NiCu Bimetallic Nanoparticles on Silica Support for Catalytic

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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NiCu Bimetallic Nanoparticles on Silica Support for Catalytic Hydrolysis of Ammonia Borane: Composition-Dependent Activity and Support Size Effect Kun Guo,†,‡ Yi Ding,§ Jun Luo,§ Minfen Gu,∥ and Zhixin Yu*,† †

Department of Energy and Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway School of Chemistry, University of Manchester, Oxford Road, M13 9PL Manchester, United Kingdom § Institute for New Energy Materials & Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China ∥ Center for Analysis and Testing, Nanjing Normal University, Nanjing 210046, China

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‡

S Supporting Information *

ABSTRACT: NiCu bimetallic nanoparticles (NPs) with different Ni/Cu compositions are controllably synthesized by tuning the ratio of Ni and Cu acetylacetonate precursors in the presence of oleylamine and trioctylphosphine. The similar particle size, monodispersity, and homogeneous alloying of the obtained NPs are confirmed by spectroscopic and microscopic analyses. When the bimetallic NPs together with the monometallic counterparts are used as catalysts for the hydrolysis of ammonia borane (AB), their catalytic activities are found to be composition-dependent. The best-performing Ni0.75Cu0.25 NPs show an activation energy of 34.2 kJ mol−1, which is among the lowest in reported non-noble-metal catalysts. This volcano-type activity trend is attributed to the alloying effect of NiCu that endows a favorable electronic structure toward the hydrolysis of AB. To investigate the catalytic effect of support particle size, a critical yet largely unexplored factor, we further deposit the Ni0.75Cu0.25 NPs onto six differently sized silica spheres in the size range 47−485 nm. It is found that the activity of NiCu/SiO2 catalysts increases progressively with decreasing SiO2 particle size, which is attributed to the less agglomeration and better stabilization of NiCu NPs enabled by SiO2 spheres with higher curvature and longer interparticle distance. Notably, the NiCu NPs supported on the smallest SiO2 exhibit a much higher turnover frequency of 1516 molH2 molmetal−1 h−1 compared to the unsupported NPs as well as an excellent reusability in the consecutive hydrolysis of AB, signifying the strong metal−support interactions. The results underline the importance of engineering alloy composition and support particle size for efficient catalytic hydrolysis of AB. KEYWORDS: composition dependent, bimetallic alloy, support size, hydrolysis, ammonia borane

1. INTRODUCTION Utilization of the clean and sustainable hydrogen fuel is largely hindered by issues stemming from the efficient hydrogen storage and production.1−4 To realize the controllable storage and release of hydrogen, researchers have targeted several key materials as potential hydrogen carriers, among which ammonia borane (AB) remains the most promising candidate due to its high gravimetric hydrogen content (19.6 wt %) and high stability under ambient conditions.5,6 More importantly, the hydrolysis of AB (eq 1) can be greatly accelerated at room temperature with 100% hydrogen yield by employing an appropriate catalyst. NH3BH3 + 2H 2O → NH4 + + BO2− + 3H 2

far, tremendous attempts have been dedicated to the investigation of various catalysts. Among reported catalysts for the hydrolysis of AB, noble metals are the most representative and have proved with excellent catalytic performance represented by low activation energy and high turnover frequencies (TOFs).7,8 Nevertheless, their commercial application is limited by the high cost and scarcity. To reduce the cost of such precious metals, researchers have proposed the use of bimetallic alloy nanoparticles (NPs) by incorporating another non-noble metal, such as Fe, Co, Cu, and Ni.9−17 It is very often found that the catalytic activity of bimetallic NPs in the hydrolysis of AB is composition-

(1) Received: May 20, 2019 Accepted: July 16, 2019 Published: July 16, 2019

Prior to implementing this technology, a consensus is that the development of catalyst requires further research efforts. So © XXXX American Chemical Society

A

DOI: 10.1021/acsaem.9b00997 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

