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Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2)
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 x x x ( 2 0 1 5 ) 1 e7
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Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2) Aysel Kantu¨rk Figen*, Bilge Cos‚kuner, Sabriye Pis‚kin Department of Chemical Engineering, Yildiz Technical University, Istanbul 34210, Turkey
article info
abstract
Article history:
In the present study, waste Mg based material (W/Mg) was used as a raw material for the
Received 25 August 2014
generation of hydrogen. Ball milling and saline solution method were performed to ensure
Received in revised form
full completion of the hydrolysis reaction. Improved texture properties were obtained by
10 December 2014
ball milling W/Mg for 15 h (2.29 m2 g1 BET area, 11.94 mm particle size). Ball milled W/Mg
Accepted 8 January 2015
was used to generate hydrogen gas in the presence of various saline solutions (1 M NiCl2,
Available online xxx
CoCl2, CuCl2, FeCl3, and MnCl2). While hydrogen generation was not observed in pure water, maximum conversion (100%) was achieved in 1 M NiCl2 solution.
Keywords:
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen generation Mg waste Saline solution method Ball milling
Introduction The prevention of environmental pollution is one of the primary goals of green energy engineering. As part of the pursuit of this goal, hydrogen (H2) is the most promising candidate for replacing traditional fossil fuels as an energy source. However, large-scale H2 production methods that use fossil fuels have such by-products as CO, CO2 and other contaminants. Hence, an economical and ‘‘green’’ method of producing H2 is required before H2 can become a truly clean energy source [1,2]. Metal utilization is an effective, user-friendly, and safe way of solving these problems for both H2 production and energy storage [3]. Many studies have investigated H2 production
from highly chemically active metallic powders. These metals, such as Al [4,5], Mg [6,7], and Li [8,9], are reacted with water and evaluated in regards to H2 production. Mg and Al would be first choices, not only due to their low toxicity but also their higher activity [10,11]. Though Al has higher H2 yield, the highly protective properties of the Al2O3 film put Mg first in the list [11e13]. H2 generation from a MgeAl based intermetallic system can be obtained through the following reactions [11]:
Mg(s) þ 2H2O(l) / Mg(OH)2(s) þ H2(g)
(1)
2Al(s) þ 6H2O(l) / 2Al(OH)3(s) þ 3H2(g)
(2)
* Corresponding author. Tel.: þ90 2123834774; fax: þ90 2123834725. E-mail addresses: [email protected], [email protected] (A. Kantu¨rk Figen). http://dx.doi.org/10.1016/j.ijhydene.2015.01.022 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kantu¨rk Figen A, et al., Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2), International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.01.022
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H2 evolved at dissolving Mg and Al surfaces, including those subjected to anodic polarization above its corrosion potential [14]. Furthermore, metal reactivity has been improved by many researchers via usage of high-energy milling procedures [7,11,12,15,16]. Also MgeAl based systems show high H2 evolution rates in presence of water and activator. The H2 evolution from aqueous solutions is based on the cationic/anodic reduction of H or H2 molecules in H2O and a highly interactive material [1,17]: The cathode reaction may be:
2H2O þ 2e / H2 þ 2OH
(3)
and/or
2Hþ þ 2e / H2
(4)
The anodic reaction should be:
Mg / Mg2þ þ 2e
(5)
Al / Al3þ þ 3e
(6)
The rate of the reaction increases while using a highly corrosive medium. For improving the H2 yield, the addition of acid [1,7] and saline [7,8,18,19] into aqueous system or the use of sea water [11,12] have been used by many researchers. On the other hand, acidic solutions are toxic and hazardous for the environment and it's inhabitants [1,10,17]. Also, Mg scraps have become a substantial problem due to challenges in recycling, reusing and waste treatment. The possibility of generating H2 from Mg scraps or low-grade Mg wastes leads to a new application area for these wastes which cannot be recycled economically due to the current lack of sufficiently efficient purification technologies [7,20e22]. Mg scraps contain certain metals such as Al, Ni and Cu in their structure [2] which accelerate H2 evolution. Zou et al. (2011) studied Mgbased hydro-reactive materials by optimizing the milling preparation conditions and investigated the reactive properties of their materials in sea water [11,12]. Yu et al. (2012) reported a method of producing H2 by mixing low-grade Mg scraps in aqueous organic acids with addition of NaCl [7]. Zou et al. (2012) indicated that performance of MgeAl based systems could be greatly improved via a high ball milling process [11]. Liu et al. achieved 100% efficiency after 3 h of milling 3 mol% AlCl3eMg powder. There are no literature reports focused on the effects different saline solutions on the H2 generation characteristics of Mg waste. In this work, W/Mg, defined as a waste Mg based material, was used in presence of various saline solutions (1 M NiCl2, CoCl2, CuCl2, FeCl3, and MnCl2) to generate H2 gas. Before the hydrolysis reaction, W/Mg15 was prepared by ball milling for 15 h in order to improve the surface characteristics of the particles. While H2 generation was not observed in pure water, maximum conversion (100%) was achieved in 1 M NiCl2 solution.
