Positive influence of minute Pt addition on the activity of Ni supported on defective graphene for hydrogenation/dehydrogenation of N-ethylcarbazole as liquid organic carrier

Abstract

[EN] Ni nanoparticles (approximate to 8 nm) have been prepared at about 15 wt% loading strongly grafted on N-doped graphitic carbon matrix [Ni@(N)C] by pyrolysis of Ni(OAc)2 adsorbed on chitosan. Ni@(N)C was used as catalyst for H2 storage/release on N-ethylcarbazole (EC). Ni@(N)C exhibits higher H2 storage capacity than analogous Ni nanoparticles (NPs) supported on silica. The addition of minute amounts (18 ppm) of Pt increases significantly the H2 storage/release activity of Ni@(N)C due to effect of Pt promoting H spill over the Ni NPs. There is no linear correlation between Pt concentration and hydrogenation/dehydrogenation activity of Ni@(N)C, the lowest Pt loading (18 ppm) resulting in the most efficient Pt/Ni@(N)C for hydrogenation-dehydrogenation of EC. This fact was attributed to the presence of small Pt clusters or single atoms, while higher concentration would correspond to a less-efficient larger cluster regime. The most efficient Pt/Ni@(N)C-18 catalyst was used for four consecutive hydrogenation/dehydrogenation cycles, exhibiting some fatigue in the H2 storage/release of the same EC batch that was attributed to the increase in the product mixture complexity upon cycling, with the formation of some intermediates that undergo more difficult hydrogenation/dehydrogenation reaction as well as partial catalyst deactivation. After four cycles, (Pt)Ni@(N)C exhibits in the fifth use a H2 storage capacity of 5.2 wt%, somewhat lower than the 5.8 wt% H2 capacity of the fresh sample, while no changes in the XRD, TEM and XPS characterization of the five-times used catalyst compared to the fresh material is observed.