- Email: [email protected]
La2O3 catalyst for hydrogen generation from hydrous hydrazine decomposition: Effect of Ni–Pt surface alloying
Journal of Power Sources 300 (2015) 294e300
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Highly efficient Ni@NiePt/La2O3 catalyst for hydrogen generation from hydrous hydrazine decomposition: Effect of NiePt surface alloying Yu-Jie Zhong b, Hong-Bin Dai a, **, Yuan-Yuan Jiang b, De-Min Chen b, Min Zhu a, Li-Xian Sun c, Ping Wang a, * a
School of Materials Science and Engineering, Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, PR China Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China c Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, Guangxi Key Laboratory of Information Materials, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Supported coreeshell structured Ni@NiePt/La2O3 catalyst is reported. The catalyst shows excellent activity for selective decomposition of N2H4$H2O to H2. The improved catalytic performance is ascribed to NiePt surface alloying. The developed catalyst may promote the application of N2H4$H2O as a viable H2 carrier.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2015 Received in revised form 14 September 2015 Accepted 17 September 2015 Available online xxx
Hydrous hydrazine has received increasing attention as a promising hydrogen carrier owing to its many favorable attributes, such as high hydrogen content, low material cost, stable liquid state at ambient conditions, and free of solid decomposition byproduct. Herein, we report the synthesis of a supported coreeshell structured Ni@NiePt/La2O3 catalyst by a combination of co-precipitation and galvanic replacement methods. The catalyst exhibits high catalytic activity and 100% selectivity towards hydrogen generation from hydrous hydrazine at mild conditions, which outperforms most reported hydrous hydrazine decomposition catalysts. The favorable catalytic performance of the Ni@NiePt/La2O3 catalyst is correlated with the Pt-induced electronic and geometric modifications on the catalyst surface. © 2015 Elsevier B.V. All rights reserved.
Keywords: Hydrous hydrazine Hydrogen generation Catalyst Surface alloying
1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.-B. Dai), [email protected] (P. Wang). http://dx.doi.org/10.1016/j.jpowsour.2015.09.071 0378-7753/© 2015 Elsevier B.V. All rights reserved.
The lack of safe and efficient means for hydrogen storage is a major impediment to the widespread use of hydrogen as an energy carrier. Extensive studies for decades on interstitial metal hydrides, complex hydrides and high-specific-area physisorbents have led to
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
no viable material that can reversibly store large amounts of hydrogen at moderate temperatures with fast kinetics [1,2]. Since around 2000, considerable efforts had been directed towards the development of chemical hydrogen storage materials as an alternative solution for vehicular and portable H2 source applications [3,4]. Among the materials of interest, hydrazine monohydrate (N2H4$H2O) is a less well explored but very promising candidate. N2H4$H2O is a water-like liquid, with high hydrogen density (8 wt%) and relatively low cost. It can be safely stored under ambient conditions and readily transported using the existing fuel infrastructure. Importantly, decomposition of N2H4$H2O does not generate any solid byproducts, which eliminates the need of regeneration and is of clear benefit to the design and construction of hydrogen source system. A combination of these favorable attributes makes N2H4$H2O an even more competitive hydrogen carrier compared with the previously extensively studied chemical hydrides, like sodium borohydride and ammonia borane [3,4].
N2 H4 /N2 þ 2H2
(1)
3N2 H4 /N2 þ 4NH3
(2)
Hydrazine (N2H4) is the effective hydrogen storage component of N2H4$H2O, which decomposes via two competitive reaction pathways following Eqns. (1) and (2), respectively. Since ammonia is a deleterious impurity that may poison low-temperature fuel cells, reaction (2) must be selectively restrained and reaction (1) promoted. To this end, a number of group Ⅷ transition metal/alloy catalysts have been prepared using chemical reduction [5e12], coprecipitation [13e17] and wetness impregnation [18e21] methods. Some of them, like NieAl2O3eHT [13], Ir/g-Al2O3 [18], and NieM alloys (M ¼ Rh, Ir, Pt, Pd, Fe, Mo) [6e12,15e21], enable selective decomposition of N2H4$H2O to generate H2 at mild conditions. But in a general view, the currently available catalysts exhibit only moderate performance in catalytic activity, selectivity and durability, which results in slow H2 generation kinetics and problematic controllability of N2H4$H2O decomposition. Novel methodologies for the controlled synthesis of catalysts with tailored catalytic property are still required for developing N2H4$H2O as a viable hydrogen carrier. In the present study, we employed a combination of alloying and basic support immobilization strategies to prepare NiePt/ La2O3 catalyst. To ensure an efficient utilization of noble metal Pt and to minimize its amount, we prepared the catalyst using a coprecipitation followed by galvanic replacement method. Our study found that the NiePt/La2O3 catalyst is composed of La2O3 support and tiny immobilized NiePt nanoparticles with, most likely, a core/shell structure. The NiePt/La2O3 catalyst shows excellent activity and satisfactory stability for selectively catalyzing the decomposition of N2H4$H2O to generate H2 at mild conditions. Its overall catalytic performance stands out as the top level of N2H4$H2O decomposition catalysts reported to date. 2. Experimental 2.1. Chemicals and preparation of the catalysts Nickel nitrate hexahydrate (Ni(NO3)3$6H2O, 98%), chloroplatinic acid (H2PtCl6$6H2O, Pt content 37%), lanthanum nitrate hexahydrate (La(NO3)3$6H2O, 99%), ethanol (C2H5OH, 99%), sodium hydroxide (NaOH, 96%) and tetramethylammonium hydroxide (TMAH) were purchased from Aladdin. N2H4$H2O (99%) was purchased from Alfa Aesar. All reagents were used as received. Deionized water was used in preparation of all the aqueous solutions.
295
The Ni/La2O3 catalyst, with a fixed 50 mol% Ni content, was prepared using a co-precipitation method. In a typical preparation procedure, 5 mmol of Ni(NO3)2 and 10 mmol of La(NO3)3 were dissolved in 60 mL of C2H5OH and another 20 mL of C2H5OH solution containing 42 mmol TMAH was then dropwise added under magnetic stirring. After standing at 60 C for 1.5 h, the solution mixture was transferred into a sealed Teflon-lined autoclave and maintained at 100 C for 12 h. The collected precipitate was first dried in an oven at 60 C for 12 h, followed by calcination at 500 C in air for 2 h, and finally reduced at 400 C under a flowing H2 atmosphere for 1.5 h. Similar procedure was also used in preparation of the Pt/La2O3 catalyst.
2Ni þ PtCl62 /Pt þ 2Ni2þ þ6Cl
(3)
The incorporation of Pt element into the Ni/La2O3 catalyst was conducted by a galvanic replacement method, which involves the reaction following Eqn. (3). The as-prepared Ni/La2O3 catalyst was impregnated with an aqueous H2PtCl6 solution at room temperature, and the Pt/Ni molar ratio was adjusted in a range from 1/80 to 1/10. After the reaction completed, the black precipitate was separated from the solution by centrifugation, followed by washing with deionized water and ethanol thoroughly to remove the residual Cle and Ni2þ ions. The catalyst samples was dried under dynamic vacuum for 12 h, and then calcined at 350 C under a flowing H2 atmosphere for 2 h. The as-prepared NiePt/La2O3 catalysts were stored in an Ar-filled glove box to minimize oxidation. 2.2. Characterization of the catalysts The catalyst samples were characterized by powder X-ray diffraction (XRD, Rigaku D/MAX-2500, Cu Ka radiation), X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Al Ka X-ray source) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai F30), which is equipped with an energy dispersive X-ray spectroscopy (EDS) analysis unit. In the XPS measurements, highresolution scans of elemental lines were recorded at 50 eV pass energy of the analyzer. All the binding energies (BEs) were calibrated using the C 1s peak (at 284.6 eV) of the adventitious carbon as an internal standard. In preparation of the TEM samples, the catalyst powder was first dispersed in ethanol by ultrasound and then deposited on a copper grid coated with a holy carbon film. Element analyses of the catalyst samples were conducted in an inductively coupled plasma-atomic emission spectrometry (ICPAES, Iris Intrepid). The N2 temperature programmed desorption (N2-TPD) measurements were conducted in a Micromeritics 2920 apparatus equipped with a thermal conductivity detector and a mass spectrometer system. 2.3. Catalyst performance testing The catalytic decomposition of N2H4$H2O was conducted in a 50 mL two-neck round flask under magnetic stirring. During the measurement, the flask was placed in a thermostat that was equipped with a water circulating system to maintain the reaction temperature, typically within ±0.5 C. In a typical measurement run, the flask containing alkaline aqueous solution and the powdery catalyst was pre-heated and held at the designated temperature, and then N2H4$H2O was injected to the flask to initiate the decomposition reaction. The gaseous products were allowed to pass through a trap containing 1.0 M HCl to absorb ammonia, if any, and then measured by a gravimetric water-displacement method using an electronic balance with an accuracy of 0.01 g. The weight data were automatically recorded by data acquisition software (one datum every 10 s) and the determined gas amount was normalized
296
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
to standard condition. In determination of reaction rate, all the Ni atoms were assumed to contribute to the catalytic performance and the time required for a 50% conversion of N2H4$H2O was used in calculation [14,16]. The selectivity towards H2 generation (X) was calculated following Eqn. (4), which can be derived from Eqns. (1) and (2).
