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19PIONEER RESEARCH AND BREAKTHROUGHS CPI IN THE FIELD OF SHS CATALYSTS, PIGMENTS AND TPS FOR SPACE APPLICATION THAT INITIATED NEW COMBUSTION SYNTHESIS RESEARCH DIRECTIONS IN THE WORLD
G. Xanthopoulou*" and G. KsandopuloA
aInstitute of Nanoscience and Nanotechnology, NCSR ''Demokritos", Agia Paraskevi Attikis 15310, Greece
bInstitute of Combustion Problems, Almaty, 050012 Republic of Kazakhstan *e-mail: g.xanthopoulou@inn.demokritos.gr
DOI: 10.24411/9999-0014A-2019-10190
• Pioneering studies on the mechanisms of structural formation during combustion synthesis of catalysts. Study of combustion wave structure of SHS systems was performed [1]. Conditions for "freezing" combustion wave were found. Mechanism of reactions and structure formation in different zones of combustions were discovered. For the first time clearly showed that, unlike the short-term processes in the combustion front (i.e. in the zone of chemical interaction), a slower process of the cooling and crystallization process (i.e. following the combustion front) is the main determinant of practical importance for physical properties of the target SHS products (this finding creates the basis for developing qualitatively new and effective SHS methods).
• For the first time in the world detailed SHS catalysts production technology developed. Now at more than 50 countries researchers follow those studies. Such interest to SHS catalyst can be explained by high activity of catalysts prepared by this methods and advantages of SHS method in comparison with traditional methods of preparation of catalysts. The main differences of SHS method for catalysts production in comparison with other methods of preparation of catalysts are: very high heating and cooling rates: 103-106oC/s, very short completion times, of the order of minutes which leads to formation of high defect structure responsible for high activity of SHS catalysts. Method is attractive for industrial production: much lower energy consumption than traditional production methods, much lower energy costs, possibility for "just-in-time" manufacturing, high productivity, cheap catalysts, relatively simple process - easily adaptable to industrial scale, controlled physico-chemical properties of the products, large range of new materials which can be used in catalysis, it has wide diapason of structural forms of products - from granules of different size to blocks of honeycomb structure and different geometric forms. SHS catalysts have low surface area (usually 0.5-5 m2) because of high combustion temperatures. The high specific catalytic activity at such low surface areas is related to both the SHS materials unique composition as well as their very high atomic defect concentration resulting from the SHS process conditions. Influence of initial batch composition, particle size of each material in the initial batch, binder concentration, pressure, drying conditions, preheating temperature and speed, initiation combustion conditions, velocity of combustion, temperature of combustion, time of cooling on the chemical composition, structure, physical properties and activity in different catalytic processes studied. The SHS method is being developed for the production of a new class of active catalyst materials based on metals, alloys, metal oxides, borides, carbides and spinels for various applications [2, 3]. In the case of heterogeneous spinel catalysts, it is well known that local distortions of the primary crystalline lattice act as active catalytic centers. The nature and extent of such distortions depends on the atomic radii of the lattice ions, the type and
parameters of the crystalline lattice and atomic bonds, the size and dispersion of the crystallites, the size and nature of the dopant ions etc. It was found that ffunctionally significant, changes in the crystal lattice parameters of the active components of a spinel catalyst are usually caused by introducing various modifying additives, which increase the number of defects. For a particular catalyst and chemical process, there appears to exist an optimum amount of lattice distortion that gives high catalytic activity. The results indicate that, it may be possible to predict or "tune" the activity of a catalytic system for a particular process, by appropriate "design" of the crystal lattice. This could be done by adding specific ions which are known to produce the needed amount of distortion. SHS appear to be ideal techniques for this, since the materials synthesised by combustion synthesis are often metastable and the atomic lattice remains distorted after synthesis due to the rapid heating and cooling rates. This has become clear during studies of the mechanisms of spinel formation in the combustion wave, secondary structure formation in the process of further reaction of components and final structure formation in the process of cooling and is particularly noticeable in the formation of intermediate and non-stoichiometric compounds. The influence of the existence of distorted atomic lattice on catalytic activity has been studied for a number of spinel systems. By comparing the published data for SHS catalysts for various processes, it was possible to confirm the above conclusion and develop predictive criteria for the composition and atomic crystal structure giving the highest catalytic activity. Specifically, the trend of crystal lattice spacing