Carbon Deposit Effect On The Structure Of Complex Oxide Adsorbents Surfaces Of C/tio2/sio2 Type
Carbon Deposit Effect on the Structure of Complex Oxide Adsorbents Surfaces of C/TiO2/SiO2 Type
R. Leboda1, V.M. Gun\’ko2, M. Marciniak1,
W. Grzegorczyk1, J. Skubiszewska-Zięba1
1Faculty of Chemistry, Maria Curie - Skłodowska University,
20-031 Lublin, Poland;
2Institute of Surface Chemistry,
31 Prospect Nauki, 252-022 Kiev, Ukraine
The current adsorbents of X/SiO2 (X = Al2O3, TiO2, ZrO2, GeO2) type can be prepared by means of the following methods: high-temperature hydrolysis (HTH) (fumed adsorbents), chemical vapour deposition (CVD) and sol-gel method.
New kind of complex adsorbents are the materials obtained from carbonization of oxide adsorbents surface with the products of various organic precursors (C/TiO2/SiO2).
The aim of this paper is to compare the surface properties of the adsorbents of C/TiO2/SiO2 type in which the mineral matrix TiO2/SiO2 was obtained by means of the CVD and HTH methods using non-porous fumed silica and porous silica gel (SBET = 377 m2/g). The system used were: TS and TfS –TiO2 deposited on the silica gel and on the fumed silica surface, respectively; fTS – TiO2 deposited on the SiO2 surface by means of the HTH TiCl4 method.
Titanium dioxide was deposited on the silica gel surface at 200 and 600° C using the CVD method, obtaining in this way two series of adsorbents of various phase composition and various TiO2 content. To obtain fumed materials there was applied highly dispersed Aerosil A-175 and A-300 with SBET = 185 and 308 m2/g and with various TiO2 content (1.7 ÷ 36 wt.%).
Carbon deposit on the surface of prepared adsorbents TiO2/SiO2 was obtained using the monitored cyclohexene pyrolysis (973K, 4-5 h) in the stream of He + 5% C6H10. The reaction was carried out in a special reactor with continuos monitoring of sample mass change.
The prepared materials were tested using the following methods: low-temperature (77K) nitrogen adsorption/desorption, IR spectroscopy,
thermogravimetry, and transmission electron microscopy. Moreover, kinetics of cyclohexene pyrolysis, surface fractability and its corresponding models were studied.
As follows from the experimental data and theoretical considerations in the case of fTS adsorbents both TiO2 phase and the interface the carbon deposit distribution on the surface of such materials. However, the TiO2 phase in the TfS sample plays a significant role because the pyrolysis reaction rate depends on both TiO2 content and specific properties of this phase (phase composition, particles topography). This results in differences in pyrocarbon distribution on the particles with different structurization level (initial particles, aggregates and agglomerates) of fumed oxides. Moreover it was stated that the value SBET for the samples of CfS (with low carbon deposit content) is greater than for the initial material contrary to the samples of CS type. Fumed SiO2, fTS, TfS and C/TiO2/SiO2 are characterized by poorly developed porous structure.
The carbon phase is characterized by greater roughness (it is more fractal) than the TiO2 phase i.e. carbon deposit is more porous than the TiO2 deposit. Besides the carbon phase is formed mainly on the outer surfaces of the initial material particles in the form of smaller particles than CVD-TiO2 ones. The phenomenon of formation of large particles using the CVD method characterized by a weak contact with the matrix surface is also observed for CVD-TiO2 synthesized on fumed silica.
As follows from the obtained data it is possible to control distribution and content of the carbon phase as well as surface heterogeneity in the adsorbents C/TiO2/SiO2 prepared using the compared methods. This is extremely useful for obtaining materials of defined parameters.
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