X¼
3Y 1 nðN2 Þ þ nðH2 Þ Y¼ 8 nðN2 H4 Þ
(4)
3. Results Heterogeneous catalysis is dominated by surface interactions rather than bulk behaviors. The study of heterogeneous catalysts should therefore focus on the surface chemical composition and physical structure. This demand becomes even more urgent for the noble-metal-containing catalysts, owing to the concerns about material cost. In the present study, we attempt to synthesize a supported coreeshell structured Ni@NiePt/La2O3 catalyst by a combination of co-precipitation and galvanic replacement methods. As schemed in Fig. 1, a supported monometallic Ni/La2O3 catalyst is first prepared by a co-precipitation method, and then the galvanic replacement reaction between Ni and H2PtCl6 solution results in the formation of Pt surface layer on the Ni particles, the resulting Ni@Pt/La(OH)3 catalyst was finally calcinated under reductive atmosphere to prepare the targeted Ni@NiePt/La2O3 catalyst with a Ni core and a thin NiePt alloy shell. Fig. 2A presents the XRD patterns of the Ni/La2O3, Ni@Pt/ La(OH)3 and Ni@NiePt/La2O3 catalysts that were collected at different stages. All the catalyst samples showed weak but detectable peak of metallic Ni, indicative of the nanophase structure of the catalysts. The metallic Pt, however, was not detected in all the Pt-containing catalysts. In addition, the conversion of La2O3 to La(OH)3 in aqueous solution and its inverse transformation in the calcination treatment at elevated temperatures were clearly evidenced by the XRD results. A close examination of the XRD patterns found that the as-synthesized Ni@Pt/La(OH)3 catalyst through the replacement reaction showed well-indexed peak of fcc Ni. But upon calcination treatment, the Ni (111) peak was observed to shift towards lower angle and the shifting degree increased with elevating the Pt/Ni molar ratio. These results clearly suggest that the alloying of Ni with Pt occurred primarily in the calcination step. As a consequence of the substitution of larger Pt atoms for the smaller Ni atoms, the lattice constant of fcc Ni increases with increasing the Pt/Ni molar ratio. In addition, the calcination treatment resulted in an appreciable grain growth of metallic Ni from around 5 to 6.5 nm, as estimated by the Scherrer equation. Fig. 2B shows a representative TEM image of the Ni@NiePt/La2O3 catalyst. The nanoparticles with an average size of around 20 nm were observed to agglomerate together into large clusters. A close examination of the microstructure by HRTEM and Fast Fourier transformation (FFT)
clearly showed the coexistence of two types of tiny nanocrystallites with random orientations and amorphous phase, as seen in Fig. 2C, D. According to the spacing of the lattice fringes, the nanocrystallites were safely assigned to NiePt alloy and La2O3, respectively. In a parallel HRTEM analysis of the Ni@Pt/La(OH)3 catalyst, however, we observed a distinct lattice fringe with a 0.203 nm spacing (See in the Support Information, Fig. S1), which is characteristic of the (111) plane of fcc Ni. These results are consistent with the XRD results, evidencing the formation of NiePt alloy in the calcination process. In an effort to determine its core/shell structure, we examined the Ni@NiePt/La2O3 catalyst using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and EDS analysis techniques. As seen in Fig. 2F, the EDS line-scanning profile clearly showed an intensified Ni signal in the central region of the nanoparticle. This result implies the presence of Ni core inside the NiePt alloy shell, which is too thin to be distinguished in this case. In view of the adopted preparation method, the low Pt concentration as well as the very close surface energies of Pt and Ni (2.475 vs 2.450 J m 2) [15], it is quite understandable for the formation of core/shell structure instead of homogeneous NiePt alloy. Furthermore, XPS analyses of the Ni/ La2O3, Pt/La2O3 and Ni@NiePt/La2O3 catalysts were conducted to characterize the elemental chemical states (See in the Supporting Information, Fig. S2). Both Ni and Pt elements showed two chemically different entities corresponding to metallic and oxide states, whereas La elements in the three catalysts were present mainly La2O3 as well as La(OH)3, which should originate from the reaction of La2O3 with H2O in the air. As compared to the monometallic Ni/ La2O3 and Pt/La2O3 catalysts, no appreciable changes in the binding energies of constitutive elements in the Ni@NiePt/La2O3 catalyst were observed. Catalytic performance of the series of Ni@NiePt/La2O3 catalyst for the decomposition reactions of N2H4,H2O were examined and compared with the relevant monometallic catalysts. As shown in Fig. 3, the Pt/La2O3 catalyst was totally inactive, and the Ni/La2O3 catalyst exhibited poor catalytic activity and moderate selectivity towards H2 generation from N2H4$H2O. But upon incorporating small amount of Pt to form a surface alloy with Ni, the resulting Ni@NiePt/La2O3 catalysts showed drastically enhanced catalytic performance. For example, the Ni@NiePt/La2O3 catalyst with a Pt/ Ni molar ratio of only 1/78 exhibited a ~3-fold enhancement in catalytic activity and an increased H2 selectivity from 72% to 92%, compared with the Ni/La2O3 catalyst. When the Pt/Ni molar ratio was increased to 1/18, the catalyst showed an optimal catalytic performance. It enabled a complete decomposition of N2H4,H2O within ~7 min at 50 C in the presence of 1.0 M NaOH. But it is noteworthy that the H2 selectivity of the catalyst was ~97%, which means a concomitant evolution of considerable amount of NH3 that is detrimental for low temperature fuel cells. In our effort to solve this problem, we fortuitously found that repeatedly treating the catalyst with H2PtCl6 solution followed by calcination treatment provides a simple but effective method for further improving the catalytic performance. As presented in Table 1, the Ni@NiePt/La2O3
Fig. 1. Schematic diagram of preparation of Ni@NiePt/La2O3 catalyst by a combination of co-precipitation and galvanic replacement methods.
