CHEMICAL ACTIOK I N T H E GLOW DISCHARGE II.* FURTHUR IIWESTIGATION ON T H E SYKTHESIS OF AMMONIA -4. KEITH BREWER AND
J. W. WESTHAVER
In a recent article' certain factors were presented governing the synthesis of ammonia from a 3H2-K2mixture in various types of glow discharge tubes. The novel thing noted in these experiments was that, when the back reaction was removed by freezing out the ammonia as fast as formed with liquid air, the rate of synthesis was completely independent of pressure. The results obtained suggested an electrochemical equivalence law for discharge tubes which may be stated: f o r a given set of operating conditions the rate of synthesis is independent of the pressure and depends only on the current passing through the tube. This may be expressed as dP/dt = cy I, where P is defined as the equivalent ammonia pressure, I is the current, and cy depends on theconditions of the experiment. I t is the purpose of the present paper to treat the various factors influencing cy,and to present further evidence regarding the reaction mechanism. Special attention will be given to the region of the discharge wherein the synthesis takes place, and to the effect of gases added to the 3H2-X2 mixture. Discharge Tubes Two general types of discharge tubes were used in these researches. Fig. I shows the tube used to test the effect of FIG.I Discharge Tube permitting Diffusion added gases on the rate of synthesis. The of Gases discharge - chamber and directlv-connected balloon flask constituted Soy0 of the volume of the entire static system. Diffusion as well as convection currents set up in the discharge prevented the accumulation of added gases around the electrodes, so the composition of the gas throughout the system at any one instant was practically uniform. Electrodes A and B were used in cracking the ammonia to produce the 3H2-Sz mixture, while elec'trodes C and D were used for synthesis. * Fertilizer and Fixed Nitrogen Investigations, Bureau of Chemistry and Soils, Washington, D. C. l Brewer and Westhaver: J. Phys. Chem., 33, 883 (1929).
154
A. KEITH BREWER AND J. W. WESTHATER
The type of discharge tube used to test the relative reactivity of rarious portions of the discharge is illustrated in Fig. 2 . The tube was 2 cm. in diameter and 7 0 cm. long; a special Dewar flask enabled the tube to be immersed to a depth of 6ocm. Exploring electrodes were placed a t suitable positions along the barrel of the tube.
+ A.
Currenf = 40 m a. Pressure = 3 . 5 m m
l!llllIillM
-
I
i
I
I FIG. 2
Distribution of S I I s i n Geissler Discharge
The method of procedure employed was identical with that described in the previous communication. Direct current from a rotary converter was substituted for the alternating currrnt previously used
Factors influencing a In the following discussion, the various factors that influence the ratr constant a will be considered separately. Efwf of Prcwicve. 'The nio.;t iiuportant property of the constant (I) 01 is its independence of presurc). l'his was shown in the previous paper to b t the case over wide ranges of pressure, both for thc positive column and the
CHEMICAL BCTIOX I?; THE GLOW DISCHARGE
155
regions surrounding the electrodes. Had it not been for this one fact, a comprehensive study of the reaction would have been impossible, since little information could be obtained of the actual gas concentration within the discharge under any such varying conditions as were present in this work. (2) Eflect of Temperature. Since the energy supplied at constant current to the discharge changes with pressure, the gas doubtless suffers considerable variation in temperature. As pointed out above, the rate of reaction was found to be constant, and hence the effect of temperature appears to be negligible, Tubes in which the electrodes were cooled by immersing in liquid air gave practically the same value for a as did similar tubes with uncooled electrodes which often became heated to the melting point of aluminum. (3) Magnetic Fields. Data showing the effect of magnetic fields on the value of a have been presented in the previous paper. I n Fig. 3 of the first paper it will be seen that a magnetic field parallel to the electric field has no effect on the value of a , while a magnetic field placed a t right angles, materially increases its value. The magnetic field when placed parallel to the electric field enabled the synthesis in tubes of the type illustrated in Fig. I to be carried on at a much lower pressure than was possible otherwise. A properly adjusted tube of this type gives with direct current a straight line time-pressure curve from 4.2 mm. to about 0.1mm. pressure, as is shown by line I of Fig. 3. The particular function of the magnetic field in this case is to confine the discharge to such a thin sheet that the amount of ammonia decomposed by electron bombardment of the walls is inappreciable. (4) Electrostatic Fields. It was pointed out in the previous paper that various electrostatic fields applied a t right angles to the path of the discharge had no effect on the rate of synthesis. ( 5 ) Portion o j Discharge where Synthesis takes Place. The reaction efficiencies for various portions