PtxNi1−x nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane

international journal of hydrogen energy 34 (2009) 8785–8791

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PtxNi1Lx nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane Xiaojing Yang, Fangyi Cheng, Jing Liang, Zhanliang Tao, Jun Chen* Institute of New Energy Material Chemistry and Engineering Research Center of Energy Storage & Conversion (Ministry of Education), Nankai University, Tianjin 300071, PR China

article info

abstract

Article history:

We report on PtxNi1x (x ¼ 0, 0.35, 0.44, 0.65, 0.75, and 0.93) nanoparticles as catalysts for

Received 30 April 2009

hydrogen generation from hydrolysis of ammonia borane (NH3BH3). The PtxNi1x catalysts

Received in revised form

were prepared through a redox replacement reaction with a reverse microemulsion tech-

24 August 2009

nique. The structure, morphology, and chemical composition of the obtained samples were

Accepted 31 August 2009

characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped

Available online 26 September 2009

with energy dispersive X-ray (EDX), and inductively coupled plasma emission spectroscopy

Keywords:

and the Pt atomic contents in the catalysts were 35%, 44%, 65%, 75%, and 93%, respectively.

(ICP). The results show that the diameters of the PtxNi1x nanoparticles are about 2–4 nm, PtxNi1x catalysts

It is found that the catalytic activity toward the hydrolysis of NH3BH3 is correlated with the

Nanoparticles

composition of the PtxNi1x catalysts. The annealing of Pt0.65Ni0.35 at 300  C for 1 h increases

Ammonia borane

the crystallinity of the nanoparticles, but shows almost the same activity as that without

Hydrogen generation

annealing. Among the as-prepared PtxNi1x nanoparticles, Pt0.65Ni0.35 displays the highest

Activation energy

catalytic performance, delivering a high hydrogen-release rate of 4784.7 mL min1 g1 and a low activation energy of 39.0 kJ mol1. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Efficient hydrogen production and storage have attracted world-wide attention because hydrogen is a potential material to satisfy the increasing demand for efficient and clean energy [1]. Recently, much attention has been paid to hydrogen generation from hydrolysis of borohydrides LiBH4 and NaBH4 that can theoretically release very high hydrogen content of 18.3 wt % [2] and 10.8 wt % [3] at room temperature, respectively. Since borohydrides are not chemically stable in water, they must be stored in alkaline solutions, producing some difficulties in the design and manufacturing of portable hydrogen generators. In comparison with borohydrides, ammonia borane (NH3BH3, AB) has higher hydrogen content (19.6 wt % H2) [4], higher solubility in water (33.6 g per 100 mL),

and higher stability in neutral aqueous solutions at room temperature [5]. Previous reports have demonstrated that AB can release considerable amount of hydrogen through the hydrolysis reaction under ambient condition [6–8]. Furthermore, several groups have studied the regeneration of AB, which helps to realize its recycling [9,10]. Thus, it is possible to use AB as a high-density hydrogen supplier [11]. Hydrolytic dehydrogenation of AB in water has been achieved by using noble metal catalysts [6], non-noble metals [7,12] and solid acids [8]. Although the hydrolysis of AB can be catalyzed by acid [8,9], the corresponding H2 release rate is relatively low and depends intensively on the concentration of acid that may cause unfavorable drawbacks in the practical application. In contrast, the metal catalysts can be effectively used in an AB aqueous solution. Pt has been proved to

* Corresponding author. Fax: þ86 22 23506808. E-mail address: [email protected] (J. Chen). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.08.075

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international journal of hydrogen energy 34 (2009) 8785–8791

Fig. 1 – (A) XRD patterns of PtxNi1Lx nanoparticles with different compositions: (a) x [ 0, (b) x [ 0.35, (c) x [ 0.44, (d) x [ 0.65, (e) x [ 0.75, and (f) x [ 0.93. (B) XRD patterns of Pt0.65Ni0.35 nanoparticles (d) before and (g) after annealing.

