis Hydrogen generation
1.
Introduction
Hydrogen as an alternative to traditional energy sources is a major topic of international importance for energy supply and environmental protection. Hydrogen is the best candidate as energy carrier. It is strongly believed that hydrogen can help to address the growing demand for energy and slow down global climate change. Hydrogen fuel cells attract more attention because hydrogen is supposed to be pollution free. 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 hydrides like sodium borohydride (NaBH4) have attracted worldwide interest as a source to supply pure hydrogen to fuel cells at room temperature. Catalytically hydrogen generation from NaBH4 solutions has many advantages: NaBH4 solutions non-flammable, the reaction products environmentally benignable, the rate of hydrogen generation easily controllable, hydrogen generatable even at low temperatures with hydrogen storage capability of 10.8 wt.% [1]. Schlesinger et al. [2] made a detailed study on the hydrolysis reaction of NaBH4 for hydrogen generation. Hydrogen is generated from NaBH4 by following hydrolysis reaction: NaBH4 þ 2H2O / NaBO2 þ 4H2[
* Corresponding author. Tel.: þ90 (484) 223 12 24; fax: þ90 (484) 223 66 31. E-mail address: [email protected] (C. Saka). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.098
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 5 2 9 5 e1 5 3 0 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 [3e5]. Therefore, the catalyst is the key factor that affects the hydrogen generation rate by hydrolysis of NaBH4. Catalysts such as Ru supported catalyst [1,6e8], cobaltbased catalyst [9e14], Ni-based catalyst [15,17], 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]. Despite several advantages of heterogeneous catalysis in hydrogen production, research comprising of the use of metal catalysts for the reactions has been limited due to pessimistic expectations for reaction rates. The limited surface area of the heterogeneous catalysts causes to a lower catalytic activity. It is well known that the catalytic properties of metal catalysts are mainly affected by their surface properties [21]. Metal catalysts, especially those based on nickel, cobalt and copper, have also been developed as economical alternatives. The low-cost metal catalysts normally show relatively low activities compared with the precious catalysts. It is thus that improving the hydrogen generation activity of the low-cost catalyst is becoming imperative and necessary. 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 is an ionized gas that can be generated by a number of methods. Generally, plasma classified in two kinds: thermal plasma called equilibrium plasma and nonthermal plasma called non-equilibrium plasma [25]. The advantages of cold plasma relate to the low temperature that will result in less energy consumption and minimum electrode erosion [26]. A major advantage of cold plasma is its rapid response without induction period that exists in the conventional catalytic reforming processes [27e30]. In nonthermal plasma, the electrical power is very low and temperature of neutral species does not change whereas the temperature of electrons is very high (up to 5000 K) [26]. The potential merit of using plasma is a highly dispersed active species, reduced energy requirements, enhanced catalyst activation, selectivity, and lifetime and shortened preparation time [31]. Among various plasmas, nonthermal plasma can be effective for assisting catalytic hydrogen production from hydrolysis of NaBH4. In earlier our study, we have investigated the hydrogen production from hydrolysis of NaBH4 by using Ni catalyst prepared in plasma reactor [18]. As well, we have reported applications of nonthermal plasma for hydrogen production from hydrolysis of NaBH4 by using Ni and Co catalyst [9,16]. All of these studies show that hydrogen production rate considerably increased by the applications of plasma. In this work, a plasma reactor was employed to examine the effect of nonthermal plasma on CoeWeB catalyst for the hydrogen production from hydrolysis of NaBH4. Besides, a comparative investigation in the absence and the presence of nonthermal plasma was made. The application of our plasma
process is expected to improve further the catalytic performance to CoeWeB catalyst.
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 CoeWeB. The solubility of NaBH4 in water at 25 C is 55 g/100 g water, but the solubility of sodium metaborate (NaBO2) is 28 g/ 100 g water. In this study, the hydrogen generation from NaBH4 hydrolysis with plasma treated CoeWeB catalyst was tested depending on N2, CO2 and Ar plasma gases, plasma applying time (5e20 min), NaBH4 (1e10%) and NaOH (2.5e10%) solutions, catalyst amount (10e75 mg) and temperature (20e50 C).
