H2 reduction of CeO2(111) surfaces via boundary Rh-O mediation

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 Ce4+ 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 CeO2 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 Rh-O. A mechanism is proposed in which Rh-O in intimate contact with ceria transfers hydrogen to ceria lattice oxygen, followed by oxygen removal from the ceria surface through water desorption. Published by Elsevier Inc.