CeO2 Loaded on Granular Activated Carbon: An Efficient

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Ni-Pt/CeO2 Loaded on Granular Activated Carbon: An Efficient Monolithic Catalyst for Controlled Hydrogen Generation from Hydrous Hydrazine Hao Dai, Yu-Ping Qiu, Hongbin Dai, and Ping Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01098 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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ACS Sustainable Chemistry & Engineering

Ni-Pt/CeO2 Loaded on Granular Activated Carbon: An Efficient Monolithic Catalyst for Controlled Hydrogen Generation from Hydrous Hydrazine Hao Dai, Yu-Ping Qiu, Hong-Bin Dai, * and 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, P.R. China ∗

Corresponding authors. Tel: +86 20 3938 0583

E-mail addresses: [email protected] (P. Wang), [email protected] (H.B. Dai).

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Abstract Hydrous hydrazine (N2H4·H2O) is considered as a promising hydrogen carrier. In the development of N2H4·H2O-based hydrogen generation system, monolithic catalysts are highly desirable owing to their reusability, ready controllability of reaction and simple design of hydrogen generator. Herein, we report the synthesis of a monolithic catalyst composed of Ni-Pt/CeO2 nanoparticles and granular activated carbon using a simple impregnation-reduction method. It was found that the activity and H2 selectivity of the resulting catalyst can be readily regulated by changing the annealing temperature and atmosphere. The optimal Ni-Pt/CeO2/GAC catalyst enabled complete and rapid decomposition of N2H4·H2O to generate H2 with a 100% selectivity at moderate temperature in the presence of 1M NaOH. Importantly, by using this monolithic catalyst, we constituted a N2H4·H2O-based HG system with a material-based hydrogen capacity as high as 6.54 wt%. This finding represents a promising step towards the development of N2H4·H2O as a viable hydrogen storage carrier.

Keywords: Hydrous hydrazine, Hydrogen generation, Monolithic catalyst, Ni-Pt alloy

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INTRODUCTION Hydrogen is an attractive renewable energy carrier, but its widespread use is severely restricted by the lack of viable means for efficient and safe storage of hydrogen.1 The extensive studies over the past decades have led to no material that can reversibly store > 6 wt% hydrogen at relevant conditions to the practical applications. Consequently, great efforts have recently been directed towards chemical hydrides as promising hydrogen storage/generation materials.2,3 The chemical hydrides under investigation include sodium borohydride,4,5 ammonia borane,6,7 hydrazine borane,8 hydrazine monohydrate (N2H4·H2O),9 formic acid,10 and so on. Among these materials of interest, N2H4·H2O is a less well explored but very promising candidate. N2H4⋅H2O has high hydrogen capacity (8 wt%), relatively low cost, and satisfactory stability under ambient conditions. Importantly, N2H4·H2O does not yield any solid byproduct in its decomposition reactions, which is an important benefit for the compact design of practical H2-source systems.3 In addition, it appears the major concern over the toxicity of hydrazine (N2H4) can be properly addressed by reacting N2H4·H2O with double-bonded carbon-oxygen carbonyl, and the resulting safe solid, hydrazone, will release liquid N2H4·H2O upon contact with warm water.11 N2H4 → N2 + 2 H2

(1)

3 N2H4 → N2 + 4 NH3

(2)

N2H4 is the effective hydrogen storage component of N2H4·H2O, which decomposes via two competitive reaction pathways: complete decomposition following Eqn. (1) and incomplete decomposition following Eqn. (2). The development of N2H4·H2O as a viable hydrogen carrier requires advanced catalysts to selectively promote H2 generation (HG) and meanwhile to restrain the formation of NH3 byproduct from N2H4·H2O decomposition.12 The last decade has witnessed

