曲文剛，趙鳳起，高紅旭

(西安近代化學(xué)研究所燃燒與爆炸技術(shù)重點實驗室，陜西西安710065)

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形貌與晶相可控的微納米金屬氧化物提升高能氧化劑熱分解性能的研究進展

曲文剛，趙鳳起，高紅旭

(西安近代化學(xué)研究所燃燒與爆炸技術(shù)重點實驗室，陜西西安710065)

摘要：綜述了近期關(guān)于金屬氧化物的形貌可控合成以及它們作為燃燒催化劑的應(yīng)用方面的一些研究。簡要介紹了微納米材料的可控合成，討論了幾種典型的具有特殊形貌金屬氧化物納米材料的結(jié)構(gòu)特征，以及它們在含能化合物催化熱分解過程中所表現(xiàn)出的良好催化性能。最后，對形貌及晶相可控的納米燃燒催化劑未來的發(fā)展趨勢以及其在固體復(fù)合推進劑應(yīng)用方面所能夠發(fā)揮的重要作用進行了總結(jié)和展望。附參考文獻57篇。

關(guān)鍵詞：材料科學(xué)；微納米結(jié)構(gòu)材料；金屬氧化物；形貌控制；燃燒催化劑；固體復(fù)合推進劑

Introduction

Thermal decomposition of energetic oxidizers is of great importance for the propellants and explosives. Several significant parameters for the thermal decomposition of energetic oxidizers, such as activation energy, the burning rate, and temperature of decomposition put great impacts on the performance of solid propellant. Various works have been pointed out that using combustion catalyst is an effective way to enhance thermal decomposition process of energetic oxidizers[1-4]. The ultimate goal of this research is to study the thermal decomposition process at the molecular level and design and synthesize catalysts with desired selectivity and activity. Previous works have been confirmed that catalysts with large specific surface areas could increase catalytic efficiency[5]. However, catalysts consisting of ultrafine powders are complex solid systems with ill-defined mixtures of surface species, which hamper the understanding of the catalytic phenomena[6].

In the last decade, significant progress has been made in nanotechnology. Nanocrystals (NCs) can be synthesized with uniform composition, size, and shape[7-8]. It has been confirmed that the physical and chemical properties of NCs are not only sensitive to crystal size but also to crystal phase and shape. For crystalline materials, different morphologies may have different geometric and electronic structures, and exhibit different physical and chemical properties. Thus, morphology-controlled synthesis of NCs is attracting more and more research interest[9].

In this review, we will give a brief overview of recent progresses in the field of surface/morphology controlled syntheses. The first section presents an introduction to the general strategies for the fabrication of the morphology- and crystal phase-controlled micro/nanocrystallites. The second section summarizes and discusses the case studies, focusing on several kinds of typical functional metal oxides which can be served as combustion catalysts, including TiO2(anatase), ZnO (wurtzite), Fe2O3(corundum-type and cubic structure), and MnO2(rutile). At the end of the minireview, we provide the future trends on this field.

1General growth mechanism of Micro/

nanocrystallites with specific morphology or

crystal phase

In crystallography, the morphology of a crystal is a result of the interplay between thermodynamics and kinetics, which can be theoretically described by the Gibbs-Wulff theorem. From the thermodynamic effect, the selective adsorption of capping or stabilizing reagents (such as organic surfactants, polymers, small molecules, and ions) onto specific faces is an effective means of reducing surface energies[10-12]. In addition, thermodynamic analysis has indicated that the supersaturation of growth species might determine the surface energy of exposed faces during crystal growth, which provides a general way for fabricating the specific shapes on micro-and nanocrystallites[13-15].

Kinetic effect is another powerful factor for tuning the shape and phase of micro/nanocrystallites. Usually, crystals often grow under conditions that differ substantially from equilibrium conditions. Under nonequilibrium conditions, the shape of crystals is not unique and strongly depends on kinetic parameters such as the crystal growth rate during the nucleation and growth of crystals[15-19]. Compared with thermodynamic control of surface structure engineering, kinetic control involves a number of factors and is therefore more complicated, and the relationship between surface structure and kinetic factors remains unclear.

