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OCR for page 85
THE SECONDARY PROCESSES IN THE PHOTODECOMPO-
SITION OF AMMONIA AND HYDRAZINEt
HUGH S. TAYLOR
Department of Chemistry, Princeton University, Princeton, New Jersey
Received May 25, 1988
Since the diffuse bands of the ammonia spectrum were interpreted by
Bonhoeffer and Farkas (1) as due to a dissociation of the molecule, the
direction of further research on the photodissociatior~ process has been
towards an elucidation of the secondary processes in such a manner that
the low quantum yield (A 0.2) might be satisfactorily explained. For
the primary process, it has been generally assumed that
NH3 + he = NH2 + H (1)
though the reaction,
NH3 + he = NH + H2
(la)
cannot be excluded on energetic grounds. It would, however, suggest an
influence of molecular hydrogen which is not found experimentally in the
photoreaction, although there is an effect in the reaction initiated by a-par-
ticles and ions, in which case, at higher pressures, NH+ is certainly formed
by secondary processes. The first reaction accounts for the presence of
atomic hydrogen, made certain by the measurements of Geib and Harteck
(6) and of Farkas and Harteck (4) on pare-hydrogen conversion in am-
monia undergoing decomposition.
Gedye and Rideal (5), using a streaming system, obtained yields of
hydrazine from the photodecomposition of ammonia of as high as 57 per
cent of the stoichiometric yield by the equation
2NH3 > N2H4 + H2
The hydrazine yield decreased rapidly as the temperature of the system
was increased. Koenig and Brings (7), and more recently Welge and
Beckman (16), have obtained positive confirmation of the formation of
hydrazine by photolysis of ammonia in a static system. The amounts
obtained, however, were very small.
Indirect evidence of the production of amine radicals may be obtained
from the photodecomposition of solutions of alkali metals in liquid am-
~ Contribution No. 8 to the Third Report of the Committee on Photochemistry,
National Research Council.
85
OCR for page 86
86
HUGH S. TAYLOR
monia (13~. Liquid ammonia does not photodecompose when pure, but
does decompose on the addition of alkali metals. The sole products are
hydrogen and amide ion, NH2-, in equivalent amounts, and the quantum
yield is well under unity. Alkali metals virtually completely dissociate
in liquid ammonia into positive metal ions and free electrons which become
strongly solvated with the ammonia. It would appear, therefore, that in
pure liquid ammonia deactivation or primary recombination effectively
retards the decomposition, while in the presence of solvated electrons the
NH2 radicals are stabilized by the formation of amide ion, thus permitting
the decomposition to proceed. The mechanism, according to this hypoth-
esis, would be
NHe + he ~ NH2 + H
NH' + H + M > NH3 + M (primary recombination)
NH2 + ~—~ > NH2
H+H+M >H2+M
The majority of workers have accounted for the low quantum yield in
ammonia decomposition by a recombination process,
NH2 + H = NH3
(3)
though Farkas and Harteck (4) pointed out certain difficulties in accepting
this, due to the low stationary state concentration of atomic hydrogen,
especially if reaction 3 requires a third body. That ammonia is regener-
ated during the secondary processes was made definite by Taylor and
Jungers (15), who showed the formation of deuteroammonias in mixtures
of decomposing NH3 and deuterium atoms produced from deuterium mole-
cules by excited mercury. No exchange occurred unless the ammonia was
undergoing photodecomposition. By investigating the photochemical
decomposition in the region of very small decompositions Welge and Beck-
mann (16) found that the quantum yield measured by hydrogen produced
approached unity; hence they concluded that the recombination reaction
(reaction 3) does not play an important role. The products in such cir-
cumstances would be substantially hydrogen and hydrazine. They further
assumed the secondary processes to be substantially heterogeneous, with
a little of the hydrazine reacting with the atomic hydrogen to give am-
monia, as shown by Dixon (2~. With larger amounts of products this
eject would be exaggerated and the non-condensable products would
approach the stoichiometric ratio due to hydrazine decomposition.
In experiments of Elgin and Taylor (3) it was shown that the total
process in both the photochemical and photosensitized decomposition
of hydrazine can be with fair accuracy expressed by the stoichiometric
equation
2N2H4 = 2NH3 + N2 + H2
(4)
OCR for page 87
PHOTODECOMPOSITION OF ~ON! ~ ED HYDRAZINE 87
With hYdrazine in the presence of ammonia, even when only a small frac-
tion of the light was absorbed by the hydrazine, it was shown, by Ogg,
Leighton, and Bergstrom (14), that hydrazine was the compound which
disappeared, with a quantum yield of 1.28 on total quanta absorbed.
Such facts provide considerable support for their conclusion, recently
restated by Mund and van Tiggelen (12), that the process which regen-
erates ammonia and causes the low quantum yield is not reaction 3 but
the reaction
N2H~ + H = NH3 + NH2
(5)
a reaction which would also explain the work of Taylor and Jungers with
deuterium atoms.
