2456
JULIANHEICKLEN
Reactions of NO(A22+) with Hydrogen, Methane, and Ethane'
by Julian Heicklen Aeroapace Corporation, El Segundo, California
(Received May 17, 1966)
Nitric oxide wag photoexcited with the 2144- and 2265-A lines of a low-pressure cadmium arc. The resultant NO(A22+)can react with unexcited NO or with Hz, CHr, or CzHato produce NZ and NzO. The relative reactivities of NO(A22+) with NO, Hz, and CH4 are in the ratio 1.00:0.32 :0.86.
I. Introduction As a continuation of our studies on the reactions of NO(Az2+), we have looked at the reactions of NO(A"+) with Hz, CHI, and CZH~.In these systems the CHaNO, CZH~NO) are produced. species RNO ("0, The photochemical investigation was performed with excess NO present (to scavenge H atoms and alkyl radicals completely), at temperatures below 200" (to prevent isomerization of RNO), at very low intensities (so that radicals would be removed only by NO), and at very small conversions (to minimize reactions involving products). Thus, complicating side reactions could be minimized.
II. Experimental Section The quartz reaction vessel was 10 cm long and 5 ern in diameter. It was jacketed in a wirewound furnace that overlapped each end by 2.5 cm. Temperature measurements were taken by a thermocouple; the temperature remained constant throughout a run to within a few degrees. Both ends of the furnace were covered with quartz plate to minimize heat loss by convection. The optical source WELT an Osram cadmium spectral lamp operated at 12 v, 1.6 a, with outer glass bulb removed to permit transmission of the 2144and 2265-A lines. Before entering the reaction cell, the radiation passed through a sodium chloride filter sandwiched between quartz plates. This effectively removed radiation below 2050 A. The gases used were from the Matheson Co. The hydrogen and methane were used directly from the cylinder without further purification. The ethane was degassed twice at -196" before use. The nitric oxide was degassed twice at - 196"; it was then warmed to -186" by liquid argon. The fraction not retained by liquid argon was collected and used. In this way, The Journal of Physical Chembtry
NO2 and most of the NzO, both of which were impurities in the NO, were excluded from the reaction mixture. The NzOwas reduced to about 0.01% of the NO. After irradiation of pure nitric oxide, the cell gases were expanded into the vacuum line through a trap frozen in solid nitrogen. On the far side of the trap was a National Research Corp. alphatron gauge that read the pressure of the noncondensable gases-nitrogen plus the vapor pressure of nitric oxide. The nitrogen then was pumped away and the residual pressure was measured. From these measurements and appropriate expansion factors between the reaction cell and the partially frozen vacuum line, the amount of nitrogen produced could be calculated. A second trap placed between the alphatron and the trap cooled in solid nitrogen was then immersed in liquid argon, and the solid nitrogen was removed from the first trap. All gases not condensable at liquid argon were pumped away. Thereafter, the liquid argon trap was replaced by a trap at -160". The noncondensable gas, NzO, was collected and measured. By correcting for the background NzO and using appropriate expansion factors, we computed the amount of NzO produced during irradiation. When mixtures of NO and either Hz or CH, were irradiated, only the NzO analysis was done. For one of the HZ-NO runs, the N20 was collected and passed through a gas chromatograph to confirm its identification. With NO-CZH~mixtures, only the Nz was measured. For one run, a mass spectrum taken of the noncondensable gas showed it to be at least 75% nitrogen, with the only possible impurity being CO. Methane was definitely absent.
(1) This work was supported by the U. S.Air Force under Contract AF 04(696)-469.
