Effect of\nLa Addition to Ceria on the Oxygen Storage\nCapacity and the Energetics of Water Adsorption

Ceria (CeO<sub>2</sub>) and doped ceria are well known\nfor their\ncatalytic surfaces that are active in various oxidation/reduction\nprocesses such as hydrogen production through thermochemical water\nsplitting and three-way catalyst in combustion engines. Doping ceria\nwith trivalent cations is expected to increase the concentration of\noxygen vacancies due to charge compensation, but its effect on oxygen\nmobility or adsorption is not straightforward and depends on the specific\ntrivalent element considered. In this study, we explore the effect\nof La addition on the bulk and surface properties of ceria by combining\nbulk (X-ray diffraction, thermogravimetry analysis, differential scanning\ncalorimetry, and temperature-programmed desorption) and surface-sensitive\ntechniques (X-ray photoelectron spectroscopy and water adsorption\ncalorimetry). Three nanosized compositions of ceria doped with La\nwere synthesized (at 5, 10, and 15% La<sup>3+</sup>) and thoroughly\ncharacterized. Compared with undoped ceria, the solid solutions obtained\nexhibited enhanced thermal stability. The solid solutions preserved\ntheir fluorite structure up to 1200 °C and exhibited a significantly\nreduced coarsening compared to pure ceria. The enhanced stability\nis attributed to the segregation of La to the surface. Doping of ceria\nwith La led to an increase in the oxygen storage capacity, with the\neffect increasing with the increasing concentration of La. This increase\nwas attributed to increased oxygen mobility with increasing La concentration.\nThe addition of a small concentration of La (5%) leads to a significant\nincrease in the amount of water adsorbed compared to pure ceria. Notably,\nwater adsorption led to an enrichment of La on the surface, most pronounced\nfor the highest La content, probably as the result of La diffusion\nfrom the subsurface to the surface. The heat of adsorption isotherms\nexhibits an unusual behavior, pointing to the need for further theoretical\nwork.