2.1.
Catalyst preparation
The CoeWeB catalyst in known method was synthesized by chemical reduction method, according to the method described at literature [5]. Catalyst in the form of CoeWeB powder was synthesized by mixing in the cobalt chloride aqueous solution. The solution was reduced by the NaBH4 under vigorous stirring. An excess amount of borohydride was used in order to completely reduce the metal cations.
2.1.1.
Plasma treatment
The plasma treatment of the CoeWeB 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. The dried precipitate sample was put into the reactor and treated by CO2 plasma. The duration of the plasma treatment was 7 min for each sample. The obtained catalyst was stored in airtight plastic container for further use.
2.2.
Characterization
XRD patterns of the samples were acquired in a Bruker D8 Advance X-ray diffractometer with Cu Ka sources. The surface morphologies of the untreated and plasma treated CoeWeB catalysts were analyzed by means of scanning electron microscopy (SEM) (Zeiss EVO 50 Model).
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 CoeWeB catalyst was dropped into the designated temperature solution to initiate hydrolysis reaction. The solution temperature was maintained at a constant 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
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 5 2 9 5 e1 5 3 0 1
subsequently converted into yield of produced hydrogen after the total amount of gas had been collected.
3.
Results and discussion
3.1.
Catalyst characterization
Fig. 1 showed X-ray diffraction (XRD) patterns of the plasma treated and untreated CoeWeP catalyst samples. For the plasma treated and untreated CoeWeB catalyst samples, the coexistence of amorphous and crystalline phases is observed: a broad dispersive peak at 2q ¼ 20 e35 indicates that sample CoeBeW still has some amorphous characteristics, while the sharp peaks at 2q ¼ 35 e40 imply an evolution of the structure to a crystalline phase. There is no significant change in the XRD patterns of the plasma treated CoeBeW catalyst. The reason probably is that during the plasma treatment could not result in the crystal change of the catalyst. After being plasma treated, only a relatively two weak diffraction peaks at 2q ¼ 110 e120 appeared indicating of the metallic species. For all the CoeWeB catalysts, no sharp peaks of Co power were detected.
3.2.
Effects of plasma upon surface topography
The surface morphologies of plasma untreated (a, b, c) and treated (d, e, f) the CoeWeB catalysts are shown in Fig. 2. Visible changes in surface topography can be observed after
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plasma treatment, as evident from SEM images. The untreated catalyst has a relatively smooth morphology, while that of plasma treated catalyst is of higher roughness. As seen by that, surface morphology of plasma treated CoeWeB catalyst is flaky particles that resulting products agglomerate with varying sizes. More agglomeration of the small particles can be seen in the SEM image of CoeWeB catalyst. This is the result of the application of plasma treatment on the surface of catalyst.
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. 3). We compared the catalytic performance of CoeWeB catalyst prepared with known method and the CoeWeB after plasma treatment. As shown in Fig. 3, plasma treated CoeWeB catalysts exhibited much greater hydrogen generation rate compared with the plasma untreated CoeWeB catalyst. The hydrogen generation rate increased in efficiency by applying three different plasma gases. This result shows that Ar, CO2 and N2 gases can be used to accelerate the hydrogen generation rate. It can be seen that the hydrogen generation rate from hydrolysis of NaBH4 with CoeWeB prepared in the presence of plasma is completed in 12 min time intervals, while the CoeWeB produced in known method let to slower hydrogen release, and the hydrolysis is completed in 23 min time intervals. We therefore conclude that changes in hydrogen generation performance are related to plasma treated catalyst surface properties. Best production rate of hydrogen is achieved by CO2 plasma gas. We used CO2 plasma gas for the hydrogen generation with CoeWeB in this study.
3.4.
