FAILURE OF HYDRATED ELECTRONS TO INITIATE NITROGEN FIXATION
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On the Failure of Hydrated Electrons to Initiate Nitrogen Fixation during
7
Radiolysis
by E. A. Shaede, B. F. P. Edwards, and D. C. Walker Chemistry Department, University of British Columbia, Vancouver 8, Canada (Received April 16,1970)
Although the hydrated electron is one of the most powerful and reactive reducing agents, it is unable to cause reduction fixation of molecular nitrogen. An upper limit of kl < 20 M - 1 sec-’ was estimated for the rate constant of the reduction reaction. Hydrated electrons were generated by y radiolysis of an aqueous solution of Hz and OH- which also contained Nz at concentrations up to 0.2 M (400 atm pressure). Significant yields of NHBwere obtained, but by completely eliminating the gas space above the solution, it was shown that all NHI arose through “direct action” of the radiation on dissolved NI. Introduction Current interest in the fixation of nitrogen, particularly under comparatively mild conditions such as exist in nature, prompted us to enquire about the possibility of direct reduction of molecular nitrogen by hydrated electrons (eaq-). The hydrated electron‘ is an extremely powerful reducing agent, its standard electrode potential being about -2.7 V, which makes it comparable in reducing strength to metallic sodium in water. In addition, it is the ideal nucleophile for simple electron transfer reactions and it shows extremely high rate constants for its reaction with a wide variety of compounds, some of which had previously been regarded as nonreducible. Furthermore, almost all of these reactions proceed with virtually no energy of activation, although in some cases the reaction rate is diminished by a small preexponential factor. Our experiments were designed to test under the most favorable possible conditions (very high convery low impurity levels, and the centrations of Kz, presence of molecular Hz) whether reaction 1 proceeds ew-
+ Nz
--j.
[Nz-,]
+ NHa (or NH20H or N2H4) (1)
at an observable rate. It seemed reasonable to assume that once the barrier to Nz reduction had been overcome in forming Nz-,, or N2H (both of which are feasible intermediates on the basis of some theoretical electron affinity studies3) then its ultimate conversion to ammonia (or at least to hydroxylamine or hydrazine) would be inevitable in this reducing environment. The conversion of N2-H2 mixtures to NH8 has been achieved in a variety of ways, including the Haber industria1 process of high-temperature and high-pressure catalysis14heterogeneous reaction with metallic lithium to give Li3N salt which can be hydrolyzed,5 the y irradiation of gaseous mixtures,6 gas discharge methods,’ sonolysisJSetc. Ammonia has also been shown to result from the y irradiation of water in the presence of
large amounts of either N2or airJgbut in all these cases the reaction was apparently initiated by direct activation or dissociation of Nz molecules. Nitrogen-fixing enzymes and the recently discovered transition metal complexes1” which coordinate hi probably represent the only examples of “simple chemical reactivity’’ shown by Nz. The experiments we report were designed to test the possible reduction-fixation of Nz by direct reaction using a chemical reagent without recourse to prior catalytic activation, coordination, or induced excitation of the nitrogen molecule. Experimental Section Apparatus and Techniques. A number of preliminary experiments utilized the stainless steel high-pressure vessel shown in Figure l a , in which the aqueous solution, contained in a Pyrex glass cell, could be saturated with the appropriate gas mixture at (1) For recent review see E. J. Hart, “Survey of Progress in Chemistry,” Vol. 5, A. F. Scott, Ed., Academic Press, New York, N. Y., 1969, p 129. (2) See, e.&, M. Anbar, Quart. Rev., 22, 579 (1968). (3) N. Bauer, J. Phys. Chem., 64, 833 (1960). (4) J. W. Mellor, “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. VIII, Supplement 1, Part 1, Wiley, New York, N. Y., 1964. (5) F. A. Cotton and G. Wilkinson, “Advanced Inorganic Chemistry,’’ Interscience, New York, N. Y., 1962. (6) (a) C. H. Cheek and V. J. Linnenbom, J. Phys. Chem., 62, 1475 (1958): (b) 5. C. Lind and D. C. Bardwell, J . Amer. Chem. Soc., 50, 745 (1928). (7) (a) R.N. Varney, J. Chem. Phys., 23, 866 (1955); (b) A. Yokohata and 9. Tsuda, Bull. Chem. Soc. Jap., 40, 1339 (1967). (8) A. I. Virtanen and N. Ellfolk, Acta Chem. Scand., 4 , 93 (1950). (9) (a) M. T. Dimitriev and S. Ya. Psheahetskii, Atomnaua Energiyap Steinberg, B.N.L. Report 13692 8, 59 (1960); (b) S. Sat0 and AM. (1962); (c) J. G. Morse, U. S. Patent 3,378,475 (1968); (d) L. Hammer, s. Forsen, 9. Hedlund, and R. Lundquist, U. s. Atomic Energy Commission CONF-213, 79 (1966). (10) (a) See, e.g., M. E. Volpin and V. B. Shur, Dolcl. Chem. Proc. Acad. Sci. USSR, 156, 1102 (1964); (b) A. D. Allen and C. V. Senoff, Chem. Commun., 621 (1965); (c) E. E. van Tamelen, G. Boche, S. W. Ela, and R. B. Fechter, J. Amer. Chem. SOC.,89, 6707 (1967).
