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Mesoporous Face-Centered-Cubic In4Ni Alloy Nanorices: Superior Catalysts for Hydrazine Dehydrogenation in Aqueous Solution Xue Miao,†,‡ Ming Ming Chen,†,‡ Wei Chu,*,§ Ping Wu,†,‡ and Dong Ge Tong*,†,‡ †
Collaborative Innovation Center of Panxi Strategic Mineral Resources Multipurpose Utilization, College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China ‡ State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China § College of Chemical Engineering and Key Laboratory of Green Chemistry & Technology of Ministry of Education, Sichuan University, Chengdu 610065, China S Supporting Information *
ABSTRACT: Mesoporous face-centered-cubic (fcc) In4Ni alloy nanorices (NRs) were successfully synthesized as superior catalysts for N2H4 dehydrogenation in aqueous solution via a facile solution plasma technique (SPT) in an ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). This incorporation introduces basic sites for dehydrogenation. Also, the synthesis of In and Ni weakens the interactions among generated adspecies such as H2 and NHx and surface metal atoms. Alongside their unique NR structure, the as-prepared fcc-In4Ni alloy NRs exhibited superior performance for N2H4 dehydrogenation in aqueous solution. The activation energy of the fcc-In4Ni alloy NRs was 38.9 ± 1.0 kJ mol−1. The NRs were also found to be stable for catalytic N2H4 dehydrogenation in aqueous solution, providing an average TOF value of 82.0 (mol of H2 (mol of active In4Ni min)−1) over 30 h reaction. These fcc-In4Ni alloy NRs have demonstrated exceptional performance, which indicates that the construction of hydrogen-producing systems from N2H4, capable of matching the performance of NaBH4 and NH3BH3 hydrogen-producing systems for fuel-cell applications, is a promising possibility. KEYWORDS: mesoporous face-centered-cubic In4Ni, bimetal catalysts, aqueous hydrazine dehydrogenation, solution plasma technique, ionic liquid
1. INTRODUCTION
is unwanted and must be prevented for any practical applications.4−53 In particular, the byproduct NH3 has the potential to contaminate fuel-cell catalysts and Nafion membranes and, therefore, increase separation costs. Hence, a pressing demand remains for effective catalysts that can selectively decompose N2H4 to H2 in aqueous solution. To this end, many efforts have been made toward the development of nickel-based catalysts for aqueous hydrazine dehydrogenation.5−7,9,10 For instance, work by Xu et al. showed that a Ni2Fe bimetallic nanocatalyst has the capability of fully decomposing hydrous hydrazine to H2 with the help of NaOH at 70 °C.9 Zhang et al. produced a graphene-supported Rh−Ni catalyst. This catalyst displayed complete H2 selectivity, which also demonstrated significantly higher catalytic efficiency than that of only Rh−Ni nanocatalysts while promoting NaOH at room temperature.11 Wang et al. produced Rh−Ni−B nanoparticles, which served as catalysts that showed great efficiency for generating hydrogen from hydrous hydrazine and promoting NaOH at room temperature.28 However, most of
Because of its high-energy density and efficiency with low environmental load, hydrogen remains a significant green energy resource.1 However, storing and generating hydrogen (H2) safely and efficiently is still a great challenge toward the construction of a “hydrogen economy” society.2 As a result, sorbent materials, metal hydrides, chemical hydrides, and organic hydrogen carriers have been explored as hydrogen carriers.3 Hydrazine in aqueous solution is still regarded as a propitious hydrogen carrier because of its high hydrogen content (12.5 wt %) of N2H4,4−43 at a relatively low cost. This solution can produce nitrogen only, in addition to hydrogen, through a reaction that causes thorough decomposition: N2H4 → N2 + 2H 2
(1)
In addition, the nitrogen that is produced could be transformed successively into ammonia and then into hydrazine by an electrolytic procedure for simple recharging. Yet, the decomposition pathway to ammonia (NH3) via the reaction 3N2H4 → 4NH3(g) + N2(g) © XXXX American Chemical Society
Accepted: September 6, 2016
(2) A
DOI: 10.1021/acsami.6b07434 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