2. EXPERIMENTAL SECTION

dependent with one specific alloy exhibiting the highest activity, better than that of each monometallic analogue. Numerous studies indicated that the superior performance of bimetallic NPs can be attributed to one or more of the ensemble, ligand, and geometric effects, involving the optimization of surface binding energy of reactants and intermediates and the fine-tuning of electronic structure.13−19 In light of this, considerable efforts are now being devoted to investigating bimetallic NPs that consist of first-row transition metals to replace noble metals while maintain comparable catalytic activity toward the hydrolysis of AB. To date, Ni and Cu are two of the state-of-the-art metal catalysts reported for hydrogen generation from AB and hydrazine, presenting fast reaction kinetics that are competitive with that of noble metals. Therefore, it is of great interest to probe the catalytic performance of bimetallic NiCu NPs in the hydrolysis of AB to achieve non-noble-metal-based catalysts with high activity, selectivity, and durability. Apart from the active metals, support materials also play a crucial role in heterogeneous catalysis. It has been found that the support not only functions to disperse the active metals and maintain the catalytic activities but also brings potential metal−support interactions that can further enhance the catalytic performance.20−23 Size-dependent properties of metal NPs have been testified in various catalytic reactions due to stronger nanoscale effects when decreasing the NP size, in particular to below 5 nm.24,25 Nonetheless, to the best of our knowledge, the effect of support particle size on the activity and selectivity of supported catalysts remains largely unexplored, especially in the hydrolysis of AB. In supported catalyst systems, discrete metal NPs are loaded on the supports that are usually several orders of magnitude larger than the metal NPs to ensure high dispersion. It is thus interesting to explore what will happen if the support size is tailored or even reduced to be comparable to that of the metal NPs, which could potentially enhance the metal−support interaction. Silica is a widely used oxide support in heterogeneous catalysis.26 The shape and morphology of silica materials can be readily engineered through controlled synthesis. Besides, the surface hydroxyl functional groups, which are normally present on the silica surface, are found instrumental to the catalytic hydrolysis of AB.27,28 Therefore, silica support serves as an ideal platform to examine the size effect of support on the hydrolysis of AB toward a better structural design of the heterogeneous catalysts. Herein, we reported bimetallic NiCu NPs with different compositions as effective catalysts for the hydrolysis of AB. Three different NiCu NPs of similar particle sizes were prepared via the coreduction of nickel acetylacetonate and copper acetylacetonate with trioctylphosphine in oleylamine by varying the ratios of metal precursors. Their catalytic activities were found to be composition-dependent in comparison with that of monometallic Ni and Cu NPs. With the Ni0.75Cu0.25 NPs presenting the best performance, reaction kinetics of these NPs were systematically analyzed in terms of the activation energy and the effect of catalyst concentration and AB concentration. As a proof-of-concept study, the optimal Ni0.75Cu0.25 NPs were further deposited on silica spheres of six different sizes ranging from 47 to 485 nm to examine the effect of support size on the catalytic activities of the supported catalysts. Mechanistic insights into the support size-dependent activity in the hydrolysis of AB were further provided by postmortem characterizations.

2.1. Chemicals. All chemicals were purchased unless otherwise indicated and used as received without further treatment. Chemicals including the ammonia borane complex (H3N·BH3, 97%), nickel(II) acetylacetonate (Ni(acac)2, 95%), copper(II) acetylacetonate (Cu(acac)2, ≥99.9%), oleylamine (OAm, technical grade, 70%), and trioctylphosphine (TOP, 97%) were received from Sigma-Aldrich. Ammonium hydroxide solution (25% NH3 basis), tetraethyl orthosilicate (TEOS, 98%), hexane (≥99%), and ethanol (≥96 vol %) were supplied by VWR International AS. Deionized (DI) water (18.2 MΩ·cm) was used in all the hydrolysis experiments. 2.2. Synthesis of NiCu NPs. NiCu bimetallic NPs were synthesized via the modified heat-up method.29,30 In the context of the synthesis of Ni0.75Cu0.25 NPs, a 250 mL round-bottom three-neck flask containing 10 mL of OAm, 5 mL of TOP, 0.9 mmol of Ni(acac)2, and 0.3 mmol of Cu(acac)2 was placed in an oil bath and heated to 80 °C. The flask was connected with a nitrogen inlet and a reflux condenser attached with a bubbler. After the flask was purged with nitrogen for at least 30 min, the flask was quickly transferred to another oil bath, which was thermostated to 230 °C. The reaction was held for 1 h, and then the flask was taken out and cooled naturally. The reaction medium was constantly stirred at 1000 rpm. A mixture of hexane and ethanol (v/v = 3/7) was added to extract the black NPs, which were separated by centrifugation at 8000 rpm for 5 min. This process was repeated three times to remove residual surfactants and impurities. The Ni0.75Cu0.25 NPs were collected by drying the sample in nitrogen at room temperature overnight. Ni, Ni0.5Cu0.5, Ni0.25Cu0.75, and Cu NPs were prepared via the same approach but changing the ratio of metal precursors. 2.3. Synthesis of SiO2 Spheres. SiO2 spheres of different sizes were synthesized via the modified Stöber process, which was based on the hydrolysis of TEOS in ethanol with ammonia as a catalyst.31 The detailed amounts of ammonia, DI water, and ethanol for the preparation of differently sized SiO2 spheres are listed in Table S1. Typically, a beaker containing ammonia, DI water, and ethanol with a total volume of 50 mL was stirred at 1500 rpm at room temperature. Another solution consisting of 45 mL of ethanol and 5 mL of TEOS was rapidly added to the beaker through a glass funnel. After sufficiently mixing for 5 min, the stirring speed was reduced to 500 rpm, and the reaction was held for 2 h. SiO2 spheres were separated by centrifugation and dried at 60 °C in an electronic oven overnight. 2.4. Synthesis of NiCu/SiO2. NiCu/SiO2 (refers to Ni0.75Cu0.25/ SiO2) catalyst with a NiCu loading of 10 wt % was prepared by mixing the Ni0.75Cu0.25 NPs and SiO2 spheres of different sizes. After the above-mentioned Ni0.75Cu0.25 NPs were separated by centrifugation for the first time, 15 mL of hexane and 0.7189 g of SiO2 were added to disperse the Ni0.75Cu0.25 NPs onto the SiO2 support. The mixture was sonicated for 30 min and 30 mL of ethanol was then slowly added. Afterward, the NiCu/SiO2 product was separated by centrifugation at 10000 rpm for 10 min and washed with a mixture of hexane and ethanol to remove surfactants and impurities. This process was repeated at least three times. The NiCu/SiO2 catalyst was collected by drying the sample in nitrogen at room temperature overnight. 2.5. Catalyst Characterization. The compositions of bimetallic NPs were measured by inductively coupled plasma−atomic emission spectroscopy (ICP-AES). The NPs were first dissolved in concentrated nitric acid and then diluted with DI water. The measurements were performed on a JY2000 Ultrace ICP-AES instrument equipped with a JY-AS 421 autosampler and 2400 g/ mm holographic grating. Powder X-ray diffraction (XRD) was performed to obtain the crystallographic information about the samples. The powder diffraction patterns were recorded on a Bruker-AXS microdiffractometer (D8 ADVANCE) using Cu Kα radiation source (λ = 1.5406 Å, 40 kV and 40 mA). Scanning angles for all samples were set in the 2θ range of 10°−90° with a step interval of 2.25°/min. Peaks were indexed according to the database established by Joint Committee on Powder Diffraction Standards (JCPDS). B


