A novel hydrolysis method to synthesize chromium hydroxide nanoparticles and its catalytic effect in the thermal decomposition of ammonium perchlorate
Highlights
► Synthesis of Cr(OH)3 nanoparticles in Cr3+–F− aqueous solution. ► The F− ion tailors coagulated materials, Cr(OH)3 nanoparticles are obtained. ► Adding nanosized Cr(OH)3, AP thermal decomposition temperature decreases to 200 °C. ► The nanosized Cr(OH)3 catalyzes NH3 oxidation, accelerating AP thermal decomposition.
Introduction
Chromium hydroxide (Cr(OH)3) nanoparticles have found potential technological applications, i.e., in nitrogen fertilizer, and petrochemical engineering catalysis, as a pigment, or as a model system in colloid science [1], [2], [3].
Cr(OH)3 nanoparticles are traditionally synthesized by hydrolysis of very diluted Cr3+ aqueous solutions (2 × 10−4–1 × 10−3 mol dm−3) in the presence of SO42− at moderate temperatures (60–90 °C) for long durations (18 h to several days) [4], [5]. Strengthening methods are also adopted to synthesize monodispersed Cr(OH)3 particles such as forced hydrolysis and microwave dielectric heating. For example, Ocaña prepared nanosized Cr(OH)3 spherical particles by forced hydrolyzing aqueous solution of Cr(NO3)3 and Na2SO4 in the presence of urea and polyvinyl pyrrolidone K-30 (PVP) [6]. Gómez et al. synthesized Cr(OH)3 sub-micro- and nanoparticles by microwave dielectric heating both chrome alum (KCr(SO4)2·12H2O) solutions and solutions of Cr(NO3)3–K2SO4 with different initial [Cr3+]/[SO42−] ratios, at the boiling temperature [7]. The role of SO42− has been proved to be crucial in the formation of Cr(OH)3 nanoparticles because the SO42−, acting as a counter ion, forms complexes such as [Cr2(OH)2SO4]2+ and Cr(OH)SO4 and others that are believed to constitute units of precursor complexes [8], [9].
Ammonium perchlorate (AP) is the most common oxidizer used in composite solid propellants. The characteristics of its thermal decomposition influence the combustion of these propellants. A specific feature of the thermal decomposition of AP is its extreme sensitivity to metal oxide additives, which accelerate the low-temperature thermal decomposition of AP [10]. The best metal oxide catalyst is CuCr2O4, which decreases thermal decomposition temperature of AP from about 450 °C to 273 °C [11].
The thermal decomposition of AP initially produces NH3 and HClO4 as intermediates which are catalyzed by metal oxides to further decompose to NH3, H2O, N2O, NO, N2, Cl2, and so on [12]. Some p-type metal oxides, such as CuO and NiO which catalyze AP thermal decomposition at 275.8 °C and 357 °C, respectively, have been shown to be efficient catalysts of the thermal decomposition of AP. In addition, recent investigations have shown that nanosized particles of metal oxides can increase the burning rate of AP. The efficiency of this catalytic process increases dramatically when nanosized oxide particles are used rather than microscale and bulk particles [13].
According to the literature, the most reactive catalyst for the decomposition of HClO4 is Cr2O3 [14]. Cr2O3 also serves as an excellent catalyst for the decomposition of NH3 by catalyzing the oxidation of NH3 to form N2 and H2O at 240 °C [15]. However, Cr2O3 is proved to be a less efficient catalyst of the thermal decomposition of AP [16] due to its n-type semiconducting structure which is believed to accept an electron from the perchlorate ion hardly.
In this work, a novel hydrolytic method, using F− instead of SO42−, was developed to prepare nanosized Cr(OH)3. The spherical Cr(OH)3 nanoparticles were synthesized by altering the molar ratio of F−/Cr3+, initial pH, and Cr3+ concentration. The catalytic effect of the prepared Cr(OH)3 nanoparticles on the thermal decomposition of AP was studied in detail.
Section snippets
Experimental
All chemicals were analytical grade reagents and used as received without further purification. Cr(NO3)3, NaF, urea and PVP were dissolved in distilled water to prepare an aqueous solution. In a typical synthesis, 100 mL deionized water, 20 mL 0.2 M Cr(NO3)3, 20 mL 0.8 M urea, 20 mL 200 g dm−3 PVP, 40 mL 0.5 M NaF aqueous solution were added to a Pyrex test tubes (250 cm3) which were tightly sealed with Teflon caps and placed in a preheated oven at 100 °C for 24 h. 0.1 M HNO3 was used to adjust initial pH.
Results and discussion
Table 1 summarizes the effects of the F−/Cr3+ molar ratio, initial pH, urea, PVP, and Cr(NO3)3 concentrations, on the morphological characteristics and composition of the particles precipitated via hydrolysis route. Low Cr(NO3)3 concentration, suitable F−/Cr3+ molar ratio, and high initial pH are beneficial to preparing nanosized particles. Through the aging of solutions with the F−/Cr3+ molar ratio at 5, the Cr(NO3)3 concentration maintained at 0.005 M, and the initial pH (5.0) constant,
Conclusions
A novel method to synthesize Cr(OH)3 nanoparticles and its catalytic effect of AP was studied, leading to the following conclusions:
- (1)
Spherical Cr(OH)3 nanoparticles could be obtained by aging at 100 °C aqueous solutions of Cr(NO3)3 (0.005 M) in the presence of NaF (F−/Cr3+ molar ratio = 5), urea (0.2 M) and PVP (20 g dm−3) with initial pH of 5. The chromium fluoride complex ions such as CrF2+, CrF2+, and CrF30 which formed with F− and Cr3+, tailored the aggregation to form the Cr(OH)3 nanoparticles.
- (2)
The
Acknowledgments
This work was financially supported by the National Nature Science Foundation of China (No. 50904058) and the National High Technology Research and Development Plan of China (863 plan, No. 2011AA060702).
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