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Highly Efficient Catalytic Hydrogen Evolution from Ammonia Borane Using the Synergistic Effect of Crystallinity and Size of Noble-MetalFree Nanoparticles Supported by Porous Metal-Organic Frameworks Penglong Liu, Xiaojun Gu, Kai Kang, Hao Zhang, Jia Cheng, and Haiquan Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01161 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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ACS Applied Materials & Interfaces

Highly Efficient Catalytic Hydrogen Evolution from Ammonia Borane Using the Synergistic Effect of Crystallinity

and

Size

of

Noble-Metal-Free

Nanoparticles Supported by Porous Metal-Organic Frameworks Penglong Liu, Xiaojun Gu,* Kai Kang, Hao Zhang, Jia Cheng, and Haiquan Su* Inner Mongolia Key Laboratory of Coal Chemistry, School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, Inner Mongolia, China. KEYWORDS: hydrogen evolution, synergistic effect, metal-organic framework, catalyst, metal nanoparticle.

ABSTRACT: A series of non-precious metal nanoparticles (NPs) supported by metal-organic framework MIL-101 were synthesized using four methods and their catalytic performance on hydrogen evolution from ammonia borane (NH3BH3) was studied. The results showed that the crystalline Co NPs with size of 4.5−8.5 and 14.5−24.5 nm had low activities featuring the total turnover frequency (TOF) values of 9.9 and 4.5 molH2⋅molcat−1⋅min−1, respectively. In contrast,

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the amorphous Co NPs with size of 1.6−2.6 and 13.5−24.5 nm exhibited high activities featuring the total TOF values of 51.4 and 22.3 molH2⋅molcat−1⋅min−1, respectively. The remarkably different activities could be ascribed to the different crystallinity and size of Co NPs in the catalysts. Moreover, the ultrasound-assisted in situ method was also successfully applied to bimetallic systems, and MIL-101-supported amorphous CuCo, FeCo and NiCo NPs had the catalytic activities with total TOF values of 51.7, 50.8 and 44.3 molH2⋅molcat−1⋅min−1, respectively, which were the highest in the values of the reported non-noble metal Co-based catalysts. The present approach, namely, using the synergistic effect of crystallinity and size of metal NPs, may offer a new prospect for high-performance and low-cost nanocatalysts.

1. INTRODUCTION Hydrogen is considered to be an attractive clean energy carrier since it can contribute to reducing the energy dependence toward fossil fuels and to lowering greenhouse-gas emissions.1–4 The development of effective hydrogen storage materials is imperative but challenging to establish a future hydrogen economy.5–9 Due to the high hydrogen content of 19.6 wt%, ammonia borane (NH3BH3) is a favorable candidate for chemical hydrogen storage applications and hydrogen can be released through its hydrolysis, which has been first reported by Xu and Chandra.10–12 To meet the practical application, the most challenge is to develop heterogeneous catalysts to boost the kinetic properties of hydrogen generation from NH3BH3 under moderate conditions.13–15 Compared to the noble-metal catalysts containing Pt, Pd and Ru, the transition metal catalysts containing Cu, Fe, Co and Ni have attracted much more attention due to their low cost and abundance in the earth.16–24 However, these non-precious catalysts always have low activities.25–


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As a result, it is highly desirable to develop new approaches to synthesize noble-metal-free

catalysts with significantly enhanced activities. Compared with the crystalline materials featuring anisotropic structures, the amorphous materials including metal nanoparticles (NPs) have isotropic structures featuring much greater structural distortion and the resulting high concentrations of unsaturated coordination sites,39–42 which can generate the superior catalytic performance such as activity.43 In addition, the size of metal NPs usually correlates with the percentages of different crystal facets and surface area, which have potential impact on the catalytic activity.44–49 Based on the above two aspects, it is envisioned that the utilization of noble-metal-free amorphous metal NPs with small size may significantly enhance the kinetic properties of catalytic hydrogen generation from NH3BH3. For synthesizing amorphous metal NPs, the methods such as the rapid quenching, electroplating and element doping have been widely used.50,51 However, these methods involve the harsh or complex processes. Recently, the rapid reduction of precursor ions of metal NPs has attracted much attention due to its simple and moderate process.52–54 For synthesizing and stabilizing small-sized metal NPs, various supports have been selected, among which metal-organic frameworks (MOFs) assembled by metal ions/clusters and organic ligands have exhibited distinctive properties owing to their high porosity, large surface area and chemical tenability.55–60 Besides their pores, which can limit the migration and aggregation of metal NPs, MOFs can also afford the distinct interactions between catalytically active metal NPs and organic ligands,61,62 which may promote the adsorption of organic catalytic substrates such as NH3BH3 on the surface of catalysts and then be beneficial for the enhanced catalytic activity. Herein, we presented four simple and moderate methods to control the synthesis of noblemetal-free amorphous or crystalline Co-based NPs supported by porous MOF MIL-101 and


