Corrugation of the external surface of multiwall carbon nanotubes by catalytic oxidative etching and its effect on their decoration with metal nanoparticles

It was established that partial combustion of carbon constituting the walls of multiwall carbon nanotubes (MWCNTs) catalyzed by previously deposited CaCO₃ nanoparticles converts parallel graphene layers in a multiwall structure to aggregates formed by nano-onions with a diameter of 5-12 nm. The areas with positive curvature of graphene layers on the external surface of air-etched MWCNTs played the role of docking stations for nickel nanoparticles inserted by sonochemical deposition after removal of the CaCO₃. The nickel nanoparticles were located exclusively at the tops of the onions. Formation of nanoscale curvature at the MWCNT support surfaces decreased the average size of Ni nanocrystals at similar loading of 50-60 wt% from 8 to 2 nm. Partial catalytic combustion did not change the concentration of surface carbonyl groups measured by titration, which attributes the observed phenomena directly to the corrugation of the MWCNT surfaces. The catalytic tests revealed a significant increase of catalytic activity of supported Ni catalyst due to corrugating of the external surface of the MWCNT support. After oxidative etching of the MWCNTs, the rate of chloroacetophenone hydrogenation measured with a Ni-MWCNT catalyst increased by a factor of 2 without change in selectivity yielding chlorophenylethanol as the main product.