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Facile synthesis of TiN decorated graphene and its enhanced catalytic effects on dehydrogenation performance of magnesium hydride† Ying Wang, Li Li, Cuihua An, Yijing Wang,* Chengcheng Chen, Lifang Jiao and Huatang Yuan TiN@rGO nanohybrids were successfully synthesized by a simple “urea glass” technique. Experimental results demonstrated that TiN nanocrystals, with an average particle size of 20 nm, were uniformly anchored onto highly reduced graphene nanosheets. The as-synthesized TiN@rGO nanohybrids showed a porous planar-like structure, which had a large surface area of 177 m2 g1. More importantly, the as-prepared TiN@rGO hybrids showed enhanced catalytic effects on the dehydrogenation of MgH2. The dehydrogenation thermodynamics and kinetics of the MgH2–TiN@rGO composites were systematically investigated and some significant improvements were confirmed. It was found that the 10 wt% TiN@rGO doped MgH2 sample started to release hydrogen at about 167 C, and roughly 6.0 wt% hydrogen was released within 18 min when isothermally heated to 300 C. In contrast, the onset dehydrogenation
Received 24th January 2014 Accepted 8th March 2014
temperature of the pure MgH2 sample was about 307 C, and only 3.5 wt% hydrogen was released even
DOI: 10.1039/c4nr00474d
after 120 min of heating under identical conditions. In addition, the catalytic mechanism of TiN@rGO on the dehydrogenation of MgH2 was discussed using the Johnson–Mehl–Avrami (JMA) model and X-ray
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diffraction equipment.
1. Introduction Due to their non-toxicity, high abundance, low cost and large hydrogen storage capacity, light metal hydrides/compounds have been regarded as indispensable hydrogen storage materials.1–3 Amongst them, MgH2 is considered one of the most ideal candidates because of its high hydrogen density (7.6 wt% in gravimetry and 110 kg m3 in volumetry) and good reversibility.4–12 However, the strong chemical bond energy of the Mg– H bonds, which needs a very large decomposition enthalpy of 75 kJ mol1, leads to an unfavorably high dehydrogenation temperature of MgH2 (usually above 300 C). Besides this, the sluggish diffusion rate of H atoms in bulk MgH2/Mg results in poor sorption kinetics. All of the above drawbacks have hampered the widespread application of MgH2. In the past decades, tremendous efforts have been made to modify the dehydrogenation performance of MgH2 and signicant progress was made.3,12–17 It is well known that ball-milling is an effective method to reduce crystallite sizes to the nanoscale, create fresh surface areas
Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Key Laboratory of Advanced Energy Materials Chemistry (MOE), Nankai University, Tianjin 300071, P. R. China. E-mail: [email protected]; Fax: +86 22 23503639; Tel: +86 22 23503639 † Electronic supplementary 10.1039/c4nr00474d
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and introduce more active sites, which play a vital role in the dissociation of hydrogen. Furthermore, metal atoms can accelerate the hydrogen decomposition and hence increase desorption kinetics dramatically. Therefore, more attention has been paid to enhance the kinetics of MgH2 by mixing MgH2 with different catalysts or additives by means of ball-milling.14,18–20 Among these, transition metal complexes, especially Ti-based compounds, have been widely used as effective catalysts to facilitate the dehydrogenation performance of MgH2. For example, Ma et al.21,22 found that adding 4 mol% TiF3 could remarkably improve the sorption kinetics of MgH2. According to rst-principles calculations, Zhang et al.23 reported the synergistic effects of Ti and F on the dehydrogenation properties of MgH2. Similarly, Lu and Choi et al.5,6 systematically studied the catalytic effects of TiH2 on MgH2. Results showed that adding TiH2 not only enhanced the sorption kinetics of MgH2, but also decreased its enthalpy and entropy. More recently, Zhou and co-workers19 investigated the effects of different Ti intermetallic alloys on the hydrogen sorption properties of MgH2, and further conrmed the positive effects of Ti based compounds. Our group previously synthesized a highly active TiN catalyst.24,25 Considering the similarity of TiN to other Ti-based compounds, we suppose that TiN may also show positive effects on the dehydrogenation of MgH2. More importantly, TiN is passivated by nitrogen so that it is more stable than Ti and not susceptible to change during the dehydrogenation. The chemically stable properties of TiN are important for us to study its catalytic mechanism.
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Graphene, one of the hottest stars in materials science, has a unique sp2-hybridized carbon structure and exhibits some exceptional characteristics, including large surface areas, high conductivity and outstanding chemical stability.26,27 All of these merits make graphene promising for applications in electronics, energy storage devices, catalysis and so on.28–36 More importantly, highly reduced graphene oxide (rGO) has plenty of channels which can facilitate the diffusion of hydrogen. Based on its outstanding performance, graphene has been considered an ideal component for the fabrication of different functional composites, which ultimately lead to new materials with superior performances. Considering the excellent properties of TiN as well as graphene, we expect that decorating rGO with TiN will result in attractive catalytic effects on the dehydrogenation performance of MgH2. Herein, we report a simple “urea glass” method to synthesize highly dispersed TiN decorated rGO (TiN@rGO) nanohybrids. Transmission electron microscopy (TEM) demonstrates that TiN nanoparticles (NPs) are anchored on the surface of the rGO, with an average size of 20 nm. Nitrogen sorption isothermals reveal a Brunauer–Emmett–Teller (BET) area of 177 m2 g1 of the as-prepared TiN@rGO sample. Furthermore, the catalytic effects of TiN@rGO on the dehydrogenation performance of MgH2 are systemically studied and some signicant improvements are conrmed.
2.
