Magnetic and optical studies of chromium oxides. Part 1.—Calcination of chromium trioxide supported on alumina

Abstract

Magnetic and optical studies were made on chromium trioxide supported at various concentrations on a porous alumina, the impregnated samples having been calcined at different temperatures over the range 50-1 150°C. The measurements of magnetic susceptibility and electron paramagnetic resonance spectra were carried out at controlled temperatures from -196 to 300°C. Optical reflectance spectra were obtained at room temperature on selected samples. The results suggest that clustering of the chromium oxide occurs, even at the lowest concentration studied (1.8 wt % Cr) ; heat treatment in air, or reaction with hydrogen, leads to the formation of Cr3+ ions, which are coupled by strong exchange interactions. Both processes of reduction occur more readily as the concentration of chromium oxide is increased.

Non-isothermal kinetics of CrO3 decomposition pathways in air

“…The redox activity of CrOx based catalysts towards H202 [10] and isopropanol [11] decomposition reactions has been found to be closely related to the exposure on the surface of intimately coupled Cr ions in different oxidation states. Such an intimate coupling has been suggested [8,12,13] to facilitate d-d electron exchange interactions and, consequently, the electron mobility across the surface. Systematic magnetic and optical spectroscopic measurements [8] allowed monitoring of the formation of such mobile-electron phases (denoted Zener phases [12]) on chromia.…”

“…These are CrO2.66, at 523-593 K, CrOz5, at 593-623 K, and CrO2.z~, at 658-703 K. X-ray diffractometry revealed [ The XRD-verified compositions for the intermediates CrO~ are indicative of composite material bulks containing mixed valency (in the range from Cr 6 § to Cr 3+) chromium-oxygen species [2,7]. Such materials are anticipated to exhibit potential surface catalysis in redox processes [8,9]. The redox activity of CrOx based catalysts towards H202 [10] and isopropanol [11] decomposition reactions has been found to be closely related to the exposure on the surface of intimately coupled Cr ions in different oxidation states.…”

“…Systematic magnetic and optical spectroscopic measurements [8] allowed monitoring of the formation of such mobile-electron phases (denoted Zener phases [12]) on chromia. It has even been claimed [8] that the availability of these phases creates the ideal environment in which not only the redox activity, but also the polymerization activity is optimized.…”

Low-temperature adsorption of oxygen on calcined chromia: IR spectroscopic and sorptiometric evidence for oxygen-assisted topochemical reduction of surface chromate species

“…These species are monomeric and/or polymeric chromates [(CrO 4 ) 2− and/or (Cr 2+x O 7+3x ) 2− ] [5,6] anchored onto surfaces of the crystalline or noncrystalline chromia phase or onto surfaces of the support material [7]. When allowed to interact electronically with nearby Cr(III)-O species [6,7], the chromates are stabilised with respect to thermal decomposition, hydrolysis or chemical reduction [3,6,8]. The availability of electronically-coupled Cr(III)-Cr(VI) species nevertheless provides the electron-mobile environment required for surface redox reactions [1,2,9].…”

“…The availability of electronically-coupled Cr(III)-Cr(VI) species nevertheless provides the electron-mobile environment required for surface redox reactions [1,2,9]. Whereas, localised adsorption occurs on coordinatively-unsaturated Cr(III) sites, the electron availability occurs through the interaction with nearby Cr(VI) ions [7,10,11]. Accordingly, calcined chromia catalysts have shown potential in a range of important redox reactions, such as CO oxidation [11], dehydrogenation of alcohols [1], and H 2 O 2 decomposition [9].…”