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Room temperature hydrogen generation from hydrolysis of ammoniaeborane over an efficient NiAgPd/C catalyst Lei Hu a, Bin Zheng a, Zhiping Lai b,*, Kuo-Wei Huang a,* a
Division of Physical Sciences & Engineering and KAUST Catalysis Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia b Division of Physical Sciences & Engineering and Advanced Membranes and Porous Material Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
article info
abstract
Article history:
NiAgPd nanoparticles are successfully synthesized by in-situ reduction of Ni, Ag and Pd
Received 15 July 2014
salts on the surface of carbon. Their catalytic activity was examined in ammonia borane
Received in revised form
(NH3BH3) hydrolysis to generate hydrogen gas. This nanomaterial exhibits a higher cata-
24 September 2014
lytic activity than those of monometallic and bimetallic counterparts and a stoichiometric
Accepted 6 October 2014
amount of hydrogen was produced at a high generation rate. Hydrogen production rates
Available online 31 October 2014
were investigated in different concentrations of NH3BH3 solutions, including in the borates saturated solution, showing little influence of the concentrations on the reaction rates. The
Keywords:
1 at room temperature hydrogen production rate can reach 3.6e3.8 mol H2 mol1 cat min
Hydrogen generation
1 (21 C). The activation energy and TOF value are 38.36 kJ/mol and 93.8 mol H2 mol1 cat min ,
Nanoparticles
respectively, comparable to those of Pt based catalysts. This nanomaterial catalyst also
Heterogeneous catalysis
exhibits excellent chemical stability, and no significant morphology change was observed
Ammoniaeborane
from TEM after the reaction. Using this catalyst for continuously hydrogen generation, the
Hydrolysis
hydrogen production rate can be kept after generating 6.2 L hydrogen with over 10,000 1 turnovers and a TOF value of 90.3 mol H2 mol1 cat min .
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is considered an ideal secondary fuel and energy carrier with a high energy density by weight and it can be used in proton exchange membrane (PEM) fuel cells to produce electrical power for vehicles and electronic devices [1]. Since hydrogen is a low density gas and flammable, how to store and use it safety becomes a major challenge [2]. Current materials such as complex hydrides [3] and chemical hydrides [4]
have been used for hydrogen storage, but most of these materials have low volumetric and/or gravimetric capacity which cannot meet the ideal requirement proposed for the on-board hydrogen storage system by the US Department of Energy (volumetric capacity >82 g/L and gravimetric capacity >9 wt.%) [5]. Boron hydrides are promising materials for this purpose [6], and in particular, ammonia borane (NH3BH3, denoted as AB) is one of the most studied boron hydrides as it stores 19.5 wt.% of hydrogen [7]. Hydrogen release from AB is typically achieved through two pathways: thermolysis [8] and
* Corresponding authors. E-mail addresses: [email protected] (Z. Lai), [email protected] (K.-W. Huang). http://dx.doi.org/10.1016/j.ijhydene.2014.10.032 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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hydrolysis [9]. Thermal dehydrogenation of AB can be catalyzed, but the resulting byproduct, BN complex, is hardly soluble in any solvent, making the reaction irreversible. Hydrolysis of AB is a different approach and hydrogen can be released at ambient temperatures in the presence of suitable catalysts, and the borate byproduct can be electrochemically reduced back to borohydride [10]. Noble metal materials as the catalysts for hydrolysis of AB were firstly reported by the Xu group [9a]. Pt and Rh supported on g-Al2O3 has shown high 1 catalytic activity (TOF ¼ 208 and 128 mol H2 mol1 cat min , respectively) [11]. As the price of novel metals reached their historical height, the search for alternative materials with similar activities yet much lower cost is the key for industry for a sustainable future. Some transition metals like Co, Ni and Cu based catalysts were also tested, and in some cases, nearly stoichiometric amount of hydrogen was produced in the AB hydrolysis system with good catalytic activity [12]. These catalysts are economical and recyclable, although the TOF values are much lower than those of Pt and Rh based nanomaterials. Bimetallic nanomaterials usually show enhanced catalytic activity, selectivity and stability compared to their monometallic counterparts [13], and there have been a number of bimetallic nanomaterials used as the catalysts for hydrogen generation from AB solutions. The observed improved catalytic activities were rationalized by the synergistic and bifunctional effects [14]. Ni and Co are the most used first-row transition metals which have been alloyed with noble metals (Au, Pt, Pd). These alloy nanoparticles can enhance the catalytic performance and reduce the consumption of the noble metals resulting in a viable approach for the development of low-cost catalysts. Liu and co-workers compared the catalytic activity of NiPt and NiPd nanomaterials and found that NiPt is more efficient than NiPd, and its monometallic counterparts (Ni or Pt) [14b]. The Xu group used metal-organic frameworks (MOFs) to immobilize AuNi nanoparticles and used them as the catalyst for AB hydrolysis with better cata1 lytic performance (TOF ¼ 66.2 mol H2 mol1 cat min ) than those of monometallic Au and Ni immobilized by MOFs [14d]. Trimetallic nanomaterials also show enhanced catalytic activity in hydrolysis of AB and some coreeshell nanoparticles were synthesized and examined [15]. Herein, we report the synthesis of a trimetallic nanomaterial supported on carbon (NiAgPd/C) by reduction of the corresponding metal salts under mild reaction conditions. This trimetallic nanomaterial shows excellent catalytic activity and stability toward hydrogen generation though AB hydrolysis at room temperature (21 C), comparable to those of Pt-based catalysts.