Experimental
2.1. Synthesis of the TiN@rGO nanohybrids Graphene oxide (GO) was synthesized using a modied Hummer's method.37 Aer that, GO was transferred into quartz tubes and thermally exfoliated at 800 C for 1 min under a continuous 10% H2–Ar gas ow. Then rGO was gained and collected for other tests. Typically, the preparation of TiN@rGO could be described as follows: TiCl4 was added slowly to absolute ethyl alcohol, and a light yellow solution was obtained. Then, rGO was dispersed in the solution by ultrasonication for 30 min. Aer that, a certain amount of solid urea was added to the mixture solution (metal to urea molar ratio was kept at 1 : 9). This solution was stirred until the urea was completely solubilized, which was followed by further standing for 12 h. Finally, the gel was transferred to a tube furnace under continuous Ar ow. The furnace was heated to 800 C at a rate of 3 C min1 and maintained at this temperature for 3 h. Pure TiN was synthesized via the same method without the addition of rGO. 2.2. Synthesis of the MgH2–TiN@rGO composites Commercially available MgH2 (Alfa Aesar, 98%) was used as received without further purication. MgH2 and the as-synthesized TiN@rGO were mixed together in a weight ratio of 90 : 10. The mixture was transferred into a 100 ml homemade steel vessel under a 0.5 MPa hydrogen pressure and ball-milled for 5 h at the speed of 450 rpm. The ball-to-powder ratio was kept at 40 : 1. For comparison, pure MgH2, MgH2–rGO and MgH2–TiN samples were prepared under identical conditions. All of the
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handling and transportation was carried out in an argon (99.999%) lled glove box with a circulatory system to keep the water and oxygen below 10 ppm. 2.3. Characterization The structures and morphologies of the as-synthesized samples were characterized by powder X-ray diffraction (XRD, Rigaku D/Max-2500, Cu Ka radiation), Raman spectrometry (Renishaw inVia, excitation 514.5 nm), nitrogen adsorption and desorption isotherms (NOVA 2200e, Quantachrome), scanning electron microscopy (SEM) and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) on a JEOL JSM7500 instrument, TEM, selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) on a JEOL JEM-2010FEF instrument. The composition of TiN@rGO was quantied by thermogravimetry (TG, Labsys evo) and X-ray photoelectron spectroscopy (XPS, PHI 5000 VerasProbe, ULVAC PHI). The decomposition performance was measured by a temperature-programmed desorption system (TPD, PX200). Typically, about 70 mg powder was loaded into the sample chamber and heated at a given rate under continuous Ar ow. Analysis of the isothermal dehydrogenation kinetics was performed on a homemade Sievert's apparatus using a volumetric method. About 100 mg sample was quickly heated to and kept at a given temperature. The dehydrogenation tests were carried out in the range of 0–0.01 MPa hydrogen pressure. Differential scanning calorimetry (DSC) and high-pressure differential scanning calorimetry (HP-DSC) were measured on a TA instrument Q20P. The results were used to determine the thermodynamic properties and the cycling stability of MgH2, respectively.
3.
Results and discussion
3.1. Characterization of the TiN@rGO nanohybrids The formation mechanism of the TiN@rGO nanohybrids is illustrated in Scheme 1. Generally, the preparation started from a TiCl4–absolute ethyl alcohol solution, which could be easily distributed onto the surface of the rGO during sonication. When urea was added, Ti4+ bonded with the urea and fabricated a metal–urea complex. Finally, the obtained gel was put into an oven and heated at 800 C for 3 h. The resulting TiN@rGO nanohybrids were dark and nely dispersed powders. It is clearly shown in Fig. 1a that the pure rGO had a layered structure with some interconnected wrinkles and channels in it. As shown in Fig. 1b, the as-prepared TiN@rGO sample retained the crumpled, plate-like structure of rGO and the layers were poorly stacked, which was favorable for the transportation of hydrogen. The TEM image, shown in Fig. 1c, further conrmed the highly crumpled character of the pure rGO. About 3–5 individual layers were found in the HRTEM image of the edge of the pure rGO, shown in the inset of Fig. 1c. The homogeneous distribution of the TiN NPs on the rGO, and their close contact was conrmed by TEM. As shown in Fig. 1d, the rGO matrix was uniformly decorated with plenty of TiN NPs with a diameter of about 20 nm. The corresponding SAED pattern of the TiN@rGO (inset
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Scheme 1 Illustration of the preparation of TiN@rGO. (1) Metal ion precursors anchor onto the surface of the rGO during the ultrasonication process. (2) Binding process of the urea with TiCl4. (3) TiN decorating the rGO layers during calcination.
To unravel the structure of the as-synthesized rGO, TiN and TiN@rGO, XRD equipment was employed and the results are shown in Fig. 2a. The broad diffraction peak at around 26 could be attributed to the (002) reection of pure rGO, which indicated a better reduction. For the as-synthesized pure TiN, all of the diffraction peaks were consistent with the JCPDS card for standard TiN no. 38-1420. As well as the standard diffraction peaks of TiN, TiN@rGO composites showed another broad peak at 26 , which was caused by the introduction of rGO. In addition, compared to pure rGO, the diffraction peak of C (002) was much smaller in the TiN@rGO sample, indicating the successful decoration of rGO with TiN. Raman spectra are shown in Fig. 2b. All of the samples displayed D, G and 2D bands at about 1337, 1585 and 2675 cm1, respectively. Generally, the D band was associated with structure defects and
Fig. 1 SEM images of (a) rGO and (b) TiN@rGO. TEM images of (c) rGO and (d) TiN@rGO. (e) EDS spectrum of TiN@rGO and (f) the corresponding atomic percentages of different elements. The HRTEM picture of pure rGO and the SAED picture of TiN@rGO are shown in insets (c) and (d), respectively.
in Fig. 1d) exhibited well-dened circles, indicating the small grain size and the polycrystalline nature of the as-synthesized TiN@rGO nanohybrids. A typical EDS spectrum of the assynthesized TiN@rGO sample, shown in Fig. 1e, conrmed that the particles consisted of Ti and N. The quantitative analysis, shown in Fig. 1f, revealed that the atomic percentages of Ti and N were 15.6% and 15.9%, respectively, which was nearly 1 : 1, in agreement with the composition of TiN.
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Fig. 2 (a) XRD curves for the as-prepared rGO, TiN and TiN@rGO samples, (b) Raman spectra for the GO, pure rGO and TiN@rGO samples, (c) nitrogen absorption–desorption isotherms and (d) the corresponding pore-size distribution calculated by the BJH method of the as-synthesized TiN@rGO.
