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A Facile Method for the Synthesis of CuO-RGO Nanocomposite for Para Nitrophenol Reduction Reaction
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 9 (2019) 587–593
www.materialstoday.com/proceedings
GMSP&NS’18
A Facile Method for the Synthesis of CuO-RGO Nanocomposite for Para Nitrophenol Reduction Reaction K. Revathia, Shajesh.Palantavida b, Baiju Kizhakkekilikoodayil Vijayana** a
Department of Nanoscience/ Chemistry, Kannur University, Swami Anandha Theertha campus, Payyannur, Edat P.O., Kerala-670 327, India b Centre for Nano and Material Sciences, Jain, Jain Global Campus, Bangalore, 52112, Karnataka, India
Abstract In this paper, we demonstrate a single and facile aqueous reduction route for the synthesis of CuO-Reduced graphene oxide (CuO-RGO) nanocomposite. The synthesized samples were characterized by XRD and SEM analysis. The catalytic activities of the CuO-RGO nanocomposite were investigated for the aqueous reduction of 4-nitrophenol (4-NP) by NaBH4. The catalytic study was monitored by UV-vis spectroscopy and it was found that the CuO-RGO nanocomposite show higher catalytic activity and catalytic recyclability compared with the pure CuO nanocrystals. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Green Methods for Separation, Purification and Nanomaterial Synthesis, GMSP&NS’18, 24–25th April 2018, Centre for Nano and Material Sciences, Jain University, Bangalore 562112, Karnataka, India. Keywords: Graphene oxide; Copper oxide;4-nitropheneol; catalysis
1. Introduction Heterostructured nanomaterials have gain increasing attention in the area of catalysis in the past two decades because they show multiple functionalities and prominent catalytic activity, selectivity, and stability over monometallic nanomaterials [1]. There are various reports on the fact that the catalytic activity depends on the surface microstructures or the surface arrangement of atoms in addition to the size and shape of the nanostructured material. It has been realized that the effective control of the surface is a powerful tool for enhancing the catalytic activity of the nanostructure material [2]. * Corresponding author: Tel: +91497 2806402 Email addresses: [email protected], [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Green Methods for Separation, Purification and Nanomaterial Synthesis, GMSP&NS’18, 24–25th April 2018, Centre for Nano and Material Sciences, Jain University, Bangalore 562112, Karnataka, India.
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Nowadays, 4-aminophenol (4-AP), one of the most valuable intermediate for drug and paint industry is produced by the hydrogenation of 4-nitro phenol [3, 4]. Usually, the catalytic hydrogenation of 4-NP is carried out at relatively higher temperature and hydrogen pressure using Pt/C or nanosized nickel as catalyst. However, such methods sometimes causes severe energy problems [5]. Copper oxide is a low cost, transition metal oxide which have demonstrated good catalytic performance due to its high oxidation-reduction potential values[6]. A crucial factor that influences the good catalytic performance of nanostructured materials is also depend on the nature of the support. Reports suggests that the large surface area of the catalysts support give rise to highly dispersed CuO active nanoparticles upon reduction which would significantly enhance the catalytic performance[7, 8]. Besides the use of such catalyst would also result in synergistic effect of both the catalyst as well as support in the catalytic property which will be different from those of each single components. The unique structure containing active functional groups[9], enormous surface area and easy surface makes graphene oxide as one of the excellent and widely accepted support for various catalysts. There are several reports on the use of GO as the support for diverse catalysts including TiO2 [10] , MnO2[11], Pt [12]and Pd[13]. In our present work we have developed a facile method for the synthesis of CuO nanocrystals and CuO-RGO nanocomposite for the efficient reduction of 4NP to 4-AP. Here graphene oxide is synthesized by the modified Hummers method which is a economically viable method among all of the reported methods of synthesis of graphene oxide. 