A Monodispersed Spherical Zr-Based Metal–Organic Framework Catalyst, Pt/Au@Pd@UIO-66, Comprising an Au@Pd Core–Shell Encapsulated in a UIO-66 Center and Its Highly Selective CO2 Hydrogenation to Produce CO

Article abstract of DOI:10.1002/smll.201702812

A Zr-based metal–organic framework (MOF) catalyst, Pt/Au@Pd@UIO-66, is assembled, where UIO-66 is Zr₆O₄(OH)₄(BDC)₆ (BDC = 1,4-benzenedicarboxylate). The gold nanoparticles (NPs) act as the core for the epitaxial

Full text of DOI:10.1002/smll.201702812

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CommuniCation
Metal–Organic Frameworks
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A Monodispersed Spherical Zr-Based Metal–Organic
Framework Catalyst, Pt/Au@Pd@UIO-66, Comprising
an Au@Pd Core–Shell Encapsulated in a UIO-66 Center
and Its Highly Selective CO Hydrogenation to Produce CO
2
Zhizhong Zheng, Haitao Xu,* Zhenliang Xu, and Jianping Ge
chemical and physical properties exhibited
by certain classes of nanoparticles.
A Zr-based metal–organic framework (MOF) catalyst, Pt/Au@Pd@UIO-66, is
[27,28]
assembled, where UIO-66 is Zr O (OH) (BDC) (BDC = 1,4-benzenedicarbo-
With the utilization of fossil fuels, the
6
4
4
6
greenhouse gas CO has been the focus
xylate). The gold nanoparticles (NPs) act as the core for the epitaxial growth
of Pd shells, and the core–shell monodispersed nanosphere Au@Pd is
encapsulated into UIO-66 to control its morphology and impart nanoparticle
functionality. The microporous nature of UIO-66 assists the adsorption of
Pt NPs, which in turn enhances the interaction between NPs and UIO-66,
favoring the formation of isolated and well-dispersed Pt NP active sites.
This MOF exhibits high catalytic activity and CO product selectivity for the
reverse-water–gas-shift reaction in a fixed-bed flow reactor.
2
of considerable attention, particularly with
regard to its removal. The CO conversion
2
for CO, CH , HCOOH, and CH COOH
4
3
[29–32]
has been widely researched.
the inert character of CO2,
Due to
the key
[
33–35]
factor in this research area is the activation
of CO . In the context of hydrogen-driven
2
CO reduction, the metal catalyst plays an
2
important role in the product selectivity.
Bimetallic core–shell nanocatalysts would
enhance the specific physical and chemical
properties of each metallocomponent.[36–38] Shape-controlled
nanocrystals provided a brand-new insight for improving the
The way a species with a particular structure assembles is an
important determinant of the resulting material’s architecture,
which may in turn lead to unexpected reactivities.[1–4] Metal-
organic framework (MOF) porous materials have permanent
microline structures and are typically characterized by large
[
39]
[40]
performance of catalysts including star-like,
flower-like, and spherical NPs.
octahedral,
[36]
[41–44]
As reported, bimetallic
Cu–Pd catalysts can achieve high selectivity for C1 and C2
chemicals, illustrating that their geometric arrangements con-
surface areas,[
5,6]
chemical stability,
[7,8]
uniform but tunable
trol the identity of the reduction product.
[45]
Optimization of
cavities, and tailorable chemistry, all of which are promising
characteristics for a variety of applications in gas storage and
the bimetallic catalyst can further enhance the selectivity and
[
46–50]
efficiency of CO reduction.
Furthermore, the core–shell
2
[9–11]
[12–15]
[16]
separation,
catalysis,
ion exchange,
and drug
nanoparticles Au@Pd up to three atomic layers exhibit excel-
delivery.[ MOFs could offer the opportunity to develop new
17]
lent catalytic activity leading to formic acid.
