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High surface area niobium oxides as catalysts for improved hydrogen sorption properties of ball milled MgH2
Journal of Alloys and Compounds 460 (2008) 507–512
High surface area niobium oxides as catalysts for improved hydrogen sorption properties of ball milled MgH2 V.V. Bhat a , A. Rougier a,∗ , L. Aymard a , G.A. Nazri b , J.-M. Tarascon a a
Laboratoire de R´eactivit´e et Chimie des Solides, UMR 6007, Universit´e de Picardie Jules Verne, 33 rue St Leu, 80039 Amiens, France b General Motors, R&D, Warren, MI, USA Received 6 April 2007; received in revised form 17 May 2007; accepted 23 May 2007 Available online 29 May 2007
Abstract We report, high surface area (up to 200 m2 /g) nanocrystalline niobium oxide (so called p-Nb2 O5 ) synthesized by ‘chimie douce’ route and its importance in enhancing the hydrogen sorption properties of MgH2 . p-Nb2 O5 induces faster kinetics than commonly used commercial Nb2 O5 (c-Nb2 O5 ) when ball milled with MgH2 (named (MgH2 )catalyst ) by reducing the time of desorption from 35 min in (MgH2 )c-Nb2 O5 to 12 min in (MgH2 )p-Nb2 O5 at 300 ◦ C. The BET surface area of as-prepared Nb2 O5 was tuned by heat treatment and its effect on sorption properties was studied. Among them, both p-Nb2 O5 and Nb2 O5 :350 (p-Nb2 O5 heated to 350 ◦ C with a BET specific surface area of 46 m2 /g) desorb 5 wt.% within 12 min, exhibiting the best catalytic activity. Furthermore, thanks to the addition of high surface area Nb2 O5 , the desorption temperature was successfully lowered down to 200 ◦ C, with a significant amount of desorbed hydrogen (4.5 wt.%). In contrast, the composite (MgH2 )c-Nb2 O5 shows no desorption at this “low” temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Magnesium hydride; Hydrogen storage; Catalysis; BET specific surface area; Niobium oxide
1. Introduction The polymer electrolyte membrane fuel cell is an efficient electrochemical energy conversion device, and it is claimed to be nearing on-road application [1]. However, the complete onroad realization can only be achieved after solving the issues of the storage and infrastructure of hydrogen fuel [2–6]. For hydrogen storage, metallic hydrides with high storage capacity are preferred to the compressed or liquid hydrogen, due to high volumetric capacity, safety concern and convenience. The U.S. Department of Energy (DOE) has suggested the need for hydrogen storage materials with more than 6 wt.% capacity and fast desorption kinetics (1.5 wt.%/min), below 150 ◦ C, for a viable hydrogen storage system in fuel cell applications [7]. Till today, most known metallic hydrides have either high capacity, fast kinetics, or suitable operating temperature but none a combination of all.
∗
Corresponding author. Tel.: +33 322 827604; fax: +33 322 827590. E-mail address: [email protected] (A. Rougier).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.05.084
Various hydrogen-containing systems [8–12] are currently under intense investigation. Among them, MgH2 [9] has emerged as a promising material due to its high theoretical hydrogen storage capacity (7.6 wt.%), abundance, low cost and environmental acceptance. However, slow kinetics and high operating temperature (>300 ◦ C), due to high thermodynamic stability (H = −75 kJ/mol H2 ), remain the two main limiting factors for its use as a practical hydrogen storage material. In recent years, significant improvements in sorption kinetics have been reported thanks to nanocrystalline MgH2 preparation using high energy ball milling [13], and the addition of a large range of materials including high surface reactive carbon [14,15], transition metal [16–20], oxides [21–23] and halides [24,25]. Among a large range of metal oxides, Barkhordian et al. identified Nb2 O5 as the most promising catalyst leading to a MgH2 composite with 0.2 mol % Nb2 O5 desorbing close to 7 wt.% of hydrogen within 130 s at 300 ◦ C [22]. Since their work, numerous studies have debated the mechanism occurring during the sorption and the role of transition metal oxides as catalyst [26,27]. Aiming at understanding the role of the chemical nature of the catalyst, we embarked in a very large screening of materials
