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Article

Boosted Catalytic Activity toward the Hydrolysis of Ammonia Borane by Mixing Co- and Cu-Based Catalysts

by
Jinyun Liao
1,2,†,
Yujie Wu
1,†,
Yufa Feng
2,
Haotao Hu
2,
Lixuan Zhang
2,
Jingchun Qiu
2,
Junhao Li
1,
Quanbing Liu
1,* and
Hao Li
2,*
1
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China
2
Guangdong Provincial Key Laboratory for Electronic Functional Materials and Devices, School of Chemistry and Materials Engineering, Huizhou University, Huizhou 516007, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2022, 12(4), 426; https://doi.org/10.3390/catal12040426
Submission received: 15 February 2022 / Revised: 31 March 2022 / Accepted: 31 March 2022 / Published: 11 April 2022
(This article belongs to the Section Catalytic Materials)
Browse Figures
Versions Notes

Abstract

:
Promoting the activity of heterogeneous catalysts in the hydrolysis of ammonia borane for hydrogen production is still a challenging topic for researchers in the hydrogen energy and catalysis fields. Herein, we present a universal, simple and efficient strategy to boost the catalytic performance toward AB hydrolysis by mixing Co- and Cu-based catalysts for the first time. Synergistic catalysts with remarkably enhanced activity can be obtained by mixing a Co-based catalyst and a Cu-based catalyst together, such as Co3O4 and Cu3(MoO4)2(OH)2, Co3O4 and Cu3(PO4)2, Co3(PO4)2 and Cu3(MoO4)2(OH)2, Co3(PO4)2 and Cu3(PO4)2, and CuO and Co3O4. For example, the turnover frequency (TOF) for the mixture catalyst of Co3O4 and Cu3(MoO4)2(OH)2 with a mass ratio of 4:1 is up to 77.3 min−1, which is approximately 11.5 times higher than that of the sum of Co3O4 and Cu3(MoO4)2(OH)2. The reasons for these findings are discussed in detail. The observations and conclusions in this work may provide a guideline for promoting the hydrolysis of ammonia borane through a simple and effective approach.

1. Introduction

Energy crises and environmental issues have hindered economic development in the past several decades and continue to do so today; thus, it is urgently required to find a green and renewable energy source to substitute traditional fossil fuel energy sources with carbon-neutral sources [1,2]. Hydrogen energy is one of the best carbon-neutral alternatives to fossil fuel energy due to its high gravimetric energy density and zero pollution. However, how to transport and store hydrogen in an efficient and safe manner is a crucial issue that needs to be well-addressed before its large-scale applications. Therefore, many efforts have been made to develop strategies for hydrogen storage and transportation. Compared with the physical approach to hydrogen storage, chemical hydrogen storage is considered more effective and safer. Ammonia borane, with the formula NH3BH3, has attracted extensive attention because of its non-toxicity, high hydrogen content of 19.6 wt% and good stability under ambient temperature [3]. Additionally, due to its mild reaction conditions and environmental friendliness, hydrogen production from NH3BH3 using hydrolysis is more attractive compared with pyrolysis and solvolysis.
However, since NH3BH3 hydrolysis is kinetically unfeasible in the absence of a catalyst, a robust catalyst is indispensable for this hydrolytic reaction [4]. In the past, it was reported that noble-metal-based catalysts were quite active to NH3BH3 hydrolysis [5,6,7]. However, due to its high cost, it is difficult to use noble metals or their alloys as catalysts to NH3BH3 hydrolysis on an industrial scale. Meanwhile, the non-precious-metal-based catalysts have attracted great interest due to their low cost [8,9,10]. However, the activity of non-precious-metal-based catalysts is still not satisfactory. Therefore, in recent years, researchers have undertaken a serious effort to improve the activity of non-precious-metal-based catalysts, and many strategies have been proposed. Among them, there are three commonly used strategies to boost the activity of the catalysts, namely, dispersing the active species on proper supports, compositing different active species together to form hybrid catalysts and controlling the morphology of nanocatalysts. Taking advantage of the first strategy, Feng et al. successfully dispersed CuxCo1−xO nanoparticles on graphene, which remarkably enhanced the performance of the catalyst in promoting NH3BH3 hydrolysis with a turnover frequency (TOF) value of 70.0 min−1 [11]. They ascribed the enhancement of the catalytic activity of the hybrid to the interfacial interaction between the nanoparticles and the graphene. There are also some successful examples for the second strategy. In our previous work [12], we fabricated Co0.8Cu0.2MoO4 composite microspheres and found that their catalytic activity was much higher than that of CoMoO4 and CuMoO4, indicating that the reactivity of the catalyst could be remarkably improved by compositing CoMoO4 and CuMoO4. The third strategy is based on the effect of the morphology of nanocatalysts. For example, Yamada et al. have successfully synthesized Co3O4 consisting of particles, cube and sheet, and compared their catalytic activity toward AB hydrolysis [13]. Their results demonstrated that Co3O4 exhibits a morphology-dependent catalytic performance. By tuning the morphology, the activity of the catalyst can be significantly enhanced.
Although the above-mentioned strategies can be applied to obtain high-performance catalysts for AB hydrolysis, the synthetic processes are often tedious, costly, or both. Additionally, these catalysts must be intricately designed and synthesized to ensure their high reactivity. For example, nanocatalysts with different morphology may exhibit different catalytic behavior. However, it is not easy to predict which kind of shape-anisotropic nanocatalysts can exhibit the highest catalytic performance and how to design and synthesize them. Thus, although highly desirable, it is still challenging to develop an economical, simple and universal method to boost the catalytic activity of inexpensive catalysts that can be easily applied in practice on a large scale. In this study, we propose a simple method to produce low-cost mixture catalysts with remarkably boosted catalytic performance toward AB hydrolysis based on mixing Co- and Cu-based compounds. First, we successfully synthesized Co3O4, Co3(PO4)2, Cu3(PO4)2, CuO and Cu3(MoO4)2(OH)2. Then, we mixed these Co- and Cu-based compounds in an arbitrary, trial-and-error manner, and synergetic mixture catalysts were obtained in this way. In particular, by controlling the mass ratio of Co- and Cu-based compounds, high-performance catalysts can be obtained. Note that Co3(PO4)2 and Cu3(PO4)2 were selected because their hybrids exhibit good catalytic activity in AB hydrolysis [14]. As we know, there has not yet been a report describing the production of high-performance synergetic catalysts towards AB hydrolysis by physically mixing two low-cost compounds. These findings provide a guideline for promoting NH3BH3 hydrolysis by a simple and effective approach.

