ИСТИНА

Mesoporous SiO2 is a porous material that can be applied as a support for catalysts due to its large surface area (up to 1000 m2/g) and low reactivity. Among the catalysts of Ox/Red processes (e.g. oxidation of CO and reduction of the nitrogen oxides) the system CexZr1-xO2 attracts high attention because of its high oxygen capacity and great oxygen conductivity that can be connected with reversible transition Се3+ ↔ Се4+. But the existing methods of synthesis do not permit to obtain complex oxides CexZr1-xO2 with large surface area. So it is very interesting to synthesize complex system CexZr1-xO2-SiO2 which can combine properties both SiO2 and CexZr1-xO2. There is a description of synthesis of such materials but the methods proposed in literature are very complicated [1]. In this work we propose a new method of synthesis of complex oxides CexZr1-xO2 – SiO2 using beta-cyclodextrin. Beta-cyclodextrin is a macrocyclic oligosugar composed of seven glucosidic units. The important property of beta-cyclodextrin is its ability to form column structures in aqueous solution. Due to this property this compound can be used as template for the preparation of porous oxide materials. The efficiency of beta-cyclodextrin using was demonstrated earlier on the example of preparation of mesoporous silica [2]. In this work we investigated the influence of the initial compounds on the properties of complex oxides CexZr1-xO2 – SiO2. The synthesis was carried out by the following way. First, tetraethoxysilane (TEOS) was hydrolyzed in the presence of 0.1 M HCl (molar ratio TEOS:HCl = 100:1) at 25 C for 2 h with stirring. Then the aqueous solution containing beta-cyclodextrin, urea and the precursors for CexZr1-xO2 was added dropwise with vigorous stirring to the solution of hydrolyzed TEOS. Cerium and zirconium nitrates and chlorides were used as precursors. Urea was used in order to increase the solubility of beta-cyclodextrin. After stirring for 1 h the colloid solution was kept at 25 C for 2 weeks which allows the gel to form and age. All samples were dried at 120 C for 24 h and then annealed at 600 ºC for 2 h. The oxide samples obtained were investigated by XRD analysis. Also measuring of their specific area was carried out by BET-method. Thermal decomposition of gels was investigated by thermogravimetric and differential thermal analysis. The temperatures of the end of gels decomposition and cerium and zirconium precursors decomposition are presented in Table 1. It can be seen that gels prepared using cerium nitrate have the lowest decomposition temperatures which are lower than the temperature of cerium nitrate decomposition. But in these cases the decomposition proceeds very fast and has almost explosive character. At the same time gel obtained using cerium and zirconium chlorides has the highest decomposition temperature which is higher than the decomposition temperatures of both cerium and zirconium chlorides. It can be suggested that the change of decomposition temperatures takes place because of mutual influence of cerium and zirconium precursors and beta-cyclodextrin and urea. Thus, in our work we proposes a new method of synthesis of complex oxides CexZr1-xO2 – SiO2. This method is simple and permits to obtain mesoporous samples with high specific surface area. References (1) R. Bacani, T.S. Martins M.C.A. Fantini, D.G. Lamas. Structural studies of mesoporous ZrO2 - CeO2 and ZrO2 - CeO2/SiO2 mixed oxides for catalytical applications. Journal of Alloys and Compounds. 2016, 671, 396–402. (2) Jin-Yu Zheng, Jie-Bin Pang, Kun-Yuan Qiu, Yen Wei. Synthesis of mesoporous silica materials via nonsurfactant templated sol-gel route by using mixture of organic compounds as template. Journal of Sol-Gel Science and Technology. 2002, 24, 81-88.