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ACS Applied Energy Materials The microstructure and morphology of metallic NPs, SiO2 spheres, and NiCu/SiO2 were characterized by transmission electron microscopy (TEM, JEOL JEM-2100F, 200 kV) and scanning electron microscopy (SEM, Gemini SUPRA 35VP). High-resolution TEM (HRTEM) images were obtained using a FEI Titan Cubed Themis 300 G2 scanning TEM (STEM) with a spherical aberration corrector, which was operated at 300 kV with an energy resolution of 0.7 eV. STEM images were also obtained using a high angle annular dark field (HAADF) mode. Elemental mapping was acquired by using a FEI Super-X energy dispersive X-ray (EDX) analyzer attached to the STEM. For the specimen preparation, one droplet of the sample suspension was dropped onto a grid coated with carbon film. X-ray photoelectron spectroscopy (XPS) analysis was performed on the ESCALAB 250Xi (Thermo Scientific) XPS system utilizing a monochromatic Al Kα source (1486.6 eV). High-resolution spectra were obtained at a pass energy of 30.0 eV, a step size of 0.1 eV, and a dwell time of 500 ms per step. All spectra were referenced to the C 1s peak (284.8 eV). Ultraviolet−visible (UV−vis) absorption spectra were recorded on a Thermo Scientific GENESYS 10S UV−vis spectrophotometer in the wavelength range of 300−900 nm. For sample preparation, 100 μL of NP suspension was diluted with 2 mL of hexane and then subject to the UV−vis analysis. Pure hexane was used for background correction. Nitrogen adsorption−desorption measurements were conducted at the liquid nitrogen temperature of 77 K on a Micromeritics TriStar II surface area and porosity analyzer after degassing under vacuum at 100 °C for 8 h using a sample degas system (Micromeritics VacPrep 061). Specific surface area (SSA) was calculated by using the Brunauer−Emmett−Teller (BET) method. Fourier-transform infrared (FTIR) spectra were recorded on a Nicolet NEXUS 670 spectrometer by using a deuterated triglycine sulfate detector. All spectra were recorded with a resolution of 4 cm−1 for 32 scans in the spectral range between ν̃ = 400 and 4000 cm−1. The background spectrum of air was measured as a single beam and used as reference. 2.6. Catalytic Dehydrogenation of AB with NiCu NPs. The activity of NiCu NP catalysts toward the hydrolysis of AB was determined by online recording the evolved H2 gas volume in a waterfilled measuring cylinder system. Typically, a 50 mL flask containing a Teflon-coated stirring bar was loaded with 20 mg of NiCu NP catalyst and 10 mL of DI water. After sonication in an ultrasound bath for 5 min, the flask was placed in a water bath, which was thermostated to 25 °C and stirred at 1000 rpm. Then, 64 mg of AB (2 mmol) was added into the flask, which was connected to a water-filled measuring cylinder. Water was displaced by evolved H2, and the time intervals were recorded for every 5 mL of H2. The reaction was considered complete when no H2 gas generation was observed. In the kinetic study, the same procedure was repeated except changing specific reaction conditions. For the study of temperature effect, the reaction was conducted in the water bath, which was thermostated to 25, 30, 35, and 40 °C. The effect of AB concentration was studied by using 16, 32, 48, and 64 mg of AB (0.5, 1, 1.5, and 2 mmol). Ni0.75Cu0.25 NPs in quantities of 8, 12, 16, and 20 mg (0.13, 0.20, 0.27, and 0.33 mmol) were used to investigate the effect of catalyst concentration. 2.7. Catalytic Dehydrogenation of AB with NiCu/SiO2. The activity of NiCu/SiO2 catalysts toward the hydrolysis of AB was determined in the same reaction system (10 mL of DI water and 64 mg of AB at 25 °C) except that 60 mg of NiCu/SiO2 was used. The supported catalyst gave a nominal NiCu metal loading of 6 mg. The reusability of NiCu/SiO2 catalyst was evaluated by refueling another equivalent 64 mg of AB into the flask after the previous dehydrogenation reaction was complete. H2 generation for the first 10 consecutive cycles was recorded to represent the reusability of NiCu/ SiO2 catalyst.

