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CeO2 catalyst for hydrogen generation from hydrous hydrazine
Journal of Alloys and Compounds 787 (2019) 1187e1194
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Study of formation mechanism of Ni-Pt/CeO2 catalyst for hydrogen generation from hydrous hydrazine Qing Shi, Yu-Ping Qiu, Hao Dai, Ping Wang* 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
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2018 Received in revised form 24 January 2019 Accepted 30 January 2019 Available online 31 January 2019
Developing advanced catalysts ideally requires fundamental knowledge of the chemical and physical transformations occurred in the preparation process of the catalysts. Herein, we report a careful study of the formation mechanism of bimetallic nickel-platinum (Ni-Pt) nanocatalyst supported on ceria (CeO2), which was prepared by co-precipitation method. A combination of phase/structure/chemical state analyses show that the preparation process of the catalyst involves the formation of [(CH3)4N]2PtCl6 and CeNi0.5Ox phases in the co-precipitation step and their phase evolution in the subsequent aging and reduction steps. The conversion of [(CH3)4N]2PtCl6 to metallic Pt in the aging step is a key event in the preparation process, which exerts profound effects on the composition, microstructural feature and accordingly the catalytic property of the catalyst. These results clearly describe the formation mechanism of Ni-Pt/CeO2 catalyst, which should be significant for the rational design and controlled synthesis of high-performance catalysts for chemical hydrogen storage/generation. © 2019 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage Hydrous hydrazine Bimetallic catalyst Alloying Phase evolution
1. Introduction Hydrogen is expected to play an important role in the future sustainable energy system. But its widespread use as an energy carrier is still severely restricted by the lack of safe and efficient means for hydrogen storage [1]. In the past decades, on-demand hydrogen generation from chemical hydrides has received considerable attention as promising hydrogen storage/generation technologies [2]. Among the chemical hydrides of interests, hydrazine monohydrate (N2H4$H2O) is a leading candidate owing to its many favorable attributes, such as high hydrogen density (8.0 wt%), relatively low cost, and satisfactory stability under ambient conditions [3,4]. Importantly, unlike boron-containing chemical hydrides [5,6], N2H4$H2O does not yield any solid byproduct in its decomposition reactions, which offers clear benefits for the compact design of practical H2-source systems. The catalytic decompose of N2H4$H2O may proceed via two competitive reaction pathways: complete decomposition to yield N2 and H2, and incomplete decomposition to yield N2 and NH3. The development of N2H4$H2O as a viable hydrogen carrier requires
* Corresponding author. E-mail address: [email protected] (P. Wang). https://doi.org/10.1016/j.jallcom.2019.01.378 0925-8388/© 2019 Elsevier B.V. All rights reserved.
highly active and selective catalysts that enable rapid H2 generation from N2H4$H2O. In the past decades, a number of transition metal nanocatalysts had been synthesized using co-precipitation [7e13], chemical reduction [14e24], evaporation induced self-assembly [25,26], colloidal solution combustion synthesis [27], galvanic replacement [28] and impregnation methods [29]. Study of these catalysts found that alloying is an effective strategy for improving catalytic activity and H2 selectivity [8e27]. Immobilization of catalyst nanoparticles onto the basic supports may render not only improved durability, but also enhanced H2 selectivity [8e13,16e18,25e27]. In a general view, considerable progresses had been achieved in developing N2H4$H2O decomposition catalysts, which enabled complete decomposition of N2H4$H2O to generate H2 at mild conditions. But from an application point of view, the catalytic performance of the known nanocatalysts remained to be substantially improved to satisfy the requirements of practical H2 sources. This can be pursued by a systematic approach, which involves the mechanistic understanding the catalytic processes, rational compositional/structural design of nanocatalysts and the development of novel technologies for controlled synthesis of the targeted catalysts. As a fundamental basis, we need to gain insights into the structure-catalytic activity relationships and the catalyst formation mechanism. But in the open literatures, most studies focused on the structural characterization and
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property examination of the targeted catalysts, whereas the indepth study of the chemical and physical transformations involved in catalyst preparation process was rather scarce. In the present work, we prepared the Ni50Pt50/CeO2 catalyst using co-precipitation method and conducted a systematic study of the phase/structure evolution in the catalyst preparation process. Here, the selection of Ni50Pt50/CeO2 as a targeted catalyst was based on its favorable catalytic performance that stands out at the top level of the N2H4$H2O decomposition catalysts. The employment of co-precipitation method is due to its advantages in obtaining tiny catalyst nanoparticles and ensuring their uniform distribution in the matrix. Specifically, the synthesis procedure of the catalyst is divided into co-precipitation, aging and reduction steps. A combination of phase/structure/chemical state analyses and control experiments were conducted to investigate the chemical and physical transformations involved in each step, and on the basis of which the formation mechanism of Ni50Pt50/CeO2 catalyst was discussed. In addition, a combination of the structural characterization and property examination results had allowed us to preliminarily establish the structure-catalytic property correlation.
manufacturer. The composition of the catalyst sample was determined by ICP-AES (Iris Intrepid). 2.3. Catalytic performance testing The catalytic decomposition of N2H4$H2O was carried out in a two-necked round-bottomed flask under magnetic stirring. Specifically, an alkaline aqueous solution and the powdery catalyst were added into a flask and preheated at the designated temperature in a water bath, and then N2H4$H2O was injected into the flask to initiate the decomposition reaction. The gaseous products were measured by a gravimetric water-displacement method as detailed in Ref. [27]. In determination of reaction rate, all the metal (Ni) atoms were assumed to participate in the catalytic reaction and the time required for a 50% conversion of N2H4$H2O was used in the calculations. The selectivity towards H2 generation from N2H4$H2O (X) was calculated from Eqn. (1).
