A facile synthesis of Cu – Ni bimetallic nanoparticle supported organo functionalized graphene oxide as a catalyst for selective hydrogenation of p -nitrophenol and cinnamaldehyde †

We report a facile and environmentally friendly protocol for the synthesis of novel mono-dispersed Cu and Ni bimetallic alloy particles supported on ﬁ brous anime functionalized graphene oxide (GO). In this protocol, we used the organic amine group to increase the binding capacity of supported metal particles. First GO was covalently functionalized by organic amine [ N -(2 amino ethyl)-3-amino propyl trimethoxy silane i.e. , (AAPTMS)] to form AAPTMS – GO and then metal ions were loaded on the surface of the AAPTMS – GO material. The metal particles supported on AAPTMS functionalized graphene oxide were named as Ni – AAPTMS – GO, Cu – AAPTMS – GO and Cu – Ni – AAPTMS – GO to re ﬂ ect the metals loaded, and all were fully characterized by various techniques including XRD, SEM, FTIR, Raman spectra, TEM and HRTEM analysis. The 5% loaded with a 1 : 1 ratio of Cu : Ni of Cu(0) – Ni(0) – AAPTMS – GO showed superb e ﬃ ciency in conversion of p -nitrophenol to p -aminophenol with 100% conversion and selectivity. Hydrogenation of cinnamaldehyde with the same catalyst gave 85% conversion and 59.8% selectivity towards cinnamal alcohol (COL) at 80 (cid:1) C. The catalyst also showed good stability in recycling tests.


Introduction
Single metal nano catalysts, such as Pd, Pt and Ru are known to be effective catalysts for reduction of aromatic compounds. 1owever, these single metal nanoparticles are less selective and get quickly deactivated. 2,3Due to the unique properties at the nanoscale, the bimetallic nanomaterials 4,5 have recorded versatile applications as catalysts 6,7 and many other elds of chemical industry. 8The excellent synergy between their constituent elements, in addition to size effects are the main reasons for the enhanced catalytic activity by bimetallic nanoparticles.Pd-Au nano bimetallic materials, for example exhibit better catalytic activity than their monometallic ones, towards the oxidation of CO, alcohols and C-H bonds, and in the synthesis of H 2 O 2 from H 2 and O 2 . 9-14 Primary drawback associated with unsupported catalysis is that metallic catalysts cannot be reusable for several catalytic runs.Whereas with supported heterogeneous catalysts, allow easy separation and reusability, which is an attractive advantage.Although some agglomeration of metal nanoparticles during catalytic runs is inevitable, a right choice of support will minimize such processes.The supports containing organic groups can greatly inuence the activity and selectivity of the catalyst material.Due to the nontoxicity and high sorption capacities, [15][16][17][18] carbon based nanomaterials have been widely used as inorganic support for catalyst materials.In material science, graphene oxides enjoy prime position among the other carbon based materials, due to their honey comb like structure and high surface area. 19,20ransition metal salts like Cu and Ni are less-expensive and readily available, relative to the most of noble metal salts, which are normally used as catalysts.Therefore, in the current study, we have constructed low-cost Cu-Ni bimetallic nano particles graed on amine functionalized graphene oxide surface.The amine functionalized graphene oxide as support material possess high surface area, which enhances the catalytic activity as well as dispersion capacity.The functionalized amine with external binding capacity will provide additional active sites and also increase its reusability.
Nitro substituted hydrocarbons are generally toxic by nature and hard to be degraded in the environment. 21In addition, the conversion of aromatic nitro-compounds to aromatic amino compounds obeys with the demands of green chemistry.Further, hydrogenation of unsaturated aldehyde to alcohol with heterogeneous catalytic processes is always a challenge, since the hydrogenation of the C]O bond, while keeping the C]C intact is not thermodynamically favourable.Thus, the hydrogenation is of supreme signicance in ne chemical synthesis and many efforts have been made to design appropriate solid catalysts.Prakash et al. have reported that Ni-Au/ TiO 2 and Ni-Ag/TiO 2 gave better activity towards hydrogenation of unsaturated aldehyde compared to their single mono metallic like Ni/TiO 2 . 22Lin et al., have also reported that Ir-Ni/ TiO 2 catalyst showed higher activity and selectivity in hydrogenation of unsaturated aldehydes compared to heterogeneous monometallic catalysts. 23n this study we developed a novel catalyst material, Cu-Ni bimetallic functionalised graphene oxide nanocomposite and investigated its activity on two organic transformations.The efficacy of the different loading of Cu and Ni on the support for selective reduction of p-nitrophenol as well as hydrogenation of cinnamaldehyde were examined and role of temperature and solvents on the reactions were assessed.

