Metal–organic framework derived Pd/ZrO2@CN as a stable catalyst for the catalytic hydrogenation of 2,3,5-trimethylbenzoquinone

Article abstract of DOI:10.1002/aoc.5233

Metal–organic frameworks (MOFs) have recently been identified as versatile sacrificing templates to construct functional nanomaterials for heterogeneous catalysis. Herein, we report a thermal transformation strategy to directly fabricate metal Pd nanoclusters inlaid within a ZrO₂@nitrogen-doped porous carbon (Pd/ZrO₂@CN) composite using Pd@NH₂-UiO-66(Zr) as a precursor that was pre-synthesized by a one-pot hydrothermal method. The developed Pd/ZrO₂@CN as a robust catalyst delivered remarkable stability and activity to the catalytic hydrogenation of 2,3,5-trimethylbenzoquinone (TMBQ) to 2,3,5-trimethylhydroquinone (TMHQ), a key reaction involved in vitamin E production. The hydrogenation was carried out at 110?°C with 1.0?MPa H₂, and it resulted in 98% TMHQ yield as the sole product over five consecutive cycles, outperforming the analogue Pd/ZrO₂@C without nitrogen doping templated from Pd@UiO-66(Zr). The excellent catalytic properties of Pd/ZrO₂@CN likely originated from the highly stable ultrafine Pd nanoclusters inlaid within ZrO₂@CN matrix on account of the strong interaction between N and Pd, as well as on the Lewis acidity of ZrO₂, which was beneficial to the hydrogenation.

Full text of DOI:10.1002/aoc.5233

Received: 4 July 2019
DOI: 10.1002/aoc.5233
Revised: 5 August 2019
Accepted: 19 August 2019
F U L L P A P E R
Metal–organic framework derived Pd/ZrO2@CN as a stable
catalyst for the catalytic hydrogenation of 2,3,5‐
trimethylbenzoquinone
Shasha Li | Jianping Pan | Xiaoxue Wu | Yanghe Fu | Qiang Xiao | Fumin Zhang
Weidong Zhu
|
Key Laboratory of the Ministry of
Metal–organic frameworks (MOFs) have recently been identified as versatile
sacrificing templates to construct functional nanomaterials for heterogeneous
catalysis. Herein, we report a thermal transformation strategy to directly fabri-
cate metal Pd nanoclusters inlaid within a ZrO2@nitrogen‐doped porous car-
bon (Pd/ZrO2@CN) composite using Pd@NH2‐UiO‐66(Zr) as a precursor that
was pre‐synthesized by a one‐pot hydrothermal method. The developed
Pd/ZrO2@CN as a robust catalyst delivered remarkable stability and activity
to the catalytic hydrogenation of 2,3,5‐trimethylbenzoquinone (TMBQ) to
2,3,5‐trimethylhydroquinone (TMHQ), a key reaction involved in vitamin E
production. The hydrogenation was carried out at 110 °C with 1.0 MPa H2,
and it resulted in 98% TMHQ yield as the sole product over five consecutive
cycles, outperforming the analogue Pd/ZrO2@C without nitrogen doping tem-
plated from Pd@UiO‐66(Zr). The excellent catalytic properties of Pd/ZrO2@CN
likely originated from the highly stable ultrafine Pd nanoclusters inlaid within
ZrO2@CN matrix on account of the strong interaction between N and Pd, as
well as on the Lewis acidity of ZrO2, which was beneficial to the
hydrogenation.
Education for Advanced Catalysis
Materials, Institute of Physical Chemistry,
Zhejiang Normal University, 321004
Jinhua, People's Republic of China
Correspondence
Fumin Zhang and Weidong Zhu, Key
Laboratory of the Ministry of Education
for Advanced Catalysis Materials, Institute
of Physical Chemistry, Zhejiang Normal
University, 321004 Jinhua, People's
Republic of China.
Email: zhangfumin@zjnu.edu.cn;
weidongzhu@zjnu.cn
Funding information
National Natural Science Foundation of
China, Grant/Award Numbers: 21576243,
No. 21576243; Zhejiang Provincial Natural
Science Foundation of China, Grant/
Award Number: LY18B060006,
LY18B030006 LY18B030006, Nos.
LY18B060006
KEYWORDS
2,3,5‐trimethylbenzoquinone, catalytic hydrogenation, MOF‐derived catalyst, NH2‐UiO‐66(Zr), Pd
nanoclusters
1 | INTRODUCTION
catalytic activity.[5–8] However, aggregation or leaching
of those noble metal species inevitably occurs during
Catalytic hydrogenation is an important type of reaction
in modern chemical industrial and petroleum refining,
in which catalysts play a central role.[1–4] Compared to
homogeneous catalysts, heterogeneous catalysts are fre-
quently used in large‐scale production processes by virtue
of their convenient recycling and repeated usability.[1–4]
So far, noble metal heterogeneous catalysts, despite their
high cost and scarcity, are still regarded as the most effi-
cient hydrogenation catalysts because of their high
the practical application of conventional supported cata-
lysts,[9] especially in liquid‐phase reactions. Therefore,
the development of highly stable and active noble
metal‐based heterogeneous catalysts for the hydrogena-
tion reaction is still highly desired, but it is a huge
challenge.
Metal–organic frameworks (MOFs), a relatively new
class of ordered porous materials built by assembling
metal ions/clusters and rigid organic linkers, have proven
Appl Organometal Chem. 2019;e5233.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5233

