FULL PAPER DOI: 10.1002/asia.201301171
In Situ Facile Synthesis of Ru-Based Core–Shell Nanoparticles Supported on Carbon Black and Their High Catalytic Activity in the Dehydrogenation of Amine-Boranes Nan Cao,[a] Jun Su,[b] Xinlin Hong,[a] Wei Luo,*[a] and Gongzhen Cheng[a] Abstract: Well-dispersed core–shell Ru@M (M = Co, Ni, Fe) nanoparticles (NPs) supported on carbon black have been synthesized via a facile in situ one-step procedure under ambient condition. Core-shell Ru@Co NPs were synthesized and characterized for the first time. The as-synthesized Ru@Co and Ru@Ni NPs exhibit superior catalytic activity in the hydrolysis of ammo-
nia borane compared with their monometallic and alloy counterparts. The Ru@Co/C NPs are the most reactive, with a turnover frequency (TOF) value of 320 (molH2 min1) molRu1 and activaKeywords: amine-boranes · hydrogen storage · nanoparticles · ruthenium
noble metals, such as Co,[25] Ni,[26] Fe,[27] Cu,[28] including monometallic,[29] bimetallic,[30] and trimetallic[31] counterparts have been evaluated. Ru-based nanoparticles (NPs) have been identified as being one of the most effective catalysts.[17, 32–34] It has been reported that a transient MH species is the key to the hydrolytic reaction because the release of hydrogen is due to the attack of water on the transient. Ru-based nanoparticles form a suitably strong MH bond that shows favorable catalytic activity.[15] However, methylsubstituted AB (methylamine borane; CH3NH2BH3 ; MeAB), with a hydrogen content of 11.1 wt %, has not been widely studied.[35] It can release hydrogen at temperatures between 120 and 210 8C within 100 min,[36] and in THF at 20 8C in the presence of Ir and Ru catalysts.[37] However, to the best of our knowledge, the hydrolysis of MeAB, which can also release three moles of hydrogen per mole of MeAB according to Equation (1), has been rarely reported.[38] Therefore, the search for suitable catalysts that meet efficient, economical, and stable requirements for hydrogen generation from amine borane systems under moderate conditions is crucial for their practical applications.
Introduction Safe storage and effective release of hydrogen are essential for the development of a hydrogen-based energy infrastructure.[1] Currently, tremendous efforts are being focused on the development of materials that have sufficient gravimetric and kinetic properties and suitable thermodynamic and kinetic properties.[2] These include metal hydrides,[3] sorbent materials,[4] and chemical hydride[5] systems.[6] Amineborane, with protic NH and hydridic BH, have attracted much attention because of their high gravimetric hydrogen densities and favorable kinetics of hydrogen release.[7] The parent ammonia borane (NH3BH3 ; AB) represents a leading material for hydrogen-storage applications because of its high hydrogen content of 19.6 wt %, high stability under ambient conditions, and environmental friendliness.[8–12] The release of hydrogen from AB can be achieved though pyrolysis in the solid state,[13] catalytic dehydrogenation in nonaqueous solvents,[14] and hydrolysis.[15] With an appropriate catalyst, hydrolysis of AB could release three moles of hydrogen gas per mole of AB under ambient conditions, which appears to be the most convenient route for portable hydrogen-storage applications.[16–19] To date, both noble-based metals, such as Pt,[20] Ru,[21] Pd,[22] Ag,[23] and Au,[24] and non-
catalyst
MeNH2 BH3 þ 2 H2 O ! ðMeNH3 ÞBO2 þ 3 H2
ð1Þ
In recent years, bimetallic core–shell NPs have attracted considerable interest owing to their unique physical and chemical properties compared with their monometallic counterparts and alloys, probably due to the interplay of electronic and lattice effects of the neighboring metals.[39] However, research into Ru-based bimetallic NPs has been mostly limited to the Pt–Ru system.[40] Recently, the synthesis of Ru@Ni core–shell NPs has been reported by a multistep spray-pyrolysis method,[41] and the reverse Ni@Ru
[a] N. Cao, X. Hong, Prof. W. Luo, Prof. G. Cheng College of Chemistry and Molecular Sciences Wuhan University Wuhan, Hubei 430072 (P. R. China) E-mail: [email protected] [b] Dr. J. Su Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan, Hubei, 430074 (P. R. China)
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tion energy (Ea) of 21.16 kJ mol1. Ru@Ni/C NPs are the next most active, whereas Ru@Fe/C NPs are almost inactive. Additionally, the as-synthesized NPs supported on carbon black exhibit higher catalytic activity than catalysts on other conventional supports, such as SiO2 and g-Al2O3.
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core–shell NPs have been synthesized by a multi-step, hightemperature method;[34] but to the best of our knowledge there are no reports on Ru@Co core–shell NPs. Therefore, the development of a facile route to construct Ru-based core–shell NPs with high catalytic activity is highly desirable. Herein, we report a facile strategy for in situ synthesis of Ru@Co, Ru@Ni, and Ru@Fe core–shell NPs supported on carbon black (Ketjen carbon black), by using AB as reductant in a one-step coreduction route at room temperature under ambient conditions. The activity of these NPs towards the catalytic hydrolysis of AB and MeAB at room temperature was studied. Ketjen carbon black was chosen as a support because it has been reported to be an efficient support for catalysts in the hydrolysis of AB.[17, 33, 42]
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tion was introduced to a round-bottom flask that contained a mixture of aqueous RuCl3, CoCl2 or NiCl2 or FeCl2, and carbon black. Considering the lower reduction potentials (E0ACHTUNGRE(Co2+/Co) = 0.28 eV vs. SHE; E0ACHTUNGRE(Ni2+/Ni) = 0.25 eV vs. SHE; E0ACHTUNGRE(Fe2+/Fe) = 0.44 eV vs. SHE), M2 + cannot be directly reduced by AB. Ru3 + , which has a high reduction potential (E0ACHTUNGRE(Ru3+/Ru) = + 0.40 eV vs. SHE), was first reduced by AB to form the Ru NPs, and serves as the in situ seeds to introduce the successive growth of the M NPs as the shell, which may be generated by a RuH species with strong reducing ability[43] to form the Ru@M core–shell NPs. The microstructure of Ru@Co/C and Ru@Ni/C NPs were characterized by transmission electron microscopy (TEM). As shown in Figure 1a and d, the as-synthesized NPs were well dispersed on carbon black. A distinct contrast of core and shell can be observed clearly in the high-resolution TEM (HRTEM) images in Figure 1b and e; the dark core is Ru and the gray shell is Co and Ni, which indicates that Ru is initially reduced by AB and subsequently acts as the seed that helps to reduce Co2 + and Ni2 + to form the shells. The d spacing of the crystallized core part is 0.205 nm, which is consistent with hexagonal RuACHTUNGRE(101) plane spacing, the out-
Results and Discussion Synthesis and Characterization In a typical synthesis of carbon-black-supported Ru@M (M = Co, Ni, Fe) core–shell NPs, the ammonia borane solu-
Figure 1. a, b) TEM image of Ru1@Co10/C at different magnifications. c) TEM image of Ru1@Co10/C NPs after five cycles. d, e) TEM image of [email protected]/ C at different magnifications. f) TEM image of [email protected]/C NPs after five cycles. g) TEM image of RuNi/C NPs reduced by NaBH4. h) EDX spectrum of Ru@Co/C. i) EDX spectrum of Ru@Ni/C.
