4 DISCUSSION
In this study, we demonstrate a chelation-anchored strategy for the synthesis of Ni@N-C SAC with a Ni loading up to 7.8 wt%. In the preparation of Ni@N-C SAC, Ni ions are first well dispersed by chelating with precursor containing OH or NH2 groups, and then a two-step pyrolysis process not only restrains the growth of Ni particles but also introducing active N species into C carrier to form atomically dispersed Ni sites by providing N coordination site between the Ni and N species. The active site of Ni@N-C is a Ni-N4 structure according to X-ray absorption fine structure analysis and computational modeling. Compared to Ni nano-cluster, the atomically dispersed Ni contributes to lower adsorption energies of Ni on the carrier, lower H2dissociation energy and energy barriers of the transition states in the hydrogenolysis process. Therefore, Ni@N-C SAC exhibits high catalytic activity in the hydrogenolysis of a lignin model compound and birch lignin. The 2 D HSQC analysis of lignin and oily products reveals that Ni@N-C SAC shows high activity not only in cleaving C-O bonds, but also in cracking C-C bonds in lignin. This methodology opens up a facile strategy for the design of Ni@N-C SAC catalyst featuring high Ni loading and excellent catalytic activity in hydrogenolysis of both C-O and C-C bonds in lignin, highlighting the application potential not only in hydrocracking of large biomass molecules, but also in other catalytic reactions.