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from oxides, halides and metals. Among acidic oxides, Nb2 O5 remains by far the “best”. Inspired from earlier work in our group on the synthesis of nano-oxides using “chimie douce” techniques [28], high surface area Nb2 O5 was successfully synthesized by precipitation route. Its specific surface area was then simply tuned by annealing at various temperatures. Herein, the influence of the BET specific surface area of the Nb2 O5 catalyst on the sorption properties of MgH2 is discussed. 2. Experimental Five batches of 2 g of commercial MgH2 ((MgH2 )com ) (Goldsmith, with ∼5% Mg) were separately ground in hardened stainless steel containers for 38 h under 2 bars hydrogen pressure using a Spex 8000 ball mill. After ball milling, all the batches were mixed together and milled again for 38 h, so that a larger batch of starting material with uniform properties could be used for catalyst addition studies. The sample is named (MgH2 )bm . High specific surface area niobium oxide-based catalysts were prepared by precipitation route, derived from Co3 O4 synthesis technique [28] as follows: 2 g of Na2 S2 O8 were added to 100 ml of a boiling aqueous solution containing 1 g NaOH. Immediately, 100 ml ethanol, in which 2 g NbCl5 were previously dissolved, were added to this solution. A white precipitate was formed. The precipitate was filtered and washed with distilled water until the precipitate was free from Cl− ions, as confirmed by AgNO3 test. Later, the powder was dried at 100 ◦ C for 4 h. This product identified as p-Nb2 O5 was heated at different temperatures (350, 450, 550, 650, and 750 ◦ C) for 1 h. Herein, the samples are named Nb2 O5 :xxx, where xxx stands for the temperature of heat treatment, e.g. the sample treated at 650 ◦ C is named Nb2 O5 :650. (MgH2 )bm was mixed with 0.2 mol% of catalyst and ball milled for two more hours. The samples are named (MgH2 )catalyst (catalyst = p-Nb2 O5 , Nb2 O5 :350, Nb2 O5 :450, Nb2 O5 :550, Nb2 O5 :650, Nb2 O5 :750 and commercial Nb2 O5 (c-Nb2 O5 )). The crystallinity and phase purity of the samples were identified by X-ray diffraction (XRD) using a Philips diffractometer (θ–2θ configura˚ tion and (Cu K␣, 1.5418 A)). The ratio of the /␥ phases was estimated by considering the area under the 100% intensity peak using a PseudoVoigt function. The BET specific surface area of heat-treated catalysts and commercial Nb2 O5 (c-Nb2 O5 ) was measured using a Micrometrics Gemini instrument. The hydrogen sorption behavior of the powders was investigated using a Hiden IGA thermobalance. Desorption was carried out under primary vacuum and absorption under 15 bar pressure of a high purity hydrogen gas at different operating temperatures (200, 250 and 300 ◦ C). The rate of sorption was calculated from the sorption curves by fitting a linear equation between 20 and 80% of sorption. In the following, the results in terms of capacity and kinetics are underestimated as the sample was handled in air for its transfer from the glove box to the Hiden apparatus. Temperature programmed desorption (TPD) of both ball milled sample and the sample obtained after sorption studies were performed using a mass spectrometer coupled with a programmable furnace under dynamic heating conditions (10 ◦ C/min).
Fig. 1. Evolution of the BET specific surface area of heat-treated niobium oxide catalysts vs. the annealing temperature.
samples heated above 550 ◦ C, XRD peaks corresponding to the well-crystallized Nb2 O5 (JCPDS No. 30-0873) appear. Further increase in temperature enhances the crystallinity. Nb2 O5 :650 consists of a single phase, while Nb2 O5 :750 is a mixture of two highly crystalline Nb2 O5 phases, with a good match with the standard XRD cards (JCPDS Nos. 30-0873 and 32-0711) for c-Nb2 O5 . 3.2. (MgH2 )catalyst characterization Ball milled MgH2 , (MgH2 )bm presents broad X-ray diffraction peaks of -MgH2 (space group: P4/2 mnm) and diffraction lines corresponding to the high pressure metastable ␥-MgH2 (space group: Pbcn) in a 85:15 ratio (Fig. 3). Around 5% Fe impurities are also detected, which is due to the high energy ball milling in stainless steel container. The grinding of MgH2 with catalyst leads to a significant decrease in the XRD peak
3. Results and discussion 3.1. Catalyst Characterization The BET specific surface area of p-Nb2 O5 sharply drops from 200 to 46 m2 /g after heat treatment at 350 ◦ C (Fig. 1). Further annealing continuously decreases the specific surface area, which diminishes to 2 m2 /g after heat treatment at 750 ◦ C, being close to that of commercial oxide (1.5 m2 /g). For p-Nb2 O5 samples heat treated up to 450 ◦ C, the XRD profiles present two broad humps in the 10–40◦ and 40–70◦ (2θ Cu ) range, indicating the nanocrystalline nature of the niobium oxides (Fig. 2). For the
Fig. 2. XRD of Nb2 O5 -based catalysts prepared by precipitation technique and heat-treated at different temperatures.
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Fig. 4. Hydrogen desorption kinetics of (MgH2 )bm at 300 ◦ C under vacuum for the first four cycles.
Fig. 3. XRD patterns of (MgH2 )bm and (MgH2 )catalyst .
3.4. Sorption properties of (MgH2 )catalyst intensity of the -MgH2 phase associated with an increase in the (/␥) ratio from 85/15 to about 70/30 (Fig. 3). As a matter of fact, the BET specific surface area of MgH2 increases from 7 to 16 m2 /g after ball milling with p-Nb2 O5 (Table 1). The diffraction patterns of (MgH2 )Nb2 O5 :650 and (MgH2 )c-Nb2 O5 show the presence of a peak located at 22.72◦ (2θ Cu ) indexed as (0 0 1) of Nb2 O5 (JCPDS No. 30-0873).
