CHEMICAL ACTION I N T H E GLOW DISCHARGE. XI
THEDECOMPOSITION OF NITROGEN DIOXIDE AND EQUILIBRIUM A. KEITH BREWER
AND
THE
NITROGEN DIOXIDE
P. D. KUECK
Fertilizer and Fixed Nitrogen Research Laboratory, B u r e a u of Chemistry and Soils, U.S. Department of Agriculture, Washington, D. C. Received M a y 9, 1983 INTRODUCTION
The present study of the decomposition of nitrogen dioxide in the glow discharge was undertaken for the purpose of determining the limiting yields as well as the principles underlying the fixation of nitrogen in the electric arc. Attention was also given to the mechanism of reaction involved. APPARATUS AND METHOD
The apparatus and method of procedure were essentially the same as those used for the decomposition of nitrous oxide (1). A discharge tube of the type illustrated in figure 4 was enclosed in an electric furnace and the temperature maintained constant a t 170°C., except in the experiment on the temperature coefficient. This temperature was chosen so that the gas would be entirely in the form of NOz. Pressure changes were read with a Nujol oil manometer; the system was kept free of mercury vapor. The nitrogen dioxide was obtained from a reservoir of liquid nitrogen dioxide made by the arc process. RESULTS
The discharge current The relative rates of decomposition of nitrogen dioxide in the negative glow and positive column, as determined by the discharge current, are shown in figure 1. Line 1 is for the negative glow and line 2 for the negative glow and positive column combined. The difference between lines 1 and 2 gives the relative rate for the positive column alone, while the difference between lines 1V and 2V gives the potential drop in this region. It will be observed that the rates are proportional to the current in both the negative glow and positive column. The yield measured from the slopes of the lines is 21 molecules decomposed per electron of current in the negative glow, 80 molecules per electron in the positive column and 889
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A. KEITH BREWER AND P. D. KUECK
negative glow combined, and 59 in the positive column alone. The proportionality between current and rate shows this reaction to behave similarly to the other reactions described in this series. The voltage curves 1V and 2V give an indication of the power consumed per molecule for various discharge currents. The conspicuous difference in the two regions of the discharge is that the rate is proportional to the
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$1
20 30 40 50 Curred (mal. FIQ.1. RELATIVE RATESOF DECOMPOSITION OF NITROGEN DIOXIDE IN THE NEGATIVE GLOWAND POSITIVE COLUMN 0
10
power input in the negative glow, while in the positive column the voltage, and hence the power consumed, drops naturally as the current increases.
The pressure This reaction is again similar to the others studied in this series in that the rate of reaction in the negative glow is independent of the initial pressure of nitrogen dioxide between wide limits. The electrochemical equivalence law, dP = ffI dt
enunciated for the discharge is still applicable.
CHEMICAL ACTION IN THE GLOW DISCHARGE. X I
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The effect of pressure in the positive column is difficult to determine since the power input and effective length of the column both vary with the pressure.
The power eficiency The power consumed per molecule decomposed in the various regions of the discharge is illustrated in figure 2.
I
1 . 1
-
IO
I
I
20
I
I
36
I
40
I I
,
3
Current (“e.)
FIG.2. P O W E R CONSUMED PER MOLECULE DECOMPOSED I N THE VARIOUSREGIONS OF THE
DISWARQE
Line 1 shows the power consumption to be constant in the negative glow from 2.5 to 50 ma., and is equivalent to approximately 28 electron volts per molecule ( V , / M ) decomposed. The results obtained in the positive column are shown by line 2. It will be noted that the efficiency changes from V,/M = 30 a t 2.5 ma. current to V,/M = 4 a t 50 ma. This increase in efficiency is due entirely to a decrease in potential gradient, since line 2 of figure 1 shows the rate to be proportional to the current. The change in efficiency with respect to potential gradient is shown by line 3, in which it will be seen that V,/M decreases linearly with the po-
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A. KEITH BREWER AND P. D. KUECK
tential gradient throughout the column; the gradients given are those obtained with the various discharge currents.
Foreign gases The effect of foreign gases on the rate of decomposition is illustrated in figure 3. Especial attention is called to the fact that nitrogen and oxygen are very similar in their effects on the rate of decomposition, oxygen if anything being slightly more efficient. The rate of decomposition is slowed down even less by helium than by oxygen.