was found to correlate with the trend of the length of the bonds of organic substances which must be adsorbed, broken on the catalyst surface and recombined with others for the synthesis of the reaction product. As a result, it is possible to "tune" the lattice spacing for maximum catalytic activity by doping the material with ions of appropriate radii, thereby distorting the crystal lattice by the correct amount. Very active SHS catalysts were developed for different processes. Development of SHS catalysts for dehydrodimerization of methane. The SHS method has been used to produce a range of active manganese-based catalysts for the oxidative dehydrodimerization of methane (for the production of ethylene). In the best case found, ethylene yield was found to reach about 26% at a selectivity of 85% and methane conversion of 30.2%. Some ethane, propene, propane and hydrogen were also obtained. The catalytic activity was found to be enhanced by careful control of SHS conditions and various post-synthesis treatments of the materials [4, 5]. The scheme of reactions during SHS discovered, supplemented by X-ray photoelectron spectroscopy measurements and by electron paramagnetic resonance (EPR), allowed for elucidation of the formation of the various products that appear in the synthesized material. Development of SHS catalysts for hydrogenation of olefins. SHS catalyst on the base of Ni-Al alloys were produced and studied in the hydrogenation of olefins with double and triple bonds. Influence of composition and different synthesis parameters on the physico-chemical characteristics studied and found out, that SHS catalysts are more active than Ni-Raney. Development of SHS catalysts for diesel fuel, petrol pyrolysis and dehydrogenation. Chemical gases and liquids, such as ethene, ethane, propene, propane, benzene and toluene, are usually industrially produced from petrol (gasoline) or diesel. It as found, that increased porosity increases the yield of liquid products and coke but decreases the yield of ethane and other olefines. Coke formation is a serious problem with catalysts with small pores. Coke tends to clog the entrance to small pores thereby quickly decreasing catalytic activity. SHS method offer simple way to regulate pores size and obtain low surface area catalysts with high specific activity. The substitution of Mg by Co in the Mg-Al-O spinel system deforms the structure changing slightly the crystal lattice parameters, depending on the amount of Co in the lattice, which can result in a significant number of lattice defects. The incorporation of Co in the spinel lattice resulted also in improved conversion efficiency. The sum of the ethene and propene yield was reaching 62 vol %. When Co was increased even further in the starting materials, excess oxide of Co appeared in the SHS product which changed the overall function of the catalyst to that of dehydrogenation
with a H2 yield of up to 80%. Such enhanced hydrogen yield suppresses the possibility of ethene production, but this catalyst is very good for dehydrogenation process. The catalytic activity of a mixed spinel material 0.6% CoAl2O4/99.4% MgAhO4 was also compared with that of a commercial catalyst (KVO3 on a mullite/ corundum carrier) used in the production of ethene. The relative yield of ethene on the Co-containing spinel SHS catalysts was measured to be 38 vol % as compared to 28 vol % for the KVO3 catalyst. Furthermore, the coke accumulation on the SHS catalyst was on average 3.7 times lower than on the commercial catalysts [6]. Development of SHS catalyst for methane reforming to synthesis gas. Co-based catalysts were developed and their activity studied in partial oxidation of methane to synthesis gas. Two section reactor was build and kinetic of oxidation was studied. Development of SHS catalyst for partial oxidation of methane to methanol. Optimization of catalyst composition and conditions of process and study influence of these parameters on the yield of product were studied. Development and study of SHS catalysts for deep methane and CO oxidation. Those processes are very important in industry as it is directly applicable to the purification of exhaust gases from a large spectrum of industries, vehicles, catalytic heat generators etc. The efficiency of catalytic oxidation in exhaust gases can be enhanced if finely comminuted catalysts are dispersed on carriers with very high surface area in contact with the gas flow. The SHS materials studied include various compositions based on the systems Al-Mn-Mg-O, Mg-Cr-O, Mg-Al-O, Mg-Cr-Al-O and Cu-Cr-O with and without the addition of Cerium oxide and an epoxide additive as catalysts and as catalysts on SHS carriers. It is evident from the results that the most active catalysts for this process are those containing chromium and especially manganese (100% conversion of methane) and Cu-Cr-O catalysts for CO oxidation (they are as active as catalysts on the base of noble metals.
• Development (for the first time) of SHS carriers of honeycomb structure by extrusion with further SHS. Honeycomb-shaped blocks were manufactured by extrusion of the green powders and in-situ SHS and were found to satisfy most of the carrier requirements for mechanical properties, surface and to have good thermal stability.Influence of number of parameters were studied for optimisation of extrusion process, drying and SHS method parameters for receiving good quality carriers.
