- Email: [email protected]
Highly efficient hydrogen production from hydrolysis of ammonia borane over nanostructured [email protected]x supported on graphene oxide
Journal Pre-proof Highly efficient hydrogen production from hydrolysis of ammonia borane over nanostructured Cu@CuCoOx supported on graphene oxide Jinlong Li, Xueying Ren, Hao Lv, Yingying Wang, Yafei Li, Ben Liu
PII:
S0304-3894(20)30187-4
DOI:
https://doi.org/10.1016/j.jhazmat.2020.122199
Reference:
HAZMAT 122199
To appear in:
Journal of Hazardous Materials
Received Date:
1 December 2019
Revised Date:
18 January 2020
Accepted Date:
27 January 2020
Please cite this article as: Li J, Ren X, Lv H, Wang Y, Li Y, Liu B, Highly efficient hydrogen production from hydrolysis of ammonia borane over nanostructured Cu@CuCoOx supported on graphene oxide, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122199
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Highly efficient hydrogen production from hydrolysis of ammonia borane over nanostructured Cu@CuCoOx supported on graphene oxide Jinlong Li, Xueying Ren, Hao Lv, Yingying Wang, Yafei Li, , Ben Liu*
Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center
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of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing
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Normal University, Nanjing 210023, China
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*Email: [email protected]
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Graphical abstract
Highlights
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A hybrid nanocatalyst composed of heterostructured Cu@CuCoO and conductive GO support is reported.
The hybrid nanocatalyst synergistically boosts catalytic performance toward hydrolysis of ammonia borane.
The design principle is applicable for high-performance nanocatalysts toward the removal of various hazardous materials.
Abstract: Designing highly efficient and cheap nanocatalysts for room-temperature hydrolysis of
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ammonia borane (AB) is of great significance for their real application in hydrogen (H2)based fuel cells. Here, we report a kind of noble metal (NM)-free hybrid nanocatalysts
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composed of heterostructured Cu@CuCoOx nanoparticles and a graphene oxide support
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(denoted as Cu@CuCoOx@GO) and demonstrate their high catalytic performance toward the hydrolysis of AB. By rationally controlling synthetic parameters, we find that optimum
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[email protected]@GO achieves a superior catalytic activity with a turnover frequency of
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44.6 molH2 molM-1 min-1 in H2O and 98.2 molH2 molM-1 min-1 in 0.2 M NaOH, better than most of previously reported NM-free nanocatalysts. This catalyst also discloses a very low
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activation energy (Ea) of 35.4 kJ mol-1. The studies on catalytic kinetics and isotopic experiments attribute the high activity to synergistically structural and compositional advantages of [email protected]@GO, which kinetically accelerates the oxidative cleavage of O-H bond in attacked H2O (the rate-determining step of the hydrolysis of AB). This study
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thus provides an opportunity for rational design of cheap NM-free nanocatalysts for H2 production from chemical H2-storage materials.
Keywords: heterogeneous catalyst, CuCo, hydrogen production, ammonia borane, catalytic kinetics
1. Introduction Fossil fuels, including coal, oil, and natural gas, are still the mostly used chemical sources of global sources. This unavoidably causes a large number of critical issues, from the release of greenhouse gas (CO2) and harmful gases (SO2, NOx, CO, etc.) to various other serious environmental problems.1 Recently, hydrogen has been recognized as an
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important alternative energy, because of high energy density and green product.2-5 However, the storage and transport of H2 gas in physical systems (compressed gas and H2
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adsorbents) are still inconvenient and dangerous.6-9 In contrast, chemical H2-storage
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materials, in which H covalently binds into stable chemicals, become more attractive to resolve these problems.9-11 Among numerous H2-storage materials, ammonia borane
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(NH3-BH3, AB) has received the special attention, due to high hydrogen content, low
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molecular weight, and good stability in solution and solid state.2,12-15 Three mole of molecular H2 can be released from one mole of AB in aqueous solution [NH3-BH3 + 3 H2O
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→ NH3-B(OH)3 + 3 H2], when suitable catalysts were used.5,16 Therefore, the development of highly efficient and cheap nanocatalysts to catalyze the hydrolysis of AB is critically important for real application of this technology. In the past 1.5 decades, a large number of nanocatalysts were fabricated for catalytic
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hydrolysis of AB to produce molecular H2, especially at room temperature.5 Despites numerous efforts, noble metals (NMs), such as Pt, Pd, Ru, and their alloys, are still used as the highly efficient catalysts for the hydrolysis of AB.17-26 Unfortunately, the high price and limited abundance greatly hindered their widespread and practical applications. In 2006, Xu’s group found that anchoring cobalt nanoparticles (Co NPs) on miscellaneous
supports can behave as NM-free nanocatalysts to catalyze the hydrolysis of AB.18 Since then, many NM-free catalysts with different nanostructures, such as, Cu, Co, Ni, and their alloys, as well as their borides, oxides and phosphides, were widely investigated for roomtemperature hydrolysis of AB.5,16,27-36 However, for a simple NM-free nanocatalyst system, the hydrolysis of AB can be dramatically hindered by the oxidative cleavage of H-
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OH bond from reaction solutions, causing a relatively low activity and poor stability. To kinetically facilitate the absorption and cleavage of the O-H bond from H2O, rational
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design and synthesis of a hybrid nanocatalyst that contains different active sites could
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kinetically improve their catalytic performance for the hydrolysis of AB.
Herein, a hybrid nanocatalyst that composed of metallic Cu, oxided CuCoOx, and a
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graphene oxide (defined as Cu@CuCoOx@GO) was designed for promoting room-
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temperature hydrolysis of AB. We proposed that oxided CuCoOx in this catalyst facilitated the adsorption of OH*, while heterostructured Cu@CuCoOx made metallic Cu with
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electron-rich surface and kinetically favored the dissociation of H-OH into H*. Meanwhile, a conductive GO support not only dispersed the Cu@CuCoOx for exposing more catalytically active sites, but also assisted the electron and mass transfer during hydrolysis. These advantages in compositions and structures of Cu@CuCoOx@GO
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synergistically boosted catalytic performance toward room-temperature hydrolysis of AB, receiving a high turnover frequency (TOF) of 44.6 molH2 molM-1 min-1 in H2O and 98.2 molH2 molM-1 min-1 in 0.2 M NaOH.
