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Novel amorphous aluminum hydroxide catalysts for aluminum–water reactions to produce H2 on demand
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Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand Anthony Newell a,b, K. Ravindranathan Thampi a,b,* a b
School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland Earth Institute, University College Dublin, Belfield, Dublin 4, Ireland
article info
abstract
Article history:
Production of aluminum (Al) through electrolysis is an energy intensive process. Al metal
Received 25 November 2016
with its very high specific energy density of 28.8 MJ kg1 can serve as an excellent energy
Received in revised form
storage vector. Al reacts with water (reaction (1)) at room temperature producing clean H2
28 April 2017
thus providing an alternative to compressed gas storage.
Accepted 29 April 2017 Available online xxx
2Al þ 6H2O / 2Al(OH)3 þ 3H2
(1)
Keywords: Aluminum Water
Reaction (1) does not proceed easily due to the presence of a 2e4 nm passive alumina
Hydrogen
surface layer. Aluminum hydroxide (Al(OH)3) catalyst disrupts this layer, sustaining reac-
Energy
tion (1). The present work focuses on the catalysis aspects of reaction (1) by amorphous
Storage Hydroxide
Al(OH)3 produced by urea hydrolysis of aluminum nitrate. H2 yields of up to 100% were obtained at 45 C. Spent reaction product mixtures are autocatalytic with successive additions of Al micropowder (MP) yielding up to 133 ml H2/minute and reaction completion within 10 min at pH z 10. There is an induction time initially due to the presence of dissolved ions, followed by production of H2. Reaction by-products are easily recyclable back to Al metal through subsequent calcining and electrolysis by the Al refining industry, which constitutes the energy storage part of the cycle. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen as a solution to renewable energy intermittency One of the biggest difficulties facing greater penetration of renewable energy applications is that of uninterrupted power
supplies [1]. The development of a suitable energy storage method is therefore a requirement for further progress in this field. Pumped water, compressed air and secondary batteries are all methods of storing energy, massive chemical storage on the other hand is an option much discussed, yet insufficiently studied and advanced. It is anticipated that H2 will serve as the most common molecular chemical energy storage
* Corresponding author. School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland. E-mail address: [email protected] (K.R. Thampi). http://dx.doi.org/10.1016/j.ijhydene.2017.04.279 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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vector [2]. Hydrogen is a clean fuel with a high energy density of 119.8 MJ kg1. However its volumetric energy density is rather poor at just 0.01 MJ L1 [3]. It can be used in a variety of applications in both stationary and transport systems as either a combustion source or as a fuel cell feedstock [4]. To ensure a minimum carbon footprint technology H2 should originate only from renewable energy sources. Currently most H2 is produced through endothermic hydrocarbon reforming, coal gasification, both producing excessive amounts of CO2. Safety is another major concern. The development of safe, easy technologies, which are also compatible with current infrastructure are therefore essential for the adoption of ‘Hydrogen Economy’; however, there are no practically and economically acceptable H2 storage technologies other than pressurized cylinders. Storage of H2 through compression or cooling incurs an energy penalty: 3 kW h/kg H2 for 700 bar, up to 13 kW h/kg H2 for liquid H2 at 20 K [5]. Except for natural gas (NG) and liquid petroleum gas (LPG), almost all our main energy vectors are either solids or liquids. NG/LPG's physicochemical properties are also suited to our current infrastructure unlike H2. Therefore solid and liquid energy vectors are most suitable for short, medium and long term renewable energy storage.
Al for producing H2/energy on demand Al has been proposed as a suitable and ideal solid energy storage vector due to its high specific energy density of 28.8 MJ kg1, 77.76 MJ L1, and is an industrially produced commodity. From an energetic point of view 1 kg Al can release up to 13.3 MJ H2 equivalent Lower Heating Value (LHV) and 15.5 MJ heat [6] according to reaction (1). 76.4 MJ kg1 is required to recycle the reaction product Bayerite back to Al metal through calcining and electro-refining in the Bayer and roult processes, respectively [7]. Energy consumed in Hall He producing metal powder through gas atomization is 23 MJ kg1 [8]. A complete cycle therefore requires z100 MJ kg1 and for meeting this requirement, renewable energy can be used. The end result is a return of about 13% on H2 basis alone and up to 29% with heat utilization, in addition. Al offers at least 3 possible means for stored energy to be released through its oxidation to Al(OH)3. 1. Alewater reaction producing H2 for electricity generation in fuel cells [9]. 2. Combined heat and power [10]. 3. Al anode for batteries [11]. The overall cycle is such that Al(OH)3 is converted to Al metal through calcining, electro-refining, then atomization to micropowder while utilizing renewable heat and energy. This is then stored as metal until needed in a controlled energy release through introduction of water to produce H2 and heat. This cycle is fully repeatable. Al can be stored as metal for long periods and at low cost unlike oil and gas reserves due to its high density. Al as an energy storage vector is of particular relevance to energy production in many countries, including Ireland. For instance, the power grid system security requires wind electricity generation limited to not more than z50% of Ireland's total
demand at any one time, thereby resulting in curtailment, and a loss of the available renewable energy resource, which is not even stored. The alumina and Al production existing in most countries thus maximize the ability to store some of this wasted clean energy when refining Alewater reaction product Al(OH)3 to alumina first and eventually to Al MP.
Activation of Al metal surface Pure Al reacts readily with water at room temperature due to its highly negative electrode potential of 1.66 V vs. SHE [12]; more negative than 0.83 V vs. SHE will effect H2 production from water. Bulk Al will also contain a dense 2e4 nm layer of alumina on the surface, which forms quickly in contact with oxygen in ambient conditions [13]. When this is disrupted, the reaction 1 proceeds by releasing H2. Continuous disruption/ removal of this layer is necessary for reaction completion and to obtain close to 100% yield of H2. Methods to initiate and ensure a sustained reaction by activation of the metal surface can be achieved in a number of different ways. The following methods are effective in sustaining Alewater reactions for H2 production. Physical activation by ball milling [14] in an inert atmosphere is used to sufficiently coarsen the surface without particle size reduction. However, the long processing times, inert atmosphere requirement and additional energy expenditure hinder its applications. Ball milling has also been studied for top-down synthesis of nanoparticles from MPs [15]. About 20 h of processing time in high-pressure inert atmosphere was required. Alloying Al with Ga, In and Sn to produce eutectic alloys have also been demonstrated as an effective oxide layer disruptor [16]. Hg has also been used for the same purpose, but less commonly owing to toxicity [17]. Ga is comparatively expensive for the quantities required for effective alloying; its effective removal before recycling in order to make the process economically feasible is another issue. Chemical activation using inorganic or hydride salts is another route. Lithium hydrides, oxides of titanium, chromium, cobalt were studied [18,19]. Their effective removal poses the same issue for recycling as with alloying. Al surface modification with g-Al2O3 effectively activated the MP with by-products compatible with simple recycling back to Al metal [20,21]. An effective method for bulk Alewater reactions has been in maintaining a sufficiently corrosive environment using NaOH or KOH. The former is more advantageous for product recycling due to the management of large quantities of NaOH in the Bayer process. However, for end-use consumer applications' corrosive conditions are undesirable. Production of nanoparticles (NP) of Al [9] was studied by decomposition of organic alane precursors in organic solvents with water and air-free processing. Oleic acid was used as a capping agent. The capped Al NPs showed exclusive reaction with water out of multiple solvents studied, polar and nonpolar. Particles were also stable in air. NPs synthesized in this reported work however contained just 40% metal content, the balance taken up by the passivation surface of oleic acid and alumina. Contrary to this, Al MP contains >99% metal content due to their favorable surface:volume ratio. Due to the
Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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presence of the surface alumina passivating layer such particles are also stable in air for carefree low cost storage. Gas atomization of liquid Al at the highest operable pressures will not produce Al particles <2 mm. Commercial methods for producing Al NPs are restricted to energy intensive processes with low production rates such as Al wire explosion and Al vapor condensation. Increased energy intensity and lower production rates result in price differences of <$20/kg compared to $10,000/kg for MPs vs. NPs [22]. Al(OH)3MP suspensions have been shown to be an effective catalyst for the Al MP-water reaction by continuous disruption of the passivating protective layer of Al surface, ensuring the exposure of pristine Al surface to water for sustained H2 production [23e25]. As well as alumina, it is the material most easily recyclable back to Al metal. Combined modification of the reaction with more basic pH using NaOH and NaAlO2 at higher reaction temperatures employing the reaction 1 exothermic output [25] and other water types like seawater [26] have been shown to be effective, but complicates the recycle process engineering. The most effective currently published methods employ high concentrations of Al(OH)3 relative to Al MP, under basic conditions (NaOH). Al(OH)3 which were synthesized by co-precipitation to achieve poorly crystalline MPs were found to effectively catalyze the Alewater reaction [25]. Their effective catalysis was attributed to their poor crystallinity, sharp edges and small size which are effective at dissociating water molecules to OH and Hþ ions [23e25,27]. The catalysis effect of amorphous Al(OH)3 may then be even better than poorly crystalline Al(OH)3 if its crystallinity is negative for Alewater catalysis. Also lower quantities of Al(OH)3 used may also prove effective, compared to the 40 g/L and 300 g/L used by Refs. [25,26]. The primary objectives of this study are to examine the nature, effectiveness and optimization of catalytic reactions involving amorphous Al(OH)3 in Alewater reactions under practically simple reaction conditions and scalable production methods.