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further used these materials as catalysts for the hydrogen evolution from NH3BH3. As we expected that the catalysts containing amorphous Co-based NPs with small size, which were synthesized using ultrasound-assisted in situ method, exhibited superior catalytic activities featuring the highest total turnover frequency (TOF) values of the reported noble-metal-free Cobased catalysts. 2. EXPERIMENTAL SECTION 2.1 Chemicals. All chemicals were obtained from commercial sources. Ammonia borane (NH3BH3, Aldrich, 97%), sodium borohydride (NaBH4, J&K Chemical, 98%), cobalt chloride hexahydrate (CoCl2⋅6H2O, Sinopharm Chemical Reagent Co., Ltd, >99%), nickel chloride hexahydrate (NiCl2·6H2O, Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd, >99%), copper chloride dehydrate (CuCl2⋅2H2O, Sinopharm Chemical Reagent Co., Ltd, >99%), iron sulfate heptahydrate (FeSO4⋅7H2O, Alfa Aesar, >99%), chromium nitrate nonahydrate (Cr(NO3)3⋅9H2O, Sinopharm Chemical Reagent Co., Ltd, >99%) and terephthalic acid (C6H4(CO2H)2, J&K Chemical, 99%) were used without further purification. 2.2 Synthesis and Catalytic Study. The first type of MIL-101-supported Co catalyst, which was labelled as Co/MIL-101-1-U, was synthesized by ultrasound-assisted in situ method and its catalytic study was as follows: Dehydrated MIL-101 (30 mg), which was obtained under vacuum at 150 oC for 12 h, was added into a two-necked round-bottom flask, and then 4.7 mL of aqueous solution of CoCl2⋅6H2O (0.0342 mmol) was introduced into the solution, which was sonicated for 1 h and then was shaken for 1 h. After that, 1.5 mL of aqueous solution of NH3BH3 (1.71 mmol) and NaBH4 (0.068 mmol) was injected into the flask through the rubber plug using a syringe, and then the reaction immediately began under shaking. The molar ratio for


4

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NH3BH3:NaBH4:Co remained constant at 1:0.04:0.02. The gas evolution was monitored using a gas burette. The atmospheric pressure in Hohhot, Inner Mongolia is 88.8 kPa. For comparison, other three monometallic Cu, Fe and Ni catalysts, which were labelled as Cu/MIL-101-1-U, Fe/MIL-101-1-U and Ni/MIL-101-1-U, were also synthesized using the same procedure as that of Co/MIL-101-1-U. The MIL-101-free Co catalyst, which was labelled as Co-1-U, was synthesized using the same procedure as that of Co/MIL-101-1-U except for the absence of MIL101. Through introduction of the second transition metal Cu, Fe or Ni into monometallic Co/MIL101-1-U, three bimetallic catalysts, which were labelled as CuCo/MIL-101-1-U, FeCo/MIL-1011-U and NiCo/MIL-101-1-U, were synthesized. The molar ratio of Cu, Fe or Ni to Co in the three catalysts was 6:94. The preparation procedure was the same as that of Co/MIL-101-1-U except for using mixed transition metals. The second type of four MIL-101-supported catalysts, which were labelled as Co/MIL-101-1, CuCo/MIL-101-1, FeCo/MIL-101-1 and NiCo/MIL-101-1, were synthesized by in situ method. The preparation procedure was the same as that of Co/MIL-101-1-U, CuCo/MIL-101-1-U, FeCo/MIL-101-1-U and NiCo/MIL-101-1-U except the absence of ultrasound. The molar ratio of Cu, Fe or Ni to Co in the three bimetallic catalysts was 6:94. The catalytic procedure was same as that of Co/MIL-101-1-U. The third type of MIL-101-supported catalyst, which was labelled as Co/MIL-101-2-U, was synthesized by ultrasound-assisted ex situ method. Dehydrated MIL-101 (30 mg) were added into a two-necked round-bottom flask, and then 4.7 mL of aqueous solution of CoCl2⋅6H2O (0.0342 mmol) was introduced into the solution, which was sonicated for 1 h and then was shaken for 1 h. After that, 0.5 mL of aqueous solution of NaBH4 (0.342 mmol) was injected into