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Figure 2. XRD patterns of Ru@Co/C and Ru@Ni/C, and RuNi/C, Ru/C, and Ni/C after annealing at 500 8C for 4 h under Ar.
Figure 3. Time plots of the catalytic dehydrogenation of AB over the Ru@Co/C catalysts under ambient conditions. Ru/AB 0.004 (molar ratio).
side shells of Co and Ni are amorphous. The EDS spectra confirm the presence of Co and Ni (Figure 1h, i), which indicates that the shell parts are amorphous Co and Ni. Figure 2 shows the power XRD pattern of the as-prepared Ru@Co/C and Ru@Ni/C NPs. The 2q peak at around 248 is associated with the carbon black, whereas the peak at around 448 is attributed to the (101) plane of metallic Ru. However, no diffraction peak for Co and Ni is observed, which may be caused by the amorphous phase of Co and Ni. Although no lattice lines for RuNi/C can be found in Figure 1g, in the XRD patterns (Figure 2) it can be seen that the peak at 448 for pure Ru/C NPs matches that of hcp-structured RuACHTUNGRE(101), and the peak at 44.58 for pure Ni/C NPs matches that of fcc-structured NiACHTUNGRE(111). The RuNi/C samples reduced by NaBH4 exhibit a peak at 44.38, which indicates the structure of the RuNi alloy.[33]
for the catalytic hydrolysis of AB by Ru-based catalysts, and higher than most other values reported for noble-metalbased catalysts if the TOF is normalized in terms of moles of noble metal (Table 1). Figures 4 and 5 shows the plots of hydrogen generation from the hydrolysis of a solution of AB or MeAB, respectively, in the presence of different Ru1@Co10/C NPs concenTable 1. Catalytic activity of different noble-metal-based catalysts used for the hydrolytic dehydrogenation of AB.
Catalytic Activities for Hydrolysis of AB and MeAB by Ru@Co/C NPs Without Ru3 + , the Co2 + cannot be reduced by AB, which results in no activity in the catalytic hydrolysis of AB, as shown in Figure 3. When pure Ru NPs were supported on carbon black, with the molar ratio of Ru/ABs kept at 0.004, AB can be completely catalytically decomposed to release three equivalents of H2 in about 12 min. Unexpectedly, when Co was introduced into the NPs to form the core–shell NPs, with the molar ratio of Ru/AB still kept at 0.004, the decomposition time of AB was decreased. When the Co/Ru molar ratio was 10, the release of H2 was complete within 2 min, and a further increase in the Co/Ru molar ratio to 12.5 resulted in a prolonged reaction time, which indicated the positive effect of the Ru@Co core–shell nanostructures on the catalytic hydrolysis of AB and showed that the best Co/Ru molar ratio in the Ru@Co core–shell NPs is 10. This is similar to results from Demirci et al. for their RuCo and RuCu systems.[21] To the best of our knowledge, this is the first report of the synthesis of Ru@Co core–shell NPs. The activity in terms of turnover frequency (TOF) is 320 (molH2 min1) molRu1 for these as-synthesized NPs, moreover, this is one of the highest values to be reported
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Catalyst
TOF [(molH2 min1) molM1] Ea [kJ mol1] Reference M = Pt, Ru, Pd, Au, Rh
Ru/C Pd@Co/graphene Ru0@MWCNT Ru@Co/C Pt/g-Al2O3 [email protected]/C Ru@Ni/C [email protected]/C Pt@MIL-101 Ni0.74Ru0.26 alloy NPs PSSA-co-MA stabilized Ru nanoclusters Rh/g-Al2O3 Ni@Ru Ru@Al2O3 after acetic acid treatment Ru/g-Al2O3 laurate-stabilized ruthenium(0) nanoclusters Ru@Al2O3 RuCo (1:1)/gAl2O3 Ru/g-Al2O3 PSSA-co-MA stabilized Pd nanoclusters RuCu (1:1)/gAl2O3 RGO/Pd
429.5 408.9 329 319.7 308 280.3 250.1 248.5 233.22 194.8
34.81 – 33 21.16 21 33 37.87 41.5 40.7 37.18
[17] [44] [32] this study [47] [20] this study [15] [48] [33]
187.6
54
[49]
178 114 83.3
21 44 46
[47] [34] [29]
77 75
23 47
[47] [45]
39.6 32.9
48 47
[29] [21]
23.05 19.9
67 44
[50] [49]
16.4
52
[21]
51
[51]
6.25
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trations at (25 0.2) 8C. The initial rate of hydrogen generation was determined from the initial nearly linear portion of each plot. The line slope of the plot of hydrogen evolution rate versus catalyst concentration in a log–log scale is 1.13 for AB and 0.98 for MeAB; this indicates that the hydrolysis of AB and MeAB catalyzed by Ru1@Co10/C NPs is first order with respect to the catalyst concentration. To get the activation energy (Ea) of AB and MeAB hydrolysis catalyzed by Ru1@Co10/C NPs, the hydrolytic reactions at different temperature range of 25–40 8C were carried out. The values of rate constant k at different temperatures were calculated from the slope of the linear part of each plot from Figures 6a and 7a. The Arrhenius plots of lnk versus 1/T for the catalysts are plotted in Figures 6b and 7b, from which the apparent activation energy was determined to be approximately 21.16 and 26.05 kJ mol1 for AB and MeAB, respectively. These values are lower than most of the reported Ea values (Table 1), which indicates the superior catalytic performance of the as-synthesized Ru1@Co10/C NPs. To better understand the hydrolysis of MeAB in the presence of Ru1@Co10/C NPs and confirm the complete decomposition of MeAB after the catalytic reaction, we performed 11 B NMR spectroscopic studies (Figure 8). The 11B peak at d = 19.8 ppm, which is assigned to MeAB, disappears after the catalytic reaction and a new peak around d = 11.4 ppm is observed, which indicates three moles of hydrogen has been generated from one mole of MeAB during the catalytic hydrolysis according to Equation (1). Furthermore, there is no change in the 11B NMR spectrum of MeAB after 7 d under ambient conditions, which indicates the catalytic activities of the as-prepared NPs in the catalytic hydrolysis of MeAB under ambient conditions.
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Figure 4. a) Plots of moles of H2 released per mole of AB vs. time for the catalytic hydrolysis of AB by different concentrations of Ru1@Co10/C at (25 0.2) 8C. b) The plot of hydrogen generation rate vs. Ru1@Co10/C NPs concentration (both in logarithmic scale).
Catalytic Activity of Ru@Ni/C NPs for Hydrolysis of AB Without Ru3 + , Ni2 + cannot be reduced by AB, which results in no activity in the catalytic hydrolysis of AB, as shown in Figure 9. When Ni was introduced into the Ru NPs to form core–shell NPs, with the molar ratio of Ru/AB kept at 0.004, the reaction time for the decomposition of AB was decreased. When the Ni/Ru molar ratio was 7.5, the release of H2 was completed in about 2 min, which indicates the positive effect of the Ru@Ni core–shell nanostructures on the catalytic hydrolysis of AB. If the Ni/Ru molar ratio was further increased to 10 and 12.5, no significant decrease in the reaction time was observed, which indicates that the best Ni/Ru molar ratio in the Ru@Ni core–shell NPs is 7.5. The activity in terms of turnover frequency (TOF) is 250 (molH2 min1) molRu1 for these as-synthesized NPs, which is lower than that of as-synthesized Ru1@Co10/C NPs but is still higher than most of the reported noble-metalbased catalysts shown in Table 1. Interestingly, the TOF value of the as-synthesized NPs is much higher than that of the reverse Ni@Ru core–shell NPs that have the more reactive Ru as the shell.[34] The results may be attributed to the positive synergistic effect between carbon black and the
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Figure 5. a) Plots of moles of H2 released per mole of MeAB vs. time for the catalytic hydrolysis of MeAB by different concentrations of Ru1@Co10/C at (25 0.2) 8C. b) The plot of hydrogen generation rate vs. Ru1@Co10/C NPs concentration (both in logarithmic scale).
