Excellent Catalytic Performance of ISOBAM Stabilized Co/Fe Colloidal Catalysts toward KBH4 Hydrolysis

Recently, developing a cost-effective and high-performance catalyst is regarded as an urgent priority for hydrogen generation technology. In this work, ISOBAM-104 stabilized Co/Fe colloidal catalysts were prepared via a co-reduction method and used for the hydrogen generation from KBH4 hydrolysis. The obtained ISOBAM-104 stabilized Co10Fe90 colloidal catalysts exhibit an outstanding catalytic activity of 37,900 mL-H2 min−1 g-Co−1, which is far higher than that of Fe or Co monometallic nanoparticles (MNPs). The apparent activation energy (Ea) of the as-prepared Co10Fe90 colloidal catalysts is only 14.6 ± 0.7 kJ mol−1, which is much lower than that of previous reported noble metal-based catalysts. The X-ray photoelectron spectroscopy results and density functional theory calculations demonstrate that the electron transfer between Fe and Co atoms is beneficial for the catalytic hydrolysis of KBH4.


Introduction
Recently, hydrogen has been widely considered as a promising clean energy source to replace the traditional fossil fuels. Chemical hydrogen storage materials have aroused tremendous interest because of their inherent advantages such as high content of hydrogen, no toxicity, low hydrogen releasing temperature, and an easily controllable hydrogen generation process [1][2][3][4][5][6][7][8]. Among those materials, potassium borohydride (KBH 4 ) stands out owing to its safe production process, harmless hydrolysis product, low activation energy and enthalpy [9][10][11][12][13][14]. Unfortunately, the low hydrogen production rate of KBH 4 self-hydrolysis hinders its large-scale practical application.
Many researchers found that metal nanoparticles (NPs) could catalyze the hydrolysis of KBH 4 and accelerate the generation rate of hydrogen [7,15,16]. For example, Kilinc et al. [7] successfully prepared the Pd complex catalysts for promoting the KBH 4 hydrolysis. The catalytic activity of the as-prepared catalysts was up to 37,900 mL-H 2 min −1 g-catalyst −1 . Recently, a series of colloidal metal catalysts were synthesized and used for catalyzing the hydrolysis of KBH 4 [17][18][19][20]. For instance, Wang et al. [19] successfully synthesized colloidal Co single-atom catalysts for the effective production of hydrogen from KBH 4 hydrolysis by using ISOBAM (isobutylene-alt maleic anhydride) as a protectant. The synthesized colloidal metal catalysts possess a clearly intrinsic catalytic activity of metal without the influence of support. Besides, those colloidal metal catalysts are stabilized by protective agents and present excellent catalytic activity and recyclability.
It has been widely accepted that the bimetallic catalysts exhibited a high catalytic activity for hydrogen production owing to the synergistic effects between different constituents [21][22][23][24][25]. In detail, the addition of another metal component could modify the electronic structure and then improve the catalytic activity [25,26]. For example, a previous report displayed that the Rh 10 Ni 90 bimetallic nanoparticles (BNPs) possessed a higher catalytic activity for the KBH 4 hydrolysis than that of Rh or Ni MNPs [27]. The catalytic activity of the reported Au/Ni BNPs was several times higher than their corresponding monometallic counterparts [28]. In addition, some non-noble metal catalysts (including Fe [29][30][31], Ni [18,32,33], Co [19,34], and Cu [35,36]) attract increasing attention owing to their considerable natural abundance, low cost, and competitive catalytic activity. However, the preparation of bimetallic catalysts with noble-free metal constituents is scarcely retrieved.
Herein, we reported a co-reduction method to prepare the ISOBAM-104 stabilized Co/Fe colloidal catalysts, which were then used for the hydrogen production from KBH 4 hydrolysis. The effects of the molar ratio of ISOBAM-104 to metal ion, concentration of metal ion, and molar ratio of Co/Fe were investigated. The as-synthesized ISOBAM-104 stabilized Co 10 Fe 90 colloidal catalysts possess an unexpected catalytic activity for hydrogen production from KBH 4 hydrolysis at room temperature. The activation energy of the as-prepared Co 10 Fe 90 colloidal catalysts towards KBH 4 hydrolysis was calculated by the Arrhenius formula. In addition, the electronic property of metal atoms was investigated based on the DFT calculations.

