Ni2P

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 1048−1054

pubs.acs.org/JPCL

Boosted Reactivity of Ammonia Borane Dehydrogenation over Ni/ Ni2P Heterostructure Yunxiang Lin,†,§ Li Yang,†,§ Hongliang Jiang,† Youkui Zhang,†,‡ Dengfeng Cao,† Chuanqiang Wu,† Guobin Zhang,† Jun Jiang,† and Li Song*,† †

J. Phys. Chem. Lett. Downloaded from pubs.acs.org by MACQUARIE UNIV on 02/21/19. For personal use only.

National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials) School of Chemistry, and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡ School of National Defense Science and Technology, Southwest University of Science and Technology, Mianyang, Sichuan 621010, P. R. China S Supporting Information *

ABSTRACT: Ammonia borane (AB) is regarded as a highly promising candidate for chemical hydrogen-storage materials. Developing low-cost yet efficient catalysts for the dehydrogenation of AB is central to achieving hydrogen conversion. Here a heterostructure of Ni/Ni2P nanoparticles deposited on a defective carbon framework for the hydrolysis of AB is developed by elaborately controlling phosphorization conditions. The electronic structure and interfacial interaction of the ternary components are probed by synchrotron-based X-ray absorption fine structure and further simulated via density functional theory. By adjusting the content of Ni and Ni2P in the hetrostructure, the optimized hybrid exhibits catalytic performance of H2 generation from the hydrolysis of AB under ambient conditions with a turnover frequency of 68.3 mol (H2) mol−1 (Cat) min−1 and an activation energy (Ea) of 44.99 kJ mol−1, implying its high potential as an efficient supplement for noble-metal-based catalysts in hydrogen energy applications.

H

properties in reactant adsorption and activation.22,23 As demonstrated in recent work, water activation is the ratedetermining step (RDS) of AB hydrolysis under neutral conditions.24,25 The formed metal−P bond exists via electron transfer from metal to P, which can directly enhance the adsorption of reactants (H2O and AB molecules).26,27 However, there is still a lack of work that studies the fabrication of heterointerfacial transition-metal nanoparticles and their phosphide-based catalysts and the structure−function relationship for AB hydrolysis. Therefore, it is highly desirable to produce a hybridized catalyst with the merit of heterostructures consisting of interfacial hybrids that perform efficient O−H bond cleavage and H2 release. Herein we developed a simple method to form novel carbon-supported Ni/Ni2P heteronanoparticles (namely, C− Ni/Ni2P) by introducing a controllable phosphatization reaction. The interfacial structure and electronic property of the hybrids were well disclosed by synchrotron-based X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The catalytic performance of the C− Ni/Ni2P catalysts was systematically evaluated, suggesting a tunable AB hydrolysis with the regulation of Ni/Ni2P ratios. Density functional theory (DFT) calculations further demonstrated that the composite system expressed higher adsorption

ydrogen storage and conversion are hot topics due to their huge potential in dealing with the energy and environmental concerns.1−6 Recently, developing convenient, cost-effective, and highly efficient ways to obtain hydrogen energy has become a hot research direction.7−9 Ammonia borane (AB), with high hydrogen content (19.6 wt %), nontoxicity, and good stability, has been recognized as the primary choice among various hydrogen-storage materials.10−12 As exhibited in eq 1 NH3BH3 (aq) + 2H 2O (aq) → NH4BO2 (aq) + 3H 2 (g) (1)

Three moles of H2 can be released per mole of AB under the catalytic reaction. Noble-metal-based (such as Pt, Ru, and Rh) catalysts have been widely studied because of their brilliant performance toward AB hydrolysis.13,14 However, their high price and scarcity hinder their widespread generalization. Therefore, non-noble transition-metal-based materials have attracted considerable attention due to their earth-abundant reserve.15,16 Carbon, serving as a matrix framework, can offer a porous structure and a larger surface area, which are highly beneficial to the exposure of active sites and the adsorption of reactants.17−19 Catalysts with heterostructures will exhibit boosted activities for a given reaction compared with a single component owing to the polarized chemical environment.20,21 Recently, transition metal phosphides (TMPs) have been introduced into AB hydrolysis because of their unique © XXXX American Chemical Society

