Graphite oxide as a novel host material of catalytically active Cu–Ni bimetallic nanoparticles

Graphite oxide as a novel host material of catalytically active Cu–Ni bimetallic nanoparticles

Catalysis Communications 10 (2009) 1529–1533 Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/l...

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Catalysis Communications 10 (2009) 1529–1533

Contents lists available at ScienceDirect

Catalysis Communications journal homepage: www.elsevier.com/locate/catcom

Graphite oxide as a novel host material of catalytically active Cu–Ni bimetallic nanoparticles J. Bian a, M. Xiao a,*, S.J. Wang a, Y.X. Lu b, Y.Z. Meng a,* a

State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, Sun Yat-Sen University, Guangzhou 510275, PR China b Department of Chemistry and Medicinal Chemistry Program, Office of Life Sciences, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore

a r t i c l e

i n f o

Article history: Received 19 January 2009 Received in revised form 25 March 2009 Accepted 4 April 2009 Available online 14 April 2009 Keywords: Graphite oxide Copper–nickel bimetallic nanoparticles Dimethyl carbonate Catalysis

a b s t r a c t Graphite oxide (GO) was readily synthesized and used as the host of the bimetallic composite catalysts containing catalytically active Cu–Ni nanoparticles. The GO and the synthesized catalysts were characterized by FTIR, TG, Ram, SEM, TEM and XRD techniques. The experimental results demonstrated that the prepared GO supported composite catalyst exhibited both excellent catalytic activity and high longevity in a probe reaction of one-step synthesis DMC. Under the optimal catalytic reaction conditions of 105 °C and 1.2 MPa, the highest CH3OH conversion of 10.12% and the DMC selectivity of 90.2% could be achieved. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Dimethyl carbonate (DMC), an environmentally building block, can be used as fuel additives, carbonylating agents, alkylating agents and polar solvents [1]. Several commercial processes have been developed for the synthesis of DMC, including the methanolysis of phosgene [2], oxidative carbonylation of methanol [3] and transesterification route [4]. However, these processes have some shortcomings. In contrast, direct synthesis of DMC from CH3OH and CO2 is a much more attractive method since such an approach is environmentally benign by nature [5,6]. A number of catalysts have been reported to be effective in this reaction, such as organometallic compounds [7], metal tetra-alkoxides [8], potassium carbonate [9], ZrO2 [10], H3PO4–V2O5 [11] Cu–Ni/VSO [12] and Cu– (Ni,V,O)/SiO2 [13]. However, the yield of DMC was reported to be low, even in the presence of dehydrates and additives such as CaCl2 [14], 2,2-dimethoxy propane (DMP) [15], molecular sieves [16] and UV-light assistance [13]. More recently, this reaction has been performed under supercritical conditions. But, the rigorous method is difficult to control.

* Corresponding authors. Address: State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, Sun Yat-Sen University, Guangzhou 510275, PR China. Tel./fax: +86 20 84114113 (Y.Z. Meng). E-mail addresses: [email protected] (M. Xiao), [email protected] (Y.Z. Meng). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.04.009

The supported transition metal particles exhibit high catalytic effects in DMC synthesis [12,13]. To develop efficient catalytic systems, more efforts have been devoted to the uses of various metals as catalysts. However, the support used in this catalytic system is much less explored. To date, the supports of loaded catalysts used in the direct synthesis of DMC are mainly inorganic oxides or compound oxides, including SiO2 [13,17], ZrO2 [10], MgO–SiO2 (MgSiO) [18], V2O5–SiO2 (VSO) [12] and ZrO2–CeO2 [15]. While these conventional materials offer some remarkable features as catalyst supports, they suffer from major shortcomings in that they have a relatively low surface area, high cost of commercial supports, low activity and short longevity of final catalysts. Therefore, development of novel supports with the required properties from inexpensive raw materials is highly desired. Carbonaceous nanomaterials and nanostructures have drawn considerable attention because of their unusual electronic transporting properties and ability to improve catalytic properties [19,20]. Of particular interest is the dispersion of metal nanoparticles on carbon nanostructures that show enhanced catalytic activity in fuel cell [21,22]. Graphene sheets – one-atom-thick twodimensional layers of sp2-bonded carbon – are predicted to have a range of unusual properties. The dispersion of metal nanoparticles on graphene sheets has potentially provided a new approach to develop catalytic, magnetic and optoelectronic materials. The p-stacked graphene sheets can be exfoliated by forming graphite oxide (GO) [23] or via manipulation of chemical functionalization [24]. It has been shown that a mild ultrasonic treatment of GO in water results in its exfoliation to form stable aqueous