Experimental
2.1 Preparation of GO 0.5 g of graphite powder, 0.5 g of NaNO 3 and con.H 2 SO 4 (23 ml) in a beaker and constant stirring in ice bath for 2 h. 3 g of KMnO 4 was added slowly and stirring continued for 70 min.Then, distilled water (50 ml) was added to the mixture.The reaction mixture was stirred at 35 C for 1 h and 98 C for another 20 min.Then, (30%) H 2 O 2 was added to the mixture and stirred for 1 h till the colour of the mixture turned dark brown to yellow.Water (50 ml) was added to the mixture and stirred for 1 h.Then, the nal mixture was sonicated for 30 min.The sonicated product was centrifuged with 10% HCl and ltered with DD water several times.Then, the nal solid material was dried at 40 C in a vacuum oven for overnight to acquire graphene oxide (GO).

AAPTMS@GO
Graphene oxide (1 g) was dissolved in 50 ml ethanol solution in a conical ask and sonicated for 1 h.1.68 mmol of AAPTMS [ (3-(2-aminoethylamino) propyl] trimethoxysilane) was drop wise added to the mixture and sonicated for 1 h.Then, the mixture was centrifuged and the product was dried at 40 C in a vacuum oven for overnight to get AAPTMS@GO.

Cu-Ni-AAPTMS@GO
Bimetallic Cu-Ni composites were prepared by incipient wetness impregnation method.The copper nitrate and nickel nitrate were mixed to get the subsequent metallic (Cu : Ni) molar ratios such as 1 : 0, 0 : 1, 1 : 1, 1 : 2, and 2 : 1.The metallic solutions were added to 4.75 g of amine functional GO solution.The liquid phase was removed by a 4 h treatment at 70 C in a rotary evaporator and then dried in vacuum overnight at 100 C.
Similar method was used for preparation of Cu-Ni bimetallic nanoparticles by using hydrazine.To the Cu and Ni containing solution of functionalized GO, 0.2 M hydrazine solution was added under stirring.Then the suspension was vigorously stirred at RT for 6 h.The suspension was washed with acetone and preserved toluene.Then, the nal solid product was vacuum dried at 100 C for overnight (Scheme 1).

Equipment and methods
The X-ray diffraction study was performed on a Bruker D8 Advance instrument with CuKa as a radiation source.The transmitted electron microscopy images were observed on a Jeol JEM-1010 electron microscope with iTEM soware.Jeol JEM 2100 Electron Microscope was used for capturing high resolution of TEM images.The JEOL JSM-6100 microscope was used for both scanning electron microscopy and EDX measurements.PerkinElmer spectrum 100 series with universal ATR accessory used for Fourier transmission infrared spectrometer (FTIR).The weight percentages of Cu and Ni in the bimetallic catalyst were determined by using inductively coupled plasma-optical emission spectroscopy (ICP-OES) (Perkin Elmer Optima 5300 DV).UV-visible spectra were recorded with high resolution spectrometer (HR2000+).Raman spectra were observed on a Perkin Elmer 1200 Fourier Transform Infrared and on a DeltaNu advantage 532™ Raman Spectrometer.The electronic structure aspects of the samples were investigated using KRATOS apparatus with Mg, Al, and Cu Ka as X-ray sources by XPS.The illustrative products were analyzed by 1 H NMR (Bruker, 400 MHz, [D 6 ] DMSO) 13 C NMR (Bruker, 400 MHz, [D 6 ] DMSO) and FTIR spectroscopies (PerkinElmer).

