Enhanced Heck reaction on flower-like Co(Mg or Ni)Al layered double hydroxide supported ultrafine PdCo alloy nanocluster catalysts: The promotional effect of Co

Article abstract of DOI:10.1039/c9dt03663f

A series of PdCo alloy nanocluster (NC) catalysts x-PdCo_r/Co(Mg or Ni)Al-LDH (x: Pd loading, r: Co/Pd molar ratio) were synthesized by immobilizing ultrafine PdCo_r-PVP NCs on flower-like layered double hydroxide (LDH) supports. The s

Full text of DOI:10.1039/c9dt03663f

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Enhanced Heck reaction on flower-like
Co(Mg or Ni)Al layered double hydroxide
supported ultrafine PdCo alloy nanocluster
catalysts: the promotional effect of Co†
Cite this: Dalton Trans., 2019, 48,
17741
Jin Li, Ying Song, Yajuan Wang and Hui Zhang
*
A series of PdCo alloy nanocluster (NC) catalysts x-PdCo
r
/Co(Mg or Ni)Al-LDH (x: Pd loading, r: Co/Pd
molar ratio) were synthesized by immobilizing ultrafine PdCo
hydroxide (LDH) supports. The sizes of PdCo alloy NCs of the catalysts can be elaborately tuned in
1.6–3.2 nm by both Co/Pd ratios and Pd loadings, and the PdCo NCs are mainly dispersed on the edge
sites of LDH nanosheets upon a flower-like morphology. The PdCo bimetallic catalysts 0.81-PdCo0.10
MgAl-LDH (2.6 ± 0.6 nm), 0.86-PdCo0.28/MgAl-LDH (2.3 ± 0.7 nm) and 0.79-PdCo0.54/MgAl-LDH
3.2 ± 0.9 nm) exhibit enhanced activity compared with the monometallic Pd catalyst for Heck coupling
r
-PVP NCs on flower-like layered double
∼
/
(
of iodobenzene with styrene. Particularly, 0.86-PdCo0.28/MgAl-LDH shows the highest activity, which can
0
be attributed to its smallest PdCo0.28 alloy NCs, and the maximum electron density of the Pd center
resulted from the electron transfer from Co and the strongest PdCo0.28 NCs – LDH synergistic effect.
0
.67-PdCo0.28/CoAl-LDH shows much better activity than those of 0.64-PdCo0.28/NiAl-LDH and
0
.86-PdCo0.28/MgAl-LDH. The lowest Pd loading sample 0.01-PdCo0.28/CoAl-LDH (1.6 ± 0.4 nm) shows
an ultrahigh turnover frequency of 163 000 h− (Pd: 1.9 × 10−5 mol%), which is the highest value obtained
so far. Meanwhile, the catalyst shows excellent adaptability for the substrates and can be reused for 12
runs without significant loss of activity. The present work may provide a new idea for the simple and
green synthesis of ultrafine Pd-based non-noble bimetallic catalysts for varied catalytic processes.
1
Received 12th September 2019,
Accepted 5th November 2019
DOI: 10.1039/c9dt03663f
rsc.li/dalton
Considering the consumption of expensive Pd, the introduc-
Introduction
16,17
18
tion of non-noble metal elements such as Ni,
Cu and
19–21
The palladium-catalyzed Heck reaction is one of the most Co
important paths for constructing C–C bonds with one step
can not only reduce costs, but also stabilize active Pd
1
,2
species, and the obtained Pd-based bimetallic catalysts can
exhibit greatly improved C–C coupling reactivity due to the adjus-
3
and has been widely used for synthesizing natural products,
4
5
pharmaceuticals, and fine chemicals. However, conventional table geometry and electronic structure involving the electron
homogeneous Pd-based catalysts such as Pd(PPh and transfer, coordination environment, lattice strain and distri-
3 4
)
6
7
16
PdCl (PPh ) , Pd-carbene complexes and cyclic Pd com- bution of surface elements. Bai et al. prepared carbon nano-
2
3 2
8
pounds often suffer from the difficulty of separation and fiber (CNF)-loaded Pd–Ni catalysts by electrospinning, then by
recovery, easily lost activity and environmental pollution. ethylene glycol reduction, followed by high-temperature carboniz-
Therefore, heterogeneous Pd-based catalysts by supporting Pd ation for Suzuki coupling of aryl halide with phenylboronic acid,
or Pd–M alloy nanoparticles (NPs) on varied supports such as of which Pd
1
Ni
4
/CNF (∼4 nm) gave the highest biphenyl yield of
9
,10
11,12
19
polymers,
carbon or silica-based materials,
metal 96.55%. Shaabani et al. reported PdCo ANP-PPI-g-graphene
1
3
14,15
oxides,
and hydrotalcites
have become
a
research (2–3 nm) via the co-complexation of PdCl and CoCl ·6H O pre-
2
2
2
hotspot in recent years.
cursors with PPI-grafted-graphene followed by NaBH
4
reduction
for Sonogashira reaction of iodobenzene with phenylacetylene,
producing a diphenylacetylene yield of 99%, which was much
higher than the monometallic ones. However, the preparation of
the above-mentioned supports, CNF and PPI-g-graphene hybrid,
suffers from tedious and time-consuming processes, and the size
of bimetallic NPs is relatively larger, which greatly limits further
improvement of the activity of these catalysts.
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, P.O. Box 98, Beijing 100029, China.
E-mail: huizhang67@gst21.com, zhanghui@mail.buct.edu.cn; Fax: +86 10 64425385;
Tel: +86 10 64425872
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
C9DT03663F
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Layered double hydroxides (LDHs), which are typical The synthesis of flower-like Co(Mg or Ni)Al-LDH supports
anionic clay materials, possess controllable morphology,
large surface area, highly ordered and tunable layer elements,
and intrinsic alkalinity along with easy preparation, and thus
Flower-like Co(Mg or Ni)Al-LDH supports were prepared by a
25
simple constant-pH coprecipitation method.
Typically,
1
0
00 mL of alkaline solution containing 0.800 g of NaOH and
.636 g of Na CO was added dropwise into 100 mL of de-
2
2
they are widely used as catalysts or supports. Choudary
2
3
1
4
et al. prepared a plate-like MgAl-LDH-supported Pd catalyst
ionized water under vigorous stirring until the pH reached
0 ± 0.1 and was kept for 5 min. Then another 100 mL
of aqueous solution containing 9 mmol Co(NO ·6H O (or
Mg(NO ·6H O or Ni(NO ·6H O) and 3 mmol Al(NO ·9H
0
2−
LDH-Pd (4–6 nm) through an ion exchange of PdCl
4
with
1
the interlayer Cl− ions of LDH followed by hydrazine hydrate
reduction for Heck olefination of 4-chloroanisole with
styrene, and the catalyst exhibited superior activity to those of
other supported Pd catalysts. Wu et al. reported simple
plate-like MgAl-LDH-loaded Au–Pd alloy NPs by using the
impregnation–reduction step for the selective photocatalytic
oxidation of benzyl alcohol, and Au –Pd /LDH (∼4.2 nm)
3
)
2
2
3
)
2
2
3
)
2
2
3
)
3
2
O
was added dropwise into the above solution by keeping a pH
of 10 ± 0.1 by simultaneously adding alkaline solution under
vigorous stirring. The resulting slurry was aged at 65 °C for
2
3
4
h, cooled to room temperature (25 °C) and washed with de-
1
9
ionized water until the pH was ∼7, and finally the obtained
precipitate was dried at 60 °C for 24 h, giving flower-like Co
showed the highest conversion rate of 91.1%, which was
much higher than those of its monometallic counterparts.