0’
I
I
I
I
IO
20
30
40
Sb
&
I
l
l
70
80
80
I 0
a m e n t NO*
FIQ3. THEEFFECT OF FOREIQN GASESON
THE RATEOF DECOMPOSITION OF NITROGEN DIOXIDE
The above results are in distinct contrast to those obtained with nitrous oxide wherein the retardations in rate of dissociation in the presence of nitrogen, oxygen, and helium were proportional to the partial pressure of nitrous oxide times the relative probabilities of ionization (e) of their gases. In the case of nitrous oxide, therefore, these foreign gases contributed nothing to the rate of decomposition. To say in the present case that activated nitrogen, for instance, did not decompose nitrogen dioxide it would be necessary to assign values of e = 1 to helium, 9 = 3 to nitrogen, and 0 = 31 to nitrogen dioxide, i.e., the probability for ionization in nitrogen dioxide must be over ten times that in nitrogen. Lind (2) assigns values to the stopping powers of these gases for a rays of 111 for
CHEMICAL ACTION I N THE GLOW DISCHARGE. XI
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nitrogen dioxide and 99 for nitrogen; thus the ionization efficiency in the two gases is nearly the same. It must be concluded, therefore, that an activated helium, oxygen, or nitrogen molecule is almost as effective in inducing decomposition as is an activated nitrogen dioxide molecule itself.
Temperature The effect of temperature on the rate of dissociation is shown in figure 4. The runs were carried out a t constant gas density from room temperature to 225°C. The voltage remained fairly constant over the entire temperature range.
50-
40-
0
30-
20
A
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FIG. 4. THE EFFECTOF TEMPERATURE ON THE RATEOF DECOMPOSITION OF NITROQEN DIOXID~
The conspicuous and surprising thing to note in these experiments is that the number of molecules decomposed per electron decreased almost linearly with temperature over the range investigated. At 225°C. about 30 electron volts are required to decompose one molecule, while a t room temperature this value drops to 10 electron volts. Equilibrium The 2N02 G Nz 202 equilibrium was measured by running the discharge in a N2.1202 mixture for about an hour at room temperature until
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A. KEITH BREWER AND P. D. KUECK
equilibrium was established. The pressures ranged from 5 to 10 mm. of mercury. Liquid air was then applied to the discharge tube and the nitrogen dioxide frozen out. The nitrogen and oxygen were next pumped off, the nitrogen dioxide was allowed to evaporate, and the pressure measured with an oil manometer. The results showed the nitrogen dioxide pressure to be 1.6 per cent of the initial pressure. The rate of synthesis of nitrogen dioxide in a liquid air-cooled tube of the type used in the decomposition experiments yielded 1.58 molecules of nitrogen dioxide per electron of current. Referring to figure 3 it will be seen that the rate of decomposition of nitrogen dioxide in a mixture containing approximately 1.5 per cent nitrogen dioxide and 98.5 per cent nitrogen or oxygen is equal to the observed rate of synthesis in a N2/202 mixture. The equilibrium value of 1.6 per cent nitrogen dioxide is, therefore, substantially correct. DISCUSSION OF RESULTS
The general interpretation to be given to these results is necessarily the same as that applied to the other reactions studied in this series, namely, that the reaction is brought about by positive ions formed in the discharge. The first step in the decomposition process is t.he formation of an NO2+, and N2+ or an 02+ ion. The second step consists of the dissociation of nitrogen dioxide by the ion so formed. A calculation of the number of molecules decomposed per ion in the negative glow can be made from an evaluation of the number of ions formed for each electron leaving the cathode. Since the positive ion current to the cathode is negligible (3), and since the electrons leaving the cathode receive an energy corresponding to the cathode potential drop, the number of positive ions formed per electron of current may be estimated directly from the results of Lehman (4)and of Anslow (5). Lehman obtained 13 ions per 600 volt electron, i.e., an expenditure of 45 volts per ion in nitrogen. Anslow, using a similar method, obtained a somewhat lower value of 5.9 ions for a 600 volt electron. Langmuir and Jones (6) obtained a value of about 85 volts per ion, which is equivalent to 7 ions per 600 volt electron. Figure 3 shows 20 molecules of nitrogen dioxide to be decomposed per electron. According to Lehman’s value, therefore, the number of molecules decomposed per NOs+ ion is M / N = 20/13 = 1.55. Anslow’s value gives M / N = 20/5.9 = 3.4, while from Langmuir and Jones the ratio M / N = 20/7 = 2.8 is obtained. It will be seen from this that an estimation of the M / N ratio necessarily involves an uncertainty factor of 2. Since the results of Lehman appear the most consistent, it is probable that t,he correct value a t 170°C. is M / N = 1.55 or possibly 2. A t
CHEMICAL ACTION I N THE GLOW DISCHARGE. X I
