Water Promotion (or Inhibition) of Condensation Reactions Depends on Exposed Cerium Oxide Catalyst Facets

Nanoparticles with well-defined facets enable quantitative correlations between surface features and catalytic activity. Here, we explore the role of geometric and acid-base properties in the mechanism of aldol condensation catalyzed by ceria nanoshapes. 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. In contrast, the (100) and (111) planes have an open structure with O on the top layer and Ce on the layer below. The former O layer interferes between adsorbates linked to Ce sites, making the C-C coupling rate-limiting. Water helps in 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.