Pt/facet-Engineered Hydroxyapatite Co-Catalyst for Highly Efficient Hydrolysis of Ammonia Borane

Abstract

Ammonia borane (AB, NH₃BH₃) is a promising candidate for a hydrogen-storage material because of its high stability as a solid state at room temperature under atmospheric pressure. This study demonstrated a novel catalyst design for highly efficient hydrolysis of AB by hybridizing Pt catalyst with hydroxyapatite (HAp, Ca₁₀(PO4)₆(OH)₂) as an “active support”, possessing Lewis-acidic Ca²⁺ sites in the a-plane and Lewis-basic PO₄^3- sites in the c-plane. Facet-engineered HAp particles were synthesized via the microwave-assisted hydrothermal reaction using Ca-EDTA chelates. Varying the Ca/P ratio of HAp precursor (from 1.5 to 1.8) affected the particle morphology, exposed facet ratio of {300} to {002}, and the percentage of phosphate anion species (PO₄^3- and HPO₄^2-). The rod-like HAp sole catalysts performed approximately 2-fold higher activity for the hydrolysis of AB, compared with spherical HAp with low crystallinity. Moreover, the Pt/rod-like HAp co-catalyst demonstrated superior catalytic performance with a turnover frequency (TOF) of 623 mol_H2 mol_Pt^-1 min^-1 than the Pt/spherical HAp (<232 mol_H2 mol_Pt^-1 min^-1). We proposed a possible mechanism of a synergistic effect in the significant enhancement of the hydrogen release rate from AB. Lewis basic PO₄^3- and Lewis acidic Ca²⁺ sites on HAp would affect preferential adsorption of electron-deficient BH₃ and electron-rich NH₃ groups in AB, catalyzing cleavage of B-N bonds. Besides, PO₄^3- sites play a critical role in anchoring Pt particles towards electron transfer from Pt to PO₄^3-, resulting in considerable enhancement of catalytic performance in dissociative adsorption of water molecules, which is a rate-determining step in the hydrolysis.