Abstract
One of the unique features of cerium oxide is its ability to be reversibly converted into oxygen-deficient structures when it is exposed to a reducing atmosphere. Chemical processes on ceria surfaces serve as gateways in controlling such oxygen transfer. In this work, we have prepared cerium oxide films on which Rh is vapor-deposited and measured the Ce oxidation state following sequential exposure to oxygen and hydrogen. The difference between the fraction of Ce 4+ state of a ceria film resulting from room temperature O 2 exposure and the fraction following sequential H 2 exposure at 400 K was measured for various conditions. Our results show that to achieve an observable reduction by H 2 exposure, O 2 preexposure of the Rh-deposited ceria surface is needed. The H 2 reduction yield increases as the dose of Rh to CeO 2 surface increases. Although this reduction process is observed in as-grown Rh-deposited ceria films, ion sputtering of Rh-dosed ceria surfaces enhances the yield of H 2 reduction. The need of both Rh dose and oxygen preexposure for reduction of ceria by hydrogen suggests that oxygen removal from ceria films is catalyzed by RhO. A mechanism is proposed in which RhO in intimate contact with ceria transfers hydrogen to ceria lattice oxygen, followed by oxygen removal from the ceria surface through water desorption.
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