Effect of 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 (5, 7, 10, 20). As shown in Fig. 4, the hydrogen generation rate is dependent on the duration of cold plasma treatment. As observed in Fig. 4, the hydrogen generation rate increased with 7, 10 and 20 min compared to plasma application time of 5 min. However, the hydrogen generation rate remained almost constant by applying three different plasma application times. It appears that longer plasma treatment on the CoeBeW catalyst was increased the hydrogen generation rate from NaBH4 hydrolysis. We were used plasma application time of 7 min for the hydrogen generation from NaBH4 hydrolysis with CoeWeB in this study.
3.5.
Fig. 1 e The comparison of XRD spectra of CoeWeB catalyst before (a) and after (b) plasma treatment.
Effect of catalyst amount
To examine the effects of CoeWeB catalyst amount, the hydrogen generation yield was measured by hydrolysis of 2.5 wt.% NaBH4 þ 2.5% NaOH solution. Fig. 5 presents the
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Fig. 2 e The comparison of SEM of CoeWeB catalyst before (a, b, c) and after (d, e, f) plasma treatment.
effect of catalyst amount that were studied by varying the amount of from 10 mg to 75 mg. The effect of catalyst amount on catalytic activity is so evident that even a small amount of catalyst is able to double the activity of CoeWeB catalyst. The hydrogen generation rate increases with catalyst amount. These results show that hydrogen generation rate can be determined by controlling the catalyst amount.
Fig. 3 e Effect of plasma gases on hydrogen production from NaBH4 with plasma treated and untreated CoeWeB catalyst.
3.6.
Effect of NaOH concentration
The concentration of NaOH important influences the hydrogen production from hydrolysis of NaBH4. Fig. 6 illustrates the hydrogen generation rate with different NaOH concentration, i.e. 0 wt.%, 2.5 wt.%, 5 wt.%, and 10 wt.%, in 2.5 wt.% NaBH4 solution with 50 mg of CoeWeB catalyst at 30 C. When the NaOH concentration increased from 0 wt.% to 2.5 wt.%, the hydrogen production rate increased. Then, when the NaOH concentration increased from 2.5 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 [1]. Ding et al. [32], by using CoeCueB -based catalyst reported an effect similar to ours with respect to NaOH concentration. Authors stated that probable reason is that OH is involved in the hydrolysis of NaBH4 and an appropriate increase of NaOH concentration can accelerate the catalyzed NaBH4 hydrolysis and enhance the hydrogen generation rate. In addition, the authors reported that excessive concentration of NaOH would lead to decrease of NaBO2 solubility and the subsequent precipitation from the solution and adherence on the catalyst surface. As a result, the hydrolysis reaction is hindered.
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Fig. 6 e Effect of NaOH concentration on hydrogen production from NaBH4 with plasma treated CoeWeB catalyst. Fig. 4 e Effect of plasma treatment time on hydrogen production from NaBH4 with plasma treated CoeWeB catalyst.
3.7.
In addition, when NaBH4 concentration is larger, the concentration of NaBO2 may exceed the solubility limit. NaBO2 may precipitate. It could be insoluble NaBO2 is coating and blocking the active site on catalyst surface catalyst [7,19,32,33].
Effect of NaBH4 concentration
The rate of hydrogen generated versus the concentration of NaBH4 in wt.% is given in Fig. 7. Effect of NaBH4 concentration on the hydrogen generation rate was measured using x wt.% NaBH4 (x ¼ 1, 2.5, 5, 10), 2.5 wt.% NaOH solutions at 30 C using 50 mg of CoeWeB catalyst. As the NaBH4 concentration increases from 1 wt.% to 2. 5 wt.%, the average hydrogen generation rate rises monotonically. Then, as the NaBH4 concentration increases from 2.5 wt.% to 10 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. 5 e Effect of catalyst amount on hydrogen production from NaBH4 with plasma treated CoeWeB catalyst.
3.8.
Effect of temperature
Influence of temperature on hydrogen generation rate in solutions containing 2.5 wt % NaBH4 and 2.5 wt % NaOH was investigated at temperatures ranging from 20 C to 50 C (Fig. 8). 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. One aim of this work is to determine an appropriate kinetic model and the activation energy of hydrolysis of NaBH4 in the presence of CoeWeB catalyst prepared by plasma. In this study, the following first-order kinetic model applied to describe the behavior of the hydrolysis reaction of hydrogen generation by using an integral method.
