Tuning hydrogen dissociation on Zn₁₂O₁₂ nanocages via first-row transition metal encapsulation: DFT insights for...

Hydrogen storage applications require efficient catalysts for the hydrogen dissociation reaction (HDR). This work evaluates the catalytic efficiency of pristine and first-row transition metal (TM)-encapsulated Zn₁₂O₁₂ nanocages (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) acting as single-atom catalysts (SACs) toward the HDR. Spin-state analysis revealed the stable configurations of the TM-Zn₁₂O₁₂ catalysts. All H₂···Zn₁₂O₁₂ and H₂···TM-Zn₁₂O₁₂ complexes exhibited negative corrected adsorption energies (-1.39 to -3.42 kcal/mol) accompanied by slight H-H bond elongation, indicating mild activation. Transition-state analysis showed further activation of H₂ over the entire catalyst surface via heterolytic hydrogen cleavage, with the Co-Zn₁₂O₁₂ catalyst displaying the most favorable predicted catalytic performance (activation barrier of 12.23 kcal/mol). During hydrogen migration, one dissociated H atom shifted to a Zn atom while the other transferred to an O atom. The Zn → H₁ and H₂ → O charge transfer identified in all complexes was the key step driving H-H bond dissociation, and the interacting species in the transition-state and product structures displayed partially covalent interactions, consistent with optimum catalytic performance. These findings provide an adequate basis for the rational design of highly effective SACs for the HDR.

The geometries, spin states, and energetics of the pristine and TM-encapsulated Zn₁₂O₁₂ nanocages and their hydrogen complexes were investigated at the M06-2X level of theory, using the 6-311+G** basis set for non-TM atoms and the LANL2DZ effective core potential for TM atoms. Multiple spin multiplicities were screened to locate the ground-state configuration of each catalyst. Adsorption energies were corrected for basis set superposition error via the counterpoise method. The HDR pathway was characterized by locating transition states and computing activation energy barriers. The nature of the charge transfer and bonding was probed through natural bond orbital (NBO) analysis, electron density difference (EDD) analysis, and the quantum theory of atoms in molecules (QTAIM). All calculations were performed using Gaussian09 with Multiwfn 3.7 for QTAIM.