Materials and methods Production of Al(OH)3 by urea hydrolysis A reported urea hydrolysis method was adapted suitably for the synthesis of Al(OH)3 [28]. 70 g of aluminum nitrate nonahydrate (VWR > 98% purity) and 152 g of urea (Sigma Aldrich 99.5%) were dissolved in 100 ml DI water with stirring and heating to account for the ureaewater reaction endothermicity until a clear and colorless solution was obtained at room temperature. The initial solution pH was measured and was found to be acidic at pH ¼ 2 as expected. The solution was then added to a round-bottomed flask and heated to 90 C by an oil bath with stirring at 1000 rpm for 12 h. CO2 was vented during hydrolysis. During the course of the reaction the solution pH and viscosity increased until a partially translucent gel was obtained of pH z 10. This was transferred into a 2 L bottle and then filled completely with DI water. The ammonium hydroxide/nitrate and other watersoluble impurities were removed by repeated washings with DI water, mixings and decantations. Once the pH reached <8
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after z6 washes and the final supernatant decanted, the gel was dried in air and crushed to a fine powder. This was referred to as ‘UH’.
Alewater reactions Al MP was sourced from Alfa Aesar, 325 mesh 99.5% purity (metals basis). PSD: Dv10 ¼ 5.63 mm, Dv50 ¼ 15.3 mm, Dv90 ¼ 30.69 mm. APS 7e15 mm. All reactions were magnetically stirred at 1000 rpm.
Experimental set 1 e Alewater reactions without catalyst 1 g Al MP was added to 50 ml DI water at initial reaction temperatures of 25, 35, 45, 55, and 60 C.
Experimental set 2 e Alewater reactions with UH catalyst 1 g UH was added to 50 ml DI water. 1 g Al MP was then added. Reaction temperatures of 35, 45 and 60 C were studied.
Experimental set 3 e Alewater reactions with modified UH catalyst mixtures 1 g UH was added to 50 ml DI water to make a water-catalyst mixture. 3 different modifications were performed for 3 separate mixtures (1e3). Mixture 1: Bubbling H2 into the mixture for 45 min with stirring at 500 rpm. T ¼ 25 C Mixture 2: Magnetic stirring of the mixture for 20 h at 1000 rpm. T ¼ 45 C Mixture 3: Ultrasonication of the mixture for 45 min, with no stirring. T ¼ 25 C. After treatment, each mixture was heated to 45 C and then 1 g Al MP was added to it. Each mixture was maintained at 45 C for the duration of the Alewater reaction by oil bath.
Experimental set 4 e Alewater reactions with a spent reaction product catalyst mixture After the experimental set 2 reaction at 45 C had completed, a white viscous milk-like slurry of 2.89 g reaction 1 product Al(OH)3 and 1 g UH was obtained. 1 g Al MP was added to this mixture at 45 C for H2 production. Once complete, another 1 g Al MP was added to the subsequent mixture, which was run at the same stirring rate and temperature again for H2 production. Once complete, furthermore, a final 1 g Al MP was added to this mixture, with the same conditions for 3 successive additions of 1 g Al MP to the previous spent reaction product mixture with accumulation of Al(OH)3. These will be referred as ‘1st, 2nd and 3rd’. The final product mixture was collected and dried in air at 50 C to obtain white Al(OH)3 powder. As this powder was obtained from a reaction product mixture it will be referred to as ‘RPM’. After the reaction from experimental set 1 at 45 C had completed, a similar white viscous milk-like slurry of Al(OH)3 byproduct was obtained. 3 additions of 1 g Al MP was added to this mixture after each completion, as previously at 45 C. This experiment will be referred to in the results as ‘1st No UH’, ‘2nd No UH’ and ‘3rd No UH.
Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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Experimental set 5 e Alewater reactions with RPM catalyst As 1 g Al reacts with water to produce 2.89 g Al(OH)3, 3.89 g RPM was added to 50 ml DI water, corresponding to the quantity of Al(OH)3 (1 g UH & 2.89 g Al(OH)3 from Alewater reaction) in the reaction product mixture obtained after reaction from experimental set 2. 1 g Al MP was then added to this, this reaction will be referred to as RPM. Once complete another 1 g Al MP was added as with experimental set 4 and reacted to completion. Similarly to experimental set 4, 3 successive additions of 1 g Al MP were made in total, each after reaction completion, and will be referred to in the results as RPM 1st, RPM 2nd, RPM 3rd. Reaction temperature ¼ 45 C. Temperature of the reaction flask was controlled by an oil bath. The product gas evolved was collected by water displacement into a 2 l graduated cylinder with bubbling through water for cooling. The reaction progress was observed using a HP 2300 recording Camera. 1360 ml corresponds to 100% yield on H2 at room temperature and pressure. The pH of solutions was measured using a Hanna Piccolo pH meter. GC Analysis was used to confirm H2 gas and determine its purity. UH samples were characterized using XRD, SEM and EDX techniques. XRD characterization was performed with a Siemens D500 X-ray diffractometer (XRD) operating at 40 kV and 30 mA with Cu Ka radiation at a wavelength of 0.1542 nm. The scan was conducted in 2q mode and spanned across a range of 10 e40 with a step resolution of 0.04 and a scanning rate of 0.8 /min. SEM and EDX characterization was performed with a Hitachi TM-1000 Scanning Electron Microscope (SEM) (Hitachi High Technologies Ltd) and APOLLO XV Silicon Drift Detector.
Results The reaction results are summarized in Table 1. Individual result profiles are presented in figures as follows.