2. Results and Discussion

The SEM images and XRD patterns of Co3O4 and Cu3(MoO4)2(OH)2 are shown in Figure 1a–f. The SEM images of Co3O4 (Figure 1a,b) reveal that our sample of Co3O4 consists of uniform microspheres with a diameter of ca. 500–1000 nm. The magnified SEM image of the Co3O4 (Figure 1b) clearly shows that it is composed of numerous nanoparticles with a size of about 80 nm. The sample surface is very coarse, and thus has a large specific surface area. The XRD pattern of Co3O4, displayed in Figure 1e, shows all the characteristic peaks of Co3O4 (JCPDS 42-1467), and no other peaks associated with impurity appear, indicating that the sample is pure Co3O4. The morphology of Cu3(MoO4)2(OH)2, displayed in Figure 1c,d, reveals that the Cu3(MoO4)2(OH)2 sample consists of an aggregated nanoplate with a mean thickness of ca. 120 nm. The XRD pattern of Cu3(MoO4)2(OH)2, shown in Figure 1f, matches well with the standard pattern of Cu3(MoO4)2(OH)2 (JCPDS 36-0405). The SEM images and XRD patterns of Cu3(PO4)2·3H2O, CuO and Co3(PO4)2 are presented in Figure S2a–i in the Supplementary Materials. The SEM images clearly show that the sphere-shaped Co3(PO4)2 particles (Figure S2a,b) are uniformly distributed with a diameter of ca. 500 nm. The XRD pattern shows no obvious peaks of Co3(PO4)2 (Figure S2c), which indicates the obtained Co3(PO4)2 is in an amorphous state. In order to further confirm the structure of Co3(PO4)2, an FTIR spectroscopy analysis was performed on the sample. The result, shown in Figure S3, revealed two obvious peaks in the fingerprint region with wavenumbers of 590.3 and 1043.4 cm−1, which are indexed to the vibration of Co-O and (PO4)3− in Co3(PO4)2, respectively [15]. It is evident that the morphology of CuO is rod-shaped with an average diameter of 80 nm (Figure S2d,e), and such rod-shaped CuO particles are uniformly distributed without aggregation. The XRD pattern, shown in Figure S2f, indicates that the phase of our product is monoclinic CuO (JCPDS 45-0937). The microstructures of Cu3(PO4)2·3H2O, shown in Figure S2g,h, clearly indicate that the obtained Cu3(PO4)2·3H2O is uniformly distributed with a three-dimensional (3D) hierarchical flower-like porous structure, which is composed of numerous interconnected nanosheets. The magnified SEM image (Figure S2h) reveals that the thickness of those nanosheets is about 60 nm. The XRD pattern of the Cu3(PO4)2·3H2O sample, displayed in Figure S2i, matches well with the standard pattern of Cu3(PO4)2·3H2O (JCPDS 22-0548), indicating that our sample is Cu3(PO4)2·3H2O instead of anhydrous Cu3(PO4)2.
The TEM images of Co3O4 and Cu3(MoO4)2(OH)2, displayed in Figure 2, show that the size of the Co3O4 microsphere is about 700 nm, which coincides with the SEM result. In addition, the inner core of the Co3O4 microsphere has a hollow interior structure with a cage size of ca. 250 nm. The formation of the hollow structure may be related to the agglomeration of the nanoparticles. The nanocrystal subunits on the surface of Co3O4 particles are displayed in the HRTEM image (Figure 2b), with interplanar spaces of 0.24, 0.47 and 0.29 nm, which are associated with the (311), (111) and (220) lattice planes of cubic Co3O4, respectively. The TEM and HRTEM images of Cu3(MoO4)2(OH)2, displayed in Figure 2c,d, show that Cu3(MoO4)2(OH)2 has a nanoplate morphology with a smooth surface, and the thickness of the obtained product is about 120 nm. The lattice fringe spacing of the sample was determined to be about 0.43 nm and was ascribed to the (021) lattice plane of monoclinic Cu3(MoO4)2(OH)2. The functional groups in the Co3O4 and Cu3(MoO4)2(OH)2 catalysts were analyzed with FTIR. The spectra of Co3O4 and Cu3(MoO4)2(OH)2, displayed in Figure 2e, show