Facile synthesis of magnetic recyclable palladium-gold alloy nanoclusters catalysts PdAur/Fe3O4@LDH and its catalytic applications in Heck reaction

Accepted Manuscript

Facile synthesis of magnetic recyclable palladium-gold alloy nanoclusters catalysts

PdAu /Fe O @LDH and its catalytic applications in Heck reaction

r

3 4

Jin Li, Yajuan Wang, Shunwang Jiang, Hui Zhang

PII:

S0022-328X(18)30687-9

10.1016/j.jorganchem.2018.10.007

JOM 20592

DOI:

Reference:

To appear in: Journal of Organometallic Chemistry

Received Date: 6 August 2018

Revised Date: 28 September 2018

Accepted Date: 14 October 2018

Please cite this article as: J. Li, Y. Wang, S. Jiang, H. Zhang, Facile synthesis of magnetic recyclable

palladium-gold alloy nanoclusters catalysts PdAu /Fe O @LDH and its catalytic applications

r

3 4

in Heck reaction, Journal of Organometallic Chemistry (2018), doi: https://doi.org/10.1016/

j.jorganchem.2018.10.007.

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ACCEPTED MANUSCRIPT

Facile synthesis of magnetic recyclable palladium-gold alloy

nanoclusters catalysts PdAu_r/Fe₃O₄@LDH and its catalytic

applications in Heck reaction

Jin Li^‡, Yajuan Wang^‡, Shunwang Jiang and Hui Zhang^*

Graphical Abstract

ACCEPTED MANUSCRIPT

Journal of Organometallic Chemistry

journal homepage: www.elsevier.com

Facile synthesis of magnetic recyclable palladium-gold alloy nanoclusters catalysts

PdAu_r/Fe₃O₄@LDH and its catalytic applications in Heck reaction

Jin Li^‡, Yajuan Wang^‡, Shunwang Jiang and Hui Zhang^*

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P.O. Box 98, Beijing, 100029; P. R. China

ARTICLE INFO

ABSTRACT

series of novel magnetic recyclable Pd-Au nanoclusters (NCs) catalysts x-

Article history:

Received

Received in revised form

Accepted

A

PdAu_r/Fe₃O₄@LDH (x: Pd loading in wt%, r: Au/Pd molar ratio, LDH: layered double

hydroxide, here refers the typical MgAl-LDH) were synthesized by a facile polyol

reduction followed sol immobilization of PdAu_r–PVP on core@shell support

Fe₃O₄@LDH. Systematic characterizations reveal that the obtained catalysts possess the

honeycomb-like core@shell, and the ultrafine PdAu alloy nanoclusters with the size in

1.70–2.55 nm highly dispersed on the LDH nanoplates shell. The obtained catalysts 1.0-

PdAu_r/Fe₃O₄@LDH (r: 0.11, 0.16 and 0.48) show higher Heck activity of iodobenzene

with styrene than monomentallic ones, especially 1.0-PdAu_0.16/Fe₃O₄@LDH displays the

highest Heck activity under the optimal condition. Moreover, the catalysts can be applied

for the Heck reaction of a variety of aryl halides with alkenes, and can be conveniently

separated by an external magnet and reused ten runs without significant loss of activity.

Available online

Keywords:

PdAu alloy nanoclusters

core@shell structure

magnetism

Heck reaction

.

1. Introduction

followed PdCl₂reduction by ascorbic acid, and the latter showed

higher yield of stilbene (90%, 3 h) in the Heck reaction of

iodobenzene with styrene than the former (80%, 3 h) due to

Au@Pd core-shell nanobranches exposing more active sites.

However, the study on PdAu alloy catalysts for the Heck reaction

is rarely published in previous reports except one [20], far from

the tiny PdAu alloy nanoclusters catalyst. Moreover, the

separation of previously reported PdAu alloy catalysts suffers

from time- and power-consuming. Therefore, development of

highly active and easily recyclable PdAu bimetallic alloy

nanoclusters catalysts for the Heck reaction is highly desired.

Recently, magnetic core@shell nanocomposite with multi-

functional combination, as an extraordinary candidate to

traditional supports for preparing high performance catalysts, has

drawn increasing attention [21-23] because of its unique

advantages of both core and shell evolving (1) magnetic

characteristic of core with efficient separation for reusability, and

(2) well-defined dispersion of active metal sites, or/and intrinsic

acid/basic sites for promoting catalytic activity. Xu et al. [21]

prepared SiO₂-coated iron oxide (Fe₃O₄/SiO₂) hybrid modified by

multicarboxylic hyperbranched polyglycerol (HPG) followed

directly growth of Pd NPs (4.0 ± 0.4 nm) from Pd(NO₃)₂reduced

by NaBH₄giving Fe₃O₄/SiO₂/HPG-Pd catalyst, which showed

high activity for the Heck reaction of iodobenzene (PhI) with

Palladium-catalyzed Heck coupling reaction are important

means for the construction of C-C bands between aryl halides

and alkenes [1-3], and is applied widely in the synthesis of

natural products [4], pharmaceuticals [5] and fine chemicals [6].

Heterogeneous Pd catalysts [7-11] applied in the Heck reaction is

favored over conventional homogeneous Pd catalysts [12-14]

because of the low-cost, easy separation for reusability, and

environment-friendliness of the former, thus becoming

a

preferred alternative. In order to develop highly active Pd-based

catalysts, alloying Pd with a second metal has proved to be a

promising research strategy [15-20] owing to their unique

physical and chemical properties which are different significantly

from their monometallic counterparts. Among the Pd-based alloy

catalysts fabricated so far, PdAu alloy catalysts are especially

fascinating due to the obviously enhanced catalytic performance

in methane oxidation [16], phenyl isocyanide oxidation [17] and

C-C coupling reactions [18-20]. Chen et al. [19] prepared AuPd

alloy nanocrystals enveloped in SiO₂nanorattles (AuPd@SiO₂)

via one-pot hydrothermal method, and the obtained

Au₃Pd₁@SiO₂(~8.1 nm) catalyst showed a higher Suzuki

reaction activity (bromobenzene conversion: 99.8% and biphenyl

selectivity: 99.3%, 30 min) than Pd@SiO₂. Khashab et al. [20]

reported oleylamine-Au₄₂Pd₅₈alloy NPs (10-15 nm) catalyst by

high temperature chemical reduction of AuCl₃and PdCl₂and

Au@Pd core-shell nanobranches one by oleylamine-Au NPs (33

nm)-mediated method with CTAB as phase transfer agent

o

styrene at 140 C using base additive NaOAc in DMF (stilbene

yield: 93%, 6 h) and slight loss of activity in the 4th run on

magnetic separation. However, though these magnetic

core@shell Pd-based catalysts show excellent catalytic property,

the preparation and surface modification of the magnetic

core@shell support suffers from long operation periods and

tedious synthetic procedures, which greatly limits the practical

application of such core@shell materials. The layered double

hydroxide (LDH), as a typical layered material, has been widely

^‡Theses authors contributed equally to this work.

*

Corresponding author. Tel: +8610-6442 5872; Fax:+8610-

6442 5385. E-mail addresses: huizhang67@gst21.com.