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Figure 7. a) Plots of moles of H2 released per mole of MeAB vs. time for the catalytic hydrolysis of MeAB by Ru1@Co10/C at different temperatures; Ru/AB 0.004. b) The Arrhenius plot (lnk vs. reciprocal absolute temperature, 1/T [K1]).
Figure 6. a) Plots of moles of H2 released per mole of AB vs. time for the catalytic hydrolysis of AB by Ru1@Co10/C at different temperatures; Ru/ AB 0.004. b) The Arrhenius plot (lnk vs. reciprocal absolute temperature, 1/T [K1]).
Ru@Ni core–shell nanostructures, and more active sites deriving from the higher amorphous Ni contents.[44] In comparison, Ru1@Fe5/C and Ru1@Fe10/C NPs were synthesized by using a similar method. As shown in Figure 9, the as-prepared Ru@Fe/C NPs show almost no reactivity toward the hydrolysis of AB and are even worse than Ru/C NPs, which indicates the negative effect of Fe in our system toward the dehydrogenation of AB under ambient conditions. For comparison, Ru1Ni7.5/C alloy NPs reduced by NaBH4 were prepared and applied to the catalytic hydrolysis of AB. As shown in Figure 10, their catalytic activity is inferior to that of [email protected]/C NPs. Additionally, to study the effects of the supporting materials on the catalytic performance of the as-synthesized core–shell NPs, [email protected]/SiO2, [email protected]/gAl2O3, and carbon-black-free [email protected] NPs were prepared and their catalytic activity in the hydrolysis of AB were studied. The inductively coupled plasma-atomic emission spectroscopy (ICP-AES) results give values of 7.5, 13.6, and 10.9 wt % for Ru@Ni NPs on C, SiO2, and g-Al2O3, respectively. However, as shown in Figure 10, the NPs supported on SiO2 and g-Al2O3 both show inferior catalytic activity to that of [email protected]/C NPs, which highlights the dominant factor of carbon black in facilitating the hydrolysis of AB in our system. The specific surface areas of all the catalysts, as determined by using the BET method, are summarized in
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Figure 8. 11B NMR spectra of a) freshly prepared aqueous MeAB, b) aqueous MeAB after 7 d in air, and c) aqueous MeAB after catalysis by [email protected]/graphene NPs.
Table 2. There are obvious decreases in the surface area after loading metals onto these supports. Figure 11 shows the plots of hydrogen generation from the hydrolysis of AB in the presence of different concentrations of [email protected]/C NPs at (25 0.2) 8C. The initial rate of hydrogen generation was determined from the initial nearly linear portion of each plot. The line slope of the plot of hydrogen evolution rate versus catalyst concentration in a log– log scale is 1.11, which indicates that the hydrolysis of AB
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Figure 9. Time plots of catalytic dehydrogenation of AB over Ru@Ni/C catalysts and Ru@Fe/C catalysts under ambient conditions; Ru/AB 0.004 (molar ratio).
Figure 11. a) Plots of moles of H2 released per mole of MeAB vs. time for the catalytic hydrolysis of AB by different concentrations of [email protected]/C at (25 0.2) 8C. (b) The plot of hydrogen generation rate vs. [email protected]/C NPs concentration (both in logarithmic scale). Figure 10. Time plots of the catalytic dehydrogenation of AB over [email protected]/C, [email protected]/SiO2, [email protected]/g-Al2O3, Ru1Ni7.5/graphene, and [email protected] NPs under ambient conditions; Ru/AB 0.004.
Recycling Stability The reusability of the catalyst is crucial for practical applications. The recycling stability of Ru1@Co10/C NPs over five runs in the hydrolysis of AB and MeAB and of [email protected]/C NPs over five runs in the hydrolysis of AB are shown in Figures 13, 14, and 15, respectively. As shown in Figures 13 and 14, the as-prepared Ru1@Co10/C catalysts retain 52.5 and 50 % of their initial catalytic activity in the fifth run for the hydrolysis of AB and MeAB, respectively. In Figure 15, the as-synthesized bimetallic core–shell [email protected]/C NPs retains 52.5 % of the initial catalytic activity for the hydrolysis of AB. Figure 1c and f show the representative TEM images of Ru1@Co10/C NPs and [email protected]/C NPs, respectively, after the fifth run of this durability test. As can be clearly seen from the TEM images, there is some agglomeration of the as-synthesized NPs on carbon black after the fifth run. The aggregation of metal nanoparticles causes a decrease in the surface area of the nanoparticles, which may further result in a decrease in the catalytic activity. Furthermore, XPS analyses of the survey scan of Ru@Co/ C NPs before and after the five cycles are displayed in Figure 16a and b. However, the overlap of the C 1s and Ru 3d peaks around 285 eV makes it difficult to analyze this range for ruthenium correctly. Therefore, Figure 16c and d show the peak of Ru 3p for the catalysts before and after five cycles. The Ru 3p feature fits well to two peaks at 461.4 and
Table 2. Characteristics of supports and catalysts tested for the hydrogen generation reaction. Support BET surface area [m2 g1]
Pore volume [cm3 g1]
Pore size [nm]
C 1030.3 380.0 SiO2 g-Al2O3 118.5
1.35 0.89 0.19
6.87 6.79 4.46
Catalyst
BET surface area [m2 g1]
Pore volume [cm3 g1]
Pore size [nm]
Ru@Ni/C Ru@Ni/SiO2 Ru@Ni/gAl2O3
254.7 216.1 23.0
0.78 0.84 0.15
8.78 7.15 7.24
catalyzed by [email protected]/C NPs is first order with respect to the catalyst concentration. To get the activation energy (Ea) of AB hydrolysis catalyzed by [email protected]/C NPs, the hydrolytic reactions at temperatures between 25 and 40 8C were carried out. As shown in Figure 12, the apparent activation energy was determined to be approximately 36.59 kJ mol1, which is higher than that of Ru1@Co10/C NPs (Table 1).
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Figure 12. (a) Time plots of mol H2 per mol MeAB vs. time for the [email protected]/C catalytic hydrolysis of AB at different temperatures, Ru/ AB = 0.004; (b) the Arrhenius plot (ln k versus the reciprocal absolute temperature, 1/T (K1)).
Figure 14. a) Time plots of the catalytic dehydrogenation of MeAB over Ru1@Co10/C catalysts from 1 to 5 cycles at (25 0.2) 8C; Ru/AB 0.004. b) Percentage of initial catalytic activity of Ru1@Co10/C in successive runs after reuse for the hydrolysis of MeAB.
Figure 13. a) Time plots of the catalytic dehydrogenation of AB over Ru1@Co10/C catalysts from 1 to 5 cycles at (25 0.2) 8C; Ru/AB 0.004. b) Percentage of initial catalytic activity of Ru1@Co10/C in successive runs after reuse for the hydrolysis of AB.