Fig. 1 e Plot of time vs the ratio of hydrogen and AB in the presence of Ni, Ag, Pd based catalysts at 21 C (nmetal:nAB ¼ 0.012).
solution to reduce the metal salts. This supported nanomaterial was conveniently obtained through filtration. The catalytic activities of the resulting NiAgPd/C and its monometallic (Ni/C, Ag/C, and Pd/C), bimetallic (NiPd/C, NiAg/ C, and AgPd/C) counterparts were investigated in hydrogen production reaction from AB aqueous solution at room temperature (Fig. 1). Obviously, the as-prepared NiAgPd/C exhibits a much better catalytic activity than its mono-metallic and bimetallic counterparts which were synthesized through similar processes. 3.0 equivalents of hydrogen were produced from the AB aqueous solution when NiAgPd/C or AgPd/C was used as the catalyst, indicating the complete hydrogen generation. The high TOF value was obtained 1 (93.8 mol H2 mol1 cat min ) from the NiAgPd/C catalyst (Fig. S1). This value is also comparable to those reported for Pt and Rh catalysts [11]. It is evidenced that Ni can enhance the catalytic activity by comparing the catalytic activity of NiAgPd/C with
Results and discussion Catalytic hydrogen generation through AB hydrolysis NiAgPd/C was synthesized by using co-reduction of meal salts in water without any surfactants. Typically, the active carbon was first dispersed in water using ultrasound and stirred under argon atmosphere. The solution of these three metal salts was then added to the well-dispersed active carbon under argon before the treatment of fresh NaBH4 aqueous
Fig. 2 e The TOF values obtained from different concentration of AB using NiAgPd/C catalyst at 21 C (nmetal:nAB ¼ 0.012).
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that of AgPd/C and the catalytic activity of NiPd/C with that of Pd/C. From its monometallic counterparts, Ni/C shows higher catalytic activity than Pd/C and Ag/C and 2.8 equivalents of hydrogen were generated in 80 min. The catalytic activity of NiPd/C is higher than those of AgPd/C and AgNi/C and 2.8 equivalents of hydrogen were generated in 18 min. The catalytic activity of NiAgPd/C was further compared with the mechanical mixture of its monometallic counterparts (molar ratio of Ni, Ag and Pd is the same as that in NiAgPd/C). While the mixture showed an improved catalytic activity than those of individual catalysts and 3 equivalents of hydrogen were generated in 70 min, the catalytic activity was significantly 1 lower (3.85 mol H2 mol1 cat min ) when compared with the NiAgPd/C catalyst. Hydrogen production rates at different AB concentrations were also studied in the presence of NiAgPd/C catalyst. As shown in Fig. 2, TOF values were relatively stable (approx. 1 90 mol H2 mol1 cat min ), and no significant change was observed from 0.1 to 8.4 mol/L AB aqueous solutions. These results indicated that the catalytic activity was not influenced by the concentration of the AB solution. From the gas production curve in the presence of NiAgPd/C, the ratio of the volume of the hydrogen versus time is constant, consistent with the observation that the catalytic activity is not concentration dependent. At high concentration of 8.4 mol/L, some solids were formed after the reaction. These solids were later confirmed to be borates due to their limited solubility in water [9b]. The catalytic activity of NiAgPd/C was further examined using a borates saturated solution under the similar reaction condition. 3.0 equivalents of hydrogen gas were 1 generated and the TOF value was 91.1 mol H2 mol1 cat min , close to those obtained from lower AB concentrations. These results suggest that NiAgPd/C is suitable for continuously generating hydrogen from AB aqueous solutions by removing the borates sediment in the reaction and the catalytic activity will not be significantly affected by changing the concentration of the AB solution. As catalytic rate can be generally accelerated by increasing the reaction temperature, different temperatures for catalytic hydrogen generation were also considered. Hydrogen can be generated completely in a shorter time in the presence of the NiAgPd/C catalyst when increasing the reaction temperature. At 80 C, 3.0 equivalents of hydrogen were obtained in 17 s and
Table 1 e H2 generation from aqueous AB solution catalyzed by Ni, Ag, Pd, Pt based catalysts at room temperature. Catalyst
2 wt.% Pt/C 20 wt.% Pt/C Pt black PtO2 Ni in starch Bare Ni NPs PVP-Ni NPs Ag@Ni/G Ag/C/Ni Ni/ZIF-8 Pd/HAP Pd black CoPd/C NiAgPd/C
Metal/AB (mol/mol) 0.018 0.018 0.018 0.018 0.1 0.1 0.1 0.05 0.022 0.016 0.02 0.018 0.024 0.012
TOF Ea (kJ/mol) (mol H2 1 mol1 cat min ) 111 83.34 13.89 20.8 5 2.5 3 77.0 5.32 14.2 6.8 0.67 22.7 93.8
e e e e e e e 49.56 38.91 e 55 ± 2 e 27.5 38.36
Ref
[9b] [9b] [9b] [9b] [17] [12a] [12a] [16b] [18] [12g] [19] [9a] [14b] This work
the corresponding TOF value could reach 1 729.5 mol H2 mol1 (Fig. S2). Under these conditions cat min (NiAgPd/C, 80 C), hydrogen could be generated continuously by re-adding AB and water into the solution, and the hydrogen production rate reached 29.8 mol H2 h1 g1 cat. These parameters indicate that in order to produce 1 kW h of electricity in a proton exchange membrane (PEM) fuel cell system, only about 0.9 g of NiAgPd/C catalyst would be sufficient. Even at room temperature (21 C), 7.3 g of NiAgPd/C catalyst would be sufficient to provide hydrogen for the PEM fuel cell system (the amount of NH3BH3 needed is 0.27 kg [11], assume the operation circuit voltage is 0.7 V for this standard PEM fuel cell). Fig. 3 shows that hydrogen production rates are nearly constants at different temperatures. These rate constants (k) can be used to calculate the activation energy for NiAgPd/C catalytic hydrogen generation reaction [14a,c,16]. With these rate constants obtained at different temperatures, Arrhenius plot (ln k vs 1/T) for the catalytic hydrogen production reaction gave the activation energy of 38.4 kJ/mol. By comparing the catalytic activity with Ni, Ag, and Pd based catalysts (Table 1), NiAgPd/C shows the optimal catalytic activity comparable to those of Pt based catalysts (Pt/C, Pt black and PtO2 catalysts) [9b].