2

Journal of Organometallic Chemistry

ACCEPTED MANUSCRIPT

applied in catalysis owing to its adjustable surface acidity-

suspension. An alkaline aq. solution (100 mL) of NaOH (0.02

basicity, high surface area and uniquely positively-charged 2-

dimensional hydroxide nanolayers [24]. Combination of magnetic

iron oxide with LDH, which contains the easy separation of

magnetic material and the advantages of LDH, is a promising

class of magnetic composites. Zhang et al. [25,26] reported a

novel core@shell hybrid Fe₃O₄@MgAl-LDH via a one-step

coprecipitation as superparamagnetic support loading nanogold

for highly efficient oxidation of 1-phenylethanol without base

additive, and it showed good recyclability upon magnetic

separation. Additionally, it is reported that the basic surface of

the LDH can greatly facilitate the oxidative addition step in the

Heck reaction [11]. Clearly, assembly of magnetic recyclable

LDH supported Pd-Au bimetallic nanoclusters catalysts for the

Heck reaction is highly desired.

Herein, a series of novel hierarchical magnetic core@shell

catalysts x-PdAu_r/Fe₃O₄@LDH (x: Pd loading in wt%, r: Au/Pd

molar ratio) were fabricated by a facile polyol reduction−sol

immobilization method and systematically characterized. The

effect of Au/Pd ratios and Pd loadings on the size and size

distribution, electron density and surface compositions of PdAu

nanoclusters on the catalysts with unique honeycomb-like

core@shell structure, thus on the Heck reactivity was deeply

studied. The reaction rate constant and apparent activation energy

of the Heck reaction over the present catalysts are determined.

Moreover, the substrate adaptability and recyclability of the

catalyst was tested.

mol) and Na₂CO₃(0.006 mol) was added dropwise into the above

suspension until pH ~10 and kept for 5 min. Then, another aq.

solution (100 mL) of Mg(NO₃)₂·6H₂O (0.009 mol) and

Al(NO₃)₃·9H₂O (0.003 mol) was added dropwise into the above

suspension under vigorously stirring with constant pH of 10 kept

by simultaneously adding above alkaline solution. The black

slurry was magnetically separated by a NdFeB magnet (0.15 T),

washed by water until pH ~7, and dried at 60 ^oC overnight giving

the support Fe₃O₄@MgAl-LDH (denoted as Fe₃O₄@LDH). The

reference support MgAl-LDH (denoted as LDH) was prepared

according to our previous work [28]

.

2.3 Synthesis of magnetic PdAu alloy nanoclusters catalysts

The synthesis of PdAu_r-PVP alloy nanoclusters (NCs) with

varied Au/Pd molar ratios were realized via a polyol reduction

method according to the previous report [29]. Typically, 0.97 g

PVP was dissolved into an aq. solution (150 mL) containing

K₂PdCl₄(0.141 mmol PdCl₂and 0.282 mmol KCl) and

HAuCl₄·4H₂O (0.0254 mmol) with Au/Pd ratio of 0.18 and

PVP/(Pd+Au) ratio of 0.1 under vigorous stirring followed

adding 50 mL EG at room temperature. Then the resultant was

refluxed at 140 ^oC for 2 h forming a dark brown colloidal

dispersion. After cooling to room temperature, the support

Fe₃O₄@LDH (1 g, the amount of support was fixed on a designed

Pd loading of 1.5 wt%) was added to the above colloidal

dispersion under vigorous stirring and kept for 1 h to immobilize

the PdAu_r-PVP colloid on the surface of support. The resultant

was magnetically separated by a magnet, washed and dried at 60

^oC for 24 h to yield a black powder denoted as 1.0-

PdAu_0.16/Fe₃O₄@LDH (both Au/Pd ratio 0.16 and Pd loading 1.0

wt% upon ICP). The catalysts with varied Pd loadings were

obtained by adjusting amount of the magnetic support resulting

in products x-PdAu_0.16/Fe₃O₄@LDH with x = 0.5, 0.1 and 0.05

(by ICP), respectively. Then, adjusting the amount of

HAuCl₄·4H₂O to 0, 0.0155 and 0.0761 mmol in 150 mL aq.

solution of K₂PdCl₄and HAuCl₄·4H₂O on Au/Pd = 0, 0.11 and

0.54 and the amount of PVP to 0.82, 0.91 and 1.26 g on

PVP/(Pd+Au) = 0.1, respectively, and keep other conditions

constant, giving the monometallic catalyst Pd/Fe₃O₄@LDH and

bimetallic catalysts 1.0-PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.48 on

ICP). The monometallic catalyst Au/Fe₃O₄@LDH was prepared

by the same method but without K₂PdCl₄. Another monometallic

catalyst Au/Fe₃O₄@LDH-1 was prepared by Au nanoclusters

precursor method according to our previous work (details in

Electronic Supplementary Information (ESI)) [26]. Scheme 1

depicts the design schematic of the magnetic PdAu alloy

nanoclusters catalysts.

2. Experimental section

2.1 Materials

All chemicals were of analytical grade and used as received

without further purification. Ethylene glycol (EG), K₂CO₃, KCl

and N, N-dimethylformamide (DMF) were supplied from Beijing

Chemical Works, and poly (N-vinyl-2-pyrrolidone) (PVP, MW =

58000) from XiLong Chemical Corporation. PdCl₂, styrene and

bromobenzene were purchased from Tianjin Fuchen Chemical

Reagents Factory, and HAuCl₄·4H₂O from Sinopharm Chemical

Reagent Co., Ltd.. Methyl acrylate, ethyl acrylate, iodobenzene

and other aryl halides were purchased from Aladdin. The

o

deionized water with a resistivity > 18.25 MΩ cm (25 C) was

employed in the whole work.

2.2 Synthesis of magnetic support

Submicrospheres Fe₃O₄(~526 nm) were prepared via a

surfactant-free solvothermal method in our previous work [27].

Next, the black Fe₃O₄powder (1.0 g) were ultrasonically

dispersed in deionized water (100 mL) to give an uniform

Scheme 1. The design schematic of the magnetic PdAu alloy nanoclusters catalysts.