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LI ET AL.
to be advanced functional materials for a wide variety of
applications.[10–12] MOFs have also attracted remarkable
attention as promising sacrificial precursors for the prep-
aration of porous carbon and/or metal oxide nanocom-
posite materials via high‐temperature thermal
transformation.[13–16] The various atoms that periodical
alternation in MOFs renders congenital conditions for
evenly atomic arrangements.[10,17] Therefore, nitrogen‐
functionalized organic linkers in well‐defined MOFs
would guarantee uniform distribution of the resulting
nitrogen atoms within the network of a porous carbon
matrix upon high‐temperature treatment under an inert
atmosphere.[18] In addition, the coexisting metal oxide
in the derived functional nanomaterials can tune the sur-
face electronic structures of noble metal nanoclusters and
thus modify the catalytic properties of the derived com-
posite catalysts.[19] Nevertheless, development of a feasi-
ble and facile methodology to direct establish noble
metal nanoclusters inlaid within a metal oxide/nitrogen‐
doped porous carbon composite as a highly stable catalyst
is seldom reported.
In a previous work, we reported a strategy for the con-
struction of a composite made of Pd nanoclusters in a
NH2‐functionalized MOF [Pd@NH2‐UiO‐66(Zr)] using
an optimized temperature control program. In this pro-
cess, chelation of Pd ions with the amine groups, in situ
assembly of ZrCl4 and 2‐aminoterephthalic acid (NH2‐
BDC) containing Pd ions, and an on‐site reduction pro-
cess were integrated into one step.[20] By virtue of the
abundant cavities in the MOF and the coordination capa-
bility of the amine groups, the Pd nanoclusters were well
distributed within the NH2‐UiO‐66 cavities over the
obtained Pd@NH2‐UiO‐66(Zr). Thus, Pd@NH2‐UiO‐
66(Zr) showed a high catalytic activity in the hydrogena-
tion of 2,3,5‐trimethylbenzoquinone (TMBQ) to 2,3,5‐
trimethylhydroquinone (TMHQ),[20] a key reaction for
the production of vitamin E.[21–23] However, compared
to conventional supports, such as metal oxides and
porous carbon, the thermal and/or chemical stabilities
of the MOF‐based catalysts are still relatively low,[10–12]
thus impeding their practical usage, particularly for cata-
lytic reactions carried out under relatively harsh
conditions.
N species and the 5d empty orbitals of the Pd ions, as well
as to the Lewis acidity of ZrO2, which promotes hydroge-
nation, the resulting Pd/ZrO2@CN exhibited good recy-
clability and reactivity.
2 | EXPERIMENTAL
2.1 | Catalyst preparation
2.1.1 | Synthesis of Pd@NH2‐UiO‐66(Zr)[20]
The precursor Pd@NH2‐UiO‐66(Zr) was synthesized by a
facile one‐pot strategy without adding any external reduc-
ing agent. Typically, ZrCl4 (1.0 g, 4.3 mmol), NH2‐BDC
(0.78 g, 4.3 mmol), acetic acid (30 ml), and deionized
water (3.0 ml) were dissolved in dimethylformamide
(DMF) (160 ml) in a glass flask (250 ml capacity). To
accelerate the dissolution of the reactants, the slurry
was first subjected to ultrasonication treatment for
10 min. Then, the Pd precursor (0.03 mol/l, 2.0 ml) was
added dropwise into the solution. The mixture was stirred
at 35 °C for 7 hr. Subsequently, the temperature was
increased to 75 °C and stirred for an additional 20 hr.
Afterwards, the temperature was elevated to 130 °C and
stirring continued for another 4 hr. The obtained solid
was then washed with DMF and methanol three times
and then dried at 150 °C for 12 hr. NH2‐UiO‐66(Zr),
UiO‐66(Zr), and Pd@UiO‐66(Zr) were also synthesized
by following the same procedure described above using
the appropriate precursors.
Preparation of Pd/ZrO2@CN
The Pd/ZrO2@CN powders were fabricated directly via a
high‐temperature thermal transformation treatment.
Typically, the Pd@NH2‐UiO‐66(Zr) crystals (2.0 g) were
transferred into a tube furnace. The furnace was heated
at 700 °C at a rate of 1 °C/min with nitrogen gas flow
(flow rate: 80 ml/min). The furnace was held at this tem-
perature for 2 hr and then allowed to cool to room tem-
perature. With the heating treatment under N2 flow, the
Pd@NH2‐UiO‐66(Zr) was converted into a Pd/ZrO2@CN
nanocomposite, which was then used as the hydrogena-
tion catalyst. ZrO2@CN, ZrO2@C, and Pd/ZrO2@C were
prepared following the same procedure.
Bearing the above discussion in mind, herein, we
report a thermal transformation strategy to directly fabri-
The methods for the catalyst characterizations are pro-
vided in the Supporting Information.
cate metal Pd nanoclusters inlaid within
a
ZrO2@nitrogen‐doped porous carbon (Pd/ZrO2@CN)
composite using Pd@NH2‐UiO‐66(Zr) as the sacrificial
precursor. After systematic characterizations, the pre-
pared Pd/ZrO2@CN was applied to the hydrogenation
of TMBQ to TMHQ under elevated temperature and pres-
sure conditions (110 °C, 1.0 MPa H2). By virtue of the
strong interaction between the lone‐pair electrons of the
2.2 | Hydrogenation of TMBQ
The catalytic hydrogenation reaction was carried out in a
high‐pressure Teflon‐lined autoclave (50 ml, NS50‐MP‐
LT‐SS1‐SV‐BS, Anhui Kemi Machinery Technology Co.

