Combustion Synthesis

Abstract

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.

The Effect of the Preparation Method of Pd-Doped Cobalt Spinel on the Catalytic Activity in Methane Oxidation Under Lean Fuel Conditions

“…Both samples were prepared with the organic fuel in sub-stoichiometric conditions, with different fuel-to-oxidizer ratio. Depending on the fuel used, in fact, the adiabatic effect can vary, as well as the nature of the combustion-flaming or smoldering (non-flaming) [2]. This could account for the slightly different morphology: on one side, more fuel implied more reducing atmosphere during the SCS synthesis.…”

“…This could account for the slightly different morphology: on one side, more fuel implied more reducing atmosphere during the SCS synthesis. On the other hand, glycine, a more complex molecule compared to urea, provides more enthalpy per unit of reacting mass [2]. This would imply a higher temperature peak reached during the combustion synthesis process when glycine is used, explaining why the spinel made from glycine appears more sintered.…”

“…Catalytic combustion of these compounds appears as the most feasible route for the gas cleaning [1]. The flameless combustion is ideal for highly diluted air/fuel streams, as the production of NO x is inhibited by the relatively low reaction temperature [2]. Typical processes involving combustion of CH 4 in lean conditions are the after-treatment of unconverted CH 4 in compressed natural gas-fed vehicle exhausts (ranging from 0.4 to 1 vol%) [3,4] or the treatment of fugitive CH 4 from leaks in coal mines [5,6], from gas transportation facilities such as pipelines and compressor stations [7,8], from upstream oil and gas production facilities [9], and from other anthropogenic sources (ranging from 0.5 to 4 vol%) [10,11].…”

“…The SCS is a technique being investigated so far to create materials with enhanced catalytic properties [2,40]. It essentially means decomposing a material (typically the nitrate of the precursors leading to the desired final product) in the presence of a reducing agent (typically an organic fuel as glycine or urea) to create new structures with different properties through a redox reaction.…”

“…It essentially means decomposing a material (typically the nitrate of the precursors leading to the desired final product) in the presence of a reducing agent (typically an organic fuel as glycine or urea) to create new structures with different properties through a redox reaction. The reaction involved in the SCS generates a significant volume of high-purity foamy elements because of the exothermicity of the redox reaction [2]. The relatively high surface area and high purity of the material are of great interest from the catalytic point of view.…”