• For the first time in the world the SHS pigments production technology developed and color regulation mechanism determined. The above method for predicting and "tuning" the functionality of various spinels made by SHS is also applicable for spinel pigments. By the addition of appropriate dopants, it is possible to produce almost any colour and hue of the spinel-based pigments, as confirmed from the results of many years' work on colour formation in SHS materials [7]. For the first time color formation mechanism in SHS pigments determined. It was found that the major factors responsible for formation of colour in specific spinel pigments, were the type of ion (Co, Ni, Mn, Cr, Fe, Cd, Cu etc.), atomic size and structure of the dopant and the lattice position in which it enters, which decides the size and type of lattice distortion. The ionic radius of the dopant metal ion was found to be correlated to the wavelength of the reflected light from each pigment, i.e. the colour of the pigment. For example, in the system Co-M-O (where M is the metal ion), the wavelength of the reflected light was found to decrease monotonically with increasing ionic radius of dopant metal ion. It is expected that the above predictive process would be applicable to all types of functional spinels produced by SHS and SCS with ordered structure and composition. The spinel structure is fairy empty and flexible in the accommodation of non-stoichiometry, which is characterized by an excess of cations, located both tetrahedral and octahedral sites. Thus, cations in the spinel structure can be substituted by many other cations to give multicomponent system, whose properties can be regulated by proper selection of the cations, appropriate adjustment of composition and structure, lattice parameters. For the first time were synthesised SHS pigments at about 1000 hues of colors. Studied their characteristics as pigments and characteristics of plastics, paints, ceramic, porceline,
cosmetics with those pigments. All characteristics were also checked in different plants. Full animal positive tests (2 years animals study) were done for SHS pigments application in cosmetic. Catalogue of 1000 pigments hues was prepared. Catalogues of tiles with SHS pigmens and glaze (tiles and glaze of each plant we work with) were also made for each plant. It was found that those pigments can be used in industry (documents-tests from ceramic, plastic, oil and emulsion paint, porcelain plants from Russia and Kazakhstan were received). Production of these pigments started in 1991 in the industrial scale.
• Basic theoretical studies of interaction of SHS pigments with glazes. Investigating of physic-chemical properties of glazes of different composition with SHS pigments on the base of vanadium system. Segregation phenomenon during combustion in this layer discovered. It was found that intermediate compositions of SHS and non-equilibrium conditions of crystallization produce mable effect in thing layers. Developed new low temperature melted glazes for high speed production of ceramic tiles [8]. Those glazes used in Uralsk plant.
• Recycling and exploitation of solid industrial wastes and rare materials by SHS - production of ceramic products. Industrial wasted of metallurgical, power plants, chemical plants production of Kazakhstan were studied and used for production ceramic tiles, wollastonite pigments and catalysts. For example, chromate concentrate was used for pyrolysis of diesel fuel. The elaborated SHS method of immobilization of radioactive and chemically dangerous components spread in grounds was developed [9].
• Refractories-TPSfor Tupolev planes. Refractories on the base of Mg-Al-O spinel + MgO were developed according to Tupolev construction buro requerements and produced 200 tiles.
• Thermal protection systems based on SHS ceramics for the "Buran Space Shuttle". the development of heat-insulation shild of high-temperature heating block for heating to temperatures 1900-2000°C was performed. At about 300 tiles with required characteristics (working temperature 2500°C, with density 0.5 g/cm3) were developed produced by SHS for "Buran" shuttle (USSR period) and placed on it, reactive bonding of high temperature ceramic (> 2500oC) also was studied [10].
1. G. Gladoun-Xanthopoulou, V. Sergienko, G. Ksandopulo, The combustion wave structure of SHS-systems based on the Compounds of iron and manganese oxides, Int. J. Self-Propag. High-Temp. Synth., 1997, vol. 6, no. 4, pp. 399-404
2. G. Xanthopoulou, I. Orynbekova, G.I. Ksandopoulo, E. Grigorian, A. Merzhanov, I. Borovinskaya, M. Nersesian, Reception method of catalysts for the synthesis of organic substances, Patent N 1729028 (USSR), 1991.
3. G. Gladoun-Xanthopoulou, Self-propagation high-temperature synthesis of catalysts and supports, Int. J. Self-Propag. High-Temp. Synth., 1994, vol. 3, no. 1, pp. 51-58.
4. G.I. Ksandopulo, G. Gladoun-Xanthopoulou, Z. Leiman, N. Yakubova, N. Sebryaeva, Batch for the catalysts reception for synthesis of ethylene, Patent N1805578 (USSR), 1992.
5. G. Xanthopoulou, Oxidative dehydrodimerization of methane using lead and samarium based catalysts made by self-propagating high-temperature synthesis, Appl. Catal. A General, 1999, vol. 185, pp. L185-L192.
6. G. Xanthopoulou, Oxide catalysts for pyrolysis of diesel fuel made by self-propagating high-temperature synthesis. Part I: cobalt-modified Mg-Al spinel catalysts, Appl. Catal. A General, 1999, vol. 182, no. 2, pp. 285-295.
7. G. Xanthopoulou, SHS pigments, Amer. Cer. Soc Bull., 1998, vol. 77, pp. 87-96.
8. G. Gladoun-Xanthopoulou, T. Chernoglazova, G. Ksandopulo, Interaction of SHS-pigments with glazes in films on the carrier, Int. J. Self-Propag. High-Temp. Synth., 1997, vol. 6, no. 1, pp. 71-76.
9. G. Gladoun-Xanthopoulou, V.D. Gladoun, L. Astapenkova, Investigation of chromate concentrates as resources for high temperature synthesis of catalysts, J. Compl. Utilisation Mineral Resour., Alma-Ata, 1990, no. 10, pp. 49-53.
10. G. Gladoun-Xanthopoulou, E. Baymouhamedov, A. Sherinhanov, SHS high-temperature light refractories, J. Eng. Phys. Thermophys., 1993, vol. 65, no. 4, pp. 1024-1025.