Fig. 3 e (A) Hydrogen generation from AB aqueous in the presence of NiAgPd/C catalyst (nmetal:nAB ¼ 0.012) at 20, 35, 50, 65 and 80 C; (B) Arrhenius plot of ln k vs (1/T).
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exhibits good stability in hydrogen generation from AB aqueous solution and has 10,216 turnovers with 6.2 L hydrogen generation by continuously adding AB solution into the reaction system. Particularly, the TOF value obtained from the last addition of AB in lifetime measuring, can also reach 1 90.3 mol H2 mol1 cat min , similar to the fresh level 1 1 (93.8 mol H2 molcat min ). These results proved that NiAgPd/C catalyst can be a promising non-Pt catalyst for hydrogen generation from AB hydrolysis.
Catalyst characterization
Fig. 4 e Plot of time vs the ratio of hydrogen and AB in the presence of NiAgPd/C catalyst during a five cycle reusability test (nmetal:nAB ¼ 0.012) at 21 C.
The stability and reusability of the as-synthesized NiAgPd/ C catalyst was also tested in the AB hydrolysis at room temperature. The volume of hydrogen was measured at different time, and re-addition of AB solutions (2.0 mmol) was carried out after the complete hydrogen generation in the previous addition. The first four cycles were obtained continually and they could all be finished in 160 s (Fig. 4). In order to evaluate the stability of the catalyst, the reaction mixture was further stirred at room temperature for 24 h after the fourth cycle was completed. Another one equivalent of AB (2.0 mmol) was then added, and complete hydrogen generation was achieved in 170 s with the TOF value remaining approximately the same 1 (88.4 mol H2 mol1 cat min , Fig. S3). The reaction solution was analyzed after the hydrogen generation, and only 0.14% of the Ni leakage was detected by ICP-MS. The size and morphology were not significantly changed during the reaction (Fig. 5B). The lifetime of NiAgPd/C was estimated by calculating the TON value and the TOF value in the reaction. NiAgPd/C
NiAgPd/C catalyst was characterized by ICP-MS to determine the ratio of these three metals and their loading. The results showed that the ratio of Ni:Ag:Pd is close to 1:1:1 with the metal loading of 6.2 wt.%. These nanoparticles were welldispersed on the surface of carbon and the sizes were in a range of 3e8 nm from TEM images and the nanoparticles size distribution (Fig. 5A). The NiAgPd/C catalyst showed good stability and no significant changes were observed in nanoparticle size and morphology after 5 cycles (Fig. 5B). Scanning TEM (STEM) and energy dispersive X-ray (EDX) were also used to characterize the structure and components of the catalyst (Fig. S4). As the nanoparticles were small and supported on carbon, the analysis was interfered by carbon and only weak peaks of Ni, Ag, and Pd were observed. The X-ray diffraction (XRD) pattern exhibited an fcc structure close to that of metallic Ag (Fig. 6A) and no impurity was observed. Together with the ICP-MS results, it was concluded that NiAgPd nanoparticles were formed to give a pure fcc structure [20]. It is predictable that the fcc structure of NiAgPd is similar to fcc Ag, as the atomic radius of Ag (0.144 nm) is bigger than Pd (0.138 nm) and Ni (0.125 nm) [21]. Pd and Ni will exist in this fcc structure. Since the lattice constant of the Ag is larger than Pd and Ni, evident strain effects on Pd, Ni are expected in this trimetallic nanoparticle, and thus a higher reactivity may be obtained [22]. This expectation was confirmed by our experiment results (Fig. 1) that NiAgPd/C has a higher catalytic activity than NiPd/C, Ni/C and Pd/C. Density functional theory (DFT) calculations (see Supporting Information) were used to
Fig. 5 e TEM images before (A) and after (B) five catalytic cycles (insert: the distribution of the nanoparticles).