(
Mg or Ni)Al-LDH supports.
2
4
Li et al.
prepared flower-like Au/NiAl-LDH catalysts
(
2.24–2.99 nm) via sol immobilization of Au-PVA NPs on NiAl-
The synthesis of PdCo
The PdCo -PVP colloidal NCs were prepared by a modified
polyol reduction method.
5.64 mmol) and 0.843 g of KCl (11.3 mmol) were totally dis-
solved in 100 mL of deionized water, resulting in a K PdCl
solution (0.056 mol L− ). 1.190 g of CoCl ·6H O (5 mmol) was
r
-PVP alloy nanoclusters (NCs)
LDH for the selective oxidation of benzyl alcohol, and the cat-
alysts showed higher activity than that of the common plate-
like Au/NiAl-LDH, which could be attributed to the confine-
ment effect of hierarchical pores improving the effective col-
lisions between substrates and active sites. However, to the
best of our knowledge, there are no reports about hierarchical
flower-like LDH supported ultrafine Pd-based non-noble bi-
metallic catalysts so far, and therefore it is highly desirable
that these catalysts are fabricated and used for the efficient
Heck reaction.
r
2
6
Firstly, 1.000
g
of PdCl2
(
2
4
1
2
2
totally dissolved in 100 mL of deionized water, resulting in a
1
CoCl
Co + Pd)/PVP = 10/1 (mol/mol), 2.5 mL of K PdCl solution
2
solution (0.050 mol L− ). A certain amount of PVP upon
(
2
4
and a certain amount of CoCl2 solution (Co/Pd molar ratio
designed as 0.1, 0.3 or 0.6) were added into 150 mL of de-
ionized water, followed by adding 50 mL of EG. Then, a NaOH
solution (1 mol L− ) was added to adjust the pH to ∼12; the
mixture was refluxed at 140 °C in an oil bath for 2 h and then
cooled to room temperature, obtaining the dark-brown col-
Herein, a series of flower-like PdCo alloy nanocluster (NC)
r
catalysts x-PdCo /Co(Mg or Ni)Al-LDH (x: Pd loading, r:
1
Co/Pd molar ratio) were fabricated via an ethylene glycol
reduction –sol immobilization method for Heck reaction.
The Co/Pd ratios, Pd loadings and elemental compositions
of LDH in the obtained catalysts have a significant effect on
Heck reactivity, and 0.01-PdCo0.28/CoAl-LDH exhibits an
ultrahigh turnover frequency of 163 000 h . Moreover, the
catalyst shows excellent adaptability for the substrates and
recyclability. Combined with systematic characterization
results and kinetic analysis, a possible mechanism of
the Heck reaction over the present catalysts was tentatively
proposed.
loidal solution named PdCo -PVP NCs. For comparison, mono-
r
metallic Pd-PVP or Co-PVP NCs were prepared by following
similar steps using K PdCl solution or CoCl solution as the
−
1
2
4
2
precursor.
The synthesis of flower-like PdCo alloy NC catalysts x-PdCor/
Co(Mg or Ni)Al-LDH
Flower-like catalysts x-PdCo /Co(Mg or Ni)Al-LDH were pre-
r
pared by a sol immobilization method. Typically, 1.500 g of
flower-like MgAl-LDH support was added into 200 mL of the
above-mentioned PdCo -PVP colloidal solution upon a theore-
tical Pd loading of 1.0 wt% under vigorous stirring for 2 h at
room temperature. The resultants were centrifuged and
r
Experimental section
Materials and chemicals
Poly(N-vinyl-2-pyrrolidone) (PVP, k-30) was purchased from washed three times with acetone and deionized water alter-
Xilong Chemical Corporation. Cobalt chloride hexahydrate nately, and finally dried at 60 °C overnight to obtain a
(CoCl
2 2 r
·6H O), ethylene glycol (EG), N,N-dimethylformamide powdery product designated as x-PdCo /MgAl-LDH (r: Co/Pd
(
DMF), K CO , and KCl were bought from Beijing Chemical ratio of 0.10, 0.28 and 0.54; x: Pd loading in wt%; ICP). For
2
3
2
Works. Palladium chloride (PdCl ), styrene and bromobenzene comparison, flower-like monometallic Pd/MgAl-LDH (Pd:
were purchased from Tianjin Fuchen Chemicals Reagent 0.80 wt%, ICP) and Co/MgAl-LDH (Co: 0.85 wt%, ICP) were
Factory. Iodobenzene and other aryl halides were purchased prepared by a similar method. By replacing MgAl-LDH
from Aladdin. All chemicals were of analytical grade and used support with CoAl-LDH and NiAl-LDH, flower-like catalysts
without further purification. The deionized water with a resis- 0.67-PdCo0.28/CoAl-LDH and 0.64-PdCo0.28/NiAl-LDH were
tivity >18.25 MΩ cm (25 °C) was used in all experiments.
obtained, respectively. Reduced Pd loading samples
17742 | Dalton Trans., 2019, 48, 17741–17751
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r
Scheme 1 The design schematic of flower-like catalysts x-PdCo /Co(Mg or Ni)Al-LDH.
0
.10-PdCo0.28/CoAl-LDH and 0.01-PdCo0.28/CoAl-LDH were Activity test
r
obtained by reducing the usage of the PdCo -PVP NC colloidal
solution to 100 mL and 20 mL, respectively. Scheme 1 depicts
the design schematic of the flower-like x-PdCo /Co(Mg or Ni)
r
Al-LDH catalysts.
