W. E. GROTH,U. SCHURATH, AND R. N. SCHINDLER
3914 ethylene and methyl radical yields in the radiolysis. For instance, process 23, which may account for an ionpair yield of CH, radicals in the radiolysis as high as 0.045 (according to the 70-eV mass spectrum), has not
The Photolysis of Ammonia at 2062
been considered in the above derivation; if this process is indeed that important in the radiolysis, the value for N,,/Ni derived from the CH3 radical yields would be lowered to -0.1.
in the Presence of Propane
by W. E. Groth, U. Schurath, and R. N. Schindler Institut far physikalische Chemie, Unioersitdt Bonn and Kernforschungsanlage Jiilich, Jiilich 1, Germany (Received M a y 7 , 1968)
Ammonia was photolyzed a t 2062 A in the presence of propane as a hydrogen atom scavenger. The only hv(2062 A) +. NHz(~BI) H (a). Hz, Nz, NzH4, photo process a t this wavelength is shown to be NH3 C3H6, C~HM, i-C3H,NH~,n-C3H&Hz, and small amounts of unidentified amines were produced in the photolyses. All products were determined quantitatively as functions of absorbed irradiation dose. A reaction mechanism is developed on the basis of the observed quantum yields. The ratio of disproportionation to addition reactions between amino and propyl radicals is obtained to be 0.21 0.07.
+
Introduction The absorpt$n spectrum of ammonia in the region between 2200 A and approximately 1660 A’ consists of a system of diffuse bands. The strong diffusiveness of the spectrum in this wavelength range is apparently produced by predissociation. Photolysis at 2062 8, corresponding to an energy of 138 kcal/einstein, should thus lead to the dissociation of ammonia molecules with a quantum yield of unity. On energetic grounds only two primary processes need be discussed NH3
+ hv(2062 A) + NH2(2B1)
NH3
+H
+ hv(2062 8) + ”(‘A) + H,
105 kcal/mol
(a)
ca. 123 kcal/mol
(b)
According to Herzberg,2 process b should yield electronically excited NH(lA) radicals, the excitation energy being in the order of 28 kcal/mol. Process a, however, yields ground-state NH2(2B1) radicals, as can be deduced from a potential diagram derived by Douglas. Energetically, the formation of electronically excited NH2(2A1) radicals would also be possible. The excitation energy is 29.2 kcal/mo!. Thisoinvestigation confirms that at a wavelength of 2062 A photodissociation of ammonia proceeds exclusively via process a. Using propane as a hydrogen atom scavenger, reactions of NH2(2BJ radicals with propyl radicals and propane were investigated. The Journal of Physical Chemistry
+
Experimental Section Materials. Ammonia was dried on KNHz and degassed before use. Impurities were not detectable. HBr gas was prepared from 48% hydrobromic acid by reaction with Pz06. Molecular bromine was removed by repeated distillation over Hg. Phillips Research grade propane was used. The propane contained 3 X lo+% ethane and 8 X lop3% n-butane. ND3 contained an unknown impurity which could be effectively removed by preirradiation of the sample before experimental use. SFe from the Matheson Co. was used without further purification. Irradiations. A microwave-powered iodine lamp served as the light source. Argon (0.8 torr) and excess iodine were sealed off in a Pyrex flask with a quartz; window. The iodine pressure could be kept constant by an ice-water bath. The lamp emits the iodine atom line at 2062 8 with high intensity. Emission lines at 1876 8, 1844 A, and shorter wavelengths were eliminated by insertion of a water-filled quartz cell of l-cm path length into the light beam. Photolyses were carried out in cylindrical quartz cells of 10-cm length and 5.5-cm diameter. Cell and lamp front windows were 5 cm apart. Irradiations were performed at ambient temperature. HBr gas (1) K. Watanabe, J . Chem. Phys., 22, 1564 (1954). (2) G.Herzberg, “Spectra of Polyatomic Molecules,” D. Van Nos-
trand Co., Inc., Princeton, N. J., 1966. (3) A. E.Douglas, Discussions Faraday Soc., 35, 158 (1963).