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Fig. 6 e (A) XRD pattern and XPS spectra (B: Pd 3d; C: Ag 3d; D: Ni 2p) of NiAgPd/C.
support this strain effects in NiAgPd system. In comparative reactions, the gas generation rate was firstly compared using D2O and H2O as the solvent, the gas generation rate in H2O is about three times than in D2O which shows that the water decomposition is the rate determining step (Fig. S5). Therefore, hydrogen atom adsorption can be used to indicate the catalytic activity [23]. The adsorption energy was firstly compared between Pd and Pd with Ag lattice. Stronger hydrogen adsorption was obtained on Pd with Ag lattice (2.97 eV) than Pd (2.90 eV) which demonstrated that hydrogen will be generated easier in Pd with Ag lattice system. Meanwhile, a higher hydrogen adsorption was observed in NiAgPd system (3.00 eV) than NiPd (2.75 eV), Ni (2.80 eV) and Pd (2.90 eV) system which is in good agreement with our experiment results that NiAgPd shows the highest catalytic activity in hydrogen generation from AB hydrolysis. The X-ray photoelectron spectroscopy (XPS) was also used to demonstrate NiAgPd/C catalyst. Fig. 6B shows the XPS peaks of Ag 3d of the catalyst: two peak at 368.1 and 373.9 eV which are in good agreement with the values for Ag0, standing for Ag 3d5/2 and Ag 3d3/2. From the XPS results in Fig. 6C, peaks with binding energies of 335.8 and 341.1 eV can be attributed to Pd 3d5/2 of the Pd 3d3/2. Peaks in Fig. 6D with binding energies of 856.8 and 874.2 eV represent oxidized Ni and a small peak at 852.9 eV corresponds to Ni0 (15%) (Fig. S6). These results indicate that Ag, Pd are stable, whereas the Ni may be transferred to outside and oxidized to certain degree after the catalyst was synthesized and during its application. Therefore, the slight decrease in the catalytic activity after stirring
under air for 24 h may be attributed to the oxidation of the Ni in that nanomaterial. According to the nature of electronegativity, the electrons may transfer from Ni (1.91) and Ag (1.93) to Pd (2.20) which can make the Pd surface negative with high catalytic activity [24]. This similar phenomenon was observed in CoAuPd/C catalyst for the formic acid decomposition [20].
Conclusion NiAgPd nanoparticles supported on carbon have been designed and successfully prepared. This nanomaterial catalyst exhibits high catalytic activity and stability towards hydrogen generation from NH3BH3 hydrolysis at room temperature. Stoichiometric amounts of hydrogen were produced under a constant 1 TOF of 93.8 mol H2 mol1 cat min . The activation energy was determined to be 38.36 kJ/mol. The process was proved to be efficient in a wide range of concentrations of the NH3BH3 solution to afford stoichiometric amounts of hydrogen with a low catalyst/NH3BH3 ratio (0.012). Hydrogen can be continually produced by adding fresh NH3BH3 solutions. This catalyst provide 10,216 turnovers by continuously adding NH3BH3 solution into the reaction system and the catalytic activity was 1 kept (TOF ¼ 90.3 mol H2 mol1 cat min ). This trimetallic nanomaterial provides a new approach to prepare the catalyst for highly efficient portable hydrogen generation system. When using this generated hydrogen to produce 1 kW h of electricity in a proton exchange membrane (PEM) fuel cell system, about 7.3 g of NiAgPd/C catalyst would be sufficient as the catalyst to
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generate hydrogen at room temperature (the amount of NH3BH3 needed is 0.27 kg, assuming the operation circuit voltage is 0.7 V for a standard PEM fuel cell).
Experimental section Synthesis of NiAgPd/C catalyst: Active carbon (400.0 mg, surface area 450 m2/g) was dispersed in water (100.0 mL) by sonication for 30 min to get a well-dispersed active carbon water solution. To this solution under argon atmosphere was added a water solution of metal salts (Ni(OAc)2$4H2O, 44.8 mg; AgNO3, 30.5 mg; Pd(NO3)2$xH2O, 50.1 mg in 10.0 mL of H2O). This reaction mixture was further ultrasonicated for 30 min before a fresh NaBH4 aqueous solution (200.0 mg NaBH4 in 10.0 mL of H2O) was added dropwise with rigorous stirring. After the addition was completed, this mixture was stirred for 18 h at room temperature. The resulting black precipitates were collected through filtration and dried in the vacuum oven at 120 C for 24 h for further use. For comparison, AgPd/C, NiPd/C, NiAg/C, Ni/C, Pd/C and Ag/C were also prepared by the same method. Hydrogen production by NiAgPd/C catalytic hydrolysis of AB aqueous solution: The catalytic activity of the catalysts was calculated by measuring the volume of the hydrogen produced by hydrolysis of AB in a water-filled gas buret system. The catalyst was first sealed in the reaction flask collect to the gas collection system. AB aqueous solution was then injected into this system under stirring. The volume of hydrogen gas generated from the system was recorded by measuring the displacement of water volume at different time (subtract the volume of the AB solution). When stoichiometric amounts of hydrogen were collected, equivalent of AB solution was readded to test the reusability of the catalyst.
Acknowledgments Financial Support is provided by King Abdullah University of Science and Technology.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.10.032.
