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UDC 546.824-31:546.763:661.183.123 DOI: 10.18698/1812-3368-2019-4-124-133
PRODUCTION OF TiO2/Cr2O3 COMPOSITE MATERIAL IN THE SPHERICAL FORM
A.O. Rogacheva roga4eva1015@yandex.ru
O.S. Khalipova chalipova@mail.ru
A.S. Brichkov anton_br@rambler.ru
V.V. Kozik vkozik@mail.ru
National Research Tomsk State University, Tomsk, Russian Federation
Abstract Keywords
Spherical TiO2/Cr2O3 oxides composites were obtained Titanium dioxide, chrome (III) by template method accompanied with sol-gel method. oxide, spherical composites, Ion exchange resins of spherical form (TOKEM-100 and template method, sol-gel method TOKEM-250) were used as an organic polymer matrix. Thermal analysis, X-ray phase analysis and micro-X-ray spectral analysis were used to identify the formation process and compositions of oxides composites. The formation of spherical oxide composite ends at 400 °C and the final products are a mixture of two oxides: &2O3 and TiO2 regardless of the structure of the used template. According SEM data prepared TiO2/Cr2O3 composites have spherical form and the size of sphere found Received 08.11.2018 to be in a range from 300 to 870 |am © Author(s), 2019
The research was support under the state assignment (no. 10.2281.2017/4.6)
Introduction. At current, mixed valence metal oxides used widely in catalysis process [1-4]. Oxide composites based on &2O3 u TiO2 are advanced catalysts in the reactions of deep and partial oxidation of hydrocarbons [5], their dehydrogenation [6], and also possess photocatalytic activity [7-9].
Scientists from all over the world pay close attention to the development of catalytically active oxide composites in spherical form. This form of catalyst granules is technologically attractive for several reasons: spherical granules are characterized by high abrasion resistance; they contribute to catalytic units stable operation with moving and fluid beds, easily moving from the reactor through the regeneration section and back into the moving bed, providing a continuous reaction and catalyst regeneration [10]. At current, these catalysts are manufactured by applying a catalytically active substance on a spherical carrier formed by suspensions moulding into granules, for example, catalysts based on alumina carrier [11-14]. As shown in the research [14], one of the most important
problems of granulation is the production of granules with targeted properties. One of the ways to solve this problem is to use template and sol-gel synthesis methods in spherical catalysts manufacturing. Thus, in works [15, 16], was shown to obtain spherical aglomarates, being the TiO2 layer applied on SiO2 spheres previously prepared with the sol-gel method using the structure-forming agent (cetyl trimethyl ammonium bromide, CTAB) [17].
The actual task for researchers is to find out alternative methods for obtaining spherical composites with targeted properties, the choice of template, the establishment of optimal parameters that contribute to the materials production with given catalytic properties. Using the example of metal oxide composites of YBa2Cu3O7-s and TiO2-SiO2 composites production in works [18-20], it was shown that ion exchange resins could act as a template. Earlier [5, 21], it was indicated that ion exchange resins could be used as a template for the preparation of catalytically active oxide composites TiO2-SiO2/Cr2O3 and TiO2-SiO2/Co3O4 with a spherical shape of aglomerates.
The purpose of the work is to obtain spherical oxide composites of TiO2/Cr2O3 using the method based on sol-gel technology and template synthesis, and to investigate their phase composition and morphology.
Experimental technique. As a template for spherical composites, ionexchange resins with various natures were used: TOKEM-100, TOKEM-250 (manufactured by TOKEM PО LLC, Russian Federation). The ion exchange resin TOKEM-250 is a polymer with a macroporous structure with an acryl-divinylbenzene matrix containing carboxyl functional groups. The components of the TOKEM-100 polymer composition have a gel-like structure and contain styrene-divinylbenzene units with sulfo functional groups.
The preparation of spherical granules was done in several stages, which was proposed in works [22]. The first stage was in placing the templates (cation exchange resins TOKEM-100, TOKEM-250) in a saturated aqueous of chromium(III) nitrate (Cr(NO3)3 • 9H2O, pure 99 %, Yugreaktiv company) for 6 hours in constant agitating conditions on a magnetic stir bar at room temperature. Then the polymer was dried for 2 hours at 60 °C in air. The second stage is the preparation of an aggregative-stable sols according to the method represented in Ref. [23]. To prepare the sol, tetrabutoxytitanium (TBT, extrapurity, Acros, USA, Ti(C4H9O)4 with concentration 0.1 mol/L), distilled water (H2O with concentration 0.4 mol/L) and nitric acid (pure 99%, UralPromPostavka LLC, Russian Federation, HNO3 with concentration 2.5 • 10-3 mol/L, using butyl alcohol as a solvent (pure 99%, "EKOS-1" JSC,
Russian Federation). The process of aging of solution took three days after preparation. Ion exchange resins with sorbed chromium(III) ions were placed into the finished sol for 12 hours. The obtained samples were marked as TBT/Cr3+(100), TBT/Cr3+(250), depending on the ion exchange resin used. Then, spherical granules of TBT/Cr3+(100), TBT/Cr3+(250) were dried for 30 minutes at a temperature of 70 °C in a loss-on-drying oven and annealed in a muffle furnace for 30 min at the temperature of 100, 200, 250, 300, 350 °C and 1 hour at a temperature of 400 °C. After heat treatment, the obtained samples were marked as TiO2/Cr2O3(100) and TiO2/Cr2O3(250) respectively.
