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Promoting hydrolysis of ammonia borane over multiwalled carbon nanotube-supported Ru catalysts via hydrogen spillover
Catalysis Communications 91 (2017) 10–15
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Promoting hydrolysis of ammonia borane over multiwalled carbon nanotube-supported Ru catalysts via hydrogen spillover Zhijie Wu a,d,⁎, Yongli Duan a, Shaohui Ge b, Alex C.K. Yip c, Fan Yang a, Yongfeng Li a, Tao Dou a a
State Key Laboratory of Heavy Oil Processing, The Key Laboratory of Catalysis of CNPC, China University of Petroleum, Beijing, 102249, China Petrochemical Research Institute, PetroChina Company Limited, Beijing 100195, China Department of Chemical and Process Engineering, University of Canterbury, Christchurch, New Zealand d Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China b c
a r t i c l e
i n f o
Article history: Received 18 September 2016 Received in revised form 24 November 2016 Accepted 7 December 2016 Available online 10 December 2016 Keywords: Ammonia borane Hydrolysis Hydrogen spillover Ru catalyst Carbon nanotube
a b s t r a c t Multi-walled carbon nanotubes were used to deposit Ru metal by electrostatic adsorption and incipient wetness impregnation methods, respectively. The electrostatic adsorption method led to small (1.8– 2.5 nm) and highly dispersed (0.51– 0.72 dispersion) Ru metal nanoparticles. The initial hydrogen generation turnover rates over Ru catalysts with different metal particle sizes showed that ammonia borane hydrolysis reaction is structure-sensitive; large Ru particles displayed high turnover rates. Multiwalled carbon nanotube-supported Ru nanoparticles exhibit high turnover rate and low activation energy for the hydrolysis of ammonia borane because of hydrogen spillover effect associated with strong interaction between Ru metal and carbon nanotubes. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Ammonia borane (AB, NH3BH3) has been recognized as a chemical hydrogen storage material with high potential [1,2]. At present, two methods are used to release hydrogen: solid thermolysis and solvolysis (hydrolysis or methanolysis) [1–3]. In general, catalytic hydrolysis of AB has been widely studied because of hydrogen capacity of up to 19.5 wt% of AB and H2O under ambient atmosphere at room temperature with the aid of efficient catalysts. The design and synthesis of efficient hydrolysis catalysts has attracted increasing attention in the last decade [4]. To date, the noble metals (Pd [5], Pt [6,7], Ru [8–25], Rh [4,26], Ag [27] and Au [28]), the transition metals [4,29–33]), and bimetallic [34] and alloy [35] catalysts have been reported to be active catalysts for the hydrolysis of AB. Among these catalysts, Ru-based catalysts have proved to be the most efficient catalysts for hydrolysis of borohydrides and are also frequently used in AB hydrolysis [8–25]. Xu et al. found that Pt nanoparticles on different supports (γ-Al2O3, carbon and SiO2) possess different hydrolysis activity, and this is mainly attributed to differences in particle size [3]. Interestingly, the activity of AB hydrolysis over Ru metal nanoparticles with similar size (~2 nm) varied with the type of support used (carbon, carbon nanotubes, metal oxides, etc.) [8–25]. In particular, carbon materials seem to show better performance ⁎ Corresponding author at: State Key Laboratory of Heavy Oil Processing, The Key Laboratory of Catalysis of CNPC, China University of Petroleum, Beijing 102249, China. E-mail address: [email protected] (Z. Wu).
http://dx.doi.org/10.1016/j.catcom.2016.12.007 1566-7367/© 2016 Elsevier B.V. All rights reserved.
for Ru catalysts [17,23]. These observations suggest an important role of supports in determining the nature and performance of Ru-based catalysts. In fact, promotion of hydrogenation by hydrogen spillover has been observed on carbon nanotube-supported metal nanoparticles [36]. Moreover, metal-doped carbon nanotubes possess good hydrogen storage capacity under ambient pressure [37,38]. We infer that hydrogen spillover associated with the excellent hydrogen storage capacity of carbon nanotubes on the supported metal catalyst would promote its catalytic performance in the hydrolysis of AB. Here, various Ru nanoparticles supported on multi-walled carbon nanotubes (MWCNTs), activated carbon (AC), and SiO2 were prepared and studied to verify our inference. 2. Experimental 2.1. Catalyst synthesis MWCNTs and AC were treated in boiling water for 12 h and then dried at 383 K for 12 h. Silica was treated in ambient air at 673 K for 3 h. Ru metal was deposited on supports using electrostatic adsorption (EA) protocols [39], which added supports to an aqueous solution of Ru(NH3)6Cl3 and NaOH. After stirring for 1 h (pH = 11), the solids were filtered and washed with deionized water to remove chloride ions. The samples were dried at 383 K for 24 h. To obtain MWCNTand AC-supported Ru catalysts, the samples were added to AB solution (0.1 mol L− 1, solid/liquid weight ratio = 1/10) with stirring
Z. Wu et al. / Catalysis Communications 91 (2017) 10–15
(500 rpm) while flowing nitrogen gas to the solution (50 mL min−1) for 1 h. The resulted catalysts were collected and washed three times with deionized water. For comparison, Ru catalysts supported on MWCNTs and AC were also prepared by a modified incipient wetness impregnation (IWI) method (See supplementary material). The catalysts prepared using EA and IWI methods are denoted as Ru/ MWCNTs-EA and Ru/MWCNTs-IWA, respectively.