2. Experimental 2.1. Materials used Cupric chloride (CuCl2, LOBA Chemie, purity 99% ), Ethylene diamine (EDA, LOBA Chemie, 98 %), Hydrazine hydrate (N2H4.H2O, Merk, Emparta, 80%,), H2O2 (Merk, 30%,), KMnO4 ( SRL, 99.5% ), H2SO4( Nice, 98.08% ), Graphite powder (LOBA Chemie, 98% ), HCl (Merk, Emplura, 35%), were purchased and used as received without further purification . All aqueous solutions were prepared with distilled water. 2.2. Synthesis of Graphene oxide Graphite oxide aqueous dispersion was prepared from the natural graphite flakes according to modified Hummer’s method[14]. In a typical reaction, 3g graphite powder was added to concentrated H2SO4 (70 ml) under magnetic stirring in an ice bath. To this 9g KMnO4 was added slowly with vigorous agitation keeping the temperature of the suspension lower that 20̊ C. This is followed by transferring the reaction solution into a 40̊C oil bath and vigorously stirred for 30 min. Then 150 ml distilled water was added and further stirred for 15 min under 95̊ C. To this 500 ml distilled water was added and followed by a slow addition of 15 ml H2O2 (30%). During this the color of the solution changes from dark brown to yellow. The mixture was then filtered and washed with1:10 HCl aqueous solution (250 ml) to remove metal ions. The resulting solid was dried in air and diluted to 600 ml to graphene oxide aqueous dispersion. This is then kept for dialysis using a dialysis membrane (Himedia, dialysis membrane-150) with a molecular weight cut-off of 8000-14,000 gmol-1. Finally it is stirred overnight and sonicated for 30 min to exfoliate to get GO. 2.3. Synthesis of CuO nanocrystals In a typical experiment, 0.4265 g CuCl2 is dissolved in to 250ml of distilled water and mixed with15 M sodium hydroxide solution. To this 7.5 ml EDA and hydrazine hydrate (80%) were added with stirring. The mixed solution was heated to 800 C and maintained for 60 min. The prepared solid sample was separated by filtration and dried using an IR lamp for 3hr [15]. 2.4. Synthesis of CuO-RGO nanocomposite In a typical experiment, 0.4265 g CuCl2 and 50 ml GO dispersion (0.002g in 1 ml) is dissolved in to 250ml of distilled water. To this 15 M sodium hydroxide solution and stirred well . To this 7.5 ml EDA and hydrazine hydrate
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(80%) were added with stirring. The mixed solution was heated to 800C and maintained for 60 min. The prepared solid sample was separated by filtration and dried using an IR lamp for 3hr [15]. 2. 5. Catalytic study To study the catalytic performance of the as-prepared CuO nanocrystals and CuO-RGO nanocomposites, the reduction of 4-nitrophenol using sodium borohydride was carried out as follows: 0.50 mg catalyst were dispersed in 10 ml of 4-nitrophenol aqueous solution (1 mM) as reactants at room temperature. Then a freshly prepared aqueous solution of NaBH4 (3 mL, 0.04 M) was added as the reducing agent. Then 2ml of the mixed solution was immediately transferred into a quartz cuvette and diluted with distilled water and the absorption spectra were measured at regular intervals of time. The change of absorption value of the solution was then monitored using a UV Vis- spectrophotometer. 2.6. Characterization X-ray diffractometer model D8 Advance (Bruker) with Cu Kα radiation ( λ= 1.54178 A ) is used for X-ray diffraction (XRD) analysis at a scanning rate of 0.02 °s-1 in the 2θ range from 20̊ - 80̊. Surface morphologies of the products were characterized using Hitachi SU8010 scanning electron microscope. UV-Vis spectrophotometer (UV2550, Shimadzu, Japan) was used for the catalytic study. 3. Results and discussion 3.1 Structural and morphological study The surface morphology and chemical composition of the as synthesized CuO nanocrystals and CuO-RGO nanocomposites respectively were shown in figure 1. As shown in figure 1a, CuO nanocrystals are of spherical in shape and were highly agglomerated. SEM image of CuO-RGO nanocomposite (figure 1b) shows that the nanostructure is in the form of flakes. Besides both the synthesized CuO nanocrystals and CuO-RGO nanocomposites were homogeneously distributed.
Fig. 1. SEM image (a) CuO nanocrystals, b) CuO-RGO nanocomposite
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Fig. 2. XRD pattern of CuO-RGO nano composite.
