English

• 0.   Introduction

  Hydrogen (H₂) evolution from ammonia borane (NH₃BH₃, AB) solution has attracted significant attention because it is a safe, convenient, and efficient H₂ feeding process for fuel cells, representing an advanced direction for a future sustainable society^[1-4]. As a class of important hydrogen storage materials, AB has a theoretical hydrogen content of 19.6 %, and can be easily transported to a far place at a low cost. The hydrogen stored in AB can be released through a dehydrogenation reaction, which has broad potential applications in the future, including hydrogen fuel cell vehicles, portable energy equipment, distributed energy systems, and industrial hydrogen sources. In this regard, platinum (Pt) has been widely investigated as an active catalyst for AB dehydrogenation, including hydrolysis (NH₃BH₃+2H₂O→NH₄BO₂+3H₂) and methanolysis (NH₃BH₃+4CH₃OH→NH₄B(OCH₃)₄+3H₂)^[5-6]. However, the practical application of AB-based H₂ feeding systems is restricted by the natural scarcity and high cost of Pt. Downsizing bulk Pt to nanoscale or even coupling Pt with a non-noble transition metal is regarded as the promising strategy to reduce Pt usage and make Pt-containing catalysts more active on account of the reasons: (a) metals can expose more active atoms on their surfaces by downsizing to nanoscale (in particular, the size of cluster is below 2 nm) compared to their bulk counterpart; (b) the coupling of Pt with a second transition metal can change its electronic structure and introduce a strong synergistic effect, leading to an enhanced catalytic activity.

  Ultrafine metal nanoclusters (MNCs) are facing a great synthetic change, as they are prone to aggregation and growth owing to their high surface energy^[7-10]. In addition, the phase separation in primary bimetallic NCs during the synthetic process usually leads to a poor catalytic performance^[11-15]. In recent years, metal-organic frameworks (MOFs), a new type of porous crystallization materials, have been demonstrated as excellent hosts for fabricating highly dispersed metal nanoparticles (MNPs)^[16-20]. MOFs have been developed for synthesizing carbon-supported MNPs by thermal transformation, which greatly extends the application scope of MOFs owing to the high thermal stability and conductivity of carbon, as well as the ultra-stable MNPs^[21-23]. However, to the best of our knowledge, there are rare reports^[24-25] on the downsizing of bulk Pt to clusters while coupling it to a second transition metal based on MOF pyrolysis.

  Herein, we report the immobilization of ultrafine, highly dispersed Pt clusters on the surface of Co NP/ porous carbon support (Pt-on-Co/C) by using an ingenious strategy of the one-step pyrolysis of Pt(Ⅱ)-encapsulating Co-MOF-74. The spatial location and amount of Pt precursors are critical to the success of such a strategy. Owing to the small size effect, particularly the enhanced synergistic interaction between Pt and Co atoms, the resultant Pt-on-Co/C400 catalysts exhibit remarkably high catalytic activity and stability for the hydrolytic dehydrogenation of AB, achieving a maximum turnover frequency (TOF) of 3 022 min^-1 under 303 K.

  1.   Experimental

  All chemicals and characterizations used were listed in the Supporting information.

  1.1   Synthesis of Co-MOF-74

  A tetrahydrofuran (THF) solution of 2, 5-dihydroxyterephthalic acid (7.5 mmol, 25 mL) and an aqueous solution of cobalt(Ⅱ) acetate (15 mmol, 25 mL) were mixed under stirring at room temperature until they became homogeneous. The resulting solution was transferred into a 100 mL Teflon-lined autoclave and heated at 110 ℃ in a preheated oven for 72 h. After cooling to room temperature, the product (Co-MOF-74) was collected by centrifugation, washed with deionized water and methanol three times. The Co-MOF-74 was activated by drying under vacuum at 150 ℃ for 24 h.