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significant progress in the development of 3d transition metals and alloy nanocatalysts for promoting HG from N2H4 ·H2O. Among these catalysts, supported bimetallic Ni-Pt, Ni-Ir, Ni-Rh nanoparticles showed the most favorable catalytic performances.13–25 Here, the use of support materials, such as metal oxides,13–15,19–22 MOFs,16–18 carbon,23 and reduced graphene oxide,24,25 may not only aid in the dispersion of catalytically active nanoparticles, but also positively impact the catalytic properties via strong metal-support interaction.26,27 But despite the use of support materials, most reported catalysts in the open literatures were in the fine powdery form. This may cause problem in the practical applications, as it should be technologically difficult to separate the fine powdery catalyst from the spent fuel solution, particularly in a continuous flowing mode. A solution for solving this problem is to develop monolithic catalysts. The inherent separability of monolithic catalysts implies important advantages on catalyst reusability, ready controllability of reaction, which are highly desirable for the design of compact hydrogen generator. In the present study, by using a simple impregnation-reduction method, we prepared a monolithic catalyst that was composed of Ni-Pt/CeO2 nanoparticles and granular activated carbon (denoted as Ni-Pt/CeO2/GAC). After being properly annealed under reductive atmosphere at elevated temperatures, the resulting catalyst enabled complete and rapid decomposition of N2H4·H2O to generate H2 at mild conditions. In particular, our study demonstrated that a material-based hydrogen capacity as high as 6.54 wt% was achievable using this monolithic catalyst and the commercially available 85 wt% N2H4·H2O solution. This finding represents a promising step towards the development of N2H4·H2O as a viable hydrogen storage carrier. EXPERIMENTAL Chemicals and Preparation of the Catalysts 4


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Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 98%), chloroplatinic acid (H2PtCl6·H2O, Pt content ≥37.5%), cerium nitrate hexahydrate (Ce(NO3)3·6H2O, 99.5%), granular activated carbon (GAC, 8~12 mesh with an particle size range of 1.7–2.3 mm), sodium hydroxide (NaOH, 97%), N2H4·H2O (85% and 99%) were purchased from Macklin. All reagents were used as received. Deionized water was used in preparation of all the aqueous solutions. Prior to use, the GAC was pretreated, which involved washing by water and C2H5OH three times, respectively, and calcination in air at 773 K for 5 h for further removal of ash. For all the catalysts, the loading amount of Ni-Pt/CeO2 on GAC was fixed at 10 wt%. The catalysts with varied Ni/Pt/CeO2 molar ratios were prepared using an impregnation-reduction method. In a typical run, the mixture of Ni(NO3)2, H2PtCl6 and Ce(NO3)3 with a certain molar ratio were first dissolved in 10 mL water, and then certain amount of pretreated GAC was poured into the solution, followed by evaporation of water at 333 K for ~ 2 h under mechanical stirring. The dried sample was transferred to a tube furnace and calcined at varied temperature for 2 h under flowing reductive atmosphere. All catalyst samples were stored in an Ar-filled glove box to minimize oxidation. Characterization of the Catalysts The catalyst samples were characterized by powder X-ray diffraction (XRD, Rigaku D/MAX-2000, Cu Kα radiation), X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-ALPHA+, Al Kα X-ray source) and high-resolution transmission electron microscopy (HRTEM, JEM-2100F) equipped with an energy dispersive X-ray spectroscopy (EDS) analysis unit. In preparation of the TEM samples, the catalyst was first ground into powder, then dispersed in ethanol by ultrasound, and finally deposited on a carbon-coated copper grid. In the XPS mesurements, the pass energies were set to 50 eV, the binding energies of the XPS spectra 5



were calibrated based on the C1s peak at 284.8 eV of the adventitious carbon as an internal standard. The curve fitting was performed by XPS PEAK 4.1 software. The specific surface area and pore size distrbution were mesured by N2 asorption/desorption isotherm at 77 K using the Bruauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) model in a Micromeritics ASAP 2460 apparatus. The post-used fuel solution was measured by an ultraviolet-visible spectrophotometer (UV/Vis, Thermo Scientific EVOLUTION 220) to identify the residual hydrazine. The composition of the catalyst sample was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Iris Intrepid).

HG Property Testing The catalytic decomposition of N2H4·H2O was carried out in a 50 mL two-neck round flask under magnetic stirring. During the measurement, the flask was placed in a thermostat, which was equipped with a water circulating system to maintain the reaction temperature, typically within ± 0.5 K. In a typical measurement run, the flask containing alkaline aqueous solution and the 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 2 s) and the determined gas amount was normalized to standard condition. The catalytic decomposition of concentrated N2H4·H2O solution was conducted in three-neck round flask under magnetic stirring. The reaction rate was measured using an online mass flow meter (Seven-star Huachang, CS 200, accuracy within ±1%) that was equipped with silica drier. The generated gas volume was calculated by integrating the measured gas generation rate over time. The reaction temperature was monitored using a 6


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thermocouple embedded in the alkaline N2H4·H2O solution and recorded using an online recorded. The experiment was repeated two times, and the determined relative error was no more than ±2%. In determination of reaction rate, all the metal (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.19 The selectivity towards HG from N2H4·H2O (X) was calculated following Eqn. (3), which can be derived from Eqns. (1) and (2). 3 N2H4 → 4 (1–X) NH3+6 X H2 + (1+2X) N2

(3)

The molar ratio Y = n(N2+H2)/n(N2H4) was obtained according to Eqn.(3); therefore, X was determined as Eqn.(4).

