Nickel nanocomposites: magnetic and catalytic properties

Abstract

In this study, we are reporting the synthesis and characterization of nanocomposites obtained from the direct reduction of nickel(II) salts on matrices of polyethylene (Pe) and chitosan (Ch) in the presence of serine under solvothermal conditions. Using different molar ratios between the metal salt (M) and the amino acid (AA), eight nanocomposites were prepared, Ni–Pe1; Ni–Pe2; Ni–Pe3; Ni–Pe4 and Ni–Ch1; Ni–Ch2; Ni–Ch3; Ni–Ch4 (M : AA = 1 : 1, (1); 0.5 : 1, (2); 0.25 : 1, (3) and 0.125 : 1, (4)). The synthesized composites were characterized by X-ray powder diffraction techniques; in all the cases, the peaks associated to the matrix (Pe or Ch) and three peaks at 2θ values of 44.5°, 51.9°, 76.4° were identified, which correspond to the Miller indices (111), (200), (220). These indices are characteristic of a face centred cubic Ni⁰ phase. The SEM images of the composites show that the use of an organic matrix changes the size and distribution of the metallic particles because in all the cases a homogenous dispersion of Ni⁰-NPs on the matrix surfaces is observed. While the spherical shape observed for isolated Ni⁰-NPs is retained on the matrices, the size of the metallic particles is smaller than 100 nm with less size variability, as compared with the isolated Ni⁰-NPs. All the composites have a weak ferromagnetic behaviour with similar hysteresis loops, presenting H_c values ranging from 120 to 226 Oe and reaching saturation at approximately 3 kOe. Preliminary catalytic properties for hydrogen transfer reaction were also investigated, showing that the composites exhibit an important activity in the transformation of acetophenone to 1-phenylethanol.

---

Introduction

Magnetic nanoparticles are widely studied because they exhibit interesting electrical, magnetic and chemical properties, which differ from those observed for bulk materials. Different synthetic techniques controlling the size, shape and morphology of nanoparticles have been reported;^1–10 among these the chemical vapour deposition (CVD),² wet chemistry,³ laser-driven aerosol,⁴ hydro/solvothermal methods,^5,6 and the microemulsion synthetic route have been used extensively.^7,8 Metallic nanoparticles are mainly used in the preparation of catalysts, batteries and magnetic materials for data storage.¹ In particular, nickel is an important magnetic material that exhibits variable magnetism at room temperature, and the magnetic properties are strongly affected by the morphology, size and shape of nickel nanoparticles.^11–15 In addition, nickel nanoparticles have interesting applications in the growth of carbon nanotubes¹⁶ and in the catalysis of the reduction reaction of different organic functional groups.¹⁷ However, the catalytic properties of nickel nanoparticles have been studied less frequently compared to those of noble metals.¹⁸ Literature data report that the nickel nanoparticles have been used as catalysts in the hydrogen transfer reaction to carbonyl groups, and hydrogenation of nitrobenzene; both being reactions of great industrial importance.^19,20 The preparation of alcohols from carbonyl compounds has numerous applications in the industrial synthesis of dyes, pharmaceuticals, agrochemicals and biologically active compounds.²¹ Therefore, the development of new and efficient catalysts for this type of reaction is a great challenge. However, the use of nickel nanoparticles as catalysts requires avoiding their oxidation. Therefore, the metallic nanoparticles have to be supported on different matrices to reach the necessary stability. Nickel nanoparticles supported on different matrices, such as carboxymethyl cellulose,²² hydrocalcite-clay,²³ TiO₂, Al₂O₃, SiO₂, zeolites/aluminosilicates, chitosan and carbon nanotubes, have been reported,^24–29 thus generating new and interesting nickel-based materials. These systems have been studied in relation to not only their role in heterogeneous catalysis, but also for the dependence of their catalytic activity on size, shape and dispersion in the used matrix.³⁰

On the other hand, polymeric nanocomposites, which are obtained from nanoparticles and polymeric matrix, are interesting because the chemical nature and the structure of polymers can change the shape and size of the nanoparticles. In addition, the intrinsic characteristic of the matrix should also have influence on the distribution of the nanoparticles in a polymeric matrix. Therefore, considering the relevance and applications of the nickel nanoparticles, in this study, we report the synthesis of nanocomposites obtained from the direct reduction of nickel(II) chloride on matrices of polyethylene (Pe) and chitosan (Ch). Both matrices were chosen considering the chemical differences between them. To evaluate the role of the organic matrices on the obtained products, the synthesized Ni–Pe and Ni–Ch composites were morphologically and magnetically characterized. In addition, the catalytic activity for the hydrogen transfer reaction was also investigated, which showed that the composites exhibit an important activity in the transformation of acetophenone to 1-phenylethanol.