tungsten electrodes were fixed in a Teflon vessel containing [BMIM][BF4], which served as a medium for the plasma reaction. The plasma discharge in the [BMIM][BF4] was generated between the two electrodes (interelectrode distance of 0.5 mm) via a pulsed direct current power supply. The applied discharge parameters were a potential of 200 V, a frequency of 6 kHz, a pulse duty of 25%, and an output current of 50 mA. Under conditions other than these, no fccIn-Ni alloy NPs were obtained. Further studies are underway to investigate the reason for this result. Previous work on glow-discharge electrolysis in aqueous solutions, which can be traced back more than 100 years, serves as the basis for the experimental approach used in this study.48,49 Other fundamental research on organic and inorganic synthesis by glow-discharge electrolysis has been reported, primarily by Hickling, which dates back to the 1950s.48,49 Since then, glow discharge applied to the reactive treatment of inorganic and organic solutions has persevered and remains an active field.8,13,48,49 In comparison with all previous works, our approach has two distinct advantages: (a) because it utilizes extremely low vapor pressure and high thermal stability, our SPT could be used in an open reactor without the need for a vacuum and without the release of harmful vapors into the atmosphere; and (b) the high ionic conductivity and low viscosity of [BMIM][BF4] allow our SPT to operate at moderate voltage, which is helpful for forming lowmelting-point metal alloy nanostructures with controllable size and shape, such as In-based alloys. In a typical synthesis, 16 mmol of InCl3 and 4.0 mmol of NiCl2 were mixed and dissolved in [BMIM][BF4] under hydrogen atmosphere. Then, 0.42 mmol of Na3PO3 and 0.80 mmol of Na2SO4 were added to the solution. Without plasma, there was not a significant reaction detected. After being wholly mixed by vigorous stirring for 30 min, SPT was performed. The electric field of the plasma in the interelectrode space was about 400 V cm−1. The precipitate was run through many consecutive washing/centrifugation cycles containing ethanol and water. Then, the collected solid was treated by SPT in KCl solution (20.0%) to desorb the sulfate ions. After which the solid was re-dispersed in hexane as a stable colloidal suspension. To provide a comparison, face-centered-cubic In and Ni NRs were also prepared by the same procedure. Body-centered tetragonal (bct) In and In4Ni nanoparticles (NPs) were put together based on the technique recorded by Cingarapu et al.51 fcc-In-Ni NRs with different In/Ni ratios were prepared using the same procedure by changing the ratio of the reactants. Conventional fcc-In4Ni NPs were prepared by the same procedure but without adding Na2SO4. 2.2. Characterization. To provide an analysis of the composition, dry samples were dissolved in boiling aqua fortis through the use of a microwave digestion system. The contents of In and Ni in the samples were ascertained through the use of inductively coupled plasma atomic emission spectroscopy (ICP-AES; Iris, Advantage, Leeds, U.K.). S and O elemental analysis was performed by using a Vario Micro cube elemental analyzer. X-ray diffraction (XRD) patterns were gathered by using an X′Pert X-ray powder diffractometer, which included a Cu Kα radiation source (λ = 0.15406 nm). Measurements of the Brunauer− Emmett−Teller (BET)-specific surface area and pore volume of the samples were taken by using a N2 adsorption−desorption method. Through this method, the samples were degassed at 200 °C for 180 min before the measurements were taken. By applying the Barrett− Joyner−Halenda (BJH) model, pore-size distribution and pore-size averages of the samples could be determined. Images from scanning transmission electron microscopy (STEM) were taken and selected-area electron diffraction patterns (SAED) of the samples were collected through the use of a JEOL-2100F microscope (Waterford, VA, USA). Sample preparation for STEM analysis consisted of depositing a single drop of diluted nanoparticle dispersion in ethanol onto an amorphous, carbon-coated copper grid. Through the use of a Bruker FT-IR spectrometer (EQUINOX55), Fourier transform infrared spectra (FTIR) were collected. X-ray photoelectron spectroscopy (XPS; PHI 5000C ESCA, PerkinElmer, Waltham, MA, USA; using Al Kα radiation) was used to determine the surface electronic states. Samples were fixed in a homemade in situ XPS reactor cell.16,17,43 After the samples were dried under an argon