REACTIONS OF NO(A21:+) WITH HYDROGEN, METHANE, AND ETHANE
III. Results The results are listed in Tables I through V and plotted in Figures 1 through 5. The only products for which analyses were performed were N2 and N20. Table I shows the (N2)/(N20) ratio for photolyses of pure NO. Except for a few runs at room temperature at low NO pressures, the ratio is approximately independent of all variables. At 23' the average value is 2.3 f 0.5, and at 195' it is 1.8 f 0.2. In all likelihood, these values should be the same. We take the average value to be 2.0. The high values at room temperature reflect high nitrogen values. The N20 values showed much less scatter. These high nitrogen Table I: Photolysis of NO Exposure time, hr
(NO), mm
(Nz)/(NzO)
2457
Table II: Photolysis of NO-HZ Mixtures
1.53 5.31 15.63 16.07 49.6 102.9 158 475
T = 23 f 1' 147.7 26.42 146.6 15.00 148.2 12.58 151.3 20.58 156 15.08 160 26.42 153.5 8.58 153.8 6.25
0.115 0.102 0.190 0.172 0.25 0.53 0.49 0.93
62.8 105.9 200.5 396 595
T = 23 f 2' 157 11.33 161 15.17 153.5 7.50 154.0 6.33 152.4 4.00
0.204 0.45 0.59 0.88 1.22
T = 23 f 1' 21.6 49.7 49.7 49.8 49.8 50.0 117.6 115.9 152.2 152.2 151.7 156.8 451 431
23.17 15.58 15.83 20.25 21.08 21.33 17.00 19.50 15.33 17.33 19.92 20.00 7.25 15.00
T
=
117.7
T 152.6 152.7 152.5 153.9 151.7 146.6 151.0 153.0
83 f 1' 20.00
1.5
120 f l o 21.33
1.55
195 f 2' 8.08 11.08 11.50 12.75 12.92 15.25 17.00 18.62
1.7 1.4 1.8 1.9 1.6 2.2 2.1 1.8
Table 111: Photolysis of NO-CHI Mixtures
20.4 73.0 150.9 364 680 13.2 55.7 97.2 101.0 211 398 610 616
=
122.4
T
2.9 6.2 5.8 2.2 3.2 4.1 1.7 1.7 1.6 2.3 2.4 2.2 3.4 2.0
=
values probably resulted from trace amounts of hydrocarbon impurities from previous runs. Only small amounts of hydrocarbons can measurably enhance the nitrogen yield, an effect that is much more pronounced at room temperature than a t more elevated temperatures.
a
T = 23 f 1' 56.3 11.17 49.9 6.17 49.7 5.08 49.4 4.08 49.8 5.58 T = 194 rt 155.6 148 165 162 150.7 152.8 152.7 152.4
2' 14.25 14.92 4.00 10.25 11.33 5.17 6.25 5.08
0.60 0.96 1.48 2.00" 1.94 0.20 0.41 0.74 0.65 1.28 1.61 1.18 1.74
Assumed.
Tables I1 and I11 give, respectively, the N20 yields, @(N20),for various mixtures of NO with H2 and CH4 at 23 and 194'. The yields increase from about 0.1 to 2.0 as the (H2)/(NO) and (CH,)/(NO) ratios rise. The absolute quantum yields were computed by assuming the N20 yield to be 2.00 for large (CH4)/ (NO) ratios, a result expected from the reaction mechanism (see Discussion). For pure NO, the N2O yield %(N20) then becomes 0.096 and the nitrogen yield 4(N2) = 0.19, independent of pressure or temperature Volume 70,Number 8 Auouat 1966
JULIANHEICKLEN
2458
Table IV : Photolysis of NO-CnHo Mixtures (CIHO), mm
(NO), mm
Exposure time, hr
T = 23 f lo,(NO) 1.38 5.73 14.83 58.2 66.0 123.9 577 668 672
53.3 55.0 59.2 49.6 49.4 49.8 49.4 49.6 48.9
T
= 195 f 2', (NO)
15.92 47.83 170 306 309 577 599
155.2 155 153.0 152.8 153.1 154.7 152.6
T
=
2.00 5.93 17.3 46.0 -730
-
50 mm 13.25 4.50 2.00 1.00 10.50 0.50 7.33 2.00 0.50
-
152 mm 15.25 11.33 7.00 7.33 4.50 4.33 6.17
-
23 i= lo,(NO) 115 mm 112.0 16.75 117.9 3.00 113.2 1.00 114.0 1.00 118.5 0.33
WNt)
23.C
