Solution Combustion as a Promising Method for the ...

Advances in Science and Technology Vol. 63 (2010) pp 187-196 Online available since 2010/Oct/27 at www.scientific.net © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AST.63.187

Solution Combustion as a Promising Method for the Synthesis of Nanomaterials Alexander S. Mukasyan 1

Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, 46556, USA [email protected]

Keywords: combustion synthesis, self-propagating high-temperature synthesis, nanopowders, catalysts, oxides, metals

Abstract. Solution–combustion is an attractive approach to synthesis of nanomaterials for a variety of applications, including catalysts, fuel cells, and biotechnology. In this paper, several novel methods based on the combustion of a reactive solution are presented. These methods include selfpropagating sol-gel combustion and combustion of impregnated inert and active supports. It was demonstrated that, based on the fundamental understanding of the considered combustion processes, a variety of extremely high surface area materials could be synthesized. The controlling process parameters are defined and discussed. Examples of materials synthesized by the above methods are presented. A continuous technology for production of nanopowders by using the solution combustion approach is also discussed. Introduction Self-propagating High-temperature Synthesis (SHS) wave represents a self-organized system where a chemical reaction localized in the vicinity of the combustion front propagates throughout the reaction media, converting initial reactants into desirable products [1,2]. The following classification, which is based on the physical nature of the used precursors, can be suggested for combustion synthesis of nanomaterials: • Synthesis of nanoparticles in flame, i.e. gas-phase combustion • Self-Propagating High-Temperature Synthesis of nanoscale materials by conventional approaches, i.e. initial reactants in solid state • Solution-Combustion Synthesis of nanopowders, i.e. initial reactive media is liquid (e.g. aqueous) solution The first approach i.e. gas-flame nano-synthesis, has a relatively long history and was overviewed in [3]. It is challenging task to produce nanomaterials by conventional solid state SHS, where the typical scale of heterogeneity for the initial solid precursors is on the order of 10-100 microns. This feature, coupled with high reaction temperatures (>2000K), makes it difficult to synthesize nanoscale structures with high surface area. However, several methods have been developed for synthesis of nanomaterials by using this approach, including SHS synthesis + intensive milling; SHS + Mechanical Activation; SHS + chemical dispersion; (iv) SHS with additives; (v) Carbon Combustion Synthesis. All these methods are described in details in recent reviews and book [4-6]. In this paper the third approach, i.e. Solution Combustion Synthesis (SCS) is briefly discussed and analyzed. While this method has relatively long history [7,8] but only recent achievements in combustion science allowed a number of important breakthroughs in this field, e.g. development of new catalysts and nano-carriers with properties better than those for similar traditional materials (see also [6,9]).

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General Definitions Typically, SCS involves a self-sustained reaction in a solution of metal containing oxidizer (e.g. metal nitrite) and fuels which can be classified based on the chemical structure of the fuel, i.e. type of reactive groups (e.g. amino, hydroxyl, carboxyl) bonded to a hydrocarbon chain. The reaction between fuel and oxygen formed during decomposition of oxidizer provides conditions for rapid high-temperature reaction. The stoichiometric equilibrium combustion reaction, for example, by using metal nitrite as an oxidizer and glycine, as a fuel can be described by the following widely accepted scheme:

where Mv is a v-valent metal, and φ is the fuel/oxidizer ratio, φ= 1 means that the initial mixture does not require atmospheric oxygen for complete oxidation of the fuel, while φ > 1 (200 m2/g). To reach this goal we 1.5 suggested φ=1 VCS 1.0 [11,12] another approach, the so-called impregnationsolution combustion synthesis method, which is 320 340 360 380 400 Initial Temperature, K discussed below. Fig. 3. Specific surface area of synthesized powders by SHS mode as a function of the initial temperature of the reactive solution.

