NiCu Bimetallic Nanoparticles on Silica Support for Catalytic Hydrolysis of Ammonia Borane: Composition-Dependent Activity and Support Size Effect

Abstract

NiCu bimetallic nanoparticles (NPs) with different Ni/Cu compositions are controllably synthesized by tuning the ratio of Ni and Cu acetylacetonate precursors in the presence of oleylamine and trioctylphosphine. The similar particle size, monodispersity, and homogeneous alloying of the obtained NPs are confirmed by spectroscopic and microscopic analyses. When the bimetallic NPs together with the monometallic counterparts are used as catalysts for the hydrolysis of ammonia borane (AB), their catalytic activities are found to be composition-dependent. The best-performing Ni0.75Cu0.25 NPs show an activation energy of 34.2 kJ mol–1, which is among the lowest in reported non-noble-metal catalysts. This volcano-type activity trend is attributed to the alloying effect of NiCu that endows a favorable electronic structure toward the hydrolysis of AB. To investigate the catalytic effect of support particle size, a critical yet largely unexplored factor, we further deposit the Ni0.75Cu0.25 NPs onto six differently sized silica spheres in the size range 47–485 nm. It is found that the activity of NiCu/SiO2 catalysts increases progressively with decreasing SiO2 particle size, which is attributed to the less agglomeration and better stabilization of NiCu NPs enabled by SiO2 spheres with higher curvature and longer interparticle distance. Notably, the NiCu NPs supported on the smallest SiO2 exhibit a much higher turnover frequency of 1516 molH2 molmetal–1 h–1 compared to the unsupported NPs as well as an excellent reusability in the consecutive hydrolysis of AB, signifying the strong metal–support interactions. The results underline the importance of engineering alloy composition and support particle size for efficient catalytic hydrolysis of AB.