these catalysts need to be promoted with alkaline solution to obtain higher H2 selectivity and kinetics. However, the use of base appears to be a barrier to practical application owing to its cost and corrosivity. To solve this problem, catalysts with base sites themselves have gained more and more research interest for the selective decomposition of N2H4. For example, Zhang’s group loaded Ni−Pt nanoparticles on the basic Al2O3 catalyst supports and improved the H2 generation rate with 100% H2 selectivity.12 Yang et al. incorporated N and H into Co−B nanowires to increase the number of basic active sites and, as such, produced an exceptional catalyst for N2H4 dehydrogenation in aqueous solution.26 Because the activities of these reported catalysts have not met the demands required for N2H4 in aqueous solution, their practical application is limited. Therefore, they cannot be considered as a feasible hydrogen carrier. Since 1990, indium (In) metal has emerged as an ideal candidate for single electron transfer (SET) reactions in most aqueous organic systems, owing to the fact that its first ionization potential (5.8 eV) is comparable to those of alkali metals but unaffected by air, oxygen, or water at room temperature.44−47 An obvious question arises: Is In active for the decomposition of hydrazine in aqueous solution? Is it possible to prepare In−Ni alloy as an efficient catalyst for hydrazine decomposition with 100% hydrogen selectivity at room temperature without the addition of an alkali? The solution plasma technique (SPT) introduces plasma to a solution and provides a cost-efficient and direct route for chemical reaction paths, e.g., active species, radicals, and ultraviolet radiation, for preparing metal NPs with well-defined nanostructures.8,13,48 For instance, Co−B honeycomb13 and monodispersed Ag nanoparticles48 have been prepared successfully on a large scale through the use of SPT within a short period of time. Also, the wide electrochemical windows of ionic liquids enable the preparation of many metal NPs that otherwise would not be obtainable in aqueous solutions (e.g., Ge, Si, Se, and Al) with the help of solution plasma.48,49 On the basis of this compelling research, this study aimed to prepare In−Ni alloy NPs through SPT by using ionic liquids as catalysts for hydrazine dehydrogenation, without a base, in aqueous solution, at room temperature. Herein, we report the synthesis of mesoporous fcc-In4Ni NRs as superior catalysts for the generation of hydrogen from hydrous hydrazine via a plasma technique in an ionic liquid, 1butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). The results indicated that sulfate ions (SO42−) are key for the successful preparation of fcc-In4Ni NRs. The as-prepared mesoporous fcc-In4Ni NRs had a surface area of 243.1 m2 g−1. Also, it showed a high level of catalytic activity with 100% H2 selectivity for the decomposition of N2H4 and showed capability under ambient conditions without any alkaline promotion. The turnover frequency (TOF) of the NRs was 83.6 mol of H2 (mol of In4Ni min)−1. This level of TOF exceeds any prior reports of catalysts that can exhibit 100% H2 selectivity for the decomposition of N2H4 at room temperature.
2. EXPERIMENTAL SECTION 2.1. Preparation of In4Ni Nanorices. Anhydrous InCl3 and NiCl2, purchased from Sigma-Aldrich (St. Louis, MO, USA), were used as received. The reactor used for the preparation of fcc-In4Ni NRs via SPT was designed in-house, and its capabilities are documented in previous studies.8,13,50 The two electrodes for the plasma discharge consisted of tungsten wires, 2 mm diameter. The B
DOI: 10.1021/acsami.6b07434 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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mL of a standardized solution (0.01000 mol L−1 HCl) at room temperature. This process ensured the absorption of any ammonia generated. After the release of gas stopped, the solution obtained was titrated with a standard solution of 0.01000 mol L−1 NaOH with phenolphthalein used as an indicator of acid−base. The quantity of ammonia gas created was ascertained by obtaining the difference in concentration in the HCl solution before and after the reaction. The acid trap was not used for mass spectral analyses. The ion source temperature of the mass spectrometer (DeltaPLUS; Finnigan MAT, Bremen, Germany) was 200 °C, the ionization energy was 70 eV, and the scan rate was 2.78 (single ion monitor). The detection limits were 33 ppb for H2, 18 ppb for N2, and 10 ppb for NH3. Through the use of a magnet the catalyst was recovered, which was then washed thoroughly with water and absolute ethanol at least three times.