The nitrogen which is formed ultimately in the photodecomposition of
ammonia must arise' therefore, from further decomposition of the hydra-
zine or NH2 radicals or both. The assumption of Ogg, Leighton, and
Bergstrom that this occurs by the reaction
N2H4 + 2NH2 = 2NH3 + N2 + H2
(6)
has been modified by Mund and van Tiggelen in a sequence of secondary
processes which more satisfactorily than any alternatives, it would appear,
accounts for the kinetics of the reaction and its quantum yield. The
sequence IS:
20(NH3 + by = NH2 + H)
17(2NH2 + M = N2H4)
16(N2H~ + H = NH3 + NH2)
1(N2H4 + 2NH2 = 2N2 + 4H2)
2(H + H + M = H2)
20h~ + 4NH3 = 2N2 + 6H2
(1)
(2)
(5)
(6a)
(7)
This sequence is based on a quantum yield of 0.2 at 1 atm. pressure, and
the numerical coefficients by which the equations are multiplied must
evidently be proportional to the velocities of the corresponding reactions.
It leads to the following expression of Mund and van Tiggelen for the
quantum yield
~ = 1
0.875 + ~/0.0156 + K~/v
where v (C~I~.bs.) is the velocity of reaction expressed as the number of mole-
cules of ammonia decomposed to nitrogen and hydrogen per cubic centi-
meter per second, P is the total pressure, and ~ the quantum yield equal
to Q/v, where Q is the number of ammonia molecules per cubic centimeter
per second primarily decomposed.
OCR for page 88
88
HUGH S. TAYLOR
This equation reproduces the tendency of ~ to vary with the pressure.
With ~ = 0.2 at 1 atm. one calculates a value of ~ = 0.314 at 0.1 atm. and
0.126 at 8.5 atm. The former is in agreement with the data of Wiig (18)
and the latter with the measurements of Ogg, Leighton, and Bergstrom.
At low pressures the influence of the walls begins to predominate and will
assist especially reactions 2 and 7. Its effect on the quantum yield will
be equivalent to that of high pressures in the gas phase, i.e., will lower the
quantum yield as Wiig observed. The influence of the intensity or the
velocity of decomposition, v, on the quantum yield is of the same order of
magnitude as the equation suggests, as elucidated in a recent research by
Mund, Brenard, and Kaertkemeyer (11), and as suggested by the work of
Ogg, Leighton, and Bergstrom, by Wiig and Kistiakowsky (20), and by
more recent measurements of Wiig (199. The Mund-van Tiggelen nu-
merical values for the relative rates of the several reactions do not consti-
tute a unique solution, as is evident from an analysis by Leighton (89.
:For the ratios 20:17:16:1:2 of the above sequence, Leighton substitutes
the sequence
lO(NH3 + ho = NH2 + H)
7~2NH2 + M = N2H4 + M)
6(H + N2H4 = NH3 + NH2)
2~2NH2 + N2H4 = 2NH3 + No + H2)
2(H + H + M = H2 + M)
It will be noted that in this sequence the nitrogen-producing mechanism
simultaneously regenerates ammonia, whereas the Mund-van Tiggelen
reaction does not. Both yield similar expressions for the relation between
quantum yield, pressure, and intensity. Leighton believes his reaction
producing nitrogen to be superior, because it accounts for the 1:1 N2:H2
ratio observed in the ammonia-sensitized decomposition of hydrazine
(see later).
It has been known since Warburg's original researches on this reaction
that neither molecular nitrogen nor hydrogen influenced the quantum
yield. The above mechanism is in accord with such inertness except in
so far as it might influence the total pressure, P; this influence is small, as
;seen. On the other hand, atomic hydrogen was shown by Melville (9)
to inhibit strongly the photodecomposition of ammonia and the same is
true of the photosensitized decomposition (10~. This effect, which Mel-
ville attributed to the recombination reaction (reaction 3), is rather to be
ascribed to the ammonia regenerative reaction with hydrazine (reaction 5),
the velocity of which, as is to be seen from the magnitude of the molecula~-
coefficient, 16, is very high. Analysis shows that in addition to the effect
OCR for page 89
PHOTODECOMPOSITION OF AMMONIA ED HYDRAZINE g9
of atomic hydrogen concentration there is also an effect due to the diminu-
tion in v, the total decomposition. The cumulative effect of these two
factors is evident in the data both of Mitchell and Dickinson and of
Melville.
Parkas and Harteck found a sharp maximum in the atomic hydrogen
concentration from the photodecomposition of ammonia in the presence of
hydrogen when the ammonia was only 10 mm. in a total pressure of 30 to
70 cm. This led these authors to the assumption of NH4 radicals in
equilibrium with NH3 + H. The maximum should bear some relation to
the maximum observed in the quantum yield but this latter, though similar
in form, is displaced to higher ammonia pressures. This discrepancy is not
yet elucidated.