Experimental Materials and characterizations W/Mg provided by a gold manufacturing factory was used as the main material in the present study. Waste Mg was molded into the form of a chip using a plastics machining process. Structural characterization of W/Mg was performed by X-ray diffraction (XRD), scanned electron microcopy-electron dispersion spectroscopy (SEM-EDS), BrunauereEmmetteTeller (BET) and X-ray fluorescence spectroscopy (XRF) techniques in our previous study [23]. It was decided that the waste Mg chip used in the study would be called “Mg-Rich Intermetallic material” as indicated by the results of the characterization analyses. In this study, W/Mg was mechanically ground using a planetary type ball-milling apparatus at a speed of 300 rpm for different grinding times (3, 6, 9, 15 and 30 h). The ball-topowder ratio was 70:1. The codes of the samples corresponded to the milling time used; W/Mg (no milling), and W/ Mg3, W/Mg6, W/Mg9, W/Mg15 and, W/Mg30, wherein the number indicates the hours of milling applied in the preparation of the sample. After grinding, SEM and BET analyses were performed on W/Mg samples in order to determine the particle size changes as a function of milling time. BET analyses of samples were done twice and average values with a maximum error of ±10% were considered as a single observation. In addition, XRD, BET, SEM-EDS and XRF analyses (Figs. 1e4, Tables 1 and 2) were carried out in order to determine the W/Mg texture properties that are associated with superior hydrogen generation.
Hydrogen generation tests H2 generation from ball milled W/Mg in various saline solutions (1 M NiCl2, CoCl2, CuCl2, FeCl3, and MnCl2) was performed based on a typical water-replacement procedure in a 10 ml glass reactor (Fig. 5). Various 1 M saline solutions were prepared from chloride salts powders (Merck, 99%) by dissolving in pure water. The reactor was connected to a waterfilled inverted burette to measure the volume of H2 gas that evolved from the 0.01 g W/Mg15 in the presence of NiCl2, CoCl2, CuCl2, FeCl3, and MnCl2 (1 M) solutions. The measurement of H2 generation was performed at moderate temperature with magnetic stirring (500 rpm). H2 generation time was
Fig. 1 e XDR patterns of W/Mg samples.
Please cite this article in press as: Kantu¨rk Figen A, et al., Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2), International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.01.022
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 x x x ( 2 0 1 5 ) 1 e7
Table 1 e Crystalline phases of the W/Mg samples. Sample W/Mg3 W/Mg6 W/Mg9 W/Mg15 W/Mg30
3
and second-order reaction kinetic models were applied for the whole reaction and for sectional regions (Table 3).
JCPDS number Mg (00-001-1148), Mg (01-089-4894), Mg (01-089-5003), Mg (01-089-4894), Mg (01-089-4894), 7752)
Fe (03-065-5099) Fe (03-006-0696) Fe (03-065-5099) Fe (01-089-7194) Fe19Ni (03-065-
Fig. 2 e Specific surface area of W/Mg samples.
measured by chronometer until no more H2 evolution was observed. For investigating effects of various saline solutions on H2 generation from W/Mg15, the H2 generation rate and reaction kinetic tendency were carried out regionally (Figs. 6e8). For reaction kinetic modeling, zero-order, first-order
Results and discussion Structural characterization of W/Mg Fig. 1 shows the XRD patterns of W/Mg samples. Main diffraction peaks were associated with Mg structure for all the samples. Fe was the secondary face of the W/Mg3, W/Mg6, W/ Mg9 and W/Mg15 samples while ironenickel (Fe19Ni) was detected in the W/Mg30 sample. Crystal phases with the matched JCPDS card numbers are given in Table 1. Fig. 2 shows multi-point BET analysis results of unmilled W/Mg and W/Mg samples. Specific surface area of unmilled W/Mg was determined as only 0.08 m2/g. 3, 6, 9, 15 and 30 h of milling increased surface areas by approximately 19, 21, 25, 29 and 32 times respectively. According to the BET surface area results, exceeding 15 h of grinding time did not significantly increase surface area. Fig. 3 shows the SEM images at 2000 magnification of W/ Mg samples. Average particle size was determined based on SEM images. After 3 h milling, the waste was in the chip form and particles with grains were not achieved. As grinding time was increased, particles were formed. Mg particles had an average size of 11.94 mm after 30 h milling. A particle size of 12.18 mm was obtained after 15 h of milling. It was decided that milling for more than 15 h did not considerably change the particle size; thus 15 h was considered the optimal time (Fig. 4). Table 2 shows the EDS analysis results. As can be seen, average compositions of W/Mg samples were determined as
Fig. 3 e SEM images (2000£ magnification) of W/Mg samples: (a) W/Mg3, (b) W/Mg6, (c) W/Mg9, (d) W/Mg15, (e) W/Mg30. Please cite this article in press as: Kantu¨rk Figen A, et al., Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2), International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.01.022
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Fig. 4 e Average particular size of W/Mg samples.