2.2. Characterization Powder X-ray diffraction (XRD) patterns of catalysts were obtained over a 2θ range of 30–80° at a scan rate of 4°min− 1 on a Rigaku D/ max 2500 X-ray diffractometer (Cu Kα, λ = 1.54178 Å) operated at 40 kV and 40 mA. The compositions of samples were determined through inductively coupled plasma atomic emission spectrometry (ICPAES) using an IRIS Intrepid spectrometer. The morphology and particle size were examined via transmission electron microscopy (TEM) using a FEI Tecnai G2 high-resolution transmission electron microscope operated at 200 kV. Ru dispersion was determined from the uptake of strongly chemisorbed hydrogen at 313 K using a Quantasorb chemisorption analyser (Quantachrome Corp.) [39].
2.3. Hydrolysis reactions The hydrolysis reaction was performed in a small glass reactor (200 mL) in a bath-type hydrogen generator [40]. The mass-transfer limitations were excluded using procedures described elsewhere [40]. A typical measurement consisted of introducing the catalyst (0.0050– 0.10 g) into the reactor along with a solution containing NH3BH3. The hydrogen generation rate (mL min−1) was measured using a hydrogen flow meter. The initial hydrogen generation rate was recorded, and the initial hydrogen generation turnover rate (per surface metal atom, as determined from the chemisorption uptake) was calculated −1 ). (molH2 mol−1 surf-metal s Typically, a certain amount of Ru catalysts was placed in a bath-type hydrogen generator (200 mL). Then, a 10% H2/He gas mixture was induced to replace the air in the generator. The Ru catalysts were then treated at 473 K for 2 h to reduce the ruthenium oxide species formed on the surface of Ru metal clusters during the transfer of the Ru catalysts into generator. After cooling to room temperature, 50 mL of deionized water was injected to disperse the Ru catalysts under stirring at 500 rpm. Then, pure He gas was induced to replace the 10% H2/He gas, and the mixture was heated to the reaction temperature. The hydrogen generator was equipped with an outlet tube connected to a column packed with silica gel to eliminate any moisture formed. A mass flow meter was used to record instantaneous hydrogen generation rates (mL min−1). The hydrolysis reaction was conducted by injecting an AB aqueous solution using a high precision syringe and the instantaneous hydrogen generation rate was recorded digitally at 1 s interval. The instantaneous hydrogen generation rate increased into a maximum value within 30 s and then stabilized for tens of minutes. The maximum hydrogen generation rate (rmax, mL min−1) was considered as the initial hydrogen generation rate for calculating initial hydrogen generation turnover rate (TOR) based on the following equation.
TOR ¼
r max −1
22:4 1000 mL∙mol
60 s∙min−1
3. Results and discussion 3.1. Catalyst properties A comparison of XRD patterns of MWCNTs and Ru/MWCNTs catalysts, presented in Fig. 1, clearly shows no change in the characteristic diffraction peaks of multiwalled carbon nanotubes after deposition of Ru metal, as found in the work of Ӧzkar et al. [12]. The diffraction peaks at 26.2 and 43.3° could be well-indexed as (002) and (110) reflections of graphite structure, respectively. The XRD patterns of MWCNTs, Ru/MWCNTs-EA (1.0 wt% Ru, Table 1) and Ru/MWCNTs-IWI-1 (0.9 wt% Ru, Table 1) are essentially the same. There are not any observable diffraction peaks attributable to Ru metal in Ru/MWCNTs-EA, Ru/MWCNTs-IWI-1 or Ru/AC-EA samples, probably as a result of low Ru loading and small Ru cluster size. A weak peak corresponding to (101) plane of Ru metal could be distinguished in the XRD pattern of Ru/MWCNTs-IWI-2 (JCPDS No. 1-1253) at higher metal loading (5.0 wt% Ru), suggesting the formation of larger Ru clusters. Table 1 shows metal loadings and metal dispersions of catalysts. Clearly, samples obtained by EA method possess a higher Ru dispersion than catalysts obtained by IWI method. This indicates an advantage in the synthesis of small metal clusters using EA method. Fig. 2 shows the TEM and HRTEM images of supported Ru catalysts. Most of Ru nanoparticles are located on the external surface of MWCNTs without incorporating into the pores of MWCNTs. Ru particles of ~ 2 nm are highly dispersed over Ru/MWCNTs-EA, and clear lattice fringes in the HRTEM images indicate the formation of metallic Ru. Compared to Ru/MWCNTs-EA samples, Ru/MWCNTs-IWI shows a homogeneous distribution of larger nanoparticles (~ 3 nm) on supports. As shown in Table 1, size of Ru nanoparticles observed in TEM images is similar to the size calculated from chemisorption results. These indicate a narrow size distribution of Ru particles over supports. In addition to MWCNT-supported Ru nanoparticles, Ru/ AC and commercial Ru/C catalysts also show a homogeneous distribution of Ru nanoparticles.
1 nsurf −Ru
where nsurf-Ru is the amount of surface active Ru metal (mol) used in each hydrolysis reaction calculated from the metal dispersion of Ru catalysts, the weight of Ru catalysts and the Ru metal loading [39].