2.4 Characterization

3. Results and discussion

ACCEPTED MANUSCRIPT

In situ diffuse reflection infrared Fourier transform

spectroscopy (DRIFTS) was carried out on a Bruker Vertex 70

3.1 The crystal Structure, composition and morphology

instrument containing

a controlled environment chamber

equipped with CaF₂windows to study the surface structure and

composition of metallic NCs on the catalyst. The DRIFTS

spectra were recorded by using wafers in the form of self-

supporting pellets of the catalysts powder mounted in a

homemade ceramic cell. The catalyst was pretreated in Ar flow

(50 mL/min) at 25 ^oC for 0.5 h. The sample was scanned to get a

background record. Then catalyst was exposed to a CO flow (50

mL/min) for 1 h, subsequently the cell was purged with Ar for 1

h. Finally measurement was conducted in Ar atmosphere at 4cm^-1

resolution by averaging 64 scans. Hydrogen temperature-

programmed reduction (TPR) was realized on a Micrometric

ChemiSorb 2750 chemisorption instrument equipped with a

thermal conductivity detector. The sample (30 mg) was loaded

into the bottom of a quartz reactor and heated from 25 ^oC to 150

o

^oC with a heating rate of 5 C/min in Ar flow (50 mL/min) and

o

kept for 1 h, then cooled to 25 C, followed by keeping a stream

o

of 10% H₂in Ar (50 cm³min^-1) with a heating rate of 5 C/min

from 25 ^oC to 300 ^oC. TGA-MS thermograms for surface

transient organometallic intermediate (named as STOI,

preparation details in ESI) of the Heck reaction were recorded on

Fig. 1 XRD patterns of 1.0-PdAu_0.11/Fe₃O₄@LDH (a)

, x-

PdAu_0.16/Fe₃O₄@LDH (x = 1.0 (b), 0.5 (b₁) and 0.1 (b₂)), 1.0-

PdAu_0.48/Fe₃O₄@LDH (c), Pd/Fe₃O₄@LDH, Au/Fe₃O₄@LDH,

the support Fe₃O₄@LDH and Fe₃O₄.

a

PerkinElmer Diamond TG/DTA/DSC ThermoStarTM

instrument to explore the Heck reaction mechanism. In the

experiment, the sample (6.4 mg) was heated from 20 ^oC to 900 ^oC

(10 ^oC/min) in a N₂flow. ¹H and ¹³C NMR spectra of the

coupling products (dissolved in CDCl₃) were obtained on an

AV600 NMR spectrometer (Germany) operating at 600.13 MHz

relative to tetramethylsilane (TMS, 0.00 ppm). The other

characterization techniques and instruments including powder X-

ray diffraction (XRD), FT-IR, inductively coupled plasma atomic

emission spectroscopy (ICP-AES), SEM, (HR)TEM, N₂

adsorption-desorption isotherm, high-angle annular dark-field

scanning transmission electron microscopy (HAADF-STEM),

vibrating sample magnetometer (VSM) and X-ray photoelectron

spectrometer (XPS) were identical to our previous reports [26,28].

Fig. 1 shows the XRD patterns of a series of magnetic catalysts

x-PdAu_r/Fe₃O₄@LDH (x = 1.0, 0.5 and 0.1 wt% of Pd, r = 0.11,

0.16 and 0.48) compared to monometallic catalysts, the support

Fe₃O₄@LDH and Fe₃O₄. The Fe₃O₄sample shows intense

diffractions indexed to (111), (220), (311), (400), (422), (511)

and (440) planes of fcc magnetite (JCPDS 19-0629), and the

particle size from Debye-Scherrer formula on (311) line is 27.7

nm (Table S1), close to the critical dimension of the single

magnetite domains (~30 nm) [30], implying superparamagnetic

characteristic of the prepared Fe₃O₄. For the series of catalysts

and the support, besides the sharp lines of Fe₃O₄phase, a series

of weak but clear lines at ~11.3^o(003), 22.8^o(006), 34.7^o(012)

and 60.6^o(110) are observed [31], which can be indexed to

typical hcp carbonate-LDH phase. Notably, the intensity ratio of

I₁₁₀/I₀₀₃for Fe₃O₄@LDH is 0.71, clearly larger than those of pure

LDH of 0.26 (Table S1), suggesting the oriented growth of LDH

with ab-face vertical to the surface of Fe₃O₄core [25,26]. Noted

that the XRD lines of all the x-PdAu_r/Fe₃O₄@LDH catalysts

clearly show a weak single peak of PdAu crystallites (enlarged

patterns in Fig. 1) at 2θ located between the peak positions of

Au(111) (38.2^o) (JCPDS 04-0784) and Pd(111) (40.1^o) (JCPDS

46-1043), implying the formation of highly dispersed PdAu

nanoalloys in the obtained catalysts.

2.5 Catalytic activity test

The Heck reaction was carried out in a 25-milliliter three-

necked flask attached a reflux condenser under magnetic stirring.

Aryl halide (1.0 mmol), alkene (1.5 mmol), K₂CO₃(3 mmol),

catalyst (0.3 mol% Pd with respect to aryl halide), DMF (12 mL)

and H₂O (4 mL) were mixed in the flask, then the reaction

proceeded at 120 ^oC under refluxing. Throughout the process, 0.2

mL mixture were pipetted every 30 min, filtered by a Nylon 66

filter (3 mm × 0.22 µm) and analyzed by gas chromatograph

(Agilent 7890A) equipped with a FID and a Agilent J&K HP-5

capillary column (5% phenyl polysiloxane, 30 m × 0.25 mm ×

The FTIR spectra of the catalysts x-PdAu_r/Fe₃O₄@LDH

compared to the support (Fig. S1) show broad and strong

absorption at ~3450 cm^-1due to the hydroxyl stretching vibration

rising from M-OH groups on the LDH nanoplates [31,32]. A

sharp absorption at 1379 cm^-1and a weak one at 868 cm^-1can be

assigned to the v₃(asymmetric stretching) and v₂(out-of-plane

o

0.25 µm). After 1 min at 70 C, the column chamber was heated

o

from 70 C to 150 C with a heating rate of 20 C/min and kept

o

for 1 min, then heated to 275 C at a heating rate of 25 C/min

and kept for 3 min. The catalytic activity of the catalyst was

evaluated on the basis of turnover frequency (TOF, moles of aryl

halide converted per moles of Pd per hour). After completion of

the reaction, the catalyst was magnetically separated from the

mixture. The filtrate was extracted by ethyl acetate. The organic

phase was thoroughly washed with saturated NaCl solution

several times and dried by anhydrous Na₂SO₄, followed rotary

evaporation (35 ^oC) to remove the solvent, and then through

recrystallization to obtain the products which were identified by

¹H and ¹³C NMR spectra (see ESI).

2-

deformation) modes of CO₃ions, respectively [32]. While a

peak at ~428 cm^-1(not shown in Fe₃O₄) can be assigned to the M-

O lattice mode of LDH [22,33], and the sharp band at 584 cm^-1to

the Fe-O mode of Fe₃O₄[27]. These observations clearly imply

the well-defined assembly of CO₃²⁻-LDH and Fe₃O₄phase. As

for weak bands at 2923 and 2853 cm^-1of all the catalysts, they

can be ascribed to the v_asand v_smodes of -CH₂of PVP molecules

[32,34]. While a strong band at 1648 cm^-1assigned to C=O of

PVP clearly downshifts compared to pure PVP (1680 cm^-1),

ascribing to the chemisorption of PVP on PdAu alloy

4

Journal of Organometallic Chemistry

ACCEPTED MANUSCRIPT

nanocrystallites through O atom of C=O group weakening the

C=O bond [34].

Fig. 2 SEM (A) and TEM (B) image of the catalyst 1.0-

PdAu_0.16/Fe₃O₄@LDH, insets: EDS.