X¼
3Y 1 nðH2 þ N2 Þ Y¼ 8 nðN2 H4 $H2 OÞ
(1)
2. Experimental 2.1. Chemicals and preparation of the catalysts Nickel nitrate hexahydrate (Ni(NO3)2$6H2O, 98%), chloroplatinic acid (H2PtCl6$6H2O, Pt content 37.5%), cerium nitrate hexahydrate (Ce(NO3)3$6H2O, 99.5%), tetramethylammonium hydroxide (TMAH, 97%), N2H4$H2O (98%) and ethanol (C2H5OH, 99.7%) were all purchased from commercial sources and used as received. Deionized (DI) water was used throughout the experiments. The Ni50Pt50/CeO2 catalyst was prepared by co-precipitation method, which involves precipitation, aging and reduction steps. In a typical run, 12 mmol of TMAH was first dissolved in 5 mL of C2H5OH, and then dropwise added to another C2H5OH solution containing 1 mmol of Ni(NO3)2$6H2O, 1 mmol of H2PtCl6$6H2O and 2 mmol of Ce(NO3)3$6H2O under magnetic stirring. After standing at 60 C for 2 h, the solution mixture was transferred into a sealed Teflon-lined autoclave and then maintained at 80 C for a duration ranging from 0 to 12 h. The collected precipitate was washed with ethanol and then dried under dynamic vacuum for 12 h. Reduction of the catalysts was conducted at 300 C in a flowing H2 atmosphere for 1 h. The ramping rate was 10 C min1. For comparison, CeNi0.5Ox and pristine CeO2 samples were prepared following the same procedure. All the reduced catalyst samples were stored in an Ar-filled glove box to minimize oxidation. 2.2. Characterization of the catalysts The phase structure of catalyst samples was investigated by powder X-ray diffraction (XRD, Rigaku RINT 2000, Cu Ka radiation). The morphology and microstructure of the catalyst samples were studied using high-resolution transmission electron microscopy (HRTEM, JEOL-2100F), which was equipped with an energy X-ray dispersive spectroscopy (EDS) analysis unit. The chemical states of the constituent elements of the catalyst samples were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific KALPHAþ, Al Ka X-ray source). In the XPS measurements, all the binding energies were calibrated using the C 1s peak (at 284.8 eV) of the adventitious carbon as an internal standard. The curve fitting was performed using XPS PEAK 4.1 software. Raman spectra were collected using a Thermo Fisher Micro DXR microscope with a HeNe laser (780 nm) excitation source at a resolution of 2 cm1. Before the measurement session, the instrument calibration was performed using the alignment/calibration tool supplied by the
3. Results and discussion Co-precipitation is a common and effective method for preparation of nanocatalysts with small particle size and uniform distribution [30e34]. In the present work, we prepared a Ni50Pt50/ CeO2 catalyst by co-precipitation method, followed by aging and reduction treatment steps, as shown in Scheme 1. Specifically, the mixture of precursor salts first reacted with TMAH to form an orange precipitate, which was then transferred into a sealed Teflonlined autoclave for aging treatment. The resulting black precipitate was finally reduced at elevated temperatures under H2 atmosphere to obtain the targeted catalyst. As determined by ICP-AES element analyses, the targeted Ni50Pt50/CeO2 catalyst has an actual composition of 50.3 mol%Ni50.4Pt49.6/49.7 mol% CeO2, close to the initially designed atomic ratio. To gain a better understanding of the phase evolution in the catalyst preparation process, we conducted a combination of phase/microstructure/chemical state analyses of the collected samples at different stages. 3.1. Characterization of the samples collected after co-precipitation step
60 C+ ; 2h H2 PtCl6 þ 2ðCH3 Þ4 Nþ OH ! ðCH3 Þ4 N 2 PtCl6 Y þ 2H2 O Ethanol
(2) XRD analysis of the collected orange precipitate suggested the formation of crystalline [(CH3)4N]2PtCl6 phase via the reaction of H2PtCl6 and TMAH following Eqn. (2) in the co-precipitation step (Fig. 1A). This phase assignment was made by comparison with the relevant tetramethylammonium hexachlorometalate compounds ([(CH3)4N]2SnCl6: JCPDS No.11-0860 and [(CH3)4N]2ZrCl6: JCPDS No.42-1935). The presence of [(CH3)4N]2PtCl6 phase was further confirmed by Raman spectroscopy analysis. As seen in Fig. 1B, the collected precipitate after the co-precipitation step displayed nearly identical Raman spectrum to the synthesized [(CH3)4N]2PtCl6 sample. Both showed the fingerprint bands of [PtCl6]2- octahedra in the low frequency region, i.e. the symmetric stretching mode at 338 cm1 and the bending modes at 313 cm1 and 164 cm1, respectively [35]. In addition, the formation of [(CH3)4N]2PtCl6 phase was also supported by the HAADF-STEM in
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Scheme 1. Schematic illustration of the preparation process of Ni50Pt50/CeO2 catalyst by co-precipitation method.
Fig. 1. (A) XRD pattern of the catalyst sample collected after co-precipitation step. (B) Raman spectra of the catalyst sample collected after co-precipitation step and the synthesized [(CH3)4N]2PtCl6 sample. (C) EDX line-scanning profile along the direction indicated by the yellow arrow in (D) HAADF-STEM image of the catalyst sample collected after coprecipitation step. (E) XRD patterns of CeO2 and CeNi0.5Ox samples. (F) A representative TEM image of CeNi0.5Ox sample. The inset gives the SAED pattern. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
combination with EDS analysis. As seen in Fig. 1C, the EDS linescanning profiles across a single particle clearly showed the synchronous changes of Pt and Cl signals. Notably, no Ni- and/or Ce-containing phase could be clearly identified in the XRD pattern of the precipitate. To eliminate the possible shadowing effect of the strong diffraction peaks of [(CH3)4N]2PtCl6 phase, we prepared another Pt-free sample using the same method. As presented in Fig. 1E, the control sample showed quite similar XRD pattern to the pure CeO2 sample, and the diffraction peaks of NiO were totally invisible. These results suggested the substitution of the larger CeIV (0.97 Å) by the smaller NiII (0.84 Å) cations in the ceria lattice to form CeNi0.5Ox solid solution [36,37]. Here, the weak and broad peaks of CeNi0.5Ox were indicative of its nanocrystalline and/or amorphous structure. Consistently, the SAED pattern of the control sample showed a series of
concentric rings, which matched well with the crystalline planes of CeNi0.5Ox (Fig. 1F). These characterization results clearly indicated that the sample collected after the co-precipitation step was composed of crystalline [(CH3)4N]2PtCl6 and highly defective CeNi0.5Ox phases. 3.2. Characterization of the samples collected after aging step In preparation of nanocatalysts by co-precipitation method, aging treatment is always adopted following the precipitation step. As a consequence of the compositional redistribution and/or structural reorganization, aging treatment may exert profound effects on the catalytic properties of the targeted catalyst. In the present study, to gain insight into the phase evolution in the aging process, we conducted a combined phase/microstructure/chemical
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state analyses of the aged samples for different periods. Fig. 2A presents the XRD patterns of the series of aged samples for 0, 1, 5, 12 h, respectively. It was observed that the diffraction peaks of [(CH3)4N]2PtCl6 phase gradually weakened with increasing the aging time. In the 5 h-aged sample, the crystalline [(CH3)4N]2PtCl6 phase completely disappeared, and the peaks of metallic Pt and another unknown phase became detectable. Upon further prolongation of the aging time to 12 h, the unknown phase disappeared and the sample showed only weak and broad peaks of Pt and CeNi0.5Ox, indicative of their nanocrystalline and/or amorphous structures. A combination of the XRD results (Figs. 1A, E and 2A) suggested that CeNi0.5Ox retained its phase structure throughout the aging process. In good agreement with the XRD results, XPS analyses of the series of the aged samples clearly showed a distinct reduction tendency of Pt species in the aging process, As seen in Fig. 2B, the sample initially showed two chemically different entities that corresponded to PtIV and PtII, respectively. After being aged for 5 h, the
sample exhibited a remarkably weakened PtIV signal, a moderately intensified PtII signal and particularly the newly appeared metallic Pt0 signal. Upon the prolongation of aging duration to 12 h, the metallic Pt0 became the dominant species, and the signals of PtII and PtIV were significantly weakened. Fig. 2C presents the energy dispersive X-Ray spectroscopy (EDS) elemental mapping results that were acquired in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mode. It was observed that the distribution state of Pt gradually changed with prolongation of the aging time, from atomic dispersion to nanoclusters/nanoparticles. Evidently, the changed distribution state of Pt was associated with the gradual reduction of [(CH3)4N]2PtCl6 by ethanol under the applied aging conditions [38]. The byproduct of this reduction reaction, tetramethyl ammonium chloride ((CH3)4NCl), is water- and ethanolsoluble and can be readily removed in the subsequent cleaning procedure. This proved an important detail in preparation of targeted catalyst, as stated below. Careful examination of the EDS
Fig. 2. (A) XRD patterns, (B) XPS spectra in the Pt 4f region and (C) HAADF-STEM images and the corresponding EDS mapping results of the aged samples for different durations. (a) 0 h, (b) 1 h, (c) 5 h, (d) 12 h.