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Fig. 1. XRD patterns of MWCNTs and supported Ru catalysts.
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Table 1 Metal loadings and dispersions of Ru-based catalysts.a Catalyst
Ru loading (wt%)
Dispersion
Mean particle size (nm)
Ru/MWCNTs-EA Ru/MWCNTs-IWI-1 Ru/MWCNTs-IWI-2 Ru/AC-EA Ru/AC-IWI Ru/C commercial Ru/SiO2-1 Ru/SiO2-2 Ru/SiO2-3 Ru/SiO2-5
1.0 0.9 5.0 1.0 1.0 3.8 1.0 1.0 1.0 1.0
0.72 0.45 0.26 0.51 0.27 0.65 0.81 0.62 0.36 0.15
1.8 2.9 5.0 2.5 4.8 2.0 1.6 2.1 3.6 8.6
a
The dispersion (D) was calculated followed reference [34].
3.2. Catalytic performance Fig. 3a and b shows the initial hydrogen generation turnover rates at 303 K for Ru/SiO2 catalysts with different Ru metal sizes and carbon material supported Ru catalysts as a function of AB concentration. For all of Ru catalysts, the initial turnover rate increased rapidly as the AB concentration increased from 0.005 to 0.015 mol L−1; the initial turnover rate then increased gradually as the concentration needed to be further increased to 0.03 mol L−1. Interestingly, the initial turnover rates of
supported Ru catalysts changed little when AB concentration was further increased from 0.03 to 0.07 mol L− 1; this is consistent with a zero-order reaction with respect to the AB concentration [4,8,11]. For AB hydrolysis over metal surfaces, the dissociative adsorption of AB cations has been corroborated by previous studies [25]. A modified hydrolysis pathway is proposed in Fig. S1. Fig. 3a and b shows zeroorder reaction kinetic behaviour over supported Ru catalysts with AB concentration from 0.3 to 0.7 mol L−1. Such kinetic behaviour does not change with the size of Ru particles. It can be clearly seen in Fig. 3c that hydrolysis rate increases when the size of the Ru particles increases, indicating that AB hydrolysis reaction is structure-sensitive. Although the large Ru particles confer a high turnover rate, smaller Ru particle size provides a higher surface concentration of active sites for reaction. Thus, the size of the Ru nanoparticles should be optimized to yield economical catalysis. For example, Ru/SiO2-4 with 8.6 nm Ru particles shows 17.9 molH2 molsurface-Ru s− 1, corresponding to 2.7 molH2 moltotal-Ru s−1, whereas Ru/SiO2-2 with 2.1 nm Ru particles possesses 6.1 molH2 molsurface-Ru s−1, corresponding to 3.5 molH2 moltotal-Ru s−1. This shows that reduction in metal size of catalyst can increase the hydrogen generation rate based on the weight of metal used, a finding that is significant for the preparation of industrial catalysts for AB hydrolysis. Fig. 3c shows high turnover rate obtained with Ru/MWCNTs compared to that provided by Ru/AC and Ru/SiO2 catalysts. For instance,
Fig. 2. TEM and HRTEM images of supported Ru catalysts.
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13
Fig. 3. Initial hydrogen generation turnover rates for AB hydrolysis over Ru catalysts at 303 K. Effect of AB concentration on (a) Ru/SiO2, and (b) carbon material supported Ru catalysts; (c) effect of the Ru particle sizes and (d) AB hydrolysis activation energies in 0.06 mol L−1 AB solution.
Ru/MWCNTs-EA (1.8 nm, Table 1) produces a turnover rate of 24.2 molH2 molsurface-Ru s−1, much higher than the rates obtained with Ru/ AC-EA (11.8 molH2 molsurface-Ru s−1, 2.5 nm, Table 1) and Ru/SiO2-2 (6.1 molH2 molsurface-Ru s−1, 2.1 nm, Table 1). The slope of Arrhenius plot (Fig. 3d) indicates an activation energy (Ea) of 36.9 kJ mol−1 for the Ru/MWCNTs-EA catalyst, similar to previously reported values [15] and much lower than the value obtained with Ru/AC-EA (44.2 kJ mol− 1) and Ru/SiO2-2 (52.6 kJ mol− 1, similar to soluble Ru clusters [8]). Clearly, Ru/MWCNTs-EA produces the highest initial turnover rate and the lowest activation energy of the tested catalysts during 0.7 mol L−1 AB hydrolysis at 303 K. These results suggest that the use of carbon materials, especially MWCNTs, as supports yields higher reaction rates in AB hydrolysis than inert oxide supports (SiO2) and that the activity is proportional to the size of metal particles on the supports. The promotion of AB hydrolysis resulted from the MWCNTs support is consistent with other reports in literature. Chen et al. [7] and Zhao et al. [30] concluded that the good performance of CNTs supported Pt or Ni nanoparticles could be attributed to the strong interface interaction between metal and nanotube surface, especially the defect-rich or oxygen group-deficient surfaces. Such interface interaction is expected to favour the formation of a tunable electronic state of metal nanoparticles which enhances the AB hydrolysis. In addition to the modification of binding energy of metal nanoparticles [7,30], promotion of