Experimental

All the starting materials were commercially available reagents of analytical grade and were used without further purification. The Ni⁰ nanoparticles were obtained using the same synthetic conditions as reported by Paredes-Garcia et al.⁶ The composites were obtained by incorporating a constant amount (0.15 g) of the polyethylene (Pe) or chitosan (Ch) matrix into a solution of nickel(II) chloride in dimethylformamide in the presence of L-serine and heating the mixture at 150 °C under solvothermal conditions. Different molar ratios of metal salt (NiCl₂·6H₂O) and amino acid (AA = L-serine) were used and the following composites were obtained: Ni–Pe1, Ni–Pe2; Ni–Pe3, Ni–Pe4 and Ni–Ch1, Ni–Ch2, Ni–Ch3, Ni–Ch4 (M : AA = (1), 1 : 1; (2), 0.5 : 1; (3), 0.25 : 1 and (4), 0.125 : 1). The polyethylene and chitosan composites were characterized by powder X-ray diffraction (PXRD) using a Bruker diffractometer, model D8 Advance with Cu Kα1 radiation and Bragg–Brentano geometry in the 5° ≤ 2θ ≤ 80° range. The data were obtained at 22 °C. A Carl Zeiss scanning electron microscope (SEM) coupled with an energy dispersive X-ray spectroscopy (EDXS), model EVO MA10, operated at 50.0 kV and a JEOL transmission electron microscope (TEM) model JEM-1001L equipment were used to characterize morphology, composition, crystal structure and size distribution of the samples.

Hydrogen transfer reaction

Catalytic hydrogen transfer reactions were performed in a magnetically stirred two necked round-bottomed 50 mL flask fitted with a condenser and placed in a temperature controlled oil bath. All the reactions were carried out under nitrogen atmosphere and repeated three times for each composite. The following reaction conditions were used for all the synthesized composites: acetophenone (2 mL), isopropanol (3 mL), catalyst (30 mg) and sodium hydroxide (100 mg), which were added into the reactor and heated at 90 °C for 3 hours. Aliquots (0.5 μL) were removed every 15 minutes and analysed by gas chromatography. Gas chromatographic analyses were carried out with a Hewlett Packard 5890 Series II instrument equipped with a flame ionization detector (FID) and a Carbowax 20M capillary (25 m × 0.2 mm × 0.2 μm) using nitrogen as the carrier gas. The products were identified by spiking with standards compounds and MS-GC.

Magnetic properties

Magnetic characterization was performed using hysteresis cycles M(H) obtained with a vibrating sample magnetometer (VSM) and PPMS Dyna Cool 9T at room temperature using a maximum applied field of 20 kOe. The magnetization saturation values (M_s) reported in this study were determined by considering the total mass of the nanocomposites (nickel nanoparticles and matrix). The amount of nickel in each composite was calculated from the obtained M_s values, taking into consideration that the surface contribution on the M_s values is neglected due to the large volume/surface ratio.

Results and discussion

Characterization of nanocomposites

X-ray powder diffraction. The chemical nature and crystallinity of the synthesized nanocomposites were identified by X-ray powder diffraction.

Fig. 1 shows the diffraction patterns obtained for the isolated matrices (Pe and Ch) compared with two nanocomposites (Ni–Pe1 and Ni–Ch1), which are representative of each matrix used. The plots exhibit peaks associated to the matrix (Pe or Ch) and three other peaks at 2θ values of 44.5°, 51.9°, and 76.4°, which correspond to the Miller indices (111), (200), (220), respectively; these peaks are characteristic of face centred cubic Ni⁰ (JCPDS Card 04-0850, cubic system, spatial group: Fm3̄m, a = 3.5238 Å). The samples were reanalysed after storage for one month, and no changes in the number and intensity of the peaks were observed, indicating that the metallic phases are stable to ambient oxidation process. Diffraction patterns corresponding to the synthesized nanocomposites show different intensities of the peaks, which can be associated with the initial concentration of metallic salt added to the reaction mixture. The plots for the remaining phases (Ni–Pe2, Ni–Pe3, Ni–Pe4 and Ni–Ch2, Ni–Ch3, Ni–Ch4) and the EDX spectra are given as ESI (Fig. 1S and 2S†).

