The Heterogeneous Thermal Decomposition of Ammonia in Strong Electric Fields

VOL. 13, 1927

CHEMISTRY: R. E. B URK

719

THE HETEROGENEOUS THERMAL DECOMPOSITION OF AMMONIA IN STRONG ELECTRIC FIELDS BY ROBBRT E. BURK* BAKER LABORATORY OF CHEMISTRY, CORNELL UNIVzRSITr Communicated August 26, 1927

At least two general methods are open by which a contact catalyst could accelerate a chemical reaction: (1) it could -assist in the energy transfer to or away from the seat of reaction, (2) it could reduce the amount of energy of activation necessary for reaction. These processes, in turn, can be imagined to occur in several ways, e.g., (2) could be brought about (a) if adsorptive forces of suitably spaced atoms in the catalyst' stretched appropriate bonds in the adsorbed molecules, thus accomplishing part of the work of separation, (b) if intermediate compounds are formed (which is not necessarily a distinct process from (a)), (c) if ap-propriate bonds in the adsorbed molecules are weakened by disturbances of the atoms themselves, i.e., distortion of the electronic orbits, by the surface fields of the catalyst. A thorough understanding of a specific heterogeneous reaction clearly involves a quantitative knowledge of the contribution of each of these mechanisms to the total increase in reaction, velocity. The present experiments constitute an attempt to estimate the contribution of (2c), the atomic disturbance effect, to the thermal decomposition of ammonia upon electrically heated wires. The method consisted of adding strong electric fields to the surface fields of the catalyst. A measured difference of potential was applied between a copper cylinder of known radius, and the wire of known radius, by means of a set of radio "B" batteries. With a molybdenum wire 0.005 cm. in diameter, fields up to 44,000 volts per cm. were obtained and with a Wollaston wire of platinum 0.0005 cm. in diameter, fields up to 150,000 volts per cm. were obtained at the surface of the wire under the conditions of decomposition. Apart from the application of elec'tric fields, the experimental procedure was very similar to that previously described.2 In no case was a change in the rate of decomposition observed when the field was applied, no matter what its direction. Perfectly smooth pressure change-time curves were obtained with the field on and off for successive quarters of the time of reaction. Field currents were obtained which reached 1 X 10-4 amperes in the case of the 150,000 volt field. For this reason, much higher fields could not be used. While a field of 150,000 volts per cm. may be small compared with those already existing at the surface of the wire, it is nevertheless as strong as those used in investigations of the Stark effect and accordingly would be

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expected to produce appreciable distortions of the electronic orbits, and to affect the chemical bonds insofar as they depend upon the exact positions and shapes of the orbits. Such a change in the chemical bond would affect the reaction velocity exponentially. The present negative results are, therefore, not without interest. They are in accord with the results of Steubing,3 who found that fields of the same magnitude did not cause shifts in certain band spectra. The writer wishes to express his best thanks to Prof. W. D. Bancroft and to Prof. F. K. Richtmeyer for their interest and for valuable suggestions. * NATIONAL RZ3SARCH F}LLOW. 1 J. Phys. Chem., 30, 1134 (1926). Proc. Nat. Acad. Sci., 13, 67 (1927).

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Steubing, Phys. Zeits., 23, 427 (1922); 26, 915 (1925).

THE PHOTOCHEMICAL DECOMPOSITION OF HYDROGENIODIDE; THE MODE OF OPTICAL DISSOCIATION' By BERNARD LzWIS2 SCHOOL Or CHUMISTRY, UNIVSRSITY Or MINNgSOTA Communicated August 30, 1927

Introduction.-;--The problem of determining by direct experiment the true mechanism of the photochemical decomposition of hydrogen-bromide or hydrogen-iodide studied by Warburg3 is of interest and importance, not only from a kinetic viewpoint, but also for the interpretation of the continuous absorption spectrum which has been found for these gases by Tingey and Gerke4 and Bonhoeffer and Steiner5 since the writer purposed studying the second of these reactions. It will be recalled that Warburg suggested the following steps in the decomposition, after carefully considering the possible secondary "dark" reactions: (1) HX+hv =H+ X, (2) H + HX = H2 + X, (3) X + X = X2, where X denotes the halogen atom. This accounts exactly for the experimentally determined quantum efficiency of two molecules decomposed for each quantum absorbed. Stern and Volmer8 have presented an entirely different point of view which does not violate the observed quantum efficiency. They conclude that the hypothesis that molecular dissociation or decomposition is the primary light process cannot be entertained; that a collision between the activated molecule and some other molecule is a necessary condition for decomposition. Accordingly, they propose the following mechanism: