C-C coupling of small oxygenates to form molecules with longer carbon backbone is a pertinent step in the production of fuels and chemicals from renewable feedstock. Ceria, among other solid oxides, is known to catalyze the self-condensation of acetaldehyde (AcH) under mild conditions, primarily forming crotonaldehyde (CrH) but also other C₃-C₄ species. On the other hand, this reaction is not typically observed in AcH adsorption experiments carried out in UHV. Through extensive first-principles DFT calculations and simulated temperature programmed desorption and infrared spectra, we have now developed a comprehensive understanding of the primary reaction mechanisms of the self-condensation of AcH on both stoichiometric and partially reduced CeO₂(111) surfaces. On stoichiometric sites, adjacent acid-base site pairs (Ce-O), like on other oxides such as TiO₂, can rapidly catalyze the reaction to yield CrH, and the reaction is desorption-limited for CrH at ambient temperature. This pathway requires the availability and enolization of weakly bound AcH, which therefore renders it inoperative in UHV. On partially reduced CeO₂(111), different pathways involving the oxygen vacancies-stabilized enolate of AcH become active depending on the reaction condition. For instance, the dynamic formation of oxygen vacancy dimers as active sites for C-C coupling is required under UHV conditions, whereas a point vacancy-promoted pathway is operative under a partial pressure of AcH. CrH desorption from an oxygen vacancy is more energetically hindered than from stoichiometric sites. With the insights into the catalytic self-condensation of AcH, we aim to advance the fundamental understanding of aldol condensation reactions on solid catalysts in general, and contribute to the design of earth-abundant catalytic materials for this important class of reaction.

Ms. Suman Bhasker Ranganath

Graduate student, LSU

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