Thermodynamic and kinetic approaches to the synthesis of micro/nanocrystallites with specific morphology both represent bottom-up processes for crystal growth. In recent years, top-down strategies have also been applied to engineer the surface structure of micro/nanocrystallites[20-21]. Directional chemical etching based on crystal anisotropy, which widely used in the semiconductor industry, has shown particular advantages in the fabrication of micro/nanocrystallites with specific surface structures.

2Typical metal oxides with well-defined

morphology

2.1Anatase-type TiO2

TiO2is an important functional material for its wide applications in ceramics, white pigment, photocatalysts, solar cells, gas sensors, etc, which exists in three main phases: rutile, anatase, and brookite[22-24]. Structurally, both rutile and anatase crystallize in a tetragonal lattice, belonging to space group of P42/mnm and I41/amd, while the brookite has orthorhombic lattice. Usually, brookite rarely occurs, exhibits bad photocatalytic activity, and thus attracts few interests. With regard to stability, rutile is thermodynamically the most stable phase of bulk TiO2for its lowest molecular volume among the three polymorphs. Anatase is less stable than rutile, but more efficient for some applications, especially in catalysis, photocatalysis, and dye-sensitized solar cells. Interestingly, theoretical calculations and experiments demonstrate that when the size of TiO2particles is below 14 nm in diameter, the stability of rutile is lower than those of anatase, and thus anatase becomes the most stable phase[25-26]. That is why most of the nanoparticles synthesized through wet-chemical methods are anatase[27].

Recently, it has been found that amorphous sol-gel derived TiO2nanoparticles improved the combustion characteristics of solid rocket propellant[28]. To further understand the properties of titania, Seal et al. have investigated the effect of anatase, rutile, and amorphous TiO2nanoparticles on the combustion of solid rocket propellant. Each additive increased the burning rate of propellant strands by 30%. Typical fast-burning propellants are unstable due to oversensitivity to pressure variations, but the anatase additive maked propellants with high yet stable burning rates over a broad pressure range (Fig. 1). Solid lines indicate the burning rates with titania additives; the surrounding dashed lines indicate the 95% confidence intervals for the linear fits; Short-dashed lines indicate a baseline generated from repeated experiments on propellants with a standard (no additive) composition. The anatase TiO2nanoparticles can also catalyze the high-temperature decomposition of ammonium perchlorate, a key component of solid propellant[29].

Fig.1　Linearized plots of solid propellant burning rates vs pressure on a log-log scale.

2.2ZnO with Polar Surfaces

The wurtzite structure is one of the most common crystal structures among inorganic materials. Among all the wurtzite-type materials, ZnO is one of the most attractive materials due to its unique dual semiconducting and piezoelectric properties[30-31].

For the solid propellant area, ZnO also attracted much attention because of its excellent performance in the catalytic decomposition of AP. For example, hierarchically complex hollow cage-like superstructures assembled by ZnO nanorods have been prepared by Yin[32]and decreased the decomposition temperature of AP to as low as 285℃, which is lower than the decomposition temperature of AP catalyzed by nanoparticles reported in recent literature reports[33-34]. However, the catalytic mechanism is not quite clear but indeed depends on the surface of ZnO, on which the products of AP decomposition will be adsorbed[30].