absorption bands centered at 3429, 1639 and 2360 cm−1, which are ascribed to the symmetrical/antisymmetric stretching vibration of free water molecules adsorbed on the surface of the catalysts, the bending vibration of HO-H, and the vibration of adsorbed atmospheric CO2, respectively [16]. Additionally, in the fingerprint region of Co3O4, the two strong absorption bands centered at 663 and 567 cm−1 are related to the vibration of Co2+-O2− and Co3+-O2− [16]. In the spectrum of the Cu3(MoO4)2(OH)2, the absorption bands centered at 962.5 cm−1 are indexed to the ν1 vibration of distorted MoO4 in Cu3(MoO4)2(OH)2 [17]. In addition, the bands present at 916.2 and 821.6 cm−1 can be attributed to the symmetric stretching and asymmetric stretching vibrations of the Mo–O–Mo [18]. The band centered at 453.3 cm−1 is related to the Cu–OH vibration in Cu3(MoO4)2(OH)2 [19]. The FTIR spectra of CuO and Cu3(PO4)2 are presented in Figure S3. In the CuO FTIR spectrum, displayed in Figure S3b, the three strong bands appearing at 420.5, 532.3 and 603.7 cm−1 can be associated with the Cu–O vibration of CuO [20]. In the fingerprint region of the Cu3(PO4)2 FTIR spectrum, displayed in Figure S3c, the bands located at 1149 cm−1 can be attributed to the p = O stretching vibration. Additionally, the bands at 1047.3, 989.5 and 630.7 cm−1 are attributed to the vibration of the asymmetrical stretching of the P–O bond, the stretching vibration of the P–O bond and the bending vibration of the P–O bond, respectively. The absorption bands at 561.3 cm−1 are indexed to the in-plane bending vibration of the phosphate ion, and the bands present at 408.9 cm−1 are indexed to the vibration of the Cu–O band. The FTIR spectroscopy analysis result of the as-obtained Cu3(PO4)2 matches with that in the literature [21].
The N2 adsorption–desorption isotherms of Co3O4 and Cu3(MoO4)2(OH)2, shown in Figure 2f, clearly reveal that both compounds have a typical hysteresis loop, which exhibits a type IV isotherm, further confirming the mesoporous structure of the material. The as-synthesized Co3O4 possesses a large BET surface area of 99.0 m2/g, which may be related to the coarse surface and hollow microstructures of Co3O4. The Cu3(MoO4)2(OH)2 nanoplates have a relatively smaller BET surface than Co3O4, which is only 6.8 m2/g. For comparison, the N2 adsorption–desorption isotherms and BET surface data of CuO, Co3(PO4)2 and Cu3(PO4)2 are displayed in Figure S4.
XPS analysis of the catalysts was performed to know the electronic structure and valence of Co3O4 and Cu3(MoO4)2(OH)2 surface. The valance of the Co element in Co3O4 was further analyzed in Figure 2g, which reveals two major peaks at 794.7 and 779.5 eV, attributed to Co2p1/2 and Co2p3/2, respectively. Notably, the gap between the two peaks is 15.2 eV, which further establishes the typical characteristic of Co3O4 phase [22]. Two deconvoluted peaks with a binding energy at 796.1 and 794.6 eV are observable in the Co2p1/2 region, which can be assigned to Co2+ and Co3+, respectively. In the Co2p3/2 region, the deconvoluted peaks at 781.1 and 779.5 eV correspond to Co2+ and Co3+, respectively. The peaks at 804.2 and 789.1 eV are satellite peaks (Sat.) of Co2p [23]. All these peaks are in good agreement with a previous report [23]. Four peaks are observable in the XPS spectrum of Cu2p for Cu3(MoO4)2(OH)2 (Figure 2h). Two of them, marked as Sat. at 962.0 and 942.3 eV, are the satellite peaks of Cu2p [23]. The other two peaks, with a binding energy at 954.3 and 934.5 eV, are attributed to Cu2+ in the Cu2p1/2 and Cu2p3/2 regions, respectively [23]. In the XPS spectrum of Mo3d, the two peaks appearing at 235.5 and 232.3 eV can be assigned to Mo6+ [12].
