- Email: [email protected]
γ-Al2O3 (M = Co, Ni) catalyst
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Hydrogen generation from Al-Water reaction promoted by M-B/g-Al2O3 (M ¼ Co, Ni) catalyst Wei-Zhuo Gai a,*, Xianghui Zhang a, Kexi Sun a, Zhen-Yan Deng b a
College of Physics and Electronic Information & Henan Key Laboratory of Electromagnetic Transformation and Detection, Luoyang Normal University, Luoyang 471934, China b Energy Materials & Physics Group, Department of Physics, Shanghai University, Shanghai 200444, China
highlights
graphical abstract
High-activity M-B/g-Al2O3 catalyst was prepared by wet chemical reduction method. M-B/g-Al2O3 greatly promotes the Al-water reaction and decreases the induction time. The catalytic activity of M-B/gAl2O3 is proportional to its active area. A possible mechanism for Alwater reaction promoted by M-B/ g-Al2O3 was proposed.
article info
abstract
Article history:
Al-water reaction promoted by catalysts is a promising hydrogen generation technology. In
Received 29 May 2019
this work, a high-activity M-B/g-Al2O3 (M ¼ Co, Ni) catalyst is prepared by wet chemical
Received in revised form
reduction method. It is found that M-B/g-Al2O3 catalyst significantly promotes the Al-water
16 July 2019
reaction and decreases the induction time. When the molar ratio of g-Al2O3 to Co-M in Co
Accepted 25 July 2019
eB/g-Al2O3 catalyst is 1:1, the induction time is only 0.43 h. The catalytic activity of M-B/g-
Available online 17 August 2019
Al2O3 is proportional to its active area. SEM analyses show that M-B particles are dispersed
Keywords:
surface of M-B/g-Al2O3, leading to a high catalytic activity. A possible mechanism is pro-
Hydrogen generation
posed, which shows that the dissociation of water molecules on g-Al2O3 surface and the
Al-water reaction
microgalvanic interaction between M-B and Al can promote the hydration process of
Catalyst
passive oxide film on Al particle surface, speeding up the Al-water reaction.
on g-Al2O3 surface, which reduces the agglomeration of M-B and increases the active
Induction time
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (W.-Z. Gai). https://doi.org/10.1016/j.ijhydene.2019.07.203 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
24378
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
Introduction Metal Al is one of the most widely studied hydrogen generation materials, which can continuously react with water and release hydrogen under ambient condition [1]. Al is the most abundant metal in the earth's crust, and has a relative low cost. Meanwhile, the byproducts of Al-water reaction are Al(OH)3 or AlOOH, which are environmentally benign and can be reduced to metal Al for recycle by a Hall-Heroult process or used to make refractory and calcium aluminate cement [2,3]. 1 g Al reacts with water to generate as much as 1245 ml H2 in the standard condition, making it a potential in-situ hydrogen generation material for small fuel cells. However, there is a long induction time before hydrogen generation for Al-water reaction due to a dense passive oxide film covering on Al particle surface [4]. The research on hydrogen generation from the Al-water reaction has been a hotspot during the past few years, and various activation methods were developed to promote the Alwater reaction and decrease the induction time. Using an alkaline solution to assist the Al-water reaction is a traditional method [5]. Martı´nez found that metal Al obtained from Al waste cans could react with NaOH solution and generate high purity hydrogen [6]. Wang proposed a mini-type hydrogen generator from Al for proton exchange membrane fuel cells, and investigated the effects of concentration, dropping rate and initial temperature of NaOH solution on Al-water reaction [7]. But strong alkalis, e.g. NaOH, KOH etc., have strong corrosiveness to reaction device, and the byproducts are also alkaline and can cause environmental pollution. In recent years, Soler and Dai used low corrosive NaAlO2 and Na2SnO3 to replace strong alkalis, and found that NaAlO2 and Na2SnO3 could promote Al-water reaction and inhibit the repassivation of Al [8e10]. Milinchuk developed a hydrogen generator based on Al-water reaction promoted by sodium metasilicate and studied the hydrogen generation kinetics [11]. Alloying of metal Al by melting or mechanical ball milling is an effective method to activate Al, because alloying process can destroy the passive oxide film on Al surface [12]. In addition, the alloying elements and Al can form microgalvanic cell, promoting the corrosion of Al. So far, plenty of Al alloys including AleLi, AleGa, AleSn, AleMg, AleIn, AleFe, Al-Ga-InSn etc., have been developed [13e27]. Different Al alloys have different activation mechanisms. For example, Ga, In, Sn in Al-Ga-In-Sn alloy would form a liquid eutectic covering on Al surface, so that inner Al atoms could diffuse to Al surface through this liquid eutectic and react with water [13,26,27]. For AleLi alloy, Li would hydrolyze and produce LiOH, which promotes the Al-water reaction [14]. Eom prepared an AleFe bulk alloy by melting method, and proposed that its fast hydrogen generation rate resulted from the combined effects of the intergranular corrosion and galvanic corrosion [15]. In addition, metal Al can be activated by mechanical ball milling with different oxide, soluble inorganic salt, graphite, hydride, polytetrafluoroethylene, BiOCl [28e38], etc. In the process of ball milling, the passive oxide film on Al surface can be destroyed, and many new and fresh surfaces are produced. In this case, the fresh Al can directly react with water to produce hydrogen. In addition, different milling mediums
play different roles in the Al-water reaction. For example, the CaO in AleCaO-salt composite prepared by the ball milling method can hydrolyze and release OH, which promotes the Al-water reaction [33]. For Al/graphite composite, the graphite can eliminate the passive film and cover on Al surface during the ball milling process. As graphite has a loose structure, H2O molecules can easily penetrate to the Al/graphite interfaces, leading to the Al-water reaction [32]. For Al/CaH2 composite prepared by ball milling, the hydrolysis of CaH2 can help to open up the structure of the Al and provide many passages for H2O molecules to penetrate through. In addition, the hydrolysis of CaH2 can release heat and OH, further promoting the Al-water reaction [34]. But ball milling consumes much energy and needs to be carried out under inert condition, which limits its wide application [28,32]. Recently, some researchers found that some special catalysts could be used to promote the Al-water reaction. For example, Teng and Newell found that Al powder could react with water and produce hydrogen in Al(OH)3 suspension [39e41]. Meng and Wang reported that FeeB, CoeB, NieB and CoeFeeB catalysts could be used to promote Al-water reaction [42,43]. In previous work, we found that a-Al2O3, g-Al2O3, TiO2 and Al(OH)3 could be used as catalysts to promote Al-water reaction [44,45]. However, agglomeration is easy to occur in the process of catalyst preparation, decreasing the activity of catalysts. In this work, a high-activity M-B/g-Al2O3 (M ¼ Co, Ni) catalyst was prepared by wet chemical reduction method. The effect of M-B/g-Al2O3 catalyst on Al-water reaction was systematically investigated, and the possible catalytic mechanisms were discussed in detail.
Experimental procedure Al powder (1.75 mm, 99.9% purity, Henan Yuanyang Powder Technology Co., Ltd., Xinxiang, China), g-Al2O3 powder (99.99% purity, Hangzhou Wan Jing New Material Co., Ltd., Hangzhou, China) and NaBH4 (97% purity, Tianjin Kemiou Chemical Reagent Co., Tianjin, China) were used in the present research. In addition, analytical reagent grade CoCl26H2O, NiCl26H2O and absolute ethanol were used in the experiment. CoeB/g-Al2O3 or NieB/g-Al2O3 catalysts were prepared by a wet chemical reduction method [46,47]. The g-Al2O3 powder was put into a beaker with 200 ml of deionized water, and then ultrasonically dispersed in an ultrasonic cleaner (Type: KQ3200E, frequency 40 kHz, power 150 W, Kunshan Ultrasonic Instruments Co., Ltd., Kunshan, China) for 1 h. Then 6 mmol CoCl26H2O or NiCl26H2O were added into above beaker and ultrasonically dispersed 0.5 h. The suspension was added into another beaker containing 12 mmol NaBH4 powders within 0.5 min and manually stirred. In this case, CoCl26H2O or NiCl26H2O can react vigorously with NaBH4 and produce a black precipitate, 2MCl2 þ 4NaBH4 þ 9H2O / M2BY þ 4NaCl þ12.5H2[ þ 3B(OH)3,
(1)
where “M” represents Co or Ni elements. The mixture suspension after reaction was filtered by a filter paper, washed with deionized water and absolute ethanol, and then dried at
24379
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
room temperature. During filtering and drying, small amount of Co2B and Ni2B would be oxidized by oxygen, 4M2B þ 3O2 / 8 M þ 2B2O3.
(2)
Therefore, the M-B/g-Al2O3 catalyst is a mixture, which consisted of g-Al2O3, M2B, and small quantities of metal M and B2O3. For comparison, CoeB and NieB catalysts were prepared using the same procedure as that to prepare M-B/g-Al2O3 catalyst, except that no g-Al2O3 was added. The weight percentage of Ni and Co in different catalysts is given in Table 1. The Al-water reaction tests promoted by M-B/g-Al2O3 catalyst were carried out in a double-necked flask, which has better air tightness. In each test, 250 ml of deionized water or M-B/g-Al2O3 suspension were used, and then 1.0 g of Al powder was added and mixed evenly. The reaction temperature was controlled by a thermostat water bath with an accuracy of ±1 C, and no agitation was used in all the tests. The hydrogen produced by Al-water reaction was collected in an inverted burette filled with tap water. The volume of hydrogen produced was measured from the water level change in the inverted burette. The hydrogen yield a can be determined a¼
VH2 100%; V0
(3)
where a is the hydrogen yield; VH2 is the volume of hydrogen; V0 is the theoretical hydrogen volume obtained by reacting all of metal Al. Each test was carried out twice in order to ensure the reproducibility of the hydrogen generation curve. X-ray diffraction (XRD, Model No. D8 Advance, Bruker Co., Germany) studies were carried out to analyse the phases in CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts. Morphology analyses of the Al powder and different catalysts were performed using scanning electron microscopy (SEM, Model No. Sigma 500/VP, Carl Zeiss Co., Germany). The specific surface area and pore size distribution of different catalysts were measured using surface area analyzer (Micromeritics ASAP 2020, USA). The specific surface area was determined by Brunauer-Emmett-Teller (BET) method, the pore size distribution was calculated by BarrettJoyner-Halenda (BJH) method, and the external surface area was determined by empirical t-plot method.