through the reduction of Ni(acac)2 and/or Cu(acac)2 in the presence of OAm and TOP at a temperature of 230 °C. TOP acts as a reductant and a cosurfactant, while OAm serves as the solvent and surfactant.29 The composition of NiCu bimetallic NPs is tuned by setting the molar ratios of Ni(acac)2 and Cu(acac)2 at 3:1, 1:1, and 1:3. Meanwhile, the same amounts of TOP and OAm are used to control the NP size. As the organic ligands could play detrimental roles in the catalytic activities of NPs, efficient ligand removal is demanded. FTIR spectra of the Ni0.75Cu0.25 NPs together with OAm, TOP, Ni(acac)2, and Cu(acac)2 are acquired to examine the efficacy of solvent washing process. As shown in Figure S1, the highlighted absorption bands at 2950−2850 cm−1 correspond to C−H stretches, which can represent the alkyl tails of organic ligands. The small C−H stretch bands of Ni0.75Cu0.25 NPs, compared to OAm and TOP, indicate that a large amount of ligands have been removed through the washing process. The specific Ni/Cu compositions of the bimetallic NPs, determined by ICP-AES, are 0.77:0.23, 0.48:0.52, and 0.24:0.76, which agree well with the nominal molar ratios of the Ni and Cu precursors. TEM characterization of the mono- and bimetallic NPs is performed to examine the NP dispersity and particle size distribution (PSD). Figure 1a is a representative TEM image of

Figure 1. Representative TEM image (a), particle size distribution curve (b), HRTEM image (c), and HAADF-STEM image and STEMEDX elemental mapping (d) of the Ni0.75Cu0.25 NPs.

the Ni0.75Cu0.25 NPs, indicating the high dispersity. Figure 1b shows an average particle size (APS) of 9.9 ± 2.2 nm for the Ni0.75Cu0.25 NPs by using a Gaussian fit of the histogram based on a statistic count of over 700 particle sizes. TEM images in low magnification and the corresponding PSD curves of all the five NPs are shown in Figure S2. They are all well dispersed with close APSs of 10.0 ± 2.3, 13.5 ± 3.3, and 11.8 ± 3.7 nm for the Ni, Ni0.5Cu0.5, and Ni0.25Cu0.75 NPs, respectively, except for the Cu NPs that have a larger APS of 19.7 ± 4.9 nm. The larger size of Cu NPs should be ascribed to the higher reducing tendency of Cu precursor and faster nucleation and growth of

3. RESULTS AND DISCUSSION 3.1. Characterization of Metal NPs. Monodispersed NiCu bimetallic and Ni, Cu monometallic NPs are prepared C


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Figure 2. XRD patterns (a), linearly fitted correlation between the (111) diffraction peak position and mole fraction of Ni (b), and high-resolution XPS spectra of Ni 2p (c) and Cu 2p (d) regions of the five mono- and bimetallic NPs with specific binding energies indicated. The inset in (b) is the enlarged XRD patterns in the selected range.

Cu than Ni in the reaction solution.20,32 Nevertheless, a similar APS of the NiCu NPs still justifies a comparison of the composition-dependent activity of these NPs. The HRTEM image (Figure 1c) of a single Ni0.75Cu0.25 NP manifests a differently oriented lattice spacing of 2.04 Å, which is between the spacings of the Ni (111) planes (2.034 Å) and Cu (111) planes (2.088 Å), revealing a polycrystalline alloy structure. Such an alloy structure is further characterized by HAADFSTEM and STEM-EDX elemental mapping. Figure 1d presents the HAADF-STEM image and the corresponding STEM-EDX elemental mapping images of multiple Ni0.75Cu0.25 NPs, both of which verify the uniform distribution of Ni and Cu in the alloy NPs. The crystal phase and crystallinity of the as-prepared monoand bimetallic NPs are analyzed by XRD. Figure 2a shows the diffraction patterns of five NPs. For the monometallic NPs, three major characteristic peaks are observed. They are indexed to the (111), (200), and (220) planes of facecentered-cubic structured Ni and Cu phases, conforming to JCPDS card nos. 04-0850 and 04-0836, respectively. The diffraction peaks of three bimetallic NPs are located between that of the Ni and Cu NPs. With increasing Ni mole fraction, each individual peak shifts from Cu toward Ni gradually, and the peak intensity decreases as well. The different variation trends of crystallinity and APS of the bimetallic NPs could be accounted to the larger Cu crystal domain sizes than that of Ni, giving higher crystallinity of Cu-rich NPs but similar particle sizes. The calculated average crystal sizes based on the (111) crystal plane by using the Scherrer equation are all smaller than the APSs determined from the TEM, as listed in Table S2, implying a polycrystalline nature of these metallic NPs.

