CATALYSIS or catalytic action has been somewhat variously defined. To give a rigid definition in sample terms is difficult. or perhaps impossible. and it is therefore preferable in an article of this kind to describe the phenomenon, rather than to attempt to define it. It is, for various reasons, advisable to follow Ostwald in grouping together under the general heading of Catalysis two type of action, which perhaps at first sight may not appear very closely related, and which we may speak of as physical and chemical catalysis respectively. The feature which is common to both is that they occur in unstable (technically metastable") systems. One or two examples will make this plain. By carefully observing certain precautions, water may be cooled below 0 deg. C. (32 deg. F.) without solidifying. In this state the system is unstable. introduce a minute crystal of ice, and immediately a growth of this solid phase spreads out from the crystal germ with which the water has been inoculated This effect of the ice crystal in bringing about the change from water to ice is an example of wl1at was spoken of above as physical catalysis. Instances of this kind might be multiplied, but one wjlJ suffice for our needs. The facts which we want to notice, as being partiCl1larly characteristic of this catalytic action exerted by the germ introduced are: 1. The amount of water which can be caused to solidify by the introduction of the ice crystal stands in no relation to the size of the Crystal. A given small crystal is capable, under suitable circumstances, of causing the freezing of an unlimited quantity of water. 2. The result that can b8 produced by the germ is absolutely limited to the establishment of the stabler (frozen) con d i t ion, from the less stable (liquid) state. The ice crystal would be quite incapable of producing, under the stated conditions, the opposite change, namely, the melting of the ice. This might be expressed in popular language by saying, that what the catalyser does, is merely to help on a change which is trying to take place also in its absence. we shall have a few more words to say of physical catalysis further on, but what has been said so far may serve as a basis which will enable us better to appreciate the nature of chemical catalyses. It is a matter of very common observation, that a chemical reaction which proceeds with difficulty or (seemingly) not at all on merely bringing together the substances consumed in the transformation, takes place With greater or less ease-sometimes tumultuously-as soon as a small quantity of some suitable material is added, This itself appears to take no part in the transfollnation. judging from ,the fact that it can be recovered unchanged from the resulting products of the action. This ehemical catalysis, as the effect is called, presents t118 same peeuliarities which we noted above with regard to physical catalysis: 1. A small am ount of the catalyzer is capable of causing the chemical transformation of an indefinitely large amount of material (though in practice there is necessarily some loss of the catalyzer through various s8condary causes, such as the impossibility of completely' separating it from the product). 2. The action of the catalyzer is strictly limited to the establishment of a “stabler” condition from a “less stable,” or, in other words, a catalyzer can only prod uce or encourage such a net' reaction, as brings the reacting system nearer to its equilibrium, and never, under any circumstances, the reverse. In the case of a chemical reaction we may deduce from this second rule a third. For we may state the second rule in this form: “A catalyzer is powerless to alter the equilibrium of a chemical reaction.'"- Since a chemical equilibrium is a condition in which two opposing reactions are going on at the same velocity, it follows immediately that: 3. If a catalyzer accelerates one of a pair of equilibrium reactions, it must aecelerate the other in exactly the same proportion. These, then, are the fundamental facts relating to catalysis. It is very natural to ask: “How -- by what mechanism - does the eatalyzer exert its effect?” To this question we can give at best but a very imperfect answer. 'e shall do well to be guided in our attempt by keeping constantly before our mind's eye the fundamental facts related above. A mechanical model will best serve to illustrate the nature of the case. 'e may lilen the supercooled water — ready at any instant to freeze on introduction of an ice crystal - or the mixture of SO, + 0 - ready to unite (say at 500 deg. C.) to SO” as soon as it comes in contact with finely-divided platinum - to “a I'qllid contained in a bowl at an elevated level. Popularly speaking, the liquid is “trying” to flow to a lower level, and will in fact do so as soon as a suitable path is furnished, either by making a hole through the side of the vessel, or by carrying a syphon over .the edge of the bowl. Once we have done the small amount of work required to start the flow, we can derive an unlimited amount of energy from the outflowing liqu'd, since the bowl may have any dimensions whatever without affecting our argument. What prevents the liquid from flowing down from the bowl, is that before it can descend it must ascend, or, in technical terms. in ord!r to pass, from a pOint of higher potential to one of lower, it would have to travel over a path passing through po:nts of still l1igher potential than the starting point. This would be contrary to the physical laws applicable to the case under consideration. Tn the' same way we may Sllppose, that the molecules of SO, and 0 are tending to unite to form mOlecules of SO,,; all but a small proportion of them however are prevented from so doing by the fact that, in the course of the change, they would have to pass through intermediate states “on the crest of the hill,” lefore they could take their downward path to the 1ore stable comtion. 'Vhen we COlLe to cCl1sider the exact manner in which the catalyzer furnishes this new path, we find ourselves in difficulties. In some eases at least a partial explanation can be given. For example, j the rpaction takes p lace betw een gases, as in the caso SO, + 0 ->- SO ,,, the action of a fnely -div ided mat fl'ial, such as spongy platinum, may perhaps be ,WCOLintm for by the fact that gases tend to form a fille film of greater conc(mtration near the surface of a solid. Such increased concentration might explain an increased J E'action velocity. In other cases it has been shown that in the presence of the catalyzer the reaction takes place in two stages, each stage singly' being more rapid than the net reaction in the absence of the catalyzer. But these are at best partial explanations, applicable to isolated instanees of a phellomenon which is extremely common, and indeed, probably universal. The eause of the mystery which still () 1Jshrouds eatalytic action is not far to seek. It is closely eonnected with the faet that our knowledge of chemical aetion is almost wholly restricted to the initial and final states. Of t11e processes which intervene between the instant when a molecule of one compound ceases to exist as such, and the moment when the molecule of the product is completely formed, of j hese proeesses we l(now praetically nothing. Referring to this subject, 8 e h ii n b e i nS has iOmarked: “Shal(8speare says that 'there are more things in heaven and earth than are dreamt of in philosophy.' Thus, presumably, intermediate be t ween the state in which two portions of nutter exist after completion of chemical combination, and the state in which they previolsly r.xistes] separately, thel is a, series of transitiol1 states of which the ehemistry of to -day knows nothing.” With all the advances which chemistry has made-since Schonbein wrote these words, they stilI remain true almost to the letter.' So long as we are left in our present ignorance of those states of matter which intervene between the initial and final states in “chemical change,” it is very unlikely that we shall be able to form an adequate idea of the nature and inner meaning of catalysis. So much for Pie facts and theories of eatalysis. Now for its hearing upon technical and vital processes. From a theoretical point of view it is a somewhat disputed question whether a catalyzer can start :J reaction. the German chemist Ostwald holding that it can merely accelerate a reaction, which would go on, however slowly, in its absence also. For praetical purposes, this fine distinction is of no interest, 101 a reaction which took thousands of years to hc(:ome noticeable would be quite worthless and might ns well be said not to take place at all. Generally speaking, the function of a eatalyzer in technical practice is 110t only to s}Jeed up a reaction, but to render it industrially possible at all. Nevertheless, it may be interesting briefly to consider the economic effect 01' catalytic a((eleration upon the process. Suppose that under given conditions one ton of raw material and material in the course of transformation must be locked up in the proeess, in order to produce a daily output of, say, one ton of the product. 'Vitll all other conditions unchanged, let us now suppose that by' the introduction of a suitable catalyzer the reaction velocities are just doubled. If we require an output of one :on, as before, we can now reduce our plant in the (Continued on page 2(0.)
This article was originally published with the title "Catalysis"