Catalytic reaction of PNP to PAP
NaBH 4 (60 mg) and 5.0 mM of p-nitrophenol (30 ml) were taken in a round bottom ask and stirred randomly to homogenize the solution.3.0 mg of catalyst was added aer getting the yellow colour.The change of p-nitrophenolate to p-aminophenol was observed by gradual discoloration.Aliquots of the mixture were removed at 4 min intervals and the absorption spectrum the reaction mixture was recorded using a UV-Vis spectrometer.While absorbance at 400 nm reecting concentration of nitrophenolate species decreased, the value of absorbance at 300 nm steadily increased indicating the formation of p-aminophenol.Aer the completion of reaction, the solid catalyst was retrieved with centrifugation and washed with DD water aer each catalytic cycle.The reaction products were also characterized by comparing the 1 H NMR, 13 C NMR and FTIR data with authentic samples.

General procedure for hydrogenation of cinnamaldehyde
150 mg of catalyst, 1.2 g of cinnamaldehyde and 16 ml of methanol were taking in a round three necked volumetric ask tted with water circulating condenser, aer purging rst with nitrogen.Then the reaction was stirred with continuous ow of hydrogen.The reactions were carried out at 80-120 C for 1 h.The resulting mixture was analysed by GC.The catalyst was recovered by the simple centrifugation and washed with D.D. water.
The Raman spectra of AAPTMS@GO (a), Cu-AAPTMS@GO (b), Ni-AAPTMS@GO (c), (1 : 1) Cu-Ni-AAPTMS@GO (d) and  and Cu 2p 1/2 are 932.3eV and 952.12 eV respectively.The binding energy of Cu 2p 3/2 was $0.5 eV lower than that in the metallic state of Cu.The binding energy of Ni 2p 3/2 and Ni 2p 1/2 are 853.5 eV and 873.3 eV respectively, which was $0.5 eV higher than that in the metallic state of Ni catalyst. 33The shiing of binding energies of Cu 2p 3/2 and Ni 2p 3/2 conrm the formation of alloy nanoparticles between Cu and Ni and also interaction between metallic alloy and support surface.
TEM images of AAPTMS@GO (a), Cu-AAPTMS@GO (b), (1 : 1) Cu-Ni-AAPTMS@GO (c), (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO (d), (1 : 2) Cu(0)-Ni(0)-AAPTMS@GO (e) and (2 : 1) Cu(0)-Ni(0)-AAPTMS@GO (f) samples, facilitate to examine the morphologies of the synthesized materials are shown in Fig. 3.The pure graphene oxide is composed of very thin sheets. 28Aer functionalization silane creates defects in the graphene oxide sheets, this morphology changes due to presence of organic solvents.The solvent appears to soen the severe attack of organic amine groups on GO surface and hinders breaking down of big sheets to smaller ones as shown in Fig. 3(a).Fig. 3(b) displays the only single metal particles, but in Fig. 3(c) displays the different phase of complex morphology of both bimetallic particles are formed, which is concurrent with the XRD study.Aer reduction of Cu and Ni metals by hydrazine in the Fig. 3(d)-(f), it can be seen that the well mono-dispersed Cu-Ni bimetallic nano particles are distributed on the functionalized graphene oxide sheets.
EDX spectroscopy gives the evidence on the type of element existing in the specic area.The SEM-EDX mapping of (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO sample is presented in the Fig. 5. From these images, the presence of silicon, carbon, nitrogen, oxygen, Cu and Ni in this material can be seen.Another most vital and valuable ability of the EDX technique is X-ray mapping of elements.The positions of particular elements emitting specic X-rays within a scrutiny eld are indicated by unique colors.The maps of distribution of elements like Si, C, N, O, Cu and Ni are exposed individually and overlapped with the original image as shown in the Fig. 5.The elemental mapping survey reveals that the most active elements like Cu and Ni were uniformly distributed throughout the sample.ICP-OES was used to determine the percentage ratio of Cu and Ni in 5 wt% of (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO catalyst and it was found to be Cu/Ni ratio 0.98 i.e., Cu is slightly lower loading than Ni.This may be Fig. 4 SEM image of (a) AAPTMS@GO (b) Cu-AAPTMS@GO (c) (1 : 1) Cu-Ni-AAPTMS@GO (d) (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO (e) (1 : 2) Cu(0)-Ni(0)-AAPTMS@GO and (f) (2 : 1) Cu(0)-Ni(0)-AAPTMS@GO.
due to Ni, more electronegative than Cu, which can more easily coordinate to the electron donating N atom of AAPTMS group present on the amine functionalized graphene oxide surface.
HRTEM was also used to further investigate the morphologies, diffraction pattern and lattice fringes of the materials.The different magnication of (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO material with spherical nano particles can be seen in Fig. 6(a From this Fig. 7, the average diameter of particle size is calculated to be $4 nm with a narrow size variation.