Experimental set 1 e Alewater reactions without catalyst Section “Experimental set 1 e Alewater reactions without catalyst” gives experimental set-up details. The results of these experiments are shown in Fig. 1. It was observed that with continuous and rapid stirring, all the reactions showed an initial induction time to commence. As shown in Fig. 1, 25 C showed the longest induction time of many days. Induction times decreased with increasing temperatures to 15 min when reactions are done at 60 C. pH of the reaction mixture was studied for these reactions and had showed an increase from 7 to 9 at its completion.
Experimental set 2 e Alewater reactions with UH catalyst Section “Experimental set 2 e Alewater reactions with UH catalyst” gives experimental set-up details. The results are shown in Fig. 2. The induction time before reaction was the longest for the lowest temperature studied of 35 C. The induction times decreased with increasing temperatures. This is the same effect of temperature observed in experimental set 1, too. The induction times before reaction from experimental
set 1 for temperatures studied of 35, 45 and 60 C were shorter than induction times for the same temperatures studied for experimental set 2. This indicates that the addition of UH caused an increase in the induction time when compared to Alewater reactions without UH. Reaction rates also increased compared to experimental set 1 and are presented in Table 1. pH of the reaction mixture was studied for these reactions and had showed an increase from 8 to 9.5 at its completion. The introduction of UH increased the alkalinity of the reaction mixtures, UH was found to introduce water soluble ion impurities present from the synthesis method, these impurities react with Al initially, consuming H2 as they are converted, hence the increased induction time, their conversion products then increase the pH. Ionic impurities have been shown to considerably affect induction time [29]. The induction time should be minimized and reaction rates maximized if ondemand production of H2 is to be realized from Alewater reactions with UH catalyst. On the other hand, induction times when adding UH offers an additional reaction control opportunity in certain applications or systems that might require delayed or programmed starts.
Experimental set 3 e Alewater reactions with modified UH catalyst mixtures Section “Experimental set 3 e Alewater reactions with modified UH catalyst mixtures” gives experimental set-up details. The results of these experiments are shown in Fig. 3. Alewater reactions with addition of UH as shown in Fig. 2 and Table 1, UHeDI water mixtures were modified as described in Section “Experimental set 3 e Alewater reactions with modified UH catalyst mixtures”. For each of these 3 different mixtures, after modification, 1 g Al MP was added for reaction with water at a single temperature of 45 C with magnetic stirring at 1000 rpm. Modifications of each mixture were performed to understand different aspects of the nature of the catalytic mechanism. Mixture 1 e Bubbling H2 into the mixture was performed as it was intended to affect the amount of H2 adsorbed on to the pristine UH surface. Mixture 2 e Stirring of the mixture for 20 h was meant to split UH aggregates and particles to increase the specific B.E.T. surface area and the resulting catalytic activity [25]. Mixture 3 e Ultrasonication was hypothesized to do the same as stirring would do for Mixture 2, but quicker and is a known method for increasing activity of Al(OH)3 suspensions [23,24]. Results shown in Fig. 3 and Table 1 indicate that modifications of Mixture 1e3 had differing effects on the H2 generation rates and induction time before reaction, when compared to the result obtained at 45 C from experimental set 2. Mixture 1 showed little effect, the induction time was lowered from 172 to 150 min; however the H2 rate was lowered from 8.5 to 6.5 ml/min. This suggests that induction of H2 release to gas phase is not due to self-adsorption of H2 on to the pristine surface of the catalyst or Al MP. Mixture 2 showed a positive effect, both increasing the H2 rate from 8.5 to 27 ml/min and lowering the induction time from 172 to 26 min. This indicates that stirring increases the catalytic effect due to splitting of UH particles. The bound ionic impurities present in UH are more easily decomposed during the Alewater reaction, also finer Al(OH)3 particles have a higher specific surface area
Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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Table 1 e Summary of experimental sets 1e5 reaction results. Reaction
Max H2 rate (ml/min)
Average H2 rate, Reaction start to z90% completion (ml/min)
Induction time (min)
H2 yield (%)
Experimental set 1 25 C 35 C 45 C 55 C 60 C
3.8 @ 280 ml 5.6 @ 430 ml 3.3 @ 600 ml 5.9 @ 70 ml 12 @ 80 ml
0.9 1.6 2.3 1.6 1.5
4582 390 71 25 15
74 79 93 88 89
Experimental set 2 35 C 45 C 60 C
21 @ 560 ml 33 @ 400 ml 15.4 @ 100 ml
9.375 8.5 4.3
585 172 29
88 96 92
Experimental set 3 Mixture 1 Mixture 2 Mixture 3
30 @ 250 ml 80 @ 550 ml 50 @ 530 ml
6.5 27 11.3
150 26 171
93 94 93
Experimental set 4 1st 2nd 3rd 1st No UH 2nd No UH 3rd No UH
158 @ 650 ml 222 @ 650 ml 200 @ 650 ml 33 @ 280 ml 46.9 @ 280 ml 51.7 @190 ml
100 131 133 10.3 9.7 17
0.5 0.5 0.5 0.5 1 0.5
99 100 88 98 100 98
Experimental set 5 RPM RPM 1st RPM 2nd RPM 3rd
7.7 @ 640 ml 26 @ 500 ml 44 @ 550 ml 56 @ 560 ml
3.8 9.53 18.4 17.1
0.5 0.5 0.5 0.5
103 100 100 101
allowing greater coverage of the Al particles for dissociation of water to OH ions, which hydrates the passive alumina layer, H2 generation follows whereby its desorption as H2 gas breaks the passivation layer allowing a fresh surface of Al to react with water, each particle can be modeled as a shrinking core for this reaction [21,23,30]. Mixture 3 showed little effect, the induction time was lowered from 172 to 171 min and the H2 rate only slightly increased from 8.5 to 11.3 ml/min. This
indicates that 45 min of ultrasonication is not a suitable replacement for 20 h stirring for increasing activity by decreasing particle size, longer ultrasonication times may effect increased activity. The pH of each of the solutions increased as in experimental set 2 from 8 to 9.5 on completion.
Experimental set 4 e Alewater reactions with a spent reaction product catalyst mixture The work reported in Refs. [24,31] that when Al MP was added to reaction product mixtures, the reaction rates were faster
Fig. 1 e Hydrogen generated for Alewater reactions without catalyst at T ¼ 25, 35, 45, 55 and 60 C; initial pH ≈ 7.
Fig. 2 e Hydrogen generated for Alewater (1 ge50 ml) reactions with 1 g UH catalyst at T ¼ 35, 45, and 60 C; initial pH ≈ 8.
Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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Fig. 4 shows that H2 generation is faster for the UH containing reaction product mixture with 100, 131 and 133 ml/min compared to 10.3, 9.7, and 17 ml/min, respectively. The induction time for all reactions is the same at 0.5 min. This faster rate indicates that without UH in the initial reaction, the corresponding successive addition reaction will not proceed as quickly. The presence of UH increases the H2 generation rate significantly. The pH of ‘1st No UH-3rd No UH’ remained at z9 even at reaction completion.
Experimental set 5 e Alewater reactions with RPM catalyst Fig. 3 e Hydrogen generated for Alewater reactions with modified 1 g UHe50 ml DI water catalyst mixtures; Mixtures 1e3. 1 g Al added to each for H2 generation at 45 C. Result from experimental set 2 at 45 C, an unmodified reactant mixture is shown for comparison; initial pH ≈ 8.
due to the presence of reaction product Al(OH)3 from the initial reaction. This reaction product Al(OH)3 was found to be autocatalytic with increasing reaction rates with each successive addition of Al MP. This effect was studied here and found to agree with the reported results. The induction time for 1st to the 3rd had decreased from 172 to 0.5 min for each reaction, compared to the initial reaction from experimental set 2, 45 C, from which the spent reaction product mixture was obtained. The H2 rate had also increased significantly from 8.5 to 100, 131, 133 ml/ min for 1st, 2nd and 3rd runs, respectively with reaction completion occurring within about 10 min. This reaction configuration is the most promising for on-demand H2 production as it starts almost immediately and gives the fastest reactions rates obtained for all of the studied experiments. The pH increased from z9.5 to 10 due to the increased quantity of reaction product Al(OH)3. A scalable reaction system is possible under these simple conditions.