At 300 ◦ C, irrespective of the catalyst presence, the hydrogen desorption initiates at around 1000 mbar (Fig. 5). In addition, the increase in the required pressure from 1 to 300 mbar to desorb
3.3. Sorption properties of (MgH2 )bm with cycling Since (MgH2 )com requires more than 100 min to desorb 0.5 wt.%, we only present the sorption kinetics of (MgH2 )bm at 300 ◦ C (Fig. 4). The first cycle requires about 90 min to desorb 4 wt.% of hydrogen. In the second cycle, nearly 3.6 wt.% hydrogen is desorbed within 35 min and 1 wt.% later. In the third and consecutive desorption cycles, desorption occurs in a single step within 35 min, accounting for 5 wt.% of hydrogen. The cause for this kind of improvement in sorption kinetics with cycling is attributed to the activation due to the formation of micro/nanocracks during cycling [25]. After cycling, the BET specific surface area of (MgH2 )bm increases from 7 to 14 m2 /g (Table 1).
Fig. 5. Pressure dependent sorption properties of (MgH2 )bm and selected (MgH2 )catalyst .
Table 1 BET specific surface area of Nb2 O5 based catalysts, absorption and desorption rates of (MgH2 )bm and (MgH2 )catalyst at 300 and 250 ◦ C Catalyst
p-Nb2 O5 Nb2 O5 :350 Nb2 O5 :450 Nb2 O5 :550 Nb2 O5 :650 Nb2 O5 :750 c-Nb2 O5
SBET (m2 /g) (±0.5)
Catalyst + MgH2 (MgH2 )cat
Desorption rate (wt.%/min)
Absorption rate ((wt.%/min)
SBET (m2 /g) (±0.5
300 ◦ C
250 ◦ C
250 ◦ C
300 ◦ C
Before cycling
After cycling
201 46 34 22 9 2 1.5
(MgH2 )bm (MgH2 )p-Nb2 O5 (MgH2 )Nb2 O5 :350 (MgH2 )Nb2 O5 :450 (MgH2 )Nb2 O5 :550 (MgH2 )Nb2 O5 :650 (MgH2 )Nb2 O5 :750 (MgH2 )c-Nb2 O5
0.15 0.54 0.71 0.64 0.48 0.50 0.48 0.30
0.01 0.17 0.20 0.20 0.14 0.12 0.13 0.08
0.06 0.17 0.27 0.26 0.26 0.23 0.29 0.20
0.18 0.22 0.23 0.23 0.22 0.23 0.24 0.22
7.3 15.7 16.1 14.1 15.1 15.2 13.6 1.02
14.2 20.9 20.1 18.3 18.8 17.2 16.7 15.6
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0.5 wt.% of hydrogen shows enhanced kinetics with the catalyst addition. For all (MgH2 )catalyst samples, the full capacity was achieved after the second cycle, corresponding to a shorter activation period than for (MgH2 )bm . Similar to (MgH2 )bm , the trend of increasing the specific surface area is also recorded in (MgH2 )catalyst , nevertheless with smaller enhancement (Table 1). For the sake of comparison, sorption properties were further considered after the third cycle. Regarding capacity, (MgH2 )bm desorbs around 4.6 wt.% of hydrogen within 35 min (Fig. 6a). Among the various catalysts, at 300 ◦ C, (MgH2 )Nb2 O5 :350 presents the best activity with a capacity of 5.2 wt.% desorbed within 12 min, whereas (MgH2 )c-Nb2 O5 shows the slowest kinetics with a desorption of 4.8 wt.% in more than 30 min (Fig. 6a). Similar behavior is observed with decreasing the temperature down to 250 ◦ C (Fig. 6b) however with slower kinetics. For instance, the complete desorption of (MgH2 )Nb2 O5 :350 requires nearly 20 min as compared to 12 min at higher temperature. The decrease in the desorption temperature from 300 to 250 ◦ C is also correlated to a decrease in the desorption initiation pressure from 1000 mbar down to 120 mbar (Fig. 6c and d). Interestingly, the corresponding pressure dependence shows that more than 50% of hydrogen desorption occur above 1 mbar representing a faster desorption in vacuum. A comparison of the desorption rates for (MgH2 )bm with (MgH2 )catalyst is given in Table 1. As expected, among all of the catalysts, Nb2 O5 :350 leads to the fastest kinetics (i.e. the highest desorption rates). The catalytic activities of p-Nb2 O5 and Nb2 O5 :450 are comparable whereas the catalytic activity of heat-treated catalysts above 550 ◦ C systematically decreases. The benefit of using nano-catalyst was
recently confirmed by Friedrichs et al. [29], who report a significant reduction of milling time and a 60 ◦ C decrease in desorption temperature using Nb2 O5 nanoparticles (15 nm) as compared to micrometrics ones. In the absorption process, the weight gain initiates around 3000 mbar at 300 ◦ C for all samples (Fig. 7a). Under dynamic pressure increase conditions, nearly 20 min are required to complete 90% of absorption, i.e. below 6500 mbar H2 pressure. At 250 ◦ C, the absorption is initiated at a pressure of 1000 mbar for both (MgH2 )bm and (MgH2 )catalyst . For the two temperatures, no significant variation in the absorption time is observed between (MgH2 )bm and (MgH2 )catalyst indicating that under dynamic pressure increase conditions, the effect of catalysts on absorption kinetics is hardly noticeable. Simultaneously, at 300 ◦ C, among (MgH2 )catalyst , (MgH2 )p-Nb2 O5 and (MgH2 )Nb2 O5 :350 exhibit the highest capacity (5.2 wt.%) and (MgH2 )c-Nb2 O5 the lowest one (∼4.5 wt.%). For temperatures, 300 and 250 ◦ C, the absorption and desorption processes are completely reversible. The XRD patterns of (MgH2 )catalyst recorded after several sorption cycles show the presence of the -MgH2 phase and Mg traces (Fig. 8). The X-ray diffraction peaks are sharper and more intense than the ones before cycling. However, the XRD profile of (MgH2 )bm still exhibits higher peak intensity than that of the (MgH2 )catalyst . After sorption cycles, a small peak related to c-Nb2 O5 is still visible on the XRD profile of (MgH2 )c-Nb2 O5 , suggesting that Nb2 O5 remains unchanged during the sorption process. Because of its promising performance, the desorption behavior of (MgH2 )Nb2 O5 :350 was investigated at a temperature as low as 200 ◦ C. At this temperature, scarcely reported for MgH2 , (MgH2 )Nb2 O5 : 350 still shows interesting properties with a
Fig. 6. Hydrogen desorption kinetics of (MgH2 )bm and selected (MgH2 )catalyst at (a) 300 ◦ C and (b) 250 ◦ C, and their corresponding pressure (c) and (d) during desorption, respectively.