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Fig. 3
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(a) XPS spectrum of the as-prepared TiN@rGO sample. Insets: high resolution XPS spectra for (b) Ti 2p and (c) C 1s.
the partially disordered structures of the sp2 domains. Additionally, the G band was assigned to sp2-hybridized carbon and can be used to explain the degree of graphitization. Furthermore, the 2D band was used to investigate the layering in the graphene. Compared to GO, the intensity ratio of ID/IG was increased in the rGO sample, and further enhanced in the TiN@rGO nanohybrids, which indicated that more sp2-hybridized carbon was formed. The 2D band of the TiN@rGO sample was broader and less intense than that of pure GO and rGO, indicating fewer layers of graphene and conrming the connection between the TiN and rGO. The absorption–desorption isotherms revealed the porous structure of the TiN@rGO. The surface area measurements and pore size distribution curve of the as-prepared TiN decorated rGO are given in Fig. 2c and d, respectively. The TiN@rGO nanohybrids have a BET value of 177 m2 g1 and an average pore diameter of 3.8 nm. The chemical composition of the TiN@rGO nanohybrids was determined by XPS and the results were shown in Fig. 3. In the wide scan spectrum of Fig. 3a, the main contaminant was oxygen, which may be caused by surface oxidation and/or the residual oxygen-containing functional group in graphene. N 1s showed a main peak at about 397.19 eV, corresponding with the Ti 2p3/2 and Ti 2p1/2 peaks at 455.6 and 461.1 eV in Fig. 3b, which indicated the formation of TiN. The weak peak at 458.3 eV could be attributed to the Ti 2p3/2 for TiO2, which was caused by the inevitable surface oxidation. In the C 1s spectrum (Fig. 3c), the peaks at around 284.5, 286.2 and 289.2 eV can be ascribed to the sp2 C]C bonds, C–O bonds and carboxylate O] C–O bonds, respectively, conrming the existence of rGO in the TiN@rGO composites. At the same time, TG analysis (ramp: 10 C min1 in air) was undertaken to determine the content of TiN in the TiN@rGO sample. Fig. S1† reveals that a 27 wt% loss
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(a) Thermally programmed hydrogen desorption capacity patterns for pure MgH2, MgH2–rGO, MgH2–TiN and MgH2–TiN@rGO composites, (b) onset dehydrogenation temperature of different samples. Heating rate is 2 C min1 under continuous Ar flow. Fig. 4
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was observed from 30 C to 800 C. In other words, 73 wt% TiN was anchored onto rGO.
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3.2. Dehydrogenation properties of the MgH2–TiN@rGO composites The catalytic effects of the as-synthesized TiN@rGO nanohybrids on the dehydrogenation performance of MgH2 were systematically investigated. The TPD curves of the as-prepared pure MgH2, MgH2–rGO, MgH2–TiN and MgH2–TiN@rGO are shown in Fig. 2S.† Fig. 4a illustrates the thermal decomposition performance of the as-synthesized samples, which was obtained by integrating the related TPD curves. During the heating treatment, about 6.97 wt%, 6.15 wt%, 6.00 wt% and 6.01 wt% hydrogen was released from the pure MgH2, MgH2–rGO, MgH2– TiN and MgH2–TiN@rGO samples, respectively. The decreased hydrogen capacity may be due to impurities and the addition of catalysts, which acted as dead weights during the dehydrogenation. Obviously, the onset dehydrogenation temperature of the pure MgH2 was about 307 C. Compared to the pure MgH2, the operating temperatures of the MgH2–rGO and MgH2–TiN samples were reduced to 269 C and 218 C, respectively. It is noteworthy that the MgH2–TiN@rGO sample showed the lowest onset dehydrogenation temperature of about 167 C, which was 140 C, 102 C, and 51 C lower than that of the pure MgH2, MgH2–rGO, and MgH2–TiN samples, respectively. Detailed information about the onset dehydrogenation temperature is revealed in Fig. 4b. The signicantly decreased operating temperature suggests that TiN and rGO have synergetic effects to facilitate the dehydriding reaction of MgH2. To further study the superior catalytic effects of the TiN@rGO nanohybrids on MgH2, a DSC experiment was employed to investigate the thermodynamic events during the dehydrogenation. Fig. 5a and b demonstrate the endothermic nature of MgH2 during the decomposition. The small shoulder
Fig. 5 DSC curves of (a) pure MgH2 and (b) the MgH2–TiN@rGO sample at different heating rates, (c) the corresponding Kissinger plots of pure MgH2 and the MgH2–TiN@rGO composite, and (d) HP-DSC traces of the MgH2–TiN@rGO sample (heating ramp: 5 C min1, cooling ramp: 5 C min1, H2 pressure: 0.5 MPa).
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peak was mainly caused by the inhomogeneous size-distribution formed during milling, which was consistent with a previous report.38 Besides this, both the peak temperature and the intensity shied to higher values as the heating rate increased from 2 to 8 C min1. Compared to pure MgH2, the temperature of the main dehydrogenation peak of the TiN@rGO doped MgH2 was reduced by 25.4 C, from 338.4 to 313.0 C, at a heating rate of 2 C min1. This, together with the thermally programmed dehydrogenation patterns (shown in Fig. 4), suggested that TiN@rGO could effectively reduce the dehydriding temperature of MgH2. The dehydrogenation energy barrier of these composites was further studied by using the Kissinger method. The equation is given below in eqn (1). b 1 Ea d ln d ¼ (1) R Tm Tm 2 Here, Tm is the peak temperature of the endothermic effect, b is the heating rate, Ea is the activation energy and R is the gas constant. Fig. 5c demonstrates that the Ea was 161 kJ mol1 for the pure MgH2 sample and 120 kJ mol1 for the MgH2– TiN@rGO sample. The decreased Ea value indicated that the
Fig. 6 Isothermal desorption kinetics of pure MgH2 at 300 C and MgH2–TiN@rGO composites at 300 C and 275 C, and (b) JMA plots for the isothermal dehydrogenation of the MgH2–TiN@rGO sample at 300 C and 275 C.
