Copper oxide alumina composite via template assisted sol–gel method for ammonium perchlorate decomposition
Graphical abstract
Introduction
Composite solid propellants (CSP) are the major source of chemical energy for satellite launch vehicles and missiles, CSP is heterogeneous mixtures consisting of an oxidizer, usually ammonium perchlorate (AP), metallic fuel like aluminum powder and a fuel-cum-binder, generally hydroxyl-terminated polybutadiene [1], [2]. In addition, they contain curing agents, plasticizers, bonding agents and deoxidizers for improving their mechanical and storage properties and burn rate modifiers for improving burning rate. The thermal decomposition and hence burning characteristics of AP is influenced by the burn rate modifiers. Mainly transition metal oxides like iron (III) oxide, cobalt (III) oxide, manganese (IV) oxide, copper (II) oxide, and mixed metal oxides like copper chromite etc. are used as these types of catalysts. The physical characteristics of the metal oxides such as particle size, surface area and defects in the crystalline structure and chemical characteristics such as oxidation state and chemical composition affect its catalytic activity. In the case of fast decomposition/combustion reaction, such as propellant combustion a catalyst with low particle size, high surface area, and high porosity is well suited [3], [4].
Since the discovery of ordered mesoporous silica (e.g., MCM-41 and SBA-15) in the early 90s, mesoporous materials have attracted much interest owing to their wide range of applications. Although numerous methods have been developed to prepare silica mesostructures, the preparation of non-silica mesoporous materials is more challenging [5], [6]. The beginning of the 21st century saw considerable progress in the preparation of ordered non-silica mesoporous metal oxides (MMO) [7], [8]. MMO have been of pivotal importance in material science and their application has been widely demonstrated in various areas, including adsorption, catalysis, energy conversion and storage, optoelectronics and biological platforms [9]. Several synthetic strategies have been developed for the preparation of MMO [10], [11], [12], [13]. Among these methods, soft-templating routes that rely on the synergistic self-assembly of surfactant and metal oxide precursor, followed by calcination, generate mesoporous metal oxides with tunable mesostructure and pore size [14], [15], [16], [17], [18].
Preparation of mesoporous transition metal oxides possessing highly porous structure and large surface area and their application in the area of catalysis are well studied [19], [20], [21], [22], [23], [24]. Among these metal oxides, copper oxide (CuO) nanoparticles attract great interest due to their outstanding and distinct features in electronics, catalysis, and photonics. However, these are also very easy to get aggregated and thereby affecting their chemical and physical properties (e.g. catalytic activity) To avoid aggregation, these nanoparticles are dispersed on a substrate which acts as an anti-agglomerating agent [25]. From the viewpoint of materials science and technology, distributing these sensitive metal oxides over the surface of an inert material would be an effective and alternative strategy [26], [27]. These dispersed nanoparticles not only acquire remarkable stability but also promote physicochemical performances, which are so attractive and fascinating that they extend the breadth of practical applications of these nanoparticles. For example, Paulose et al. have studied the effect of graphite oxide support on the distribution and catalytic activity of Fe2O3 nanoparticles and concluded that dispersing iron oxide nanoparticles on graphite oxide surface not only decreases the tendency for aggregation of iron oxide but also increases its catalytic activity [28].
However, to the best of our knowledge, the use of mesoporous copper oxide dispersed on alumina as combustion catalysts for AP decomposition has not been studied well to date. Herein, we report the synthesis of mesoporous copper oxide, dispersed on alumina by the sol–gel process using block copolymer as a template. A series of mesoporous oxides of Cu and Al were prepared with different Cu:Al ratios. These mesoporous copper oxides dispersed on alumina (MCO) were characterized by different techniques and their catalytic activity for the thermal decomposition of AP was evaluated. Attempts were made to correlate the properties of MCO's with their catalytic activity.
Section snippets
Chemicals
All chemicals used in this study are of analytical grade. Copper nitrate trihydrate, aluminum nitrate nonhydrate, (SD Fine Chemicals, India) Citric acid (Merck, India) Pluronic 123 co-polymer (Sigma-Aldrich, USA Mol. Wt 5800) were used for the synthesis of catalysts. Ammonium perchlorate made in house with purity >99% was used for studying the catalytic activity.
Preparation of copper oxide dispersed on alumina
MCO was synthesized by a solvent-evaporation-induced self-assembly method using metal nitrates as the metal source and triblock
Elemental composition
Elemental composition of the samples was estimated by volumetric titration methods and the results are tabulated in Table 1 [29]. From Table 1 it can be seen that the experimental results are well matching with the theoretical values.
Powder X-ray diffraction
The crystal phase of the mesoporous CuO nanoparticles dispersed on Al2O3 was determined by powder XRD analysis and the diffractograms are shown in Fig. 1. From Fig. 1 it can be seen that the patterns and relative intensities of all diffraction peaks are matching
Conclusions
Copper oxide–alumina composites were prepared by polymer template assisted sol–gel method. Copper oxide particle prepared by this route are mesoporous in nature with a particle size of 20–30 nm. Effect of alumina on the catalytic activity of the synthesized copper oxide was studied in detail. The final products and distribution of copper oxide particles were strongly dependent on the alumina concentration. FESEM analysis shows that as the concentration of alumina increases the size of copper
Acknowledgments
The authors thank Director, VSSC, Deputy Director (PCM), VSSC for granting permission to publish this work and colleagues in Analytical and Spectroscopy Division, VSSC for their support. One of the authors (Sanoop Paulose) acknowledges ISRO for the fellowship.
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