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Fig. 3. (A) XRD patterns, (B) Representative HRTEM images and (C) HAADF-STEM images and the corresponding EDS mapping results of the reduced catalysts that were aged for different durations. (a) 0 h, (b) 1 h, (c) 5 h, (d) 12 h. The XRD patterns of Ni/CeO2 and Pt/CeO2 reference samples were included in (A) for comparison. Insets give the enlarged view of the selected regions in (A).
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mapping results also found that, in all the samples, the spatial distribution of Ni and Ce elements statistically matched well with each other. This is indicative of the phase stability of CeNi0.5Ox in the aging process, which is consistent with the XRD results. 3.3. Characterization of the catalyst samples collected after reduction step and structure-catalytic property correlation The aged sample was subsequently reduced at elevated temperature under H2 atmosphere to prepare targeted catalyst. Superficially, precipitation of metallic Ni from CeNi0.5Ox and its alloying with Pt are the only central issues of this final step. But in practice, the status of Pt-containing phase formed in the aging step may also play a key role in influencing the phase/structure evolution in the reduction step. Therefore, an overall consideration of the aging and reduction steps are always required in understanding the compositional and structural features of the catalysts. Fig. 3A presents the XRD patterns of the reduced catalysts and the relevant Ni/CeO2 and Pt/CeO2 samples. It was found that, depending upon the aging time, the reduced sample possessed substantially different phase structures. For the shortly aged samples, the resulting catalyst from reduction treatment showed the well-indexed peaks of CeOCl (JCPDS No.74-2033). This phase assignment was further supported by the SAED analysis (Fig. S1, Supporting Information) and HRTEM observation. As shown in Fig. 3B, the lattice fringes with interplanar distances of 0.68 and 0.26 nm can be safely assigned to the (001) and (102) planes of CeOCl. Presumably, CeOCl is formed via an oxygen vacancy mechanism as described by Eqns. (3) and (4) [39,40]. CeO2 first react with H2 to generate oxygen vacancies, which then accommodate Cl anions to yield CeOCl. Here, CeO2 was generated via precipitation of Ni from CeNi0.5Ox phase. Cl anions came from the unreacted [(CH3)4N]2PtCl6 that otherwise would be converted to the ethanolsoluble (CH3)4NCl and metallic Pt in the aging process.
O2 Ce4þ O2 þ H2 /O2 Ce3þ ,þH2 O
(3)
O2 Ce3þ , þ Cl /CeOCl
(4)
The XRD results indicated that the reduction treatment of the 5 h- and 12 h-aged samples under H2 atmosphere resulted in the formation of CeO2 and Ni-Pt alloy. As seen in the inset of Fig. 3A, the (111) peak of CeO2 showed slight low-angle shift compared to the CeNi0.5Ox phase, suggesting the precipitation of metallic Ni from CeNi0.5Ox. Here, the invisibility of the diffraction peaks of metallic Ni should be attributed to its nanophase and/or amorphous structure. A close examination of the XRD patterns also found that Pt (111) peak slightly shifted to high-angle in the bimetallic catalyst, suggesting the occurrence of Ni-Pt alloying in the reduction process. Evidences supporting the Ni-Pt alloying were further obtained from the microstructural observation and EDS elemental analysis. As seen in the HRTEM image (Fig. 3C), the crystalline nanoparticles with lattice fringe spacings of 0.214e0.218 nm could be safely assigned to Ni-Pt alloy since the values fall between those of the (111) plane of fcc Ni (0.203 nm, JCPDS No.04-0850) and the (111) plane of fcc Pt (0.227 nm, JCPDS No. 04-0802). The EDS linescanning profiles across a single particle clearly showed the presence of Ni and Pt elements and particularly the synchronous changes of their signals (Fig. S2, Supporting Information). A close examination of the HRTEM images shown in Fig. 3B found that the two samples with different aging durations differed significantly from each other not only in the type of Ce-containing phases, but also in the microstructural features. In comparison with the sample experiencing an aging treatment of 1 h, the reduced 5 haged sample showed much improved dispersion state of Ni-Pt alloy
phase, that is, smaller size and larger amounts of nanoparticles. This is consistent with the XRD results (Fig. 3A), which clearly showed the variation of diffraction intensity of Pt-Ni peaks upon changing the aging time. In addition, the EDS elemental mapping analyses also provide direct and strong evidences showing the effect of aging time on the dispersion state of Pt-Ni alloy phase in the reduced samples. As seen from Fig. 3C, in all the samples, the spatial distribution of Pt and Ni elements matched well with each other, suggesting the formation of Pt-Ni alloy. With increasing the aging time, the Pt-Ni nanoparticles became smaller and more homogeneously dispersed throughout the reduced samples. Presumably, the improved dispersion state of Pt-Ni alloy in the reduced sample should be associated with the Pt-containing phase evolution in the aging step. As seen in Fig. 2A, the shortly aged samples contained [(CH3)4N]2PtCl6 phase with large grain sizes, as supported by its sharp and strong diffraction peaks. The in situ reduction of such [(CH3)4N]2PtCl6 phase in the subsequent step was expected to generate metallic Pt with large particle size. In the samples with longer aging time, [(CH3)4N]2PtCl6 was largely reduced by ethanol to generate tiny Pt nanoclusters/nanoparticles in the aging process (Fig. 2C and (d)). The finely dispersed metallic Pt may promote the precipitation of Ni from CeNi0.5Ox via hydrogen spillover effect [41], and thereby positively impact the Pt-Ni alloying in the subsequent reduction step. For a given catalyst, microstructural feature is a key factor influencing the catalytic properties. Our study on the preparation process of Ni50Pt50/CeO2 catalyst by co-precipitation method had demonstrated that variation of the aging time may result in significantly different dispersion state of Pt-Ni nanoparticles on the ceria support. As a consequence, depending upon the aging time, the prepared Ni50Pt50/CeO2 catalyst is expected to exhibited remarkably different catalytic properties towards N2H4$H2O decomposition for hydrogen generation. This was confirmed by the property measurement. Fig. 4 presents the kinetic curves of N2H4$H2O decomposition over the series of Ni50Pt50/CeO2 catalysts. The catalysts with an aging time of 5e12 h showed a reaction rate of 465e500 h1 at 30 C, which is 2.5 times as high as those of the catalysts with an aging time of 0e1 h. These study results had allowed us to preliminarily establish a structure-property correlation of the Ni50Pt50/CeO2 catalysts, and may lay foundation for the
Fig. 4. The kinetic curves of N2H4$H2O decomposition over the Ni50Pt50/CeO2 catalysts with different aging durations. (a) 0 h, (b) 1 h, (c) 5 h, (d) 12 h. The catalytic decomposition of N2H4$H2O was conducted in a solution (2 mL) of 0.5 M N2H4$H2O þ 2.0 M NaOH at 30 C with a fixed catalyst/N2H4$H2O molar ratio of 1:20.
