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A bottom-up approach to prepare cobalt-based bimetallic supported catalysts for hydrolysis of ammonia borane
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 5 6 2 7 e5 6 3 7
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A bottom-up approach to prepare cobalt-based bimetallic supported catalysts for hydrolysis of ammonia borane O. Akdim, U.B. Demirci*, P. Miele IEM (Institut Europeen des Membranes), UMR 5635 (CNRS-ENSCM-UM2), Universite Montpellier 2, Place E. Bataillon, F-34095 Montpellier, France
article info
abstract
Article history:
Catalysis is an important research topic in the field of hydrogen generation by hydrolysis of
Received 5 September 2012
boron-based hydrides. A typical example is hydrolysis of ammonia borane (AB, NH3BH3),
Received in revised form
1 mol of which is able to liberate up to 3 mol H2 at temperatures lower than 80 C. However,
21 February 2013
the presence of a catalyst, generally metal-based, is necessary. The present work was thus
Accepted 23 February 2013
conducted in this framework. Herein, we propose a bottom-up approach to prepare cobalt-
Available online 30 March 2013
based bimetallic supported catalysts. The general idea was: first, to screen cobalt-based bimetallic nanoparticles and select the best combination, which was found to be CoCu
Keywords:
with a weight ratio 70:30 e its reactivity was discussed in terms of electronic and geometric
Ammonia borane
effects; second, to prepare Ni foam-supported CoCu through a 2-stage process e CoCu/Ni
Cobalt catalyst
showed a hydrogen generation rate of ca. 25 mL min1, which almost 5 times better than
Copper catalyst
that observed for the monometallic counterparts Co/Ni and Cu/Ni; third, to propose a new
Hydrolysis
concept of CoCu supported catalysts using a plastic film (light, easy to handle and to
Hydrolytic dehydrogenation
prepare) e it showed to be stable and, despite a low hydrogen generation rate (because most of the nanoparticles were embedded in the film), totally converted AB. Our main results are reported and discussed herein. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Since the past decade, ammonia borane NH3BH3 (AB) is presented as being one of the most promising chemical hydrogen storage materials owing to its low molecular weight (30.8 g mol1 NH3BH3) and subsequent high gravimetric/ volumetric hydrogen storage capacities (19.5 wt% H and 146 g H L1). Hence, it has been intensely investigated and huge research efforts have been dedicated to its dehydrogenation, especially since the mid-2000s [1e4]. Ammonia borane is constituted of three protonic hydrogens (Hdþ) and three hydridic hydrogens (Hd). The release of dihydrogen can then be devised through two different approaches. In the first one, the idea is to make the protonic and
hydridic hydrogens of AB react together through intra- and/or inter-molecular interactions. This is achieved at high temperatures (>100 C), by thermolysis [5]. In the second approach, the protonic hydrogens are provided by another molecule, typically water or methanol, which reacts with the hydridic hydrogens of AB, by hydrolysis or methanolysis [6,7]. Two alternative approaches were more recently reported. The first is called hydrothermolysis; it is a hydrolysis taking place at high temperatures (w100 C) [8]. The second is based on aluminum/water combustion; this reaction provides heat for the AB dehydrogenation and releases additional H2 from water [9]. Hydrolytic dehydrogenation of AB, the hydride being in aqueous solution or as a solid, takes place at room
* Corresponding author. Tel.: þ33 467149160; fax: þ33 467149119. E-mail addresses: [email protected], [email protected] (U.B. Demirci). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.110
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temperature in the presence of an excess of hydrolytic water and of a catalyst [10e12]: NH3 BH3 ðaq or sÞ þ ð4 þ xÞH2 OðIÞ/NHþ 4 ðaqÞ þ BðOHÞ4 ðaqÞ
þ 3H2 ðgÞ þ ðx H2 OðIÞÞ
(1)
Though this reaction suffers from few drawbacks (i.e. gravimetric hydrogen storage capacities lower than 8 wt% H, release of NH3 during hydrolysis, and difficult recyclability of the B(OH) 4 ) [4], recent literature surveys showed that the aspect that has been the most widely investigated concerns catalyst [3,7,13,14]. Research mainly focused on finding reactive and durable catalysts. While the utilization of acids or alkaline metal hydrides were also envisaged [11,15], most of the proposed catalysts are metal-based. Of the transition metals, the following were used and tested as homogenous or heterogenous catalysts: Fe [16], Co [17], Ni [18], Cu [19], Ru [20], Rh [21], Pd [22], Ir [23], Pt [24], and Au [25]. To go further in improving the catalytic abilities of these metals, bimetallic combinations such as e.g. PtNi [26] or CoPt [27] as well as addition of doping elements such as B [28] or P [29] were considered. Other examples will be given throughout the following sections. In hydrolysis of ammonia borane, most of the metal catalysts were investigated in powdery form, which is not practical for a reaction in liquid state. The difficulty lies in the separation of the powder (if not magnetic) from the liquid spent fuel and in the loss of catalytic material. Hence, it is of importance to support the catalytically active phase on a substrate like e.g. plate, foam or monolith [28,30,31]. In the present work, we developed Co-based bimetallic combinations, with the objective of improving catalytic activity. Then, the best combination was considered to prepare two different kinds of supported catalysts. For the first kind, we used Ni foam as substrate, also to compare to a previous work using only Co [32]. For the second kind, we tentatively prepared plastic thin films to disperse and support the Co-based catalyst. Our results are reported hereafter.
2.
Experimental
Cobalt chloride hexahydrate (CoCl2.6H2O, SigmaeAldrich), copper sulfate pentahydrate (CuSO4.5H2O, Fisher), chromium chloride hexahydrate (CrCl3.6H2O, Acros), zirconium chloride
(ZrCl4, SigmaeAldrich) and HfCl4 (Fluka) were used as received. The bimetallic catalysts, CoCr, CoCu, CoZr and CoHf, were prepared with various mol ratios Co:M (with M ¼ Cr, Cu, Zr, and Hf). The ratios were set as being 95:5, 90:10, 80:20, 70:30 and 50:50. The preparation was as follows. An aqueous solution (15 mL) of both chlorides (total of 1 mmol) and of cetyl trimethyl ammonium bromide (CTAB, CH3(CH2)15N(Br)(CH3)3, 98%, SigmaeAldrich) is prepared by dissolution, subsequent sonication, and stirring. In parallel, a sodium hydroxide-stabilized aqueous solution of 1 M sodium borohydride (NaBH4, SigmaeAldrich) was prepared. Millipore milli-q water with a resistivity >18 MU cm, which was degassed by bubbling argon for 30 min, was used. Then, 6 mL of the aqueous solution of the reducing agent was added dropwise to the chlorides solution. Hydrogen evolved vigorously. The slurry was aged one hour. Finally, the catalyst was separated by centrifugation, washed several times, centrifuged at each step, and finally dried at 80 C. The preparation of the Ni foam-supported catalysts was performed as follows. The substrate Ni foam (10 20 mm2) was first pretreated in an acid solution (H2SO4 at 10%) under a voltage of 7 V for 1 min, to remove the oxide layer on surface. Then, it was washed with deionized water several times and dried at 80 C. The electrodeposition of Co was then performed according to the procedure described elsewhere [32]. The asobtained catalyst is denoted Co/Ni. The insertion of Cu in the Co layer was performed by electroless deposition. Typically, Co/Ni was immersed in an aqueous solution of Cu2þ (2 g L1) for various times to vary the Cu content and get the targeted one, washed with deionized water, dried, and stored to be used in hydrolysis of AB. The preparation of the plastic film-supported catalysts was performed as follows. The nanoparticles were prepared according to the procedure described above. The plastic film was prepared in parallel. Polyvinylchloride (PVC, SigmaeAldrich) and polymethyl methacrylate (PMMA, SigmaeAldrich) were dissolved in dimethylacetamide (34 g, DMA, SigmaeAldrich). The weight ratio DMA/(PVC þ PMMA) was 6. Then, 15 mg of nanoparticles were added in 120 mg of the polymer slurry and vigorously stirred. The as-formed mixture was spread over a watch glass and dried at 60 C. The nanoparticles were characterized by X-ray diffraction (XRD, Bruker D5005 powder diffractometer, CuKa radiation, l ¼ 1.5406 A), scanning electron microscopy (SEM, Hitachi S800
Fig. 1 e Hydrogen evolution for the Co nanoparticles.
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Fig. 2 e Hydrogen evolution for the various CoCr nanoparticles.
