Coverage-Dependent Hydrogen Adsorption, Spillover, and Vacancy Formation in Ni/ZrO₂ Systems: A First-Principles DFT...

Hydrogen spillover represents a fundamental mechanism in supported metal catalysts, yet its coverage-dependent behavior and relationship to oxygen vacancy formation remain not fully understood. In this line, the present theoretical investigation systematically examines the adsorption of hydrogen molecules (H₂), the spillover of dissociated species (H*) and vacancy generation in a Ni/ZrO₂ model system through varying hydrogen coverages. The results show that clean zirconia surfaces adsorb H₂ weakly, showing a minor tendency for spontaneous bond cleavage. Conversely, in the presence of Ni, exothermic and spontaneous dissociative adsorption is observed. The results also reveal an interesting transformation in spillover energetics as a function of hydrogen coverage, transitioning from endothermic at low to exothermic at high coverages. This transformation coincides with a systematic decrease in oxygen vacancy formation energies, which is eventually facilitated by spillover. Structural analysis indicates that the Ni nanoparticle maintains stability up to some degree of hydrogen coverage before undergoing a more significant geometric reorganization at higher coverages. Electronic structure analysis indicates that spillover is governed by structural rearrangements rather than by charge transfer processes. Relationships were identified between hydrogen coverage and adsorption energies, spillover barriers, and vacancy formation energies, providing a quantitative framework under varying H₂ conditions.