X =

3Y − 1 n( N 2 ) + n( H 2 ) [Y = ] 8 n( N 2H 4 )

(4)

RESULTS AND DISCUSSION In the present study, we prepared a series of monolithic catalysts composed of Ni-Pt/CeO2 nanoparticles and GAC using a simple impregnation method followed by reduction treatment at elevated temperatures, as shown in Scheme 1. The choice of the component phases of the catalyst is based on the considerations as follows. It has been well documented that the Ni-Pt alloy nanoparticles supported on the basic metal oxides possess favorable catalytic properties towards HG from N2H4·H2O.19–21 The selection of GAC as support material is due to its high porosity, low density, good thermal and satisfactory chemical stability under the reaction conditions. It was expected that the catalyst precursors were simultaneously introduced into the pores and/or onto the surface of the GAC support in the wetness impregnation step, and then the catalyst precursors would transform to the Ni-Pt alloy and CeO2 in the calcination step under reductive atmosphere. 7



Figure 1A gives the N2 adsorption/desorption isotherms and the pore size distributions for the pretreated GAC and Ni60Pt40/CeO2/GAC-573H2 samples. It was observed that both samples showed the type I isotherms, which are characteristic for microporous materials.28 According to measurement results of the BJH pore size distributions (Figure 1A, inset), both samples were primarily composed of micropores with sizes less than 2 nm. The measured BET surface area and pore volume (812 m2 g–1 and 0.377 cm3 g–1) of the Ni60Pt40/CeO2/GAC-573H2 sample were lower than the values of the pretreated GAC (1014 m2 g–1 and 0.417 cm3 g–1), suggesting the incorporation of catalyst nanoparticles into the pores of GAC support during the preparation process. The phase structures, morphologies and microstructures of the catalyst samples were analyzed by XRD and TEM techniques. As shown in Figure 1B, two broad and weak diffraction peaks corresponding to the (100) and (101) plane of carbon (PDF#03-0401) were observed in all the samples, indicative of the amorphous structure of AC. The Ni60Pt40/CeO2/GAC-573H2 sample showed only two peaks at 39.76 and 46.24°, which can be well indexed to the (111) and (200) planes of fcc Pt (PDF#04-0802). The invisibility of diffraction peaks of CeO2 and metallic Ni or its alloy is presumably due to their fine grain size and/or amorphous structure. After being annealed at 873 K under H2 or NH3 atmosphere, the catalyst samples showed new diffraction peaks at 28.55, 33.08, 47.48 and 56.33o, which correspond to the (111), (200), (220) and (311) planes of fluorite CeO2 (PDF#34-0394), respectively. Notably, another new peak at around 42.73 o was also observed, which showed significant low-angle shift compared to the (111) peak of metallic Ni. This result clearly suggests the formation of Ni-Pt alloy. TEM observation of the Ni60Pt40/CeO2/GAC-573H2 catalyst sample found that the catalyst nanoparticles with an average size of about 3.5 nm uniformly dispersed on the surface of AC substrate (Figure 2A). Close examination by HRTEM showed that the catalyst was composed of the tiny nanocrystallites with random orientations and amorphous 8