The catalytic activity toward AB hydrolysis of those Co- and Cu-based catalysts, shown in Figure 3a, indicates that all the catalysts are active for NH3BH3 hydrolysis. Clearly, the Co-based compound catalysts exhibit higher activity than their Cu-based counterparts. It should be noted that, for the Co-based catalysts, there is an undesirable induction period (220 s for Co3O4, 130 s for Co3(PO4)2), when no hydrogen is produced. In contrast to Co-based catalysts, Cu-based catalysts can react with AB immediately, which is superior to Co-based catalysts in terms of the reduced induction time. However, Cu-based catalysts generally show poor activity compared to Co-based catalysts, which cannot be ignored when acting as catalysts. The TOF values, which represent the catalytic activity, are calculated and compared in Figure 3b. The TOF values follow the order of: Co3(PO4)2 > Co3O4 > Cu3(MoO4)2(OH)2 > Cu3(PO4)2·3H2O > CuO. Considering that the TOF value of Co3(PO4)2 is the highest among these catalysts, Co3(PO4)2 was characterized with XPS, EDS-mapping and H2-TPR, and the results are shown in Figure S5. Note that these five catalysts possess different surface areas (please see Figure 2f and Figure S4). If the BET surface areas are taken into consideration, the BET surface areas normalized catalytic activity following the order of: Cu3(MoO4)2(OH)2 > Co3(PO4)2 > Co3O4 > Cu3(PO4)2 > CuO.
The catalytic performance of all the single-component catalysts is not high in AB hydrolysis; thus, we tested the catalytic behavior of a mixture of Cu-based and Co-based compounds. To our surprise, an unexpected synergetic effect was observed when the mixture was used as a catalyst with an arbitrary mass ratio, as shown in Figure 3c and Figure S6a–h. Specifically, Figure 3c shows the hydrogen release curves when the mixture of Co3O4 and Cu3(MoO4)2(OH)2 was used as a catalyst at various mass ratios. These curves reveal that hydrogen generation is significantly improved in the presence of the mixture catalyst, without an induction period. The TOF values for the mixture catalysts of Co3O4 and Cu3(MoO4)2(OH)2 at various mass ratios are shown in Figure 3d. Importantly, the TOF value reaches 77.3 min−1 when the mass ratio of Co3O4 and Cu3(MoO4)2(OH)2 is 8:2, while those for single Co3O4 and Cu3(MoO4)2(OH)2 are only 7.5 and 3.7 min−1, respectively. The TOF value for the mixture is about 11.5 times higher than the sum of those for Co3O4 and Cu3(MoO4)2(OH)2 individually. The hydrogen release curves for other mixture catalysts (Co3O4 and Cu3(PO4)2·3H2O, Co3O4 and CuO, Co3(PO4)2 and Cu3(MoO4)2(OH)2, and Co3(PO4)2 and Cu3(PO4)2·3H2O) with different mass ratios are shown in Figure S6a–h. All these results indicate that catalytic activity can be markedly enhanced after the Co-based and Cu-based compounds are physically mixed. Figure 3e displayed the best TOF value, corresponding to the mixture catalysts of such Co-based and Cu-based compounds with their optimal ratios. For comparison, we list the TOF values of some non-noble-metal-based catalysts in Table 1, which show that our mixture catalyst is one of the most robust catalysts in terms of its high TOF value.
In an industrial scale application, a catalyst with both high stability and good reusability is highly desired. The H2 evolution at different catalytic cycles in the case of the mixture of Co3O4 and Cu3(MoO4)2(OH)2 with the mass ratio of 8:2 as the catalyst is displayed in Figure 3f. There is only a slight decrease in the activity for the mixture catalyst after five catalytic cycles. Moreover, the mole ratio of H2/AB was still 3, which is indicative of complete hydrolytic conversion. These observations suggested that the mixture catalysts have good reusability and relatively high stability. The SEM images of the used mixture catalyst are shown in Figure S7, indicating that the microspheres (Co3O4) and the nanoplate (Cu3(MoO4)2(OH)2) still exist in the sample. According to the following chemical equation: NH3BH3 + 2H2O → NH4+ + BO2− + 3H2, the by-product of BO2− will be adsorbed on the surface of the catalysts and occupy some active sites, thus causing the slight decrease in the successive catalytic tests.