2. Results and Discussion
The Cuy@Cu1-yCoOx@GO was synthesized by a two-step method with copper acetate [Cu(Ac)2] and cobalt acetate [Co(Ac)2] as metal precursors and graphene oxide (GO) as a support. Briefly, we first synthesized oxided CuCoOx@GO by hydrothermal treatment of Cu(Ac)2 and Co(Ac)2 (1:1) in the mixed solution of ethanol/H2O (10:1) containing a predominant amount of GO. Then, the sample was further treated with the reducing gas
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(H2/N2 = 5 % / 95%) under different temperatures to obtain Cuy@Cu1-yCoOx@GO. We found, with increasing the reducing temperatures, CuCoOx@GO was gradually converted
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to Cuy@Cu1-yCoOx@GO (150-300 oC), and finally to Cu@CoO@GO (400 oC). Wide-angle X-
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ray diffraction (XRD) was first characterized to reveal the crystalline structures of final materials (Figure 1). Before the treatment with H2, XRD pattern indicated that the sample
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mostly composed of CuO and Co3O4 (defined as CuCoOx). Metallic Cu gradually appeared
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after treating with H2. When the reduction temperature was increased to 200 oC, XRD signal can be ascribed to metallic Cu, and oxided Cu2O and CoO, respectively. We carefully
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investigated the ratio of Cu/Cu2O by etching with diluted HCl. Inductively coupled plasma mass spectrometry (ICP-MS) indicated the molar ratio of Cu for Cu/Cu2O was 0.28 : 0.72. Considering that the ratio of Cu/Co is 1:1, we defined the sample as [email protected]@GO. When further increasing the reduction temperature to 400
o
C, Cu2O in
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[email protected]@GO was totally reduced to metallic Cu (Cu@CoO@GO). We emphasized that the ratios of Cu/Cu2O can be rationally tuned by changing the reduction temperatures (Figure S1). The nanostructures and crystallines of the samples were revealed by transmission electron microscope (TEM). [email protected]@GO was thoroughly characterized as an
typical example, since it exhibited the best catalytic activity toward room-temperature hydrolysis of AB (see below). The low-magnification TEM image displayed that [email protected] homogeneously dispersed on the surface of GO (Figure 2a). [email protected] are nearly spherical NPs with a diameter ranging from 5-20 nm (Figure 2b). The high-angle annular dark-field scanning TEM (HAADF-STEM) image further
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exhibited uniform [email protected] NPs dispersed on GO (Figure 2c). High-resolution TEM image disclosed two different lattice fringes, which can be ascribed to (111) plane of Cu
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and (111) plane of CuCoOx, respectively (Figure 2d). This suggested the co-existence of
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metallic Cu and oxided CuCoOx in the sample, which is well matched to the result observed from XRD. STEM elemental mappings displayed partially overlapped signals of
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Cu, Co and O on C (Figure 2e), suggesting the formation of the [email protected]@GO again.
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The nanostructures and crystallines of CuCoOx@GO and Cu@CoO@GO were also studied. Similar to [email protected]@GO, CuCoOx@GO was also uniform NPs loaded on
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the surface of GO (Figure S2a). The lattice spacing of 0.24 nm that ascribed to (111) plane of CuCoOx implied oxided states of the sample (Figure S2b). By comparison, Cu@CoO@GO disclosed a nanostructure of hollow NPs on GO, in which clear lattice spacing of 0.24 nm suggested the presence of metallic Cu (Figure S2c,d). Hollow
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nanostructure is possibly because of different reduction potentials of Cu and Co that gave rise to galvanic replacement reactions between different compositions. The high-resolution X-ray photoelectron spectroscopy (XPS) was further carried out
to investigate surface electronic states of [email protected]@GO. XPS spectrum of Cu 2p exhibited two pairs of signals, assigned to 2p1/2 and 2p3/2, respectively (Figure 2f). Among
them, the peaks at 932.1 and 951.9 eV were attributed to metallic Cu0 and/or monovalent Cu+, while the peaks at 933.7 and 953.5 eV were assigned to bivalent Cu2+. The ratios of Cu0/Cu+ : Cu2+ is approximately 35 : 65. Meanwhile, two strong satellite (Sat.) signals of oxided Cu2+ were seen. These results further indicated the presences of metallic Cu and oxided Cu+/Cu2+ in the sample. In contrast, no XPS signal of Co 2p assigned to metallic Co
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can be observed (Figure 2g). Bivalent Co2+ and trivalent Co3+ as well as their satellite signals indicated oxided states of CoOx in the sample. In addition to the data from XRD
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and TEM, we confirmed [email protected]@GO was successfully fabricated. Meanwhile,
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high-resolution XPS Cu 2p and Co 2p were also compared with CuCoOx@GO, Cu@CoO@GO, and [email protected] (Figure S3), further implying the controllability of
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our method in precisely engineering surface electronic states of the samples.37
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Catalytic performance of the [email protected]@GO, CuCoOx@GO and Cu@CoO@GO was carefully evaluated for room temperature hydrolysis of AB. Catalytic reaction was
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carried out with 2.4 mg of catalysts and 20 mg of AB at 25 oC, and the gas produced during catalysis was collected by a classic H2O-displacement method. [email protected] (without GO support) (Figure S4) and pure GO (without loading any metals and/or their oxides) were also investigated and compared as controls. Figure 3a showed time-dependent plots
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of H2 produced for above five catalysts. Almost no H2 produced for GO even after reacted for 20 min, indicating GO is inactive for the hydrolysis of AB. In contrast, other four catalysts were catalytically active and released H2 immediately, implying the catalytic sites are Cu/Co and/or their oxides. All four catalysts produced ~42.1 mL of H2, which is approximately 3 equivalent amount of AB. This indicated total release of H2 from AB
catalyzed by these catalysts. Among them, [email protected]@GO exhibited the highest catalytic rate with a total H2 release time of 8.0 min. The release time of H2 was increased to 9.5 min for Cu@CoO@GO and 11.0 min for CuCoOx@GO. Besides, without GO support, [email protected] showed a slightly longer release time of 16.0 min, due to the aggregate nature of material that dramatically decreased the catalytically active sites. These results
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correlated that both ratios of Cu/CuCoOx and GO support are important for promoting their catalytic activity. To clearly compare catalytic activities of catalysts, the turnover
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frequency (TOF) was further calculated and summarized based on initial 20 mL of H2
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produced and the amounts of metals (Cu and Co). Specifically, the TOF values are 44.6 molH2 molM-1 min-1 for [email protected]@GO, 28.1 molH2 molM-1 min-1 for CuCoOx@GO,
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30.3 molH2 molM-1 min-1 for Cu@CoO@GO, and 9.1 molH2 molM-1 min-1 for [email protected],
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respectively (Figure 3b). The results, again, highlighted that structural and compositional merits of the [email protected]@GO promoted room-temperature hydrolysis of AB.