of the discharge in a 3H2-Szmixture are represented graphically in Fig. 2 . Relative reaction rates are plotted against the centimeter length of the tube. The cross-hatched areas represent the relative amounts of ammonia formed in the different portions of the tube. The actual reaction rates were measured for several depths of immersion, and from these data the relative amounts of ammonia frozen out on the walls were calculated. The time-pressure curves from which the rates were obtained were straight lines. X qualitative estimate of the reactivity throughout the tube was made by a direct observation of the distribution of ammonia frozen out on the walls. The white films of ammonia were quite sharply defined, showing little evidence of diffusion from the uncooled portions of the tube. It will be seen from the graph that appreciable synthesis takes place only in the glowing parts of the discharge. Maximum synthesis was found to occur in the negative glow, while no synthesis was noted in the Crookes and Faraday dark spaces. The positive column, including the anode glow, produced a uniform synthesis throughout its length. Under special conditions, however, striated deposits have been observed in the positive column, in-
156
4.KEITH BREWER AND J. W. WESTHAVER
dicating that each striation with its following dark space behaves much the same as the negative glow and Faraday dark space. The potential distribution along the tube was measured by means of exploring electrodes connected to a specially designed electrostatic voltmeter. This distribution of potential was found to vary with changes in pressure and current. The volt,age drops shown on the graph were measured a t 3. j mm. pressure and 40 mm. current. The rate of synthesis in the negative glow was 33 times that in a unit length of positive column, and the ratio of voltage drops for the above values of current and pressure was 2 2 j / 3 7 . The rate per unit of power input under these conditions is 33 X 3 7 j 2 2 5 or j . 4 times as great for the negative glow as for the positive column. At a pressure of I mm. the power efficiencies of the two regions are nearly equal due to an increase in the cathode fall of potential and a decrease of the potential gradient in the positive column. Below I mm. pressure the power efficiency favors the positive column. Although the power input to the two regions changes with pressure, the rate of synthesis for both remains constant provided the current is unchanged. For the positive column the value of a , which may be termed the current efficiency, is proportional to the length of the tube immersed; the power efficiency, however, due to the uniform potential gradient, is independent of the length of tube immersed. In the positive column the value of a is smaller for tubes of small bore. I n the negative glow CY varies but slightly with the size of the cathode; such variations as were noted could be attributed to effects produced by the proximity of the glass walls t o the electrode. For seven tubes in which the negative glow alone was responsible for the synthesis, the mean value of CY was found to be .74 molecules per electron of current. (6) E f e c t of Added Gases. The effect of various gases on the rate of synthesis when added in different proportions to the 3H2-Nz mixture is illustrated in Figs. 3-5. Fig. 3 shows a set of time-pressure curves obtained from a series of runs in which nitrogen was in excess of that necessary for a 3HrN2 ratio. Similar series of curves were obtained in which hydrogen, argon and helium were added to the combining mixture. Line I shows the time-pressure relationship obtained for the 3H2-Nz mixture, It will be seen that the rate of synthesis as given by the slope of the line is unchanged from 4.2 mm. to 0 . 2 mm. The slowing down of the rate a t the low pressures, as pointed out in the previous paper, is due largely if not entirely to the removal of ammonia from the walls by electronic bombardment, and possibly by radiation. The other lines in Fig. 3 represent the effect of various quantities of added nitrogen on the rate of synthesis. Line 2 is interesting in that it first shows a speeding up of the reaction rate as the percent of nitrogen increases and, then a slowing down as the percent of nitrogen passes that for maximum rate. When the curves were plotted on a large scale it was seen that only line I was straight, the rest curving to a degree depending on the excess nitrogen
CHEMICAL ACTION IN THE GLOW DISCHARGE
I57
present. Similar curved lines were obtained for excess hydrogen, helium and argon, From this it will be seen that the rate of reaction is independent of pressure as long as the composition of the gas remains unchanged. The effect of helium is less marked than that of the other gases. The variation in CY with change of composition is plotted in Figs. 4 and 5 . The rates plotted were taken from the time-pressure curves a t 3.5 mm. Since the lines obtained with added gases become slightly curved with increasing added gas content, the slope of the tangent to the curve at 3.5 mm. was taken as a convenient measure of the rate, and the composition of the gas a t this pressure was calculated.
