Localization Of V2o5 Phase In Silica Gels
LOCALIZATION OF V2O5 PHASE IN SILICA GELS
M. S. Mel’gunov1, V. B. Fenelonov1, R. Leboda2, B. Charmas2
1Boreskov Institute of Catalysis,
Prospekt Akad. Lavrentieva 5, 630090, Novosibirsk,
Russian Federation, e-mail: max@catalysis.nsk.su
2Faculty of Chemistry, Maria Curie-Skladowska University,
20031, Lublin, Poland
Recently, а new textural approach to study localization of carbon in various porous supports was offered [1-4]. The considered approach is based on study of unspecific adsorption interactions of different fluids with all components of tested composite porous solids. These studies include measuring of both the specific surface area of composite material, ACM and the specific accessible surface area AC of guest component (carbon) in this material. Knowing these values of ACM and AC as well as the specific surface area A0 of the initial support and the amount of guest component deposited, Z, one can calculate the number of empirical parameters, which values carry the important information about guest component textural properties. It was shown by means of this approach that both organization of initial support porous space as well as mobility of carbon precursors over support surface significantly influence on carbon localization. Restricted mobility of carbon precursors in supports with narrow mesopores as well as not restricted mobility over the smooth surface of the support without narrow mesopores leads to homogeneous distribution of carbon clusters over the surface if the entire support surface is equiaccessible. However, not restricted mobility of carbon precursors in dense packed porous materials seems to be the reason of carbon precursors pulling into and, so, carbon deposition in narrow mesopores. These peculiarities of carbon deposition influence on the carbon / mineral (hydrophobic / hydrophilic) composition of the composite surface. Trapping of carbon in narrow mesopores leads to decrease and, contrary, homogeneous dispersion of carbon over the whole support surface leads to increase in the part of the composite surface, which can be appropriated to carbon.
In this paper the considered approach is used for study of V2O5 guest component in different silica gels. Two sets of V2O5 /SiO2 catalysts with various vanadia loading were extensively studied elsewhere [5,6]. Accordingly to [5,6] the catalysts were prepared by wet impregnation of SiO2 supports (Ketjen Si-6-5P; A0 = 597 m2/g, pore volume, 1.22 cm3/g (Series 1)) and (Ketjen F-5; A0 = 672 m2/g, pore volume, 1.1 cm3/g (Series 2)) with aqueous solutions of ammonium metavanadate. The impregnated samples were air-dried, calcined in air at 500 0C and then reduced in the flow of purified hydrogen at 500 0C for 6 h. The appropriate data on basic textural parameters (ACM, AC, A0 and Z) are presented elsewhere [5,6]. These data was used for calculation of degree of support surface coverage, q, part of V2O5 surface from the whole surface of composite, c, and accessibility of V2O5 surface, l. The results of the calculations are presented on Figure 1. For comparison the similar data for carbon localization in silica gel supports from [4] is also presented on Figure 1.
Figure 1. Textural parameters of vanadia and carbon localization in silica gels.
As one can see the considered textural parameters and so type of vanadia localization in porous space of silica gel is similar to that of carbon. Such similarity is probably related with the feature of deposition general for both vanadia and carbon. At the considered conditions of synthesis (500 0C) vanadia phase should be in liquid and, so, mobile form (vanadia melting point is 417 0C). As in the case of carbon, vanadia is possibly pulled and deposited in silica gel mesopores. Some deviations in particular values of textural parameters for vanadia and carbon are discussed in the paper.
References
1. Fenelonov,V.B. J. Porous Matter. 2, (1996) 263.
2. Fenelonov,V.B., Procudina,N.A. and Okkel’,L.G. J. Porous Matter. 3, (1996) 23.
3. Mel’gunov,M.S., Fenelonov,V.B., Leboda,R and Charmas, B. Carbon (2000) in press.
4. Mel’gunov,M.S., Fenelonov,V.B., Gorodetskaya,T.A., Leboda,R and Charmas, B. J. Colloid&Interface Sci. (2000) in press.
5. Reddy,B.M., Narsimha,K., Rao, P.K. and Mastikhin,V.M., J. Catal. 118, (1989) 22.
6. Nag,N.K., Chary,K.V.R., Rao,B.R. and Subrahmanyam,V.S., Appl. Catal. 31, (1984) 73.
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