83.81% Mg, 10.45% Al, 0.85 Mn%, 0.56% Zn, 3.21% Fe, 0.05% Co, 0.30 Ti%, 0.38% V, 0.60% Cr, 0.80% Cu and 0.07 Ni %. In previous studies, researchers investigated H2 generation properties of energy ball-milling prepared alloys of Mg and Al with the addition of different elements [24,25]. Fan et al. obtained a complete theoretical conversion yield from Al alloys that had been prepared by alloying. In this case, we did not need to add further elements to our composites which is ideal given that preparation of such alloys is expensive and time consuming. Decreasing the Mg particle size and increasing the specific exposed surface area of powders improves the contact between solution and metal particles. The ball milling process provided these properties depending on the metal properties. The pitting corrosion process on surface of metals forms fresh surfaces and defects. By controlling the milling time, oxidation of active surface and specific surface area of powders can be controlled, which has a crucial role on the H2 generation reaction [3,26,27].
Hydrogen generation tests The theoretical H2 generation capacity of W/Mg15 was calculated as 89.39 l g1W/Mg15 according to eqs. (1), (2) and (7) by using EDS results for the sample. In Eq. (7), weight of generated H2 is mH2 , weight ratio of generated H2 is xH2 , molecular weight of H2 is MH2 , and molecular weight and weight ratio of I compound are Mi and, xi, respectively [28]. % mH2 ¼
xH2 MH2 100 xH2 MH2 þ xi Mi
(7)
Fig. 5 e Hydrogen generation system: a-Temperature controlled magnetic stirrer, b-Three necked reactor, c-W/ Mg, D-Saline solution, e-Injector, f-Silicon pipe for H2 gas exit, g-Thermocouple, h-Beaker with full of water, lGenerating H2 gas volume, j-Laboratory stand, k-Jacket.
According to the characterization results, W/Mg15 was selected as a metallic powder for hydrogen generation tests. Firstly, pure water was used to generate H2 from metallic powder as a control reaction. No interaction was observed in the H2 generation tests under pure water in accordance with the findings of Fan et al. [24]. Mg based systems show high H2 evolution rate in presence of water and activator. Under pure water, due to lack of ions in the medium, the corrosion potential of systems is not enough to generate H2. Also, pure water is non-conductive, but a solution of salt in water is readily conductive. Consequently, addition of electrolytes to our Mg system was required [16]. Fig. 6 shows the H2 generation data from W/Mg15 in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, and MnCl2) that were performed by a typical water-replacement procedure. It can be clearly seen that only the NiCl2 solution provided 100% conversion in 25 s. MnCl2 also showed good activity but the H2 generation time was longer than that for NiCl2. CoCl2 and FeCl3 had similar conversion rates. The most inactive saline solution was CuCl2 with 60% conversion. Acids, bases and, salts are called electrolytes when they are dissolved in water. Electrolyte solutions conduct electricity due to the mobility of the positive and negative ions. The ionization is completed according to the type of electrolyte solution. Electrolyte molecules may be ionized fully or in part
Table 2 e EDS analysis results of W/Mg samples. Samples
W/Mg3 W/Mg6 W/Mg9 W/Mg15 W/Mg30
Elements (%) Mg
Al
Mn
Zn
Fe
Co
Ti
V
Cr
Cu
Ni
83.11 84.00 84.74 83.20 83.96
10.95 13.91 9.77 13.15 4.46
0.77 0.96 0.78 0.79 0.93
0.46 0.76 0.45 0.54 0.58
0.99 0.40 3.18 2.27 9.20
e e 0.07 e 0.02
0.78 e 0.21 0.06 0.13
0.72 e 0.04 e e
0.96 e 0.24 e e
1.27 e 0.42 e 0.72
e 0.07 e e e
Please cite this article in press as: Kantu¨rk Figen A, et al., Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2), International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.01.022
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Fig. 6 e W/Mg15 conversion (%) in various saline solutions.
when they are dissolved in water. This phenomenon affects the electrolytic potential of the solutions. Consequently, it is necessary to focus on H2 production in different saline solutions [16,29,30]. Fig. 7 shows H2 generation rate versus time under various saline solutions for W/Mg15. Despite the observation that NiCl2 provided the best activity for H2 generation, FeCl3 had the highest H2 generation rate, 150 ml min1 g1 at the beginning of the reaction. Furthermore, FeCl3 and CoCl2 had the same conversion tendencies although their H2 generation rates were different. The H2 generation rate of CoCl2 was 38% lower than that of FeCl3. Although CuCl2 solution yielded a very low conversion percentage, its H2 generation rate within the full time span of the reaction was same as that of NiCl2. MnCl2 solution's H2 generation rate changed slowly but not as intensively as other saline solutions. For all saline solutions, it is clear that interaction between the saline and waste Mg based material changes with time. Fig. 8 illuminates the regional H2 generation rate per W/ Mg15 weight in various solutions. Time spans for calculating regional H2 rate were determined according to the saline solution's tendency. Table 3 shows selected time spans for each
Fig. 8 e Hydrogen generation rate per W/Mg15 weight for different regions in various saline solutions.