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More control catalysis was investigated to reveal the importance of compositional ratios of catalysts for the hydrolysis of AB. First, we studied the effect of metallic Cu amounts of final catalysts in the hydrolysis of AB by changing initial reduction temperatures (Figure S5a). Increasing reduction temperatures would enlarge the
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amounts of metallic Cu in final Cuy@Cu1-yCoOx@GO. We found, with the increase of reduction temperatures, catalytic activity was initially increased, and reached the peak at 200 oC ([email protected]@GO). Then, it gradually decreased with increasing reduction temperature to 400 oC (Figure S5b). The result displayed that the amount of metallic Cu and ratios of Cu/CuCoOx changed their catalytic performance. Second, Cu amounts of
[email protected]@GO were evaluated by changing the addition ratios of Cu(Ac)2 and Co(Ac)2 during synthesis (Figure S5c). A clear Volcano-type activity was obtained for different Cu amounts (Figure S5d). Both monometallic Cu and Co exhibited worse catalytic activities, implying bimetallic CuCo-alloyed catalysts promoted the hydrolysis. Similar tendency was also observed when tuning the total amounts of metals (Figure
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S5e,f). These results definitely indicated, only under the optimal compositional ratios, catalytic activity of Cuy@Cu1-yCoOx@GO can be maximally improved for room-
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temperature hydrolysis of AB.
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To further reveal how the hydrolysis of AB catalyzed by [email protected]@GO, we disclosed the catalytic kinetics and further compared with its counterpart catalysts of
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CuCoOx@GO and Cu@CoO@GO. The amounts of AB and catalysts were thoroughly
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investigated first. As shown in Figure 4a, when increasing the amounts of AB added, the final production amounts of H2 for [email protected]@GO were increased correspondingly.
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However, initial H2 production curves (20 mL) were totally overlapped, although addition amount of AB were very different. Similar phenomenon also happened for CuCoOx@GO, and Cu@CoO@GO (Figure S6). We further plotted the relationship between In TOF and In [AB] ([AB] indicates the concentration of AB). A nearly horizontal relationship for all
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three catalysts disclosed a zero-order reaction for AB toward the hydrolysis of AB. However, we found that increasing the amounts of catalysts promoted the H2 production rates (Figure 4c and Figure S7). Interestingly, when taking the relationship between In rate and In [Metal], a linear relationship with the slopes of 1.05-1.19 was achieved, indicating that the hydrolysis of AB is first-order in the concentration of metals (Figure
4d). Among them, [email protected]@GO had a highest slop value of 1.19, suggesting it is more kinetically favorable for the hydrolysis of AB. The activation energy (Ea) was also evaluated by taking the catalytic kinetics under different temperatures (Figure 4e and Figure S8). The increase of test temperatures obviously boosted the catalytic rates for all three samples, resulting in a linear
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relationship between In TOF and 1000/T (Figure 4f). According to the Arrhenius plots, Ea value of [email protected]@GO was calculated to be 35.4 kJ mol-1, lower than that of
catalytic
hydrolysis
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AB
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be
energetically
activated
by
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indicated
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CuCoOx@GO (60.5 kJ mol-1) and Cu@CoO@GO (45.3 kJ mol-1) (insets in Figure 4f). This
[email protected]@GO easily. We should emphasize that this is also one of the lowest Ea
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value, when compared with previously reported NM-free catalysts (Table S1).
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There are three key steps for catalyzing room-temperature hydrolysis of AB.28,33,38 First, both H2O and AB molecules were adsorbed on the surfaces of catalysts. Second, the
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H-OH bond of H2O was cleaved, and HO* and H* were released. OH* then attacked the B-N bond of AB, and further released H* and formed NH3-BH2OH (and NH3-BH(OH)2 and NH3-B(OH)3). Third, one H* from H2O and another H* from AB reacted and produced molecular H2. Considering the above results, we deduced that the rate-determining step
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of the hydrolysis of AB for our catalysts is the cleavage of O-H bond of H2O, as reported in most of metal-based catalysts. Two more series of catalytic experiments were designed and carried out carefully to confirm the deduction for [email protected]@GO. First, the isotopic experiment was conducted by using D2O instead of H2O. The kinetic isotope effect (KIE) has widely been evaluated on the roles of H2O in the rate-determining step
for catalytic hydrolysis of AB. In comparison to H2O, H2 production rate was remarkably decreased in the presence of D2O (Figure 5a), due to the difficulty for the cleavage of DOD. It suggested that H2O also contributed as the reactant for the hydrolysis of AB to produce H2. Meanwhile, a high KIE value of 3.4 was calculated. These definitely implied that the cleavage of O-H bond from H2O is the rate-determining step for the hydrolysis of
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AB, when catalyzed by [email protected]@GO. Second, we also investigated the role of OHon the hydrolysis of AB in the presence of [email protected]@GO. Figure 5b showed the
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catalytic activities of [email protected]@GO in different NaOH concentrations from 0.01
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to 0.4 M. Obviously, H2 production rates increased with the addition of NaOH. A highest catalytic rate with a TOF of 98.2 molH2 molM-1 min-1 for [email protected]@GO was
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obtained when taking the hydrolysis of AB in 0.2 M NaOH, which surpassed the activities
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of most reported NM-free catalysts (Table S1). This indicated that OH- can behave as a catalyst promoter that makes the catalysts more electron-rich and also contributes more
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adsorbed O-H. Both of them thus accelerated the rate-limiting oxidative reaction of O-H bond. However, the catalytic activities were slightly decreased when the higher concentrations of NaOH were used, possibly because excessive OH- can cover the active sites for the coordination of H and the dissociation of H2. Similarly, the promoting effect
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of OH- in the hydrolysis of AB was also observed for CuCoOx@GO and Cu@CoO@GO (Figure S9, S10), further indicating the rate-determining step of our catalysts is the oxidative cleavage of O-H of H2O. Based on compositional and structural features of [email protected]@GO, a possibly catalytic mechanism can be proposed toward room-temperature hydrolysis of AB. On the
one hand, oxided CuCoOx in the catalyst facilitated the adsorption of OH*, while metallic Cu accelerated the dissociation of H*.31,39 Especially, alloying CoO into Cu2O provided more active sites for the coordination of OH- on the catalysts, which had been proven by a lower activity catalyzed by Cu@Cu2O@GO (Figure S5c,d). In contrast, heterostructured Cu@CuCoOx also made metallic Cu with electron-rich surface, that kinetically favored the
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dissociation of H-OH into H*. Therefore, compositional advantages kinetically accelerated the cleavage of O-H of H2O (the rate-determining step), thus enhanced the catalytic
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activity of catalysts. On the other hand, loading Cu@CuCoOx on GO remarkably dispersed
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catalytically active NPs, and thus exposed more catalytically active sites. Meanwhile, GO also accelerated the electron and mass transfer during the hydrolysis, enhancing their
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3. Conclusion
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activity also.