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
297
Fig. 2. (A) XRD patterns of Ni/La2O3, Ni@Pt/La(OH)3 and Ni@NiePt/La2O3 catalysts. Note: Small amounts of LaOOH, La(OH)3 and LaO$CO2 phases were detected in the Ni/La2O3 and Ni@NiePt/La2O3 catalyst samples due to the reactions of La2O3 support with H2O or CO2 from the air; (B) TEM image of the Ni@NiePt/La2O3 catalyst with a Pt/Ni molar ratio of 1/8; (C) HRTEM image (inset: FFT spectrum); (D) HRTEM image of the region indicated by the red rectangle in (C); (E) HAADF-STEM image, and (F) EDX line-scanning profile along the direction indicated by the red line in (D). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
catalyst exhibits remarkable improvements on both catalytic activity and H2 selectivity after being subjected to the second time replacement reaction and calcination treatment at 350 C under a reductive atmosphere. Meanwhile, it was noted that improperly controlling calcination temperature may negatively and significantly impact the catalytic performance of the catalyst. Currently, the mechanistic reasons for the property improvement arising upon “second time alloying” as well as the strong property dependence on the calcination condition are unclear. Among the tested Ni@NiePt/La2O3 catalysts, the best one has a nominal Pt/Ni molar ratio of 1/8 and an authentic chemical composition of 48.4 mol% Ni88.4Pt11.6/51.6 mol% La2O3, as determined by elemental analyses. It enabled a complete decomposition of N2H4,H2O to generate H2 within 2.6 min at 50 C in the presence of 1.0 M NaOH, corresponding to a reaction rate of 312 h1, which is assumed that all Ni atoms act as active sites. This catalytic performance stands out
as the top level of the N2H4,H2O decomposition catalysts reported to date, as seen in the Supporting Information, Table S1. Alkali is an effective promoter for H2 generation from N2H4$H2O in the presence of catalyst, whereas its usage alone has no appreciable effect. As shown in Fig. 4, addition of appropriate amount of NaOH exerts remarkable promoting effects on both reaction rate and H2 selectivity. Similar phenomenon was repeatedly observed in the catalytic decomposition of N2H4$H2O over the Ni-based alloy catalysts [9e12,17,19e21]. Currently, the mechanism underlying the promoting effects of alkali is still unclear. One possibility is that a highly basic environment restrains the formation of undesirable NH3 or N2Hþ 5 ions and thereby selectively promoting H2 generation from N2H4$H2O [9,19]. Alternatively, we surmised that OHe ions may interact with the catalyst surface, resulting in modification of the electronic structure of the catalyst, as discussed below. The catalytic decomposition behaviors of N2H4$H2O over the
298
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
Fig. 4. Effect of NaOH concentration on reaction rate and H2 selectivity of the system composed of 4 mL of 0.5 M N2H4$H2O þ x M NaOH solution (x ¼ 0, 0.5, 1, 1.5, 2 M, respectively) and the Ni@NiePt/La2O3 catalyst (Pt/Ni molar ratio ¼ 1/8) at 50 C. The catalyst/N2H4$H2O molar ratio was fixed at 1:10.
Fig. 3. Time course profiles (top) and Pt-content dependence of reaction rate and H2 selectivity of the system (bottom) composed of 4 mL of 0.5 M N2H4$H2O, 1.0 M NaOH and the Ni@NiePt/La2O3 catalyst at 50 C. The catalyst/N2H4$H2O molar ratio was fixed at 1:10.
which is comparable to the values for the systems using the NiePt [17,20] catalyst. In addition, the newly developed Ni@NiePt/La2O3 catalyst was tested in terms of durability in cyclic usage. As shown in Fig. 6, the catalyst retained 82% of its initial activity and showed no appreciable degradation on H2 selectivity after ten times usage. Presumably, the degradation of catalytic activity is due primarily to the transformation of La2O3 to La(OH)3 in aqueous solution, which may induce surface morphology change of the catalyst. As a consequence, some Ni-based nanoparticles on the catalyst surface might be shielded from contact with N2H4$H2O, thus becoming deactivated. Our effort to solve this problem is currently underway.
Table 1 A comparison of catalytic performance between the Ni@NiePt/La2O3 catalyst and those subjected to the second time replacement reaction and calcinations at varied temperatures. Catalysts
Calcination temperature ( C)
Reaction rate (h1)
H2 Selectivity (%)
1st time replacement
350 No calcination 250 350 450 600
105 41 84 312 150 104
97 95 95 100 95 90
2nd time replacement
Note: The measurements were conducted in 4 mL of aqueous solution containing 0.5 M N2H4$H2O þ 1.0 M NaOH at 50 C; The catalyst/N2H4$H2O molar ratio was fixed at 1:10. The bold is the optimal catalytic performance of the catalyst that subjected to the second time replacement and calcination at 350 C.
Ni@NiePt/La2O3 catalyst were further examined at varied temperatures. As expected, N2H4$H2O decomposes faster at higher temperatures, e.g. elevating the temperature from 30 to 60 C resulted in a ~6 fold increase of reaction rate, as seen in Fig. 5. Interestingly, the temperature variation exerts no effect on the H2 selectivity. It was observed that the decomposition of N2H4$H2O over the Ni@NiePt/La2O3 catalyst exhibits 100% H2 selectivity at a temperature range of 30e60 C. This is in sharp contrast to the cases over the NiePt [7] or NiePd [8], NieAl2O3eHT [13] or Ni/CeO2 [14] catalysts, which showed positive or negative temperature dependence of the H2 generation property. From a practical point of view, high and temperature-independent H2 selectivity is clearly a desirable attribute for hydrogen source applications. On the basis of the temperature-dependent rate data, the apparent activation energy of the catalytic decomposition of N2H4$H2O over the Ni@NiePt/La2O3 catalyst was determined to be 56.2 ± 1.9 kJ mol1,
4. Discussion The Ni@NiePt/La2O3 catalyst has proven highly effective for selectively promoting H2 generation from N2H4$H2O. A key factor contributing to the favorable catalytic performance is the formation of NiePt surface alloy. Better understanding the Pt-induced modifications of catalytic activity and reaction selectivity is clearly of significance for the rational design of novel catalysts with finely tuned chemical properties. According to the d-band theory [22e24], catalytic performance of a metal catalyst is closely related to its electronic structure and, more exactly, the closer the d-band center is to the Fermi level, the chemically more active the metal is. In the present study, we utilized the Auger electron spectroscopy (AES) technique to probe the electronic states of the catalysts. As seen in Fig. 7, the Auger spectrum for the Ni L2M2,3M4,5 electronic transition is narrowed in the
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
Fig. 5. Decomposition kinetics curves of N2H4$H2O over the Ni@NiePt/La2O3 catalyst (Pt/Ni molar ratio of 1/8) at varied temperatures. The catalyst/N2H4$H2O molar ratio was fixed at 1:10. The inset gives the Arrhenius treatment of temperature-dependent rate data for determination of the apparent activation energy.
Fig. 6. Cyclic performance of the Ni@NiePt/La2O3 catalyst (Pt/Ni molar ratio of 1/8) in catalyzing decomposition of N2H4$H2O at 50 C. The catalyst/N2H4$H2O molar ratio was fixed at 1:10.
Ni@NiePt/La2O3 catalyst compared with the Ni/La2O3 catalyst, indicating that the width of the Ni d-band just below the Fermi level becomes narrower upon the formation of NiePt surface alloy [25]. Owing to the belief that the transfer of electronic charge between Ni and Pt is negligible, the narrowed width of the Ni d-band below the Fermi level implies an upward shift of the Ni d-band center towards the Fermi level. As a consequence, the antibonding Ni-adsorbate states formed in the Ni@NiePt/La2O3 catalyst are higher in energy than the corresponding antibonding states formed in the Ni/La2O3 catalyst. This means that the antibonding Niadsorbate states are less populated in the Ni@NiePt/La2O3 catalyst compared with the monometallic Ni/La2O3 catalyst, which ultimately strengthens the interaction between the adsorbate and the Ni sites on the Ni@NiePt/La2O3 catalyst [25]. Evidence supporting this contention was obtained from N2-TPD measurements. As shown in Fig. 8, the N2 desorption temperatures for the Pt/La2O3, Ni/La2O3 and Ni@NiePt/La2O3 catalysts were determined at around 152 C, 162 C and 173 C, respectively, indicating an increase of the metaleN bond strength in the order of Pt/La2O3 < Ni/
299
Fig. 7. Ni L2M2,3M4,5 AES spectra for the Ni/La2O3 and Ni@NiePt/La2O3 catalysts.
