Herein, we report a novel carbothermal
welding strategy to prepare
atomically dispersed Pd sites anchored on a three-dimensional (3D)
ZrO2 nanonet (Pd1@ZrO2) via two-step
pyrolysis, which were evolved from isolated Pd sites anchored on linker-derived
nitrogen-doped carbon (Pd1@NC/ZrO2). First,
the NH2–H2BDC linkers and Zr6-based [Zr6(μ3-O)4(μ3-OH)4]12+ nodes of UiO-66-NH2 were transformed into amorphous N-doped carbon skeletons (NC) and
ZrO2 nanoclusters under an argon atmosphere, respectively.
The NC supports can simultaneously reduce and anchor the Pd sites,
forming isolated Pd1–N/C sites. Then, switching
the argon to air, the carbonaceous skeletons are gasified and the
ZrO2 nanoclusters are welded into a rigid and porous nanonet.
Moreover, the reductive carbon will result in abundant oxygen (O*)
defects, which could help to capture the migratory Pd1 species,
leaving a sintering-resistant Pd1@ZrO2 catalyst
via atom trapping. This Pd1@ZrO2 nanonet can
act as a semi-homogeneous catalyst to boost the direct synthesis of
indole through hydrogenation and intramolecular condensation processes,
with an excellent turnover frequency (1109.2 h–1) and 94% selectivity.
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