FIG. 3 Time-Preeaure Curves for Excess Nitrogen
The maximum rate of synthesis for the hydrogen-nitrogen combinations was obtained a t approximately a zH2-N2mixture rather than a t 3H2-N2 as would be expected from the combining ratio. Thus, in Fig. 5 it will be seen that the addition of nitrogen to the 3H-2iV2 mixture increases the rate of synthesis until a 2H2-N2 ratio is reached. The addition of hydrogen to the 3H2-Nz mixture always slows down the reaction. Since the rate as given by the slopes of the lines in Fig. 3 is independent of the absolute pressure of the gas, depending only on the ratio of nitrogen to hydrogen, values similar to those obtained from the slopes of all the lines a t 3.5 mm. could be obtained from the tangents to line 2 alone, taken a t points representing the corresponding gas ratios. The use of a complete set of curves
4.K E I T H B R E W E R A S D J. TV. R ' E a T H A V E R
p
Nt frogen
'
40
I
o-c j
20
30
1
Jo
% Hydrogen
I '
$0
'
80
EIG 4 Rate C'urve for \ arious Sitrogen-H) drogen JIiutures
90
~
,do
CHEXICAL ACTION I N T H E GLOW DISCHARGE
159
rather than a single curve enabled a more reliable determination of the reaction rates and corresponding gas ratios. I n Fig. 3 the points on either side of the maximum fall fairly well on straight lines. I t is interesting to note that the slopes of these lines have a numerical ratio to one another of I to 3 . This indicates that, over the range of compositions to the right, the addition of nitrogen to the hydrogen-nitrogen mixture has three times the effect in speeding up the rate as a corresponding addition of hydrogen has when added to a nitrogen-hydrogen mixture in the range of compositions to the left. The effect of argon and helium on the rates of synthesis is plotted in Fig. 5 . Helium when added in any quantity up to ;o% of the total quantity of gas present had no effect whatsoever on the rate of synthesis. Argon, however, when added even in small amounts slowed down the rate in a very pronounced manner. Since the argon contained traces of nitrogen, the actual effect is doubtless more pronounced than that indicated by the curve. An attempt was made to find the effect of mercury vapor on the rate by using a U tube in which the usual aluminum electrodes were replaced by mercury. The results obtained, however, were the same in both cases because the mercury vapor pressure in the cooled portion of the tube was too low to give an observable effect. Discussion of Results In the previous paper it was shown that the synthesis of ammonia was initiated by the positive ions formed in the discharge, the reaction going to completion in the gas phase. This conclusion resulted from the facts that the rate of synthesis for a given set of operative conditions is proportional to the current and is independent of the pressure, the spectrum, or the intensity of the glow emitted. The spectrum changed completely throughout the course of a single run; a pure band spectrum was obtained a t the high pressures, while the emission a t low pressures was almost entirely lines. Not only did the intensity of the emitted light vary greatly from pressure to pressure, but the total glow in the tube could be considerably reduced in intensity and completely confined to a thin sheet between the electrodes by a magnetic field parallel to the electric field without producing any effect whatsoever on the rate of synthesis. Crew and Hulburt' have shown that the concentration of nitrogen atoms is very low in a discharge of this type, also that the concentration of hydrogen atoms varies markedly with the pressure, rising sharply from a very low concentration a t pressures above 2 mm. to a high maximum in the neighborhood of 0.4 mm. Their results show further that the concentration of atoms in the discharge varies but slightly with the power input and is, therefore, nearly independent of the current. The rate of production of excited molecules is also a complex function of the pressure and temperature, the maximum concentration being reached a t *Crew and Hulburt: Phys. Rev.,
(2) 29, 843
(1927); 30, 124 (1927).