According to Vegard’s law, the (111) peak positions (inset of Figure 2b) are correlated to the Ni mole fraction. A linear relationship is confirmed with a high correlation coefficient of R2 = 0.988 (Figure 2b), demonstrating the homogeneous alloy structure of the bimetallic NPs.33 High-resolution XPS is performed to examine the element chemical states and surface compositions of the mono- and bimetallic NPs. Figure 2c shows the XPS spectra of Ni 2p region for the Ni-containing NPs. Apart from the shakeup satellite peaks, two spin−orbit doublets assigned to Ni 2p1/2 and Ni 2p3/2 peaks can be resolved. With decreasing Ni mole fraction in the NPs, the peak intensity decreases as expected, whereas the positions for both peaks show a gradual shift to slightly lower binding energies (from 856.6 to 855.8 eV for Ni 2p3/2 and from 874.3 to 873.2 eV for Ni 2p1/2). However, the XPS peak positions of Cu 2p1/2 and Cu 2p3/2 shift gradually to higher binding energies with decreasing Cu content (from 934.2 to 935.1 eV for Cu 2p3/2 and from 954.0 to 954.8 eV for Cu 2p1/2), as illustrated in Figure 2d. The opposite variation in binding energy of Ni 2p and Cu 2p reveals the balancing regulation of near-surface electronic structure of the NiCu alloy NPs, which is an important alloying effect.20,34−36 The atomic Ni/Cu ratios of the bimetallic NPs derived from the XPS, as detailed in Table S3, are in good agreement with the ICP results. Considering the unique optical features of Cu, we further analyze the mono- and bimetallic NPs using UV−vis spectroscopy. Because of the surface plasmon resonance (SPR) originated from coherent oscillation of the free conduction-band electrons induced by incident light, Cu nanostructures display an absorption in the UV−vis region.37 D


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Figure 3. (a) Plot of time vs volume of H2 produced from the hydrolysis of AB catalyzed by five mono- and bimetallic NPs ([AB] = 200 mM, [metal] = 33 mM, 25 °C). (b) Plot of TOF vs mole fraction of Ni.

Figure 4. (a) Plot of time vs volume of H2 produced from the hydrolysis of AB catalyzed by Ni0.75Cu0.25 NPs at different temperatures of 25, 30, 35, and 40 °C ([AB] = 200 mM, [NiCu] = 33 mM) and (b) corresponding Arrhenius plot of ln k vs 1/T. (c) Plot of time vs volume of H2 produced from the hydrolysis of AB catalyzed by Ni0.75Cu0.25 NPs in different AB concentrations of 200, 150, 100, and 50 mM ([NiCu] = 33 mM, 25 °C) and (d) corresponding plot of AB concentration vs reaction rate in logarithmic scale. (e) Plot of time vs volume of H2 produced from the hydrolysis of AB catalyzed by different Ni0.75Cu0.25 NP concentrations of 33, 27, 20, and 13 mM ([AB] = 200 mM, 25 °C) and (f) corresponding plot of Ni0.75Cu0.25 NP concentration vs reaction rate on a logarithmic scale.

Figure S3 shows the UV−vis spectra of the five types of NPs. The SPR band at the wavelength of 586 nm is clearly observed for the Cu NPs. By alloying with increasing content of Ni, this SPR band experiences a blue-shift to shorter wavelengths of 570 and 564 nm coupled with weaker peak intensity for the Ni0.25Cu0.75 and Ni0.5Cu0.5 NPs, respectively. Interestingly, further increase of the Ni content results in disappearance of the SPR band, which should be attributed to the damping effect of SPR-inactive Ni. Such damping effect has also been observed by previous studies.38,39 These results again confirm the alteration of electronic structure for the NiCu alloy NPs. 3.2. Hydrolysis of AB with Unsupported NPs. The catalytic activities of mono- and bimetallic NPs in the

hydrolysis of AB are studied by adding these NPs into an aqueous AB solution at ambient temperature (25 °C). The released H2 is collected in an inverted, water prefilled measuring cylinder. H2 volume is monitored by recording the displaced volume of water on a regular basis. Figure 3a shows the time course of H2 generation from the hydrolysis of AB catalyzed by five different NPs. If fully hydrolyzed, 2 mmol of AB complex generates 6 mmol of H2, which approximates to a volume of 134 mL. The faster the hydrolysis is completed, the more active the metal NPs are. We can see that all the metal NPs can catalyze the complete hydrolysis of AB with a final H2 volume of 134 mL, despite at different reaction rates. Cu NPs take the longest completion time of almost 20 min to E


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Figure 5. Representative TEM images and PSD curves of six SiO2 spheres with APSs of 47 (a, b), 97 (c, d), 195 (e, f), 333 (g, h), 391 (i, j), and 485 nm (k, l).