  1.2   Synthesis of Pt(Ⅱ)@Co-MOF-74

  200 mg of activated Co-MOF-74 was dispersed into 50 mL of dry n-hexane under sonication for 1 h. Then, an aqueous K₂PtCl₄ solution (66 μL) with the desired concentration (0.3 mol·L^-1) as a hydrophilic solvent was added dropwise over 30 min with vigorous stirring. The mixture was further stirred continuously for another 3 h. Finally, the hexane was removed, and the product (Pt(Ⅱ)@Co-MOF-74) was dried in a vacuum at 298 K for 24 h.

  1.3   Synthesis of Pt(Ⅱ)/Co-MOF-74

  200 mg of activated Co-MOF-74 was dispersed in 50 mL of water under sonication for 1 h. An aqueous solution of 0.3 mol·L^-1 K₂PtCl₄ (66 μL) was added to the above dispersion and stirred at 298 K for 3 h. The solid materials (Pt(Ⅱ)/Co-MOF-74) were collected by centrifugation and dried under vacuum at 298 K for 24 h.

  1.4   Synthesis of the Pt-on-Co/C400 catalysts

  200 mg of Pt(Ⅱ)@Co-MOF-74 was transferred into a ceramic boat and placed into a temperature-programmed furnace. The materials were heated at 400 ℃ for 6 h under Ar gas with a heating rate of 2 ℃·min^-1 and then allowed to cool naturally. The resulting black powders (Pt-on-Co/CT, T (K) was the heating temperature, here was 400) were washed with water and methanol, and dried in a vacuum oven at 298 K. In addition, Pt(Ⅱ)@Co-MOF-74 was annealed at other temperatures (300, 350, 450, and 500 ℃) under analogous conditions. The resulting materials, denoted as Pt-on-Co/C300, Pt-on-Co/C350, Pt-on-Co/C450, and Pt-on-Co/C500, respectively.

  1.5   Synthesis of Pt/C400

  30 mg of Pt-on-Co/C400 was dispersed in 20 mL of aqueous H₃PO₄ (0.5 mL) solution and stirred at 298 K for 2 h. The resulting materials (Pt/C400) were collected by centrifugation, washed with water, and dried under vacuum at 298 K for 24 h.

  2.   Results and discussion

  Co-MOF-74 was chosen as the precursor, which can be readily synthesized by the reaction of cobalt(Ⅱ) acetate and 2, 5-dihydroxyterephthalic acid in a water/THF mixture under hydrothermal conditions (Fig.S1). The Pt-on-Co/C was synthesized using the one-step pyrolysis of Pt(Ⅱ)-encapsulating Co-MOF-74, as illustrated in Fig. 1. Firstly, according to the stoichiometry and double solvents approach (DSA)^[26], a small amount of hydrophilic aqueous solution containing K₂PtCl₄ was added dropwise into a large amount of hydrophobic solvent hexane containing activated/dispersed Co-MOF-74. Then the above mixture was continuously stirred for another 3 h, and Pt precursors were adsorbed into the hydrophilic pores of Co-MOF-74 to form the Pt(Ⅱ)@Co-MOF-74 composites, driven by capillary force. Finally, the Pt(Ⅱ)@Co-MOF-74 composites were heated for 6 h, and the Pt precursors would be in-situ reduced to Pt clusters (small blue ball in Fig. 1) and be anchored onto the surfaces of Co NPs (big yellow ball in Fig. 1) derived from Co-nodes of Co-MOF-74 and porous carbons derived from the ligands of Co-MOF-74 (Pt-on-Co/CT). During the heat treatment, the organic ligands of Pt(Ⅱ)@Co-MOF-74 precursors can be decomposed, releasing Co and Pt species. Due to the fast mobility of Pt atoms and their good affinity with Co, the surfaces of Co NPs preferentially absorbed Pt atoms to form Pt clusters. In addition, Co NPs can provide active sites to reduce the activation energy of Pt reduction, and then accelerate the deposition of Pt atoms.