La2O3 < Ni@NiePt/La2O3. An integrated analysis of these results together with the observed catalytic performance of the serial catalysts further suggests that the metaleN bond strength is a suitable activity descriptor for the N2H4$H2O decomposition catalysts [26,27]. The Pt/La2O3 catalyst binds N too weakly and consistently, shows inability to activate N2H4. The monometallic Ni/La2O3 catalyst binds N with insufficient strength and thereby, exhibits low catalytic activity. Notably, the alloying of Ni with Pt results in strengthening of the NieN bond. But overly strengthened binding of adsorbates will cause poisoning of the active Ni sites. This may account for the observed volcano-type dependence of the catalytic activity on the Pt amount, as seen in Fig. 3. For an optimized N2H4$H2O decomposition catalyst, the compromise between the ability to activate N2H4 and the ability to avoid poisoning by N2H4 and reaction intermediates must be achieved. The improved activity of the Ni@NiePt/La2O3 catalyst towards N2H4$H2O decomposition is ascribed to the Pt-induced modification of electronic structure, while the increase of H2 selectivity is believed to originate from a Pt-induced geometric effect. In general, N2H4 can be adsorbed on metal surface in anti, cis and gauche conformations and undergoes decomposition following intramolecular or intermolecular reaction pathways, depending upon the nature and structure of metal catalysts as well as reaction conditions [28e30]. A recent theoretical study of N2H4 adsorption on Ni (111) suggested that anti conformation is the most stable configuration, followed by gauche and cis conformations [29]. In the anti and gauche conformations, N2H4 is bonded to the Ni surface through one N atom with its NeN axis tilting from the Ni surface, whereas the cis conformation prefers a bridging configuration with both N atoms bonding to the Ni surface and its NeN axis paralleling to the Ni surface. From a perspective of hydrogen storage, the latter bridging configuration is undesirable as it may allow the dissociation of N2H4 via NeN bond cleavage, and the resulting amide radicals (NH2) can readily react with another N2H4 to yield NH3 and N2 [30]. For the monometallic Ni catalyst with contiguous active sites, it is impossible to suppress the formation of bridging configuration, since this energetically unfavorable configuration may exist as a transition state in the decomposition of N2H4 [29]. But upon incorporating Pt to form NiePt surface alloy, the highly symmetric Ni active sites might be disturbed and as a consequence of the modified surface geometry, the adsorption of N2H4 in the undesirable bridging configuration might be suppressed throughout the reaction process.
300
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
N2H4,H2O to generate H2 within ~10 min at 30 C in the presence of 1.0 M NaOH, which outperforms most reported N2H4,H2O decomposition catalysts. The synthesis of high-performance catalyst and particularly the development of novel methodology for controlling the surface structure and composition may greatly promote the utilization of N2H4$H2O as a viable hydrogen carrier for vehicular and portable applications. Acknowledgments The financial supports for this research from the National Outstanding Youth Science Foundation of China (Grant No. 51125003), National Natural Science Foundation of China (Grant No. 51471168) and 985 Project of South China University of Technology are gratefully acknowledged. Appendix A. Supplementary data
Fig. 8. N2-TPD results of the Pt/La2O3, Ni/La2O3 and Ni@NiePt/La2O3 catalyst samples.
The promoting effects of alkali on the reaction rate and H2 selectivity can be understood from a combination of geometric and electronic effects. It is speculated that, in the alkaline aqueous solution, considerable amount of Ni active sites are occupied or intercepted by OHe ions, thus making them unavailable for adsorption of N2H4 molecules. This is in favor of the N2H4 adsorption through one bonded N atom to the catalyst surface, thereby contributing to the increase of H2 selectivity. Meanwhile, the coadsorbed OHe ion may act as an electron donor and induce charge transfer from the lone-pair orbital of OHe ion to the d-band of Ni atom. As a consequence of increased electron density, the charge transfer from Ni catalyst to the N2H4-antibonding orbital is expected to increase, which facilitates the decomposition of N2H4. The excellent catalytic performance of the Ni@NiePt/La2O3 catalyst has been correlated with the geometric and electronic structure changes in response to the NiePt surface alloying. But the mechanistic reasons for the remarkable property improvements arising upon the “second time alloying” as well as the strong property dependence on the calcination condition are still unclear. These observations suggest that subtle compositional and/or structural changes in the surface or near-surface region may exert profound effects on the catalytic performance of the catalysts [26,27]. In this regard, coupled theoretical and experimental studies are required to obtain atomic- and molecular-level understanding of the physical and chemical processes involved in the catalytic decomposition of N2H4$H2O. 5. Conclusions A supported coreeshell structured Ni@NiePt/La2O3 catalyst was prepared using a co-precipitation followed by galvanic replacement method. As a consequence of the Pt-induced electronic and geometric modifications on the catalyst surface, the Ni@NiePt/La2O3 catalyst exhibits high activity and selectivity towards H2 generation from N2H4,H2O at mild conditions. For example, the catalyst with an optimal composition enabled complete decomposition of
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.09.071. References [1] S. Satyapal, J. Petrovic, C. Read, G. Thomas, G. Ordaz, Catal. Today 120 (2007) 246e256. [2] S.I. Orimo, Y. Nakamori, J.R. Eliseo, A. Züttel, C.M. Jensen, Chem. Rev. 107 (2007) 4111e4132. [3] U.B. Demirci, P. Miele, Energy Environ. Sci. 4 (2011) 3334e3341. [4] M. Yadav, Q. Xu, Energy Environ. Sci. 5 (2012) 9698e9725. [5] S.K. Singh, X.B. Zhang, Q. Xu, J. Am. Chem. Soc. 131 (2009) 9894e9895. [6] S.K. Singh, Q. Xu, J. Am. Chem. Soc. 131 (2009) 18032e18033. [7] S.K. Singh, Z.H. Lu, Q. Xu, Eur. J. Inorg. Chem. (2011) 2232e2237. [8] S.K. Singh, Y. Iizuka, Q. Xu, Int. J. Hydrogen Energy 36 (2011) 11794e11801. [9] S.K. Singh, A.K. Singh, K. Aranishi, Q. Xu, J. Am. Chem. Soc. 133 (2011) 19638e19641. [10] J. Wang, Y. Li, Y. Zhang, Adv. Funct. Mater 24 (2014) 7073e7077. [11] H.L. Wang, J.M. Yan, Z.L. Wang, S.I. O, Q.J. Jiang, J. Mater. Chem. A 1 (2013) 14957e14962. [12] H.L. Wang, J.M. Yan, S.J. Li, X.W. Zhang, Q. Jiang, J. Mater. Chem. A 3 (2015) 121e124. [13] L. He, Y.Q. Huang, A.Q. Wang, X.D. Wang, X.W. Chen, J.J. Delgado, T. Zhang, Angew. Chem. Int. Ed. 51 (2012) 6191e6194. [14] L. He, B. Liang, L. Li, X. Yang, Y. Huang, A. Wang, X. Wang, T. Zhang, ACS Catal. 5 (2015) 1623e1628. [15] L. He, Y. Huang, X.Y. Liu, L. Li, A. Wang, X. Wang, C.-Y. Mou, T. Zhang, Appl. Catal. B Environ. 147 (2014) 779e788. [16] L. He, Y.Q. Huang, A.Q. Wang, Y. Lu, X.Y. Liu, X.W. Chen, J.J. Delgado, X.D. Wang, T. Zhang, J. Catal. 298 (2013) 1e9. [17] Y.Y. Jiang, Q. Kang, J.J. Zhang, H.B. Dai, P. Wang, J. Power Sources 273 (2015), 554e550. [18] S.J. Cho, J. Lee, Y.S. Lee, D.P. Kim, Catal. Lett. 109 (2006) 181e186. [19] J. Wang, X.B. Zhang, Z.L. Wang, L.M. Wang, Y. Zhang, Energy Environ. Sci. 5 (2012) 6885e6888. [20] N. Cao, J. Su, W. Luo, G.Z. Cheng, Int. J. Hydrogen Energy 39 (2014) 9726e9734. [21] N. Cao, L. Yang, C. Du, J. Su, W. Luo, G.Z. Cheng, J. Mater. Chem. A 2 (2014) 14344e14347. [22] B. Hammer, J.K. Nørskov, Surf. Sci. 343 (1995) 211e220. [23] B. Hammer, Y. Morikawa, J.K. Nørskov, Phys. Rev. Lett. 76 (1996) 2141e2144. [24] B. Hammer, J.K. Nørskov, Nature 376 (1995) 238e240. [25] E. Nikolla, J. Schwank, S. Linic, J. Am. Chem. Soc. 131 (2009) 2747e2754. [26] F. Calle-Vallejo, M.T.M. Koper, A.S. Bandarenka, Chem. Soc. Rev. 42 (2013) 5210e5230. [27] A. Holewinski, H.L. Xin, E. Nikolla, S. Linic, Curr. Opin. Chem. Eng. 2 (2013) 312e319. [28] M.K. Agusta, M. David, H. Nakanishi, H. Kasai, Surf. Sci. 604 (2010) 245e251. [29] M.K. Agusta, H. Kasai, Surf. Sci. 606 (2012) 766e771. [30] P.X. Zhang, Y.G. Wang, Y.Q. Huang, T. Zhang, G.S. Wu, J. Li, Catal. Today 165 (2011) 80e88.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Highly efficient Ni@NiePt/La2O3 catalyst for hydrogen generation from hydrous hydrazine decomposition: Effect of NiePt surface alloying Yu-Jie Zhong b, Hong-Bin Dai a, **, Yuan-Yuan Jiang b, De-Min Chen b, Min Zhu a, Li-Xian Sun c, Ping Wang a, * a
School of Materials Science and Engineering, Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, PR China Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China c Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, Guangxi Key Laboratory of Information Materials, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Supported coreeshell structured Ni@NiePt/La2O3 catalyst is reported. The catalyst shows excellent activity for selective decomposition of N2H4$H2O to H2. The improved catalytic performance is ascribed to NiePt surface alloying. The developed catalyst may promote the application of N2H4$H2O as a viable H2 carrier.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2015 Received in revised form 14 September 2015 Accepted 17 September 2015 Available online xxx
Hydrous hydrazine has received increasing attention as a promising hydrogen carrier owing to its many favorable attributes, such as high hydrogen content, low material cost, stable liquid state at ambient conditions, and free of solid decomposition byproduct. Herein, we report the synthesis of a supported coreeshell structured Ni@NiePt/La2O3 catalyst by a combination of co-precipitation and galvanic replacement methods. The catalyst exhibits high catalytic activity and 100% selectivity towards hydrogen generation from hydrous hydrazine at mild conditions, which outperforms most reported hydrous hydrazine decomposition catalysts. The favorable catalytic performance of the Ni@NiePt/La2O3 catalyst is correlated with the Pt-induced electronic and geometric modifications on the catalyst surface. © 2015 Elsevier B.V. All rights reserved.