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TiN@rGO nanohybrids could decrease the dehydriding energy barrier of MgH2. HP-DSC was used to explore the reversibility of the as-synthesized MgH2–TiN@rGO sample. As shown in Fig. 5d, the endothermic and exothermic peaks represented the dehydrogenation and re-hydrogenation reactions of MgH2, respectively. The dehydrogenation peak temperatures were higher under H2 pressure than Ar atmosphere. With successive cycles, both the dehydrogenation and the re-hydrogenation peak areas did not diminish signicantly, indicating that there was little loss of the hydrogen storage capacity. So the MgH2– TiN@rGO sample had good cycle stability. Isothermal dehydrogenation experiments could provide clear evidence for the desorption kinetics. As shown in Fig. 6a, lowering the heating temperature resulted in slower desorption kinetics. Obviously, pure MgH2 showed a sluggish desorption rate and only released 3.5 wt% hydrogen even aer 120 min heated at 300 C. However, the MgH2–TiN@rGO sample released about 6.0 wt% hydrogen within 18 min at 300 C, which was dozens of times more than that of pure MgH2 under identical conditions. Even at 275 C, about 5.8 wt% hydrogen was released within 60 min by the MgH2–TiN@rGO sample. Thus, we believe that TiN@rGO exhibits superior catalytic performances and signicantly improves the dehydrogenation kinetics of MgH2.
temperature of MgH2. Therefore, TiN@rGO demonstrated enhanced catalytic effects on the dehydrogenation of MgH2. The microstructures of the milled MgH2–TiN@rGO composites are shown in Fig. S3.† Clearly, the Mg element was homogeneously distributed in the composites, which indicated good dispersion of the MgH2. Additionally, the Ti and C mappings of the MgH2–TiN@rGO sample indicated that the TiN@rGO hybrids were uniformly distributed on the MgH2 matrix. The high density distribution of the C element in the area at the edge was caused by the conducting resin, which was used to attach the samples. SEM images of the samples, shown in Fig. S4a† and 4c, showed that the particle size in the MgH2–TiN@rGO sample did not obviously change before and aer dehydrogenation. All of the samples consisted of small particles, in the range of tens to hundreds of nanometers. The very large particles were due to the agglomeration of the pulverized particles. This could well explain the shoulder peak observed in the DSC curves. The planar-like nanosheet structure of TiN@rGO can be clearly observed in the TEM images of both the milled and post-dehydrogenation samples (Fig. S4b† and 4d), indicating that the TiN@rGO was maintained during the dehydriding. XRD was employed to investigate the chemical process, occurring during the dehydrogenation. As shown in Fig. 7a, the diffraction peaks of the milled MgH2–TiN@rGO sample were
3.3. Catalytic mechanism Typically, the dehydriding reaction in solid-state can be classied as isotropic diffusion and random nucleation, or preferential nucleation along certain crystal axes, such as defects or active catalytic sites.2,3 The Johnson–Mehl–Avrami (JMA) equation is widely used to determine the solid-state dehydrogenation mechanism. Herein, isothermal dehydriding data for the MgH2–TiN@rGO sample at different temperatures is used to t the JMA model. The JMA equation can be expressed as below: ln[ln(1 a(t))] ¼ nln(t) + nln(k)
(2)
Generally, t is the reaction time, a(t) is the fraction that has already reacted at time t, n is the Avrami exponent that is related to the transformation mechanism, and k is the rate constant. As shown in Fig. 6b, when a(t) was varied from 0.2 to 0.8, ln[ln(1 a(t))] is a straight line when plotted against ln(t). Clearly, the Avrami exponent n (1.47 and 1.34) was close to 1.5 for the dehydriding reaction of the MgH2–TiN@rGO sample at 300 C and 275 C. Previous reports have conrmed that when n is close to 1.5, the phase transformation of MgH2 to Mg has a nucleation rate of zero, which is a diffusion-limited growth reaction.39 In addition, the porous structure of the as-prepared TiN@rGO nanohybrids (discussed in Fig. 2c and d) provides plenty of channels and active sites for the diffusion of hydrogen in MgH2. More importantly, the good affinity of hydrogen for TiN can facilitate the breaking of Mg–H bonds,40–42 which always needs high energy. So hydrogen is subsequently transferred from the bulk MgH2 to the TiN@rGO, and nally released under moderate conditions. On the other hand, the good thermal conductivity of rGO may also contribute to the decrease in the operating
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Fig. 7 XRD patterns of the pure MgH2 and MgH2–TiN@rGO samples (a) as-synthesized, (b) after dehydrogenation.
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much broader than that of pure MgH2, which implied a smaller grain size. In addition, a small Mg diffraction peak was also observed for the as-prepared pure MgH2 and TiN@rGO doped MgH2. Aer dehydrogenation at 300 C for 5 h, the products were collected for XRD analysis. As shown in Fig. 7b, the MgH2– TiN@rGO sample showed a main hexagonal Mg phase which was transformed from MgH2 during the dehydrogenation. However, a certain amount of MgH2 still existed in the pure MgH2 sample, which explained its lower dehydrogenation capacity. The XRD pattern of the re-hydrogenated MgH2– TiN@rGO sample is shown in Fig. S5;† almost all Mg phases were re-hydrogenated into MgH2, indicating that the dehydrogenated MgH2 could be almost fully re-hydrogenated. It is noteworthy that the TiN@rGO survived during the de/rehydrogenation cycles, which illustrates the good reversibility of the MgH2–TiN@rGO sample.
4. Conclusions In summary, TiN@rGO nanohybrids are synthesized through a simple “urea glass” method. TEM and SEM experiments demonstrate that TiN nanocrystals, with an average size of 20 nm, are uniformly anchored onto the surface of rGO. The porous planar-like structure of the as-prepared TiN@rGO nanohybrids has a large surface area of 177 m2 g1 and an average pore diameter of 3.8 nm. This material exhibits excellent catalytic effects on the dehydrogenation performance of MgH2. Thermal decomposition results show that adding TiN@rGO to MgH2 can signicantly decrease the onset dehydrogenation temperature from 307 C for pure MgH2 to about 167 C. Isothermal dehydrogenation experiments reveal the enhanced dehydrogenation kinetics of the MgH2–TiN@rGO sample, which releases 6.0 wt% hydrogen within 18 min at 300 C, dozens of times more than that of pure MgH2. JMA tting demonstrates that the phase transformation of MgH2 to Mg has a nucleation rate of zero, which is a diffusion-limited growth reaction. The porous structure of the TiN@rGO hybrids can provide more active sites and channels for hydrogen diffusion. Additionally, TiN@rGO has a strong affinity to hydrogen. All of the above results can explain the enhanced catalytic effects of TiN@rGO nanohybrids on the dehydrogenation of MgH2. More importantly, TiN@rGO remains stable during de/re-hydrogenation, leading to stable catalytic effects.
Acknowledgements The authors of this work gratefully acknowledge the nancial support received from MOST projects (2010CB631303, 2012AA051901), NSFC (5117108), 111Project (B12015) and MOE (IRT-13R30).
Notes and references 1 L. Schlapbach and A. Z¨ uttel, Nature, 2001, 414, 353–358. 2 K. F. Aguey-Zinsou and J. R. Ares-Fernandez, Energy Environ. Sci., 2010, 3, 526–543.
