Reaction Mechanisms for Aldol Condensation Determined By Topology of CeO2 Catalyst Facets

Metal oxides with cooperative acid/base sites can effectively catalyze a wide variety of reactions relevant to chemical, pharmaceutical, and biomass conversion processes. It is well accepted that the performance of a metal oxide depends not only on particle size, but also on particle shape. The preferentially exposed certain facets may be crucial for structure-sensitive reactions. Therefore, producing particles with controlled shapes at the nanoscale that result in different surface atom arrangements has shown to be an effective tool for direct identification and quantification of active sites under realistic reaction conditions.

Here, we explore the role of geometric and acid-base properties on the mechanism of the aldol condensation reaction catalyzed by CeO₂ nanoshapes, combining experimental kinetics and DFT calculations. In the crucial C–C coupling step, the two carbonyl adsorbates are bound to two adjacent cations. On the CeO₂ (110) plane, all atoms lie on the same layer, favoring the interaction between adsorbates and facilitating the bimolecular C–C coupling. Thus, the initial unimolecular deprotonation is rate limiting. By contrast, the (100) and (111) planes have open structures with O on the top layer and Ce on the layer below. The O layer interferes between adsorbates linked to Ce sites, making the C–C coupling rate limiting. As one of the major products, water helps overcoming this spatial hindrance by remote bond polarization via H-bonds. Therefore, water promotion is only observed on those planes for which the bimolecular step is rate limiting.