Evidence from the Earlier Literature

Special attention should be directed to the horizontal lines for 273° K. indicating immediate coverage and coverage after long contact. These are different. This arises firstly from the magnitude of the activation energies. 

Iron Synthetic Ammonia Catalysts Brunauer and Emmett (10) in their careful studies of iron catalysts had already observed (see especially Tables IV, X and XIII of the paper cited) the phenomenon of increasing adsorption with decreasing temperature that we have indicated as occurring in our studies, as recorded in Fig. 8, on other preparations of synthetic ammonia catalysts. 

In a study of the effect of gas pressure on the chemisorption of hydrogen on chromium oxide gel, Taylor and Burwell (6f) found it necessary to resort to an arbitrary process of subtraction from experimentally observed amounts of adsorbed gas in order to display a relatively uniform area of gel for hydrogen chemisorption. Within close limits, at temperatures of 457 and 491° K. and at pressures of 1, 0.5 and 0.25 atmospheres, the energy of activation for the adsorption process was nearly independent of the amounts of adsorption (0-35 cc.) and the activation energy was about 21 kcal. In the temperature range 383 to 457° K. no such uniformity was observed. For example, if the amounts adsorbed with time at 405 and 427° K. be used to calculate activation energies the values received increase from 0 to 18.5 kcal. as the amount of gas adsorbed on the surface increases from 1.6 to 8.5 cc. Similar results can be drawn from the earlier measurements of Kohlschutter (17). 

As Burwell and Taylor observed, the data as a whole appear to reveal (a) at higher temperatures, an activated adsorption with an activation energy and a pressure effect nearly independent of the amount of hydrogen adsorbed and (b) at somewhat lower temperatures, an activation energy and a pressure effect varying markedly with the amount adsorbed. 

They go on to suggest that the amounts of gas which we have been led to subtract from the observed data in the lower temperature range, 383.5-457° K., represent activated adsorption occurring with loweractiva-tion energies than that characterizing substantially all the adsorption above 457° K. Since the amounts subtracted (see Table I) diminish with increas- 

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