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In conclusion, we reported a two-step method for synthesizing and supporting welldispersed Cu@CuCoOx NPs on GO, and demonstrated their catalytic performance toward room-temperature hydrolysis of AB. Compositional and structural features, including metallic Cu, oxided CuCoOx and a GO support, provided important contributions for
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enhancing their catalytic activity. We found, among more than ten catalysts, [email protected]@GO exhibited best catalytic activity with a high TOF of 44.6 molH2 molM1
min-1 and a low Ea of 35.4 kJ mol-1. More importantly, the hydrolysis of AB can be
activated by NaOH; a very high TOF of 98.2 molH2 molM-1 min-1 was achieved in the presence of 0.2 M NaOH. This is superior to the activities of mostly reported NM-free
nanocatalysts. Catalytic kinetics and isotopic experiments indicated synergistically compositional and structural advantages of [email protected]@GO accelerated the oxidative cleavage of O-H of H2O (the rate-determining step), thus improving their catalytic performance toward the hydrolysis of AB. With the high catalytic performance, low cost, and easy synthesis step, this protocol would be extended to synthesize other
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hybrid nanocatalysts for H2 production from various chemical fuels. We expect that highly
for the removal of various hazardous materials.40-42
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Declaration of interests
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effective hybrid nanocatalysts based on this design strategy would exhibit great potential
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Credit author statement
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B. Liu conceived the project and designed the experiments. Y. Li and B. Liu supervised the project. J. Li and Y. Ren prepared the materials and conducted catalytic experiments. H. Lv performed TEM imaging and XPS analysis. Y. Wang assisted the experiments. All the authors discussed the results and co-wrote the paper.
Acknowledgements
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The authors thank the financial supports from Jiangsu Specially Appointed Professor Plan, Natural Science Foundation of Jiangsu Province (No. BK20180723), Priority Academic Program Development of Jiangsu Higher Education Institutions, National and Local Joint Engineering Research Center of Biomedical Functional Materials.
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(21) Yan, J.-M.; Zhang, X.-B.; Akita, T.; Haruta, M.; Xu, Q. One-step seeding growth of magnetically recyclable Au@ Co core− shell nanoparticles: highly efficient catalyst for hydrolytic dehydrogenation of ammonia borane. J. Am. Chem. Soc. 2010, 132, 5326-5327. (22) Fu, F.; Wang, C.; Wang, Q.; Martinez-Villacorta, A. M.; Escobar, A.; Chong, H.; Wang, X.; Moya, S.; Salmon, L.; Fouquet, E.; Ruiz, J.; Astruc, D. Highly Selective and Sharp Volcano-type Synergistic Ni2Pt@ZIF8-Catalyzed Hydrogen Evolution from Ammonia Borane Hydrolysis. J. Am. Chem. Soc. 2018, 140, 1003410042. (23) Li, Z.; He, T.; Matsumura, D.; Miao, S.; Wu, A.; Liu, L.; Wu, G.; Chen, P. Atomically dispersed Pt on the surface of Ni particles: Synthesis and catalytic function in hydrogen generation from aqueous ammonia– borane. ACS Catal. 2017, 7, 6762-6769. (24) Mori, K.; Miyawaki, K.; Yamashita, H. Ru and Ru–Ni nanoparticles on TiO2 support as extremely active catalysts for hydrogen production from ammonia–borane. ACS Catal. 2016, 6, 3128-3135. (25) Wang, S.; Zhang, D.; Ma, Y.; Zhang, H.; Gao, J.; Nie, Y.; Sun, X. Aqueous solution synthesis of Pt–M (M= Fe, Co, Ni) bimetallic nanoparticles and their catalysis for the hydrolytic dehydrogenation of ammonia borane. ACS Appl. Mater. Inter. 2014, 6, 12429-12435. (26) Ren, X.; Lv, H.; Yang, S.; Wang, Y.; Li, J.; Wei, R.; Xu, D.; Liu, B. Promoting Effect of Heterostructured NiO/Ni on Pt Nanocatalysts toward Catalytic Hydrolysis of Ammonia Borane. J. Phys. Chem. Lett. 2019, 7374-7382. (27) Yan, J.; Liao, J.; Li, H.; Wang, H.; Wang, R. Magnetic field induced synthesis of amorphous CoB alloy nanowires as a highly active catalyst for hydrogen generation from ammonia borane. Catal. Commun. 2016, 84, 124-128. (28) Wang, C.; Tuninetti, J.; Wang, Z.; Zhang, C.; Ciganda, R.; Salmon, L.; Moya, S.; Ruiz, J.; Astruc, D. Hydrolysis of Ammonia-Borane over Ni/ZIF-8 Nanocatalyst: High Efficiency, Mechanism, and Controlled Hydrogen Release. J. Am. Chem. Soc. 2017, 139, 11610-11615. (29) Peng, C. Y.; Kang, L.; Cao, S.; Chen, Y.; Lin, Z. S.; Fu, W. F. Nanostructured Ni2P as a robust catalyst for the hydrolytic dehydrogenation of ammonia–borane. Angew. Chem. Int. Ed. 2015, 54, 15725-15729. (30) Bulut, A.; Yurderi, M.; Ertas, İ. E.; Celebi, M.; Kaya, M.; Zahmakiran, M. Carbon dispersed coppercobalt alloy nanoparticles: a cost-effective heterogeneous catalyst with exceptional performance in the hydrolytic dehydrogenation of ammonia-borane. Appl. Catal. B-Environ. 2016, 180, 121-129. (31) Feng, K.; Zhong, J.; Zhao, B.; Zhang, H.; Xu, L.; Sun, X.; Lee, S.-T. CuxCo1−xO Nanoparticles on Graphene Oxide as A Synergistic Catalyst for High-Efficiency Hydrolysis of Ammonia–Borane. Angew. Chem. Int. Ed. 2016, 55, 11950-11954. (32) Liao, J.; Li, H.; Zhang, X.; Feng, K.; Yao, Y. Fabrication of a Ti-supported NiCo2O4 nanosheet array and its superior catalytic performance in the hydrolysis of ammonia borane for hydrogen generation. Catal. Sci. Technol. 2016, 6, 3893-3899. (33) Li, Z.; He, T.; Liu, L.; Chen, W.; Zhang, M.; Wu, G.; Chen, P. Covalent triazine framework supported non-noble metal nanoparticles with superior activity for catalytic hydrolysis of ammonia borane: from mechanistic study to catalyst design. Chem. Sci. 2017, 8, 781-788. (34) Liu, Q.; Zhang, S.; Liao, J.; Feng, K.; Zheng, Y.; Pollet, B. G.; Li, H. CuCo2O4 nanoplate film as a low-cost, highly active and durable catalyst towards the hydrolytic dehydrogenation of ammonia borane for hydrogen production. J. Power Sources 2017, 355, 191-198. (35) Yamada, Y.; Yano, K.; Xu, Q.; Fukuzumi, S. Cu/Co3O4 nanoparticles as catalysts for hydrogen evolution from ammonia borane by hydrolysis. J. Phys. Chem. C 2010, 114, 16456-16462. (36) Lu, D.; Li, J.; Lin, C.; Liao, J.; Feng, Y.; Ding, Z.; Li, Z.; Liu, Q.; Li, H. A Simple and Scalable Route to Synthesize CoxCu1−xCo2O4@ CoyCu1−yCo2O4 Yolk–Shell Microspheres, A High‐Performance Catalyst to Hydrolyze Ammonia Borane for Hydrogen Production. Small 2019, 15, 1805460. (37) Fierro, G.; Jacono, M. L.; Inversi, M.; Dragone, R.; Porta, P. TPR and XPS study of cobalt–copper mixed oxide catalysts: evidence of a strong Co–Cu interaction. Top. Catal. 2000, 10, 39-48. (38) Lin, Y.; Yang, L.; Jiang, H.; Zhang, Y.; Cao, D.; Wu, C.; Zhang, G.; Jiang, J.; Song, L. Boosted Reactivity of Ammonia Borane Dehydrogenation over Ni/Ni2P Heterostructure. J. Phys. Chem. Lett. 2019, 10, 10481054. (39) Zheng, H.; Feng, K.; Shang, Y.; Kang, Z.; Sun, X.; Zhong, J. Cube-like CuCoO nanostructures on reduced graphene oxide for H2 generation from ammonia borane. Inorg. Chem. Front. 2018, 5, 1180-1187.