Fig. 7 e Effect of NaBH4 concentration on hydrogen production from NaBH4 with plasma treated CoeWeB catalyst.
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Table 1 e Kinetic parameters for hydrogen production from hydrolysis of sodium borohydride with CoeWeB catalyst prepared in the presence plasma for first-order model. First-order
Fig. 8 e Effect of temperature on hydrogen production from NaBH4 with plasma treated CoeWeB catalyst.
The reaction rate based on first-order kinetics can be described as rNaBH4 ¼
dCNaBH4 ¼ kCNaBH4 dt
(2)
By integrating Equation (2), following equation is obtained CNaBH4 ðt¼0Þ ¼ kt ln CNaBH4 ðt¼tÞ
(3)
A plot of lnðCNaBH4 ðt¼0Þ =CNaBH4 ðt¼tÞ Þ as a function of time should give a straight line and the slope of line can be used to calculate the first-order rate constant, k. The plot of lnðCNaBH4 ðt¼0Þ =CNaBH4 ðt¼tÞ Þ versus time at temperatures of 30, 40 and 50 C has a good linear regression with a correlation coefficient of 0.999. In order to find Arrhenius constant activation energy, E, for first-order reaction model, the plot of ln(k) versus 1/T for the temperatures of 30, 40 and 50 C was obtained (Fig. 9). The activation energy of first-order can be obtained from the slope and intercept of the regression line. Arrhenius plot of the hydrogen production rate using CoeWeB catalyst gives the activation energy of about 29.12 kJ mol1 for the plasma treated catalyst. This value is lower than the activation energy of 41 kJ mol1 found in known method with no plasma treatment [5].
T
k (s1)
R2
20 30 40 50
0.000488 0.007075 0.011202 0.01447
0.993 0.997 0.996 0.993
Table 1 summarizes the following regression data for firstorder at temperatures range of 30e50 C, the reaction rate constants found by the slope of linear regression and the correlation coefficients for regression. The kinetic parameter (k) obtained at different temperatures is given in Table 1.
4.
Conclusions
In this study, the hydrogen production from hydrolysis of NaBH4 in the presence of the catalytic properties of plasma treated CoeWeB catalyst was investigated based on NaBH4 concentration, NaOH concentration, catalyst amount, plasma treatment time, plasma gases and temperature. The catalytic activity of CoeWeB catalyst samples was enhanced greatly by plasma treatment, as compared to pure CoeWeB prepared with untreated plasma. The hydrogen generation rate from hydrolysis of NaBH4 with CoeWeB prepared in the presence of plasma is completed in 12 min time intervals, while the CoeWeB produced in known method let to slower hydrogen release, and the hydrolysis is completed in 23 min time intervals. Characteristics of this plasma treated CoeWeB-based catalyst were carried out by using SEM and XRD. Hydrolysis kinetics of NaBH4 was investigated at a temperature range of 20e50 C and first-order kinetic was applied to the obtained data. The activation energy was found to be 29.12 kJ mol1 for first-order. It can be concluded that the plasma may be used in the hydrogen generation from hydrolysis of NaBH4 in the presence of CoeWeB catalyst.
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Fig. 9 e Arrhenius equation according to first-order.
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
The effects of plasma treatment on electrochemical activity of CoeWeB catalyst for hydrogen production by hydrolysis of NaBH4 Arzu Ekinci a, O¨mer S‚ahin b, Cafer Saka c,*, Tu¨lin Avci b a
Faculty of Science and Letters, Siirt University, 56100 Siirt, Turkey Faculty of Engineering and Architecture, Siirt University, 56100 Siirt, Turkey c School of Healthy, Siirt University, 56100 Siirt, Turkey b
article info
abstract
Article history:
A plasma treatment of CoeWeB catalyst increases the rate of hydrogen generation from
Received 17 July 2013
the hydrolysis of NaBH4. The catalytic properties of CoeWeB prepared in the presence of
Received in revised form
plasma have been investigated as a function of NaBH4 concentration, NaOH concentration,
10 September 2013
temperature, plasma applying time, catalyst amount and plasma gases. The CoeWeB
Accepted 16 September 2013
catalyst prepared with cold plasma effect hydrolysis in only 12 min, where as the CoeWeB
Available online 9 October 2013
catalyst prepared in known method with no plasma treatment in 23 min. The activation energy for first-order reaction is found to be 29.12 kJ mol1.