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phase (Figure 2B). The lattice fringes with spacing of 0.227 nm correspond well to the (111) plane of fcc Pt, and the lattice fringes with distances of 0.270 and 0.312 nm to the (200) and (111) planes of CeO2. In particular, the observed lattice fringe with spacing of 0.207 nm was assigned to fcc Ni-Pt alloy, since the spacing value falls between those of the (111) plane of fcc Ni (0.203 nm) and the (111) plane of fcc Pt (0.227 nm). This assignment agrees well with the XRD result. The detection of fcc Pt by XRD and HRTEM analyses implies that part of Pt atoms did not participate in the alloying with Ni. But according to the XRD results, the degree of Ni-Pt alloying increased with elevating the annealing temperatures. Consistently, HRTEM observations found that the lattice fringe spacing (0.217 nm) of Ni-Pt alloy in the Ni60Pt40/CeO2/GAC-873NH3 catalyst was larger than that (0.207 nm) observed in the Ni60Pt40/CeO2/GAC-573H2 sample (Figure 2D). The distribution states of the constituent elements of the Ni60Pt40/CeO2/GAC-873NH3 catalyst were further investigated by the high-angle annular dark-field scanning TEM (HAADF-STEM), in combination with EDS elemental mapping analysis (Figure 2E and 2F). It was observed that the Ni and Pt elements uniformly dispersed on the CeO2 surface, and the spatial distribution of Pt element statistically matched well with that of Ni. We first examined the catalytic decomposition properties of N2H4·H2O over the series of Ni-Pt/CeO2/GAC catalysts that were annealed under H2 atmosphere at 573 K. It was found that the Ni-Pt/CeO2/GAC catalysts show strong performance dependence on the Ni, Pt, and CeO2 contents (Figure 3A and 3B). The optimal catalytic performance was achieved in the catalyst with a nominal Ni/Pt/CeO2 ratio of 0.6/0.4/1. According to the ICP-AES measurement, this catalyst has an authentic composition of 48.57 mol% Ni58.54Pt41.46/51.43 mol% CeO2. But in terms of the H2 selectivity in N2H4·H2O decomposition, all the catalysts showed problematic performance. For example, the optimized Ni60Pt40/CeO2/GAC-573H2 catalyst exhibited a H2 selectivity of only ~ 88%. 9



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The concomitant release of NH3 represents a serious challenge to the N2H4·H2O-based HG system, since the byproduct NH3 could not only poison the Nafion membrane, but also reduce fuel efficiency. Our study found that the H2 selectivity of Ni60Pt40/CeO2/GAC-573H2 catalyst can be improved by regulating the annealing temperature and atmosphere. As seen in Figure 3C, the H2 selectivity was enhanced from 85 to 95% with increasing the annealing temperature from 573 to 873 K under H2 atmosphere and particularly, the H2 selectivity reached up to 100% upon changing the annealing atmosphere from H2 to NH3 at 873 K. But meanwhile, the activities of the catalysts were observed

to

decline

with

increasing

the

annealing

temperature.

For

example,

the

Ni60Pt40/CeO2/GAC-873NH3 catalyst exhibited a reaction rate of 286 h–1, which was just one-third of the value of the Ni60Pt40/CeO2/GAC-573H2 catalyst. This is not unexpected in view of the catalyst nanoparticles aggregation during annealing treatment at elevated temperatures. As shown in Figure 2A and 2C, the average size of catalyst particles increased from 3.5 to 5.4 nm with increasing the annealing temperature from 573 to 873 K. In the open literatures, it has been well documented that the alloying of Ni with Pt was a prerequisite for the improved H2 selectivity and activity in the catalytic decomposition of N2H4·H2O.19-21 But a careful comparison of the XRD results (Figure 1B) found that changing the annealing atmosphere from H2 to NH3 resulted in a reduced intensity of the peak at around 42.73o, suggesting a lowered degree of Ni-Pt alloying. These results clearly suggested that the improved H2 selectivity arising upon changing annealing atmosphere should not be correlated with the variation of the degree of Ni-Pt alloying. In our effort to understand this interesting phenomenon, we examined the Ni60Pt40/CeO2/GAC-873H2 and Ni60Pt40/CeO2/GAC-873NH3 samples by the surface-sensitive XPS technique to determine the chemical states of the constituent elements of the

10


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catalysts. In the two catalyst samples (Figure S1, Supporting Information), the Ni 2p spectra can be well fitted with two doublets assigned to metallic Ni0 and NiO together with the satellite features,20,21,26,29 and the Pt 4f spectra show two chemically different entities that correspond to metallic Pt0 and PtO, respectively. Similarly, the Ce 3d spectra can still be fitted with two doublets assigned to CeO2 and Ce2O3 as well as some satellite features.20,21,26 No significant changes in the binding energies of the constituent elements were observed in the two examined samples. Notably, XPS analysis of the Ni-Pt/CeO2/GAC-873NH3 sample clearly detected N signals. As seen in Figure 3D, the N 1s spectrum could be well fitted with four peaks assigned to pyridinic N (398.3 eV), pyrrolic N (400.1 eV), graphitic N (401.3 eV), and oxidic N (403.3 eV), respecively.30 Since it was generally accepted that the basic sites may facilitate the breakage of N-H bonds to form N2 and H2,26,31 the incorporation of these N-containing species with Lewis basic nature might be a possible reason for the enhancement of H2 selectivity of the catalyst arising upon changing the annealing atmosphere from NH3 to H2. In this regard, sophisticated control experiments and theoretical study are still required to gain insight into this phenomenon, which might shed light on the compositional design of advanced catalyst for N2H4·H2O decomposition. The reaction temperature and NaOH concentration also exert important influences on the catalytic decomposition behaviors of N2H4·H2O. In the present study, we examined the Ni-Pt/CeO2/GAC-873NH3 monolithic catalyst towards N2H4·H2O decomposition at temperatures a ranging from 303 to 343 K. As shown in Figure 4A, the reaction rate increased with increasing reaction temperature. According to the temperature-dependent reaction rates, the apparent activation energy of N2H4·H2O decomposition over the Ni-Pt/CeO2/GAC-873NH3 catalyst was determined to be 38.72 kJ mol−1. Notably, the catalyst showed 100% H2 selectivity at the examined temperature