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Impregnated Layer Combustion Mode The so-called impregnated layer support combustion (ILSC), involves several steps (Fig. 4). First, based on the needs of application, one has to choose solid pellets with the right composition and microstructure for use as an inert support. Typically, it is oxide (e.g. Al2O3, ZrO2, etc.) with high SA (>100 m2 /g). During the solution impregnation step I the porous pellets are dipped into the desired reactive solution under vacuum conditions, which leads to infiltration of the solution into the bulk of the material. Second is a drying step (not shown in Fig.4), which is followed by step III, i.e. initiation and propagation of the reaction. The combustion front moves along the heterogeneous media of impregnated solids similar to that in the SSGC mode. The final product consists of pellets loaded by the thin surface layer of the desired catalyst. Step I: Solution Impregnation Initial Porous Support

Step III: Combustion Reaction Propagation

Catalytic Layer

Combustion Product: Supported Catalyst

Fig. 4. General scheme for ILSC synthesis. Table 3 illustrates the characteristics of iron oxide catalysts supported on different substrates by using ILSC method [11]. It can be seen that typically the BET specific surface are of the catalyst is comparable with that of initial support. However, in special case of activated Al2O3 the BET of catalyst is twice above of the support. For example, recently extremely active catalysts were produced by this method for oxidative reforming of methanol [33]. Table 3. BET of the different supported catalysts synthesized by ILSC method BET of the support BET of catalyst BET of the supported Support 2 2 without support, m /g catalyst, m2/g m /g 5 4.5 5.8 α-Al2O3 244 37 197 γ-Al2O3 Al2O3-activated 149 40 225 ZrO2 125 22 112 A uniform adherent coating of Pd-substituted ceria (Ce 0.98 Pd 0.02 O 2-δ) on cordierite honey-comb monolith has been demonstrated in a single step by ILSC of ceric-ammonium nitrate, oxalyldihydrazide and PdCl2 redox mixture [34]. This material is used as a three-way catalyst in automobiles. Also, LaCoO3 catalyst deposited by in situ SCS directly over a ceramic honeycomb monolith and tested in a lab-scale test rig gave 50% of N2O conversion performance for Gas Hourly Space Velocity values of industrial interest (10,000–30,000 h-1) [35]. These simple and in-expensive processes for preparation of the supported catalysts hold a great promise for automobile exhaust remediation.

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Another modification of the ILSC involves usage of the reactive porous media [12]. The idea of the method is that the reactive solution can be impregnated into porous media that are also reactive and thus can assist self-propagation of low exothermic reactions. However, in order to synthesize powders of the desired composition, it is preferable that the products of the supportive reaction be in the gas phase. Also, because we need production of nanoparticles, the process temperature should be relatively low and its duration as short as possible. All of the above suggest using an active support element, for example, a thin layer of pure cellulose paper. Indeed, such a media has excellent infiltration characteristics and can be easily impregnated with an aqueous solution. Also, the experiments show that residual ash after burning of the paper in air is less than 0.2 wt %. Finally, burning of the thin layer, owing to large heat loss, leads to the so-called quenching effect, i.e. rapid temperature drop after the reaction front, which favors the formation of nanopowders. It is more important that such approach allows the continuous method for synthesis of the nanopowders [12, 13], which scheme is shown in Fig. 5. This safe, energy-efficient, and effective apparatus was used for continuous synthesis of a variety of complex oxides allowing production up to 0.5 kg/h of nanomaterials. These powders were used as the catalysts for direct ethanol and methanol fuel cells [29, 30], as well as for reforming of the kerosene-type fuels [36] and methanol [33] to produce hydrogen.