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Cite this: Nanoscale, 2014, 6, 6684
Facile synthesis of TiN decorated graphene and its enhanced catalytic effects on dehydrogenation performance of magnesium hydride† Ying Wang, Li Li, Cuihua An, Yijing Wang,* Chengcheng Chen, Lifang Jiao and Huatang Yuan TiN@rGO nanohybrids were successfully synthesized by a simple “urea glass” technique. Experimental results demonstrated that TiN nanocrystals, with an average particle size of 20 nm, were uniformly anchored onto highly reduced graphene nanosheets. The as-synthesized TiN@rGO nanohybrids showed a porous planar-like structure, which had a large surface area of 177 m2 g1. More importantly, the as-prepared TiN@rGO hybrids showed enhanced catalytic effects on the dehydrogenation of MgH2. The dehydrogenation thermodynamics and kinetics of the MgH2–TiN@rGO composites were systematically investigated and some significant improvements were confirmed. It was found that the 10 wt% TiN@rGO doped MgH2 sample started to release hydrogen at about 167 C, and roughly 6.0 wt% hydrogen was released within 18 min when isothermally heated to 300 C. In contrast, the onset dehydrogenation
Received 24th January 2014 Accepted 8th March 2014
temperature of the pure MgH2 sample was about 307 C, and only 3.5 wt% hydrogen was released even
DOI: 10.1039/c4nr00474d
after 120 min of heating under identical conditions. In addition, the catalytic mechanism of TiN@rGO on the dehydrogenation of MgH2 was discussed using the Johnson–Mehl–Avrami (JMA) model and X-ray
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diffraction equipment.
1. Introduction Due to their non-toxicity, high abundance, low cost and large hydrogen storage capacity, light metal hydrides/compounds have been regarded as indispensable hydrogen storage materials.1–3 Amongst them, MgH2 is considered one of the most ideal candidates because of its high hydrogen density (7.6 wt% in gravimetry and 110 kg m3 in volumetry) and good reversibility.4–12 However, the strong chemical bond energy of the Mg– H bonds, which needs a very large decomposition enthalpy of 75 kJ mol1, leads to an unfavorably high dehydrogenation temperature of MgH2 (usually above 300 C). Besides this, the sluggish diffusion rate of H atoms in bulk MgH2/Mg results in poor sorption kinetics. All of the above drawbacks have hampered the widespread application of MgH2. In the past decades, tremendous efforts have been made to modify the dehydrogenation performance of MgH2 and signicant progress was made.3,12–17 It is well known that ball-milling is an effective method to reduce crystallite sizes to the nanoscale, create fresh surface areas
Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Key Laboratory of Advanced Energy Materials Chemistry (MOE), Nankai University, Tianjin 300071, P. R. China. E-mail: [email protected]; Fax: +86 22 23503639; Tel: +86 22 23503639 † Electronic supplementary 10.1039/c4nr00474d
information
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available.
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DOI:
and introduce more active sites, which play a vital role in the dissociation of hydrogen. Furthermore, metal atoms can accelerate the hydrogen decomposition and hence increase desorption kinetics dramatically. Therefore, more attention has been paid to enhance the kinetics of MgH2 by mixing MgH2 with different catalysts or additives by means of ball-milling.14,18–20 Among these, transition metal complexes, especially Ti-based compounds, have been widely used as effective catalysts to facilitate the dehydrogenation performance of MgH2. For example, Ma et al.21,22 found that adding 4 mol% TiF3 could remarkably improve the sorption kinetics of MgH2. According to rst-principles calculations, Zhang et al.23 reported the synergistic effects of Ti and F on the dehydrogenation properties of MgH2. Similarly, Lu and Choi et al.5,6 systematically studied the catalytic effects of TiH2 on MgH2. Results showed that adding TiH2 not only enhanced the sorption kinetics of MgH2, but also decreased its enthalpy and entropy. More recently, Zhou and co-workers19 investigated the effects of different Ti intermetallic alloys on the hydrogen sorption properties of MgH2, and further conrmed the positive effects of Ti based compounds. Our group previously synthesized a highly active TiN catalyst.24,25 Considering the similarity of TiN to other Ti-based compounds, we suppose that TiN may also show positive effects on the dehydrogenation of MgH2. More importantly, TiN is passivated by nitrogen so that it is more stable than Ti and not susceptible to change during the dehydrogenation. The chemically stable properties of TiN are important for us to study its catalytic mechanism.
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Graphene, one of the hottest stars in materials science, has a unique sp2-hybridized carbon structure and exhibits some exceptional characteristics, including large surface areas, high conductivity and outstanding chemical stability.26,27 All of these merits make graphene promising for applications in electronics, energy storage devices, catalysis and so on.28–36 More importantly, highly reduced graphene oxide (rGO) has plenty of channels which can facilitate the diffusion of hydrogen. Based on its outstanding performance, graphene has been considered an ideal component for the fabrication of different functional composites, which ultimately lead to new materials with superior performances. Considering the excellent properties of TiN as well as graphene, we expect that decorating rGO with TiN will result in attractive catalytic effects on the dehydrogenation performance of MgH2. Herein, we report a simple “urea glass” method to synthesize highly dispersed TiN decorated rGO (TiN@rGO) nanohybrids. Transmission electron microscopy (TEM) demonstrates that TiN nanoparticles (NPs) are anchored on the surface of the rGO, with an average size of 20 nm. Nitrogen sorption isothermals reveal a Brunauer–Emmett–Teller (BET) area of 177 m2 g1 of the as-prepared TiN@rGO sample. Furthermore, the catalytic effects of TiN@rGO on the dehydrogenation performance of MgH2 are systemically studied and some signicant improvements are conrmed.
2.