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range. This temperature-independent H2 selectivity is clearly of benefit to the practical application. NaOH is an effective promoter for the catalytic decomposition of N2H4·H2O for HG. As presented in Figure 4B, both reaction rate and H2 selectivity increased with increasing NaOH concentration, and reached the maximum value, 286 h−1 and 100%, at a NaOH concentration of 1.0 M. Similar phenomena were repeatedly observed in catalytic decomposition of N2H4·H2O over different catalysts.12,13,16,21,23-25 The mechanistic reason for the pronounced effects of alkali in improving the HG properties of N2H4·H2O is still unclear. One possibility is that the presence of alkali results in accelerated kinetics of the rate-determining step of N2H4 decomposition (N2H4 → N2H3· + H·) and meanwhile, inhibits the formation of the undesired N2H5+ ions in the solution.12,16,23-25 The newly developed Ni60Pt40/CeO2/GAC-873NH3 monolithic catalyst was further subjected to cyclic usage in the N2H4·H2O solution to test its durability. Notably, the catalyst showed satisfactory cyclic stability. As seen in Figure 4C, the catalyst well retained 100% H2 selectivity and 84% of its initial activity after 10 cycles. TEM observation and XRD analysis found that the cyclic usage caused no appreciable changes on the morphology, microstructure and phase structure of the Ni60Pt40/CeO2/GAC-873NH3 catalyst (Figure S2, Supporting Information). Presumably, the observed degradation of catalytic activity was caused by the over-strongly bound N-containing species that were generated in the N2H4·H2O decomposition on the catalyst surface.22 Hydrogen capacity is a key parameter in evaluating the practicability of H2 source systems. But currently, most of reported works focused on dilute N2H4·H2O solutions with a limited hydrogen capacity to 2 wt%, which was well below the practical requirements. In the present work, by using the developed monolithic catalyst, we constituted a concentrated N2H4·H2O-based HG system to evaluate its hydrogen capacity. The fuel solution was composed of the commercially available 85 12


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wt% N2H4·H2O solution and 1.0 M NaOH promoter. As seen in the top of Figure 4D, upon immersing the monolithic catalyst into the fuel solution, the decomposition reaction of N2H4·H2O was immediately initiated with no appreciable lag time. At the initial reaction stage, the reaction rate rapidly increased and reached its maximum value, ~850 mL (N2+H2) min−1. Then, the system maintained a reaction rate at 50~90 mL (N2+H2) min−1 for over 300 min. At late reaction stage, the rate gradually decreased with consuming N2H4·H2O reactant. After the reaction was completed, the post-used fuel solution was examined by UV/Vis spectroscopy (Figure S3, Supporting Information). The absence of N2H4 signal confirmed the completeness of the decomposition reaction of N2H4·H2O.32 This, together with the precisely determined 3 equivalents of gaseous products, clearly suggests the complete decomposition of N2H4·H2O with 100% H2 selectivity. According to the measured gaseous products amount and fuel conversion, the constituted system yields a material-based hydrogen capacity of 6.54 wt%. This is the highest hydrogen capacity for N2H4·H2O-based HG system reported up to date, which is promising for the practical applications. CONCLUSIONS By using a simple impregnation method followed by reduction treatment at elevated temperatures, we prepared a monolithic Ni-Pt/CeO2/GAC catalyst for promoting hydrogen generation from N2H4·H2O. The catalytic activity and selectivity towards hydrogen generation from N2H4·H2O can be improved by regulating the annealing temperature and atmosphere of the catalyst. The optimal Ni-Pt/CeO2/GAC catalyst enabled complete decomposition of N2H4·H2O to generate H2 at a reaction rate of 286 h−1 at 323K in the presence of 1M NaOH. Importantly, by using this monolithic catalyst, we constituted a N2H4·H2O-based HG system with a material-based hydrogen capacity as