hydrogenation by hydrogen spillover occurring on carbon nanotubesupported metal nanoparticles has been demonstrated [36,41]. A series of experiment (Table 2) was conducted to reveal the role of hydrogen spillover in AB hydrolysis. Ru/SiO2-2 (2.1 nm Ru metal) yields 6.1 molH2 molsurface-Ru s−1, which is much lower than that of Ru/ACEA (2.0 nm, 10.1 molH2 molsurface-Ru s− 1) and Ru/MWCNTs-EA (1.8 nm, 24.2 6.1 molH2 molsurface-Ru s−1). Ru/SiO2-2 was diluted with SiO2 powder (1:4 weight ratio) to form 38–48 μm pellets and then mixed with AC and MWCNTs to form an interpellet mixture. The results show that there was no obvious promotion of turnover rate in either toluene hydrogenation or AB hydrolysis (Tables 2 and S1). However, when an interpellet mixture was formed by directly mixing Ru/SiO2-2 with AC or MWCNTs (1:4 weight ratio) an apparent increase of 26–30% (Table S1) in turnover rate of toluene hydrogenation was measured. Moreover, the rate increased significantly by ~90% when Ru/SiO2-2 was directly diluted with AC or MWCNTs (1:4 weight ratio) to form 38–48 μm intrapellets (Table S1), yielding a hydrogenation turnover rate similar to those obtained with Ru/AC-EA and Ru/MWCNTs-EA. These results indicate that even without the strong interaction between Ru metal clusters with carbon supports, a promotion phenomenon still occurs by introducing carbon nanotubes into the metal catalysts. Hydrogenation can be promoted by hydrogen spillover when there is sufficient contact with metal sites that the use of supports enables hydrogen spillover, and
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Table 2 The initial hydrogen generation turnover rate of intrapellet and interpellet mixtures of Ru/ SiO2 and carbon supports with 1:4 weight ratio.a Catalyst
Mixture
Turnover rate (molH2 molsurface-Ru s−1)
Ru/SiO2-2 Ru/SiO2-2 + SiO2 (1:4) intrapelletb Ru/SiO2-2 Ru/SiO2-2 Ru/AC-EA Ru/SiO2-2 + SiO2 (1:4) interpelletb Ru/SiO2-2
– AC, interpellet
6.1 6.3
AC, interpellet AC, intrapellet – MWCNTs, interpellet MWCNTs, interpellet MWCNTs, intrapellet –
7.5 9.5 10.1 6.5
Ru/SiO2-2 Ru/MWCNTs-EA
7.8 15.5 24.2
a
The intrapellet mixture was prepared by mixing powdered Ru/SiO2-2 with AC and MWCNTs using a mortar and pestle until homogenous (30 min) under ambient conditions, respectively. The interpellet mixture was obtained by directly adding 38–48 μm Ru/SiO2-2 powders to 38–48 μm AC and MWCNTs powders. b The Ru/SiO2-2 + SiO2 (1:4) intrapellet with 38–48 μm was prepared by mixing powdered Ru/SiO2-2 with SiO2.
AC and MWCNTs provide a similar enhancement of hydrogenation. For AB hydrolysis, the initial turnover rate of Ru/SiO2-2 also increased from 6.1 to 9.5 molH2 molsurface-Ru s−1, which is similar to that obtained with Ru/AC-EA (10.1 molH2 molsurface-Ru s−1), when AC was used as a diluent to form intrapellets, suggesting a promoting effect of hydrogen spillover on AB hydrolysis. Interestingly, a higher turnover rate (15.5 molH2 molsurface-Ru s− 1) was observed after diluting Ru/SiO2-2 with MWCNTs to form intrapellets, indicating an enhanced promotion effect when MWCNTs were used. Although some reports pointed out that the presence of the metal on or inside carbon nanotubes can enhance hydrogen (gas) storage capacity at ambient pressure [37,38] without obvious promotion in hydrogen storage (i.e., b1%, [41]), atomic hydrogen can be stored by chemisorption under ambient conditions in the presence of metal nanoparticles owing to the spillover of hydrogen from metal to the defects of carbon nanotubes [41]. Thus, our results, together with other results reported in the literature, show that the promotion of AB hydrolysis by MWCNTs can be explained by the hydrogen spillover effect that is associated with strong interface interaction between metal and MWCNTs. As shown in Table 2, the use of Ru/MWCNTs-EA yields a still higher turnover rate (24.2 molH2 molsurface-Ru s− 1) than the rate achieved using diluted intrapellets of Ru/SiO2-2 and MWCNTs. In our view, this can be attributed to better contact or interaction of metal sites with MWCNTs supports. 4. Conclusions We report a general strategy for synthesis of homogeneously distributed Ru nanoparticles on MWCNTs using electrostatic adsorption method. The AB hydrolysis was shown to be a structure-sensitive reaction in which large metal particles possess a high reaction rate. Ru/MWCNTs exhibited high reaction rate and low activation energy during hydrolysis. This can be attributed to the hydrogen spillover effect associated with strong interface interaction between metal and MWCNTs. The results reported herein verified the practical role of carbon nanostructures in the hydrolysis of AB and other boron-containing compounds. Acknowledgments This work was supported by the Science Foundation of China University of Petroleum, Beijing (C201603), the NSF of China (21206192), and the Open Project of Key Lab Adv Energy Mat Chem (Nankai Univ).