The Heck reaction was carried out using a 25 mL three-necked
round-bottom flask attached to a reflux condenser under mag-
netic stirring. Aryl halide (1.0 mmol), alkene (1.5 mmol),
K CO (3 mmol), catalyst (an appropriate amount of Pd with
2
3
2
respect to aryl halide), DMF (12 mL) and H O (4 mL) were
Characterization
mixed in the flask, and then the reaction was allowed to
proceed at 120 °C under magnetic stirring at atmospheric
pressure. The products were analyzed by using the Agilent
The X-ray diffraction (XRD) analysis of the samples was
carried out using a Shimadzu XRD-6000 X-ray powder diffract-
ometer equipped with Cu Kα radiation (1.5418 Å, 40 kV,
7
890A gas chromatograph equipped with an Agilent J&K HP-5
capillary column (5% phenyl polysiloxane, 30 m × 0.25 mm ×
.25 μm) and a flame ionization detector for quantitative ana-
3
0 mA). The PdCo
r
-PVP NC colloidal solution was dried at
8
0 °C in a vacuum for 24 h and ground into the powder for
0
the XRD test. The FT-IR spectra were recorded with a Bruker
Vector-22 FT-IR spectrophotometer using the KBr pellet tech-
nique (sample/KBr = 1/100). The surface morphology of the
samples was analyzed by using a Zeiss supra 55 Scanning
electron microscope (SEM), and the surface elements and
contents were detected by the DXGENSIS60 energy scattering
spectrometer. High resolution transmission electron
lysis using the peak area normalized method. After reaction,
the catalyst was separated by centrifugation and thoroughly
washed 4 times with DMF and water for the next run. The fil-
trate was extracted by ethyl acetate to obtain an organic phase
which was dried by adding anhydrous Na SO , followed by
2
4
rotary evaporation (45 °C) to remove the solvent, and then
1
13
recrystallized to obtain the products for H and C NMR ana-
microscopy (HRTEM) was performed by using
a JEM
lysis (see the ESI†).
2
010 high resolution transmission electron microscope.
High-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM) with energy-dispersive X-ray
spectroscopy was performed using a JEOL JEM-2100F trans-
Results and discussion
mission electron microscope. The contents of Pd, Co, Ni, Mg The ultrafine bimetallic PdCo -PVP NCs were first synthesized
r
and Al in the samples were analyzed using the Shimadzu by ethylene glycol reduction at 140 °C, and their XRD patterns
ICPS-7500 inductively coupled plasma atomic emission are shown in Fig. 1A. Clearly, with increasing Co/Pd ratios,
spectrometer (ICP-AES). The surface electronic structure and three PdCo -PVP NCs (r = 0.10, 0.28 and 0.54 by ICP) show
r
composition of the samples were analyzed using
a
gradually upshifted diffraction peaks at 2θ ca. 40.60°, 41.13°
VG-ESCALAB 250 X-ray photoelectron spectrometer (XPS) at a and 41.59° indexed to a reduced d(111) of 0.222, 0.219 and
−
9
base pressure in the analysis chamber of 2 × 10 Pa using a 0.217 nm, respectively, all between 0.225 nm of Pd-PVP (the
standard Al Kα source (1486.6 eV). The binding energy was same as fcc Pd of JCPDS 46-1043) and 0.205 nm of Co-PVP NCs
referenced to the C 1s line of surface contamination carbon C (the same as fcc Co of JCPDS 15-0806), implying the formation
1
s (284.9 eV). CO adsorption in situ FT-IR spectra were of the PdCo alloy phase, which is consistent with Vegard’s
r
2
7
recorded on a Nicolet 380 instrument containing a controlled law. These gradually reduced d(111) values can be ascribed to
environment chamber equipped with CaF windows. 30 mg the lattice contraction of Pd due to the incorporation of Co
of samples were compressed into tablets and fixed in a into the Pd fcc structure.
ceramic sample chamber. Prior to testing, the samples were and C reveal the average size of PdCo0.28-PVP NCs as 1.5 ±
2
2
8–30
The HRTEM images in Fig. 1B
−
1
kept for 1 h in N
2
flow (50 mL min ) and scanned to obtain 0.5 nm, and the (111) plane lattice fringes with spacings of
a background spectrum. Then it was exposed into CO flow 0.220 nm, further confirming the formation of the alloy state
−
1
(
50 mL min ) for 1 h, and finally purged with N
2
(50 mL of ultrafine PdCo0.28 NCs.
−
1
min ) for 0.5 h. The IR spectrum was recorded in absor- Then a series of PdCo alloy NC catalysts x-PdCo
bance mode with a resolution of 4 cm
r
/Co(Mg or
−
1
.
Ni)Al-LDH (x = 0.01–0.86 wt% by ICP) were fabricated by sup-
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r
Fig. 1 XRD patterns (A) of PdCo -PVP (r = 0.10 (a), 0.28 (b) and 0.54 (c)) NCs compared with Pd-PVP and Co-PVP NCs, and HRTEM images (B and
C) of PdCo0.28-PVP NCs (insets: histogram of particle size distribution and fast Fourier transform image (FFT)).
porting PdCo -PVP NCs on flower-like Co(Mg or Ni)Al-LDH PdCo alloy NCs are observed, implying the ultrafine size and
r
supports via the sol immobilization method (Scheme 1), and high dispersion of PdCo alloy NCs in the as-obtained catalysts.
their XRD patterns are shown in Fig. 2. All the catalysts show
The FT-IR spectra (Fig. S1†) of a series of flower-like PdCo
quite similar diffractions to their supports except for a slightly alloy NC catalysts x-PdCo /Co(Mg or Ni)Al-LDH compared with
r
weakened peak intensity at 2θ ca. 11.5°, 23.2°, 34.9°, 39.6°, monometallic catalysts and their corresponding supports as
−
1
4
6.8°, 60.6° and 61.8° indexed to the (003), (006), (012), (015), well as PVP show broad and strong absorptions at ∼3500 cm
(
018), (110) and (113) planes of typical hexagonal LDH due to the stretching vibration of –OH groups of the LDH lami-
2
5,31
31
phase,
d
respectively. The sharp and strong (003) peaks with nate or physically adsorbed water molecules. The PVP shows
003 in 0.754–0.766 nm (Table S1†) indicate the presence of the wide peaks in 2950–2850 cm− due to the ν (symmetric stretch-
1
2
−
31
−1
CO3 -LDH phase. It can be seen from Table S1† that ing) mode of CH or CH , a strong peak at 1680 cm due to
x-PdCo /MgAl-LDH possesses a larger D110 (∼58 nm) and D003 the ν(–CvO) mode, and sharp peaks at 1290 cm due to the
(
1
3
2
−
1
r
3
2
∼14.5 nm) than those of 0.64-PdCo0.28/NiAl-LDH (25.4 nm, ν(–CN) mode. For all the catalysts and supports, sharp peaks
1.5 nm) and x-PdCo0.28/CoAl-LDH (∼49.8 nm, ∼14.7 nm), at 1370 cm− and weak ones at 860 cm can be assigned to
1
−1
2
−
probably due to the combinational effect of the electro- the ν
3 2 3
and ν (out-of-plane deformation) modes of CO
−
1
negativity and cation radii of different LDH layer metals. It is anions, respectively, and sharp peaks at 665 and 420 cm can
noteworthy that no characteristic diffraction peaks related to be attributed to the lattice vibration of the Co(Ni, Mg and Al)–
O bond and the Co(Ni, Mg and Al)–OH bond in the LDH lami-
3
2
nates, respectively, further confirming the existence of the
2
−
CO3 -LDH phase. It is to be noted that, the blueshift of the
−
1
bending vibration of the LDH interlayer water to ∼1655 cm
−1
compared to supports (1640 cm ) and obvious broaden band
in 2950–2850 cm− from ν(–CH /CH ) of PVP, imply the poss-
1
3
2
ible interaction between PdCo
r
NCs and LDH supports via
hydrogen bonds between tertiary amino groups or carbonyl
groups of PVP in PVP-protected PdCo alloy NCs and hydroxyl
r
groups on the LDH laminates causing the change of the
chemical environment of water molecules in LDH interlayer.