Results and discussion Characterization of M-B/g-Al2O3 catalyst Fig. 1 shows the morphologies of as-received pure Al powder and g-Al2O3 powder. It is found that Al particles are spherical,
and their surfaces are dense and smooth. The Al powder has a particle size distribution, and the Al particles range from nano-sized to micro-sized (Fig. 1(a)). For g-Al2O3, the grains are very fine and about several nanometers, but there are some agglomerates (Fig. 1(b)). Fig. 2 shows the X-ray diffraction patterns of as-received pure Al powder, CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts. It can be seen that the Al powder has a high purity, both CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts are amorphous. Fig. 3 shows the morphologies of CoeB, NieB, CoeB/gAl2O3 and NieB/g-Al2O3 catalysts. It can be seen that all the four catalysts have porous structure, but the pore size distribution of different catalysts is very different. Obviously, the pores in CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts are larger than that in CoeB and NieB catalysts. In addition, there are many agglomerates in all of catalysts, and it seems that the agglomeration of CoeB and NieB catalysts is more serious than that of CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts. For CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts, the CoeB or NieB particles cover on the surface of g-Al2O3 grains or agglomerates, indicating that the existence of g-Al2O3 reduces the agglomeration of CoeB and NieB particles. Fig. 4(a) shows the particle size distribution of as-received Al powder. It can be seen that the Al particles range from 0.2 mm to 4.3 mm, and the average particle size is about 1.75 mm. Fig. 4(b) shows the pore size distribution of different catalysts. It is noticed that CoeB catalyst has low porosity. For NieB catalyst, the pores are mainly mesoporous, and the pore sizes vary between 2 nm and 20 nm. However, both CoeB/gAl2O3 and NieB/g-Al2O3 catalysts have a wider pore size distribution, and they contain mesopores and macropores. The BET surface area, external surface area, pore volume and average pore size of different catalysts are given in Table 1. The pore volumes of CoeB, NieB, CoeB/g-Al2O3 and NieB/gAl2O3 catalysts are 0.135, 0.190, 0.465 and 0.468 cm3g-1, respectively. The addition of g-Al2O3 greatly increases the surface area of CoeB catalyst, while it slightly increases the surface area of NieB catalyst.
Al-water reaction promoted by M-B/g-Al2O3 catalyst Fig. 5(a) shows the hydrogen evolution from deionized water using pure Al powder with and without the addition of 10 wt% (the weight fraction of catalyst in Al powder þ catalyst) gAl2O3, CoeB, CoeBþg-Al2O3 (the mixture of CoeB and g-Al2O3) or CoeB/g-Al2O3 catalyst at 40 C. It is found that the pure Al powder could react with water and generate hydrogen at 40 C, but there is a long induction time (3.0 h) for the beginning of Al-water reaction. When the catalysts were added into
Table 1 e The metal percentage, BET surface area, external surface area, pore volume and average pore size of different catalysts. Catalysts CoeB NieB CoeB/g-Al2O3a NieB/g-Al2O3a a
Co or Ni weight percentage
Average pore size (nm)
Pore volume (cm3g1)
BET surface area (m2g1)
External surface area (m2g1)
91.6% 91.6% 35.4% 35.3%
35.7 9.8 22.6 23.1
0.135 0.190 0.465 0.468
14.9 80.6 81.4 81.9
14.9 59.9 74.7 63.2
The molar ratio of g-Al2O3 to M (M ¼ Co, Ni) in M-B/g-Al2O3 catalysts is 1:1.
24380
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
Fig. 1 e SEM micrographs of (a) pure Al and (b) g-Al2O3 powders.
Fig. 2 e X-ray patterns for (a) as-received pure Al powder, (b) CoeB/g-Al2O3 and (c) NieB/g-Al2O3 catalysts.
water, the induction time of Al-water reaction was greatly decreased. The catalysts significantly promoted the Al-water reaction, but different catalysts have different effects on the Al-water reaction dynamics. For g-Al2O3 and CoeB catalysts, the reaction dynamics of Al with water is almost the same except that the hydrogen yield of Al-water reaction promoted by CoeB is higher than that promoted by g-Al2O3. The CoeB/gAl2O3 catalyst has the best catalytic activity, and could considerably promote the Al-water reaction and hydrogen generation. In order to reveal the catalytic mechanism of CoeB/g-Al2O3 catalyst, a mixture of CoeB and g-Al2O3 (CoeBþg-Al2O3) was used as catalyst to promote the Al-water reaction, and the hydrogen generation curve was also given in Fig. 5(a). Obviously, the hydrogen generation performance of Al-water reaction promoted by CoeBþg-Al2O3 catalyst is better than that promoted by CoeB or g-Al2O3 catalyst, implying that CoeB and g-Al2O3 have a synergistic effect on Al-water reaction. Table 2 gives the induction time and the maximum hydrogen generation rate (MHGR) of Al-water reaction with and without the addition of 10 wt% different catalysts at 40 C. It can be seen that g-Al2O3, CoeB, CoeBþg-Al2O3 and CoeB/gAl2O3 decreased the induction time from 3.0 h to 1.17 h, 1.08 h, 0.77 h and 0.43 h, respectively. This means that the activity of
Fig. 3 e SEM micrographs of (a) CoeB, (b) NieB, (c) CoeB/g-Al2O3 and (d) NieB/g-Al2O3 catalysts.
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
24381
Fig. 4 e (a) Particle size distribution of as-received Al power and (b) pore size distribution of different catalysts.
the catalysts is in the order of CoeB/g-Al2O3 > CoeBþg-Al2O3 > CoeB > g-Al2O3. The Al-water reaction promoted by CoeB/g-Al2O3 catalyst has the shortest induction time, and >85% of Al was consumed and generated H2 within 3.5 h, indicating that CoeB/g-Al2O3 has high activity and the Alwater reaction promoted by CoeB/g-Al2O3 is a promising hydrogen generation technology for portable fuel cells. Fig. 5(b) shows the hydrogen evolution from deionized water using pure Al powder with and without the addition of 10 wt% g-Al2O3, NieB, NieBþg-Al2O3 (the mixture of Ni eB and g-Al2O3) or NieB/g-Al2O3 catalyst at 40 C. It can be seen that all the catalyst could promote the Al-water reaction and decrease the induction time. The activity of catalysts is in the order of NieB/g-Al2O3 > NieBþg-Al2O3 > NieB, which is similar to the results of Fig. 5(a). The Al-water reaction promoted by NieB/g-Al2O3 catalyst has the best hydrogen generation performance, but its induction time is longer than that of the Al-water reaction promoted by CoeB/g-Al2O3 catalyst. Fig. 5 indicates that M-B and g-Al2O3 have a synergistic effect on Al-water reaction, so the ratio of g-Al2O3 to M-B might have important impact on Al-water reaction dynamics. Fig. 6 shows the hydrogen evolution from Al-water reaction promoted by CoeB/g-Al2O3 catalyst with different molar ratio of g-Al2O3 to Co at 40 C. It can be seen that the ratio of g-Al2O3 to Co has great effect on the hydrogen generation performance of the Al-water reaction promoted by CoeB/g-Al2O3, implying that the catalytic activity of M-B/g-Al2O3 is closely related to the ratio of g-Al2O3 to Co. The CoeB/g-Al2O3 catalyst decreased the induction time of the Al-water reaction, but the relation between the induction time and the ratio of g-Al2O3 to
Fig. 5 e H2 evolution from deionized water using pure Al powder with and without the addition of 10 wt% different catalysts at 40 C, where the M-Bþg-Al2O3 (M ¼ Co, Ni) catalysts are the mixtures of M-B and g-Al2O3, and the molar ratio of g-Al2O3 to M in M-B/g-Al2O3 and M-BþgAl2O3 catalysts is 1:1.
Co is not monotonic. There is a shortest induction time (0.43 h) for Al-water reaction promoted by CoeB/g-Al2O3 when the ratio of g-Al2O3 to Co is 1:1. Fig. 7 shows the hydrogen evolution from deionized water using Al powder with the addition of different weight contents CoeB/g-Al2O3 catalysts at 40 C. It can be seen that the content of CoeB/g-Al2O3 has little effect on Al-water reaction dynamics. The induction time of Al-water reaction decreased with increasing the CoeB/g-Al2O3 content. Increasing the content of CoeB/g-Al2O3 from 5 wt% to 20 wt%, the induction time decreased from 0.77 h to 0.32 h, and the hydrogen yield increased from 81.2% to 90.0%. However, the Al-water reaction dynamics has little change when the content of CoeB/g-Al2O3 increases from 20 wt% to 30 wt%, indicating that the effect of CoeB/g-Al2O3 content on Al-water reaction tends to be saturated. Fig. 8(a) shows the hydrogen evolution from deionized water using Al powder with the addition of 10 wt% CoeB/gAl2O3 catalyst at different temperatures. When the reaction temperature increases, the hydrogen generation rate
24382
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
Table 2 e Hydrogen generation data of Al powder in deionized water with and without the addition of 10 wt% different catalysts at 40 C. catalysts Without catalyst g-Al2O3 CoeB CoeBþg-Al2O3 CoeB/g-Al2O3 NieB NieBþg-Al2O3 NieB/g-Al2O3 a
Induction time (hours)
MHGR (ml$min1$g1-Al)a
3.00 1.17 1.08 0.77 0.43 1.12 0.85 0.67
5.40 11.28 9.87 11.33 13.30 8.75 12.06 13.40
MHGR is the maximum hydrogen generation rate, which is obtained by the hydrogen volume divided by the time interval and the Al powder weight.
increases and the induction time decreases obviously. At 60 C, the induction time of Al-water reaction is just 0.15 h, and over 90% of Al powder was consumed within 2.5 h, implying that it could satisfy the requirement of portable fuel cells. Increasing the temperature from 35 C to 60 C, the MHGR increases from 6.6 to 51.3 ml min1$g1-Al, so the hydrogen generation rate could be controlled by adjusting the reaction temperature in the practical application. In order to quantify the temperature effect, the Arrhenius equation [1] was used, k ¼ A expð Ea =RTÞ
(4)
where A is a pre-exponential factor, Ea is the activation energy, R is the gas constant, T is the reaction temperature, k is the rate constant related to the surface chemical reaction and can be determined by fitting the experimental data of Fig. 8(a) using the following equation [1]: 1 ð1 aÞ1=3 ¼ kt:
(5)
The activation energy of Al-water reaction was calculated from the plot of the reaction rate constant k against the reciprocal temperature using Eq. (4), as shown in Fig. 8(b). The plot has a good linear fitting and the activation energy for Alwater reaction promoted by CoeB/g-Al2O3 catalyst was determined to be 50.9 kJ/mol, which is lower than the activation energy for the reaction of pure Al with water (68.1 kJ/ mol for 2.25 mm Al powder) [1]. This indicates that the CoeB/gAl2O3 catalyst could greatly decrease the activation energy and promote the Al-water reaction.
Physicochemical mechanisms Fig. 6 e H2 evolution from Al-water reaction promoted by CoeB/g-Al2O3 catalyst with different molar ratio of g-Al2O3 to Co at 40 C, where the weight contents of CoeB/g-Al2O3 catalysts in Al þ CoeB/g-Al2O3 is 10 wt%.
As is known, the surfaces of Al particles are covered by a dense passive oxide film, which inhibits the reaction of Al with water. When Al powders are added into water, the passive oxide film on Al surface hydrates with water molecules [48,49], Al2O3 þ 3H2O / 2Al(OH)3.
(6)
AlOOH or Al(OH)3 are produced during hydration process, because these oxyhydroxide or hydroxide phases are thermodynamically more stable than oxide phases. When the hydration reaction is completed, the hydroxide phases contact the inner surface of Al particles. In this case, a condensation reaction between hydroxide phases and Al occurs, because aluminum hydroxide is thermodynamically unstable in contact with metal Al [49], 2Al(OH)3 þ 2Al / 2Al2O3 þ 3H2[.
Fig. 7 e H2 evolution from deionized water using Al powder with the addition of different weight contents CoeB/gAl2O3 catalyst at 40 C, where the molar ratio of g-Al2O3 to Co in CoeB/g-Al2O3 catalysts is 1:1. The inset is the local magnification.