Experimental
2.1. Synthesis of the TiN@rGO nanohybrids Graphene oxide (GO) was synthesized using a modied Hummer's method.37 Aer that, GO was transferred into quartz tubes and thermally exfoliated at 800 C for 1 min under a continuous 10% H2–Ar gas ow. Then rGO was gained and collected for other tests. Typically, the preparation of TiN@rGO could be described as follows: TiCl4 was added slowly to absolute ethyl alcohol, and a light yellow solution was obtained. Then, rGO was dispersed in the solution by ultrasonication for 30 min. Aer that, a certain amount of solid urea was added to the mixture solution (metal to urea molar ratio was kept at 1 : 9). This solution was stirred until the urea was completely solubilized, which was followed by further standing for 12 h. Finally, the gel was transferred to a tube furnace under continuous Ar ow. The furnace was heated to 800 C at a rate of 3 C min1 and maintained at this temperature for 3 h. Pure TiN was synthesized via the same method without the addition of rGO. 2.2. Synthesis of the MgH2–TiN@rGO composites Commercially available MgH2 (Alfa Aesar, 98%) was used as received without further purication. MgH2 and the as-synthesized TiN@rGO were mixed together in a weight ratio of 90 : 10. The mixture was transferred into a 100 ml homemade steel vessel under a 0.5 MPa hydrogen pressure and ball-milled for 5 h at the speed of 450 rpm. The ball-to-powder ratio was kept at 40 : 1. For comparison, pure MgH2, MgH2–rGO and MgH2–TiN samples were prepared under identical conditions. All of the
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handling and transportation was carried out in an argon (99.999%) lled glove box with a circulatory system to keep the water and oxygen below 10 ppm. 2.3. Characterization The structures and morphologies of the as-synthesized samples were characterized by powder X-ray diffraction (XRD, Rigaku D/Max-2500, Cu Ka radiation), Raman spectrometry (Renishaw inVia, excitation 514.5 nm), nitrogen adsorption and desorption isotherms (NOVA 2200e, Quantachrome), scanning electron microscopy (SEM) and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) on a JEOL JSM7500 instrument, TEM, selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) on a JEOL JEM-2010FEF instrument. The composition of TiN@rGO was quantied by thermogravimetry (TG, Labsys evo) and X-ray photoelectron spectroscopy (XPS, PHI 5000 VerasProbe, ULVAC PHI). The decomposition performance was measured by a temperature-programmed desorption system (TPD, PX200). Typically, about 70 mg powder was loaded into the sample chamber and heated at a given rate under continuous Ar ow. Analysis of the isothermal dehydrogenation kinetics was performed on a homemade Sievert's apparatus using a volumetric method. About 100 mg sample was quickly heated to and kept at a given temperature. The dehydrogenation tests were carried out in the range of 0–0.01 MPa hydrogen pressure. Differential scanning calorimetry (DSC) and high-pressure differential scanning calorimetry (HP-DSC) were measured on a TA instrument Q20P. The results were used to determine the thermodynamic properties and the cycling stability of MgH2, respectively.
3.
Results and discussion
3.1. Characterization of the TiN@rGO nanohybrids The formation mechanism of the TiN@rGO nanohybrids is illustrated in Scheme 1. Generally, the preparation started from a TiCl4–absolute ethyl alcohol solution, which could be easily distributed onto the surface of the rGO during sonication. When urea was added, Ti4+ bonded with the urea and fabricated a metal–urea complex. Finally, the obtained gel was put into an oven and heated at 800 C for 3 h. The resulting TiN@rGO nanohybrids were dark and nely dispersed powders. It is clearly shown in Fig. 1a that the pure rGO had a layered structure with some interconnected wrinkles and channels in it. As shown in Fig. 1b, the as-prepared TiN@rGO sample retained the crumpled, plate-like structure of rGO and the layers were poorly stacked, which was favorable for the transportation of hydrogen. The TEM image, shown in Fig. 1c, further conrmed the highly crumpled character of the pure rGO. About 3–5 individual layers were found in the HRTEM image of the edge of the pure rGO, shown in the inset of Fig. 1c. The homogeneous distribution of the TiN NPs on the rGO, and their close contact was conrmed by TEM. As shown in Fig. 1d, the rGO matrix was uniformly decorated with plenty of TiN NPs with a diameter of about 20 nm. The corresponding SAED pattern of the TiN@rGO (inset
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Scheme 1 Illustration of the preparation of TiN@rGO. (1) Metal ion precursors anchor onto the surface of the rGO during the ultrasonication process. (2) Binding process of the urea with TiCl4. (3) TiN decorating the rGO layers during calcination.
To unravel the structure of the as-synthesized rGO, TiN and TiN@rGO, XRD equipment was employed and the results are shown in Fig. 2a. The broad diffraction peak at around 26 could be attributed to the (002) reection of pure rGO, which indicated a better reduction. For the as-synthesized pure TiN, all of the diffraction peaks were consistent with the JCPDS card for standard TiN no. 38-1420. As well as the standard diffraction peaks of TiN, TiN@rGO composites showed another broad peak at 26 , which was caused by the introduction of rGO. In addition, compared to pure rGO, the diffraction peak of C (002) was much smaller in the TiN@rGO sample, indicating the successful decoration of rGO with TiN. Raman spectra are shown in Fig. 2b. All of the samples displayed D, G and 2D bands at about 1337, 1585 and 2675 cm1, respectively. Generally, the D band was associated with structure defects and
Fig. 1 SEM images of (a) rGO and (b) TiN@rGO. TEM images of (c) rGO and (d) TiN@rGO. (e) EDS spectrum of TiN@rGO and (f) the corresponding atomic percentages of different elements. The HRTEM picture of pure rGO and the SAED picture of TiN@rGO are shown in insets (c) and (d), respectively.
in Fig. 1d) exhibited well-dened circles, indicating the small grain size and the polycrystalline nature of the as-synthesized TiN@rGO nanohybrids. A typical EDS spectrum of the assynthesized TiN@rGO sample, shown in Fig. 1e, conrmed that the particles consisted of Ti and N. The quantitative analysis, shown in Fig. 1f, revealed that the atomic percentages of Ti and N were 15.6% and 15.9%, respectively, which was nearly 1 : 1, in agreement with the composition of TiN.
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Fig. 2 (a) XRD curves for the as-prepared rGO, TiN and TiN@rGO samples, (b) Raman spectra for the GO, pure rGO and TiN@rGO samples, (c) nitrogen absorption–desorption isotherms and (d) the corresponding pore-size distribution calculated by the BJH method of the as-synthesized TiN@rGO.