outperform other non-noble metals. However, its high price and limited abundance are disadvantageous to its widespread application. For practical use, the development of low-cost and highly efficient catalysts is required. A few studies have been focused on the PtxNi1x alloys as catalysts for AB hydrolysis. Yang’s group has prepared the Pt3Ni nanoparticles with the diameters of about 5–20 nm by a chemical impregnation method and employed them for hydrogen generation from AB solution [13]. Our previous work has reported on the template-replacement synthesis of nest-like PtxNi1x (x ¼ 0.03, 0.06, 0.09, and 0.12) submicrometer-size hollow spheres and their application as the catalysts for hydrolysis and thermolysis of AB [14]. It should be noted that the reported Pt3Ni

nanoparticles exhibited a relatively wide size distribution, and our previous PtxNi1x hollow spheres were in the size range of several hundreds nanometers. The wide distribution of the particle sizes of the PtxNi1x alloy catalysts resulted in the limited rates of hydrogen generation from AB hydrolysis. Therefore, downsizing the PtxNi1x nanoparticles with uniform size distribution helps to improve their catalytic activities. In this work, PtxNi1x (x ¼ 0, 0.35, 0.44, 0.65, 0.75, and 0.93) nanoparticles, which possess small sizes of several nanometers and narrow size distribution, have been synthesized through a reverse microemulsion method by employing a nonionic surfactant. The reverse microemulsion technique

Fig. 2 – TEM images of PtxNi1Lx nanoparticles: (a) x [ 0, (b) x [ 0.35, (c) x [ 0.44, (d) x [ 0.65, (e) x [ 0.75, and (f) x [ 0.93.

international journal of hydrogen energy 34 (2009) 8785–8791

based on solution chemical reactions is relatively simple and environmentally benign. In the reverse microemulsion system, the nucleation and crystal growth are confined in the ‘‘micro reactor’’ that is formed in oil medium, and the particle aggregation is suppressed by the surrounding surfactants that act as capping agents. Thus, the reverse microemulsion synthetic strategy is suitable for producing small and monodispersive bimetallic nanoparticles with controlled compositions [15]. The as-prepared PtxNi1x (x ¼ 0.35, 0.44, 0.65, 0.75, and 0.93) alloys exhibit favorable catalytic activities for the hydrolysis of AB. Therefore, the PtxNi1x nanoparticles should find possible applications in fuel cells with the hydrogen supply from catalytic hydrolysis of AB.

2.

Experimental

2.1.

Preparation of PtxNi1x catalysts

All the reagents were of analytical grade and used without further purification. The initial nanoparticles were prepared by the reduction of Ni2þ with NaBH4, and the molar ratio of water to surfactant (u) was fixed at 7 to prepare the initial Ni nanoparticles [16]. In a typical synthesis, a 3 mL of NiCl2 (0.5 M) aqueous solution was added to a 36 mL mixture composed of 9 mL of nonionic surfactant polyoxyethylene lauryl ether (Brij30) and 27 mL of n-octane to form the first reverse microemulsion solution. The second reverse microemulsion solution containing 1 mL of NaBH4 (3 M) with proportions identical to those in the first one was then added

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in drops. The solution immediately turned black upon the formation of nickel nanoparticles. The reaction of the redox process can be expressed as [14],

 þ Ni2þ þ BH 4 þ 2H2O / Ni þ BO2 þ 2H þ 3H2[

(1)

The PtxNi1x alloy nanoparticles were chemically prepared through a redox reaction with a reverse microemulsion method. The reverse microemulsion solution (11 mL) containing 1.5 mL of H2PtCl6 (1.93 mM) was slowly dripped to the solution containing the initial Ni nanoparticles. The molar ratio of Ni to Pt was chosen at 3, 5, 7, 11, and 14. The solution was kept under argon for 12 h to ensure the completeness of the redox reaction that can be described as

2þ þ Ni1xPtx þ 6xCl (1 þ x)Ni þ xPtCl2 6 / 2xNi

(2)

The resultant PtxNi1x nanoparticles in the solution were precipitated by centrifugation and washed with a mixture of ethanol and acetone. Finally, the nanoparticles were dried in vacuum at 60 C for 12 h.

2.2.

Catalyst characterization

The as-synthesized catalysts were characterized by powder X-ray diffraction (XRD, Rigaku D/max-2500 X-ray generator, Cu Ka radiation), inductively coupled plasma emission

Fig. 3 – Size distributions of PtxNi1Lx nanoparticles: (a) x [ 0, (b) x [ 0.35, (c) x [ 0.44, (d) x [ 0.65, (e) x [ 0.75, and (f) x [ 0.93.