LI ET AL.
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Ltd., Anhui, China). Typically, TMBQ (0.54 g), catalyst
(0.05 g), and i‐propanol (15 ml) as the solvent were placed
in the autoclave and repeatedly purged with H2 (10 times)
at 25 °C. The autoclave was then heated quickly to the
given reaction temperature (110 °C), after which hydro-
gen was introduced and a constant pressure (1.0 MPa)
was maintained in the reactor. Finally, agitation was ini-
tiated with a stirring speed of 1000 rpm. The reaction
mixtures were analyzed regularly by using a gas chroma-
tography (Shimadzu 2014) equipped with a flame ioniza-
tion detector and an InertCap 5 capillary column. The
recyclability of the Pd/ZrO2@CN catalyst was tested after
its separation from the reaction system by centrifugation,
rinsing with a large quantity of ethanol, and drying in a
vacuum desiccator at 150 °C for 8 hr.
inert atmosphere, the organic linkers and inorganic Zr‐
based anodes of NH2‐UiO‐66(Zr) were converted into
nitrogen‐doped porous carbon and ZrO2 nanoparticles,
respectively. The ‐NH2 groups in the formed MOFs are
vital for the fabrication of ultra‐small Pd nanoclusters as
these groups can largely impede the migration of Pd
nanoclusters at high temperatures owing to the strong
interaction between the lone‐pair electrons of the N spe-
cies and the 5d empty orbitals of the Pd ions.[26–29] In
contrast, without the assistance of the ‐NH2 groups, the
Pd species were inclined to aggregate into relatively large
nanoparticles upon pyrolysis of the counterpart Pd@UiO‐
66(Zr).
The powder XRD patterns of Pd@NH2‐UiO‐66(Zr)
were almost identical to those of the calculated UiO‐
66(Zr) (Figure 1a),[23–25] demonstrating that Pd@NH2‐
UiO‐66(Zr) was successfully synthesized via the tempera-
ture control program described above. Upon carbonizing
treatment at 700 °C under N2 flow, the peaks of NH2‐
UiO‐66(Zr) completely vanished and new peaks ascribed
to tetragonal ZrO2 appeared (Figure 1a). No characteristic
peaks of Pd nanoclusters were observed, implying that
the Pd nanoclusters were likely highly dispersed in the
derived ZrO2@CN. In addition, the absence of XRD peaks
associated with crystallized carbon demonstrated that the
generated carbon species were amorphous. In contrast,
for the corresponding Pd/ZrO2@C derived from
3 | RESULTS AND DISCUSSION
Scheme 1 shows a schematic illustration of the prepara-
tion of Pd/ZrO2@CN. The MOF chosen for accommodat-
ing the Pd nanoclusters was NH2‐UiO‐66(Zr)
[Zr6O4(OH)4(NH2‐BDC)6], which features substantial
coordination sites (amine groups), abundant cavities,
and a large specific surface area.[24,25] Pd@NH2‐UiO‐
66(Zr) can be conveniently synthesized by a one‐pot strat-
egy. By further pyrolysis at high temperature under an
SCHEME 1 Schematic illustration for preparation of the Pd/ZrO2@CN catalyst.
FIGURE 1 Powder XRD patterns of various samples

