Bulletin of the Chemical Society of Japan
Online ISSN : 1348-0634
Print ISSN : 0009-2673
ISSN-L : 0009-2673
A Study of Zinc Oxide-Chromium Oxide Catalysts. VI. The Changes Brought about by Heat Treatment in the Activity for Methanol Synthesis and in the Structure of the Raw Catalysts
Hiroshi UchidaMasaaki ObaMichio Araki
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1965 Volume 38 Issue 11 Pages 1993-2002

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Abstract

The activity for methanol synthesis has been determined for two series of catalysts prepared from zinc oxide and chromic acid with and without subsequent heat treatment in the air. Also, the correlations between catalytic activity and other catalyst properties have been studied. Each catalyst series has consisted of four catalysts, three of them prepared from slurries of zinc oxide powder in aqueous chromic acid, and the other prepared directly from zinc oxide powder. The chromium/zinc ratios in the catalysts made from the slurries are 0.1, 0.5, and 1.0. The catalysts of one series have been heated in the air at 400°C after their preparation (the “heated series”).
In both series, the activity and the specific activity have been found to be influenced by the chromium/zinc ratio. The catalysts not heated in the air after their preparation (the “unheated series”) show higher activities than the corresponding catalysts of the heated series, though the former catalysts have much smaller surface areas.
The results of electron microscopic examination show that the two series of catalysts, after having been used for methanol synthesis, are structually different. The structural difference has been ascribed to the difference in the way of the decomposition of the layer-structure of zinc hydroxychromate. In the case of the unheated series, the layer-structure is decomposed into crystallites of zinc oxide and zinc chromite by reduction in the synthesis gas, whereas in the case of the heated series the layer-structure is decomposed by heat treatment in the air.
X-Ray diffraction data show that, in all cases, the lattice constant for the zinc oxide in catalysts is normal, whereas the lattice constant for the zinc chromite is larger than the normal value.
The average particle diameter as calculated from the surface area and density of catalysts does not agree with the size as estimated by electron microscopic observations.
The difference in specific activity between the two series may be due to the difference in the degree of the mutual penetration of zinc oxide and zinc chromite crystallites rather than to the variation in the zinc chromite lattice constant.
In addition, the effects upon the activity and upon the catalyst structure of catalyst treatment inhydrogen at 500°C after methanol synthesis have been discussed.

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