atmosphere they were transferred to an analysis chamber, and their XPS spectra were recorded. The binding energies were referenced to the C 1s peak (binding energy of 284.6 eV) of the surface adventitious carbon. Studies of time-of-flight secondary ion mass spectrometry (ToF-SIMS) were performed through the use of a PHI TRIFT II (Chanhassen, MN, USA), which was equipped with a pulsed liquidmetal ion gun. Spectra, over a range of mass of 1−280 Da, were obtained in positive ion mode, and Ga+ was used as a primary source with a 100 × 100 mm2 raster size. In addition, a voltage of 15 kV was applied with an aperture current of 600 pA with an acquisition time of 10 min. Analyses were performed on at least three different spots on each sample, and the most representative data were utilized. Data acquisition and element composition analyses were performed through the use of the IonSpec software (Lafe Forest, CA, USA). Also through the use of the Quantachrome CHEMBET 3000 system, experiments on temperature-programmed desorption (TPD) experiments were conducted. First, surface air was taken out from the sample by an argon stream at 423 K for 1 h. By using the same argon stream, the surface was cooled to room temperature. For conducting tests on H2 TPD, pure H2 was injected to the point of saturation at 333 K. For testing on NH3-TPD, NH3 was injected to the point of saturation at 293 K. After flushing with Ar until the baseline became stable, the TPD process was performed until reaching 673 K at a rate of 1 K min−1. For testing CO2-TPD, CO2 was preadsorbed by the catalyst at 383 K for 2 h. After which, the CO2 stream was replaced with the carrier gas. The gas was maintained at 383 K for 2 h, which enabled the gas to be purged and for the CO2 to be physically absorbed from the surface of the catalyst. After the surface was cooled to room temperature, CO2 desorption was performed by increasing the temperature to 773 K at 1 K min−1. The TPD process, which used a thermal conductivity detector and a quadrupole mass spectrometer, enabled the monitoring of the desorbed products. . Measurements of the microcalorimetric H2 were performed by using a BT 2.15 heat−flux calorimeter (SETARAM, Caluire, France) at 313 K. The use of MKS 698A baratron capacitance manometers for precision pressure measurements (±1.33 × 10−2 Pa) enabled the calorimeter to be linked to both gas-handling and volumetric systems. The maximum leak rate of the volumetric system was detectable at 10−4 Pa min−1, and the system volume was approximately 80 cm3. The ultimate dynamic vacuum of the system was approximately 10−5 Pa. Recordings of the ultraviolet−visible (UV−vis) spectra were taken by using a UV5800 modal spectrophotometer (Fivephase, Shanghai, China). To measure UV−vis, a chromogenic agent, which consisted of a mixture of 0.5000 g of p-(dimethylamino)benzaldehyde, 25.00 mL of of ethanol, and 2.50 mL of hydrochloric acid (1.000 mol L−1), was applied to the N2H4, after which a mixture of hydrous hydrazine solution (0.30 mL of 0.50 mol L−1), chromogenic agent (0.10 mL), and deionized water (3.40 mL) was stored for 30 min before testing. A vibrating sample magnetometer (MagLab-12; Oxford Instruments, Abingdon, U.K.) was used to take magnetic measurements at room temperature. For varying concentrations of N2H4 in aqueous solution, the viscosity was determined by using a DV2TTM Brookfield viscometer (Middlesboro, MA, USA). Through the use of a homemade electrolytic cell (200 mm long, 75 mm wide, and 60 mm high) and an isotope tracer method, the ion motilities of various anions in [BMIM][BF4] could be determined. Tungsten plates (20 mm × 20 mm) were used as the two electrodes. The measurement of radioactive isotopes was conducted using a scintillation counter (Phonix, Chengdu, China), while the analysis of stable isotopes was performed on a MAT 253 isotope mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). 2.3. Catalytic Hydrolysis of Hydrazine in Aqueous Solution. Assessments of sample performance for hydrazine dehydrogenation in aqueous solution were performed by measuring the hydrogen release rate of the reaction with an online gas chromatography system (HP Series 6890, Palo Alto, CA, USA), as reported previously.8,13,16,17 Initiation of the reaction occurred by introducing the hydrazine aqueous solution into a three-neck round-bottom flask containing the catalyst, which was prepared by magnetic stirring. Gas that was generated during the reaction passed through a trap that contained 15
3. RESULTS AND DISCUSSION 3.1. Material Characterization. Figure 1a shows fcc-In4Ni NRs, with an average particle size 50 nm long and 20 nm wide,
Figure 1. (a) Low-magnification STEM image and (b) enlarged STEM images of single NR of the as-prepared mesoporous fcc-In4Ni NRs.