Keywords:
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Plasma Sodium borohydride CobalteWolframeBoron catalyst Hydrolysis Hydrogen generation
1.
Introduction
Hydrogen as an alternative to traditional energy sources is a major topic of international importance for energy supply and environmental protection. Hydrogen is the best candidate as energy carrier. It is strongly believed that hydrogen can help to address the growing demand for energy and slow down global climate change. Hydrogen fuel cells attract more attention because hydrogen is supposed to be pollution free. 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 hydrides like sodium borohydride (NaBH4) have attracted worldwide interest as a source to supply pure hydrogen to fuel cells at room temperature. Catalytically hydrogen generation from NaBH4 solutions has many advantages: NaBH4 solutions non-flammable, the reaction products environmentally benignable, the rate of hydrogen generation easily controllable, hydrogen generatable even at low temperatures with hydrogen storage capability of 10.8 wt.% [1]. Schlesinger et al. [2] made a detailed study on the hydrolysis reaction of NaBH4 for hydrogen generation. Hydrogen is generated from NaBH4 by following hydrolysis reaction: NaBH4 þ 2H2O / NaBO2 þ 4H2[
* Corresponding author. Tel.: þ90 (484) 223 12 24; fax: þ90 (484) 223 66 31. E-mail address: [email protected] (C. Saka). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.098
(1)
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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 [3e5]. Therefore, the catalyst is the key factor that affects the hydrogen generation rate by hydrolysis of NaBH4. Catalysts such as Ru supported catalyst [1,6e8], cobaltbased catalyst [9e14], Ni-based catalyst [15,17], 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]. Despite several advantages of heterogeneous catalysis in hydrogen production, research comprising of the use of metal catalysts for the reactions has been limited due to pessimistic expectations for reaction rates. The limited surface area of the heterogeneous catalysts causes to a lower catalytic activity. It is well known that the catalytic properties of metal catalysts are mainly affected by their surface properties [21]. Metal catalysts, especially those based on nickel, cobalt and copper, have also been developed as economical alternatives. The low-cost metal catalysts normally show relatively low activities compared with the precious catalysts. It is thus that improving the hydrogen generation activity of the low-cost catalyst is becoming imperative and necessary. 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 is an ionized gas that can be generated by a number of methods. Generally, plasma classified in two kinds: thermal plasma called equilibrium plasma and nonthermal plasma called non-equilibrium plasma [25]. The advantages of cold plasma relate to the low temperature that will result in less energy consumption and minimum electrode erosion [26]. A major advantage of cold plasma is its rapid response without induction period that exists in the conventional catalytic reforming processes [27e30]. In nonthermal plasma, the electrical power is very low and temperature of neutral species does not change whereas the temperature of electrons is very high (up to 5000 K) [26]. The potential merit of using plasma is a highly dispersed active species, reduced energy requirements, enhanced catalyst activation, selectivity, and lifetime and shortened preparation time [31]. Among various plasmas, nonthermal plasma can be effective for assisting catalytic hydrogen production from hydrolysis of NaBH4. In earlier our study, we have investigated the hydrogen production from hydrolysis of NaBH4 by using Ni catalyst prepared in plasma reactor [18]. As well, we have reported applications of nonthermal plasma for hydrogen production from hydrolysis of NaBH4 by using Ni and Co catalyst [9,16]. All of these studies show that hydrogen production rate considerably increased by the applications of plasma. In this work, a plasma reactor was employed to examine the effect of nonthermal plasma on CoeWeB catalyst for the hydrogen production from hydrolysis of NaBH4. Besides, a comparative investigation in the absence and the presence of nonthermal plasma was made. The application of our plasma
process is expected to improve further the catalytic performance to CoeWeB catalyst.