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Fig. 7. Hydrogen absorption kinetics of (MgH2 )bm and selected (MgH2 )catalyst at (a) 300 ◦ C and (b) 250 ◦ C, and their corresponding pressure (c) and (d) respectively.
desorption of 4.5 wt.% in 175 min (Fig. 9). As expected, the desorption rate is slower than the one recorded at higher operating temperatures (inset Fig. 9). A similar activity was recorded for (MgH2 )p-Nb2 O5 , whereas (MgH2 )c-Nb2 O5 does not present any activity. 3.5. Mechanism As it is well-known, hydrogen desorption from metal hydrides involves mainly two different processes: (i) Mg H
Fig. 8. XRD profiles of (MgH2 )catalyst after sorption cycling at 300 ◦ C.
bond breaking and hydrogen diffusion inside the lattice followed by (ii) desorption through the surface, which requires to overcome a high energy barrier [30]. In the bulk, Mg H bond breaking depends on the thermodynamics of MgH2 and therefore cannot be altered. The diffusion path-length, however, can be decreased by decreasing the grain-size (increasing the specific surface area), thus enhancing the kinetics. Indeed it was clearly demonstrated earlier that Nb2 O5 additives do not alter the bulk properties of MgH2 , but only enhance the kinetics by interfa-
Fig. 9. Hydrogen desorption properties of (MgH2 )Nb2 O5 :350 under vacuum at various temperatures. The inset presents the evolution of the desorption rate with decreasing operating temperature.
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cial interaction. The limiting step is temperature dependent. At high temperature, the surface energy controls the desorption. On the contrary, at lower temperature, the limiting step is the hydrogen diffusion inside the lattice. Here at 300 ◦ C, we only consider desorption of hydrogen through the surface, emphasizing the importance of surface area and morphology of both the catalyst and MgH2 . Indeed, for the latter the increased surface area upon cycling (from 7 to 14 m2 /g) (i.e. activation) is correlated to enhanced kinetics. As expected, the highest surface area in all (MgH2 )catalysts is observed for p-Nb2 O5 (200 m2 /g) and Nb2 O5 :350 (46 m2 /g) added samples and this results in speedy kinetics. Having demonstrated the importance of the specific surface area of both catalyst and MgH2 , the questions of the chemical nature of the catalyst should be addressed. Interestingly in the literature, for both Nb and Nb2 O5 catalysts, authors tend to agree on the formation of an intermediate oxide phase with MgO present over MgH2 surface [27–29]. These oxides are presumed to transport hydrogen readily compared to MgO and help to enhance the desorption rate. Our work [25] on the additions of halides shows a higher catalytic activity for Nb-X (X = O, Cl, F) compounds as the electronegativity of X increases suggesting the consideration of other mechanisms. One assumption lies on the formation of an intermediate Hδ− Nbδ+ bond favored by the electronic delocalization of the Nb-X one, leading to a further weakening of the Mgδ+ Hδ− bond at the surface. However, the possibility of an intermediate phase formation is under intense investigation, with for instance testing the stability of NbF5 towards Mg [31].
and assisting in experimentation, and M. Nelson for her English support. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
4. Conclusions Addition of p-Nb2 O5 , which was prepared using a solution route in order to favor high specific surface area catalyst provides promising sorption properties for MgH2 at a temperature as low as 200 ◦ C. The accelerated hydrogen sorption kinetics with the catalyst addition may be associated with various parameters. First, the presence of the catalyst leads to a significant decrease in the average particle size and an increase in the BET specific surface area of the (MgH2 )catalyst . Besides, the catalytic activity of p-Nb2 O5 and Nb2 O5 :350 with high surface area is superior to that of low surface area c-Nb2 O5 indicating that the high surface area of the catalysts is an important criterion. We also conclude that the degree of charge distribution of “M-X” bond of the catalysts has a major effect on hydrogen sorption properties of hydrides.