  Figure 1

  

  Figure 1.  Schematic illustration of the synthesis of Pt(Ⅱ)@Co-MOF-74 and Pt-on-Co/C400

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  The crystallinity and structural integrity of Pt(Ⅱ)@Co-MOF-74 were confirmed by powder X-ray diffraction (PXRD) and scanning electron microscope (SEM) measurements. As shown in Fig.S2 and S3, Pt(Ⅱ)@Co-MOF-74 samples showed similar characteristic peaks and morphologies to pure Co-MOF-74 (Fig.S1). For the N₂ adsorption-desorption isotherms of pure Co-MOF-74, a Brunauer-Emmett-Teller (BET) surface area of 1 163 m²·g^-1 was obtained (Fig.S4). However, a significant decrease was shown in N₂ uptake, surface area, and pore volume of Pt(Ⅱ)@Co-MOF-74 samples, indicating that the pores of Co-MOF-74 were occupied by the encapsulation of Pt(Ⅱ) ions (Fig.S4). Therefore, it can be confirmed that Pt precursors were successfully encapsulated into the pores of MOFs by using the DSA.

  Pt(Ⅱ)@Co-MOF-74 precursors were further annealed to form Pt clusters. Thermo-gravimetric analyses (TGA) reveal that Co-MOF-74 was thermally stable up to 500 ℃ (Fig.S5). However, if Pt precursor was introduced into Co-MOF-74, the as-synthesized Pt(Ⅱ)@Co-MOF-74 quickly decomposed when the temperature increased to 400 ℃ (Fig.S6), probably due to the Pt catalysis. If the heating time increased to 3 h at 400 ℃, most of Pt(Ⅱ)@Co-MOF-74 was decomposed to form Pt-on-Co/C composite (Fig.S7). Thus, an optimal annealing temperature was selected as 400 ℃. Under this condition, these Pt ions inside the pores of Co-MOF-74 can be reduced to Pt clusters while Co nodes of MOF-74 can be reduced to Co NPs under an Ar atmosphere via electron transfer from ligand to metal center, which was confirmed by the PXRD measurements. As shown in Fig. 2a, the Pt-on-Co/C300 showed analogous characteristic peaks to Co-MOF-74. With the temperature increased to 350 ℃, the characteristic peaks corresponding to Co-MOF-74 disappeared, while the characteristic peaks corresponding to hexagonal close-packed Co NPs appeared, indicating the complete transformation of MOF to metal/carbon. There was a weak characteristic peak at 40° for Pt clusters owing to their ultrasmall sizes. The appearance of separate peaks for both the Co and Pt attests to the formation of Pt clusters on Co NPs rather than Pt-Co alloys. The further increase in heat temperatures resulted in the formation of larger Co and Pt NPs (Fig. 2a). Such observations can be further confirmed by Fourier-transform infrared spectroscopy (FTIR) spectra. As shown in Fig. 2b, the appearance of coordination of Co-MOF-74 indicated the existence of MOF architecture in the catalysts obtained at 300 and 350 ℃. Further heat treatments led to the disappearance of the MOF structure, indicating the complete conversion of Co-MOF-74 to metal/carbon composites. Raman analyses of Pt-on-Co/C400 showed a clear D band at 1 299 cm^-1 and G band at 1 563 cm^-1 with I_D/I_G (where I_D and I_G are the D-band and G-band Raman intensities) ratio of 1.03, indicating the formation of graphitic carbon at 400 ℃ (Fig. 2c). N₂ adsorption-desorption isotherm indicated that the porous structures of Co-MOF-74 collapsed under the heating at 350 ℃, and there were no clear changes in their specific surface area owing to the existence of a large amount of Co NPs (Fig. 2d and S8). The mass fractions of Pt of 3.8% and Co of 59.2% in the obtained Pt-on-Co/C400 were measured by the inductively coupled plasma (ICP) analysis. Notably, the control experiments that, the direct pyrolysis of Co-MOF-74, Pt/Co-MOF-74 and Pt_0.01@Co-MOF-74 (by encapsulating 0.01 mmol of Pt precursor within Co-MOF-74) at 400 ℃, were performed to prepare Pt clusters on Co NP/porous carbon support. All the resultant samples were not completely carbonized, as illustrated by the PXRD measurements (Fig.S9-S11), indicating that the encapsulation of an appropriate amount of Pt precursor inside the pores of Co-MOF-74 is the key factor for anchoring Pt clusters on Co NPs and the carbonization process.

