Spectroscopic studies of Cr-doped silica gels

Hexavalent chromium was introduced into silica gel monoliths prepared via sols obtained from tetra ethyl orthosilicate and metal salts at a different final sol pH. Gels were heat-treated at temperatures to 1000 degrees C. The UV-visible spectra of the 60 degrees C dried gels indicated a change in the chromium species from Cr3+ to a combination of Cr3+ and Cr6+ With increase in the final sol pH. With increase in the temperature of heat treatment, the conversion C3+ --> C6+ was promoted. Electronic paramagnetic resonance studies showed Cr5+ in increasing concentration with increase in the final sol pH and heat treatment temperature. The results indicated that, depending on the final sol pH value and heat treatment temperature, the chromium ions were incorporated as Cr3+, Cr5+ and Cr6+ in the gel and densified gel matrix.