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Fig. 3
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(a) XPS spectrum of the as-prepared TiN@rGO sample. Insets: high resolution XPS spectra for (b) Ti 2p and (c) C 1s.
the partially disordered structures of the sp2 domains. Additionally, the G band was assigned to sp2-hybridized carbon and can be used to explain the degree of graphitization. Furthermore, the 2D band was used to investigate the layering in the graphene. Compared to GO, the intensity ratio of ID/IG was increased in the rGO sample, and further enhanced in the TiN@rGO nanohybrids, which indicated that more sp2-hybridized carbon was formed. The 2D band of the TiN@rGO sample was broader and less intense than that of pure GO and rGO, indicating fewer layers of graphene and conrming the connection between the TiN and rGO. The absorption–desorption isotherms revealed the porous structure of the TiN@rGO. The surface area measurements and pore size distribution curve of the as-prepared TiN decorated rGO are given in Fig. 2c and d, respectively. The TiN@rGO nanohybrids have a BET value of 177 m2 g1 and an average pore diameter of 3.8 nm. The chemical composition of the TiN@rGO nanohybrids was determined by XPS and the results were shown in Fig. 3. In the wide scan spectrum of Fig. 3a, the main contaminant was oxygen, which may be caused by surface oxidation and/or the residual oxygen-containing functional group in graphene. N 1s showed a main peak at about 397.19 eV, corresponding with the Ti 2p3/2 and Ti 2p1/2 peaks at 455.6 and 461.1 eV in Fig. 3b, which indicated the formation of TiN. The weak peak at 458.3 eV could be attributed to the Ti 2p3/2 for TiO2, which was caused by the inevitable surface oxidation. In the C 1s spectrum (Fig. 3c), the peaks at around 284.5, 286.2 and 289.2 eV can be ascribed to the sp2 C]C bonds, C–O bonds and carboxylate O] C–O bonds, respectively, conrming the existence of rGO in the TiN@rGO composites. At the same time, TG analysis (ramp: 10 C min1 in air) was undertaken to determine the content of TiN in the TiN@rGO sample. Fig. S1† reveals that a 27 wt% loss
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(a) Thermally programmed hydrogen desorption capacity patterns for pure MgH2, MgH2–rGO, MgH2–TiN and MgH2–TiN@rGO composites, (b) onset dehydrogenation temperature of different samples. Heating rate is 2 C min1 under continuous Ar flow. Fig. 4
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was observed from 30 C to 800 C. In other words, 73 wt% TiN was anchored onto rGO.
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3.2. Dehydrogenation properties of the MgH2–TiN@rGO composites The catalytic effects of the as-synthesized TiN@rGO nanohybrids on the dehydrogenation performance of MgH2 were systematically investigated. The TPD curves of the as-prepared pure MgH2, MgH2–rGO, MgH2–TiN and MgH2–TiN@rGO are shown in Fig. 2S.† Fig. 4a illustrates the thermal decomposition performance of the as-synthesized samples, which was obtained by integrating the related TPD curves. During the heating treatment, about 6.97 wt%, 6.15 wt%, 6.00 wt% and 6.01 wt% hydrogen was released from the pure MgH2, MgH2–rGO, MgH2– TiN and MgH2–TiN@rGO samples, respectively. The decreased hydrogen capacity may be due to impurities and the addition of catalysts, which acted as dead weights during the dehydrogenation. Obviously, the onset dehydrogenation temperature of the pure MgH2 was about 307 C. Compared to the pure MgH2, the operating temperatures of the MgH2–rGO and MgH2–TiN samples were reduced to 269 C and 218 C, respectively. It is noteworthy that the MgH2–TiN@rGO sample showed the lowest onset dehydrogenation temperature of about 167 C, which was 140 C, 102 C, and 51 C lower than that of the pure MgH2, MgH2–rGO, and MgH2–TiN samples, respectively. Detailed information about the onset dehydrogenation temperature is revealed in Fig. 4b. The signicantly decreased operating temperature suggests that TiN and rGO have synergetic effects to facilitate the dehydriding reaction of MgH2. To further study the superior catalytic effects of the TiN@rGO nanohybrids on MgH2, a DSC experiment was employed to investigate the thermodynamic events during the dehydrogenation. Fig. 5a and b demonstrate the endothermic nature of MgH2 during the decomposition. The small shoulder
Fig. 5 DSC curves of (a) pure MgH2 and (b) the MgH2–TiN@rGO sample at different heating rates, (c) the corresponding Kissinger plots of pure MgH2 and the MgH2–TiN@rGO composite, and (d) HP-DSC traces of the MgH2–TiN@rGO sample (heating ramp: 5 C min1, cooling ramp: 5 C min1, H2 pressure: 0.5 MPa).
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peak was mainly caused by the inhomogeneous size-distribution formed during milling, which was consistent with a previous report.38 Besides this, both the peak temperature and the intensity shied to higher values as the heating rate increased from 2 to 8 C min1. Compared to pure MgH2, the temperature of the main dehydrogenation peak of the TiN@rGO doped MgH2 was reduced by 25.4 C, from 338.4 to 313.0 C, at a heating rate of 2 C min1. This, together with the thermally programmed dehydrogenation patterns (shown in Fig. 4), suggested that TiN@rGO could effectively reduce the dehydriding temperature of MgH2. The dehydrogenation energy barrier of these composites was further studied by using the Kissinger method. The equation is given below in eqn (1). b 1 Ea d ln d ¼ (1) R Tm Tm 2 Here, Tm is the peak temperature of the endothermic effect, b is the heating rate, Ea is the activation energy and R is the gas constant. Fig. 5c demonstrates that the Ea was 161 kJ mol1 for the pure MgH2 sample and 120 kJ mol1 for the MgH2– TiN@rGO sample. The decreased Ea value indicated that the
Fig. 6 Isothermal desorption kinetics of pure MgH2 at 300 C and MgH2–TiN@rGO composites at 300 C and 275 C, and (b) JMA plots for the isothermal dehydrogenation of the MgH2–TiN@rGO sample at 300 C and 275 C.
