By W G Frankenberg
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Extra resources for Advances in Catalysis and Related Subjects, Volume 13
Mole) is rather too great for a molecular chemisorption and this species could then become converted to O:&, presumed inactive. Studies of semiconductivity have shed additional light on this problem, and these are discussed in Section IV,A. Oxygen exchange studies are also valuable, and Winter has admirably summarized his views on the various states of chemisorbed oxygen on oxides in the review already mentioned (21). The quantities of oxygen adsorbed by zinc oxide are very small, much less than 1% coverage, but the uptake can be conveniently studied a t low pressures using a Pirani gauge.
VI. Photoadsorption and Photocatalysis A. INTRODUCTION A further way in which the relative concentrations of electrons and holes in a n oxide may be changed is by irradiation in the fundamental absorption band of the solid. One may visualize, using the band picture, that an absorbed quantum excites an electron from the valence band across the energy gap which separates it from the conduction band, and the independent existence of an electron liberated into the conduction band and a hole free to move in the valence band manifests itself in a n applied electric field as photoconductivity.
For trivalent additions, Hauffe and Block (90) have shown that the incorporation of small amounts of Cr3+ions decreases the conductivity of nickel oxide: one infers accordingly that the hole concentration is decreased and that the Fermi level is raised. This is therefore an attractive situation with which to examine the influence of the height of the Fermi level on catalytic activity. The most appropriate n-type oxide for analogous studies is zinc oxide. OF CO B. THEOXIDATION OVER DOPEDNICKELOXIDE CATALYSTS This has been the most widely studied of all reactions on doped oxides.