[51]
types of composite materials that display enhanced or even
with respect to the parent
MOFs. The incorporation of nanoparticles (NPs) in MOFs,
Catalyst support does significantly influence the catalytic
activity. The metal NP catalysts on support require O vacan-
cies at the surface of catalyst oxide component where metal
NPs facilitate hydrogenation of carbonate intermediates formed
unprecedented behaviors[
16,18–20]
either using MOFs as templates to generate nanoparticles
within their cavities[
21–24]
or encapsulating presynthesized
encompasses the benefits of novel
after oxidative adsorption of CO on these vacancies, such as
2
nanoparticles in MOFs,[
25,26]
TiO and CeO carriers for the creation of oxygen vacancy facile
2
2
[
52–55]
on their surface.
because the secondary building units (SBUs) similar to TiO or
MOF would be an advantageous support
2
Prof. H. Xu, Prof. Z. Xu, Dr. Z. Zheng
State Key Laboratory of Chemical Engineering
Membrane Science and Engineering R&D Lab
East China University of Science and Technology
Shanghai 200237, China
CeO were also metal-oxide component. The Zr(IV)-based MOF
2
UIO-66 was chosen as a support owing to its high thermal sta-
[7]
bility (approaching 500 °C), porosity, and high surface area.
Based on the abovementioned viewpoint, we designed a catalyst
comprising supported Au@Pd and Pt NPs, which uses UIO-66
E-mail: xuhaitao@ecust.edu.cn
Prof. J. Ge
as a support, to study gas-phase CO hydrogenation for the
2
Shanghai Key Laboratory of Green Chemistry and Chemical Processes
School of Chemistry and Molecular Engineering
East China Normal University
reverse-water–gas-shift (RWGS) reaction.
Our strategy for the catalyst assembly is shown in Figure 1,
which combines the excellent properties of MOFs and Au@
Pd and Pt nanoparticles, and involves the functionalization of
bimetallic NP surfaces with polyvinylpyrrolidone (PVP) and
the optimization of the crystallization of UIO-66. We designed
Shanghai 200237, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201702812.
DOI: 10.1002/smll.201702812
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Figure 1. Synthetic route for the production of Au@Pd NPs and other nanocomposites.
−
and prepared the catalyst Pt/Au@Pd@UIO-66 that includes an
encapsulated Au@Pd core–shell in its center and Pt nanopar-
ticles well dispersed onto the surface of UIO-66 to study gas-
undergo oxidative etching caused by Cl or the reaction mixture
can turn pale yellow over time at the reaction temperature. To
promote the formation of the heterostructure of core–shell
Au@UIO-66, additional PVP can extensively be used to control
morphology and dispersity of Au@UIO-66, which can be selec-
tively absorbed on crystal planes to reduce their growth rate and
enhance the generation of heterostructures characterized by
good crystallinity.
phase CO hydrogenation through RWGS.
2
As shown in Figure 2, the monodispersed metal NP sizes
were about 16 nm for Au NPs, 17 nm for Au@Pd NPs, and
3
nm for Pt NPs, respectively. UIO-66 reported by Cavka
et al.[ has an isoreticular structure, whereby Zr oxide SBU
7]
Zr O (OH) CO ) binds to 12-BDC to form a 3D porous mate-
rial featuring large cavities (7.2 Å) and small apertures (6.8 Å).
Clearly, in the absence of PVP, the Au particles embedded in
Au@UIO-66 aggregate with each other, and this observation is
largely in contrast to what happens when a certain amount of
PVP is added (Figure S3, Supporting Information). When the
amount of added PVP is up to 200 mg, products are typically
monodispersed and spherical, with a well-defined core–shell
structure (Figure 2c). Results indicated the evolution of a specific
morphology within certain crystal planes. Powder X-ray diffrac-
tion (PXRD) patterns confirmed the presence of MOFs UIO-66
and Au nanoparticles (Figure S4d, Supporting Information). The
Brunauer–Emmett–Teller (BET) surface area of Au@UIO-66 is
6
4
4
2 12
The synthesis of NPs@UiO-66 was similar to that reported
in literature,[
56,57]
with some modifications to obtain mono-
disperse nanospheres encapsulating these NPs in UIO-66 in
a one-to-one fashion. In a typical procedure, 25 mg of tereph-
thalic acid, 33.4 mg of ZrCl , and 0.7 mL of acetic acid and PVP
4
were mixed together with PVP-stabilized Au nanoparticles in
1
0 mL of N,N-dimethylformamide, and the resulting reaction
mixture was heated at 120 °C for 24 h.
