Effect of Cobalt on Reduction Characteristics of Ceria under Ethanol Steam Reforming Conditions: AP-XPS and XANES Studies

The surface and bulk reduction characteristics of bare ceria and ceria with supported cobalt nanoparticles were investigated under ethanol steam reforming conditions using AP-XPS and XANES techniques. Ceria particles were prepared in two different particle sizes, one in nano and the other in micron size (termed CeO2-NP and CeO2-MP), with average particle sizes of 4 and 120 nm, respectively. It was found that particle size affects surface reducibility of ceria particles; smaller particle size leads to a higher extent of surface reduction. Supported cobalt nanoparticles have a significant effect on the surface reducibility of both CeO2-NP and CeO2-MP. Compared to bare ceria particles, the presence of fully oxidized cobalt nanoparticles on the surface of ceria support retards surface reducibility of ceria since reduction of the cobalt oxide phases (Co3O4 and CoO) takes precedence over that of ceria. The degree of reduction of the cobalt phase during ethanol steam reforming determines the effect of cobalt on the reduction process of ceria, i.e., whether it retards or facilitates the reduction of ceria support. AP-XPS studies show that the surface of cobalt nanoparticles consists of both metallic Co and CoOx. The reduction of the surface region of CoOx to metallic Co forms a metallic Co-based shell and CoOx-based core. The anchor of metallic Co on CoOx make metallic Co shell well dispersed on CeO2 without sintering. In addition, the reforming reaction takes place primarily at the interface of metallic Co and CeO2. The much larger difference between Co/CeO2-NP and Co/CeO2-MP than the difference between CeO2-NP and CeO2-MP suggests the significance of metallic Co in catalyzing the reforming reaction, although bare ceria support shows some dehydration activity in its own right.