Fig. 4 e Hydrogen generated for Alewater reactions with 2 spent reaction catalyst mixtures. ‘1st No UH’, ‘2nd No UH’, ‘3rd No UH’, 1 g Al added to the experimental set 1 45 C product mixture, 3 additions, T ¼ 45 C. 1st, 2nd and 3rd additions to experimental set 2 45 C spent reaction product mixture. pH of reaction product mixtures ≈9.
Section “Experimental set 5 e Alewater reactions with RPM catalyst” provides experimental set-up details. Results are shown in Fig. 5. The spent reaction product mixture from experimental set 4 3rd was collected and dried in air at 50 C as described in Section “Experimental set 5 e Alewater reactions with RPM catalyst”. This was conducted to test whether it retains its catalytic effect in dried form, which would allow for simpler storage compared to the viscous slurry. 3.89 g RPM was added initially to match the quantity of Al(OH)3 present in the reaction flask at experimental set 2 45 C reaction completion. The reaction started after an induction time of just 0.5 min but its H2 rate was much slower compared to results obtained for ‘1st, 2nd and 3rd’ at 3.8 ml/min compared to 100, 131 and 133 ml/min, respectively. This indicates that the catalysis effect is not retained after drying. With successive additions of 1 g Al MP to the flask after RPM had completed, up to 3 successive additions, RPM 1st, RPM 2nd, RPM 3rd, the induction time remained the same but the H2 rate increased from 3.8 to 9.53, 18.1 and 17.4 ml/min. This increase is not as great as observed in Fig. 4. This indicates that the UH added for experimental set 2 45 C increased the reaction rate for successive additions of 1 g Al to the spent reaction product mixture to a much greater extent than RPM did for successive additions of 1 g Al shown in Fig. 5, likely due to agglomeration of the catalyst from drying resulting in lower activity. This suggests that if a highest reaction rate is desired then the spent reaction product catalyst mixtures are to be stored within the liquid to retain their catalytic effect. Again, this is vital to scale-up of systems.
Fig. 5 e Hydrogen generated for Alewater reactions with RPM catalyst. T ¼ 45 C. 1 g Al MP, 3.89 g RPM and 50 ml DI water. To spent mixture, 1 g Al was added for RPM 1st, RPM 2nd and RPM 3rd, totaling 3 successive additions. pH was not measured.
Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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SEM imaging Fig. 6 shows the SEM images obtained for UH Al(OH)3 catalyst. UH Al(OH)3 shows an irregular morphology with particles and aggregates ranging in size from 10 nm to 200 mm.
EDX analysis Energy Dispersive X-Ray Spectroscopy is used to determine the elemental composition of samples. The results will be further discussed in Section “Discussion”.
XRD analysis Fig. 7 shows the diffractograms for UH Al(OH)3 and RPM Al(OH)3. UH Al(OH)3 shows no distinct Bayerite peaks. It is an amorphous material. RPM Al(OH)3 is however crystalline and is indicative of Bayerite. There is some peak broadening observed from the presence of UH in this sample also indicating the Al(OH)3 is very fine.
Discussion The Alewater reaction proceeds even without the addition of UH catalyst with over 90% H2 yield. This finding demonstrates
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that stirring and moderate temperatures are sufficient to disrupt the alumina layer and the reaction indeed proceeds even at 25 C but, after many days. The induction time decreases with an increase in the reaction temperatures. The addition of fresh UH with Al MP in water increased the induction time to 172 min, but, the reaction rate also showed an increase. EDX analysis of the UH Al(OH)3 sample shows qualitatively the presence of carbon and nitrogen on the surface, with the expected Al and oxygen, indicating that there are remaining impurities (Table 2). An explanation for this is that ionic nitrogen based impurities present in UH react with the Al surface until they are consumed, also increasing the pH; from then, Al reacts with water at a higher rate due to greater availability of OH from both the pH increase and water dissociation of the catalytic Al(OH)3 reaction product which in turn hydrate the Al surface. The UHeDI mixture induction time may be lowered and reaction rate increased for the initial reaction by stirring for 20 h. This may indicate that the catalytic effect of UH depends on particle size where continued stirring leads to attrition of particles and aggregates, imaged by SEM. These smaller particles possess greater catalytic activity. The role of stirring indicates attrition by short ultrasonic treatment was not sufficiently effective. The reaction 1 product Al(OH)3 also shows autocatalytic properties. Al MP reactions with Al(OH)3 product slurries were observed to start almost immediately. However, the product mixtures without an initial addition of UH exhibited
Fig. 6 e Dry Al(OH)3 UH powder catalyst sample after preparation as outlined in Section “Production of Al(OH)3 by urea hydrolysis”. SEM images at various magnifications as shown in the micrographs.
Fig. 7 e (a) & (b). (a) UH Al(OH)3 powder sample diffractogram, prepared as described in Section “Production of Al(OH)3 by urea hydrolysis”. (b) RPM Al(OH)3 powder sample diffractogram, prepared as described in Section “Experimental set 4 e Alewater reactions with a spent reaction product catalyst mixture”. Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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Table 2 e EDX analysis of UH powder. Element Carbon Nitrogen Oxygen Aluminum
Wt.%
At%
9.94 1.9 7.35 80.8
18.74 3.07 10.4 67.78
inferior catalytic properties (Fig. 4), the reaction product mixtures with UH were more gelatinous and of a higher pH from the decomposed impurities. This catalytic effect is only present in the slurry and is decreased when dried due to agglomeration and aggregation of particles, which lowers specific surface area and water dissociation activity of Al(OH)3 (Fig. 5). This decrease was also observed in other work [23]. The product hydrogen gas showed 97% purity through GC measurement, with remaining showing Ar and air arising from gas sampling and contamination occurring while sample collection, as well as air dissolved in water used for displacement when collecting the product gas.
Conclusions Al can serve as an energy storage vector for renewable energy through a repeatable energy storage and recovery cycle. When reacted with water at moderate temperatures Al particles produce H2 and it is possible to operate that process on demand. Alewater reactions were modified with novel amorphous Al(OH)3 produced by urea hydrolysis to increase reaction rates and disrupt the passivating alumina layer. Reaction product mixture slurries remaining from UH modified Alewater reactions showed the fastest hydrogen generation capability due to complementary auto-catalytic effects, allowing continuous addition of Al MP to the reactor for fast H2 generation. On-demand production of H2 in this way, when coupled with a fuel cell could be used in principle to power electrical devices or vehicles. The study offers possibility for scale up of Al based energy storage systems in future.