Keywords: Hydrous hydrazine Hydrogen generation Catalyst Surface alloying
1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.-B. Dai), [email protected] (P. Wang). http://dx.doi.org/10.1016/j.jpowsour.2015.09.071 0378-7753/© 2015 Elsevier B.V. All rights reserved.
The lack of safe and efficient means for hydrogen storage is a major impediment to the widespread use of hydrogen as an energy carrier. Extensive studies for decades on interstitial metal hydrides, complex hydrides and high-specific-area physisorbents have led to
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
no viable material that can reversibly store large amounts of hydrogen at moderate temperatures with fast kinetics [1,2]. Since around 2000, considerable efforts had been directed towards the development of chemical hydrogen storage materials as an alternative solution for vehicular and portable H2 source applications [3,4]. Among the materials of interest, hydrazine monohydrate (N2H4$H2O) is a less well explored but very promising candidate. N2H4$H2O is a water-like liquid, with high hydrogen density (8 wt%) and relatively low cost. It can be safely stored under ambient conditions and readily transported using the existing fuel infrastructure. Importantly, decomposition of N2H4$H2O does not generate any solid byproducts, which eliminates the need of regeneration and is of clear benefit to the design and construction of hydrogen source system. A combination of these favorable attributes makes N2H4$H2O an even more competitive hydrogen carrier compared with the previously extensively studied chemical hydrides, like sodium borohydride and ammonia borane [3,4].
N2 H4 /N2 þ 2H2
(1)
3N2 H4 /N2 þ 4NH3
(2)
Hydrazine (N2H4) is the effective hydrogen storage component of N2H4$H2O, which decomposes via two competitive reaction pathways following Eqns. (1) and (2), respectively. Since ammonia is a deleterious impurity that may poison low-temperature fuel cells, reaction (2) must be selectively restrained and reaction (1) promoted. To this end, a number of group Ⅷ transition metal/alloy catalysts have been prepared using chemical reduction [5e12], coprecipitation [13e17] and wetness impregnation [18e21] methods. Some of them, like NieAl2O3eHT [13], Ir/g-Al2O3 [18], and NieM alloys (M ¼ Rh, Ir, Pt, Pd, Fe, Mo) [6e12,15e21], enable selective decomposition of N2H4$H2O to generate H2 at mild conditions. But in a general view, the currently available catalysts exhibit only moderate performance in catalytic activity, selectivity and durability, which results in slow H2 generation kinetics and problematic controllability of N2H4$H2O decomposition. Novel methodologies for the controlled synthesis of catalysts with tailored catalytic property are still required for developing N2H4$H2O as a viable hydrogen carrier. In the present study, we employed a combination of alloying and basic support immobilization strategies to prepare NiePt/ La2O3 catalyst. To ensure an efficient utilization of noble metal Pt and to minimize its amount, we prepared the catalyst using a coprecipitation followed by galvanic replacement method. Our study found that the NiePt/La2O3 catalyst is composed of La2O3 support and tiny immobilized NiePt nanoparticles with, most likely, a core/shell structure. The NiePt/La2O3 catalyst shows excellent activity and satisfactory stability for selectively catalyzing the decomposition of N2H4$H2O to generate H2 at mild conditions. Its overall catalytic performance stands out as the top level of N2H4$H2O decomposition catalysts reported to date. 2. Experimental 2.1. Chemicals and preparation of the catalysts Nickel nitrate hexahydrate (Ni(NO3)3$6H2O, 98%), chloroplatinic acid (H2PtCl6$6H2O, Pt content 37%), lanthanum nitrate hexahydrate (La(NO3)3$6H2O, 99%), ethanol (C2H5OH, 99%), sodium hydroxide (NaOH, 96%) and tetramethylammonium hydroxide (TMAH) were purchased from Aladdin. N2H4$H2O (99%) was purchased from Alfa Aesar. All reagents were used as received. Deionized water was used in preparation of all the aqueous solutions.
295
The Ni/La2O3 catalyst, with a fixed 50 mol% Ni content, was prepared using a co-precipitation method. In a typical preparation procedure, 5 mmol of Ni(NO3)2 and 10 mmol of La(NO3)3 were dissolved in 60 mL of C2H5OH and another 20 mL of C2H5OH solution containing 42 mmol TMAH was then dropwise added under magnetic stirring. After standing at 60 C for 1.5 h, the solution mixture was transferred into a sealed Teflon-lined autoclave and maintained at 100 C for 12 h. The collected precipitate was first dried in an oven at 60 C for 12 h, followed by calcination at 500 C in air for 2 h, and finally reduced at 400 C under a flowing H2 atmosphere for 1.5 h. Similar procedure was also used in preparation of the Pt/La2O3 catalyst.
2Ni þ PtCl62 /Pt þ 2Ni2þ þ6Cl
(3)
The incorporation of Pt element into the Ni/La2O3 catalyst was conducted by a galvanic replacement method, which involves the reaction following Eqn. (3). The as-prepared Ni/La2O3 catalyst was impregnated with an aqueous H2PtCl6 solution at room temperature, and the Pt/Ni molar ratio was adjusted in a range from 1/80 to 1/10. After the reaction completed, the black precipitate was separated from the solution by centrifugation, followed by washing with deionized water and ethanol thoroughly to remove the residual Cle and Ni2þ ions. The catalyst samples was dried under dynamic vacuum for 12 h, and then calcined at 350 C under a flowing H2 atmosphere for 2 h. The as-prepared NiePt/La2O3 catalysts were stored in an Ar-filled glove box to minimize oxidation. 2.2. Characterization of the catalysts The catalyst samples were characterized by powder X-ray diffraction (XRD, Rigaku D/MAX-2500, Cu Ka radiation), X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Al Ka X-ray source) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai F30), which is equipped with an energy dispersive X-ray spectroscopy (EDS) analysis unit. In the XPS measurements, highresolution scans of elemental lines were recorded at 50 eV pass energy of the analyzer. All the binding energies (BEs) were calibrated using the C 1s peak (at 284.6 eV) of the adventitious carbon as an internal standard. In preparation of the TEM samples, the catalyst powder was first dispersed in ethanol by ultrasound and then deposited on a copper grid coated with a holy carbon film. Element analyses of the catalyst samples were conducted in an inductively coupled plasma-atomic emission spectrometry (ICPAES, Iris Intrepid). The N2 temperature programmed desorption (N2-TPD) measurements were conducted in a Micromeritics 2920 apparatus equipped with a thermal conductivity detector and a mass spectrometer system. 2.3. Catalyst performance testing The catalytic decomposition of N2H4$H2O was conducted in a 50 mL two-neck round flask under magnetic stirring. During the measurement, the flask was placed in a thermostat that was equipped with a water circulating system to maintain the reaction temperature, typically within ±0.5 C. In a typical measurement run, the flask containing alkaline aqueous solution and the powdery catalyst was pre-heated and held at the designated temperature, and then N2H4$H2O was injected to the flask to initiate the decomposition reaction. The gaseous products were allowed to pass through a trap containing 1.0 M HCl to absorb ammonia, if any, and then measured by a gravimetric water-displacement method using an electronic balance with an accuracy of 0.01 g. The weight data were automatically recorded by data acquisition software (one datum every 10 s) and the determined gas amount was normalized
296
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
to standard condition. In determination of reaction rate, all the Ni atoms were assumed to contribute to the catalytic performance and the time required for a 50% conversion of N2H4$H2O was used in calculation [14,16]. The selectivity towards H2 generation (X) was calculated following Eqn. (4), which can be derived from Eqns. (1) and (2).