specific activity of these NP catalysts. The TOF calculation is detailed in the Supporting Information. Figure 3b shows the plot of TOF vs Ni mole fraction of the five types of NPs. A volcano-type activity trend is observed with Ni0.75Cu0.25 NPs achieving the highest TOF of 197 molH2 molNi−1 h−1, demonstrating the composition-dependent catalytic activity of NiCu alloy NPs in the hydrolytic dehydrogenation of AB. Given that the Ni0.75Cu0.25 NPs present the highest catalytic activity in the hydrolysis of AB, a kinetic study is further done by conducting the hydrolysis reactions at different temperatures, AB concentrations and NiCu concentrations. The temperature effect is studied by running the reaction at temperatures ranging from 25 to 40 °C. Figure 4a shows the plots of time vs released H2 volume at four temperatures. With the elevation of temperature from 25 to 40 °C, H2 is generated more rapidly, signifying a faster reaction kinetics. The reaction rate constant, k, is calculated based on eq eq 2:

fully hydrolyze the AB, indicating the most sluggish reaction kinetics among the NPs. As the Ni mole fraction increases from Ni0.25Cu0.75 to Ni0.75Cu0.25, the hydrolysis of AB is accelerated with the completion time shortened from 14 to 7 min. The enhanced reaction kinetics by alloying with Ni manifests the superior role of Ni than Cu in the hydrolysis of AB. However, the catalytic activity of Ni NPs is found to be inferior to two of the bimetallic NPs, as reflected by a completion time of 11 min. According to the d-band theory, d-band electrons of the metals play a decisive role in the catalytic performance. Ni and Cu have distinct d-band centers; thus, the NiCu alloy would have a d-band center positioned in between that of Ni and Cu.35,36 According to the Sabatier principle, this d-band shift could optimize the interaction between metal active sites and reacting species involved in catalysis. For the catalytic hydrolysis of AB, although the detailed mechanism remains elusive, previous studies have proposed a plausible reaction pathway.13,40 The initial activation of AB molecules by active metals with the formation of complex species is regarded as the key step. H2 is then produced from the hydrolysis of a BH3 intermediate generated from the reaction between the complex species and H2O molecules. Figure S4 illustrates the potential reaction mechanism. In light of the aforementioned alteration of electronic structure manifested by the XRD, XPS, and UV− vis analysis, the activity enhancement of bimetallic NPs compared to the monometallic NPs should be attributed to the NiCu alloying effect, which better activates the AB molecules. Furthermore, the TOF is calculated to compare the

−

d[NH3BH3] d [H 2 ] = =k 3 dt dt

(2)

The activation energy is then derived from the Arrhenius equation (eq 3) between ln k and 1/T: ln k = −

Ea ij 1 yz jj zz + ln A R kT {

(3)

where Ea is the activation energy, R is the gas constant, T is the absolute temperature in kelvin, and A is the pre-exponential factor. By linear fitting, an activation energy of 34.2 ± 1.6 kJ F


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Figure 6. (a) Plot of time vs volume of H2 produced from the hydrolysis of AB catalyzed by six SiO2-supported Ni0.75Cu0.25 NPs ([AB] = 200 mM, [NiCu] = 10 mM, 25 °C). (b) Plot of TOF vs SiO2 particle size. (c) TOFs of NiCu/47-SiO2 and other reported catalysts in the literature. The detailed TOFs and corresponding references are listed in Table S5.

mol−1 is calculated for the hydrolysis of AB catalyzed by NiCu NPs, as displayed in Figure 4b. This activation energy is very competitive among the reported catalysts and even lower than some noble-metal-based catalysts, as listed in Table S4. It is thus concluded that the energy barrier can be substantially reduced by engineering the composition of NiCu alloy catalysts. Figure 4c presents the time course of H2 production from the hydrolysis using the same amount of Ni0.75Cu0.25 NPs at 25 °C but different AB concentrations. The largely overlapped H2 generation implies a similar reaction rate. Figure 4d shows the correlation between reaction rate and AB concentration in logarithmic scale. The slope after linear fitting is calculated to be 0.09. The hydrolysis of AB catalyzed by NiCu NPs is therefore a pseudo-zero-order reaction with regard to AB concentration. The effect of Ni0.75Cu0.25 NP concentration is displayed in Figure 4e. The completion time is extended as the NP concentration decreases from 33 to 13 mM. Figure 4f further shows the plot of reaction rate vs NiCu concentration on a logarithmic scale. A slope of 1.05 is determined for the fitted line, indicating that the hydrolysis of AB catalyzed by

NiCu NPs is a pseudo-first-order reaction with respect to NiCu concentration. 3.3. Hydrolysis of AB with SiO2-Supported NPs. Catalyst support is crucial in stabilizing the NPs anchored on the support surface. Besides, the metal−support interaction could further enhance the catalytic performance of supported NPs. It has been well documented that active NPs often show size-dependent activities. Specifically, with decreasing NP size, more coordinatively unsaturated sites are exposed on the surface to better activate the reactant molecules than the saturated sites in the bulk. However, the size effect of supports remain largely unexplored. Hypothetically, one would expect that with decreasing support particle size the support surface becomes more energetically unstable and can firmly stabilize the adsorbed metal NPs. As a proof-of-concept study, we employ SiO2 spheres with different particle sizes to validate this hypothesis. The use of SiO2 instead of carbon-based materials rules out the impacts of other structural parameters such as the support shape, surface functional groups, and structural defects. The SiO2 spheres are prepared by revisiting the famous Stöber method. Notably, the precise control of ammonia and G


