Thermal stability of bulk and silica supported chromium trioxide

Abstract

Thermal behavior of bulk CrO 3 and supported CrO 3/SiO 2 catalysts on heating up to 1000 °C in inert (Ar), oxidative (air, O 2) and reductive (H 2, CO) atmospheres was investigated by thermogravimetry (TG), differential thermal analysis (DTA) and mass spectroscopy (MS, SIMS). A two or three stage reduction pathway of unsupported CrO 3 is or may be accompanied by simultaneous decomposition of intermediate CrO x phases (Cr 3O 8 and Cr 5O 12), which are eventually transformed into crystalline Cr 2O 3 at about 500 °C. Entirely different behavior of supported CrO 3/SiO 2 is observed especially for low chromium content (up to 3% Cr). Oxidation at 500 °C leads to catalyst surface covered with thermally stable chromate-like species CrO 4 2−. These highly dispersed Cr-oxo species are anchored to silica substrate in the form of chromate esters. The increase in Cr content leads gradually to di- or polychromate species, amorphous and finally crystalline α-chromia phases. For low loaded CrO x /silica reversible interconversions Cr(VI) ⇔ Cr(III) and Cr(VI) ⇔ Cr(II) seem to be accepted when hydrogen or carbon monoxide are used as the reducing agents. In the case of CO-reduced sample the reactivity of Cr(II) leads to Cr(III) formation as a result of interaction with water or hydroxyl groups. Catalytically active Cr(II) and Cr(III) sites are involved in water gas shift reaction above 500 °C.