The SEM images of 0.86-PdCo0.28/MgAl-LDH, 0.64-PdCo0.28
/
NiAl-LDH and 0.67-PdCo0.28/CoAl-LDH (Fig. S2a–c†) clearly
show flower-like structures, quite similar to the corresponding
supports (Fig. S2a –c †), assembled from staggered LDH
0
0
nanosheets with sizes of ∼196, 186, and 210 nm and thick-
nesses of ∼17.0, 11.1 and 14.0 nm, respectively, implying
approximately a side by side arrangement of ∼4, 8, 4 primary
LDH nanoplates upon Scherrer dimensions (Table S1†). The
corresponding EDS spectra (Fig. S2a′–c′†) clearly show the
existence of Pd and Co elements besides Mg (or Ni or Co), Al,
C and O signals.
The HRTEM images of 0.81-PdCo0.10/MgAl-LDH, 0.86-
PdCo0.28/MgAl-LDH and 0.79-PdCo0.54/MgAl-LDH (Fig. 3a –a )
Fig. 2 XRD patterns of the 0.81-PdCo0.10/MgAl-LDH (a
PdCo0.28/MgAl-LDH (a ), 0.79-PdCo0.54/MgAl-LDH (a ), 0.64-PdCo0.28
NiAl-LDH (b), x-PdCo0.28/CoAl-LDH (x = 0.67 (c ), 0.10 (c ), 0.01 (c ))
compared with Pd/MgAl-LDH, Co/MgAl-LDH and related supports
–c ).
1
), 0.86-
1
3
2
3
/
show the sizes of PdCo NCs of 2.6 ± 0.6, 2.3 ± 0.7, and 3.2 ±
.9 nm, respectively, between Pd/MgAl-LDH (2.0 ± 0.8 nm,
1
2
3
0
(
a
0
0
Fig. S3a†) and Co/MgAl-LDH (4.8 ± 1.2 nm, Fig. S3b†), indicat-
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Fig. 4 HADDF-STEM images and the corresponding EDX line scanning
profiles of 0.81-PdCo0.10/MgAl-LDH (a
and 0.79-PdCo0.54/MgAl-LDH (a ).
1
), 0.86-PdCo0.28/MgAl-LDH (a
2
)
3
Fig. 5 shows HRTEM images of the catalysts x-PdCo0.28
/
CoAl-LDH (x = 0.67, 0.10 or 0.01 wt%) and 0.64-PdCo0.28/NiAl-
LDH. Clearly, the average size of PdCo0.28 NCs in 0.67-
PdCo0.28/CoAl-LDH is 2.3 ± 0.6 nm (Fig. 5a), which is almost
equal to that of 0.86-PdCo0.28/MgAl-LDH, but slightly smaller
than that of 0.64-PdCo0.28/NiAl-LDH (2.5 ± 0.6 nm, Fig. 5d).
Meanwhile, the PdCo0.28 NCs in 0.67-PdCo0.28/CoAl-LDH are
prominently dispersed on the edge sites of CoAl-LDH
nanosheets as flower-like structures, and this edge effect is see-
mingly more obvious in the x-PdCo0.28/CoAl-LDH catalysts
1 1
Fig. 3 HRTEM images of 0.81-PdCo0.10/MgAl-LDH (a , a
’), 0.86- than in the 0.86-PdCo0.28/MgAl-LDH and 0.64-PdCo0.28/NiAl-
PdCo0.28/MgAl-LDH (a
2
,
a
2
’) and 0.79-PdCo0.54/MgAl-LDH (a
3
,
a
3
’) LDH catalysts, suggesting the existence of a possibly stronger
interaction between PdCo0.28 NCs and CoAl-LDH nanoplates.
Upon reducing Pd loadings, both 0.10-PdCo0.28/CoAl-LDH
(
insets: particle size distribution and FFT).
(
Fig. 5b) and 0.01-PdCo0.28/CoAl-LDH (Fig. 5c), show greatly
ing that the sizes of PdCo alloy NCs can be effectively tuned by decreased NC sizes of 1.8 ± 0.5 and 1.6 ± 0.4 nm, respectively.
Co/Pd ratios in obtained catalysts. The high-magnified The corresponding high-magnified HRTEM images (Fig. 5a′–
HRTEM images (Fig. 3a ′–a ′) show the lattice fringe spacings d′) show the lattice fringes with spacings of 0.220 nm corres-
1 3
of the PdCo NCs (111) plane of 0.223, 0.220 and 0.217 nm, ponding to the PdCo0.28 alloy NC (111) planes, and the lattice
respectively, between Pd/MgAl-LDH (0.225 nm, Fig. S3a′†) and fringes with spacings of 0.197 nm corresponding to the hexag-
Co/MgAl-LDH (0.205 nm, Fig. S3b′†), further indicating the for- onal LDH (018) plane. It is noteworthy that the lowest Pd
mation of the PdCo alloy phase. Meanwhile, lattice fringes loading sample 0.01-PdCo0.28/CoAl-LDH shows the smallest
with spacings of 0.197 nm are also observed in three samples, size of PdCo0.28 NCs, which is approximately equal to that of
indexed to the hexagonal LDH (018) plane. It is noteworthy pure PdCo0.28 colloidal NCs (∼1.5 nm). The above results show
that the PdCo alloy NCs are predominantly anchored on the that the sizes of PdCo alloy NCs can be significantly tuned by
edge sites of LDH nanosheets, suggesting the possible exist- both Co/Pd ratios and Pd loadings, and all the PdCo NCs are
ence of strong interaction between PdCo alloy NCs and LDH mainly dispersed on the edge of isolated LDH nanoplates in
3
3,34
nanosheets in the catalysts.
the form of flower-like morphologies, implying the strong
The HADDF-STEM-EDX line scanning analysis (Fig. 4) of interaction between PdCo NCs and LDH support; especially
0
.81-PdCo0.10/MgAl-LDH, 0.86-PdCo0.28/MgAl-LDH and 0.79- the x-PdCo0.28/CoAl-LDH system may show an even stronger
PdCo0.54/MgAl-LDH (selecting larger NCs to meet the detection interaction between PdCo0.28 NCs and CoAl-LDH, which plays
limit of the device) reveals the presence of both Pd and Co a crucial role in catalytic reaction. Additionally, the flower-like
signals at the same position, confirming the formation of the structures of the catalysts promote the rapid transportation
PdCo alloy phase and gradually enhanced Co signals with and diffusion of reactant molecules and expose more accessi-
increasing Co/Pd ratio. The HADDF-STEM image (Fig. S4a†) of ble active sites, which is beneficial for catalytic activity.