H2 generation region. The time spans were selected from the beginning of the H2 generation and according to the rate changes in Fig. 7, the regions (1, 2 and, 3) were clarified. The rate decreases were very prominent in the presence of all saline solutions. NiCl2, CoCl2 and, CuCl2 had two different regions for H2 generation from W/Mg15. The decreasing H2 generation rates of NiCl2, CoCl2 and CuCl2 were 55.85, 86.59 and, 88.76H2 ml s1 g1 W/Mg15, respectively. In spite of the fact that FeCl3 and MnCl2 affect the H2 generation behavior in a different way, the H2 generation rates of both saline solutions contained three steps. H2 generation rate per W/Mg weight in FeCl3 solution decreased at 92.08%. The decrease in the second
Table 3 e Reaction kinetic model fitting values for zero, first and, second order model for H2 generation from W/ Mg15 under various saline solutions. Saline solution
NiCl2
CoCl2
CuCl2
FeCl3
MnCl2
Fig. 7 e Hydrogen generation rate in various saline solutions of W/Mg15.
Time span (s)
1 2 Full 1 2 Full 1 2 Full 1 2 3 Full 1 2 3 Full
0e7.8 7.8e24.6 0e24.6 0e4 4e17.7 0e17.7 0e9 9e18.7 0e18.7 0e4.3 4.3e8 8e17.3 0e17.3 0e10.66 10.66e22.33 22.33e46.66 0e46.66
R2 values for kinetic modeling Zero order
First order
Second order
0.9166 0.9915 0.9004 0.8979 0.8492 0.6592 0.8923 0.9704 0.724 0.8923 0.9992 0.9662 0.6273 0.9066 0.9969 0.9831 0.7153
0.9408 0.9882 0.9398 0.9187 0.8637 0.702 0.9187 0.9717 0.7576 0.9176 0.9992 0.968 0.6692 0.9312 0.9966 0.9855 0.7706
0.9582 0.9835 0.9668 0.9377 0.8775 0.7922 0.9418 0.9729 0.7885 0.9392 0.9992 0.9697 0.709 0.9523 0.9959 0.9876 0.8207
The best fitting model for H2 generation tendencies of each saline solution is indicated by bold characters.
Please cite this article in press as: Kantu¨rk Figen A, et al., Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2), International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.01.022
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step was very intense, 75.16%. The saline solution that presented the highest activity, NiCl2 solution, decreased the H2 generation rate per W/Mg weight by 55.85%, which was the lowest decrease ratio. Grosjean et al. (2006) evolved H2 gas from high energy ball-milled (0e10 h) Mg-based materials in 1 M KCl solution with 88e118 ml min1 (g of powder)1 in a time span of 0e1 min and only 0.76e8.82 ml min1 (g of powder)1 in a time period of 15e60 min in two steps: fast and lower step. Al powder in NaOH solutions also generate H2 with a similar tendency as Mg, the reaction rate increased up to 540 s, after which it decreased immediately [31]. With the aim of illuminating the H2 generation rate tendency, zero-order, first-order and second-order reaction kinetic models were applied to regional data (1, 2 and, 3) and to the full time span. The best fitting model for H2 generation tendencies of each saline solution is indicated by bold characters in Table 3. Almost all the regions of all saline solutions showed a second-order reaction kinetic behavior. Only the NiCl2' and MnCl2's second regions showed first order reaction kinetics. This means that the concentration of waste Mg based material affected the H2 generation intensely. While the H2 generation rate of W/Mg15 changed in presence of various saline solutions, Table 3 shows that there was not any relationship between the H2 generation rate and kinetic behavior of the hydrolysis reaction. The formation of hydroxide layers on the surface of powder particles can interrupt the H2 generation reaction. Nontoxic hydroxide compound byproducts (Mg(OH)2 and Al(OH)3) on the surface of powder particles blocked the interactions between water and active powder [1,19,32]. Another by-product, chloride ions, comes from salines that are not oxidized to chloride gas [29]. H2 generation from waste Mg based material may provide an efficient means for the recycling of end-of-life magnesium products.
Conclusion In this study, waste Mg based material from the waste stream of a gold manufacturing factory production process was used as a H2 production source. This waste is proposed as a future green energy source. To improve the efficiency of this process, W/Mg surface characteristics were improved by ball milling (3, 6, 9, 15 and, 30 h) to improve the interaction between the saline solution and W/Mg. W/Mg15 prepared by ball milling for 15 h showed the best internal and external properties with 2.29 m2 g1 BET area and 11.94 mm particle size. H2 generation from W/Mg15 was carried out with various saline solutions (1 M NiCl2, CoCl2, CuCl2, FeCl3, and MnCl2). 1 M NiCI2 solution provided 100% of the total waste material H2 generation capacity. According to the kinetic investigation, almost all the regions of all salines showed second-order reaction kinetic behavior. Only the NiCl2 and MnCl2 second regions showed first order reaction kinetics.
Acknowledgments The authors would like to thank the Yildiz Technical University Research Foundation (Project no: 2014-07-01-GEP02) for its financial support.
