Hollow hierarchical Ni@C nanocomposites with highly dispersed Ni nanoparticles (NPs) embedded in wellgraphitized carbon matrix have been synthesized by solid-state pyrolysis of simple, well-defined organicinorganic layered nickel hydroxide. The integration of highly dispersed Ni NPs, high Ni NPs content (up to $88.01 wt%), well-graphitized carbon as well as strong Ni/carbon interaction in the Ni@C make them display excellent catalytic activity and stable magnetic recyclability toward the reduction of 4-nitrophenol by NaBH 4 .The development of more efficient and stable catalysts has been an increasingly important goal for chemists and materials scientists for both economic and environmental reasons. As efficient and cost-effective non-noble metal, Ni NPs have attracted much attention owing to their potential applications in electronic or optical materials, as well as in catalysis.1-3 In general, the overall performance of Ni NPs depends heavily upon the size, shape and dispersion. It is widely accepted that a higher catalytic activity can be achieved by increasing the surface area of the specic active phase of the catalysts through reducing the size of the corresponding catalytic particles.
4-7However, due to their high surface area to volume ratio, Ni NPs are typically unstable and tend to sinter into larger species, especially at high metal contents, which results in a dramatic decrease in catalytic activity and selectivity. It is thus highly important to prepare stable Ni NPs with uniform dispersion and narrow size distribution to promote their catalytic activities.Over the past decades, embedding of Ni NPs inside the porous supports has been proved to enhance catalytic activity and impede nanoparticle sintering. [8][9][10][11][12][13][14][15] The advantages of carbon supports with respect to conventional oxidic supports, like silica and zirconia, involve the high specic surface area, high stability, intrinsic high electrical conductivity, as well as easy recovery of metal active phases from spent catalysts by burning away carbon. To date, various methods for preparing carbon-supported Ni NPs have been developed. [16][17][18][19][20][21][22] In particular, incipient wetness impregnation and coprecipitation are commonly used methods to prepare these supported catalysts. It is well known that traditional carbons (including activated carbon, carbon nanotubes, graphene and carbon nanobers) are poor in functional groups and a complex functionalization pre-treatment (such as acid oxidation, ionic liquid linking or polymer wrapping) is always required. In addition, the excess reducing regents are also needed for the reduction of Ni NPs. Although a high dispersion of Ni NPs on porous carbon can be achieved by employing this methods, this tedious post-synthetic method renders instable catalysts with dispersed Ni NPs on the external surface or near pore mouths and Ni NPs still have a tendency to be sintered, especially at high metal loading or high temperature, resulting in a signicant loss of reactivity because of...
