(754f) Oxygen Vacancy-Promoted Formation of Enolate Species On Reduced CeO2-x(111) Surfaces

Enolate species are suggested to be the key intermediates in a number of organic reactions including aldol condensation, which involves C-C bond coupling and has been proposed as an important reaction in the conversion of biomass-derived feedstock to fuel and chemicals.[1]  It is difficult to detect and characterize these elusive species due to their high reactivity.  In this study we use a combination of reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations to demonstrate that oxygen vacancies can enable the CeO₂(111) surface to activate acetaldehyde (CH₃CHO), a representative ≥C₂ aldehyde species, and convert it into the enolate form (CH₂CHO).  Acetaldehyde adsorbs only weakly on the stoichiometric CeO₂(111) surface at low temperatures and desorbs without further reaction around 210 K.  When the surface is partially reduced (CeO_2-x(111)), however, acetaldehyde interacts strongly with the surface at low temperatures and loses its carbonyl bond character upon adsorption.  Annealing the surface to 400 K leads to the desorption of some of the strongly adsorbed species as acetaldehyde and the appearance of hydroxyl and another organic species.  The identities of the intermediate species on CeO_2-x(111) are determined on the basis of the characteristic IR signatures, previous XPS and NEXAFS results,[2] and DFT calculations, and provide significant insight for constructing a temperature-dependent reaction energy profile for the reaction of acetaldehyde on CeO_2-x(111), which finds good agreement with the TPD of acetaldehyde from CeO_2-x(111).[2]  Our results are consistent with vacancy-promoted dehydrogenation in the original methyl position of acetaldehyde producing the enolate species.  The surface reactions that lead to the formation of the enolate are elucidated, and the further reactivity of this species will be discussed.

Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.

References

[1] Chheda, J.N.; Huber, G.W.; Dumesic, J.A. Angew. Chem. Inter. Ed. 2007, 46, 7164.

[2] Chen, T.L.; Mullins, D.R. J Phys Chem C 2011, 115, 3385.