I 60
A. KEITH BREWER AND J. W. WESTHAVER
pressures of several millimeters, depending on the gases and the character of the discharge. The proportionality between current and rate, and t,he independence of pressure, therefore, not only eliminate atoms andexcitedmolecules asinitiators of chemical action, but also eliminate a mechanism involving an interaction between either of them and postive ions. I n the present paper the results obtained by the addition of various gases to the 3H2-Nz mixture enable a comparison to be made of the relative abilities of the H2+and N2+ions to initiate the reaction. The rate of production of N2+ and Hz+ions in the discharge is given directly from the mixing ratios and the stopping powers,' Le., the relative number of ions formed per electron per cm. of path, taking helium as 1.0. The stopping powers of pitrogen and hydrogen are 8.72 and 2.84, respectively. The ratio of N2+to H2+is, therefore, 8.72/3 : 2.84, or 1.02 : 1.0, in a 3 H 2 - x ~mixture. Since the total positive ion production does not change materially with a change in the nitrogen-hydrogen ratio, the effect of changing the ratio of Nz+ to Hz+is illustrated by Fig. 4. It will be seen that the rate of synthesis increases with increasing iXz+ production, reaching a maximum a t an approximate zH2-N2ratio, where the ratio of Nz+ to H2+ is about I.; : 1.0. Thus, while nitrogen and hydrogen combine in a 3-1 ratio and should be expected to give a maximum yield when mixed in these proportions, the fact that the maximum occurs near a 2H2-Y2mixture indicates that X2+ions are relatively much more efficient in initiating chemical action than are H2+ ions. The relative efficiencies of the N2+ and H2+ ions are brought out in a much more emphatic manner when the rates of synthesis in the presence of helium and argon are considered in connection with the relative ionization potentials of these gases. The values are as follows: Ionization potential Stopping power
Sp
Hz
A
He
16.8 v 8.72
'5.3 v 2.84
15.7"
24.5 V
8.87
I
The effect of adding argon to the 3Hz-N2 mixture is to materially change the ratio of N2+to Hz+ ions in the discharge. This results directly from its high stopping power and from the fact that its ionization potential falls between that of nitrogen and hydrogen. Argon, due t o its high stopping power, will markedly decrease the rate at which X2+and H2+ions are initially formed in the discharge. The ionization potential of argon is, however, 0.4 v higher than that for hydrogen, so the chance of the A+ ion transferring its charge to hydrogen forming an Hz+ ion becomes very appreciable, especially since an A' ion will make on the average about 1 0 3 collisions with hydrogen molecules before reaching the walls or becoming neutralized. The net result of additions of argon is, therefore, t o cut down the production of S2+and to increase that of Hn+. The decrease in 1 Compton and \'an Voorhis: Phys. Rev., (2) 27, 724 (1926). (Note:-We have chosen the values of Hughes and Klein, corrected by Compton and Van Voorhis, as representing the most accurate values for the stopping power).
CHEMICAL ACTION I N THE GLOW DISCHARGE
161
the rate of synthesis upon addition of argon can be entirely accounted for by the decrease in N2+ion production. The zero effect of helium on the rate of synthesis may be accounted for also on the basis of the same general considerations as those used in the case of argon. The low stopping power of helium, as compared to nitrogen, prevents the accumulation of large quantities of He+ ions in the discharge, except a t very large helium concentrations. Then, since the ionization potential of helium is some 8.0 v higher than that of either nitrogen or hydrogen, it is possible for the He+ ion to transfer its charge upon collision with nitrogen and hydrogen molecules to form Nz+and Hz+ions. The net result, therefore, is no appreciable change in the number of Nz+or Hz+ions formed in the discharge, and hence no change in the rate of synthesis. From the above considerations it must be concluded that no marked synthesis can be attributed to Hz+ ions; Nz+ ions alone of all the products formed in the discharge, seem capable of initiating the synthesis of ammonia.
