Nature of the mixed-oxide interface in ceria-titania catalysts: Clusters, chains, and nanoparticles

The ceria-titania mixed metal oxide is an important component of catalysts active for the production of hydrogen through the water-gas shift reaction (CO + H2O → H2 + CO2) and the photocatalytic splitting of water (H2O + hv → H2 + 0.5O 2). We have found that ceria-titania catalysts prepared through wet chemical methods have a unique hierarchal architecture. Atomic resolution imaging by high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) reveals that ceria supported on titania exhibits a range of morphologies. One can clearly identify ceria structures involving clusters, chains, and nanoparticles, which are distributed inhomogeneously on the titania support. These structures are often below the sensitivity limit of techniques such as X-ray diffraction (XRD), which in this case identifies the average particle size of the ceria and titania nanoparticles (via the Debye-Scherer equation) to be 7.5 and 36 nm, respectively. The fluorite-structured ceria grows epitaxially on the anatase-structured titania, and this epitaxial growth influences the morphology of the nanoparticles. The presence of defects in the ceria - such as dislocations and surface steps - was routinely observed in HAADF STEM. Density functional theory (DFT) calculations indicate an energetic preference for the formation of O vacancies and the corresponding Ce 3+ sites at the ceria-titania interface. Experimental corroboration by soft X-ray absorption spectroscopy (SXAS) does suggest the presence of Ce3+ sites at the interface. © 2013 American Chemical Society.The research carried out at the Center for Functional Nanomaterials and the Chemistry Department of Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. The theoretical studies were funded by the Ministry of Economy and Competitiveness (Spain, grants MAT2012-31526 and CSD2008-0023) and EU FEDER. Computational resources were provided by the Barcelona Supercomputing Center/Centro Nacional de Supercomputación (Spain).Peer Reviewe