(7)
In condensation reaction, the inner Al would react with Al(OH)3 and generate hydrogen. Then the hydrogen accumulates and forms small hydrogen bubbles at the Al:Al2O3 interface due to the limited hydrogen dissolving capacity of Al particles and the low permeability of the hydrated Al2O3 toward hydrogen (Fig. 1 in Ref. [49]). With the process of reaction, the pressure in hydrogen bubbles increases gradually until the critical gas pressure that the hydrated oxide film can
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
24383
Al-water reaction (Figs. 5e8), implying that M-B/g-Al2O3 could promotes the hydration process of passive film on Al particle surface. M-B/g-Al2O3 catalyst is a mixture of M-B and g-Al2O3, so the effect of M-B/g-Al2O3 on the hydration process is complex, as schematically shown in Fig. 9. The effect of M-B/g-Al2O3 catalyst on the hydration process can be attributed to the following three reasons: (1) g-Al2O3 has a defect spinel structure and some cation sites are vacant. Therefore, g-Al2O3 has high activity, and can store and release water liking a “reactive sponge”. When g-Al2O3 is contact with H2O molecules, it can break and dissociate H2O molecules into Hþ and OH ions [50,51]. Therefore, when M-B/g-Al2O3 was added into water, some g-Al2O3 grains and aggregates are contact with Al particles. In this case, the ions dissociated from H2O molecules on g-Al2O3 surfaces are easy to hydrate with the passive oxide film on Al particle surfaces and promote the hydration reaction. (2) The electrode potential of M-B is higher than that of Al, so M-B and Al can form microgalvanic cell when M-B contacts with Al. For microgalvanic cell, metal Al is anode, and M-B is cathode. The cell reactions are as following:
Fig. 8 e (a) H2 evolution from deionized water using Al powder with the addition of 10 wt% CoeB/g-Al2O3 catalyst at different temperatures, and (b) Arrhenius plots for Alwater reaction promoted by 10 wt% CoeB/g-Al2O3 catalyst, where the molar ratio of g-Al2O3 to Co in CoeB/g-Al2O3 catalyst is 1:1. sustain. In this case, the hydrated oxide film on Al surfaces will be broken, and the inner Al comes into direct contact with water. Then the inner Al continuously reacts with water and generates hydrogen. Based on the above analysis, it is concluded that there is an induction time before hydrogen release from the Al-water reaction, which arises from the following three processes: (1) the hydration process of the passive oxide film on Al surface, (2) the hydrogen diffusion process in bulk Al until saturation and (3) the accumulation process of hydrogen molecules in hydrogen bubbles until the critical gas pressure. The results of Figs. 5 and 6 confirm the existence of the induction time. For the Al-water reaction in deionized water with and without the addition of M-B/g-Al2O3 catalyst, the time taken by the processes (2) and (3) should have no big difference, because the same Al powder was used in Al-water reaction. Therefore, the difference in induction time mainly comes from the hydration process. Any phenomena that can promote or inhibit the hydration reaction of passive film on Al particle surface could have significant impact on Al-water reaction. In this research, M-B/g-Al2O3 catalyst greatly decreases the induction time of
M-B cathode: 2H2O þ 2e ¼ 2OH þ H2,
(8)
Al anode: Al ¼ Al3þ þ 3e.
(9)
The hydroxide ions formed on the M-B cathode could hydrate with the passive oxide film on Al particle surfaces and promote the hydration process. (3) There is a synergistic effect between M-B and g-Al2O3. In M-B/g-Al2O3 suspension, the ions dissociated from H2O molecules on g-Al2O3 surfaces increase the local ion concentration, accelerating the work of Al/M-B microgalvanic cell. In addition, M-B might promote the dissociation of H2O molecules on g-Al2O3 surfaces [52,53]. For M-B catalyst, only the second effect is effective in the Al-water reaction, while g-Al2O3 has only the first effect on Al-water reaction. This explains why the induction time of the Alwater reaction promoted by M-Bþg-Al2O3 catalyst is shorter than that promoted by M-B or g-Al2O3 catalyst (Fig. 5). From the above analyses, it is clear that the contact between M-B/g-Al2O3 and Al particle is the key factor to affect the catalytic effect of M-B/g-Al2O3 catalyst on Al-water reaction. Therefore, the catalytic activity of M-B/g-Al2O3 is closely
Fig. 9 e Schematic representation of the hydration process of the passive oxide film on Al particle surface promoted by M-B/g-Al2O3 catalyst.
24384
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
related to its active area, which refers to the surface area of MB/g-Al2O3 that can be contact with Al particle. Increasing the active area of catalyst could improve its catalytic activity. Both M-B and M-B/g-Al2O3 catalysts are porous materials and have large specific surface area, but only the surface area attributed from the pores larger than Al particles contributes to the active area, because the surface attributed from the pores smaller than Al particles can not be contact with Al particles. For M-B and M-B/g-Al2O3 catalysts, the pore size ranges from several nanometers to hundreds of nanometers (Fig. 4). As 1.75 mm Al powder was used for Al-water reaction in this research, only the external surface area of the catalysts contributes to their active area. The external surface area of M-B/ g-Al2O3 catalyst is larger than that of M-B catalyst (Table 1), so the active area of M-B/g-Al2O3 catalyst is larger than that of MBþg-Al2O3 or M-B catalyst. This is the reason why the hydrogen generation performance of Al-water reaction promoted by M-B/g-Al2O3 catalyst is better than that promoted by M-Bþg-Al2O3 or M-B catalyst (Fig. 5). When the M-B/g-Al2O3 content increases in the suspension, the active surface of M-B/ g-Al2O3 increases so that the hydration process of the passive oxide film is speeded up, leading to a decrease of the induction time (Fig. 7). In addition, g-Al2O3 and M-B can inhibit the Al repassivation during the Al-water reaction, increasing the hydrogen generation rate. This explains why the hydrogen generation rate of Al-water reaction in M-B, g-Al2O3, M-B/gAl2O3 or M-B/g-Al2O3 suspension is obviously larger than that in deionized water (Fig. 5 and Table 2).
Conclusions In this work, a high-activity M-B/g-Al2O3 (M ¼ Co, Ni) catalyst was prepared by wet chemical reduction method, and it was used as a catalyst for the Al-water reaction. The results showed that M-B/g-Al2O3 catalyst could considerably promote the Al-water reaction and decrease the induction time. The effects of the molar ratio of g-Al2O3 to Co in CoeB/g-Al2O3 catalyst, the weight content of M-B/g-Al2O3 and reaction temperature on Al-water reaction were investigated. When the molar ratio of g-Al2O3 to Co is 1:1, CoeB/g-Al2O3 catalyst has the highest activity. The induction time is only 0.43 h, and the MHGR is as high as 13.3 ml min1$g1-Al. The catalytic activity of M-B/g-Al2O3 is proportional to its active area. The mechanism analyses reveal that the catalytic effect of M-B/gAl2O3 on Al-water reaction can be attributed to three reasons: (1) the ions dissociated from H2O molecules on g-Al2O3 surfaces promote the hydration of passive oxide film on Al surface; (2) M-B and Al form microgalvanic cell, promoting the Alwater reaction; (3) M-B and g-Al2O3 have a synergistic effect on Al-water reaction. The present work provides a new way to generate hydrogen from Al-water reaction for portable fuel cells.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 51872181 and 51801093), the
Key scientific research projects of Henan Province Colleges and Universities (No. 18A150039) and the Henan Provincial Key Science and Technology Research Project (No. 182102311047).
references
[1] Gai WZ, Liu WH, Deng ZY, Zhou JG. Reaction of Al powder with water for hydrogen generation under ambient condition. Int J Hydrogen Energy 2012;37(17):13132e40. https://doi.org/10.1016/j.ijhydene.2012.04.025. [2] Huang XN, Gao T, Pan XL, Wei D, Lv CJ, Qin LS, Huang YX. A review: feasibility of hydrogen generation from the reaction between aluminum and water for fuel cell applications. J Power Sources 2013;229:133e40. https://doi.org/10.1016/j. jpowsour.2012.12.016. [3] Haupin WE. Electrochemistry of the Hall-Heroult process for aluminum smelting. J Chem Educ 1983;60(4):279e82. https:// doi.org/10.1021/ed060p279. [4] Wang YQ, Gai WZ, Zhang XY, Pan HY, Cheng ZX, Xu PG, Deng ZY. Effect of storage environment on hydrogen generation by the reaction of Al with water. RSC Adv 2017;7(4):2103e9. https://doi.org/10.1039/C6RA25563A. [5] Belitskus D. Reaction of aluminum with sodium hydroxide solution as a source of hydrogen. J Electrochem Soc 1970;117(8):1097e9. https://doi.org/10.1149/1.2407730. nchez LA, Gallegos AAA, Sebastian PJ. [6] Martı´nez SS, Sa Coupling a PEM fuel cell and the hydrogen generation from aluminum waste cans. Int J Hydrogen Energy 2007;32(15):3159e62. https://doi.org/10.1016/j.ijhydene.2006. 03.015. [7] Wang ED, Shi PF, Du CY, Wang XR. A mini-type hydrogen generator from aluminum for proton exchange membrane fuel cells. J Power Sources 2008;181(1):144e8. https://doi.org/ 10.1016/j.jpowsour.2008.02.088. s J, Mun ~ oz M, Casado J. In situ [8] Soler L, Candela AM, Macana generation of hydrogen from water by aluminum corrosion in solutions of sodium aluminate. J Power Sources 2009;192(1):21e6. https://doi.org/10.1016/j.jpowsour.2008.11. 009. s J, Mun ~ oz M, Casado J. [9] Soler L, Candela AM, Macana Hydrogen generation from water and aluminum promoted by sodium stannate. Int J Hydrogen Energy 2010;35(3):1038e48. https://doi.org/10.1016/j.ijhydene.2009. 11.065. [10] Dai HB, Ma GL, Xia HJ, Wang P. Reaction of aluminum with alkaline sodium stannate solution as a controlled source of hydrogen. Energy Environ Sci 2011;4(6):2206e12. https://doi. org/10.1039/C1EE00014D. [11] Milinchuk VK, Klinshpont ER, Belozerov VI. Standalone hydrogen generator based on chemical decomposition of water by aluminum. Nucl Energy Technol 2015;1(4):259e66. https://doi.org/10.1016/j.nucet.2016.02.013. [12] Xu S, Zhao X, Liu J. Liquid metal activated aluminum-water reaction for direct hydrogen generation at room temperature. Renew Sustain Energy Rev 2018;92:17e37. https://doi.org/10.1016/j.rser.2018.04.052. [13] Ziebarth JT, Woodall JM, Kramer RA, Choi G. Liquid phaseenabled reaction of Al-Ga and Al-Ga-In-Sn alloys with water. Int J Hydrogen Energy 2011;36(9):5271e9. https://doi.org/10. 1016/j.ijhydene.2011.01.127. [14] Fan MQ, Mei DS, Chen D, Lv CJ, Shu KY. Portable hydrogen generation from activated Al-Li-Bi alloys in water. Renew Energy 2011;36(11):3061e7. https://doi.org/10.1016/j.renene. 2011.03.029.