0
.86-PdCo0.28/MgAl-LDH shows the average size of PdCo0.28
The catalytic performances of the Heck reaction between
alloy NCs of 2.2 ± 0.6 nm, and the mapping results show the iodobenzene (PhI) and styrene over the flower-like PdCo alloy
almost overlapped Pd and Co signals besides the evenly dis- NCs catalysts x-PdCo /Co(Mg or Ni)Al-LDH (r = 0.10, 0.28 and
r
persed Mg and Al elements, further demonstrating the alloy 0.54; x = 0.01–0.86 wt%) and their related samples are listed in
state of PdCo NCs. The EDX (Fig. S4b†) spectrum shows a Co/ Table 1. The Pd/MgAl-LDH shows a PhI conversion of 38.0%
Pd ratio of 0.29, in line with ICP result.
(Table 1, entry 1), while the Co/MgAl-LDH (Table 1, entry 2)
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entries 8 and 9) display a remarkably increased TOF of
1
−1
2
405 h− and 3317 h , respectively. Specifically, the 0.01-
−
1
PdCo0.28/CoAl-LDH exhibits an extra high TOF of 163 000 h
at the Pd usage of 1.9 × 10 mol% (Table 1, entry 10), which
can be ascribed to the shift of the homeopathic Pd catalysts to
−
5
3
5,36
the catalytic cycle at a lower Pd dosage.
This activity is far
higher than that of the currently reported bimetallic hierarchi-
1
7
−1
cal PdNi-Ni(OH)2 (147 959 h , Table 1, entry 11), (RS) Au
1
(
SL)
0 2
(SL-PdCl )1.2 with the Pd-complex immobilized on the
3
7
−1
surface of Au NPs (48 700 h , Table 1, entry 12) and mono-
metallic Si nanowire array-stabilized Pd NPs (40 000 h
Table 1, entry 13), and even higher than that of the hierarchi-
cal nanosheet array-like catalyst Pd0.0098/CoAl-LDH/rGO at
2
−
1
,
3
8
0 °C higher temperature (160 000 h− , Table 1, entry 14).
1
39
The above results strongly suggest that the present flower-like
PdCo alloy NC catalysts possess excellent activity for the Heck
reaction, especially the 0.01-PdCo0.28/CoAl-LDH with the smal-
lest PdCo NCs of ∼1.6 nm shows the highest activity, which is
probably related to its smallest PdCo0.28 alloy NCs and the
strong interaction between PdCo0.28 alloy NCs and CoAl-LDH.
Moreover, the substrates’ adaptability for the Heck reaction
of various aryl halides with olefins over 0.01-PdCo0.28/CoAl-
LDH is shown (Table 2). Obviously, the catalyst shows excellent
catalytic activity for the Heck reaction of styrene and aryl
iodides with electron-rich or electron-poor substituents
(
(
Table 2, entries 1–5) in an order of TOF values as IPh-NO
2
10 000 h− ) > IPh-COCH
1
(9100 h ) > IPh-H (3317 h ) > IPh-
(3083 h−1), which is consistent
−1
−1
3
(3170 h− ) > IPh-OCH
1
CH
3
3
1
4
with the order of the nucleophilicity of the aromatic ring.
Additionally, the Heck reaction of PhI with ethyl acrylate or
butyl acrylate (Table 2, entries 6 and 7) can be catalyzed
efficiently with a TOF of 3260 or 3301 h− , respectively.
Meanwhile, the high activities for the Heck reaction of bromo-
benzene with styrene or ethyl acrylate are obtained with a TOF
of 927 or 1167 h− , respectively (Table 2, entries 8 and 9),
1
Fig. 5 HRTEM images of 0.67-PdCo0.28/CoAl-LDH (a and a’), 0.10-
PdCo0.28/CoAl-LDH (b and b’), 0.01-PdCo0.28/CoAl-LDH (c and c’) and
1
which is much higher than that of the currently reported
0
FFT).
.64-PdCo0.28/NiAl-LDH (d and d’) (insets: particle size distribution and
Pd@Co/CNT-50 (CNT = carbon nanotube, 163 h−
1 40
)
and Pd
1 41
(
2.5)/NT (NT = hydrogen titanate nanotubes, 43.2 h− ). It also
shows some activity for the Heck reaction of the inactive chloro-
shows no activity after 12 h, indicating that metal Pd is an benzene with styrene with a TOF of 323 h− (Table 2, entry 10).
1
active species for the Heck reaction, which is consistent with These results suggest that the present flower-like PdCo alloy
2
the previous report. After alloying with Co, the catalysts 0.81- NC catalysts possess excellent adaptability for substrates.
PdCo0.10/MgAl-LDH, 0.86-PdCo0.28/MgAl-LDH and 0.79-
Furthermore, the recyclability of 0.67-PdCo0.28/CoAl-LDH
PdCo0.54/MgAl-LDH (Table 1, entries 3–5) exhibit a much was carried out. After the reaction, the catalyst was recovered
higher activity with 1.67, 2.33 and 1.60 times the turnover fre- by centrifugation, washed with ethyl acetate and H O alter-
2
quency (TOF) of Pd/MgAl-LDH, respectively, of which the 0.86- nately, and dried at 60 °C for the next run under the same con-
PdCo0.28/MgAl-LDH with a middle Co/Pd ratio shows the ditions. The recovered sample shows a PhI conversion of
highest activity, indicating that the incorporation of Co can 94.7% and a trans-stilbene selectivity of 98.0% (Fig. S5A†) after
significantly improve the Heck reactivity. Upon altering LDH 12 runs. The SEM (Fig. S5B†) and HRTEM images (Fig. S5C
compositions, the 0.67-PdCo0.28/CoAl-LDH (Table 1, entry 7) and D†) of the recovered catalyst clearly show the well-kept
−
1
exhibits the TOF of 661 h , which is higher than the 0.86- nanosheets’ flower-like structure. The average size of the
−
1
PdCo0.28/MgAl-LDH (589 h ) and 0.64-PdCo0.28/NiAl-LDH PdCo0.28 alloy NCs of 2.4 ± 0.9 nm is almost the same as the
537 h− , Table 1, entry 6), probably due to the combinational fresh one (2.3 ± 0.6 nm) with the PdCo0.28 NCs predominantly
1
(
effect of ultrafine PdCo0.28 NCs and strong interaction between dispersed on the edge sites of the CoAl-LDH nanosheets. The
PdCo0.28 NCs and CoAl-LDH. Upon reducing Pd loadings, both lattice fringes with spacings of 0.220 and 0.197 nm indexed to
0
.10-PdCo0.28/CoAl-LDH and 0.01-PdCo0.28/CoAl-LDH (Table 1, the PdCo0.28 alloy NCs (111) plane and hexagonal LDH (018)
17746 | Dalton Trans., 2019, 48, 17741–17751
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a
Table 1 Catalytic activities of various catalysts for Heck reaction
b
−1
Entry
Catalysts
Pd/mol%
t/h
Conv./%
Yield/%
TOF /h
Ref.