The processes of oxide systems formation during temperature treatment of samples TBT/Cr3+(100), TBT/Cr3+(250) were examined by means of thermal analysis. The research was done on a synchronous thermal analyzer STA 449 C Jupiter (Netzsch-Geratebau GmbH, Germany) in the temperature range of 30-900 °C in air. Sample heating rate was 5 °/min. Heating was done in corundum (Al2O3) crucibles.
The phase composition of samples obtained after annealing was determined by X-ray phase analysis (XRD) on a MiniFlex 600 diffractometer (Rigaku, Japan) with CuKa radiation. The shooting was carried out in the range of angles 20 10-80°. Identification of the synthesis products was done according to the international data bank PDF-2.
The surface morphology of the obtained oxide composite materials was examined on a TM-3000 scanning electron microscope (SEM) (Hitachi, Japan) with an accelerating voltage of 15 kV (electron gun 5 ■ 10-2 Pa, camera for the sample 30-50 Pa). X-ray microanalysis was performed on the Shift ED 3000 console using energy dispersive X-ray.
Results and discussion. The most important step in the production of spherical oxide composites by the proposed method based on sol-gel technology and template synthesis is a stepwise temperature treatment of polymer matrices treated with chromium(III) nitrate solution and a tetrabutoxytitanium-based sol. The temperature treatment modes for TBT/Cr3+(100), TBT/Cr3+(250) samples were selected based on the results of the thermal analysis performed: thermogravimetric (TG) and differential scanning calorimetry (DSC) (Fig. 1).
The thermal decomposition of examined samples, despite of the cation exchanger selected as a template, proceeds in three main stages, which is confirmed by the presence of three areas of sample mass alteration on TG curves (Fig. 1, a). This process is accompanied with low endothermic effects (Fig. 1, b),
TG, %
100 3.24 % ""ч- /
90 Л/- и.10%
80 _ 12.87
70 13.83 %\ \
60 - \ / 59.64%
50 - \ 1
40 - \ 1
30 45.59% \1
70 , , , , , , , 1
100
200 300
400 500 a
DSC, |iV/mg
600 700 800 T,°С DSC, цУ/mg
400 500 600 700 800 T, °C b
Fig. 1. Thermal analysis results:
a TG-curves for TBT/Cr3+(100) (1) and TBT/Cr3+(250) (2) samples; b DSC-curves for TBT/Cr3+(100) (1) and TBT/Cr3+(250) (2) samples
which is typical for the process of water desorption from the surface of the samples. Ion exchange resins are wet polymeric materials, their dehydration is complicated by high pore volume, advanced specific surface, content of polar groups and contaminations of metal ions. According to the data of [24, 25], ion-exchange resins, saturated with metal ions, are able to sorb water and retain it in the form of weakly hydrated water due to a weak bond with counter-ions — metal ions. At temperatures of 209.2 and 219.7 °C, small exothermic effects are observed for samples TBT/Cr3+(100) and
TBT/ Cr3+(250) (see Fig. 1, b), which can be associated with chemically bound water desorption in the second stage of their decomposition within the temperature range of 85-250 °C. At the third stage, within the range of temperatures of 250-400 °C, the complete thermal destruction of examined samples happens, proceeding with a large heat release, which is associated with thermal destruction of TBT based sol, and to a greater extent with the destruction of ion exchange resins, oxidation and removal of volatile decomposition products. For the TBT/Cr3+(100) sample, exothermic effects are observed at the temperature of 396.2 °C, and for the TBT/Cr3+(250) sample are observed at the temperature of 318.2 °C (see Fig. 1, b). According to the TG curves (see Fig. 1, a) and the XRD results (Fig. 2), decomposition of the studied samples, regardless of the cationite used structure, is completed at a temperature of approx. 400 °C.
10 20 30 40 50 60 70 20, deg.