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the flask. After the bubble generation ceased, 1.0 mL of aqueous solution of NH3BH3 (1.71 mmol) was injected into the flask, and then the reaction immediately began under shaking. Through introduction of the second transition metal Cu, Fe or Ni into monometallic Co/MIL101-2-U, three bimetallic catalysts, which were labelled as CuCo/MIL-101-2-U, FeCo/MIL-1012-U and NiCo/MIL-101-2-U, were synthesized. The molar ratio of Cu, Fe or Ni to Co in the three bimetallic catalysts was 6:94. The preparation procedure was the same as that of Co/MIL101-2-U except for using mixed transition metals. Another MIL-101-free Co catalyst, which was labelled as Co-2-U, was synthesized using the same procedure as that of Co/MIL-101-2-U except for the absence of MIL-101. The fourth type of four MIL-101-supported catalysts, which were labelled as Co/MIL-101-2, CuCo/MIL-101-2, FeCo/MIL-101-2 and NiCo/MIL-101-2, were synthesized by ex situ method. The preparation procedure was the same as that of the above Co/MIL-101-2-U, CuCo/MIL-1012-U, FeCo/MIL-101-2-U and NiCo/MIL-101-2-U except for the absence of ultrasound. The molar ratio of Cu, Fe or Ni to Co in the three bimetallic catalysts was 6:94. The catalytic procedure was same as that of Co/MIL-101-2-U. For the experiments about recycle stability of catalysts, once the first hydrogen generation reaction completed, the aqueous solution containing equivalent NH3BH3 (1.14 M, 1.5 mL) was added into the reaction flask. The evolution of gas was monitored using the gas burette. Such cycle experiments were repeated for many times under ambient atmosphere at room temperature. 2.3 Calculation methods. The TOF value was calculated from the following equation. TOF =

3nNH3BH3 nmetal t

In the equation, nmetal is the total molar amount of metal species in the catalyst, t is reaction time, and nNH3BH3 is the total molar amount of NH3BH3 in the dehydrogenation reaction.


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2.4 Characterization. The morphologies of all the samples were observed using a transmission electron microscope (TEM, JEM-2010) equipped with an energy-dispersive X-ray (EDX) analysis unit. The X-ray photoelectron spectra (XPS) were acquired with an ESCALAB250 (Thermo VG Corp.) equipped with an Al-Kα X-ray excitation source. Powder Xray diffraction (PXRD) studies were conducted using a Panalytical X-Pert X-ray diffractometer with

a

Cu-Kα

source.