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dispersions that consist almost entirely of 1-nm-thick sheets. GO is a layered and oxygen-abundant material produced by the controlled oxidation of graphite. As related to its pronounced cation exchange capacity and intercalation ability, GO has been employed as a host material for the accommodation of long chain aliphatic hydrocarbons and polymers [25,26]. It may also be regarded as a catalyst support material and a promising host for the intercalation of catalytically active transition and noble metal nanoparticles [27–29]. GO, inexpensive and available in large quantity, is widely used as supporting material in heterogeneous catalysis, but there is little information available for DMC synthesis. Previous studies by our team have shown that carbonaceous materials can be used as novel host materials of catalytically active Cu–Ni bimetallic nanoparticles [30–32]. In this communication we report the novel use of Cu–Ni catalysts supported on GO for onestep synthesis of DMC, which has proven to be excellent. The synthesis procedure is based on the generation of Cu–Ni particles in a micellar system, by applying the cationic surfactant hexadecyl trimethyl ammonium bromide (C19TABr) as a stabilizer. This forms the first report on DMC synthesis using Cu–Ni bimetal supported on GO catalysts.

2. Experimental 2.1. Catalyst preparation The methodology to obtain Cu–Ni/GO composites was depicted in Fig. SM1 (see Supplementary Material). The GO used was synthesized from microcrystalline natural graphite powder (NGP) by Hummers and Offeman’s method [23] with a chemical formula of C4O2.4H2.5 by elemental analysis. For the generation of Cu–Ni bimetallic nanoparticles in GO, an ammonia solution of the precursors Cu(NO3)2  3H2O and Ni(NO3)2  6H2O was first added to C19TABr solution under vigorous stirring, upon which a stable colloidal system was formed through solubilization. Simultaneously, GO was added into excessive ammonia solution and the resulting mixture was sonicated for 6 h, exfoliating of GO took place, which resulted in the formation of a stable GO sol. Sonication was maintained at room temperature and then the surfactant-stabilized Cu–Ni sol was added dropwise to a dilute aqueous suspension of GO at pH 10. On addition of the Cu–Ni sol to GO, cation exchange took place, which resulted in the formation of a hydrophobic Cu–Ni/GO organocomplex. After being dried at 90 °C, the fully dried samples were calcined at 600 °C for 2 h, and then activated by pure H2 flow under 600 °C for 3 h to yield Cu–Ni/GO bimetallic composite catalysts.

scan spectra in the binding energy range of 1100–0 eV were recorded with constant pass energy of 20 eV. The inductively coupled plasma-atomic emission spectrometry (ICP-AES) measurement was carried out on a TJA IRIS (HR) analyzer. The catalytic activity measurements of the catalysts were evaluated using a continuous tubular fixed-bed micro-gaseous reactor. A typical procedure was given as follows: 1 g of fresh catalyst was first charged into the reactor and the reactor was sealed and purged using CO2 gas flow for 10 min to exhaust the air inside. When the reaction system was heated to desired temperature, CH3OH and CO2 were introduced into the system to afford the experimental pressures. A 2:1 ratio of CH3OH (0.08 ml/min) and CO2 (30 ml/min) was kept during all the experiments, which was controlled via vaporization of CH3OH and the flux of CO2. Enough residence time was afforded until an equilibrium mixture was obtained. Amounts of CH3OH and CO2 were accurately controlled by gas-flow meter. The catalytic reaction was carried out at different temperatures (80–130 °C) and at a constant pressure of 1.2 MPa. The resulting mixture was introduced into an on-line GC (GC7890F) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID) to analyze the composition and concentration. The condensed liquid from the cut-off valve was collected and analyzed by the gas chromatograph mass spectrometer (GCMS-QP2010 plus) to confirm the DMC formation in the reaction system. Catalyst activity was mainly indicated by CH3OH conversion and DMC selectivity. These parameters were calculated according to the following equations:

½CH3 OH reacted  100 ½CH3 OH total ½DMC DMC selectivity ð%Þ ¼  100 ½DMC þ ½by  products CH3 OH conversion ð%Þ ¼

ð1Þ ð2Þ

3. Results and discussion 3.1. Surface properties of hydrophilic GO Fig. 1 shows the FTIR spectra and the corresponding digital images of samples. The change in color from black to brown-yellow indicated that the oxidation was complete. NGP (Fig. 1A) only showed absorbed and free water at 3447 and 1631 cm1, while GO (Fig. 1B) showed five sharp peaks at 3531, 1851, 1722, 1357 and 1184 cm1, which corresponded to free water (–OH), C@C, C@O, C–O and C–O–C group respectively [33,34]. The thermogravimetric analysis showed that the starting graphite oxidized at 700 °C (Fig. 2A); after acid oxidation reaction, the bulk material

