SOME ASPECTS OF CATALYSIS JEROME ALEXANDER,50 EAST 41s~STREET, NEWYORK CITY In its broadest sense, catalysis is the speeding or direction of chemical change, whereby a relatively small quantity of a substance, the catalyst, determines, under certain conditions, the building up or the breaking down of relatively large numbers of molecules or molecular complexes. The catalyst functions somewhat like a judge, uniting in the bonds of chemical or electrostatic matrimony selected particles that come within its jurisdiction, or else severing the bonds that b i d others, and continuing its active term for a long time, if not indefinitely. The ordinary "zipper" or "talon" closure, now in common use on bags, tobacco pouches, arctics, sweaters, and similar articles of clothing, has a "key," which, when drawn in one direction, massages the opposing "hooks" into a position where they engage each other and hold firmly. Reversing the motion of the key, the opposing hooks are disengaged, and as the key slides along, the closure is opened. The action of the key is analogous to that of a catalyst and, in fact, as was shown by Croft-Hill as far back as 1898,' the action of catalysts may be a reversible one. He worked with an enzyme, yeast maltase, and built up from dextrose a disaccharide, a t first thought to be maltose, but later shown to be isomaltose. The analogy with the "key" may be carried farther. There is a mechanical specificity between the key and the hooks which it can influence. At temperatures approaching the softening points of the metal comprising the key or hooks, we would expect the opening and closing processes to fail. Prolonged use of the key might wear it to such an extent that it could no longer function, so that there wCiuld be some limit to the number of hooks which any key could open and close. Again, an adhering bit of solder, or paint, or chewing gum might prevent the key from moving, or some deformation of the hooks might hold i t fast. In like manner, catalysts are specific in their action, and have a "life." They may be worn out, temporarily or permanently masked, or poisoned. Protagonists of the various views as to the nature of catalysis will, I think, all agree that close-range attractions between particles of any kind, be they electrons, protons, atoms, molecules, or small molecular groups, are consequent upon the outwardly directed unsatisfied residual electrostatic fields of force, emanating ultimately from the protons and electrons whose complexes constitute the chemical elements of which molecules and larger particles are made. Berzelius, to whom we owe the tenn catalysis, anticipated this aspect, for he saidZkeenly:
It is then proved that several simple and compound bodies, soluble and insoluble, have the property of exercising on other bodies an action very diierent from chemical affinity. By means of this action A. CROFT-HILL, J. Chem. Soc., 73, 634 (1898). a BERZBLIIJS, Jahresberichle, 15, 237 (1836). 2026
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they produce, in these bodies, decompositions of these elements and different recombinations of these same elements to which they themselves remain indifferent. This new force, which was hitherto unknown, is common to organic and inorganic nature. I do not believe that it is a force quite independent of the electrochemical affinities of matter; I believe, on the contrary, that it is only a new manifestation of them; but, since we cannot see their connecion, and mutual dependence, it will be more convenient to designate the force by a separate name. I will therefore call this force the catalytic force, and I will call catalysis the decomposition of bodies by this force, in the same way that one calls by the name analysis the decomposition of bodies by chemical affinity. Most books on catalysis distinguish between homogeneous catalysis, occurring in homogeneous dispersions, and heterogeneous catalysis, which occurs a t demonstrable interfaces. A moment's consideration will show, however, that since gases and liquids consist of discrete particles (atoms, molecules, or groups of these), all catalysis is essentially heterogeneous. In gases and liquids we simply reach a limiting case in the degree of dispersion of the catalyst and/or reactants, so that the temporary or transient intermediate groupings of catalyst and reactants are, in these cases, generally stoichiometric, and may be regarded as unstable chemical compounds. As catalysts become complex and appear in larger and still larger masses, the sharp definiteness of chemical combination between catalyst and reactants begins to give way to the dominance of specific electronic surface contours of the catalyst, which represent a mosaic of the outwardly directed unsatisfied electronic fields of the atoms and mglecules in the active interface. The sharp definiteness of stoichiometric relations tends to be replaced by a statistical or mosaic average relationship, which may a t times simulate some stoichiometric compound. The X-ray spectrometer should be helpful in differentiating between the two classes of compounds, though this will be largely a matter of our own definitions, rather than of any basic natural difference. But whether the fixation be regarded as adsorption or as chemical combination, the