Hydrogenation of unsaturated aldehydes
The scope of these novel nanomaterials was investigated on hydrogenation of unsaturated aldehyde to form an unsaturated alcohol product.Initially, to compare the activity and choice of different metal and different ratio of bimetallic modied functionalized GO were screened and the results are presented in the Table 1.A perusal of results in Table 1 shows that (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO nano materials gave excellent results compare to mono metallic and other bimetallic catalysts, due to synergetic effect and uniform distribution as well as sufficient amount of metal particles.In case of (1 : 2) and (2 : 1) wt% composites, i.e., with one metal in excess than other, and their activity was lower for same reaction.That excess presence of one metal causes imbalance in the even distribution of active sites of the metals on the catalyst surface for the hydrogenation of unsaturated aldehyde.Hence, the activity of the catalyst decreased.With increase in the temperature (90-120 C), the conversion of unsaturated aldehyde increased from 85% up to 98% and but selectivity towards cinnamyl alcohol (COL) decreased signicantly (Table 2).This may be due to weakly adsorbed hydrogen on the catalyst surface, which enhances the conversion of COL to hydrocinnamyl alcohol (HCOL) at higher rate.Hence, the selectivity to cinnamyl alcohol decreases with rise in temperature.
Table 3 presents the activity/selectivity of hydrogenation of unsaturated aldehyde by (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO nano materials under same reaction conditions, but in different solvent media, such as water, methanol and isopropyl alcohol.The hydrogenation reaction with methanol as solvent gave better conversion and selectivity towards COL compared to other solvents.This could be due to higher dipole moment as well as higher solubility of hydrogen in methanol.
of nitro phenol was investigated with NaBH 4 as hydrogen source and water as solvent.The (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO as catalyst gave highly impressive results with 100% conversion and selectivity towards p-amino phenol and reaction nished in 16 min at RT, while the 1 : 2 and 2 : 1 compositions recorded lower selectivity and conversions.These results prove superior to the literature reported results in terms of both conversion and selectivity efficiency and reaction conditions and time needed.
The hydrogen abstraction from BH 4 À and transferring to pnitrophenolate anion occurred efficiently by the bimetallic alloy particles on the amino functionalised graphene oxide surface.The reduction of aromatic nitro compound was monitored by UV-Vis study.In UV-Vis, the peak at 400 nm indicates p-nitrophenolate anion and at 300 nm indicates the product of p-aminophenol with (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO catalyst shown in ESI Fig. S3.† In Fig. S3(A), † shows the decrease in absorbance at 400 nm, due to the consumption of p-nitrophenolate anion intermediate and the increase of intensity at 300 nm due to the formation of product, p-aminophenol.The representative products were characterized by 1 H NMR, 13 C NMR and FTIR spectroscopies for conformational study in ESI Fig. S4(a-c).† (1 : 1) Cu-Ni-AAPTMS@GO catalyst showed very less activity due to the presence of different form of metallic phases, which was conrmed from the XRD results.However, the (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO catalyst displayed the best catalytic activity towards p-amino phenol reduction, when compared to (1 : 2) Cu(0)-Ni(0)-AAPTMS@GO and (2 : 1) Cu(0)-Ni(0)-AAPTMS@GO.The high activity of (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO was due to good dispersion of the (1 : 1) ratio of Cu-Ni bimetallic alloy nanoparticles on the functionalized graphene oxide sheets and the enhanced adsorption ability of graphene oxide for p-nitrophenol stems from the p-p stacking interactions.From Fig. S3(B) and (C), † it can be deduced that  the reduction of nitro compound to amino compound over (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO follow pseudo rst-order kinetics for the chosen conditions.Best features of ideal heterogeneous catalyst are high selectivity, high activity, long lifetime and cost effectiveness.The recyclability of the heterogeneous catalyst is vital parameter and it depends on its stability and the easy separability.The activity can change due to leaching of some metal particles into the reaction medium or may be the coke formation on the catalyst surface in the high temperature reactions.To test its robustness, the recovered catalyst was reused aer regeneration followed by wash with water and calcination.No noticeable change up to 6 th cycle was observed with activity remaining intact (Fig. 8).The catalyst did not leach out in this medium showing strong interaction between nanoparticles and amino group.The decrease in 7th circle could be due to some leaching Cu metal as compared to Ni.The binding capacity of Ni is higher than Cu, which can be more easily coordinated to the electron donating N atom of organic group present on the amine functionalized graphene oxide surface.The catalytic activity of hydrogenation of unsaturated aldehyde gives good conversion and selectivity and also the catalyst activity decreased (8%) in 5 th cycle.
The reduction of PNP to PAP performance of (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO was evaluated and compared with the state of the art in the corresponding eld in Table 4

