Journal of Non-Crystalline Solids 273 (2000) 36±40
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Chromium-containing silica materials R.H.M. Godoi a, M. Jafelicci Jr. a,*, M.R. Davolos a, L. Fernandes a, M.A.U. Martines a, J. Portillo b a
S~ ao Paulo State University ± Chemistry Institute, Campus of Araraquara, Araraquara, Brazil b Seveis Cientõ®co Tecnics, University of Barcelona, Barcelona, Spain
Abstract Chromium-containing silica samples were obtained from soluble sodium silicate solutions in the presence of dierent chromium nitrate concentrations. Precipitation was carried out in ethanolic media. Gel precipitate was dried by liophylization and samples measured by transmission electron microscopy (TEM), X-ray energy-dispersive spectrometer, X-ray mapping, X-ray photoemission spectroscopy (XPS), and particle size analysis. Spherical chromium containing silica particles with 3.5% and 4.8% (at.%) of chromium were obtained. Particle size analysis results showed that with increased addition of chromium in sodium silicate solutions produces agglomerates whose sizes range from 1 to 0.2 lm. Chromium mapping and XPS results show that chromium oxide is preferentially segregated on particle surfaces. Chromium oxide was detected on particle surface with a binding energy of 576:77 0:05 eV as obtained from XPS analysis. During the hydrolysis and condensation processes chromium oxide precipitates on the silica surface and it aects the silica chain size. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Sol±gel method is studied both as the method of producing gels and glasses in the form of bulk, ®lm, and ®ber, as well as colloidal particles as precursors of glasses and ceramics [1±4]. These materials are currently attracting attention because of their potential applications in catalysis [5± 7], ceramic processing, and optics [8,9]. Sol±gel route can be manipulated to form oxide materials with dierent properties by controlling hydrolysis± * Corresponding author. Address: Universidade Estadual Paulista, Instituto de Quimica, Rua Professor Francisco Degni, s/no, C.P. 355 14801-970, Araraquara ± SP, Brazil. Tel.: +55-16 201 6651; fax: +55-16 222 7932. E-mail addresses:
[email protected] (R.H.M. Godoi),
[email protected] (M. Jafelicci Jr.).
condensation reaction of the precursor that produces, successively, dimers, oligomers, polymers, and gels [10,11] and/or by adding other metal elements to the system. Catalytic properties of chromium oxide based systems are due to surface chromium oxide species on surfaces of inert particles instead of chromium oxides, such as crystalline CrO3 or Cr2 O3 [5,6]. This knowledge has led to interest in the structure of supported chromium oxide species, and the factors, which determine the chromium oxide surface structure [5]. This work describes a procedure to prepare spherical silica± chromium particles from soluble sodium silicate solutions in presence of dierent chromium nitrate concentrations and it shows the eect of the chromium incorporation in silica. Transmission electron microscopy (TEM), X-ray energy-dispersive spectra, X-ray mapping, X-ray photoemission
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 1 2 3 - X
R.H.M. Godoi et al. / Journal of Non-Crystalline Solids 273 (2000) 36±40
spectra (XPS) and particle size analysis of particles were measured to determine chromium properties on the surface of silica. 2. Experimental Soluble sodium silicate solution was prepared by diluting sodium silicate [12] in deionized water at proportion of 1:20. Chromium nitrate was dissolved in ethanol in dierent percentages, 1, 3, 5, and 7 mol% of Cr2 O3 . Aqueous sodium silicate solution was added to chromium ethanolic solution causing geli®cation. Gel dialysis was made by using water to remove any soluble silicate and impurities and dried in a vacuum dissicator. Samples were identi®ed according to the chromium at.% as X0 , X1 , X3 , X5 , and X7 . The samples were investigated by TEM (Hitachi H-800-MT), X-ray energy-dispersive spectra (Kevex 8000 analytical system, model Quantum), X-ray mapping (H-810 STEM), XRS (Physical Eletronics 5500), and particle size analysis (Leeds & Northrup SRA150). 3. Results We observed a sudden turbidity in the system in which particle nucleation and growth take place in less than 1 s when aqueous sodium silicate solution is added to ethanolic chromium media. The average particle sizes were determined from particle size analysis and are listed in Table 1. Fig. 1 shows the silica±chromium XPS spectra for samples obtained from dierent chromium percentages. XPS spectra were taken with exciter radiation AlKa1 1486:6 eV, 300 W. Binding energy (b.e.) was measured with reference to the C 1s peak with b.e. of about 284.8 eV and the Si 2p and Cr 2p3=2 peaks with b.e. of about 103.3 and 576.84 eV, respectively. The measurement preciTable 1 Average particle sizes of silica±chromium samples Sample
X0
X1
X3
X5
X7
Size (lm)
2.045
1.61
1.54
1.33
1.50
37
sion for binding energy of the skeleton levels was 0.2 eV. XPS spectra show that the chromium was detected in the sample X5 and X7 . The chromium concentrations on solid samples of X5 and X7 were 3.5 and 4.8 at.%, respectively, and the stoichiometric relation of silicon±oxygen was maintained. We detected that sodium is present in the silica± chromium sample. TEM results in Fig. 2 show spherical particles of silica±chromium. The samples were also examined combining TEM with X-ray energy-dispersive spectrometer, and X-ray mapping. Fig. 3 shows the spectra of silica±chromium system by two representative samples and Fig. 4 shows the mapping of chromium and silicon in sample X5 . 4. Discussion Silica precipitation from soluble sodium silicate solution is used for preparation of silica ®ne particles [10,13]. This process is based on hydrolysis and condensation reactions that occurs in metal solution. Spherical silica±chromium was obtained from ethanolic solution containing chromium nitrate and sodium silicate. Particle size changed as a function of the chromium concentration. Hydrolysis and condensation reactions depend on pH, temperature, solvent, and concentration. Controlled sample preparation was carried out in different chromium concentrations. Both silicon and chromium species hydrolysis takes place, as well as the silicic acid polymerization and chromium oxy± aquacomplexes condensation precursors produce metal±oxygen inorganic polymers. However, chromium oxycomplexes species condensation [11] stops silica polymerization once such chromium complexes at the end of silica chain stop growing at the tetramer species because their polycondensation chemical reactivities. Chromium oxide was preferentially detected on silica surface and silica particle sizes decrease in presence of chromium. Chromium, silicon, oxygen, sodium, and carbon were detected by photoelectron spectra of silica±chromium samples. Samples with 1% and 3% of chromium do not show remanescent chromium [14]. This absence was also observed by
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R.H.M. Godoi et al. / Journal of Non-Crystalline Solids 273 (2000) 36±40
Mikolaichuk et al. [14] in their investigation of samples with small amounts of chromium. In this work, we found chromium oxide islands on silica samples obtained from solutions containing 5% and 7% of chromium. For larger chromium concentration solutions, the chromium (III) hydroxide
complex reacts with silanol groups on silica surface, and it forms Si±O±Cr bonds. At low chromium concentration (
