The structure and activity of chromium oxide catalysts I. Structure analysis
1972; Elsevier BV; Volume: 25; Issue: 3 Linguagem: Inglês
10.1016/0021-9517(72)90239-4
ISSN1090-2694
Autores Tópico(s)Thermal and Kinetic Analysis
ResumoTwo forms of activated chromia catalysts are recognized: (1) an amorphous form, catalyst A, prepared by heating under vacuum or in an inert atmosphere, and (2) a microcrystalline form, catalyst C, prepared by heating in a reducing atmosphere. The structural properties of these catalysts have been studied by X-ray line-broadening, radial distribution analysis, electron microscopy, infrared spectroscopy, and low temperature nitrogen adsorption. Catalyst C, prepared by heating in hydrogen, is composed of crystallites of α-chromium oxide supported on amorphous chromia. Detailed analysis of the X-ray diffraction powder pattern indicates little contribution to the linewidth from strain. The shape of the crystallites (characterized by X-ray line-broadening of the (104), (110), and (116) diffraction peaks) is oblong. The dimension along the α-axes is roughly twice that along the c-axis. The electron micrographs are consistent with this picture. Attempts to prepare crystallites with dimensions between 15 and 70 Å were unsuccessful. Typically a catalyst activated at 400 °C has an effective crystallite size of 80 × 90 Å while a 450 °C activated catalyst has an effective crystallite size of 110 × 200 Å. The percentage crystalline phase estimated from infrared absorption in the lattice vibration region in catalysts activated for 12 hr at a temperature between 400–450 °C are in the range 5–30%. Catalyst A, prepared by heating in helium, does not give rise to a discernible X-ray diffraction pattern, but does result in a well defined radial distribution curve. Analysis of the radial distribution curve indicates that there exists local order over approximately a 10 Å dimension of the same kind as found in α-Cr2O3. A model for this local order is proposed.
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