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rational design and controlled synthesis of high-performance nanocatalysts for hydrogen generation from N2H4$H2O. Finally, the Ni50Pt50/CeO2 catalyst was subjected to cyclic usage in the N2H4$H2O solution to test its durability. As seen in Fig. S3 of the Supporting Information, the catalytic activity gradually decreased with increasing the cycle number. Similar activity degradation was repeatedly observed in other Ni-based catalysts [7,8,11,16,17]. Currently, our studies to ascertain the mechanism underlying the activity degradation and to solve this problem are underway. 4. Conclusions We prepared the Ni50Pt50/CeO2 catalyst by co-precipitation method and conducted a combination of phase/structure/chemical state analyses to investigate its formation mechanism. It was found that the preparation process of the catalyst involves the formation of [(CH3)4N]2PtCl6 and CeNi0.5Ox phases in the coprecipitation step and their evolution in the subsequent aging and reduction steps. The conversion of [(CH3)4N]2PtCl6 to metallic Pt in the aging step is a key event in the preparation process, which exerts profound effects on the composition, microstructural feature and accordingly the catalytic property of the catalyst. Our study, for the first time, gave a clear picture of the phase/structure evolution in the preparation process of Ni50Pt50/CeO2 catalyst. This may lay foundation for the rational design and controlled synthesis of highperformance catalysts for promoting H2 generation from N2H4$H2O and other chemical hydrides. Acknowledgments We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (Grant No. 51671087), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51621001), the Foundation for Research Groups of the Natural Science Foundation of Guangdong Province (Grant No. 2016A030312011), and the Special Support Plan for National 10000-talents Program. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.01.378. References [1] S. Orimo, Y. Nakamori, J.R. Eliseo, A. Züttel, C.M. Jensen, Complex hydrides for hydrogen storage, Chem. Rev. 107 (2007) 4111e4132. [2] U.B. Demirci, P. Miele, Sodium borohydride versus ammonia borane, in hydrogen storage and direct fuel cell applications, Energy Environ. Sci. 2 (2009) 627e637. [3] Q.-L. Zhu, Q. Xu, Liquid organic and inorganic chemical hydrides for highcapacity hydrogen storage, Energy Environ. Sci. 8 (2015) 478e512. [4] M. Yadav, Q. Xu, Liquid-phase chemical hydrogen storage materials, Energy Environ. Sci. 5 (2012) 9698e9725. [5] P. Wang, X.-D. Kang, Hydrogen-rich boron-containing materials for hydrogen storage, Dalton Trans. 40 (2008) 5400e5413. [6] U.B. Demirci, P. Miele, Chemical hydrogen storage: ‘material’ gravimetric capacity versus ‘system’ gravimetric capacity, Energy Environ. Sci. 4 (2011) 3334e3341. [7] L. He, Y. Huang, A. Wang, X. Wang, X. Chen, J.J. Delgado, T. Zhang, A noblemetal-free catalyst derived from Ni-Al hydrotalcite for hydrogen generation from N2H4$H2O decomposition, Angew. Chem. Int. Ed. 51 (2012) 6191e6194. [8] L. He, Y. Huang, X.Y. Liu, L. Li, A. Wang, X. Wang, C.-Y. Mou, T. Zhang, Structural and catalytic properties of supported NieIr alloy catalysts for H2 generation via hydrous hydrazine decomposition, Appl. Catal. B Environ. 147 (2014) 779e788. [9] T. Liu, J. Yu, H. Bie, Z. Hao, Highly efficient hydrogen generation from hydrous hydrazine using a reduced graphene oxide-supported NiPtP nanoparticle catalyst, J. Alloys Compd. 690 (2017) 783e790.
