By W G Frankenberg

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Sample text

An increase in the p-typeness of the oxide by this means increases the activity, in agreement with the activity pattern discussed in Section IV,C. The same trend of increasing activity for this reaction in the direction n -+p (decrease of Fermi level) is also shown by the results of Schwab and Block (92) for small additions of lithium and gallium to zinc oxide. Doped zinc oxide has also been studied by Molinari and Parravano ( i O 2 ) in Hz-DZ exchange, and by Cimino et al. (103) in hydrogen and deuterium chemisorption.

The reaction proceeds more slowly than CO oxidation, and it is interesting that the greater resistance to catalysis is reflected entirely in the increased activation energies. The values of the frequency factor are in fact about an order of magnitude greater than in CO oxidation. Regarding the mechanism, the same difficulties of interpretation arise as in the high-temperature CO oxidation, and we shall not speculate on them further. For the oxidation of ethylene to COZ and water, which was studied at, temperatures above 400”, no systematic influence of dopent was found.

Below lOO”, for example, hydrogen chemisorbed on zinc oxide (60) and on nickel oxide (68, 61) does not affect the semiconductivity, but a t higher temperatures there is a marked change in conductivity when the gas is chemisorbed. With this having been said, it remains to emphasize that conductivity studies during chemisorption can often have a very important qualitative significance. Thus an observed conductivity change in the prcscnce of a gas amounts to proof of chemisorption. Also we have already drawn attention in Fig.

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