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ScienceDirect journal homepage: www.elsevier.com/locate/he
Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2) Aysel Kantu¨rk Figen*, Bilge Cos‚kuner, Sabriye Pis‚kin Department of Chemical Engineering, Yildiz Technical University, Istanbul 34210, Turkey
article info
abstract
Article history:
In the present study, waste Mg based material (W/Mg) was used as a raw material for the
Received 25 August 2014
generation of hydrogen. Ball milling and saline solution method were performed to ensure
Received in revised form
full completion of the hydrolysis reaction. Improved texture properties were obtained by
10 December 2014
ball milling W/Mg for 15 h (2.29 m2 g1 BET area, 11.94 mm particle size). Ball milled W/Mg
Accepted 8 January 2015
was used to generate hydrogen gas in the presence of various saline solutions (1 M NiCl2,
Available online xxx
CoCl2, CuCl2, FeCl3, and MnCl2). While hydrogen generation was not observed in pure water, maximum conversion (100%) was achieved in 1 M NiCl2 solution.
Keywords:
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen generation Mg waste Saline solution method Ball milling
Introduction The prevention of environmental pollution is one of the primary goals of green energy engineering. As part of the pursuit of this goal, hydrogen (H2) is the most promising candidate for replacing traditional fossil fuels as an energy source. However, large-scale H2 production methods that use fossil fuels have such by-products as CO, CO2 and other contaminants. Hence, an economical and ‘‘green’’ method of producing H2 is required before H2 can become a truly clean energy source [1,2]. Metal utilization is an effective, user-friendly, and safe way of solving these problems for both H2 production and energy storage [3]. Many studies have investigated H2 production
from highly chemically active metallic powders. These metals, such as Al [4,5], Mg [6,7], and Li [8,9], are reacted with water and evaluated in regards to H2 production. Mg and Al would be first choices, not only due to their low toxicity but also their higher activity [10,11]. Though Al has higher H2 yield, the highly protective properties of the Al2O3 film put Mg first in the list [11e13]. H2 generation from a MgeAl based intermetallic system can be obtained through the following reactions [11]:
Mg(s) þ 2H2O(l) / Mg(OH)2(s) þ H2(g)
(1)
2Al(s) þ 6H2O(l) / 2Al(OH)3(s) þ 3H2(g)
(2)
* Corresponding author. Tel.: þ90 2123834774; fax: þ90 2123834725. E-mail addresses: [email protected], [email protected] (A. Kantu¨rk Figen). http://dx.doi.org/10.1016/j.ijhydene.2015.01.022 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kantu¨rk Figen A, et al., Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2), International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.01.022
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H2 evolved at dissolving Mg and Al surfaces, including those subjected to anodic polarization above its corrosion potential [14]. Furthermore, metal reactivity has been improved by many researchers via usage of high-energy milling procedures [7,11,12,15,16]. Also MgeAl based systems show high H2 evolution rates in presence of water and activator. The H2 evolution from aqueous solutions is based on the cationic/anodic reduction of H or H2 molecules in H2O and a highly interactive material [1,17]: The cathode reaction may be:
2H2O þ 2e / H2 þ 2OH
(3)
and/or
2Hþ þ 2e / H2
(4)
The anodic reaction should be:
Mg / Mg2þ þ 2e
(5)
Al / Al3þ þ 3e
(6)
The rate of the reaction increases while using a highly corrosive medium. For improving the H2 yield, the addition of acid [1,7] and saline [7,8,18,19] into aqueous system or the use of sea water [11,12] have been used by many researchers. On the other hand, acidic solutions are toxic and hazardous for the environment and it's inhabitants [1,10,17]. Also, Mg scraps have become a substantial problem due to challenges in recycling, reusing and waste treatment. The possibility of generating H2 from Mg scraps or low-grade Mg wastes leads to a new application area for these wastes which cannot be recycled economically due to the current lack of sufficiently efficient purification technologies [7,20e22]. Mg scraps contain certain metals such as Al, Ni and Cu in their structure [2] which accelerate H2 evolution. Zou et al. (2011) studied Mgbased hydro-reactive materials by optimizing the milling preparation conditions and investigated the reactive properties of their materials in sea water [11,12]. Yu et al. (2012) reported a method of producing H2 by mixing low-grade Mg scraps in aqueous organic acids with addition of NaCl [7]. Zou et al. (2012) indicated that performance of MgeAl based systems could be greatly improved via a high ball milling process [11]. Liu et al. achieved 100% efficiency after 3 h of milling 3 mol% AlCl3eMg powder. There are no literature reports focused on the effects different saline solutions on the H2 generation characteristics of Mg waste. In this work, W/Mg, defined as a waste Mg based material, was used in presence of various saline solutions (1 M NiCl2, CoCl2, CuCl2, FeCl3, and MnCl2) to generate H2 gas. Before the hydrolysis reaction, W/Mg15 was prepared by ball milling for 15 h in order to improve the surface characteristics of the particles. While H2 generation was not observed in pure water, maximum conversion (100%) was achieved in 1 M NiCl2 solution.