In a previous report, Cu(II) and Co(II) themselves were found to be inert toward NH3BH3 hydrolysis [37]. However, NH3BH3 in a reaction medium could reduce Cu(II) and Co(II) to metallic Cu and Co, which are the real active species for AB hydrolysis. Considering the relatively high standard reduction potential of Cu2+/Cu (0.337 V vs. SHE) in contrast to that of Co3+/Co (-0.28 V vs. SHE), Cu(II) can be reduced more readily than Co(II) [38]. Therefore, it is understandable that Cu-based catalysts can initiate NH3BH3 hydrolysis instantly, while an induction time is needed in the case of Co-based catalysts. Nevertheless, according to a study reported by Xu [39], Co shows better catalytic performance in NH3BH3 dehydrogenation than Cu. So, the Co-based catalysts possess higher reactivity than Cu-based catalysts, as observed in Figure 3a.
As mentioned above, the formed Co(0) and Cu(0) reduced by AB are the real active species toward AB hydrolysis. To confirm the formation of Co(0) and Cu(0) in our catalyst, XPS analysis was performed on the used mixed catalyst of Co3O4 and Cu3(MoO4)2(OH)2. The results, presented in Figure 4a,b, show deconvoluted peaks at 793.8 and 778.6 eV in Co2p, as well as 952.9 and 932.9 eV in Cu2p, which correspond to Co(0) [40] and Cu(0) [23], respectively, thus demonstrating their formation. Accordingly, the formation rates of Co(0) and Cu(0) have a significant impact on the H2 production rate. In other words, the reducibility of Co- and Cu-based catalysts has an important effect on their catalytic activity toward AB hydrolysis. Therefore, the reducibility of Co3O4, Cu3(MoO4)2(OH)2 and their mixture were characterized by H2-TPR analysis. Figure 4c reveals that the TPR profile of Co3O4 has two evident hydrogen consumption peaks centered at 287.6 and 387.5 °C. The two peaks are associated with the reduction of Co3+ to Co2+ and Co2+ to Co0, respectively [41]. The chemical equation can be expressed as follows:
Co3O4 + H2 → 3CoO + H2O
CoO + H2 → Co + H2O
The TPR profile of Cu3(MoO4)2(OH)2 has three strong reduction peaks. The first peak at about 306.1 °C is attributed to a copper oxide reduction (Cu(II)→Cu(0)) [22], and the higher temperature centered at 388.7 °C is indexed to the reduction process of Mo(Ⅵ) to Mo(Ⅳ) [22]. As the temperature increases to 678.1 °C, a broad peak appears, which is ascribed to the reduction of Mo(Ⅳ) to Mo(0) [23]. The reduction process of Cu3(MoO4)2(OH)2 is expressed as follows:
CuMoO4(s) + H2(g)→Cu(s) + MoO3(s) + H2O(g)
MoO3(s) + H2(g)→MoO2(s) + H2O(g)
MoO2(s) + 2H2(g)→Mo(s) + 2H2O(g)
Remarkably, all the peaks in the TPR curve of the mixture of Co3O4 and Cu3(MoO4)2(OH)2 shifted to a lower temperature under similar conditions compared to Co3O4 and Cu3(MoO4)2(OH)2, which further confirms that the mixture can be more easily reduced. The decreased reduction temperature may be caused by the active intermediate Cu-H species, which is formed during the reduction of the Cu species by hydrogen. The active intermediate Cu-H possesses very strong reducibility [25]. When they come into contact with Cu3(MoO4)2(OH)2 or Co3O4, they can easily reduce Cu3(MoO4)2(OH)2 or Co3O4 to the counterparts with zero valence. Thus, the active metallic Cu and Co are formed and immediately act as catalysts to hydrolyze NH3BH3. In this case, the catalytic activity of the mixture is markedly improved.