X¼
3Y 1 nðN2 Þ þ nðH2 Þ Y¼ 8 nðN2 H4 Þ
(4)
3. Results Heterogeneous catalysis is dominated by surface interactions rather than bulk behaviors. The study of heterogeneous catalysts should therefore focus on the surface chemical composition and physical structure. This demand becomes even more urgent for the noble-metal-containing catalysts, owing to the concerns about material cost. In the present study, we attempt to synthesize a supported coreeshell structured Ni@NiePt/La2O3 catalyst by a combination of co-precipitation and galvanic replacement methods. As schemed in Fig. 1, a supported monometallic Ni/La2O3 catalyst is first prepared by a co-precipitation method, and then the galvanic replacement reaction between Ni and H2PtCl6 solution results in the formation of Pt surface layer on the Ni particles, the resulting Ni@Pt/La(OH)3 catalyst was finally calcinated under reductive atmosphere to prepare the targeted Ni@NiePt/La2O3 catalyst with a Ni core and a thin NiePt alloy shell. Fig. 2A presents the XRD patterns of the Ni/La2O3, Ni@Pt/ La(OH)3 and Ni@NiePt/La2O3 catalysts that were collected at different stages. All the catalyst samples showed weak but detectable peak of metallic Ni, indicative of the nanophase structure of the catalysts. The metallic Pt, however, was not detected in all the Pt-containing catalysts. In addition, the conversion of La2O3 to La(OH)3 in aqueous solution and its inverse transformation in the calcination treatment at elevated temperatures were clearly evidenced by the XRD results. A close examination of the XRD patterns found that the as-synthesized Ni@Pt/La(OH)3 catalyst through the replacement reaction showed well-indexed peak of fcc Ni. But upon calcination treatment, the Ni (111) peak was observed to shift towards lower angle and the shifting degree increased with elevating the Pt/Ni molar ratio. These results clearly suggest that the alloying of Ni with Pt occurred primarily in the calcination step. As a consequence of the substitution of larger Pt atoms for the smaller Ni atoms, the lattice constant of fcc Ni increases with increasing the Pt/Ni molar ratio. In addition, the calcination treatment resulted in an appreciable grain growth of metallic Ni from around 5 to 6.5 nm, as estimated by the Scherrer equation. Fig. 2B shows a representative TEM image of the Ni@NiePt/La2O3 catalyst. The nanoparticles with an average size of around 20 nm were observed to agglomerate together into large clusters. A close examination of the microstructure by HRTEM and Fast Fourier transformation (FFT)
clearly showed the coexistence of two types of tiny nanocrystallites with random orientations and amorphous phase, as seen in Fig. 2C, D. According to the spacing of the lattice fringes, the nanocrystallites were safely assigned to NiePt alloy and La2O3, respectively. In a parallel HRTEM analysis of the Ni@Pt/La(OH)3 catalyst, however, we observed a distinct lattice fringe with a 0.203 nm spacing (See in the Support Information, Fig. S1), which is characteristic of the (111) plane of fcc Ni. These results are consistent with the XRD results, evidencing the formation of NiePt alloy in the calcination process. In an effort to determine its core/shell structure, we examined the Ni@NiePt/La2O3 catalyst using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and EDS analysis techniques. As seen in Fig. 2F, the EDS line-scanning profile clearly showed an intensified Ni signal in the central region of the nanoparticle. This result implies the presence of Ni core inside the NiePt alloy shell, which is too thin to be distinguished in this case. In view of the adopted preparation method, the low Pt concentration as well as the very close surface energies of Pt and Ni (2.475 vs 2.450 J m 2) [15], it is quite understandable for the formation of core/shell structure instead of homogeneous NiePt alloy. Furthermore, XPS analyses of the Ni/ La2O3, Pt/La2O3 and Ni@NiePt/La2O3 catalysts were conducted to characterize the elemental chemical states (See in the Supporting Information, Fig. S2). Both Ni and Pt elements showed two chemically different entities corresponding to metallic and oxide states, whereas La elements in the three catalysts were present mainly La2O3 as well as La(OH)3, which should originate from the reaction of La2O3 with H2O in the air. As compared to the monometallic Ni/ La2O3 and Pt/La2O3 catalysts, no appreciable changes in the binding energies of constitutive elements in the Ni@NiePt/La2O3 catalyst were observed. Catalytic performance of the series of Ni@NiePt/La2O3 catalyst for the decomposition reactions of N2H4,H2O were examined and compared with the relevant monometallic catalysts. As shown in Fig. 3, the Pt/La2O3 catalyst was totally inactive, and the Ni/La2O3 catalyst exhibited poor catalytic activity and moderate selectivity towards H2 generation from N2H4$H2O. But upon incorporating small amount of Pt to form a surface alloy with Ni, the resulting Ni@NiePt/La2O3 catalysts showed drastically enhanced catalytic performance. For example, the Ni@NiePt/La2O3 catalyst with a Pt/ Ni molar ratio of only 1/78 exhibited a ~3-fold enhancement in catalytic activity and an increased H2 selectivity from 72% to 92%, compared with the Ni/La2O3 catalyst. When the Pt/Ni molar ratio was increased to 1/18, the catalyst showed an optimal catalytic performance. It enabled a complete decomposition of N2H4,H2O within ~7 min at 50 C in the presence of 1.0 M NaOH. But it is noteworthy that the H2 selectivity of the catalyst was ~97%, which means a concomitant evolution of considerable amount of NH3 that is detrimental for low temperature fuel cells. In our effort to solve this problem, we fortuitously found that repeatedly treating the catalyst with H2PtCl6 solution followed by calcination treatment provides a simple but effective method for further improving the catalytic performance. As presented in Table 1, the Ni@NiePt/La2O3
Fig. 1. Schematic diagram of preparation of Ni@NiePt/La2O3 catalyst by a combination of co-precipitation and galvanic replacement methods.
