Formation of textural and mechanical properties of extruded ceramic honeycomb monoliths: An 1H NMR imaging study
Introduction
Today monolith supports with honeycomb structure and catalysts on their basis are widely used in various branches of industry [1], [2], [3], [4]. Most actively monolithic catalysts are applied for purification of automobile exhaust gases [1], [5] and waste gases of chemical, metallurgical and energy producing plants [2], [3] from pollutants, such as hydrocarbons, carbon monoxide, nitrogen oxides, hydrogen sulfide, volatile organic compounds, etc. The development of monolithic catalysts increases their technical merit due to ease of operation and regeneration owing to low pressure drop, small dust and attrition sensitivity [1], [2], [5].
Extrusion on vacuum presses is the most widely used method for preparation of honeycomb monolithic supports. The production of honeycomb monolithic substrate includes the following technological stages: preparation of the forming mass, its extrusion through a die, dry-curing, drying and thermal treatment [5].
The rational compositions of the extrusion mass containing the main oxide component, the binder and the plasticizer are selected to provide both optimum rheological properties of the mass and optimum physicochemical properties of the final monolithic substrate. Various oxide powders, such as titania, zirconia, silica and alumina [2], mullite, spodumene, cordierite [1], [2], [5], kaolin, montmorillonite, halloysite clays [2] and their compositions, are widely used. A binder and a plasticizer varying in concentration and preparation method are added to the forming mass to give them plasticity [1], [5], [6]. Clays from different deposits, kaolin, montmorillonite, and the product of thermal dispersion of alumina hydrate are usually used as the binder.
The thermal treatment is the final stage in the production of honeycomb monoliths, which determines their operational properties [1], [2], such as mechanical strength, specific surface area and pore structure. Chemical and phase transformations and sintering processes forming the support texture and physicochemical properties can take place during the thermal treatment of ceramic supports. Therefore, it is important to have detailed information on the formation of the above properties at each technological stage.
In the current study, the formation of the textural and physicochemical properties of ceramic monoliths based on alumina, titania and aluminosilicates has been investigated by 1H NMR microimaging in combination with a number of traditionally used methods (XRD, SEM, mercury porosimetry and others) during dry-curing, drying and thermal treatment. The non-destructive character of the 1H NMR microimaging method is attractive because it makes possible visualization of the dynamics of the substrate texture formation at various preparation stages, which is important for optimization of the technology for synthesis of substrate with desired operational characteristics.
Section snippets
Preparation of monoliths
The masses for extrusion were composed by mixing Al2O3, TiO2 or clay powders with aluminum hydroxide sol having pseudoboehmite structure or Ca-form of montmorillonite (Tagan deposit) in a Z-shape mixer. The water content of the extrusion masses was 33–35%. The masses had the following compositions (calculated for the dry products):
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alumina: γ-Al2O3 (60 wt.%), α-Al2O3 (10 wt.%) and γ-Al2O3 or Ca-montmorillonite (30 wt.%);
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titania: anatase (60 wt.%), rutile (10 wt.%) and Ca-montmorillonite (30 wt.%);
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Results and discussion
The most important properties of air-dry ceramic supports, such as pore structure, specific surface area, texture, mechanical strength, density, water resistance, etc., are known to be formed during their thermal treatment [3]. The results of the investigation of physicochemical properties of monolith supports on the basis of alumina, titania and aluminosilicate dried at 100 °C and calcined at temperatures from 600 (700) to 1100–1300 °C indicate that independent of the type of the oxide an
Conclusions
The main regularities of formation of honeycomb monoliths (alumina, titania and aluminosilicates) texture have been revealed using 1H NMR imaging, SEM, XRD, adsorption technique, and others. The texture changes may be tentatively subdivided into several temperature regions.
At the temperature range of 20–100 °C, i.e. during dry-curing and drying of the samples, changes of the texture of the samples are related to removal of capillary water from the pore space of the material. Then the interaction
Acknowledgements
This work was supported by INCO—COPERNICUS contract no ICA2-CT-1999-10028, INTAS-00413, NWO-RFBR 047.015.012.
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