[20] [21] [22] [23] [24]
[25] [26] [27] [28] [29]
Acknowledgements The authors also express their gratitude to X. Darok, R. Herrera-Urbina, M. Kandavel and N. Recham for discussion
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High surface area niobium oxides as catalysts for improved hydrogen sorption properties of ball milled MgH2 V.V. Bhat a , A. Rougier a,∗ , L. Aymard a , G.A. Nazri b , J.-M. Tarascon a a
Laboratoire de R´eactivit´e et Chimie des Solides, UMR 6007, Universit´e de Picardie Jules Verne, 33 rue St Leu, 80039 Amiens, France b General Motors, R&D, Warren, MI, USA Received 6 April 2007; received in revised form 17 May 2007; accepted 23 May 2007 Available online 29 May 2007
Abstract We report, high surface area (up to 200 m2 /g) nanocrystalline niobium oxide (so called p-Nb2 O5 ) synthesized by ‘chimie douce’ route and its importance in enhancing the hydrogen sorption properties of MgH2 . p-Nb2 O5 induces faster kinetics than commonly used commercial Nb2 O5 (c-Nb2 O5 ) when ball milled with MgH2 (named (MgH2 )catalyst ) by reducing the time of desorption from 35 min in (MgH2 )c-Nb2 O5 to 12 min in (MgH2 )p-Nb2 O5 at 300 ◦ C. The BET surface area of as-prepared Nb2 O5 was tuned by heat treatment and its effect on sorption properties was studied. Among them, both p-Nb2 O5 and Nb2 O5 :350 (p-Nb2 O5 heated to 350 ◦ C with a BET specific surface area of 46 m2 /g) desorb 5 wt.% within 12 min, exhibiting the best catalytic activity. Furthermore, thanks to the addition of high surface area Nb2 O5 , the desorption temperature was successfully lowered down to 200 ◦ C, with a significant amount of desorbed hydrogen (4.5 wt.%). In contrast, the composite (MgH2 )c-Nb2 O5 shows no desorption at this “low” temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Magnesium hydride; Hydrogen storage; Catalysis; BET specific surface area; Niobium oxide
1. Introduction The polymer electrolyte membrane fuel cell is an efficient electrochemical energy conversion device, and it is claimed to be nearing on-road application [1]. However, the complete onroad realization can only be achieved after solving the issues of the storage and infrastructure of hydrogen fuel [2–6]. For hydrogen storage, metallic hydrides with high storage capacity are preferred to the compressed or liquid hydrogen, due to high volumetric capacity, safety concern and convenience. The U.S. Department of Energy (DOE) has suggested the need for hydrogen storage materials with more than 6 wt.% capacity and fast desorption kinetics (1.5 wt.%/min), below 150 ◦ C, for a viable hydrogen storage system in fuel cell applications [7]. Till today, most known metallic hydrides have either high capacity, fast kinetics, or suitable operating temperature but none a combination of all.
∗
Corresponding author. Tel.: +33 322 827604; fax: +33 322 827590. E-mail address: [email protected] (A. Rougier).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.05.084
Various hydrogen-containing systems [8–12] are currently under intense investigation. Among them, MgH2 [9] has emerged as a promising material due to its high theoretical hydrogen storage capacity (7.6 wt.%), abundance, low cost and environmental acceptance. However, slow kinetics and high operating temperature (>300 ◦ C), due to high thermodynamic stability (H = −75 kJ/mol H2 ), remain the two main limiting factors for its use as a practical hydrogen storage material. In recent years, significant improvements in sorption kinetics have been reported thanks to nanocrystalline MgH2 preparation using high energy ball milling [13], and the addition of a large range of materials including high surface reactive carbon [14,15], transition metal [16–20], oxides [21–23] and halides [24,25]. Among a large range of metal oxides, Barkhordian et al. identified Nb2 O5 as the most promising catalyst leading to a MgH2 composite with 0.2 mol % Nb2 O5 desorbing close to 7 wt.% of hydrogen within 130 s at 300 ◦ C [22]. Since their work, numerous studies have debated the mechanism occurring during the sorption and the role of transition metal oxides as catalyst [26,27]. Aiming at understanding the role of the chemical nature of the catalyst, we embarked in a very large screening of materials