PVP is an amphiphilic, nonionic polymer used extensively
not only as a “general” surfactant to stabilize various nanopar-
ticles in polar solvents but also as a capping agent to assist in
2
−1
2
−1
810 m g , which is less than that of UIO-66 (1150 m g ). This
decrease was attributed to the Au nanoparticles being embedded
in the MOF matrices. The pore size distributions of Au@UIO-66
remained nearly unchanged relative to UIO-66. Using magni-
fied transmission electron microscopy (TEM; Figure S4b, Sup-
porting Information), the multilayered nature of the area at the
edge of composite was clear, which was surprisingly consistent
[
58,59]
controlling the size and shape of certain nanoparticles.
The amount of PVP was the key factor that influenced the
morphology of NPs@UIO-66. Without PVP, the deep red Au
nanoparticles dispersed in DMF containing ZrCl or acetic acid
4
rapidly turned purple, indicating Au nanoparticles aggregating
together. A sufficient amount of PVP is required to produce
the desired functionalization of Au; otherwise, gold atoms can
[58]
with that reported for Pd@Irmof-3. These phenomena could
be explained considering that PVP is a perfect stabilizer once
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Figure 2. Transmission electron microscopy (TEM) images of the NPs: a) Au, b) UIO-66, c) Au@UIO-66, d) Au@Pd, e) Au@Pd@UIO-66, f) Pt/
Au@UIO-66, g) Pt, h) Pt/UIO-66, and i) Pt/Au@Pd@UIO-66.
Au nanoparticles have been functionalized by it, and PVP also
confers to the nanoparticles the necessary affinity to be adsorbed
onto UIO-66 precursors via coordination interaction between the
carboxyl group in the aromatic rings and Zr2+ ions and also via
hydrophobic interaction between the apolar groups of PVP and
−
−
−
Au + 2Cl → AuCl
2
+ e
E
0
= 1.15 V
= 1.002 V
(3)
−
−
−
Au + 4Cl → AuCl
4
+ 3e
E
0
(4)
−
From these equations, we know the fact that E(AuCl /
Au) and E(AuCl /Au) have lower values than E(O /OH )
[60]
4−
−
the H BDC organic linkers.
2
2
−
When the reaction solution includes dissolved O and Cl ,
accounted for the etching of Au. Therefore, by synthesizing
bimetallic core–shell Au@Pd nanoparticles that have Pd as
a uniform shell and Au as a core, we adopt a facile approach
to prevent the etching of Au. Synthesis of these nanoparticles
involves the Au-catalyzed reduction of Pd ions. In particular,
2
−
due to the high redox potential of the O /Cl pair, the oxidation
2
+
3+
+
of Au to Au or Au can occur. HAuCl can release H in solu-
4
tion, which can, in turn, promote the oxidative ability of the O /
2
−
Cl pair, as can be clarified by the following standard electrode
reaction equations[
61]
Pd ions are reduced by sodium citrate to form a thin Pd shell
2+
[
51]
through catalysis by Au seed particles. The size of Au@Pd
NPs is nearly the same as that of Au NPs (Figure 2). As already
alluded to above, the Pd shell thus produced can prevent the
etching of Au. In the absence of additional PVP, only need to
adjust the amount of Au@Pd NP solution, the Au@Pd@MOF
+
−
O
2
+ 4H + 4e → 2H
2
O
E
0
= 1.229 V
(1)
(2)
−
−
O
2
+ 2H
2
O+ 4e → 4OH
0
E = 1.401 V
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core–shell nanocomposites with heterostructure were syn-
thesized, composed of single bimetal nanocrystal individually
encased in single UIO-66 nanocrystal. Energy-dispersive X-ray
spectroscopy (EDS) elemental mapping clearly revealed that
elemental Au and Pd are distributed only in the bimetal core–
shell Au@Pd and that the elements C, O, and Zr of UIO-66 are
homogenously distributed throughout the entire nanoparticle
critical influence in downsizing the UIO-66 spheres to a large
extent. This phenomenon may be ascribed to the synergistic
effects of Pd shell and Au core, which could lead to a reduc-
tion of Au etching, thus regulating crystal size. This evidence
confirms that this is a significant breakthrough in the fabrica-
tion of core–shell nanocomposites, as the present approach is
simpler, more rapid, and more direct than previous methods.
