Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 123, Issue 13, Pages 8273-8286Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcc.8b08535
Keywords
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Funding
- U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division [DE-AC05-00OR22725]
- US-DOE, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS), under the Science Undergraduate Laboratory Internships Program (SULI) program at Oak Ridge National Laboratory
- U.S. National Science Foundation [CHE-1664984]
- Office of Science of US-DOE [DE-AC02-05CH11231]
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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.
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