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Room temperature hydrogen generation from hydrolysis of ammoniaeborane over an efficient NiAgPd/C catalyst Lei Hu a, Bin Zheng a, Zhiping Lai b,*, Kuo-Wei Huang a,* a
Division of Physical Sciences & Engineering and KAUST Catalysis Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia b Division of Physical Sciences & Engineering and Advanced Membranes and Porous Material Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
article info
abstract
Article history:
NiAgPd nanoparticles are successfully synthesized by in-situ reduction of Ni, Ag and Pd
Received 15 July 2014
salts on the surface of carbon. Their catalytic activity was examined in ammonia borane
Received in revised form
(NH3BH3) hydrolysis to generate hydrogen gas. This nanomaterial exhibits a higher cata-
24 September 2014
lytic activity than those of monometallic and bimetallic counterparts and a stoichiometric
Accepted 6 October 2014
amount of hydrogen was produced at a high generation rate. Hydrogen production rates
Available online 31 October 2014
were investigated in different concentrations of NH3BH3 solutions, including in the borates saturated solution, showing little influence of the concentrations on the reaction rates. The
Keywords:
1 at room temperature hydrogen production rate can reach 3.6e3.8 mol H2 mol1 cat min
Hydrogen generation
1 (21 C). The activation energy and TOF value are 38.36 kJ/mol and 93.8 mol H2 mol1 cat min ,
Nanoparticles
respectively, comparable to those of Pt based catalysts. This nanomaterial catalyst also
Heterogeneous catalysis
exhibits excellent chemical stability, and no significant morphology change was observed
Ammoniaeborane
from TEM after the reaction. Using this catalyst for continuously hydrogen generation, the
Hydrolysis
hydrogen production rate can be kept after generating 6.2 L hydrogen with over 10,000 1 turnovers and a TOF value of 90.3 mol H2 mol1 cat min .
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is considered an ideal secondary fuel and energy carrier with a high energy density by weight and it can be used in proton exchange membrane (PEM) fuel cells to produce electrical power for vehicles and electronic devices [1]. Since hydrogen is a low density gas and flammable, how to store and use it safety becomes a major challenge [2]. Current materials such as complex hydrides [3] and chemical hydrides [4]
have been used for hydrogen storage, but most of these materials have low volumetric and/or gravimetric capacity which cannot meet the ideal requirement proposed for the on-board hydrogen storage system by the US Department of Energy (volumetric capacity >82 g/L and gravimetric capacity >9 wt.%) [5]. Boron hydrides are promising materials for this purpose [6], and in particular, ammonia borane (NH3BH3, denoted as AB) is one of the most studied boron hydrides as it stores 19.5 wt.% of hydrogen [7]. Hydrogen release from AB is typically achieved through two pathways: thermolysis [8] and
* Corresponding authors. E-mail addresses: [email protected] (Z. Lai), [email protected] (K.-W. Huang). http://dx.doi.org/10.1016/j.ijhydene.2014.10.032 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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hydrolysis [9]. Thermal dehydrogenation of AB can be catalyzed, but the resulting byproduct, BN complex, is hardly soluble in any solvent, making the reaction irreversible. Hydrolysis of AB is a different approach and hydrogen can be released at ambient temperatures in the presence of suitable catalysts, and the borate byproduct can be electrochemically reduced back to borohydride [10]. Noble metal materials as the catalysts for hydrolysis of AB were firstly reported by the Xu group [9a]. Pt and Rh supported on g-Al2O3 has shown high 1 catalytic activity (TOF ¼ 208 and 128 mol H2 mol1 cat min , respectively) [11]. As the price of novel metals reached their historical height, the search for alternative materials with similar activities yet much lower cost is the key for industry for a sustainable future. Some transition metals like Co, Ni and Cu based catalysts were also tested, and in some cases, nearly stoichiometric amount of hydrogen was produced in the AB hydrolysis system with good catalytic activity [12]. These catalysts are economical and recyclable, although the TOF values are much lower than those of Pt and Rh based nanomaterials. Bimetallic nanomaterials usually show enhanced catalytic activity, selectivity and stability compared to their monometallic counterparts [13], and there have been a number of bimetallic nanomaterials used as the catalysts for hydrogen generation from AB solutions. The observed improved catalytic activities were rationalized by the synergistic and bifunctional effects [14]. Ni and Co are the most used first-row transition metals which have been alloyed with noble metals (Au, Pt, Pd). These alloy nanoparticles can enhance the catalytic performance and reduce the consumption of the noble metals resulting in a viable approach for the development of low-cost catalysts. Liu and co-workers compared the catalytic activity of NiPt and NiPd nanomaterials and found that NiPt is more efficient than NiPd, and its monometallic counterparts (Ni or Pt) [14b]. The Xu group used metal-organic frameworks (MOFs) to immobilize AuNi nanoparticles and used them as the catalyst for AB hydrolysis with better cata1 lytic performance (TOF ¼ 66.2 mol H2 mol1 cat min ) than those of monometallic Au and Ni immobilized by MOFs [14d]. Trimetallic nanomaterials also show enhanced catalytic activity in hydrolysis of AB and some coreeshell nanoparticles were synthesized and examined [15]. Herein, we report the synthesis of a trimetallic nanomaterial supported on carbon (NiAgPd/C) by reduction of the corresponding metal salts under mild reaction conditions. This trimetallic nanomaterial shows excellent catalytic activity and stability toward hydrogen generation though AB hydrolysis at room temperature (21 C), comparable to those of Pt-based catalysts.