3. Materials and Methods

3.1. Preparation of Samples

In the preparation of the Co3O4 hollow microsphere, a mixture solution of 2.0 mmol Co(NO3)2·6H2O (Aladdin, 99%) and 2.0 mmol potassium sodium tartrate tetrahydrate (C4H4O6KNa·4H2O, Taishan Yueqiao Reagent Plastic Co. Ltd., Jiangmen, China, >99%) were prepared in deionized water (40.0 mL) under stirring. Then, 40 mL of a 0.125 M NaHCO3 (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, ≥99.5%) solution was added into the above solution drop by drop. Stirring for half an hour, the mixed solution was poured into a Teflon-lined autoclave (100 mL), sealed and heated to 120 °C for 8 h. After the reaction, the final product was thoroughly rinsed with ethanol and deionized water and subsequently calcined at 500 °C for 2 h.
In the preparation of the Cu3(MoO4)2(OH)2 nanoplate, an aqueous solution (40 mL) containing 4 mmol CuCl2 (Taishan Yueqiao Reagent Plastic Co. Ltd., >99 %) was prepared. Subsequently, 40 mL of 0.1 M Na2MoO4 (Tianjin Damao Chemical Reagent Co. Ltd., Tianjin, China, >99%) solution was dropped into the CuCl2 solution. The resultant solution was hydrothermally heated at 120 °C for 8 h. The final product was flushed thoroughly with ethanol and deionized water. Finally, the sample was dehydrated in air at 80 °C for 6 h. The synthetic processes for Co3(PO4)2, Cu3(PO4)2, and CuO can be seen in the supporting information.

3.2. Characterization

Powder X-ray diffraction (XRD) patterns were obtained on a Rigaku Ultima IV diffractometer (Rigaku Corporation, Kyoto, Japan) with Cu Kα radiation (λ = 1.5406 Å at 40 kV and 40 mA). The morphology of the catalysts was investigated by scanning electron microscopy (SEM) using a Hitachi Su-8010 scanning electron microscope (Hitachi Ltd., Tokyo, Japan). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using an FEI Tecnai G2 F20 S-TWIN transmission electron microscope (FEI Company, Hillsboro, OR, USA). X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Kratos Axis Ultra DLD X-ray photoelectron spectrometer (Kratos Analytical Ltd., Manchester, UK) with Al Kα radiation. A Micromeritics ASAP 2020 nitrogen adsorption analyzer (Micromeritics Instruments, Norcross, GA, USA) was used to obtain the nitrogen adsorption–desorption isotherms and the Brunauer–Emmett–Teller (BET) surface areas of the samples. Fourier transform infrared (FTIR) spectroscopy analysis was performed on a Bruker Tensor 27 FTIR spectrometer (Bruker Optik GmbH, Ettlingen, Germany) to obtain the FTIR spectra of the samples.

3.3. Hydrogen Production Experiments

The hydrogen production experiment was performed in a round-bottom flask sealed and connected to a glass burette. Unless specified, the catalytic activity was measured at 25 °C. The reaction flask was placed in a water bath to maintain the temperature at 25 °C under an ambient atmosphere. Typically, the catalyst (10.0 mg) was ultrasonically dispersed in deionized water (10 mL). Afterwards, a 10 mL mixed solution of NH3BH3 (0.3 M) and NaOH (2 M) was quickly injected into the round-bottom flask. The reaction time counting was started when the first bubble appeared. The volume of the generated gas could be monitored by recording the displacement of water in the gas burette. The experimental set-up for hydrolysis can be seen in Figure S1 in the Supplementary Materials.