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Wang, Tuning the surface composition of Ni/meso-CeO2 with iridium as an efficient catalyst for hydrogen generation from hydrous hydrazine, Chem. Eur. J. 24 (2018) 4902e4908. [28] J. Chen, Z.-H. Lu, W. Huang, Z. Kang, X. Chen, Galvanic replacement synthesis of NiPt/graphene as highly efficient catalysts for hydrogen release from hydrazine and hydrazine borane, J. Alloys Compd. 695 (2017) 3036e3043. [29] K. Yang, K. Yang, S. Zhang, Y. Luo, Q. Yao, Z.-H. Lu, Complete dehydrogenation of hydrazine borane and hydrazine catalyzed by MIL-101 supported NiFePd nanoparticles, J. Alloys Compd. 732 (2018) 363e371. [30] G. Qi, R.T. Yang, R. Chang, MnOx-CeO2 mixed oxides prepared by coprecipitation for selective catalytic reduction of NO with NH3 at low temperatures, Appl. Catal. B Environ. 51 (2004) 93e106. [31] G. Yi, H. Lu, S. Zhao, Y. Ge, W. Yang, D. Chen, L.-H. Guo, Synthesis, characterization, and biological application of size-controlled nanocrystalline NaYF4: Yb,Er infrared-to-visible up-conversion phosphors, Nano Lett. 4 (2004) 2191e2196. [32] L. Zhou, D. Zhao, X.W. Lou, Double-shelled CoMn2O4 hollow microcubes as high-capacity anodes for lithium-ion batteries, Adv. Mater. 24 (2012) 745e748. [33] C.D. Wessells, R.A. Huggins, Y. Cui, Copper hexacyanoferrate battery electrodes with long cycle life and high power, Nat. Commun. 2 (2011) 550. [34] D. Lopes, F. Zotin, L.A. Palacio, Copper-nickel catalysts from hydrotalcite precursors: the performance in NO reduction by CO, Appl. Catal. B Environ. 237 (2018) 327e338. [35] M. Autillo, R.E. Wilson, Phase transitions in tetramethylammonium
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hexachlorometalate compounds (TMA)2MCl6 (M ¼ U, Np, Pt, Sn, Hf, Zr), Eur. J. Inorg. Chem. 41 (2017) 4834e4839. [36] Z. Hu, S. Qiu, Y. You, Y. Guo, Y. Guo, L. Wang, W. Zhan, G. Lu, Hydrothermal synthesis of NiCeOx nanosheets and its application to the total oxidation of propane, Appl. Catal. B Environ. 225 (2018) 110e120. [37] J. Xu, B. Xue, Y.-M. Liu, Y.-X. Li, Y. Cao, K.-N. Fan, Mesostructured Ni-doped ceria as an efficient catalyst for styrene synthesis by oxidative dehydrogenation of ethylbenzene, Appl. Catal. A Gen. 405 (2011) 142e148. [38] C. Dong, C. Lian, S. Hu, Z. Deng, J. Gong, M. Li, H. Liu, M. Xing, J. Zhang, Sizedependent activity and selectivity of carbon dioxide photocatalytic reduction
over platinum nanoparticles, Nat. Commun. 9 (2018) 1252. ski, J. Okal, Occurrence and mechanism of formation of CeOCl in Pd/ [39] L. Ke˛ pin CeO2 catalysts, J. Catal. 192 (2000) 48e53. [40] F. Fajardie, J.-F. Tempere, J.-M. Manoli, G. Djega-Mariadassou, G. Blanchard, Ceria lattice oxygen ion substitution by Cl- during the reduction of Rh(Cl)/ CeO2 catalysts. formation and stability of CeOCl, J. Chem. Soc. Faraday Trans. 94 (1998) 3727e3735. [41] A. Wong, Q. Liu, S. Griffin, A. Nicholls, J.R. Regalbuto, Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supports, Science 358 (2017) 1427e1430.
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Study of formation mechanism of Ni-Pt/CeO2 catalyst for hydrogen generation from hydrous hydrazine Qing Shi, Yu-Ping Qiu, Hao Dai, Ping Wang* 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
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2018 Received in revised form 24 January 2019 Accepted 30 January 2019 Available online 31 January 2019
Developing advanced catalysts ideally requires fundamental knowledge of the chemical and physical transformations occurred in the preparation process of the catalysts. Herein, we report a careful study of the formation mechanism of bimetallic nickel-platinum (Ni-Pt) nanocatalyst supported on ceria (CeO2), which was prepared by co-precipitation method. A combination of phase/structure/chemical state analyses show that the preparation process of the catalyst involves the formation of [(CH3)4N]2PtCl6 and CeNi0.5Ox phases in the co-precipitation step and their phase evolution in the subsequent aging and reduction steps. The conversion of [(CH3)4N]2PtCl6 to metallic Pt in the aging step is a key event in the preparation process, which exerts profound effects on the composition, microstructural feature and accordingly the catalytic property of the catalyst. These results clearly describe the formation mechanism of Ni-Pt/CeO2 catalyst, which should be significant for the rational design and controlled synthesis of high-performance catalysts for chemical hydrogen storage/generation. © 2019 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage Hydrous hydrazine Bimetallic catalyst Alloying Phase evolution
1. Introduction Hydrogen is expected to play an important role in the future sustainable energy system. But its widespread use as an energy carrier is still severely restricted by the lack of safe and efficient means for hydrogen storage [1]. In the past decades, on-demand hydrogen generation from chemical hydrides has received considerable attention as promising hydrogen storage/generation technologies [2]. Among the chemical hydrides of interests, hydrazine monohydrate (N2H4$H2O) is a leading candidate owing to its many favorable attributes, such as high hydrogen density (8.0 wt%), relatively low cost, and satisfactory stability under ambient conditions [3,4]. Importantly, unlike boron-containing chemical hydrides [5,6], N2H4$H2O does not yield any solid byproduct in its decomposition reactions, which offers clear benefits for the compact design of practical H2-source systems. The catalytic decompose of N2H4$H2O may proceed via two competitive reaction pathways: complete decomposition to yield N2 and H2, and incomplete decomposition to yield N2 and NH3. The development of N2H4$H2O as a viable hydrogen carrier requires
* Corresponding author. E-mail address: [email protected] (P. Wang). https://doi.org/10.1016/j.jallcom.2019.01.378 0925-8388/© 2019 Elsevier B.V. All rights reserved.
highly active and selective catalysts that enable rapid H2 generation from N2H4$H2O. In the past decades, a number of transition metal nanocatalysts had been synthesized using co-precipitation [7e13], chemical reduction [14e24], evaporation induced self-assembly [25,26], colloidal solution combustion synthesis [27], galvanic replacement [28] and impregnation methods [29]. Study of these catalysts found that alloying is an effective strategy for improving catalytic activity and H2 selectivity [8e27]. Immobilization of catalyst nanoparticles onto the basic supports may render not only improved durability, but also enhanced H2 selectivity [8e13,16e18,25e27]. In a general view, considerable progresses had been achieved in developing N2H4$H2O decomposition catalysts, which enabled complete decomposition of N2H4$H2O to generate H2 at mild conditions. But from an application point of view, the catalytic performance of the known nanocatalysts remained to be substantially improved to satisfy the requirements of practical H2 sources. This can be pursued by a systematic approach, which involves the mechanistic understanding the catalytic processes, rational compositional/structural design of nanocatalysts and the development of novel technologies for controlled synthesis of the targeted catalysts. As a fundamental basis, we need to gain insights into the structure-catalytic activity relationships and the catalyst formation mechanism. But in the open literatures, most studies focused on the structural characterization and
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property examination of the targeted catalysts, whereas the indepth study of the chemical and physical transformations involved in catalyst preparation process was rather scarce. In the present work, we prepared the Ni50Pt50/CeO2 catalyst using co-precipitation method and conducted a systematic study of the phase/structure evolution in the catalyst preparation process. Here, the selection of Ni50Pt50/CeO2 as a targeted catalyst was based on its favorable catalytic performance that stands out at the top level of the N2H4$H2O decomposition catalysts. The employment of co-precipitation method is due to its advantages in obtaining tiny catalyst nanoparticles and ensuring their uniform distribution in the matrix. Specifically, the synthesis procedure of the catalyst is divided into co-precipitation, aging and reduction steps. A combination of phase/structure/chemical state analyses and control experiments were conducted to investigate the chemical and physical transformations involved in each step, and on the basis of which the formation mechanism of Ni50Pt50/CeO2 catalyst was discussed. In addition, a combination of the structural characterization and property examination results had allowed us to preliminarily establish the structure-catalytic property correlation.
manufacturer. The composition of the catalyst sample was determined by ICP-AES (Iris Intrepid). 2.3. Catalytic performance testing The catalytic decomposition of N2H4$H2O was carried out in a two-necked round-bottomed flask under magnetic stirring. Specifically, an alkaline aqueous solution and the powdery catalyst were added into a flask and preheated at the designated temperature in a water bath, and then N2H4$H2O was injected into the flask to initiate the decomposition reaction. The gaseous products were measured by a gravimetric water-displacement method as detailed in Ref. [27]. In determination of reaction rate, all the metal (Ni) atoms were assumed to participate in the catalytic reaction and the time required for a 50% conversion of N2H4$H2O was used in the calculations. The selectivity towards H2 generation from N2H4$H2O (X) was calculated from Eqn. (1).