Fig. 1 e Plot of time vs the ratio of hydrogen and AB in the presence of Ni, Ag, Pd based catalysts at 21 C (nmetal:nAB ¼ 0.012).
solution to reduce the metal salts. This supported nanomaterial was conveniently obtained through filtration. The catalytic activities of the resulting NiAgPd/C and its monometallic (Ni/C, Ag/C, and Pd/C), bimetallic (NiPd/C, NiAg/ C, and AgPd/C) counterparts were investigated in hydrogen production reaction from AB aqueous solution at room temperature (Fig. 1). Obviously, the as-prepared NiAgPd/C exhibits a much better catalytic activity than its mono-metallic and bimetallic counterparts which were synthesized through similar processes. 3.0 equivalents of hydrogen were produced from the AB aqueous solution when NiAgPd/C or AgPd/C was used as the catalyst, indicating the complete hydrogen generation. The high TOF value was obtained 1 (93.8 mol H2 mol1 cat min ) from the NiAgPd/C catalyst (Fig. S1). This value is also comparable to those reported for Pt and Rh catalysts [11]. It is evidenced that Ni can enhance the catalytic activity by comparing the catalytic activity of NiAgPd/C with
Results and discussion Catalytic hydrogen generation through AB hydrolysis NiAgPd/C was synthesized by using co-reduction of meal salts in water without any surfactants. Typically, the active carbon was first dispersed in water using ultrasound and stirred under argon atmosphere. The solution of these three metal salts was then added to the well-dispersed active carbon under argon before the treatment of fresh NaBH4 aqueous
Fig. 2 e The TOF values obtained from different concentration of AB using NiAgPd/C catalyst at 21 C (nmetal:nAB ¼ 0.012).
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that of AgPd/C and the catalytic activity of NiPd/C with that of Pd/C. From its monometallic counterparts, Ni/C shows higher catalytic activity than Pd/C and Ag/C and 2.8 equivalents of hydrogen were generated in 80 min. The catalytic activity of NiPd/C is higher than those of AgPd/C and AgNi/C and 2.8 equivalents of hydrogen were generated in 18 min. The catalytic activity of NiAgPd/C was further compared with the mechanical mixture of its monometallic counterparts (molar ratio of Ni, Ag and Pd is the same as that in NiAgPd/C). While the mixture showed an improved catalytic activity than those of individual catalysts and 3 equivalents of hydrogen were generated in 70 min, the catalytic activity was significantly 1 lower (3.85 mol H2 mol1 cat min ) when compared with the NiAgPd/C catalyst. Hydrogen production rates at different AB concentrations were also studied in the presence of NiAgPd/C catalyst. As shown in Fig. 2, TOF values were relatively stable (approx. 1 90 mol H2 mol1 cat min ), and no significant change was observed from 0.1 to 8.4 mol/L AB aqueous solutions. These results indicated that the catalytic activity was not influenced by the concentration of the AB solution. From the gas production curve in the presence of NiAgPd/C, the ratio of the volume of the hydrogen versus time is constant, consistent with the observation that the catalytic activity is not concentration dependent. At high concentration of 8.4 mol/L, some solids were formed after the reaction. These solids were later confirmed to be borates due to their limited solubility in water [9b]. The catalytic activity of NiAgPd/C was further examined using a borates saturated solution under the similar reaction condition. 3.0 equivalents of hydrogen gas were 1 generated and the TOF value was 91.1 mol H2 mol1 cat min , close to those obtained from lower AB concentrations. These results suggest that NiAgPd/C is suitable for continuously generating hydrogen from AB aqueous solutions by removing the borates sediment in the reaction and the catalytic activity will not be significantly affected by changing the concentration of the AB solution. As catalytic rate can be generally accelerated by increasing the reaction temperature, different temperatures for catalytic hydrogen generation were also considered. Hydrogen can be generated completely in a shorter time in the presence of the NiAgPd/C catalyst when increasing the reaction temperature. At 80 C, 3.0 equivalents of hydrogen were obtained in 17 s and
Table 1 e H2 generation from aqueous AB solution catalyzed by Ni, Ag, Pd, Pt based catalysts at room temperature. Catalyst
2 wt.% Pt/C 20 wt.% Pt/C Pt black PtO2 Ni in starch Bare Ni NPs PVP-Ni NPs Ag@Ni/G Ag/C/Ni Ni/ZIF-8 Pd/HAP Pd black CoPd/C NiAgPd/C
Metal/AB (mol/mol) 0.018 0.018 0.018 0.018 0.1 0.1 0.1 0.05 0.022 0.016 0.02 0.018 0.024 0.012
TOF Ea (kJ/mol) (mol H2 1 mol1 cat min ) 111 83.34 13.89 20.8 5 2.5 3 77.0 5.32 14.2 6.8 0.67 22.7 93.8
e e e e e e e 49.56 38.91 e 55 ± 2 e 27.5 38.36
Ref
[9b] [9b] [9b] [9b] [17] [12a] [12a] [16b] [18] [12g] [19] [9a] [14b] This work
the corresponding TOF value could reach 1 729.5 mol H2 mol1 (Fig. S2). Under these conditions cat min (NiAgPd/C, 80 C), hydrogen could be generated continuously by re-adding AB and water into the solution, and the hydrogen production rate reached 29.8 mol H2 h1 g1 cat. These parameters indicate that in order to produce 1 kW h of electricity in a proton exchange membrane (PEM) fuel cell system, only about 0.9 g of NiAgPd/C catalyst would be sufficient. Even at room temperature (21 C), 7.3 g of NiAgPd/C catalyst would be sufficient to provide hydrogen for the PEM fuel cell system (the amount of NH3BH3 needed is 0.27 kg [11], assume the operation circuit voltage is 0.7 V for this standard PEM fuel cell). Fig. 3 shows that hydrogen production rates are nearly constants at different temperatures. These rate constants (k) can be used to calculate the activation energy for NiAgPd/C catalytic hydrogen generation reaction [14a,c,16]. With these rate constants obtained at different temperatures, Arrhenius plot (ln k vs 1/T) for the catalytic hydrogen production reaction gave the activation energy of 38.4 kJ/mol. By comparing the catalytic activity with Ni, Ag, and Pd based catalysts (Table 1), NiAgPd/C shows the optimal catalytic activity comparable to those of Pt based catalysts (Pt/C, Pt black and PtO2 catalysts) [9b].
