Size-Dependent Catalytic Activity of Monodispersed Nickel Nanoparticles for the Hydrolytic Dehydrogenation of Ammonia Borane.

Nickel (Ni) nanoparticles (NPs) with controlled sizes in the range of 4.9-27.4 nm are synthesized by tuning the ratio of the nickel acetylacetonate precursor and trioctylphosphine in the presence of oleylamine. X-ray diffraction and transmission electron microscopy confirm the formation of the metallic Ni crystal phase and their monodispersed nature. These Ni NPs are found to be effective catalysts for the hydrolytic dehydrogenation of ammonia borane, and their catalytic activities are size-dependent. A volcano-type activity trend is observed with 8.9 nm Ni NPs presenting the best catalytic performance. The activation energy and turnover frequency (TOF) of the 8.9 nm NP catalyst are further calculated to be 66.6 kJ·mol-1 and 154.2 molH2·molNi-1·h-1, respectively. Characterization of the spent catalysts indicates that smaller-sized NPs face severe agglomeration, resulting in poor stability and activity. Three carbon support materials are thus used to disperse and stabilize the Ni NPs. It shows that 8.9 nm Ni NPs supported on Ketjenblack (KB) exhibit higher activity than that supported on carbon nanotubes and graphene nanoplatelets. The agglomeration-induced activity loss is further illustrated by immobilizing 4.9 nm Ni NPs onto KB, which exhibits significantly enhanced activity with a high TOF of 447.9 molH2·molNi-1·h-1 as well as an excellent reusability in the consecutive dehydrogenation of ammonia borane. The high catalytic performance can be attributed to the intrinsic activity of nanoparticulate Ni and the improved activity and stability due to the strong Ni/KB metal-support interactions.