3.3 Mechanism and universality of chelation-anchored strategy for preparing Ni@N-C SAC
The above-described morphology and electronic structure studies demonstrate the successful synthesis of Ni@N-C SAC. To investigate the role of chelating agent in synthesizing Ni@N-C SAC, D-glucosamine hydrochloride was first treated at 800 °C to remove its amino/hydroxyl groups, then the generated sample was co-pyrolyzed with nickel acetate and melamine by a standard process used in the synthesis of Ni@N-C SAC (Figure 3a), obtaining a catalyst denoted as Ni@C. SEM image of Ni@C suggests that carbon nanotubes are obtained probably via a Ni-catalyzed growth during the decomposition of melamine46,47(Figure 3b). Moreover, TEM images show that a large proportion of Ni atoms aggregate to clusters rather than atomically dispersed Ni (Figure 3 c, d). The diffraction peaks of Ni@C in XRD profile at 2θ of 43.2°, 44.5°, 49.6°, 51.8° and 76.4° are attributed to NiO (012), Ni (111), Ni2O3 (112), Ni (200), Ni (220), respectively (Figure S4). The above results indicate that the abundant amino/hydroxyl groups of D-glucosamine hydrochloride play crucial roles to coordinate with Ni ions, which prevent the aggregation of Ni for obtaining single atom Ni.
Meanwhile, a comparison catalyst denoted as Ni@NC was prepared without the addition of melamine (Figure 3e). The SEM characterization showed that the morphology of Ni@NC (Figure 3f) was non-porous. Contrarily, Ni@N-C SAC (Figure 1b) and Ni@C (Figure 3b) display the wrinkled and porous morphology. The XRD spectrum of Ni@NC (Figure S4) shows Ni peaks obviously, and its TEM images (Figure 3 g, h) reveal that Ni clusters are formed on the carbon support.
Relying on the above studies relating the roles of chelating agent and soft-template, it could be deduced that the pyrolysis of D-glucosamine hydrochloride formed a carbon skeleton in the interlayer of the g-C3N4 at the lower temperature stage of pyrolysis (600 °C), The layer-by-layer stacking structure similar to sandwiches may inhibit the growth of Ni particles NPs. The further pyrolysis at a higher temperature (800 °C) produced volatile gases from the decomposition of the g-C3N4 to form wrinkled and porous morphology. Meanwhile, decomposition of g-C3N4 resulted in the formation of active N radicals, doping into carbon skeleton and inducing the coordination of Ni with N to form Ni-N4 structure. It should be noted that although every molecule of D-glucosamine hydrochloride contains one amino group, it cannot provide enough N to chelate Ni for hindering the aggregation of Ni in the absence of melamine. For this reason, the addition of melamine is also essential for preparing the atomically dispersed Ni species. Therefore, during the process of forming Ni SAC, the chelating agent prevents the aggregation of Ni2+, and the soft template provides enough N to coordinate and anchor Ni by forming Ni-N4 structure.