Published: Jan 1963
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In the present symposium, the surfaces of solids are being discussed from many different points of view. The variety of approaches to the study of surfaces attests to the many divergent aspects of surface chemistry that have become important in modern research and technology. The present paper will attempt to outline some of the many ways in which chemical adsorption on and activity of the surfaces of various types of catalysts may be helpful in building up a picture of the over-all properties of the surfaces of solids.
The adsorption of a gas on a solid may be either physical or chemical in nature. Physical adsorption is being defined and discussed in one of the other papers in this symposium.2 It will therefore suffice to emphasize here that through physical adsorption the surface area (1,2) 3 and roughness factors of surfaces can now be determined as well as the pore size and pore size distribution (3) of cracks and crevices that may occur on the surface of a solid. The way in which such surface area and pore size measurements have become important in catalysis is an interesting story in itself. However, it relates more to the details of catalytic mechanism studies than to the appraisal of the chemical properties of surfaces so it will not be considered here in detail. This portion of the subject has been extensively discussed in excellent summaries by Wheeler (4,5), by Weisz (6,7), and by Theile (8). These authors have amply demonstrated that the size of pores or cracks in the surfaces of solids can influence not only the extent to which the area along these cracks and crevices contributes to catalytic surface reactions but also can influence the order and temperature coefficient of the reaction, the poisoning characteristics of the surface for one reaction compared to another, and finally the extent to which a porous solid can build up temperature and pressure gradients during catalytic reactions. For details, one should refer to some of the above mentioned summaries or to some of the current articles that are being published on the subject.
The second type of adsorption, chemisorption, involves forces similar to those occurring in chemical reactions. Such chemisorptions together with catalytic reactions to which they frequently lead will now be discussed. As pointed out above, the emphasis throughout will be directed toward the manner in which our knowledge relative to the phenomena of chemisorption and catalysis may be important to those interested in the properties of solids.
Emmett, P. H.
W. R. Grace Professor of Chemistry, The Johns Hopkins University, Baltimore, Md