Preparation of Co/Fe Colloidal Catalysts and Hydrogen Generation
Firstly, certain concentrations of Co(NO 3 ) 2 ·6H 2 O and Fe(NO 3 ) 3 ·9H 2 O solution were mixed together in a three-neck flask ( Figure S2). Next, a certain amount of ISOBAM-104 was added into the flask and it was then filled with deionized water to 50 mL. After that, the mixed solution was continuously stirred for 24 h at room temperature. Subsequently, the configured KBH 4 and NaOH solution were rapidly added into the above solution to obtain ISOBAM-104 protected Co/Fe BNPs.
The influence of the molar ratio of ISOBAM-104 to metal ion concentration (denoted as R ISO , from 10 to 80), metal ion concentration (from 0.6 to 1.5 mM), and chemical composition (Fe, Co 10 Fe 90 , Co 30 Fe 70 , Co 50 Fe 50 , Co 70 Fe 30 , Co 90 Fe 10 , and Co) were investigated. The detailed batch compositions are shown in the Table S1. The volume of generated H 2 was measured by an electronic balance, which was automatically recorded based on the displacement level of water every two seconds. During this process, the generated gas was passed through a trap containing concentrated H 2 SO 4 to remove H 2 O and any NH 3 that might have been generated. The rate of hydrogen generation (k, mL-H 2 ·min −1 ) could be obtained from the slope of H 2 volume-time curve in the initial stage of the reaction.
The catalytic activity (mL-H 2 ·min −1 ·g-cat −1 ) could be calculated by the ratio of the hydrogen generation rate (k) to the mass of catalyst (m). It should be noted that the ISOBAM-104 used in this work contains the NH 4 + group, which also possesses a catalytic effect for KBH 4 hydrolysis [19,37]. Therefore, under the same condition, the catalytic activities of ISOBAM-104 stabilized Co/Fe colloidal catalysts and ISOBAM-104 (NH 4 + group) were measured. The intrinsic catalytic activity value of Co/Fe colloidal catalysts were obtained by subtracting the value of ISOBAM-104 from that of ISOBAM-104 stabilized catalysts. All the catalytic experiments were repeated no less than three times under the identical condition. The average values, which were normalized to mL-H 2 min −1 g-Co −1 , were used to determine the catalytic activity (detailed calculation procedures are provided in the supporting information).

Material Characterization
UV-vis absorption spectra were recorded at 200-800 nm by a Shimadzu UV-2550 spectrophotometer (Shimadzu Company, Kobe, Japan). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected by using a JEM-2100F (JEOL Company, Tokyo, Japan). The average size of the nanoparticles in each sample was estimated by measuring at least 200 particles from different parts of the grid. Fourier transform infrared (FTIR) spectra were obtained on a FTIR spectrometer (VERTEX 70, Bruker Corporation, Karlsruhe, Germany), and the samples were embedded in KBr pellet. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG MultiLab 2000 instrument (Thermo Electron Corporation, Massachusetts, USA) equipped with a 300 W Al Kα excitation source. The obtained XPS spectra were calibrated using a reference energy of 284.6 eV for the C 1s level and analyzed by Avantage software.

Density Functional Theory (DFT) Calculation
The spin-polarized density functional theory (DFT) calculations were carried out using a generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [38], as implemented in the DMol 3 package (BIOVIA Company, San Diego, CA, USA) [39]. The double numerical basis set and polarization functions (DNP) were carried out to describe the valence electrons, and an electron relativistic core treatment was used to perform full optimization of the investigated cluster model of Co 6 Fe 49 BNP without symmetry constraint. The convergence criteria were set to medium quality with a tolerance for the self-consistent field (SCF), optimization energy, maximum force, and maximum displacement of 10 −5 Ha, 2 × 10 −5 Ha, 0.004 Ha/Å and 0.005 Å, respectively. The charge analysis was performed on the basis of the Mulliken population distribution scheme [40,41].