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Pd@UiO‐66(Zr) under the same conditions, a sharp dif-
fraction peak at around 40 °C ascribed to the Pd species
was clearly observed (Figures S1 and S2 in the Supporting
Information), suggesting that the Pd species significantly
aggregated during the thermal transformation procedure.
This comparison demonstrates that amine groups are
indispensable for the dispersion of ultra‐small Pd
nanoclusters.[26–29] Further calculations using the
Scherrer equation revealed that the size of the Pd nano-
particles in Pd/ZrO2@C was about 13 nm. ICP‐AES anal-
ysis showed that the actual Pd loadings in Pd/ZrO2@CN
and Pd/ZrO2@C were 1.5 wt% and 1.1 wt%, respectively
(Table S1 in the Supporting Information).
The specific surface areas of various samples were
measured via the N2 adsorption experiment at −196 °C
(Figure 2 and Table S1).[24,25] Upon high‐temperature
pyrolysis, the precursor Pd@NH2‐UiO‐66(Zr) was con-
verted into Pd/ZrO2@CN, mainly induced by the thermal
transformation of the organic linkers into nitrogen‐doped
porous carbon and the Zr‐based nodes into ZrO2. The
derived Pd/ZrO2@CN lost most its porosity, resulting in
a relatively low specific surface area of 290.5 m2/g as com-
pared to 906 m2/g of Pd@NH2‐UiO‐66(Zr). Scanning
electron microscopy (SEM) photos revealed a relatively
uniform octahedral morphology with a diameter of
approximately 70–80 nm for the precursor Pd@NH2‐
UiO‐66(Zr) (Figure 3).[23] As expected, the average crystal
size of the Pd/ZrO2@CN slightly decreased as compared
to the precursor due to the shrinkage during thermolysis
treatment. EDS analysis suggested Pd, N, and Zr were
well distributed
throughout
the
Pd/ZrO2@CN
(Figure 4). The embedded ZrO2 nanoparticles, with an
average size of 4–6 nm, were uniformly embedded within
the ZrO2@CN (Figure 5), which corresponds well with
the XRD results. The high‐resolution transmission elec-
tron (HRTEM) microscopy image manifested some regu-
lar fringes, corresponding to the (111) planes of the
tetragonal ZrO2 phase (Figure 5).[30] Notably, no Pd
nanoclusters were detected, revealing the high distribu-
tion of ultra‐small Pd nanoclusters within ZrO2@CN.
The slightly higher dispersion of Pd in Pd/ZrO2@CN than
that in Pd/ZrO2@C was confirmed by the CO chemisorp-
tion experiments (Table S1).
XPS analyses of Pd/ZrO2@CN and Pd/ZrO2@C further
indicated the existence of ZrO2, where the Zr 3p3/2 and
Zr 3p1/2 peaks at 333.1 eV and 346.7 eV, respectively,
are assigned Zr4+ (Figure 6a and Figure 6b).[30] Moreover,
the signals at 335.3 eV and 340.9 eV are attributed to the
Pd 3d5/2 and Pd 3d3/2 levels, respectively, of Pd0 in
Pd/ZrO2@C (Figure 6a).[23] However, these peaks slightly
negative shifted in the Pd 3d spectra of Pd/ZrO2@CN
(Figure 6b),[23–25] mostly due to the strong interaction
between the Pd species and N during the high‐
temperature pyrolysis. To further verify this interaction,
the N species in Pd/ZrO2@C and Pd/ZrO2@CN were also
characterized by XPS (Figure 6c and Figure 6d). The peak
at 401.1 eV was assigned to graphitic N. The pyridinic N
(400.1 eV) and pyrrolic N (398.5 eV) peaks were domi-
nant in the Pd/ZrO2@CN (Figure 6d). These two types
of N atoms were coexisted in a π‐conjugated system,
which as anchor centers could stabilize the Pd
nanoclusters.[28] Previous studies implied that N doping
in porous carbon could effectively generate a large
FIGURE 2 N2 isotherms of various samples
FIGURE 3 SEM photos of (a) Pd@NH2‐UiO‐66(Zr) and (b) Pd/ZrO2@CN