1
2
3
4
5
6
7
Pd/MgAl-LDH
Co/MgAl-LDH
0.81-PdCo0.10/MgAl-LDH
0.86-PdCo0.28/MgAl-LDH
0.79-PdCo0.54/MgAl-LDH
0.64-PdCo0.28/NiAl-LDH
0.67-PdCo0.28/CoAl-LDH
0.30
0.30
0.30
0.30
0.30
0.30
0.30
0.5/1.3
0.5/12
0.5/1.2
0.5/1
0.5/1.2
0.5
0.5
2
2
1.5
3
24
2
48
38.0/96.0
n.d.
63.4/99.7
88.3/99.6
60.6/99.1
80.6
99.1
68.8
96.2
99.5
37.2/94.0
n.d.
61.8/98.4
86.6/97.2
59.4/96.6
76.3
96.2
68.0
93.2
96.9
9.0
87
97.3
95
253/246
n.d.
423/277
589/332
404/275
537
661
1720
2405
3317
163 000
147 959
48 700
40 000
160 000
This
This
This
This
This
This
This
This
This
This
This
17
0.02
8
9
0.10-PdCo0.28/CoAl-LDH
0.01-PdCo0.28/CoAl-LDHc
0.01-PdCo0.28/CoAl-LDH
Hierarchical PdNi-Ni(OH)
0.02
0.02
1.9 × 10
2.45 × 10
1.0 × 10
4.9 × 10
1.0 × 10
−5
1
1
1
1
1
0
1
2
3
4
9.3
—
—
—
d
−5
2
e
−5
(RS)
Si nanowire array-stabilized Pd NPs
Hierarchical nanosheet array-like Pd0.0098/CoAl-LDH/rGO
1 0
Au(SL) (SL-PdCl
)
2 1.2
37
38
39
f
−5
−4
g
1
16
—
a
b
Reaction conditions: PhI (1 mmol), styrene (1.5 mmol), vDMF : vH O = 12 mL : 4 mL, K
Turnover frequency (TOF): the converted moles of PhI per mole of Pd per hour. Reaction conditions: PhI (50 mmol), styrene (75 mmol),
2
CO
3
(3 mmol), 120 °C under atmospheric conditions.
2
c
d
v
DMF : vH O = 120 mL : 40 mL, K
2
CO
3
N (150 mmol), TBAA (10 mmol), 160 °C. Reaction conditions: PhI (10 mmol), butyl acrylate (10 mmol), Et N (1.5 mL), DMSO
g
3
(100 mmol), 120 °C under atmospheric conditions. Reaction conditions: PhI (50 mmol), butyl acrylate
2
e
(100 mmol), Et
3
f
(1 mL), 115 °C. Reaction conditions: PhI (50 mmol), butyl acrylate (100 mmol), Et
3
N (150 mmol), TBAA (10 mmol), 160 °C. Reaction con-
ditions: PhI (50 mmol), styrene (60 mmol), vDMF : vH O = 120 mL : 40 mL, K
2
CO (100 mmol), 140 °C under atmospheric conditions.
3
2
Table 2 Catalytic activities for the Heck reaction of different aryl temperatures on Heck reactivity. It can be seen clearly from
a
halides with various alkenes over 0.01-PdCo0.28/CoAl-LDH
Fig. S6† that the conversion (C) of PhI increases linearly up to
∼70% with a prolonged time (t) over these three catalysts,
implying the weak inhibition of the product. As shown in
Fig. 6a –a , the plots of −ln(1 − C) against t for three catalysts
1
3
show a linear correlation, indicating a first-order reaction
characteristic with respect to PhI. The Heck reaction rate con-
stant (k) values for three catalysts (Table S2†), obtained by cal-
culating the slope of the −ln(1 − C) versus t plots, show a
decrease in an order of 0.86-PdCo0.28/MgAl-LDH > 0.81-
PdCo0.10/MgAl-LDH > 0.79-PdCo0.54/MgAl-LDH. Meanwhile,
the plots of ln k versus the inverse of absolute temperature
Substrate (1)
Alkene (2)
b
c
−1
Entry
X
R
1
R
2
t/h
0.5
Conv. /%
TOF /h
1
2
3
4
5
6
7
8
9
I
I
I
NO
COCH
H
2
Ph
Ph
Ph
Ph
Ph
CO
CO
Ph
CO
Ph
100 (3a)
91.0 (3b)
98.1 (3c)
95.1 (3d)
92.5 (3e)
97.8 (3f)
99.1 (3g)
92.7 (3c)
93.9 (3f)
64.6 (3c)
10 000
9100
3317
3170
3083
3260
3301
927
3
0.5
1.5
1.5
1.5
1.5
1.5
5.0
4.0
10.0
I
I
I
CH
3
OCH
H
3
2
CH
3
I
Br
Br
H
H
H
H
2 4 9
C H
2
CH
3
1167
323
d
1
0
Cl
a
Reaction conditions: 0.01-PdCo0.28/CoAl-LDH (Pd: 0.02 mol% based
on the amount of aryl halide), aryl halide (1.0 mmol), alkene
1.2 mmol), vDMF : vH O = 12 mL : 4 mL, K CO (3.0 mmol), 120 °C
under atmospheric conditions. Determined By GC, identified by
(
2
3
2
b
1
H
1
c
and C NMR. Turnover frequency (TOF): the converted moles of PhI
d
(
PhBr or PhCl) per mole of Pd per hour. Reaction conditions: 0.01-
PdCo0.28/CoAl-LDH (Pd: 0.02 mol%), aryl halide (1.0 mmol), alkene
1.2 mmol), vDMF : vH O = 12 mL : 4 mL, K CO (3.0 mmol), 140 °C.
(
2
3
2
plane (Fig. S5E†), respectively, are still observed clearly. The
ICP analysis of the supernatant (detection limit: 0.01 ppm)
after 12 runs shows no leaching of Pd and Co species. These
results clearly suggest that the present PdCo alloy NC catalysts
possess excellent structural stability and recyclable efficiency.
In order to get an insight into the nature of the Heck reac-
tion over the flower-like PdCo alloy NC catalysts, the macro-
Fig. 6 −ln(1 − C) against time (a
Heck reaction over 0.81-PdCo0.10/MgAl-LDH (a
LDH (a ) and 0.79-PdCo0.54/MgAl-LDH (a ) at different temperatures.
Reaction conditions: catalyst (Pd: 0.3 mol% upon PhI), PhI (1 mmol),
conducted, including the effect of Co/Pd ratios and reaction styrene (1.5 mmol), vDMF : vH O = 12 mL : 4 mL, K
CO (3 mmol).