A Probable Mechanism The most logical interpretation of the data presented in this paper is that the synthesis of ammonia takes place around Nz+ ions formed in the discharge. The reaction process, therefore, may be divided into two distinct sections: part I , the formation of the Nz+ ion, and part 2, the combining of this ion with hydrogen to form ammonia. A consideration of the energies involved leads to the conclusion that the entire expenditure of energy in bringing about the reaction occurs in part I . The loss of energy is necessarily appreciable, since the excited molecules, atoms, and Hz+ions formed in the discharge apparently play no part in the reaction mechanism. The positive ion current, which may be appreciable, also represents an energy loss. While the data throw no direct light on the nature of the second part of the reaction mechanism, it is nevertheless possible to draw certain conclusions concerning the interaction between Nz+ ions and hydrogen molecules. As has been pointed out, the average ion in these experiments will make many collisions with neutral molecules before reaching the walls. Sir J. J. Thornson’ has given the attraction between a positive ion and a neutral molecule as e2b3/21.4,where b is the molecular radius and r is the distance separating the two. He assumes attachment will occur when e2b3/2+ > 3 1 2 kT, i.e., the force of attraction is greater than the kinetic agitation of the gas. These expressions were derived on the assumption that only ordinary electrostatic forces are in evidence and that the ions and molecules behave as charged and uncharged metallic spheres. This assumption, however, does not apply in the cases where chemical action can take place between the ions and the attached molecules, for under these conditions it is probable that an electronic rearrangement will take place in which the forces of attraction are much greater than those calculated by the Thomson equation. Thus large clusters l
Phil. Mag., ( 6 ) 47, 337 (1924).
162
A. KEITH BREWER A S D J. W. WESTHATER
may be built up by the addition of neutral molecules before a point is reached where the force of attraction is balanced by the kinetic agitation of the gas. Since an N?+ion will make in the neighborhood of 103collisions with hydrogen molecules before reaching the walls, the possibility for the formation of relatively large clusters exists. While it is possible to set up many types of equations which might represent the mechanism of interaction between Sz+and H P Jonly three will be considered here, which seem the most probable. The first two mechanisms will be based on the cluster ideas just presented, while the third will be based on considerations of the energy of combination as put forward by Franck. Mechanism I . This mechanism involves the idea of intermediate compound formation, and represents an attempt to correlate the hypothesis’ already put forward by chemists with the results obtained in this work. The reaction process may be represented by the following equations: ( I ) Sz+ HP = (S?H*)+ (NzHz)+ e = zNH (2) Hz = NH3 (3) S H I n postulating a mechanism of this type, it is necessary to assume that the probability of the N H complex combining with hydrogen to form ammonia is 1.0, otherwise the rate would not be independent of the pressure. This endows the intermediate S H with such stable properties that its loss to the system, by processes other than combination with hydrogen, is negligible. Mechanism 2 . This mechanism is based on the large cluster theory in which the Nz+ion, because of it’s charge and its ability to enter into chemical combination with hydrogen, is capable of associating itself with several hydrogen molecules forming an aggregate which upon neutralization yields two molecules of ammonia. This may be expressed by the following equations. 3Hz = (XzHs)+ ( I ) N2+ (N2Hs)+ e = 2XH3 (2) Fortunately, these two mechanisms are sufficiently different so that the more probable one can be picked with reasonable certainty. According to the first mechanism the rate is determined by the formation of a complex K H in which the nitrogen and hydrogen combine in the ratio I :I. According to the second mechanism, the rate is determined by the formation of the cluster (K2H6)+,in which the combining ratio is I :3. By reference to Fig. 4 it will be seen that nitrogen is three times as effective as hydrogen in increasing the rate. It appears, therefore, that the second mechanism is in better agreement with the facts than is the first. Mechanism 3 . A third type of mechanism is possible which involves neither gas phase neutralization of a positive ion by an electron or negative ion, nor the formation of large clusters, This is illustrated by the following equations: Hz = NHz+ N ( I ) Sz+ SHz+ HP = NH3 Hf (2)
+ + +
+
+
+ +
Bernard Lewis: J. Am. Chrm. Soc., 50, 27 (1928).