FEG) equipped with energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM, JEOL 1200 EXII), and N2 adsorption/desorption to determine the specific surface area by the BET method. X-ray photoelectron spectroscopy (XPS, performed at “Science et Surface”, in Ecully, France; PHI Quantera SXM spectrometer equipped with an Al Ka, hn ¼ 280 eV, 47.7 W) was also envisaged but the magnetic properties of the particles made the experiment difficult. The Ni foam supported catalysts were characterized by XRD and EDS. Ammonia borane (SigmaeAldrich) was stored in an argonfilled glove box (MBraun M200B, H2O < 0.1 ppm, O2 < 0.1 ppm) to prevent the sample from moisture and air. The hydrogen generation measurement by hydrolysis of AB was performed as follows. The catalyst (15 mg) was introduced into the reactor consisting of a glass tube (27.5 mL) sealed with a silicon septum. The reactor was then placed in a water bath thermostated at 50 C and connected to a water-filled inverted burette. An acidic and a cold trap were put between the reactor and the burette to condensate ammonia and steam. In parallel, an aqueous solution of NH3BH3 (1 M) was prepared. To start the hydrolysis, 4 mL of the NH3BH3 solution (ca. 123 mg of hydride) was injected into the reactor and the H2 evolution was measured. The experiment was video-recorded in order that the H2 generation was analyzed afterwards. The hydrogen generation rates were calculated by dividing the H2 volume by the time.
evolution curve (mol H2 per mol NH3BH3 as a function of time) obtained with Co is shown in Fig. 1. It will be used for comparison purpose, that is, as reference to screen the various bimetallic systems. Note that all of the catalysts tested in the present work totally converted AB, leading to the evolution of 3 mol H2 per mol NH3BH3.
3.1.2.
3.1.3.
3.
Results and discussion
3.1.
Screening
3.1.1.
Co as reference
To dope the catalytic activity of Co, we pitched on adding a second transition metal to get bimetallic nanoparticles. Several metals from the 1st row of the periodic table as well as from the column IVB were selected. Our first attempts focused on Cr, Cu, Zr and Hf. We chose Cr and Cu because computational works showed that Co would segregate and antisegregate, respectively, when combined with them [33]. We chose Zr and Hf because in preliminary works [30] we observed positive effects on the catalytic reactivity of Co in their presence. The bimetallic CoCr, CoCu, CoZr and CoHf were then prepared and their reactivity in hydrolysis of AB was compared to that of Co. The H2
CoCr
Fig. 2 shows the results obtained with the catalysts CoCr. All suffers from an induction period of several minutes that delays the starting of the hydrolysis (Table 1). This induction period is typical of catalysts which surface has been oxidized. During this period, the catalytically active phase (surface) of the catalyst forms by reduction in the presence of the borane [34]. Compared to Co, the addition of Cr does not reduce or hinder the occurrence of such a period; it seems to make it longer. The reactivity of the various CoCr was compared by calculating the hydrogen generation rate. This was done by dividing the volume of generated H2 by time. The ranges 0e0.5 and 2.5e3 mol H2 per mol AB were not taken into account in the calculation. The rates are presented in Table 2. Except for CoCr 50:50, the hydrogen generation rates are higher than that of Co (11.5 mL H2 min1 or 0.8 L H2 min1 g1 Co). Combining Cr with Co has thus a positive effect.
CoCu
Fig. 3a shows the results obtained with the catalysts CoCu. The addition of Cu to Co is beneficial to the catalytic reactivity (Table 2) and in reducing the induction time (Table 1). As shown in Table 2, hydrogen generation rates varying from 14 to 41.6 mL H2 min1 (or ca. 0.9 to 2.8 L H2 min1 g1 CoCu) were found, with the best performance found for CoCu 70:30. This catalyst has a hydrogen generation rate 3.6
Table 1 e Induction times in min measured for the different cobalt-based bimetallic catalysts.
CoCr CoCu CoZr CoHf
min min min min
95:5
90:10
80:20
70:30
50:50
6.3 1 0.7 2.3
2.7 0.3 1.3 2.3
1.8 0.1 1.7 1.7
3.7 0.5 3.3 1.3
7.3 0.2 3.8 1.3
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Table 2 e Hydrogen generation rates in mL H2 minL1 and mL H2 minL1 gL1 CoM (M [ Cr, Cu, Zr or Hf) measured for the different cobalt-based bimetallic catalysts.
CoCr CoCu CoZr CoHf
mL mL mL mL mL mL mL mL
min1 min1 g1 min1 min1 g1 min1 min1 g1 min1 min1 g1
95:5
90:10
80:20
70:30
50:50
20.4 1.4 14 0.9 27.5 1.8 10.9 0.7
17.3 1.5 27.5 1.8 10.5 0.7 14.4 1
19.5 1.3 34.6 2.3 16.3 1.1 15.6 1
26.2 1.7 41.6 2.8 15.3 1 17.4 1.2
12 0.8 22.2 1.5 14 0.9 19.6 1.3
times higher than that of Co. The rate is comparable to that of Co35Pd65 reported by Sun et al. [35] though the experiments were realized at slightly different temperatures (40 vs. 50 C in our case). The reactivity is also high with CoCu 80:20, having a hydrogen generation rate 3 times higher than that of Co. Fig. 3b reports the hydrogen generation rates and shows their variation as a function of the Cu (or Co) content. The variation has a volcano-shape. Such behavior is related to the Sabatier principle, stating that there is a strong dependence between optimal catalytic activity (and thus the best bimetallic catalyst) and intermediate binding energies for reaction intermediates on a catalytic surface [36]. Such a behavior was also reported for CoPd bimetallic systems [35]. In other words,
the presence of Cu in CoCu 70:30 would provide the optimum surface and catalytic features in terms of activation barrier and intermediates adsorption. This result is a further evidence of that combining two metals may lead to synergy and thus to significant reactivity improvement. Another attractive feature with CoCu is that the induction period is almost suppressed.
3.1.4.
CoZr
Fig. 4 shows the results obtained with the catalysts CoZr. Like for Cr, the presence of Zr does not permit to hinder the induction period (Table 1). At best, it is reduced of few tens of seconds, as it is the case for CoZr 90:10. With the other bimetallic combinations, the induction period in rather lengthen. For example, it is of ca. 3 min for CoZr 70:30 whereas it is of 1.5 min for Co. With respect to the reactivity, the results are mitigated. The calculations of the hydrogen generation rates gave the values in Table 2. The results are nevertheless globally improved in comparison to the hydrogen generation rate found for Co (11.5 mL H2 min1).
3.1.5.
CoHf
Fig. 5 shows the results obtained with the catalysts CoHf. In a first approximation, we can notice that the situation is quite similar to that observed with the catalysts CoZr. The addition of Hf to Co has a positive effect, but it is slight. The induction period is as long as with Co (Table 1). It is however lengthened with CoHf 90:10. During the evolution of the 1st mol H2 per mol
Fig. 3 e Hydrogen evolution for the various CoCu nanoparticles (a) and evolution of the hydrogen generation rates as a function of the metals content.
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Fig. 4 e Hydrogen evolution for the various CoZr nanoparticles.
AB, Co and CoHf show quite similar kinetics. However, after that, the H2 evolution in the presence of Co slows down whereas it remains almost constant with CoHf, except for CoHf 50:50. The hydrogen generation rates found are reported in Table 2.
3.1.6.
Analysis and selection
The results reported heretofore represent a further evidence of how the catalytic reactivity of Co can be improved by adding a second metal. In the open literature, Lu et al. [37] recently showed that the bimetallic AuCo supported over silica nanospheres could be more reactive, owing to a synergetic interaction, than the monometallic counterparts in hydrolysis of AB. It was suggested that Au in AuCo@SiO2 plays a role in the activation of the catalyst. Qiu et al. [38] reported the synthesis and reactivity of a series of FeCo nano-alloys that displayed improved dehydrogenation properties in comparison to monometallic Co and Fe. It was shown that in the alloys, Co is electron-enriched while Fe is electron-deficient, indicating that the better reactivity might be ascribed to electronic effects. As commented by Yang et al. [26], the bimetallic catalysts have the ability to weaken the bonding of hydrogen and ammonium to the surface atoms, leading to an accelerated release of hydrogen. This is related to the Sabatier principle. Combining one metal with another one, ideally alloying them, may lead to improved reactivity and this may be attributed to electronic effects. Hammer and Nørskov [33] showed that the
D-band
center of a given metal could be shifted up or down when combined with another one. An up-shift leads to stronger adsorption of molecules and a down-shift to the reverse. For example, Co combined with Au, Fe or Cu should have higher-lying d states than pure Co and thus should bind more strongly the adsorbate. This is consistent with the electron-enrichment reported by Qiu et al. [38] for the FeCo nano-alloys. In the present work, electronic effects may account to the improved reactivity of CoCu. Another effect that may help to explain an improved reactivity (but also a deteriorated reactivity) is of geometric nature. This is segregation. Nørskov and co-workers [33,36] reported a useful table to predict the segregation of a given metal when combined or alloyed with another one. It is shown that Cr should moderately antisegregate in the presence of Co, implying then that the surface would be Co-enriched. This does not really help to explain the discrepancy in the reactivity of the CoCr combinations. On the other hand, Cu, Zr and Hf should segregate in, respectively, moderate, strong and moderate ways, then favoring the formation of smaller Co particles on the surface. This may rationalize the improved performance of CoCu. This prediction is also rather consistent with the improvement of the reactivity of CoHf when the Hf content is increased. The more Hf is, the smaller the Co particles on the surface should be, and the more active Co sites there should be. With respect to CoZr, no trend stands out and the segregation phenomenon does not seem to be the critical
Fig. 5 e Hydrogen evolution for the various CoHf nanoparticles.