Fig. 1 X-ray powder diffraction pattern of nickel nanocomposites (a) Ni⁰-NPs; (b) polyethylene (Pe) matrix; (c) Ni–Pe1 composite; (d) chitosan (Ch) matrix; (e) Ni–Ch1 composite.

SEM and TEM microscopy. The morphology and size of the Ni⁰-NPs were determined with the purpose to compare the structural and dispersion changes of the metallic nanoparticles as a consequence of the morphological and chemical characteristics of the organic matrix. Fig. 2 shows the SEM and TEM images obtained for the Ni⁰-NPs. The micrographs permit the observation of the metal agglomerates with quasi spherical shape, similar to those reported by Jia et al.³¹ The size of the metallic agglomerates measured by SEM ranges from 100 to 800 nm, while in the TEM image (Fig. 2b), the presence of NPs with different sizes is observed. Besides, all the particles have a thick organic coating with values between 5 and 7 nm. This coating was associated to the amino acid used as starting materials in a previous study, which was also responsible for the stability of the Ni⁰-NPs and avoided their oxidation.⁶

Fig. 2 (a) SEM image of Ni⁰-NPs; (b) TEM image of Ni⁰-NPs.

Fig. 3 shows the SEM images obtained for the nanocomposites Ni–Pe1, Ni–Pe3, Ni–Ch1 and Ni–Ch3. The SEM images for composites Ni–Pe2, Ni–Pe4, Ni–Ch2 and Ni–Ch4 are given as ESI.† The micrographs show that the use of the organic matrix changes the size and distribution of the metallic particles, showing in all cases a more homogenous dispersion as compared with the isolated Ni⁰-NPs. While the spherical shape observed for the Ni⁰-NPs is retained on the matrices, the size agglomerates are smaller than 100 nm and with less size variability, as compared with the Ni⁰-NPs synthesized without the presence of the organic matrices. The SEM micrographs show that in the case of nanocomposites obtained from most concentrated solutions of metallic salt, metallic nanoparticles forming agglomerates with larger sizes can be detected (Fig. 3 and 3S†). With a higher concentration of metallic nanoparticles (Ni–Pe1 and Ni–Ch1), the dispersion in both matrices appears very similar, and considering the morphological characteristics of each matrix, no significant changes in the size or shape of Ni⁰-NPs grafted onto the matrix surface are observed. However, as the concentration of the nickel nanoparticles decreases, some differences among the composites become evident. Thus, Ni–Pe3 and Ni–Ch3 with the same size and shape of the nanoparticles have a completely different dispersion. Furthermore, Ni–Ch3 retains the same dispersion distribution as Ni–Ch1, while Ni–Pe3 shows important changes as compared with Ni–Pe1. The presence of metallic domains (islands) can be observed for this polyethylene composite when an M : AA molar ratio of 0.25 : 1 is used. To the best of our knowledge, this island type dispersion for nickel nanoparticles has not been reported to date in the literature. Although Suzuki et al.³² and Byeon et al.³³ have used the term metallic islands or nanoscaled islands for silver and gold nanoparticles deposited on a silicon substrate, the form of the nickel island deposited in the polyethylene matrix is completely different from those reported by these authors. Comparing the samples with the lower amount of metallic nanoparticles (Ni–Pe4 and Ni–Ch4 (M : AA = 0.125 : 1), it is possible to observe some differences among the composites. No metallic islands are observed, and Ni–Pe4 shows a more homogenous dispersion than Ni–Ch4. Moreover, Ni–P4 and Ni–Ch4 composites are characterized by not presenting large agglomerates of nickel nanoparticles. Another important aspect is the amount of Ni⁰-NPs on the organic matrices (Table 1). According to the data, the chitosan matrix doubles the concentration of metallic nickel, as compared with the polyethylene composite. This can be related to the fact that chitosan is rich in functional groups that can interact with the nickel ions and promote the reduction process, leading to the formation of the nanoparticles.

