Apr.,
1 9 1j
T H E J O U R N A L OF I N D C S T R I A L .I .VIl EiVGI'YEERIXG C H E M I S T R I . ADDRESS OF ACCEPTANCE By IRYZNU LANOM~ZS
hln. CHAIRMAN: I deeply appreciate the honor lhat hasbeen conferred upon me by the Ne%,York Section of the American Chemical Society. and I thank you for the kind words you have spoken. The credit for the work upon which this award is based, should really be shared by many. In the first place, these investigations would never have been possible if it had not been for the activc encouragement given by llr. Whitnry and ior thc wonderful spirit of cobperation and enthusiasm which he has instilled into the research laboratory. In the second place, the work is the result of thc very tunusual ability of my assistant, Mr. Sweetser, who has carried out fully 90 per cent of all the experiments. hlr. Swcetser has made a quantitative study of B very large number of chemical reactions and other phenomena, and thr data arc preserved in thc form of about 3 w o large pages of notes which record all the details of the experiments. These data constitute a veritable mine of information on low pressure reactions. The results of only about one-fifth of the material have been published thus far. Gradually, I am working through t.he remaining four-fifths, trying to analyze the results and trying to develop theories of the phenomena observed. Besides Mr. Sweetser, thcre have been many other members of the laboratory who have actively contributed to this work. Among these I would mention especially Mr. Mackay and Mr. Rogers.
3 49
According to thc "theory of molecular films" the velocity of heterogeneous reactions is in general limited by the rate a t which the gas molecules can come into contact with the active portion of the stirlace. The fundamental conception back of this theory is that the uunilier of gas molei.ules which strike a given surface per second, according to the kinctic theory, is strictly limited a t any given pressure. Thus, in ii gas of molecular weight M a t a pressure 9, and absolute temprrature T.the total number, 11, of molecules which strike a sq. em. of the walls per second is ?z
=
2.6.32
x
1
"
~
P
~
ViMT Ilcrc p is expressed io b u s (dynes per sq. cm.). For air at room temperature and atmospheric pressure, this corresponds to about 2 . 9 x iog3molecules per second; (in other words, 13.8 grams, or r a liters of air per second). Although this ratc seems extremely high, yet if the active swjace of the solid is very small the actual rate a t which the gas is able to come into coiitact with it may slso b c sniall. The portion of the surface which is active in a give,, rraction is in general determined by the exlent to which the suriace is covered by a layer of adsorbed gas. In some cases, the active surface is in the uncovered portion of the surface, while in others it may be the covered I,ortion. The adsorption film is thus lookcd upon us a layer of molccules, usually only one molecule deep, which covexs the surface more or less completely. This film is not thought to be a layer of highly compressed gas, as in
CHEMICAL REACTIONS AT LOW PRESSURES BY IS-VIINO L~wcnrnn
If small quantities of almost any gas arc introduced into an evacuated bulb containing a highly heated tungsten filament. it is found that the gas gradually disappears. In the great majority of cases this action proves to he oi a purely chemical nature. A large number of reactions have been studied quantitatively in this way. using many different gases and several diffcrent filament materials. In each case the actual rate was determined at which the gas disappeared, under given conditions oi pressure, filament tcmperatore and bulb temperature. Pressures ranging ~ 0.05 mm. of mercury were employed. from 0 . m to The phenomena observed were studied from the view-point of the kinetic 68s theory, arid in this way it has been possible in the majority of cases to draw some morc or less definite conclusions as to the mechanism of the rfactioils involved. In particukr, t h r statistics of the rcactions hsvc been determined wherever possible. Thus. in a reaction betwpeo a gas and i i solid body, the question is raised: (hit of all the molecules which strike the surface of the body, what iractiori enters into reaction with it? Similarly, in a reaction between gasrs, the question becomes: Among all the collisions hetwcen moIccuIcs of the two sisen. what fraction results in combination? The rractions occurring between heated filaments and thr surrounding rases are heterogeneous. Itractions of this type have often been investigated inthe past and several theories of the mechanism of such reiictions have been advanced. The theory that hnr met with most favor is that oi Badenstein and Fink, according to which the velocity of the reaction is limitpd by the rate a t which the reacting substances can diffuse through an adsorbed film of gases on the surface. The rate of diffusion is inversely proportional to the thickncss of this film and the thickness of the film is in turn dependent on the partial pressure of the gas from which it is formed. As a result of several years of study of heterogeneous reaction between solids and gases a t low pressures, the writer concludes t h a t under the conditions of these experiments, Bodenstein and Fink's theory does not apply. A new theory, which may be termed the "theory of molecular films," is proposed and is found fxtremely useful in all t h e law pressure reactions.
