Microscopic Scaling\nRelation of Ti-Based Catalysts\nin De/Hydrogenation Reactions of Mg/MgH<sub>2</sub>

While some early transition metals, such as Ti, can efficiently\nadsorb and dissociate hydrogen, they have rarely been utilized in\nhydrogenation and dehydrogenation (de/hydrogenation) reactions because\ntheir strong Cat–H bond results in a high hydrogen diffusion\nbarrier. This limitation is known as the macroscopic scaling relation.\nHerein, using de/hydrogenation reactions of Mg/MgH<sub>2</sub> as\nthe example, we report that the hydrogen dissociation and diffusion\nbarrier can be scaled by the Ti valence state, leading to the establishment\nof a “microscopic” scaling relation. The reaction rates\nof Ti<i>TM</i>-MgO/MgH<sub>2</sub> are improved by 69–72\ntimes compared to that of MgH<sub>2</sub> under the same conditions,\nwhich are even 10 times higher than those of Pd- and Pt-based catalysts.\nKinetic analyses and density functional theory (DFT) calculations\nconfirm that the electron transfer properties between catalysts and\nhydrogens can be systematically controlled as a function of Ti valence\nstates, optimizing the Ti–H bond stability. Significantly,\nthe chemical and structural properties of the Ti<i>TM</i>-MgO catalyst remained largely unchanged during and after de/hydrogenation\nreactions. Our results revealed a “microscopic” scaling\nrelation within a single element governed by its valence state, offering\na blueprint for the application of early transition metals in de/hydrogenation\nreactions.