Acknowledgments This project is funded by the Irish Research Council Enterprise Partnership Scheme (IRC contract No. EPSPG/2013/591) with support from RUSAL Aughinish Alumina Ltd., Ireland.
references
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aluminum hydroxide catalyst. Int J Chem Eng Appl 2015;6:146. s J, Mun ~ oz M, Casado J. [26] Soler L, Candela AM, Macana Hydrogen generation by aluminum corrosion in seawater promoted by suspensions of aluminum hydroxide. Int J Hydrogen Energy 2009;34:8511e8. [27] Gai W-Z, Shi Y, Deng Z-Y, Zhou J-G. Clarification of activation mechanism in oxide-modified aluminum. Int J Hydrogen Energy 2015;40:12057e62. [28] Macedo MI, Osawa CC, Bertran CA. Solegel synthesis of transparent alumina gel and pure gamma alumina by urea
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand Anthony Newell a,b, K. Ravindranathan Thampi a,b,* a b
School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland Earth Institute, University College Dublin, Belfield, Dublin 4, Ireland
article info
abstract
Article history:
Production of aluminum (Al) through electrolysis is an energy intensive process. Al metal
Received 25 November 2016
with its very high specific energy density of 28.8 MJ kg1 can serve as an excellent energy
Received in revised form
storage vector. Al reacts with water (reaction (1)) at room temperature producing clean H2
28 April 2017
thus providing an alternative to compressed gas storage.
Accepted 29 April 2017 Available online xxx
2Al þ 6H2O / 2Al(OH)3 þ 3H2
(1)
Keywords: Aluminum Water
Reaction (1) does not proceed easily due to the presence of a 2e4 nm passive alumina
Hydrogen
surface layer. Aluminum hydroxide (Al(OH)3) catalyst disrupts this layer, sustaining reac-
Energy
tion (1). The present work focuses on the catalysis aspects of reaction (1) by amorphous
Storage Hydroxide
Al(OH)3 produced by urea hydrolysis of aluminum nitrate. H2 yields of up to 100% were obtained at 45 C. Spent reaction product mixtures are autocatalytic with successive additions of Al micropowder (MP) yielding up to 133 ml H2/minute and reaction completion within 10 min at pH z 10. There is an induction time initially due to the presence of dissolved ions, followed by production of H2. Reaction by-products are easily recyclable back to Al metal through subsequent calcining and electrolysis by the Al refining industry, which constitutes the energy storage part of the cycle. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen as a solution to renewable energy intermittency One of the biggest difficulties facing greater penetration of renewable energy applications is that of uninterrupted power
supplies [1]. The development of a suitable energy storage method is therefore a requirement for further progress in this field. Pumped water, compressed air and secondary batteries are all methods of storing energy, massive chemical storage on the other hand is an option much discussed, yet insufficiently studied and advanced. It is anticipated that H2 will serve as the most common molecular chemical energy storage
* Corresponding author. School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland. E-mail address: [email protected] (K.R. Thampi). http://dx.doi.org/10.1016/j.ijhydene.2017.04.279 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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vector [2]. Hydrogen is a clean fuel with a high energy density of 119.8 MJ kg1. However its volumetric energy density is rather poor at just 0.01 MJ L1 [3]. It can be used in a variety of applications in both stationary and transport systems as either a combustion source or as a fuel cell feedstock [4]. To ensure a minimum carbon footprint technology H2 should originate only from renewable energy sources. Currently most H2 is produced through endothermic hydrocarbon reforming, coal gasification, both producing excessive amounts of CO2. Safety is another major concern. The development of safe, easy technologies, which are also compatible with current infrastructure are therefore essential for the adoption of ‘Hydrogen Economy’; however, there are no practically and economically acceptable H2 storage technologies other than pressurized cylinders. Storage of H2 through compression or cooling incurs an energy penalty: 3 kW h/kg H2 for 700 bar, up to 13 kW h/kg H2 for liquid H2 at 20 K [5]. Except for natural gas (NG) and liquid petroleum gas (LPG), almost all our main energy vectors are either solids or liquids. NG/LPG's physicochemical properties are also suited to our current infrastructure unlike H2. Therefore solid and liquid energy vectors are most suitable for short, medium and long term renewable energy storage.
Al for producing H2/energy on demand Al has been proposed as a suitable and ideal solid energy storage vector due to its high specific energy density of 28.8 MJ kg1, 77.76 MJ L1, and is an industrially produced commodity. From an energetic point of view 1 kg Al can release up to 13.3 MJ H2 equivalent Lower Heating Value (LHV) and 15.5 MJ heat [6] according to reaction (1). 76.4 MJ kg1 is required to recycle the reaction product Bayerite back to Al metal through calcining and electro-refining in the Bayer and roult processes, respectively [7]. Energy consumed in Hall He producing metal powder through gas atomization is 23 MJ kg1 [8]. A complete cycle therefore requires z100 MJ kg1 and for meeting this requirement, renewable energy can be used. The end result is a return of about 13% on H2 basis alone and up to 29% with heat utilization, in addition. Al offers at least 3 possible means for stored energy to be released through its oxidation to Al(OH)3. 1. Alewater reaction producing H2 for electricity generation in fuel cells [9]. 2. Combined heat and power [10]. 3. Al anode for batteries [11]. The overall cycle is such that Al(OH)3 is converted to Al metal through calcining, electro-refining, then atomization to micropowder while utilizing renewable heat and energy. This is then stored as metal until needed in a controlled energy release through introduction of water to produce H2 and heat. This cycle is fully repeatable. Al can be stored as metal for long periods and at low cost unlike oil and gas reserves due to its high density. Al as an energy storage vector is of particular relevance to energy production in many countries, including Ireland. For instance, the power grid system security requires wind electricity generation limited to not more than z50% of Ireland's total
demand at any one time, thereby resulting in curtailment, and a loss of the available renewable energy resource, which is not even stored. The alumina and Al production existing in most countries thus maximize the ability to store some of this wasted clean energy when refining Alewater reaction product Al(OH)3 to alumina first and eventually to Al MP.
Activation of Al metal surface Pure Al reacts readily with water at room temperature due to its highly negative electrode potential of 1.66 V vs. SHE [12]; more negative than 0.83 V vs. SHE will effect H2 production from water. Bulk Al will also contain a dense 2e4 nm layer of alumina on the surface, which forms quickly in contact with oxygen in ambient conditions [13]. When this is disrupted, the reaction 1 proceeds by releasing H2. Continuous disruption/ removal of this layer is necessary for reaction completion and to obtain close to 100% yield of H2. Methods to initiate and ensure a sustained reaction by activation of the metal surface can be achieved in a number of different ways. The following methods are effective in sustaining Alewater reactions for H2 production. Physical activation by ball milling [14] in an inert atmosphere is used to sufficiently coarsen the surface without particle size reduction. However, the long processing times, inert atmosphere requirement and additional energy expenditure hinder its applications. Ball milling has also been studied for top-down synthesis of nanoparticles from MPs [15]. About 20 h of processing time in high-pressure inert atmosphere was required. Alloying Al with Ga, In and Sn to produce eutectic alloys have also been demonstrated as an effective oxide layer disruptor [16]. Hg has also been used for the same purpose, but less commonly owing to toxicity [17]. Ga is comparatively expensive for the quantities required for effective alloying; its effective removal before recycling in order to make the process economically feasible is another issue. Chemical activation using inorganic or hydride salts is another route. Lithium hydrides, oxides of titanium, chromium, cobalt were studied [18,19]. Their effective removal poses the same issue for recycling as with alloying. Al surface modification with g-Al2O3 effectively activated the MP with by-products compatible with simple recycling back to Al metal [20,21]. An effective method for bulk Alewater reactions has been in maintaining a sufficiently corrosive environment using NaOH or KOH. The former is more advantageous for product recycling due to the management of large quantities of NaOH in the Bayer process. However, for end-use consumer applications' corrosive conditions are undesirable. Production of nanoparticles (NP) of Al [9] was studied by decomposition of organic alane precursors in organic solvents with water and air-free processing. Oleic acid was used as a capping agent. The capped Al NPs showed exclusive reaction with water out of multiple solvents studied, polar and nonpolar. Particles were also stable in air. NPs synthesized in this reported work however contained just 40% metal content, the balance taken up by the passivation surface of oleic acid and alumina. Contrary to this, Al MP contains >99% metal content due to their favorable surface:volume ratio. Due to the
Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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presence of the surface alumina passivating layer such particles are also stable in air for carefree low cost storage. Gas atomization of liquid Al at the highest operable pressures will not produce Al particles <2 mm. Commercial methods for producing Al NPs are restricted to energy intensive processes with low production rates such as Al wire explosion and Al vapor condensation. Increased energy intensity and lower production rates result in price differences of <$20/kg compared to $10,000/kg for MPs vs. NPs [22]. Al(OH)3MP suspensions have been shown to be an effective catalyst for the Al MP-water reaction by continuous disruption of the passivating protective layer of Al surface, ensuring the exposure of pristine Al surface to water for sustained H2 production [23e25]. As well as alumina, it is the material most easily recyclable back to Al metal. Combined modification of the reaction with more basic pH using NaOH and NaAlO2 at higher reaction temperatures employing the reaction 1 exothermic output [25] and other water types like seawater [26] have been shown to be effective, but complicates the recycle process engineering. The most effective currently published methods employ high concentrations of Al(OH)3 relative to Al MP, under basic conditions (NaOH). Al(OH)3 which were synthesized by co-precipitation to achieve poorly crystalline MPs were found to effectively catalyze the Alewater reaction [25]. Their effective catalysis was attributed to their poor crystallinity, sharp edges and small size which are effective at dissociating water molecules to OH and Hþ ions [23e25,27]. The catalysis effect of amorphous Al(OH)3 may then be even better than poorly crystalline Al(OH)3 if its crystallinity is negative for Alewater catalysis. Also lower quantities of Al(OH)3 used may also prove effective, compared to the 40 g/L and 300 g/L used by Refs. [25,26]. The primary objectives of this study are to examine the nature, effectiveness and optimization of catalytic reactions involving amorphous Al(OH)3 in Alewater reactions under practically simple reaction conditions and scalable production methods.