The Journal of Physical Chemistry, Vol. 74, No. 17, 1970
E. A. SHAEDE, B. F. P. EDWARDS,
3218
I
AND
D. C. WALKER
t
(b)
Figure 1. High-pressure irradiation cells: (a) with irradiated gas space; (b) syringe arrangement which eliminated gas space.
pressures up to 6000 psig (400 atm). In typical experiments the solution volume varied from 5 to 15 rnl with the remaining gas volume above the solution equal to 25 to 15 ml. The solutions were deoxygenated by bubbling with high-purity Nzbefore being pressurized with the experimental gas mixture. After irradiation the solutions were analyzed for ammonia using the Nessler spectrophotometric method.” This cell had the disadvantage tliat there was always a considerable gas space above the solution and thus gas-phase radiolysis was occurring at a significant rate because of the high gas pressures used. In addition, the gas was in contact with the stainless steel surfaces of the highpressure vessel which may have resulted in catalytic enhancement of the gas-phase radiolysis. Any ammonia so formed would dissolve in the solution. In the final series of experiments, the apparatus shown in Figure l b was used. This apparatus eliminated the gas space above the solution by containing the pressurized solution in a modified 5 0 4 all-glass syringe. In a typical experiment, the solution to be used was first deoxygenated in the glass apparatus shown in Figure 2 by bubbling high-purity hydrogen through the solution and irradiating it for 30 min (-2 X lo5 r a b ) under the hydrogen atmosphere. Subsequently the experimental gas mixture (composition determined by gas chromatography) was bubbled through the solution and flushed through the syringe which was attached via a standard B7 ground-glass joint and lubricated with the solution. After the system was well purged (-2-3 hr), the syringe was filled with 30-35 ml of the gas mixture followed by -20 ml of solution. The syringe was then quickly removed from the filling apparatus and while a portion of the The J o u d of P h y d Chcm&tw, Vd.74,No. 17,1870
Figure 2. Apparatus used for bubbling with prescribed NrHI mixture and for filling the syringe with gas and liquid samples. By rotating the whole apparatus about 30’ counterclockwise B gas sample could be admitted to the syringe prior to taking up a liquid sample.
solution was flushed from the syringe, a ground-glass cap was put over the end of the syringe. In this way, it was possible to fill the syringe without any atmospheric oxygen contamination. When the syringe was placed in the high-pressure housing and pressurized by Nzgas to 3000 psig (200 atm), virtually all of the gas in the syringe dissolved in the solution since the amount of gas available was chosen to correspond almost exactly to the amount that would dissolve in the solution a t 200 atm pressure. As a result, the plunger was in contact with the liquid surface. The solution of gas was made homogeneous by use of the mixing plate in the syringe and by allowing at least 1 hr for equilibration before irradiation. Following irradiation (irradiation times from 60 to 12,000 min were used at 2500 rads/min dose rate) the solution was analyzed for ammonia using the sensitive indophenol-blue method of Tetlow and Wilson.’z The sensitivity limit of this method was about 5 X lo-’ M NHa+. It was noted that hydrogen peroxide (which was formed in irradiated solutions containing oxygen) interfered drastically with the analysis. A hydrogen peroxide concentration of only 20 ppm (6 X M ) gave an orange colored solution which had an absorbance of -1.0 a t 630 nm in a 5-cm cell. The absorption was due to the tail of an intense band which had a maximum at
