Co0.8Cu0.2MoO4 Microspheres Composed of Nanoplatelets as a

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Co0.8Cu0.2MoO4 microspheres composed of nanoplatelets as a robust catalyst for the hydrolysis of ammonia borane Jinyun Liao, Dongsheng Lu, Guiqiang Diao, Xibin Zhang, Minna Zhao, and Hao Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03994 • Publication Date (Web): 18 Mar 2018 Downloaded from http://pubs.acs.org on March 18, 2018

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ACS Sustainable Chemistry & Engineering

Co0.8Cu0.2MoO4 microspheres composed of nanoplatelets as a robust catalyst for the hydrolysis of ammonia borane Jinyun Liao, Dongsheng Lu, Guiqiang Diao, Xibin Zhang, Minna Zhao, Hao Li* School of chemistry and Materials Engineering, Huizhou University, No.46 Yanda Avenue, Huizhou 516007, China. * Corresponding author: [email protected] (Hao Li), Tel/Fax: +86 752 2527229

Abstract: Catalytic hydrolytic dehydrogenation of chemical hydrides is a safe and efficient way to provide hydrogen fuel for new energy vehicles and portable electronic devices. Currently, cost-efficient and high performance catalysts must be developed before large-scale application of this hydrogen production technology can proceed. In this work, we prepared a series of CoxCu1-xMoO4 microspheres composed of nanoplatelets by a green approach free of any organics, such as surfactant, complexing agent or organic solvent. For the first time, the catalytic activity of CoxCu1-xMoO4 in the hydrolytic dehydrogenation of ammonia borane (AB) was investigated. It is found that there is a synergistic effect between CoMoO4 and CuMoO4 in CoxCu1-xMoO4 towards the hydrolytic dehydrogenation of AB. Among those CoxCu1-xMoO4 catalysts, Co0.8Cu0.2MoO4 exhibits the highest catalytic activity with a turnover frequency of 55.0 molH2 min−1 molcatalyst−1, which is one of the most active noble-metal-free catalysts towards AB hydrolysis. It is revealed that the active species are the CoCu alloy nanoparticles supported on the Co0.8Cu0.2MoO4 microsphere. Additionally, the Co0.8Cu0.2MoO4 catalyst demonstrates good reusability, making it a robust catalyst for AB hydrolysis to generate hydrogen in practical applications. 1

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Keywords: Molybdate; Microspheres; Nanosheets; Hydrogen production; Ammonia borane.

Introduction Due to its environmentally benign nature and high energy density, hydrogen is regarded as a clean and promising energy carrier 1, 2 For example, hydrogen may find large-scale and wide applications in proton exchange membrane fuel cells that will be incorporated into new energy vehicles and electronic devices in the near future.3,4 However, the production, storage and transportation of hydrogen are still great challenges, especially when it is used for mobile devices and vehicles, which is the key barrier that limits its practical applications.5 Over the past decade, catalytic hydrolysis of chemical hydrides has been intensively investigated because it provides a facile, safe and efficient way to produce, storage and transport hydrogen.6-8 For example, ammonia borane (AB) possesses a high hydrogen storage capacity of 19.6 wt%, which is much higher than the targeted value for the year 2020 (5.5 wt%) set by the U.S. DOE (Department of Energy).9 Additionally, AB is nontoxic and very stable in the state of a solid or aqueous solution, and these can be easily and safely storied and transported. Although it is thermodynamically spontaneous at room temperature, the hydrolysis of AB will not occur actually because of the slow reaction kinetics. Thus, a proper catalyst is indispensable to lower the energy barrier of the hydrolytic reaction and therefore enhance the hydrogen production. In 2006, Xu’s group firstly found that transition metals were active to AB hydrolysis for hydrogen production.10 Since then, a number of noble metals and their alloys have been developed, which exhibit high activity in catalyzing AB hydrolysis for hydrogen production.11-16 However, the high cost of the noble-metal-based catalysts is a large obstacle for their commercial applications. In order to decrease the cost of catalysts for AB hydrolysis, 2