2.8 10.1 13.6 15.0 23.4 27.2 35.4 41.6 48.8 0.33 0.65 0.94 0.93 1.39 1.43 2.62 2.9 9.3 15.0 31 83
Table V: @-(N2)for NO-C2Hs Mixtures T, OC
(NO), mm
(C2Hs), mm
23 23 23 23 23 23 47 84 121 152 195 195
21.6 21.9 48.9 49.6 49.4 118.5 116.7 121.8 116.7 117.7 154.7 152.6
337 266 672 668 577 -730 -730 -730 -730 -730 577 599
Exposure time, hr
WNS
0.516 1.00 0.50 2.00 7.33 0.33 1.00 1.00 1.50 3.00 4.33 6.17
12.6 20 49 42 35 83 45 25 8.8 6.3 1.43 2.6
as expected from earlier work.2 The quantum yield of NO disappearance in pure NO can be computed to be 1.05. (Four molecules of NO disappear for every Nz formed, and 3 molecules of NO disappear for every N20 formed.) This value of 1.05 is similar to that of 1.45 i 0.10 found by MacDonalda for NO disappearance with more energetic radiation. From the quanThe J O U Tof~Physical Chmi8lTy
t IO-' IO'*
1
10-1
10
(CH4)1(NO)
Figure 2. Plot of { [@(NaO)/%(NzO)]- 1)/ [2
- @(NzO)]US. (CH,)/(NO).
tum yields, the absorbed intensities used in these studies were computed to be between 8 and 52 equivalent p / h r or about 2 X 10" quanta/cc sec. In Table IV are shown the quantum yields of nitrogen formation @(Nz)at 23 and 195' for various C2Hs NO mixtures and exposure times. The yields rise ~~
(2) J. J. McGee and J. P.
~
Heicklen, J . C h m . Phys., 41,2974 (1964).
(3) J. Y. MacDonald, J . Chem. SOC.,1 (1928).
REACTIONS OF NO(A22+)WITH HYDROGEN, METHANE, AND ETHANE
2459
1-
-
I
0 0
IO"
T:23*C,NO-I15mm
NO, mm
0 T 3 195*C, NO-152 mm
Figure 3. Plot of @,(N*)us. NO pressure a t 23".
from about 15 to 83 as the NO pressure rises from 20 to 118 mm. At high NO pressures, k ( N 2 ) drops as the temperature is enhanced.
IV. Discussion The reaction in the absence of foreign gases is2
+N20 IO2
2.0
2.4
2.8
3.2
IOVT, OK-' Figure 4. Plot of @,(N*)/(NO)us. !l'-'.
markedly with (C2He)/(NO), the effect being more pronounced a t room temperature were Qi(Nt) reaches 83. The duration of exposure has almost no effect on the results. Increasing the length of irradiation by a factor of nearly 15 for mixtures of about 600 mm of C2He and 50 mm of NO at 23" only reduces @(N2) by about 25%. For all the runs reported in this study, the amount of products formed was less than in the 7.33-hr run a t 23". Thus, the products do not play an important role in the reaction mechanism. Table V shows Qim(N2), the N2 yield in a large excess of hydrocarbon, at various temperatures and NO pressures. At room temperature, Qi, (Nt) increases
+ 0 -% NO2
(2c)
where NO* is theA22+state of NO. Reaction 2c was not reported in ref 2, but is clearly needed to explain N2O formation. The data in Table I show that @(N2)/@(N20) is independent of pressure and thus (2c) and (2b) must be of the same order. This can be contrasted with the case when NO* is the a4ri state. For that system @(N20)/@(N2)increases linearly with NO pre~sure.~Presumably, with a4ri molecules reaction 2c is not sufficiently rapid to proceed unless an additional NO molecule is present so that the products are N20 and NO2. With the more energetic AZZ+ state, reaction 2c proceeds smoothly. I n the presence of H2, CH4, or GHa, the following reactions (3a-c) can also occur. (4) 0.P. Strausz and H.
E. Gunning, Can. J . Chem., 39, 2549
(1961).