Received: January 13, 2019 Accepted: February 19, 2019 Published: February 19, 2019 1048

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Figure 1. (a) XRD of the C−Ni/Ni2P heterohybrids and control sample. (b,c) TEM images with different scales for C−Ni/Ni2P-2 (insert of Figure 1b: size distribution of C−Ni/Ni2P nanoparticles). (d) HRTEM of C−Ni/Ni2P-2 (inset: SAED image). (e) STEM EDX mapping of the C−Ni/ Ni2P-2.

whereas the peaks of Ni (011) and (210) lattice planes decreased, indicating that the main components of the heterostructure are changed from Ni to Ni2P. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were also collected to reveal the morphology and lattice information on the samples. As illustrated in Figure 1b,c, the Ni/Ni2P heteronanoparticles were uniformly dispersed in the carbon framework. Besides, the average diameter of nanoparticles has been counted as 5.2 nm by analyzing the size distribution in the inset of Figure 1b. Furthermore, the size of Ni/Ni2P heteronanoparticles has no visible difference compared with the pristine Ni nanoparticles in C−Ni hybrids and other control samples (Figure S1), indicating that the subsequent phosphorization process did not influence the growth process of nanoparticles. As illustrated in Figure 1d, an obvious interface can be found in the heteronanoparticle, and the lattice spacing of the two sides is 0.192, 0.204, and 0.178 nm, corresponding to the (111) facet of Ni2P and (111) and (200) facets of Ni, respectively. In addition, an obvious larger lattice fringe with a distance of 0.341 nm on the margin of nanoparticles can be attributed to the C (002) facets. This result can also be verified from the selected area electron diffraction (SAED) pattern (inset of Figure 1d). Except for the diffraction ring of Ni2P (111) and Ni (200) mentioned in XRD analysis, the diffraction ring of the Ni (220) plane can be observed at the diffraction angle of

energies of H2O and AB molecules, which could facilitate the subsequent catalytic reactions. More importantly, the Ni/Ni2P heterostructure exhibited a lower energy barrier of RDS (H2O activation), which helps to promote the attack of the AB molecule. Hence, the C−Ni/Ni2P with ideal turnover frequency (TOF) and activation energy (Ea) has been successfully fabricated, provides an efficient supplement for noble-metal-free catalysts toward AB hydrolysis. The ternary C−Ni/Ni2P hybrids were obtained by reacting carbon-supported Ni nanoparticles (C−Ni) with different amounts of NaH2PO2 under an Ar atmosphere at 350 °C (marked as C−Ni/Ni2P-1, C−Ni/Ni2P-2, C−Ni/Ni2P-3, and C−Ni/Ni2P, respectively; see the Supporting Information). The X-ray diffraction (XRD) was first carried out to verify the crystalline structure of the samples. The C−Ni precursor shows sharp peaks at 44.5 and 51.8° (Figure 1a), corresponding to the (011) and (200) lattice planes of metal Ni (JCPDS: 04-0850). As demonstrated in previous work, there is ∼5 wt % NiO on the surface of Ni nanoparticles of the C−Ni precursor, which is beneficial to the phosphorization process.28,29 After reacting with NaH2PO2, three new peaks located at 40.8, 44.6, and 47.3° can be found in C−Ni/Ni2P hybrids because of the formation of the new phase, corresponding to the (111), (201), and (300) lattice planes of Ni2P (JCPDS: 03-0953), respectively. Moreover, with the NaH2PO2 amount increased, the peaks of Ni2P increased, 1049

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Figure 2. Normalized (a) C K-edge and (b) Ni L-edge XANES spectra of C−Ni/Ni2P-2 and control samples. (c) Normalized Ni K-edge XANES spectra of C−Ni/Ni2P heterohybrids. (d) Fourier-transformed extended XAFS spectra of Ni K-edge for C−Ni/Ni2P-2 and contrast samples (insert: enlarged Ni−P bond peak locate at 1.6 Å).

