Thermionic Emission from Catalyst for Ammonia Synthesis. I. Electron Microscopic Images of the Catalysts

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Abstract
The electronic images have been observed on 5 types of the catalysts for ammonia synthesis, which are different in promotercomposition. On heating the oxides from 750 to 900°C, the images of different kinds which were characterized by their respective promoters compositions appeared after a few minutes. On reduction, some of the images remained, while the others disappeared, and during the periods of five to thirty minutes of the reduction, the images turned to the respective characteristic ones again, sometimes being quite different from those of the oxides. On the additional observations, a part of the images weakened or disappeared and rather the homogeneous images over the whole sections became to be observed by raising thetemperature. Comparing the image with the optical, the net work of the emission after the reduction corresponds to cracks on the section. When the microscopic structure of the section thoroughly polished and etched with hydrochloric acid is adopted for comparison with the above optical image, grain boundaries in the oxide always correspond to the cracks. Therefore, the electronic images on the oxide are not always related to the grain boundaries. Qualitative explanations on respective promoting effect of alumina, lime and silica are yielded from view point of the electronic image. It has been proved from the observations of the electronic as well as the optical images that there are the following 3 types of regions which are rich in potash, i.e. the grain boundaries, particular interfacial grains and some of the sub-grains into which the parent grains are divided. When references are made to the results of the activity for ammonia synthesis, surface area and etc., together with that by H. Kobayashi who has described that the activity is greatly enhanced by the larger amount of potash insoluble in water, the estimation shows that probably potash existing in the sub-grains above may most contribute to the activity for ammonia synthesis.

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