Fig. 2 shows the SEM (A) and TEM (B) image of the catalyst

1.0-PdAu_0.16/Fe₃O₄@LDH, while those of other catalysts x-

PdAu_r/Fe₃O₄@LDH,

1.0-PdAu_0.16/LDH,

the

support

Fe₃O₄@LDH and Fe₃O₄are given in Fig. S2. Fig. S2A clearly

shows nearly monodispersed submicrospheres of Fe₃O₄with a

smooth surface and a mean diameter of 526 ± 32 nm. After

coating with LDH, Fe₃O₄@LDH (Fig. S2B) clearly shows a

honeycomb-like morphology holding many voids with size of

25−70 nm, and interestingly, the LDH nanoplates with the

dimensions of ~200 × 20 nm grow preferably in the orientation of

c-axis parallel to and ab-face vertical to the surface of Fe₃O₄core,

and its TEM image (insets in Fig. S2B) shows typical core@shell

structure. Moreover, there is no aggregation of single LDH plates

(Fig. S2G) in this structure, implying that Fe₃O₄core can greatly

suppress the particle–particle interactions often occurred among

the LDH nanoparticles [35]. The SEM images of the magnetic

catalysts (Fig. 2A and Fig. S2(C-F)) all show similar morphology

to its support, which is quite different from the aggregation of

pure LDH supported PdAu catalyst (Fig. S2G) and may be in

favor of the exposure of the active sites.

Fig. 3 shows the HRTEM of the x-PdAu_r/Fe₃O₄@LDH

catalysts, while those of monometallic catalysts are shown in Fig.

S3. The Pd particle size of monometallic Pd/Fe₃O₄@LDH

catalyst rangs from 1 to 7 nm with mean size of 2.00 ± 0.42 nm,

clearly illustrating the heterogeneity of size of mono-Pd catalyst

(Fig. S3a). As for monometallic Au/Fe₃O₄@LDH prepared by the

same method, it shows obviously aggregated and much larger Au

particles (> 20 nm) (Fig. S3b), similar to Ebitani’s result [29],

while another one Au/Fe₃O₄@LDH-1 prepared by Au

nanoclusters precursor method depicts well-dispersed Au

nanoparticles of 4.26 ± 0.96 nm (Fig. S3c). After alloying with

Au, the PdAu catalysts 1.0-PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16

and 0.48) (insets in Fig. 3a, b and c) present obviously

homogeneous dispersion of metal nanoclusters, with mean sizes

of 2.35 ± 0.47, 1.81 ± 0.33 and 2.55 ± 0.49 nm, respectively,

clearly smaller than 1.0-PdAu_0.16/LDH (2.87 ± 0.50 nm in Fig.

S2H), and the 1.0-PdAu_0.16/Fe₃O₄@LDH has the minimum PdAu

NCs’ size and the narrowest size distribution (1.0 – 3.0 nm).

These results strongly demonstrate that size and size distribution

of PdAu NCs can be tuned effectively by incorporating

appropriate amount of gold. Further reducing Pd loading with

Au/Pd ratio of 0.16, the obtained x-PdAu_0.16/Fe₃O₄@LDH (x =

0.5 and 0.1) (Fig. 3b₁and b₂) exhibit further reduced NCs’ sizes

of 1.75 ± 0.40 and 1.70 ± 0.30 nm, respectively, indicating that

the Pd loadings has a slight effect on the size and size distribution

of PdAu NCs.

Fig. 3 The low- and high-magnified HRTEM images of 1.0-

PdAu_0.11/Fe₃O₄@LDH (a, a’), x-PdAu_0.16/Fe₃O₄@LDH (x = 1.0

(b, b’), 0.5 (b₁, b₁’) and 0.1 (b₂, b₂’)) and 1.0-

PdAu_0.48/Fe₃O₄@LDH (c, c’) (insets: histograms of particle size

distribution and FFT).

possibly strong interaction between PdAu NCs and the support.

The lattice spacings for PdAu nanocrystallites of the 1.0-

PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16 and 0.48) are 0.226, 0.227

and 0.229 nm, respectively, in line with FFT images (insets in

Fig. 3a’–c’), which are between the values of bulk Pd(111)

(0.225 nm, JCPDS 46-1043) and bulk Au(111) (0.235 nm,

JCPDS 04-0784), indexed to the (111) planes of the PdAu alloy

phase and consistent with the d-spacing calculated from Vegard’s

law [36]. Moreover, (018) lattice fringes of the shell LDH can

The magnified HRTEM images of the series of catalysts x-

PdAu_r/Fe₃O₄@LDH clearly show that the PdAu NCs are tightly

anchored on the margin of LDH nanosheets (arrows marked in

Fig. 3a’–c’) or embedded deeply into LDH laminate, implying

36.1 and 41.8 emu·g⁻¹, respectively, though lower than Fe₃O₄

ACCEPTED MANUSCRIPT

(76.5 emu·g⁻¹) owing to the coating of nonmagnetic shell LDH

[25,26]. Consequently, the obtained magnetic core@shell

catalysts are easily separated from reaction system by using an

external magnet.

3.3 Catalytic performance

The Heck reaction of iodobenzene (PhI) with styrene is

selected as a probe reaction to study the catalytic performance of

the magnetic catalysts. The 1.0-PdAu_0.48/Fe₃O₄@LDH is first

employed to optimize Heck reaction conditions involving solvent

composition and additive base type (since pure PdAu_0.48-PVP

colloid or Fe₃O₄@LDH without base additive shows no product),

and the results are summarized in Table S3 . The volume ratio of

DMF and H₂O is adjusted to explore the suitable solvent

composition, and the best yield (87.9%) is obtained in the case of

Fig. 4 The line profiles of EDS spectra from HAADF-STEM for

the specially selected PdAu NCs on the 1.0-

PdAu_0.11/Fe₃O₄@LDH (a), 1.0-PdAu_0.16/Fe₃O₄@LDH (b), 1.0-

PdAu_0.48/Fe₃O₄@LDH (c) and STEM image of 1.0-

PdAu_0.16/Fe₃O₄@LDH (d) with particle size distribution.

also be found in the HRTEM images with corresponding

diffraction dots in FFT images. The HAADF-STEM and STEM-

EDS analyses of the specially selected PdAu NCs in three PdAu

alloy catalysts (larger particles selected for instrument’s detection

limitations) (Fig. 4a-c) demonstrate that the NCs consist of both

Pd and Au, which indicate forcefully that the Pd and Au coexist

in the form of alloy state in the catalysts as XRD and HRTEM

indicated. For 1.0-PdAu_0.16/Fe₃O₄@LDH, the point analyses of

the randomly selected NCs also verify that most of obtained NCs

contain both Pd and Au, and only a very few fraction are pure Pd,

as also evidenced by the mostly spatial coincidence of the Au and

Pd in EDS mapping analysis (Fig. S4). The quantitative analysis

of the EDS profile shows that the catalyst is composed of mixed

Au and Pd atoms with r of 0.18, in good agreement with the ICP

data (0.16). Moreover, the particle size distribution from STEM

of 1.0-PdAu_0.16/Fe₃O₄@LDH (Fig. 4d) gives the mean size of

1.88 ± 0.30 nm, in line with the HRTEM data.