The

surface

area

measurements

were

performed

by

N2

adsorption/desorption at liquid N2 temperature (77 K) using automatic volumetric adsorption equipment (Autosorb-iQ2-MP). Inductively coupled plasma-atomic emission spectroscopy (ICPAES) measurement was performed on a Thermo Jarrell Ash (TJA) Atomscan Advantage instrument. The Raman spectra were recorded on a micro-Raman spectrometer (LabRAM HR Evolution) at room temperature. 3. RESULTS AND DISCUSSION 3.1. Synthesis and Catalyst Characterization. One of porous MOFs, MIL-101 with a formula of Cr3F(H2O)2O[C6H4(CO2)2]3·nH2O (n ≈ 25), was chosen as catalyst support because of its stability in solution environment, large cavity size (2.9 to 3.4 nm) and high surface area,63,64 which may facilitate the immobilization of metal NPs and the adsorption of catalytic molecule NH3BH3 into the nanopores. The reduction ways of metal ions have affected the microstructures of metal NPs.7,8 Among the various reduction ways, the in situ method, which has been testified to be effective in the construction of amorphous state of metal NPs and has been selected to synthesize amorphous metal NPs.50,51 For comparison, the ex situ method was used to synthesize crystalline metal NPs. Moreover, the ultrasound was selected to prompt the metal ions to enter the nanopores of MIL-101 and/or to disperse metal ions on the surface of MIL-101, resulting in the formation of MIL-101-supported metal NPs with small size.65,66 On the basis of the above


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considerations, in order to explore the effect of crystallinity and size of Co NPs on the catalytic activity in the hydrogen generation from NH3BH3, four types of catalysts were synthesized using four methods (Figure 1). When the ultrasound-assisted in situ method and the in situ method were selected in the synthesis of catalysts, amorphous Co NPs with small size and big size were constructed, respectively. In contrast, when the ultrasound-assisted ex situ method and the ex situ method were selected, crystalline Co NPs with small size and big size were constructed, respectively.

Figure 1. Schematic illustration for the synthesis of four types of MIL-101-supported Co NPs: (a) Co/MIL-101-1-U; (b) Co/MIL-101-1; (c) Co/MIL-101-2-U; (d) Co/MIL-101-2. The morphologies and crystallinity of MIL-101-supported Co NPs were characterized by TEM and selective area electron diffraction (SAED), respectively. The TEM images showed that the Co NPs in Co/MIL-101-1-U were highly dispersed without obvious aggregation and the size of the Co NPs was in the range of 1.6−2.6 nm (Figures 2a and 2b), while the size of Co NPs in Co/MIL-101-1 was in the range of 13.5−24.5 nm (Figures 2d and 2e). The SAED patterns for Co/MIL-101-1-U and Co/MIL-101-1 demonstrated the amorphous nature of Co NPs. The results of TEM and SAED exhibited that the size of Co NPs in Co/MIL-101-2-U and Co/MIL-101-2 was in the range of 4.5−8.5 and 14.5−24.5 nm, respectively, and the Co NPs in the two catalysts


8

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featured crystalline state (Figures 2g, 2h, 2j and 2k). In addition, the size distribution regularity of Co-based NPs in the bimetallic catalysts CuCo/MIL-101-1-U, CuCo/MIL-101-1, CuCo/MIL101-2-U, FeCo/MIL-101-1-U and FeCo/MIL-101-2-U were similar to that of monometallic Co catalysts (Figures S1–S5). The EDX spectra showed the presence of Co species in the assynthesized catalysts (Figures 2c, 2f, 2i, 2l and S6–S10). The above results indicated that the in situ and ex situ reduction of metal ions could induce the formation of amorphous and crystalline Co NPs, respectively, and the ultrasound could decrease the size of Co NPs. These different crystallinity and size of Co NPs in the catalysts might lead to the different catalytic activities. The ICP analyses showed that the metal NPs was not significantly lost in the as-synthesized catalysts compared to the theoretical amounts of metal species in the experiments (Table S1). The PXRD patterns showed no loss of crystallinity of MIL-101, suggesting that the framework of MIL-101 was maintained in the catalysts (Figure S11). All the catalysts did not exhibit the apparent characteristic diffractions for Co species due to that the Co species in the catalysts had low loadings (Table S1). Moreover, the amorphous state of the Co species in the in situsynthesized catalysts (Figures 2a and 2d) also leaded to the unapparent characteristic diffractions. Due to the oxidation of metallic Co species on the surface of Co NPs exposed in the air, the structure information of Co oxides on the surface of Co NPs, which could be provided through Raman spectra, was used to speculate the structure information of metallic Co NPs featuring crystalline or amorphous state caused by the change of Co−Co bonds before oxidation. In order to eliminate the influence of inorganic-organic hybrid MIL-101 on Raman signals of Co oxides, MIL-101-free Co-1-U and Co-2-U were selected as probes. The results of Raman spectra show that that there were different peaks featuring Co–O bonds in Co-1-U and Co-2-U (Figure S12). Especially, compared with the ex situ-synthesized Co-2-U, the in situ-synthesized Co-1-U had