  Figure 2

  

  Figure 2.  PXRD patterns and (b) FTIR spectra of Pt(Ⅱ)@Co-MOF-74, Pt-on-Co/C300, Pt-on-Co/C350, Pt-on-Co/C400, and Pt-on-Co/C450 samples; (c) Raman spectrum and (d) N₂ adsorption-desorption isotherm of Pt-on-Co/C400

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  Transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) techniques were performed to better analyse the structure of Pt-on-Co/C400, particularly, the spatial relationship between Pt and Co. As shown in Fig. 3a and S12, Co NPs and Pt clusters were well dispersed with an average particle size of 20.80 and 0.72 nm, respectively. The elemental mapping images showed that ultrasmall Pt clusters were uniformly distributed on the surface of Co NPs (Fig. 3b-3f). In addition, a portion of the formed Pt clusters was immobilized on the porous carbon support. X-ray photoelectron spectroscopy (XPS) analysis (Fig.S13) was performed to identify the chemical states of Co and Pt in Pt-on-Co/C400. The peaks at 792.19 and 777.05 eV are ascribed to the Co2p_3/2 and 2p_1/2 levels of Co⁰, while successful formation of Pt⁰ clusters is confirmed by the signals at 74.28 and 70.59 eV, corresponding to Pt4f_5/2 and 4f_7/2, respectively.

  Figure 3

  

  Figure 3.  TEM image, (b) HAADF-STEM image, and (c-f) the corresponding elemental mappings of Pt-on-Co/C400

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  Finally, the obtained Pt-on-Co/C samples were investigated for hydrogen generation from AB in aqueous solution. The molar ratio of Pt and AB (n_Pt/n_AB) was fixed at 0.002∶1 for all the catalytic reactions. Under our evaluation conditions, the Pt-on-Co/C400 catalyst with Pt loading 3.8% exhibited excellent activity for AB hydrolysis with an extremely high turnover frequency (TOF) of 3 022 min^-1 at 303 K (Fig. 4a and 4b), which was superior to most of the reported catalysts (Table S2)^[27-31]. In contrast, the Pt-on-Co/C300, Pt-on-Co/C350, Pt-on-Co/C450, and Pt-on-Co/C500 catalysts showed lower catalytic activities toward AB dehydrogenation (Fig. 4a and 4b). In addition, the control samples, including the pure Co/C400 derived from the pyrolysis of pristine Co-MOF-74, the PtCo/C400 derived from the pyrolysis of Pt(Ⅱ)/Co-MOF-74, and Pt/C400 derived from the etch of Co in Pt-on-Co/C400, were observed for the poor catalytic activities (Fig.S14-S16). It can be concluded that the one-step pyrolysis of Pt(Ⅱ)-encapsulating Co-MOF-74 is important for anchoring Pt clusters on Co particle surfaces. The synergistic interactions between Pt clusters and Co NPs strongly promote the AB dehydrogenation: (a) Co NPs provide a large number of sites for anchoring highly dispersed Pt clusters, and then Pt clusters expose more active sites for catalytic reactions; (b) the atomic arrangement at the Pt-Co interface may form unique active sites and reduce the reaction energy barrier of AB dehydrogenation; (c) the electronic structure of Pt can be changed by the Co interfaces due to the its low work function, causing the increase in the adsorption strength of Pt on reactants.

