Mechanistic Study of C-C Coupling of Acetaldehyde on Partially Reduced CeO2-x(111)

SAVARA, ADITYA; CALAZA, FLORENCIA C.; ZHAO, CHUANLIN; XU, YE

Santa Fe

Conferencia; VI SAN LUIS CONFERENCE ON SURFACES, INTERFACES AND CATALYSIS; 2018

CONICET - HUMBOLDT STIFTUNG - UNL - MINCTYP

Selective C-C coupling catalyzed by solid catalysts is highly pertinent to the manufacture of fuel and chemical products from environmentally benign feedstock such as biomass, methane and CO2.1 Here we report a combined temperature programmed desorption (TPD) and density functional theory (DFT) study of acetaldehyde (AcH, or ethanal), a representative carbonyl compound, as a probe reaction to elucidate the mechanism of aldol condensation on ceria, a representative reducible oxide. Under UHV, on CeO2(111) thin film surfaces, AcH does not undergo aldol condensation in a typical TPD procedure,2 even though the enolate of AcH (CH2CHO), a key intermediate, is clearly identifiable.3 Under reactor conditions (continuous AcH flow), on ceria nano-octahedra that primarily exposes (111) facet, crotonaldehyde (CrA, or 2-butenal), can be readily produced upon AcH adsorption at room temperature.4 We demonstrate that, based on the mechanistic insights from previous studies, a tailored ?double-ramping? TPD procedure can successfully capture the formation of CrA on partially reduced CeO2-x(111). Combined with DFT calculations and microkinetic modeling,5 we propose and explore several possible C-C coupling pathways in great detail, and demonstrate that a surface oxygen vacancy dimer formed dynamically in the double-ramping procedure is required to enable aldol addition between enolate and AcH or another enolate under UHV conditions.