To study the influence of PVP on the yolk–shell structure, a
series of experiments were conducted that consisted of adding
different amounts of PVP to the precursor solution. However,
we found that PVP had little effect on the yolk–shell size; even
in the absence of PVP, the yolk–shell structure remained uni-
form and the size of nanocomposites remained approximately
110 nm (Figure S7, Supporting Information). With the amount
of PVP added to the precursor solution in the lower range, up to
100 mg, the multilayer structure of core–shell Au@Pd@UIO-66
nanocomposites was rather obvious (Figure S7b, Supporting
(Figure 3d). As shown in Figure S7 (Supporting Information),
the product is composed of monodisperse uniform core–shell
spheres, with no evidence of aggregation. Even in the absence
of PVP, no obvious etching was detected in Au@Pd. Under the
same condition, the size of the Au@Pd@UIO-66 spheres thus
produced is about 114 ± 11 nm, much smaller than UIO-66
nanospheres, which are about 409 ± 18 nm (Figure S2, Sup-
porting Information), and much smaller than Au@UIO-66,
which are about 405 ± 23 nm (Figure S4, Supporting Informa-
tion). It was very interesting that core–shell Au@Pd NPs had a
Figure 3. HAADF-STEM and TEM images of the core–shell nanoparticles: a) Au@Pd@UIO-66, b) Pt/Au@Pd@UIO-66, c) magnified TEM image of
Pt/Au@Pd@UIO-66, and d) EDX elemental mapping image of Au@Pd@UIO-66.
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Information). However, as the amount of PVP in the precursor
solution increased to 200 mg, the multilayer structure pro-
gressively disappeared (Figure S7c, Supporting Information).
Polymeric molecular layers can bridge the interaction between
bimetals and MOFs via the polymers’ hydrophobic tail, and they
highest CO conversion among the catalysts tested, whereas
2
UIO-66 did not catalyze CO2 reduction in the conditions
described above. Bimetallic core–shell Au@Pd in MOF proved
superior catalysts to core Au in MOFs. The catalysts showed
low activity and relatively stable at low temperature, attributing
to endothermic nature and equilibrium limitation of the RWGS
[
62]
can stabilize specific facets of the MOFs and metals. Based
on experiments conducted on yolk–shell products isolated at
different reaction times, we concluded that the UIO-66 shells
formed on the Au@Pd NP cores at a rapid rate (Figure S6, Sup-
porting Information). After the core–shell Au@Pd@UIO-66
nanocomposites were fabricated, Pt NPs were situated on the sur-
face of their different crystals by stirring them in the ethanol solu-
tion of Pt NPs to get the catalyst Pt/Au@Pd@UIO-66 (Figure 3b).