Conclusions
In conclusion, in an environmentally friendly route different ratios of bimetallic Cu-Ni nanoparticles supported on organic functionalized graphene oxide were effectively prepared with an average particle size of 4 nm by a simple procedure.The 1 : 1 ratio of the nano catalyst was found to be highly efficient (100% conversion and selectivity in <20 min at RT) and reusable and towards reduction of nitro compounds with water as solvent.The same proved ideal for hydrogenation of cinnamaldehyde    with good conversion and selectivity with methanol as solvent.
The activity was due to synergistic interactions between two metals involving charge transfer from one metal to another metal to form a Cu-Ni hetero structures with specic geometry in addition to the amino functionalized GO surface.The formation of bimetallic Cu-Ni particles was conrmed from XRD and XPS studies.From FT-IR and TEM analysis, covalent binding of the organic group of AAPTMS onto graphene oxide layer was conrmed.TEM and HR-TEM also conrmed that bimetallic Cu-Ni nanoparticles were uniformly distributed on the functionalized graphene oxide surface.The recovered catalyst can be recycled aer activation.Bimetallic materials show higher CAL conversion and selectivity towards COL at lower temperatures, 80 C and at higher temperatures (90-120 C), while the conversion of CAL increased, selectivity towards COL got decreased due to weak adsorption of hydrogen on bimetallic catalysts surface at those temperatures.
)-(e).From Fig. 6(g) showing the SAED pattern of the polycrystalline diffraction rings, (111) and (200) planes were indexed as the brighter inner diffraction ring and dimmer outer ring.The lattice fringe presented in Fig. 6(f) shows the single crystalline nature of the bimetallic alloy nano particles with characteristic d-spacings of 0.20 nm for (111) planes of both Ni and Cu.Therefore, it was conrmed that the solid solution constitutes both Cu and Ni.The TEM and HRTEM images of mono dispersed bimetallic nano particles with histogram of particle size distribution plot for the Cu-Ni nanoparticles are shown in the [Fig.7(a) and (b)].
. The Pd/C and Pd/G catalysts 46 the activity decreased in every cycle reaching 85% in the 5 th cycle.With CuFe 2 O 4 MNPs catalyst 50 catalyst was stable up to 2 nd cycles only.So, that our (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO catalyst has better recyclability and reusability.The XRD, FTIR, SEM, TEM, HRTEM analysis of sixth cycle of reused catalyst are shown in ESI Fig. S5.† In the XRD spectra of reused (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO catalyst, the strong and active diffraction peak of nano particle at 2q ¼ 43.51 for (111) plane, disappeared in the 7 th cycle, although it can be seen aer recycling it for the 6 th cycle catalyst [Fig.S5(a) and (b) †].From FTIR also it can be seen that the stretching vibrations of Si-O-C peak vanished aer 6 th cycle in [Fig.S5 (c) and (d) †].The SEM, TEM and HRTEM images [Fig.S5(e)-(g) †] of the 6 th cycle reused catalyst show no obvious change in morphology of the nanoparticles i.e., well-dispersed nanoparticles as like the fresh sample.Agglomerated bimetallic Cu-Ni nano particles were observed in the SEM, TEM and HRTEM images [Fig.S5(h)-(j) †] of the (1 : 1) Cu(0)-Ni(0)-AAPTMS@GO catalyst reused aer six times.