Fig. 3 SEM images of nickel nanocomposites. (a) Ni–Pe1; (b) Ni–Pe3; (c) Ni–Ch1; (d) Ni–Ch3.

Table 1 Magnetic parameters of nickel nanocomposites

Nanocomposite	Hc [Oe]	Ms [emu g^−1]	Mr/Ms	Ni [%]
Ni–Pe1	126	9.2	0.14	16.7
Ni–Pe2	122	2.1	0.13	3.8
Ni–Pe3	145	0.5	0.18	0.9
Ni–Pe4	146	0.1	0.14	0.2
Ni–Ch1	137	13.1	0.20	23.8
Ni–Ch2	140	4.8	0.18	8.7
Ni–Ch3	170	0.7	0.19	1.3
Ni–Ch4	226	0.1	0.14	0.2

---

Magnetic characterization. The magnetic characterization of Ni⁰-NPs, using M(H) loops, was performed with the purpose of comparing the changes in the magnetic properties when the metallic particles are deposited on the matrices. The as-synthesized Ni⁰-NPs have a soft ferromagnetic behavior with a coercivity (H_c) value of 127 Oe, saturation remanence (M_r) of 9.2 emu g⁻¹ and saturation magnetization (M_s) of 46 emu g⁻¹. The values are very similar to those reported by Paredes-Garcia et al.⁶ and Jia et al.³¹ The lower value of saturation magnetization as compared to that of macroscopic nickel (55 emu g⁻¹) is attributed to the presence of the organic material, which acts as a coating of the nanoparticles.

Taking into account that the M_r values depend on particle elongation, interaction effects, thermal activation, cubic magnetocrystalline and uniaxial components or formation of domain structures,³⁴ as well as considering the relatively large size of the particles (>60 nm) and the quasi spherical form evidenced from SEM images, it is possible to suggest that the lower remanence value observed for the synthesized nickel nanoparticles is produced mainly by interaction effects or domain structure formation. The last is also consistent with the particle aggregation observed through the SEM images, where it is possible to observe that the distance between particles is the same as the diameter.

Fig. 4 shows the magnetic hysteresis observed for the nickel composites. All composites are characterized to have a weak ferromagnetic behaviour with H_c values similar to the one obtained for Ni⁰-NPs⁶ and with similar hysteresis loops reaching saturation at approximately 3 kOe. The magnetic parameters are given in Table 1. In the case of polyethylene composites, Ni–Pe1 and Ni–Pe2, similar values of H_c (≈120 Oe) are observed. However, Ni–Pe3 and Ni–Pe4, which were prepared using a lower M : AA ratio present slightly higher coercivity values (ca. 140 Oe). The same behaviour is also observed for chitosan composites with Ni–Ch4 composite exhibiting the highest H_c value (226 Oe). The higher values of the coercivity observed at the lower ratio of M : AA can be associated to the different distribution of the magnetic particles on the matrices, and therefore to the interactions between them. C. Cruzat et al.³⁵ also synthesized nickel nanoparticles with sizes between 10 and 80 nm using chemical liquid deposition and solvated metal atom dispersive techniques and deposited them on chitosan; however, unlike this work, the authors reported a superparamagnetic behaviour for the particles. Besides, Hui et al.³⁶ reported nickel nanoparticles on a carbon matrix (Ni@C) with M_s values similar to those reported for Ni–Pe1 and Ni–Ch1 (9.2 and 13.1 emu g⁻¹ respectively). The authors explain that the lower value obtained for M_s (11.82 emu g⁻¹) with Ni@C, compared with that reported for bulk Ni⁰, is a consequence of the matrix contribution. Considering this fact, and disregarding any change of M_s on the surface, a saturation value of 55 emu g⁻¹ was taken as reference to estimate the amount of magnetic material present in the as-synthesized nickel nanoparticles. The calculated value was ca. 84%, which is in the range obtained from TEM images, if 10% to 19% of organic coating is considered. The same procedure was used to calculate the amount of magnetic material in the composites (Table 1).

Fig. 4 M(H) plots of (a) Ni⁰-NPs, (b) Ni–Pe1, and (c) Ni–Ch1.

