Elsevier

Polyhedron

Volume 72, 18 April 2014, Pages 1-7
Polyhedron

Comparative investigation of nickel aluminate (NiAl2O4) nano and microstructures for the structural, optical and catalytic properties

https://doi.org/10.1016/j.poly.2014.01.013Get rights and content

Abstract

Nickel aluminate (NiAl2O4) nano and microstructures were synthesized by both microwave combustion method (MCM) and conventional combustion method (CCM) using aloe vera as the plant extract. The synthesized NiAl2O4 nanoparticles were characterized by X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FT-IR) studies, high resolution scanning electron microscopy (HR-SEM), high resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray analysis (EDX), diffuse reflectance spectroscopy (DRS), and photoluminescence (PL) studies. The XRD results confirmed the formation of a cubic NiAl2O4. The formation of NiAl2O4 nano and microstructures were confirmed by HR-SEM and HR-TEM. The optical absorption and photoluminescence emissions were determined by DRS and PL spectra, respectively. The effect of catalyst and solvent on the catalytic oxidation of benzyl alcohol using the NiAl2O4 catalyst prepared by both CCM and MCM was also investigated.

Graphical abstract

This in morphology from nanoparticles from the conventional method to microwave method is clearly shown by HR-TEM. The particle size distribution histogram is shown in the inset of Fig. (a, b), it was observed that both the obtained nanoparticles were spherical like shape and the nanoparticle size is in the range (sample A and B) of 15–18 and 10–12 nm, respectively. The results from XRD suggested that the crystallite size for sample B is slightly lower than that of sample A. This is in good agreement with the results of XRD analysis.

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Introduction

Mixed metal oxide spinels have long been a topic of interest, because of their usefulness as magnetic materials, pigments, catalysts, and refractory materials. Transition metal spinels also find applications in sensor technology [1], [2]. The oxidation of alcohols into their corresponding aldehydes and ketones are of significant importance in both laboratory and industrial synthetic chemistry [3]. Nickel aluminate, (NiAl2O4) is a transition metal spinel, which falls under the category of normal spinel with aluminium in octahedral sites and nickel in tetrahedral sites. NiAl2O4 acts as a catalytic support, which can provide a stabilizing effect, while maintaining a high degree of chemical inertia [4].

High surface area NiAl2O4 is of prime importance for the catalytic purposes. In general, high surface area materials will have the small particle size. Hence, the synthesis of nanosized nickel aluminate nanoparticles is worth to be studied [5]. NiAl2O4 can be prepared by different methods, such as, conventional impregnation, impregnation, co-precipitation, and sol–gel method [6], [7], [8], [9].

Microwave combustion processing of materials is fundamentally different from the conventional combustion processing, due to its different heating mechanism. In a microwave oven, due to the interaction of microwaves with the material, the heat is generated within the sample itself. In conventional method, the heat is generated by heating the elements, and then it is transferred to the sample surface. The use of microwave in the combustion synthesis has several advantages. The foremost of which are the shortened periods of synthesis, enhanced reaction kinetics, and the reactant selectivity, during the energy transfer from the microwave field, which assists the stabilization of the metastable phase of the material. Therefore, it is possible to control both kinetic and thermodynamic factors of the chemical reactions using microwave in the combustion process.

Recently, plant extracts have been used as both reducing and capping agent for the synthesis of nanomaterials. The plant extract plays not only a fuel role, but also has a coordinating action, capturing the involved metal ions in the amylose helix of the extract in well-defined sites, and impeding the separation of metal oxides [10]. An appealing topic of this research field is the non-polluting and controlled synthesis of oxide materials, which involves a low cost and natural compound as raw materials and also as active ingredients in the nanosized metal oxide particles [11].

Aloe vera gel may be used as a bio-reducing agent in the preparation of metal oxide precursors. Recently, aloe vera plant extract has been successfully used to synthesize single nanocrystalline triangular gold nanoparticles and silver nanoparticles in high yield by the reaction of aqueous metal source ions (chloroaurate ions for Au and silver ions for Ag) and the aloe vera plant extract [12], [13], [14]. Most recently, Maensiri et al. reported for the first time, the synthesis of In2O3 nanoparticles using aloe vera plant extract [15]. This is because of the long chain polysaccharides present in the aloe vera plant extract, which affords the homogeneous distribution of mixed metal oxides. The advantage of this method includes (i) use of inexpensive, nontoxic and environmentally sympathetic precursors, and (ii) simple and time saving procedure [16]. Many nanoparticles show the particle size of 30 nm with a distinct cap, which may be due to the presence of flavonoids, proteins, starch and other functional groups present in the leaf broth of aloe vera and they are likely to be responsible for the formation of metal oxide nanoparticles in smaller size [17]. Hence in the present study, we prepared NiAl2O4 by two different methods viz. CCM and MCM using aloe vera plant extract for the comparative investigation of their structural, optical, and catalytic properties. The prepared NiAl2O4 nanoparticles were characterized by X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FT-IR), high resolution scanning electron microscopy (HR-SEM), energy dispersive X-ray analysis (EDX), high resolution transmission electron microscopy (HR-TEM), diffuse reflectance spectroscopy (DRS), and photoluminescence studies (PL). Finally, the samples were tested for their catalytic activity towards the oxidation of benzyl alcohol.

Section snippets

Synthesis of NiAl2O4 by CCM and MCM

Nickel nitrate (99% purity) and aluminum nitrate (98% purity) were used as the starting materials, (Merck chemicals, India) and were used without further purification. The aloe vera leaves were collected from the local agricultural fields, Chennai, Tamilnadu. A 20 g portion of thoroughly washed aloe vera leaves were finely cut and the obtained gel was dissolved in 100 ml of de-ionized water, and stirred for 45 min to obtain a clear solution. The resulting extract was used as an aloe vera plant

X-ray diffraction (XRD) studies

The XRD patterns were recorded twice on two batches of samples for reproducible results. The XRD patterns of the prepared samples were given in Fig. 1(a, b). All the diffraction peaks matched well with the characteristic reflections of the cubic spinel structure. Fig. 1a showed the XRD pattern of the sample A (prepared by CCM), and Fig. 1b displayed the XRD pattern of sample B (prepared by MCM). It revealed that both the samples A and B had the same crystalline structure. The diffraction peaks

Conclusions

In summary, nickel aluminate spinel was prepared by both CCM and MCM. The effect of the preparation method on the structural, morphological, optical, and catalytic activity for the selective oxidation of benzyl alcohol was investigated. In comparison with CCM, MCM shortens the reaction time. The heating is not only quick but also uniformly spread through the entire bulk of the reaction mixture. This may result in the formation of nanoparticles structure with narrow distribution. It was found

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