produced successfully on a large scale. Figure 1a shows fccIn4Ni NRs, with an average particle size 50 nm long and 20 nm wide, produced successfully on a large scale. The enlarged STEM image depicts the fcc-In4Ni NRs consisting of numerous small particles attached to each other (Figures 1b and S1). This assembly of NPs shows the obtained fcc-In4Ni NRs as a rough surface. The lattice spacing at different parts of the NRs was measured to be around 0.274 nm, which is between the {111} lattice spacings of pure Ni (0.203 nm) and In (0.290 nm), indicating the successful incorporation of Ni atoms into the In lattice. This result also indicated that each attached nanoparticle in the fcc-In4Ni NRs shared the {111} growth direction. Similar fast Fourier transform (FFT) patterns were also gathered at the various highlighted areas of the fcc-In4Ni NRs (Figure S1). The high-resolution STEM (HR-STEM) images and FFT patterns also confirmed the single-crystalline nature of the assembled NPs in the fcc-In4Ni NRs (Figure S2). Numerous nanopores found between the nanoparticles on the surface of the fcc-In4Ni NRs could be verified further through the nitrogen adsorption−desorption isotherms (Figure 2a). The average pore diameter was 3.2 nm for the as-prepared fcc-In4Ni NRs; their specific surface area was 243.1 m2 g−1, which is higher than that of the conventional fcc-In4Ni and bctIn4Ni NPs (20.9 and 22.1 m2 g−1, respectively; Figures S3 and S4). Therefore, the as-prepared fcc-In4Ni NRs were expected to demonstrate elevated activity toward the decomposition of N2H4 because both their high specific surface area and their mesoporous structure would benefit the permeation of reaction molecules. Analysis of the cross-sectional compositional line for the fccIn4Ni NRs was performed during high-angle annular dark field C
DOI: 10.1021/acsami.6b07434 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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agreement with the anticipated values of the profile factor (Rp = 7.34%), the weighted profile factor (Rwp =5.29%), and the Bragg factor (Rb = 4.33%). The small Rf ( 120 mL were not used owing to the limitations of our plasma equipment. These results indicate that the role of [BMIM][BF4] in this work was mainly to provide a medium for the plasma reaction. Figure 3 shows a possible mechanism to provide a clearer comprehension of the synthesis of fcc-In4Ni NRs via SPT in
Figure 4. N(H2 + N2)/n(N2H4) and H2/N2 molar ratio vs time for 40.0 mg of fcc-In4Ni NRs during the decomposition of 50.0 mL of hydrous hydrazine with a concentration of 1.00 mol L−1 at 293 K.
active and that it completed the hydrazine dehydrogenation within only 15.5 min at room temperature, whereas the H2/N2 ratio was nearly constant throughout the experiment. This finding indicates that reactions 1 and 2 proceeded simultaneously over the In4Ni NRs. As such, eq 10 can be applied to express the hydrazine decomposition reaction in this work.
Figure 3. Synthetic protocol of the as-prepared mesoporous fcc-In4Ni NRs.
this ongoing research. At the initial stage, the extremely high energy of the plasma enabled the generation of H* radicals from H2, which participated in the reduction of Ni and In ions to form fcc-In4Ni NPs. Indeed, with only plasma without H2 or with H2 without plasma, no Ni or In was obtained. The reaction is shown in eqs 6−9. H 2 → 2H*
(6)
Ni 2 + + 2H* → Ni
(7)
In 3 + + 3H* → In + 3H+
(8)
Ni + 4In → In4Ni
3N2H4 → (1 + 2x)N2 + 6x H 2 + 4(1 − x)NH3
(10)
where x represents H2 selectivity. The activity of the In4Ni NRs in terms of the TOF (mol of H2 (mol of In4Ni min)−1) was 83.6. This level was higher (as much as 110%) than that of the most active 100% H2 selective catalysts that have been reported for N2H4 dehydrogenation (Table S2). Reaction completeness and H2 selectivity were determined as well by UV−vis spectroscopy (no N2H4 after reaction, Figure S24) and mass spectrometry (no NH3, H2/N2 = 2.0, and 100% H2 selectivity, Figure S25), respectively. Figure S26 illustrates hydrogen volume produced during the N2H4 dehydrogenation over the fcc-In4Ni NRs as a function of time, at different temperatures that ranged from 293 to 333 K. The rate constant k was obtained from the slope of the linear part of each plot (Figure S27). According to the Arrhenius plot (ln k vs 1/T), shown in Figure S27, activation energy for N2H4