Fig. 2. X-ray pattern of samples ТЮ2/Сг20з(100) (1) and ТЮ2/Сг20з(250) (2)
The samples of ТЮ2/Сг20з(100) and ТЮ2/Сг20з(250) obtained after annealing are &2O3 with a corundum structure. The X-ray patterns of the examined samples (see Fig. 2) correspond to the data of the crystal peaks of the PDF-2 database (maps no. 00-038-1479), where a = 4.960 A and с = 13.597 A (20 = 24.40; 33.56; 36.16; 39.67; 41.48; 44.17; 50.24; 54.87; 58.38; 63.42; 65.09; 72.90; 76.74; 79.02). The dimensions of the coherent scattering areas are 16.8 nm (sample Ti02/Cr203(100)) and 18.4 nm (sample Ti02/Cr203(250)). Reflexes corresponding to the Ti02 phase were not indicated on any of X-ray patterns, which could be due to the fact that the amount of titanium dioxide formed is below the detection limit of this method (less than 5 wt. %).
The presence of the titanium phase in the obtained spherical composites is confirmed by the results of the qualitative (Fig. 3) and quantitative micro-X-ray
spectral analysis. The results of quantitative X-ray microanalysis show that the content of the titanium compound in the samples is less than 4 wt. %, which correspond with the XRD data.
keV
keV
Fig. 3. Results of X-ray microanalysis of samples: Ti02/Cr203(100) (a) and Ti02/Cr203(250) (b)
The spectrum of both samples contains emission lines specific for chromium (at 0.5, 5.4, and 5.9 keV), titanium (at 0.39, 4.5, and 4.9 eV) and oxygen (at 0.6 keV). The spectrum of the sample Ti02/Cr203(250) also contains a spectral line at 1.1 keV, specific for sodium. This may be due to the fact that the T0KEM-250 cation exchanger used as an organic matrix was used in sodium form and as a result of sorption, not all sodium ions were replaced
by chromium (III) ions. According to researches of such systems [5, 21, 23] and based on XRD data and micro-X-ray spectral analysis, we assume that the samples obtained are oxides TiO2 and &2O3.
SEM data indicated that TiO2/Cr2O3(100) and TiO2/Cr2O3(250) (Fig. 4, a, c) samples have a spherical form. The size of the spheres of all samples presented lies in the range of values of 300-870 ^m.
2 mm 100 цш
с d
Fig. 4. SEM image of samples: TiO2/Cr2O3(100) (a, b) and TiO2/Cr2O3(250) (c, d)
The surface morphology of the TiO2/Cr2O3(100), TiO2/Cr2O3(250) sample spheres is a relief (Fig. 4, b, d), consisting of herringbone convexes and dimples distributed over the entire surface of a spherical granule. On separate parts of the surface, larger clusters of oxide crystals are formed, closely adjacent to each other.
Conclusion. Spherical shape TiO2/Cr2O3 oxide composites were obtained with the method based on sol-gel technology and template synthesis, using ionexchange resins with various structures as templates. It is found out that staged
heat treatment leads to the decomposition of cation exchangers and synthesis precursors with the formation of oxide composites that retain the spherical shape of the template. Despite the structure of used ion-exchange resin, the formation of Ti02/Cr203 oxide composites is completed at the temperature of 400 °C and obtained samples have a similar surface morphology of spherical granules. The resulting spherical composites Ti02/Cr203(100) and Ti02/Cr203(250) could be used as catalysts in hydrocarbons oxidation.
Translated by K. Zykova
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Rogacheva A.O. — Post-Graduate Student, Department of Inorganic Chemistry, National Research Tomsk State University (Lenina prospekt 36, Tomsk, 634050 Russian Federation).
Khalipova O.S. — Cand. Sc. (Eng.), Researcher, Department of New Materials for the Electrical and Chemical Industry, National Research Tomsk State University (Lenina prospekt 36, Tomsk, 634050 Russian Federation).
Brichkov A.S. — Cand. Sc. (Eng.), Researcher, Department of New Materials for the Electrical and Chemical Industry, National Research Tomsk State University (Lenina prospekt 36, Tomsk, 634050 Russian Federation).
Kozik V.V. — Dr. Sc. (Eng.), Professor, Head of the Department of Inorganic Chemistry, National Research Tomsk State University (Lenina prospekt 36, Tomsk, 634050 Russian Federation).
Please cite this article as:
Rogacheva A.O., Khalipova O.S., Brichkov A.S., et al. Production of TiO2/Cr2O3 composite material in the spherical form. Herald of the Bauman Moscow State Technical University, Series Natural Sciences, 2019, no. 4, pp. 124-133. DOI: 10.18698/1812-3368-2019-4-124-133
