Synthesis of metal aluminate nanoparticles by sol–gel method and studies on their reactivity

Abstract

Nanosized metal aluminates (CoAl₂O₄, NiAl₂O₄ and CuAl₂O₄) were prepared by sol–gel method. The formation of metal aluminate nanoparticles and their particle size were found to depend upon the calcination temperature. CoAl₂O₄ nanoparticles were obtained at 700 °C, which is much lower than that required for its preparation through solid state reactions. The formation of NiAl₂O₄ and CuAl₂O₄ nanoparticles occurs at 900 °C and 800 °C, respectively. Characterization of the metal aluminate nanoparticles were carried out by X-ray diffraction, thermal gravimetric analysis, UV–vis diffuse reflectance spectroscopy, FT-IR spectroscopy, scanning electron microscopy and transmission electron microscopy. The chemical reactivity of the synthesized metal aluminate nanoparticles were also tested using paraoxon destructive adsorption and catalytic reduction of 4-nitro phenol.

Introduction

Nanocrystalline metal aluminates possess important applications in various fields such as heterogeneous catalysis, pigments, sensors and ceramics [1], [2], [3], [4], [5], [6]. Aluminate spinels have been used as catalysts in the decomposition of methane, steam reforming, dehydration of saturated alcohols to olefins, dehydrogenation of alcohols, etc. [7], [8], [9], [10], [11]. Aluminate nanoparticles have also been reported as good photocatalysts, e.g. for the degradation of methyl orange [12].

The general formula of spinels is AB₂O₄. In the spinel structure, the anions are arranged in a cubic close packed array with the cations arranged in the holes of the array. There are eight tetrahedral and four octahedral holes per molecule. In the case of normal spinels, the A²⁺ ions occupy tetrahedral holes and B³⁺ ions are present in the octahedral holes. In the case of inverse spinels, one half of A²⁺ ions occupy tetrahedral holes and the remainder of A²⁺ ions and all B³⁺ ions occupy the octahedral holes [13]. It is known that the nature of occupancy of tetrahedral and octahedral sites depends on the calcination temperature [14].

Metal aluminate nanocrystalline powders are generally prepared using solid state high temperature reactions. The temperature is usually greater than 1000 °C which influences the particle size [15], [16]. One of the disadvantages of the high temperature method is that the product obtained usually possesses low surface area. Solution based methods that have been reported for the preparation of metal aluminate nanoparticles include co-precipitation, polymeric precursor method, combustion and sol–gel method [17], [18], [19], [20]. The preparation of spinels by co-precipitation often leads to non-uniform materials since it is difficult to precipitate, homogenously, materials in large batches. Metal aluminate nanoparticles have been prepared by other methods such as solvothermal synthesis [21], [22], supercritical method [23], spray pyrolysis [24], flame synthesis [25], thermolysis [26], chemical vapour deposition [27] and sonochemical synthesis [28].

Sol–gel method has been a good option to produce homogenous materials since in this method, the chemical elements become uniformly distributed during the gel formation step [17]. Sol–gel method also offers good stoichiometric control and production of ultrafine particles with narrow size distribution at comparatively low temperatures [29], [30]. Sol–gel method has been used to prepare the metal aluminate nanoparticles by different authors. For example, Stranger and Orel have reported the synthesis of CoAl₂O₄ nanoparticles using ethylacetoacetate as the chelating agent [31]. Cui et al. have reported the preparation of NiAl₂O₄ nanoparticles using propylene oxide as the gelation agent [32]. The synthesis of nickel, copper and cobalt aluminate nanoparticles using single source heterobimetallic alkoxides as precursors is also known in the literature [30]. In the present study, metal aluminate nanoparticles such as CoAl₂O₄, NiAl₂O₄ and CuAl₂O₄ have been synthesized without using any chelating/gelating agent. Two different metal precursors were used instead of bimetallic alkoxide precursors during the sol–gel process.