PHOTOLYSIS OF AMMONIA IN
THE
PRESENCE OF PROPANE
at a pressure of 60 torr was used as an actinometer. Light intensities were calculated from the amount of = 1.00. I n all experihydrogen evolved, assuming ments the ammonia pressure was at least 37.5 torr, providing for total absorption of the incident radiation. The absorption coefficients of HBr and NH3 are 15 cm-' and 23.8 cm-L5 respectively. C3Hs and SF6 are transparent at 2062 A. Analyses. The yields of the noncondensable gases were determined by pV measurement and subsequent mass spectrometric analysis. Hydrocarbons and amines were analyzed gas chromatographically. Products were identified by retention time. Separation of hydrocarbons was achieved on a temperature-programmed silica gel column. Ammonia and amines were retained quantitatively under working conditions. The amines were separated on chromosorb W, 45/60, treated with 15 wt % KOH and 15 wt % polyglycole 1000. A flame ionization detector was used. Because of its very low sensitivity for ammonia, the amines could be determined in spite of heavy tailing of the ammonia peak. The determination of hydrazine was carried out photometrically in solution.6
Results (a) Photolysis of Pure Ammonia. Pure ammonia was photolyzed at a pressure of 37.5 torr. The yields of the permanent gases and of hydrazine as functions of quanta absorbed are given in Table I. The ammonia quantum yield was 0.325 after absorption of 3 X 10ls quanta. A pressure dependence has not been investigated. Hydrazine yields are plotted in Figure 1. Table I : Product Quantum Yields as Functions of Quanta Absorbed in the Photolysis of Pure Ammonia (PNHs = 37.5 torr)" Quanta absorbed x 10-17
5.58 12.7 20.7 32.4 103 20.3 19.5 5.75 25.9 a
% Nz
QNz
4Hz
18.8 19.7 22.5 24.5 25
0.103 0.122 0,143 0.157 0.163
0.443 0.495 0.483 0.485 0.490
4 ~ ~ 8 4
0.036 0.034 0.003 0.0005 0.014 0.014 0.023 0.0066
The hydrazine quantum yields are plotted in Figure 1.
(b) Photolyses of NH3-CsH8 Mixtures. Efect of Propan,e Pressure. I n a series of runs the quantum yields of permanent gases and of hydrazine were measured. The results are given in Figures 2 and 3. At light intensities of 5.5 to 8.8 X 1Ol6 quanta/sec about 2 X 10ls quanta were absorbed per irradiation. The
3915
4
3
2 37.5 torr NH3* 50 torr C3Hl
1
50
100
abs.Light buanta x 10 171 Figure 1. Hydrazine formation in the photolysis of pure ammonia and of ammonia-propane mixtures.
hydrogen yield rises from 0.49 in pure ammonia to 0.90 at a propane pressure of 700 torr. On the other hand, the nitrogen quantum yield is reduced by the addition of propane and reaches a limiting value of 50.01 at high propane pressures. The hydrazine quantum yield rises from ca. 0.01 in pure ammonia to a distinct maximum of 0.068 a t 16 torr propane pressure and then falls off again. To investigate the influence of total pressure on product formation, one irradiation was performed in the presence of 500 torr of SFB. The results of this run are included in Figures 2 and 3. It is seen that the addition of 500 torr SF6does not influence the quantum yields measurably. E$ect of Radiation Dose. The following experiments were carried out at a propane pressure of 50 torr. Gas chromatographic analyses deteriorated when higher propane pressures were used. Quantum yields of H2 and Nz are plotted in Figure 4 as functions of the number of quanta absorbed. Hydrogen quantum yields tend to fall off toward longer irradiation times. The limiting value of $H$ at short irradiation times is about 0.77. The nitrogen quantum (4) E. Warburg, Berl. Akad. Ber., 300 (1918); R. M. Martin and J. E. Willard, J. Chem. Phys., 40, 2999 (1964). (5) P. Harteck, R. R. Reeves, and B. A. Thompson, Z.Naturforsch., 19a, 2 (1964). (6) C. W. Watt and D. Chrisp, Anal. Chem., 24, 2006 (1952).
