Hall effect studies of carbon monoxide oxidation over doped zinc oxide catalysts

Abstract

The role of the conduction band electrons in the chemisorption of oxygen and the oxidation of carbon monoxide on indium-doped polycrystalline zinc oxide was studied by a dual a.c. Hall effect method that employed a 52 c/sec a.c. sample current and a 60 c/sec a.c. magnetic field to give an 8 c/sec Hall voltage. This method seems to be particularly well suited for measuring the concentration of current carries in semiconductor catalysis studies. The results of this study support the conclusions that the chemisorption of oxygen on indium-doped zinc oxide in the temperature range 200–350°C, is associated with an electronic charge transfer from the zinc oxide to the surface oxygen species, and that O^– plays a prominent role in this adsorption. The chemisorption of carbon monoxide in the temperature range 300–350°C, on indium-doped zinc oxide containing pre-chemisorbed oxygen, gives an increase in carrier concentration corresponding to one electron per carbon monoxide molecule chemisorbed. The measurements of the changes in carrier concentration during carbon monoxide oxidation at 350°C by this doped zinc oxide indicate that the conduction band electrons are intimately involved in the reaction, and that O^– is an important transient surface species therein. A comparison of the results obtained from reaction studies at 460°C over lithium-doped and indium-doped zinc oxide supplies additional evidence that a strong correlation exists between the concentration of conduction band electrons in these materials and their level of catalytic activity for carbon monoxide oxidation.