4. Conclusions

In this study, a universal, simple and efficient strategy for enhancing catalytic activity toward AB hydrolysis is proposed. By simply mixing Co- and Cu-based compounds, synergistic catalysts with markedly enhanced catalytic activity can be obtained. We evaluated the catalytic performance of five couples of catalysts in AB hydrolysis. The results revealed that the activity of the mixture catalysts was much higher than the sum of that of the activity of the individual compounds. In particular, the mixture of Cu3(MoO4)2(OH)2 and Co3O4 at the mass ratio of 8:2 exhibited a robust catalytic performance with a TOF of 77.3 min−1, which is one of the most active non-precious-metal-based catalysts ever reported in the literature. The reason for the enhancement of the activity of the mixture catalysts is believed to be closely related to their enhanced reducibility. The findings in this work provide a guideline for promoting NH3BH3 hydrolysis through a simple and effective approach.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12040426/s1, Preparation of Co3(PO4)2, Cu3(PO4)2 and CuO; Figure S1: Illustration of the set-up for the hydrolysis experiments; Figure S2: SEM images of Co3(PO4)2 (a,b), CuO (d,e) and Cu3(PO4)2·3H2O (g,h), and XRD patterns of Co3(PO4)2 (c), CuO (f) and Cu3(PO4)2·3H2O (i); Figure S3: FT-IR spectra of Co3(PO4)2 (a), CuO (b) and Cu3(PO4)2 (c); Figure S4: Catalytic performance of mixtures of Co- and Cu-based catalysts with various mass ratios and the comparison of their TOF values: Co3O4 and Cu3(PO4)2 (a,b), Co3(PO4)2 and Cu3(MoO4)2(OH) (c,d), Co3(PO4)2 and Cu3(PO4)2 (e,f), Co3O4 and CuO (g,h); Figure S5: XPS spectra in Co 2p and P 2p regions (a,b), elemental mapping (c) and H2-TPR profile (d) of Co3(PO4)2; Figure S6: Catalytic performance of mixture of Co- and Cu-based catalyst with various mass ratio and the comparison of their TOF Value: Co3O4 and Cu3(PO4)2 (a,b), Co3(PO4)2 and Cu3(MoO4)2(OH) (c,d), Co3(PO4)2 and Cu3(PO4)2 (e,f), Co3O4 and CuO (g,h); Figure S7: Low-magnification (a) and high-magnification (b) SEM images of the catalysts after 5th catalytic run. The Co3O4 microspheres and Cu3(MoO4)2(OH)2 nanoplates in Figure S7b are marked with blue circles and yellow rectangles, respectively.

Author Contributions

Conceptualization, J.L. (Jinyun Liao), Y.W. and Y.F.; methodology, J.L. (Jinyun Liao), Y.W., H.H. and L.Z.; investigation, J.L. (Jinyun Liao), Y.W., H.H., L.Z. and J.Q.; data curation, J.L. (Junhao Li) and Y.W.; writing—original draft preparation, J.L. (Jinyun Liao); writing—review and editing, Q.L. and H.L.; supervision, Q.L.; project administration, H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Key-Area Research and Development Program of Guangdong Province (No. 2020B090919005), the National Natural Science Foundation of China (Nos. U1801257, 21975056), the Major and Special Project in the Field of Intelligent Manufacturing of the Universities in Guangdong Province (No. 2020ZDZX2067), Pearl River Science and Technology New Star Project (No. 201806010039), the Science & Technology project of Huizhou City (No. 2020SD0404032), the Natural Science Foundation of Huizhou University (No. 20180927172750326), and the Professorial and Doctoral Scientific Research Foundation of Huizhou University (Nos. 2018JB036, 2019JB023 and 2019JB034).

