Theoretical Insights\ninto H<sub>2</sub> Activation\nover Anatase TiO<sub>2</sub> Supported Metal Adatoms

H₂ activation is fundamental in catalysis.\nSingle-atom\ncatalysts (SACs) can be highly selective hydrogenation catalysts due\nto their tunable geometric and electronic properties. In this work,\nH₂ activation (adsorption, splitting, and diffusion) on\nthe anatase TiO₂-supported SAC has been modeled in detail.\nThe stable configurations of 14 transition metals from 3d to 5d (Fe,\nCo, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, and Au) and Sn have\nbeen screened. We compared H and H₂ adsorption and H₂ heterolytic and homolytic splitting on SA/TiO₂. H on the SAC in neutral, hydridic, and proton forms and the preferred\nH₂ dissociation paths are revealed. We found that the metal\nadatoms strengthen the Brønsted acids via forming the SA-O bonds\nand promote the H adsorption on Ti sites via forming the Ti³⁺ sites. The electronic descriptor using the energy level of the frontier <i>d</i> orbital, referenced to vacuum, can predict the single\nH and H₂ dissociative adsorption energies on the metal\nsite. As the SA-H^δ‑ interaction is stronger\nthan Ti-H^δ‑, the activation barriers for heterolytic\npaths over SA-O sites are lower than over Ti-O sites. H₂ adsorption is activated on Au, Ru, Rh, Pd, and Ir in a dihydrogen\ncomplex structure with an elongated H-H bond. Homolytic splitting\nover SA sites is favored thermodynamically and kinetically on Rh,\nPd, Os, Ir, and Pt. In contrast, for the remaining SA/TiO₂, H-H splitting at the SA-O is kinetically favored compared to the\nTi-O sites, but the products are less thermodynamically favored.