Activation in heterogeneous reactions

That some degree of adsorption of the reacting molecules is a necessary preliminary to heterogeneous catalysis is almost obvious, and has been accepted since the time of Faraday,5 or even earlier. Through the last century two principal lines of thought are discernible first, that adsorption results in an increased concentration of the reacting molecules at, or near, the surface, so that the velocity of reaction is increased by the law of mass action second, that intermediate compounds might be formed at the surface of the solid. 

Neither of these views has survived in the original form, though the 

The distances between the atoms3 in each molecule will vary periodically, as the molecules vibrate, and consequently each molecule vibrates through states of different potential energy. These different states have different reactivities, which are probably determined largely by quite simple geometrical considerations. Exchange of partners can only occur 

8 Other coordinates of the molecules may be treated similarly. The theory of transition states has been treated by Evans and Polanyi, Trans. Faraday Soc., 31, 876 (1935), and refs, therein cf. also Faraday Society Discussion, Sept. 1937. 

Clearly the chance of attaining the transition state depends on many factors on the geometry of the molecules, on the amount of potential energy which must be added to them in the course of their vibrations, to bring them to the transition state also on the manner in which the molecules are oriented when close to each other and on the time during which they are in contact. 

In order to take part in a heterogeneous reaction molecules must be adsorbed by a catalyst. But attachment to the surface is not by itself sufficient to cause their transformation. They require activation just as in homogeneous reactions. 

A systematic attempt to correlate the catalytic effect of different surfaces with their adsorptive capacity was made by Taylor and his collaborators. Taylor and Burns, for example, investigated the adsorption of hydrogen, carbon dioxide, and ethylene by the six metals nickel, cobalt, palladium, platinum, iron, and copper. All these metals are able to catalyse the hydrogenation of ethylene to ethane, while nickel, cobalt, and palladium also catalyse the reduction of carbon monoxide and of carbon dioxide to methane. 

There is a general correlation between adsorptive capacity and catalytic effect to the extent that all the metals show some capacity for adsorbing those gases the interaction of which they are able to promote. Beyond this the correlation does not extend. 

There is no quantitative proportionality between degree of adsorption and rate of reaction. Nor is any such close relation to be expected. Indeed, at temperatures where reaction attains a measurable speed adsorption is often quite small. Thus, although the adsorption of ethylene by certain kinds of copper catalyst can be demonstrated at lower temperatures, the velocity of interaction of ethylene and hydrogen only attains an appreciable speed at temperatures where the adsorption becomes almost too small to measure. 

It is clear, therefore, that thermal activation plays a part in surface reactions as important as that which we have seen it to play in homogeneous changes. 

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ACTIVATION IN HETEROGENEOUS REACTIONS internal changes both in the adsorbed molecules and in the molecules of the surface. 

ACTIVATION IN HETEROGENEOUS REACTIONS the reactant when its concentration in the gas phase is unity. 

ACTIVATION IN HETEROGENEOUS REACTIONS 245 measurably slow even at temperatures in the region of 1,000° C., must have a heat of activation greater than 70,000 or 80,000 calories (page 97). The heterogeneous decomposition on the surface of tungsten has a heat of activation of 39,000 calories. 

ACTIVATION IN HETEROGENEOUS REACTIONS 257 fluences. If these specific influences alone are operative, further progress must await the solution of the wider problem of the general relation between mode of chemical union and properties. In the meantime, by empirical means, a special inorganic chemistry of interfaces is being built up. 

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