Hierarchically alloyed Pd–Cu microarchitecture with tunable shapes: Morphological engineering, and catalysis for hydrogen evolution reaction of ammonia borane

Abstract

Some uniquely microstructured bi/poly-metal alloys containing lesser or no noble metal have been testified to be cost-effective catalysts suitable for solvolytic dehydrogenation of ammonia borane, which expects the intensive study on their morphological engineering and synergistic mechanism. To this, we here introduce a facile and flexible fabricating protocol, i.e. stepwise wet-chemical reduction route, to purposefully modulate the orientated growth of Pd–Cu alloyed crystals via joint operation of adding suitable additives and adjusting temperature. The characterization of phase composition and evaluation of catalytic ability regarding as-fabricated Pd–Cu nanocrystals with diverse morphological features (e.g. concave tetrahedron, cube, polyhedron, nanosphere, nanowire, worm-like, seaurchin-like, flower-like, etc.) demonstrates that the nanocrystals have homogeneous alloyed phase, holding significantly enhanced catalytic activity and durability. The formation of synergistic effect due to charge transfer from Cu to Pd gives rise to electron enrichment around Pd atomic nucleus, facilitating the adsorption of H to form metal-H species, thus promoting the dehydrogenation of ammonia borane. The difference of catalytic activity of the Pd–Cu nanocrystals with different shapes reveals their morphologic dependent nature, endowing the uniquely shaped bimetal nanocrystals with exceptional catalytic performance. The concave tetrahedron shaped Pd–Cu alloyed nanocrystals hold the excellent catalytic activity almost equivalent to that of noble metal Pd, catalyzing hydrolytic reaction of ammonia borane at 298 K with the apparent activation energy of 31.18 kJ/mol, and maintaining 89% of the incipient catalytic ability after 5 recycling runs. Theoretically, the suggested morphologic tuning strategy can be also applied in fabricating other bi/poly metal alloy catalysts, with great practical potential and development prospects.