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Fig. 4 – EDX spectra of PtxNi1Lx nanoparticles: (a) x [ 0, (b) x [ 0.35, (c) x [ 0.44, (d) x [ 0.65, (e) x [ 0.75, and (f) x [ 0.93.

spectroscopy (ICP-9000, Thermo Jarrell-Ash Corp.), transmission electron microscopy and energy dispersive X-ray spectroscopy (TEM and EDX, Philips Tecnai F20, 200 kV).

2.3.

Hydrogen generation measurement

In a typical experiment, AB solution was placed in a sealed flask fitted with an outlet tube for collecting the evolved H2. The outlet tube exhaust was placed under an inverted, waterfilled gas burette that was situated in a water-filled vessel. A certain amount of catalysts was added into the solution under mild stirring. The hydrolysis reaction was carried out at controlled temperatures. The volume of the generated H2 was measured by the water displacement method. The process of AB hydrolysis was described as following [17]  NH3BH3 þ 2H2O / NHþ 4 þ BO2 þ 3H2[

Fig. 1A shows the XRD patterns of the nanoparticles with various Pt content. Without annealing, the catalysts are almost amorphous with several broadened diffraction peaks. In curve a, the diffraction peak of Pt0Ni at 2q value of 44.5 corresponds to the (111) plane of face centered cubic (fcc) Ni crystal (JCPDS No.87–712). The particles with deposited Pt in curve b, c, d, e, and f display broad XRD peaks with the centers around 2q ¼ 39.8 , which corresponds to pure Pt (111) (JCPDS No.87–647). All the PtxNi1x (x ¼ 0.35, 0.44, 0.65, 0.75, and 0.93) samples exhibit similar diffraction patterns to that of pure Pt. However, the diffraction peaks shift to higher angles compared to that of pure Pt, demonstrating the substitution of the Pt atoms for the Ni atoms in Pt crystal lattice. Fig. 1B shows the XRD patterns of Pt0.65Ni0.35 before and after annealing at

(3)

When the hydrolysis reaction was completed, the residual solution was filtered and the catalyst was reserved.

3.

Results and discussion

3.1.

Catalyst characterization

The PtxNi1x alloys were prepared by an in situ redox reaction without any additional reducing between the Ni and PtCl2 6 agent. The first step is to prepare Ni nanoparticles in reverse microemulsion when Ni2þ was reduced by NaBH4 Eq. (1). The formation of PtxNi1x was attained in the second step through the replacement reaction between H2PtCl6 and Ni Eq. (2), where the driving force is from the large standard reduction potential gap between the Ni2þ/Ni (0.250 V vs standard hydrogen electrode (SHE)) and the PtCl2 6 /Pt (0.735 V vs SHE) redox pairs [14].

Fig. 5 – Hydrogen generation from AB (0.5 wt %, 10 ml) containing different PtxNi1Lx catalysts (9 mg) at 25 8C: (a) x [ 0, (b) x [ 0.35, (c) x [ 0.44, (d) x [ 0.65, (e) x [ 0.75, (f) x [ 0.93, and (g) x [ 0.65 after annealing at 300 8C.

international journal of hydrogen energy 34 (2009) 8785–8791

Table 1 – The hydrogen generation rate, total H2 generation volume, and reaction time of the as-prepared PtxNi1Lx (x [ 0, 0.35, 0.44, 0.65, 0.75, and 0.93). Catalyst

Ni Pt0.35Ni0.65 Pt0.44Ni0.56 Pt0.65Ni0.35 Pt0.75Ni0.25 Pt0.93Ni0.07 Pt0.65Ni0.35 annealing

H2 generation ration (mL min1 g1)

Total H2 generation volume (mL)

Reaction time (min)

357.14 826.45 2010.8 4784.7 3787.9 2272.7 4334.4

31.0 67.6 85.2 94.0 92.1 89.4 92.2

79 11 6 4 5 6 5

300  C for 1 h. It can be noticed that the peaks become sharper and more distinguishable after annealing. The peaks do not belong to pure Ni or Pt, indicating the increase of the crystallinity of alloy upon heating treatment. The morphology and composition of the obtained products were investigated by TEM, EDX, and ICP analysis. Fig. 2a, b, c, d, e, and f show the typical TEM images of the obtained PtxNi1x samples with x value of 0, 0.35, 0.44, 0.65, 0.75, and 0.93, respectively. The inset of Fig. 2d displays the typical HRTEM image of a single nanoparticle, revealing the interlaced lattice fringes of the Pt0.65Ni0.35. The size distribution of the samples is shown in the diameter histograms (Fig. 3) obtained by statistically measuring the sizes of more than 100