The effectiveness of the catalysts was measured with respect
to the reaction of hydrogenation of CO2 (RWGS) in a con-
tinuous fixed-bed flow reactor. The reaction products were
analyzed using a gas chromatograph (Model GC7890II) with
a thermal conductivity detector. The outlet gas flow was deter-
mined by the external standard method, in which the amounts
of CO, CH , and CO were calculated. The catalytic efficien-
0
−1
reaction (CO + H → CO + H O, ∆H = 42.1 kJ mol ). If
2
2
2
298
changing conditions as a support and electric field, etc., their
[
63]
catalytic activities would be enhanced. Interestingly, Pd in the
core–shell Au@Pd enhanced the catalytic effect, as Figure 4a
demonstrates Pt/Au@Pd@UIO-66 and Au@Pd@UIO-66
more effective than Pt/Au@UIO-66 and Au@UIO-66, respec-
tively. Pt-based species were generally effective catalysts due to
their high hydrogenation activity, and MOFs have affinity with
especially dispersed Pt nanoparticles on the
surface of UIO-66 promoting their effect on hydrogenation of
CO . When the reaction temperature is gradually increased
[
59,64]
bitty Pt NPs,
2
to 300 °C and above (Figure 4b), it is clear that Pt/Au@Pd@
UIO-66 is an effective catalyst for CO reduction. CO conver-
2
2
sion can be 27.3% with Pt/Au@Pd@UIO-66 at 400 °C. CO2
conversion would enlarge with increasing the molar ratio
(R = H :CO ). When R was 4:1, CO conversion was 35.3%,
4
2
cies of UIO-66, Pt/UIO-66, Au@UIO-66, Pt/Au@UIO-66,
Au@Pd@UIO-66, and Pt/Au@Pd@UIO-66 are shown in
Figure 4. At 300 °C, 2 MPa, H :CO molar ratio 3:1, and hourly
2
2
2
which was a high conversion in contrast to common catalysts
2
2
−1
−1
[65–70]
space velocity of gas of 24 000 mL g
h
for 4 h, CO conver-
reported with different support at higher temperature.
2
sion was 7.30% for Pt/Au@Pd@UIO-66, 6.20% for Pt/Au@
UIO-66, 5.20% for Pt/UIO-66, 5.00% for Au@Pd@UIO-66,
However, the CO selectivity decreased when the temperature
increased owing to the increased amount of CH . The stability
4
4
.20% for Au@UIO-66, and 0.00% for UIO-66. These results
performance of Pt/Au@Pd@UIO-66 catalyst was also excel-
show that Pt/@Au@Pd@UIO-66 was associated with the
lent through a longevity test at 400 °C; it maintained its initial
Figure 4. a) CO reduction achieved with different catalysts, b) CO -reduction catalyzed by Pt/Au@Pd@UIO-66 at different temperatures, c) stability
2
2
performance of Pt/Au@Pd@UIO-66 catalyst for RWGS reaction in a longevity test at 400 °C, d) CO product selectivity of catalysts Au@Pd@UIO-66
and Pt/Au@Pd@UIO-66.
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activity after more than 30 h (Figure 4c), and at high tempera-
ture, Pt nanoparticles still remain highly dispersed and Au@
Pd remain localized in UIO-66 center adopting a one-to-one
fashion. At 400 °C, the framework of UIO-66 collapsed; powder
XRD showed it had turned into ZrO , and their size of the
aggregating Pt NPs is about 5.9 nm on the surface of ZrO2
Au NP sol was cooled to room temperature slowly in oil bath, 400 mg
PVP (0.5 g, Mw = 58 000) was added to the Au NP sol with stirring at
room temperature for 24 h. The PVP-stabilized Au NPs were collected by
centrifugation at 13 000 rpm for 30 min, washed three times with DMF,
and finally dispersed in DMF.
2
Synthesis of Pt NPs: Pt NPs were synthesized according to the
literature with minor modifications, 20 mg PVP (Mw = 55 000) was
dissolved in 45 mL ethanol, and then 5.0 mL H2PtCl6 aqueous solution
6.0 × 10 m) was added dropwise. After stirring for about 2 min at
room temperature, the solution was refluxed in a 100 mL flask for 3 h
under air to synthesize the PVP-stabilized Pt NPs. The concentration of
(Figures S11 and S12, Supporting Information). The aggre-
−
3
(
gating Pt NPs were still small, so they still sustained excellent
catalytic activity. The bimetal core Au@Pd still maintained
good crystalline and stability, accounting for the observed
persisting activity (Figure S12e,f, Supporting Information).
Pt/Au@Pd@UIO-66 did also exhibit a high CO selectivity at
low temperature, and UIO-66 as a support played an impor-
tant role in dispersing Pt NPs on its surface and encapsu-
lating Au@Pd NPs in its center to prevent their aggregation.