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from oxides, halides and metals. Among acidic oxides, Nb2 O5 remains by far the “best”. Inspired from earlier work in our group on the synthesis of nano-oxides using “chimie douce” techniques [28], high surface area Nb2 O5 was successfully synthesized by precipitation route. Its specific surface area was then simply tuned by annealing at various temperatures. Herein, the influence of the BET specific surface area of the Nb2 O5 catalyst on the sorption properties of MgH2 is discussed. 2. Experimental Five batches of 2 g of commercial MgH2 ((MgH2 )com ) (Goldsmith, with ∼5% Mg) were separately ground in hardened stainless steel containers for 38 h under 2 bars hydrogen pressure using a Spex 8000 ball mill. After ball milling, all the batches were mixed together and milled again for 38 h, so that a larger batch of starting material with uniform properties could be used for catalyst addition studies. The sample is named (MgH2 )bm . High specific surface area niobium oxide-based catalysts were prepared by precipitation route, derived from Co3 O4 synthesis technique [28] as follows: 2 g of Na2 S2 O8 were added to 100 ml of a boiling aqueous solution containing 1 g NaOH. Immediately, 100 ml ethanol, in which 2 g NbCl5 were previously dissolved, were added to this solution. A white precipitate was formed. The precipitate was filtered and washed with distilled water until the precipitate was free from Cl− ions, as confirmed by AgNO3 test. Later, the powder was dried at 100 ◦ C for 4 h. This product identified as p-Nb2 O5 was heated at different temperatures (350, 450, 550, 650, and 750 ◦ C) for 1 h. Herein, the samples are named Nb2 O5 :xxx, where xxx stands for the temperature of heat treatment, e.g. the sample treated at 650 ◦ C is named Nb2 O5 :650. (MgH2 )bm was mixed with 0.2 mol% of catalyst and ball milled for two more hours. The samples are named (MgH2 )catalyst (catalyst = p-Nb2 O5 , Nb2 O5 :350, Nb2 O5 :450, Nb2 O5 :550, Nb2 O5 :650, Nb2 O5 :750 and commercial Nb2 O5 (c-Nb2 O5 )). The crystallinity and phase purity of the samples were identified by X-ray diffraction (XRD) using a Philips diffractometer (θ–2θ configura˚ tion and (Cu K␣, 1.5418 A)). The ratio of the /␥ phases was estimated by considering the area under the 100% intensity peak using a PseudoVoigt function. The BET specific surface area of heat-treated catalysts and commercial Nb2 O5 (c-Nb2 O5 ) was measured using a Micrometrics Gemini instrument. The hydrogen sorption behavior of the powders was investigated using a Hiden IGA thermobalance. Desorption was carried out under primary vacuum and absorption under 15 bar pressure of a high purity hydrogen gas at different operating temperatures (200, 250 and 300 ◦ C). The rate of sorption was calculated from the sorption curves by fitting a linear equation between 20 and 80% of sorption. In the following, the results in terms of capacity and kinetics are underestimated as the sample was handled in air for its transfer from the glove box to the Hiden apparatus. Temperature programmed desorption (TPD) of both ball milled sample and the sample obtained after sorption studies were performed using a mass spectrometer coupled with a programmable furnace under dynamic heating conditions (10 ◦ C/min).
Fig. 1. Evolution of the BET specific surface area of heat-treated niobium oxide catalysts vs. the annealing temperature.
samples heated above 550 ◦ C, XRD peaks corresponding to the well-crystallized Nb2 O5 (JCPDS No. 30-0873) appear. Further increase in temperature enhances the crystallinity. Nb2 O5 :650 consists of a single phase, while Nb2 O5 :750 is a mixture of two highly crystalline Nb2 O5 phases, with a good match with the standard XRD cards (JCPDS Nos. 30-0873 and 32-0711) for c-Nb2 O5 . 3.2. (MgH2 )catalyst characterization Ball milled MgH2 , (MgH2 )bm presents broad X-ray diffraction peaks of -MgH2 (space group: P4/2 mnm) and diffraction lines corresponding to the high pressure metastable ␥-MgH2 (space group: Pbcn) in a 85:15 ratio (Fig. 3). Around 5% Fe impurities are also detected, which is due to the high energy ball milling in stainless steel container. The grinding of MgH2 with catalyst leads to a significant decrease in the XRD peak
3. Results and discussion 3.1. Catalyst Characterization The BET specific surface area of p-Nb2 O5 sharply drops from 200 to 46 m2 /g after heat treatment at 350 ◦ C (Fig. 1). Further annealing continuously decreases the specific surface area, which diminishes to 2 m2 /g after heat treatment at 750 ◦ C, being close to that of commercial oxide (1.5 m2 /g). For p-Nb2 O5 samples heat treated up to 450 ◦ C, the XRD profiles present two broad humps in the 10–40◦ and 40–70◦ (2θ Cu ) range, indicating the nanocrystalline nature of the niobium oxides (Fig. 2). For the
Fig. 2. XRD of Nb2 O5 -based catalysts prepared by precipitation technique and heat-treated at different temperatures.
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Fig. 4. Hydrogen desorption kinetics of (MgH2 )bm at 300 ◦ C under vacuum for the first four cycles.
Fig. 3. XRD patterns of (MgH2 )bm and (MgH2 )catalyst .
3.4. Sorption properties of (MgH2 )catalyst intensity of the -MgH2 phase associated with an increase in the (/␥) ratio from 85/15 to about 70/30 (Fig. 3). As a matter of fact, the BET specific surface area of MgH2 increases from 7 to 16 m2 /g after ball milling with p-Nb2 O5 (Table 1). The diffraction patterns of (MgH2 )Nb2 O5 :650 and (MgH2 )c-Nb2 O5 show the presence of a peak located at 22.72◦ (2θ Cu ) indexed as (0 0 1) of Nb2 O5 (JCPDS No. 30-0873).