Fig.5. General scheme for continuous synthesis of nanopowders by impregnated active layer combustion (IALC) method. Pure Metals and Alloys by Solution Combustion According to reaction (1) the final combustion product is a metal oxide. Indeed, during last decade it was proved that SCS is an effective and versatile method for synthesis of almost any binary and many complex nano-oxides. The question is: can compounds other than oxides e.g. pure metals or their alloys, be produced by this approach? This issue was partially addressed experimentally in publications [37, 38]. It was shown that by using micro-waved induced and mist ultrasonic pyrolysis combustion methods and optimizing solution composition one may produce metals and alloys. In our recent work this issue was investigated in details by using conventional VCS and SHS modes of the solution combustion [39]. Nanoparticles of pure metals (nickel, cooper) and their alloy were for the first time directly synthesized by these conventional approaches. Moreover the specific

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reaction pathways were proposed to describe the phase formation of such powders in combustion wave. It was shown that the exothermic reaction between NH3 and HNO3 species that formed during the decomposition of glycine and metal nitrates, act as a source of energy that is required to achieve the self sustained reaction regime. A thermodynamic analysis of the reaction system suggests that increase in glycine concentration leads to establishing of a hydrogen rich reducing environment in the combustion wave that in turn results in the formation of pure metals and metal alloys. It was experimentally proved that the formation of oxide phases occurs in the reaction front, followed by oxides gradual reduction to pure metallic phases in the post-combustion zone. For example, Fig. 6 shows the XRD patterns obtained for the final products in nickel nitrateglycine ( ) system as a function of fuel/oxidizer ratio in the range of φ from 0 to 1.25 in conventional SHS mode. It can be seen that for φ = 0 i.e. the case of simple nickel nitrate decomposition, the final product involves only NiO. Increasing φ, as predicted above, leads to the increased amount of Ni phase and pure metal was synthesized when φ = 1.25 by both SCS modes. SEM micrograph of the as synthesized pure Ni for φ = 1.25 (Fig. 7a) presents a typical morphology of the SC products, i.e. highly porous structure with tightly agglomerated nanocrystallites with individual size well below 50 nm (see insert). As it was mentioned above, it is difficult to investigate the sequence of phase transformations during VCS due to the explosive nature of the reaction. Careful examination of a typical temperature time profile for combustion wave in SHS mode shows the existence of two temperature peaks (see Figure 7b). In turn relatively slow velocities of the reaction front propagation (~5mm/s) allows one to quench the process. XRD results of the combustion products for a Ni nitrate-glycine solution with φ = 1.75 prepared by SHS mode and quenched before and after appearance of the second temperature peak unequally showed that intermediate products formed in the combustion front (after the first peak in Figure 7b) consists primarily of NiO phase. After the second reaction, only pure Ni phase was detected. It is interesting that two peaks were also observed during DTA analysis of the same composition. These results provide additional evidences for the proposed mechanism for pure metal formation during SC in this system. Similar results were obtained for cooper nitrate – glycine and Ni(NO3)2·6H2O/ Cu(NO3)2 ·6H2O – glycine systems. A methodology for solution combustion synthesis of pure metals and metal alloys is also suggested [39].

Figure 6:XRD pattern of SCS products of glycine-nickel nitrate system with varying φ

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Figure 7: (a) Typical microstructure as-synthesized pure Ni (b) Temperature – time history of Ni synthesis by SHS mode in iron nitrate-glycine solution

Concluding Remarks In recent years, solution combustion synthesis has not only opened new opportunities for the synthesis of various novel nano-oxides and composites, but also succeeded in preparation of nonoxide compound, including metal and metal alloys. It is even more important that continuous synthesis methods of nanopowders and approaches for various supported catalysts and coatings have been developed. As a result, conditions have been established for breakthrough in this area over the next several years. Indeed, now it is time to commercialize SCS method by selling the reality of ‘simpler is smarter and better’. Acknowledgement We gratefully acknowledge funding from NSF Grant 0730190 to support this work. This work was also partially supported by Notre Dame Integrated Imaging Facility. References [1] [2] [3] [4]

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12th INTERNATIONAL CERAMICS CONGRESS PART B doi:10.4028/www.scientific.net/AST.63 Solution Combustion as a Promising Method for the Synthesis of Nanomaterials doi:10.4028/www.scientific.net/AST.63.187