2.2. Catalyst characterization FTIR spectra of samples (4000–400 cm1) were recorded on an Analect RFX-65A type FTIR spectrophotometer using the KBr pallet. Thermogravimetric (TG–DTG) analyses of samples were performed on a Perkin Elmer Pyris Diamond SII thermal analyzer (dry air, 10 °C/min). Raman spectra of samples (4000–50 cm1) were recorded on Renishaw inVia instrument. The crystal size and morphology of the samples were determined by a JEOL JSM-6380LA scanning electron microscopy (SEM) and a JEOL JSM-2010 transmission electron microscopy (TEM), and the operating voltage was 15 and 200 kV, respectively. The XRD patterns of the samples were recorded on a D/Max-IIIA power diffractometer in a step mode between 3° and 60° using Cu (Ka) radiation (0.15406 nm), operated at 35 kV and 25 mA. The X-ray photoelectron spectrum (XPS) measurements of sample were conducted with an ESCALAB 250 (Thermo-VG Scientific) analyzer spectrometer using monochromatized Al (Ka) excitation (1486.6 eV, 15 kV, 150 W). Survey

Fig. 1. FTIR spectra and the corresponding digital images (inset) of (A) NGP and (B) GO.

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Fig. 2. Thermogravimetric analysis data. (A) Graphite starting material had an oxidation onset at 700 °C. (B) Acid-oxidized graphite suffered significant weight loss at 600 °C. Residual water was removed at 120 °C, and acidic residues and functional groups were oxidized between 180 and 220 °C.

oxidized at 600 °C, with substantial weight loss around 200 °C due to the loss of the acidic functional groups and residues (Fig. 2B). FTIR and TG results implied that the prepared GO had well hydrophilicity, which has proven to be useful to swell and exfoliate GO in aqueous solution. Raman spectra (Fig. SM2, see Supplementary Material) showed that the prominent features of graphitic materials were the well-known D and G bands. The sharp D band around 1350 cm1 and G band around 1600 cm1 showed the defective, crystalline structure of graphene nanosheets [35]. The changes in Ram shifts indicated that some changes in the support surface had taken place after oxidation and impregnation. And also, reduction of catalyst had somewhat effect on the structure and surface of graphite. 3.2. Structure study of the catalysts Fig. 3 shows the typical micro-structural images of samples. It was evident from Fig. 3A that the chemical functionalization and ultrasonic exfoliating methods were satisfactory in separating individual graphene sheets. These 2-D sheets served as a foundation to disperse the Cu–Ni nanoparticles as seen from Fig. 3B. Furthermore, although the layered structure of pristine NGP was maintained well in GO, the value of the gallery spacing (0.335 nm) among carbon nano-sheets increased because the H2O molecules

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and oxygen-containing functional groups were intercalated into the graphite layers (Fig. 3A). It could be readily observed for the catalyst (Fig. 3B) that quasi-spherical, monodispersed nanoparticles were formed. This indicated an efficient size control, exerted by the C19TABr surfactant stabilizer. When the metal particles were deposited on GO, no obvious aggregation was detected (Fig. 3C). These results confirmed our success in achieving fairly uniform dispersion of metal nanoparticles on graphene sheets. Based on the TEM analysis, the average particle diameter was estimated to be 24.8 nm. The XRD patterns of samples are displayed in Fig. 4. After oxidation, the graphitic (0 0 2, 2h = 26.6° and 0 0 4, 2h = 54.8°) diffraction peaks (Fig. 4A) completely disappeared and, instead, were replaced by a well defined peak at a lower diffraction angle (Fig. 4B, 2h = 13.0°). The absence of the characteristic 0 0 2 diffraction of graphite at 26.6° confirmed that complete oxidation took place, resulting in the formation of a well-ordered, lamellar structure, which was more open than that of graphite (dL = 0.335 nm) and thus more susceptible to intercalation. The XRD pattern of GO revealed a sharp (0 0 1) peak at 2h = 13.0°, indicative of a good layer regularity with repeating interplanar distance (dL) of 0.858 nm, which was in the range of the reported values (0.6–1.1 nm). The increase of the basal spacing of GO in the course of oxidation is owing to the accommodation of various oxygen species and the changes of carbon hexahedron grid plane. The weak peak at 2h = 20.6° indicated the existence of some stacking structures. After being reduced by H2 (Fig. 4C), the diffraction peak of GO disappeared and the graphitic (0 0 2) diffraction peak appeared again although not as sharp as the original graphite, indicating that GO structure changed as the reduction proceeded. Two new diffraction peaks at 2h = 43.4° and 50.7° could be assigned to diffraction of metal phase Cu and cubic phase Cu–Ni alloy overlap [12,36] (Fig. 4C). The absence of Ni species diffraction peak was an indication of well-dispersed Ni microcrystalline phase. XRD results indicated that metallic phases of Cu, Ni and alloy phase of Cu–Ni were formed in the catalyst. The XPS results of catalyst were shown in Fig. SM3. The survey scan spectrum indicated the presence of C, O, Cu and Ni on sample surface (Fig. SM3A). Oxygen content in catalyst was much higher due to the oxidative treatment and a slight atmospheric oxidation. As shown in Fig. SM3B and C, the binding energy (BE) of Cu (sp2/3) and Ni (sp2/3) was centered at 933.5 eV (Cu0) and at 855.6 eV (Ni0), respectively, suggesting that Cu and Ni metals were formed under the reduction and activation step, these results were in accordance with XRD observations. Based on the XPS results, molar ratio of Cu/ Ni was 1.67/0.76, which was nearly equal to the composition in the starting mixture. The ICP-AES measurement indicated that the total metal content was 15.74% (obtained from the emission intensity by means of a calibration curve), which is a little lower than the starting loading.

