Gas Reactions and Adsorption on Surfaces According to Langmuir1

N. A. Shishakov.

The study of the kinetics of gas reactions when working with ordinary pressures is associated with great difficulties, arising chiefly from the sharp temperature gradient, convection currents, and the slowness of diffusion. Conversely, at low pressures, when collisions between molecules become sufficiently rare, when, consequently, diffusion proceeds with almost infinitely great velocity, and when the rate even of the fastest reactions is greatly slowed, the matter becomes extraordinarily simplified. Under such conditions it becomes possible to apply the statistical method to the study of reactions and thus to form a very simple picture of the mechanism of reaction.

Langmuir’s work on physicochemical processes in incandescent lamps, on the dissociation of hydrogen, and on electron emission concerns chiefly low pressures and, thanks to this, leads to very simple conceptions of the essence of reaction.

The theory of a monomolecular adsorbed layer, which occupies a central place in the works of Langmuir just named and which makes it possible to explain an enormous number of heterogeneous reactions, was precisely the result of such statistical conceptions of reactions at low pressures.

Already in one of his first works on chemical reactions in the tungsten lamp (the reaction of oxygen with tungsten), Langmuir was compelled to assume the existence of a special kind of oxygen film on the surface of tungsten even at temperatures close to its melting point. At temperatures above \(1200^\circ\ \mathrm{K}\), when the oxide \(\mathrm{WO_3}\) formed in the reaction volatilizes immediately after its formation, the rate of disappearance of oxygen in the lamp at a given temperature of the filament is proportional to the pressure, i.e., this is a reaction of the first order. In the reaction equation

\[ \frac{dp}{dt}=-\varepsilon \frac{A}{V}p,\ . . . . . . . . . . . . . . . . \tag{1} \]

where \(\frac{A}{V}\) is constant, the quantity \(\varepsilon\) depends only on the temperature. On the basis of the kinetic theory of gases, one may calculate the rate at which gas molecules come into contact with the filament. If this rate is expressed in \(\mathrm{g/cm^2}\), and if \(M\) and \(T\) denote the molecular weight and the absolute temperature, then the rate is expressed as:

\[ m=\sqrt{\frac{M}{2\pi RT}}\cdot p =43.74\cdot 10^{-6}\sqrt{\frac{M}{T}}\cdot p\ . . . . . . . . . . . . . \tag{2} \]

N. SHISHAKOV

This rate is proportional to the pressure, just like the experimentally found rate of the reaction of oxygen itself with tungsten. Calculating from this last equation the rate for some pressure \(p\) and dividing by it the experimentally found reaction rate at the same pressure, we obtain a number expressing the fraction of reacting molecules among all the molecules striking the filament in the given interval of time. It turns out that this fraction is very small—within the temperature range from \(1270^\circ\) K to \(2770^\circ\) K it has values from 0.0011 to 0.15. Even at such a high temperature as \(2770^\circ\) K, only 15% of all the molecules impinging on the filament react.

In the formation of the compound \(\mathrm{WO}_3\), at least two molecules of \(\mathrm{O}_2\) must take part. Calculation shows that the probability of the simultaneous encounter of two free oxygen molecules on the surface of tungsten is so small that it leads to values of \(\varepsilon\) differing considerably from those found experimentally. Moreover, under such an assumption \(\varepsilon\) would not be independent of the pressure. It must therefore be assumed that, by the moment when an oxygen molecule strikes the surface of the filament, another molecule or atom of oxygen is already present at the point of impact. In favor of such an assumption also speaks the fact that the reaction rate proves to be independent of the bulb temperature, i.e., that it is affected by the conditions existing specifically on the surface of the filament.

Fig. 1.

The study of a whole series of other reactions at low pressures likewise led Langmuir to the necessity of recognizing the existence of a gaseous layer on the surface of incandescent bodies. But the most striking indication of this must be considered the phenomenon of the decrease of thermionic emission in the presence of gases.

The curve (A, Fig. 1), expressing the thermionic current in a high vacuum, only within known limits (I) increases with temperature according to Richardson’s equation; the other part of it (II) is independent of temperature. However, both parts of this curve change strongly in the presence of small amounts of gas, the second part, owing to the formation of positive ions and the neutralization of the space charge, being increased, while the first, which represents the true electron emission, is very greatly decreased (B, Fig. 1). Whereas the intensification of the thermionic current due to positive ionization occurs for all gases, only certain gases—namely, chemically active ones—exert an influence on the decrease of the emission.