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
[15] Eom KS, Kwon JY, Kim MJ, Kwon HS. Design of Al-Fe alloys for fast on-board hydrogen production from hydrolysis. J Mater Chem 2011;21(34):13047e51. https://doi.org/10.1039/ C1JM11329A. [16] Zhao ZW, Chen XY, Hao MM. Hydrogen generation by splitting water with Al-Ca alloy. Energy 2011;36(5):2782e7. https://doi.org/10.1016/j.energy.2011.02.018. [17] Ilyukhina AV, Ilyukhin AS, Shkolnikov EI. Hydrogen generation from water by means of activated aluminum. Int J Hydrogen Energy 2012;37(21):16382e7. https://doi.org/10. 1016/j.ijhydene.2012.02.175. [18] Hu XY, Zhu GZ, Zhang YJ, Wang YM, Gu MS, Yang S, Song PX, Li XJ, Fang HJ, Jiang GS, Wang ZF. Hydrogen generation through rolling using Al-Sn alloy. Int J Hydrogen Energy 2012;37(15):11012e20. https://doi.org/10.1016/j.ijhydene.2012. 04.141. [19] Zou MS, Guo XY, Huang HT, Yang RJ, Zhang P. Preparation and characterization of hydro-reactive Mg-Al mechanical alloy materials for hydrogen production in seawater. J Power Sources 2012;219:60e4. https://doi.org/10.1016/j.jpowsour. 2012.07.008. [20] Wang CP, Yang T, Liu YH, Ruan JJ, Yang SY, Liu XJ. Hydrogen generation by the hydrolysis of magnesium-aluminum-iron material in aqueous solutions. Int J Hydrogen Energy 2014;39(21):10843e52. https://doi.org/10.1016/j.ijhydene.2014. 05.047. [21] He TT, Wang W, Chen DM, Yang K. Effect of Ti on the microstructure and Al-water reactivity of Al-rich alloy. Int J Hydrogen Energy 2014;39(2):684e91. https://doi.org/10.1016/j. ijhydene.2013.10.095. [22] Chen XY, Zhao ZW, Liu XH, Hao MM, Chen AL, Tang ZY. Hydrogen generation by the hydrolysis reaction of ballmilled aluminum-lithium alloys. J Power Sources 2014;254:345e52. https://doi.org/10.1016/j.jpowsour.2013.12. 113. [23] Elitzur S, Rosenban V, Gany A. Study of hydrogen production and storage based on aluminum-water reaction. Int J Hydrogen Energy 2014;39(12):6328e34. https://doi.org/10. 1016/j.ijhydene.2014.02.037. [24] Huang TP, Gao Q, Liu D, Xu SN, Guo CB, Zou JJ, Wei CD. Preparation of Al-Ga-In-Sn-Bi quinary alloy and its hydrogen production via water splitting. Int J Hydrogen Energy 2015;40(5):2354e62. https://doi.org/10.1016/j.ijhydene.2014. 12.034. [25] Preez SPD, Bessarabov DG. Hydrogen generation by the hydrolysis of mechanochemically activated aluminum-tinindium composites in pure water. Int J Hydrogen Energy 2018;43(46):21398e413. https://doi.org/10.1016/j.ijhydene. 2018.09.133. [26] An Q, Hu HY, Li N, Liu D, Xu SN, Liu Z, Wei CD, Luo F, Xia MS, Gao Q. Effects of Bi composition on microstructure and Alwater reactivity of Al-rich alloys with low-In. Int J Hydrogen Energy 2018;43(24):10887e95. https://doi.org/10.1016/j. ijhydene.2018.05.009. [27] Qiao DX, Lu YP, Tang ZY, Fan XS, Wang TM, Li TJ, Liaw PK. The superior hydrogen-generation performance of multicomponent Al alloys by the hydrolysis reaction. Int J Hydrogen Energy 2019;44(7):3527e37. https://doi.org/10.1016/ j.ijhydene.2018.12.124. [28] Alinejad B, Mahmoodi K. A novel method for generating hydrogen by hydrolysis of highly activated aluminum nanoparticles in pure water. Int J Hydrogen Energy 2009;34(19):7934e8. https://doi.org/10.1016/j.ijhydene.2009. 07.028. [29] Czech E, Troczynski T. Hydrogen generation through massive corrosion of deformed aluminum in water. Int J Hydrogen Energy 2010;35(3):1029e37. https://doi.org/10.1016/ j.ijhydene.2009.11.085.
24385
[30] Wang HW, Chung HW, Teng HT, Cao GZ. Generation of hydrogen from aluminum and water e effect of metal oxide nanocrystals and water quality. Int J Hydrogen Energy 2011;36(23):15136e44. https://doi.org/10.1016/j.ijhydene.2011. 08.077. [31] Dupiano P, Stamatis D, Dreizin EL. Hydrogen production by reacting water with mechanically milled composite aluminum-metal oxide powders. Int J Hydrogen Energy 2011;36(8):4781e91. https://doi.org/10.1016/j.ijhydene.2011. 01.062. [32] Huang XN, Lv CJ, Wang Y, Shen HY, Chen D, Huang YX. Hydrogen generation from hydrolysis of aluminum/graphite composite with a core-shell structure. Int J Hydrogen Energy 2012;37(9):7457e63. https://doi.org/10.1016/j.ijhydene.2012. 01.126. [33] Chen XY, Zhao ZW, Hao MM, Wang DZ. Research of hydrogen generation by the reaction of Al-based materials with water. J Power Sources 2013;222:188e95. https://doi.org/ 10.1016/j.jpowsour.2012.08.078. [34] Liu YA, Wang XH, Liu HZ, Dong ZH, Li SQ, Ge HW, Yan M. Study on hydrogen generation from the hydrolysis of a ball milled aluminum/calcium hydride composite. RSC Adv 2015;5(74):60460e6. https://doi.org/10.1039/C5RA09200K. [35] Razavi-Tousi SS, Szpunar JA. Effect of addition of watersoluble salts on the hydrogen generation of aluminum in reaction with hot water. J Alloys Compd 2016;679:364e74. https://doi.org/10.1016/j.jallcom.2016.04.038. [36] Irankhah A, Fattahi SMS, Salem M. Hydrogen generation using activated aluminum/water reaction. Int J Hydrogen Energy 2018;43(33):15739e48. https://doi.org/10.1016/j. ijhydene.2018.07.014. [37] Xiao F, Yang RJ, Li JM. Preparation and characterization of mechanically activated aluminum/polytetrafluoroethylene composites and their reaction properties in high temperature water steam. J Alloys Compd 2018;761:24e30. https://doi.org/10.1016/j.jallcom.2018.05.087. [38] Zhao C, Xu F, Sun LX, Chen J, Guo XL, Yan E, Yu F, Chu HL, Peng HL, Zou YJ, Liu ZW, Li FW. A novel Al-BiOCl composite for hydrogen generation from water. Int J Hydrogen Energy 2019;44(13):6655e62. https://doi.org/10.1016/j.ijhydene.2018. 12.165. [39] Teng HT, Lee TY, Chen YK, Wang HW, Cao GZ. Effect of Al(OH)3 on the hydrogen generation of aluminum-water system. J Power Sources 2012;219:16e21. https://doi.org/10. 1016/j.jpowsour.2012.06.077. [40] Newell A, Thampi KR. Novel amorphous aluminum hydroxide catalysts for aluminum-water reactions to produce H2 on demand. Int J Hydrogen Energy 2017;42(37):23446e54. https://doi.org/10.1016/j.ijhydene.2017. 04.279. [41] Wen YC, Huang WM, Wang HW. Kinetics study on the generation of hydrogen from an aluminum/water system using synthesized aluminum hydroxides. Int J Energy Res 2018;42(4):1615e24. https://doi.org/10.1002/er.3955. [42] Meng HX, Wang N, Dong YM, Jia ZL, Gao LJ, Chai YJ. Influence of M-B (M ¼ Fe, Co, Ni) on aluminum-water reaction. J Power Sources 2014;268:550e6. https://doi.org/10.1016/j.jpowsour. 2014.06.094. [43] Wang N, Meng HX, Dong YM, Jia ZL, Gao LJ, Chai YJ. Cobaltiron-boron catalyst-induced aluminum-water reaction. Int J Hydrogen Energy 2014;39(30):16936e43. https://doi.org/10. 1016/j.ijhydene.2014.07.180. [44] Gai WZ, Fang CS, Deng ZY. Hydrogen generation by the reaction of Al with water using oxides as catalysts. Int J Energy Res 2014;38(7):918e25. https://doi.org/10.1002/er.3093. [45] Yang Y, Gai WZ, Deng ZY, Zhou JG. Hydrogen generation by the reaction of Al with water promoted by an ultrasonically prepared Al(OH)3 suspension. Int J Hydrogen Energy
24386
[46]
[47]
[48]
[49]
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
2014;39(33):18734e42. https://doi.org/10.1016/j.ijhydene.2014. 09.085. Glavee GN, Klabunde KJ, Sorensen CM, Hadjipanayis GC. Borohydride reduction of cobalt ions in water e chemistry leading to nanoscale metal, boride, or borate particles. Langmuir 1993;9(1):162e9. https://pubs.acs.org.ccindex.cn/ doi/abs/10.1021/la00025a034. Glavee GN, Klabunde KJ, Sorensen CM, Hadjipanayis GC. Borohydride reduction of nickel and copper ions in aqueous and nonaqueous media e controllable chemistry leading to nanoscale metal and metal boride particles. Langmuir 1994;10(12):4726e30. https://pubs.acs.org.ccindex.cn/doi/ abs/10.1021/la00024a055. Bunker BC, Nelson GC, Zavadil KR, Barbour JC, Wall FD, Sullivan JP, Windisch CF, Engelhardt MH, Baer DR. Hydration of passive oxide films on aluminum. J Phys Chem B 2002;106(18):4705e13. https://pubs.acs.org.ccindex.cn/doi/ abs/10.1021/jp013246e. Deng ZY, Ferreira JMF, Tanaka Y, Ye JH. Physicochemical mechanism for the continuous reaction of g-Al2O3 modified aluminum powder with water. J Am Ceram Soc 2007;90(5):1521e6. https://doi.org/10.1111/j.1551-2916.2007. 01546.x.