Figure 15. a) Time plots of the catalytic dehydrogenation of AB over [email protected]/C catalysts from 1 to 5 cycles at (25 0.2) 8C; Ru/AB 0.004. b) Percentage of initial catalytic activity of [email protected]/C in successive runs after reuse for the hydrolysis of AB.
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463.5 eV, which are readily assigned to Ru0 3p and RuIV 3p, respectively.[45] There are almost no differences between the two peaks before and after the catalytic hydrolysis of AB. RuIV oxide present on the surface of Ru likely forms during the sample preparation process for XPS measurements. Figure 16e and f shows the peak of Co 2p before and after five cycles. There are three peaks with peak tops at 778.1, 781.5, and 786.3 eV, which were assigned to Co0 and oxidized Co.[38] In Figure 16f, there are almost no peaks for Co0, which indicates that the outside shell Co is oxidized during the catalytic process, which may cause a decrease in the catalytic activity. Moreover, the increased viscosity of the solution or deactivation effect of the increasing metaborate concentration during the hydrolysis of AB/MeAB should be taken into account.[46]
Fe) core–shell NPs. The Ru@Co core–shell NPs were synthesized and characterized first. The as-synthesized Ru@Co and Ru@Ni NPs exhibit superior catalytic activity compared with monometallic and alloy counterparts in the hydrolysis of ammonia borane. The Ru@Co/C NPs exhibit superior catalytic activity, with a TOF value of 320 (molH2 min1) molRu1 and an activation energy (Ea) of 21.16 kJ mol1; followed by Ru@Ni/graphene NPs, whereas the Ru@Fe/graphene NPs are almost inactive. The carbonblack-supported Ru@Co NPs exhibit higher catalytic activity than the counterparts supported on SiO2 or g-Al2O3 or without carbon black. Moreover, this simple synthetic method can be extended to other carbon-black-supported core–shell NPs systems for further applications.
Experimental Section
Conclusion
Chemicals and Materials
We have developed a facile in situ one-step method for the synthesis of carbon-black-supported Ru@M (M = Co, Ni,
Ammonia borane (NH3BH3, AB, Aldrich, 90 %), sodium borohydride (Sinopharm Chemical Reagent Co., 96 %), ruthenium chloride hydrate (RuCl3·n H2O, The Wuhan Xinsirui Technology Co., 99.9 %), cobalt chloride hexahydrate (CoCl2·6 H2O, Sinopharm Chemical Reagent Co., 99 %), nickel chloride hexahydrate (NiCl2·6 H2O, Sinopharm Chemical Reagent Co., 99 %), ferrous chloride tetrahydrate (FeCl2·4 H2O, Sinopharm Chemical Reagent Co., 99 %), Ketjenblack EC-300 J (Triquo Chemical Co.), neutral silica power (SiO2, Branch of Qingdao Haiyang Chemical Co.), Aluminum oxide neutral (gAl2O3, Sinopharm Chemical Reagent Co.), methylamine hydrochloride (CH3NH2·HCl, Sinopharm Chemical Reagent Co., 96 %), tetrahydrofuran (C4H8O, THF, Sinopharm Chemical Reagent Co., 99 %), dimethyl ether anhydrous (C4H10O, Sinopharm Chemical Reagent Co., Ltd., 99.7 %). All chemicals were used as obtained. We used ordinary distilled water as the reaction solvent. Preparation of Methyl Ammonia Borane (CH3NH2BH3, MeAB)
Figure 16. XPS spectra of a, b) the survey scan of Ru@Co/C NPs before the first cycle and after five cycles; c, d) the Ru 3p levels of Ru@Co/C NPs before the first cycle and after five cycles; e, f) the Co 2p levels of Ru@Co/C NPs before the first cycle and after five cycles.
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MeAB was synthesized by using the method reported in the literature.[35] Sodium borohydride (1.8925 g, 0.05 mol) and methylamine hydrochloride (3.376 g, 0.05 mol) were added to a 250 mL two-neck round-bottom flask with a neck connected to a condenser. THF (100 mL) was added to the flask and the contents were vigorously stirred at 25 8C. The reaction was carried out at RT under a nitrogen atmosphere. After 12 h, the resultant solution was filtered by suction filtration and the filtrate was concentrated under vacuum at RT. The product was purified by using diethyl ether.
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In Situ Synthesis of Ru@Co/C, Ru@Ni/C, and Ru@Fe/C Catalysts
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In a typical experiment for producing Ru@Co/C NPs, carbon black (8 mg) was dispersed in water (1.6 mL) and kept in a two-necked roundbottom flask. Ultrasonication was required to get a uniform dispersion. An aqueous ruthenium chloride solution (0.4 mL, 0.01 mol L1) and an aqueous cobalt chloride solution (4 mL, 0.01 mol L1) were added to this flask. One neck was connected to a gas burette and the other was connected to a pressure equalizer to introduce AB. An aqueous solution containing AB (62 mg in 4 mL; 90 %) was kept in the pressure equalizer. The reaction was started when the AB solution was added to the flask with vigorous stirring and the evolution of gas was monitored by using the gas burette. After the hydrogen generation reaction was complete, an aqueous solution of AB (30.8 mg in 2 mL) was added to the flask and the evolution of gas was monitored. A water bath was used to control the temperature of the reaction solution. For Ru@Ni/C and Ru@Fe/C, the same preparation procedure as described above for Ru@Co was applied, except that CoCl2·6 H2O was replaced by NiCl2·6 H2O and FeCl2·4 H2O, respectively. For comparison, Ru@Ni/g-Al2O3 NPs, Ru@Ni/SiO2 NPs, and Ru@Ni NPs without C reduced by AB, and RuNi/C reduced by NaBH4 were synthesized by using a similar method. Catalytic Hydrolysis of AB and MeAB Sets of experiments with different concentrations of Ru@Ni (0.017, 0.0225, 0.034, 0.0425 mmol) or Ru@Co (0.022, 0.033, 0.044, 0.055 mmol) were performed at RT ((25 0.2) 8C) with the AB (or MeAB) concentration kept the same (1 mmol) to determine the rate law of the catalytic hydrolysis of AB (or MeAB). The temperature used were (25 0.2), (30 0.2), (35 0.2), and (40 0.2) 8C, and the concentration of Ru (0.004 mmol) and AB or MeAB (1 mmol) were kept constant to obtain the activation energy (Ea). Recycling Stability Test For the recycling stability test, catalytic reactions were repeated five times by adding another equivalent of AB or MeAB (1 mmol) to the mixture after the previous cycle. Characterization Transmission electron microscope (TEM) and energy-dispersive X-ray spectroscopy (EDS) images were obtained by using a FEI Tecnai G20 UTwin TEM instrument operating at 200 kV. Powder X-ray diffraction (XRD) patterns were measured by using a Bruker D8-Advance X-ray diffractometer with a CuKa radiation source (l = 0.154178 nm) with a velocity of 68 min1. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Kratos XSAM 800 spectrophotometer. 11 B NMR spectra were recorded at ambient temperature by using a Varian-VX 300 spectrometer and externally referenced to BF3·Et2O (d = 0). Elemental composition was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and was performed by using an IRIS Intrepid II XSP. The surface area measurements were performed by N2 adsorption at the temperature of liquid N2 by using a Micromeritics ASAP 2020 BET analyzer.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21201134), the Ph. D Programs Foundation of the Ministry of Education of China (20120141120034), and the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.