exhibited in Figure 2a, there are two main peaks at around 284.0 and 291.0 eV in C K-edge spectra, corresponding to π* and σ* structures of C−C bond, respectively. Comparing the C K-edge spectrum of HCl-treated C−Ni/Ni2P with that of other samples, a distinguishable peak B between π* and σ* could be found in C−Ni and C−Ni/Ni2P hybrids, which is mainly contributed by functional groups (e.g., CO, C−O, (COOH)) or a mutual effect toward Ni/Ni2P nanoparticles and the carbon framework, indicating the strong interaction between nanoparticles and carbon.32 Moreover, doping P into the C framework directly caused the peak B of C−Ni/Ni2P to be much stronger than that of C−Ni, which is consistent with the Raman results mentioned before.32−34 As reported in previous work, the bridge bond (e.g., C−N−Co, C−O−Ni) in carbon-based materials may be beneficial to the electrontransfer process and thus promote the catalytic performance.28,35,36 The Ni L-edge excitations were tested to uncover the valence states and the charge transfer among the heterostructures (Figure 2b). The peak located at ∼846 eV can be attributed to the core electrons excited from the L3 edge of Ni, and the peak located at ∼864 eV is signed to the L2-edge excitation.37,38 As can be seen, C−Ni expressed the lowest L2 peak, whereas C−Ni/Ni2P-3 expressed the highest L2 peak, indicating the increasing valence state of Ni in C−Ni/Ni2P hybrids caused by the additional phosphatized process. The electron transfer from Ni to P can also be verified from the XPS results (Figure S5).39,40 The Ni K-edge absorption information is further exhibited in Figure 2c,d. It is widely accepted that the XANES spectrum, which is often used to distinguish the material symmetry and elemental chemical environment, is highly correlated with the crystal structure and chemical state of samples.41,42 As exhibited in Figure 2c, XANES spectra of C−Ni, C−Ni/Ni2P-1, and C−Ni/Ni2P-2

76.37 (Figure S2). Hence, ternary interfacial structures can be verified among Ni, Ni2P, and C layers from the microzone structure analysis. The energy-dispersive X-ray (EDX) mapping of C−Ni/Ni2P-2 was investigated to further probe the element composition and distribution. As shown in Figure 1e, the P and Ni were uniformly dispersed throughout the C framework, resulting from the C−Ni precursor and the bonding of Ni−P. In particular, the EDX mapping of C is almost covering the hole zone, which mainly results from the carbonous substrate of TEM testing. Raman spectra of samples were investigated to uncover the formation of carbon. As shown in Figure S3, there are two main peaks located at around 1340 and 1590 cm−1, corresponding to the D band and G band of carbon. The intensity ratio of the D and G bands (ID/IG) increased from 1.02 to 1.08 with the increase in P content, indicating that the doping level of P can introduce more defects in the carbon framework.30,31 Thermogravimetric analysis (TGA), element analysis, and X-ray fluorescence (XRF) spectra were carried out to further investigate the elemental content in the C−Ni/Ni2P hybrids. As shown in Figure S4 and Table S1, the mass content of C, Ni, P, N, and H is 20.96, 51.7, 22.03, 1.4, and 3.2 wt %, respectively. Thus the molar mass of C−Ni/Ni2P can be obtained as 119.28 g mol−1 through calculating the content of the constituent elements. X-ray absorption fine structure (XAFS) technologies were further employed to expound the electronic structure of the asprepared samples. As shown in Figure 2, the C K-edge, Ni Ledge, and Ni K-edge excitation information was detected by XAS. To further understand the underlying interaction between carbon and nanoparticles, the as-prepared C−Ni/ Ni2P hybrids were stirred in 0.5 M HCl solution under 80 °C for 6 h to remove the incompletely wrapped nanoparticles. As 1050

