Surface Areas by Low Temperature Adsorption Isotherms

Measurement of Surface Areas BY Low Temperature Adsorption Isotherms 

Physical adsorption has long been known and in a general way has been recognized as a means of obtaining some idea as to the relative surface areas of solid materials. For example, Russell and Taylor (5) compared promoted and unpromoted nickel catalysts by noting the relative amounts of COs adsorbed by them at — 78°C. However, physical adsorption, like chemical adsorption, to be effective in measuring surface areas must be used in such a way as to enable one to be sure that he can determine the amount of gas required to form a complete monolayer on the sol id adsorbent. 

Only within the last 15 years has serious attention been directed toward ascertaining ways and means of selecting on physical adsorption isotherms the point corresponding to the amount of gas required to form a monolayer on catalysts or other adsorbents. 

Emmett, 28, 33) that added to the conviction that the point B method could be used for these gases to measure the surface afeas of contact catalysts provided the measurements were made at temperatures close to the boiling points of the respective adsorbates. However, similar adsorption experiments with butane soon showed that for many isotherms the point B method was inapplicable. Such a curve for butane is shown in Fig. 2, together with isotherms of other gases. Clearly it would be nearly impossible to select a point B on an isotherm that is so nearly a straight line through the origin as is the butane curve. 

In view of the apparent failure of the point B method to cover all adsorbates and adsorbents, an attempt was made to develop a theory for the multilayer adsorption that might enable one to select the point on 

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