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room temperature, however, the M / N ratio is approximately three times that for 170°C. An estimation of the M / N ratio in the positive column is speculative, since there exists no reliable method for computing the number of ions formed per electron of current. Assuming the same voltage efficiency in the positive column as in the negative glow a ratio of M / N = 10 is obtained a t 50 ma. current. This value is necessarily high but serves as an upper limit. At 2.5 ma. the ratio falls to M / N = 1.5. It is reasonable to assume, therefore, that the true M / N ratio is essentially the same in the two regions of the discharge. The results presented in the series of studies on the synthesis and decomposit,ion of nitrogen dioxide in the discharge show directly the limits that may be obtained in the arc process for the h a t i o n of nitrogen. In the synthesis (7) it has been shown that it is possible to remove the nitrogen dioxide from the arc as fast as formed by cooling the discharge tube with liquid air. I n this case the reaction N2 2 0 2 = 2 N 0 2 goes to completion. The great source of energy loss results from the fact that only Nz+ ions contribute to the formation. On the other hand it is to be seen and NO2+ions all contribute to disfrom the present paper that N2+,02+, sociation, hence the rate of dissociation is rapid compared to synthesis. For this reason the equilibrium value is low, being of the order of 1.6 per cent nitrogen dioxide. Thus the lower limit for the above equation is 1.6 per cent on the side of nitrogen dioxide where the rates of synthesis and decomposition are equal, and 100 per cent when decomposition is eliminated. In practice, therefore, the yield must depend on bhe rapidity with which nitrogen dioxide is removed from the discharge. It is interesting to note that the equilibrium will be practically the same in air as in an N2/202 mixture, since both the rates of synthesis and decomposition are the same under these conditions.
+
MECHANISM OF REACTION
There are two probable mechanisms for the reaction about the ion when once formed. One is the step or quasi molecule mechanism, generally considered in photochemical reactions. It involves dissociation according to the equation: NO2
4
NO
+0
This step is impossible in the present case since it would necessitate a retardation in rate by oxygen; the possibility of nitric oxide being decomposed before being carried out of the discharge by convection is small. Figure 3 shows the dissociation in oxygen to be slightly greater than in nitrogen. Again such a mechanism would demand a positive and not a negative temperature coefficient since the rate of oxidation of nitric oxide
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increases with decreasing temperature. It must be concluded, therefore, that the production of nitric oxide is not a step in the reaction mechanism. The second possible mechanism for decomposition is one involving ion clusters. An ion cluster of the form (NO2+ NOz) or (Nzf. 2N02) is con202. This mechanism is in sidered as decomposing directly into Nz accord with the observed negative temperature coefficient since the dielectric constant, and hence the clustering ability of nitrogen dioxide, increases with decreasing temperature. It is interesting to note that the presence of such clusters has been shown to exist in mixtures of nitrogen and oxygen by mass spectrograph measurements by Luhr (8). The cluster mechanism will be discussed in more detail in a later communication.
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SUMMARY
The decomposition of nitrogen dioxide in the glow discharge is shown to obey the same electrochemical equivalence law given for the other reactions studied in this series. The decomposition is shown to be brought about with almost equal ease by NO2+,N2+,02+,and He+ ions. The M / N ratio at 170°C. is calculated for the negative glow and posi, = 1.55 and M / N = 3. The tive column. The ratio lies between M” lower value is the more probable. The decomposition has a strong negative temperature coefficient, the M / N ratio being three times as large at room temperature as at 225°C. A value of 1.6 per cent is obtained for the amount of nitrogen dioxide in the arc a t room temperature under equilibrium conditions. This is discussed in connection with the possible yields that can be obtained in the arc process for nitrogen fixation. The results are interpreted as indicating that the decomposition cannot be by step mechanism involving the formation of nitric oxide. It is suggested that the cluster type mechanism will best account for both the rate decomposition and the negative temperature coefficient. REFERENCES (1) KUECR,P . D., AND BREWER,A. K . : J. Phys. Chem. 34,2395 (1932). (2) LIND, S. C. : Chemical Effects of Alpha Particles and Electrons. The Chemical Catalog Co., New York (1921). (3) BREWER,A. K . , AND MILLER,R. R . : Phys. Rev. 42, 786 (1932). (4) LEHMAN,J. F . : Proc. Roy. SOC.London 116, 624 (1927). (5) ANSLOW,G . A.: Phys. Rev. 26, 484 (1925). (6) LANGMUIR, I., AND JONES,H . A.: Phys. Rev. 31, 357 (1928). J. W., AND BREWER,A. K . : J. Phys. Chem. 34, 554 (1930). (7) WESTHAVER, (8) LUHR,0 . :Phys. Rev. 38, 1937 (1931).