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Figure 7. (a) Plot of time vs volume of H2 produced from the hydrolysis of AB catalyzed by NiCu/47-SiO2 for the first consecutive ten cycles and a new cycle (designated as New-first) in a fresh AB solution ([AB] = 200 mM, [NiCu] = 10 mM, 25 °C). TEM images of the NiCu/47-SiO2 catalyst before the reaction (b), after ten catalytic cycles (c), and after the New-first cycle (d). The scale bar is 90 nm.

infra, Figure 7b) and NiCu/485-SiO 2 (Figure S10a) demonstrate that the metal NPs are well dispersed on the SiO2 surface without obvious agglomeration. The catalytic activity of SiO2-supported NiCu NPs in the hydrolysis of AB is then evaluated. Prior to this, blank tests of the SiO2 spheres for 2 h show no H2 production, meaning that pure SiO2 is inactive to the hydrolysis (Figure S11). Figure 6a shows the plots of time vs volume of H2 generated from the hydrolysis of AB catalyzed by six supported catalysts. The NiCu/485-SiO2 gives a completion time of 11 min, longer than that of the unsupported NiCu NPs (Figure 3a). The lower activity of supported catalyst could be accounted to the poor mass transfer stemming from the relatively large APS of SiO2 spheres. The effect of mass transfer in the hydrolysis of AB has been previously unveiled and will be discussed later as well.29 When decreasing the SiO2 APS to 391 and 333 nm, the hydrolysis becomes faster and the completion time is comparable to that of the unsupported NPs. Further downsizing the SiO 2 endows a significantly reduced completion time, thus a much enhanced reaction kinetics. For instance, the NiCu/47-SiO2 completes the hydrolysis in 3 min; it is worth noting that the NiCu content of supported catalyst (10 mg) is only half of the unsupported NPs (20 mg). A direct comparison of TOFs of the unsupported Ni0.75Cu0.25 NPs and six supported catalysts is shown in Figure S12, in which we can see that after loading on the SiO2 support the TOFs are all higher than that of unsupported NPs. The activity enhancement thus could be attributed to the better dispersion and stabilization of NiCu NPs enabled by the relatively small sized SiO2 supports and favorable metal−support interactions.

DI water concentrations has enabled us to obtain six differently sized SiO2 spheres (Table S1). Representative TEM images and PSD curves in Figure 5 confirm the highly dispersed SiO2 spheres with six different APSs of 47 ± 9, 97 ± 31, 195 ± 36, 333 ± 46, 391 ± 29, and 485 ± 18 nm, designated thereafter as x-SiO2 where x refers to the APS. The spherical shape and dispersity are also visually observed from the SEM images in Figure S5. The broad diffraction peaks in the XRD patterns of all SiO2 spheres point to the same quasi-amorphous nature (Figure S6). Surface functional groups of the SiO2 spheres are examined by the FTIR spectroscopy. From the FTIR spectra in Figure S7, the same type and amount of hydroxyl groups are identified on the surface of SiO2 spheres. The hydroxyl groups have been reported to play a positive role in the hydrolysis of AB. The BET SSAs of the SiO2 spheres are measured based on the nitrogen adsorption−desorption isotherms illustrated in Figure S8. Undoubtedly, the SSAs of SiO2 spheres increase with decreasing APS and vary in the range of 11−195 m2 g−1. Note that the isotherms of large sized SiO2 follow the characteristics of the IUPAC type III classification, suggesting a nonporous nature. The hysteresis loops appeared in 47- and 97-SiO2 should be attributed to the open interspace between the spheres rather than in the spheres since the loop sits mainly in the high relative pressure region. Therefore, the SiO2 spheres possess the same nonporous structure. Using the same synthetic receipt, we deposit the best-performed Ni0.75Cu0.25 NPs on the six types of SiO2 spheres. Taking the NiCu/47-SiO2 as an example, its XRD pattern (Figure S9) confirms the successful deposition of metal NPs on the SiO2 support. Meanwhile, the TEM images of NiCu/47-SiO2 (vide H


