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2139
permit the two oxygen atoms to separate as 0 2 . The thermodynamics of the decomposition of N203 in the gas phase indicates the reaction is exothermic by about 1 kcal./mole. The activated complex (N203)* shown in this mcchanism is identical with an excited state of dinitrogen trioxide. Dinitrogen trioxide is formed when an equimolar mixture of nitrogen dioxide and nitric oxide is condensed. Dinitrogen trioxide has never been observed in the gas phase. From the standpoint of the principle of least motion,8 this mechanism would be expected to be more important in the solid phase than in the gas phase. The principle of least motion may be summarized as follows: a low activation energy for an elementary reaction is favored when the motion of the nuclei of the reactant molecules or atoms is minimized. One would thus expect this reaction to be enhanced in the solid phase where the formation of dinitrogen trioxide is ensured. Under experimental conditions which favor reaction in the solid phase, the same products are obtained as in the case of the gas phase photolysis of nitrogen dioxide. The similarity of products is an indication that there may exist reactions in the solid phase analogous to the reactions occurring in the gas phase
+
N204(~) hv +2NO(s)
+ 02(g)
+ hv+ NO + 0 NOz(s) + 0 +NO +
(1)
NOa(s)
0 2
+ NzOa(S) + hv NO2(s)
+N2Oa(s) +N20(g)
(11) (111)
(IV)
+ 02(g)
(VI
Reaction I1 is less probable than I since the frozen condensate consists mainly of the dimeric species of nitrogen dioxide. Reaction IV is known to occur whenever a gaseous mixture of nitrogen dioxide and nitric oxide is condensed. Reaction V is analogous to reactions 6 and 7 of the postulated gas phase mechanism. Reaction V is more important than reaction 7 of the gas phase mechanism since dinitrogen trioxide is stable in the solid phase. The over-all consecutive reaction in the gas phase is 3N02
+ 2hv +NO + 202 + NZO
The analogous consecutive reaction in the solid phase may be written
Nz04
+ 3N02 + 3hv +3N0 + 302 + NzO
The reaction postulated by Kistiakowski involving excited nitric oxide molecules and nitrogen dioxide in
the gas phase has as an analogous reaction in the solid phase photochemical decomposition of dinitrogen trioxide.
Acknowledgment. This work was supported in part by the National Science Foundation, Grant G-17434, and in part by the Atomic Energy Commission, Contract At(40-1) 2590. (8) F. Rice and E. Teller, J. Chem. Phys., 6,489 (1938).
F. 0. RICE RADIATIONLABORATORY OF NOTREDAME UNPLERSITY NOTREDAME,INDIANA DEPARTMENT OF PHYSICS F. J. WUNDERLICH GEORQETOWN UNIVERSITY D. C. WASHINGTON, RECEIVED APRIL19, 1965
Infrared Spectra of Molecules Adsorbed on Metal Powders Obtained from Electrically Exploded Wires Sir: It has been shown that the electrical explosion of metal wires in an atmosphere (p -760 mm.) of a
rare gas produces a metal aerosol which, under the proper conditions, consists of spherical particles having a mean diameter of -200 A.lt2 We find that -20cm. lengths of ca. 0.2-mm. diameter wires may be exploded safely in an argon atmosphere using energies up to 1400 joules in a Pyrex chamber having a 4-1. volume. The condenser bank employs 28.4 pf. at voltages up to 10 kv. in a circuit having an inductance of 0.3 phenry.3 The explosion chamber terminates in an 8 cm. 0.d. X 20 cm. closed tube which has been fitted with two diametrically opposed sodium chloride windows. The aerosol is permitted to sediment (-2 hr.) onto a salt block which may be moved by magnetic means into the path of an infrared beam which passes through the windows. Spectra were obtained with a PerkinElmer Model 521 infrared spectrophotometer. We have confirmed the particle sizes cited by Karioris, Fish, and Royster2 with electron photomicro(1) F.G. Karioris and B. R. Fish,J. Colloid Sci., 17, 155 (1962). (2) F. G. Karioris, B. R. Fish, and G. W. Royster in “Exploding Wires,” Vol. 2, W. G. Chace and H. K. Moore, Ed., Plenum Press, New York, N. Y.,1962, p. 299. (3) c. P. NMh and c. W. OIsen in ref. 2, p. 5.