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Fig. 6 e Arrhenius treatment for CoCu 70:30 over the temperature range 30e55 C.
factor. A systematic study focusing on the analysis of the electronic configuration of the metals and the surface composition of the nanoparticles might help to gain insight about such electronic and geometric effects. This could be envisaged in another work as this is out of the scope of the present paper. From the results reported heretofore, we selected CoCu to be used in the continuation of the present work, and the catalyst with the composition 70:30 was characterized.
3.2.
Characterization of CoCu
In addition to the H2 generation experiment performed at 50 C with CoCu 70:30, three other ones were carried out at 30, 40 and 55 C in order to determine the apparent activation energy by Arrhenius treatment (Fig. 6). Upon the completion of the induction period, the H2 evolution is linear, implying zero-order kinetics, that is, a reaction rate independent on the concentration of the AB. The hydrogen generation rates were determined over the range 0.5e2.5 mol H2 per mol AB and the Neperian logarithm of these values were plotted as a function of the reverse of the temperature (in K). The slope (a) was exploited to calculate the apparent activation energy (such as a ¼ Ea/R). An apparent activation energy of 60.8 kJ mol1 was found. Taken into consideration the experimental error on the volume of H2 (3 mL) as well as that on the reaction
temperature (1 C), an error of 1.6 kJ mol1 was determined for the apparent activation energy. This energy value was compared to some reported recently in the literature for bimetallic catalysts: i.e. 16e34 kJ mol1 for FeCo nano-alloys [38]; 27.5 kJ mol1 for Co35Pd65 [35]; 39e44 kJ mol1 for PtNi [26]; 41.5 kJ mol1 for carbon supported Co core-Pt shell nanoparticles [39]; 51.5 kJ mol1 for NiAg [40]; 52.8 kJ mol1 for Co nanoparticles inside hydrogen networks prepared from 2acryl-amido-2-methyl-1-propansulfonic acid [41]; and 54.7 kJ mol1 for CoeNieP/PdeTiO2 [42]. It is likely that the improvement in terms of activation energy is less pronounced with CoCu but the discrepancies in the experimental conditions might also explain the difference. The crystallinity of CoCu 70:30 was analyzed by powder XRD (Fig. 7). Unlike Co [43], few diffraction peaks, with the most intense centered at 2q ¼ 36.6 , were observed. Using the software EVA, they were ascribed to copper (I) oxide Cu2O (cuprite, ICDD 00-003-0898). Note that the peak at 2q ¼ 36.6 could also be attributed to cobalt oxide (Co2.62O4 ICDD 01-00785625 or Co2.87O4 ICDD 01-078-5630) and/or copper cobalt oxide ((Cu0.3Co0.7(Co2O4)) ICDD 00-025-0270) but the other smaller peaks were shifted of few degrees compared to the reference patterns. Nevertheless, their presence, in small amounts, is not completely discarded. The surface morphology of CoCu 70:30 was scrutinized by SEM and compared to that of Co (Fig. 8). No appreciable
Fig. 7 e XRD pattern for CoCu 70:30.
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Fig. 8 e SEM (a, b) and TEM (c, d) images of Co and CoCu 70:30, respectively.
difference was noticed. For both samples, large particles were observed, certainly consisting in aggregated particles of much smaller sizes. This was confirmed by TEM (Fig. 8). The nanoparticles of Co and CoCu have a size of about 20 nm, are slightly agglomerated and are embedded in a gangue, certainly consisting of borates as generally reported for Co nanoparticles [43]. Surface specific areas of 133 and 106 m2 g1 were found for Co and CoCu. Such higher areas may be explained by the presence of the gangue. The sample CoCu 70:30 was in addition analyzed by EDS (Fig. 9). The mol ratio 70:30 was confirmed and the mapping showed that the distribution of both metals was homogenous.
3.3.
Supported CoCu
3.3.1.
CoCu supported on Ni foam
In this work, Ni foam was selected as it is an ideal material for mass transportation owing to its porous structure. CoCu was deposited over pretreated Ni foam by applying a two-stage procedure. Co was first electrodeposited. Then, Cu was deposited by electroless deposition over the surface of Co/Ni. For comparison purpose, Cu/Ni and Co/Ni were also prepared by electrodeposition. Simultaneous electrodeposition of CoCu using an aqueous solution containing both metal salts was avoided because of the immiscibility of both metals by this
preparation way [44]. Here, our objective was to assess the scalability and the shaping of our selected bimetallic catalyst. The underlying objective was thus to validate our bottom-up approach to prepare supported, shaped catalysts. The as-obtained supported catalysts as well as pretreated Ni foam (as reference) were analyzed by XRD (Fig. 10). To start, the peaks of Ni foam were indexed. Co/Ni displays the same peaks and the pattern is similar to a previous report [32]. The diffraction peaks at 2q ¼ 44.1, 51.4 and 75.6 may be attributed to Ni (ICDD 04-010-6148) but also to face-centered Co (00-015-0806). With respect to Cu/Ni, 6 diffraction peaks were observed. The substrate Ni foam diffracted as characterized by the peaks at 2q ¼ 44.3, 51.7 and 76.2 . The three other peaks, at 2q ¼ 43.2, 50.2 and 73.5 , are ascribed to facecentered Cu (ICDD 04-009-2090) though there is a down-shift of ca. 0.38 compared to the reference pattern. These patterns were then compared to that of CoCu/Ni. On the one hand, in terms of diffraction peaks, the pattern of CoCu/Ni shows all of the peaks observed in Cu/Ni, suggesting the presence of face-centered Cu. Nevertheless, the presence of facecentered Co could not be discarded. On the other hand, in terms of diffraction angles, CoCu/Ni shows a shift of 04e07 to lower angles for all of the 6 main peaks. This may be indicative of the strain effect of e.g. Co atoms in the Cu layers and therefore of the formation of a solid solution. The
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Fig. 9 e Presence of Co and Cu in CoCu 70:30 determined by EDS. The image at left shows the area analyzed.
Fig. 10 e XRD patterns of Ni foam, Co/Ni, Cu/Ni and CoCu/Ni.
presence of both Co and Cu in CoCu/Ni was otherwise verified by EDS (Fig. 11). The catalytic activity of CoCu/Ni was assessed. Fig. 12 reports the results. They are compared to those of Co/Ni and Cu/ Ni. Consistently with the experiments using the powdery catalysts, CoCu/Ni, where both metals are present, is more active than the monometallic counterparts Co/Ni and Cu/Ni. It is showed that 3 mol H2 are released within 15, 60 and 80 min, respectively. The hydrogen generation rates were calculated and found to be 25.1, 4.4, and 5.6 mL H2 min1, respectively. Surprisingly, the hydrogen generation rate of Cu/Ni is almost 4 times higher than that of Cu. This suggests that the presence of Ni in the supported catalyst has an effect on the catalytic activity of Cu, perhaps due to electronic effects [33,36]. With respect to CoCu/Ni and Co/Ni, the hydrogen generation rates are lower than those found for the powdery catalysts, but remain quite attractive, especially in the case of CoCu/Ni. The
hydrogen generation rate of the bimetallic system is comparable to that of CoCu 90:10. The difference in reactivity may be ascribed to the difference in morphology as the nanoparticles should show more surface active sites than a layer over the Ni foam. In our conditions, the reactivity of CoCu/Ni is comparable to that of CoePeB/Ni reported elsewhere [45] though the temperature for the latter system is 10 C lower. The authors suggested that the role of B was to transfer electronic density to Co and that of P to favor the segregation and surface enrichment of Co. There are electronic and geometric effects, respectively. Compared to CoeWeBeP/Ni prepared by Yang et al. [46], the hydrogen generation rate displayed by CoCu/Ni is slightly lower, namely 25.1 at 50 C vs. 28 mL H2 min1 and 34 mL H2 min1 at 45 and 55 C. In fact, the presence of B and P is crucial to create more Co active sites and the addition of W is important to prevent Co from oxidation. Improved
Fig. 11 e Presence of Co and Cu in CoCu 70:30 determined by EDS. The image at left shows the area analyzed.