Fig.2　Crystal structures

Structurally, wurtzite ZnO consists of tetrahedrally coordinated zinc and oxygen atoms that are stacked alternately along thecaxis. Such a structural feature results in a spontaneous polarization of the {0 0 0 1} or {1 0 1 1} faces and a divergence in surface energy (Fig. 2)[30]. Compared with the high-energy {0 0 0 1} surfaces, the {10-10} surfaces are non-polar and exhibit the lowest surface energy. Accordingly, ZnO facilitates to grow along thecaxis and finally form hexagonal prisms bounded by nonpolar {10-10} facets to minimize the surface energy, meaning that the (0 0 0 1) plane of ZnO should be much more reactive than these thermodynamically stable {10-10} facets, which may be the dominant active sites for various applications[35-37].Based on this principle, ZnO nanosheets with exposed (0001) facets have been prepared, and the correlation between exposed facets and photocatalytic/gassensing properties has also been investigated. Recently, Tang et al[38]. prepared ZnO micro/nanocrystals with different percentages of the exposed (0 0 0 1) facets by a facile chemical bath deposition method and applied in promoting the thermal decomposition of AP. The different samples showed a morphology dependent activity: ZnO nanocrystals with a higher percentage of exposed (0 0 0 1) facets are more catalytically active for the reaction. The activation energy of AP decomposition decreased from (154.0±13.9)kJ/mol to (90.8±11.4)kJ/mol, (83.7±15.1)kJ/mol, and (63.3±3.7)kJ/mol for the ZnO crystals with ca. 18.6%, 20.3%, and 39.3% of exposed (0 0 0 1) facets. The HTD temperature of AP was reduced by 125℃ and the heat release was increased by 449J/g in the presence of ZnO nanocrystals with 39.3% percentage of exposed (0 0 0 1) facets (Fig. 3).

Fig.3　Morphology dependent catalytic activity of ZnO on thermal decomposition of AP

2.3Iron oxides

Fe2O3has been widely used in catalysts as either the main component or an effective support. The size, shape, and crystal phase of Fe2O3is generally considered to substantially alter the reaction efficiency[39-41]. Fe2O3has four polymorphs based on the atomic arrangements of the Fe3+and O2-ions:α-,β-,γ-, andε-phases. Of these phases, the highly crystallineα-Fe2O3is the most popular and widely used.α-Fe2O3has a corundum-type structure with lattice parameters of 0.50352 and 1.37508nm[42-43]. The Fe3+ions occupy two-thirds of the octahedral sites confined by the nearly ideal hexagonal close-packed O lattice. Fig. 4 exhibit the atomic configurations of the crystal facets forα-Fe2O3.

Fig.4　Atomic configurations of the crystal facetsfor α-Fe2O3

α-Fe2O3nanorods are the most popular nanostructured iron oxides as their shape anisotropy greatly affects their chemical properties. Fe2O3nanorods are mostly derived fromα-orγ-FeOOH via a thermal treatment. These precursors were usually prepared using an aqueous-phase precipitation and hydrothermal synthesis. The hydrolysis of Fe2+or Fe3+by OH-is the most convenient route for preparing FeOOH crystals[44], and the temperature and pH values of the aqueous solution govern the crystalline phase and FeOOH particle shape. Heating in air easily dehydrated the rod-shaped iron oxyhydroxides to α-Fe2O3nanorods, 5-80 nm in diameter and 150-800nm long[45-47].

For example, Ma et al. have prepared hollow Fe2O3nanorods using a hydrothermal method, and then combined with Al nanoparticles to form superthermite Al/ Fe2O3by ultrasonic mixing. The superthermite Al/Fe2O3affects the thermal decomposition of RDX and greatly enhances its secondary gas-phase reaction. The main influences of superthermite Al/Fe2O3, Al nanopowders, and Fe2O3nanorods on the thermal decomposition of RDX are that the secondary decomposition peaks become distinct and the peak temperature decreases (Fig. 5 and Fig. 6)[48].

Fig.5　SEM image of superthermite Al/Fe2O3

Fig.6　DSC curves of RDX and RDX/superthermite(Al/Fe2O3) mixture

2.4Manganese oxides

Manganese oxides have various chemical states (Mn2+, Mn3+, and Mn4+) and diverse crystalline phases[49-50]. MnO has a face-centred cubic structure (rock-salt type), Mn2O3possesses a body-centred cubic structure, and Mn3O4has a tetragonal spinel structure with metal cations occupying one-eighth of the tetrahedral sites and half of the octahedral sites. Among all the oxides, MnO2is the most attractive material and can exist as theα-(monoclinic),β-(rutile),δ-(layered), orε-(hexagonal close packing) phase depending on the arrangement of [MnO6] octahedra.