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metallic Co, Ni and their alloys have been developed as noble-metal-free catalysts over the past years.17 However, their catalytic activity is still far from satisfactory and can not meet the requirement of fast hydrogen release in practical applications. Thus, the development of novel noble-metal-free catalysts with both low cost and high activity is of extreme importance for the commercialization of this hydrogen production technology. It has been reported that CoCu nanoalloys are active catalysts to AB hydrolysis and the introduction of elemental Mo into the catalyst can further enhance the catalytic activity.18,19 Additionally, the oxides of Co and Cu on the sample surface can be reduced by AB to form CoCu alloy. Thus, it is likely that CoxCu1-xMoO4 is a highly active catalyst for AB hydrolysis. Motivated by these ideas, we have prepared a series of CoxCu1-xMoO4 microspheres composed of nanoplatelets in this work. To the best of our knowledge, nanostructured CoxCu1-xMoO4 microspheres have not been previously reported. It should be noted that the applications of CuMoO4 and CoMoO4 as supercapacitor and anode materials for lithium-ion batteries have been widely reported in the literature.20-23 However, their applications as catalysts for AB hydrolysis have not been documented thus far. In this work, the Co0.8Cu0.2MoO4 microspheres showed the turnover frequency (TOF) of 55.0 molhydrogen min-1 molcat-1, which is one of the most active noble-metal-free catalysts towards AB hydrolysis. More importantly, it was found that a significant a synergistic effect exists between CuMoO4 and CoMoO4 in the hydrolytic reaction. These observations may provide new insight into the design of novel catalysts for AB hydrolysis.

Experimental Preparation of catalysts All reagents were of analytic grade. In a typical procedure, x mmol CoCl2 (x= 3



0.4, 0.8, 1.2, 1.6, 1.8, and 1.9) and (2-x) mmol CuCl2 were dissolved in 20 mL water under magnetic stirring. Then, 2.0 mmol molybdic acid was dispersed into 20 mL water under ultrasonication to form a white suspension. After that suspension was blended with the Co2+/Cu2+ mixture solution, 40 mL urea solution (0.5 mol L-1) was added to the resultant mixture under stirring. The final mixture was transferred into a Teflon-lined stainless steel autoclave, which was sealed and then placed into a drying oven for a hydrothermal treatment at 165 °C for 7 h. The obtained powder was then collected, washed, and calcined at 500 °C for 2 h. Characterization The powder X-ray diffraction (XRD) analysis was carried out using a Rigaku TTR3 X-ray powder diffractometer with Cu Kα radiation (λ = 1.5406Å). The morphology of the samples was observed using a JEOL 7800 scanning electron microscope. TEM and HRTEM images were obtained with a Tecnai G2 F20 S-TWINT transmission electron microscope. The elements and oxidation state of the samples were analyzed by using a Kratos Axis Ultra DLD X-ray photoelectron spectrometer. The specific surface area was measured by the Brunauer-Emmett-Teller (BET) method based on the sorption isotherms obtained with a Quantachrome Autosorb-1 volumetric analyzer. A Varian 720 Inductively Coupled Plasma-Optical Emission Spectrometers (ICP-OES) was applied to analyze the compositions of CoxCu1-xMoO4. Catalytic experiments The catalytic experiments were carried out at 298 K if not specified. Catalytic reactions were performed in a water bath to control the temperature. In a typical process, 10.0 mg catalyst powder was added to a reaction vessel containing 5.0 mL water, which was then subjected to an ultrasonication treatment for 30 min. Subsequently, 15 mL of reaction solution containing AB (3.0 mmol) and NaOH (20.0 4


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mmol) was added into the vessel, which was connected to a gas burette to determine the volume of generated hydrogen. A certain amount of sodium hydroxide was added into the reaction system because it could reduce induction time of catalytic hydrolysis of AB and enhance the release of hydrogen from AB solution.24 For the purpose of investigating the reusability and stability of our catalyst, the hydrolytic dehydrogenation of AB using the same catalyst was repeated 5 times. When the previous run of the hydrolytic reaction was finished, the catalyst was isolated from the reaction solution and rinsed. Then, freshly prepared AB and NaOH solution was blended with the collected catalyst in the way described above, and a new run of hydrolysis was initiated.