Experimental Materials and characterizations W/Mg provided by a gold manufacturing factory was used as the main material in the present study. Waste Mg was molded into the form of a chip using a plastics machining process. Structural characterization of W/Mg was performed by X-ray diffraction (XRD), scanned electron microcopy-electron dispersion spectroscopy (SEM-EDS), BrunauereEmmetteTeller (BET) and X-ray fluorescence spectroscopy (XRF) techniques in our previous study [23]. It was decided that the waste Mg chip used in the study would be called “Mg-Rich Intermetallic material” as indicated by the results of the characterization analyses. In this study, W/Mg was mechanically ground using a planetary type ball-milling apparatus at a speed of 300 rpm for different grinding times (3, 6, 9, 15 and 30 h). The ball-topowder ratio was 70:1. The codes of the samples corresponded to the milling time used; W/Mg (no milling), and W/ Mg3, W/Mg6, W/Mg9, W/Mg15 and, W/Mg30, wherein the number indicates the hours of milling applied in the preparation of the sample. After grinding, SEM and BET analyses were performed on W/Mg samples in order to determine the particle size changes as a function of milling time. BET analyses of samples were done twice and average values with a maximum error of ±10% were considered as a single observation. In addition, XRD, BET, SEM-EDS and XRF analyses (Figs. 1e4, Tables 1 and 2) were carried out in order to determine the W/Mg texture properties that are associated with superior hydrogen generation.
Hydrogen generation tests H2 generation from ball milled W/Mg in various saline solutions (1 M NiCl2, CoCl2, CuCl2, FeCl3, and MnCl2) was performed based on a typical water-replacement procedure in a 10 ml glass reactor (Fig. 5). Various 1 M saline solutions were prepared from chloride salts powders (Merck, 99%) by dissolving in pure water. The reactor was connected to a waterfilled inverted burette to measure the volume of H2 gas that evolved from the 0.01 g W/Mg15 in the presence of NiCl2, CoCl2, CuCl2, FeCl3, and MnCl2 (1 M) solutions. The measurement of H2 generation was performed at moderate temperature with magnetic stirring (500 rpm). H2 generation time was
Fig. 1 e XDR patterns of W/Mg samples.
Please cite this article in press as: Kantu¨rk Figen A, et al., Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2), International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.01.022
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Table 1 e Crystalline phases of the W/Mg samples. Sample W/Mg3 W/Mg6 W/Mg9 W/Mg15 W/Mg30
3
and second-order reaction kinetic models were applied for the whole reaction and for sectional regions (Table 3).
JCPDS number Mg (00-001-1148), Mg (01-089-4894), Mg (01-089-5003), Mg (01-089-4894), Mg (01-089-4894), 7752)
Fe (03-065-5099) Fe (03-006-0696) Fe (03-065-5099) Fe (01-089-7194) Fe19Ni (03-065-
Fig. 2 e Specific surface area of W/Mg samples.
measured by chronometer until no more H2 evolution was observed. For investigating effects of various saline solutions on H2 generation from W/Mg15, the H2 generation rate and reaction kinetic tendency were carried out regionally (Figs. 6e8). For reaction kinetic modeling, zero-order, first-order
Results and discussion Structural characterization of W/Mg Fig. 1 shows the XRD patterns of W/Mg samples. Main diffraction peaks were associated with Mg structure for all the samples. Fe was the secondary face of the W/Mg3, W/Mg6, W/ Mg9 and W/Mg15 samples while ironenickel (Fe19Ni) was detected in the W/Mg30 sample. Crystal phases with the matched JCPDS card numbers are given in Table 1. Fig. 2 shows multi-point BET analysis results of unmilled W/Mg and W/Mg samples. Specific surface area of unmilled W/Mg was determined as only 0.08 m2/g. 3, 6, 9, 15 and 30 h of milling increased surface areas by approximately 19, 21, 25, 29 and 32 times respectively. According to the BET surface area results, exceeding 15 h of grinding time did not significantly increase surface area. Fig. 3 shows the SEM images at 2000 magnification of W/ Mg samples. Average particle size was determined based on SEM images. After 3 h milling, the waste was in the chip form and particles with grains were not achieved. As grinding time was increased, particles were formed. Mg particles had an average size of 11.94 mm after 30 h milling. A particle size of 12.18 mm was obtained after 15 h of milling. It was decided that milling for more than 15 h did not considerably change the particle size; thus 15 h was considered the optimal time (Fig. 4). Table 2 shows the EDS analysis results. As can be seen, average compositions of W/Mg samples were determined as
Fig. 3 e SEM images (2000£ magnification) of W/Mg samples: (a) W/Mg3, (b) W/Mg6, (c) W/Mg9, (d) W/Mg15, (e) W/Mg30. Please cite this article in press as: Kantu¨rk Figen A, et al., Hydrogen generation from waste Mg based material in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, MnCl2), International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.01.022
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Fig. 4 e Average particular size of W/Mg samples.