X¼
3Y 1 nðH2 þ N2 Þ Y¼ 8 nðN2 H4 $H2 OÞ
(1)
2. Experimental 2.1. Chemicals and preparation of the catalysts Nickel nitrate hexahydrate (Ni(NO3)2$6H2O, 98%), chloroplatinic acid (H2PtCl6$6H2O, Pt content 37.5%), cerium nitrate hexahydrate (Ce(NO3)3$6H2O, 99.5%), tetramethylammonium hydroxide (TMAH, 97%), N2H4$H2O (98%) and ethanol (C2H5OH, 99.7%) were all purchased from commercial sources and used as received. Deionized (DI) water was used throughout the experiments. The Ni50Pt50/CeO2 catalyst was prepared by co-precipitation method, which involves precipitation, aging and reduction steps. In a typical run, 12 mmol of TMAH was first dissolved in 5 mL of C2H5OH, and then dropwise added to another C2H5OH solution containing 1 mmol of Ni(NO3)2$6H2O, 1 mmol of H2PtCl6$6H2O and 2 mmol of Ce(NO3)3$6H2O under magnetic stirring. After standing at 60 C for 2 h, the solution mixture was transferred into a sealed Teflon-lined autoclave and then maintained at 80 C for a duration ranging from 0 to 12 h. The collected precipitate was washed with ethanol and then dried under dynamic vacuum for 12 h. Reduction of the catalysts was conducted at 300 C in a flowing H2 atmosphere for 1 h. The ramping rate was 10 C min1. For comparison, CeNi0.5Ox and pristine CeO2 samples were prepared following the same procedure. All the reduced catalyst samples were stored in an Ar-filled glove box to minimize oxidation. 2.2. Characterization of the catalysts The phase structure of catalyst samples was investigated by powder X-ray diffraction (XRD, Rigaku RINT 2000, Cu Ka radiation). The morphology and microstructure of the catalyst samples were studied using high-resolution transmission electron microscopy (HRTEM, JEOL-2100F), which was equipped with an energy X-ray dispersive spectroscopy (EDS) analysis unit. The chemical states of the constituent elements of the catalyst samples were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific KALPHAþ, Al Ka X-ray source). In the XPS measurements, all the binding energies were calibrated using the C 1s peak (at 284.8 eV) of the adventitious carbon as an internal standard. The curve fitting was performed using XPS PEAK 4.1 software. Raman spectra were collected using a Thermo Fisher Micro DXR microscope with a HeNe laser (780 nm) excitation source at a resolution of 2 cm1. Before the measurement session, the instrument calibration was performed using the alignment/calibration tool supplied by the
3. Results and discussion Co-precipitation is a common and effective method for preparation of nanocatalysts with small particle size and uniform distribution [30e34]. In the present work, we prepared a Ni50Pt50/ CeO2 catalyst by co-precipitation method, followed by aging and reduction treatment steps, as shown in Scheme 1. Specifically, the mixture of precursor salts first reacted with TMAH to form an orange precipitate, which was then transferred into a sealed Teflonlined autoclave for aging treatment. The resulting black precipitate was finally reduced at elevated temperatures under H2 atmosphere to obtain the targeted catalyst. As determined by ICP-AES element analyses, the targeted Ni50Pt50/CeO2 catalyst has an actual composition of 50.3 mol%Ni50.4Pt49.6/49.7 mol% CeO2, close to the initially designed atomic ratio. To gain a better understanding of the phase evolution in the catalyst preparation process, we conducted a combination of phase/microstructure/chemical state analyses of the collected samples at different stages. 3.1. Characterization of the samples collected after co-precipitation step
60 C+ ; 2h H2 PtCl6 þ 2ðCH3 Þ4 Nþ OH ! ðCH3 Þ4 N 2 PtCl6 Y þ 2H2 O Ethanol
(2) XRD analysis of the collected orange precipitate suggested the formation of crystalline [(CH3)4N]2PtCl6 phase via the reaction of H2PtCl6 and TMAH following Eqn. (2) in the co-precipitation step (Fig. 1A). This phase assignment was made by comparison with the relevant tetramethylammonium hexachlorometalate compounds ([(CH3)4N]2SnCl6: JCPDS No.11-0860 and [(CH3)4N]2ZrCl6: JCPDS No.42-1935). The presence of [(CH3)4N]2PtCl6 phase was further confirmed by Raman spectroscopy analysis. As seen in Fig. 1B, the collected precipitate after the co-precipitation step displayed nearly identical Raman spectrum to the synthesized [(CH3)4N]2PtCl6 sample. Both showed the fingerprint bands of [PtCl6]2- octahedra in the low frequency region, i.e. the symmetric stretching mode at 338 cm1 and the bending modes at 313 cm1 and 164 cm1, respectively [35]. In addition, the formation of [(CH3)4N]2PtCl6 phase was also supported by the HAADF-STEM in
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Scheme 1. Schematic illustration of the preparation process of Ni50Pt50/CeO2 catalyst by co-precipitation method.