Volume 70,Number 8 August 1966
JULIAN HEICKLEN
2460
+ H2+ H + H + NO NO* + CH, H + CH3 + NO NO* + C2H6 +H + C2H5 + NO NO*
The evidence for steps 5a and 6a is not as conclusive. Arden and P h i l l i p l6 ~ ~examined ~~ the diethyl peroxide4 (3b) nitric oxide system at 95 and 135". At 95" they found ( 3 ~ ) large amounts of Nz which increased with the NO pressure. In their earlier communication, they explained Reactions 3 might lead directly to HNO formation, the N2 formation to be a result of reaction 5a, the HNO but this is kinetically indistinguishable from the subcoming from sequent addition steps CzH60 NO +CHsCHO HNO H NO M-HNO M (44 In their later work, a more careful analysis of their CH3 NO (+M) +CH3NO (+M) (4b) data indicated to them that reaction 5b was the principal source of N2. However, their results still C2H5 NO +C2HsNO (44 required that NO2 be produced from the reaction of NO with HNO. Under our conditions of low intensity and high NO presSrinivasan17 found that when 15NH3was photolyzed sure, all the hydrogen atoms and alkyl radicals will in the presence of 4-12 mm of 14N0, a small amount be scavenged by reactions 4. of the nitrogen produced was I4N2and that its rate of There are a number of possible fates for the RNO production was proportional to the NO pressure. The species. At temperatures above 200", RNO can 14N2 can only be produced from the termolecular reisomerize, and the isomer can decompose both homogenaction of HNO with two molecules of NO. Since its eously and heterogeneously.6 However, for CHaNO, rate of production was linear with (NO), the step reboth the isomerization reaction and the subsequent moving HNO must be 6a. decomposition have been reported to occur with an A stronger argument for reactions 5a and 6a comes activation energy of about 40 k ~ a l / m o l e . ~Con~~ from this study. Our results show that (P,(N20) is sequently, they cannot be important at lower temperathe same in both the H2 and CH, systems. Since one tures. Strauz and Gunnin@ found that C2H6NO does N20 molecule is produced for every CHI radical, then not isomerize at room temperature. However, Woodone N20 must be produced for every hydrogen atom. all and Gunnings found that n-C3H?NO did isomerize Additional evidence is obtained from a consideraat room temperature. Thus at our higher temperation of k ( N 2 ) in the C2H6-N0 system. Christie, tures there is some possibility that C2HSNO might Gilbert, and Voisey12 added NO to 5.3 p of C2HsN0 have isomerized. at room temperature and found values of y/(NO) of The chain mechanism involving RNO is 0.45 mm-l where y is the number of NO2 molecules HNO 2N0 + H N2 NO3 2N02 (5a) produced per molecule of C2H5N0. The amount of NO2 should be between one and two times the amount CHSNO 2N0 + of N2.14 Consequently, from Christie, Gilbert, and CH3 N2 NO1 -% 2N02 (5b) (5) (a) G. L. Pratt and J. H. Purnell, Trans. Faraday SOC.,60, 371 C2HsNO 2N0 +
+ +
+
(34
+
+
+
+
+ +
+ +
2
+ +
+
~
+ + NO3 -% 2N02
C2H5 N2 and the chain-breaking step is
+ +
+
(5c)
HNO NO +HO N2O (64 CH3NO NO +CH3O N20 (6b) C2HhNO NO +C2H50 NzO (6~) The chain steps 5b and 5c are well known.7,8,10-14 The chain-ending step 6b has recently been measured.l 4 Under the conditions Of this study, it iS much more important than the dimerization of CHaNO. The fact that @,(N2) (see Figure 3) is linear with (NO) is clear evidence that ( 6 ~ )is the important chain-terminating step in the C2HsNO system and that the dkflerizacan be neglected. tion of C~HSNO
+
The Journal of Physical Chemistry
+
~
(1964); (b) L. Batt and B . G. Gowenlock, ibid., 56, 682 (1960). (6) B . G. Gowenlock, L. Batt, and J. Trotman, Special Publication No. 10, The Chemical Society, London, 1957, p 75. (7) M. I. Christie, PTOC.Roy. SOC.(London), A249, 258 (1958). (8) 0.P. Strausa and H. E. Gunning, Can. J . Chem., 41, 1207 (1963). (9) G. N. C. Woodall and H . E. Gunning, Bull. SOC.C h i n . Belges, 71, 725 (1962). (10) J. F. Brown, Jr., J. A m . Chem. SOC.,79, 2480 (1957). (11) E. J. Burrell, Jr., J . Phys. Chem., 66,401 (1962). (12) M. I. Christie, C. Gilbert, and M. A. Voisey, J. Chem. SOC.,3147 (1964). (13) M. I. Christie, J. S. Frost, and M. A. Voisey, Trans. Faraday SOC.1 61,674 (1965). (14) T . Johnston and J. Heicklen, Aerospace Report No. TR669(6250-40)-8 (1966). (15) E. A. Arden and L. Phillips, PTOC. Chem. SOC.,3% (1962). (16) E. A. Arden and L. Phillips, J. Chem. Soc., 5118 (1964). (17) R. Srinivasan, J . Phys. Chem., 64, 679 (1960).