The XRD pattern of CuO-RGO nanocomposite is shown in figure 2. It is clear from figure 2 that the broad diffraction peak at 17.28 and 75.38 is attributed to the (002) and (220) planes of hexagonal graphite structure(JCPDS No 89-8488). The peaks at 32.77̊, 35.99̊, 39.12̊, 48.82̊, 53.86̊, 58.58̊, 61.98̊, 66.23̊ and 68.33̊ corresponds to (110), (111), (111), (202), (020), (202), (113), (311) and (220) of monoclinic CuO (JCPDS file no. 48-1548) respectively. 3.2 Catalytic study In this study, NaBH4 reduction of p-Nitrophenol to p-aminopenol was selected as a model reaction in order to analyze the catalytic activity efficiency of the synthesized CuO nanocrystals and CuO-RGO nanocomposite. p-NP shows a strong absorption peak at 400 nm. Preliminary analysis indicates that conversion of p-NP to p-AP will not occur in the absence of a catalyst. Figure 3a and 3b shows that UV absorption spectra of the p-NP reduction reaction in the using CuO and CuO-RGO nanocomposites. It can be clearly observed from the figure 3a that as time passes, there is gradual decrease in the absorption peak of 4-NP at 400 nm when our prepared nanostructures were used as the catalyst. The rate constants for the above catalytic reaction using CuO and CuO-RGO nanocomposites were found to be 0.101 and 0.15 min-1 respectively. When the concentration of NaBH4 is high the rate of reaction depends only upon the concentration of 4-NP and thus the reaction follows a pseudo first order kinetics [16, 17]. The kinetic equation or the conversion of 4-NP to 4-AP can be represented as follows; Kt= lnC0 – lnC = ln A0-ln A Here C0 and C represents the concentration of 4-NP at initial t=0 and at time t respectively, k is the apparent rate constant. The concentration ratio C/C0 of 4-NP can be obtained from the ratio of absorbance A at 400nm at time t to its initial value at t=0. We also studied the recyclability of the as prepared catalyst for further two cycles. The rate constant values for the first, second and third cycles of the reaction were 0.101 min-1, 0.029 min-1 and 0.001 min-1 for CuO and 0.15 min-1, 0.13min-1 and 0.058 min-1for CuO –RGO respectively. Figure 3c and 3d shows the plot of ln (At/A0 ) Vs time for the catalytic reduction of 4-NP using Na BH4 in the presence of copper oxide and CuO-RGO nanocomposites in the first and second cycle respectively. Fig.3c and 3d illustrated that this reduction reaction displayed good linearity and the correlation coefficient of R2 value were equal to 0.925 and0.938 for copper oxide and 0.947 and 0.982 for CuO -RGO nanocomposites in the first and second
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cycle respectively. This suggests that there was a great agreement between the first order kinetics with the experimental value .
Fig. 3. UV absorption spectra of 4-NP reduction using a) CuO nanocrystals , b) CuO-RGO nanocomposites, c) lnC/C0 versus time plot of 4-NP reduction using the prepared catalysts in the first and d) second cycle respectively.