[50] Ionescu A, Allouche A, Aycard JP, Rajzmann M, Hutschka F. Study of g-alumina surface reactivity: adsorption of water and hydrogen sulfide on octahedral aluminum sites. J Phys Chem B 2002;106(36):9359e66. https://pubs.acs.org.ccindex. cn/doi/abs/10.1021/jp020145n. [51] Gai WZ, Shi Y, Deng ZY, Zhou JG. Clarification of activation mechanism in oxide-modified aluminum. Int J Hydrogen Energy 2015;40(36):12057e62. https://doi.org/10.1016/j. ijhydene.2015.07.102. n DL, Liu ZY, Duchon T, Evans J, [52] Carrasco J, Dura Senanayake SD, Crumlin EJ, Matolı´n V, Rodrı´guez JA, Pirovano MVG. In situ and theoretical studies for the dissociation of water on an active Ni/CeO2 catalyst: importance of strong metal-support interactions for the cleavage of O-H bonds. Angew Chem Int Ed 2015;54(13):3917e21. https://doi.org/10.1002/anie. 201410697. [53] Choong CKS, Huang L, Zhong ZY, Lin JY, Hong L, Chen LW. Effect of calcium addition on the catalytic ethanol steam reforming of Ni/Al2O3: II. Acidity/basicity, water adsorption and catalytic activity. Appl Catal A: Gen 2011;407(1):155e62. https://doi.org/10.1016/j.apcata.2011.08.038.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Hydrogen generation from Al-Water reaction promoted by M-B/g-Al2O3 (M ¼ Co, Ni) catalyst Wei-Zhuo Gai a,*, Xianghui Zhang a, Kexi Sun a, Zhen-Yan Deng b a
College of Physics and Electronic Information & Henan Key Laboratory of Electromagnetic Transformation and Detection, Luoyang Normal University, Luoyang 471934, China b Energy Materials & Physics Group, Department of Physics, Shanghai University, Shanghai 200444, China
highlights
graphical abstract
High-activity M-B/g-Al2O3 catalyst was prepared by wet chemical reduction method. M-B/g-Al2O3 greatly promotes the Al-water reaction and decreases the induction time. The catalytic activity of M-B/gAl2O3 is proportional to its active area. A possible mechanism for Alwater reaction promoted by M-B/ g-Al2O3 was proposed.
article info
abstract
Article history:
Al-water reaction promoted by catalysts is a promising hydrogen generation technology. In
Received 29 May 2019
this work, a high-activity M-B/g-Al2O3 (M ¼ Co, Ni) catalyst is prepared by wet chemical
Received in revised form
reduction method. It is found that M-B/g-Al2O3 catalyst significantly promotes the Al-water
16 July 2019
reaction and decreases the induction time. When the molar ratio of g-Al2O3 to Co-M in Co
Accepted 25 July 2019
eB/g-Al2O3 catalyst is 1:1, the induction time is only 0.43 h. The catalytic activity of M-B/g-
Available online 17 August 2019
Al2O3 is proportional to its active area. SEM analyses show that M-B particles are dispersed
Keywords:
surface of M-B/g-Al2O3, leading to a high catalytic activity. A possible mechanism is pro-
Hydrogen generation
posed, which shows that the dissociation of water molecules on g-Al2O3 surface and the
Al-water reaction
microgalvanic interaction between M-B and Al can promote the hydration process of
Catalyst
passive oxide film on Al particle surface, speeding up the Al-water reaction.
on g-Al2O3 surface, which reduces the agglomeration of M-B and increases the active
Induction time
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (W.-Z. Gai). https://doi.org/10.1016/j.ijhydene.2019.07.203 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
24378
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
Introduction Metal Al is one of the most widely studied hydrogen generation materials, which can continuously react with water and release hydrogen under ambient condition [1]. Al is the most abundant metal in the earth's crust, and has a relative low cost. Meanwhile, the byproducts of Al-water reaction are Al(OH)3 or AlOOH, which are environmentally benign and can be reduced to metal Al for recycle by a Hall-Heroult process or used to make refractory and calcium aluminate cement [2,3]. 1 g Al reacts with water to generate as much as 1245 ml H2 in the standard condition, making it a potential in-situ hydrogen generation material for small fuel cells. However, there is a long induction time before hydrogen generation for Al-water reaction due to a dense passive oxide film covering on Al particle surface [4]. The research on hydrogen generation from the Al-water reaction has been a hotspot during the past few years, and various activation methods were developed to promote the Alwater reaction and decrease the induction time. Using an alkaline solution to assist the Al-water reaction is a traditional method [5]. Martı´nez found that metal Al obtained from Al waste cans could react with NaOH solution and generate high purity hydrogen [6]. Wang proposed a mini-type hydrogen generator from Al for proton exchange membrane fuel cells, and investigated the effects of concentration, dropping rate and initial temperature of NaOH solution on Al-water reaction [7]. But strong alkalis, e.g. NaOH, KOH etc., have strong corrosiveness to reaction device, and the byproducts are also alkaline and can cause environmental pollution. In recent years, Soler and Dai used low corrosive NaAlO2 and Na2SnO3 to replace strong alkalis, and found that NaAlO2 and Na2SnO3 could promote Al-water reaction and inhibit the repassivation of Al [8e10]. Milinchuk developed a hydrogen generator based on Al-water reaction promoted by sodium metasilicate and studied the hydrogen generation kinetics [11]. Alloying of metal Al by melting or mechanical ball milling is an effective method to activate Al, because alloying process can destroy the passive oxide film on Al surface [12]. In addition, the alloying elements and Al can form microgalvanic cell, promoting the corrosion of Al. So far, plenty of Al alloys including AleLi, AleGa, AleSn, AleMg, AleIn, AleFe, Al-Ga-InSn etc., have been developed [13e27]. Different Al alloys have different activation mechanisms. For example, Ga, In, Sn in Al-Ga-In-Sn alloy would form a liquid eutectic covering on Al surface, so that inner Al atoms could diffuse to Al surface through this liquid eutectic and react with water [13,26,27]. For AleLi alloy, Li would hydrolyze and produce LiOH, which promotes the Al-water reaction [14]. Eom prepared an AleFe bulk alloy by melting method, and proposed that its fast hydrogen generation rate resulted from the combined effects of the intergranular corrosion and galvanic corrosion [15]. In addition, metal Al can be activated by mechanical ball milling with different oxide, soluble inorganic salt, graphite, hydride, polytetrafluoroethylene, BiOCl [28e38], etc. In the process of ball milling, the passive oxide film on Al surface can be destroyed, and many new and fresh surfaces are produced. In this case, the fresh Al can directly react with water to produce hydrogen. In addition, different milling mediums
play different roles in the Al-water reaction. For example, the CaO in AleCaO-salt composite prepared by the ball milling method can hydrolyze and release OH, which promotes the Al-water reaction [33]. For Al/graphite composite, the graphite can eliminate the passive film and cover on Al surface during the ball milling process. As graphite has a loose structure, H2O molecules can easily penetrate to the Al/graphite interfaces, leading to the Al-water reaction [32]. For Al/CaH2 composite prepared by ball milling, the hydrolysis of CaH2 can help to open up the structure of the Al and provide many passages for H2O molecules to penetrate through. In addition, the hydrolysis of CaH2 can release heat and OH, further promoting the Al-water reaction [34]. But ball milling consumes much energy and needs to be carried out under inert condition, which limits its wide application [28,32]. Recently, some researchers found that some special catalysts could be used to promote the Al-water reaction. For example, Teng and Newell found that Al powder could react with water and produce hydrogen in Al(OH)3 suspension [39e41]. Meng and Wang reported that FeeB, CoeB, NieB and CoeFeeB catalysts could be used to promote Al-water reaction [42,43]. In previous work, we found that a-Al2O3, g-Al2O3, TiO2 and Al(OH)3 could be used as catalysts to promote Al-water reaction [44,45]. However, agglomeration is easy to occur in the process of catalyst preparation, decreasing the activity of catalysts. In this work, a high-activity M-B/g-Al2O3 (M ¼ Co, Ni) catalyst was prepared by wet chemical reduction method. The effect of M-B/g-Al2O3 catalyst on Al-water reaction was systematically investigated, and the possible catalytic mechanisms were discussed in detail.
Experimental procedure Al powder (1.75 mm, 99.9% purity, Henan Yuanyang Powder Technology Co., Ltd., Xinxiang, China), g-Al2O3 powder (99.99% purity, Hangzhou Wan Jing New Material Co., Ltd., Hangzhou, China) and NaBH4 (97% purity, Tianjin Kemiou Chemical Reagent Co., Tianjin, China) were used in the present research. In addition, analytical reagent grade CoCl26H2O, NiCl26H2O and absolute ethanol were used in the experiment. CoeB/g-Al2O3 or NieB/g-Al2O3 catalysts were prepared by a wet chemical reduction method [46,47]. The g-Al2O3 powder was put into a beaker with 200 ml of deionized water, and then ultrasonically dispersed in an ultrasonic cleaner (Type: KQ3200E, frequency 40 kHz, power 150 W, Kunshan Ultrasonic Instruments Co., Ltd., Kunshan, China) for 1 h. Then 6 mmol CoCl26H2O or NiCl26H2O were added into above beaker and ultrasonically dispersed 0.5 h. The suspension was added into another beaker containing 12 mmol NaBH4 powders within 0.5 min and manually stirred. In this case, CoCl26H2O or NiCl26H2O can react vigorously with NaBH4 and produce a black precipitate, 2MCl2 þ 4NaBH4 þ 9H2O / M2BY þ 4NaCl þ12.5H2[ þ 3B(OH)3,
(1)
where “M” represents Co or Ni elements. The mixture suspension after reaction was filtered by a filter paper, washed with deionized water and absolute ethanol, and then dried at
24379
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
room temperature. During filtering and drying, small amount of Co2B and Ni2B would be oxidized by oxygen, 4M2B þ 3O2 / 8 M þ 2B2O3.
(2)
Therefore, the M-B/g-Al2O3 catalyst is a mixture, which consisted of g-Al2O3, M2B, and small quantities of metal M and B2O3. For comparison, CoeB and NieB catalysts were prepared using the same procedure as that to prepare M-B/g-Al2O3 catalyst, except that no g-Al2O3 was added. The weight percentage of Ni and Co in different catalysts is given in Table 1. The Al-water reaction tests promoted by M-B/g-Al2O3 catalyst were carried out in a double-necked flask, which has better air tightness. In each test, 250 ml of deionized water or M-B/g-Al2O3 suspension were used, and then 1.0 g of Al powder was added and mixed evenly. The reaction temperature was controlled by a thermostat water bath with an accuracy of ±1 C, and no agitation was used in all the tests. The hydrogen produced by Al-water reaction was collected in an inverted burette filled with tap water. The volume of hydrogen produced was measured from the water level change in the inverted burette. The hydrogen yield a can be determined a¼
VH2 100%; V0
(3)
where a is the hydrogen yield; VH2 is the volume of hydrogen; V0 is the theoretical hydrogen volume obtained by reacting all of metal Al. Each test was carried out twice in order to ensure the reproducibility of the hydrogen generation curve. X-ray diffraction (XRD, Model No. D8 Advance, Bruker Co., Germany) studies were carried out to analyse the phases in CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts. Morphology analyses of the Al powder and different catalysts were performed using scanning electron microscopy (SEM, Model No. Sigma 500/VP, Carl Zeiss Co., Germany). The specific surface area and pore size distribution of different catalysts were measured using surface area analyzer (Micromeritics ASAP 2020, USA). The specific surface area was determined by Brunauer-Emmett-Teller (BET) method, the pore size distribution was calculated by BarrettJoyner-Halenda (BJH) method, and the external surface area was determined by empirical t-plot method.