1
–a
3
) and Arrhenius plots (b) for the
1
), 0.86-PdCo0.28/MgAl-
2
3
r
scopic kinetics of the catalysts x-PdCo /MgAl-LDH has been
2
2
3
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0
1 3
show a good linear relationship (Fig. 6b), and the calculated LDH (Fig. 7A(a –a )) show the Pd 3d5/2 BE values at 335.25,
apparent activation energy (E ) from the slope with the least- 334.69 and 334.92 eV, respectively, shifting to a low BE with
a
square fit is listed in Table S2.† Clearly, the 0.86-PdCo0.28
/
,
the corresponding offsets of 0.45, 1.01, and 0.78 eV compared
value of 48.9 kJ mol−
1
with Pd/MgAl-LDH, indicating the increased electron density
MgAl-LDH possesses the smallest E
a
0
15,39,42
which is significantly lower than those of 0.81-PdCo0.10/MgAl- of the Pd center.
The Co 2p3/2 BE values of three cata-
−
1
LDH (58.7 kJ mol ) and 0.79-PdCo0.54/MgAl-LDH (66.8 kJ lysts (Fig. 7B(a –a )) at 778.25, 777.80 and 778.15 eV, respect-
1 3
−
1
mol ). The above results indicate that 0.86-PdCo0.28/MgAl- ively, with decreases of 0.55, 1.00 and 0.65 eV compared to Co/
0
LDH possesses the highest k and the lowest E value among MgAl-LDH indicate the decreased electron density of Co
a
1
9,42
the x-PdCo
r
/MgAl-LDH catalysts, in line with its highest Heck species.
The negative shift of both Pd 3d5/2 and Co 2p3/2
reactivity.
clearly demonstrates the electron transfer from Co to Pd atom,
To explore the relationship between structure and perform- especially the 0.86-PdCo0.28/MgAl-LDH shows the most elec-
ance on the flower-like PdCo alloy NCs catalysts, XPS and tron-rich Pd species, suggesting its strongest interaction
0
in situ CO FT-IR analyses were performed for a series of the between Pd and Co. Meanwhile, the percentages of Pd species
PdCo alloy NC catalysts.
of 90.9, 100 and 86.0% (Table S3†) for 0.81-PdCo0.10/MgAl-LDH,
The XPS results (Fig. 7) reveal that Pd/MgAl-LDH shows two 0.86-PdCo0.28/MgAl-LDH and 0.79-PdCo0.54/MgAl-LDH, respect-
distinct broad peaks at binding energies (BEs) of ∼336.30 and ively, are higher than that for Pd/MgAl-LDH (78.0%), probably
0
∼
340.5 eV, which can be attributed to Pd 3d and Pd 3d3/2 due to the proper Co-doping inhibiting the oxidation of Pd
5/2
4
2
core levels, respectively. The deconvolution BE values of species especially for the sample 0.86-PdCo0.28/MgAl-LDH.
Pd 3d5/2 at 335.70 and 337.30 eV can be assigned to Pd and
0
The O 1s spectra of 0.81-PdCo0.10/MgAl-LDH, 0.86-PdCo0.28
/
II
0
Pd species, respectively, and the percentage of Pd is 78.0%. MgAl-LDH and 0.79-PdCo0.54/MgAl-LDH (Fig. S7A(a –a )†)
1
3
The Co/MgAl-LDH displays two broad peaks at ∼778.80 and show that an apparent asymmetric envelope peak in 527–537
785.70 eV, which can be attributed to Co 2p3/2 and Co 2p1/2 eV can be fitted into three peaks, which can be attributed to
∼
4
2
2−
core levels, respectively, indicating the existence of metallic the surface lattice O (530–531 eV), OH (531–532 eV) and the
0
42
Co species. After alloying with Co, the catalysts 0.81-PdCo0.10
MgAl-LDH, 0.86-PdCo0.28/MgAl-LDH and 0.79-PdCo0.54/MgAl- surface OH content in the three catalysts decreases to 68.1%–
9.2% compared with MgAl-LDH (73.3%, Table S3†),
/
2
adsorbed or interlayer H O (532–534 eV), respectively. The
6
suggesting the existence of a strong interaction between PdCo
NCs and the surface OH groups of LDH. Particularly, the 0.86-
PdCo0.28/MgAl-LDH with the largest surface OH content of
6
9.2% possesses the strongest PdCo0.28 NCs – MgAl-LDH inter-
action. Meanwhile, the Mg 2p BE values (Fig. S7B(a –a )†) are
0.16, 50.19 and 50.14 eV for 0.81-PdCo0.10/MgAl-LDH, 0.86-
1
3
5
PdCo0.28/MgAl-LDH, and 0.79-PdCo0.54/MgAl-LDH, respectively,
which are slightly higher than those of Pd/MgAl-LDH (50.05
eV) and Co/MgAl-LDH (50.09 eV), and all these values are
slightly shifted toward high BE values compared with MgAl-
LDH (50.01 eV), suggesting the existence of a proper inter-
action between PdCo NCs and MgAl-LDH. Specifically, the
0.86-PdCo0.28/MgAl-LDH with the largest offset (0.18 eV) exhi-
bits the strongest PdCo0.28 NCs – MgAl-LDH synergistic inter-
action, which is consistent with Pd 3d and Co 2p XPS data.
The above results show that the incorporation of Co not
0
only provides electrons to Pd species, but also inhibits the oxi-
0
dation of Pd species, of which 0.86-PdCo0.28/MgAl-LDH with a
Co/Pd ratio of 0.28 possesses the highest electron density of
0
0
the Pd centre and the largest content of Pd species and the
strongest PdCo0.28 NCs – MgAl-LDH interaction, in line with its
highest Heck reactivity.
Furthermore, the Pd 3d XPS spectra of the 0.67-PdCo0.28
/
CoAl-LDH with an even higher activity (Fig. 7A(b )) clearly
1
0
show two distinct peaks assigned to Pd 3d5/2 (334.50 eV) and
3/2 (339.80 eV), and the Pd 3d5/2 peak shifts to a low BE
0
3d
Fig. 7 Pd 3d (A) and Co 2p (B) XPS spectra of 0.81-PdCo0.10/MgAl-LDH
), 0.86-PdCo0.28/MgAl-LDH (a ), 0.79-PdCo0.54/MgAl-LDH (a ), com-
pared with monometallic Pd/MgAl-LDH and Co/MgAl-LDH; and 0.67-
PdCo0.28/CoAl-LDH (b ), and 0.01-PdCo0.28/CoAl-LDH (b ) compared
with CoAl-LDH support.
with an offset of 0.50 eV compared with bulk Pd (335.00 eV),
(
a
1
2
3
0
indicating the increased electron density of the Pd centre of
1
5,39
the catalyst.
