Physicochemical investigation of calcined chromia-coated silica and alumina catalysts: Characterization of chromium-oxygen species

Abstract

Chromia catalysts were prepared by coating silica and alumina supports by the pentahydrated Cr 3+ gel at various levels (0.53 – 10.15 wt% Cr 2O 3). Physicochemical characterization of these materials in their calcined states was carried out by thermoanalytical techniques (DTA, TGA), X-ray diffraction, surface excess charge determination, infrared absorption and Raman spectroscopy, and by diffuse reflectance spectroscopy, and their catalytic activity for hydrogen peroxide decomposition was measured. Unsupported chromia was also studied for reference purposes. On silica, polychromates, chromium chromates and α-Cr 2O 3 are detected. The α-Cr 2O 3 content increases with increasing coating, and the relative abundance of chromium chromates is simultaneously enhanced at the expense of the polychromates. In contrast, chromate species are formed on alumina at low coating levels (<5.20 wt% Cr 2O 3), while α-Cr 2O 3 is additionally formed with increasing coating. Evidence is provided for the existence of coupled Cr 3+–Cr 6+ species and it is suggested that these are responsible for the catalytic activity towards H 2O 2 decomposition.