Catalytic activity of nickel nanoparticles and nanocomposites. The hydrogen transfer reactions were performed using Ni–Pe2 and Ni–Ch2 as catalysts, which contain ca. 4% and 9% of the metallic component, respectively. The catalysts were employed for heterogeneous hydrogen-transfer reactions of acetophenone using isopropanol as hydrogen donor in the presence of NaOH (Scheme 1).

Scheme 1

The catalytic study was performed taking into account the characteristics of the composites that act as a heterogeneous catalyst. All the experiments were performed using the same amount of composite (30 mg). Fig. 5 shows the conversion as a function of the reaction time for the used catalysts. In all the cases, the catalytic activity starts after 15 minutes of reaction time (induction time); then, the transformation of acetophenone to 1-phenylethanol gradually increases. For short reaction times, the used composites have higher conversion compared with the nickel nanoparticles, reaching values of ca. 55%, 50% and 40% for Ni–Ch2, Ni–Pe2 and Ni⁰–NPs, respectively, at 60 minutes. The obtained results show that the nickel nanoparticles forming part of a composite are better catalysts in the studied reaction, compared with the coated Ni⁰–NPs. At 90 minutes, the conversion reaches values of 65%, 48% and 55% for Ni–Ch2, Ni–Pe2 and Ni⁰-NPs, respectively. The selectivity for all the studied catalysts was 100%, as compared to the commercial catalyst Nickel RANEY®, which produces ethylbenzene instead of 1-phenylethanol under the same experimental conditions.³⁷ Furthermore, other nickel catalysts such as Ni–Al, Ni/TiO₂, Ni/SiO₂–Al₂O₃ and NiO were studied by Alonso et al.,¹⁹ but these catalysts did not present catalytic conversion towards any product. However, in this study, the reaction was carried out without the presence of a base. The conversion value of 40% at 60 min of reaction time, observed for the as-synthesized Ni⁰-NPs with agglomerates in the range of 100 to 800 nm, is lower than that reported by Alonso et al. for spherical Ni⁰-NPs with a diameter of <2 nm.¹⁹ Thus, the importance of the size of the Ni-NPs becomes relevant, when comparing catalytic activities.

Fig. 5 Conversion percentage as function of reaction time for the heterogeneous hydrogen-transfer reaction using Ni⁰-NPs, Ni–Pe2 and Ni–Ch2 as catalysts.

According to previous studies, when transition metals are involved, the catalytic hydrogen transfer proceeds through the hydridic route. However, Alonso et al. reported that, for Ni⁰-NPs, the dihydride-type mechanism is more in agreement with the data obtained with deuterated isopropanol.³⁸ Thus, the induction time evidenced during the first 15 min can be related with the coating of the superficial metallic particles of the composite, which must be removed to permit the formation of the nickel dihydride. According to the proposed mechanism, this species would be necessary for the transformation of acetophenone to 1-phenylethanol.

As can be observed in Fig. 5, the Ni–Ch2 composite presents a higher conversion value at 60 min (65%), compared to Ni–Pe2 and Ni⁰-NPs. As discussed above, the morphological characteristics of both composites Ni–Ch2 and Ni–Pe2 should be responsible for the difference in the catalytic activities. It is interesting to observe that at 30 min, the polyethylene composite has the best catalytic activity, as compared with that of Ni–Ch2 and Ni⁰-NPs. Moreover, the same composite reaches the maximum conversion at 40 minutes, whereas the chitosan composite continues showing an increase in the conversion percentage at 90 minutes. Thus, it can be concluded that as the reaction time advances, the Ni–Ch2 composite becomes a better catalyst. It must be taken into account that the dispersion of the NPs and molecular structure of the matrix are different in both composites; therefore, the catalytic behaviour is expected to be dissimilar. Dutta et al.³⁹ discussed the effect of the used amount of catalyst on the conversion values, and their conclusions are in accordance with those reported in this study. These authors synthesized Ni⁰-NPs, supported on montmorillonite, using the pores of the matrix to obtain spherical Ni⁰-NPs with sizes smaller than 8 nm. The authors report a very high catalytic activity (using similar reaction conditions to those of this work), obtaining a conversion of about 100% for 4 hours of reaction time. However, the conversion is only 17% after 1 hour of reaction time, which is much lower than that obtained in this study (ca. 50%) for both studied composites. In addition, yolk–shell-type Ni⁰-NPs (Ni@SiO₂) have been studied for the same reaction at 150 °C by Park et al.⁴⁰ These authors report a conversion of 90% for 1 hour of reaction. The same authors also report that the conversion value is lower when the temperature is 100 °C (68%) or 80 °C (61%); these values are in the range of those obtained in this study. Moreover, Park et al. also showed that the particle size affects the catalytic process; 90% of conversion was obtained at 150 °C with particles of 3 nm. This conversion decreased 10% when the particle size was increased to 30 nm.