Results and discussion Figure 1a shows the low-magnification SEM image of the Co0.8Cu0.2MoO4 sample, indicating that it consists of microspheres with a typical diameter of 3-5 µm. Figure 1b shows the morphology of a single Co0.8Cu0.2MoO4 microsphere, demonstrating that the microsphere is composed of numerous nanoplatelets with arrangement radiating from the center of the microsphere. These nanoplatelets with a uniform thickness of approximately 20 nm are not loosely connected, but closely cross-linked each other (Figure 1c). Note that there are numerous pores on the surface of the microsphere that are derived from the gaps among the nanoplatelets. In general, this type of architecture is desirable for a heterogeneous catalyst because it can provide more active sites for a catalytic reaction. The SEM images of other CoxCu1-xMoO4 samples with different compositions are displayed in Figure S1 in the SI (Supporting Information). As can be seen, Co0.95Cu0.05MoO4, Co0.9Cu0.1MoO4, Co0.6Cu0.4MoO4, and Co0.4Cu0.6MoO4 are also microspheres composed of nanoplatelets, which are very similar to those of Co0.8Cu0.2MoO4 in both morphology and size. However, 5



Co0.2Cu0.8MoO4 sample consists of aggregated nanobelts, which are morphologically different from the other CoxCu1-xMoO4 samples. These nanobelts are very long in size, up to 10 µm. It is difficult for these nanobelts to form microspheres with radiating architectures due to the large steric hindrance of the long nanobelts. Figure 1d shows the TEM image of a single microsphere, further confirming the architecture of the Co0.8Cu0.2MoO4 sample. Figure 1e shows the TEM image of part of the Co0.8Cu0.2MoO4 microsphere, in which the lateral morphology of the nanoplatelet can be seen. The thickness of the nanoplatelet is approximately 20 nm, which well matches the SEM results. As shown in the HRTEM image of Co0.8Cu0.2MoO4 in Figure 1f, the lattice spacing of 0.357 nm is between the distance of the (002) planes of monoclinic crystal phase of CoMoO4 (0.34 nm, JCPDS 21-0868) and triclinic crystal phase of CuMoO4 (0.38 nm, JCPDS 85-1529). This observation indicates that the doped Cu has changed the lattice spacing of CoMoO4. The compositions of these CoxCu1-xMoO4 are analyzed with ICP-OES and the molar ratios of Co, Cu and Mo are shown in Table S1 (See Supporting Information). Clearly, the corresponding molar ratios of different CoxCu1-xMoO4 are very close to the targeted values. Figure 2 displays the XRD patterns of CoxCu1-xMoO4 samples. It should be mentioned that there is no standard XRD pattern of CoxCu1-xMoO4 in the International Centre for Diffraction Data (ICDD). Thus, the standard XRD patterns of both CoMoO4 and CuMoO4 are also shown in Figure 2 for comparison. As can be seen, all the diffraction peaks of CoxCu1-xMoO4 samples well match standard patterns of CoMoO4 and CuMoO4. Compared with that of Co0.95Cu0.05MoO4, the highest peak in the XRD pattern of Co0.2Cu0.8MoO4 shifts negatively approximately 0.12o. This is understandable because increase the content of Cu will result in a small increase of the corresponding lattice spacing of CoxCu1-xMoO4 and thus cause negative shift of 6


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(a)

(b)

1 µm

10 µm (c)

(d)

100 nm

1 µm

(e)

(f)

0.357 nm

Figure 1 SEM images (a, b, c), TEM images (d, e) and HRTEM image (f) of Co0.8Cu0.2MoO4 sample. 7



the peak. Notably, it is difficult to determine whether CoMoO4 and CuMoO4 are presented as a compound (viz. CoxCu1-xMoO4) or they are a mixture based on their XRD patterns. Hence, the formation of a compound cannot be confirmed beyond doubt by the XRD results alone. For this reason, elemental mapping analysis is carried out to verify this. Figure 3 shows the corresponding element distribution of a single Co0.8Cu0.2MoO4 microsphere. It is observed that elements of Co, Cu, Mo and O are homogeneously distributed in the microsphere, demonstrating that the sample is a compound rather than a mixture of CoMoO4 and CuMoO4.

(a) ♦◊

♦ ◊

♦ ◊

♦ ◊

♦ ◊

♦ ◊

◊

(b)

♦ CuMoO4 ◊ CoMoO4

♦ ◊

◊ CoMoO4 ♦ CuMoO4

(A) Intensity (a.u.)