At 300 ◦ C, irrespective of the catalyst presence, the hydrogen desorption initiates at around 1000 mbar (Fig. 5). In addition, the increase in the required pressure from 1 to 300 mbar to desorb
3.3. Sorption properties of (MgH2 )bm with cycling Since (MgH2 )com requires more than 100 min to desorb 0.5 wt.%, we only present the sorption kinetics of (MgH2 )bm at 300 ◦ C (Fig. 4). The first cycle requires about 90 min to desorb 4 wt.% of hydrogen. In the second cycle, nearly 3.6 wt.% hydrogen is desorbed within 35 min and 1 wt.% later. In the third and consecutive desorption cycles, desorption occurs in a single step within 35 min, accounting for 5 wt.% of hydrogen. The cause for this kind of improvement in sorption kinetics with cycling is attributed to the activation due to the formation of micro/nanocracks during cycling [25]. After cycling, the BET specific surface area of (MgH2 )bm increases from 7 to 14 m2 /g (Table 1).
Fig. 5. Pressure dependent sorption properties of (MgH2 )bm and selected (MgH2 )catalyst .
Table 1 BET specific surface area of Nb2 O5 based catalysts, absorption and desorption rates of (MgH2 )bm and (MgH2 )catalyst at 300 and 250 ◦ C Catalyst
p-Nb2 O5 Nb2 O5 :350 Nb2 O5 :450 Nb2 O5 :550 Nb2 O5 :650 Nb2 O5 :750 c-Nb2 O5
SBET (m2 /g) (±0.5)
Catalyst + MgH2 (MgH2 )cat
Desorption rate (wt.%/min)
Absorption rate ((wt.%/min)
SBET (m2 /g) (±0.5
300 ◦ C
250 ◦ C
250 ◦ C
300 ◦ C
Before cycling
After cycling
201 46 34 22 9 2 1.5
(MgH2 )bm (MgH2 )p-Nb2 O5 (MgH2 )Nb2 O5 :350 (MgH2 )Nb2 O5 :450 (MgH2 )Nb2 O5 :550 (MgH2 )Nb2 O5 :650 (MgH2 )Nb2 O5 :750 (MgH2 )c-Nb2 O5
0.15 0.54 0.71 0.64 0.48 0.50 0.48 0.30
0.01 0.17 0.20 0.20 0.14 0.12 0.13 0.08
0.06 0.17 0.27 0.26 0.26 0.23 0.29 0.20
0.18 0.22 0.23 0.23 0.22 0.23 0.24 0.22
7.3 15.7 16.1 14.1 15.1 15.2 13.6 1.02
14.2 20.9 20.1 18.3 18.8 17.2 16.7 15.6
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0.5 wt.% of hydrogen shows enhanced kinetics with the catalyst addition. For all (MgH2 )catalyst samples, the full capacity was achieved after the second cycle, corresponding to a shorter activation period than for (MgH2 )bm . Similar to (MgH2 )bm , the trend of increasing the specific surface area is also recorded in (MgH2 )catalyst , nevertheless with smaller enhancement (Table 1). For the sake of comparison, sorption properties were further considered after the third cycle. Regarding capacity, (MgH2 )bm desorbs around 4.6 wt.% of hydrogen within 35 min (Fig. 6a). Among the various catalysts, at 300 ◦ C, (MgH2 )Nb2 O5 :350 presents the best activity with a capacity of 5.2 wt.% desorbed within 12 min, whereas (MgH2 )c-Nb2 O5 shows the slowest kinetics with a desorption of 4.8 wt.% in more than 30 min (Fig. 6a). Similar behavior is observed with decreasing the temperature down to 250 ◦ C (Fig. 6b) however with slower kinetics. For instance, the complete desorption of (MgH2 )Nb2 O5 :350 requires nearly 20 min as compared to 12 min at higher temperature. The decrease in the desorption temperature from 300 to 250 ◦ C is also correlated to a decrease in the desorption initiation pressure from 1000 mbar down to 120 mbar (Fig. 6c and d). Interestingly, the corresponding pressure dependence shows that more than 50% of hydrogen desorption occur above 1 mbar representing a faster desorption in vacuum. A comparison of the desorption rates for (MgH2 )bm with (MgH2 )catalyst is given in Table 1. As expected, among all of the catalysts, Nb2 O5 :350 leads to the fastest kinetics (i.e. the highest desorption rates). The catalytic activities of p-Nb2 O5 and Nb2 O5 :450 are comparable whereas the catalytic activity of heat-treated catalysts above 550 ◦ C systematically decreases. The benefit of using nano-catalyst was
recently confirmed by Friedrichs et al. [29], who report a significant reduction of milling time and a 60 ◦ C decrease in desorption temperature using Nb2 O5 nanoparticles (15 nm) as compared to micrometrics ones. In the absorption process, the weight gain initiates around 3000 mbar at 300 ◦ C for all samples (Fig. 7a). Under dynamic pressure increase conditions, nearly 20 min are required to complete 90% of absorption, i.e. below 6500 mbar H2 pressure. At 250 ◦ C, the absorption is initiated at a pressure of 1000 mbar for both (MgH2 )bm and (MgH2 )catalyst . For the two temperatures, no significant variation in the absorption time is observed between (MgH2 )bm and (MgH2 )catalyst indicating that under dynamic pressure increase conditions, the effect of catalysts on absorption kinetics is hardly noticeable. Simultaneously, at 300 ◦ C, among (MgH2 )catalyst , (MgH2 )p-Nb2 O5 and (MgH2 )Nb2 O5 :350 exhibit the highest capacity (5.2 wt.%) and (MgH2 )c-Nb2 O5 the lowest one (∼4.5 wt.%). For temperatures, 300 and 250 ◦ C, the absorption and desorption processes are completely reversible. The XRD patterns of (MgH2 )catalyst recorded after several sorption cycles show the presence of the -MgH2 phase and Mg traces (Fig. 8). The X-ray diffraction peaks are sharper and more intense than the ones before cycling. However, the XRD profile of (MgH2 )bm still exhibits higher peak intensity than that of the (MgH2 )catalyst . After sorption cycles, a small peak related to c-Nb2 O5 is still visible on the XRD profile of (MgH2 )c-Nb2 O5 , suggesting that Nb2 O5 remains unchanged during the sorption process. Because of its promising performance, the desorption behavior of (MgH2 )Nb2 O5 :350 was investigated at a temperature as low as 200 ◦ C. At this temperature, scarcely reported for MgH2 , (MgH2 )Nb2 O5 : 350 still shows interesting properties with a
Fig. 6. Hydrogen desorption kinetics of (MgH2 )bm and selected (MgH2 )catalyst at (a) 300 ◦ C and (b) 250 ◦ C, and their corresponding pressure (c) and (d) during desorption, respectively.
