(1942). THE KINETICS OF THE OXIDATION OF ORGANIC COMPOUNDS BY POTASSIUM PERMANGANATE (1) Transactions of the Royal Society of South Africa: Vol. 29, No. 4, pp. 309-317.
The surface-potential changes that occur during the reaction of hydrogen and oxygen on an evaporated platinum film have been recorded by means of the static capacitor method. A sequence of reaction steps has been proposed for the reactions of hydrogen gas with an oxygen-adatom monolayer, and oxygen gas with a hydrogen-adatom layer. The chemisorption of water vapour, by both a clean film and one covered with an oxygen or hydrogen monolayer, has also been investigated. From separate measurements of the s.p. of monolayers of oxygen, hydrogen and water, that of a hydroxyl layer has been calculated; from these values and the measured uptakes of oxygen and hydrogen, the complete s.p. plots have been calculated with consistent agreement with the experimental values. The presence of groups of surface sites of different reactivity previously suggested is confirmed. Explanations for the lack of stoichiometry of the hydrogen titration results reported in the literature and for lack of agreement with infrared results on oxide-supported metal are outlined.
A continuous recording (static capacitor) device, with a response time of 0.1 s, has been used to measure the surface potential changes occurring during oxygen chemisorption on clean nickel films. At 77 and 90 °K, the surface potential is decreased to – 1.6 V, followed by an increase of 0.1 V at an oxygen pressure of ca . 10 -2 torr. This increase is the result of physical adsorption of oxygen molecules in a second layer. Oxygen doses added at higher temperatures cause a surface potential decrease, followed immediately by a slow decrease due to incorporation of oxygen in the metal lattice. The kinetics of the incorporation process are affected by the surface electric field formed by the oxygen-nickel double layer. Analysis of the kinetic curves, corrected for this field effect, shows that the average charge per oxygen atom is 0.3 ± 0.04 of an electronic charge. The experimental rate of incorporation is smaller, by a factor of approximately 10 -12 , than that calculated from the absolute rate equation and the use of the experimental value of the activation energy of 1 kcal/mole. This discrepancy is attributed to the very small probability of creating a low-energy barrier by the simultaneous movement apart of surface nickel atoms, allowing passage inwards of the oxygen adatoms. The calculated probability for this concerted motion is in agreement with the experimental value; the magnitude of the effective surface energy barrier is then evaluated as 7.3 ±1 kcal/mole. At temperatures between 350 and 403 °K, the kinetics of incorporation change, and their autocatalytic character corresponds to that of a nucleation and growth process. At these temperatures the surface potential increases to +0.7 V, and the adsorptive capacity is markedly reduced. These changes are attributed to the formation of oxide-like patches containing positively charged nickel atoms in their external surface.
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