(B)

Intensity (a.u.)

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

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(C) (D) (E)

◊

♦ ◊

♦♦ ♦ ◊◊ ◊

♦ ◊

♦ ♦ ◊ ◊

♦ ◊

♦ ◊

◊

(F)

20

30

40

CoMoO4 JCPDS 21-0868

CoMoO4 JCPDS 21-0868

CuMoO4 JCPDS 85-1529

CuMoO4 JCPDS 85-1529

50 60 2Theta (degree)

70

80

20

30

40 50 2Theta (degree)

60

Figure 2 (a) XRD patterns of Co0.95Cu0.05MoO4 (A), Co0.9Cu0.1MoO4 (B), Co0.8Cu0.2MoO4 (C), Co0.6Cu0.4MoO4 (D), Co0.4Cu0.6MoO4 (E), and Co0.2Cu0.8MoO4 (F); (b) Enlarged XRD pattern of Co0.8Cu0.2MoO4.

8


70

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Co

Cu

Mo

O

Figure 3 Elemental mappings of a Co0.8Cu0.2MoO4 microsphere.

To determine the elements and their chemical states on the catalyst surface, XPS analysis of the Co0.8Cu0.2MoO4 sample was carried out. Figure 4a, 4c, and 4e display the XPS spectra of the Co0.8Cu0.2MoO4 sample before catalytic reaction. In the spectrum of Co2p (Figure 4a), in addition to two satellite peaks (denoted as “Sat.” in Figure 4), two peaks at 779.7 eV and 794.8 eV are observed, both of which can be indexed to the Co2+ state.25 In the magnified spectrum of Cu2p (Figure 4c), two peaks centered at 954.2 and 934.4 eV are observed. In addition, there are two satellite peaks at 962.0 and 941.8 eV. All these peak values are in good agreement with those of Cu2+ in previous report.26 In the spectrum of Mo3d (Figure 4e), there are two peaks centered at 232.6 and 235.7 eV, both of which are characteristic peaks of Mo(VI).27, 28 9



In summary, the XPS results demonstrates the coexistence of Co2+, Cu2+ and Mo6+ on the surface of Co0.8Cu0.2MoO4 microspheres before catalytic reaction.

Figure 4 XPS spectra of the Co0.8Cu0.2MoO4 sample in the Co2p, Cu2p and Mo3d regions before (a, c, e) and after (b, d, f) catalytic reaction.

The porous structure and BET surface area of the CoxCu1-xMoO4 samples were 10


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analyzed by N2 adsorption-desorption isotherms, and the results are displayed in Figure S2 and S3 in the Supporting Information. As shown, all six samples possess similar adsorption-desorption isotherms, which can be classified as type III isotherm according to International Union of Pure and Applied Chemistry (IUPAC) classification. This indicates the presence of the disorder and multi-scale pores in these samples, which is confirmed by the corresponding pore size distribution shown in the inset in Figure S2. The pores may be derived from the gaps among the nanoplatelets, which can be observed in the SEM image (Figure 1c). It is worth noting that all six samples show a small adsorption-desorption hysteresis at the relative pressure range of 0.7~1, which is indicative of the interstitial porosity in the samples. The catalytic activity of CoxCu1-xMoO4 towards the hydrolysis of AB was investigated, and Figure 5a shows the hydrogen evolution vs. reaction time in the presence of different CoxCu1-xMoO4 samples as catalysts. The molar ratio of generated hydrogen to AB can reach 3 for all these catalysts, indicative of a complete hydrolytic reaction. The turnover frequency (TOF) is often applied to evaluate the activity of a catalyst in AB hydrolysis. In this study, the TOF values for different catalysts can be calculated from the linear part of the plots in Figure 5a, which are depicted as a histogram in Figure 5b. Evidently, both CoMoO4 and CuMoO4 possess poor catalytic activity in AB hydrolysis. However, the catalytic activity is significantly improved when CoMoO4 and CuMoO4 are combined as compound catalysts. As shown in Figure 5b, the TOF values for CoxCu1-xMoO4 are significantly higher than the sum of the TOF values for CoMoO4 and CuMoO4. This confirms a synergistic effect exists between CoMoO4 and CuMoO4 in the CoxCu1-xMoO4. In our preliminary experiments, we found that an appropriate amount of NaOH in the reaction medium conduced to AB hydrolysis. As shown in Figure S4, the TOF values will rise at first and then 11



decrease slightly as the increase of NaOH concentration. The highest TOF value is achieved at NaOH concentration of 1 M. It should be mentioned that ammonia will be produced in basic medium via. the following reaction equation, NH4+ + OH- → NH3·H2O So, the hydrogen gas may be contaminated by ammonia gas. However, the collected gas can be easily purified by an acid solution or even water. 3.0