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FIGURE 4 EDS mapping for Pd/ZrO2@CN
FIGURE 5 TEM images of Pd/ZrO2@CN
number of topological defects, and the corresponding
density is associated with the intensity ratio of the D
and G bands (ID/IG) in the Raman spectra.[28,31] The ID/
IG values for Pd/ZrO2@CN and Pd/ZrO2@C were calcu-
lated to be 1.3 and 0.9, respectively (Figure 7). This sup-
ports the hypothesis that there would be more
dispersive defects in Pd/ZrO2@CN, which might help sta-
bilize the Pd nanoclusters. Based on the above analysis, it
can be concluded that the Pd/ZrO2@CN catalyst with a
uniform dispersion of ultra‐small Pd nanoclusters was
successfully prepared by the thermal transformation
method. As a comparison, Pd/ZrO2@C with a similar
morphology, Pd loading, and specific surface area but
with a severe aggregation of Pd nanoparticles was also
obtained by the same method (Table S1 and Figures S1–
S6).
With the Pd/ZrO2@CN catalyst in hand, we then eval-
uated its catalytic performance in the catalytic hydroge-
nation of TMBQ to TMHQ, an important transformation
for vitamin E production, as a model reaction. When
the initial concentration of TMBQ increased from
0.23 mol/l to 2.33 mol/l (i.e., the molar ratio of TMBQ
to metal Pd was set to be 500, 2,000, and 5,000, respec-
tively), the conversion rate of TMBQ decreased gradually
(Figure 8). This may have been due to the fact that the
total number of hydrogenation active sites in the fixed
catalyst dosage (0.05 g) remained constant; that is, the
conversion rate of the reactant per unit time did not

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FIGURE 6 High‐resolution XPS spectra: Zr and Pd spectra of (a) Pd/ZrO2, (b) Pd/ZrO2@CN; N spectrum of (c) ZrO2@C and (d) Pd/
ZrO2@CN
improve upon reaching the maximum. Further increases
in the initial concentration of TMBQ caused the relative
consumption of the reactants per unit time to decrease,
and thus the overall conversion rate dropped with the
increase of the initial TMBQ concentration.
As can be seen from Figure 9a, the conversion rate of
TMBQ increased significantly with increasing reaction
temperature, mainly owing to the fact that the high
temperature helped accelerate the activation of the reac-
tants. Because the reaction time needed for complete con-
version at the lower temperature was relatively longer,
110 °C is selected as the optimum temperature in further
studies. As shown in Figure 9b, when the hydrogen pres-
sure was increased from 0.5 to 1.0 MPa, the conversion
FIGURE 8 Influence of TMBQ initial concentrations on the
conversion of TMBQ as a function of reaction time. Reaction
conditions: TMBQ (0.54 g), 1.5 wt% Pd/ZrO2@CN (0.05 g), i‐
propanol (15 ml), 1.0 MPa H2, 110 °C
FIGURE 7 Raman spectra of Pd/ZrO2@CN and Pd/ZrO2@C

