Combustion Synthesis

Combustion synthesis (CS) is becoming one of the most important ways to produce a wide range of advanced porous ceramic or metallic materials. These include ceramic oxide like nanostructured catalysts. In CS, the heat released from the redox chemical reaction is used to produce useful materials. Depending upon the nature of the reactants, and the amount of heat made available during reaction, CS is in general mainly described as self-propagating high temperature synthesis (SHS), solution combustion synthesis (SCS), and flame synthesis (FS). Among these, SCS is an attractive alternative for the production of particular materials of high value compared to the more conventional and expensive preparation routes.

SCS processes are characterized by relatively medium heating oven temperatures (350–600 °C), fast heating rates, and short reaction times. SCS, in fact, offers a unique synthesis route via a highly exothermic redox reaction between metal nitrates and an organic fuel to produce multi-component oxides. The reaction is self-propagating and able to sustain high temperatures for a time long enough to complete the synthesis reaction and form the desired product. Therefore, the reaction is over in a few minutes without the use of high temperature furnaces. The final product is usually of high purity and well crystallized with nanometric size clusters, owing to the very small residence time at high temperature, a condition allowing the formation of a very large number of particles without huge growing effects. As a consequence, it is characterized by high values of both surface area and specific volume. Moreover, the significant gas rate produced during the synthesis process breaks up the large agglomerates and yields a porous mass that is easily crumbled into a fine powder. Furthermore, SCS, suitably adapted, is easily tunable to complex systems to produce directly in situ structured catalysts. In recent years this technique has been used to produce very fine, crystalline powders, without the intermediate steps that other conventional synthesis routes would require. These features make CS an attractive alternative for the production of ceramic materials compared to the conventional more expensive and time consuming processes. Examples of SCS and in situ SCS for the production of structured catalysts for combustion of natural gas and preferential oxidation of carbon monoxide are described.

Keywords: advanced materials; ceramics; mixed oxides; combustion synthesis; nanostructured catalysts; solution combustion synthesis