electronic fields of the fixed particle adjacent to the catalyst surface will be materially modified by the fixation, with the result that a redistribution of electronic charges must take place, whereby the outwardly directed fields of the particle present to the milieu an electronic contour quite diflerent from what existed before fixation. If this new type of specific interface is capable of attaching to itself some other particle, we are confronted with two possibilities: (1) if the fust and the second particle both remain fixed a t the catalyst surface, activity there ceases; (2) if, in the readjustment of electronic fields of the three units concerned, the bond between the catalyst and the now duplex particle breaks, while the bond between the united particles continues to hold, the duplex particle
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will be set free into the milieu, all surfaces involved undergoing a readjustment of their fields. The behavior just described would result in a synthesis; but a reversal of the process would result in a breaking-down catalysis, or analysis. It is, of course, quite possible that corresponding reactions could be brought about if both reacting particles are fixed at the catalyst surface, side by side, or, in the case of an analysis, if adjoining parts of the particle undergoing splitting were both subjected to electronic distortion by the catalyst. The diagrams in Figure 1illustrate the mechanism described. Because of the complicated and thermolabile structures both of catalysts and of the substances on which they act, this simple diagram is merely iudicative of the gross joining or severance of any two particulate units by a catalyst. In any actual practical case we are confronted with the effects of many other factors, e. g., variations in the surface contour of the catalyst and of the reactants yielding areas of optimum and of null activity, the presence of myriads of potentially reactive particles, of diluent molecules of the dispersion medium (solvent, air), and of effective "impurities" (promotors, poisons). From what the spectroscope reveals regarding the numerous, extensive, and specific activities in the internal kinetics of atoms at high temperatures, we are justified in believing that eveu at lower temperatures a considerable portion of the energy of thermal agitation is absorbed within atoms and molecules, producing specific changes in the energy positions of their constituent electrons. No doubt the energy thus absorbed bears a most intimate relation to the specific heat of any substance. Apart from the internal electronic activities of all particles, which may result in different outwardly directed fields of force eveu in the case of identical molecules, we must also consider the kinetic motion, both rotational and translational, of the particles regarded as units; and this applies to the catalyst itself, if it consists, not of fixed surfaces, but of free-moving particles, as in the case of enzymes, or colloidal metals, or labile molecules. Temperature is an important factor in catalysis; but what we measure and call the temperature of any system represents an integration of the kinetic activities of all its constituent particles over a period of time sufficient to affect some recording instrument (thermometer, thermopile). During this interval, and indeed at any instant, the internal electronics and the particulate rotation and free path motion of individual particles will be
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widely different. The activities of any one particle will vary widely from one instant to another. The question naturally arises as to whether we can speak of any indiniduul particle as having any measurable temperature, or, in fact, any lasting temperature a t all. At any instant, only a portion of the particles would exist in a state of "excitation" permissive of certain specific changes. It is interesting to note that various types of radiation (light, X-rays, ultra-violet rays) may produce marked internal particulate activation without determinable increase in temperature. Perforce, we rely on statistical averages in our measurements of the net results of catalysis as well as of temperature, but a correct understanding of the mechanism whereby these results are attained is essential to the control of the operative factors. The average wealth per capita is no measure of the efficiency of a government or of the happiness of its people, for some different distribution might lead to greatly increased total.wealth and individual happiness. When a particle approaches a catalyst, two aspects are of outstanding importance: (1) the momentary electronic contour of the surfaces in apposition; (2) the particulate kinetic velocity of translation and of rotation. The former is evidently determinative of the possibility of union, and so is the latter, as may be illustrated by a simple observation. In the course of an address a t the Toronto meeting of the British Association for the Advancement of Science (1924), Sir Ernest Rutherford illustrated the effectof positively charged atomic nuclei upon positively charged alpha particles emitted by radium by swinking a magnet suspended from the ceiling toward another magnet fixed on a tarble, the positive