When the nanocomposite collapses at high temperature, it
can still bolster its previous morphology and monodispersed
NPs, abating NP sintering and agglomeration. The nature is
propitious to keep good activity at high temperature for long
time, implying the new UIO-66 promising catalyst support for
RWGS besides the metal oxide.
In summary, a monodisperse spherical catalyst, Pt/Au@
Pd@UIO-66, encapsulating bimetallic core–shell Au@Pd in
its center and loading Pt NPs onto its surface, was success-
fully assembled, which imparting the NPs’ functionality. It
allows the incorporation of multiple nanoparticles in nonag-
glomerated fashion and is capable of controlling the spatial
distribution of nanoparticles in MOFs. The as-obtained Pt/Au@
Pd@UIO-66 composites encompass the benefits of porous and
molecular sieving behavior characterized by the UIO-66 matrix
together with the functional behavior characteristic of isolated
Au@Pd and Pd NPs. Furthermore, the catalysts for the RWGS
with a fixed-bed flow reactor exhibited high catalytic activity
and CO selectivity, and the concept of inserting nanoparticles
into microporous MOFs will revolutionize future industrial
applications.
−3
as-synthesized Pt NPs was about 0.6 × 10 m and used directly without
further treatment.
Synthesis of Au@Pd Core–Shell NPs: A seed-mediated growth method
was used to prepare bimetallic Au–Pd nanoparticles with an Au core and
a Pd shell structure. 20 mL as-synthesized Au solution was added to a
round bottom flask fitted with a reflux condenser and kept stirring in
an oil bath. Subsequently, the Au hydrosol was brought to 100 °C, and
−
3
then 1 mL 10 × 10 m H PdCl was added dropwise. 2 h later, the color
2
4
of the solution turned red wine to brownish black, and then the Au@Pd
solution was collected by centrifugation at 13 000 rpm for 20 min and
was washed twice with water and twice with DMF. Finally, Au@Pd was
dispersed in 1 mL DMF for further use.
Synthesis of UIO-66: 25 mg H BDC and 33.4 mg ZrCl were dissolved
2
4
in 10 mL DMF, 0.7 mL acetic acid was added subsequently, and the
resulting solution was transferred into a 30 mL Teflon-lined stainless-
steel autoclave and heated at 120 °C for 24 h. After being cooled
naturally, the obtained product was collected by centrifugation at 8000 g
for 5 min and washed several times with DMF and ethanol. Finally, the
powder product was dried in a drying oven at 120 °C for 12 h.
Synthesis of Pt/UIO-66 NPs: In a typical procedure, 30 mg obtained
UIO-66 was dispersed in 10 mL ethanol, 30 mL as-synthesized Pt NP
solution was added dropwise to the ethanol of UIO-66 under violent
stirring, and the mixture solution was stirred for further 12 h. At last,
Pt/UIO-66 was collected by centrifugation at 8000 rpm for 10 min and
was washed twice with DMF and twice with ethanol. Finally, Pt/UiO-66
(Zr) was dried in a drying oven at 120 °C for 12 h.
Synthesis of Au@UIO-66: 25 mg H BDC and 33.4 mg ZrCl4 were
dissolved in 9 mL DMF, 0.7 mL acetic acid was added subsequently,
.1 g PVP was added and kept for 5 min under ultrasound, 1 mL Au in
2
0
DMF was introduced into the mixture solution, and then the resulting
solution was transferred into a 30 mL Teflon-lined stainless-steel
autoclave and heated at 120 °C for 24 h. After being cooled naturally,
the obtained product was collected by centrifugation at 8000 g for 5 min
and washed several times with DMF and ethanol. Finally, the powder
product was dried in a drying oven at 120 °C for 12 h.