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 CoeWeB. The solubility of NaBH4 in water at 25 C is 55 g/100 g water, but the solubility of sodium metaborate (NaBO2) is 28 g/ 100 g water. In this study, the hydrogen generation from NaBH4 hydrolysis with plasma treated CoeWeB catalyst was tested depending on N2, CO2 and Ar plasma gases, plasma applying time (5e20 min), NaBH4 (1e10%) and NaOH (2.5e10%) solutions, catalyst amount (10e75 mg) and temperature (20e50 C).
2.1.
Catalyst preparation
The CoeWeB catalyst in known method was synthesized by chemical reduction method, according to the method described at literature [5]. Catalyst in the form of CoeWeB powder was synthesized by mixing in the cobalt chloride aqueous solution. The solution was reduced by the NaBH4 under vigorous stirring. An excess amount of borohydride was used in order to completely reduce the metal cations.
2.1.1.
Plasma treatment
The plasma treatment of the CoeWeB 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. The dried precipitate sample was put into the reactor and treated by CO2 plasma. The duration of the plasma treatment was 7 min for each sample. The obtained catalyst was stored in airtight plastic container for further use.
2.2.
Characterization
XRD patterns of the samples were acquired in a Bruker D8 Advance X-ray diffractometer with Cu Ka sources. The surface morphologies of the untreated and plasma treated CoeWeB catalysts were analyzed by means of scanning electron microscopy (SEM) (Zeiss EVO 50 Model).
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 CoeWeB catalyst was dropped into the designated temperature solution to initiate hydrolysis reaction. The solution temperature was maintained at a constant 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
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subsequently converted into yield of produced hydrogen after the total amount of gas had been collected.
3.
Results and discussion
3.1.
Catalyst characterization
Fig. 1 showed X-ray diffraction (XRD) patterns of the plasma treated and untreated CoeWeP catalyst samples. For the plasma treated and untreated CoeWeB catalyst samples, the coexistence of amorphous and crystalline phases is observed: a broad dispersive peak at 2q ¼ 20 e35 indicates that sample CoeBeW still has some amorphous characteristics, while the sharp peaks at 2q ¼ 35 e40 imply an evolution of the structure to a crystalline phase. There is no significant change in the XRD patterns of the plasma treated CoeBeW catalyst. The reason probably is that during the plasma treatment could not result in the crystal change of the catalyst. After being plasma treated, only a relatively two weak diffraction peaks at 2q ¼ 110 e120 appeared indicating of the metallic species. For all the CoeWeB catalysts, no sharp peaks of Co power were detected.
3.2.
Effects of plasma upon surface topography
The surface morphologies of plasma untreated (a, b, c) and treated (d, e, f) the CoeWeB catalysts are shown in Fig. 2. Visible changes in surface topography can be observed after
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plasma treatment, as evident from SEM images. The untreated catalyst has a relatively smooth morphology, while that of plasma treated catalyst is of higher roughness. As seen by that, surface morphology of plasma treated CoeWeB catalyst is flaky particles that resulting products agglomerate with varying sizes. More agglomeration of the small particles can be seen in the SEM image of CoeWeB catalyst. This is the result of the application of plasma treatment on the surface of catalyst.
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. 3). We compared the catalytic performance of CoeWeB catalyst prepared with known method and the CoeWeB after plasma treatment. As shown in Fig. 3, plasma treated CoeWeB catalysts exhibited much greater hydrogen generation rate compared with the plasma untreated CoeWeB catalyst. The hydrogen generation rate increased in efficiency by applying three different plasma gases. This result shows that Ar, CO2 and N2 gases can be used to accelerate the hydrogen generation rate. It can be seen that the hydrogen generation rate from hydrolysis of NaBH4 with CoeWeB prepared in the presence of plasma is completed in 12 min time intervals, while the CoeWeB produced in known method let to slower hydrogen release, and the hydrolysis is completed in 23 min time intervals. We therefore conclude that changes in hydrogen generation performance are related to plasma treated catalyst surface properties. Best production rate of hydrogen is achieved by CO2 plasma gas. We used CO2 plasma gas for the hydrogen generation with CoeWeB in this study.