THB ! V ~ . L I , ~ &1 X % . ~ ' ~ C I I O L ?Ulr:u~r. S
the usual theory of adsorptioii, but is considered to be a layer of rnolccules held on the stirface by the same kind of forces as those that hold the atoms of a solid body together. The adsorption film i s taken to be in a state of kinetic cquilibiium with the gases around it. Thus it is assumed that the majority ol gas molecules striking the hare surface of a filament do not rebound from the surface by elastic collisions, but are held hy coltcsive forces until thy? evnpmnte from the surface. According to this vica-point. which has been based on much evperimrntal evidence, the rate of formation of the adsorption film i s proportional to the prcssnre of the gas and also to the area of that part of the suriacc remaining u n c a v e r d On the other hand, the rate of evaporation of the adsorption film is a fiinction of the temperature and is proportional t o the extent of the surface covered by the film. I n a steady state the rate of iormation and the rate oi evaporation must be equal. According to this theory, the adsorption elm is in a state of constant changr. One by om, the molecules are evaporating and thus exposing the hiire suriace of the metal of the filament. Other molecules then soon fill up these gaps, but at any given instant there will alyays be a certain small fraction of the surface exposed. This theory of "molecular films'' has been in good quantitative agrcemcnt with the experiments in all the cas- studied thus far. In iact, examination of the published data on the velocity of heterogeneous reactions a t ordinary pressures, makes it appear probablc that even a t these higher pressures thio theory is more
T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
350
nearly in accord with the experimental facts than is the theory which postulates that the velocity is limited by the rate of dijusion through an adsorption film. The application of this theory to particular cases will be considered more in detail in connection with the experimental results. The experiments on the clean-up (disappearance) of gases by heated filaments have shown that there are many advantages in studying heterogeneous reactions a t very low pressures. In the first place, a t low pressures convection currents in the gas are entirely absent and diffusion takes place so rapidly that the reaction products moving away from the filament, do not in any way interfere with the movement of the reacting gases towards the filament. In fact, a t very low pressures the collisions between gas molecules become relatively so infrequent that one may consider the gas to consist merely of a swarm of totally independent molecules which move in straight lines between different points on the bulb and filament. Under these conditions the temperature of the gas is determined by that of the bulb and there is no temperature gradient in the gas, in the ordinary sense, even very close to the filament. Thus the filament may react with a gas a t a totally different temperature from itself-a thing impossible a t ordinary pressures. A study of the effect on the reaction velocity, of the separate variation of gas temperature and filament temperature, operis up a new and powerful method for arriving a t a better understanding of the mechanism of such reactions. Another advantage in working a t low pressures lies in the fact that the molecules of the products of the reaction, after once leaving the filament, do not return to it again until after having made many collisions with the walls of the bulb. If the bulb is maintained a t such low temperature that the products condense, it i s possible to prevent these products from coming into contact with the filament a t all, except a t the moment of their formation. The reactions that have been observed in the clean-up of gases at low pressures may be divided into four classes: 1-The filament is attacked by the gas. 2-The gas reacts with vapor given off by the filament. 3-The filament acts catalytically on the gas, producing a chemical change in the gas without any permanent change in the filament. .+-The gas is chemically changed or reacts with the filament as the result of electrical discharges through the gas. These may be termed electrochemical reactions. In this paper only a few examples of the first three types of reactions will be given. The consideration of the fourth type is reserved for a future paper. I -DIRECT
ATTACK O F THE FILAMENT
BY A TUNGSTES FILAifExT-Even a t very low pressures,.oxygen attacks tungsten a t high temperatures to form R703, which distils off the filament, leaving the surface clean and bright. A study of the rate of clean-up under various conditions and an analysis of the results from the viewpoint of the “molecular film theory” has led to the following picture of the mechanism of the reaction: A large fraction (certainly over 15 per cent) of all the oxygen molecules striking the bare surface of tungsten sticks to the surface or is adsorbed, forming a layer which is probably only one molecule deep. This layer exists in two modifications which are in chemical equilibrium with each other. One of these modifications is actiae and reacts immediately with oxygen molecules which may strike it, to form W 0 3 , while the other is inective and cannot so react with oxygen. For example, it may be that the oxygen is first adsorbed by the metal as WOz,but that this reacts with more tungsten to form 2 WO, so that these two substances are in equilibrium with each other on the surface. Oxygen molecules striking PU’O would react to form WOS, while CLEAN-UP
OF
OXYGEN
1701.