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Fig. 7. Hydrogen absorption kinetics of (MgH2 )bm and selected (MgH2 )catalyst at (a) 300 ◦ C and (b) 250 ◦ C, and their corresponding pressure (c) and (d) respectively.
desorption of 4.5 wt.% in 175 min (Fig. 9). As expected, the desorption rate is slower than the one recorded at higher operating temperatures (inset Fig. 9). A similar activity was recorded for (MgH2 )p-Nb2 O5 , whereas (MgH2 )c-Nb2 O5 does not present any activity. 3.5. Mechanism As it is well-known, hydrogen desorption from metal hydrides involves mainly two different processes: (i) Mg H
Fig. 8. XRD profiles of (MgH2 )catalyst after sorption cycling at 300 ◦ C.
bond breaking and hydrogen diffusion inside the lattice followed by (ii) desorption through the surface, which requires to overcome a high energy barrier [30]. In the bulk, Mg H bond breaking depends on the thermodynamics of MgH2 and therefore cannot be altered. The diffusion path-length, however, can be decreased by decreasing the grain-size (increasing the specific surface area), thus enhancing the kinetics. Indeed it was clearly demonstrated earlier that Nb2 O5 additives do not alter the bulk properties of MgH2 , but only enhance the kinetics by interfa-
Fig. 9. Hydrogen desorption properties of (MgH2 )Nb2 O5 :350 under vacuum at various temperatures. The inset presents the evolution of the desorption rate with decreasing operating temperature.
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cial interaction. The limiting step is temperature dependent. At high temperature, the surface energy controls the desorption. On the contrary, at lower temperature, the limiting step is the hydrogen diffusion inside the lattice. Here at 300 ◦ C, we only consider desorption of hydrogen through the surface, emphasizing the importance of surface area and morphology of both the catalyst and MgH2 . Indeed, for the latter the increased surface area upon cycling (from 7 to 14 m2 /g) (i.e. activation) is correlated to enhanced kinetics. As expected, the highest surface area in all (MgH2 )catalysts is observed for p-Nb2 O5 (200 m2 /g) and Nb2 O5 :350 (46 m2 /g) added samples and this results in speedy kinetics. Having demonstrated the importance of the specific surface area of both catalyst and MgH2 , the questions of the chemical nature of the catalyst should be addressed. Interestingly in the literature, for both Nb and Nb2 O5 catalysts, authors tend to agree on the formation of an intermediate oxide phase with MgO present over MgH2 surface [27–29]. These oxides are presumed to transport hydrogen readily compared to MgO and help to enhance the desorption rate. Our work [25] on the additions of halides shows a higher catalytic activity for Nb-X (X = O, Cl, F) compounds as the electronegativity of X increases suggesting the consideration of other mechanisms. One assumption lies on the formation of an intermediate Hδ− Nbδ+ bond favored by the electronic delocalization of the Nb-X one, leading to a further weakening of the Mgδ+ Hδ− bond at the surface. However, the possibility of an intermediate phase formation is under intense investigation, with for instance testing the stability of NbF5 towards Mg [31].
and assisting in experimentation, and M. Nelson for her English support. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
4. Conclusions Addition of p-Nb2 O5 , which was prepared using a solution route in order to favor high specific surface area catalyst provides promising sorption properties for MgH2 at a temperature as low as 200 ◦ C. The accelerated hydrogen sorption kinetics with the catalyst addition may be associated with various parameters. First, the presence of the catalyst leads to a significant decrease in the average particle size and an increase in the BET specific surface area of the (MgH2 )catalyst . Besides, the catalytic activity of p-Nb2 O5 and Nb2 O5 :350 with high surface area is superior to that of low surface area c-Nb2 O5 indicating that the high surface area of the catalysts is an important criterion. We also conclude that the degree of charge distribution of “M-X” bond of the catalysts has a major effect on hydrogen sorption properties of hydrides.
[20] [21] [22] [23] [24]
[25] [26] [27] [28] [29]
Acknowledgements The authors also express their gratitude to X. Darok, R. Herrera-Urbina, M. Kandavel and N. Recham for discussion
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