The role of the cationic Pt sites in the adsorption properties of water and ethanol on the Pt₄/Pt(111) and Pt₄/CeO₂(111) substrates: A density functional theory investigation

Finite site platinum particles, Pt_n, supported on reduced or unreduced cerium oxide surfaces, i.e., CeO_2-x(111) (0 < x < 1 2), have been employed and studied as catalysts for a wide range of applications, which includes hydrogen production using the ethanol steam reforming processes. Our atomic-level understanding of the interaction of Pt with CeO_2-x has been improved in the last years; however, the identification of the active sites on the Pt_n/CeO_2-x(111) substrates is still far from complete. In this work, we applied density functional theory based calculations with the addition of the on-site Coulomb interactions (DFT+U) for the investigation of the active sites and the role of the Pt oxidation state on the adsorption properties of water and ethanol (probe molecules) on four selected substrates, namely, Pt(111), Pt₄/Pt(111), CeO₂(111), and Pt₄/CeO₂(111). Our results show that water and ethanol preferentially bind in the cationic sites of the base of the tetrahedron Pt₄ cluster instead of the anionic lower-coordinated Pt atoms located on the cluster-top or in the surface Ce (cationic) and O (anionic) sites. The presence of the Pt₄ cluster contributes to increase the adsorption energy of both molecules on Pt(111) and CeO₂(111) surfaces; however, its magnitude increases less for the case of Pt₄/CeO₂(111). Thus, the cationic Pt sites play a crucial role in the adsorption properties of water and ethanol. Both water and ethanol bind to on-top sites via the O atom and adopt parallel and perpendicular configurations on the Pt(111) and CeO₂(111) substrates, respectively, while their orientation is changed once the Pt₄ cluster is involved, favoring H binding with the surface sites.