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In Situ Facile Synthesis of Ru-Based Core–Shell Nanoparticles Supported on Carbon Black and Their High Catalytic Activity in the Dehydrogenation of Amine-Boranes Nan Cao,[a] Jun Su,[b] Xinlin Hong,[a] Wei Luo,*[a] and Gongzhen Cheng[a] Abstract: Well-dispersed core–shell Ru@M (M = Co, Ni, Fe) nanoparticles (NPs) supported on carbon black have been synthesized via a facile in situ one-step procedure under ambient condition. Core-shell Ru@Co NPs were synthesized and characterized for the first time. The as-synthesized Ru@Co and Ru@Ni NPs exhibit superior catalytic activity in the hydrolysis of ammo-
nia borane compared with their monometallic and alloy counterparts. The Ru@Co/C NPs are the most reactive, with a turnover frequency (TOF) value of 320 (molH2 min1) molRu1 and activaKeywords: amine-boranes · hydrogen storage · nanoparticles · ruthenium
noble metals, such as Co,[25] Ni,[26] Fe,[27] Cu,[28] including monometallic,[29] bimetallic,[30] and trimetallic[31] counterparts have been evaluated. Ru-based nanoparticles (NPs) have been identified as being one of the most effective catalysts.[17, 32–34] It has been reported that a transient MH species is the key to the hydrolytic reaction because the release of hydrogen is due to the attack of water on the transient. Ru-based nanoparticles form a suitably strong MH bond that shows favorable catalytic activity.[15] However, methylsubstituted AB (methylamine borane; CH3NH2BH3 ; MeAB), with a hydrogen content of 11.1 wt %, has not been widely studied.[35] It can release hydrogen at temperatures between 120 and 210 8C within 100 min,[36] and in THF at 20 8C in the presence of Ir and Ru catalysts.[37] However, to the best of our knowledge, the hydrolysis of MeAB, which can also release three moles of hydrogen per mole of MeAB according to Equation (1), has been rarely reported.[38] Therefore, the search for suitable catalysts that meet efficient, economical, and stable requirements for hydrogen generation from amine borane systems under moderate conditions is crucial for their practical applications.
Introduction Safe storage and effective release of hydrogen are essential for the development of a hydrogen-based energy infrastructure.[1] Currently, tremendous efforts are being focused on the development of materials that have sufficient gravimetric and kinetic properties and suitable thermodynamic and kinetic properties.[2] These include metal hydrides,[3] sorbent materials,[4] and chemical hydride[5] systems.[6] Amineborane, with protic NH and hydridic BH, have attracted much attention because of their high gravimetric hydrogen densities and favorable kinetics of hydrogen release.[7] The parent ammonia borane (NH3BH3 ; AB) represents a leading material for hydrogen-storage applications because of its high hydrogen content of 19.6 wt %, high stability under ambient conditions, and environmental friendliness.[8–12] The release of hydrogen from AB can be achieved though pyrolysis in the solid state,[13] catalytic dehydrogenation in nonaqueous solvents,[14] and hydrolysis.[15] With an appropriate catalyst, hydrolysis of AB could release three moles of hydrogen gas per mole of AB under ambient conditions, which appears to be the most convenient route for portable hydrogen-storage applications.[16–19] To date, both noble-based metals, such as Pt,[20] Ru,[21] Pd,[22] Ag,[23] and Au,[24] and non-
catalyst
MeNH2 BH3 þ 2 H2 O ! ðMeNH3 ÞBO2 þ 3 H2
ð1Þ
In recent years, bimetallic core–shell NPs have attracted considerable interest owing to their unique physical and chemical properties compared with their monometallic counterparts and alloys, probably due to the interplay of electronic and lattice effects of the neighboring metals.[39] However, research into Ru-based bimetallic NPs has been mostly limited to the Pt–Ru system.[40] Recently, the synthesis of Ru@Ni core–shell NPs has been reported by a multistep spray-pyrolysis method,[41] and the reverse Ni@Ru
[a] N. Cao, X. Hong, Prof. W. Luo, Prof. G. Cheng College of Chemistry and Molecular Sciences Wuhan University Wuhan, Hubei 430072 (P. R. China) E-mail: [email protected] [b] Dr. J. Su Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan, Hubei, 430074 (P. R. China)
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tion energy (Ea) of 21.16 kJ mol1. Ru@Ni/C NPs are the next most active, whereas Ru@Fe/C NPs are almost inactive. Additionally, the as-synthesized NPs supported on carbon black exhibit higher catalytic activity than catalysts on other conventional supports, such as SiO2 and g-Al2O3.
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core–shell NPs have been synthesized by a multi-step, hightemperature method;[34] but to the best of our knowledge there are no reports on Ru@Co core–shell NPs. Therefore, the development of a facile route to construct Ru-based core–shell NPs with high catalytic activity is highly desirable. Herein, we report a facile strategy for in situ synthesis of Ru@Co, Ru@Ni, and Ru@Fe core–shell NPs supported on carbon black (Ketjen carbon black), by using AB as reductant in a one-step coreduction route at room temperature under ambient conditions. The activity of these NPs towards the catalytic hydrolysis of AB and MeAB at room temperature was studied. Ketjen carbon black was chosen as a support because it has been reported to be an efficient support for catalysts in the hydrolysis of AB.[17, 33, 42]
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tion was introduced to a round-bottom flask that contained a mixture of aqueous RuCl3, CoCl2 or NiCl2 or FeCl2, and carbon black. Considering the lower reduction potentials (E0ACHTUNGRE(Co2+/Co) = 0.28 eV vs. SHE; E0ACHTUNGRE(Ni2+/Ni) = 0.25 eV vs. SHE; E0ACHTUNGRE(Fe2+/Fe) = 0.44 eV vs. SHE), M2 + cannot be directly reduced by AB. Ru3 + , which has a high reduction potential (E0ACHTUNGRE(Ru3+/Ru) = + 0.40 eV vs. SHE), was first reduced by AB to form the Ru NPs, and serves as the in situ seeds to introduce the successive growth of the M NPs as the shell, which may be generated by a RuH species with strong reducing ability[43] to form the Ru@M core–shell NPs. The microstructure of Ru@Co/C and Ru@Ni/C NPs were characterized by transmission electron microscopy (TEM). As shown in Figure 1a and d, the as-synthesized NPs were well dispersed on carbon black. A distinct contrast of core and shell can be observed clearly in the high-resolution TEM (HRTEM) images in Figure 1b and e; the dark core is Ru and the gray shell is Co and Ni, which indicates that Ru is initially reduced by AB and subsequently acts as the seed that helps to reduce Co2 + and Ni2 + to form the shells. The d spacing of the crystallized core part is 0.205 nm, which is consistent with hexagonal RuACHTUNGRE(101) plane spacing, the out-
Results and Discussion Synthesis and Characterization In a typical synthesis of carbon-black-supported Ru@M (M = Co, Ni, Fe) core–shell NPs, the ammonia borane solu-
Figure 1. a, b) TEM image of Ru1@Co10/C at different magnifications. c) TEM image of Ru1@Co10/C NPs after five cycles. d, e) TEM image of [email protected]/ C at different magnifications. f) TEM image of [email protected]/C NPs after five cycles. g) TEM image of RuNi/C NPs reduced by NaBH4. h) EDX spectrum of Ru@Co/C. i) EDX spectrum of Ru@Ni/C.
