On the Utility of Ce3+ Spin−Orbit Transitions in the Interpretation of Rate Data in Ceria Catalysis: Theory, Validation, and Application

Abstract

Cerium oxide is a ubiquitous component in catalytic systems of industrial and academic interest, and the precise mechanistic function of oxygen vacancies/Ce3+ ions a persistent open question in this regard. Existing methods for quantifying vacancy densities are for the most part indirect in nature, and they either have to necessarily be applied ex situ or, if applicable in situ, are oftentimes limited in applicability or quantitative rigor. We present herein a theoretical and experimental analysis of the utility of 2F5/2–2F7/2 electronic transitions in the quantitative estimation of oxygen vacancy or Ce3+ densities. Application of crystal field theory reveals the nonforbidden nature of 2F5/2–2F7/2 transitions for Ce3+ ions present in a C3v site symmetry environment, unlike free Ce3+ ions in the gas phase that exhibit La Porte forbidden transitions between states of equivalent parity. The molar extinction coefficient for the infrared feature of interest is estimated using three reductants─H2, CO, and ethanol─and applied toward interpretation of ceria-catalyzed H2–D2 exchange and tert-butanol dehydration catalysis rate data. Linear correlations between H2–D2 exchange rates and infrared spectroscopy-derived vacancy densities point to the sole involvement of reduced domains in H2 activation steps prevalent in hydrogenation and hydrodeoxygenation catalysis. Contrastingly, areal tert-butanol dehydration rates that trend inversely with vacancy densities both in the presence and absence of α-hydrogen-containing oxygenates carrying a propensity toward stoichiometric oxidative routes suggest the participation of oxidized, not reduced domains in dehydrative turnovers producing isobutene. The nonforbidden nature of the Ce3+2F5/2–2F7/2 transition─evidenced here using symmetry considerations─likely extends beyond the C3v symmetry environment under consideration here, implying that its fortuitous appearance within the infrared region of the electromagnetic spectrum may render it amenable to quantitative application in relating local structure to catalytic function in ceria catalysis more broadly.