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
297
Fig. 2. (A) XRD patterns of Ni/La2O3, Ni@Pt/La(OH)3 and Ni@NiePt/La2O3 catalysts. Note: Small amounts of LaOOH, La(OH)3 and LaO$CO2 phases were detected in the Ni/La2O3 and Ni@NiePt/La2O3 catalyst samples due to the reactions of La2O3 support with H2O or CO2 from the air; (B) TEM image of the Ni@NiePt/La2O3 catalyst with a Pt/Ni molar ratio of 1/8; (C) HRTEM image (inset: FFT spectrum); (D) HRTEM image of the region indicated by the red rectangle in (C); (E) HAADF-STEM image, and (F) EDX line-scanning profile along the direction indicated by the red line in (D). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
catalyst exhibits remarkable improvements on both catalytic activity and H2 selectivity after being subjected to the second time replacement reaction and calcination treatment at 350 C under a reductive atmosphere. Meanwhile, it was noted that improperly controlling calcination temperature may negatively and significantly impact the catalytic performance of the catalyst. Currently, the mechanistic reasons for the property improvement arising upon “second time alloying” as well as the strong property dependence on the calcination condition are unclear. Among the tested Ni@NiePt/La2O3 catalysts, the best one has a nominal Pt/Ni molar ratio of 1/8 and an authentic chemical composition of 48.4 mol% Ni88.4Pt11.6/51.6 mol% La2O3, as determined by elemental analyses. It enabled a complete decomposition of N2H4,H2O to generate H2 within 2.6 min at 50 C in the presence of 1.0 M NaOH, corresponding to a reaction rate of 312 h1, which is assumed that all Ni atoms act as active sites. This catalytic performance stands out
as the top level of the N2H4,H2O decomposition catalysts reported to date, as seen in the Supporting Information, Table S1. Alkali is an effective promoter for H2 generation from N2H4$H2O in the presence of catalyst, whereas its usage alone has no appreciable effect. As shown in Fig. 4, addition of appropriate amount of NaOH exerts remarkable promoting effects on both reaction rate and H2 selectivity. Similar phenomenon was repeatedly observed in the catalytic decomposition of N2H4$H2O over the Ni-based alloy catalysts [9e12,17,19e21]. Currently, the mechanism underlying the promoting effects of alkali is still unclear. One possibility is that a highly basic environment restrains the formation of undesirable NH3 or N2Hþ 5 ions and thereby selectively promoting H2 generation from N2H4$H2O [9,19]. Alternatively, we surmised that OHe ions may interact with the catalyst surface, resulting in modification of the electronic structure of the catalyst, as discussed below. The catalytic decomposition behaviors of N2H4$H2O over the
298
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
Fig. 4. Effect of NaOH concentration on reaction rate and H2 selectivity of the system composed of 4 mL of 0.5 M N2H4$H2O þ x M NaOH solution (x ¼ 0, 0.5, 1, 1.5, 2 M, respectively) and the Ni@NiePt/La2O3 catalyst (Pt/Ni molar ratio ¼ 1/8) at 50 C. The catalyst/N2H4$H2O molar ratio was fixed at 1:10.
Fig. 3. Time course profiles (top) and Pt-content dependence of reaction rate and H2 selectivity of the system (bottom) composed of 4 mL of 0.5 M N2H4$H2O, 1.0 M NaOH and the Ni@NiePt/La2O3 catalyst at 50 C. The catalyst/N2H4$H2O molar ratio was fixed at 1:10.
which is comparable to the values for the systems using the NiePt [17,20] catalyst. In addition, the newly developed Ni@NiePt/La2O3 catalyst was tested in terms of durability in cyclic usage. As shown in Fig. 6, the catalyst retained 82% of its initial activity and showed no appreciable degradation on H2 selectivity after ten times usage. Presumably, the degradation of catalytic activity is due primarily to the transformation of La2O3 to La(OH)3 in aqueous solution, which may induce surface morphology change of the catalyst. As a consequence, some Ni-based nanoparticles on the catalyst surface might be shielded from contact with N2H4$H2O, thus becoming deactivated. Our effort to solve this problem is currently underway.
Table 1 A comparison of catalytic performance between the Ni@NiePt/La2O3 catalyst and those subjected to the second time replacement reaction and calcinations at varied temperatures. Catalysts
Calcination temperature ( C)
Reaction rate (h1)
H2 Selectivity (%)
1st time replacement
350 No calcination 250 350 450 600
105 41 84 312 150 104
97 95 95 100 95 90
2nd time replacement
Note: The measurements were conducted in 4 mL of aqueous solution containing 0.5 M N2H4$H2O þ 1.0 M NaOH at 50 C; The catalyst/N2H4$H2O molar ratio was fixed at 1:10. The bold is the optimal catalytic performance of the catalyst that subjected to the second time replacement and calcination at 350 C.
Ni@NiePt/La2O3 catalyst were further examined at varied temperatures. As expected, N2H4$H2O decomposes faster at higher temperatures, e.g. elevating the temperature from 30 to 60 C resulted in a ~6 fold increase of reaction rate, as seen in Fig. 5. Interestingly, the temperature variation exerts no effect on the H2 selectivity. It was observed that the decomposition of N2H4$H2O over the Ni@NiePt/La2O3 catalyst exhibits 100% H2 selectivity at a temperature range of 30e60 C. This is in sharp contrast to the cases over the NiePt [7] or NiePd [8], NieAl2O3eHT [13] or Ni/CeO2 [14] catalysts, which showed positive or negative temperature dependence of the H2 generation property. From a practical point of view, high and temperature-independent H2 selectivity is clearly a desirable attribute for hydrogen source applications. On the basis of the temperature-dependent rate data, the apparent activation energy of the catalytic decomposition of N2H4$H2O over the Ni@NiePt/La2O3 catalyst was determined to be 56.2 ± 1.9 kJ mol1,
4. Discussion The Ni@NiePt/La2O3 catalyst has proven highly effective for selectively promoting H2 generation from N2H4$H2O. A key factor contributing to the favorable catalytic performance is the formation of NiePt surface alloy. Better understanding the Pt-induced modifications of catalytic activity and reaction selectivity is clearly of significance for the rational design of novel catalysts with finely tuned chemical properties. According to the d-band theory [22e24], catalytic performance of a metal catalyst is closely related to its electronic structure and, more exactly, the closer the d-band center is to the Fermi level, the chemically more active the metal is. In the present study, we utilized the Auger electron spectroscopy (AES) technique to probe the electronic states of the catalysts. As seen in Fig. 7, the Auger spectrum for the Ni L2M2,3M4,5 electronic transition is narrowed in the
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
Fig. 5. Decomposition kinetics curves of N2H4$H2O over the Ni@NiePt/La2O3 catalyst (Pt/Ni molar ratio of 1/8) at varied temperatures. The catalyst/N2H4$H2O molar ratio was fixed at 1:10. The inset gives the Arrhenius treatment of temperature-dependent rate data for determination of the apparent activation energy.
Fig. 6. Cyclic performance of the Ni@NiePt/La2O3 catalyst (Pt/Ni molar ratio of 1/8) in catalyzing decomposition of N2H4$H2O at 50 C. The catalyst/N2H4$H2O molar ratio was fixed at 1:10.
Ni@NiePt/La2O3 catalyst compared with the Ni/La2O3 catalyst, indicating that the width of the Ni d-band just below the Fermi level becomes narrower upon the formation of NiePt surface alloy [25]. Owing to the belief that the transfer of electronic charge between Ni and Pt is negligible, the narrowed width of the Ni d-band below the Fermi level implies an upward shift of the Ni d-band center towards the Fermi level. As a consequence, the antibonding Ni-adsorbate states formed in the Ni@NiePt/La2O3 catalyst are higher in energy than the corresponding antibonding states formed in the Ni/La2O3 catalyst. This means that the antibonding Niadsorbate states are less populated in the Ni@NiePt/La2O3 catalyst compared with the monometallic Ni/La2O3 catalyst, which ultimately strengthens the interaction between the adsorbate and the Ni sites on the Ni@NiePt/La2O3 catalyst [25]. Evidence supporting this contention was obtained from N2-TPD measurements. As shown in Fig. 8, the N2 desorption temperatures for the Pt/La2O3, Ni/La2O3 and Ni@NiePt/La2O3 catalysts were determined at around 152 C, 162 C and 173 C, respectively, indicating an increase of the metaleN bond strength in the order of Pt/La2O3 < Ni/
299
Fig. 7. Ni L2M2,3M4,5 AES spectra for the Ni/La2O3 and Ni@NiePt/La2O3 catalysts.