The BET analyses show that the catalysts 1.0-

PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16 and 0.48) possess similar

specific surface areas of 49.9-52.9 m²g^-1to the support

Fe₃O₄@LDH (53.1 m²g^-1) (Fig. S5 and Table S2) and typical

mesoporosity with mean pore sizes of 2.1-2.7 nm, along with the

hierarchical core@shell structure with abundant accessible edge

and junctions as SEM indicated, being in favor of the exposure of

catalytic active sites [26].

v_DMF:v_H2O= 3:1 at 2 h (Table S3, entry 2), while the lower or

even no yield is detected under other solvent compositions.

Moreover, the weak base K₂CO₃is found to be the preferred

choice for the reaction to both the stronger base NaOH and

weaker bases (NaAc and N(C₂H₅)₃) upon the highest selectivity

and yield for the Heck reaction (Table S3, entries 2, 6–8).

Therefore, the optimized reaction condition using K₂CO₃as base

additive and DMF-H₂O with v/v=3:1 as solvent is achieved.

Then, the effect of Au/Pd ratios on the Heck reaction is studied

over the series of magnetic PdAu alloy NCs catalysts 1.0-

PdAu_r/Fe₃O₄@LDH under the optimized reaction conditions (Table

1). Not surprisingly, monometallic Au/Fe₃O₄@LDH-1 shows no

activity, while the Pd/Fe₃O₄@LDH presents high activity with the

Yield of 81.8% in 2 h, clearly indicating that Pd species are the

intrinsic active sites for the Heck reaction as previously reported

[1,2]. However, after alloying with Au, three alloy catalysts 1.0-

PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16 and 0.48) all show higher activity

than the Pd/Fe₃O₄@LDH. Especially, the 1.0-PdAu_0.16/Fe₃O₄@LDH

with smallest PdAu NCs’ size (~1.81 nm) exhibits the highest

activity with the Yield of 87.6% in 1 h. These results strongly

suggest that Au/Pd ratios can greatly influence the catalytic

performance of the magnetic catalysts 1.0- PdAu_r/Fe₃O₄@LDH for

the Heck reaction. It is also noted that under no base additive, 1.0-

PdAu_0.48/Fe₃O₄@LDH shows quite low yield (4%) at 2 h but

increased to 53.5% at 9 h (Table S3, entry 9), implying that the

magnetic support with intrinsic basic LDH layers serves to the

base-required Heck reaction to some extent.

3.2 Magnetic property

Furthermore, the effect of the Pd loadings of the magnetic

PdAu NCs catalysts on the Heck reaction is studied (Table 1).

The low Pd-loading samples x-PdAu_0.16/Fe₃O₄@LDH (x = 0.5

and 0.1) exhibit the Yield of 94.1% in 2.0 h and 89.9% in 1.5 h

with Pd dosage of 0.05 mol% respectively, while 1.0-

PdAu_0.16/Fe₃O₄@LDH is 94.0% in 3.5 h. However, further

reduced Pd loading sample 0.05-PdAu_0.16/Fe₃O₄@LDH show

decreased Heck activity probably due to their sequentially

reduced active sites. The above results clearly demonstrate that

the catalyst 0.1-PdAu_0.16/Fe₃O₄@LDH possesses even higher

Heck coupling activity, implying the predominant size effect of

the present magnetic PdAu alloy NCs catalysts. Actually, the

catalyst 0.1-PdAu_0.16/Fe₃O₄@LDH presents much better Heck

reactivity, even at lower temperature with stilbene yield of 92.3%

at 90 ^oC, than previously reported magnetic Fe₃O₄/SiO₂/HPG-Pd,

[21] Fe₃O₄@SiO₂-Dendrimer-Pd [23], Pd-MNPSS (magnetic

NPs−starch substrate) [37] and Fe₃O₄@PUNP (Poly

(undecylenic acid-co-N-isopropylacrylamide-co-potassium 4-

Fig. 5 The magnetization curves of Fe₃O₄(a), Fe₃O₄@LDH (b)

and the catalyst 1.0-PdAu_0.16/Fe₃O₄@LDH (c).

Fig. 5 depicts the magnetization curves of the catalyst 1.0-

PdAu_0.16/Fe₃O₄@LDH compared with the support Fe₃O₄@LDH

and Fe₃O₄. The magnetization curves of the three samples exhibit

typically superparamagnetic characteristics, also verified by their

weak coercive force and remanence, in line with the D₃₁₁value of

the magnetic core Fe₃O₄. The 1.0-PdAu_0.16/Fe₃O₄@LDH and

support exhibit considerable saturation magnetization (Ms) of

acryloxyoyl-pyridine-2,

6-dicarboxylate))-Pd

[38]

with

complicated preparation under similar reaction conditions (Table

1).

6

Journal of Organometallic Chemistry

Table 1 Catalytic activity of the various catalysts for the Heck reaction of iodobenzene with styrene. ^a

ACCEPTED MANUSCRIPT

Catalysts

D_Pd/nm^b

Pd/ mol%

t /h

Conv./ %

Yield/%

Ref.

1.0-PdAu_0.11/Fe₃O₄@LDH

1.0-PdAu_0.16/Fe₃O₄@LDH

0.5-PdAu_0.16/Fe₃O₄@LDH

0.1-PdAu_0.16/Fe₃O₄@LDH

0.1-PdAu_0.16/Fe₃O₄@LDH^c

1.0-PdAu_0.48/Fe₃O₄@LDH

1.0-PdAu_0.16/LDH

2.35±0.47

1.81±0.33

1.75±0.40

1.70±0.30

2.55±0.49

2.87±0.50

2.00±0.42

4.26±0.96

-

0.30

0.05

0.30

0.05

3.0

0.5/1.5

0.5/1.0

0.5/3.5

0.5/2.0

0.5/1.5

0.5/3.0

0.5/2.0

0.5/1.0

0.5/2.0

0.5/4.0

6.0

39.9/92.1

50.6/92.9

16.0/99.5

30.1/99.9

31.1/93.0

22.4/96.4

28.0/91.6

13.5/39.5

21.0/86.8

n.d.

36.0/86.1

45.8/87.6

15.0/94.0

27.9/94.1

28.2/89.9

22.0/92.3

25.9/87.9

12.7/37.6

19.3/81.8

n.d.

This work

21

Pd/Fe₃O₄@LDH

Au/Fe₃O₄@LDH-1

0.05-PdAu_0.16/Fe₃O₄@LDH

Fe₃O₄/SiO₂/HPG-Pd^d

12.6/90.7

-

12.6/88.9

93

-

Fe₃O₄@SiO₂-Dendrimer-Pd^e

Pd-MNPSS^f

-

0.009

0.36

0.1

2.0

-

92

23

-

4.0

-

90

37

Fe₃O₄@PUNP-Pd^g

-

8.0

-

95

38

a

Reaction conditions: PhI (1 mmol), styrene (1.5 mmol), v_DMF:v_H2O= 12 mL:4 mL, K₂CO₃(3 mmol), 120 ^oC, catalysts x-

c

PdAu_r/Fe₃O₄@LDH under atmospheric conditions. ^bMean values upon 200 individual NCs in TEM images. PhI (50 mmol), styrene

o

(75 mmol), v_DMF:v_H2O=120 mL:40 mL, K₂CO₃(100 mmol), 90 C. ^dPhI (0.1 mmol), styrene (0.2 mmol), DMF (8 mL), NaAc (0.18

o

e

o

f

mmol), 140 C. PhI (1 mmol), styrene (2 mmol), DMF (2 mL), Et₃N (3 mmol), 120 C. PhI (1 mmol), styrene (1.2 mmol), H₂O ( 5

mL), K₂CO₃(2.0 mmol), 100 ^oC. ^gPhI (1 mmol), styrene (1.5 mmol), v_H2O:v_DMF=1 mL:1 mL, K₂CO₃(3 mmol), Ar atmosphere, 94 ^oC.