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Fig. 12 e Hydrogen evolution for Co/Ni, Cu/Ni and CoCu/Ni.
performance might be achieved with our CoCu/Ni by considering addition of such elements. This is a prospect of catalyst development.
3.3.2.
CoCu dispersed in a plastic thin film
Among the various strategies we have in finding reactive supported, shaped catalysts, we present herein a new one. The present experiment had the objective to validate our concept. Generally, the active phase, i.e. Co or CoCu, is supported over a plate, foam, membrane or monolithic structure [30]. These are the strategies we use to develop in the lab. Recently, we envisaged a novel strategy. Our main idea was to exploit the aforementioned CoCu particles. After synthesis, the batch was divided into two samples. The first one was dedicated to be studied directly as catalytic particles in hydrolysis of AB. The second was used to prepare a shaped catalyst using plastic films. Plastic films are attractive materials owing to their lightness, easy-handling, relative thermal stability, relative chemical stability and solubility in various organic solvents. After testing several polymeric materials, a mixture of polymethyl methacrylate and polyvinylchloride was chosen. Their respective proportion was varied and optimized. We found that a weight ratio 50:50 in dimethylacetamide was a good compromise in terms of viscosity, handling, spreading and drying. Hence, according to the preparation procedure reported
in the Experimental section, we prepared a shaped metalplastic catalyst where the CoCu particles were dispersed into the plastic film. We preferred to disperse the nanoparticles in the slurry (before drying) rather than impregnating the metal salts onto the dried plastic film. The as-formed catalyst is denoted CoCu/PMMA-PVC. After evaporation of the solvent, the catalyst was tested in hydrolysis of AB. Fig. 13 shows the evolution of H2 (mol per mol AB) as a function of time. The CoCu/PMMA-PVC material is reactive in hydrolysis of AB, with 3 mol H2 being released within 120 min. The hydrogen generation rate is 1.5 mL H2 min1. This low rate is easily explained by the features of the film. In this first attempt, the film was smooth, without porosity. Most of the particles were embedded in the film, making the accessibility of these catalytically active sites hard for the AB molecules. This first attempt was in fact a feasibility test of such catalytic films. The main advantages are as follows. No degradation of the film was observed during the reaction. It was easily separated from the reaction medium, washed and dried. Works are in progress to enhance the catalytic activity by avoiding the formation of embedded particles and favoring their deposition on the film surface. Plastic films with micrometric holes are also under development. Our ultimate goal is to propose a supported catalyst very easy to handle, as reactive as the nanoparticles it may contains, and that can be used in various catalytic reactions using liquid media.
Fig. 13 e Hydrogen evolution for CoCu 70:30 embedded in the plastic film.
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4.
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Conclusion
In the present work, we have reported, for the first time in hydrolysis of ammonia borane, a bottom-up approach to elaborate cobalt-based bimetallic catalysts. The study was performed according to three-stage consisting in synthetizing cobalt-based bimetallic catalysts, using the best bimetallic combination to prepare a Ni foam-supported catalyst (i.e. shaped catalyst), and using the best bimetallic combination to elaborate a new type of shaped, plastic-based catalyst. Shaped catalysts are of high importance for reactor scale. The first objective was thus first to screen bimetallic nanoparticles using Cr, Cu, Zr and Hf as the second metal M. Depending on the second metal, the gains in reactivity for CoM in comparison to Co were different. The best results were found for CoCu. The reactivity, plotted as a function of the Co content, showed to have a volcano shape peaking for CoCu with a ratio 70:30. The improved reactivity was ascribed to electronic and geometric effects. The second objective was thus to prepare a supported, shaped catalyst while using Ni foam as support. CoCu supported over Ni foam was prepared according to a two-step process consisting of the electrodeposition of cobalt followed by electroless deposition of copper. The reactivity of CoCu/Ni in hydrolysis was compared to the monometallic counterparts. It was found that by combining both metals the dehydrogenation properties in terms of hydrogen generation rates could be substantially improved. Better results could afterwards achieved by adding a third metal or making the surface rougher or porous. Therefore, Ni foam may be considered as a material of choice for elaborating shaped catalysts. The third objective was thus to prepare another shaped catalyst, using a plastic film as matrix of the CoCu nanoparticles. Though the reactivity was found to be lower than for the other catalysts investigated, the conversion of ammonia borane was of 100%. The present experiment had the objective to validate our concept. The under-lying objective was to show that a plastic film, which is light, easy to handle and to prepare, could act as a stable support. No degradation was observed during the hydrolysis reaction and while handling it after reaction. The challenge is now to improve the reactivity of the supported catalyst by better controlling the insertion of the nanoparticles and then providing porosity to the film. The following strategies could be explored: e.g. deposition of the nanoparticles on the plastic film surface by different techniques (impregnation, precipitation.); microemulsion to prepare through a one-stage process the nanoparticles-doped plastic film; use of sacrificial reagents or structure to get the expected porosity in the plastic film. In our opinion, this is all the more important that such supported films could be envisaged in various catalytic reactions using liquid or gaseous reaction media.
Acknowledgments The present work was conducted within the frame of the project Jeunes Chercheurs et Jeunes Chercheuses BoraHCx
and was funded by the Agence Nationale pour la Recherche (ANR). UBD is grateful.
references
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[20] Brockman A, Zheng Y, Gore J. A study of catalytic hydrolysis of concentrated ammonia borane solutions. Int J Hydrogen Energy 2010;35:7350e6. ¨ zkar S. Water soluble laurate[21] Durap F, Zahmakiran M, O stabilized rhodium(0) nanoclusters catalyst with unprecedented catalytic lifetime in the hydrolytic dehydrogenation of ammonia-borane. Appl Catal A 2009;369:53e9. ¨ , Sahin S, O ¨ zkar S. Water-soluble poly(4[22] Metin O styrenesulfonic acid-co-maleic acid) stabilized ruthenium(0) and palladium(0) nanoclusters as highly active catalysts in hydrogen generation from the hydrolysis of ammoniaborane. Int J Hydrogen Energy 2009;34:6304e13. [23] Ciganda R, Garralda MA, Ibarlucea L, Pinilla E, Torres MR. A hydridoirida-b-diketone as an efficient and robust homogeneous catalyst for the hydrolysis of ammoniaborane or amine-borane adducts in air to produce hydrogen. Dalton Trans 2010;39:7226e9. [24] Mohajeri N, R-Raissi A, Adebiyi O. Hydrolytic cleavage of ammonia-borane complex fro hydrogen production. J Power Sources 2007;167:482e5. [25] Chandra M, Xu Q. Room temperature hydrogen generation from aqueous ammonia-borane using noble metal nanoclusters as highly active catalysts. J Power Sources 2007;168:135e42. [26] Yang X, Cheng F, Liang J, Tao Z, Chen J. PtxNi1x nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane. Int J Hydrogen Energy 2009;34:8785e91. [27] Qin W, Yang C, Ma X, Lai S. Selective synthesis and characterization of metallic cobalt, cobalt/platinum, and platinum microspheres. J Alloys Compd 2011;509:338e42. [28] Dai HB, Gao LL, Liang Y, Kang XD, Wang P. Promoted hydrogen generation from ammonia borane aqueous solution using cobaltemolybdenumeboron/nickel foam catalyst. J Power Sources 2010;195:307e12. [29] Eom K, Cho K, Kwon H. Hydrogen generation from hydrolysis of NH3BH3 by an electroplated CoeP catalyst. Int J Hydrogen Energy 2009;35:181e6. [30] Demirci UB, Akdim O, Hannauer J, Chamoun R, Miele P. Cobalt, a reactive metal in releasing hydrogen from sodium borohydride by hydrolysis: a short review and a research perspective. Sci China Chem 2010;53:1870e9. [31] Akdim O, Chamoun R, Demirci UB, Zaatar Y, Khoury A, Miele P. Anchored cobalt film as stable supported catalyst for hydrolysis of sodium borohydride for chemical hydrogen storage. Int J Hydrogen Energy 2011;36:14527e33. [32] Akdim O, Demirci UB, Miele P. Deactivation and reactivation of cobalt in hydrolysis of sodium borohydride. Int J Hydrogen Energy 2011;36:13669e75.
