The mechanism of activation at catalytic surfaces

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A considerable body of experimental evidence is now available with reference to the properties of the catalytic surface, showing that such a surface is not uniformly active and that chemical reactions occur over portions of the surface which may vary in extent from a very small fraction of the surface to the whole area. A theory which takes account of these facts was recently put forward by the writer and has received a very favourable acceptance by workers in this field. For reactions in which only a relatively small fraction of the surface is catalytically active, the activity is associated with occasional groups of atoms fixed in metastable positions associated with high energy and chemical unsaturation relative to the atoms in the regular lattice of the catalyst granule. This concept, however, of the catalyst surface, satisfactory though it be, does not answer what, in the last analysis, must be the central problem of the studyof contact catalysis, namely, the nature of the activation induced in the reactants, by association with these activating centres of the surface. It is this problem which has called forth, in the recent literature, vague and ill-defined possibilities which lack any experimental foundation. A series of recent experimental investigations, carried out in the Princeton Laboratories, provide, however, a very concrete picture as to the nature of the activation process in several different examples. It is the object of the present communication to summarise these researches and point out the conclusions to which they logically lead. Hinshelwood and Pritchard, in considering this problem, believed that adsorption of the reactant might increase as often as it decreased the stability of the adsorbed molecule. They see in excessive momentum of parts of the adsorbed molecule a possible cause of chemical change. Norrish writes of a change of configuration or distortion of the molecule consequent on an association with a polar catalyst. His observation that such a change must occur with the absorption of energy needs further qualification when it is remembered that the absorption process is often accompanied by the liberation of large amounts of heat energy. Bodenstein refers to the deformation of adsorbed molecules. Rideal uses the same expression in a recent series of lectures. Constable is more specific. He speaks of a distortion of the OH group in an adsorbed alcohol molecule and of a straining of the adsorbed molecule by the electrostatic and electromagnetic fields of force of the atoms which constitute the centres of activity of the surface. He believes that there must be a limit to the distortion of the adsorbed molecule produced by the centres of activity of a particular substance, though he is not specific as to what that limit may be.