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individual nanoparticles on TEM images. The Ni particles are well distributed with an average diameter of about 5 nm (Fig. 3a), while PtxNi1x particles possess the average size of 3 nm (Fig. 3b–f). ICP elemental analysis of the synthesized samples has been performed to detect the exact composition of PtxNi1x. EDX data (Fig. 4) of the nanoparticles confirm average elemental compositions of the alloys, and the molar ratios of Ni and Pt determined by the EDX analysis are consistent with that obtained from ICP result.

3.2.

Catalytic activities

We have investigated the catalytic activities of the synthesized PtxNi1x (x ¼ 0, 0.35, 0.44, 0.65, 0.75, and 0.93) for hydrogen generation from the hydrolysis of AB solution. The catalytic performances of the prepared nickel powders, PtxNi1x (x ¼ 0, 0.35, 0.44, 0.65, 0.75, and 0.93) nanoparticles without annealing, and Pt0.65Ni0.35 nanoparticles with annealing at 300  C have been tested. Fig. 5 shows the H2 amount generated as a function of time for a 10 mL AB solution with 9 mg of catalysts. The synthesized nickel nanoparticles exhibit almost no catalytic activity during the hydrolysis of AB, while the release of hydrogen is greatly quickened in the presence of PtxNi1x (x ¼ 0.35, 0.44, 0.65, 0.75, and 0.93) nanoparticles, denoting their favorable catalytic activities. The hydrolysis reaction of AB with the as-synthesized PtxNi1x (x ¼ 0.35, 0.44, 0.65, 0.75, and 0.93) nanoparticles without annealing and Pt0.65Ni0.35 after annealing is completed in approximately 10 min. The assynthesized Pt0.65Ni0.35 displays the highest catalytic activity

Fig. 6 – Temperature effect on hydrogen generation rate using 4 mg PtxNi1Lx catalysts in AB solution (0.5 wt %, 10 mL): (a) x [ 0, (b) x [ 0.35, (c) x [ 0.65, (d) x [ 0.93.

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Table 2 – H2 generation rate for the hydrolysis of AB catalyzed by 4 mg catalyst in 10 mL 0.5 wt % AB solutions at different temperatures. Temperature( C)

H2 generation rate (mL min1 g1) with different catalysts Ni

25 30 35 40

357.1 576.7 801.3 1131.2

Pt0.35Ni0.65 Pt0.65Ni0.35 Pt0.97Ni0.03 925.9 1389.6 1713.1 2141.2

3896.1 5833.3 6788.3 7900.5

3000.0 3895.4 5324.5 7882.5

because the hydrolysis reaction of AB is completed in merely 4 min. The Pt0.65Ni0.35 after annealing shows almost the same activity as that of Pt0.65Ni0.35 without annealing. After the completion of hydrogen release, the molar ratio of the generated H2 to the initial AB is approximately 3.0. The hydrogen generation rate, total H2 generation volume, and reaction time for various catalysts are summarized in Table 1. The rate of hydrogen generation of Pt0.65Ni0.35 is nearly 4784.7 mL min1 g1. This value is much larger than that of Pt3Ni (about 1388 mL min1 g1) reported by Yang et al. [13], and also higher than our pervious results for submicrometer-size Ni0.88Pt0.12 hollow spheres (about 2333 mL min1 g1) [14]. Fig. 6 shows the effect of temperature on the hydrogen generation rate of the as-prepared PtxNi1x (x ¼ 0, 0.35, 0.65, and 0.93) catalysts. In order to save materials and reagents, the dosage of the PtxNi1x (x ¼ 0, 0.35, 0.65, and 0.93) catalysts used herein is 4 mg less than that used beforehand. As expected, the rates of hydrogen generation rise dramatically