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the red shifts of characteristic peaks of Co–O bonds, which in turn indicated that the Co–Co bonds in the Co NPs in Co-1-U and Co-2-U before oxidation were different and thus Co NPs in the both catalysts had different structure information. On the basis of the results of SAED, PXRD and Raman spectra of catalysts, it could be concluded that the in situ-synthesized Co NPs had amorphous state and the ex situ-synthesized Co NPs had crystalline state.

Figure 2. TEM images, the SAED patterns (insets), the corresponding size distribution diagrams of Co NPs and the EDX spectra of (a, b, c) Co/MIL-101-1-U, (d, e, f) Co/MIL-101-1, (g, h, i) Co/MIL-101-2-U and (j, k, l) Co/MIL-101-2.


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The XPS investigations showed that the peaks of Co 2p3/2 and Co 2p1/2 before Ar etching were observed with binding energies at around 781.5 and 797.3 eV corresponding to Co 2p of CoO in the catalysts (Figure 3), which was caused by oxidation during the exposure of catalyst to the air. However, the peaks (778.3 and 793.4 eV) of metallic Co species were detected after Ar etching. The XPS results of bimetallic catalysts also exhibited the partial oxidation of metallic Co species (Figures S13 and S14). The appreciable decreases in the amount of N2 sorption of the catalysts indicated that the pores of MIL-101 were occupied by Co NPs and/or blocked by the Co NPs located at its surface (Figure S15).60,61

Figure 3. XPS patterns of catalysts before and after Ar etching: (a, b) Co/MIL-101-1-U; (c, d) Co/MIL-101-1; (e, f) Co/MIL-101-2-U; (g, h) Co/MIL-101-2. 3.2 Hydrogen Evolution Performance. For determining the effect of crystallinity and size of Co NPs on the activity, the catalysis of Co/MIL-101-1-U, Co/MIL-101-1, Co/MIL-2-U and Co/MIL-101-2 was performed. Strikingly, Co/MIL-101-1-U containing amorphous Co NPs with size of 1.6−2.6 nm exhibited the highest activity with total TOF value of 51.4 molH2⋅molcat−1⋅min−1 (Figure 4), which was the highest in all the reported noble-metal-free Co-


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based catalysts (Table 1). Co/MIL-101-1 containing amorphous Co NPs with size of 13.5−24.5 nm also exhibited the high activity with total TOF value of 22.3 molH2⋅molcat−1⋅min−1. Compared to the above two catalysts containing amorphous Co NPs, Co/MIL-101-2-U and Co/MIL-101-2 containing crystalline Co NPs with size of 4.5−8.5 and 14.5−24.5 nm exhibited much lower activities with total TOF values of 9.9 and 4.5 molH2⋅molcat−1⋅min−1, respectively. From these results, it could be concluded that (i) the amorphous state of Co NPs played a decisive role in the enhanced activity, and (ii) when the crystallinity of Co NPs was same, their small size was beneficial for the high activity. This above conclusion, especially the role of amorphous metal NPs in the catalysis, was also testified by the reported amorphous Fe-based NPs having excellent catalytic activity.16,20 Table 1 Catalytic activities of catalysts for the hydrogen evolution from NH3BH3 at 298 K. Catalyst

TOF

Reference

(molH2⋅molcat-1⋅min-1) Co/MIL-101-1-U

51.4

This work

CuCo/MIL-101-1-U

51.7

This work

FeCo/MIL-101-1-U

50.8

This work

NiCo/MIL-101-1-U

44.3

This work

Co nanoclusters

25.7

[31]

Co nanoparticles

39.8

[52]

Co/PEI-GO

39.9

[54]

CuCo@MIL-101

19.6

[29]

Cu@FeCo

10.5

[30]

Cu@Co

15

[33]

AuCo/CNT-1

36.05

[7]

AuCo/NXC-1

42.1

[8]