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Figure 2. XRD patterns of Ru@Co/C and Ru@Ni/C, and RuNi/C, Ru/C, and Ni/C after annealing at 500 8C for 4 h under Ar.
Figure 3. Time plots of the catalytic dehydrogenation of AB over the Ru@Co/C catalysts under ambient conditions. Ru/AB 0.004 (molar ratio).
side shells of Co and Ni are amorphous. The EDS spectra confirm the presence of Co and Ni (Figure 1h, i), which indicates that the shell parts are amorphous Co and Ni. Figure 2 shows the power XRD pattern of the as-prepared Ru@Co/C and Ru@Ni/C NPs. The 2q peak at around 248 is associated with the carbon black, whereas the peak at around 448 is attributed to the (101) plane of metallic Ru. However, no diffraction peak for Co and Ni is observed, which may be caused by the amorphous phase of Co and Ni. Although no lattice lines for RuNi/C can be found in Figure 1g, in the XRD patterns (Figure 2) it can be seen that the peak at 448 for pure Ru/C NPs matches that of hcp-structured RuACHTUNGRE(101), and the peak at 44.58 for pure Ni/C NPs matches that of fcc-structured NiACHTUNGRE(111). The RuNi/C samples reduced by NaBH4 exhibit a peak at 44.38, which indicates the structure of the RuNi alloy.[33]
for the catalytic hydrolysis of AB by Ru-based catalysts, and higher than most other values reported for noble-metalbased catalysts if the TOF is normalized in terms of moles of noble metal (Table 1). Figures 4 and 5 shows the plots of hydrogen generation from the hydrolysis of a solution of AB or MeAB, respectively, in the presence of different Ru1@Co10/C NPs concenTable 1. Catalytic activity of different noble-metal-based catalysts used for the hydrolytic dehydrogenation of AB.
Catalytic Activities for Hydrolysis of AB and MeAB by Ru@Co/C NPs Without Ru3 + , the Co2 + cannot be reduced by AB, which results in no activity in the catalytic hydrolysis of AB, as shown in Figure 3. When pure Ru NPs were supported on carbon black, with the molar ratio of Ru/ABs kept at 0.004, AB can be completely catalytically decomposed to release three equivalents of H2 in about 12 min. Unexpectedly, when Co was introduced into the NPs to form the core–shell NPs, with the molar ratio of Ru/AB still kept at 0.004, the decomposition time of AB was decreased. When the Co/Ru molar ratio was 10, the release of H2 was complete within 2 min, and a further increase in the Co/Ru molar ratio to 12.5 resulted in a prolonged reaction time, which indicated the positive effect of the Ru@Co core–shell nanostructures on the catalytic hydrolysis of AB and showed that the best Co/Ru molar ratio in the Ru@Co core–shell NPs is 10. This is similar to results from Demirci et al. for their RuCo and RuCu systems.[21] To the best of our knowledge, this is the first report of the synthesis of Ru@Co core–shell NPs. The activity in terms of turnover frequency (TOF) is 320 (molH2 min1) molRu1 for these as-synthesized NPs, moreover, this is one of the highest values to be reported
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Catalyst
TOF [(molH2 min1) molM1] Ea [kJ mol1] Reference M = Pt, Ru, Pd, Au, Rh
Ru/C Pd@Co/graphene Ru0@MWCNT Ru@Co/C Pt/g-Al2O3 [email protected]/C Ru@Ni/C [email protected]/C Pt@MIL-101 Ni0.74Ru0.26 alloy NPs PSSA-co-MA stabilized Ru nanoclusters Rh/g-Al2O3 Ni@Ru Ru@Al2O3 after acetic acid treatment Ru/g-Al2O3 laurate-stabilized ruthenium(0) nanoclusters Ru@Al2O3 RuCo (1:1)/gAl2O3 Ru/g-Al2O3 PSSA-co-MA stabilized Pd nanoclusters RuCu (1:1)/gAl2O3 RGO/Pd
429.5 408.9 329 319.7 308 280.3 250.1 248.5 233.22 194.8
34.81 – 33 21.16 21 33 37.87 41.5 40.7 37.18
[17] [44] [32] this study [47] [20] this study [15] [48] [33]
187.6
54
[49]
178 114 83.3
21 44 46
[47] [34] [29]
77 75
23 47
[47] [45]
39.6 32.9
48 47
[29] [21]
23.05 19.9
67 44
[50] [49]
16.4
52
[21]
51
[51]
6.25
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trations at (25 0.2) 8C. The initial rate of hydrogen generation was determined from the initial nearly linear portion of each plot. The line slope of the plot of hydrogen evolution rate versus catalyst concentration in a log–log scale is 1.13 for AB and 0.98 for MeAB; this indicates that the hydrolysis of AB and MeAB catalyzed by Ru1@Co10/C NPs is first order with respect to the catalyst concentration. To get the activation energy (Ea) of AB and MeAB hydrolysis catalyzed by Ru1@Co10/C NPs, the hydrolytic reactions at different temperature range of 25–40 8C were carried out. The values of rate constant k at different temperatures were calculated from the slope of the linear part of each plot from Figures 6a and 7a. The Arrhenius plots of lnk versus 1/T for the catalysts are plotted in Figures 6b and 7b, from which the apparent activation energy was determined to be approximately 21.16 and 26.05 kJ mol1 for AB and MeAB, respectively. These values are lower than most of the reported Ea values (Table 1), which indicates the superior catalytic performance of the as-synthesized Ru1@Co10/C NPs. To better understand the hydrolysis of MeAB in the presence of Ru1@Co10/C NPs and confirm the complete decomposition of MeAB after the catalytic reaction, we performed 11 B NMR spectroscopic studies (Figure 8). The 11B peak at d = 19.8 ppm, which is assigned to MeAB, disappears after the catalytic reaction and a new peak around d = 11.4 ppm is observed, which indicates three moles of hydrogen has been generated from one mole of MeAB during the catalytic hydrolysis according to Equation (1). Furthermore, there is no change in the 11B NMR spectrum of MeAB after 7 d under ambient conditions, which indicates the catalytic activities of the as-prepared NPs in the catalytic hydrolysis of MeAB under ambient conditions.
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Figure 4. a) Plots of moles of H2 released per mole of AB vs. time for the catalytic hydrolysis of AB by different concentrations of Ru1@Co10/C at (25 0.2) 8C. b) The plot of hydrogen generation rate vs. Ru1@Co10/C NPs concentration (both in logarithmic scale).
Catalytic Activity of Ru@Ni/C NPs for Hydrolysis of AB Without Ru3 + , Ni2 + cannot be reduced by AB, which results in no activity in the catalytic hydrolysis of AB, as shown in Figure 9. When Ni was introduced into the Ru NPs to form core–shell NPs, with the molar ratio of Ru/AB kept at 0.004, the reaction time for the decomposition of AB was decreased. When the Ni/Ru molar ratio was 7.5, the release of H2 was completed in about 2 min, which indicates the positive effect of the Ru@Ni core–shell nanostructures on the catalytic hydrolysis of AB. If the Ni/Ru molar ratio was further increased to 10 and 12.5, no significant decrease in the reaction time was observed, which indicates that the best Ni/Ru molar ratio in the Ru@Ni core–shell NPs is 7.5. The activity in terms of turnover frequency (TOF) is 250 (molH2 min1) molRu1 for these as-synthesized NPs, which is lower than that of as-synthesized Ru1@Co10/C NPs but is still higher than most of the reported noble-metalbased catalysts shown in Table 1. Interestingly, the TOF value of the as-synthesized NPs is much higher than that of the reverse Ni@Ru core–shell NPs that have the more reactive Ru as the shell.[34] The results may be attributed to the positive synergistic effect between carbon black and the
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Figure 5. a) Plots of moles of H2 released per mole of MeAB vs. time for the catalytic hydrolysis of MeAB by different concentrations of Ru1@Co10/C at (25 0.2) 8C. b) The plot of hydrogen generation rate vs. Ru1@Co10/C NPs concentration (both in logarithmic scale).