7 , NO. 4
those striking WOZ would not react. The WOa distils off as fast as formed, but the rate a t which the adsorption film evaporates is small, compared to that a t which it is removed by combining with more oxygen. This theory is in splendid quantitative agreement with the results of the experiments, REACTIONS BETWEEN OXYGEK AND CARBOX-.kt 1 2 0 0 ’ K, part of the oxygen reacts with carbon to form COS but another part gradually forms an extremeiy stable adsorption film which greatly retards the progress of the reaction that leads to the formation of C02. At 1700’ K the adsorption film slowly, and a t 2100’ K rapidly, decomposes in vacuum giving off CO. Thus when oxygen acts on a filament a t n700°, which has previously been heated in vacuum a t a high temperature, a t first only COZ is formed, but gradually, as the adsorption film grows, more and more CO is produced. The adsorption film may also be formed by heating the carbon in Con. In this case the volume of CO liberated is equal to the COPconsumed, showing that half of the oxygen is retained by the. carbon to Form the film. Carbon monoxide, on the other hand, is not absorbed by the carbon a t any temperature, showing that although the adsorption film gives up carbon monoxide on heating it cannot be formed from this gas. There are good reasons for believing that this adsorption layer consists of oxygen atoms chemically combined with the carbon atoms forming the surface layer of the filament. These carbon atoms in turn form endless cuvbon chains or lattices with all the other carbon atoms of the filament. According to this viewpoint it is not possible to assign a chemical formula to the oxygen “compound” on the surface. It is thought probable that the adsorption of oxygen by a tungsten filament at high temperatures is to be explained in a similar manner. 2-REACTION
WITH VAPOR PROM THE FILAMENT
C L E A S - U P O F XITROGES BY TUNGSTEN-It has been previously shown that nitrogen does not combine with solid tungsten, but that every collision between an atom of tungsten vapor and a molecule of nitrogen, results in the formation of the compound WKz. In this case the temperature of the bulb is without effect on the velocity of the reaction. CLEAN-UP OF NITROGEN BY MOLYBDENUM-A study of this reaction shows that nitrogen does not combine with solid molybdenum, and that every collision between an atom of molybdenum and a molecule of nitrogen results in the two “sticking” together, sometiriles in the form of a stable compound MoNz, but in other cases as an unstable “adsorption compound,” which breaks up again when it strikes the bulb. The relative proportions of the stable and unstable compounds is dependent on the relative velocity of the nitrogen molecules and molybdenum atoms a t the moment of their collision. The lower the relative velocity (lower temperature), the greater is the proportion of the stable modification. CLEAS-UP O F CARBON X O S O X I D E BY TUsGsTEN-With the bulb a t room temperature this reaction follows exactly the same mechanism as the reaction between nitrogen and tungsten and leads to the formation of a compound WCO. On the other hand, when the bulb i s cooled below-70“ C., the CO actually uttucks the tuizgsteiz jilement and forms an adsorption film of this compound on the surface of the filament. The rate of attack of the filament is thus limited by the rate a t which the compound can distil off, and is independent of the pressure of the CO or the C . ) . This bulb temperature (provided this remains below -70’ is an interesting examplc of a chemical reaction having a reaction velocity with a negative temperature coefficient. CLBAN-UP OF OXYGEN BY PLATIxuM-This case is analogous I
K (Kelvin) is used t o denote absolute temperatures.
Apr., ‘
1915
T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
t o that of the clean-up of nitrogen by tungsten, but a t higher pressures of oxygen (over I mm.), the oxygen in addition to combining with the vapor from the platinum, also attacks the platinum a t a rate that increases with the pressure. 3-CATALYTIC
REACTIONS INTO ATOMS-This
has been fully described in recent papers in the Journal of the American Chemical Society. 1 DISSOCIATION OF CHLORINE INTO ATOMS-A highly heated tungsten filament in chlorine a t low pressures dissociates this largely into atoms. This leads to some interesting results. For example, consider two tungsten filaments mounted side by side in a bulb containing chlorine a t low pressure. If one of the filaments be heated to a high temperature, while the other is kept cold, the cold $lament gradually grows thinner and may finally disappear, while the hot filament may grow in size by the decomposition of the vapor of the WCle formed by the attack of the cold filament by the atomic chlorine. DISSOCIATION
OF HYDROGEN
REACTION BETWEEN OXYGEN AND CARBON MONOXIDE I N CONTACT WITH PLATINUM-It is found that this reaction takes
351
molecules which are present in an adsorption film on the surface. At low temperatures the carbon monoxide adsorbed on the surface prevents the oxygen molecules from being adsorbed. As the temperature is raised, the carbon monoxide evaporates more rapidly and thus exposes the bare surface of the platinum. The oxygen and the carbon monoxide molecules then compete with each other t o reach these bare spots. If oxygen strikes such a spot it is adsorbed and thus reacts with the next CO molecule which strikes it. At higher temperatures the CO and 0: distil off more rapidly and finally a point is reached where the velocity of the reaction decreases with further rise in temperature because the oxygen adsorbed on the surface distills off before the CO molecules have a chance to react with it. The experiments a t all temperatures and pressures are in good quantitative agreement with this theory. REACTION BETWEEN OXYGEN AND HYDROGEN I N CONTACT WITH PLATINUM-The mechanism of this reaction is very simi-