The objection that might be raised to the Mund-van Tiggelen develop-
ment, namely, that nitrogen only results from the termolecular process
(from 2NH2 + N2H4, reaction 6) and not from such a process as suggested,
among others, by Wiig,
NH2 + NH2 = N2 + 2H2
(8)
is discussed in the original communication of Mund and van Tiggelen and
more recently by them in a comment on the newer work of Wiig. They
point out that reaction 8 always leads to kinetic expressions from which
the observed increase of quantum yield with velocity of decomposition
cannot be deduced, and, further, it fails to account for the small variation
of yield between 1 and 8.5 atm. found by Ogg, Leighton, and Bergstrom.
One might expect that other molecules than NH could act each with a
particular efficiency in the recombination-decomposition process (reaction
6) of the Mund-va~ Tiggelen scheme. To introduce such possibilities
into the kinetic scheme would further complicate the equation derived'
and the experimental data at present are not accurate enough to justify,
such further refinements.
In the ammonia-sensitized hydrazine decomposition experiments of
Ogg, Leighton, and Bergstrom, the two initial stages are quite clear, namely'
reactions 1 and 5 of the ammonia scheme above.
NH3 + hi = NH2 + H
H + N2H4 = NH3 + NH2
The Mund-van Tiggelen mechanism would then give
N2H4 + 2NH2 = 2N2 + 4H2
(1)
(5)
(6a)
which would mean a quantum yield of 2 at a maximum diminished by the
recombination process,
2NH2 + M = 2N2H4
(2)
/
OCR for page 90
90
HUGH S. TAYLOR
The product gas should be 33 per cent nitrogen and 66 per cent hydrogen.
The Ogg, Leighton, and Bergstrom mechanism would give
N2H4 + 2NH2 = 2NH3 + N2 + H2
(6)
again a maximum quantum yield of 2 on the hydrazine Hi.~.nn~.rin~ hilt.
a 1:1 ratio of nitrogen and hydrogen. This ratio was actually found by
these authors and also by Elgin and Taylor. Wenner and Beckmann (17)
found quantum yields ranging from 1 at low pressures to 1.7 at higher
pressures and hydrogen concentrations ranging from 58 to 64 per cent in
the same pressure range of 2 to 14 mm., decreasing toward 50 per cent with
· .
ncreasmg pressure.
The primary mechanism in hydrazine decomposition is still uncertain.
If it be
___w=,< ~ ~ v
N2H4 + he = 2NH2
then it would be necessary to fall back on reaction 6 to explain the ammonia
formation observed. On the other hand, a primary mechanism
NH + he = N2H3 + H
could by reaction 5 regenerate one-half the observed ammonia with t.he
atomic hydrogen. The NH2 and the N2H3 thus produced could, by several
~ 1 1 ~ 1 _ 1 · ~ 1 ~ ~ . ~ ~ · ~ . . .
alternative mechanisms already proposed, but for which there is as yet no
experimental test, yield NH3 + N2 + H2 in approximate accord with the
observations. The pare-hydrogen conversion reaction might be employed
to test these two alternative mechanisms for the primary photoprocess.
REFERENCES
(l) BONHOEFFER AND FAREAS: z. physik. Chem. 136, 337 (1928).
(2) DIXON: J. Am. Chem. soc. 64, 4262 (1932).
(3) ELGIN AND TAYLOR: J. Am. Chem. soc. 61, 2059 (1929).
(4) AREAS AND HARTECE: z. physik. Chem. B26, 257 (1934).
(5) GEDYE AND RIDEAL: J. Chem. soc. 1932, 1160.
(6) GEIB AND HARTECE: z. physik. Chem., Bodenstein Festband, p. 849 (1931).
(7) KOENIG AND BRINGS: z. physik. Chem., Bodenstein Festband, p. 595 (1931).
(8) LEIGHTON: Actualites Scientifiques. Hermann et Pie, Paris (1938). In press.
(9) MELVILLE: Trans. Faraday soc. 28, 885 (1932); Proc. Roy. Soc. (London)
A162, 323 (1935).
(10) MITCHELL AND DICKINSON: J. Am. Chem. soc. 49, 1478 (1927).
(ll) MIJND, BRENARD, AND KAERTEEMEYER: Bull. sac. chim. Belg. 46, 211 (1937).
(12) MEND AND VAN TIGGELEN: Bull. soc. chim. Belg. 46, 104 (1937).
(13) OGG, LEIGHTON, AND BERGSTROM: J. Am. Chem. soc. 66, 1754 (1933).
(14) OGG, LEIGHTON, AND BERGSTROM: J. Am. Chem. soc. 66,318 (1934).
(15) TAYLOR AND JUNGERS: J. Chem. Phys. a, 373,452 (1934~.
(16) WELGE AND BECRMANN: J. Am. Chem. soc. 68, 2462 (1936).
(17) WENNER AND BECEMANN: J. Am. Chem. soc. 54,2787 (1932).
(18) WI1G: J. Am. Chem. soc. 67, 1559 (1935~.
(19) WI]G: J. Am. Chem. soc. 69,827 (1937).
(20) WI1G AND KISTIAEOWSEY: J. Am. Chem. soc. 64, 1806 (19321.
Representative terms from entire chapter:
atomic hydrogen