83.81% Mg, 10.45% Al, 0.85 Mn%, 0.56% Zn, 3.21% Fe, 0.05% Co, 0.30 Ti%, 0.38% V, 0.60% Cr, 0.80% Cu and 0.07 Ni %. In previous studies, researchers investigated H2 generation properties of energy ball-milling prepared alloys of Mg and Al with the addition of different elements [24,25]. Fan et al. obtained a complete theoretical conversion yield from Al alloys that had been prepared by alloying. In this case, we did not need to add further elements to our composites which is ideal given that preparation of such alloys is expensive and time consuming. Decreasing the Mg particle size and increasing the specific exposed surface area of powders improves the contact between solution and metal particles. The ball milling process provided these properties depending on the metal properties. The pitting corrosion process on surface of metals forms fresh surfaces and defects. By controlling the milling time, oxidation of active surface and specific surface area of powders can be controlled, which has a crucial role on the H2 generation reaction [3,26,27].
Hydrogen generation tests The theoretical H2 generation capacity of W/Mg15 was calculated as 89.39 l g1W/Mg15 according to eqs. (1), (2) and (7) by using EDS results for the sample. In Eq. (7), weight of generated H2 is mH2 , weight ratio of generated H2 is xH2 , molecular weight of H2 is MH2 , and molecular weight and weight ratio of I compound are Mi and, xi, respectively [28]. % mH2 ¼
xH2 MH2 100 xH2 MH2 þ xi Mi
(7)
Fig. 5 e Hydrogen generation system: a-Temperature controlled magnetic stirrer, b-Three necked reactor, c-W/ Mg, D-Saline solution, e-Injector, f-Silicon pipe for H2 gas exit, g-Thermocouple, h-Beaker with full of water, lGenerating H2 gas volume, j-Laboratory stand, k-Jacket.
According to the characterization results, W/Mg15 was selected as a metallic powder for hydrogen generation tests. Firstly, pure water was used to generate H2 from metallic powder as a control reaction. No interaction was observed in the H2 generation tests under pure water in accordance with the findings of Fan et al. [24]. Mg based systems show high H2 evolution rate in presence of water and activator. Under pure water, due to lack of ions in the medium, the corrosion potential of systems is not enough to generate H2. Also, pure water is non-conductive, but a solution of salt in water is readily conductive. Consequently, addition of electrolytes to our Mg system was required [16]. Fig. 6 shows the H2 generation data from W/Mg15 in various saline solutions (NiCl2, CoCl2, CuCl2, FeCl3, and MnCl2) that were performed by a typical water-replacement procedure. It can be clearly seen that only the NiCl2 solution provided 100% conversion in 25 s. MnCl2 also showed good activity but the H2 generation time was longer than that for NiCl2. CoCl2 and FeCl3 had similar conversion rates. The most inactive saline solution was CuCl2 with 60% conversion. Acids, bases and, salts are called electrolytes when they are dissolved in water. Electrolyte solutions conduct electricity due to the mobility of the positive and negative ions. The ionization is completed according to the type of electrolyte solution. Electrolyte molecules may be ionized fully or in part
Table 2 e EDS analysis results of W/Mg samples. Samples
W/Mg3 W/Mg6 W/Mg9 W/Mg15 W/Mg30
Elements (%) Mg
Al
Mn
Zn
Fe
Co
Ti
V
Cr
Cu
Ni
83.11 84.00 84.74 83.20 83.96
10.95 13.91 9.77 13.15 4.46
0.77 0.96 0.78 0.79 0.93
0.46 0.76 0.45 0.54 0.58
0.99 0.40 3.18 2.27 9.20
e e 0.07 e 0.02
0.78 e 0.21 0.06 0.13
0.72 e 0.04 e e
0.96 e 0.24 e e
1.27 e 0.42 e 0.72
e 0.07 e e e
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Fig. 6 e W/Mg15 conversion (%) in various saline solutions.
when they are dissolved in water. This phenomenon affects the electrolytic potential of the solutions. Consequently, it is necessary to focus on H2 production in different saline solutions [16,29,30]. Fig. 7 shows H2 generation rate versus time under various saline solutions for W/Mg15. Despite the observation that NiCl2 provided the best activity for H2 generation, FeCl3 had the highest H2 generation rate, 150 ml min1 g1 at the beginning of the reaction. Furthermore, FeCl3 and CoCl2 had the same conversion tendencies although their H2 generation rates were different. The H2 generation rate of CoCl2 was 38% lower than that of FeCl3. Although CuCl2 solution yielded a very low conversion percentage, its H2 generation rate within the full time span of the reaction was same as that of NiCl2. MnCl2 solution's H2 generation rate changed slowly but not as intensively as other saline solutions. For all saline solutions, it is clear that interaction between the saline and waste Mg based material changes with time. Fig. 8 illuminates the regional H2 generation rate per W/ Mg15 weight in various solutions. Time spans for calculating regional H2 rate were determined according to the saline solution's tendency. Table 3 shows selected time spans for each
Fig. 8 e Hydrogen generation rate per W/Mg15 weight for different regions in various saline solutions.