Fig. 1. (A) XRD pattern of the catalyst sample collected after co-precipitation step. (B) Raman spectra of the catalyst sample collected after co-precipitation step and the synthesized [(CH3)4N]2PtCl6 sample. (C) EDX line-scanning profile along the direction indicated by the yellow arrow in (D) HAADF-STEM image of the catalyst sample collected after coprecipitation step. (E) XRD patterns of CeO2 and CeNi0.5Ox samples. (F) A representative TEM image of CeNi0.5Ox sample. The inset gives the SAED pattern. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
combination with EDS analysis. As seen in Fig. 1C, the EDS linescanning profiles across a single particle clearly showed the synchronous changes of Pt and Cl signals. Notably, no Ni- and/or Ce-containing phase could be clearly identified in the XRD pattern of the precipitate. To eliminate the possible shadowing effect of the strong diffraction peaks of [(CH3)4N]2PtCl6 phase, we prepared another Pt-free sample using the same method. As presented in Fig. 1E, the control sample showed quite similar XRD pattern to the pure CeO2 sample, and the diffraction peaks of NiO were totally invisible. These results suggested the substitution of the larger CeIV (0.97 Å) by the smaller NiII (0.84 Å) cations in the ceria lattice to form CeNi0.5Ox solid solution [36,37]. Here, the weak and broad peaks of CeNi0.5Ox were indicative of its nanocrystalline and/or amorphous structure. Consistently, the SAED pattern of the control sample showed a series of
concentric rings, which matched well with the crystalline planes of CeNi0.5Ox (Fig. 1F). These characterization results clearly indicated that the sample collected after the co-precipitation step was composed of crystalline [(CH3)4N]2PtCl6 and highly defective CeNi0.5Ox phases. 3.2. Characterization of the samples collected after aging step In preparation of nanocatalysts by co-precipitation method, aging treatment is always adopted following the precipitation step. As a consequence of the compositional redistribution and/or structural reorganization, aging treatment may exert profound effects on the catalytic properties of the targeted catalyst. In the present study, to gain insight into the phase evolution in the aging process, we conducted a combined phase/microstructure/chemical
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state analyses of the aged samples for different periods. Fig. 2A presents the XRD patterns of the series of aged samples for 0, 1, 5, 12 h, respectively. It was observed that the diffraction peaks of [(CH3)4N]2PtCl6 phase gradually weakened with increasing the aging time. In the 5 h-aged sample, the crystalline [(CH3)4N]2PtCl6 phase completely disappeared, and the peaks of metallic Pt and another unknown phase became detectable. Upon further prolongation of the aging time to 12 h, the unknown phase disappeared and the sample showed only weak and broad peaks of Pt and CeNi0.5Ox, indicative of their nanocrystalline and/or amorphous structures. A combination of the XRD results (Figs. 1A, E and 2A) suggested that CeNi0.5Ox retained its phase structure throughout the aging process. In good agreement with the XRD results, XPS analyses of the series of the aged samples clearly showed a distinct reduction tendency of Pt species in the aging process, As seen in Fig. 2B, the sample initially showed two chemically different entities that corresponded to PtIV and PtII, respectively. After being aged for 5 h, the
sample exhibited a remarkably weakened PtIV signal, a moderately intensified PtII signal and particularly the newly appeared metallic Pt0 signal. Upon the prolongation of aging duration to 12 h, the metallic Pt0 became the dominant species, and the signals of PtII and PtIV were significantly weakened. Fig. 2C presents the energy dispersive X-Ray spectroscopy (EDS) elemental mapping results that were acquired in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mode. It was observed that the distribution state of Pt gradually changed with prolongation of the aging time, from atomic dispersion to nanoclusters/nanoparticles. Evidently, the changed distribution state of Pt was associated with the gradual reduction of [(CH3)4N]2PtCl6 by ethanol under the applied aging conditions [38]. The byproduct of this reduction reaction, tetramethyl ammonium chloride ((CH3)4NCl), is water- and ethanolsoluble and can be readily removed in the subsequent cleaning procedure. This proved an important detail in preparation of targeted catalyst, as stated below. Careful examination of the EDS
Fig. 2. (A) XRD patterns, (B) XPS spectra in the Pt 4f region and (C) HAADF-STEM images and the corresponding EDS mapping results of the aged samples for different durations. (a) 0 h, (b) 1 h, (c) 5 h, (d) 12 h.
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Fig. 3. (A) XRD patterns, (B) Representative HRTEM images and (C) HAADF-STEM images and the corresponding EDS mapping results of the reduced catalysts that were aged for different durations. (a) 0 h, (b) 1 h, (c) 5 h, (d) 12 h. The XRD patterns of Ni/CeO2 and Pt/CeO2 reference samples were included in (A) for comparison. Insets give the enlarged view of the selected regions in (A).
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mapping results also found that, in all the samples, the spatial distribution of Ni and Ce elements statistically matched well with each other. This is indicative of the phase stability of CeNi0.5Ox in the aging process, which is consistent with the XRD results. 3.3. Characterization of the catalyst samples collected after reduction step and structure-catalytic property correlation The aged sample was subsequently reduced at elevated temperature under H2 atmosphere to prepare targeted catalyst. Superficially, precipitation of metallic Ni from CeNi0.5Ox and its alloying with Pt are the only central issues of this final step. But in practice, the status of Pt-containing phase formed in the aging step may also play a key role in influencing the phase/structure evolution in the reduction step. Therefore, an overall consideration of the aging and reduction steps are always required in understanding the compositional and structural features of the catalysts. Fig. 3A presents the XRD patterns of the reduced catalysts and the relevant Ni/CeO2 and Pt/CeO2 samples. It was found that, depending upon the aging time, the reduced sample possessed substantially different phase structures. For the shortly aged samples, the resulting catalyst from reduction treatment showed the well-indexed peaks of CeOCl (JCPDS No.74-2033). This phase assignment was further supported by the SAED analysis (Fig. S1, Supporting Information) and HRTEM observation. As shown in Fig. 3B, the lattice fringes with interplanar distances of 0.68 and 0.26 nm can be safely assigned to the (001) and (102) planes of CeOCl. Presumably, CeOCl is formed via an oxygen vacancy mechanism as described by Eqns. (3) and (4) [39,40]. CeO2 first react with H2 to generate oxygen vacancies, which then accommodate Cl anions to yield CeOCl. Here, CeO2 was generated via precipitation of Ni from CeNi0.5Ox phase. Cl anions came from the unreacted [(CH3)4N]2PtCl6 that otherwise would be converted to the ethanolsoluble (CH3)4NCl and metallic Pt in the aging process.