Results and discussion Characterization of M-B/g-Al2O3 catalyst Fig. 1 shows the morphologies of as-received pure Al powder and g-Al2O3 powder. It is found that Al particles are spherical,
and their surfaces are dense and smooth. The Al powder has a particle size distribution, and the Al particles range from nano-sized to micro-sized (Fig. 1(a)). For g-Al2O3, the grains are very fine and about several nanometers, but there are some agglomerates (Fig. 1(b)). Fig. 2 shows the X-ray diffraction patterns of as-received pure Al powder, CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts. It can be seen that the Al powder has a high purity, both CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts are amorphous. Fig. 3 shows the morphologies of CoeB, NieB, CoeB/gAl2O3 and NieB/g-Al2O3 catalysts. It can be seen that all the four catalysts have porous structure, but the pore size distribution of different catalysts is very different. Obviously, the pores in CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts are larger than that in CoeB and NieB catalysts. In addition, there are many agglomerates in all of catalysts, and it seems that the agglomeration of CoeB and NieB catalysts is more serious than that of CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts. For CoeB/g-Al2O3 and NieB/g-Al2O3 catalysts, the CoeB or NieB particles cover on the surface of g-Al2O3 grains or agglomerates, indicating that the existence of g-Al2O3 reduces the agglomeration of CoeB and NieB particles. Fig. 4(a) shows the particle size distribution of as-received Al powder. It can be seen that the Al particles range from 0.2 mm to 4.3 mm, and the average particle size is about 1.75 mm. Fig. 4(b) shows the pore size distribution of different catalysts. It is noticed that CoeB catalyst has low porosity. For NieB catalyst, the pores are mainly mesoporous, and the pore sizes vary between 2 nm and 20 nm. However, both CoeB/gAl2O3 and NieB/g-Al2O3 catalysts have a wider pore size distribution, and they contain mesopores and macropores. The BET surface area, external surface area, pore volume and average pore size of different catalysts are given in Table 1. The pore volumes of CoeB, NieB, CoeB/g-Al2O3 and NieB/gAl2O3 catalysts are 0.135, 0.190, 0.465 and 0.468 cm3g-1, respectively. The addition of g-Al2O3 greatly increases the surface area of CoeB catalyst, while it slightly increases the surface area of NieB catalyst.
Al-water reaction promoted by M-B/g-Al2O3 catalyst Fig. 5(a) shows the hydrogen evolution from deionized water using pure Al powder with and without the addition of 10 wt% (the weight fraction of catalyst in Al powder þ catalyst) gAl2O3, CoeB, CoeBþg-Al2O3 (the mixture of CoeB and g-Al2O3) or CoeB/g-Al2O3 catalyst at 40 C. It is found that the pure Al powder could react with water and generate hydrogen at 40 C, but there is a long induction time (3.0 h) for the beginning of Al-water reaction. When the catalysts were added into
Table 1 e The metal percentage, BET surface area, external surface area, pore volume and average pore size of different catalysts. Catalysts CoeB NieB CoeB/g-Al2O3a NieB/g-Al2O3a a
Co or Ni weight percentage
Average pore size (nm)
Pore volume (cm3g1)
BET surface area (m2g1)
External surface area (m2g1)
91.6% 91.6% 35.4% 35.3%
35.7 9.8 22.6 23.1
0.135 0.190 0.465 0.468
14.9 80.6 81.4 81.9
14.9 59.9 74.7 63.2
The molar ratio of g-Al2O3 to M (M ¼ Co, Ni) in M-B/g-Al2O3 catalysts is 1:1.
24380
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
Fig. 1 e SEM micrographs of (a) pure Al and (b) g-Al2O3 powders.
Fig. 2 e X-ray patterns for (a) as-received pure Al powder, (b) CoeB/g-Al2O3 and (c) NieB/g-Al2O3 catalysts.
water, the induction time of Al-water reaction was greatly decreased. The catalysts significantly promoted the Al-water reaction, but different catalysts have different effects on the Al-water reaction dynamics. For g-Al2O3 and CoeB catalysts, the reaction dynamics of Al with water is almost the same except that the hydrogen yield of Al-water reaction promoted by CoeB is higher than that promoted by g-Al2O3. The CoeB/gAl2O3 catalyst has the best catalytic activity, and could considerably promote the Al-water reaction and hydrogen generation. In order to reveal the catalytic mechanism of CoeB/g-Al2O3 catalyst, a mixture of CoeB and g-Al2O3 (CoeBþg-Al2O3) was used as catalyst to promote the Al-water reaction, and the hydrogen generation curve was also given in Fig. 5(a). Obviously, the hydrogen generation performance of Al-water reaction promoted by CoeBþg-Al2O3 catalyst is better than that promoted by CoeB or g-Al2O3 catalyst, implying that CoeB and g-Al2O3 have a synergistic effect on Al-water reaction. Table 2 gives the induction time and the maximum hydrogen generation rate (MHGR) of Al-water reaction with and without the addition of 10 wt% different catalysts at 40 C. It can be seen that g-Al2O3, CoeB, CoeBþg-Al2O3 and CoeB/gAl2O3 decreased the induction time from 3.0 h to 1.17 h, 1.08 h, 0.77 h and 0.43 h, respectively. This means that the activity of
Fig. 3 e SEM micrographs of (a) CoeB, (b) NieB, (c) CoeB/g-Al2O3 and (d) NieB/g-Al2O3 catalysts.
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
24381
Fig. 4 e (a) Particle size distribution of as-received Al power and (b) pore size distribution of different catalysts.
the catalysts is in the order of CoeB/g-Al2O3 > CoeBþg-Al2O3 > CoeB > g-Al2O3. The Al-water reaction promoted by CoeB/g-Al2O3 catalyst has the shortest induction time, and >85% of Al was consumed and generated H2 within 3.5 h, indicating that CoeB/g-Al2O3 has high activity and the Alwater reaction promoted by CoeB/g-Al2O3 is a promising hydrogen generation technology for portable fuel cells. Fig. 5(b) shows the hydrogen evolution from deionized water using pure Al powder with and without the addition of 10 wt% g-Al2O3, NieB, NieBþg-Al2O3 (the mixture of Ni eB and g-Al2O3) or NieB/g-Al2O3 catalyst at 40 C. It can be seen that all the catalyst could promote the Al-water reaction and decrease the induction time. The activity of catalysts is in the order of NieB/g-Al2O3 > NieBþg-Al2O3 > NieB, which is similar to the results of Fig. 5(a). The Al-water reaction promoted by NieB/g-Al2O3 catalyst has the best hydrogen generation performance, but its induction time is longer than that of the Al-water reaction promoted by CoeB/g-Al2O3 catalyst. Fig. 5 indicates that M-B and g-Al2O3 have a synergistic effect on Al-water reaction, so the ratio of g-Al2O3 to M-B might have important impact on Al-water reaction dynamics. Fig. 6 shows the hydrogen evolution from Al-water reaction promoted by CoeB/g-Al2O3 catalyst with different molar ratio of g-Al2O3 to Co at 40 C. It can be seen that the ratio of g-Al2O3 to Co has great effect on the hydrogen generation performance of the Al-water reaction promoted by CoeB/g-Al2O3, implying that the catalytic activity of M-B/g-Al2O3 is closely related to the ratio of g-Al2O3 to Co. The CoeB/g-Al2O3 catalyst decreased the induction time of the Al-water reaction, but the relation between the induction time and the ratio of g-Al2O3 to
Fig. 5 e H2 evolution from deionized water using pure Al powder with and without the addition of 10 wt% different catalysts at 40 C, where the M-Bþg-Al2O3 (M ¼ Co, Ni) catalysts are the mixtures of M-B and g-Al2O3, and the molar ratio of g-Al2O3 to M in M-B/g-Al2O3 and M-BþgAl2O3 catalysts is 1:1.
Co is not monotonic. There is a shortest induction time (0.43 h) for Al-water reaction promoted by CoeB/g-Al2O3 when the ratio of g-Al2O3 to Co is 1:1. Fig. 7 shows the hydrogen evolution from deionized water using Al powder with the addition of different weight contents CoeB/g-Al2O3 catalysts at 40 C. It can be seen that the content of CoeB/g-Al2O3 has little effect on Al-water reaction dynamics. The induction time of Al-water reaction decreased with increasing the CoeB/g-Al2O3 content. Increasing the content of CoeB/g-Al2O3 from 5 wt% to 20 wt%, the induction time decreased from 0.77 h to 0.32 h, and the hydrogen yield increased from 81.2% to 90.0%. However, the Al-water reaction dynamics has little change when the content of CoeB/g-Al2O3 increases from 20 wt% to 30 wt%, indicating that the effect of CoeB/g-Al2O3 content on Al-water reaction tends to be saturated. Fig. 8(a) shows the hydrogen evolution from deionized water using Al powder with the addition of 10 wt% CoeB/gAl2O3 catalyst at different temperatures. When the reaction temperature increases, the hydrogen generation rate
24382
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
Table 2 e Hydrogen generation data of Al powder in deionized water with and without the addition of 10 wt% different catalysts at 40 C. catalysts Without catalyst g-Al2O3 CoeB CoeBþg-Al2O3 CoeB/g-Al2O3 NieB NieBþg-Al2O3 NieB/g-Al2O3 a
Induction time (hours)
MHGR (ml$min1$g1-Al)a
3.00 1.17 1.08 0.77 0.43 1.12 0.85 0.67
5.40 11.28 9.87 11.33 13.30 8.75 12.06 13.40
MHGR is the maximum hydrogen generation rate, which is obtained by the hydrogen volume divided by the time interval and the Al powder weight.
increases and the induction time decreases obviously. At 60 C, the induction time of Al-water reaction is just 0.15 h, and over 90% of Al powder was consumed within 2.5 h, implying that it could satisfy the requirement of portable fuel cells. Increasing the temperature from 35 C to 60 C, the MHGR increases from 6.6 to 51.3 ml min1$g1-Al, so the hydrogen generation rate could be controlled by adjusting the reaction temperature in the practical application. In order to quantify the temperature effect, the Arrhenius equation [1] was used, k ¼ A expð Ea =RTÞ
(4)
where A is a pre-exponential factor, Ea is the activation energy, R is the gas constant, T is the reaction temperature, k is the rate constant related to the surface chemical reaction and can be determined by fitting the experimental data of Fig. 8(a) using the following equation [1]: 1 ð1 aÞ1=3 ¼ kt:
(5)
The activation energy of Al-water reaction was calculated from the plot of the reaction rate constant k against the reciprocal temperature using Eq. (4), as shown in Fig. 8(b). The plot has a good linear fitting and the activation energy for Alwater reaction promoted by CoeB/g-Al2O3 catalyst was determined to be 50.9 kJ/mol, which is lower than the activation energy for the reaction of pure Al with water (68.1 kJ/ mol for 2.25 mm Al powder) [1]. This indicates that the CoeB/gAl2O3 catalyst could greatly decrease the activation energy and promote the Al-water reaction.
Physicochemical mechanisms Fig. 6 e H2 evolution from Al-water reaction promoted by CoeB/g-Al2O3 catalyst with different molar ratio of g-Al2O3 to Co at 40 C, where the weight contents of CoeB/g-Al2O3 catalysts in Al þ CoeB/g-Al2O3 is 10 wt%.
As is known, the surfaces of Al particles are covered by a dense passive oxide film, which inhibits the reaction of Al with water. When Al powders are added into water, the passive oxide film on Al surface hydrates with water molecules [48,49], Al2O3 þ 3H2O / 2Al(OH)3.
(6)
AlOOH or Al(OH)3 are produced during hydration process, because these oxyhydroxide or hydroxide phases are thermodynamically more stable than oxide phases. When the hydration reaction is completed, the hydroxide phases contact the inner surface of Al particles. In this case, a condensation reaction between hydroxide phases and Al occurs, because aluminum hydroxide is thermodynamically unstable in contact with metal Al [49], 2Al(OH)3 þ 2Al / 2Al2O3 þ 3H2[.
Fig. 7 e H2 evolution from deionized water using Al powder with the addition of different weight contents CoeB/gAl2O3 catalyst at 40 C, where the molar ratio of g-Al2O3 to Co in CoeB/g-Al2O3 catalysts is 1:1. The inset is the local magnification.