Au@Co

13.7

[14]

AuCo@MIL-101

23.5

[19]

PdCo/C

35.7

[17]

Pd@Co@MIL-101

51

[21]

Pd@Co/graphene

40.9

[27]

Ru@Co/graphene

40.46

[28]

Ag@Co/graphene

10.23

[53]


12

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Figure 4. Plots of time versus volume of generated H2 from NH3BH3 aqueous solution over (a) Co/MIL-101-1-U, (b) Co/MIL-101-1, (c) Co/MIL-101-2-U and (d) Co/MIL-101-2 at 298 K. Inset: Corresponding TOF values of the catalysts. In order to further lower the cost of monometallic Co catalysts, other much less expensive metals Cu, Fe and Ni were used to partially replace Co to construct bimetallic catalysts CuCo/MIL-101-1-U,

FeCo/MIL-101-1-U

and

NiCo/MIL-101-1-U,

and

their

catalytic

performance was studied. The results showed that the three bimetallic catalysts and monometallic Co/MIL-101-1-U had the similar activities (Figure 5), indicating that the present bimetallic strategy was effective for further lowering the cost of Co catalysts and meantime their activities were maintained. It should be noted that Ni/MIL-101-1-U had low activity and Cu/MIL-101-1-U and Fe/MIL-101-U almost had no activity. However, compared with the monometallic catalysts, the bimetallic catalysts exhibited high activities, which might be caused by the synergistic interaction between two different metals in terms of the ensemble effect and the ligand effect.67,68 It is worth noting that the reported Cu@SiO2, Cu/RGO and mesoporous Cu catalysts are found to have good activities.34-36 These catalytic differences indicated that the microstructures of Cu NPs and supports also affected the activities of catalysts. Moreover, in


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order to confirm whether the orderliness of catalytic activities of different bimetallic Co-based catalysts was similar to that of the four types of monometallic Co catalysts, the catalytic performance on the twelve MIL-101-supported CuCo, FeCo and NiCo catalysts was systematically studied. The results showed that ultrasound-assisted in situ synthesized bimetallic catalysts and ex situ synthesized bimetallic catalysts had the highest and the lowest activities in the three series of bimetallic catalysts, respectively (Figures 6, S16 and S17).

Figure 5. (Top) Plots of time versus volume of generated H2 from NH3BH3 aqueous solution over (a) CuCo/MIL-101-1-U, (b) FeCo/MIL-101-1-U, (c) NiCo/MIL-101-1-U, (d) Ni/MIL-1011-U, (e) Fe/MIL-101-1-U and (f) Cu/MIL-101-1-U catalysts at 298 K and (bottom) the TOF values of the above six catalysts.


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Figure 6. Plots of time versus volume of generated H2 from NH3BH3 aqueous solution over (a) CuCo/MIL-101-1-U, (b) CuCo/MIL-101-1, (c) CuCo/MIL-101-2-U and (d) CuCo/MIL-101-2 at 298 K. Inset: Corresponding TOF values of the catalysts. Different supports had different compositions and structures, resulting in the different catalytic activities.69 Then porous inorganic materials Al2O3 and SiO2 were used as supports to synthesize Co catalysts using the same procedure as that for Co/MIL-101-1-U. The TEM images showed that the Co NPs in Co/Al2O3-1-U and Co/SiO2-1-U also had the small size (Figures S18 and S19), but they were bigger than those of the Co NPs in Co/MIL-101-1-U. The catalytic results showed that Co/Al2O3-1-U and Co/SiO2-1-U had lower activities than Co/MIL-101-1-U (Figure 7), but their activities were still high among the reported noble-metal-free catalysts (Table 1). These different activities might be caused by the different supports, among which the high surface area of MIL-101 and the strong adsorption of NH3BH3 into its pores leaded to the fast catalytic dehydrogenation of NH3BH3. Moreover, these results involving different types of supports also indicated the generality of enhanced catalytic performance over amorphous metal NPs synthesized using the ultrasound-assisted in situ method.