MnO2have been recognized as effective catalysts in the catalytic combustion of low hydrocarbons like C3H8, and monopropellants like H2O2for a long time[51-52]. It is also one of the first active catalysts reported for AP decomposition, and perceived as a burning rate enhancer for AP-based composite propellants. In recent years, mixed valent manganese oxide molecular sieves have possess notable catalytic activities on the decomposition of AP[53]. The excellent catalytic activity of mesoporousβ-MnO2, particularly in gas-phase oxidation-reduction of small molecules, has also been reported[54-56]. It means that mesoporous metal oxides, particularly mesoporousβ-MnO2could have potential applications as burning rate enhancers for AP-based composite rocket propellants.

This method was confirmed by Chandru and his co-workers recently[57]. They prepared mesoporousβ-MnO2using ordered mesoporous silica (KIT-6) as the template, involving the impregnation of Mn(NO3)2in KIT-6, followed by calcination at 350℃ to form the desiredβ-phase, and subsequent selective dissolution of the template using NaOH. The XRD pattern of the prepared catalyst (Fig.7(a)) shows the presence of both amorphous and crystalline phases. All the observed diffraction peaks can be readily indexed to theβ-MnO2tetragonal crystal phase. A representative TEM image shown in Fig. 7(b) reveals the presence of ordered mesopores expected for a negative replica structure of the KIT-6 template.

Fig.7　XRD pattern (a) and TEM image (b) of theprepared mesoporous β-MnO2

The catalysts were evaluated for the thermal sensitization of AP using various standard thermo-analytical techniques. Fig.8 shows the DSC curves of pure AP and mixtures of AP with the prepared MnO2catalysts. All the MnO2catalysts markedly decrease the temperature of the HTD process. The addition of merely 0.25% (mass fraction) mesoporousβ-MnO2brings the HTD peak temperature from 426.3℃ down to 301.0℃. Intense activity, manifested as a larger exotherm at a lower temperature, is observed as the conc. of mesoporous catalyst is increased to 2% and 5%; wherein the decomposition process commences even before the phase transition of AP and concludes before the LTD temperature regime.

Fig.8　DSC curves of (a) pure AP, (b) AP+2%micro-sized MnO2, (c) AP+0.25% mesoporousMnO2, (d) AP+2% mesoporous MnO2and (e)AP+5% mesoporous MnO2(heating rate: 5℃/min-1)

3Conclusions

Nanocatalysis is a rapidly growing field because of its important contributions to both materials science and practical chemical reactions. Extensive studies on nanocatalysis have readily demonstrated that the size and shape of the catalyst on the nanometre scale profoundly affect catalytic performance. The particle size effect has typically been explained on the basis of variations in the number of active sites. In the last decade, increasing evidence suggested that the shape of the catalyst particle is equally important for obtaining the desired catalytic activity and selectivity. This morphological dependence becomes more significant with decreasing size along specific dimensions.

Controlling the phase and shape of NCs provides a powerful tool for tailoring their catalytic properties (reactivity and selectivity). By varying the shape of NCs, the properties of the materials and their catalytic performance are influenced. This is significant because even small improvements in catalytic properties can lead to a tremendous increase in efficiency. Future generations of catalysts will have well-defined structures and be highly selective, meeting the standards of sustainable chemistry. Although the last decade has witnessed significant progress in the shape and phase-controlled synthesis of NCs, the next requisite for successful nanocatalysis is the development of more versatile, reliable, and simple synthetic strategies for the tailored surface of NCs with desired components.

Finally, it is expected that such studies will contribute considerably to the design and synthesis of nanocrystals with specific morphologies and to the advancement of relevant

industries.

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中圖分類號：TJ55;O645

文獻標(biāo)志碼：A

文章編號：1007-7812(2015)05-0018-06