Fig. 3 e (A) Hydrogen generation from AB aqueous in the presence of NiAgPd/C catalyst (nmetal:nAB ¼ 0.012) at 20, 35, 50, 65 and 80 C; (B) Arrhenius plot of ln k vs (1/T).
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exhibits good stability in hydrogen generation from AB aqueous solution and has 10,216 turnovers with 6.2 L hydrogen generation by continuously adding AB solution into the reaction system. Particularly, the TOF value obtained from the last addition of AB in lifetime measuring, can also reach 1 90.3 mol H2 mol1 cat min , similar to the fresh level 1 1 (93.8 mol H2 molcat min ). These results proved that NiAgPd/C catalyst can be a promising non-Pt catalyst for hydrogen generation from AB hydrolysis.
Catalyst characterization
Fig. 4 e Plot of time vs the ratio of hydrogen and AB in the presence of NiAgPd/C catalyst during a five cycle reusability test (nmetal:nAB ¼ 0.012) at 21 C.
The stability and reusability of the as-synthesized NiAgPd/ C catalyst was also tested in the AB hydrolysis at room temperature. The volume of hydrogen was measured at different time, and re-addition of AB solutions (2.0 mmol) was carried out after the complete hydrogen generation in the previous addition. The first four cycles were obtained continually and they could all be finished in 160 s (Fig. 4). In order to evaluate the stability of the catalyst, the reaction mixture was further stirred at room temperature for 24 h after the fourth cycle was completed. Another one equivalent of AB (2.0 mmol) was then added, and complete hydrogen generation was achieved in 170 s with the TOF value remaining approximately the same 1 (88.4 mol H2 mol1 cat min , Fig. S3). The reaction solution was analyzed after the hydrogen generation, and only 0.14% of the Ni leakage was detected by ICP-MS. The size and morphology were not significantly changed during the reaction (Fig. 5B). The lifetime of NiAgPd/C was estimated by calculating the TON value and the TOF value in the reaction. NiAgPd/C
NiAgPd/C catalyst was characterized by ICP-MS to determine the ratio of these three metals and their loading. The results showed that the ratio of Ni:Ag:Pd is close to 1:1:1 with the metal loading of 6.2 wt.%. These nanoparticles were welldispersed on the surface of carbon and the sizes were in a range of 3e8 nm from TEM images and the nanoparticles size distribution (Fig. 5A). The NiAgPd/C catalyst showed good stability and no significant changes were observed in nanoparticle size and morphology after 5 cycles (Fig. 5B). Scanning TEM (STEM) and energy dispersive X-ray (EDX) were also used to characterize the structure and components of the catalyst (Fig. S4). As the nanoparticles were small and supported on carbon, the analysis was interfered by carbon and only weak peaks of Ni, Ag, and Pd were observed. The X-ray diffraction (XRD) pattern exhibited an fcc structure close to that of metallic Ag (Fig. 6A) and no impurity was observed. Together with the ICP-MS results, it was concluded that NiAgPd nanoparticles were formed to give a pure fcc structure [20]. It is predictable that the fcc structure of NiAgPd is similar to fcc Ag, as the atomic radius of Ag (0.144 nm) is bigger than Pd (0.138 nm) and Ni (0.125 nm) [21]. Pd and Ni will exist in this fcc structure. Since the lattice constant of the Ag is larger than Pd and Ni, evident strain effects on Pd, Ni are expected in this trimetallic nanoparticle, and thus a higher reactivity may be obtained [22]. This expectation was confirmed by our experiment results (Fig. 1) that NiAgPd/C has a higher catalytic activity than NiPd/C, Ni/C and Pd/C. Density functional theory (DFT) calculations (see Supporting Information) were used to
Fig. 5 e TEM images before (A) and after (B) five catalytic cycles (insert: the distribution of the nanoparticles).
