Coupling of Acetaldehyde to Crotonaldehyde on CeO2-x(111): Bifunctional Mechanism and Role of Oxygen Vacancies

Selective C-C coupling of oxygenates is pertinent to the manufacture of fuel and chemical products from biomass and from derivatives of C-1 compounds (i.e., oxygenates produced from methane and CO2). Here we report a combined experimental and theoretical study on the temperature programmed reaction (TPR) of acetaldehyde (AcH) on a partially reduced CeO2-x(111) thin film surface. The experiments have been carried out under ultra-high-vacuum conditions without continuous gas exposure, allowing better isolation of active sites and reactive intermediates than in flow reaction conditions. AcH does not undergo aldol condensation in a typical TPR procedure, even though the enolate form of AcH (CH2CHO) is readily produced on CeO2-x (111) with oxygen vacancies. We find however that a tailored double-ramp TPR procedure is successfully produce an aldol adduct, crotonaldehyde (CrA). Using density functional theory calculations and microkinetic modeling we explore several possible C-C coupling pathways. We conclude that the double-ramp procedure allows surface oxygen vacancy dimers, stabilized by adsorbate occupation, to form dynamically during the TPR. The vacancy dimers in turn enable C-C coupling to occur between an enolate and an adjacent AcH molecule via a bifunctional enolate keto mechanism that is distinct from conventional acid- or base-catalyzed aldol condensation reactions. The proposed mechanism indicates that CrA desorption is rate-limiting while C-C coupling is facile.