magnetic poles being opposed. The mutual repulsion of the like poles made the swinging magnet take a parabolic path in any off-center approach. To show the relatively rare recoil which occurs when an alpha particle directly hits an atomic nucleus, Sir Ernest took careful aim-but the swinging magnet passed completely over the fixed one, probably giving a slight "jump" as it did so. Quickly retrieving the moving magnet, he swungit again from a lesser distance, and the expected recoil occurred. We thus see that if a certain critical velocity is exceeded, particles which have the power of cohering may not cohere even on close approach. In catalysis, apart from the rotation and the angle of approach of particles or surfaces, the particulate velocity of kinetic translation is obviously vital. With increase in thermal agitation the total number of encounters between catalyst and reactant areas likewise increases, with the tendency to increase the number of fruitful encounters. When, however, the temperature of the system reaches a certain point, the increasing average particulate velocity reaches a point where the number of unfruitful encounters dominates, and the catalyst efficiency per unit of time tends to diminish. The word " t e n d is used advisedly, for there are other intercurrent factors
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influencing the thermal optimum in catalysis; e. g., internal changes in catalyst, reactants, and end-products which may lead to particulate disruption or decomposition; the extent to which the reversibility of the catalytic reaction may be influenced. Another group of factors are those affecting the degree of dispersion of reactants, and in some cases of the catalyst. These vary from the grosser deflocculations effected by protective colloids (glue, gum arabic) or by specific salts (silicates with clay, calcium chloride with glue, thiocyanates with cellulose) and the finer colloidal or quasi-molecular dispersions accompanying changes in pH, illustrated by the followingepitome of some of the work of The Svedberg: A number of proteins have been found to be monodisperse (homogeneous with regard to molecular weight). They can be divided into two groups: the first group containing the proteins with molecular weights from 34,500 to 208,000 and the second group proteins with molecular weights of the order of millions (the hzmocyanins). The proteins of the first group fall into four classes: with molecular groups 1, 2, 3, 6 times 34,500. Each of these monodisperse proteins has a fairly wide pH-stability region including the isoelectric point of the protein. Measurements of the molecular weight and of the sedimentation constant a t different pH values show that the isoelectric point is not a singular point. It is never situated in the middle of the stability region but always more or less shifted in the direction of low pH values. The protein molecules containing more than one group of weight 34,500 are as a rule dissociated into molecules of lower number of groups of 34,500 when the pH of the solution is raised over a certain value. Thus the proteins of molecular weight 6 X 34,500 split up into molecules of 4, $, and 5 of the original molecule but never into molecules of or 'j of the original. This is in line with the fact that proteins possessing these latter weights a t or near their isoelectric point have not been met with. At a sufficiently high alkalinity all proteins have the same molecule weight, kz., 34,500. These integer submolecules of a certain protein are electrochemically identical with the original molecule (proved by electrophoresis experiments carried out by A. Tiselius). On the other hand, chemically different proteins having molecules of the same weight are electrochemically different (A. Tiselius). If we continue to raise or to lower the pH of the solution we finally arrive at a point where the molecule is completely broken down into fragments of low molecular weight, and of an electrochemically different behavior. This complete decomposition is in many cases reversible, i. e., if the mixture is restored to a pH situated within the stability region molecules of the original weight are built up again out of the fragments. In conclusion, i t might be pointed out that a proper understanding of the mechanism of catalysis may elucidate many of the mysteries of living matter and of life itself. Thus, a catalyst which could catalyze its own
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formation (an autocatalytic catalyst) would have to be considered as being alive, a t least potentially. The smallest living units we now know, genes and the smaller ultrafiltrable "viruses," approximate the size of large molecules or small molecular groups. I n addition to the power of selfduplication or autocatalysis, they also possess the power of directing other, and generally many other, chemical changes. Within each chromosome which the microscope reveals in every germ cell, lie a multitude of genes, those highly specific, exceedingly small catalytic and autocatalytic units which determine into what the germ will develop, given suitable environment-a rose or a sunflower, a mouse or an elephant. Viewed as camers of heredity, gene catalysts constitute the most precious substance in the world.