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Promoting hydrolysis of ammonia borane over multiwalled carbon nanotube-supported Ru catalysts via hydrogen spillover Zhijie Wu a,d,⁎, Yongli Duan a, Shaohui Ge b, Alex C.K. Yip c, Fan Yang a, Yongfeng Li a, Tao Dou a a
State Key Laboratory of Heavy Oil Processing, The Key Laboratory of Catalysis of CNPC, China University of Petroleum, Beijing, 102249, China Petrochemical Research Institute, PetroChina Company Limited, Beijing 100195, China Department of Chemical and Process Engineering, University of Canterbury, Christchurch, New Zealand d Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China b c
a r t i c l e
i n f o
Article history: Received 18 September 2016 Received in revised form 24 November 2016 Accepted 7 December 2016 Available online 10 December 2016 Keywords: Ammonia borane Hydrolysis Hydrogen spillover Ru catalyst Carbon nanotube
a b s t r a c t Multi-walled carbon nanotubes were used to deposit Ru metal by electrostatic adsorption and incipient wetness impregnation methods, respectively. The electrostatic adsorption method led to small (1.8– 2.5 nm) and highly dispersed (0.51– 0.72 dispersion) Ru metal nanoparticles. The initial hydrogen generation turnover rates over Ru catalysts with different metal particle sizes showed that ammonia borane hydrolysis reaction is structure-sensitive; large Ru particles displayed high turnover rates. Multiwalled carbon nanotube-supported Ru nanoparticles exhibit high turnover rate and low activation energy for the hydrolysis of ammonia borane because of hydrogen spillover effect associated with strong interaction between Ru metal and carbon nanotubes. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Ammonia borane (AB, NH3BH3) has been recognized as a chemical hydrogen storage material with high potential [1,2]. At present, two methods are used to release hydrogen: solid thermolysis and solvolysis (hydrolysis or methanolysis) [1–3]. In general, catalytic hydrolysis of AB has been widely studied because of hydrogen capacity of up to 19.5 wt% of AB and H2O under ambient atmosphere at room temperature with the aid of efficient catalysts. The design and synthesis of efficient hydrolysis catalysts has attracted increasing attention in the last decade [4]. To date, the noble metals (Pd [5], Pt [6,7], Ru [8–25], Rh [4,26], Ag [27] and Au [28]), the transition metals [4,29–33]), and bimetallic [34] and alloy [35] catalysts have been reported to be active catalysts for the hydrolysis of AB. Among these catalysts, Ru-based catalysts have proved to be the most efficient catalysts for hydrolysis of borohydrides and are also frequently used in AB hydrolysis [8–25]. Xu et al. found that Pt nanoparticles on different supports (γ-Al2O3, carbon and SiO2) possess different hydrolysis activity, and this is mainly attributed to differences in particle size [3]. Interestingly, the activity of AB hydrolysis over Ru metal nanoparticles with similar size (~2 nm) varied with the type of support used (carbon, carbon nanotubes, metal oxides, etc.) [8–25]. In particular, carbon materials seem to show better performance ⁎ Corresponding author at: State Key Laboratory of Heavy Oil Processing, The Key Laboratory of Catalysis of CNPC, China University of Petroleum, Beijing 102249, China. E-mail address: [email protected] (Z. Wu).
http://dx.doi.org/10.1016/j.catcom.2016.12.007 1566-7367/© 2016 Elsevier B.V. All rights reserved.
for Ru catalysts [17,23]. These observations suggest an important role of supports in determining the nature and performance of Ru-based catalysts. In fact, promotion of hydrogenation by hydrogen spillover has been observed on carbon nanotube-supported metal nanoparticles [36]. Moreover, metal-doped carbon nanotubes possess good hydrogen storage capacity under ambient pressure [37,38]. We infer that hydrogen spillover associated with the excellent hydrogen storage capacity of carbon nanotubes on the supported metal catalyst would promote its catalytic performance in the hydrolysis of AB. Here, various Ru nanoparticles supported on multi-walled carbon nanotubes (MWCNTs), activated carbon (AC), and SiO2 were prepared and studied to verify our inference. 2. Experimental 2.1. Catalyst synthesis MWCNTs and AC were treated in boiling water for 12 h and then dried at 383 K for 12 h. Silica was treated in ambient air at 673 K for 3 h. Ru metal was deposited on supports using electrostatic adsorption (EA) protocols [39], which added supports to an aqueous solution of Ru(NH3)6Cl3 and NaOH. After stirring for 1 h (pH = 11), the solids were filtered and washed with deionized water to remove chloride ions. The samples were dried at 383 K for 24 h. To obtain MWCNTand AC-supported Ru catalysts, the samples were added to AB solution (0.1 mol L− 1, solid/liquid weight ratio = 1/10) with stirring
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(500 rpm) while flowing nitrogen gas to the solution (50 mL min−1) for 1 h. The resulted catalysts were collected and washed three times with deionized water. For comparison, Ru catalysts supported on MWCNTs and AC were also prepared by a modified incipient wetness impregnation (IWI) method (See supplementary material). The catalysts prepared using EA and IWI methods are denoted as Ru/ MWCNTs-EA and Ru/MWCNTs-IWA, respectively.
