Comparing direct pyrolysis and post-impregnation in the synthesis of atomic Fe active sites for solvent-free aerobic coupling of benzylamine

Abstract

Both direct pyrolysis and post-impregnation are employed for the synthesis of a 3D macroporous carbon-supported Fe/N-C catalyst in this study. By comparing the microstructure, physicochemical properties, and local electronic structure of the products obtained from both preparation methods, the underlying formation mechanisms of atomic Fe sites are proposed based on the degree of carbonization of carbon-based precursors. As embedded Fe ions significantly facilitate the carbonization of carbon-based precursors to form highly stable carbon supports with a certain degree of graphitization, Fe–N_x active sites are the predominant active sites in the catalysts prepared by the direct pyrolysis method, as expected. In contrast, the absence of metal salts in the precursor results in an inadequate carbonization process, leading to pyrolyzed products with limited resistance to concentrated NaOH. This deficiency accounts for the presence of a significant number of oxygen defects in the bare carbon supports. Since metal impregnation and subsequent reduction treatment possess a limited capacity to recover these O-defects, the catalysts obtained via the post-impregnation method exhibit distinct Fe–O_yN_x configurations. The solvent-free oxidative coupling of benzylamine was used as a probe reaction to evaluate the catalytic activity of these atomic Fe sites. The catalytic results demonstrate the superior catalytic activity and recyclability of Fe–N_x active sites compared with Fe–O_yN_x moieties, thereby confirming the superiority of the direct pyrolysis approach over the post-impregnation method.