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[33] Hammer B, Nørskov JK. Theoretical surface science and catalysis e calculations and concepts. Adv Catal 2000;45:71e129. [34] Hannauer J, Demirci UB, Geantet C, Herrmann JM, Miele P. Enhanced hydrogen release by catalyzed hydrolysis of sodium borohydride-ammonia borane mixtures: a solutionstate 11B NMR study. Phys Chem Chem Phys 2011;13:3809e18. ¨ , Sun S. Catalytic hydrolysis of [35] Sun D, Mazunder V, Metin O ammonia borane via cobalt palladium nanoparticles. ACS Nano 2011;5:6458e64. [36] Greeley J, Jaramillo TF, Bonde J, Chorkendorff I, Nørskov JK. Computational high-throughput screening of electrocatalytic materials for hydrogen evolution. Nat Mater 2006;5:909e13. [37] Lu ZH, Jiang HL, Yadav M, Aranishi K, Xu Q. Synergetic catalysis of AueCo@SiO2 nanospheres in hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage. J Mater Chem 2012;12:5065e71. [38] Qiu FY, Wang YJ, Wang YP, Li L, Liu G, Yan C, et al. Dehydrogenation of ammonia borane catalyzed by in situ synthesized FeeCo nano-alloy in aqueous solution. Catal Today 2011;170:64e8. [39] Yang X, Cheng F, Tao Z, Chen J. Hydrolytic dehydrogenation of ammonia borane catalyzed by carbon supported Co corePt shell nanoparticles. J Power Sources 2011;196:1785e9. [40] Yao CF, Zhuang L, Cao YL, Ai XP, Yang HX. Hydrogen release from hydrolysis of borazine on Pt- and Ni-based alloy catalysts. Int J Hydrogen Energy 2008;33:2462e7. [41] Ozay O, Inger E, Aktas N, Sahiner N. Hydrogen production from ammonia borane via hydrogen template synthesized Cu, Ni, Co composites. Int J Hydrogen Energy 2011;36:8209e16. ¨ zkar. Hydrogen generation from [42] Rakap M, Kalu EE, O hydrolysis of ammonia-borane using PdePVBeTiO2 and CoeNieP/PdeTiO2 under stirred conditions. J Power Sources 2012;210:184e90. [43] Demirci UB, Miele P. Cobalt in NaBH4 hydrolysis. Phys Chem Chem Phys 2010;12:14651e65. [44] Karaagac O, Alper M, Kockar H. Characterizations of CoCu films electrodeposited at different cathode potentials. J Magn Magn Mater 2010;322:1098e101. [45] Patel N, Kale A, Miotello A. Improved dehydrogenation of ammonia borane over CoePeB coating on Ni: a single catalyst for both hydrolysis and thermolysis. Appl Catal B 2012;111e112:178e84. [46] Yang J, Cheng F, Liang J, Chen J. Hydrogen generation by hydrolysis of ammonia borane with a nanoporous cobaltetungsteneboronephosphorus catalyst supported on Ni foam. Int J Hydrogen Energy 2011;36:1411e7.
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A bottom-up approach to prepare cobalt-based bimetallic supported catalysts for hydrolysis of ammonia borane O. Akdim, U.B. Demirci*, P. Miele IEM (Institut Europeen des Membranes), UMR 5635 (CNRS-ENSCM-UM2), Universite Montpellier 2, Place E. Bataillon, F-34095 Montpellier, France
article info
abstract
Article history:
Catalysis is an important research topic in the field of hydrogen generation by hydrolysis of
Received 5 September 2012
boron-based hydrides. A typical example is hydrolysis of ammonia borane (AB, NH3BH3),
Received in revised form
1 mol of which is able to liberate up to 3 mol H2 at temperatures lower than 80 C. However,
21 February 2013
the presence of a catalyst, generally metal-based, is necessary. The present work was thus
Accepted 23 February 2013
conducted in this framework. Herein, we propose a bottom-up approach to prepare cobalt-
Available online 30 March 2013
based bimetallic supported catalysts. The general idea was: first, to screen cobalt-based bimetallic nanoparticles and select the best combination, which was found to be CoCu
Keywords:
with a weight ratio 70:30 e its reactivity was discussed in terms of electronic and geometric
Ammonia borane
effects; second, to prepare Ni foam-supported CoCu through a 2-stage process e CoCu/Ni
Cobalt catalyst
showed a hydrogen generation rate of ca. 25 mL min1, which almost 5 times better than
Copper catalyst
that observed for the monometallic counterparts Co/Ni and Cu/Ni; third, to propose a new
Hydrolysis
concept of CoCu supported catalysts using a plastic film (light, easy to handle and to
Hydrolytic dehydrogenation
prepare) e it showed to be stable and, despite a low hydrogen generation rate (because most of the nanoparticles were embedded in the film), totally converted AB. Our main results are reported and discussed herein. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Since the past decade, ammonia borane NH3BH3 (AB) is presented as being one of the most promising chemical hydrogen storage materials owing to its low molecular weight (30.8 g mol1 NH3BH3) and subsequent high gravimetric/ volumetric hydrogen storage capacities (19.5 wt% H and 146 g H L1). Hence, it has been intensely investigated and huge research efforts have been dedicated to its dehydrogenation, especially since the mid-2000s [1e4]. Ammonia borane is constituted of three protonic hydrogens (Hdþ) and three hydridic hydrogens (Hd). The release of dihydrogen can then be devised through two different approaches. In the first one, the idea is to make the protonic and
hydridic hydrogens of AB react together through intra- and/or inter-molecular interactions. This is achieved at high temperatures (>100 C), by thermolysis [5]. In the second approach, the protonic hydrogens are provided by another molecule, typically water or methanol, which reacts with the hydridic hydrogens of AB, by hydrolysis or methanolysis [6,7]. Two alternative approaches were more recently reported. The first is called hydrothermolysis; it is a hydrolysis taking place at high temperatures (w100 C) [8]. The second is based on aluminum/water combustion; this reaction provides heat for the AB dehydrogenation and releases additional H2 from water [9]. Hydrolytic dehydrogenation of AB, the hydride being in aqueous solution or as a solid, takes place at room
* Corresponding author. Tel.: þ33 467149160; fax: þ33 467149119. E-mail addresses: [email protected], [email protected] (U.B. Demirci). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.110
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temperature in the presence of an excess of hydrolytic water and of a catalyst [10e12]: NH3 BH3 ðaq or sÞ þ ð4 þ xÞH2 OðIÞ/NHþ 4 ðaqÞ þ BðOHÞ4 ðaqÞ
þ 3H2 ðgÞ þ ðx H2 OðIÞÞ
(1)
Though this reaction suffers from few drawbacks (i.e. gravimetric hydrogen storage capacities lower than 8 wt% H, release of NH3 during hydrolysis, and difficult recyclability of the B(OH) 4 ) [4], recent literature surveys showed that the aspect that has been the most widely investigated concerns catalyst [3,7,13,14]. Research mainly focused on finding reactive and durable catalysts. While the utilization of acids or alkaline metal hydrides were also envisaged [11,15], most of the proposed catalysts are metal-based. Of the transition metals, the following were used and tested as homogenous or heterogenous catalysts: Fe [16], Co [17], Ni [18], Cu [19], Ru [20], Rh [21], Pd [22], Ir [23], Pt [24], and Au [25]. To go further in improving the catalytic abilities of these metals, bimetallic combinations such as e.g. PtNi [26] or CoPt [27] as well as addition of doping elements such as B [28] or P [29] were considered. Other examples will be given throughout the following sections. In hydrolysis of ammonia borane, most of the metal catalysts were investigated in powdery form, which is not practical for a reaction in liquid state. The difficulty lies in the separation of the powder (if not magnetic) from the liquid spent fuel and in the loss of catalytic material. Hence, it is of importance to support the catalytically active phase on a substrate like e.g. plate, foam or monolith [28,30,31]. In the present work, we developed Co-based bimetallic combinations, with the objective of improving catalytic activity. Then, the best combination was considered to prepare two different kinds of supported catalysts. For the first kind, we used Ni foam as substrate, also to compare to a previous work using only Co [32]. For the second kind, we tentatively prepared plastic thin films to disperse and support the Co-based catalyst. Our results are reported hereafter.
2.