Article

ACS Applied Energy Materials Similar to the stronger binding strength of reactant molecules to smaller sized metal NPs, support particles with higher curvatures may better stabilize the attached NPs.41 Meanwhile, the distinct SSAs of SiO2 spheres suggest the different interparticle distance between the supported NiCu NPs. The long interparticle distance could also inhibit the NPs from agglomeration.42−45 These aspects highlight the critical role of support size in maximizing the stabilization of metal NPs and improving the activity of supported catalysts. For a clear comparison, the total TOF of supported catalysts is plotted versus the SiO2 support size, as shown in Figure 6b. We can see that the TOF increases monotonically with decreasing support size. The NiCu/47-SiO2 catalyst delivers the highest TOF of 1516 molH2 molmetal−1 h−1, which is better than many reported catalysts (Figure 6c and Table S5). The spent NiCu/485-SiO2 is selected for post-mortem characterization to provide insights of the size-dependent activity of the SiO2-supported catalysts. Figures S10a and S10b show the TEM images of NiCu/485-SiO2 before and after the hydrolysis of AB, respectively. After the reaction, the supported NiCu NPs face severe aggregation, leading to reduced catalytic activity. It is assumed that the insufficient stabilization from the large sized SiO2 spheres cannot stop the NPs from moving along and/or detaching from the support surface and agglomerating with each other. On the contrary, the spent NiCu/47-SiO2 maintains high dispersity of the NiCu NPs and thus ensures their superior catalytic activity (vide infra, Figure 7). The simplified schematic diagram of the NP migration on the support surface is illustrated in Figure S13. These results testify the importance of controlling the support size to improve the activity and stability of supported catalysts. Furthermore, the reusability of NiCu/47-SiO2 is assessed by testing the same catalyst for ten consecutive reaction cycles by simply adding equivalent amount of AB. Figure 7a presents the cumulative time course of H2 production. After seven cycles, no major activity loss is observed since the completion time is slightly prolonged, verifying the excellent stability of the NiCu/ 47-SiO2 catalyst. After that, the activity degradation become obvious. Our early study has revealed that the accumulation of byproducts (metaborate and its hydrated products) and the increase of solution viscosity along the tests deteriorate the hydrolysis of AB by hindering the mass transfer.29 Thus, we retrieve the NiCu/47-SiO2 catalyst and apply them in a fresh AB solution (designated as New-first in Figure 7a). TEM images of the NiCu/47-SiO2 catalyst before the test (Figure 7b), after ten cycles (Figure 7c), and after the New-first cycle (Figure 7d) show no obvious agglomeration of the NiCu NPs and no major change of the particle size throughout the reusability test, again demonstrating the strong stabilizing effect of the smallest SiO2 sphere.

conclusion is supported by the significantly improved activity and reusability exhibited by the smallest SiO2 supported Ni0.75Cu0.25/47-SiO2 catalyst. This study not only signifies the development of cost-effective, non-noble-metal catalysts by engineering the alloy composition but also provides important understanding to the support structure-dependent activities of heterogeneous catalysts.

4. CONCLUSIONS Mono- and bimetallic Ni, Cu, and NiCu NPs with close APSs are controllably synthesized, and their catalytic activities in the hydrolysis of AB are found to be composition-dependent. Ni0.75Cu0.25 NPs exhibit the highest catalytic activity in the hydrolysis of AB at ambient conditions with a low activation energy of 34.2 kJ mol−1, which is ascribed to the optimal electronic structure by alloying. Using six differently sized SiO2 supports as an ideal platform, we observe an important support size-dependent activity trend. Catalyst characterizations reveal that the small sized SiO2 better stabilize the dispersed NPs and maintain superior stability than large sized SiO2 supports. This

(1) Cohen, R. L.; Wernick, J. H. Hydrogen Storage Materials: Properties and Possibilities. Science 1981, 214, 1081−1087. (2) Schlapbach, L.; Zuttel, A. Hydrogen-Storage Materials for Mobile Applications. Nature 2001, 414, 353−358. (3) Eberle, U.; Felderhoff, M.; Schuth, F. Chemical and Physical Solutions for Hydrogen Storage. Angew. Chem., Int. Ed. 2009, 48, 6608−6630. (4) Schlapbach, L. Technology: Hydrogen-Fuelled Vehicles. Nature 2009, 460, 809−811. (5) Dalebrook, A. F.; Gan, W.; Grasemann, M.; Moret, S.; Laurenczy, G. Hydrogen Storage: Beyond Conventional Methods. Chem. Commun. 2013, 49, 8735−8751. (6) Lai, Q.; Paskevicius, M.; Sheppard, D. A.; Buckley, C. E.; Thornton, A. W.; Hill, M. R.; Gu, Q.; Mao, J.; Huang, Z.; Liu, H. K.;

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00997. TOF calculation method, preparation of SiO2 (Table S1), sizes of NPs by TEM and XRD (Table S2), elemental contents by XPS and ICP-AES (Table S3), activation energy and TOF values of catalysts (Tables S4 and S5), FTIR spectra (Figure S1), TEM images (Figure S2) and UV−vis spectra (Figure S3) of NPs, reaction mechanism (Figure S4), SEM images (Figure S5), XRD patterns (Figure S6), FTIR spectra (Figure S7), and N2 adsorption−desorption isotherms (Figure S8) of SiO2, XRD patterns of SiO2, NiCu NPs and NiCu/SiO2 (Figure S9), TEM images of fresh and spent catalyst (Figure S10), blank experiments of SiO2 (Figure S11), TOFs of unsupported and supported catalysts (Figure S12), and illustration of NP migration (Figure S13) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel +47 51 83 22 38. Fax +47 51 83 20 50. E-mail zhixin.yu@ uis.no. ORCID

Kun Guo: 0000-0002-4822-5984 Yi Ding: 0000-0002-1347-2811 Jun Luo: 0000-0001-5084-2087 Zhixin Yu: 0000-0003-2446-6537 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Vidar F. Hansen and Dr. Wakshum M. Tucho, University of Stavanger, for the TEM characterization. The authors also acknowledge the Research Council of Norway and the industrial partners of The National IOR Centre of Norway for financial support.

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