To verify the universal adaptability of the present PdAu NCs

catalysts x-PdAu_r/Fe₃O₄@LDH for the Heck reaction, the range

of aryl halides and alkenes is explored over the catalyst 0.1-

PdAu_0.16/Fe₃O₄@LDH under the optimal reaction conditions

(Table 2). The catalyst exhibits excellent catalytic performance

for the Heck reaction of aryl iodides with electron-withdrawing

groups or electron-donating groups and styrene, to give the

desired Heck reaction products 1a-1e in 89.7–99.9% yield (Table

2, entries 1–5). PhI reacts with methyl acrylate and ethyl acrylate,

to give 1f in 99.1% yield and 1g in 99.0% yield, respectively

(Table 2, entries 6 and 7). As for the reaction of 4-PhI-OH and 4-

PhI-CHO with styrene, instead of Heck product but an Ullmann

reaction product 2 in 98.0% yield (Table 2, entry 8) and a

hydrogenation product 3 in 99.3% yield (Table 2, entry 9) are

obtained, respectively. Then for the reaction of bromobenzene

with styrene and ethyl acrylate, which give the Heck product 1c

in 92.2% yield and 1g in 92.4% yield, respectively (Table 2,

entries 10 and 11). While the reaction of 4-PhBr-CHO with

styene gives the Heck product 1h in 87.7% yield along with

hydrogenation product 3 in 12.1% yield (Table 2, entry 12), and

the reaction of 4-PhBr-OH with styene gives the Heck product 4-

hydroxystilbene in 98.6% yield (Table 2, entry 13), probably

attributed to the different dissociation energy of C-I and C-Br

bond. These results are much higher than those of previously

reported catalyst SiNA-Pd (SiNA: silicon nanowire array) [39]

(Yield: 82%, 48.0 h) and Pd(2.5)/NT (NT: hydrogen titanate

nanotubes) [40] with lower Heck product yield (43.2%, 24.0 h),

and higher than those of Pd@Co/CNT-50 (Yield: 88%, 6 h) [41]

under similar catalytic reaction conditions (Table 2, entries 14-

16), implying the higher Heck activities of the present 0.1-

PdAu_0.16/Fe₃O₄@LDH for aryl bromides and alkenes. The above

results clearly demonstrate that the present magnetic PdAu alloy

NCs catalysts x-PdAu_r/Fe₃O₄@LDH possess an extensive

substrate adaptability.

Table 2 Catalytic activity of the 0.1-PdAu_0.16/Fe₃O₄@LDH for

the Heck reaction of varied aryl halides with varied alkenes. ^a

Substrate

Conv.

No.

t/h

product ^b

/ %

X

R₁

R₂

1

2

3

4

5

6

7

8

9

I

NO₂

COCH₃

H

CH₃

OCH₃

H

Ph

0.67

1.5

1.67

2.0

1.5

1.0

2.0

10

99.9

96.9

93.0

99.9

90.7

99.6

99.0

98.1

99.5

95.0

93.3

1a (99.9)

1b (95.9)

1c (89.7)

1d (96.4)

1e (90.3)

1f (99.1)

1g (99.0)

2 (98.0)

3 (99.3)

1c (92.2)

1g (92.4)

1h (87.7),

3 (12.1)

98.6

CO₂CH₃

CO₂C₂H₅

Ph

CO₂C₂H₅

H

OH

CHO

H

10 Br

11 Br

H

7.0

12 Br

CHO

Ph

2.5

99.8

13 Br

14^cBr

15^dBr

16^eBr

OH

H

Ph

CO₂C₄H₉

Ph

12

48

6.0

24

98.9

-

82

88

43

H

Ph

a

Reaction conditions: 0.1-PdAu_0.16/Fe₃O₄@LDH (0.05 mol%

Pd), aryl halide (1.0 mmol), alkene (1.5 mmol), v_DMF:v_H2O=12

mL:4 mL, K₂CO₃(3.0 mmol), 120 ^oC. ^bThe varied products were

identified by ¹³C and ¹H NMR (see ESI) and the data in

parentheses are yield of corresponding product by GC. ^cPhBr,

butyl acrylate, NaAc, TBAB. ^dPhBr, styrene, Et₃N, DMF, N₂. ^e

PhBr-CHO, styrene, Et₃N, DMF, tri-(o-tolyl)phosphine, N₂

.

3.4 The kinetics study

ACCEPTED MANUSCRIPT

Fig. 6 -ln(1-C) against time (a-c) and Arrhenius plots (d) for the

Heck reaction over 1.0-PdAu_r/Fe₃O₄@LDH (r = 0.11 (a), 0.16 (b)

o

and 0.48 (c)) at temperatures of 100, 110, 120, 130 and 140 C.

Fig. 7 Pd 3d and Au 4f XPS spectra of 1.0-PdAu_0.11/Fe₃O₄@LDH

(a), 1.0-PdAu_0.16/Fe₃O₄@LDH (b), 1.0-PdAu_0.48/Fe₃O₄@LDH (c),

Pd/Fe₃O₄@LDH and Au/Fe₃O₄@LDH.

Reaction conditions: catalyst (Pd: 0.3 mol% based on PhI), PhI

(1.0 mmol), styrene (1.5 mmol), v_DMF:v_H2O= 12 mL:4 mL,

K₂CO₃(3.0 mmol).

The XPS analysis is first employed to explore the surface

chemical composition and electron structure of the series of

magnetic PdAu alloy NCs catalysts. The Pd 3d and Au 4f XPS

spectra are shown in Fig. 7 and the corresponding parameters are

listed in Table S5. The Pd 3d XPS spectrum of Pd/Fe₃O₄@LDH

shows two distinct asymmetrical peaks for Pd 3d_5/2at binding

energy (BE) of ~335.60 eV and Pd 3d_3/2at BE of ~341.02 eV,

respectively. The deconvolution BE values of Pd 3d_5/2are 335.50

and 337.45 eV, which can be assigned to the Pd⁰and Pd^IIspecies,

respectively [42]. The Au 4f XPS spectrum of Au/Fe₃O₄@LDH

shows two asymmetrical peaks for Au 4f_7/2at 83.55 eV and Au

4f_5/2at 86.47 eV, which is overlapped by Mg 2s at 89.60 eV. The

deconvolution BE values of Au 4f_7/2are 83.55 and 84.45 eV,

attributable to surface metallic Au and Au⁺species, respectively

[42,43]. After alloying with Au, the Pd⁰3d_5/2BE values of three

catalysts 1.0-PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16 and 0.48) are