Experimental
Cobalt chloride hexahydrate (CoCl2.6H2O, SigmaeAldrich), copper sulfate pentahydrate (CuSO4.5H2O, Fisher), chromium chloride hexahydrate (CrCl3.6H2O, Acros), zirconium chloride
(ZrCl4, SigmaeAldrich) and HfCl4 (Fluka) were used as received. The bimetallic catalysts, CoCr, CoCu, CoZr and CoHf, were prepared with various mol ratios Co:M (with M ¼ Cr, Cu, Zr, and Hf). The ratios were set as being 95:5, 90:10, 80:20, 70:30 and 50:50. The preparation was as follows. An aqueous solution (15 mL) of both chlorides (total of 1 mmol) and of cetyl trimethyl ammonium bromide (CTAB, CH3(CH2)15N(Br)(CH3)3, 98%, SigmaeAldrich) is prepared by dissolution, subsequent sonication, and stirring. In parallel, a sodium hydroxide-stabilized aqueous solution of 1 M sodium borohydride (NaBH4, SigmaeAldrich) was prepared. Millipore milli-q water with a resistivity >18 MU cm, which was degassed by bubbling argon for 30 min, was used. Then, 6 mL of the aqueous solution of the reducing agent was added dropwise to the chlorides solution. Hydrogen evolved vigorously. The slurry was aged one hour. Finally, the catalyst was separated by centrifugation, washed several times, centrifuged at each step, and finally dried at 80 C. The preparation of the Ni foam-supported catalysts was performed as follows. The substrate Ni foam (10 20 mm2) was first pretreated in an acid solution (H2SO4 at 10%) under a voltage of 7 V for 1 min, to remove the oxide layer on surface. Then, it was washed with deionized water several times and dried at 80 C. The electrodeposition of Co was then performed according to the procedure described elsewhere [32]. The asobtained catalyst is denoted Co/Ni. The insertion of Cu in the Co layer was performed by electroless deposition. Typically, Co/Ni was immersed in an aqueous solution of Cu2þ (2 g L1) for various times to vary the Cu content and get the targeted one, washed with deionized water, dried, and stored to be used in hydrolysis of AB. The preparation of the plastic film-supported catalysts was performed as follows. The nanoparticles were prepared according to the procedure described above. The plastic film was prepared in parallel. Polyvinylchloride (PVC, SigmaeAldrich) and polymethyl methacrylate (PMMA, SigmaeAldrich) were dissolved in dimethylacetamide (34 g, DMA, SigmaeAldrich). The weight ratio DMA/(PVC þ PMMA) was 6. Then, 15 mg of nanoparticles were added in 120 mg of the polymer slurry and vigorously stirred. The as-formed mixture was spread over a watch glass and dried at 60 C. The nanoparticles were characterized by X-ray diffraction (XRD, Bruker D5005 powder diffractometer, CuKa radiation, l ¼ 1.5406 A), scanning electron microscopy (SEM, Hitachi S800
Fig. 1 e Hydrogen evolution for the Co nanoparticles.
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Fig. 2 e Hydrogen evolution for the various CoCr nanoparticles.
FEG) equipped with energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM, JEOL 1200 EXII), and N2 adsorption/desorption to determine the specific surface area by the BET method. X-ray photoelectron spectroscopy (XPS, performed at “Science et Surface”, in Ecully, France; PHI Quantera SXM spectrometer equipped with an Al Ka, hn ¼ 280 eV, 47.7 W) was also envisaged but the magnetic properties of the particles made the experiment difficult. The Ni foam supported catalysts were characterized by XRD and EDS. Ammonia borane (SigmaeAldrich) was stored in an argonfilled glove box (MBraun M200B, H2O < 0.1 ppm, O2 < 0.1 ppm) to prevent the sample from moisture and air. The hydrogen generation measurement by hydrolysis of AB was performed as follows. The catalyst (15 mg) was introduced into the reactor consisting of a glass tube (27.5 mL) sealed with a silicon septum. The reactor was then placed in a water bath thermostated at 50 C and connected to a water-filled inverted burette. An acidic and a cold trap were put between the reactor and the burette to condensate ammonia and steam. In parallel, an aqueous solution of NH3BH3 (1 M) was prepared. To start the hydrolysis, 4 mL of the NH3BH3 solution (ca. 123 mg of hydride) was injected into the reactor and the H2 evolution was measured. The experiment was video-recorded in order that the H2 generation was analyzed afterwards. The hydrogen generation rates were calculated by dividing the H2 volume by the time.
evolution curve (mol H2 per mol NH3BH3 as a function of time) obtained with Co is shown in Fig. 1. It will be used for comparison purpose, that is, as reference to screen the various bimetallic systems. Note that all of the catalysts tested in the present work totally converted AB, leading to the evolution of 3 mol H2 per mol NH3BH3.
3.1.2.
3.1.3.
3.
Results and discussion
3.1.
Screening
3.1.1.
Co as reference
To dope the catalytic activity of Co, we pitched on adding a second transition metal to get bimetallic nanoparticles. Several metals from the 1st row of the periodic table as well as from the column IVB were selected. Our first attempts focused on Cr, Cu, Zr and Hf. We chose Cr and Cu because computational works showed that Co would segregate and antisegregate, respectively, when combined with them [33]. We chose Zr and Hf because in preliminary works [30] we observed positive effects on the catalytic reactivity of Co in their presence. The bimetallic CoCr, CoCu, CoZr and CoHf were then prepared and their reactivity in hydrolysis of AB was compared to that of Co. The H2
CoCr
Fig. 2 shows the results obtained with the catalysts CoCr. All suffers from an induction period of several minutes that delays the starting of the hydrolysis (Table 1). This induction period is typical of catalysts which surface has been oxidized. During this period, the catalytically active phase (surface) of the catalyst forms by reduction in the presence of the borane [34]. Compared to Co, the addition of Cr does not reduce or hinder the occurrence of such a period; it seems to make it longer. The reactivity of the various CoCr was compared by calculating the hydrogen generation rate. This was done by dividing the volume of generated H2 by time. The ranges 0e0.5 and 2.5e3 mol H2 per mol AB were not taken into account in the calculation. The rates are presented in Table 2. Except for CoCr 50:50, the hydrogen generation rates are higher than that of Co (11.5 mL H2 min1 or 0.8 L H2 min1 g1 Co). Combining Cr with Co has thus a positive effect.
CoCu
Fig. 3a shows the results obtained with the catalysts CoCu. The addition of Cu to Co is beneficial to the catalytic reactivity (Table 2) and in reducing the induction time (Table 1). As shown in Table 2, hydrogen generation rates varying from 14 to 41.6 mL H2 min1 (or ca. 0.9 to 2.8 L H2 min1 g1 CoCu) were found, with the best performance found for CoCu 70:30. This catalyst has a hydrogen generation rate 3.6
Table 1 e Induction times in min measured for the different cobalt-based bimetallic catalysts.
CoCr CoCu CoZr CoHf
min min min min
95:5
90:10
80:20
70:30
50:50
6.3 1 0.7 2.3
2.7 0.3 1.3 2.3
1.8 0.1 1.7 1.7
3.7 0.5 3.3 1.3
7.3 0.2 3.8 1.3
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Table 2 e Hydrogen generation rates in mL H2 minL1 and mL H2 minL1 gL1 CoM (M [ Cr, Cu, Zr or Hf) measured for the different cobalt-based bimetallic catalysts.
CoCr CoCu CoZr CoHf
mL mL mL mL mL mL mL mL
min1 min1 g1 min1 min1 g1 min1 min1 g1 min1 min1 g1
95:5
90:10
80:20
70:30
50:50
20.4 1.4 14 0.9 27.5 1.8 10.9 0.7
17.3 1.5 27.5 1.8 10.5 0.7 14.4 1
19.5 1.3 34.6 2.3 16.3 1.1 15.6 1
26.2 1.7 41.6 2.8 15.3 1 17.4 1.2
12 0.8 22.2 1.5 14 0.9 19.6 1.3
times higher than that of Co. The rate is comparable to that of Co35Pd65 reported by Sun et al. [35] though the experiments were realized at slightly different temperatures (40 vs. 50 C in our case). The reactivity is also high with CoCu 80:20, having a hydrogen generation rate 3 times higher than that of Co. Fig. 3b reports the hydrogen generation rates and shows their variation as a function of the Cu (or Co) content. The variation has a volcano-shape. Such behavior is related to the Sabatier principle, stating that there is a strong dependence between optimal catalytic activity (and thus the best bimetallic catalyst) and intermediate binding energies for reaction intermediates on a catalytic surface [36]. Such a behavior was also reported for CoPd bimetallic systems [35]. In other words,
the presence of Cu in CoCu 70:30 would provide the optimum surface and catalytic features in terms of activation barrier and intermediates adsorption. This result is a further evidence of that combining two metals may lead to synergy and thus to significant reactivity improvement. Another attractive feature with CoCu is that the induction period is almost suppressed.