(a) n(H2) / n(AB)

3.0

2.5

n(H2) / n(AB)

2.5 2.0

2.0

1.5

1.5

Co0.95Cu0.05MoO4

CoMoO4 CuMoO4

1.0 0.5

Co0.9Cu0.1MoO4

1.0

0.0

Co0.8Cu0.2MoO4

0

20

40

60

80

100

120

Time (min)

Co0.6Cu0.4MoO4

0.5

Co0.4Cu0.6MoO4 0.0

Co0.2Cu0.8MoO4 0

5

10

15

20

25

30

Time (min) 60

(b) 50

Co0.8Cu0.2MoO4

Co0.9Cu0.1MoO4

Co0.95Cu0.05MoO4

2

Co0.6Cu0.4MoO4

1

0

Co0.4Cu0.6MoO4

10

Co0.2Cu0.8MoO4

20

CuMoO4

30

3

4

5

6

7

CoMoO4

40

-1

-1

TOF (molH2 min molcat. )

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

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8

Samples Figure 5 (a) Hydrogen evolution from AB solution in the presence of different catalysts; (b) comparison of TOF values comparison for different catalysts. 12


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There are two concerns regarding the catalytic activity of CoxCu1-xMoO4 that should be addressed. To the best of our knowledge, there has been no published report on the catalytic hydrolysis of AB with CoMoO4, CuMoO4, or CoxCu1-xMoO4 as a catalyst. Therefore, the first concern is the identification of the real active species in the catalysts. According to the literature, Co(II) and Cu(II) themselves are inactive to AB hydrolysis.29 However, AB in the hydrolysis system can also act as a reducing agent with mild reducibility and reduce Co(II) and Cu(II) to metallic Co and Cu, which exhibit catalytic activity in AB hydrolysis. This is confirmed by the XPS results for the used Co0.8Cu0.2MoO4 catalyst, in which the catalyst surface was found to be partially covered with Co(0) and Cu(0) (see Figure 4b and 4d). After reaction, the weight percentages of the Co(0) among the Co element and Cu(0) among the Cu element were approximately 35.8% and 52.9 %, respectively. Considering that elements Co and Cu are homogenously distributed in Co0.8Cu0.2MoO4 microspheres with the molar ratio of 4:1, the molar ratio of Co to Cu in CoCu alloy is approximately 2.7:1. The TEM, HRTEM images and XRD pattern of the catalyst after reaction are shown in Figure S5. As can be seen in Figure S5a, the architecture of microsphere is still maintained. Notably, some nanoparticles are observable on the nanoplatelet. In addition, the HRTEM image shows a lattice spacing of 0.213 nm, indexing to CoCu alloy structure.18 This observation further demonstrates the presence of CoCu alloy on the surface of the used catalyst. As shown in Figure S5c, characteristic peaks of CoCu alloy, besides those of Co0.8Cu0.2MoO4, are observable in the XRD pattern of the used catalyst. Generally speaking, XPS can only detect the elements on the top surface (at a depth of 5-10 nm) of a sample. Considering all these results together, it is rational to conclude that the real catalyst in the hydrolysis system is the Co0.8Cu0.2MoO4 microsphere supported CoCu alloy. The possible formation 13