Apparently, what plays a role here is a surface layer of gas bound to the metal by chemical forces. That the surrounding gas as such has nothing to do with it is evident from the example of nitrogen. Nitrogen ions falling upon an incandescent tungsten filament combine with it. At a sufficiently high anode potential, when many ions are formed and the reaction between tungsten and nitrogen becomes noticeable, the emission decreases. If the filament is cooled and the remaining nitrogen is pumped out, then upon secondary heating of the filament the emission will at first be the same as in the presence of nitrogen, and only later will it begin to decrease. Obviously this is connected with the gradual evaporation of the gas sheath.

In order to obtain a noticeable effect, sometimes quite insignificant quantities of gas are sufficient, so that such a gaseous layer can hardly have a thickness of more than one molecule. The degree of influence of this sheath on the electron emission must depend on the extent to which the cathode is covered by it. The magnitude of the covered surfac-

...depend on the rate of formation and on the rate of disintegration of this layer: when both these rates are equal, the thickness of the layer remains constant; conversely, at different rates, when the degree of coverage changes, the modern emission must also change.

Anomalies of this kind in electron emission, the so-called retarding effects (Verzögerungseffekt), which occur in the presence of negligible traces of gases, speak especially convincingly in favor of the existence of a surface film. Let us take an example. With a sudden increase in the temperature of the cathode, the emission does not increase at once; in exactly the same way, when the temperature is lowered, the decrease of emission requires a certain interval of time. It is evident that when the temperature is raised the gas layer tends to volatilize rather than to form, and when it is lowered—the reverse. It must be added to this that gases diminish electron emission at low temperatures considerably more strongly than at high ones, which again indicates a tendency of gas films, when the temperature is raised, rather to evaporate. From the rates of decrease of emission upon admission of gas and of its increase upon raising the temperature, one may also obtain a judgment as to the rate of evaporation of the film at different temperatures. Since even at high temperatures the film evaporates rather slowly, it is evident that it possesses an extraordinary stability.

With the aid of this theory it is possible elegantly to explain more complicated facts as well. At \(1500^\circ \mathrm{K}\), when a tungsten filament is surrounded by a mixture of \(\mathrm{H_2}\) and \(\mathrm{O_2}\), the filament reacts with oxygen as though there were no hydrogen. So long as oxygen is present, hydrogen does not dissociate and does not react with oxygen. When, however, all the oxygen in the form of oxide \(\mathrm{WO_3}\) has flown over onto the glass, dissociation of the hydrogen suddenly begins, which entails its reaction with \(\mathrm{WO_3}\). At this moment the emission begins to increase. It is evident that even at \(0.01\) bar1 of oxygen pressure the surface of tungsten is almost wholly covered with oxygen, and the oxygen sits so firmly on the surface that even at \(1500^\circ \mathrm{K}\) hydrogen does not react with it. Later Langmuir showed that hydrogen dissociates only after it has been adsorbed by the surface of tungsten.

Exactly the same is observed also in some other reactions.

Still more striking is the behavior of thorium on the surface of tungsten. If thoriated tungsten is heated to \(2300^\circ \mathrm{K}\), the thorium diffuses to the surface, owing to which the emission increases extraordinarily strongly. At lower temperatures this diffusion is imperceptible, and at higher ones the thorium evaporates. It turns out that however long the tungsten is kept at \(2300^\circ\), evaporation of the thorium accumulated in this way at high temperatures occurs almost instantaneously, and this testifies to the fact that the layer has a thickness of no more than one atom. The surface energy of thorium is lower than that of tungsten; consequently thorium, as is prescribed also by Gibbs’s rule, must be adsorbed on the surface of tungsten. It is interesting to note that oxygen, while lowering the electron emission on a thoriated filament, leaves it low even after its removal. Apparently, here the great mutual affinity of Th and \(\mathrm{O_2}\) makes itself felt, owing to which the surface layer of thorium is seized by oxygen.

Some anomalies in the phenomena of the photoelectric effect are explained in a similar manner.