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Figure 3. (a) Stoichiometric hydrogen evolution in aqueous solution (8 mL) containing 0.645 mmol AB catalyzed by C−Ni/Ni2P-2 and control sample at mole ratio (nCat/nAB) of 0.02 at 293 K. (b) Stoichiometric hydrogen evolution in aqueous solution (8 mL) containing 0.645 mmol AB at various mole ratios of C−Ni/Ni2P-2 and AB. (c) Relationship between hydrogen-generating rate and AB concentration at a fixed amount (0.0129 mmol) of C−Ni/Ni2P-2 at 293 K. (d) Hydrogen generation of C−Ni/Ni2P-2 under different temperatures from 293 to 313 K at mole ratio (nCat/ nAB) of 0.02.

performances of these samples were first compared at a mole ratio of 0.02 for catalysts and AB at temperature of 293 K. The released hydrogen was collected and monitored by a waterfilled buret. There is no H2 generated from the AB aqueous solution in the blank contrast experiment because of the high stability of AB under ambient conditions. As shown in Figure 3a, the C−Ni/Ni2P-2 spends ∼355 s to fully dehydrogenize the AB, whereas the full reaction time is 760 s for C−Ni, 583 s for C−Ni/Ni2P-1, and 470 s for C−Ni/Ni2P-3. In addition, the C−Ni/Ni2P pure sample also shows the activity of 508 s to fully hydrolyze AB, which is a bit better than that of C−Ni/ Ni2P-1, indicating that the pure Ni2P supported in the P-doped carbon matrix can also catalyze the AB hydrolysis. The calculated TOFs of these samples are summarized in Figure S6. Accordingly, the C−Ni/Ni2P-2 catalyst exhibited the highest activity for AB hydrolysis among these C−Ni−Ni2P catalysts with the TOF of 23.04 mol (H2) mol−1 (Cat) min−1. As compared in Figure 3a and Figure S6, the metallic Ni content in the heterohybrids has distinguishable influence toward AB hydrolysis. The catalytic performance is first increased with decreasing Ni content, then decreases because of heavy phosphatization. Accordingly, we suggest that the synergistic effect between Ni and Ni2P in the carbon framework efficiently improves the catalytic ability. In addition, during the phosphorization reaction, P could also be introduced into the carbon framework, which might be further beneficial to the adsorption of the AB molecule.23 Moreover, the reaction kinetics of AB hydrolysis has been further revealed via a series of tests by adjusting the reaction temperature and the mole ratio of catalysts to AB (nCat/nAB) in our experiments. The relationship between the hydrogengenerating rate and the catalyst amount was first demonstrated by regulating the nCat/nAB from 0.02 to 0.08 (Figure 3b). The

hybrids show a similar shape in the XANES region, indicating that the main phase of these three samples was Ni. It can be easily found that the C−Ni/Ni2P-3 shows a different XANES spectrum from other samples, implying that the main phase was changed from Ni to Ni2P after the P content was increased to a certain degree. Fourier-transformed extended XAFS (FTEXAFS) of Ni K-edge spectra were employed to further investigate the local geometric structure of C−Ni/Ni2P hybrids. As shown in Figure 2d, there are two main peaks located at 1.63 and 2.17 Å in the R-space range, which correspond to the Ni−P and Ni−Ni bonds, respectively.43−45 The C−Ni sample exhibits the strongest Ni−Ni peak, along with a slight shoulder peak, responding to the Ni−O bond because of the surface oxidation of the Ni nanoparticles. It is noteworthy that the bond length of Ni−Ni is slightly shifted to low R in the C−Ni/Ni2P hybrids, which is mainly due to the surface structural disorder caused by the import of P.46 Moreover, the introduction of P has not only caused the surface disorder of nanoparticles but also changed the chemical environment of Ni in C−Ni/Ni2P. The change of coordination number of Ni−P and Ni−Ni bonds can be obviously observed with the increase in the Ni−P bond peak and the decrease in the Ni−Ni bond peak, which is according to the XRD and XPS results mentioned above. To understand the proposed synergistic effect of our ternary heterohybrids between Ni2P, Ni, and C frameworks, the dehydrogenation of AB under the catalysis of our samples was demonstrated as the proof of concept. Typically, the catalysts and AB were immersed in 4 mL of water separately and dispersed under ultrasound for 30 min to get homogeneous ink. Small bubbles could be observed when the AB solution was added to the catalysts ink, and the reaction time was recorded by stopwatch at the same time. The catalytic 1051