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(40) Esmaeilirad, M.; Zabihi, M.; Shayegan, J.; Khorasheh, F. Oxidation of toluene in humid air by metal oxides supported on γ-alumina. J. Hazard. Mater. 2017, 333, 293-307. (41) Jacukowicz-Sobala, I.; Ociński, D.; Mazur, P.; Stanisławska, E.; Kociołek-Balawejder, E. Evaluation of hybrid anion exchanger containing cupric oxide for As(III) removal from water. J. Hazard. Mater. 2019, 370, 117-125. (42) Hua, M.; Zhang, S.; Pan, B.; Zhang, W.; Lv, L.; Zhang, Q. Heavy metal removal from water/wastewater by nanosized metal oxides: a review. J. Hazard. Mater. 2012, 211, 317-331.
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Figures
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Cu@CoO@GO.
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Figure 1. Wide-angle XRD patterns of CuCoOx@GO, [email protected]@GO, and
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Figure 2. (a, b) Low-magnification TEM and (c) HADDF-STEM images, (d) high-resolution TEM images, (e) STEM elemental mappings, high-resolution XPS spectra of (f) Cu 2p and
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(g) Co 2p.
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Figure 3. (a) H2 production curves and (b) corresponding TOF values toward the hydrolysis
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of AB catalyzed by [email protected]@GO, CuCoOx@GO, Cu@CoO@GO, [email protected],
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and GO.
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Figure 4. H2 production curves catalyzed by [email protected]@GO under different (a) AB amounts, (c) catalysts amounts, and (e) test temperatures. The relationships between
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TOF and (b) AB amounts, (d) catalysts amounts, and (f) test temperatures for
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[email protected]@GO, CuCoOx@GO, and Cu@CoO@GO. Inset in (f) is the Ea values of [email protected]@GO, CuCoOx@GO, and Cu@CoO@GO for the hydrolysis of ammonia
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borane.
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Figure 5. (a) H2 production curves and (inset) TOF values catalyzed by
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[email protected]@GO for the hydrolysis of AB in H2O and D2O. (b) H2 production curves
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and (inset) TOF values catalyzed by [email protected]@GO for the hydrolysis of AB under
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different concentrations of NaOH.
PII:
S0304-3894(20)30187-4
DOI:
https://doi.org/10.1016/j.jhazmat.2020.122199
Reference:
HAZMAT 122199
To appear in:
Journal of Hazardous Materials
Received Date:
1 December 2019
Revised Date:
18 January 2020
Accepted Date:
27 January 2020
Please cite this article as: Li J, Ren X, Lv H, Wang Y, Li Y, Liu B, Highly efficient hydrogen production from hydrolysis of ammonia borane over nanostructured Cu@CuCoOx supported on graphene oxide, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122199
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.
Highly efficient hydrogen production from hydrolysis of ammonia borane over nanostructured Cu@CuCoOx supported on graphene oxide Jinlong Li, Xueying Ren, Hao Lv, Yingying Wang, Yafei Li, , Ben Liu*
Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center
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of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing
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Normal University, Nanjing 210023, China
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*Email: [email protected]
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Graphical abstract
Highlights
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A hybrid nanocatalyst composed of heterostructured Cu@CuCoO and conductive GO support is reported.
The hybrid nanocatalyst synergistically boosts catalytic performance toward hydrolysis of ammonia borane.
The design principle is applicable for high-performance nanocatalysts toward the removal of various hazardous materials.
Abstract: Designing highly efficient and cheap nanocatalysts for room-temperature hydrolysis of
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ammonia borane (AB) is of great significance for their real application in hydrogen (H2)based fuel cells. Here, we report a kind of noble metal (NM)-free hybrid nanocatalysts
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composed of heterostructured Cu@CuCoOx nanoparticles and a graphene oxide support
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(denoted as Cu@CuCoOx@GO) and demonstrate their high catalytic performance toward the hydrolysis of AB. By rationally controlling synthetic parameters, we find that optimum
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[email protected]@GO achieves a superior catalytic activity with a turnover frequency of
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44.6 molH2 molM-1 min-1 in H2O and 98.2 molH2 molM-1 min-1 in 0.2 M NaOH, better than most of previously reported NM-free nanocatalysts. This catalyst also discloses a very low
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activation energy (Ea) of 35.4 kJ mol-1. The studies on catalytic kinetics and isotopic experiments attribute the high activity to synergistically structural and compositional advantages of [email protected]@GO, which kinetically accelerates the oxidative cleavage of O-H bond in attacked H2O (the rate-determining step of the hydrolysis of AB). This study
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thus provides an opportunity for rational design of cheap NM-free nanocatalysts for H2 production from chemical H2-storage materials.