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high as 6.54 wt%. These results are of clear significance for promoting the practical use of N2H4·H2O as a viable hydrogen storage carrier. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge in the ACS Publications website at DOI: . XPS spectra, TEM images, XRD patterns and UV-Vis spectra. AUTHOR INFORMATION Corresponding Authors *Ping Wang. E-mail: [email protected]. *Hong-Bin Dai. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The financial supports for this research from the National Natural Science Foundation of China (Grant Nos. 51471168 and 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 are gratefully acknowledged. REFERENCES (1) Schlapbach, L.; Züttel, A. Hydrogen-storage materials for mobile applications. Nature 2001, 414 (6861), DOI 10.1038/35104634. (2) Singh, A. K.; Xu, Q. Synergistic catalysis over bimetallic alloy nanoparticles. ChemCatChem 14


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2013, 5 (3), DOI 10.1002/cctc.201200591. (3) Zhu, Q. L.; Xu, Q. Liquid organic and inorganic chemical hydrides for high-capacity hydrogen storage. Energy Environ. Sci. 2015, 8 (2), DOI 10.1039/c4ee03690e. (4) Liu, B. H.; Li, Z. P. A review: Hydrogen generation from borohydride hydrolysis reaction. J. Power Sources 2009, 187 (2), DOI 10.1016/j.jpowsour.2008.11.032. (5) Demirci, U. B.; Akdim, O.; Miele, P. Ten-year efforts and a no-go recommendation for sodium borohydride for on-board automotive hydrogen storage. Int. J. Hydrogen Energy 2009, 34 (6), DOI 10.1016/j.ijhydene.2009.01.038. (6) Zhan, W. W.; Zhu, Q. L.; Xu, Q. Dehydrogenation of ammonia borane by metal nanoparticle catalysts. ACS Catal. 2016, 6 (10), DOI 10.1021/acscatal.6b02209. (7) Demirci, U. B.; Miele, P. Sodium borohydride versus ammonia borane in hydrogen storage and direct fuel cell applications. Energy Environ. Sci. 2009, 2 (6), DOI 10.1039/b900595a. (8) Moury, R.; Demirci, U. B. Hydrazine borane and hydrazinidoboranes as chemical hydrogen storage materials. Energies 2015, 8 (4), DOI 10.3390/en8043118. (9) Singh, S. K.; Xu, Q. Nanocatalysts for hydrogen generation from hydrazine. Catal. Sci. Technol. 2013, 3 (8), DOI 10.1039/c3cy00101f. (10) Singh, A. K.; Singh, S.; Kumar, A. Hydrogen energy future with formic acid: a renewable chemical storage system. Catal. Sci. Technol. 2016, 6 (1), DOI 10.1039/c5cy01276g. (11) Asazawa, K.; Yamada, K.; Tanaka, H.; Oka, A.; Taniguchi, M.; Kobayashi, T. A platinum-free zero-carbon-emission easy fuelling direct hydrazine fuel cell for vehicles. Angew. Chem. Int. Ed. 2007, 46 (42), DOI 10.1002/anie.200701334. (12) Singh, A. K.; Xu, Q. Complete conversion of hydrous hydrazine to hydrogen at room temperature for chemical hydrogen storage. J. Am. Chem. Soc. 2009, 131 (50), DOI 15



