Baxter, Warren P. (1928) Activation of molecular hydrogen by electron impact. Quantum yield in the photochemical decomposition of nitrogen dioxide. Mechanism of the photochemical decomposition of nitrogen pentoxide. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-02252005-131836
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ACTIVATION OF MOLECULAR HYDROGEN BY ELECTRON IMPACT
In this research we have undertaken to study the activation of hydrogen molecules by electron impact under conditions in which we know the energies of the impinging electrons. The experiments of Cario and Franck show that hydrogen molecules can be activated by excited mercury atoms by collisions of the second kind and that copper oxide and tungstic oxide can then be reduced. In their experiments the mercury atoms receive energy of 4.9 volts from the light source, which is sufficient to dissociate hydrogen molecules, the heat of dissociation of hydrogen being 3 to 4 volts. Furthermore, it has been known for some time that in a discharge tube hydrogen will disappear when a discharge is passed. Hughes, in particular, has investigated the electrical clean-up of hydrogen and nitrogen, and finds a definite decrease in the hydrogen pressure at 13.3 volts and higher. He adopts Langmuir’s conclusion that hydrogen is dissociated under these conditions, and that the decrease in pressure is due to the freezing out of atomic hydrogen on surfaces cooled by liquid air. His results will be referred to later in connection with our own experiments. In Hughes’ investigations no copper oxide was present, and the minimum electron energy at which hydrogen disappears was not accurately determined. A number of experimenters also have investigated the chemical reactivity of hydrogen activated by an electric discharge, but the energies of the impinging electrons were not known.
We shall discuss four possible mechanisms by which electrons may be expected to activate hydrogen molecules.
First, it might be that an electron having kinetic energy of 3 to 4 volts could transfer its energy to the hydrogen molecule and cause its dissociation into atoms. These in turn could then react with other substances. However, it is known that no kink occurs in current-potential curves of hydrogen near 4 volts, and it seems, therefore, that electrons having kinetic energy equal to the dissociation energy of hydrogen molecules cannot transfer their energy to these molecules. We then should expect to find no evidence of reaction when hydrogen is bombarded with 4-volt electrons, and our experiments actually do give no indication of reaction. This is in agreement with the commonly held idea that the dissociation of molecules does not occur as the direct result of electron impact.
Second, electrons may have to possess sufficient energy to resonate the molecule, which may then dissociate if its heat of dissocitation is less than its resonance potential. The hydrogen molecule has, according to the latest results of spectroscopy, a resonance potential at 11.6 volts. Electrons of this energy can raise hydrogen molecules into an upper quantum state. These activated molecules would ordinarily return to the normal state after a short time. However, they may either dissociate into atoms upon impact with other molecules and the atoms thus react, or the resonated molecules may act directly on any oxide or other substance on which they impinge.
A third mechanism of causing activation by electron impact might be that where the impinging electron transfers enough energy to the hydrogen molecule to cause its dissociation and resonance of one of the atoms. This process may be expected at 13 to 14 volts, which is the sum of the heat of dissociation and the resonance potential of the hydrogen atom. This is the mechanism postulated by Hughes.
A fourth possibility may be considered. It may be necessary that an electron ionize a hydrogen molecule before the latter can be made to react. It was found by Anderson and Storch and Olson that nitrogen and hydrogen reacted to form ammonia when bombarded by 17-volt electrons. This voltage is near the ionization potentials of these molecules.
We may state at once the results of our experiments. Electrons of 11.4 volts’ energy can activate hydrogen molecules, for we find that there is a definite pressure decrease when the accelerating voltage applied to our tube has this value. At the same voltage we also obtain a kink in the current-potential curves, using the Franck method.
This research was aided financially by a grant made to Professor A. A. Noyes by the Carnegie Institution of Washington.
QUANTUM YIELD IN THE PHOTOCHEMICAL DECOMPOSITION OF NITROGEN DIOXIDE
In a paper on "Photochemical Equilibrium in Nitrogen Peroxide,' Norrish has described experiments in which a pressure increase was observed when NO2 in a quartz vessel was illuminated by a quartz mercury vapor lamp. Under illumination the pressure increased rapidly at first, then more slowly, and reached a sensibly constant value after about fifteen minutes. When the illumination was cut off, the pressure dropped rapidly at first and then approached its original value comparatively slowly. The pressure increase could not be accounted for by heating alone. Norrish assumed that NO2 was decomposed photochemically into NO and 02; constancy of pressure was then attained when the rate of recombination of NO and O2 became equal to the rate of photodecomposition of NO2. This assumption received confirmation from the results of experiments carried out at various NO2 pressures and with NO or 02 initially present. Both of these gases cut down the pressure increase, and the NO did so more effectively than the 02, as was expected from the fact that the rate of recombination is proportional to [...].
In the present paper are described first some qualitative experiments which test further the correctness of Norrish's view that NO2 is photochemically decomposed into NO and 02, and then some measurements on the quantum yield of the reaction with monochromatic light. In contrast with the previous work, the present experiments were carried out under conditions where recombination should be negligible. Since the recombination is a third-order reaction, its rate becomes small at low pressures: for example, at [...] = 0.04 mm. and [...] = 0.08 mm., the rate of decrease of [...] may be calculated from the measurements of Bodenstein and Lindner to be only 2.4 x [...] mm./hour at 22 [degrees]. Hence, by keeping the pressures of the reaction products sufficiently small, it might be possible to treat the reaction simply as a photochemical decomposition rather than as a photochemical equilibrium. In order to measure the small amount of reaction product present in the larger amount of NO2 used to secure sufficient light absorption, the N2 was frozen out with liquid air, and the residual gas measured with a quartz fiber gage, the use of this procedure under similar circumstances having already been found convenient. Measurements of a small amount of product also facilitate the use of relatively weak monochromatic light sources. This investigation was aided financially from a grant made to Professor A. A. Noyes by the Carnegie Institution of Washington.
MECHANISM OF THE PHOTOCHEMICAL DECOMPOSITION OF NITROGEN PENTOXIDE
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|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Chemistry and Chemical Engineering|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||1 January 1928|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||28 Feb 2005|
|Last Modified:||26 Dec 2012 02:32|
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