process of the in situ generated supported catalyst is illustrated in Figure 6. In contrast to the unsupported metal or alloy catalysts, the supported counterparts often exhibit higher catalytic activity, which may result from the metal-support interaction in heterogeneous catalysis.30-32 It is noted in Figure 5a that a deviation from the linear dependence of the accumulated hydrogen volume with the reaction time is observed, which may result from the activity change of our catalyst. As discussed above, the active species are CoCu alloy and its content may change during the reaction. Considering the fact that it is difficult to determine accurate amount of CuCo alloy during the of hydrolysis reaction, the calculation of TOF values in this work is based on the mass of the CoxCu1-xMoO4 rather than that of CoCu alloy. The second concern is why a synergistic effect occurs between CoMoO4 and CuMoO4 in the CoxCu1-xMoO4. For the CoMoO4 sample, the reduction electrode potential of Co2+/Co is -0.28 V vs. the SHE (standard hydrogen electrode) and the reduction process of Co(II) with AB is very slow, which will affect the formation of active center and thus lower the overall hydrolytic reaction rate. Therefore, the catalytic activity of CoMoO4 is poor. However, although the reduction process of Cu(II) with AB is fast because of higher reduction electrode potential (0.337 V vs. SHE), Cu possesses significantly lower catalytic activity in contrast to Co.33, 34 Thus, the catalytic activity of CuMoO4 is also poor. However, when the CoxCu1-xMoO4 samples are used as catalysts, metallic Cu firstly formed on the surface of the catalysts by the reduction process can catalyze the reduction of Co(II),35 which is favorable for the formation of active CuCo alloy on the catalyst surface. It has been well demonstrated that bimetallic alloy nanoparticles have exhibited synergistic effect in many catalytic reactions.36 As far as our catalysts are concerned, the modification of the surface electronic structure and chemical properties of the Cu-Co nanoalloy through the strain and ligand effects between Cu 14


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and Co can improve the catalytic activity.37 In this study, the catalytic activity of the samples can be significantly enhanced by incorporating CuMoO4 into CoMoO4, and thus a synergistic effect is observed between CuMoO4 and CoMoO4 in CoxCu1-xMoO4.

Figure 6 An illustration of the possible formation process of an in situ generated supported catalyst.

Among these CoxCu1-xMoO4 samples, Co0.8Cu0.2MoO4 shows the highest activity in AB hydrolysis. It is known that many factors, such as BET surface, the morphology of nanostructures, the composition, and the sizes of the nanostructured heterogeneous catalyst, will influence their catalytic performance. As discussed above, except for Co0.2Cu0.8MoO4, these CoxCu1-xMoO4 samples have similar morphology and sizes. Therefore, the morphology and size are unlikely to be the key factor resulting in the activity difference among these CoxCu1-xMoO4 catalysts. As shown in Figure S3 in the Supporting Information, the BET surface area of CoxCu1-xMoO4 catalysts is in the range from 14.5 to 36.8 m2 g-1. Note that the TOF values of these catalysts are not directly proportional to the BET surface area. This suggests that there are other factors in addition to BET surface area that affects their catalytic activity. As far as our CoxCu1-xMoO4 catalysts are concerned, it is very likely that the composition plays a crucial role in determining their activity. As shown in Figure 5b, the catalytic activity 15



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of CoxCu1-xMoO4 will increase at first and then decrease as the x value increases. At x=0.8, the highest catalytic activity is achieved. As discussed above, metallic Co is more active than metallic Cu in AB hydrolysis, and the main active centers on the catalyst surface is Co, which is formed by the reduction of Co(II) with AB. Therefore, a higher content of Co in CoxCu1-xMoO4 will lead to more active sites on the surface of catalysts and thus improve their catalytic activity. However, when the content of Co is too high and the content of Cu is too low, it is likely that the reduction of Co(II) catalyzed by the metallic Cu slows down, which will retard the overall catalytic activity of CoxCu1-xMoO4. For comparison, the TOF values for our Co0.8Cu0.2MoO4 sample and some other non-noble-metal catalysts towards AB hydrolysis are listed in Table 1. As can be seen, our Co0.8Cu0.2MoO4 sample is one of the most active catalysts for AB hydrolysis in terms of TOF value. Table 1 Comparison of TOF value of some noble-metal-free catalysts towards AB hydrolysis. TOF (molhydrogenmin-1 molcat-1)

