Microscopic Scaling Relation of Ti-Based Catalysts in De/Hydrogenation Reactions of Mg/MgH2

While some early transition metals, such as Ti, can efficiently adsorb and dissociate hydrogen, they have rarely been utilized in hydrogenation and dehydrogenation (de/hydrogenation) reactions because their strong Cat-H bond results in a high hydrogen diffusion barrier. This limitation is known as the macroscopic scaling relation. Herein, using de/hydrogenation reactions of Mg/MgH2 as the example, we report that the hydrogen dissociation and diffusion barrier can be scaled by the Ti valence state, leading to the establishment of a "microscopic" scaling relation. The reaction rates of TiTM-MgO/MgH2 are improved by 69-72 times compared to that of MgH2 under the same conditions, which are even 10 times higher than those of Pd- and Pt-based catalysts. Kinetic analyses and density functional theory (DFT) calculations confirm that the electron transfer properties between catalysts and hydrogens can be systematically controlled as a function of Ti valence states, optimizing the Ti-H bond stability. Significantly, the chemical and structural properties of the TiTM-MgO catalyst remained largely unchanged during and after de/hydrogenation reactions. Our results revealed a "microscopic" scaling relation within a single element governed by its valence state, offering a blueprint for the application of early transition metals in de/hydrogenation reactions.