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10.1021/ja908037t. (13) Zhong, Y. J.; Dai, H. B.; Zhu, M.; Wang, P. Catalytic decomposition of hydrous hydrazine over Ni-Pt/La2O3 catalyst: A high-performance hydrogen storage system. Int. J. Hydrogen Energy 2016, 41 (26), DOI 10.1016/j.ijhydene.2016.03.207. (14) He, L.; Huang, Y. Q.; Liu, X. Y.; Li, L.; Wang, A. Q.; Wang, X. D.; Mou, C. Y.; Zhang, T. Structural and catalytic properties of supported Ni-Ir alloy catalysts for H2 generation via hydrous hydrazine decomposition. Appl. Catal. B-Environ. 2014, 147, DOI 10.1016/j.apcatb.2013.10.022. (15) Dai, H. B.; Zhong, Y. J.; Wang, P. Hydrogen generation from decomposition of hydrous hydrazine over Ni-Ir/CeO2 catalyst. Prog. Nat. Sci-Mater. 2017, 27 (1), DOI 10.1016/ j.pnsc.2016.12.012. (16) Zhao, P. P.; Cao, N.; Su, J.; Luo, W.; Cheng, G. Z. NiIr Nanoparticles immobilized on the pores of MIL-101 as highly efficient catalyst toward hydrogen generation from hydrous hydrazine. ACS Sustain. Chem. Eng. 2015, 3 (6), DOI 10.1021/acssuschemeng.5b00009. (17) Zhao, P. P.; Cao, N.; Luo W.; Cheng, G. Z. Nanoscale MIL-101 supported RhNi nanoparticles: an efficient catalyst for hydrogen generation from hydrous hydrazine. J. Mater. Chem. A 2015, 3 (23), DOI 10.1039/c5ta02201k. (18) Cao, N.; Su, J.; Luo, W.; Cheng G. Z. Ni-Pt nanoparticles supported on MIL-101 as highly efficient catalysts for hydrogen generation from aqueous alkaline solution of hydrazine for chemical hydrogen storage. Int. J. Hydrogen Energy 2014, 39 (18), DOI 10.1016/j.ijhydene.2014.04.075. (19) He, L.; Huang, Y. Q.; Wang, A. Q.; Liu, Y.; Liu, X. Y.; Chen, X. W.; Delgado, J. J.; Wang, X. D.; Zhang, T. Surface modification of Ni/Al2O3 with Pt: highly efficient catalysts for H2 generation via

selective

decomposition

of

hydrous

hydrazine.

10.1016/j.jcat.2012.10.012. 16


J.

Catal.

2013,

298,

DOI

Page 17 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(20) Wang, H. L.; Yan, J. M.; Wang, Z. L.; Song-Il, O.; Jiang, Q. Highly efficient hydrogen generation from hydrous hydrazine over amorphous Ni0.9Pt0.1/Ce2O3 nanocatalyst at room temperature. J. Mater. Chem. A 2013, 1 (47), DOI 10.1039/c3ta13259e. (21) Jiang, Y. Y.; Kang, Q.; Zhang, J. J.; Dai, H. B.; Wang, P. High-performance nickel-platinum nanocatalyst supported on mesoporous alumina for hydrogen generation from hydrous hydrazine. J. Power Sources 2015, 273, DOI 10.1016/j.jpowsour.2014.09.119. (22) Dai, H.; Qiu, Y. P.; Dai, H. B.; Wang, P. A study of degradation phenomenon of Ni-Pt/CeO2 catalyst towards hydrogen generation from hydrous hydrazine. Int. J. Hydrogen Energy 2017, 42 (26), DOI 10.1016/j.ijhydene.2017.05.086. (23) Oliaee, S. N.; Zhang, C. L.; Hwang, S. Y.; Cheung, H. M.; Peng, Z. M. Hydrogen production via hydrazine decomposition on model platinum-nickel nanocatalyst with a single (111) facet. J. Phys. Chem. C 2016, 120 (18), DOI 10.1021/acs.jpcc.6b00815. (24) Wang, J.; Zhang, X. B.; Wang, Z. L.; Wang, L. M.; Zhang, Y. Rhodium-nickel nanoparticles grown on graphene as highly efficient catalyst for complete decomposition of hydrous hydrazine at room temperature for chemical hydrogen storage. Energy Environ. Sci. 2012, 5 (5), DOI 10.1039/c2ee03344e. (25) Song, F. Z.; Zhu, Q. L.; Xu, Q. Monodispersed PtNi nanoparticles deposited on diamine-alkalized graphene for highly efficient dehydrogenation of hydrous hydrazine at room temperature. J. Mater. Chem. A 2015, 3 (46), DOI 10.1039/c5ta05664k. (26) He, L.; Liang, B. L.; Li, L.; Yang, X. F.; Huang, Y. Q.; Wang, A. Q.; Wang, X. D.; Zhang, T. Cerium-oxide-modified nickel as a non-noble metal catalyst for selective decomposition of hydrous hydrazine to hydrogen. ACS Catal. 2015, 5 (3), DOI 10.1021/acscatal.5b00143. (27) Zhu, Q. L.; Xu, Q. Immobilization of ultrafine metal nanoparticles to high-surface-area 17



