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1 Polish Journal of Chemistry

1 2002

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Characterization of Chromium Oxide Supported on Silica

Jóźwiak W., Ignaczak W., Kincel D., Góralski J., Paryjczak T.

Polish Journal of Chemistry

2002

Vol. 76 / nr 6

857-873

Thermal stability and redox behavior of the surface Cr-oxo structures supported upon silica were investigated by: TPR, TPO, XRD, IR and XPS techniques. During reduction in hydrogen, the surface Cr(VI) oxide phase is completely transformed into the Cr(III) oxidation state. For low loaded Cr/SiO2 samples almost fully reversible interconversion Cr(VI)Cr(III) was found during TPR - TPO cycling up to 900_C. For higher loaded samples, temperature depended oxygen - hydrogen cycling strongly influences the relative composition of chromium oxide phase supported on silica. The coexistence of three major forms of chromium oxide phase in supported CrO3/SiO2 system has been confirmed: monomolecularly dispersed chromates strongly bonded to silica surface, relatively inactive stable crystallites of _-chromia and intermediate metastable amorphous aggregates of CrO3. The relative contribution of each form strongly depends on the chromium content, method of preparation, treatment conditions (temperature, atmosphere and water traces). The anchoring of chromate-like species requires suitable support adsorption sites and their concentration is governed by hydroxyl group population. Mobile hydroxo species of CrO3 may migrate on the silica surface, anchoring to preferable adsorption sites and forming stable chromate-like species. It was also found that water is involved in the hydrolytic process of monochromated species destabilization, resulting in partial transition Cr(VI) _ Cr(III). Thermodynamically favored chromium sintering process, chromates CrO4 2_ _crystalline _-Cr2O3, requires water as destabilization agent for hydrolytic splitting of Cr - O - Si linkages and chromium oxide agglomeration.