Exploring the Adsorption Properties of Small Molecules on CeZr-Based Nanoclusters

An atomistic understanding of the interactions between the molecules and surfaces is crucial to optimize the catalytic performance in heterogeneous systems. In this investigation, calculations based on density functional theory are used to characterize the adsorption of a set of catalytically motivated molecules (CO, CO₂, CH₄, NH₃, H₂O, SO₂) on ceria (Ce₁₅O₃₀), zirconia (Zr₁₅O₃₀), and mixed ceria-zirconia (Ce₈Zr₇O₃₀) nanoclusters, which were selected based on their relevance in a variety of catalytic reactions. To obtain an improved analysis of all optimized adsorbed structures, we developed an automated algorithm to characterize the adsorption modes, covering the orientation and site preferences, based on the combination of Coulomb matrix representations with k-means clustering, using Silhouette scores to define the number of representative structures. From our calculations and analysis, we found that the 6 closest substrate atoms to the adsorbed molecule provide an optimal representation for the characterization of orientation and site preference of the selected molecules. The adsorption modes of CO, CO₂, CH₄, NH₃, H₂O, and SO₂ were grouped into distinct classes, showing consistent orientation patterns, such as parallel or inclined geometries relative to the substrate. In general, the adsorption process does not induce large deformations in the oxide nanoclusters. In the lowest energy structures, the specific interaction preferences of the molecules with the oxide clusters follow the pattern: CO and NH₃ form bonds via lone pairs on the C and N atoms, respectively; CH₄ assumes the umbrella configuration; and H₂O and SO₂ interact through their O atoms. In particular, the SO₂ molecule undergoes large changes in the bond angle, indicating a possible deformation toward the SO₃ molecule, especially in ceria nanoclusters. With the exception of SO₂, all remaining molecules contribute electron density to the substrate upon adsorption, whereas SO₂ functions as an electron acceptor (Lewis acid).