lar to that of carbon monoxide and oxygen. RESEARCH LABORATORY, GENERAL ELECTRIC COMPANY
place only when carbon monoxide molecules strike oxygen
SCHENECTADY, N E W
YORK
CURRENT INDUSTRIAL NEWS By M. L. HAMLIN
GAS PROGRESS IN THE UNITED STATES In a speech before the convention of the “Investment Bankers’ Association of America” on “The Modern Gas Company as Security for Bonds” (quoted in the J . Gas Lighting, 129 (1915), 2 1 2 ) , Rufus C. Dawes of Chicago outlined the growth of the gas industry in this country. As early as 1 8 3 5 there were at least six gas plants in the largest cities of the United States, and the business may be considered as well established a t that time. The oldest tradition of the business is to maintain an uninterrupted supply of gas. It was a proud advertisement made by the Consolidated Gas Company that “New York City’s gas supply has never failed in eighty-seven years.” But it is substantially true of all gas companies. This determination to establish a dependable service has had no small part in the development of the business, for the gas business has, from the start, had many things t o contend against. The inventive genius of mankind has exhausted itself in an effort to supply some substitute for the service it has rendered; but, in spite of many obstacles and contrary t o many fears, the gas industry has steadily grown. The obstacles have been overcome. The fears have subsided. The industry has entered a new era, and has more than doubled its volume of business in the last decade. Invention now works for, not against, its future growth. The price a t which its product has been sold has steadily declined. In New York City, for instance, the price in 1 8 2 6 was $10; in 1 8 4 6 , $6; in 1866, $3.50; in 1 8 8 6 , $ 1 . 2 5 ; in 1 9 0 6 , $0.80. Each reduction in the selling price of gas has opened up new fields for its use. Gas carries heat units in a form more available for use than any of its competitors. Whenever a new field is invaded, the genius of inventors perfects the methods of burning gas; and the demand for such inventions has only recently been felt. The response is most encouraging, and the double effect of lower prices and more efficient burners has already been apparent, and supports the strongest confidence for further success in the heating field. Herein lies our future; and we are not so far as some have supposed from our great goal-the use of gas exclusively for domestic heating. Bonds issued by modern gas companies are secured by a natural monopoly in the sense that their property is the only 1 Langmuir and Mackay, J. A . C. S., S6 (1914). 1708; and Langmuir, I b i d . , 37 (1915). 417.
one capable of supplying exactly the same service. Moreover, under present legal adjustments, which provide for regulation and control with due regard to the protection of money invested, such property can never be duplicated or abandoned, but must be devoted to supplying this service, exclusively and perpetually. Yet these companies are engaged in the greatest of competitions-the competition of fuels. At least we have a t last learned to conduct our business in this conviction; and we do believe that we can deliver the elusive and highly prized heat units in safer, cheaper, and more available form than our competitors, for an annually increasing number of purposes. In support of our confidence, I may submit some statistics compiled by Mr. John W. Lansley, from a United States Government report relating to the gas industry. These figures are the best available, and appear to be substantially correct: Year
Plants
1850
30
.......... ,. . . ,.. . 1890.................. 1900.. . . . . , . . . . . . , . . , , 1910..................
742 877 1,296
Capital $ 6,674,000 258,772,000 567,001,000 915,537,000
Annual product $ 1,921,746 56,987,000 75,717,000 166,814,000
Between 1890 and 1900 the increase in capital appears, from these statistics, to be 1 1 9 per cent, while the increase in annual gross income appears to have been only 33 per cent. Between 1 9 0 0 and 1910, on the contrary, capital appears to have increased 60 per cent and the annual gross income 1 2 0 per cent. The increase in the amount of capital between 1890 and 1900 was about the same as between 1900 and 1 9 1 0 , but the increase in income between 1900 and 1910 was five times that between 1890 and 1900. This increase in gross annual income was brought abodt by a reduction of 32 per cent in the price a t which the product was sold. In other words, a gradual reduction of 3 2 per cent in the price of the product resulted in ten years in an increase of 1 2 0 per cent in the gross annual revenue of an industry eighty years old, and necessitated an increase of only 60 per cent in capitalization. The most interesting incident in this extraordinary accomplishment is the increase in the consumption of gas for fuel purposes. It is difficult to determine these proportions accurately, but the United States Geologioal Survey gives figures from which it may be estimated that in 1 9 0 0 the proportion of gas used for fuel was about 20 per cent of the total, and in 1910 about 50 per cent. From these conditions and these tendencies
