Adsorption Studies on Evaporated Metal Films

Sheridan, Margaret H (1966) Adsorption Studies on Evaporated Metal Films. PhD thesis, University of Glasgow.

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The adsorption of hydrogen on evaporated nickel films has been studied under a variety of experimental conditions. It has been shown that, in comparison to the uptake of hydrogen on a vacuum deposited film, enhancement of the amount of hydrogen adsorbed can be achieved by depositing the metal film in the presence of the adsorbate. Some such slight degree of enhancement is observed at 273 K, but this is very much more marked at 77 K. At this low temperature, uptakes approaching one hydrogen atom adsorbed per nickel atom have been observed. The phenomenon can be explained by postulating that, in the case of a vacuum deposited film, the nickel atoms in the bulk of the structure are capable of adsorbing hydrogen but do not do so simply because hydrogen does not have access to them; in the case of a film formed in the presence of hydrogen, on the other hand, the gas has the opportunity to be adsorbed on each nickel atom as it is deposited. It is thought that the absence of greatly enhanced adsorption on films deposited in hydrogen at 273 K is due to the fact that both the adsorbed hydrogen and the nickel atoms are mobile at that temperature, and thermal movement of both species leads to elimination of hydrogen from the internal surfaces of the structure. The manner in which the hydrogen held by a film deposited at 77 K is released from its state of enhanced adsorption as the temperature rises has been studied. This has shown that the additional amount of hydrogen which is adsorbed when the temperature is decreased has reached a saturation value at a temperature above 77 K, and consequently that the measurements made at liquid nitrogen temperature have a fundamental significance as being the maximum possible. To gain information as to the manner in which the hydrogen is held by the nickel, exchange experiments have been carried out. Using tritium labelled hydrogen as the adsorbate, a radioactive tracer technique has been developed to study its exchange with gas phase inactive hydrogen. It has been found that a quantity of hydrogen equivalent to 0. 6 hydrogen atom per metal atom does not take part in the exchange process. Similar adsorption and exchange experiments have been carried out on palladium films for comparison purposes. These have shown that, although hydrogen sorption occurs to a greater extent on palladium than on nickel (owing to an additional mode of incorporation of hydrogen in palladium for which access of the adsorbate is not rate determining), the non-exchangeable ratio is still 0.6. This figure is significant in that it is in excellent agreement with the amount of hydrogen which, if bonded to the d-band of either metal, would exactly fill the electron vacancies there. It is suggested that the total filling of the d-band with electrons from the hydrogen in this way results in the absence of a type C chemisorbed component to act as an intermediate in exchange. The extent of the present work has not enabled any definite conclusions to be drawn as to the manner in which the exchangeable hydrogen is held by the metal, but experimental observations have led to the elimination of certain possibilities (i. e. physical adsorption and type C chemisorption to the d-band) as being unlikely.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Condensed matter physics
Date of Award: 1966
Depositing User: Enlighten Team
Unique ID: glathesis:1966-78430
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 28 Feb 2020 12:09
Last Modified: 28 Feb 2020 12:09

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