2.2. Characterization Powder X-ray diffraction (XRD) patterns of catalysts were obtained over a 2θ range of 30–80° at a scan rate of 4°min− 1 on a Rigaku D/ max 2500 X-ray diffractometer (Cu Kα, λ = 1.54178 Å) operated at 40 kV and 40 mA. The compositions of samples were determined through inductively coupled plasma atomic emission spectrometry (ICPAES) using an IRIS Intrepid spectrometer. The morphology and particle size were examined via transmission electron microscopy (TEM) using a FEI Tecnai G2 high-resolution transmission electron microscope operated at 200 kV. Ru dispersion was determined from the uptake of strongly chemisorbed hydrogen at 313 K using a Quantasorb chemisorption analyser (Quantachrome Corp.) [39].
2.3. Hydrolysis reactions The hydrolysis reaction was performed in a small glass reactor (200 mL) in a bath-type hydrogen generator [40]. The mass-transfer limitations were excluded using procedures described elsewhere [40]. A typical measurement consisted of introducing the catalyst (0.0050– 0.10 g) into the reactor along with a solution containing NH3BH3. The hydrogen generation rate (mL min−1) was measured using a hydrogen flow meter. The initial hydrogen generation rate was recorded, and the initial hydrogen generation turnover rate (per surface metal atom, as determined from the chemisorption uptake) was calculated −1 ). (molH2 mol−1 surf-metal s Typically, a certain amount of Ru catalysts was placed in a bath-type hydrogen generator (200 mL). Then, a 10% H2/He gas mixture was induced to replace the air in the generator. The Ru catalysts were then treated at 473 K for 2 h to reduce the ruthenium oxide species formed on the surface of Ru metal clusters during the transfer of the Ru catalysts into generator. After cooling to room temperature, 50 mL of deionized water was injected to disperse the Ru catalysts under stirring at 500 rpm. Then, pure He gas was induced to replace the 10% H2/He gas, and the mixture was heated to the reaction temperature. The hydrogen generator was equipped with an outlet tube connected to a column packed with silica gel to eliminate any moisture formed. A mass flow meter was used to record instantaneous hydrogen generation rates (mL min−1). The hydrolysis reaction was conducted by injecting an AB aqueous solution using a high precision syringe and the instantaneous hydrogen generation rate was recorded digitally at 1 s interval. The instantaneous hydrogen generation rate increased into a maximum value within 30 s and then stabilized for tens of minutes. The maximum hydrogen generation rate (rmax, mL min−1) was considered as the initial hydrogen generation rate for calculating initial hydrogen generation turnover rate (TOR) based on the following equation.
TOR ¼
r max −1
22:4 1000 mL∙mol
60 s∙min−1
3. Results and discussion 3.1. Catalyst properties A comparison of XRD patterns of MWCNTs and Ru/MWCNTs catalysts, presented in Fig. 1, clearly shows no change in the characteristic diffraction peaks of multiwalled carbon nanotubes after deposition of Ru metal, as found in the work of Ӧzkar et al. [12]. The diffraction peaks at 26.2 and 43.3° could be well-indexed as (002) and (110) reflections of graphite structure, respectively. The XRD patterns of MWCNTs, Ru/MWCNTs-EA (1.0 wt% Ru, Table 1) and Ru/MWCNTs-IWI-1 (0.9 wt% Ru, Table 1) are essentially the same. There are not any observable diffraction peaks attributable to Ru metal in Ru/MWCNTs-EA, Ru/MWCNTs-IWI-1 or Ru/AC-EA samples, probably as a result of low Ru loading and small Ru cluster size. A weak peak corresponding to (101) plane of Ru metal could be distinguished in the XRD pattern of Ru/MWCNTs-IWI-2 (JCPDS No. 1-1253) at higher metal loading (5.0 wt% Ru), suggesting the formation of larger Ru clusters. Table 1 shows metal loadings and metal dispersions of catalysts. Clearly, samples obtained by EA method possess a higher Ru dispersion than catalysts obtained by IWI method. This indicates an advantage in the synthesis of small metal clusters using EA method. Fig. 2 shows the TEM and HRTEM images of supported Ru catalysts. Most of Ru nanoparticles are located on the external surface of MWCNTs without incorporating into the pores of MWCNTs. Ru particles of ~ 2 nm are highly dispersed over Ru/MWCNTs-EA, and clear lattice fringes in the HRTEM images indicate the formation of metallic Ru. Compared to Ru/MWCNTs-EA samples, Ru/MWCNTs-IWI shows a homogeneous distribution of larger nanoparticles (~ 3 nm) on supports. As shown in Table 1, size of Ru nanoparticles observed in TEM images is similar to the size calculated from chemisorption results. These indicate a narrow size distribution of Ru particles over supports. In addition to MWCNT-supported Ru nanoparticles, Ru/ AC and commercial Ru/C catalysts also show a homogeneous distribution of Ru nanoparticles.
1 nsurf −Ru
where nsurf-Ru is the amount of surface active Ru metal (mol) used in each hydrolysis reaction calculated from the metal dispersion of Ru catalysts, the weight of Ru catalysts and the Ru metal loading [39].