La2O3 < Ni@NiePt/La2O3. An integrated analysis of these results together with the observed catalytic performance of the serial catalysts further suggests that the metaleN bond strength is a suitable activity descriptor for the N2H4$H2O decomposition catalysts [26,27]. The Pt/La2O3 catalyst binds N too weakly and consistently, shows inability to activate N2H4. The monometallic Ni/La2O3 catalyst binds N with insufficient strength and thereby, exhibits low catalytic activity. Notably, the alloying of Ni with Pt results in strengthening of the NieN bond. But overly strengthened binding of adsorbates will cause poisoning of the active Ni sites. This may account for the observed volcano-type dependence of the catalytic activity on the Pt amount, as seen in Fig. 3. For an optimized N2H4$H2O decomposition catalyst, the compromise between the ability to activate N2H4 and the ability to avoid poisoning by N2H4 and reaction intermediates must be achieved. The improved activity of the Ni@NiePt/La2O3 catalyst towards N2H4$H2O decomposition is ascribed to the Pt-induced modification of electronic structure, while the increase of H2 selectivity is believed to originate from a Pt-induced geometric effect. In general, N2H4 can be adsorbed on metal surface in anti, cis and gauche conformations and undergoes decomposition following intramolecular or intermolecular reaction pathways, depending upon the nature and structure of metal catalysts as well as reaction conditions [28e30]. A recent theoretical study of N2H4 adsorption on Ni (111) suggested that anti conformation is the most stable configuration, followed by gauche and cis conformations [29]. In the anti and gauche conformations, N2H4 is bonded to the Ni surface through one N atom with its NeN axis tilting from the Ni surface, whereas the cis conformation prefers a bridging configuration with both N atoms bonding to the Ni surface and its NeN axis paralleling to the Ni surface. From a perspective of hydrogen storage, the latter bridging configuration is undesirable as it may allow the dissociation of N2H4 via NeN bond cleavage, and the resulting amide radicals (NH2) can readily react with another N2H4 to yield NH3 and N2 [30]. For the monometallic Ni catalyst with contiguous active sites, it is impossible to suppress the formation of bridging configuration, since this energetically unfavorable configuration may exist as a transition state in the decomposition of N2H4 [29]. But upon incorporating Pt to form NiePt surface alloy, the highly symmetric Ni active sites might be disturbed and as a consequence of the modified surface geometry, the adsorption of N2H4 in the undesirable bridging configuration might be suppressed throughout the reaction process.
300
Y.-J. Zhong et al. / Journal of Power Sources 300 (2015) 294e300
N2H4,H2O to generate H2 within ~10 min at 30 C in the presence of 1.0 M NaOH, which outperforms most reported N2H4,H2O decomposition catalysts. The synthesis of high-performance catalyst and particularly the development of novel methodology for controlling the surface structure and composition may greatly promote the utilization of N2H4$H2O as a viable hydrogen carrier for vehicular and portable applications. Acknowledgments The financial supports for this research from the National Outstanding Youth Science Foundation of China (Grant No. 51125003), National Natural Science Foundation of China (Grant No. 51471168) and 985 Project of South China University of Technology are gratefully acknowledged. Appendix A. Supplementary data
Fig. 8. N2-TPD results of the Pt/La2O3, Ni/La2O3 and Ni@NiePt/La2O3 catalyst samples.
The promoting effects of alkali on the reaction rate and H2 selectivity can be understood from a combination of geometric and electronic effects. It is speculated that, in the alkaline aqueous solution, considerable amount of Ni active sites are occupied or intercepted by OHe ions, thus making them unavailable for adsorption of N2H4 molecules. This is in favor of the N2H4 adsorption through one bonded N atom to the catalyst surface, thereby contributing to the increase of H2 selectivity. Meanwhile, the coadsorbed OHe ion may act as an electron donor and induce charge transfer from the lone-pair orbital of OHe ion to the d-band of Ni atom. As a consequence of increased electron density, the charge transfer from Ni catalyst to the N2H4-antibonding orbital is expected to increase, which facilitates the decomposition of N2H4. The excellent catalytic performance of the Ni@NiePt/La2O3 catalyst has been correlated with the geometric and electronic structure changes in response to the NiePt surface alloying. But the mechanistic reasons for the remarkable property improvements arising upon the “second time alloying” as well as the strong property dependence on the calcination condition are still unclear. These observations suggest that subtle compositional and/or structural changes in the surface or near-surface region may exert profound effects on the catalytic performance of the catalysts [26,27]. In this regard, coupled theoretical and experimental studies are required to obtain atomic- and molecular-level understanding of the physical and chemical processes involved in the catalytic decomposition of N2H4$H2O. 5. Conclusions A supported coreeshell structured Ni@NiePt/La2O3 catalyst was prepared using a co-precipitation followed by galvanic replacement method. As a consequence of the Pt-induced electronic and geometric modifications on the catalyst surface, the Ni@NiePt/La2O3 catalyst exhibits high activity and selectivity towards H2 generation from N2H4,H2O at mild conditions. For example, the catalyst with an optimal composition enabled complete decomposition of
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.09.071. References [1] S. Satyapal, J. Petrovic, C. Read, G. Thomas, G. Ordaz, Catal. Today 120 (2007) 246e256. [2] S.I. Orimo, Y. Nakamori, J.R. Eliseo, A. Züttel, C.M. Jensen, Chem. Rev. 107 (2007) 4111e4132. [3] U.B. Demirci, P. Miele, Energy Environ. Sci. 4 (2011) 3334e3341. [4] M. Yadav, Q. Xu, Energy Environ. Sci. 5 (2012) 9698e9725. [5] S.K. Singh, X.B. Zhang, Q. Xu, J. Am. Chem. Soc. 131 (2009) 9894e9895. [6] S.K. Singh, Q. Xu, J. Am. Chem. Soc. 131 (2009) 18032e18033. [7] S.K. Singh, Z.H. Lu, Q. Xu, Eur. J. Inorg. Chem. (2011) 2232e2237. [8] S.K. Singh, Y. Iizuka, Q. Xu, Int. J. Hydrogen Energy 36 (2011) 11794e11801. [9] S.K. Singh, A.K. Singh, K. Aranishi, Q. Xu, J. Am. Chem. Soc. 133 (2011) 19638e19641. [10] J. Wang, Y. Li, Y. Zhang, Adv. Funct. Mater 24 (2014) 7073e7077. [11] H.L. Wang, J.M. Yan, Z.L. Wang, S.I. O, Q.J. Jiang, J. Mater. Chem. A 1 (2013) 14957e14962. [12] H.L. Wang, J.M. Yan, S.J. Li, X.W. Zhang, Q. Jiang, J. Mater. Chem. A 3 (2015) 121e124. [13] L. He, Y.Q. Huang, A.Q. Wang, X.D. Wang, X.W. Chen, J.J. Delgado, T. Zhang, Angew. Chem. Int. Ed. 51 (2012) 6191e6194. [14] L. He, B. Liang, L. Li, X. Yang, Y. Huang, A. Wang, X. Wang, T. Zhang, ACS Catal. 5 (2015) 1623e1628. [15] L. He, Y. Huang, X.Y. Liu, L. Li, A. Wang, X. Wang, C.-Y. Mou, T. Zhang, Appl. Catal. B Environ. 147 (2014) 779e788. [16] L. He, Y.Q. Huang, A.Q. Wang, Y. Lu, X.Y. Liu, X.W. Chen, J.J. Delgado, X.D. Wang, T. Zhang, J. Catal. 298 (2013) 1e9. [17] Y.Y. Jiang, Q. Kang, J.J. Zhang, H.B. Dai, P. Wang, J. Power Sources 273 (2015), 554e550. [18] S.J. Cho, J. Lee, Y.S. Lee, D.P. Kim, Catal. Lett. 109 (2006) 181e186. [19] J. Wang, X.B. Zhang, Z.L. Wang, L.M. Wang, Y. Zhang, Energy Environ. Sci. 5 (2012) 6885e6888. [20] N. Cao, J. Su, W. Luo, G.Z. Cheng, Int. J. Hydrogen Energy 39 (2014) 9726e9734. [21] N. Cao, L. Yang, C. Du, J. Su, W. Luo, G.Z. Cheng, J. Mater. Chem. A 2 (2014) 14344e14347. [22] B. Hammer, J.K. Nørskov, Surf. Sci. 343 (1995) 211e220. [23] B. Hammer, Y. Morikawa, J.K. Nørskov, Phys. Rev. Lett. 76 (1996) 2141e2144. [24] B. Hammer, J.K. Nørskov, Nature 376 (1995) 238e240. [25] E. Nikolla, J. Schwank, S. Linic, J. Am. Chem. Soc. 131 (2009) 2747e2754. [26] F. Calle-Vallejo, M.T.M. Koper, A.S. Bandarenka, Chem. Soc. Rev. 42 (2013) 5210e5230. [27] A. Holewinski, H.L. Xin, E. Nikolla, S. Linic, Curr. Opin. Chem. Eng. 2 (2013) 312e319. [28] M.K. Agusta, M. David, H. Nakanishi, H. Kasai, Surf. Sci. 604 (2010) 245e251. [29] M.K. Agusta, H. Kasai, Surf. Sci. 606 (2012) 766e771. [30] P.X. Zhang, Y.G. Wang, Y.Q. Huang, T. Zhang, G.S. Wu, J. Li, Catal. Today 165 (2011) 80e88.