Interplay of Product\nSelectivity and Isolated Ru-Oxo\nSites in the Solvent-Free Oxidization of Benzyl Alcohol

Understanding the\nstructure–reactivity relationships of\nheterogeneous single-atom catalysts (SACs) is indispensable for advancing\ntheir potential in industrial applications. In this study, we assessed\nthe catalytic performance of isolated Ru-oxo sites and RuO<sub><i>x</i></sub> clusters supported on high-specific-surface-area\nTiO<sub>2</sub> (≥242 m<sup>2</sup> g<sup>–1</sup>)\nin the solvent-free aerobic oxidation of benzyl alcohol, a key intermediate\nstep in a wide range of chemical transformations. RuO<sub><i>x</i></sub>/TiO<sub>2</sub> catalysts were characterized by\na suite of techniques, including X-ray diffraction, H<sub>2</sub>-temperature-programmed\nreduction, electron microscopy, diffuse reflectance UV–vis,\nand X-ray photoelectron spectroscopy, together with X-ray absorption\nspectroscopy at the near (XANES) and extended (EXAFS) Ru K-edge. The\nproduct selectivity was found to decisively depend on the nature of\nRu species. While the catalysts rich in the surface-isolated Ru-oxo\nsites showed the highest selectivity toward benzaldehyde (83.6%),\ncatalysts rich in high RuO<sub><i>x</i></sub> nanostructures\n(dimers, subnanometer clusters, or particles) promoted lower benzaldehyde\nselectivity (25.6%) and a concomitant formation of other side products\n(dibenzyl ether and benzoic acid). Differences in the catalytic behavior\ncould be correlated to the extent of interaction between TiO<sub>2</sub> and Ru species, which depends on the dispersion of Ru on the oxide\nsurface. Cyclic stability measurements indicated stable catalytic\nperformance after 4 consecutive runs, with postreaction structural\ncharacterizations indicating that RuO<sub><i>x</i></sub>/TiO<sub>2</sub> catalysts do not suffer any measurable metal leaching\nor significant structural modulation during reaction, which together\nmakes them excellent candidates for oxidation catalysis at liquid/solid\ninterface.