DFT Insights into Hydrogen Spillover Mechanisms: Effects of Metal Species, Size, and Support

Hydrogen spillover is a crucial mechanism in heterogeneous catalysis. Herein, density functional theory calculations were conducted to study the metal–support interactions (MSI) and the hydrogen spillover mechanisms in terms of metal size, metal species, and support effects using single-atom (M₁) and four-atom cluster (M₄) models of Ru, Ni, and Pt, supported on anatase TiO₂(101), rutile TiO₂(110), MgO(100), MgO(110), and graphene. For M₁ systems, the binding energies (E_b) vary widely across different M₁ species and substrate surfaces. In contrast to M₁ systems whose MSI are affected by metal type, those supported M₄ models are determined primarily by the support: r-TiO₂(110) > a-TiO₂(101) > MgO(110) > MgO(100) > graphene. Thermodynamically favorable hydrogen spillover on oxide-supported M₁ models required hydrogen coverages (θ) of ∼6 ML, whereas counterpart M₄ systems require ∼3 ML. Therefore, oxide-supported cluster catalysts can facilitate favorable hydrogen spillover better than single-atom catalysts; hydrogen spillover to TiO₂ is more favorable than to MgO. In contrast, no favorable hydrogen spillover was observed on graphene-supported M₁ and M₄ models. Despite considering the same route, different hydrogen spillover mechanisms are observed depending on the support: (i) on reducible TiO₂, hydrogen spills over as a proton with the electron transferred to the support; (ii) on nonreducible MgO, hydrogen spills over as a proton but the electron remains localized to the bound metal; (iii) on graphene, hydrogen spills over as a neutral hydrogen atom. Notably, supported M₄ models with stronger MSI are predicted to exhibit a more facile hydrogen spillover from both thermodynamic and kinetic perspectives, particularly when considering the same metal species across different supports. These detailed insights are expected to advance the understanding of hydrogen spillover on catalysts, which will be valuable for their future design and development.