N₂ as an Efficient IR Probe Molecule for the Investigation of Ceria-Containing Materials

Ceria and ceria-based catalysts are very important in redox and acid-base catalysis. Nanoceria have also been found to be important in biomedical applications. To design efficient materials, it is necessary to thoroughly understand the surface chemistry of ceria, and one of the techniques that provides such information about the surface is the vibrational spectroscopy of probe molecules. Although the most commonly used probe is CO, it has some disadvantages when applied to ceria and ceria-based catalysts. CO can easily reduce the material, forming carbonate-like species, and can be disproportionate, thus modifying the surface. Here, we offer a pioneering study of the adsorption of ¹⁵N₂ at 100 K, demonstrating that dinitrogen can be more advantageous than CO when studying ceria-based materials. As an inert gas, N₂ is not able to oxidize or reduce cerium cations and does not form any surface anionic species able to modify the surface. It is infrared and transparent, and thus there is no need to subtract the gas phase spectrum, something that often increases the noise level. Being a weaker base than CO, N₂ has a negligible induction effect. By using stoichiometric nano-shaped ceria samples, we concluded that ¹⁵N₂ can distinguish between surface Ce⁴⁺ sites on different, low index planes; with cations on the {110} facets and on some of the edges, Ce⁴⁺-¹⁵N₂ species with IR bands at 2258-2257 cm^-1 are formed. Bridging species, where one of the N atoms from the molecule interacts with two Ce⁴⁺ cations, are formed on the {100} facets (2253-2252 cm^-1), while the interaction with the {111} facets is very weak and does not lead to the formation of measurable amounts of complexes. All species are formed by electrostatic interaction and disappear during evacuation at 100 K. In addition, N₂ provides more accurate information than CO on the acidity of the different OH groups because it does not change the binding mode of the hydroxyls.