| Abstract
| - Competitive ethene oxidation pathways are presented for a p(4 × 4) surface-oxide phase onAg(111) obtained from density functional theory (DFT) calculations. Both parallel routes are found to proceedfrom a common oxametallacycle intermediate (OMME) in agreement with previous mechanistic studies onlow coverage O adatom phase, although acetaldehyde (AcH) is favored by almost 2 kcal/mol. An evenmore striking difference with pure metal surface appears with the oxide regeneration pathways, which arefound non-rate controlling. Furthermore, a kinetic model is developed on the basis of these DFT calculationsand yields 96% selectivity in favor of AcH for a simulation in realistic catalytic conditions (600 K and respectivepartial pressures of 1 atm for ethene and oxygen reactants). As a key finding, this low ethene epoxideselectivity is proposed to be directly linked to the conformational barrier of the pivotal intermediate. In fact,the elasticity of the ultrathin oxide adlayer enables a twisted OMME structure as a true minimum, whichagrees well with orbital prerequisite of the concerted H migration toward AcH. On the contrary, the desiredselective ring closure forming ethene epoxide (EO) requires conformational inversion although the eclipsedform lies 2 kcal/mol above.
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