Keywords: heterogeneous catalyst, CuCo, hydrogen production, ammonia borane, catalytic kinetics
1. Introduction Fossil fuels, including coal, oil, and natural gas, are still the mostly used chemical sources of global sources. This unavoidably causes a large number of critical issues, from the release of greenhouse gas (CO2) and harmful gases (SO2, NOx, CO, etc.) to various other serious environmental problems.1 Recently, hydrogen has been recognized as an
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important alternative energy, because of high energy density and green product.2-5 However, the storage and transport of H2 gas in physical systems (compressed gas and H2
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adsorbents) are still inconvenient and dangerous.6-9 In contrast, chemical H2-storage
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materials, in which H covalently binds into stable chemicals, become more attractive to resolve these problems.9-11 Among numerous H2-storage materials, ammonia borane
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(NH3-BH3, AB) has received the special attention, due to high hydrogen content, low
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molecular weight, and good stability in solution and solid state.2,12-15 Three mole of molecular H2 can be released from one mole of AB in aqueous solution [NH3-BH3 + 3 H2O
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→ NH3-B(OH)3 + 3 H2], when suitable catalysts were used.5,16 Therefore, the development of highly efficient and cheap nanocatalysts to catalyze the hydrolysis of AB is critically important for real application of this technology. In the past 1.5 decades, a large number of nanocatalysts were fabricated for catalytic
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hydrolysis of AB to produce molecular H2, especially at room temperature.5 Despites numerous efforts, noble metals (NMs), such as Pt, Pd, Ru, and their alloys, are still used as the highly efficient catalysts for the hydrolysis of AB.17-26 Unfortunately, the high price and limited abundance greatly hindered their widespread and practical applications. In 2006, Xu’s group found that anchoring cobalt nanoparticles (Co NPs) on miscellaneous
supports can behave as NM-free nanocatalysts to catalyze the hydrolysis of AB.18 Since then, many NM-free catalysts with different nanostructures, such as, Cu, Co, Ni, and their alloys, as well as their borides, oxides and phosphides, were widely investigated for roomtemperature hydrolysis of AB.5,16,27-36 However, for a simple NM-free nanocatalyst system, the hydrolysis of AB can be dramatically hindered by the oxidative cleavage of H-
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OH bond from reaction solutions, causing a relatively low activity and poor stability. To kinetically facilitate the absorption and cleavage of the O-H bond from H2O, rational
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design and synthesis of a hybrid nanocatalyst that contains different active sites could
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kinetically improve their catalytic performance for the hydrolysis of AB.
Herein, a hybrid nanocatalyst that composed of metallic Cu, oxided CuCoOx, and a
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graphene oxide (defined as Cu@CuCoOx@GO) was designed for promoting room-
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temperature hydrolysis of AB. We proposed that oxided CuCoOx in this catalyst facilitated the adsorption of OH*, while heterostructured Cu@CuCoOx made metallic Cu with
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electron-rich surface and kinetically favored the dissociation of H-OH into H*. Meanwhile, a conductive GO support not only dispersed the Cu@CuCoOx for exposing more catalytically active sites, but also assisted the electron and mass transfer during hydrolysis. These advantages in compositions and structures of Cu@CuCoOx@GO
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synergistically boosted catalytic performance toward room-temperature hydrolysis of AB, receiving a high turnover frequency (TOF) of 44.6 molH2 molM-1 min-1 in H2O and 98.2 molH2 molM-1 min-1 in 0.2 M NaOH.
2. Results and Discussion
The Cuy@Cu1-yCoOx@GO was synthesized by a two-step method with copper acetate [Cu(Ac)2] and cobalt acetate [Co(Ac)2] as metal precursors and graphene oxide (GO) as a support. Briefly, we first synthesized oxided CuCoOx@GO by hydrothermal treatment of Cu(Ac)2 and Co(Ac)2 (1:1) in the mixed solution of ethanol/H2O (10:1) containing a predominant amount of GO. Then, the sample was further treated with the reducing gas
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(H2/N2 = 5 % / 95%) under different temperatures to obtain Cuy@Cu1-yCoOx@GO. We found, with increasing the reducing temperatures, CuCoOx@GO was gradually converted
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to Cuy@Cu1-yCoOx@GO (150-300 oC), and finally to Cu@CoO@GO (400 oC). Wide-angle X-
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ray diffraction (XRD) was first characterized to reveal the crystalline structures of final materials (Figure 1). Before the treatment with H2, XRD pattern indicated that the sample
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mostly composed of CuO and Co3O4 (defined as CuCoOx). Metallic Cu gradually appeared
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after treating with H2. When the reduction temperature was increased to 200 oC, XRD signal can be ascribed to metallic Cu, and oxided Cu2O and CoO, respectively. We carefully
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investigated the ratio of Cu/Cu2O by etching with diluted HCl. Inductively coupled plasma mass spectrometry (ICP-MS) indicated the molar ratio of Cu for Cu/Cu2O was 0.28 : 0.72. Considering that the ratio of Cu/Co is 1:1, we defined the sample as [email protected]@GO. When further increasing the reduction temperature to 400
o
C, Cu2O in
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[email protected]@GO was totally reduced to metallic Cu (Cu@CoO@GO). We emphasized that the ratios of Cu/Cu2O can be rationally tuned by changing the reduction temperatures (Figure S1). The nanostructures and crystallines of the samples were revealed by transmission electron microscope (TEM). [email protected]@GO was thoroughly characterized as an
typical example, since it exhibited the best catalytic activity toward room-temperature hydrolysis of AB (see below). The low-magnification TEM image displayed that [email protected] homogeneously dispersed on the surface of GO (Figure 2a). [email protected] are nearly spherical NPs with a diameter ranging from 5-20 nm (Figure 2b). The high-angle annular dark-field scanning TEM (HAADF-STEM) image further
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exhibited uniform [email protected] NPs dispersed on GO (Figure 2c). High-resolution TEM image disclosed two different lattice fringes, which can be ascribed to (111) plane of Cu
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and (111) plane of CuCoOx, respectively (Figure 2d). This suggested the co-existence of
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metallic Cu and oxided CuCoOx in the sample, which is well matched to the result observed from XRD. STEM elemental mappings displayed partially overlapped signals of
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Cu, Co and O on C (Figure 2e), suggesting the formation of the [email protected]@GO again.
