The sorption of hydrogen on copper. Part I.—Adsorption and the heat of adsorption

Abstract

A knowledge of the adsorption of gases on solid surfaces and the energy changes resulting is of great value in investigating the structures of surfaces and understanding their catalytic action. Evidence about the energy changes is usually acquired from experimental determinations of the heat of adsorption, but it must be clearly understood that this will only equal the total energy of adsorption if the adsorbed molecule is in the same condition on the surface as it was in the gas phase, which, in general, will not be true. If the molecule is distorted when adsorbed, the energy of adsorption will be the sum of the energy of distortion and the heat liberated. It is probable that an increasing total energy of adsorption will correspond to an increasing heat of adsorption, and that the heat liberated may be taken as a rough indication of the activity of the surface, but it must be wrong to use the two terms as quantitatively equivalent. Lennard-Jones and Dent have calculated the field outside the 100 plane of a crystal of KC1, and estimated the energy of adsorption of argon to be 2000-2500 cals. per gram-molecule. If it were possible to extend this type of theoretical treatment to the more heterogeneous catalytic surfaces where heats of adsorption are known, the amount of distortion of the adsorbed molecules could be readily determined. Much attention has been directed to the variation of the heat of adsorption of successive amounts of gas. For charcoal adsorbents it seems well established that earlier portions of gas produce a greater heat evolution. The evidence as to the initial heats of adsorption with metals is less complete. Foresti found 11,431 cals, per gram-molecule for hydrogen on activated nickel, and for CO on copper Beebe§ found an initial high heat of from 20,000 to 30,000 cals. per gram-molecule which fell after about 2-3 c. c. adsorbed to about 9000 cals. per gram-molecule. Frylingjj with different copper catalysts found heats varying from 8000 to 30,000 cals. per gram-molecule and for smaller amounts of gas adsorbed than gave the initial high heat, he found in some of his promoted catalysts an initial low heat before the region of high heat. Taylor and Kistiakowsky and Kistiakowsky, Flosdorf and Taylor repeated the work and also found a low heat, which they explained by saying that the most active portion of the surface has sufficient energy to distort the hydrogen molecule endothermically.