(9)
Yet, reduction of NiCl2 by H2 or in the plasma field, which formed Ni atoms (or seeds) at a rate that was nearly simultaneous and the resulting PO43− reacted with Ni0. This mechanism was comparable to a buffer that could tune the subsequent nucleation of Ni0 and In−Ni competition of surface segregation. Uniform fcc-In4Ni NRs formed through the reinforcement of the SO42− surface binding. Interactions PO33−
F
DOI: 10.1021/acsami.6b07434 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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adsorption was 322.3 mol g−1 for fcc-In4Ni NRs. This amount is notably lower than that of the fcc-In NRs (480.5 mol g−1) and fcc-Ni NRs (636.9 mol g−1). At the same time, the corresponding initial heats of the H2 absorbance were 79.0, 68.0, and 56.2 kJ mol g−1 for the fcc-Ni, fcc-In, and fcc-Ni4In nanorices, respectively. These findings suggest that the adsorption abilities were much weaker on the surface of the fcc-In4Ni nanorices. Histograms depicting the heat distribution of the active sites (Figure S33) were obtained through a method presented by Cortright and Dumesic.62 The results indicated that Ni and In NRs had differential heat of H2 adsorptions above 60 kJ mol−1, 88.9 and 64.2%, of the active sites, respectively. By comparison, 100.0% of the active sites on the surface of the fcc-In4Ni NRs had a differential heat between 30 and 60 kJ mol−1, a notably weaker H2 adsorption. It is obvious that the alloying of In with Ni significantly weakened the strength of the M−H bond during N2H4 hydrolysis. Additional H2-TPD studies aimed to further affirm the distinctive aspects of the fcc-In4Ni NRs, (Figure S34). Only one peak, at approximately 474 and 463 K, attributed to H2 desorption, could be detected for the Ni and In NRs, respectively. This notable decrease in the H2 desorption temperature of the fcc-In4Ni NRs could further affirm that the alloying of In with Ni significantly weakened the M−H bond during N2H4 decomposition.62−65 Considering that the hydrazine dehydrogenation experiments were performed at 298 K in this work, it is possible that hydrogen desorption could be one of the rate-determining steps. Hence, the hydrogen that was produced was released more readily from the surface of the fcc-In4Ni NRs than from the surface of the Ni and In NRs. to investigate the bonding strength of the N species that were adsorbed onto the In4Ni NRs, NH3 TPD was performed. Figure S35 illustrates the mass spectra of the desorbed NH3 and N2. All samples examined indicated a NH3 desorption peak and a N2 desorption peak, and these peaks originated from an adsorbed ammonia intermediate. These findings verify a high level of N−H bond breakage.66−68 However, the desorption temperature and amount of NH3 and N2 desorbed from the fcc-In4Ni NRs was significantly lower than those observed for the Ni and In NRs. This condition could possibly be attributable to the M−N bond caused by the alloying of Ni and In atoms, which is relatively weaker. Under these mild conditions, the N species were hard to desorb, and catalyst deactivation occurred. Weaker levels of adsorption of the N species on the surface of the fcc-In4Ni NRs decreased the extent of deactivation and also enhanced the stability of the catalyst.15,26,43 From the results given, it is apparent that the alloying of In with Ni weakened both the bonds of both M−H and M−N during the N2H4 dehydrogenation in aqueous solution. This type of change diminished the energy barrier for molecular N2H4 desorption from the surface of the catalysts. Also, such a change might be the source of the improved conversion rate seen for the fcc-In4Ni NRs.15,26,43 Because it is established that NaOH promotes the activity and selectivity of catalysts toward N2H4 decomposition,9,22−35 the presence of NaOH had no effect on the catalytic performance of fcc-In4Ni NRs (Figure S36). As such, the improved basic sites that arose from the alloying of In with Ni (Figure 6) were the reason for their 100% H2 selectivity for N2H4 decomposition in comparison with previously reported Ni catalysts.28−37 CO2-TPD was applied to conduct quantitative measurement of both the number and strength of the basic