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The nanostructures and crystallines of CuCoOx@GO and Cu@CoO@GO were also studied. Similar to [email protected]@GO, CuCoOx@GO was also uniform NPs loaded on
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the surface of GO (Figure S2a). The lattice spacing of 0.24 nm that ascribed to (111) plane of CuCoOx implied oxided states of the sample (Figure S2b). By comparison, Cu@CoO@GO disclosed a nanostructure of hollow NPs on GO, in which clear lattice spacing of 0.24 nm suggested the presence of metallic Cu (Figure S2c,d). Hollow
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nanostructure is possibly because of different reduction potentials of Cu and Co that gave rise to galvanic replacement reactions between different compositions. The high-resolution X-ray photoelectron spectroscopy (XPS) was further carried out
to investigate surface electronic states of [email protected]@GO. XPS spectrum of Cu 2p exhibited two pairs of signals, assigned to 2p1/2 and 2p3/2, respectively (Figure 2f). Among
them, the peaks at 932.1 and 951.9 eV were attributed to metallic Cu0 and/or monovalent Cu+, while the peaks at 933.7 and 953.5 eV were assigned to bivalent Cu2+. The ratios of Cu0/Cu+ : Cu2+ is approximately 35 : 65. Meanwhile, two strong satellite (Sat.) signals of oxided Cu2+ were seen. These results further indicated the presences of metallic Cu and oxided Cu+/Cu2+ in the sample. In contrast, no XPS signal of Co 2p assigned to metallic Co
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can be observed (Figure 2g). Bivalent Co2+ and trivalent Co3+ as well as their satellite signals indicated oxided states of CoOx in the sample. In addition to the data from XRD
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and TEM, we confirmed [email protected]@GO was successfully fabricated. Meanwhile,
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high-resolution XPS Cu 2p and Co 2p were also compared with CuCoOx@GO, Cu@CoO@GO, and [email protected] (Figure S3), further implying the controllability of
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our method in precisely engineering surface electronic states of the samples.37
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Catalytic performance of the [email protected]@GO, CuCoOx@GO and Cu@CoO@GO was carefully evaluated for room temperature hydrolysis of AB. Catalytic reaction was
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carried out with 2.4 mg of catalysts and 20 mg of AB at 25 oC, and the gas produced during catalysis was collected by a classic H2O-displacement method. [email protected] (without GO support) (Figure S4) and pure GO (without loading any metals and/or their oxides) were also investigated and compared as controls. Figure 3a showed time-dependent plots
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of H2 produced for above five catalysts. Almost no H2 produced for GO even after reacted for 20 min, indicating GO is inactive for the hydrolysis of AB. In contrast, other four catalysts were catalytically active and released H2 immediately, implying the catalytic sites are Cu/Co and/or their oxides. All four catalysts produced ~42.1 mL of H2, which is approximately 3 equivalent amount of AB. This indicated total release of H2 from AB
catalyzed by these catalysts. Among them, [email protected]@GO exhibited the highest catalytic rate with a total H2 release time of 8.0 min. The release time of H2 was increased to 9.5 min for Cu@CoO@GO and 11.0 min for CuCoOx@GO. Besides, without GO support, [email protected] showed a slightly longer release time of 16.0 min, due to the aggregate nature of material that dramatically decreased the catalytically active sites. These results
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correlated that both ratios of Cu/CuCoOx and GO support are important for promoting their catalytic activity. To clearly compare catalytic activities of catalysts, the turnover
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frequency (TOF) was further calculated and summarized based on initial 20 mL of H2
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produced and the amounts of metals (Cu and Co). Specifically, the TOF values are 44.6 molH2 molM-1 min-1 for [email protected]@GO, 28.1 molH2 molM-1 min-1 for CuCoOx@GO,
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30.3 molH2 molM-1 min-1 for Cu@CoO@GO, and 9.1 molH2 molM-1 min-1 for [email protected],
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respectively (Figure 3b). The results, again, highlighted that structural and compositional merits of the [email protected]@GO promoted room-temperature hydrolysis of AB.
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More control catalysis was investigated to reveal the importance of compositional ratios of catalysts for the hydrolysis of AB. First, we studied the effect of metallic Cu amounts of final catalysts in the hydrolysis of AB by changing initial reduction temperatures (Figure S5a). Increasing reduction temperatures would enlarge the
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amounts of metallic Cu in final Cuy@Cu1-yCoOx@GO. We found, with the increase of reduction temperatures, catalytic activity was initially increased, and reached the peak at 200 oC ([email protected]@GO). Then, it gradually decreased with increasing reduction temperature to 400 oC (Figure S5b). The result displayed that the amount of metallic Cu and ratios of Cu/CuCoOx changed their catalytic performance. Second, Cu amounts of
[email protected]@GO were evaluated by changing the addition ratios of Cu(Ac)2 and Co(Ac)2 during synthesis (Figure S5c). A clear Volcano-type activity was obtained for different Cu amounts (Figure S5d). Both monometallic Cu and Co exhibited worse catalytic activities, implying bimetallic CuCo-alloyed catalysts promoted the hydrolysis. Similar tendency was also observed when tuning the total amounts of metals (Figure
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S5e,f). These results definitely indicated, only under the optimal compositional ratios, catalytic activity of Cuy@Cu1-yCoOx@GO can be maximally improved for room-
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temperature hydrolysis of AB.
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To further reveal how the hydrolysis of AB catalyzed by [email protected]@GO, we disclosed the catalytic kinetics and further compared with its counterpart catalysts of
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CuCoOx@GO and Cu@CoO@GO. The amounts of AB and catalysts were thoroughly
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investigated first. As shown in Figure 4a, when increasing the amounts of AB added, the final production amounts of H2 for [email protected]@GO were increased correspondingly.
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However, initial H2 production curves (20 mL) were totally overlapped, although addition amount of AB were very different. Similar phenomenon also happened for CuCoOx@GO, and Cu@CoO@GO (Figure S6). We further plotted the relationship between In TOF and In [AB] ([AB] indicates the concentration of AB). A nearly horizontal relationship for all
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three catalysts disclosed a zero-order reaction for AB toward the hydrolysis of AB. However, we found that increasing the amounts of catalysts promoted the H2 production rates (Figure 4c and Figure S7). Interestingly, when taking the relationship between In rate and In [Metal], a linear relationship with the slopes of 1.05-1.19 was achieved, indicating that the hydrolysis of AB is first-order in the concentration of metals (Figure
4d). Among them, [email protected]@GO had a highest slop value of 1.19, suggesting it is more kinetically favorable for the hydrolysis of AB. The activation energy (Ea) was also evaluated by taking the catalytic kinetics under different temperatures (Figure 4e and Figure S8). The increase of test temperatures obviously boosted the catalytic rates for all three samples, resulting in a linear
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relationship between In TOF and 1000/T (Figure 4f). According to the Arrhenius plots, Ea value of [email protected]@GO was calculated to be 35.4 kJ mol-1, lower than that of
catalytic
hydrolysis
of
AB
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be
energetically
activated
by
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indicated
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CuCoOx@GO (60.5 kJ mol-1) and Cu@CoO@GO (45.3 kJ mol-1) (insets in Figure 4f). This
[email protected]@GO easily. We should emphasize that this is also one of the lowest Ea
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value, when compared with previously reported NM-free catalysts (Table S1).