Volume 78, Number 11 October 1068
3916
W. E. GROTH,U. SCHURATH, AND R. N. SCHINDLER
a I
v
U 3
c
e G.!
A
with 500 ( e r r SF6Wdd
I
!
02
0
100
200
300
,400
500
600
700
OJ’l
P c ~ H[torr] ~
*
_-____._._ ,. * ;. . ,. .., . . !N2, .
1
Figure 2. Quantum yields of noncondensable gases as functions of propane pressure. Absorbed dose, 1.5-2.5 X quanta.
./
100
10
abs.Light buanta x
io1’]
Figure 4. Quantum yields of noncondensable gases as a function of absorbed dose. P N H ~ = 37.5 torr; pcSas = 50 torr.
100
200
300
400
5&
Sbo
&
‘C3He [torr]
i
1’0
100
abs.Light buanta x i o 1 V
Figure 3. Hydrazine quantum yields as a function of propane pressure. Absorbed dose, 1.5-2.5 X 1018quanta.
Figure 5 . Propylene and hexane quantum yields as a function of absorbed dose. ~ N =H 37.5 ~ torr; ~ c =~ 50H torr. ~
yield is within experimental error independent of irradiation time equal to 0.045 k 0.01. Quantum yields for propylene and 2.3-dimethylbutane formation are plotted in Figure 5 . The limiting ~ short H ~ irradiation times is 0.16. Absorpvalue ~ c for tion of 5 X 1OI8 quanta yields a concentration of 1.5 X 10l6 molecules of CIHB within the reaction cell cortorr. responding to a partial pressure of 2 X The quantum yield of CSH14is 0.105 at low conver~~ at longer irradiation times. sion. $ c ~ Hdecreases Propylene, 2.&dimethylbutane, and an isomer hexane were the only detectable hydrocarbon products when conversion was kept low. The isomer hexane yield was in the order of 5% of the 2.3-dimethylbutane. Quantum yields for amine formation are plotted in Figure 6. A typical gas chromatogram of a sample after absorption of 5.95 X 10l8quanta is reproduced in Figure 7. It can be seen that in addition to iso- and n-propylamine, several further products are formed.’ D is still not identified. E is assumed to be an isomer
hexene diamine. Both retention times are shorter than that of isopropylhydrazine. Hydrazine yields as a function of quanta absorbed are plotted in Figure 1. The quantum yield decreases with prolonged irradiation. After absorption of 1.5 X lo9 quanta the limiting hydrazine concentration had not yet been reached. A steady-state pressure of ea. 6 X 10-2 torr of NzH4may be estimated from these data. ( e ) Photolysis o f ND3-C3H8 Mixtures. Mixtures of 40 to 41 torr of ND3and 47 t o 51 torr of C3H8and a mixture of 41 torr of ND3 and 620 torr of C3H8 were irradiated. Quantum yields for the formation of permanent gases may be taken from Table 11. H, and Dt are produced in negligible amounts only compared with HD. The ratio H2:HD was (5.9 f 0.4) X
The Journal of Phgsical Chemistry
(7) .Quantum yields of unidentified products were calculated on the basis of calibrations with isopropylamine. Thus for product D a quantum yield of 0.01 is found at d0.2% conversion. E and other unidentified products are 0.04 under the same conditions, E being the predominant species at higher conversion.
PHOTOLYSIS OF AMMONIA IN
THE
PRESENCE OF PROPANE
R41
39 17
Table 11: Photolysis of NDa-CaHs Mixtures (640 torr of SFBAdded in One Experiment) Quanta absorbed
tn
X
lo-’’
+(HD+D~)“
4 E
+N2
40 torr of NDa 0.763 0.01 0.742 0.01
6.2 5.9
ab5.Light [,,anta
+DZ
HdHD
Dz/HD
102
102
x
x
+ 51 torr of C3He 0.04 0.04
5.6 5.5
1.5 1.6
3.25 10.4
41 torr of ND3 4- 51 torr of CaHs 0.01 0.035 5.7 0.813 0.791 0.01 0.04 5.5
1.6 1.6
4.35 17.2 16.8
41 torr of ND3 620 torr of CaHe >0.923 60.003 0.001 6.1 20.941 0.003 0.004 6.3 30.934 0.006 0.0035 6.2
6.1
41 torr of ND3 47.5 torr of C3Hs 0.811 0.008 0.03 5.9
6.1
Same Mixture 640 torr of SFe >0.70 0.008 >0.02 5.6
+
x IO”]
Figure 6. Amine quantum yields as a function of absorbed dose.