dehydrogenation in aqueous solution over the fcc-In4Ni NRs was 38.9 ± 1.0 kJ mol−1. This activation energy is bigger than that reported for Rh−Ni/MIL-101 (33 kJ mol−1 at 50 °C);29 however, it is comparable with the values of 39.9 ± 1.5, 41.6 ± 1.2, 42.3, 46.7 ± 1.3, 48.1 ± 2.0, 49.3 ± 3.2, 54.3 ± 2.7, and 55.1 ± 1.8 kJ mol−1 reported for Co−B−N−H nanowires,26 Rh2Ni octahedra,43 amorphous Ni0.9Pt0.1/Ce2O3,19 Co−B honeycomb,13 monodisperse Ni3Fe nanospheres/C,16 Fe−B/ MWCNTs,17 Ni−Al2O 3,12 and amorphous NiMoB−La(OH)318 catalysts, respectively. Also, the activation energy was smaller than that observed for conventional fcc-In4Ni NPs (60.7 ± 1.7 kJ mol−1, Figures S28 and S29) and bct-In4Ni NPs (100.9 ± 2.2 kJ mol−1, Figures S30 and S31). The findings are attributable to the high specific surface area and the unique NR structure of the fcc-In4Ni NRs. Furthermore, the activity of the fcc-phase for N2H4 dehydrogenation was found to be higher than those observed for the bct-phase for both In and In4Ni NPs (Table S2). Theoretical studies on this interesting phenomenon are currently underway. Therefore, the composition−activity dependence was examined by utilizing fcc-phase structured NRs having various In/Ni ratios (Table S2). All told, the bimetallic nanoalloy catalysts showed greater activity than that of their monometallic counterparts, in the order In4Ni > In2Ni ≈ InNi3 > InNi > InNi2 > InNi5 > In > Ni, taking into consideration both thermodynamic and kinetic factors. The XRD patterns of fccInNi NRs with different In/Ni ratios were shown in Figure S32. It is apparent that improvements in the activity and hydrogen selectivity of In−Ni alloy occurred because of the synergistic effect of In and Ni and because alloying In and Ni tunes their interactions with N−N and N−H bonds in addition to the stability of the reaction intermediates on the surface compared with their monometallic counterparts.5−8 Meanwhile, the activity and H2 selectivity of our fcc-In4Ni NRs was better than those of core−shell structured fcc-In4Ni prepared without the addition of Na3PO3 (Table S2). To identify factors attributable to the pronounced enhancement achieved by incorporating In with Ni, investigation focused on the structure and surface adsorption qualities of the fcc-In4Ni NRs. Figure 5 illustrates the differential heat of H2 adsorption at 313 K, which serves in H2 uptake over fcc-In4Ni NRs, fcc-In NRs, and fcc-Ni NRs. The saturated amount of H2
Figure 5. Differential heat of H2 adsorption on (a) fcc-Ni NRs, (b) fcc-In NRs, and (c) fcc-In4Ni NRs. G
DOI: 10.1021/acsami.6b07434 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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sizable surface area of the In4Ni NRs.16,17,26,43 However, the slope of the line in the second part (C(N2H4) = 0.500−10.0 mol L−1) of the plot was 0. This finding shows that the reaction reached zero order with respect to N2H4 concentration when C(N2H4) ≥ 0.100 mol L−1 and that the rate reduced gradually with an increase in the C(N2H4) concentration in the range of 12.5−20.0 mol L−1. This condition was caused by the high viscosity of these high-concentration solutions (Figure S40), which slowed the mass transport of N2H4 molecules during the reaction. A similar phenomenon was noted during the N2H4 hydrolysis over Fe−B/MWCNTs,16 Co−B−N−H nanowires,26 and Rh2Ni nanooctahedrons/C.43 Figure S41 shows the solution of the gravimetric hydrogen density (mass of hydrogen/mass of solution) collected at various N2H4 concentrations. Hydrogen density grew with N2H4 concentration to a maximum value of 4.00 wt % at C(N2H4) = 10.0 mol L−1. Such findings show that the N2H4 system could serve as a useful hydrogen storage system for a fuel-cell uninterrupted power supply (FCUPS) or another emergency power source once the related experimental parameters are optimized in the coming years.16,17,26,43 Investigation also focused on the catalytic lifetime of the Ni4In NRs. Over 30 h at 298 K, the catalyst provided 1.50 × 105 turnovers in the dehydrogenation of N2H4 in aqueous solution. Determination of the average TOF value (ATOF) was 82.0 min−1, 98.1% of the initial TOF. The rate of hydrogen production slowed as the reaction proceeded and then ultimately stopped. Elemental analysis showed that Ni and In species loss from the catalyst after the reaction was
