NOTE ON CONTACT CATALYSIS BY WILDER D. BANCROFT
I n a previous paper1 I tried to show that it was not a theoretical impossibility for a solid catalytic agent to cause a displacement of equilibrium. Since writing that paper, I have come across a passage2 to the effect that J. J. Thomson3 had shown that both the velocity and the equilibrium relations could be changed markedly in surface films. This proves to be the case and Thomson’s reasoning is so interesting that I reproduce selected portions of it. “The first effect we shall consider is that due to the surface tension of the solution. We know that the surface tension depends on the strength and nature of the solution, so that since the composition changes as chemical action goes on the surface tension of the solvent and therefore its mean Lagrangian function will change; and therefore, by the principle we have just stated, the conditions for equilibrium will be altered by the surface tension.. . . If the solution be spread out in a film of thickness t , 6k/k is of the order 6/10 t where k is the coefficient of chemical combination, as Thomson calls it. Thus, if the thickness of the film is l/looo~ of a centimeter [I p ] the value of k is altered by about 0 . 6 percent. If the thickness of the layer is comparable with molecular dis] 8k/k might be as large as 6. tance, say about IO-’ [ ~ p p then This of course implies that the conditions of equilibrium would be altered completely. Thus in very thin films the influence of capillarity might be sufficient to modify completely the nature of chemical equilibrium, though we should not expect it to do much in the body of a fluid. “If the surface tension increases as the chemical action goes on, the capillarity will tend to stop the action, while if the surface tension diminishes as the action goes on, the capil1 2
3
Bancroft: Jour. Phys. Chem., 21, 580 (1917). Ehrenberg: “Die Bodenkolloide,” 2 j 2 (1915 ) . “Applications of Dynamics t o Physics and Chepistry,” 203, 234 (1888).
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W i l d e r D. Balzcrojt
larity will tend to increase the action. Thus the chemical action in a space such as a thin film, throughout which the forces producing the capillary phenomenon are active, might be very different from the chemical action in the same substance in bulk when most of it would be free from the action of such forces.” These remarks refer to solutions, but Thomson has also considered the case of gases. “Though the effects of surface tension are not nearly so prominent in gases as in liquids, still since there is perfect continuity from the liquid to the gaseous state, we should expect that the outer layer of molecules of a gas which was not in the perfect condition would, like the outer layer in a liquid, be under different conditions from the other molecules, and would therefore not possess the ssme amount of energy as the same number. of molecules in the midst of the gas. In van der Waals’ theory of the relation between the pressure and volume in an imperfect gas, the result of which is expressed by the relation
(p-5)
(v-b)
=
Re,
the term a b 2 is due to the action of the surface tension of the gas. “Though it is much more difficult to detect that existence of the action of surface tension experimentally in gases than in liquids, there is still some evidence of its existence from experiments such as those of Bosscha on the forms of clouds of fog and tobacco smoke. If the surface tension diminishes as dissociation goes on, in which case a T / a t is positive, the dissociation will be greater the larger the surface of the g a s 2 We should expect a priori that the surface tension of the surface tension of the dissociated gas would be smaller than that of the undissociated, for in most cases the dissociated gas approaches more nearly than the other to the Van der Waals’: “Die Continuitat des gasformigen und fliissigen Zustands,” 34. T is the quantity corresponding to the surface tension of the gas. For ether Thornson calculates T = 3.24 X I O - ~ E is the mass of the substance.
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435
state of a perfect gas: thus in most cases sT/s E will be positive, so that dissociation will be facilitated by increasing the surface of the gas. “If the gas be supposed to be a film of thickness t, then
ax _
x -
I .2
x
t
IO*
[where X is a measure of the dissociation],
so that if the thickness of the film were comparable with molecular dimensions, say t = IO-’ [I p p ] then the surface tension would produce very large effects. This example may be sufficient to show that, if we have the gas in thin films, surface tension may produce a very considerable effect ; such films occur adhering to glass fibers or t o matter in a fine state of division, such as spongy platinum or charcoal. The value of T given above is only part of the surface tension 3f the surface of contact of the gas and the solid. The surface tension of the surfaces separating A and B [gas and solid] is due to the energy of thin layers of A and B next their junction differing by a finite amount from the energy possessed by equally thin layers in their interior. The abnormal energy of these layers is due to the want of symmetry of the action on the two sides. In the preceding investigation we have calculated the part of the energy of the layer of one of these substances arising from the effects produced by its own molecules; in addition to this there is the energy arising from the action of the glass on the gas as well as the energy in the thin film of glass. Thus the value of the surface tension may be much greater than that given above and the effects due to it may therefore be greater than our estimate. “The value of T may depend upon the substance to which the film adheres, and thus the nature of the walls of vessels used for chemical experiments may affect the chemical comVan’t Hoffl has debination which goes on inside of them. scribed some experiments which seem to show that effects of this kind do exist. He shows that the rate at which the polymerization of cyanic acid goes on is increased by increasing the area of the walls of the vessel in which it is contained, the ’ Etudes de Dynamique Chimique,” 56.