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Figure 7. a) Plots of moles of H2 released per mole of MeAB vs. time for the catalytic hydrolysis of MeAB by Ru1@Co10/C at different temperatures; Ru/AB 0.004. b) The Arrhenius plot (lnk vs. reciprocal absolute temperature, 1/T [K1]).
Figure 6. a) Plots of moles of H2 released per mole of AB vs. time for the catalytic hydrolysis of AB by Ru1@Co10/C at different temperatures; Ru/ AB 0.004. b) The Arrhenius plot (lnk vs. reciprocal absolute temperature, 1/T [K1]).
Ru@Ni core–shell nanostructures, and more active sites deriving from the higher amorphous Ni contents.[44] In comparison, Ru1@Fe5/C and Ru1@Fe10/C NPs were synthesized by using a similar method. As shown in Figure 9, the as-prepared Ru@Fe/C NPs show almost no reactivity toward the hydrolysis of AB and are even worse than Ru/C NPs, which indicates the negative effect of Fe in our system toward the dehydrogenation of AB under ambient conditions. For comparison, Ru1Ni7.5/C alloy NPs reduced by NaBH4 were prepared and applied to the catalytic hydrolysis of AB. As shown in Figure 10, their catalytic activity is inferior to that of [email protected]/C NPs. Additionally, to study the effects of the supporting materials on the catalytic performance of the as-synthesized core–shell NPs, [email protected]/SiO2, [email protected]/gAl2O3, and carbon-black-free [email protected] NPs were prepared and their catalytic activity in the hydrolysis of AB were studied. The inductively coupled plasma-atomic emission spectroscopy (ICP-AES) results give values of 7.5, 13.6, and 10.9 wt % for Ru@Ni NPs on C, SiO2, and g-Al2O3, respectively. However, as shown in Figure 10, the NPs supported on SiO2 and g-Al2O3 both show inferior catalytic activity to that of [email protected]/C NPs, which highlights the dominant factor of carbon black in facilitating the hydrolysis of AB in our system. The specific surface areas of all the catalysts, as determined by using the BET method, are summarized in
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Figure 8. 11B NMR spectra of a) freshly prepared aqueous MeAB, b) aqueous MeAB after 7 d in air, and c) aqueous MeAB after catalysis by [email protected]/graphene NPs.
Table 2. There are obvious decreases in the surface area after loading metals onto these supports. Figure 11 shows the plots of hydrogen generation from the hydrolysis of AB in the presence of different concentrations of [email protected]/C NPs at (25 0.2) 8C. The initial rate of hydrogen generation was determined from the initial nearly linear portion of each plot. The line slope of the plot of hydrogen evolution rate versus catalyst concentration in a log– log scale is 1.11, which indicates that the hydrolysis of AB
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Figure 9. Time plots of catalytic dehydrogenation of AB over Ru@Ni/C catalysts and Ru@Fe/C catalysts under ambient conditions; Ru/AB 0.004 (molar ratio).
Figure 11. a) Plots of moles of H2 released per mole of MeAB vs. time for the catalytic hydrolysis of AB by different concentrations of [email protected]/C at (25 0.2) 8C. (b) The plot of hydrogen generation rate vs. [email protected]/C NPs concentration (both in logarithmic scale). Figure 10. Time plots of the catalytic dehydrogenation of AB over [email protected]/C, [email protected]/SiO2, [email protected]/g-Al2O3, Ru1Ni7.5/graphene, and [email protected] NPs under ambient conditions; Ru/AB 0.004.
Recycling Stability The reusability of the catalyst is crucial for practical applications. The recycling stability of Ru1@Co10/C NPs over five runs in the hydrolysis of AB and MeAB and of [email protected]/C NPs over five runs in the hydrolysis of AB are shown in Figures 13, 14, and 15, respectively. As shown in Figures 13 and 14, the as-prepared Ru1@Co10/C catalysts retain 52.5 and 50 % of their initial catalytic activity in the fifth run for the hydrolysis of AB and MeAB, respectively. In Figure 15, the as-synthesized bimetallic core–shell [email protected]/C NPs retains 52.5 % of the initial catalytic activity for the hydrolysis of AB. Figure 1c and f show the representative TEM images of Ru1@Co10/C NPs and [email protected]/C NPs, respectively, after the fifth run of this durability test. As can be clearly seen from the TEM images, there is some agglomeration of the as-synthesized NPs on carbon black after the fifth run. The aggregation of metal nanoparticles causes a decrease in the surface area of the nanoparticles, which may further result in a decrease in the catalytic activity. Furthermore, XPS analyses of the survey scan of Ru@Co/ C NPs before and after the five cycles are displayed in Figure 16a and b. However, the overlap of the C 1s and Ru 3d peaks around 285 eV makes it difficult to analyze this range for ruthenium correctly. Therefore, Figure 16c and d show the peak of Ru 3p for the catalysts before and after five cycles. The Ru 3p feature fits well to two peaks at 461.4 and
Table 2. Characteristics of supports and catalysts tested for the hydrogen generation reaction. Support BET surface area [m2 g1]
Pore volume [cm3 g1]
Pore size [nm]
C 1030.3 380.0 SiO2 g-Al2O3 118.5
1.35 0.89 0.19
6.87 6.79 4.46
Catalyst
BET surface area [m2 g1]
Pore volume [cm3 g1]
Pore size [nm]
Ru@Ni/C Ru@Ni/SiO2 Ru@Ni/gAl2O3
254.7 216.1 23.0
0.78 0.84 0.15
8.78 7.15 7.24
catalyzed by [email protected]/C NPs is first order with respect to the catalyst concentration. To get the activation energy (Ea) of AB hydrolysis catalyzed by [email protected]/C NPs, the hydrolytic reactions at temperatures between 25 and 40 8C were carried out. As shown in Figure 12, the apparent activation energy was determined to be approximately 36.59 kJ mol1, which is higher than that of Ru1@Co10/C NPs (Table 1).
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Figure 12. (a) Time plots of mol H2 per mol MeAB vs. time for the [email protected]/C catalytic hydrolysis of AB at different temperatures, Ru/ AB = 0.004; (b) the Arrhenius plot (ln k versus the reciprocal absolute temperature, 1/T (K1)).
Figure 14. a) Time plots of the catalytic dehydrogenation of MeAB over Ru1@Co10/C catalysts from 1 to 5 cycles at (25 0.2) 8C; Ru/AB 0.004. b) Percentage of initial catalytic activity of Ru1@Co10/C in successive runs after reuse for the hydrolysis of MeAB.
Figure 13. a) Time plots of the catalytic dehydrogenation of AB over Ru1@Co10/C catalysts from 1 to 5 cycles at (25 0.2) 8C; Ru/AB 0.004. b) Percentage of initial catalytic activity of Ru1@Co10/C in successive runs after reuse for the hydrolysis of AB.