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Fig. 1. XRD patterns of MWCNTs and supported Ru catalysts.
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Table 1 Metal loadings and dispersions of Ru-based catalysts.a Catalyst
Ru loading (wt%)
Dispersion
Mean particle size (nm)
Ru/MWCNTs-EA Ru/MWCNTs-IWI-1 Ru/MWCNTs-IWI-2 Ru/AC-EA Ru/AC-IWI Ru/C commercial Ru/SiO2-1 Ru/SiO2-2 Ru/SiO2-3 Ru/SiO2-5
1.0 0.9 5.0 1.0 1.0 3.8 1.0 1.0 1.0 1.0
0.72 0.45 0.26 0.51 0.27 0.65 0.81 0.62 0.36 0.15
1.8 2.9 5.0 2.5 4.8 2.0 1.6 2.1 3.6 8.6
a
The dispersion (D) was calculated followed reference [34].
3.2. Catalytic performance Fig. 3a and b shows the initial hydrogen generation turnover rates at 303 K for Ru/SiO2 catalysts with different Ru metal sizes and carbon material supported Ru catalysts as a function of AB concentration. For all of Ru catalysts, the initial turnover rate increased rapidly as the AB concentration increased from 0.005 to 0.015 mol L−1; the initial turnover rate then increased gradually as the concentration needed to be further increased to 0.03 mol L−1. Interestingly, the initial turnover rates of
supported Ru catalysts changed little when AB concentration was further increased from 0.03 to 0.07 mol L− 1; this is consistent with a zero-order reaction with respect to the AB concentration [4,8,11]. For AB hydrolysis over metal surfaces, the dissociative adsorption of AB cations has been corroborated by previous studies [25]. A modified hydrolysis pathway is proposed in Fig. S1. Fig. 3a and b shows zeroorder reaction kinetic behaviour over supported Ru catalysts with AB concentration from 0.3 to 0.7 mol L−1. Such kinetic behaviour does not change with the size of Ru particles. It can be clearly seen in Fig. 3c that hydrolysis rate increases when the size of the Ru particles increases, indicating that AB hydrolysis reaction is structure-sensitive. Although the large Ru particles confer a high turnover rate, smaller Ru particle size provides a higher surface concentration of active sites for reaction. Thus, the size of the Ru nanoparticles should be optimized to yield economical catalysis. For example, Ru/SiO2-4 with 8.6 nm Ru particles shows 17.9 molH2 molsurface-Ru s− 1, corresponding to 2.7 molH2 moltotal-Ru s−1, whereas Ru/SiO2-2 with 2.1 nm Ru particles possesses 6.1 molH2 molsurface-Ru s−1, corresponding to 3.5 molH2 moltotal-Ru s−1. This shows that reduction in metal size of catalyst can increase the hydrogen generation rate based on the weight of metal used, a finding that is significant for the preparation of industrial catalysts for AB hydrolysis. Fig. 3c shows high turnover rate obtained with Ru/MWCNTs compared to that provided by Ru/AC and Ru/SiO2 catalysts. For instance,
Fig. 2. TEM and HRTEM images of supported Ru catalysts.
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Fig. 3. Initial hydrogen generation turnover rates for AB hydrolysis over Ru catalysts at 303 K. Effect of AB concentration on (a) Ru/SiO2, and (b) carbon material supported Ru catalysts; (c) effect of the Ru particle sizes and (d) AB hydrolysis activation energies in 0.06 mol L−1 AB solution.
Ru/MWCNTs-EA (1.8 nm, Table 1) produces a turnover rate of 24.2 molH2 molsurface-Ru s−1, much higher than the rates obtained with Ru/ AC-EA (11.8 molH2 molsurface-Ru s−1, 2.5 nm, Table 1) and Ru/SiO2-2 (6.1 molH2 molsurface-Ru s−1, 2.1 nm, Table 1). The slope of Arrhenius plot (Fig. 3d) indicates an activation energy (Ea) of 36.9 kJ mol−1 for the Ru/MWCNTs-EA catalyst, similar to previously reported values [15] and much lower than the value obtained with Ru/AC-EA (44.2 kJ mol− 1) and Ru/SiO2-2 (52.6 kJ mol− 1, similar to soluble Ru clusters [8]). Clearly, Ru/MWCNTs-EA produces the highest initial turnover rate and the lowest activation energy of the tested catalysts during 0.7 mol L−1 AB hydrolysis at 303 K. These results suggest that the use of carbon materials, especially MWCNTs, as supports yields higher reaction rates in AB hydrolysis than inert oxide supports (SiO2) and that the activity is proportional to the size of metal particles on the supports. The promotion of AB hydrolysis resulted from the MWCNTs support is consistent with other reports in literature. Chen et al. [7] and Zhao et al. [30] concluded that the good performance of CNTs supported Pt or Ni nanoparticles could be attributed to the strong interface interaction between metal and nanotube surface, especially the defect-rich or oxygen group-deficient surfaces. Such interface interaction is expected to favour the formation of a tunable electronic state of metal nanoparticles which enhances the AB hydrolysis. In addition to the modification of binding energy of metal nanoparticles [7,30], promotion of