Effect of R ISO on the Activity of Co/Fe Colloidal Catalysts
To explore the optimized reaction condition, the effect of R ISO on the preparation and catalytic activity of the Co/Fe BNPs was systematically investigated. The TEM images ( Figure 1) and size distribution histograms ( Figure S3) indicate that the average particle sizes of Co 50 Fe 50 BNPs are about 4.6 nm (R ISO = 10), 3.7 nm (R ISO = 30), 3.2 nm (R ISO = 50), and 2.3 nm (R ISO = 80), respectively. Obviously, the average particle size decreases with the increase of R ISO value, which may be ascribed to the fact that the increase of the protective agents could provide a large number of −COO − and −NH 2 groups to prevent the agglomeration of particles. Figure 2 displays the catalytic activities of the obtained Co 50 Fe 50 colloidal catalysts for hydrogen production at different R ISO . It can be clearly observed that the Co 50 Fe 50 colloidal catalysts with R ISO = 50 possess a higher catalytic value (17,500 mL-H 2 min −1 g-Co −1 ) than those synthesized at R ISO = 10, 30, and 80 (6800, 6600, and 5500 mL-H 2 min −1 g-Co −1 , respectively). This result may be attributed to the fact that Co 50 Fe 50 nanoparticles cannot receive effective protection at low R ISO and are prone to agglomeration, leading to a low catalytic activity. Comparatively, when R ISO was superfluous, the surface of the nanoparticles would be covered by ISOBAM-104, resulting in the decrease of active sites and catalytic activity [28]. Thus, based on the above results, the Co 50 Fe 50 catalysts with moderate particle size and high catalytic activity could be synthesized when R ISO = 50.

Effect of Metal Ion Concentration on the Activity of Co/Fe Colloidal Catalysts
The effect of ion concentration on the preparation and catalytic activity of Co 50 Fe 50 colloidal catalysts was also investigated. TEM morphologies and size distribution histograms of the as-prepared Co 50 Fe 50 BNPs are presented in Figure 3 and Figure S4. The average particle sizes are about 2.3, 3.2, 2.6, and 3.4 nm at the metal ion concentrations of 0.6, 0.9, 1.2, and 1.5 mM, respectively. It is found that the metal ion concentration exerts a significant influence on the particle size of the obtained catalysts. Although the Co 50 Fe 50 BNPs with the smaller particle sizes are obtained at the metal ion concentrations of 0.6 mM, the low concentration of metal ion impedes the large-scale preparation of catalysts. Hence, the concentration of metal ion is set as 1.2 mM in the following discussion.

Effect of Chemical Composition on the Activity of Co/Fe Colloidal Catalysts
The UV-vis spectra of the obtained Co/Fe BNPs with various compositions are shown in Figure S5. It was found that no surface plasma resonance peak of Fe or Co nanoparticles could be detected, which agrees with the previous reports [26,27,42]. The spectra of the dispersed Co/Fe nanoparticles BNPs with a featureless absorbance were located between the spectra of single Co and Fe nanoparticles, exhibiting a featureless absorbance. These obvious differences of the absorbance at various Fe content suggest the formation of alloystructured Co/Fe BNPs. Figure 4 presents the TEM images of the obtained Co/Fe BNPs at various Co/Fe atomic ratios. It can be clearly seen that the particles possessed a sphere-like morphology. The average sizes of ISOBAM-104 stabilized Fe, Co 10 Fe 90 , Co 30 Fe 70 , Co 50 Fe 50 , Co 70 Fe 30 , Co 90 Fe 10 , and Co colloidal catalysts are respectively about 3.0, 3.2, 2.6, 2.6, 2.2, 2.5, and 1.8 nm ( Figure S6). The corresponding catalytic activities of the above colloidal catalysts are displayed in Figure 5. By comparison, the above-mentioned Co/Fe BNPs presented a superior catalytic activity than that of Co or Fe MNPs. More importantly, the catalytic activity of the Co 10 Fe 90 colloidal catalysts reaches up to 37,900 mL-H 2 min −1 g-Co −1 , which is about 5 and 4 times higher than that of Fe (7400 mL-H 2 min −1 g-Fe −1 ) and Co (9600 mL-H 2 min −1 g-Co −1 ), respectively. Base on the above results, the desirable Co/Fe colloidal catalysts with high catalytic performance can be synthesized at the chemical composition of Co 10 Fe 90 , R ISO = 50, and ion concentrations of 1.2 mM.  The structure of the obtained Co 10 Fe 90 colloidal catalysts was further characterized by the high-resolution transmission electron microscope (HRTEM). As shown in Figure 6, the interplanar spacings of the four individual randomly-chosen Co/Fe BNPs are measured as 0.168, 0.172, 0.174, and 0.169 nm, respectively. These values are inconsistent with the theoretical interplanar spacing values of Co and Fe (Table S2). However, it is worth noting that this measured interplanar spacing located between the interplanar distance of Co (200) and Fe (200) (Table S3), suggests the alloy structure of the formed Co/Fe BNPs. In order to understand the protecting role of ISOBAM-104 in the catalysts stabilization, the FTIR spectra of ISOBAM-104 stabilized Co/Fe catalysts, ISOBAM-104, Co(NO 3 ) 2 , and Fe(NO 3 ) 3 are displayed in Figure S7. The absorption peak at 1400, 1680, 2300, and 3400 cm −1 , respectively, correspond to the stretching vibration of -OH, -COOH, -CO 2 , and the -NH 2 group of ISOBAM-104. By comparison, it can be clearly seen that the -COOH group of ISOBAM-104 disappeared, while the -OH and -NH 2 group still appeared in the ISOBAM-104 stabilized Co/Fe catalysts, demonstrating that the -NH 2 group in ISOBAM-104 should play a protective role on the as-prepared metal catalyst [18].

