On the Real Nature of Rh Single-Atom Catalysts Dispersed on the ZrO2 Surface

The nature of a Rh single-atom catalyst (SAC) stabilized on the surface of tetragonal zirconia, t-ZrO2, is investigated here by performing extensive DFT calculations on various possible structural models and comparing the resulting spectral properties with existing data from the literature. The models considered include a Rh atom adsorbed on the clean surface, (Rh)(ads), Rh atoms stabilized by the reaction with surface OH groups to form direct Rh-O bonds, (RhO)(ads), and (RhO2)(ads), the interaction of a Rh atom with an OH group, (RhOH)(ads), and a Rh atom replacing Zr in the lattice, (Rh)(subZr). Also a t-ZrO2 supported Rh-6 cluster has been considered for comparison with Rh single atoms. Surface heterogeneity has been taken into account by computing the various Rh species at terraces and at step sites of the zirconia surface. Based on the calculated adsorption energies, Rh 3d core level binding energies, and stretching frequencies of adsorbed CO and comparison with the experimental data we conclude that the potential candidates for Rh SAC on t-ZrO2 are (RhO)(ads) and (RhOH)(ads) species.