“
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Wilder D. Bancroft
volume being kept constant. Thus when the area of the walls was increased six times, the rate of polymerization was increased i n the rktio of 4 to 3 . He also found that when the walls of the vessel were covered with a deposit of cyamelide the rate of polymerization of cyanic wid was increased three-fold. Victor Meyer too found that the decomposition of carbonic acid tpkes place in a porcelzin vessel a t a temperature several hundred degrees lower than jn a platinum vessel. When the effects produced are of this magnitude, it is doubtful whether they can be due to the effect of surface tension; but it is probable that in the case of many catalytic actions, where we have thin films of gas, the effects observed might be explained by considerations of this kind.” Thomsonl also discusses the effect of 2 neutral gas. “If the properties of the neutral gas are not affected in any way by the presence of the gas which is dissociating, the value of the mean Lagrangian function af the neutral gas will not change as dissociation goes on. The presence of this gas will therefore not affect the maximum amount of dissociation. The presence of a foreign gas certainly alters the rate of dissociation, and in some cases the experiments seem to show that it does alter the maximum amount of dissociation. This is contrary to the result we have just arrived at, and the only way of reconciling the two is to suppose that the gas is not perfectly neutral hut has its properties affected to some extent by the presence of the other gases. If the dissociation were at all catalytic, we might explain the action of the neutral gas by supposing that by itself forming a film on the surface of the vessel i t prevented to some extent the dissociating gas from doing so.” J. J. Thomson has brought out clearly the fact that there is nothing thermodynamically impossible about the displacement of equilibrium at the surface of a solid which acts as a catalytic agent. If he had put in the further assumption that the reaction took place with appreciable velocity only at the surface of the solid catalytic agent, his argument would “Application of Dynamics to Physics and Chemistry,”
207
(1888).
Note
0%
Contact Catalysis
437
have been the same as mine in substance1 though not in form. Thomson considers t h a t the reaction will proceed so as to lower the surface tension. T h a t is not an especially helpful way of putting it when we are dealing with a solid. If we adopt Freundlich’s generalization that adsorption is always accompanied by lowering of surface tension, then the greatest decrease in surface tension will be caused by the substance which is adsorbed the most, provided other things zre equal, Under these circumstances Thomson’s criterion becomes essentially the same as the one adopted in the paper referred to. Gibbs2 has said the same thing in regard to liquid films, though not quite so clearly. “ I f the tension of the film is less than that of any other film of the same components which can exist between the same homogeneous masses (whjch has therefore the same values of t , pa, pb, etc.) and which, moreover, has the same values of the potentials pLg,~ h etc., , so far as it contains the substances to which these relate, then the first film will be stable. But the film will be practically unstable if any other such film has a less tension.. It is, however, evidently necessary for the stability of the surface of discontinuity with respect to deformatiow, that the value of the superficial tension should be positive. ” It is interesting also to note that Thomson had formulated clearly in 1888 the statement that an apparently neutral gas must cut down the adsorption of a reacting gas by the solid catalytic agent, if the first gas is to affect the equilibrium relations of the second. It is not quite clear why J. J. Thornson considered that the experiments of van’t Hoff and of V. LMeyer probably involved other factors. In t h e first place thermodynamics tells us nothing about reaction velocities which is what these men were studying. In the second place, the relative effect of walls may be almost anything if the reaction in the mass of the gas takes place sufficiently slowly. While J. J. Thomson has obtained the same results from a conBancroft: Jour. Phys. Chern., 21, 580 (19x7). Scientific Papers, I , 240 (1906).
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D.Ba?zcroft
sideration of surface tension which have been obtained by a study of adsorption, it does not follow that the two methods of attack are equally good. Adsorption can be measured, while the surface tension of a solid cannot. It seems to me, therefore, that the consideration of adsorption relations is the safest and best way of attacking these problems even though it gives only qualitative results for the present. The general results of this paper are as follows: I, The statement that it was thermodynamically impossible for a solid catalytic agent to displace an equilibrium was disproved years before i t was formulated. 2 . Back in 1888 J. J. Thomson showed that a solid catalytic reagent would displace an equilibrium, and pointed out one criterion for determining the sign of the change. Cornell University