Figure 4 shows the catalytic activity for the bare copper oxide as well as CuO-RGO catalyst for the three cycles of the reaction. It is well clear that the catalytic activity of the CuO-RGO composite is higher than that of bare CuO nanocrystals. In the second and third catalytic cycles, the catalytic activity of bare CuO nanocrystas decreases tremendously from 0.101min-1 to 0.001 min-1. But there is only slight decrease in the catalytic efficiency of CuORGO composite in the second and third cycles respectively. The rate of reaction at different cycles is plotted in the Figure 4. It was found that the catalytic efficiency of CuO-RGO composite was higher than that of CuO nanocrystals, indicating the efficient catalytic performance of the CuO-RGO composite. Also the CuO-RGO composite shows good catalytic stability for over three cycles of the reaction. Table 1 shows the rate constant values for 4-NP reduction using different catalysts reported in the literature The improved catalytic performance of CuO-RGO nanocomposite can be atribute to the following reasons: (1) The enormous surface area of the RGO sheets which favours the effective anchoring of CuO nanocrystals. (2) Synergistic effect between CuO nanocrystals and graphene oxide sheets play an important role. The high adsorption capacity of RGO towards 4-NP via π-π stacking interactions facilitates the flow of high amount of 4-NP towards CuO nanocrystals. In addition, the local electron concentration near CuO catalyst is increased by the electron transfer from the graphene to CuO nanocrystals which further facilitates the electron uptake of 4-NP molecules[3]. (3)The existence of reduced graphene oxide further stabilizes the catalysts and prevents the particles from
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agglomeration of CuO nanocrystals. This leads to further increase in the active sites available for 4-NP reduction compared to bare CuO nanocrystals. Table 1. Comparison of obtained reaction constant (K) value with different catalytic systems for the reduction of 4-Nitrophenol Catalyst
Rate constant (k in s-1)
Pd/GO nanocomposites
0.034
[18]
Au/graphene hydrogel
0.003
[3]
Ag-Au-rGO
0.003
[19]
Pt-PDA/rGO
0.003
[20]
Hollow porous AuNP
0.007
[21]
CuO-RGO
0.15 (K in min-1) 0.0025 (K in s-1)
Present work
Reference
Fig. 4. Relative catalytic activity for the bare copper oxide as well as CuO-RGO catalyst for the three cycles of catalytic reaction.
4. Conclusion We have developed a facile method for the synthesis of CuO-RGO composite by an aqueous reduction method. The as prepared CuO-RGO nanocomposite shows good catalytic stability over three cycles of the reaction. The rate constant of the 4-NP reduction reaction using CuO-RGO composite was determined to be 0.15 min-1, and after reacting for over three cycles the rate constant will be 0.058min-1. The developed economical method for the synthesis of CuO-RGO nanocomposite together with improved catalytic efficiency provides a potential new approach or the design of nanostructured catalysts with excellent performance. Acknowledgements RK and BKV acknowledge the funding from KSCSTE project ((Order No.1562/2016/KSCSTE) and UGC start up grant No. F 30-384/2017 (BSR).The authors acknowledge Nanomission project "SR/NM/NS-20/2014" for the FESEM facility.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Toshima, N. and T. Yonezawa, Bimetallic nanoparticles-novel materials for chemical and physical applications. New Journal of Chemistry, 1998. 22(11): p. 1179-1201. Wu, B. and N. Zheng, Surface and interface control of noble metal nanocrystals for catalytic and electrocatalytic applications. Nano Today, 2013. 8(2): p. 168-197. Li, J., C.-y. Liu, and Y. Liu, Au/graphene hydrogel: synthesis, characterization and its use for catalytic reduction of 4-nitrophenol. Journal of Materials Chemistry, 2012. 22(17): p. 8426-8430. Zhang, P., et al., In situ assembly of well-dispersed Ag nanoparticles (AgNPs) on electrospun carbon nanofibers (CNFs) for catalytic reduction of 4-nitrophenol. Nanoscale, 2011. 3(8): p. 3357-3363. Lu, H., et al., Influence of support on catalytic activity of Ni catalysts in p-nitrophenol hydrogenation to p-aminophenol. Catalysis Communications, 2008. 10(3): p. 313-316. Dhandapani, B. and S.T. Oyama, Gas phase ozone decomposition catalysts. Applied Catalysis B: Environmental, 1997. 11(2): p. 129-166. Cao, J.-L., et al., Preparation, characterization and catalytic behavior of nanostructured mesoporous CuO/Ce0.8Zr0.2O2 catalysts for lowtemperature CO oxidation. Applied Catalysis B: Environmental, 2008. 78(1): p. 120-128. Luo, M.