Multi-shell nanostructures minimize diffusion pathways and dual active sites decouple activation for efficient ammonia borane hydrolysis

Abstract

The development of efficient non-noble metal catalysts for low-energy, high-safety hydrolysis of ammonia borane (AB) is critical to advancing hydrogen generation technologies. Current catalysts face two main challenges: sluggish diffusion and competitive activation of liquid-phase molecules (H₂O and AB) on single metal sites, causing low efficiency. To address these limitations, we adopt two strategies: (1) the fabrication of multi-shell hollow sphere architectures to minimize reactant diffusion pathways and (2) the integration of dual reaction sites to decouple the activation of reactants. This study diverges from trial-and-error experimentation by employing integrated theoretical and experimental methodologies. Theoretical modeling shows that optimal shell thicknesses are below 0.98 and 12.49 µm for AB and H₂O, respectively. We selected Zn, Mn, Ni, and Cu from seven common metal oxides as secondary reaction sites for Co₃O₄ catalysts based on their affinity for reactants. Experiments revealed that the multi-shell structure increased the catalyst's specific surface area from 25 to 98.26 m² g⁻¹, reducing reactant diffusion distance. Dual reaction sites enabled selective activation of reactants, with activity order Cu > Ni > Mn > Zn for modified Co₃O₄ catalysts. Enhanced catalytic performance correlates with improved charge localization and electron transfer, lowering H₂O and AB dissociation energy barriers by 8-fold and 1.5-fold, respectively. Reactant binding energies align linearly with the catalyst's d-band center, and catalytic activity shows a volcano-shaped correlation with dopant-induced d-band center shifts, consistent with the Sabatier principle. The optimal dopant d-band center is between −1.4 and −1.8 eV. In general, this work offers a design reference for mass transfer and chemical reaction enhancement of AB hydrolysis catalysts.