H2 generation region. The time spans were selected from the beginning of the H2 generation and according to the rate changes in Fig. 7, the regions (1, 2 and, 3) were clarified. The rate decreases were very prominent in the presence of all saline solutions. NiCl2, CoCl2 and, CuCl2 had two different regions for H2 generation from W/Mg15. The decreasing H2 generation rates of NiCl2, CoCl2 and CuCl2 were 55.85, 86.59 and, 88.76H2 ml s1 g1 W/Mg15, respectively. In spite of the fact that FeCl3 and MnCl2 affect the H2 generation behavior in a different way, the H2 generation rates of both saline solutions contained three steps. H2 generation rate per W/Mg weight in FeCl3 solution decreased at 92.08%. The decrease in the second
Table 3 e Reaction kinetic model fitting values for zero, first and, second order model for H2 generation from W/ Mg15 under various saline solutions. Saline solution
NiCl2
CoCl2
CuCl2
FeCl3
MnCl2
Fig. 7 e Hydrogen generation rate in various saline solutions of W/Mg15.
Time span (s)
1 2 Full 1 2 Full 1 2 Full 1 2 3 Full 1 2 3 Full
0e7.8 7.8e24.6 0e24.6 0e4 4e17.7 0e17.7 0e9 9e18.7 0e18.7 0e4.3 4.3e8 8e17.3 0e17.3 0e10.66 10.66e22.33 22.33e46.66 0e46.66
R2 values for kinetic modeling Zero order
First order
Second order
0.9166 0.9915 0.9004 0.8979 0.8492 0.6592 0.8923 0.9704 0.724 0.8923 0.9992 0.9662 0.6273 0.9066 0.9969 0.9831 0.7153
0.9408 0.9882 0.9398 0.9187 0.8637 0.702 0.9187 0.9717 0.7576 0.9176 0.9992 0.968 0.6692 0.9312 0.9966 0.9855 0.7706
0.9582 0.9835 0.9668 0.9377 0.8775 0.7922 0.9418 0.9729 0.7885 0.9392 0.9992 0.9697 0.709 0.9523 0.9959 0.9876 0.8207
The best fitting model for H2 generation tendencies of each saline solution is indicated by bold characters.
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step was very intense, 75.16%. The saline solution that presented the highest activity, NiCl2 solution, decreased the H2 generation rate per W/Mg weight by 55.85%, which was the lowest decrease ratio. Grosjean et al. (2006) evolved H2 gas from high energy ball-milled (0e10 h) Mg-based materials in 1 M KCl solution with 88e118 ml min1 (g of powder)1 in a time span of 0e1 min and only 0.76e8.82 ml min1 (g of powder)1 in a time period of 15e60 min in two steps: fast and lower step. Al powder in NaOH solutions also generate H2 with a similar tendency as Mg, the reaction rate increased up to 540 s, after which it decreased immediately [31]. With the aim of illuminating the H2 generation rate tendency, zero-order, first-order and second-order reaction kinetic models were applied to regional data (1, 2 and, 3) and to the full time span. The best fitting model for H2 generation tendencies of each saline solution is indicated by bold characters in Table 3. Almost all the regions of all saline solutions showed a second-order reaction kinetic behavior. Only the NiCl2' and MnCl2's second regions showed first order reaction kinetics. This means that the concentration of waste Mg based material affected the H2 generation intensely. While the H2 generation rate of W/Mg15 changed in presence of various saline solutions, Table 3 shows that there was not any relationship between the H2 generation rate and kinetic behavior of the hydrolysis reaction. The formation of hydroxide layers on the surface of powder particles can interrupt the H2 generation reaction. Nontoxic hydroxide compound byproducts (Mg(OH)2 and Al(OH)3) on the surface of powder particles blocked the interactions between water and active powder [1,19,32]. Another by-product, chloride ions, comes from salines that are not oxidized to chloride gas [29]. H2 generation from waste Mg based material may provide an efficient means for the recycling of end-of-life magnesium products.
Conclusion In this study, waste Mg based material from the waste stream of a gold manufacturing factory production process was used as a H2 production source. This waste is proposed as a future green energy source. To improve the efficiency of this process, W/Mg surface characteristics were improved by ball milling (3, 6, 9, 15 and, 30 h) to improve the interaction between the saline solution and W/Mg. W/Mg15 prepared by ball milling for 15 h showed the best internal and external properties with 2.29 m2 g1 BET area and 11.94 mm particle size. H2 generation from W/Mg15 was carried out with various saline solutions (1 M NiCl2, CoCl2, CuCl2, FeCl3, and MnCl2). 1 M NiCI2 solution provided 100% of the total waste material H2 generation capacity. According to the kinetic investigation, almost all the regions of all salines showed second-order reaction kinetic behavior. Only the NiCl2 and MnCl2 second regions showed first order reaction kinetics.
Acknowledgments The authors would like to thank the Yildiz Technical University Research Foundation (Project no: 2014-07-01-GEP02) for its financial support.
references
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