Materials and methods Production of Al(OH)3 by urea hydrolysis A reported urea hydrolysis method was adapted suitably for the synthesis of Al(OH)3 [28]. 70 g of aluminum nitrate nonahydrate (VWR > 98% purity) and 152 g of urea (Sigma Aldrich 99.5%) were dissolved in 100 ml DI water with stirring and heating to account for the ureaewater reaction endothermicity until a clear and colorless solution was obtained at room temperature. The initial solution pH was measured and was found to be acidic at pH ¼ 2 as expected. The solution was then added to a round-bottomed flask and heated to 90 C by an oil bath with stirring at 1000 rpm for 12 h. CO2 was vented during hydrolysis. During the course of the reaction the solution pH and viscosity increased until a partially translucent gel was obtained of pH z 10. This was transferred into a 2 L bottle and then filled completely with DI water. The ammonium hydroxide/nitrate and other watersoluble impurities were removed by repeated washings with DI water, mixings and decantations. Once the pH reached <8
3
after z6 washes and the final supernatant decanted, the gel was dried in air and crushed to a fine powder. This was referred to as ‘UH’.
Alewater reactions Al MP was sourced from Alfa Aesar, 325 mesh 99.5% purity (metals basis). PSD: Dv10 ¼ 5.63 mm, Dv50 ¼ 15.3 mm, Dv90 ¼ 30.69 mm. APS 7e15 mm. All reactions were magnetically stirred at 1000 rpm.
Experimental set 1 e Alewater reactions without catalyst 1 g Al MP was added to 50 ml DI water at initial reaction temperatures of 25, 35, 45, 55, and 60 C.
Experimental set 2 e Alewater reactions with UH catalyst 1 g UH was added to 50 ml DI water. 1 g Al MP was then added. Reaction temperatures of 35, 45 and 60 C were studied.
Experimental set 3 e Alewater reactions with modified UH catalyst mixtures 1 g UH was added to 50 ml DI water to make a water-catalyst mixture. 3 different modifications were performed for 3 separate mixtures (1e3). Mixture 1: Bubbling H2 into the mixture for 45 min with stirring at 500 rpm. T ¼ 25 C Mixture 2: Magnetic stirring of the mixture for 20 h at 1000 rpm. T ¼ 45 C Mixture 3: Ultrasonication of the mixture for 45 min, with no stirring. T ¼ 25 C. After treatment, each mixture was heated to 45 C and then 1 g Al MP was added to it. Each mixture was maintained at 45 C for the duration of the Alewater reaction by oil bath.
Experimental set 4 e Alewater reactions with a spent reaction product catalyst mixture After the experimental set 2 reaction at 45 C had completed, a white viscous milk-like slurry of 2.89 g reaction 1 product Al(OH)3 and 1 g UH was obtained. 1 g Al MP was added to this mixture at 45 C for H2 production. Once complete, another 1 g Al MP was added to the subsequent mixture, which was run at the same stirring rate and temperature again for H2 production. Once complete, furthermore, a final 1 g Al MP was added to this mixture, with the same conditions for 3 successive additions of 1 g Al MP to the previous spent reaction product mixture with accumulation of Al(OH)3. These will be referred as ‘1st, 2nd and 3rd’. The final product mixture was collected and dried in air at 50 C to obtain white Al(OH)3 powder. As this powder was obtained from a reaction product mixture it will be referred to as ‘RPM’. After the reaction from experimental set 1 at 45 C had completed, a similar white viscous milk-like slurry of Al(OH)3 byproduct was obtained. 3 additions of 1 g Al MP was added to this mixture after each completion, as previously at 45 C. This experiment will be referred to in the results as ‘1st No UH’, ‘2nd No UH’ and ‘3rd No UH.
Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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Experimental set 5 e Alewater reactions with RPM catalyst As 1 g Al reacts with water to produce 2.89 g Al(OH)3, 3.89 g RPM was added to 50 ml DI water, corresponding to the quantity of Al(OH)3 (1 g UH & 2.89 g Al(OH)3 from Alewater reaction) in the reaction product mixture obtained after reaction from experimental set 2. 1 g Al MP was then added to this, this reaction will be referred to as RPM. Once complete another 1 g Al MP was added as with experimental set 4 and reacted to completion. Similarly to experimental set 4, 3 successive additions of 1 g Al MP were made in total, each after reaction completion, and will be referred to in the results as RPM 1st, RPM 2nd, RPM 3rd. Reaction temperature ¼ 45 C. Temperature of the reaction flask was controlled by an oil bath. The product gas evolved was collected by water displacement into a 2 l graduated cylinder with bubbling through water for cooling. The reaction progress was observed using a HP 2300 recording Camera. 1360 ml corresponds to 100% yield on H2 at room temperature and pressure. The pH of solutions was measured using a Hanna Piccolo pH meter. GC Analysis was used to confirm H2 gas and determine its purity. UH samples were characterized using XRD, SEM and EDX techniques. XRD characterization was performed with a Siemens D500 X-ray diffractometer (XRD) operating at 40 kV and 30 mA with Cu Ka radiation at a wavelength of 0.1542 nm. The scan was conducted in 2q mode and spanned across a range of 10 e40 with a step resolution of 0.04 and a scanning rate of 0.8 /min. SEM and EDX characterization was performed with a Hitachi TM-1000 Scanning Electron Microscope (SEM) (Hitachi High Technologies Ltd) and APOLLO XV Silicon Drift Detector.
Results The reaction results are summarized in Table 1. Individual result profiles are presented in figures as follows.
