FTIR Detection of Ce3+ Sites on Shape-Controlled Ceria Nanoparticles Using Adsorbed 15N2 as a Probe Molecule

Abstract

Ceria is an important redox catalyst due to the facile Ce3+/Ce4+ switching at its surface. Therefore, in situ determination of the oxidation state of surface cerium cations is of significant interest. Infrared spectroscopy of probe molecules such as CO holds great potential for this purpose. However, the ability of CO to reduce Ce4+ cations is an important drawback as it alters the initial cerium speciation. Dinitrogen (N2), due to its chemical inertness, presents an attractive alternative. We recently demonstrated that low-temperature 15N2 adsorption on stoichiometric ceria leads to the formation of complexes with Ce4+ cations on the (110) and (100) planes (bands at 2257 and 2252 cm−1, respectively), while the (111) plane is inert. Here, we report results on the low-temperature 15N2 adsorption on reduced ceria nanoshapes (cubes, polyhedra, and rods). A main band at 2255 cm−1, with a weak shoulder at 2254 cm−1, was observed. We attributed these bands to 15N2 adsorbed on Ce3+ sites located on edges and corners as well as on {100} facets. In conclusion, 15N2 adsorbs on the most acidic surface Ce3+ sites and enables their distinction from Ce4+ cations.