Figure 7. Gas chromatogram of an irradiated sample of 37.7 torr of ammonia and 50 torr of propane. Absorbed dose, 6 X 1018 quanta: A, ammonia; B, isopropylamine; C, n-propylamine; D and E, not identified.
Table 111: Product Quantum Yields in the Photolysis of NHa-Isopropylamine Mixtures Quanta absorbed
x
practically independent of propane pressure, while the ratio Dz:H D fell off toward higher propane pressure. Hydrogen quantum yields were found to be somewhat higher, nitrogen yields somewhat lower than in ”3 experiments a t equal pressures. The irradiated samples were also analyzed by gas chromatography. The results were identical with those obtained with light ammonia. I n one further run, 41 torr of ND3 and 47.5 torr of C3H8were irradiated in the presence of 640 torr of SFe. Permanent gas yields were not significantly changed by the addition of SFe. Moreover, Hz :H D and DZ:H D ratios remained unchanged within experimental error. ( d ) Photolysis of A m m o n i a in the Presence of Isopropylamine. Mixtures of ammonia and isopropylamine 1000:4 and 1000:2 were irradiated. Under these conditions direct light absorption by the amine is negligible. The results are shown in Table I11 and Figure 8. Isopropylamine was nearly exclusively transferred into the species E which is assumed to be an isomer hexene diamine.
Discussion The following discussion is based on the assumption that the photodecomposition of ammonia at 2062 A
10-l’
18.3 23.7
PNHa,
Pi-CBHlNHa
torr
torr
41.4 46.4
0.16 0.10
BHz
#Na
+E
Bi-C8HlNHz
0.60 0.03 -0.34(?) 0.53 0.04 -0.22(?) Mean value -0.28
0.28 0.28 0.28
occurs with a quantum yield of unity. Results from an investigation of the ammonia photolysis in the presence of ethylene at the same wavelength seem to support this assumption experimentally.* When ND3 is photolyzed in the presence of C3Hs as a D-atom scavenger, the Dz quantum yield represents an upper limit for the relative probability of process b ND3
+ hv(2062 A) +ND(’A) 3. Dz
(b)
From the fact that the photolysis of 41 torr of ND3 and 620 torr of C3H8 produced Dz at a quantum yield of (b~L $ 0.003 (cf. Table II), it can be concluded that the photodecomposition proceeds only via ND,
+ hv(2062 8) +ND2(2B1) + D(%)
(a)
Reactions of hydrogen atoms with propaneg as well as (8) U. Schurath, P. Tiedemann, and R. N. Schindler, J. Phys. Chem., in press. (9) K. Yang, J. Amer. Chem. Soc., 84, 719 (1962).
Volume 79.Number 11 October 1968
W. E. GROTH,U. SCHURATH, AND R. N. SCHINDLER
3918
Figure 8.
Gas chromatogram of a n irradiated
sample of 46.5 torr of ammonia and 0.1 torr of isopropylamine. Absorbed dose, 2.4 X 1018quanta.
reactions between propyl radicals'O are sufficiently well known. Considering the reaction products detected in our experiments, the following mechanism appears to be adequate to describe the photolysis of ammonia in the pesence of propane.