  Figure 4

  

  Figure 4.  (a) Plots of time vs volume of the hydrogen evolved on hydrolysis of AB (2.0 mmol) at 303 K catalysed by Pt-on-Co/CT and (b) the corresponding TOF values; (c) Plots of time vs volume of the hydrogen evolved on hydrolysis of AB (2.0 mmol) catalysed by Pt-on-Co/C400 at different temperatures and (d) the corresponding Arrhenius plot (ln TOF vs 1 000/T); (e) Durability test for AB hydrolysis over Pt-on-Co/C400 at 303 K for 10 cycles

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  The catalytic hydrolysis of AB over Pt-on-Co/C400 at different temperatures (293-313 K) was also performed. Fig. 4c indicates that the reactions were completed within 1.67, 0.67, 0.50, 0.41, and 0.33 min at 293, 298, 303, 308, and 313 K, respectively, with enormously high TOF values (Fig.S17). The Arrhenius plot fitted on ln TOF vs 1 000/T yielded activation energy (E_a) of 55.6 kJ·mol^-1 (Fig. 4d). Moreover, the durability of Pt-on-Co/C400 was checked by the successive addition of aqueous AB solution into the reaction mixture after completion of one cycle. The reactivity of the catalyst remained almost unchanged after 10 cycles (Fig. 4e). The XRD analyses of Pt-on-Co/C400 after 10 cycles of catalytic reaction revealed no significant changes in the size, distribution, and structure of the catalyst, indicating the high stability and durability of the catalyst (Fig.S18).

  3.   Conclusions

  In summary, for the first time, ultrasmall Pt NCs were anchored on the surface of Co NPs by one-step pyrolysis of Pt(Ⅱ)-encapsulating Co-MOF-74. The as-prepared Pt-on-Co/C400 exhibited extremely high catalytic activity toward hydrogen generation from aqueous AB solution with an enormously high TOF value of 3 022 min^-1 at 303 K. The remarkably high activity of Pt-on-Co/C400 is mainly attributed to the high dispersibility and small size effects of Pt clusters, as well as the strongly enhanced synergistic coupling between Pt and Co atoms. The present strategy offers new opportunities to develop ultrafine metal clusters as efficient catalysts for numerous reactions.

  
  Acknowledgments: This work was supported by funds provided by the National Natural Science Foundation of China (Grant No.22201294), Guangdong Basic and Applied Basic Research Foundation (Grant No.2023A1515012370), Guangdong Pearl River Talent Program (Grant No.2023QN10C361), High-level Talents Program of the CAS (Grant No.E344021001), Clean Energy Joint International Laboratory (Grant No.E3G1041001), Shenzhen Science and Technology Program (Grant No.KQTD20221101093647058), and SIAT Innovation Fund for Excellent Young Scientists (Grant No.E3G0071001). Declaration of competing interest:   The authors declare no competing interest.
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Schematic illustration of the synthesis of Pt(Ⅱ)@Co-MOF-74 and Pt-on-Co/C400

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  Figure 2  PXRD patterns and (b) FTIR spectra of Pt(Ⅱ)@Co-MOF-74, Pt-on-Co/C300, Pt-on-Co/C350, Pt-on-Co/C400, and Pt-on-Co/C450 samples; (c) Raman spectrum and (d) N₂ adsorption-desorption isotherm of Pt-on-Co/C400

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  Figure 3  TEM image, (b) HAADF-STEM image, and (c-f) the corresponding elemental mappings of Pt-on-Co/C400

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  Figure 4  (a) Plots of time vs volume of the hydrogen evolved on hydrolysis of AB (2.0 mmol) at 303 K catalysed by Pt-on-Co/CT and (b) the corresponding TOF values; (c) Plots of time vs volume of the hydrogen evolved on hydrolysis of AB (2.0 mmol) catalysed by Pt-on-Co/C400 at different temperatures and (d) the corresponding Arrhenius plot (ln TOF vs 1 000/T); (e) Durability test for AB hydrolysis over Pt-on-Co/C400 at 303 K for 10 cycles

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