-F., et al., High-surface area CuO–CeO2 catalysts prepared by a surfactant-templated method for low-temperature CO oxidation. Journal of Catalysis, 2007. 246(1): p. 52-59. Park, C.-H. and S.G. Louie, Tunable Excitons in Biased Bilayer Graphene. Nano Letters, 2010. 10(2): p. 426-431. Perera, S.D., et al., Hydrothermal Synthesis of Graphene-TiO2 Nanotube Composites with Enhanced Photocatalytic Activity. ACS Catalysis, 2012. 2(6): p. 949-956. He, Y., et al., Freestanding Three-Dimensional Graphene/MnO2 Composite Networks As Ultralight and Flexible Supercapacitor Electrodes. ACS Nano, 2013. 7(1): p. 174-182. Yoo, E., et al., Enhanced Electrocatalytic Activity of Pt Subnanoclusters on Graphene Nanosheet Surface. Nano Letters, 2009. 9(6): p. 2255-2259. Li, Y., et al., CO oxidation over graphene supported palladium catalyst. Applied Catalysis B: Environmental, 2012. 125: p. 189-196. Hummers, W.S. and R.E. Offeman, Preparation of Graphitic Oxide. Journal of the American Chemical Society, 1958. 80(6): p. 1339-1339. Liu, Y., et al., From Copper Nanocrystalline to CuO Nanoneedle Array: Synthesis, Growth Mechanism, and Properties. The Journal of Physical Chemistry C, 2007. 111(13): p. 5050-5056. Bhui, D.K. and A. Misra, Synthesis of worm like silver nanoparticles in methyl cellulose polymeric matrix and its catalytic activity. Carbohydrate Polymers, 2012. 89(3): p. 830-835. Imura, Y., et al., Preparation and Catalytic Activity of Pd and Bimetallic Pd–Ni Nanowires. Langmuir, 2014. 30(17): p. 5026-5030. Sun, W., et al., Fabrication of highly dispersed palladium/graphene oxide nanocomposites and their catalytic properties for efficient hydrogenation of p-nitrophenol and hydrogen generation. International Journal of Hydrogen Energy, 2014. 39(17): p. 9080-9086. K, H., et al., Anchoring of Ag-Au alloy nanoparticles on reduced graphene oxide sheets for the reduction of 4-nitrophenol. Applied Surface Science, 2016. 389: p. 1050-1055. Ye, W., et al., Green synthesis of Pt–Au dendrimer-like nanoparticles supported on polydopamine-functionalized graphene and their high performance toward 4- nitrophenol reduction. Applied Catalysis B: Environmental, 2016. 181: p. 371-378. Guo, M., et al., One-step synthesis of hollow porous gold nanoparticles with tunable particle size for the reduction of 4-nitrophenol. Journal of Hazardous Materials, 2016. 310: p. 89-97.
ScienceDirect Materials Today: Proceedings 9 (2019) 587–593
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GMSP&NS’18
A Facile Method for the Synthesis of CuO-RGO Nanocomposite for Para Nitrophenol Reduction Reaction K. Revathia, Shajesh.Palantavida b, Baiju Kizhakkekilikoodayil Vijayana** a
Department of Nanoscience/ Chemistry, Kannur University, Swami Anandha Theertha campus, Payyannur, Edat P.O., Kerala-670 327, India b Centre for Nano and Material Sciences, Jain, Jain Global Campus, Bangalore, 52112, Karnataka, India
Abstract In this paper, we demonstrate a single and facile aqueous reduction route for the synthesis of CuO-Reduced graphene oxide (CuO-RGO) nanocomposite. The synthesized samples were characterized by XRD and SEM analysis. The catalytic activities of the CuO-RGO nanocomposite were investigated for the aqueous reduction of 4-nitrophenol (4-NP) by NaBH4. The catalytic study was monitored by UV-vis spectroscopy and it was found that the CuO-RGO nanocomposite show higher catalytic activity and catalytic recyclability compared with the pure CuO nanocrystals. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Green Methods for Separation, Purification and Nanomaterial Synthesis, GMSP&NS’18, 24–25th April 2018, Centre for Nano and Material Sciences, Jain University, Bangalore 562112, Karnataka, India. Keywords: Graphene oxide; Copper oxide;4-nitropheneol; catalysis
1. Introduction Heterostructured nanomaterials have gain increasing attention in the area of catalysis in the past two decades because they show multiple functionalities and prominent catalytic activity, selectivity, and stability over monometallic nanomaterials [1]. There are various reports on the fact that the catalytic activity depends on the surface microstructures or the surface arrangement of atoms in addition to the size and shape of the nanostructured material. It has been realized that the effective control of the surface is a powerful tool for enhancing the catalytic activity of the nanostructure material [2]. * Corresponding author: Tel: +91497 2806402 Email addresses: [email protected], [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Green Methods for Separation, Purification and Nanomaterial Synthesis, GMSP&NS’18, 24–25th April 2018, Centre for Nano and Material Sciences, Jain University, Bangalore 562112, Karnataka, India.