335.05, 334.69 and 335.22 eV, with 0.45, 0.81 and 0.28 eV

decrease compared with Pd/Fe₃O₄@LDH, respectively, while the

Au 4f_7/2BE values are 83.08, 82.63 and 83.11 eV, similarly with

0.47, 0.92 and 0.44 eV decrease compared with Au/Fe₃O₄@LDH,

respectively. The observed negative shift for both Pd 3d_5/2and

Au 4f_7/2BE values are in line with the net charge transferring into

palladium (gaining d electrons from gold) and gold (increased Au

s or p electron cloud densities) [19,44], suggesting the electronic

structure of the surface Pd atoms to be greatly modified by

alloying with Au, besides the role of support (see later O 1s, Fe

To reveal the nature reasons for different Heck reaction

performance of the series of magnetic PdAu alloy nanoclusters

catalysts, the macroscopic kinetic experiment of the obtained 1.0-

PdAu_r/Fe₃O₄@LDH catalysts including the effect of the different

Au/Pd ratios and reaction temperatures (100, 110, 120, 130 and

140 ^oC) on the Heck reaction rate of PhI with styrene was studied.

As shown in Fig. S6, the Conv.-time (t) plots are linearly up to ca.

60% Conv. of PhI, implying that the product inhibition is very

small. It can be clearly seen in Fig. 6a-c that the term -ln(1-C) is

linearly increased with the reaction time for three catalysts 1.0-

PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16 and 0.48), fitting quite well to

the first order with respect to PhI. The rate constant, k, is

calculated upon the slope of the -ln(1-C) verse t plot. The k

values (Table S5) of the Heck reaction of PhI with styrene for

three catalysts at the same temperature are found to be in an order

of 1.0-PdAu_0.16/Fe₃O₄@LDH > 1.0-PdAu_0.11/Fe₃O₄@LDH > 1.0-

PdAu_0.48/Fe₃O₄@LDH.

A good linear relationship is achieved by plotting of lnk verse

1/T (T: temperature), and the apparent activation energy (E_a) is

calculated upon the slope by the least-square fit analysis (Fig. 6d

and Table S4). The catalyst 1.0-PdAu_0.16/Fe₃O₄@LDH shows the

lowest E_aof 19.36 kJ/mol among the three catalysts with an order

of 1.0-PdAu_0.16/Fe₃O₄@LDH < 1.0-PdAu_0.11/Fe₃O₄@LDH (27.58)

< 1.0-PdAu_0.48/Fe₃O₄@LDH (32.29). The highest k and the

lowest E_aof the catalyst 1.0-PdAu_0.16/Fe₃O₄@LDH are consistent

with its smallest PdAu NCs’ size.

2p,

Mg

2p

XPS

analyses).

Particularly,

1.0-

3.5 The structure-performance relationship.

PdAu_0.16/Fe₃O₄@LDH presents the maximum Pd⁰3d_5/2downshift,

corresponding to the most negatively charged Pd⁰, that is the

largest electron density of surface Pd⁰.

The above results clearly suggest that the magnetic PdAu alloy

nanoclusters catalysts x-PdAu_r/Fe₃O₄@LDH exhibit efficiently

catalytic performance for the Heck reaction. To understand and

reveal the structure-performance relationship, XPS, In situ CO-

DRIFTS and H₂-TPR analyses are performed to explore the

electronic and geometric structure of the as-obtained catalysts.

Though the XPS detective depth is up to 10 nm, which is

higher than the size of the PdAu alloy NCs in the obtained

magnetic catalysts, the XPS data may strongly disclose the

surface composition of the PdAu alloy catalysts [45,46]. As

shown in Table S5, the XPS-derived Au/Pd and Pd⁰/(Pd^II+Pd⁰)

molar ratios reveal the surface metal composition is affected by

the addition of Au in the as-obtained magnetic PdAu alloy NCs

catalysts. It is found that surface Au/Pd ratios of the alloy

catalysts by XPS are obviously different from those of bulk ratios

by ICP. In detail, the surface Au/Pd ratios of 1.0-

8

Journal of Organometallic Chemistry

₃ACCEPTED MANUSCRIPT

4

PdAu_0.11/Fe₃O₄@LDH and 1.0-PdAu_0.16/Fe O @LDH are 0.62

and 0.73, ca. 5.5 and 4.6 times those of the corresponding bulk

ratios, respectively, while the surface Au/Pd ratio for the 1.0-

PdAu_0.48/Fe₃O₄@LDH is only 0.22. Furthermore, the XPS-

derived Pd⁰/(Pd⁰+Pd^II) values of three catalysts 1.0-

PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16 and 0.48) upon deconvolution

are 85.0%, 93.1% and 77.9%, respectively, much higher than that

of Pd/Fe₃O₄@LDH (69.1%). It can be attributed to the influence

of alloying with Au to various degrees, in line with the findings

of Liao et al. [45] and Huang et al.[47], on the protecting Pd⁰

species from being oxidized to Pd^II. These results indicate that

the surface metallic composition of magnetic PdAu alloy NCs

catalysts can be effectively tuned by the initial Au/Pd ratio, and

the 1.0-PdAu_0.16/Fe₃O₄@LDH has the maximum Pd⁰percentage

and Au-rich surface which might form properly isolated Pd

atoms or small ensembles by Au.

To get more insight into the surface characteristics of the

series of magnetic PdAu alloy NCs catalysts, O 1s, Fe 2p and Mg

2p spectra (Fig. S7) are carefully analyzed for the catalysts

compared with the support and Fe₃O₄, and the parameters are

listed in Table S6. The curve fittings of O 1s spectra of three

catalysts 1.0-PdAu_r/Fe₃O₄@LDH (r=0.11, 0.16 and 0.48) and the

support reveal that there are three oxygen species as surface OH

(531–532 eV), lattice O^2-(520–531 eV) and adsorbed H₂O and/or

intercalated carbonate (532–534 eV) species [42]. The BE values

of OH group in the three catalysts all shift to higher levels

compared to the support, implying that the occurrence of electron

transfer from the surface OH groups related to LDH layer Mg²⁺

ions to PdAu NCs. The 1.0-PdAu_0.16/Fe₃O₄@LDH kept the

largest fraction of M-OH (70.3%) shows the highest BE increase,

suggesting the most electron transfer from M-OH to PdAu NCs,

consistent with its highest Heck reactivity.