Taking into account the reported data, the catalytic activity of the synthesized composites is in agreement with the fact that the larger size of the NPs decreases the performance of the used catalysts. However, the fact that the amount of Ni⁰-NPs on the studied matrices is ca. half for the Ni–Pe2 composite compared to that of Ni–Ch2 could explain the better performance of the latter at longer times of reaction if a deactivation process is considered to take place.

Conclusions

Using a solvothermal technique, it was possible to synthesize Ni⁰-NPs that were stable to air oxidation; besides, these NPs were also dispersed on chitosan and polyethylene matrices. The resulting composites present smaller-size metallic particles as compared with the isolated Ni⁰-NPs, making evident the role of the organic matrices on the dispersion of the magnetic particles. All the composites exhibited a weak ferromagnetic behaviour with coercivity values ranging from 120 to 220 Oe, and can classified as soft magnets. Ni⁰-NPs and the composites Ni–Pe2 and Ni–Ch2 were also employed as catalysts for the heterogeneous hydrogen-transfer reaction for acetophenone, reaching conversion values between 35% and 65%, depending on the reaction time. The highest conversion value was observed for Ni–Ch2 composite at 90 minutes, which may be due to the dispersion of the NPs and molecular structure of the matrix.

Acknowledgements

The authors acknowledge CONICYT-FONDEQUIP/PPMS/EQM130086 grant and financial support from Proyecto Basal CEDENNA, Financiamiento Basal FB0807, and Laboratorio de Caracterização Magnética, LMCMM-UFSC, Departamento de Física and Centro de Microscopia Electrónica, CME-UFSC, Universidade Federal do Santa Catarina. C. C. thanks CONICYT for the Doctoral fellowship 21110032.