Leaving aside many other proofs, given by Langmuir, of the existence of a monomolecular layer, let us say a few more words about how Langmuir understands its structure. Atoms or molecules of gas are held on the surface of a solid body by forces entirely analogous to chemical valences and differing from them only in magnitude. The concept of these weaker (“secondary”) valences may be obtained from consideration of the structure of crystals. Just as in a crystal of common salt each sodium atom is surrounded by six chlorine atoms equally distant from it, and chlorine is surrounded by six sodium atoms, so—

in all other crystals there is such a fragmentation of valences, and the number of these small valences in the atom of a given element may, in different cases, be, generally speaking, different. The surfaces of metals and of many other solid bodies which are crystals, as well as any internal plane of a crystal, must in structure resemble a chessboard. On it, in a definite order, the free valences of the surface atoms must be arranged. Owing to these unsaturated valences, the space between the surface atoms and above them is surrounded by a field of electric forces, more intense than inside the crystal. It is precisely by the existence of such a field that the phenomena of adsorption can be explained. From this point of view, adsorption is nothing other than the binding, by the free valences of molecules or atoms reaching the surface, of the bonds issuing from the outer atoms or molecules.

Gas molecules striking a surface are in general not elastically reflected from it, but condense if, during one oscillation of the molecule on the surface, they manage to lose part of their energy. The evaporation of adsorbed molecules taking place simultaneously with this proceeds as an independent process. If there is a difference between the rates of condensation and evaporation, then the temporary retention of gas molecules on the surface that results from this is adsorption. Adsorption, consequently, is the result of the kinetics of equilibrium.

The adsorbed layer may be represented visually as follows:

\[ \begin{array}{ccc@{\qquad}c@{\qquad}ccc} \mathrm{O} & \mathrm{O} & \mathrm{O} & & \mathrm{O} & \mathrm{O} & \mathrm{O} \\ \Vert & \Vert & \Vert & & \Vert & \Vert & \Vert \\ \mathrm{W} & \mathrm{W} & \mathrm{W} & \text{or} & \mathrm{C} & \mathrm{C} & \mathrm{C} \\ /\backslash & /\backslash & /\backslash & & /\backslash & /\backslash & /\backslash \\ & \mathrm{W} & \mathrm{W} & \mathrm{W} & & \mathrm{C} & \mathrm{C} \end{array} \]

The degree of adsorption may depend on two causes. First, on how great the forces on the surface are: if the forces are weak, the “life” of the adsorbed molecule is short; if they are large, then the rate of evaporation is very slow, and the surface may be completely covered with a layer one molecule thick. In addition, adsorption depends on temperature, which plays the role that only the rate of evaporation depends on it. It is evident that the higher the temperature, the smaller the amount of adsorption.

Up to now we have been speaking of adsorption on the surface of metals. According to Langmuir, on the surface of amorphous bodies, and also of liquids, there also exists a monomolecular layer. Studying the phenomena of the surface tension of liquids, Langmuir found that thin oil films on the surface of a liquid, when the oil is dissolved in the liquid, usually do not exceed one molecule in thickness.

In the case of amorphous bodies, for which Langmuir likewise regards the forces of cohesion as chemical forces, there must also exist on their surface free valences, which also determine adsorption.

Direct determinations of the quantities of gases adsorbed by glass, as well as by mica and platinum, lead Langmuir to the conviction that even at very low temperatures \((-183^\circ \mathrm{C})\) adsorption does not exceed a monomolecular thickness. In almost all cases adsorption proved to be a completely and readily reversible process. If adsorption is regarded as a mobile equilibrium between the rates of condensation and evaporation, then one can calculate the number of molecules which fall in one second on a given surface, and the number of molecules which at a given moment evaporate—and from this compute the adsorption isotherm. The elementary cells formed by the surface atoms of a solid body can, generally speaking, hold gas molecules in different ways; but if we take the simplest case, when each cell receives one gas molecule, then the dependence between the quantity of adsorbed gas \(q\) and the pressure \(p\) will be as follows:

\[ q=\frac{abp}{1+bp}, \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots (3) \]

where \(a\) and \(b\) are constants. The results of experiments with glass, mica, and platinum fit this equation very well.

It should be noted here that all the cases discussed so far have concerned only smooth surfaces. In the case of porous surfaces the relationships will be different; there absorption will take place, i.e., retention of the gas in capillary spaces. The same must be said of glass, where the thickness of the adsorbed layer proves to be considerably greater than one molecule and where, consequently, factors other than adsorption in the form in which Langmuir understands it must play their role.

1. Bar \(=\) \(\text{dynes}/\text{cm}^2 = 0.00075\) mm Hg. ↩↩