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The Journal of Physical Chemistry Letters initial TOF of C−Ni/Ni2P-2 ternary hybrids was calculated as 23.04 mol (H2) mol−1 (Cat) min−1 at the nCat/nAB of 0.02, whereas the TOFs were increased to the highest value of 68.3 mol (H2) mol−1 (Cat) min−1 at the nCat/nAB of 0.08, which is competitive among the previous reported Ni-based catalysts toward AB hydrolysis under ambient conditions in water solution (Table S2). With increasing nCat/nAB, the hydrogengenerated rate increased sharply due to the increasing catalyst concentration. Figure S7 shows the logarithmic relationship between the hydrogen-generating rate and the AB concentration. The logarithmic TOF versus the logarithmic C−Ni/ Ni2P-2 concentration possesses a slope of 0.78 calculated by linear fitting, indicating the first-order kinetics toward the hydrolysis reaction and the catalyst concentration. Furthermore, we also studied the effect of AB on the hydrogen generation by changing the AB concentration to adjust nCat/ nAB from 0.020 to 0.035 (Figure 3c). The volume of H2 decreased with declining AB concentration, apparently with a similar hydrogen-generating rate. The slope of the hydrogengenerating rate versus AB concentration in a log−log range is −0.075 (Figure S8), indicating that the catalytic reaction with respect to AB concentration follows zero-order kinetics. In general, the activation energy of the chemical reaction is often used to reflect the difficulty level of the activating reactant toward the chemical reaction. Here the hydrolysis reaction was also carried out under various temperatures in the range from 293 to 313 K with nCat/nAB of 0.02 to further understand the activation energy of C−Ni/Ni2P-2. It can be easily found in Figure 3d that the hydrogen-generating rate increased with increasing ambient temperature. The activation energy of the catalytic hydrolysis of AB can be calculated from Arrhenius eq 2 ln rate = −Ea /RT + C

energy barrier for H2O cleavage (ΔEac(H2O)). Our DFT calculations found that the Ni/Ni2P heterointerface possesses a higher H2O adsorption energy of −0.52 eV compared with that on the Ni surface (−0.32 eV), indicating the Ni/Ni2P system is more ready for H2O adsorption (Figures S13 and S14, Table S3). In addition, the adsorption of AB on the surface of the Ni/Ni2P compound is also easier than that of pristine Ni metal with an Ead(AB) difference of 0.11 eV (Figure 4a,b). This may be attributed to the introduction of P for

Figure 4. Optimized AB molecule adsorption geometries on (a) Ni surface and (b) Ni/Ni2P interface from the top view. (c) DFTcalculated energy potential profile of water cleavage on the Ni surface (black line) and the Ni/Ni2P surface (blue line) from the side view. The gray-blue, red, white, pink, and blue balls represent Ni, O, H, P, and N atoms, respectively.

(2)

As demonstrated in eq 2, the slope of the graphic logarithmic TOF and T−1 can be attributed to −Ea·R−1 (R is ideal gas constant: 8.314). Thus, in this work, the activation energy of the catalytic reaction is calculated to be 44.99 kJ·mol−1 through the Arrhenius equation (Figure S9). The stability of C−Ni/ Ni2P-2 was tested at nCat/nAB of 0.08 under temperature of 293 K. As shown in Figure S10, the C−Ni/Ni2P-2 keeps a favorable performance after five full cycles with ∼0.72 min to fully hydrolyze the AB, whereas the first cycle takes 0.63 min. The XRD pattern of C−Ni/Ni2P-2, with main components of Ni and Ni2P, indicates that the main phase of this heterostructure did not changed after the catalytic reaction (Figure S11). Besides, the mesoporous morphology with carbon-coating Ni/ Ni2P can be observed from scanning electron microscopy (SEM) images (Figure S12), indicating that no collapse aggregation appeared toward the AB hydrolysis process. DFT calculations have been carried out to give a deep insight into the promotion effects of the Ni/Ni2P interface on the AB hydrolysis. The dehydrogenation process of AB contains three key steps. First, the adsorption of H2O and the AB molecule occurs on the surface of catalysts. Second, the cleavage of the O−H bond happens in the adsorbed H2O, triggering the release of OH*, which further attacks the B−N bond of the AB molecule and then produces BH3OH*.24−26 Third, the H* exits from the intermediate, and the H2 molecule is generated. In general, an ideal catalyst of AB hydrolysis should possess a specific free energy for AB and H2O adsorption (Ead(AB), Ead(H2O)) as well as a low activation