REACTIONS OF NO(A2L:+)WITH HYDROGEN, METHANE, AND ETHANE
Voisey's work, a minimum value corresponding to am(N2)/(NO) of 0.23 mm-I is obtained. This value can be low for three reasons. (1) The NO2 may not be twice the N2. (2) The CzHsNO pressure was estimated from the I2 yield (C2HsI was photolyzed in the presence of NO to produce the C2HsNO). Since C2H6N0 is unstable and is removed, its estimated concentration is a maximum. (3) Some of the C2HsNO may be removed by dimerization rather than by reaction 6c. That this may occur is indicated by the fact that when the C2HsN0 pressure was doubled, am(N2)/(NO)dropped by one-third. In any event, k.(N2)/(NO) must be at least 0.23 mm-1, is probably greater than 0.3, and may be as high as 0.4 or 0.5 mm-1, but no higher. In the work reported here for the C2Hg.N0 system, am(N2)/(N0) is 0.7 mm-l a t room temperature. Thus reaction 5a must be operative. On the other hand, Strausz and Gunning18 believe that reaction 5a is not a chain step and should be written HNO
+ 2N0 +
"03
+ N2
The N20would be formed from the well-known 2HNO +H2O
+
+ NO
--f
RON0
(7)
The possible competing reaction
RO
k3c -(C2H6) -_ k2c
(NO)
(8c)
Figures 1 and 2 are log-log plots of the left-hand side of eq 8a and 8b us. (H2)/(NO) and (CH4)/(NO), respectively. The plots are linear with unit slope and are independent of temperature. They yield values of k 3 a / h c and k 3 b / b of 3.3 and 8.8, respectively.
Table VI: Rate Constant Ratios Ratio
Value
kdkz kdkz k2o/k2 kdk2 kablkz kslk2
0.71 0.19 0.096 0.32 0.86 0.67a t 195' 2.45 at 23"
Source
1 - %(N2O) - @o(Nz) @dN2) @dN2O ) Eq 8a, Figure 1 Eq 8b, Figure 2 Eq 10, Figure 5
+ N2O
Such a mechanism predicts that for the HNO system @(N2) 2@(N20)= 1.0, a result approached at high NO pressures in Strausz and Gunning's studies. It is difficult to reconcile the Strausz and Gunning mechanism with our findings or with those of Srinivasan. As pointed out by Arden and Phillips, such a mechanism is contrary to their own (Arden and Phillips') findings.16 Even if the Strausz and Gunning mechanism is correct, and every H atom does not lead to an N20 molecule, then the quantum yields reported here will be somewhat high (perhaps as much as a factor of 2), and the first three entries of Table VI will need modification. However, the last three entries of Table VI will be unaffected. The RO species is always removed by
RO
2461
+ NO -+ R'O + HNO
is Only 0'15 as important "(7) in the isOProPY1system.23 Since isopropyl has an easily abstractable hydrogen atom, the latter reaction is surely unimportant for HO, CHIC), or C2HsO. Furthermore, for CH30, the reaction has been shown to be negligible.'4*24 The mechanism predicts eq 8a-c.