O2 Ce4þ O2 þ H2 /O2 Ce3þ ,þH2 O
(3)
O2 Ce3þ , þ Cl /CeOCl
(4)
The XRD results indicated that the reduction treatment of the 5 h- and 12 h-aged samples under H2 atmosphere resulted in the formation of CeO2 and Ni-Pt alloy. As seen in the inset of Fig. 3A, the (111) peak of CeO2 showed slight low-angle shift compared to the CeNi0.5Ox phase, suggesting the precipitation of metallic Ni from CeNi0.5Ox. Here, the invisibility of the diffraction peaks of metallic Ni should be attributed to its nanophase and/or amorphous structure. A close examination of the XRD patterns also found that Pt (111) peak slightly shifted to high-angle in the bimetallic catalyst, suggesting the occurrence of Ni-Pt alloying in the reduction process. Evidences supporting the Ni-Pt alloying were further obtained from the microstructural observation and EDS elemental analysis. As seen in the HRTEM image (Fig. 3C), the crystalline nanoparticles with lattice fringe spacings of 0.214e0.218 nm could be safely assigned to Ni-Pt alloy since the values fall between those of the (111) plane of fcc Ni (0.203 nm, JCPDS No.04-0850) and the (111) plane of fcc Pt (0.227 nm, JCPDS No. 04-0802). The EDS linescanning profiles across a single particle clearly showed the presence of Ni and Pt elements and particularly the synchronous changes of their signals (Fig. S2, Supporting Information). A close examination of the HRTEM images shown in Fig. 3B found that the two samples with different aging durations differed significantly from each other not only in the type of Ce-containing phases, but also in the microstructural features. In comparison with the sample experiencing an aging treatment of 1 h, the reduced 5 haged sample showed much improved dispersion state of Ni-Pt alloy
phase, that is, smaller size and larger amounts of nanoparticles. This is consistent with the XRD results (Fig. 3A), which clearly showed the variation of diffraction intensity of Pt-Ni peaks upon changing the aging time. In addition, the EDS elemental mapping analyses also provide direct and strong evidences showing the effect of aging time on the dispersion state of Pt-Ni alloy phase in the reduced samples. As seen from Fig. 3C, in all the samples, the spatial distribution of Pt and Ni elements matched well with each other, suggesting the formation of Pt-Ni alloy. With increasing the aging time, the Pt-Ni nanoparticles became smaller and more homogeneously dispersed throughout the reduced samples. Presumably, the improved dispersion state of Pt-Ni alloy in the reduced sample should be associated with the Pt-containing phase evolution in the aging step. As seen in Fig. 2A, the shortly aged samples contained [(CH3)4N]2PtCl6 phase with large grain sizes, as supported by its sharp and strong diffraction peaks. The in situ reduction of such [(CH3)4N]2PtCl6 phase in the subsequent step was expected to generate metallic Pt with large particle size. In the samples with longer aging time, [(CH3)4N]2PtCl6 was largely reduced by ethanol to generate tiny Pt nanoclusters/nanoparticles in the aging process (Fig. 2C and (d)). The finely dispersed metallic Pt may promote the precipitation of Ni from CeNi0.5Ox via hydrogen spillover effect [41], and thereby positively impact the Pt-Ni alloying in the subsequent reduction step. For a given catalyst, microstructural feature is a key factor influencing the catalytic properties. Our study on the preparation process of Ni50Pt50/CeO2 catalyst by co-precipitation method had demonstrated that variation of the aging time may result in significantly different dispersion state of Pt-Ni nanoparticles on the ceria support. As a consequence, depending upon the aging time, the prepared Ni50Pt50/CeO2 catalyst is expected to exhibited remarkably different catalytic properties towards N2H4$H2O decomposition for hydrogen generation. This was confirmed by the property measurement. Fig. 4 presents the kinetic curves of N2H4$H2O decomposition over the series of Ni50Pt50/CeO2 catalysts. The catalysts with an aging time of 5e12 h showed a reaction rate of 465e500 h1 at 30 C, which is 2.5 times as high as those of the catalysts with an aging time of 0e1 h. These study results had allowed us to preliminarily establish a structure-property correlation of the Ni50Pt50/CeO2 catalysts, and may lay foundation for the
Fig. 4. The kinetic curves of N2H4$H2O decomposition over the Ni50Pt50/CeO2 catalysts with different aging durations. (a) 0 h, (b) 1 h, (c) 5 h, (d) 12 h. The catalytic decomposition of N2H4$H2O was conducted in a solution (2 mL) of 0.5 M N2H4$H2O þ 2.0 M NaOH at 30 C with a fixed catalyst/N2H4$H2O molar ratio of 1:20.
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rational design and controlled synthesis of high-performance nanocatalysts for hydrogen generation from N2H4$H2O. Finally, the Ni50Pt50/CeO2 catalyst was subjected to cyclic usage in the N2H4$H2O solution to test its durability. As seen in Fig. S3 of the Supporting Information, the catalytic activity gradually decreased with increasing the cycle number. Similar activity degradation was repeatedly observed in other Ni-based catalysts [7,8,11,16,17]. Currently, our studies to ascertain the mechanism underlying the activity degradation and to solve this problem are underway. 4. Conclusions We prepared the Ni50Pt50/CeO2 catalyst by co-precipitation method and conducted a combination of phase/structure/chemical state analyses to investigate its formation mechanism. It was found that the preparation process of the catalyst involves the formation of [(CH3)4N]2PtCl6 and CeNi0.5Ox phases in the coprecipitation step and their evolution in the subsequent aging and reduction steps. The conversion of [(CH3)4N]2PtCl6 to metallic Pt in the aging step is a key event in the preparation process, which exerts profound effects on the composition, microstructural feature and accordingly the catalytic property of the catalyst. Our study, for the first time, gave a clear picture of the phase/structure evolution in the preparation process of Ni50Pt50/CeO2 catalyst. This may lay foundation for the rational design and controlled synthesis of highperformance catalysts for promoting H2 generation from N2H4$H2O and other chemical hydrides. Acknowledgments We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (Grant No. 51671087), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51621001), the Foundation for Research Groups of the Natural Science Foundation of Guangdong Province (Grant No. 2016A030312011), and the Special Support Plan for National 10000-talents Program. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.01.378. References [1] S. Orimo, Y. Nakamori, J.R. Eliseo, A. Züttel, C.M. Jensen, Complex hydrides for hydrogen storage, Chem. Rev. 107 (2007) 4111e4132. [2] U.B. Demirci, P. Miele, Sodium borohydride versus ammonia borane, in hydrogen storage and direct fuel cell applications, Energy Environ. Sci. 2 (2009) 627e637. [3] Q.-L. Zhu, Q. Xu, Liquid organic and inorganic chemical hydrides for highcapacity hydrogen storage, Energy Environ. Sci. 8 (2015) 478e512. [4] M. Yadav, Q. Xu, Liquid-phase chemical hydrogen storage materials, Energy Environ. Sci. 5 (2012) 9698e9725. [5] P. Wang, X.-D. Kang, Hydrogen-rich boron-containing materials for hydrogen storage, Dalton Trans. 40 (2008) 5400e5413. [6] U.B. Demirci, P. Miele, Chemical hydrogen storage: ‘material’ gravimetric capacity versus ‘system’ gravimetric capacity, Energy Environ. Sci. 4 (2011) 3334e3341. [7] L. He, Y. Huang, A. Wang, X. Wang, X. Chen, J.J. Delgado, T. Zhang, A noblemetal-free catalyst derived from Ni-Al hydrotalcite for hydrogen generation from N2H4$H2O decomposition, Angew. Chem. Int. Ed. 51 (2012) 6191e6194. [8] L. He, Y. Huang, X.Y. Liu, L. Li, A. Wang, X. Wang, C.-Y. Mou, T. Zhang, Structural and catalytic properties of supported NieIr alloy catalysts for H2 generation via hydrous hydrazine decomposition, Appl. Catal. B Environ. 147 (2014) 779e788. [9] T. Liu, J. Yu, H. Bie, Z. Hao, Highly efficient hydrogen generation from hydrous hydrazine using a reduced graphene oxide-supported NiPtP nanoparticle catalyst, J. Alloys Compd. 690 (2017) 783e790.
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