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There are three key steps for catalyzing room-temperature hydrolysis of AB.28,33,38 First, both H2O and AB molecules were adsorbed on the surfaces of catalysts. Second, the
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H-OH bond of H2O was cleaved, and HO* and H* were released. OH* then attacked the B-N bond of AB, and further released H* and formed NH3-BH2OH (and NH3-BH(OH)2 and NH3-B(OH)3). Third, one H* from H2O and another H* from AB reacted and produced molecular H2. Considering the above results, we deduced that the rate-determining step
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of the hydrolysis of AB for our catalysts is the cleavage of O-H bond of H2O, as reported in most of metal-based catalysts. Two more series of catalytic experiments were designed and carried out carefully to confirm the deduction for [email protected]@GO. First, the isotopic experiment was conducted by using D2O instead of H2O. The kinetic isotope effect (KIE) has widely been evaluated on the roles of H2O in the rate-determining step
for catalytic hydrolysis of AB. In comparison to H2O, H2 production rate was remarkably decreased in the presence of D2O (Figure 5a), due to the difficulty for the cleavage of DOD. It suggested that H2O also contributed as the reactant for the hydrolysis of AB to produce H2. Meanwhile, a high KIE value of 3.4 was calculated. These definitely implied that the cleavage of O-H bond from H2O is the rate-determining step for the hydrolysis of
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AB, when catalyzed by [email protected]@GO. Second, we also investigated the role of OHon the hydrolysis of AB in the presence of [email protected]@GO. Figure 5b showed the
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catalytic activities of [email protected]@GO in different NaOH concentrations from 0.01
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to 0.4 M. Obviously, H2 production rates increased with the addition of NaOH. A highest catalytic rate with a TOF of 98.2 molH2 molM-1 min-1 for [email protected]@GO was
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obtained when taking the hydrolysis of AB in 0.2 M NaOH, which surpassed the activities
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of most reported NM-free catalysts (Table S1). This indicated that OH- can behave as a catalyst promoter that makes the catalysts more electron-rich and also contributes more
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adsorbed O-H. Both of them thus accelerated the rate-limiting oxidative reaction of O-H bond. However, the catalytic activities were slightly decreased when the higher concentrations of NaOH were used, possibly because excessive OH- can cover the active sites for the coordination of H and the dissociation of H2. Similarly, the promoting effect
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of OH- in the hydrolysis of AB was also observed for CuCoOx@GO and Cu@CoO@GO (Figure S9, S10), further indicating the rate-determining step of our catalysts is the oxidative cleavage of O-H of H2O. Based on compositional and structural features of [email protected]@GO, a possibly catalytic mechanism can be proposed toward room-temperature hydrolysis of AB. On the
one hand, oxided CuCoOx in the catalyst facilitated the adsorption of OH*, while metallic Cu accelerated the dissociation of H*.31,39 Especially, alloying CoO into Cu2O provided more active sites for the coordination of OH- on the catalysts, which had been proven by a lower activity catalyzed by Cu@Cu2O@GO (Figure S5c,d). In contrast, heterostructured Cu@CuCoOx also made metallic Cu with electron-rich surface, that kinetically favored the
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dissociation of H-OH into H*. Therefore, compositional advantages kinetically accelerated the cleavage of O-H of H2O (the rate-determining step), thus enhanced the catalytic
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activity of catalysts. On the other hand, loading Cu@CuCoOx on GO remarkably dispersed
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catalytically active NPs, and thus exposed more catalytically active sites. Meanwhile, GO also accelerated the electron and mass transfer during the hydrolysis, enhancing their
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3. Conclusion
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activity also.
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In conclusion, we reported a two-step method for synthesizing and supporting welldispersed Cu@CuCoOx NPs on GO, and demonstrated their catalytic performance toward room-temperature hydrolysis of AB. Compositional and structural features, including metallic Cu, oxided CuCoOx and a GO support, provided important contributions for
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enhancing their catalytic activity. We found, among more than ten catalysts, [email protected]@GO exhibited best catalytic activity with a high TOF of 44.6 molH2 molM1
min-1 and a low Ea of 35.4 kJ mol-1. More importantly, the hydrolysis of AB can be
activated by NaOH; a very high TOF of 98.2 molH2 molM-1 min-1 was achieved in the presence of 0.2 M NaOH. This is superior to the activities of mostly reported NM-free
nanocatalysts. Catalytic kinetics and isotopic experiments indicated synergistically compositional and structural advantages of [email protected]@GO accelerated the oxidative cleavage of O-H of H2O (the rate-determining step), thus improving their catalytic performance toward the hydrolysis of AB. With the high catalytic performance, low cost, and easy synthesis step, this protocol would be extended to synthesize other
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hybrid nanocatalysts for H2 production from various chemical fuels. We expect that highly
for the removal of various hazardous materials.40-42
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Declaration of interests
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effective hybrid nanocatalysts based on this design strategy would exhibit great potential
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Credit author statement
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B. Liu conceived the project and designed the experiments. Y. Li and B. Liu supervised the project. J. Li and Y. Ren prepared the materials and conducted catalytic experiments. H. Lv performed TEM imaging and XPS analysis. Y. Wang assisted the experiments. All the authors discussed the results and co-wrote the paper.
Acknowledgements
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The authors thank the financial supports from Jiangsu Specially Appointed Professor Plan, Natural Science Foundation of Jiangsu Province (No. BK20180723), Priority Academic Program Development of Jiangsu Higher Education Institutions, National and Local Joint Engineering Research Center of Biomedical Functional Materials.
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Figures
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Cu@CoO@GO.
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Figure 1. Wide-angle XRD patterns of CuCoOx@GO, [email protected]@GO, and
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Figure 2. (a, b) Low-magnification TEM and (c) HADDF-STEM images, (d) high-resolution TEM images, (e) STEM elemental mappings, high-resolution XPS spectra of (f) Cu 2p and
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(g) Co 2p.
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Figure 3. (a) H2 production curves and (b) corresponding TOF values toward the hydrolysis
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of AB catalyzed by [email protected]@GO, CuCoOx@GO, Cu@CoO@GO, [email protected],
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and GO.
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Figure 4. H2 production curves catalyzed by [email protected]@GO under different (a) AB amounts, (c) catalysts amounts, and (e) test temperatures. The relationships between
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TOF and (b) AB amounts, (d) catalysts amounts, and (f) test temperatures for
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[email protected]@GO, CuCoOx@GO, and Cu@CoO@GO. Inset in (f) is the Ea values of [email protected]@GO, CuCoOx@GO, and Cu@CoO@GO for the hydrolysis of ammonia
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Figure 5. (a) H2 production curves and (inset) TOF values catalyzed by
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[email protected]@GO for the hydrolysis of AB in H2O and D2O. (b) H2 production curves
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and (inset) TOF values catalyzed by [email protected]@GO for the hydrolysis of AB under
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different concentrations of NaOH.