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Figure 7. Plots of time versus volume of generated H2 from NH3BH3 aqueous solution over (a) Co/MIL-101-1-U, (b) Co/Al2O3-1-U and (c) Co/SiO2-1-U at 298 K. Inset: Corresponding TOF values of the catalysts. The kinetics of Co/MIL-101-1-U and CuCo/MIL-101-1-U for hydrogen generation was studied at different reaction temperatures. From the catalytic results, it could be clearly seen that the hydrogen evolution rate increased gradually with increasing solution temperatures from 298 to 323 K, indicating that the high temperature was beneficial for enhancing the dehydrogenation rate of NH3BH3 (Figure 8). According to the Arrhenius plot of ln r versus 1/T, the obtained apparent activation energy (Ea) values of the catalytic processes of Co/MIL-101-1-U and CuCo/MIL-101-1-U were 31.3 and 30.5 kJ/mol, respectively, which were comparable to the reported values for the same reaction using various catalysts.4 However, the low Ea value of a catalytic process could not mean that the corresponding catalyst necessarily had the high catalytic activity.70


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Figure 8. Plots of time versus volume of generated H2 from NH3BH3 aqueous solution over (a) Co/MIL-101-1-U and (b) CuCo/MIL-101-1-U at different temperatures. Inset: Corresponding Arrhenius plot of ln r vs 1/T. The stability results of catalysts showed that the activities of Co/MIL-101-1-U and CuCo/MIL-101-1-U had no significant change after five runs of catalysis without any treatment (Figure S20), which could be attributed to the formation of small-sized Co and CuCo NPs dispersed and immobilized by MIL-101. In addition, there was no significant increase in the size of Co and CuCo NPs in the spent catalysts Co/MIL-101-1-U and CuCo/MIL-101-1-U (Figures S21 and S22), which could also explain the reason for the high catalytic stability of both catalysts. However, the activities of Co/MIL-101-1 and Co/MIL-101-2-U with big size of Co


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NPs decreased after five runs of catalysis (Figures S23 and S24). In order to the check the longtime durability of catalysts, Co/MIL-101-1U and Co/MIL-101 were selected as examples. The catalytic results show that the H2 productivity remained unchanged and the activity of Co/MIL101-1-U with small size of Co NPs had no significant change after 30 runs (480 min) (Figure 9a). The H2 productivity of Co/MIL-101-1 with big size of Co NPs remained unchanged after 17 runs (615 min), but its activity remarkably decreased (Figure 9b). These above results illustrated that the generation of small-sized metal NPs using the strong confinement effect of porous MOFs and ultrasound-assisted in situ method could lead to the high stability/durability of catalysts.

Figure 9. Long-time durability characterization of (a) Co/MIL-101-1-U and (b) Co/MIL-101-1 for the generation of H2 from NH3BH3 aqueous solution at 298 K.


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4. CONCLUSION In summary, from the viewpoint of regulating and optimizing the performance of supported catalysts, four simple and moderate methods, which were used to synthesize amorphous or crystalline noble-metal-free Co-based NPs supported by porous MIL-101, were presented. The as-synthesized catalysts exhibited drastically different activities in the hydrogen evolution from NH3BH3, which could be ascribed to the different crystallinity and size of Co NPs. Among the catalysts, the ultrasound-assisted in situ synthesized catalysts with amorphous and small-sized Co-based NPs exhibited the highest activities among all the reported noble-metal-free Co-based catalysts. The present approach, namely, controlling the crystallinity and size of active metal NPs, may bring a new opportunity toward the development of high-performance supported noble-metal-free nanocatalysts with tremendous application prospect in various fields including energy storage and production. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. TEM images, EDX, PXRD, Raman spectra, XPS, N2 adsorption-desorption isotherms, ICP results and TOF data of catalysts as well as the experimental results for hydrogen evolution from ammonia borane. AUTHOR INFORMATION Corresponding Authors


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*E-mail: [email protected]. *E-mail: [email protected]. ORCID Xiaojun Gu: 0000-0002-3877-4373 Haiquan Su: 0000-0003-2164-3219 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Program for New Century Excellent Talents in University of the Ministry of Education of China (grant no. NCET-13-0846), and the Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (grant no. NJYT-13-A01).

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