(7)
In condensation reaction, the inner Al would react with Al(OH)3 and generate hydrogen. Then the hydrogen accumulates and forms small hydrogen bubbles at the Al:Al2O3 interface due to the limited hydrogen dissolving capacity of Al particles and the low permeability of the hydrated Al2O3 toward hydrogen (Fig. 1 in Ref. [49]). With the process of reaction, the pressure in hydrogen bubbles increases gradually until the critical gas pressure that the hydrated oxide film can
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
24383
Al-water reaction (Figs. 5e8), implying that M-B/g-Al2O3 could promotes the hydration process of passive film on Al particle surface. M-B/g-Al2O3 catalyst is a mixture of M-B and g-Al2O3, so the effect of M-B/g-Al2O3 on the hydration process is complex, as schematically shown in Fig. 9. The effect of M-B/g-Al2O3 catalyst on the hydration process can be attributed to the following three reasons: (1) g-Al2O3 has a defect spinel structure and some cation sites are vacant. Therefore, g-Al2O3 has high activity, and can store and release water liking a “reactive sponge”. When g-Al2O3 is contact with H2O molecules, it can break and dissociate H2O molecules into Hþ and OH ions [50,51]. Therefore, when M-B/g-Al2O3 was added into water, some g-Al2O3 grains and aggregates are contact with Al particles. In this case, the ions dissociated from H2O molecules on g-Al2O3 surfaces are easy to hydrate with the passive oxide film on Al particle surfaces and promote the hydration reaction. (2) The electrode potential of M-B is higher than that of Al, so M-B and Al can form microgalvanic cell when M-B contacts with Al. For microgalvanic cell, metal Al is anode, and M-B is cathode. The cell reactions are as following:
Fig. 8 e (a) H2 evolution from deionized water using Al powder with the addition of 10 wt% CoeB/g-Al2O3 catalyst at different temperatures, and (b) Arrhenius plots for Alwater reaction promoted by 10 wt% CoeB/g-Al2O3 catalyst, where the molar ratio of g-Al2O3 to Co in CoeB/g-Al2O3 catalyst is 1:1. sustain. In this case, the hydrated oxide film on Al surfaces will be broken, and the inner Al comes into direct contact with water. Then the inner Al continuously reacts with water and generates hydrogen. Based on the above analysis, it is concluded that there is an induction time before hydrogen release from the Al-water reaction, which arises from the following three processes: (1) the hydration process of the passive oxide film on Al surface, (2) the hydrogen diffusion process in bulk Al until saturation and (3) the accumulation process of hydrogen molecules in hydrogen bubbles until the critical gas pressure. The results of Figs. 5 and 6 confirm the existence of the induction time. For the Al-water reaction in deionized water with and without the addition of M-B/g-Al2O3 catalyst, the time taken by the processes (2) and (3) should have no big difference, because the same Al powder was used in Al-water reaction. Therefore, the difference in induction time mainly comes from the hydration process. Any phenomena that can promote or inhibit the hydration reaction of passive film on Al particle surface could have significant impact on Al-water reaction. In this research, M-B/g-Al2O3 catalyst greatly decreases the induction time of
M-B cathode: 2H2O þ 2e ¼ 2OH þ H2,
(8)
Al anode: Al ¼ Al3þ þ 3e.
(9)
The hydroxide ions formed on the M-B cathode could hydrate with the passive oxide film on Al particle surfaces and promote the hydration process. (3) There is a synergistic effect between M-B and g-Al2O3. In M-B/g-Al2O3 suspension, the ions dissociated from H2O molecules on g-Al2O3 surfaces increase the local ion concentration, accelerating the work of Al/M-B microgalvanic cell. In addition, M-B might promote the dissociation of H2O molecules on g-Al2O3 surfaces [52,53]. For M-B catalyst, only the second effect is effective in the Al-water reaction, while g-Al2O3 has only the first effect on Al-water reaction. This explains why the induction time of the Alwater reaction promoted by M-Bþg-Al2O3 catalyst is shorter than that promoted by M-B or g-Al2O3 catalyst (Fig. 5). From the above analyses, it is clear that the contact between M-B/g-Al2O3 and Al particle is the key factor to affect the catalytic effect of M-B/g-Al2O3 catalyst on Al-water reaction. Therefore, the catalytic activity of M-B/g-Al2O3 is closely
Fig. 9 e Schematic representation of the hydration process of the passive oxide film on Al particle surface promoted by M-B/g-Al2O3 catalyst.
24384
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
related to its active area, which refers to the surface area of MB/g-Al2O3 that can be contact with Al particle. Increasing the active area of catalyst could improve its catalytic activity. Both M-B and M-B/g-Al2O3 catalysts are porous materials and have large specific surface area, but only the surface area attributed from the pores larger than Al particles contributes to the active area, because the surface attributed from the pores smaller than Al particles can not be contact with Al particles. For M-B and M-B/g-Al2O3 catalysts, the pore size ranges from several nanometers to hundreds of nanometers (Fig. 4). As 1.75 mm Al powder was used for Al-water reaction in this research, only the external surface area of the catalysts contributes to their active area. The external surface area of M-B/ g-Al2O3 catalyst is larger than that of M-B catalyst (Table 1), so the active area of M-B/g-Al2O3 catalyst is larger than that of MBþg-Al2O3 or M-B catalyst. This is the reason why the hydrogen generation performance of Al-water reaction promoted by M-B/g-Al2O3 catalyst is better than that promoted by M-Bþg-Al2O3 or M-B catalyst (Fig. 5). When the M-B/g-Al2O3 content increases in the suspension, the active surface of M-B/ g-Al2O3 increases so that the hydration process of the passive oxide film is speeded up, leading to a decrease of the induction time (Fig. 7). In addition, g-Al2O3 and M-B can inhibit the Al repassivation during the Al-water reaction, increasing the hydrogen generation rate. This explains why the hydrogen generation rate of Al-water reaction in M-B, g-Al2O3, M-B/gAl2O3 or M-B/g-Al2O3 suspension is obviously larger than that in deionized water (Fig. 5 and Table 2).
Conclusions In this work, a high-activity M-B/g-Al2O3 (M ¼ Co, Ni) catalyst was prepared by wet chemical reduction method, and it was used as a catalyst for the Al-water reaction. The results showed that M-B/g-Al2O3 catalyst could considerably promote the Al-water reaction and decrease the induction time. The effects of the molar ratio of g-Al2O3 to Co in CoeB/g-Al2O3 catalyst, the weight content of M-B/g-Al2O3 and reaction temperature on Al-water reaction were investigated. When the molar ratio of g-Al2O3 to Co is 1:1, CoeB/g-Al2O3 catalyst has the highest activity. The induction time is only 0.43 h, and the MHGR is as high as 13.3 ml min1$g1-Al. The catalytic activity of M-B/g-Al2O3 is proportional to its active area. The mechanism analyses reveal that the catalytic effect of M-B/gAl2O3 on Al-water reaction can be attributed to three reasons: (1) the ions dissociated from H2O molecules on g-Al2O3 surfaces promote the hydration of passive oxide film on Al surface; (2) M-B and Al form microgalvanic cell, promoting the Alwater reaction; (3) M-B and g-Al2O3 have a synergistic effect on Al-water reaction. The present work provides a new way to generate hydrogen from Al-water reaction for portable fuel cells.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 51872181 and 51801093), the
Key scientific research projects of Henan Province Colleges and Universities (No. 18A150039) and the Henan Provincial Key Science and Technology Research Project (No. 182102311047).
references
[1] Gai WZ, Liu WH, Deng ZY, Zhou JG. Reaction of Al powder with water for hydrogen generation under ambient condition. Int J Hydrogen Energy 2012;37(17):13132e40. https://doi.org/10.1016/j.ijhydene.2012.04.025. [2] Huang XN, Gao T, Pan XL, Wei D, Lv CJ, Qin LS, Huang YX. A review: feasibility of hydrogen generation from the reaction between aluminum and water for fuel cell applications. J Power Sources 2013;229:133e40. https://doi.org/10.1016/j. jpowsour.2012.12.016. [3] Haupin WE. Electrochemistry of the Hall-Heroult process for aluminum smelting. J Chem Educ 1983;60(4):279e82. https:// doi.org/10.1021/ed060p279. [4] Wang YQ, Gai WZ, Zhang XY, Pan HY, Cheng ZX, Xu PG, Deng ZY. Effect of storage environment on hydrogen generation by the reaction of Al with water. RSC Adv 2017;7(4):2103e9. https://doi.org/10.1039/C6RA25563A. [5] Belitskus D. Reaction of aluminum with sodium hydroxide solution as a source of hydrogen. J Electrochem Soc 1970;117(8):1097e9. https://doi.org/10.1149/1.2407730. nchez LA, Gallegos AAA, Sebastian PJ. [6] Martı´nez SS, Sa Coupling a PEM fuel cell and the hydrogen generation from aluminum waste cans. Int J Hydrogen Energy 2007;32(15):3159e62. https://doi.org/10.1016/j.ijhydene.2006. 03.015. [7] Wang ED, Shi PF, Du CY, Wang XR. A mini-type hydrogen generator from aluminum for proton exchange membrane fuel cells. J Power Sources 2008;181(1):144e8. https://doi.org/ 10.1016/j.jpowsour.2008.02.088. s J, Mun ~ oz M, Casado J. In situ [8] Soler L, Candela AM, Macana generation of hydrogen from water by aluminum corrosion in solutions of sodium aluminate. J Power Sources 2009;192(1):21e6. https://doi.org/10.1016/j.jpowsour.2008.11. 009. s J, Mun ~ oz M, Casado J. [9] Soler L, Candela AM, Macana Hydrogen generation from water and aluminum promoted by sodium stannate. Int J Hydrogen Energy 2010;35(3):1038e48. https://doi.org/10.1016/j.ijhydene.2009. 11.065. [10] Dai HB, Ma GL, Xia HJ, Wang P. Reaction of aluminum with alkaline sodium stannate solution as a controlled source of hydrogen. Energy Environ Sci 2011;4(6):2206e12. https://doi. org/10.1039/C1EE00014D. [11] Milinchuk VK, Klinshpont ER, Belozerov VI. Standalone hydrogen generator based on chemical decomposition of water by aluminum. Nucl Energy Technol 2015;1(4):259e66. https://doi.org/10.1016/j.nucet.2016.02.013. [12] Xu S, Zhao X, Liu J. Liquid metal activated aluminum-water reaction for direct hydrogen generation at room temperature. Renew Sustain Energy Rev 2018;92:17e37. https://doi.org/10.1016/j.rser.2018.04.052. [13] Ziebarth JT, Woodall JM, Kramer RA, Choi G. Liquid phaseenabled reaction of Al-Ga and Al-Ga-In-Sn alloys with water. Int J Hydrogen Energy 2011;36(9):5271e9. https://doi.org/10. 1016/j.ijhydene.2011.01.127. [14] Fan MQ, Mei DS, Chen D, Lv CJ, Shu KY. Portable hydrogen generation from activated Al-Li-Bi alloys in water. Renew Energy 2011;36(11):3061e7. https://doi.org/10.1016/j.renene. 2011.03.029.
