Evolution of Ni coordination configuration during one-pot pyrolysis synthesis of Ni-g-C3N4 single atom catalyst

In this paper, the unclear Ni atom coordination structure evolution in one-pot pyrolysis method synthesizing Ni single atom catalyst using graphitic carbon nitride as support (Ni-g-C3N4) was deeply discussed. In-situ X-ray absorption fine structure spectra and other assisted methods were used to characterize the structural evolution. The results demonstrate that the pyrolysis process can be divided into three stages. In the first stage from room temperature to 183 °C (Stage I), the NiCl2⋅6H2O was mixed with the supporting material (dicyandiamide) to form the precursor with Ni-(H2O)6 octahedral coordination configuration. In the second stage from 183 to 350 °C (Stage II), the H2O molecules have been removed and the supporting material was converted to melamine. The melamine molecules were polycondensed to melem during 308–350 °C. The intermediates labeled Ni-melamine and Ni-melem was formed with Ni-N4 planar quadrilateral coordination configuration. In the third stage from 350 to 600 °C (Stage III), the supporting material (melem) was gradually converted to g-C3N4, and the Ni-N4 coordination configurations were replaced by Ni-N6 planar hexagon coordination configuration one by one. The three Ni coordination configurations were further confirmed by Reverse Monte Carlo simulations and ab initio calculations.