3.4.
Effect of 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 (5, 7, 10, 20). As shown in Fig. 4, the hydrogen generation rate is dependent on the duration of cold plasma treatment. As observed in Fig. 4, the hydrogen generation rate increased with 7, 10 and 20 min compared to plasma application time of 5 min. However, the hydrogen generation rate remained almost constant by applying three different plasma application times. It appears that longer plasma treatment on the CoeBeW catalyst was increased the hydrogen generation rate from NaBH4 hydrolysis. We were used plasma application time of 7 min for the hydrogen generation from NaBH4 hydrolysis with CoeWeB in this study.
3.5.
Fig. 1 e The comparison of XRD spectra of CoeWeB catalyst before (a) and after (b) plasma treatment.
Effect of catalyst amount
To examine the effects of CoeWeB catalyst amount, the hydrogen generation yield was measured by hydrolysis of 2.5 wt.% NaBH4 þ 2.5% NaOH solution. Fig. 5 presents the
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Fig. 2 e The comparison of SEM of CoeWeB catalyst before (a, b, c) and after (d, e, f) plasma treatment.
effect of catalyst amount that were studied by varying the amount of from 10 mg to 75 mg. The effect of catalyst amount on catalytic activity is so evident that even a small amount of catalyst is able to double the activity of CoeWeB catalyst. The hydrogen generation rate increases with catalyst amount. These results show that hydrogen generation rate can be determined by controlling the catalyst amount.
Fig. 3 e Effect of plasma gases on hydrogen production from NaBH4 with plasma treated and untreated CoeWeB catalyst.
3.6.
Effect of NaOH concentration
The concentration of NaOH important influences the hydrogen production from hydrolysis of NaBH4. Fig. 6 illustrates the hydrogen generation rate with different NaOH concentration, i.e. 0 wt.%, 2.5 wt.%, 5 wt.%, and 10 wt.%, in 2.5 wt.% NaBH4 solution with 50 mg of CoeWeB catalyst at 30 C. When the NaOH concentration increased from 0 wt.% to 2.5 wt.%, the hydrogen production rate increased. Then, when the NaOH concentration increased from 2.5 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 [1]. Ding et al. [32], by using CoeCueB -based catalyst reported an effect similar to ours with respect to NaOH concentration. Authors stated that probable reason is that OH is involved in the hydrolysis of NaBH4 and an appropriate increase of NaOH concentration can accelerate the catalyzed NaBH4 hydrolysis and enhance the hydrogen generation rate. In addition, the authors reported that excessive concentration of NaOH would lead to decrease of NaBO2 solubility and the subsequent precipitation from the solution and adherence on the catalyst surface. As a result, the hydrolysis reaction is hindered.
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Fig. 6 e Effect of NaOH concentration on hydrogen production from NaBH4 with plasma treated CoeWeB catalyst. Fig. 4 e Effect of plasma treatment time on hydrogen production from NaBH4 with plasma treated CoeWeB catalyst.
3.7.
In addition, when NaBH4 concentration is larger, the concentration of NaBO2 may exceed the solubility limit. NaBO2 may precipitate. It could be insoluble NaBO2 is coating and blocking the active site on catalyst surface catalyst [7,19,32,33].
Effect of NaBH4 concentration
The rate of hydrogen generated versus the concentration of NaBH4 in wt.% is given in Fig. 7. Effect of NaBH4 concentration on the hydrogen generation rate was measured using x wt.% NaBH4 (x ¼ 1, 2.5, 5, 10), 2.5 wt.% NaOH solutions at 30 C using 50 mg of CoeWeB catalyst. As the NaBH4 concentration increases from 1 wt.% to 2. 5 wt.%, the average hydrogen generation rate rises monotonically. Then, as the NaBH4 concentration increases from 2.5 wt.% to 10 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. 5 e Effect of catalyst amount on hydrogen production from NaBH4 with plasma treated CoeWeB catalyst.