3.1.4.
CoZr
Fig. 4 shows the results obtained with the catalysts CoZr. Like for Cr, the presence of Zr does not permit to hinder the induction period (Table 1). At best, it is reduced of few tens of seconds, as it is the case for CoZr 90:10. With the other bimetallic combinations, the induction period in rather lengthen. For example, it is of ca. 3 min for CoZr 70:30 whereas it is of 1.5 min for Co. With respect to the reactivity, the results are mitigated. The calculations of the hydrogen generation rates gave the values in Table 2. The results are nevertheless globally improved in comparison to the hydrogen generation rate found for Co (11.5 mL H2 min1).
3.1.5.
CoHf
Fig. 5 shows the results obtained with the catalysts CoHf. In a first approximation, we can notice that the situation is quite similar to that observed with the catalysts CoZr. The addition of Hf to Co has a positive effect, but it is slight. The induction period is as long as with Co (Table 1). It is however lengthened with CoHf 90:10. During the evolution of the 1st mol H2 per mol
Fig. 3 e Hydrogen evolution for the various CoCu nanoparticles (a) and evolution of the hydrogen generation rates as a function of the metals content.
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Fig. 4 e Hydrogen evolution for the various CoZr nanoparticles.
AB, Co and CoHf show quite similar kinetics. However, after that, the H2 evolution in the presence of Co slows down whereas it remains almost constant with CoHf, except for CoHf 50:50. The hydrogen generation rates found are reported in Table 2.
3.1.6.
Analysis and selection
The results reported heretofore represent a further evidence of how the catalytic reactivity of Co can be improved by adding a second metal. In the open literature, Lu et al. [37] recently showed that the bimetallic AuCo supported over silica nanospheres could be more reactive, owing to a synergetic interaction, than the monometallic counterparts in hydrolysis of AB. It was suggested that Au in AuCo@SiO2 plays a role in the activation of the catalyst. Qiu et al. [38] reported the synthesis and reactivity of a series of FeCo nano-alloys that displayed improved dehydrogenation properties in comparison to monometallic Co and Fe. It was shown that in the alloys, Co is electron-enriched while Fe is electron-deficient, indicating that the better reactivity might be ascribed to electronic effects. As commented by Yang et al. [26], the bimetallic catalysts have the ability to weaken the bonding of hydrogen and ammonium to the surface atoms, leading to an accelerated release of hydrogen. This is related to the Sabatier principle. Combining one metal with another one, ideally alloying them, may lead to improved reactivity and this may be attributed to electronic effects. Hammer and Nørskov [33] showed that the
D-band
center of a given metal could be shifted up or down when combined with another one. An up-shift leads to stronger adsorption of molecules and a down-shift to the reverse. For example, Co combined with Au, Fe or Cu should have higher-lying d states than pure Co and thus should bind more strongly the adsorbate. This is consistent with the electron-enrichment reported by Qiu et al. [38] for the FeCo nano-alloys. In the present work, electronic effects may account to the improved reactivity of CoCu. Another effect that may help to explain an improved reactivity (but also a deteriorated reactivity) is of geometric nature. This is segregation. Nørskov and co-workers [33,36] reported a useful table to predict the segregation of a given metal when combined or alloyed with another one. It is shown that Cr should moderately antisegregate in the presence of Co, implying then that the surface would be Co-enriched. This does not really help to explain the discrepancy in the reactivity of the CoCr combinations. On the other hand, Cu, Zr and Hf should segregate in, respectively, moderate, strong and moderate ways, then favoring the formation of smaller Co particles on the surface. This may rationalize the improved performance of CoCu. This prediction is also rather consistent with the improvement of the reactivity of CoHf when the Hf content is increased. The more Hf is, the smaller the Co particles on the surface should be, and the more active Co sites there should be. With respect to CoZr, no trend stands out and the segregation phenomenon does not seem to be the critical
Fig. 5 e Hydrogen evolution for the various CoHf nanoparticles.
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Fig. 6 e Arrhenius treatment for CoCu 70:30 over the temperature range 30e55 C.
factor. A systematic study focusing on the analysis of the electronic configuration of the metals and the surface composition of the nanoparticles might help to gain insight about such electronic and geometric effects. This could be envisaged in another work as this is out of the scope of the present paper. From the results reported heretofore, we selected CoCu to be used in the continuation of the present work, and the catalyst with the composition 70:30 was characterized.
3.2.
Characterization of CoCu
In addition to the H2 generation experiment performed at 50 C with CoCu 70:30, three other ones were carried out at 30, 40 and 55 C in order to determine the apparent activation energy by Arrhenius treatment (Fig. 6). Upon the completion of the induction period, the H2 evolution is linear, implying zero-order kinetics, that is, a reaction rate independent on the concentration of the AB. The hydrogen generation rates were determined over the range 0.5e2.5 mol H2 per mol AB and the Neperian logarithm of these values were plotted as a function of the reverse of the temperature (in K). The slope (a) was exploited to calculate the apparent activation energy (such as a ¼ Ea/R). An apparent activation energy of 60.8 kJ mol1 was found. Taken into consideration the experimental error on the volume of H2 (3 mL) as well as that on the reaction
temperature (1 C), an error of 1.6 kJ mol1 was determined for the apparent activation energy. This energy value was compared to some reported recently in the literature for bimetallic catalysts: i.e. 16e34 kJ mol1 for FeCo nano-alloys [38]; 27.5 kJ mol1 for Co35Pd65 [35]; 39e44 kJ mol1 for PtNi [26]; 41.5 kJ mol1 for carbon supported Co core-Pt shell nanoparticles [39]; 51.5 kJ mol1 for NiAg [40]; 52.8 kJ mol1 for Co nanoparticles inside hydrogen networks prepared from 2acryl-amido-2-methyl-1-propansulfonic acid [41]; and 54.7 kJ mol1 for CoeNieP/PdeTiO2 [42]. It is likely that the improvement in terms of activation energy is less pronounced with CoCu but the discrepancies in the experimental conditions might also explain the difference. The crystallinity of CoCu 70:30 was analyzed by powder XRD (Fig. 7). Unlike Co [43], few diffraction peaks, with the most intense centered at 2q ¼ 36.6 , were observed. Using the software EVA, they were ascribed to copper (I) oxide Cu2O (cuprite, ICDD 00-003-0898). Note that the peak at 2q ¼ 36.6 could also be attributed to cobalt oxide (Co2.62O4 ICDD 01-00785625 or Co2.87O4 ICDD 01-078-5630) and/or copper cobalt oxide ((Cu0.3Co0.7(Co2O4)) ICDD 00-025-0270) but the other smaller peaks were shifted of few degrees compared to the reference patterns. Nevertheless, their presence, in small amounts, is not completely discarded. The surface morphology of CoCu 70:30 was scrutinized by SEM and compared to that of Co (Fig. 8). No appreciable
Fig. 7 e XRD pattern for CoCu 70:30.
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Fig. 8 e SEM (a, b) and TEM (c, d) images of Co and CoCu 70:30, respectively.
difference was noticed. For both samples, large particles were observed, certainly consisting in aggregated particles of much smaller sizes. This was confirmed by TEM (Fig. 8). The nanoparticles of Co and CoCu have a size of about 20 nm, are slightly agglomerated and are embedded in a gangue, certainly consisting of borates as generally reported for Co nanoparticles [43]. Surface specific areas of 133 and 106 m2 g1 were found for Co and CoCu. Such higher areas may be explained by the presence of the gangue. The sample CoCu 70:30 was in addition analyzed by EDS (Fig. 9). The mol ratio 70:30 was confirmed and the mapping showed that the distribution of both metals was homogenous.
3.3.
Supported CoCu
3.3.1.