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Nowadays, 4-aminophenol (4-AP), one of the most valuable intermediate for drug and paint industry is produced by the hydrogenation of 4-nitro phenol [3, 4]. Usually, the catalytic hydrogenation of 4-NP is carried out at relatively higher temperature and hydrogen pressure using Pt/C or nanosized nickel as catalyst. However, such methods sometimes causes severe energy problems [5]. Copper oxide is a low cost, transition metal oxide which have demonstrated good catalytic performance due to its high oxidation-reduction potential values[6]. A crucial factor that influences the good catalytic performance of nanostructured materials is also depend on the nature of the support. Reports suggests that the large surface area of the catalysts support give rise to highly dispersed CuO active nanoparticles upon reduction which would significantly enhance the catalytic performance[7, 8]. Besides the use of such catalyst would also result in synergistic effect of both the catalyst as well as support in the catalytic property which will be different from those of each single components. The unique structure containing active functional groups[9], enormous surface area and easy surface makes graphene oxide as one of the excellent and widely accepted support for various catalysts. There are several reports on the use of GO as the support for diverse catalysts including TiO2 [10] , MnO2[11], Pt [12]and Pd[13]. In our present work we have developed a facile method for the synthesis of CuO nanocrystals and CuO-RGO nanocomposite for the efficient reduction of 4NP to 4-AP. Here graphene oxide is synthesized by the modified Hummers method which is a economically viable method among all of the reported methods of synthesis of graphene oxide. 2. Experimental 2.1. Materials used Cupric chloride (CuCl2, LOBA Chemie, purity 99% ), Ethylene diamine (EDA, LOBA Chemie, 98 %), Hydrazine hydrate (N2H4.H2O, Merk, Emparta, 80%,), H2O2 (Merk, 30%,), KMnO4 ( SRL, 99.5% ), H2SO4( Nice, 98.08% ), Graphite powder (LOBA Chemie, 98% ), HCl (Merk, Emplura, 35%), were purchased and used as received without further purification . All aqueous solutions were prepared with distilled water. 2.2. Synthesis of Graphene oxide Graphite oxide aqueous dispersion was prepared from the natural graphite flakes according to modified Hummer’s method[14]. In a typical reaction, 3g graphite powder was added to concentrated H2SO4 (70 ml) under magnetic stirring in an ice bath. To this 9g KMnO4 was added slowly with vigorous agitation keeping the temperature of the suspension lower that 20̊ C. This is followed by transferring the reaction solution into a 40̊C oil bath and vigorously stirred for 30 min. Then 150 ml distilled water was added and further stirred for 15 min under 95̊ C. To this 500 ml distilled water was added and followed by a slow addition of 15 ml H2O2 (30%). During this the color of the solution changes from dark brown to yellow. The mixture was then filtered and washed with1:10 HCl aqueous solution (250 ml) to remove metal ions. The resulting solid was dried in air and diluted to 600 ml to graphene oxide aqueous dispersion. This is then kept for dialysis using a dialysis membrane (Himedia, dialysis membrane-150) with a molecular weight cut-off of 8000-14,000 gmol-1. Finally it is stirred overnight and sonicated for 30 min to exfoliate to get GO. 2.3. Synthesis of CuO nanocrystals In a typical experiment, 0.4265 g CuCl2 is dissolved in to 250ml of distilled water and mixed with15 M sodium hydroxide solution. To this 7.5 ml EDA and hydrazine hydrate (80%) were added with stirring. The mixed solution was heated to 800 C and maintained for 60 min. The prepared solid sample was separated by filtration and dried using an IR lamp for 3hr [15]. 2.4. Synthesis of CuO-RGO nanocomposite In a typical experiment, 0.4265 g CuCl2 and 50 ml GO dispersion (0.002g in 1 ml) is dissolved in to 250ml of distilled water. To this 15 M sodium hydroxide solution and stirred well . To this 7.5 ml EDA and hydrazine hydrate
