Angewandte
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might not only avoid the use of a capping agent, but could also
enable straightforward control over the particle size and
spatial distribution of Pt by adjusting the reduction rate of Pt
ions and/or the growth rate of the MOF.
Initially, we investigated the effect of the reduction rate of
Pt ions on the encapsulation process. Considering the mild
reduction ability of DMF, H2 was introduced into the
synthetic system as a stronger reductant to promote the
reduction of Pt ions. In a typical experiment, ZrCl4, 1,4-
benzene dicarboxylic acid, and acetic acid were dissolved in
DMF and kept at 1208C for 24 h under H2/air (0:1, 1:2, 1:1,
and 2:1 v/v). On completion of the reaction, the supernatant
was dark gray in the absence of H2 (i.e., H2/air 0:1), and the
content of Pt doped on UiO-66 was only 0.7 wt% (ca. 35% of
the total Pt added). With the introduction of H2, the reaction-
mixture supernatant was transparent and colorless, and
analysis of the powder and supernatant by atomic absorption
spectrometry (AAS) indicated that essentially all of the Pt
was incorporated in the UiO-66 (2 wt%) regardless of the H2/
air ratio.
It has been documented that electron-beam irradiation
during transmission electron microscopy (TEM) might
destroy MOF structures and lead to agglomeration of the
initially loaded small metal clusters.[9] To minimize this effect,
we used a low-intensity TEM electron beam (< 200 keV),
thus maximizing the use of a defocused beam to limit the local
beam current. TEM images were also recorded as quickly as
possible to shorten the exposure time to the electron beam.
We did not observe any significant changes in the size and
distribution of metal clusters or the morphology of UiO-66
during our TEM measurements. TEM images showed highly
monodisperse octahedral crystals composed of UiO-66 and Pt
NPs for all the as-prepared Pt@UiO-66 materials (Figure 2).
The results are consistent with the crystal morphology of the
parent UiO-66, thus indicating that the incorporation of Pt
did not affect MOF formation. The Pt NPs were unevenly
dispersed and slight aggregation was observed in the absence
of H2 (Figure 2a). It was interesting to note that when H2 was
introduced with a 1:2 H2/air ratio, Pt clusters with a mean size
of (1.7 Æ 0.3) nm were fully embedded into UiO-66 with an
approximately 5 nm thick MNP-free MOF shell (Figure 2b;
see also Figure S1 in the Supporting Information). As the H2
volume ratio was increased, the outer MNP-free UiO-66 shell
gradually became thicker, whereas the size and dispersion of
the Pt clusters remained almost unchanged (see Table S1 in
the Supporting Information). Typical Pt dispersion in the
materials was in the range of 53–71%. Analysis of the cores
by high-resolution TEM showed that the interplanar spacing
of the particle lattice was about 0.226 nm (Figure 2g), which
corresponds to the spacing of the (111) planes of face-
centered cubic (fcc) Pt. HAADF-STEM imaging and corre-
sponding EDX elemental mapping (Figure 2i) further dem-
onstrated that Pt was homogeneously distributed throughout
the MOF.
Figure 1. Incorporation of Pt NPs in MOFs through a) an in situ one-
step strategy[6c] and b) a kinetically modulated in situ one-step strategy
(this study).
of formation of Pt NPs as compared to the MOF, most of the
Pt NPs were not well embedded within the MOF, thus leading
to uneven and uncontrollable distribution of Pt NPs. Herein,
we disclose a novel and efficient kinetically modulated one-
step protocol for the controlled embedding of “clean” Pt
clusters within MOF crystals. Control over the size and spatial
distribution of Pt NPs is achieved by the use of acetic acid as
a MOF-formation modulator and/or H2 as an assistant
reducing agent (Figure 1b) to increase the reduction rate of
Pt and/or decrease the growth rate of the MOF, respectively.
The as-prepared Pt clusters embedded within MOFs exhib-
ited high activity and stability in the aerobic oxidation of
cinnamyl alcohol, as well as excellent size selectivity in olefin
hydrogenation.
UiO-66,[7] a representative carboxylate-based MOF with-
out any potential protecting groups for MNPs, features a large
surface area and high physicochemical stability. We chose this
MOF as an example to investigate the controllable incorpo-
ration of Pt clusters. Clearly different from the previously
reported cumbersome methods, this one-step strategy
involved the direct mixing of both the Pt (i.e., H2PtCl6) and
MOF precursors in DMF in the present of acetic acid under
a mixed H2/air atmosphere. During the synthetic process,
H2PtCl6 was first reduced to Pt clusters in a short time at the
synthesis temperature of UiO-66 (i.e., 1208C) with H2 and
DMF as reducing agents. On the basis of the well-established
hard and soft acid and base (HSAB) principle,[8] the Pt
clusters generated in situ might preferentially coordinate to
4+
À
the soft C N group of DMF, whereas the hard oxophilic Zr
cations for the MOF would show a preferential interaction
=
with the hard C O group of DMF. This coordination would
provide a “bridge” (i.e., DMF) between the MOF precursors
and Pt nanoclusters, thus inducing preferential anisotropic
growth of the MOF on the Pt surface rather than self-
nucleation. Subsequently, MOF microcrystals would be
produced and spontaneously grow around the surface of the
Pt clusters, which are generated in situ by reduction. The
formed MOF shell around the Pt clusters could serve as
a protective layer to restrict the tiny Pt clusters from growing
and aggregating during the synthesis. Hence, this strategy
The characteristic XRD peaks of all of the above
Pt@UiO-66 composites matched well with those of the
parent UiO-66 (see Figure S2), thus suggesting that the
incorporation of Pt did not affect the integrity of the MOF.
Furthermore, no identifiable peaks associated with Pt NPs
2
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!