For the C2HsN0 system there is another reaction that must be c ~ n s i d e r e d . ~ ~ C2H5
+ NO +C2H4 + HNO
Strausz and Gunning actually found CzH4, but its source was not definitely established. On the other hand, C3Hewas not found in the C 3 H r N 0 s y ~ t e m . ~ The reaction effectively converts C~HSNOto HNO. Except in the unlikely event that these species have similar reactivities, this reaction complicates the analysis. The importance of the reaction is not known. For the purpose of simplicity in the ensuing discussion, it will be ignored, though it is not clear that to do so is correct. Omitting the reaction leads to the eq 9. (18) 0.P.Strausa and H. E. Gunning, Trans. Faraday SOC.,60,347 (1964). (19) H.A. Taylor and C. Tanford, J. Chem. Phys., 12,47 (1944). (20) A. Serewica and W. A. Noyes, Jr., J . Phys. Chem., 63, 843 (1959). (21) 0.Husain and R. G. W. Norrish, Proc. Roy. SOC.(London), A273, 145 (1963). (22) F. C, Kohout and F. W. Lampe, J , A m . Chem, Sot., 87, 5795 (1~65). (23) G. R. McMillan, ibid., 83, 3018 (1961). (24) A. R. Knight and H. E. Gunning, Can. J. Chem., 39, 1231 (1961). (25)E. L. Metcalfeand A. F. Trotman-Dickenson, J. Chem. SOC., 3833 (1962).
Volume 70,Number 8 Auguat 1966
2462
F. E. DIEBOLD AND C. L. HILTROP
(9)
For the room-temperature runs, @-(N2) is plotted against (NO) in Figure 3. The log-log plot is reasonably fitted by a straight line of slope 1. Thus, the results extend those of Srinivasanl' to higher NO pressures. The log of @,(Nz)/(NO) is plotted vs. the reciprocal temperature in Figure 4. The Arrhenius plot is not linear, which is not too surprising. In the first place, the sum of two rate constant ratios is being plotted; if these ratios have different activation energies, then deviation from linearity would be expected.' Secondly, reactions 5 are probably not so simple as written, but may proceed via an R(N0)2 intermediate. In any event, however, the mechanism does predict that relationship 10 above
+ +
where h = ha k2b k2c. The left-hand side of (10) is plotted vs. (CzHe)/(NO) in Figure 5 at 23 and 195". The plots are linear with unit slope, in conformance with eq 10. However, the lines that best represent the data points yield values of k k / h that differ by a factor of 3.7 for the two temperatures. Such a result is contrary to our expectations and diffcult to believe. Perhaps the mechanism is more involved than outlined here. All of the rate constant ratios are tabulated, for convenience, in Table VI.
Achowledgment. The author wishes to thank Mrs. Barbara Peer for assistance with the manuscript and Professors Harry Gunning and Otto Strausz for stimulating and useful discussions.
The Prediction of Stable States
by F. E. Diebold and C. L. Hiltrop Department of Chemistry, Colorado School of Mines, Golden, Colorado
(Received July 23,1966)
A technique is presented for the determination of the stable phases and calculation of their respective amounts for any specified bulk composition in multicomponent systems. The required experimental information is a list of the potentially stable phases and their free energies of formation at the temperature and pressure of interest. All combinations of potentially stable phases are initially considered as being possible stable states, but some of those possibilities are rejected by considering the list of existent compounds. Others are eliminated on the basis of the specified bulk composition and the stoichiometry of the reactions producing each assemblage of phases. Finally, that combination which results from those reactions producing the greatest over-all decrease in free energy of the system is selected as the stable state.
Introduction Research in the field of phase relationships is limited to system which have a maximum of four or five cornponents. This limitation is not only due to the tedious experimental work required for such multicomponent systems, but is also affected by the difficulty in portrayThe Journal of Physical Chemistry
ing the experimental results. For the unary, binary, and ternary systems, act description of the stable states can be accomplished by graphical means. HOWever, various approximations must be introduced to represent the stable states for other multicomponent systems on a plane diagram.
