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 O2 exposure and the fraction following sequential H2 exposure at 400 K was measured for various conditions. Our results show that to achieve an observable reduction by H2 exposure, O2 preexposure of the Rh-deposited ceria surface is needed. The H2 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 H2 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.