Novel Method for Synthesis of Nano-Materials: Combustion of Active Impregnated Layers†

Nanomaterials with characteristic structural dimensions on the order of few nanometers (1–100 nm) attract wide attention owing to their unique properties. They found a variety of applications in different branches of science and industry including catalysis, micro and biotechnologies, energy storage and conversion devices, such as fuel cells. Different techniques have been developed for nanomaterial preparations, solid-state, wet-type and CVD methods, mechano-, plasmaand combustion synthesis. A combination of combustion and reactive solution approaches leads to socalled solution (aqueous) combustion synthesis (SCS) method. Typically SCS involves a self-sustained reaction in solutions of metal nitrates and different fuels, which can be classified based on their chemical structure, i.e. type of reactive groups (e.g. amino, hydroxyl, carboxyl) bonded to the hydrocarbon chain. The reaction between fuel and oxygen containing species provides high temperature rapid interaction. In selfsustained propagation mode after local initiation the combustion front steadily moves along the sol-gel media forming extremely fine solid products with tailored composition. Several distinctive features of SCS contribute to the unique properties of the synthesized products. First, the initial reaction media being a liquid solution allows mixing the reactants on the molecular level thus permitting precise and uniform formulation of the desired composition on nano scale. Second, the high combustion temperatures ensure the formation of the desired phase composition directly in SCS wave. This feature allows one to skip additional step, i.e. high-temperature product calcination, which typically follows the conventional sol-gel approach. Third, short process duration (seconds) and formation large amount of gases during SCS, inhibit solid particle size growth and favor synthesis of nano-size powders with high specific surface area. Forth, almost no external energy is required to produce materials, since SCS occurs owing solely to the self-sustained highly exothermic reactions. Finally, simple equipments are typically used for this process. A wide variety of nano-materials was synthesized by the SCS approach, including unique pigments and catalysts, materials for SOFC and lithium batteries, oxygen sensors and storages, novel dielectric and piezoelectric compositions. Recently we reported method for synthesis of high surface area supported catalysts by reactive solution impregnation to the inert porous solid support, followed by initiation of the combustion reaction in such complex heterogeneous media. An unusual effect was found. By using this, so-called Impregnated Support Combustion (ISC) method, materials with very high specific surface area (> 200 m/g) were synthesized. However, not every material can be obtained by above techniques. Many low exothermic systems, which allow synthesize of valuable compositions, do not provide self-sustained reaction mode. To overcome this obstacle a novel, so-called, Impregnated Active Layer Combustion (IALC) method was developed and described below. IALC allows not only one step high yield production of powders with high surface area but also suggests an approach for continuous synthesis of such materials.