1
In contrast, Co 2p XPS (Fig. 7B(b )) fails to
1
2
0
show the peaks for the Co species from the PdCo0.28 NCs
17748 | Dalton Trans., 2019, 48, 17741–17751
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probably due to the overlapping of a strong signal originating 2040 cm− for Pd/MgAl-LDH can be attributed to the CO mole-
1
2
+
from Co 2p3/2 from the CoAl-LDH as the peaks at ∼780.80 cule adsorbed on the Pd atom in the bridge mode and linear
2
+
0
45
−1
(
Co 2p3/2) indicated. The higher offset for Pd 3d5/2 BE com- mode, respectively, while a broad peak at 2052 cm for Co/
pared with the bulk Pd in 0.67-PdCo0.28/CoAl-LDH than in MgAl-LDH is due to the CO molecule adsorbed on the metallic
4
6
0
.86-PdCo0.28/MgAl-LDH (0.31 eV) is reasonable for the higher Co sites in a linear mode. For three bimetallic catalysts
1 3
–a
), the intensity of the peak at ∼1914 cm− gradually
1
activity of the former. The lower surface OH content (Fig. 8a
Table S3†) of 75.5% than CoAl-LDH (78.3%) indicates the weakens with increasing Co/Pd ratios, indicating that more
(
existence of a strong PdCo NCs – CoAl-LDH interaction, in line and more CO molecules are adsorbed on Pd sites in a linear
with the Pd 3d and Co 2p XPS results. Meanwhile, the XPS mode (∼2033 cm− ). Meanwhile, the peak areas of CO linearly
data of the recovered samples of 0.67-PdCo0.28/CoAl-LDH after adsorbed on Pd atoms in these three catalysts account for
1
4
, 8 and 12 runs (Fig. S5F†) also reveal the microstructure 83.1, 95.4 and 98.2% of the total area of linear and bridged CO
stability of the catalyst during the catalytic process (details are peaks, respectively, which is significantly higher than that of
in the ESI†). Pd/MgAl-LDH (60.5%), suggesting that surface continuous Pd
Then for 0.01-PdCo0.28/CoAl-LDH with the highest activity, sites can be isolated by the doped Co atoms. In detail, the
the Pd 3d signal (Fig. 7A(b )) cannot be observed probably due linear CO adsorbed peaks of Pd atoms in the three catalysts
to its ultralow Pd loading of 0.01 wt%, and the Co 2p XPS only are located at 2033, 2024 and 2035 cm− , respectively, showing
2
1
2
+
shows the peaks of the Co species from support, contrarily
a
remarkable redshift compared with Pd/MgAl-LDH
−
1
confirming the ultrahigh dispersion of the PdCo NCs in this (2040 cm ). Particularly, the 0.86-PdCo0.28/MgAl-LDH shows
catalyst. It should be noted that the Co 2p3/2 BE value shows the largest redshift of 16 cm−1, indicating the highest electron
2+
0
a larger downshift of 0.80 eV compared to CoAl-LDH, than density of Pd species, in line with XPS results. The aforemen-
0
.67-PdCo0.28/CoAl-LDH (0.60 eV), indicating more electron tioned results indicate that the incorporation of Co signifi-
2+
transfer from the Co –OH group of the support to PdCo NCs, cantly isolates the surface continuous Pd sites and the adja-
and indirectly implying a stronger PdCo0.28 NCs – CoAl-LDH cent Co atoms modify the electronic structure of the active Pd
synergistic interaction in 0.01-PdCo0.28/CoAl-LDH with
a
species in the obtained PdCo alloy NC catalysts. Specifically,
slightly higher surface OH content (76.1%, Table S3†), in line 0.86-PdCo0.28/MgAl-LDH exhibits the highest electron density
0
with its highest Heck coupling activity.
Fig. 8 shows the CO in situ FTIR spectra of the catalysts which is consistent with its highest Heck reactivity.
.81-PdCo0.10/MgAl-LDH, 0.86-PdCo0.28/MgAl-LDH and 0.79- In order to confirm whether the catalytic mechanism of the
of the Pd centre in the three x-PdCo /MgAl-LDH catalysts,
r
0
PdCo0.54/MgAl-LDH compared with the monometallic present catalysts follows a homogeneous or heterogeneous
samples. For all the samples, a distinctly strong peak at catalytic reaction path, hot filtration experiments have been
170 cm− which is commonly observed can be attributed to conducted. Two parallel tests have been carried out over
1
14
2
the stretching frequency of CO adsorbed on Brønsted acid 0.67-PdCo0.28/CoAl-LDH (0.3 mol%). The first one is proceeded
δ+
sites related to the AlO–H centre on LDH in a linear until the completion of the reaction (99.2%, 35 min), and the
4
3,44
A sharp strong peak at 1914 cm−1 and a weak one at plot of PhI conversion versus reaction time is shown curves a
in Fig. S8.† For the second test (curves b, Fig. S8†), the catalyst
mode.
was separated from the reaction mixture at a PhI conversion of
56.3% by hot filtration. The ICP analysis of the filtrate shows
only 0.02 ppm Pd (corresponding to 1.5 wt% of the total
amount of Pd) leached. The filtrate was allowed to continue
reacting for another 1.2 h under the same conditions, and the
PhI conversion remained almost unchanged. These results
suggest that only the Pd species from PdCo NCs bound to
CoAl-LDH can catalyze the Heck reaction, that is, the reaction
occurs on the surface of the present catalysts.
Based on the above experiments and XPS as well as CO
in situ FTIR analyses combined with previous findings about
4
7–50
the generally accepted mechanism,
a possible Heck coup-
ling mechanism on the present flower-like PdCo alloy NC cata-
lysts (Scheme 2) was tentatively proposed. First, the oxidation
addition of PdCo alloy NCs with aryl halides is formed to
49
surface organometallic intermediates. It is noteworthy that
both Co atoms adjacent to the Pd atoms and surface –OH
groups of the flower-like LDH support contributed to the
increased electron density of the Pd center to promote the oxi-
dative addition of the aryl halides in this step. Meanwhile, the
flower-like structure of the present catalysts also contributed to
Fig. 8 CO FTIR spectra of 0.81-PdCo0.10/MgAl-LDH (a
1
), 0.86-
PdCo0.28/MgAl-LDH (a ), 0.79-PdCo0.54/MgAl-LDH (a ) compared with
2
3
the monometallic Pd/MgAl-LDH and Co/MgAl-LDH.
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alloy NCs and strongest PdCo0.28 NCs–CoAl-LDH synergistic
interaction. Meanwhile, the catalyst possesses excellent sub-
strate universality for the Heck reaction and can be reused for
12 runs without significant loss of activity. The current work
may provide a new idea for the simple and green synthesis of
ultrafine Pd-based low-cost non-noble bimetallic catalysts for
various catalytic processes.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21878007, 21576013 and 21521005).
Scheme 2 Plausible mechanism of Heck coupling of aryl halides with
r
alkenes over the x-PdCo /Co(Mg or Ni)Al-LDH catalysts.
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