Experimental set 1 e Alewater reactions without catalyst Section “Experimental set 1 e Alewater reactions without catalyst” gives experimental set-up details. The results of these experiments are shown in Fig. 1. It was observed that with continuous and rapid stirring, all the reactions showed an initial induction time to commence. As shown in Fig. 1, 25 C showed the longest induction time of many days. Induction times decreased with increasing temperatures to 15 min when reactions are done at 60 C. pH of the reaction mixture was studied for these reactions and had showed an increase from 7 to 9 at its completion.
Experimental set 2 e Alewater reactions with UH catalyst Section “Experimental set 2 e Alewater reactions with UH catalyst” gives experimental set-up details. The results are shown in Fig. 2. The induction time before reaction was the longest for the lowest temperature studied of 35 C. The induction times decreased with increasing temperatures. This is the same effect of temperature observed in experimental set 1, too. The induction times before reaction from experimental
set 1 for temperatures studied of 35, 45 and 60 C were shorter than induction times for the same temperatures studied for experimental set 2. This indicates that the addition of UH caused an increase in the induction time when compared to Alewater reactions without UH. Reaction rates also increased compared to experimental set 1 and are presented in Table 1. pH of the reaction mixture was studied for these reactions and had showed an increase from 8 to 9.5 at its completion. The introduction of UH increased the alkalinity of the reaction mixtures, UH was found to introduce water soluble ion impurities present from the synthesis method, these impurities react with Al initially, consuming H2 as they are converted, hence the increased induction time, their conversion products then increase the pH. Ionic impurities have been shown to considerably affect induction time [29]. The induction time should be minimized and reaction rates maximized if ondemand production of H2 is to be realized from Alewater reactions with UH catalyst. On the other hand, induction times when adding UH offers an additional reaction control opportunity in certain applications or systems that might require delayed or programmed starts.
Experimental set 3 e Alewater reactions with modified UH catalyst mixtures Section “Experimental set 3 e Alewater reactions with modified UH catalyst mixtures” gives experimental set-up details. The results of these experiments are shown in Fig. 3. Alewater reactions with addition of UH as shown in Fig. 2 and Table 1, UHeDI water mixtures were modified as described in Section “Experimental set 3 e Alewater reactions with modified UH catalyst mixtures”. For each of these 3 different mixtures, after modification, 1 g Al MP was added for reaction with water at a single temperature of 45 C with magnetic stirring at 1000 rpm. Modifications of each mixture were performed to understand different aspects of the nature of the catalytic mechanism. Mixture 1 e Bubbling H2 into the mixture was performed as it was intended to affect the amount of H2 adsorbed on to the pristine UH surface. Mixture 2 e Stirring of the mixture for 20 h was meant to split UH aggregates and particles to increase the specific B.E.T. surface area and the resulting catalytic activity [25]. Mixture 3 e Ultrasonication was hypothesized to do the same as stirring would do for Mixture 2, but quicker and is a known method for increasing activity of Al(OH)3 suspensions [23,24]. Results shown in Fig. 3 and Table 1 indicate that modifications of Mixture 1e3 had differing effects on the H2 generation rates and induction time before reaction, when compared to the result obtained at 45 C from experimental set 2. Mixture 1 showed little effect, the induction time was lowered from 172 to 150 min; however the H2 rate was lowered from 8.5 to 6.5 ml/min. This suggests that induction of H2 release to gas phase is not due to self-adsorption of H2 on to the pristine surface of the catalyst or Al MP. Mixture 2 showed a positive effect, both increasing the H2 rate from 8.5 to 27 ml/min and lowering the induction time from 172 to 26 min. This indicates that stirring increases the catalytic effect due to splitting of UH particles. The bound ionic impurities present in UH are more easily decomposed during the Alewater reaction, also finer Al(OH)3 particles have a higher specific surface area
Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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Table 1 e Summary of experimental sets 1e5 reaction results. Reaction
Max H2 rate (ml/min)
Average H2 rate, Reaction start to z90% completion (ml/min)
Induction time (min)
H2 yield (%)
Experimental set 1 25 C 35 C 45 C 55 C 60 C
3.8 @ 280 ml 5.6 @ 430 ml 3.3 @ 600 ml 5.9 @ 70 ml 12 @ 80 ml
0.9 1.6 2.3 1.6 1.5
4582 390 71 25 15
74 79 93 88 89
Experimental set 2 35 C 45 C 60 C
21 @ 560 ml 33 @ 400 ml 15.4 @ 100 ml
9.375 8.5 4.3
585 172 29
88 96 92
Experimental set 3 Mixture 1 Mixture 2 Mixture 3
30 @ 250 ml 80 @ 550 ml 50 @ 530 ml
6.5 27 11.3
150 26 171
93 94 93
Experimental set 4 1st 2nd 3rd 1st No UH 2nd No UH 3rd No UH
158 @ 650 ml 222 @ 650 ml 200 @ 650 ml 33 @ 280 ml 46.9 @ 280 ml 51.7 @190 ml
100 131 133 10.3 9.7 17
0.5 0.5 0.5 0.5 1 0.5
99 100 88 98 100 98
Experimental set 5 RPM RPM 1st RPM 2nd RPM 3rd
7.7 @ 640 ml 26 @ 500 ml 44 @ 550 ml 56 @ 560 ml
3.8 9.53 18.4 17.1
0.5 0.5 0.5 0.5
103 100 100 101
allowing greater coverage of the Al particles for dissociation of water to OH ions, which hydrates the passive alumina layer, H2 generation follows whereby its desorption as H2 gas breaks the passivation layer allowing a fresh surface of Al to react with water, each particle can be modeled as a shrinking core for this reaction [21,23,30]. Mixture 3 showed little effect, the induction time was lowered from 172 to 171 min and the H2 rate only slightly increased from 8.5 to 11.3 ml/min. This
indicates that 45 min of ultrasonication is not a suitable replacement for 20 h stirring for increasing activity by decreasing particle size, longer ultrasonication times may effect increased activity. The pH of each of the solutions increased as in experimental set 2 from 8 to 9.5 on completion.
Experimental set 4 e Alewater reactions with a spent reaction product catalyst mixture The work reported in Refs. [24,31] that when Al MP was added to reaction product mixtures, the reaction rates were faster
Fig. 1 e Hydrogen generated for Alewater reactions without catalyst at T ¼ 25, 35, 45, 55 and 60 C; initial pH ≈ 7.
Fig. 2 e Hydrogen generated for Alewater (1 ge50 ml) reactions with 1 g UH catalyst at T ¼ 35, 45, and 60 C; initial pH ≈ 8.
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Fig. 4 shows that H2 generation is faster for the UH containing reaction product mixture with 100, 131 and 133 ml/min compared to 10.3, 9.7, and 17 ml/min, respectively. The induction time for all reactions is the same at 0.5 min. This faster rate indicates that without UH in the initial reaction, the corresponding successive addition reaction will not proceed as quickly. The presence of UH increases the H2 generation rate significantly. The pH of ‘1st No UH-3rd No UH’ remained at z9 even at reaction completion.