3.8.
Effect of temperature
Influence of temperature on hydrogen generation rate in solutions containing 2.5 wt % NaBH4 and 2.5 wt % NaOH was investigated at temperatures ranging from 20 C to 50 C (Fig. 8). 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. One aim of this work is to determine an appropriate kinetic model and the activation energy of hydrolysis of NaBH4 in the presence of CoeWeB catalyst prepared by plasma. In this study, the following first-order kinetic model applied to describe the behavior of the hydrolysis reaction of hydrogen generation by using an integral method.
Fig. 7 e Effect of NaBH4 concentration on hydrogen production from NaBH4 with plasma treated CoeWeB catalyst.
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Table 1 e Kinetic parameters for hydrogen production from hydrolysis of sodium borohydride with CoeWeB catalyst prepared in the presence plasma for first-order model. First-order
Fig. 8 e Effect of temperature on hydrogen production from NaBH4 with plasma treated CoeWeB catalyst.
The reaction rate based on first-order kinetics can be described as rNaBH4 ¼
dCNaBH4 ¼ kCNaBH4 dt
(2)
By integrating Equation (2), following equation is obtained CNaBH4 ðt¼0Þ ¼ kt ln CNaBH4 ðt¼tÞ
(3)
A plot of lnðCNaBH4 ðt¼0Þ =CNaBH4 ðt¼tÞ Þ as a function of time should give a straight line and the slope of line can be used to calculate the first-order rate constant, k. The plot of lnðCNaBH4 ðt¼0Þ =CNaBH4 ðt¼tÞ Þ versus time at temperatures of 30, 40 and 50 C has a good linear regression with a correlation coefficient of 0.999. In order to find Arrhenius constant activation energy, E, for first-order reaction model, the plot of ln(k) versus 1/T for the temperatures of 30, 40 and 50 C was obtained (Fig. 9). The activation energy of first-order can be obtained from the slope and intercept of the regression line. Arrhenius plot of the hydrogen production rate using CoeWeB catalyst gives the activation energy of about 29.12 kJ mol1 for the plasma treated catalyst. This value is lower than the activation energy of 41 kJ mol1 found in known method with no plasma treatment [5].
T
k (s1)
R2
20 30 40 50
0.000488 0.007075 0.011202 0.01447
0.993 0.997 0.996 0.993
Table 1 summarizes the following regression data for firstorder at temperatures range of 30e50 C, the reaction rate constants found by the slope of linear regression and the correlation coefficients for regression. The kinetic parameter (k) obtained at different temperatures is given in Table 1.
4.
Conclusions
In this study, the hydrogen production from hydrolysis of NaBH4 in the presence of the catalytic properties of plasma treated CoeWeB catalyst was investigated based on NaBH4 concentration, NaOH concentration, catalyst amount, plasma treatment time, plasma gases and temperature. The catalytic activity of CoeWeB catalyst samples was enhanced greatly by plasma treatment, as compared to pure CoeWeB prepared with untreated plasma. The hydrogen generation rate from hydrolysis of NaBH4 with CoeWeB prepared in the presence of plasma is completed in 12 min time intervals, while the CoeWeB produced in known method let to slower hydrogen release, and the hydrolysis is completed in 23 min time intervals. Characteristics of this plasma treated CoeWeB-based catalyst were carried out by using SEM and XRD. Hydrolysis kinetics of NaBH4 was investigated at a temperature range of 20e50 C and first-order kinetic was applied to the obtained data. The activation energy was found to be 29.12 kJ mol1 for first-order. It can be concluded that the plasma may be used in the hydrogen generation from hydrolysis of NaBH4 in the presence of CoeWeB catalyst.
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Fig. 9 e Arrhenius equation according to first-order.
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