Experimental Section
Materials and Reagents: 1,4-Benzenedicarboxylate (H BDC, 98 wt%),
Synthesis of Au@Pd@UIO-66: 25 mg H BDC and 33.4 mg ZrCl were
2
2
4
zirconium chloride (ZrCl , 98%), acetic Acid (CH COOH, 36%), N,N-
dissolved in 9 mL DMF, 0.7 mL acetic acid was added subsequently,
0.1 g PVP was added and kept for 5 min under ultrasound, 1 mL Au@Pd
in DMF was introduced into the mixture solution, and then the resulting
solution was transferred into a 30 mL Teflon-lined stainless-steel
autoclave and heated at 120 °C for 24 h. After being cooled naturally,
the obtained product was collected by centrifugation at 8000 g for 5 min
and washed several times with DMF and ethanol. Finally, the powder
product was dried in a drying oven at 120 °C for 12 h.
Synthesis of Pt/Au@UIO-66: In a typical procedure, 30 mg obtained
Au@UIO-66 was dispersed in 5 mL ethanol, 30 mL as-synthesized
Pt NP solution was added dropwise to ethanol of Au@UIO-66 under
violent stirring, and the mixture solution was stirred for further 12 h.
At last, Pt/Au@UIO-66 was collected by centrifugation at 8000 rpm for
10 min and was washed twice with DMF and twice with ethanol. Finally,
Pt/Au@UiO-66 was dried in a drying oven at 120 °C for 12 h.
4
3
dimethylformamide (DMF, 99.5 wt%), ethanol (CH CH OH, 95%),
3
2
hydrochloric acid (HCl, 37%), trichlorogold hydrate hydrochloride
(
HAuCl ·4H O, 99.9%), chloroplatinic acid hexahydrate (H PtCl ·6H O,
4
2
2
6
2
3
8 wt% Pt), palladium chloride (PdCl , 98%), cetyltrimethylammonium
2
bromide (CTAB, 98%), ascorbic acid (AA, 99.7%), PVP (MW = 58 000), AA
(
l-ascorbic acid, 99.7%), trisodium citrate dehydrate (Na C H O ·2H O,
3 6 5 7 2
9
9%), and all the chemicals were used without further purification.
Synthesis of UiO-66: 25 mg H BDC and 33.4 mg ZrCl were dissolved
2
4
in 10 mL DMF, and 0.7 mL acetic acid was added subsequently and the
compounds were placed in autoclave at 120 °C for 24 h. The resulting
UiO-66 was collected by centrifugation at 6000 rpm for 5 min and
washed with DMF and ethanol. Finally, the obtained UiO-66(Zr) was
dried in a drying oven at 120 °C for 12 h.
Synthesis of Au NPs: In a typical procedure for the synthesis of
1
5 nm Au NPs, an aqueous solution of HAuCl4 (0.01%, 100 mL) was
Synthesis of Pd/Au@UIO-66: The Pd/Au@UIO-66 NPs were
synthesized following the same procedure as that of Pt/Au@UIO-66 by
using the 30 mL as-synthesized Pd NPs instead of as-synthesized Pt.
Synthesis of Pt/Au@Pd@UIO-66: In a typical procedure, 30 mg
obtained Au@Pd@UIO-66 was dispersed in 5 mL ethanol, 30 mL
as-synthesized Pt NP solution was added dropwise to ethanol of
added to a round bottom flask fitted with a reflux condenser and kept
stirring. When the solution was heated up to 100 °C, an aqueous
solution of trisodium citrate (1%, 3 mL) was added. Then, the mixture
was stirred for another 20 min. The resulting deep red suspension
was obtained. Then, the compound was removed from heat; after the
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Au@Pd@UIO-66 under violent stirring, and the mixture solution was
stirred for further 12 h. At last, Pt/Au@Pd@UIO-66 was collected
by centrifugation at 8000 rpm for 10 min and was washed twice with
DMF and twice with ethanol. Finally, Pt/Au@Pd@UiO-66 was dried in a
drying oven at 120 °C for 12 h.
Characterization: Scanning electron microscopy measurement was
performed on a Hitachi S4800 scanning electron microscope at 6.0 kV.
PXRD data were collected and recorded in the 2θ range of 5°–80°. TEM
and high-resolution TEM imaging was carried out using JEM-1400 at
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