hydrogenation by hydrogen spillover occurring on carbon nanotubesupported metal nanoparticles has been demonstrated [36,41]. A series of experiment (Table 2) was conducted to reveal the role of hydrogen spillover in AB hydrolysis. Ru/SiO2-2 (2.1 nm Ru metal) yields 6.1 molH2 molsurface-Ru s−1, which is much lower than that of Ru/ACEA (2.0 nm, 10.1 molH2 molsurface-Ru s− 1) and Ru/MWCNTs-EA (1.8 nm, 24.2 6.1 molH2 molsurface-Ru s−1). Ru/SiO2-2 was diluted with SiO2 powder (1:4 weight ratio) to form 38–48 μm pellets and then mixed with AC and MWCNTs to form an interpellet mixture. The results show that there was no obvious promotion of turnover rate in either toluene hydrogenation or AB hydrolysis (Tables 2 and S1). However, when an interpellet mixture was formed by directly mixing Ru/SiO2-2 with AC or MWCNTs (1:4 weight ratio) an apparent increase of 26–30% (Table S1) in turnover rate of toluene hydrogenation was measured. Moreover, the rate increased significantly by ~90% when Ru/SiO2-2 was directly diluted with AC or MWCNTs (1:4 weight ratio) to form 38–48 μm intrapellets (Table S1), yielding a hydrogenation turnover rate similar to those obtained with Ru/AC-EA and Ru/MWCNTs-EA. These results indicate that even without the strong interaction between Ru metal clusters with carbon supports, a promotion phenomenon still occurs by introducing carbon nanotubes into the metal catalysts. Hydrogenation can be promoted by hydrogen spillover when there is sufficient contact with metal sites that the use of supports enables hydrogen spillover, and
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Table 2 The initial hydrogen generation turnover rate of intrapellet and interpellet mixtures of Ru/ SiO2 and carbon supports with 1:4 weight ratio.a Catalyst
Mixture
Turnover rate (molH2 molsurface-Ru s−1)
Ru/SiO2-2 Ru/SiO2-2 + SiO2 (1:4) intrapelletb Ru/SiO2-2 Ru/SiO2-2 Ru/AC-EA Ru/SiO2-2 + SiO2 (1:4) interpelletb Ru/SiO2-2
– AC, interpellet
6.1 6.3
AC, interpellet AC, intrapellet – MWCNTs, interpellet MWCNTs, interpellet MWCNTs, intrapellet –
7.5 9.5 10.1 6.5
Ru/SiO2-2 Ru/MWCNTs-EA
7.8 15.5 24.2
a
The intrapellet mixture was prepared by mixing powdered Ru/SiO2-2 with AC and MWCNTs using a mortar and pestle until homogenous (30 min) under ambient conditions, respectively. The interpellet mixture was obtained by directly adding 38–48 μm Ru/SiO2-2 powders to 38–48 μm AC and MWCNTs powders. b The Ru/SiO2-2 + SiO2 (1:4) intrapellet with 38–48 μm was prepared by mixing powdered Ru/SiO2-2 with SiO2.
AC and MWCNTs provide a similar enhancement of hydrogenation. For AB hydrolysis, the initial turnover rate of Ru/SiO2-2 also increased from 6.1 to 9.5 molH2 molsurface-Ru s−1, which is similar to that obtained with Ru/AC-EA (10.1 molH2 molsurface-Ru s−1), when AC was used as a diluent to form intrapellets, suggesting a promoting effect of hydrogen spillover on AB hydrolysis. Interestingly, a higher turnover rate (15.5 molH2 molsurface-Ru s− 1) was observed after diluting Ru/SiO2-2 with MWCNTs to form intrapellets, indicating an enhanced promotion effect when MWCNTs were used. Although some reports pointed out that the presence of the metal on or inside carbon nanotubes can enhance hydrogen (gas) storage capacity at ambient pressure [37,38] without obvious promotion in hydrogen storage (i.e., b1%, [41]), atomic hydrogen can be stored by chemisorption under ambient conditions in the presence of metal nanoparticles owing to the spillover of hydrogen from metal to the defects of carbon nanotubes [41]. Thus, our results, together with other results reported in the literature, show that the promotion of AB hydrolysis by MWCNTs can be explained by the hydrogen spillover effect that is associated with strong interface interaction between metal and MWCNTs. As shown in Table 2, the use of Ru/MWCNTs-EA yields a still higher turnover rate (24.2 molH2 molsurface-Ru s− 1) than the rate achieved using diluted intrapellets of Ru/SiO2-2 and MWCNTs. In our view, this can be attributed to better contact or interaction of metal sites with MWCNTs supports. 4. Conclusions We report a general strategy for synthesis of homogeneously distributed Ru nanoparticles on MWCNTs using electrostatic adsorption method. The AB hydrolysis was shown to be a structure-sensitive reaction in which large metal particles possess a high reaction rate. Ru/MWCNTs exhibited high reaction rate and low activation energy during hydrolysis. This can be attributed to the hydrogen spillover effect associated with strong interface interaction between metal and MWCNTs. The results reported herein verified the practical role of carbon nanostructures in the hydrolysis of AB and other boron-containing compounds. Acknowledgments This work was supported by the Science Foundation of China University of Petroleum, Beijing (C201603), the NSF of China (21206192), and the Open Project of Key Lab Adv Energy Mat Chem (Nankai Univ).
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