Figure 15. a) Time plots of the catalytic dehydrogenation of AB over [email protected]/C catalysts from 1 to 5 cycles at (25 0.2) 8C; Ru/AB 0.004. b) Percentage of initial catalytic activity of [email protected]/C in successive runs after reuse for the hydrolysis of AB.
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463.5 eV, which are readily assigned to Ru0 3p and RuIV 3p, respectively.[45] There are almost no differences between the two peaks before and after the catalytic hydrolysis of AB. RuIV oxide present on the surface of Ru likely forms during the sample preparation process for XPS measurements. Figure 16e and f shows the peak of Co 2p before and after five cycles. There are three peaks with peak tops at 778.1, 781.5, and 786.3 eV, which were assigned to Co0 and oxidized Co.[38] In Figure 16f, there are almost no peaks for Co0, which indicates that the outside shell Co is oxidized during the catalytic process, which may cause a decrease in the catalytic activity. Moreover, the increased viscosity of the solution or deactivation effect of the increasing metaborate concentration during the hydrolysis of AB/MeAB should be taken into account.[46]
Fe) core–shell NPs. The Ru@Co core–shell NPs were synthesized and characterized first. The as-synthesized Ru@Co and Ru@Ni NPs exhibit superior catalytic activity compared with monometallic and alloy counterparts in the hydrolysis of ammonia borane. The Ru@Co/C NPs exhibit superior catalytic activity, with a TOF value of 320 (molH2 min1) molRu1 and an activation energy (Ea) of 21.16 kJ mol1; followed by Ru@Ni/graphene NPs, whereas the Ru@Fe/graphene NPs are almost inactive. The carbonblack-supported Ru@Co NPs exhibit higher catalytic activity than the counterparts supported on SiO2 or g-Al2O3 or without carbon black. Moreover, this simple synthetic method can be extended to other carbon-black-supported core–shell NPs systems for further applications.
Experimental Section
Conclusion
Chemicals and Materials
We have developed a facile in situ one-step method for the synthesis of carbon-black-supported Ru@M (M = Co, Ni,
Ammonia borane (NH3BH3, AB, Aldrich, 90 %), sodium borohydride (Sinopharm Chemical Reagent Co., 96 %), ruthenium chloride hydrate (RuCl3·n H2O, The Wuhan Xinsirui Technology Co., 99.9 %), cobalt chloride hexahydrate (CoCl2·6 H2O, Sinopharm Chemical Reagent Co., 99 %), nickel chloride hexahydrate (NiCl2·6 H2O, Sinopharm Chemical Reagent Co., 99 %), ferrous chloride tetrahydrate (FeCl2·4 H2O, Sinopharm Chemical Reagent Co., 99 %), Ketjenblack EC-300 J (Triquo Chemical Co.), neutral silica power (SiO2, Branch of Qingdao Haiyang Chemical Co.), Aluminum oxide neutral (gAl2O3, Sinopharm Chemical Reagent Co.), methylamine hydrochloride (CH3NH2·HCl, Sinopharm Chemical Reagent Co., 96 %), tetrahydrofuran (C4H8O, THF, Sinopharm Chemical Reagent Co., 99 %), dimethyl ether anhydrous (C4H10O, Sinopharm Chemical Reagent Co., Ltd., 99.7 %). All chemicals were used as obtained. We used ordinary distilled water as the reaction solvent. Preparation of Methyl Ammonia Borane (CH3NH2BH3, MeAB)
Figure 16. XPS spectra of a, b) the survey scan of Ru@Co/C NPs before the first cycle and after five cycles; c, d) the Ru 3p levels of Ru@Co/C NPs before the first cycle and after five cycles; e, f) the Co 2p levels of Ru@Co/C NPs before the first cycle and after five cycles.
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MeAB was synthesized by using the method reported in the literature.[35] Sodium borohydride (1.8925 g, 0.05 mol) and methylamine hydrochloride (3.376 g, 0.05 mol) were added to a 250 mL two-neck round-bottom flask with a neck connected to a condenser. THF (100 mL) was added to the flask and the contents were vigorously stirred at 25 8C. The reaction was carried out at RT under a nitrogen atmosphere. After 12 h, the resultant solution was filtered by suction filtration and the filtrate was concentrated under vacuum at RT. The product was purified by using diethyl ether.
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In Situ Synthesis of Ru@Co/C, Ru@Ni/C, and Ru@Fe/C Catalysts
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In a typical experiment for producing Ru@Co/C NPs, carbon black (8 mg) was dispersed in water (1.6 mL) and kept in a two-necked roundbottom flask. Ultrasonication was required to get a uniform dispersion. An aqueous ruthenium chloride solution (0.4 mL, 0.01 mol L1) and an aqueous cobalt chloride solution (4 mL, 0.01 mol L1) were added to this flask. One neck was connected to a gas burette and the other was connected to a pressure equalizer to introduce AB. An aqueous solution containing AB (62 mg in 4 mL; 90 %) was kept in the pressure equalizer. The reaction was started when the AB solution was added to the flask with vigorous stirring and the evolution of gas was monitored by using the gas burette. After the hydrogen generation reaction was complete, an aqueous solution of AB (30.8 mg in 2 mL) was added to the flask and the evolution of gas was monitored. A water bath was used to control the temperature of the reaction solution. For Ru@Ni/C and Ru@Fe/C, the same preparation procedure as described above for Ru@Co was applied, except that CoCl2·6 H2O was replaced by NiCl2·6 H2O and FeCl2·4 H2O, respectively. For comparison, Ru@Ni/g-Al2O3 NPs, Ru@Ni/SiO2 NPs, and Ru@Ni NPs without C reduced by AB, and RuNi/C reduced by NaBH4 were synthesized by using a similar method. Catalytic Hydrolysis of AB and MeAB Sets of experiments with different concentrations of Ru@Ni (0.017, 0.0225, 0.034, 0.0425 mmol) or Ru@Co (0.022, 0.033, 0.044, 0.055 mmol) were performed at RT ((25 0.2) 8C) with the AB (or MeAB) concentration kept the same (1 mmol) to determine the rate law of the catalytic hydrolysis of AB (or MeAB). The temperature used were (25 0.2), (30 0.2), (35 0.2), and (40 0.2) 8C, and the concentration of Ru (0.004 mmol) and AB or MeAB (1 mmol) were kept constant to obtain the activation energy (Ea). Recycling Stability Test For the recycling stability test, catalytic reactions were repeated five times by adding another equivalent of AB or MeAB (1 mmol) to the mixture after the previous cycle. Characterization Transmission electron microscope (TEM) and energy-dispersive X-ray spectroscopy (EDS) images were obtained by using a FEI Tecnai G20 UTwin TEM instrument operating at 200 kV. Powder X-ray diffraction (XRD) patterns were measured by using a Bruker D8-Advance X-ray diffractometer with a CuKa radiation source (l = 0.154178 nm) with a velocity of 68 min1. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Kratos XSAM 800 spectrophotometer. 11 B NMR spectra were recorded at ambient temperature by using a Varian-VX 300 spectrometer and externally referenced to BF3·Et2O (d = 0). Elemental composition was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and was performed by using an IRIS Intrepid II XSP. The surface area measurements were performed by N2 adsorption at the temperature of liquid N2 by using a Micromeritics ASAP 2020 BET analyzer.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21201134), the Ph. D Programs Foundation of the Ministry of Education of China (20120141120034), and the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.
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