Experimental set 5 e Alewater reactions with RPM catalyst Fig. 3 e Hydrogen generated for Alewater reactions with modified 1 g UHe50 ml DI water catalyst mixtures; Mixtures 1e3. 1 g Al added to each for H2 generation at 45 C. Result from experimental set 2 at 45 C, an unmodified reactant mixture is shown for comparison; initial pH ≈ 8.
due to the presence of reaction product Al(OH)3 from the initial reaction. This reaction product Al(OH)3 was found to be autocatalytic with increasing reaction rates with each successive addition of Al MP. This effect was studied here and found to agree with the reported results. The induction time for 1st to the 3rd had decreased from 172 to 0.5 min for each reaction, compared to the initial reaction from experimental set 2, 45 C, from which the spent reaction product mixture was obtained. The H2 rate had also increased significantly from 8.5 to 100, 131, 133 ml/ min for 1st, 2nd and 3rd runs, respectively with reaction completion occurring within about 10 min. This reaction configuration is the most promising for on-demand H2 production as it starts almost immediately and gives the fastest reactions rates obtained for all of the studied experiments. The pH increased from z9.5 to 10 due to the increased quantity of reaction product Al(OH)3. A scalable reaction system is possible under these simple conditions.
Fig. 4 e Hydrogen generated for Alewater reactions with 2 spent reaction catalyst mixtures. ‘1st No UH’, ‘2nd No UH’, ‘3rd No UH’, 1 g Al added to the experimental set 1 45 C product mixture, 3 additions, T ¼ 45 C. 1st, 2nd and 3rd additions to experimental set 2 45 C spent reaction product mixture. pH of reaction product mixtures ≈9.
Section “Experimental set 5 e Alewater reactions with RPM catalyst” provides experimental set-up details. Results are shown in Fig. 5. The spent reaction product mixture from experimental set 4 3rd was collected and dried in air at 50 C as described in Section “Experimental set 5 e Alewater reactions with RPM catalyst”. This was conducted to test whether it retains its catalytic effect in dried form, which would allow for simpler storage compared to the viscous slurry. 3.89 g RPM was added initially to match the quantity of Al(OH)3 present in the reaction flask at experimental set 2 45 C reaction completion. The reaction started after an induction time of just 0.5 min but its H2 rate was much slower compared to results obtained for ‘1st, 2nd and 3rd’ at 3.8 ml/min compared to 100, 131 and 133 ml/min, respectively. This indicates that the catalysis effect is not retained after drying. With successive additions of 1 g Al MP to the flask after RPM had completed, up to 3 successive additions, RPM 1st, RPM 2nd, RPM 3rd, the induction time remained the same but the H2 rate increased from 3.8 to 9.53, 18.1 and 17.4 ml/min. This increase is not as great as observed in Fig. 4. This indicates that the UH added for experimental set 2 45 C increased the reaction rate for successive additions of 1 g Al to the spent reaction product mixture to a much greater extent than RPM did for successive additions of 1 g Al shown in Fig. 5, likely due to agglomeration of the catalyst from drying resulting in lower activity. This suggests that if a highest reaction rate is desired then the spent reaction product catalyst mixtures are to be stored within the liquid to retain their catalytic effect. Again, this is vital to scale-up of systems.
Fig. 5 e Hydrogen generated for Alewater reactions with RPM catalyst. T ¼ 45 C. 1 g Al MP, 3.89 g RPM and 50 ml DI water. To spent mixture, 1 g Al was added for RPM 1st, RPM 2nd and RPM 3rd, totaling 3 successive additions. pH was not measured.
Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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 7 ) 1 e9
SEM imaging Fig. 6 shows the SEM images obtained for UH Al(OH)3 catalyst. UH Al(OH)3 shows an irregular morphology with particles and aggregates ranging in size from 10 nm to 200 mm.
EDX analysis Energy Dispersive X-Ray Spectroscopy is used to determine the elemental composition of samples. The results will be further discussed in Section “Discussion”.
XRD analysis Fig. 7 shows the diffractograms for UH Al(OH)3 and RPM Al(OH)3. UH Al(OH)3 shows no distinct Bayerite peaks. It is an amorphous material. RPM Al(OH)3 is however crystalline and is indicative of Bayerite. There is some peak broadening observed from the presence of UH in this sample also indicating the Al(OH)3 is very fine.
Discussion The Alewater reaction proceeds even without the addition of UH catalyst with over 90% H2 yield. This finding demonstrates
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that stirring and moderate temperatures are sufficient to disrupt the alumina layer and the reaction indeed proceeds even at 25 C but, after many days. The induction time decreases with an increase in the reaction temperatures. The addition of fresh UH with Al MP in water increased the induction time to 172 min, but, the reaction rate also showed an increase. EDX analysis of the UH Al(OH)3 sample shows qualitatively the presence of carbon and nitrogen on the surface, with the expected Al and oxygen, indicating that there are remaining impurities (Table 2). An explanation for this is that ionic nitrogen based impurities present in UH react with the Al surface until they are consumed, also increasing the pH; from then, Al reacts with water at a higher rate due to greater availability of OH from both the pH increase and water dissociation of the catalytic Al(OH)3 reaction product which in turn hydrate the Al surface. The UHeDI mixture induction time may be lowered and reaction rate increased for the initial reaction by stirring for 20 h. This may indicate that the catalytic effect of UH depends on particle size where continued stirring leads to attrition of particles and aggregates, imaged by SEM. These smaller particles possess greater catalytic activity. The role of stirring indicates attrition by short ultrasonic treatment was not sufficiently effective. The reaction 1 product Al(OH)3 also shows autocatalytic properties. Al MP reactions with Al(OH)3 product slurries were observed to start almost immediately. However, the product mixtures without an initial addition of UH exhibited
Fig. 6 e Dry Al(OH)3 UH powder catalyst sample after preparation as outlined in Section “Production of Al(OH)3 by urea hydrolysis”. SEM images at various magnifications as shown in the micrographs.
Fig. 7 e (a) & (b). (a) UH Al(OH)3 powder sample diffractogram, prepared as described in Section “Production of Al(OH)3 by urea hydrolysis”. (b) RPM Al(OH)3 powder sample diffractogram, prepared as described in Section “Experimental set 4 e Alewater reactions with a spent reaction product catalyst mixture”. Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279
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Table 2 e EDX analysis of UH powder. Element Carbon Nitrogen Oxygen Aluminum
Wt.%
At%
9.94 1.9 7.35 80.8
18.74 3.07 10.4 67.78
inferior catalytic properties (Fig. 4), the reaction product mixtures with UH were more gelatinous and of a higher pH from the decomposed impurities. This catalytic effect is only present in the slurry and is decreased when dried due to agglomeration and aggregation of particles, which lowers specific surface area and water dissociation activity of Al(OH)3 (Fig. 5). This decrease was also observed in other work [23]. The product hydrogen gas showed 97% purity through GC measurement, with remaining showing Ar and air arising from gas sampling and contamination occurring while sample collection, as well as air dissolved in water used for displacement when collecting the product gas.
Conclusions Al can serve as an energy storage vector for renewable energy through a repeatable energy storage and recovery cycle. When reacted with water at moderate temperatures Al particles produce H2 and it is possible to operate that process on demand. Alewater reactions were modified with novel amorphous Al(OH)3 produced by urea hydrolysis to increase reaction rates and disrupt the passivating alumina layer. Reaction product mixture slurries remaining from UH modified Alewater reactions showed the fastest hydrogen generation capability due to complementary auto-catalytic effects, allowing continuous addition of Al MP to the reactor for fast H2 generation. On-demand production of H2 in this way, when coupled with a fuel cell could be used in principle to power electrical devices or vehicles. The study offers possibility for scale up of Al based energy storage systems in future.
Acknowledgments This project is funded by the Irish Research Council Enterprise Partnership Scheme (IRC contract No. EPSPG/2013/591) with support from RUSAL Aughinish Alumina Ltd., Ireland.
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Please cite this article in press as: Newell A, Thampi KR, Novel amorphous aluminum hydroxide catalysts for aluminumewater reactions to produce H2 on demand, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.279