NH3
+h .+ ~ NH2 + H ; H
+ CsHs
-
+C3H7
2C3H7
2"
+ C3H7
=
+ H2
1.00
C6H14
2C3H7 +CsHa NH2
+(I)
----t
+ C3H7 +
"3
+ CsHs
CaH7NHz
+ C~HB
2NHz +NzH4
(1) (2)
(3)
(4) (5)
(6)
(7)
However, hydrogen scavenging does not occur exclusively via step 2 in our system. Figure 2 shows that the hydrogen quantum yield is never unity and reaches a value of only 0.9 at a pressure of 700 torr. Consequently, addition reactions of hydrogen atoms must also be considered in our system." The following steps will be discussed
H H
+ NH2 +NH3
(8)
+C3Hs
(9)
+ C3H7
Reaction 8 is postulated in the photolysis of pure ammonia to account for the low quantum yields of decomposition. It is generally assumedi2 that process 8 proceeds in termolecular collisions. This assumption is based on the observed pressure dependence of the quantum yields. As shown in Figures 2 and 3, however, addition of 500 torr of SF6 does not change the product quantum yields in our experiments. Most significantly, the hydrazine quantum yield remains unchanged. Reaction 9 cannot be excluded in our system. It has been shown10 to occur in the mercuryThe Journal of Physical Chemistrzl
sensitized photolysis of isopropyl ketone in the presence of excess Dz. Recombination of hydrogen atoms via triple collisions can be neglected in the presence of propane. The experiments with ND3 indicate (cf. Table 11) that molecular hydrogen is nearly exclusively formed by reaction 2. The low Dz quantum yield remains unchanged when 640 torr of SFs is added to the mixture. This excludes triple collision recombination of D atoms as the main source of Dz. The mechanism discussed so far holds only under the limiting condition of low conversion. As shown in Figures 5 to 7 , several product quantum yields fall off after absorption of less than 6 X 1017quanta corresponding to a conversion of about 0.2% ammonia. Direct light absorption by the products can be excluded under these conditions. Thus secondary attack of the products must be responsible for the reduction of quantum yields. Propylene is known to be very sensitive to hydrogen atom a t t a ~ k . The ~ reduction of hydrogen quantum yields at higher irradiation times could result from this reaction. Secondary decomposition of propylamines is not understood at present. As shown in Figures 7 and 8, the same products are formed in extended photolysis of NH3-C3Hsmixtures and in the photolysis of ammonia in the presence of isopropylamine. Also, the products are formed in similar relative amounts. I n the latter case an unknown substance E is produced in quantities approximately equal to the amount of isopropylamine consumed. When NH3-C3H8 mixtures are submitted to prolonged irradiation, however, the yield of E does not compensate for the reduction of the yield of isopropylamine. Therefore, it is assumed that further secondary products are formed from isopropylamine which elude detection. The dependence of hydrazine quantum yield on irradiation time is of interest in connection with the production of nitrogen. As shown in Figure 1, the hydrazine concentration is a linear function of irradiation dose up to about 2 X 1OI8 quanta absorbed. Deviation from linearity at higher conversion results from secondary decomposition of hydrazine and/or from a reduced rate of hydrazine production.
(10) C. A. Heller and A. 6 . Gordon, J. Phga. Chem., 64, 390 (1960). (11) In the radiolysis of ammonia a t atmospheric pressure, Johnson and Simic (Nature, 216, 479 (1967)) found no detectable dependence of G(H2) on propane pressure in the region of 1.5-6 mol %. From this observation complete scavenging of H atoms was concluded. In our system approximately 1016 H atomdm1 min are produced in the reaction zone as compared with about 1.6 X 1014 H rttoms/ml min in the radiolysis experiments of Johnson and Simic. Since the completeness of H-atom scavenging depends mainly on the ratio of the partial pressures of H atoms and CaHs molecules, considerably higher propane pressure would be necessary t o achieve complete H atom scavenging in our system. (12) W. Groth and J. Rommel, 2.Phys. Chem. (Frankfurt am Main), 45, 96 (1965).
PHOTOLYSIS OF AMMONIA IN
THE
PRESENCE OF PROPANE
3919
Table IV : Material Balance
__---___ H
Products observed
hoduat
Hz
0.77
COH14 CsHe
0.105 0.164
CsH7NHn NzH4 Nz
Reaction
consumed
2 12 3 4 6 12
0.77-+iz