Data Availability Statement

The data presented in this study are available in Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Catalysts 12 00426 g001 550
Figure 1. SEM images (a–d), and XRD pattern (e) of Co3O4, and XRD pattern (f) of Cu3(MoO4)2(OH)2.
Figure 1. SEM images (a–d), and XRD pattern (e) of Co3O4, and XRD pattern (f) of Cu3(MoO4)2(OH)2.
Catalysts 12 00426 g001
Catalysts 12 00426 g002 550
Figure 2. TEM and high-resolution TEM images of Co3O4 (a,b) and Cu3(MoO4)2(OH)2 (c,d); FT-IR spectra of Co3O4 and Cu3(MoO4)2(OH)2 (e); N2 adsorption–desorption isotherms of Co3O4 and Cu3(MoO4)2(OH)2 (f); high-resolution XPS spectra of Co2p (g) of Co3O4 and Cu2p (h), Mo3d (i) of Cu3(MoO4)2(OH)2.
Figure 2. TEM and high-resolution TEM images of Co3O4 (a,b) and Cu3(MoO4)2(OH)2 (c,d); FT-IR spectra of Co3O4 and Cu3(MoO4)2(OH)2 (e); N2 adsorption–desorption isotherms of Co3O4 and Cu3(MoO4)2(OH)2 (f); high-resolution XPS spectra of Co2p (g) of Co3O4 and Cu2p (h), Mo3d (i) of Cu3(MoO4)2(OH)2.
Catalysts 12 00426 g002
Catalysts 12 00426 g003 550
Figure 3. Catalytic activity of single Co-based and Cu-based catalysts (a) and their corresponding TOF values (b); catalytic performance of Co3O4 and Cu3(MoO4)2(OH)2 at various mass ratios (c) and their corresponding TOF values (d); the best activity of mixture catalysts of such Co-based and Cu-based compounds with their optimal mass ratios (e) and the reusability of the mixture catalyst of Co3O4 and Cu3(MoO4)2(OH)2 with a mass ratio of 8:2 (f).
Figure 3. Catalytic activity of single Co-based and Cu-based catalysts (a) and their corresponding TOF values (b); catalytic performance of Co3O4 and Cu3(MoO4)2(OH)2 at various mass ratios (c) and their corresponding TOF values (d); the best activity of mixture catalysts of such Co-based and Cu-based compounds with their optimal mass ratios (e) and the reusability of the mixture catalyst of Co3O4 and Cu3(MoO4)2(OH)2 with a mass ratio of 8:2 (f).
Catalysts 12 00426 g003
Catalysts 12 00426 g004 550
Figure 4. XPS spectra of used mixed catalyst of Co3O4 and Cu3(MoO4)2(OH)2 (a,b) and H2-TPR curves of Co3O4 (I), Cu3(MoO4)2(OH)2 (II) and their mixture (III) (c).
Figure 4. XPS spectra of used mixed catalyst of Co3O4 and Cu3(MoO4)2(OH)2 (a,b) and H2-TPR curves of Co3O4 (I), Cu3(MoO4)2(OH)2 (II) and their mixture (III) (c).
Catalysts 12 00426 g004
Table
Table 1. TOF values of some representative non-noble-metal-based catalysts toward AB hydrolysis.
Table 1. TOF values of some representative non-noble-metal-based catalysts toward AB hydrolysis.
CatalystsTOF(molH2·molcat.−1·min−1)ConditionsRef.
Cu0.6Ni0.4Co2O4 nanowires119.5T = 298 K, nAB = 3 mmol, Mcat. = 5 mg[24]
CoxCu1−xCo2O4@CoyCu1−yCo2O4 yolk–shell microspheres81.8T = 298 K, nAB = 3 mmol, Mcat. = 10 mg[25]
Ni0.5Cu0.5Co2O4 nanoplatelets80.2T = 298 K, nAB = 3 mmol, Mcat. = 5 mg[16]
Co3O4 and Cu3(MoO4)2(OH)2 mixture77.3T = 298 K, nAB = 3 mmol, Mcat. = 10 mgThis work
CuCo2O4 nanoplatelets73.4T = 298 K, nAB = 2.6 mmol, Mcat. = 10 mg[26]
Cu0.8Ni0.1Co0.1@MIL–10172.1T = 298 K, nAB = 1 mmol, Mcat. = 50 mg[27]
Cu0.2Co0.8 nanoparticles/RGO50.6T = 298 K, nAB = 1.3 mmol, ncat. = 0.065 mmol[28]
Cu@MoCo core–shell nanoparticles49.6T = 298 K, nAB = 2 mmol, ncat. = 0.08 mmol[29]
Cu(OH)2@Co2CO3(OH)2/CF39.72T = 298 K, nAB = 0.63 mmol, ncat. = 0.0055 mmol[30]
Cu-Co/PDDA–HNTs30.8T = 298 K, nAB = 1.04 mmol, Vcat. = 4 ml[31]
CoCu/Ni foam30.5T = 298 K, nAB = 2.6 mmol,
Scat. = 5 × 3 cm		[32]
CuCo19.6T = 298 K, nAB = 1 mmol, Mcat. = 100 mg[33]
Co–Cu NWs6.17T = 298 K, nAB = 0.76 mmol, Mcat. = 5 mg[34]
Co@N–C–7005.6T = 298 K, nAB = 1.3 mmol, Mcat. = 20 mg[35]
Co0.52Cu0.483.4T = 298 K, nAB = 0.16 mmol, Mcat. = 29.7 mg[36]
Cu/SiO23.24T = 298 K, nAB = 1 mmol, ncat. = 0.09 mmol[10]
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Liao, J.; Wu, Y.; Feng, Y.; Hu, H.; Zhang, L.; Qiu, J.; Li, J.; Liu, Q.; Li, H. Boosted Catalytic Activity toward the Hydrolysis of Ammonia Borane by Mixing Co- and Cu-Based Catalysts. Catalysts 2022, 12, 426. https://doi.org/10.3390/catal12040426

AMA Style

Liao J, Wu Y, Feng Y, Hu H, Zhang L, Qiu J, Li J, Liu Q, Li H. Boosted Catalytic Activity toward the Hydrolysis of Ammonia Borane by Mixing Co- and Cu-Based Catalysts. Catalysts. 2022; 12(4):426. https://doi.org/10.3390/catal12040426

Chicago/Turabian Style

Liao, Jinyun, Yujie Wu, Yufa Feng, Haotao Hu, Lixuan Zhang, Jingchun Qiu, Junhao Li, Quanbing Liu, and Hao Li. 2022. "Boosted Catalytic Activity toward the Hydrolysis of Ammonia Borane by Mixing Co- and Cu-Based Catalysts" Catalysts 12, no. 4: 426. https://doi.org/10.3390/catal12040426

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