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(80%) were added with stirring. The mixed solution was heated to 800C and maintained for 60 min. The prepared solid sample was separated by filtration and dried using an IR lamp for 3hr [15]. 2. 5. Catalytic study To study the catalytic performance of the as-prepared CuO nanocrystals and CuO-RGO nanocomposites, the reduction of 4-nitrophenol using sodium borohydride was carried out as follows: 0.50 mg catalyst were dispersed in 10 ml of 4-nitrophenol aqueous solution (1 mM) as reactants at room temperature. Then a freshly prepared aqueous solution of NaBH4 (3 mL, 0.04 M) was added as the reducing agent. Then 2ml of the mixed solution was immediately transferred into a quartz cuvette and diluted with distilled water and the absorption spectra were measured at regular intervals of time. The change of absorption value of the solution was then monitored using a UV Vis- spectrophotometer. 2.6. Characterization X-ray diffractometer model D8 Advance (Bruker) with Cu Kα radiation ( λ= 1.54178 A ) is used for X-ray diffraction (XRD) analysis at a scanning rate of 0.02 °s-1 in the 2θ range from 20̊ - 80̊. Surface morphologies of the products were characterized using Hitachi SU8010 scanning electron microscope. UV-Vis spectrophotometer (UV2550, Shimadzu, Japan) was used for the catalytic study. 3. Results and discussion 3.1 Structural and morphological study The surface morphology and chemical composition of the as synthesized CuO nanocrystals and CuO-RGO nanocomposites respectively were shown in figure 1. As shown in figure 1a, CuO nanocrystals are of spherical in shape and were highly agglomerated. SEM image of CuO-RGO nanocomposite (figure 1b) shows that the nanostructure is in the form of flakes. Besides both the synthesized CuO nanocrystals and CuO-RGO nanocomposites were homogeneously distributed.
Fig. 1. SEM image (a) CuO nanocrystals, b) CuO-RGO nanocomposite
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Fig. 2. XRD pattern of CuO-RGO nano composite.
The XRD pattern of CuO-RGO nanocomposite is shown in figure 2. It is clear from figure 2 that the broad diffraction peak at 17.28 and 75.38 is attributed to the (002) and (220) planes of hexagonal graphite structure(JCPDS No 89-8488). The peaks at 32.77̊, 35.99̊, 39.12̊, 48.82̊, 53.86̊, 58.58̊, 61.98̊, 66.23̊ and 68.33̊ corresponds to (110), (111), (111), (202), (020), (202), (113), (311) and (220) of monoclinic CuO (JCPDS file no. 48-1548) respectively. 3.2 Catalytic study In this study, NaBH4 reduction of p-Nitrophenol to p-aminopenol was selected as a model reaction in order to analyze the catalytic activity efficiency of the synthesized CuO nanocrystals and CuO-RGO nanocomposite. p-NP shows a strong absorption peak at 400 nm. Preliminary analysis indicates that conversion of p-NP to p-AP will not occur in the absence of a catalyst. Figure 3a and 3b shows that UV absorption spectra of the p-NP reduction reaction in the using CuO and CuO-RGO nanocomposites. It can be clearly observed from the figure 3a that as time passes, there is gradual decrease in the absorption peak of 4-NP at 400 nm when our prepared nanostructures were used as the catalyst. The rate constants for the above catalytic reaction using CuO and CuO-RGO nanocomposites were found to be 0.101 and 0.15 min-1 respectively. When the concentration of NaBH4 is high the rate of reaction depends only upon the concentration of 4-NP and thus the reaction follows a pseudo first order kinetics [16, 17]. The kinetic equation or the conversion of 4-NP to 4-AP can be represented as follows; Kt= lnC0 – lnC = ln A0-ln A Here C0 and C represents the concentration of 4-NP at initial t=0 and at time t respectively, k is the apparent rate constant. The concentration ratio C/C0 of 4-NP can be obtained from the ratio of absorbance A at 400nm at time t to its initial value at t=0. We also studied the recyclability of the as prepared catalyst for further two cycles. The rate constant values for the first, second and third cycles of the reaction were 0.101 min-1, 0.029 min-1 and 0.001 min-1 for CuO and 0.15 min-1, 0.13min-1 and 0.058 min-1for CuO –RGO respectively. Figure 3c and 3d shows the plot of ln (At/A0 ) Vs time for the catalytic reduction of 4-NP using Na BH4 in the presence of copper oxide and CuO-RGO nanocomposites in the first and second cycle respectively. Fig.3c and 3d illustrated that this reduction reaction displayed good linearity and the correlation coefficient of R2 value were equal to 0.925 and0.938 for copper oxide and 0.947 and 0.982 for CuO -RGO nanocomposites in the first and second
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cycle respectively. This suggests that there was a great agreement between the first order kinetics with the experimental value .
