Nature of Atomically Dispersed Ru on Anatase TiO2: Revisiting Old Data Based on DFT Calculations

In recent years, a lot of emphasis has been given to the concept of single-atom catalyst. In this paper, we show that some of these species have been observed long time ago. Active catalysts based on highly dispersed Ru species on the surface of TiO2 have been widely studied in the past and various techniques have been used to characterize the active species, but so far an atomistic assignment is missing. We have performed an extensive set of DFT + U calculations, considering various potential candidates for stable Ru single-atom catalysts on the TiO2(101) regular and (145) stepped surfaces. The sites include a Ru adatom, (Ru)ad, and (RuO)ad and (RuO2)ad units, which are formally derived from the interaction of Ru with one or two hydroxyl groups, respectively, and Ru atoms substituting lattice Ti and O atoms. The properties of isolated Ru species have been compared to those of a supported Ru4 cluster to mimic formation of small Ru nanoparticles. Studying the dependence of the vibrational and binding properties of CO probe molecules on the various Ru/TiO2 species and comparing the density functional theory (DFT) results with temperature-dependent IR spectra, we come to the conclusion that the active species present on the surface of Ru/TiO2 catalysts, and observed since a long time, consist of single Ru atoms stabilized at the surface via interaction with a surface O probably originating from an OH group.