Physicochemical and catalytic properties of the system chromium oxides-oxygen-water

Abstract

This paper describes investigations of various chemical, electrical, and catalytic properties of materials, obtained by calcination of chromic oxide gel. Samples of four different preparations of chromic oxide gel, whereof one contained additions of magnesium, were annealed for 5 hr in air at various temperatures between 100 ° and 700 °. As shown by differential thermal, thermogravimetric, and X-ray analyses, dehydration first results in the formation of X-ray-amorphous monohydrate, which is stable over a wide range of temperatures between 280–360 °. Simultaneously with dehydration the oxidation of the material takes place. The oxidation increases with increasing temperature of annealing. The O Cr ratio, corresponding to maximal oxidation at 350 °, amounts to about 2. It has been shown by applying various methods of chemical analysis that the excess charges are present in such material in the form of hexavalent chromium, the proper formula being thus Cr 2O 3 · CrO 3 and not CrO 2. At 400 ° crystallization of Cr 2O 3 occurs, the oxide being at first considerably oxidized on the surface. This oxidation decreases on annealing at still higher temperatures. Addition of magnesium has no influence on the oxidation state of the amorphous material, but greatly increases the amount of excess charges in crystalline Cr 2O 3. Measurements of the electrical conductivity and thermoelectric power indicate that the conduction mechanism is similar in the amorphous material and in the crystalline oxide. Changes of electrical conductivity and of its activation energy are related to the changes of concentration of Cr 6+ ions. The catalytic activity in H 2O 2 decomposition has been determined and specific rate constants, k, frequency factors k 0, and activation energies, E, of the reaction have been computed. The reaction exhibits a compensation effect, which hints at the changes of energy states of active centers with the temperature of annealing. This is consistent with the results of electrical conductivity measurements, since changes of activation energy of electrical conductivity are parallel to changes of activation energy of the reaction. The experiments reveal a correlation between rate constants, k, and surface concentration of Cr 6+ ions, whereas discussion of the reaction mechanism shows that frequency factors k 0 rather than rate constants k should be related to this concentration.