Fig. 3. UV absorption spectra of 4-NP reduction using a) CuO nanocrystals , b) CuO-RGO nanocomposites, c) lnC/C0 versus time plot of 4-NP reduction using the prepared catalysts in the first and d) second cycle respectively.
Figure 4 shows the catalytic activity for the bare copper oxide as well as CuO-RGO catalyst for the three cycles of the reaction. It is well clear that the catalytic activity of the CuO-RGO composite is higher than that of bare CuO nanocrystals. In the second and third catalytic cycles, the catalytic activity of bare CuO nanocrystas decreases tremendously from 0.101min-1 to 0.001 min-1. But there is only slight decrease in the catalytic efficiency of CuORGO composite in the second and third cycles respectively. The rate of reaction at different cycles is plotted in the Figure 4. It was found that the catalytic efficiency of CuO-RGO composite was higher than that of CuO nanocrystals, indicating the efficient catalytic performance of the CuO-RGO composite. Also the CuO-RGO composite shows good catalytic stability for over three cycles of the reaction. Table 1 shows the rate constant values for 4-NP reduction using different catalysts reported in the literature The improved catalytic performance of CuO-RGO nanocomposite can be atribute to the following reasons: (1) The enormous surface area of the RGO sheets which favours the effective anchoring of CuO nanocrystals. (2) Synergistic effect between CuO nanocrystals and graphene oxide sheets play an important role. The high adsorption capacity of RGO towards 4-NP via π-π stacking interactions facilitates the flow of high amount of 4-NP towards CuO nanocrystals. In addition, the local electron concentration near CuO catalyst is increased by the electron transfer from the graphene to CuO nanocrystals which further facilitates the electron uptake of 4-NP molecules[3]. (3)The existence of reduced graphene oxide further stabilizes the catalysts and prevents the particles from
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agglomeration of CuO nanocrystals. This leads to further increase in the active sites available for 4-NP reduction compared to bare CuO nanocrystals. Table 1. Comparison of obtained reaction constant (K) value with different catalytic systems for the reduction of 4-Nitrophenol Catalyst
Rate constant (k in s-1)
Pd/GO nanocomposites
0.034
[18]
Au/graphene hydrogel
0.003
[3]
Ag-Au-rGO
0.003
[19]
Pt-PDA/rGO
0.003
[20]
Hollow porous AuNP
0.007
[21]
CuO-RGO
0.15 (K in min-1) 0.0025 (K in s-1)
Present work
Reference
Fig. 4. Relative catalytic activity for the bare copper oxide as well as CuO-RGO catalyst for the three cycles of catalytic reaction.
4. Conclusion We have developed a facile method for the synthesis of CuO-RGO composite by an aqueous reduction method. The as prepared CuO-RGO nanocomposite shows good catalytic stability over three cycles of the reaction. The rate constant of the 4-NP reduction reaction using CuO-RGO composite was determined to be 0.15 min-1, and after reacting for over three cycles the rate constant will be 0.058min-1. The developed economical method for the synthesis of CuO-RGO nanocomposite together with improved catalytic efficiency provides a potential new approach or the design of nanostructured catalysts with excellent performance. Acknowledgements RK and BKV acknowledge the funding from KSCSTE project ((Order No.1562/2016/KSCSTE) and UGC start up grant No. F 30-384/2017 (BSR).The authors acknowledge Nanomission project "SR/NM/NS-20/2014" for the FESEM facility.
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