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.