Volume 69, Number 6 June 1966
2140
COMMUNICATIONS TO THE EDITOR
graphs of a palladium dispersion and an X-ray lineIf Eischens and Pliskin’s4identification of the bound broadening study of a copper dispersion. When product is correct, we are effecting, spontaneously, a about 5 mg. of aerosol is deposited uniformly on a 6 - ~ m . ~ high degree of conversion of adsorbed acetylene to salt block, the background transmission in the infrared n-butyl radicals and carbides. It is clear that subis -10%. stantial amounts of carbide must be present, since a follow-up exposure of our sample to hydrogen produced After the explosion has occurred, the adsorbing gas is introduced, the system is allowed to equilibrate, and an increase in the intensity of the spectrum by a factor the chamber is then evacuated. Systems we have inof about 5, without much alteration in the band shapes. vestigated in our initial survey include carbon monoxide When explosively dispersed nickel is treated with on palladium, molybdenum, and copper; and acetylene acetylene, we obtain a spectrum which agrees with on copper, nickel, and palladium. that reported by Eischens and Pliskin4 for this system The spectra we observe for carbon monoxide on and which they attribute to adsorbed ethyl radicals. palladium are in excellent agreement with those d i e In our system, as in theirs, hydrogenation leads to an cussed by Izischens and Pliskin14with the single excepapparent increase in the CH2:CH3 ratio. Exposure of a palladium dispersion to acetylene gives rise to tion that we fail to observe the weak peak at 2065 cm.-l which these authors find for their silica-supported adsorbed species which show very weak bands a t 2865, samples near monolayer coverage. The entire spec2925, and 2965 cm.-l. We then agree with the recent trum of carbon monoxide adsorbed on palladium is report of Dunken, Schmidt, and Hobert18rather than shifted below the low frequency wing of gaseous CO, the earlier results of Little, Sheppard, and Yates,’ so that we were here able to obtain the spectrum of CO who found only olefinic species until hydrogen gas was added. adsorbed OIL aerosols as well as on sedimented powders. Our spectrum for carbon monoxide on molybdenum, From the foregoing, it is clear that the use of exfor which a precedent does not exist in the literature, ploding wires provides a new and simple means to shows a broad, unsymmetrical absorption quite similar produce dispersions of any conducting material, whose to that found in the CO-Pd system, except that the adsorptive properties may be studied in a manner sharp drop in transmission on the high wave number which should be quite free from extraneous contribuside begins a t about 2070 cm.-l on molybdenum vs. tions from a supporting medium. That these effects 1980 cm.-l on Pd. exist may be inferred from the fact that our studies on We fail to find any peak which may be attributed to copper surfaces yield in both cases results which differ CO adsorbed on copper. This result is in disagreement markedly from previous investigations. with the results of Eischens, Pliskin, and Francis: and Acknowledgment. This work was supported by the Gardner arid Petrucci,6 all of whom studied silicaU. S. Atomic Energy Commission through the Lawrence supported copper. Radiation Laboratory, Livermore, Calif. The surfaces of our copper samples are far from inactive, however, as evidenced by their behavior toward (4) R. P. Eischens and W. A. Pliskin, Advan. Catalysis, 10, 1 (1958). acetylene. We observe here, in total disagreement (5) R. P. Eischens, W. A. Pliskin, and S. A. Francis, J . Chem. Phys., with Little, Sheppard, and Yates7 (for supported cop22, 1786 (1954). per), a spectrum having three absorption maxima at (6) R. A. Gardner and R. H. Petrucci, J . Phys. Chem., 67, 1376 2959, 2924, and 2865 cm.-’, with no others between (1963). 3400 and 2700 cm. -I. The striking feature of our result (7) L. H.Little, N. Sheppard, and D. J. C. Yates, Proc. Roy. SOC. (London), A259, 242 (1961). is that both the positions and relative intensities of (8) H. Dunken, K. Schmidt, and H. Hobert, 2. C h a . , 4, 312 these peaks are in excellent agreement with the results (1964). of Little, et aZ.,’ for the adsorbed product obtained when ethylene adsorbed on Vycor-supported palladium DEPARTMENT OF CHEMISTRY CHARLES P. NASH was treated with hydrogen. This Same spectrum was OF CALIFORNIA UNIVERSITY ROBERT P. DESIENO also obtained by Eischens and Pliskin4 by hydrogenat95616 DAVIS,CALIFORNIA RECEIVED APRIL12, 1965 ing ethylene adsorbed on silica-supported nickel.
The Journal of P h g s k l Chemistry