Fig. 3. SEM images of (A) before and (B) after deposition with Cu–Ni nanoparticles. (C) TEM of Cu–Ni/GO bimetallic catalyst.

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given metal content and molar ratio. Fig. 5 illustrates that the catalytic reaction performed at 105 °C and 1.2 MPa had the highest activity, capable of achieving 10.12% conversion of CH3OH and 90.2% selectivity of DMC over the Cu–Ni/GO bimetallic catalyst. This was perhaps due to the synergetic effects of Cu, Ni and Cu– Ni alloy in the activation of CH3OH and CO2. When reaction temperature was below 105 °C, the conversion of CH3OH increased with increasing temperature. When reaction temperature exceeded 105 °C, the DMC content reduced and the by-products increased. The one-step direct synthesis of DMC investigated in present work using Cu–Ni/GO as a catalyst produced more than just DMC. By-products such as H2O, CO, CH2O, and dimethyl ether (DME) were also produced. The stability tests performed under the optimal catalytic conditions (n (CH3OH)/n (CO2) = 2/1, catalyst weight 1.0 g, T = 105 °C, P = 1.2 MPa, time on stream 10 h) showed that the conversion of CH3OH only decreased 3.2% (from 10.12% to 9.8%) after 10-h reactions, indicating that the catalyst had an excellent durability. The activity tests for the regenerated catalysts showed that the activity did not change obviously. 4. Conclusions

Fig. 4. XRD patterns of (A) NGP, (B) GO and (C) Cu–Ni/GO bimetallic catalyst.

Well-dispersed Cu–Ni nanoparticles were immobilized in graphite oxide, a layer-structured host material. The particle size can be effectively controlled by using a cationic surfactant stabilizer. The resulting Cu–Ni/GO bimetallic catalysts exhibited both high activity and selectivity for the one-step direct synthesis of DMC. It was ascertained that both interlamellar and external Cu, Ni and Cu–Ni alloy nanoparticles participated in the catalytic reactions as active sites.

3.3. Catalyst activity and selectivity

Acknowledgments

In an effort to confirm the catalytic application of the prepared catalyst, a probe reaction of one-step direct synthesis of DMC was carried out. For comparison purpose, the catalytic activities when using only GO, Cu/GO and Ni/GO monometallic catalysts were also investigated. The effects of catalytic reaction temperature on the activities of catalysts are shown in Fig. 5. It could be seen that GO was inert to the target reaction, while Cu/GO or Ni/GO monometallic catalyst alone used was catalytic active, although the activity was relatively low. However, when Cu–Ni/GO bimetallic catalyst was used, the activity was enhanced obviously under the

The authors would like to thank the China High-Tech Development 863 Program, Guangdong Province Sci. and Tech. Bureau (Key Strategic Project Grant No. 2006B12401006), and Guangzhou Sci. and Tech. Bureau (2005U13D2031, 2007Z2-D2031) for financial support of this work. Y.L. thanks National University of Singapore for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2009.04.009. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

Fig. 5. The dependence of CH3OH conversion and DMC selectivity on the reaction temperature over the Cu–Ni/GO bimetallic catalysts. Reaction conditions: n (CH3OH)/n (CO2) = 2/1, catalyst weight 1.0 g, P = 1.2 MPa, time on stream 3 h.

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