Kinetic Study and Catalytic Mechanism of Co/Fe Colloidal Catalysts
To calculate the apparent activation energy (E a ), the catalytic performance of Co 10 Fe 90 colloidal catalysts were evaluated under the perturbation of the reaction temperature. As shown in Figure S8, it can be seen that the catalytic activity of the Co 10 Fe 90 colloidal catalysts increases from 8400 to 15,200 mL-H 2 min −1 g-catalyst −1 as the temperature increases from 293 to 308 K. The E a is calculated by using the Arrhenius method [43]. As shown in Figure 7, the slope of the linear curve between the natural logarithm of catalytic activity and the reciprocal of temperature is −E a /R, where R is the universal gas constant. The calculated E a of Co 10 Fe 90 colloidal catalysts is 14.6 ± 0.7 kJ mol −1 , which is much lower than most of the reported metal-based catalysts (Table 1). Interestingly, the corresponding catalytic activity of the Co 10 Fe 90 colloidal catalysts is much higher than these metal-based catalysts. Thus, it can be confirmed that the excellent catalytic activity of Co 10 Fe 90 colloidal catalysts is closely related to the lower activation energy towards KBH 4 hydrolysis.  An XPS measurement was subsequently carried out to clarify the elemental composition and valence state of the Co 10 Fe 90 BNPs. In Figure S9a, the element of Co, Fe, O, N, C, and B are detected in the obtained Co/Fe colloidal catalysts. The high-resolution XPS spectra of Co 2p ( Figure S9b) shows that the electron binding energy of Co 0 2p 3/2 (776.0 eV) is about 2.3 eV lower than that of the bulk Co (778.3 eV), indicating a negatively-charged characteristic of Co atoms in Co 10 Fe 90 BNPs. Meanwhile, the electron binding energy of Fe 0 2p 3/2 (708.5 eV) was about 1.8 eV higher than that of the bulk Fe (706.7 eV), suggesting that the Fe atoms were positively charged ( Figure S9c). The negative shift of the Co 0 2p 3/2 binding energy and positive shift of the Fe 0 2p 3/2 binding energy might be ascribed to the electron charge transfer occurring between Fe and Co atoms [23,24,26,50,51]. To further confirm the electron transfer effect, DFT calculations were employed to investigate the electronic states of each atom in the Co 6 Fe 49 alloy nanoparticles [52]. As shown in Figure 8a, the Co atoms are negatively charged (−0.091 eV), while the Fe atoms are positively charged (0.029 or 0.021 eV), which is matched well with the above XPS result. Based on above discussions and the related literature [23,27], a plausible mechanism for the high catalytic performance of Co/Fe colloidal catalysts could be proposed. Due to the charge transfer between Fe atoms and Co atoms (Figure 8b

Conclusions
In summary, the ISOBAM-104 stabilized Co/Fe colloidal catalysts are successfully synthesized for hydrogen generation by a simple co-reduction method via using ISOBAM-104 as a protective agent, and Co(NO 3 ) 2 ·6H 2 O, Fe(NO 3 ) 3 ·9H 2 O, and KBH 4 as starting materials. The catalytic activities of the obtained Co/Fe colloidal catalysts could reach up to 37,900 mL-H 2 min −1 g-Co −1 at the chemical composition of Co 10 Fe 90 , R ISO = 50, and ion concentrations of 1.2 mM, which is superior to their corresponding monometallic nanoparticles. The excellent catalytic activity of Co 10 Fe 90 colloidal catalysts is mainly attributed to their lower activation energy towards KBH 4 hydrolysis, and the charge transfer effect between Fe and Co atoms. This finding could provide a deeper insight for developing the economic, highly active, and recyclable bimetallic catalysts.