+ +
CHEMICAL ACTION I N T H E GLOW DISCHARGE
163
This type of mechanism is as satisfactory as Mechanism z in providing a means for the removal of the energy liberated in the reaction process, accomplished in this case by the splitting off of nitrogen atoms and H+ ions. While Mechanisms z and 3 may be equally probable as far as the general character of the data and energy considerations are concerned, they differ markedly in that Mechanism z yields two molecules of ammonia per Nz+ ion, while Mechanism 3 yields but one. A determination of the ratio of ammonia molecules to Kz+formed in the discharge would, therefore, serve to distinguish between these two reaction processes. Unfortunately the number of positive ions formed in the negative glow cannot be determined closer than a factor of two, because of the uncertainties involved in an understanding of discharge tube phenomena. I n the first place, the amount of current carried by the electrons and the positive ions is not known with certainty. While it has been conceded generally that the current is divided between the electrons and positive ions according to their respective mobilities, Aston' has shown in a series of experiments, wherein the conditions were quite similar to those in the present work, that the current is divided equally between the two types of carriers, half being carried by electrons and half by positive ions. I n case only half the current in the discharge is carried by electrons, the total number of positive ions capable of initiating the reaction is but one-fourth what it would be were the positive ion current negligible. This uncertainty, therefore, introduces a maximum possible factor of four in an evaluation of the total number of positive ions reacting. Another uncertainty arises in determining what fraction of the positive ion current is carried by the Hz+ ions and by the N2+ ions. The greater mobility of the Hz+ ions will doubtless result in the ratio of H2+to Nz+ion currents being greater than that of their rates of formation. This would tend to increase the concentrations of Nz+ ions in gas mixtures of high hydrogen concentration. An exact evaluation of the effective voltage of the electrons entering the negative glow presents further uncertainties. This voltage, however, cannot be far from the optimum ionizing potential of the gases,2 which in the present case is in the neighborhood of IOO volts. The best that can be said, therefore, is that the number of positive ions formed per electron entering the negative glow is in the neighborhood of two, perhaps slightly less.3 I n making the computations for Figs. 4 and 5, i t was assumed that the entire current is carried by electrons. If this assumption is correct, then a maximum yield of very nearly one molecule of ammonia per Ns+ ion is obtained. This yield is in fair agreement with the values of Lind and Bardwell4 who found an M , Y H ~ / ~ ;ratio N ~ +of about 0.5, which is doubtless slightly
'
F. R. Aston: Proc. Roy. SOC., 96,200 (1919). Hughes and Klein: Phys. Rev., (2) 23, 450 (1924);Compton and Van Voorhis: 27, 724 (1926). Langmuir and Jones: Phys. Rev., (2) 31, 403 (1928). Lind and Bardwell: J. Am. Chem. Soc., 50,745 (1928).
164
A. KEITH BREWER AND J. W. W'ESTHAVER
low due to the experimental difficulties involved. Such a yield is in complete agreement with the demands of Mechanism 3. On the other hand, if Aston's results are correct, approximately four molecules of ammonia are obtained per N2+ion, which is more in accord with hlechanism z . Possibly the most accurate ratio of molecules to ions (&I,%)can be had by computing the number of X+ ions on the basis that the positive ion current is negligible. If this assumption is correct, the yields given in Fig. 4 are verj close to two molecules of ammonia for each Y2+ion in gas mixtures containing less than 5Yc nitrogen. For higher nitrogen concentrations the yield per 7S2+ ion falls off almost linearly with the partial pressure of nitrogen, over a wide composition range, due probably to the loss of K2- ions to the walls and electrodes. I n conclusion it may be said that, while the yields at low nitrogen concentrations appear to favor a mechanism giving M K H ~ / S S ~=+ 2 , the uncertainties are so great that a choice cannot be made between Mechanisms z and 3 for the second part of the reaction process. The writers wish to express their appreciation to hlr. J. Reuter, instrument maker, for the construction of the electrostatic voltmeter used in these experiments.
Summary The various factors contributing to the rate constant a for the synthesis of ammonia in the glow discharge have been considered separately. Especial attention has been given to the portion of the discharge wherein the synthesis takes place, and to the effects of various added gases on the rate. The data when interpreted from the standpoint of the ionization potential and stopping power of the different gases, indicate that the synthesis is initiated, principally if not entirely, by Sl+ ions; the production of atoms, excited molecules and H2+ions represents energy lost to the system. The reaction mechanism is divided into two steps: first, the production of K2+ ions, and second, the union of the K2+ ion with hydrogen to form animonia. Three mechanisms for the second step are considered. The first two involve the clustering of hydrogen around the Sz+ ion yielding on neutralization two molecules of S H , for each available S2+ ion. The third mechanism allows for the dissipation of energy and dots not involve an electron neutralization; the yield is one molecule of K H Qfor each available S2+ion. A quantitative analysis of the data shows that a distinction between these mechanisms cannot be made on the basis of the observed yield because of lack of knowledge concerning discharge tube phenomena. ' Z