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Fig. 6 e (A) XRD pattern and XPS spectra (B: Pd 3d; C: Ag 3d; D: Ni 2p) of NiAgPd/C.
support this strain effects in NiAgPd system. In comparative reactions, the gas generation rate was firstly compared using D2O and H2O as the solvent, the gas generation rate in H2O is about three times than in D2O which shows that the water decomposition is the rate determining step (Fig. S5). Therefore, hydrogen atom adsorption can be used to indicate the catalytic activity [23]. The adsorption energy was firstly compared between Pd and Pd with Ag lattice. Stronger hydrogen adsorption was obtained on Pd with Ag lattice (2.97 eV) than Pd (2.90 eV) which demonstrated that hydrogen will be generated easier in Pd with Ag lattice system. Meanwhile, a higher hydrogen adsorption was observed in NiAgPd system (3.00 eV) than NiPd (2.75 eV), Ni (2.80 eV) and Pd (2.90 eV) system which is in good agreement with our experiment results that NiAgPd shows the highest catalytic activity in hydrogen generation from AB hydrolysis. The X-ray photoelectron spectroscopy (XPS) was also used to demonstrate NiAgPd/C catalyst. Fig. 6B shows the XPS peaks of Ag 3d of the catalyst: two peak at 368.1 and 373.9 eV which are in good agreement with the values for Ag0, standing for Ag 3d5/2 and Ag 3d3/2. From the XPS results in Fig. 6C, peaks with binding energies of 335.8 and 341.1 eV can be attributed to Pd 3d5/2 of the Pd 3d3/2. Peaks in Fig. 6D with binding energies of 856.8 and 874.2 eV represent oxidized Ni and a small peak at 852.9 eV corresponds to Ni0 (15%) (Fig. S6). These results indicate that Ag, Pd are stable, whereas the Ni may be transferred to outside and oxidized to certain degree after the catalyst was synthesized and during its application. Therefore, the slight decrease in the catalytic activity after stirring
under air for 24 h may be attributed to the oxidation of the Ni in that nanomaterial. According to the nature of electronegativity, the electrons may transfer from Ni (1.91) and Ag (1.93) to Pd (2.20) which can make the Pd surface negative with high catalytic activity [24]. This similar phenomenon was observed in CoAuPd/C catalyst for the formic acid decomposition [20].
Conclusion NiAgPd nanoparticles supported on carbon have been designed and successfully prepared. This nanomaterial catalyst exhibits high catalytic activity and stability towards hydrogen generation from NH3BH3 hydrolysis at room temperature. Stoichiometric amounts of hydrogen were produced under a constant 1 TOF of 93.8 mol H2 mol1 cat min . The activation energy was determined to be 38.36 kJ/mol. The process was proved to be efficient in a wide range of concentrations of the NH3BH3 solution to afford stoichiometric amounts of hydrogen with a low catalyst/NH3BH3 ratio (0.012). Hydrogen can be continually produced by adding fresh NH3BH3 solutions. This catalyst provide 10,216 turnovers by continuously adding NH3BH3 solution into the reaction system and the catalytic activity was 1 kept (TOF ¼ 90.3 mol H2 mol1 cat min ). This trimetallic nanomaterial provides a new approach to prepare the catalyst for highly efficient portable hydrogen generation system. When using this generated hydrogen to produce 1 kW h of electricity in a proton exchange membrane (PEM) fuel cell system, about 7.3 g of NiAgPd/C catalyst would be sufficient as the catalyst to
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generate hydrogen at room temperature (the amount of NH3BH3 needed is 0.27 kg, assuming the operation circuit voltage is 0.7 V for a standard PEM fuel cell).
Experimental section Synthesis of NiAgPd/C catalyst: Active carbon (400.0 mg, surface area 450 m2/g) was dispersed in water (100.0 mL) by sonication for 30 min to get a well-dispersed active carbon water solution. To this solution under argon atmosphere was added a water solution of metal salts (Ni(OAc)2$4H2O, 44.8 mg; AgNO3, 30.5 mg; Pd(NO3)2$xH2O, 50.1 mg in 10.0 mL of H2O). This reaction mixture was further ultrasonicated for 30 min before a fresh NaBH4 aqueous solution (200.0 mg NaBH4 in 10.0 mL of H2O) was added dropwise with rigorous stirring. After the addition was completed, this mixture was stirred for 18 h at room temperature. The resulting black precipitates were collected through filtration and dried in the vacuum oven at 120 C for 24 h for further use. For comparison, AgPd/C, NiPd/C, NiAg/C, Ni/C, Pd/C and Ag/C were also prepared by the same method. Hydrogen production by NiAgPd/C catalytic hydrolysis of AB aqueous solution: The catalytic activity of the catalysts was calculated by measuring the volume of the hydrogen produced by hydrolysis of AB in a water-filled gas buret system. The catalyst was first sealed in the reaction flask collect to the gas collection system. AB aqueous solution was then injected into this system under stirring. The volume of hydrogen gas generated from the system was recorded by measuring the displacement of water volume at different time (subtract the volume of the AB solution). When stoichiometric amounts of hydrogen were collected, equivalent of AB solution was readded to test the reusability of the catalyst.
Acknowledgments Financial Support is provided by King Abdullah University of Science and Technology.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.10.032.
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