References

1. (a) M. R. Knecht, J. C. García-Martinez and R. M. Crooks, Chem. Mater., 2006, 18, 5039; (b) N. C. Nelson, T. Purnima, A. Ruberu and M. D. Reichert, J. Phys. Chem. C, 2013, 117, 25826.
2. C. Rui-ying, Trans. Nonferrous Met. Soc. China, 2006, 16, 1223.
3. Q. Zhen-Ping, G. Hong-Xia, L. Dong-Sheng and S. Xue-Ping, J. Funct. Mater. Devices, 2004, 10, 95.
4. H. Yuan-Qing, L. Xue-Geng and M. Swihart, Chem. Mater., 2005, 17, 1017.
5. L. Zhao-Ping, L. Shu, Y. You, P. Sheng, H. Zhao-Kang and Q. Yi-Tai, Adv. Mater., 2003, 15, 1946.
6. V. Paredes-Garcia, C. Cruz, J. Denardín, D. Venegas-Yazigi, C. Castillo, E. Spodine and Z. Luo, New J. Chem., 2014, 38, 837.
7. Y. Lan, W. Luo and X. Wang, West Leacher, 2002, 2, 20.
8. G. Bao-Jiao, G. Jian-Feng and Z. Jia-Qi, Chin. J. Inorg. Chem., 2001, 17, 491.
9. D. Chen and S. Wu, Chem. Mater., 2000, 12, 1354.
10. D. Shi, P. He, P. Zhao, F. Guo, F. Wang, C. Huth, X. Chaud, S. Bud'ko and J. Lian, Composites, Part B, 2011, 1532.
11. K. Nielsch, R. B. Wehrspohn, J. Barther, J. Kirschner, S. F. Fischer, H. Kronmuller, T. Schweinbock, D. Weiss and U. Gosele, J. Magn. Magn. Mater., 2002, 249, 234.
12. J. C. Bao, C. Y. Tie, Z. Xu, Q. F. Zhou, D. Shen and Q. Ma, Adv. Mater., 2001, 13, 1631.
13. L. Dayong, R. Shan, W. Hui, Z. Qingtang and W. Lishi, J. Mater. Sci., 2008, 43, 1974.
14. R. Akbarzadeh and H. Dehghani, Dalton Trans., 2014, 5474.
15. J. Zhang, K. Yanagisawa, S. Yao and H. Wong, J. Mater. Chem. A, 2015, 3, 7877.
16. Y. Qiua, H. Zhenga, J. Geng, B. Johnson, B. Zhou, S. Hermans and G. Somorjai, Nanocompos. Sci. Technol., 2004, 1, 159.
17. F. Alonso and M. Yus, Chem. Soc. Rev., 2004, 33, 284.
18. J. Geng and B. Johnson, Nanocompos. Sci. Technol., 2004, 1, 159.
19. F. Alonso, P. Riente and M. Yus, Tetrahedron, 2008, 64, 1847.
20. F. Alonso, P. Riente and M. Yus, Tetrahedron Lett., 2008, 49, 1939.
21. R. Larock, Comprenhensive Organic Transformation: a Guide to Funtional Group Preparations, Wiley-VCH, New York, 2nd edn, 1999.
22. M. Harrad, B. Boualy, L. Firdoussi, A. Mehdi, C. Santi, S. Giovagnoli, M. Nocchetti and M. Ait, Catal. Commun., 2013, 32, 92.
23. T. Subramanian and K. Pitchumani, Catal. Commun., 2012, 29, 109.
24. V. Rodriguez-Gonzales, S. Alfaro, A. Zaldivar-Cadena and S. Lee, Catal. Today, 2011, 166, 166.
25. J. Sun and X. Bao, Chem.–Eur. J., 2008, 14, 7478.
26. R. White, R. Luque, V. Budarin, J. Clark and D. Macquarrie, Chem. Soc., 2009, 38, 481.
27. J. Campelo, D. Luna, R. Luque, J. Marinas and A. Romero, ChemSusChem, 2009, 2, 18.
28. J. De'Rogatis, M. Cargnello, V. Gombac, B. Lorenzut, T. Montini and P. Fornasiero, ChemSusChem, 2010, 3, 24.
29. H. Sakurai and A. Murugadoss, J. Mol. Catal. A: Chem., 2011, 341, 1.
30. M. Khairy, S. El-Safty, M. Ismael and H. Kawarada, Appl. Catal., B, 2012, 127, 1.
31. F. Jia, L. Zhang, X. Shang and Y. Yang, Adv. Mater., 2008, 20, 1050.
32. Y. Suzuki, Y. Sumi, K. Kita, T. Miyanaga and K. Sagisaka, Surf. Interface Anal., 2006, 38, 1296.
33. J. Byeon and J. Roberts, Langmuir, 2014, 30, 8770.
34. J. Geshev, A. Viegas and J. Echmidt, J. Appl. Phys., 1998, 84, 1488.
35. C. Cruzat, O. Peña, M. Meléndrez, J. Díaz-Visurraga and G. Cárdenas, Colloid Polym. Sci., 2011, 289, 21.
36. Z. Hui, R. Zhang, X. Ying, X. Liang and T. Zhoub, Appl. Surf. Sci., 2012, 263, 795.
37. M. Andrews and C. Pillai, Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem., 1978, 16, 465.
38. (a) J. E. Bäckvall, J. Organomet. Chem., 2002, 652, 105; (b) J. Trocha and H. B. Henbest, Chem. Commun., 1967, 545.
39. D. Dutta, B. Borah, L. Saikia and M. Pathak, Appl. Clay Sci., 2011, 53, 650.
40. X. J. Park, H. Lee, J. Kim, K. Park and H. Song, J. Phys. Chem. C, 2010, 114, 6381.

---

Footnote

† Electronic supplementary information (ESI) available: Fig. 1S X-ray powder diffraction pattern of nickel nanocomposites. Fig. 2S EDX spectra of (a) Ni–Pe1; (b) Ni–Ch1. Fig. 3S SEM images of nickel nanocomposites (a) Ni–P2; (b) Ni–P4; (c) Ni–C2; (d) Ni–C4. See DOI: 10.1039/c5ra09622g

---