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
[15] Eom KS, Kwon JY, Kim MJ, Kwon HS. Design of Al-Fe alloys for fast on-board hydrogen production from hydrolysis. J Mater Chem 2011;21(34):13047e51. https://doi.org/10.1039/ C1JM11329A. [16] Zhao ZW, Chen XY, Hao MM. Hydrogen generation by splitting water with Al-Ca alloy. Energy 2011;36(5):2782e7. https://doi.org/10.1016/j.energy.2011.02.018. [17] Ilyukhina AV, Ilyukhin AS, Shkolnikov EI. Hydrogen generation from water by means of activated aluminum. Int J Hydrogen Energy 2012;37(21):16382e7. https://doi.org/10. 1016/j.ijhydene.2012.02.175. [18] Hu XY, Zhu GZ, Zhang YJ, Wang YM, Gu MS, Yang S, Song PX, Li XJ, Fang HJ, Jiang GS, Wang ZF. Hydrogen generation through rolling using Al-Sn alloy. Int J Hydrogen Energy 2012;37(15):11012e20. https://doi.org/10.1016/j.ijhydene.2012. 04.141. [19] Zou MS, Guo XY, Huang HT, Yang RJ, Zhang P. Preparation and characterization of hydro-reactive Mg-Al mechanical alloy materials for hydrogen production in seawater. J Power Sources 2012;219:60e4. https://doi.org/10.1016/j.jpowsour. 2012.07.008. [20] Wang CP, Yang T, Liu YH, Ruan JJ, Yang SY, Liu XJ. Hydrogen generation by the hydrolysis of magnesium-aluminum-iron material in aqueous solutions. Int J Hydrogen Energy 2014;39(21):10843e52. https://doi.org/10.1016/j.ijhydene.2014. 05.047. [21] He TT, Wang W, Chen DM, Yang K. Effect of Ti on the microstructure and Al-water reactivity of Al-rich alloy. Int J Hydrogen Energy 2014;39(2):684e91. https://doi.org/10.1016/j. ijhydene.2013.10.095. [22] Chen XY, Zhao ZW, Liu XH, Hao MM, Chen AL, Tang ZY. Hydrogen generation by the hydrolysis reaction of ballmilled aluminum-lithium alloys. J Power Sources 2014;254:345e52. https://doi.org/10.1016/j.jpowsour.2013.12. 113. [23] Elitzur S, Rosenban V, Gany A. Study of hydrogen production and storage based on aluminum-water reaction. Int J Hydrogen Energy 2014;39(12):6328e34. https://doi.org/10. 1016/j.ijhydene.2014.02.037. [24] Huang TP, Gao Q, Liu D, Xu SN, Guo CB, Zou JJ, Wei CD. Preparation of Al-Ga-In-Sn-Bi quinary alloy and its hydrogen production via water splitting. Int J Hydrogen Energy 2015;40(5):2354e62. https://doi.org/10.1016/j.ijhydene.2014. 12.034. [25] Preez SPD, Bessarabov DG. Hydrogen generation by the hydrolysis of mechanochemically activated aluminum-tinindium composites in pure water. Int J Hydrogen Energy 2018;43(46):21398e413. https://doi.org/10.1016/j.ijhydene. 2018.09.133. [26] An Q, Hu HY, Li N, Liu D, Xu SN, Liu Z, Wei CD, Luo F, Xia MS, Gao Q. Effects of Bi composition on microstructure and Alwater reactivity of Al-rich alloys with low-In. Int J Hydrogen Energy 2018;43(24):10887e95. https://doi.org/10.1016/j. ijhydene.2018.05.009. [27] Qiao DX, Lu YP, Tang ZY, Fan XS, Wang TM, Li TJ, Liaw PK. The superior hydrogen-generation performance of multicomponent Al alloys by the hydrolysis reaction. Int J Hydrogen Energy 2019;44(7):3527e37. https://doi.org/10.1016/ j.ijhydene.2018.12.124. [28] Alinejad B, Mahmoodi K. A novel method for generating hydrogen by hydrolysis of highly activated aluminum nanoparticles in pure water. Int J Hydrogen Energy 2009;34(19):7934e8. https://doi.org/10.1016/j.ijhydene.2009. 07.028. [29] Czech E, Troczynski T. Hydrogen generation through massive corrosion of deformed aluminum in water. Int J Hydrogen Energy 2010;35(3):1029e37. https://doi.org/10.1016/ j.ijhydene.2009.11.085.
24385
[30] Wang HW, Chung HW, Teng HT, Cao GZ. Generation of hydrogen from aluminum and water e effect of metal oxide nanocrystals and water quality. Int J Hydrogen Energy 2011;36(23):15136e44. https://doi.org/10.1016/j.ijhydene.2011. 08.077. [31] Dupiano P, Stamatis D, Dreizin EL. Hydrogen production by reacting water with mechanically milled composite aluminum-metal oxide powders. Int J Hydrogen Energy 2011;36(8):4781e91. https://doi.org/10.1016/j.ijhydene.2011. 01.062. [32] Huang XN, Lv CJ, Wang Y, Shen HY, Chen D, Huang YX. Hydrogen generation from hydrolysis of aluminum/graphite composite with a core-shell structure. Int J Hydrogen Energy 2012;37(9):7457e63. https://doi.org/10.1016/j.ijhydene.2012. 01.126. [33] Chen XY, Zhao ZW, Hao MM, Wang DZ. Research of hydrogen generation by the reaction of Al-based materials with water. J Power Sources 2013;222:188e95. https://doi.org/ 10.1016/j.jpowsour.2012.08.078. [34] Liu YA, Wang XH, Liu HZ, Dong ZH, Li SQ, Ge HW, Yan M. Study on hydrogen generation from the hydrolysis of a ball milled aluminum/calcium hydride composite. RSC Adv 2015;5(74):60460e6. https://doi.org/10.1039/C5RA09200K. [35] Razavi-Tousi SS, Szpunar JA. Effect of addition of watersoluble salts on the hydrogen generation of aluminum in reaction with hot water. J Alloys Compd 2016;679:364e74. https://doi.org/10.1016/j.jallcom.2016.04.038. [36] Irankhah A, Fattahi SMS, Salem M. Hydrogen generation using activated aluminum/water reaction. Int J Hydrogen Energy 2018;43(33):15739e48. https://doi.org/10.1016/j. ijhydene.2018.07.014. [37] Xiao F, Yang RJ, Li JM. Preparation and characterization of mechanically activated aluminum/polytetrafluoroethylene composites and their reaction properties in high temperature water steam. J Alloys Compd 2018;761:24e30. https://doi.org/10.1016/j.jallcom.2018.05.087. [38] Zhao C, Xu F, Sun LX, Chen J, Guo XL, Yan E, Yu F, Chu HL, Peng HL, Zou YJ, Liu ZW, Li FW. A novel Al-BiOCl composite for hydrogen generation from water. Int J Hydrogen Energy 2019;44(13):6655e62. https://doi.org/10.1016/j.ijhydene.2018. 12.165. [39] Teng HT, Lee TY, Chen YK, Wang HW, Cao GZ. Effect of Al(OH)3 on the hydrogen generation of aluminum-water system. J Power Sources 2012;219:16e21. https://doi.org/10. 1016/j.jpowsour.2012.06.077. [40] Newell A, Thampi KR. Novel amorphous aluminum hydroxide catalysts for aluminum-water reactions to produce H2 on demand. Int J Hydrogen Energy 2017;42(37):23446e54. https://doi.org/10.1016/j.ijhydene.2017. 04.279. [41] Wen YC, Huang WM, Wang HW. Kinetics study on the generation of hydrogen from an aluminum/water system using synthesized aluminum hydroxides. Int J Energy Res 2018;42(4):1615e24. https://doi.org/10.1002/er.3955. [42] Meng HX, Wang N, Dong YM, Jia ZL, Gao LJ, Chai YJ. Influence of M-B (M ¼ Fe, Co, Ni) on aluminum-water reaction. J Power Sources 2014;268:550e6. https://doi.org/10.1016/j.jpowsour. 2014.06.094. [43] Wang N, Meng HX, Dong YM, Jia ZL, Gao LJ, Chai YJ. Cobaltiron-boron catalyst-induced aluminum-water reaction. Int J Hydrogen Energy 2014;39(30):16936e43. https://doi.org/10. 1016/j.ijhydene.2014.07.180. [44] Gai WZ, Fang CS, Deng ZY. Hydrogen generation by the reaction of Al with water using oxides as catalysts. Int J Energy Res 2014;38(7):918e25. https://doi.org/10.1002/er.3093. [45] Yang Y, Gai WZ, Deng ZY, Zhou JG. Hydrogen generation by the reaction of Al with water promoted by an ultrasonically prepared Al(OH)3 suspension. Int J Hydrogen Energy
24386
[46]
[47]
[48]
[49]
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 4 4 ( 2 0 1 9 ) 2 4 3 7 7 e2 4 3 8 6
2014;39(33):18734e42. https://doi.org/10.1016/j.ijhydene.2014. 09.085. Glavee GN, Klabunde KJ, Sorensen CM, Hadjipanayis GC. Borohydride reduction of cobalt ions in water e chemistry leading to nanoscale metal, boride, or borate particles. Langmuir 1993;9(1):162e9. https://pubs.acs.org.ccindex.cn/ doi/abs/10.1021/la00025a034. Glavee GN, Klabunde KJ, Sorensen CM, Hadjipanayis GC. Borohydride reduction of nickel and copper ions in aqueous and nonaqueous media e controllable chemistry leading to nanoscale metal and metal boride particles. Langmuir 1994;10(12):4726e30. https://pubs.acs.org.ccindex.cn/doi/ abs/10.1021/la00024a055. Bunker BC, Nelson GC, Zavadil KR, Barbour JC, Wall FD, Sullivan JP, Windisch CF, Engelhardt MH, Baer DR. Hydration of passive oxide films on aluminum. J Phys Chem B 2002;106(18):4705e13. https://pubs.acs.org.ccindex.cn/doi/ abs/10.1021/jp013246e. Deng ZY, Ferreira JMF, Tanaka Y, Ye JH. Physicochemical mechanism for the continuous reaction of g-Al2O3 modified aluminum powder with water. J Am Ceram Soc 2007;90(5):1521e6. https://doi.org/10.1111/j.1551-2916.2007. 01546.x.
[50] Ionescu A, Allouche A, Aycard JP, Rajzmann M, Hutschka F. Study of g-alumina surface reactivity: adsorption of water and hydrogen sulfide on octahedral aluminum sites. J Phys Chem B 2002;106(36):9359e66. https://pubs.acs.org.ccindex. cn/doi/abs/10.1021/jp020145n. [51] Gai WZ, Shi Y, Deng ZY, Zhou JG. Clarification of activation mechanism in oxide-modified aluminum. Int J Hydrogen Energy 2015;40(36):12057e62. https://doi.org/10.1016/j. ijhydene.2015.07.102. n DL, Liu ZY, Duchon T, Evans J, [52] Carrasco J, Dura Senanayake SD, Crumlin EJ, Matolı´n V, Rodrı´guez JA, Pirovano MVG. In situ and theoretical studies for the dissociation of water on an active Ni/CeO2 catalyst: importance of strong metal-support interactions for the cleavage of O-H bonds. Angew Chem Int Ed 2015;54(13):3917e21. https://doi.org/10.1002/anie. 201410697. [53] Choong CKS, Huang L, Zhong ZY, Lin JY, Hong L, Chen LW. Effect of calcium addition on the catalytic ethanol steam reforming of Ni/Al2O3: II. Acidity/basicity, water adsorption and catalytic activity. Appl Catal A: Gen 2011;407(1):155e62. https://doi.org/10.1016/j.apcata.2011.08.038.