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TiN@rGO nanohybrids could decrease the dehydriding energy barrier of MgH2. HP-DSC was used to explore the reversibility of the as-synthesized MgH2–TiN@rGO sample. As shown in Fig. 5d, the endothermic and exothermic peaks represented the dehydrogenation and re-hydrogenation reactions of MgH2, respectively. The dehydrogenation peak temperatures were higher under H2 pressure than Ar atmosphere. With successive cycles, both the dehydrogenation and the re-hydrogenation peak areas did not diminish signicantly, indicating that there was little loss of the hydrogen storage capacity. So the MgH2– TiN@rGO sample had good cycle stability. Isothermal dehydrogenation experiments could provide clear evidence for the desorption kinetics. As shown in Fig. 6a, lowering the heating temperature resulted in slower desorption kinetics. Obviously, pure MgH2 showed a sluggish desorption rate and only released 3.5 wt% hydrogen even aer 120 min heated at 300 C. However, the MgH2–TiN@rGO sample released about 6.0 wt% hydrogen within 18 min at 300 C, which was dozens of times more than that of pure MgH2 under identical conditions. Even at 275 C, about 5.8 wt% hydrogen was released within 60 min by the MgH2–TiN@rGO sample. Thus, we believe that TiN@rGO exhibits superior catalytic performances and signicantly improves the dehydrogenation kinetics of MgH2.
temperature of MgH2. Therefore, TiN@rGO demonstrated enhanced catalytic effects on the dehydrogenation of MgH2. The microstructures of the milled MgH2–TiN@rGO composites are shown in Fig. S3.† Clearly, the Mg element was homogeneously distributed in the composites, which indicated good dispersion of the MgH2. Additionally, the Ti and C mappings of the MgH2–TiN@rGO sample indicated that the TiN@rGO hybrids were uniformly distributed on the MgH2 matrix. The high density distribution of the C element in the area at the edge was caused by the conducting resin, which was used to attach the samples. SEM images of the samples, shown in Fig. S4a† and 4c, showed that the particle size in the MgH2–TiN@rGO sample did not obviously change before and aer dehydrogenation. All of the samples consisted of small particles, in the range of tens to hundreds of nanometers. The very large particles were due to the agglomeration of the pulverized particles. This could well explain the shoulder peak observed in the DSC curves. The planar-like nanosheet structure of TiN@rGO can be clearly observed in the TEM images of both the milled and post-dehydrogenation samples (Fig. S4b† and 4d), indicating that the TiN@rGO was maintained during the dehydriding. XRD was employed to investigate the chemical process, occurring during the dehydrogenation. As shown in Fig. 7a, the diffraction peaks of the milled MgH2–TiN@rGO sample were
3.3. Catalytic mechanism Typically, the dehydriding reaction in solid-state can be classied as isotropic diffusion and random nucleation, or preferential nucleation along certain crystal axes, such as defects or active catalytic sites.2,3 The Johnson–Mehl–Avrami (JMA) equation is widely used to determine the solid-state dehydrogenation mechanism. Herein, isothermal dehydriding data for the MgH2–TiN@rGO sample at different temperatures is used to t the JMA model. The JMA equation can be expressed as below: ln[ln(1 a(t))] ¼ nln(t) + nln(k)
(2)
Generally, t is the reaction time, a(t) is the fraction that has already reacted at time t, n is the Avrami exponent that is related to the transformation mechanism, and k is the rate constant. As shown in Fig. 6b, when a(t) was varied from 0.2 to 0.8, ln[ln(1 a(t))] is a straight line when plotted against ln(t). Clearly, the Avrami exponent n (1.47 and 1.34) was close to 1.5 for the dehydriding reaction of the MgH2–TiN@rGO sample at 300 C and 275 C. Previous reports have conrmed that when n is close to 1.5, the phase transformation of MgH2 to Mg has a nucleation rate of zero, which is a diffusion-limited growth reaction.39 In addition, the porous structure of the as-prepared TiN@rGO nanohybrids (discussed in Fig. 2c and d) provides plenty of channels and active sites for the diffusion of hydrogen in MgH2. More importantly, the good affinity of hydrogen for TiN can facilitate the breaking of Mg–H bonds,40–42 which always needs high energy. So hydrogen is subsequently transferred from the bulk MgH2 to the TiN@rGO, and nally released under moderate conditions. On the other hand, the good thermal conductivity of rGO may also contribute to the decrease in the operating
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Fig. 7 XRD patterns of the pure MgH2 and MgH2–TiN@rGO samples (a) as-synthesized, (b) after dehydrogenation.
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much broader than that of pure MgH2, which implied a smaller grain size. In addition, a small Mg diffraction peak was also observed for the as-prepared pure MgH2 and TiN@rGO doped MgH2. Aer dehydrogenation at 300 C for 5 h, the products were collected for XRD analysis. As shown in Fig. 7b, the MgH2– TiN@rGO sample showed a main hexagonal Mg phase which was transformed from MgH2 during the dehydrogenation. However, a certain amount of MgH2 still existed in the pure MgH2 sample, which explained its lower dehydrogenation capacity. The XRD pattern of the re-hydrogenated MgH2– TiN@rGO sample is shown in Fig. S5;† almost all Mg phases were re-hydrogenated into MgH2, indicating that the dehydrogenated MgH2 could be almost fully re-hydrogenated. It is noteworthy that the TiN@rGO survived during the de/rehydrogenation cycles, which illustrates the good reversibility of the MgH2–TiN@rGO sample.
4. Conclusions In summary, TiN@rGO nanohybrids are synthesized through a simple “urea glass” method. TEM and SEM experiments demonstrate that TiN nanocrystals, with an average size of 20 nm, are uniformly anchored onto the surface of rGO. The porous planar-like structure of the as-prepared TiN@rGO nanohybrids has a large surface area of 177 m2 g1 and an average pore diameter of 3.8 nm. This material exhibits excellent catalytic effects on the dehydrogenation performance of MgH2. Thermal decomposition results show that adding TiN@rGO to MgH2 can signicantly decrease the onset dehydrogenation temperature from 307 C for pure MgH2 to about 167 C. Isothermal dehydrogenation experiments reveal the enhanced dehydrogenation kinetics of the MgH2–TiN@rGO sample, which releases 6.0 wt% hydrogen within 18 min at 300 C, dozens of times more than that of pure MgH2. JMA tting demonstrates that the phase transformation of MgH2 to Mg has a nucleation rate of zero, which is a diffusion-limited growth reaction. The porous structure of the TiN@rGO hybrids can provide more active sites and channels for hydrogen diffusion. Additionally, TiN@rGO has a strong affinity to hydrogen. All of the above results can explain the enhanced catalytic effects of TiN@rGO nanohybrids on the dehydrogenation of MgH2. More importantly, TiN@rGO remains stable during de/re-hydrogenation, leading to stable catalytic effects.
Acknowledgements The authors of this work gratefully acknowledge the nancial support received from MOST projects (2010CB631303, 2012AA051901), NSFC (5117108), 111Project (B12015) and MOE (IRT-13R30).
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