(411a) Quantitative in-Situ FTIR Analysis of Ce3+ Densities and the Role of Oxygen Vacancies in Catalysis over Ceria Surfaces

Cerium oxide catalyzes a range of redox and acid catalyzed reactions, with the facile reduction of Ce⁴⁺ to Ce³⁺ and concurrent oxygen vacancy formation purported to play a key role in many of these reactions. Characterization of oxygen vacancies (or Ce³⁺ densities) is critical in most cases to the interpretation of catalytic phenomena involving the same. Spectroscopic techniques employed in prior literature to probe oxygen vacancies, however, each have specific limitations, such as complex and overlapping X-ray photoelectron spectra peaks corresponding to Ce³⁺ and Ce⁴⁺ ions, and the effect of particle size on Raman quantification. In this work, we show, using group and crystal-field theory, that not only do La Porte forbidden electronic transitions of free Ce³⁺ ions from ²F_5/2 to ²F_7/2 electronic states become symmetrically allowed through the action of electrostatic field of crystalline CeO_2-x, but that these transitions, coincidentally, appear in the infrared region at a wavenumber of approximately 2150 cm^-1. Integrated molar extinction coefficients measured for this transition were found to be insensitive to both the identity of the reductant (H₂, CO, and C₂H₅OH) as well as the temperature range of measurement, and were used to decipher active site requirements for two separate reactions. Firstly, H₂-D₂ exchange rates per oxygen vacancy at 373K were found to be independent of degree of reduction, suggesting the exclusive occurrence of exchange over reduced metal sites. Secondly, inverse trends between isobutene formation rates and the density of vacancies resulting from CO₂ formation during tert-butanol dehydration suggest the sole involvement of oxidized ceria domains in catalyzing isobutene formation, the rates of which can be controlled using ethanol co-feeds. The analysis of this electronic transition may be more broadly applicable to other bulk metal oxides, and could provide a means for clarifying the mechanistic function of understoichiometric domains in oxide catalysis more generally.