Catalysts

Reference

Ni/ZIF‑8

85.7

38

CoP

72.7

39

Cu0.8Co0.2O/Graphene oxide

70.0

40

Ni0.9Mo0.1/Graphene

66.7

41

Co0.8Cu0.2MoO4 microspheres

55.0

CuCo/diamine-functionalized

51.5

42

NiCo2O4/Ti

50.1

43

CuCo2O4(nanoplates)/Ti

44.0

44

CoP NA/Ti

42.8

45

Ni@3D-(N)GFs

41.7

46

Cu0.49Co0.51/C

28.7

18

Ni/CNTs

26.2

47

This work

reduced graphene oxide

16


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MnCo2O4/Ti

24.3

48

CuCo@MIL-101

19.6

49

CoNi/Graphene

16.4

50

Cotton-like CoB alloy

14.7

51

Co/Graphene

13.8

52

CoB nanowires

10.4

24

Cu/Co3O4

7.0

53

Co/Hydroxyapatite

5.8

54

Co@N-C

5.6

55

Skeletal Ni

5.3

56

Co/Intrazeolite

5.3

57

Cu/RGO

3.61

58

The influence of hydrolytic temperature on the hydrogen release from AB in the presence of Co0.8Cu0.2MoO4 as the catalyst was investigated and the results are shown in Figure S6a. As displayed, the hydrogen release accelerates as the temperature increases from 298 to 318 K. This observation is consistent with previously published literatures, in which high temperature was found to be favorable for fast hydrogen generation in AB hydrolysis.41 Figure S6b displays the plot of lnk vs. 1/T. The apparent activated energy of the AB hydrolysis catalyzed by Co0.8Cu0.2MoO4 is calculated to be 39.6 kJ mol-1. The recyclability and reusability of a heterogeneous catalyst are especially important in practical applications. Figure 7 shows the hydrogen release curves in different catalytic cycles when Co0.8Cu0.2MoO4 is used as a catalyst. Even in the 6th catalytic run, the molar ratio of hydrogen to AB is 3, demonstrating that the hydrolysis efficiency can still reach 100%. The hydrolysis rate in the 6th catalytic run only slightly decreases in contrast to that in the first run. This implies that our catalyst possesses good reusability. 17



3.0 2.5

n(H2)/n(AB)

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

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1st

3st

2st

4st

5st

6st

2.0 1.5 1.0 0.5 0.0 0

10

20

30

40

50

60

Time (min) Figure 7 Hydrogen evolutions at different catalytic cycles in the presence of Co0.8Cu0.2MoO4 as a catalyst.

CONCLUSIONS In summary, CoxCu1-xMoO4 microspheres composed of nanoplatelets were prepared by a facile approach free of any organics, such as surfactant, complexing agent and organic solvent. The catalytic activity of CoxCu1-xMoO4 in AB hydrolysis was investigated for the first time. It was found that a synergistic effect existed between CoMoO4 and CuMoO4 in the CoxCu1-xMoO4 samples towards AB hydrolysis. Among the CoxCu1-xMoO4 catalysts, Co0.8Cu0.2MoO4 shows the highest activity with a TOF value of 55.0 molH2 min−1 molcatalyst−1, which was one of the most active noble-metal-free catalysts towards AB hydrolysis. It is also revealed that the active species is CoCu alloy supported on the Co0.8Cu0.2MoO4 microspheres. In addition, the Co0.8Cu0.2MoO4 catalyst demonstrates good reusability, making it a robust catalyst towards AB hydrolysis for rapid hydrogen generation in practical applications.

ACKNOWLEDGEMENT This work was financially supported by the Natural Science Foundation of 18


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Guangdong Province (No. 2016A030313120), the Excellent Youth Foundation of the University in Guangdong Province (No. YQ2015154), Natural Science Foundation of Huizhou University (No. 20160226013501332) and the China-Ukraine Technology Park Platform Project (No. 2014C050012001).

SUPPORTING INFORMATION SEM images, N2 adsorption-desorption isotherms and the corresponding pore size distribution, ICP data, the comparison of BET surface areas of different samples, the hydrogen release curves at different temperatures, and the TEM/HRTEM images and the XRD pattern of the Co0.8Cu0.2MoO4 after reaction.

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27



Abstract Graphic

TOF = 55.0 molH2 min−1 molcatalyst−1

Co0.8Cu0.2MoO4 microspheres

Hydrogen

NH3BH3 solution

Synopsis: Co0.8Cu0.2MoO4 microspheres act as a low-cost and highly active catalyst in the hydrolysis of ammonia borane for fast hydrogen generation.

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Co0.8Cu0.2MoO4 Microspheres Composed of Nanoplatelets as a

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