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FIGURE 9 Conversion of TMBQ as a function of reaction time under different (a) reaction temperatures and (b) hydrogen pressures.
Reaction conditions: (a) TMBQ (0.54 g), 1.5 wt% Pd/ZrO2@CN (0.05 g), i‐propanol (15 ml), 1.0 MPa H2; (b) TMBQ (0.54 g), 1.5 wt% Pd/
ZrO2@CN (0.05 g), i‐propanol (15 ml), 110 °C
rate of TMBQ increased significantly. With a further
increase of the hydrogen pressure up to 1.5 MPa,
although the initial conversion rate of TMBQ increased
to some extent, this trend became less pronounced after
reaction of 30 min. Therefore, in the further investiga-
tions, the reactions were performed at a hydrogen pres-
sure of 1.0 MPa.In addition, the catalytic activity of
Pd/ZrO2@CN was also compared with the analogue
Pd/ZrO2@C without nitrogen atom doping (Figure 10).
Only a trace amount of TMBQ was converted when the
reaction was conducted in the absence of a catalyst (blank
experiment) or over the bare ZrO2@CN support (not
shown). Pd/ZrO2@CN showed a TMBQ conversion of
66%, with TMHQ as the sole product at 110 °C and
1.0 MPa of hydrogen pressure for 15 min. Furthermore,
95% TMBQ conversion was achieved when the reaction
time was increased to 45 min (Figure 10). In contrast,
Pd/ZrO2@C exhibited a much lower catalytic activity of
TMBQ under a same reaction condition. In addition to
the appropriate acidity of ZrO2 that promote the hydroge-
nation,[22,23] the superior catalytic activity of
Pd/ZrO2@CN could be mainly ascribed to the highly dis-
persed Pd nanoclusters within the ZrO2@CN support
compared to that of Pd/ZrO2@C,[21] as confirmed by the
XRD and TEM analyses.
The recyclability of the heterogeneous catalysts is cru-
cial for their practical application. We further tested the
stability of 1.5 wt% Pd/ZrO2@CN. There was no obvious
decrease in catalytic activity over five cycles, regardless
whether the reaction was performed for 15 or 45 min,
demonstrating the excellent stability of the prepared
1.5 wt% Pd/ZrO2@CN catalyst for the hydrogenation of
TMBQ (Figure 11). The recovered catalysts were further
analyzed by N2 adsorption (Table S1), Pd dispersion
(Table S1), TEM (Figure 12), EDS mapping (Figure 13),
FIGURE 10 Activity comparison between 1.5 wt% Pd/ZrO2@CN
and 1.1 wt% Pd/ZrO2@C. Reaction conditions: TMBQ (0.54 g),
molar ratio of TMBQ to Pd in the catalyst is 500, i‐propanol (15 ml),
1.0 MPa H2, 110 °C
FIGURE 11 Recyclability of 1.5 wt% Pd/ZrO2@CN. Reaction
conditions: TMBQ (0.54 g), catalyst (50 mg), 110 °C, 1.0 MPa H2

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FIGURE 12 SEM and EDS mapping of the used 1.5 wt% Pd@ZrO2@CN
FIGURE 13 TEM and HRTEM images of the used 1.5 wt% Pd@ZrO2@CN
and Raman spectroscopy (Figure S7). The results revealed
that there was no obvious difference in morphology, par-
ticle size, or distribution of Pd nanoclusters as compared
to the fresh one, consistently confirming that
Pd/ZrO2@CN was stable under the current reaction
conditions.
66(Zr). The derived Pd/ZrO2@CN catalyst exhibited good
reusability and activity in the catalytic hydrogenation of
TMBQ to TMHQ under the investigated reaction condi-
tions (110 °C, 1.0 MPa H2), attributable to the strong
interaction between N and Pd, as well as to the Lewis
acidity of ZrO2, which promotes hydrogenation. This
work demonstrates the feasibility of using various inlaid
metal nanoclusters within metal oxide@nitrogen‐doped
porous carbon composites as robust catalysts with excel-
lent stability for the various catalytic applications. How-
ever, the catalytic of Pd/ZrO2@CN was lower than that
of Pd@NH2‐UiO‐66(Zr),[23] probably owing to the fact
that there were carbon coatings on the outer surfaces of
4 | CONCLUSIONS
In summary, highly dispersed Pd nanoclusters embedded
within a matrix of ZrO2@CN were successfully prepared
via direct pyrolysis of the precursor Pd@NH2‐UiO‐

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[15] H. R. Kim, T. U. Yoon, S. I. Kim, J. An, Y. S. Bae, C. Y. Lee,
the Pd nanoclusters during the high‐temperature hydro-
lysis,[32] which on one hand could stabilize Pd
nanoclusters against sintering and leaching under harsh
reaction conditions, but on the other could partial block
the active sites. In further work, we will explore efficient
strategies to remove the as‐called carbon layer without
affecting the size or morphology of the metal
nanoclusters to boost the catalytic activity of the Pd
nanoclusters while retaining their stability.
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ACKNOWLEDGEMENTS
[21] D. Su, Z. Wei, S. Mao, J. Wang, Y. Li, H. Li, Z. Chen, Y. Wang,
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Financial support from the National Natural Science
Foundation of China (No. 21576243), and Zhejiang Pro-
vincial Natural Science Foundation of China (Nos.
LY18B060006 and LY18B030006) are greatly appreciated.
[22] X. Cai, J. Pan, G. Tu, Y. Fu, F. Zhang, W. Zhu, Catal. Commun.
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