with the increase of temperature. The total volumes of hydrogen generated from the hydrolysis of AB grow slightly as the temperature increases. The reaction rate is found to be almost unchanged when the AB concentration decreases as the reaction process is prolonged, indicating a zero order reaction kinetics of AB hydrolysis. The hydrogen generation rates at various temperatures are summarized in Table 2. Fig. 7 shows the Arrhenius plots of lnk vs the reciprocal absolute temperature (1/T) for different catalysts. The slope of the straight line gives apparent activation energy of 39.0 kJ mol1 for Pt0.65Ni0.35. In comparison, the activation energy for Ni, Pt0.97Ni0.03, and Pt0.35Ni0.65 catalysts are 57.0, 43.7, and 39.3 kJ mol1, respectively. The activation energy of Pt0.65Ni0.35 is the lowest in the PtxNi1x (x ¼ 0, 0.35, 0.44, 0.65, 0.75, and 0.93) series, being consistent with the results of the hydrogen-release rate. The above results indicate that the synthesized PtxNi1x (x ¼ 0.35, 0.44, 0.65, 0.75, and 0.93) alloy catalysts are superior to pure Pt and Ni metals, and Pt0.65Ni0.35 nanoparticles exhibit the highest catalytic activity. The preliminary interpretation follows. Pt and Ni belong to a group of metals with low hydrogen overpotential and catalytic activity, which can break the N–B bond in AB and also provide strong adsorption affinity for hydrogen and ammonium species. When AB is catalytically hydrolyzed to produce hydrogen and ammonium, the reaction intermediates may adsorb intimately on Pt and Ni surfaces, leading to a sluggish desorption of hydrogen molecules [13]. Since there is a much lower overpotential for hydrogen evolution reaction on Pt than that on Ni, Pt is easily to react with AB and to adsorb hydrogen, while Ni is less active. In the case of PtxNi1x alloys, Pt and Ni interact with

Fig. 7 – Arrhenius plots of lnk vs the reciprocal absolute temperature 1/T in the temperature range of 25–40 8C: (a) x [ 0, (b) x [ 0.35, (c) x [ 0.65, (d) x [ 0.93.

international journal of hydrogen energy 34 (2009) 8785–8791

each other. The bimetallic catalysts must weaken the bonding of hydrogen and ammonium to the surface atoms, leading to an accelerated release of gaseous hydrogen [13]. Therefore, PtxNi1x (x ¼ 0.35, 0.44, 0.65, 0.75, and 0.93) alloys are more efficient than pure Pt and Ni in catalyzing the hydrolysis reaction. It has been reported that a volcano shape is found when the catalytic activity of a catalyst for the hydrogen evolution reaction is plotted as a function of the strength of hydrogen-metal bond [18,19]. This behavior is related to the Sabatier principle, a general explanatory paradigm in heterogeneous catalysis, which states that optimal catalytic activity can be achieved on a catalytic surface with median binding energies (or free energies of adsorption) for reactive intermediates [20]. If the intermediates bind too weakly, it is difficult for the surface to activate them, but if they bind too strongly, they will occupy all available surface sites and poison the catalyst [21]. Thus, there is an optimal ratio between Pt and Ni in PtxNi1x alloy to show the highest catalytic activity. In this work, the optimal composition has been found to be Pt0.65Ni0.35.

4.

Conclusions

In summary, PtxNi1x (x ¼ 0, 0.35, 0.44, 0.65, 0.75, and 0.93) catalysts with particle diameters of about 3 nm have been synthesized via a redox replacement process in reverse microemulsion. Furthermore, the as-prepared PtxNi1x nanoparticles were investigated as the catalysts for the hydrogen generation from AB hydrolysis. The results show that the assynthesized PtxNi1x catalysts outperform either pure Pt or Ni catalyst with a higher H2 release rate. In the PtxNi1x (x ¼ 0, 0.35, 0.44, 0.65, 0.75, and 0.93) series, the catalytic activities depend on the Pt content and Pt0.65Ni0.35 catalyst shows the best performance considering the hydrogen generation rate, hydrogen generation efficiency, and apparent activation energy. The reaction system consisting of AB and PtxNi1x nanoparticles may find applications in the field of on-board hydrogen supply.

Acknowledgments This work was supported by the National Programs of NSFC (20701021 and 50631020) and MOST (2007AA05Z108 and 2009AA05Z106).

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