regulating the 3d electronic state of Ni by charge transfer, which is beneficial to the adsorption of H2O and AB molecule.23,27 Considering that the activation of H2O has been demonstrated as the RDS of AB hydrolysis, we further investigated the activation energy barriers of H2O dissociation (Eac(H2O)). As illustrated in Figure 4c and Figure S15, the reaction pathway of activating the H2O molecule in AB hydrolysis has been simulated. The energy barrier for the activation of H2O on the Ni/Ni2P and Ni surface is 0.94 and 1.13 eV, respectively. Lower ΔEac(H2O) on the Ni/Ni2P surface indicates that the heterostructures have a befitting property to compromise the reaction barriers of the RDS for AB hydrolysis. Thus it could be expected that the formation of Ni−P bonds and the electronic transfer from Ni to Ni2P would efficiently promote the H2O activation.24,47 On the basis of previous reports, carbon-supported Ni nanoparticles could facilitate the desorption of H* in the catalytic H2 evolution process.28 Hence we can confirm that the synergistic effects among C, Ni, and Ni/Ni2P in the obtained heterostructure hybrids simultaneously promote the reactants’ adsorption and activation and the generation of final products during AB hydrolysis process. 1052

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In summary, we have designed a new type of ternary heteronickel-based catalyst as a feasible candidate for AB hydrolysis. Notably, profiting from the polarized chemical environment of interfacial components, the C−Ni/Ni2P ternary heterocatalysts achieved an enhanced AB hydrolysis performance under neutral conditions. More importantly, this work offers a deep insight into the electronic structure of carbon-coated metal/metal phosphides through the combination of advanced experimental characterizations and theoretical simulations and may provide a rational design of ternary heterohybrids for AB hydrolysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00122.

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Letter

Experimental Section. Figure S1. Morphology of C−Ni precursor. Figures S2−S5. XRD, Raman, TGA, and XPS results of different samples. Figure S6. Initial TOF of different samples. Figures S7−S9. Reaction kinetics of the samples toward AB hydrolysis. Figure S10. Stability test for C−Ni/Ni2P-2. Figures S11 and S12. XRD and SEM of C−Ni/Ni2P-2 after reaction. Figures S13 and S14. Configurations of adsorbed H2O from different views. Figure S15. Optimized AB molecule adsorption geometries and calculated energy potential profile of water cleavage. Table S1. Contents of different elements. Table S2. TOF values in previous reports for comparison. Table S3. Adsorption energy of H2O and AB molecules and activation energy of H2O (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hongliang Jiang: 0000-0002-5243-3524 Jun Jiang: 0000-0002-6116-5605 Li Song: 0000-0003-0585-8519 Author Contributions §

Y.L. and L.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by National Key R&D Program of China (2017YFA0303500), National Natural Science Foundation of China (U1532112, 11574280, 21706248), Innovative Research Groups of NSFC (11621063), NSFC-MAECI (51861135202), CAS Interdisciplinary Innovation Team and CAS Key Research Program of Frontier Sciences (QYZDB-SSW-SLH018), and Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2016FXCX003). We thank Shanghai Synchrotron Radiation Facility (14W1, SSRF), Hefei Synchrotron Radiation Facility (Photoemission and MCD Endstations, NSRL), and USTC Center for Micro and Nanoscale Research and Fabrication for help with characterizations. 1053

DOI: 10.1021/acs.jpclett.9b00122 J. Phys. Chem. Lett. 2019, 10, 1048−1054

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DOI: 10.1021/acs.jpclett.9b00122 J. Phys. Chem. Lett. 2019, 10, 1048−1054