CoCu supported on Ni foam
In this work, Ni foam was selected as it is an ideal material for mass transportation owing to its porous structure. CoCu was deposited over pretreated Ni foam by applying a two-stage procedure. Co was first electrodeposited. Then, Cu was deposited by electroless deposition over the surface of Co/Ni. For comparison purpose, Cu/Ni and Co/Ni were also prepared by electrodeposition. Simultaneous electrodeposition of CoCu using an aqueous solution containing both metal salts was avoided because of the immiscibility of both metals by this
preparation way [44]. Here, our objective was to assess the scalability and the shaping of our selected bimetallic catalyst. The underlying objective was thus to validate our bottom-up approach to prepare supported, shaped catalysts. The as-obtained supported catalysts as well as pretreated Ni foam (as reference) were analyzed by XRD (Fig. 10). To start, the peaks of Ni foam were indexed. Co/Ni displays the same peaks and the pattern is similar to a previous report [32]. The diffraction peaks at 2q ¼ 44.1, 51.4 and 75.6 may be attributed to Ni (ICDD 04-010-6148) but also to face-centered Co (00-015-0806). With respect to Cu/Ni, 6 diffraction peaks were observed. The substrate Ni foam diffracted as characterized by the peaks at 2q ¼ 44.3, 51.7 and 76.2 . The three other peaks, at 2q ¼ 43.2, 50.2 and 73.5 , are ascribed to facecentered Cu (ICDD 04-009-2090) though there is a down-shift of ca. 0.38 compared to the reference pattern. These patterns were then compared to that of CoCu/Ni. On the one hand, in terms of diffraction peaks, the pattern of CoCu/Ni shows all of the peaks observed in Cu/Ni, suggesting the presence of face-centered Cu. Nevertheless, the presence of facecentered Co could not be discarded. On the other hand, in terms of diffraction angles, CoCu/Ni shows a shift of 04e07 to lower angles for all of the 6 main peaks. This may be indicative of the strain effect of e.g. Co atoms in the Cu layers and therefore of the formation of a solid solution. The
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Fig. 9 e Presence of Co and Cu in CoCu 70:30 determined by EDS. The image at left shows the area analyzed.
Fig. 10 e XRD patterns of Ni foam, Co/Ni, Cu/Ni and CoCu/Ni.
presence of both Co and Cu in CoCu/Ni was otherwise verified by EDS (Fig. 11). The catalytic activity of CoCu/Ni was assessed. Fig. 12 reports the results. They are compared to those of Co/Ni and Cu/ Ni. Consistently with the experiments using the powdery catalysts, CoCu/Ni, where both metals are present, is more active than the monometallic counterparts Co/Ni and Cu/Ni. It is showed that 3 mol H2 are released within 15, 60 and 80 min, respectively. The hydrogen generation rates were calculated and found to be 25.1, 4.4, and 5.6 mL H2 min1, respectively. Surprisingly, the hydrogen generation rate of Cu/Ni is almost 4 times higher than that of Cu. This suggests that the presence of Ni in the supported catalyst has an effect on the catalytic activity of Cu, perhaps due to electronic effects [33,36]. With respect to CoCu/Ni and Co/Ni, the hydrogen generation rates are lower than those found for the powdery catalysts, but remain quite attractive, especially in the case of CoCu/Ni. The
hydrogen generation rate of the bimetallic system is comparable to that of CoCu 90:10. The difference in reactivity may be ascribed to the difference in morphology as the nanoparticles should show more surface active sites than a layer over the Ni foam. In our conditions, the reactivity of CoCu/Ni is comparable to that of CoePeB/Ni reported elsewhere [45] though the temperature for the latter system is 10 C lower. The authors suggested that the role of B was to transfer electronic density to Co and that of P to favor the segregation and surface enrichment of Co. There are electronic and geometric effects, respectively. Compared to CoeWeBeP/Ni prepared by Yang et al. [46], the hydrogen generation rate displayed by CoCu/Ni is slightly lower, namely 25.1 at 50 C vs. 28 mL H2 min1 and 34 mL H2 min1 at 45 and 55 C. In fact, the presence of B and P is crucial to create more Co active sites and the addition of W is important to prevent Co from oxidation. Improved
Fig. 11 e Presence of Co and Cu in CoCu 70:30 determined by EDS. The image at left shows the area analyzed.
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Fig. 12 e Hydrogen evolution for Co/Ni, Cu/Ni and CoCu/Ni.
performance might be achieved with our CoCu/Ni by considering addition of such elements. This is a prospect of catalyst development.
3.3.2.
CoCu dispersed in a plastic thin film
Among the various strategies we have in finding reactive supported, shaped catalysts, we present herein a new one. The present experiment had the objective to validate our concept. Generally, the active phase, i.e. Co or CoCu, is supported over a plate, foam, membrane or monolithic structure [30]. These are the strategies we use to develop in the lab. Recently, we envisaged a novel strategy. Our main idea was to exploit the aforementioned CoCu particles. After synthesis, the batch was divided into two samples. The first one was dedicated to be studied directly as catalytic particles in hydrolysis of AB. The second was used to prepare a shaped catalyst using plastic films. Plastic films are attractive materials owing to their lightness, easy-handling, relative thermal stability, relative chemical stability and solubility in various organic solvents. After testing several polymeric materials, a mixture of polymethyl methacrylate and polyvinylchloride was chosen. Their respective proportion was varied and optimized. We found that a weight ratio 50:50 in dimethylacetamide was a good compromise in terms of viscosity, handling, spreading and drying. Hence, according to the preparation procedure reported
in the Experimental section, we prepared a shaped metalplastic catalyst where the CoCu particles were dispersed into the plastic film. We preferred to disperse the nanoparticles in the slurry (before drying) rather than impregnating the metal salts onto the dried plastic film. The as-formed catalyst is denoted CoCu/PMMA-PVC. After evaporation of the solvent, the catalyst was tested in hydrolysis of AB. Fig. 13 shows the evolution of H2 (mol per mol AB) as a function of time. The CoCu/PMMA-PVC material is reactive in hydrolysis of AB, with 3 mol H2 being released within 120 min. The hydrogen generation rate is 1.5 mL H2 min1. This low rate is easily explained by the features of the film. In this first attempt, the film was smooth, without porosity. Most of the particles were embedded in the film, making the accessibility of these catalytically active sites hard for the AB molecules. This first attempt was in fact a feasibility test of such catalytic films. The main advantages are as follows. No degradation of the film was observed during the reaction. It was easily separated from the reaction medium, washed and dried. Works are in progress to enhance the catalytic activity by avoiding the formation of embedded particles and favoring their deposition on the film surface. Plastic films with micrometric holes are also under development. Our ultimate goal is to propose a supported catalyst very easy to handle, as reactive as the nanoparticles it may contains, and that can be used in various catalytic reactions using liquid media.
Fig. 13 e Hydrogen evolution for CoCu 70:30 embedded in the plastic film.
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4.
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Conclusion
In the present work, we have reported, for the first time in hydrolysis of ammonia borane, a bottom-up approach to elaborate cobalt-based bimetallic catalysts. The study was performed according to three-stage consisting in synthetizing cobalt-based bimetallic catalysts, using the best bimetallic combination to prepare a Ni foam-supported catalyst (i.e. shaped catalyst), and using the best bimetallic combination to elaborate a new type of shaped, plastic-based catalyst. Shaped catalysts are of high importance for reactor scale. The first objective was thus first to screen bimetallic nanoparticles using Cr, Cu, Zr and Hf as the second metal M. Depending on the second metal, the gains in reactivity for CoM in comparison to Co were different. The best results were found for CoCu. The reactivity, plotted as a function of the Co content, showed to have a volcano shape peaking for CoCu with a ratio 70:30. The improved reactivity was ascribed to electronic and geometric effects. The second objective was thus to prepare a supported, shaped catalyst while using Ni foam as support. CoCu supported over Ni foam was prepared according to a two-step process consisting of the electrodeposition of cobalt followed by electroless deposition of copper. The reactivity of CoCu/Ni in hydrolysis was compared to the monometallic counterparts. It was found that by combining both metals the dehydrogenation properties in terms of hydrogen generation rates could be substantially improved. Better results could afterwards achieved by adding a third metal or making the surface rougher or porous. Therefore, Ni foam may be considered as a material of choice for elaborating shaped catalysts. The third objective was thus to prepare another shaped catalyst, using a plastic film as matrix of the CoCu nanoparticles. Though the reactivity was found to be lower than for the other catalysts investigated, the conversion of ammonia borane was of 100%. The present experiment had the objective to validate our concept. The under-lying objective was to show that a plastic film, which is light, easy to handle and to prepare, could act as a stable support. No degradation was observed during the hydrolysis reaction and while handling it after reaction. The challenge is now to improve the reactivity of the supported catalyst by better controlling the insertion of the nanoparticles and then providing porosity to the film. The following strategies could be explored: e.g. deposition of the nanoparticles on the plastic film surface by different techniques (impregnation, precipitation.); microemulsion to prepare through a one-stage process the nanoparticles-doped plastic film; use of sacrificial reagents or structure to get the expected porosity in the plastic film. In our opinion, this is all the more important that such supported films could be envisaged in various catalytic reactions using liquid or gaseous reaction media.
Acknowledgments The present work was conducted within the frame of the project Jeunes Chercheurs et Jeunes Chercheuses BoraHCx
and was funded by the Agence Nationale pour la Recherche (ANR). UBD is grateful.
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