materials and their catalytic applications. Chem 2016, 1 (2), DOI 10.1016/j.chempr.2016.07.005. (28) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K. S. W. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87 (9-10), DOI 10.1515/pac-2014-1117. (29) Wang, H. L.; Yan, J. M.; Li, S. J.; Zhang, X. W.; Jiang, Q. Noble-metal-free NiFeMo nanocatalyst for hydrogen generation from the decomposition of hydrous hydrazine. J. Mater. Chem. A 2015, 3 (1), DOI 10.1039/c4ta05360e. (30) Guo, D. H.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016, 351 (6271), DOI 10.1126/science.aad0832. (31) He, L.; Huang, Y. Q.; Wang, A. Q.; Wang, X. D.; Chen, X. W.; Delgado, J. J.; Zhang, T. A noble-metal-free catalyst derived from Ni-Al hydrotalcite for hydrogen generation from N2H4·H2O decomposition. Angew. Chem. Int. Ed. 2012, 51 (25), DOI 10.1002/anie.201201737. (32) Wang, J.; Li, W.; Wen, Y. R.; Gu, L.; Zhang, Y. Rh-Ni-B nanoparticles as highly efficient catalysts for hydrogen generation from hydrous hydrazine. Adv. Energy Mater. 2015, 5 (10), DOI 10.1002/aenm.201401879.

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Figures and Figure Captions

Scheme 1. Schematic illustration of the preparation of Ni-Pt/CeO2/GAC monolithic catalyst.

Figure 1. (A) N2 adsorption/desorption isotherms and pore size distributions of the pretreated GAC and

Ni60Pt40/CeO2/GAC-573H2

samples;

(B)

XRD

patterns

of

the

pretreated

GAC,

Ni60Pt40/CeO2/GAC-573H2, Ni60Pt40/CeO2/GAC-873H2 and Ni60Pt40/CeO2/GAC-873NH3 samples; The diffraction peak positions and relative intensities of pristine Ni, Pt and CeO2 were given below for reference.

19



Figure 2. (A) A typical TEM image (inset: size distribution of the catalyst particles) and (B) HRTEM image of the Ni60Pt40/CeO2/GAC-573H2 sample; (C) TEM image (inset: size distribution of catalyst particles) and (D) HRTEM image of the Ni60Pt40/CeO2/GAC-873NH3 sample; (E) and (F) HAADF-STEM image and the corresponding EDS mapping images of Ni, Pt, Ce and O elements for the Ni60Pt40/CeO2/GAC-873NH3 catalyst. 20


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Figure 3. Dependence of reaction rate and H2 selectivity on (A) Pt-content and (B) CeO2-content of the Ni-Pt/CeO2/GAC catalysts; (C) Effects of annealing temperature and atmosphere on the catalytic properties of the Ni60Pt40/CeO2/GAC catalyst towards N2H4·H2O decomposition. Reaction conditions: 4 mL 0.50 M N2H4·H2O + 1.0 M NaOH solution at 323 K, the catalyst/N2H4·H2O molar ratio was fixed at 1:20. The legends give the annealing temperature, atmosphere, reaction rate, and H2

selectivity;

(D) N1s

XPS

spectra

of

the

Ni60Pt40/CeO2/GAC-873NH3

Ni60Pt40/CeO2/GAC-873H2 (below) samples.

21


(top) and


Figure 4. Catalytic decomposition properties of N2H4·H2O over the Ni60Pt40/CeO2/GAC-873NH3 catalyst, the catalyst/N2H4·H2O molar ratio was fixed at 1:20. (A) Catalytic decomposition kinetics curves of N2H4·H2O in the presence of 1.0 M NaOH at various temperatures. The inset shows the Arrhenius treatment of the temperature-dependent rate data for determination of the apparent activation energy; (B) Effect of NaOH concentration on reaction rate and H2 selectivity of N2H4·H2O decomposition at 323 K; (C) Cyclic kinetics curves of N2H4·H2O decomposition in the presence of 1.0 M NaOH at 323 K. The legends give the cycle numbers, reaction rate and retained value relative to the initial rate; (D) Time-course profiles of reaction rate (top) and fuel conversion (bottom) of the system. The fuel solution was composed of 22 mL of 17.5 M N2H4·H2O and 1.0 M NaOH with a density of 1.07 g cm−3, the reaction was performed at 323 K over 950 mg of the catalyst.

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Table of Contents (TOC)

The system composed of a concentrated N2H4·H2O solution and a high-performance monolithic catalyst yielded a hydrogen capacity of 6.54 wt%, which is promising for on-board application.

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CeO2 Loaded on Granular Activated Carbon: An Efficient

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