The Fe 2p XPS spectra of three PdAu alloy NCs catalysts

1.0-PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16 and 0.48) compared with

Pd/Fe₃O₄@LDH, Au/Fe₃O₄@LDH and Fe₃O₄@LDH show two

distinct asymmetrical peaks at ~710.40 and ~724.50 eV for Fe

2p_3/2and Fe 2p_1/2, and the deconvolution BE values of Fe 2p_3/2

are 709.75 and 711.85 eV, 709.90 and 711.95 eV, 709.61 and

711.59 eV, 709.62 and 711.49 eV, 709.74 and 711.18 eV as well

as 709.60 and 711.00 eV, which can be ascribed to the Fe²⁺and

Fe³⁺species of Fe₃O₄core [42], respectively. The Fe²⁺2p_3/2and

Fe³⁺2p_3/2BE values of the support Fe₃O₄@LDH are 0.16 and

0.30 eV downshift from the Fe₃O₄(709.76 and 711.30 eV),

respectively. The slight BE downshift of Fe²⁺and Fe³⁺species in

Fe₃O₄@LDH support compared to the Fe₃O₄can be attributed to

partial charge transfer from the LDH shell to Fe₃O₄core on

various electronegativity of Fe (1.83) and Mg (1.31) elements

[48], in line with a slight upshift (0.18 eV) of Mg 2p (50.38 eV)

of the support Fe₃O₄@LDH compared to pure LDH (50.20 eV)

[28]. For the PdAu NCs catalysts 1.0-PdAu_r/Fe₃O₄@LDH (r =

0.11, 0.16 and 0.48) and Pd/Fe₃O₄@LDH and Au/Fe₃O₄@LDH,

the BE values of Fe²⁺and Fe³⁺species show varied upshift (0.15

and 0.85 eV, 0.30 and 0.95 eV, 0.01 and 0.59 eV, 0.02 and 0.49

eV as well as 0.14 and 0.18 eV, respectively) compared to

Fe₃O₄@LDH after loading PdAu alloy NCs with greater

electronegative Pd (2.20) and Au (2.54) [48], while the Mg 2p

BE values show negligible shift compared to the support. These

observations can be attributed to partial electron transfer from

Fe₃O₄core to the LDH shell, then to PdAu alloy NCs, in line

with the downshift of ~0.29 eV of Pd⁰3d_5/2and ~0.19 eV of Au

4f_7/2in 1.0-PdAu_0.16/Fe₃O₄@LDH compared to non-magnetic 1.0-

PdAu_0.16/LDH (Fig. S8 and Table S5). The above XPS data

strongly demonstrate the presence of remarkable PdAu NCs –

LDH – Fe₃O₄three-phase synergistic effect in the present

magnetic PdAu alloy NCs catalysts.

Fig. 8 In situ DRIFT spectra of CO adsorption on 1.0-

PdAu_0.11/Fe₃O₄@LDH (a), 1.0-PdAu_0.16/Fe₃O₄@LDH (b), 1.0-

PdAu_0.48/Fe₃O₄@LDH (c), Pd/Fe₃O₄@LDH and Au/Fe₃O₄@LDH.

In situ DRIFTS of CO adsorption is an important surface-

sensitive technique to probe the geometric and electronic effects

arising from the addition of a second metal, which are considered

to be the key factor influencing the catalytic activity of bimetallic

catalysts. Fig. 8 depicts DRIFT spectra of CO adsorption on the

obtained catalysts 1.0-PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16 and

0.48) compared to Pd/Fe₃O₄@LDH and Au/Fe₃O₄@LDH. For the

Au/Fe₃O₄@LDH, no band detected may be owing to the weaker

CO adsorption strength [49]. However, the Pd/Fe₃O₄@LDH

shows a sharp peak at ~2040 cm^-1and a weak one at ~1910 cm^-1,

which could be assigned to linear and two fold-bridged adsorbed

CO on metallic Pd atoms, respectively [50,51]. After alloying

with Au, the peaks of two fold-bridged adsorbed CO on Pd of the

three PdAu alloy NCs catalysts almost disappear. The peak area

ratios of linear adsorbed CO (A_l) to total of linear and two fold-

bridged CO (A_t) are 93.9%, 100% and 99.7% for the three

catalysts with r of 0.11, 0.16 and 0.48, respectively, obviously

higher than that of Pd/Fe₃O₄@LDH (83.1%). These results

illustrate that the magnetic PdAu alloy NCs catalysts are

preferred to adsorb CO molecules in linear pattern, and the 1.0-

PdAu_0.16/Fe₃O₄@LDH presents the largest A_l/A_tvalue probably

originating from isolated Pd atoms or small ensembles by Au

doping, which are favorable for the Heck reaction [50]. It is noted

that the linear adsorbed CO bands on Pd at 2033, 2015 and 2030

cm^-1of the three catalysts 1.0-PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16

and 0.48) show remarkable redshift of 7, 25 and 10 cm^-1,

respectively, compared to Pd/Fe₃O₄@LDH, suggesting the

increased electron density of the surface Pd atoms via alloying

with Au. Particularly, 1.0-PdAu_0.16/Fe₃O₄@LDH exhibits the

maximum redshift, corresponding to the most negatively charged

Pd⁰, in line with the XPS data, which explains its best Heck

activity from the electronic effect. The above analyses strongly

illustrate that alloying Pd with Au can greatly influence the

geometric structure (the added Au isolates the continuous Pd

sites) and electronic structure (neighboring Au atoms increase the

electron density of Pd) of the present PdAu alloy NCs catalysts.

strongest PdAu_rNCs–LDH–Fe₃O₄three-phase synergistic effect

ACCEPTED MANUSCRIPT

along with its smallest PdAu alloy NCs’ size, thus afford to its

best Heck reaction activity.

In order to study whether the catalytic mechanism of the

catalysts x-PdAu_r/Fe₃O₄@LDH follow

a homogeneous or

heterogeneous catalysis path, the hot filtration experiments are

performed [11,50]. In detail, two parallel tests are realized for the

Heck reaction of PhI with styrene on the catalyst 1.0-

PdAu_0.16/Fe₃O₄@LDH (Pd: 0.3 mol%) under the optimized

reaction condition. The first test proceeds until the reaction is

completed. After separating catalyst from the reaction system by

an NdFeB magnet, ICP analysis of the filtrate shows that only

0.01 ppm (corresponding to 0.05 wt% of the initial amount) Pd

leached into the solution at the end of reaction. The second test is

stopped after 0.4 h with PhI Conv. of 47.2%. After magnetic

separation, the ICP analysis of the filtrate shows that 0.30 ppm

(1.5 wt% of the initial amount) Pd leached into the solution. The

reaction is continued for an additional 1.2 h with the filtrate, and

the PhI Conv. almost remains constant. These results verify that

only Pd bound to the Fe₃O₄@LDH support during the reaction is

active, and the Heck reaction occurs on the heterogeneous

surface of the present magnetic PdAu alloy nanoclusters catalysts.

On the basis of all the Heck reaction results and related

catalysts characterization data along with previous findings

[2,11], we tentatively propose

a possible Heck coupling

mechanism of aryl halides (Ar-X) with alkenes on the present

magnetic PdAu alloy NCs catalysts as shown in Scheme 2.

Firstly, the PdAu alloy NCs undergo oxidative addition with aryl

halides to form a surface transient organometallic intermediate

(STOI) Ar-PdAu_r-X/Fe₃O₄@LDH, as is evident from XPS and

TGA-DTA-MS on the intermediate. The Pd 3d, I 3d and C 1s

XPS spectra of a typical STOI CH₃Ph-PdAu_0.16-I/Fe₃O₄@LDH

(experimental details see ESI) are shown in Fig. S9. The Pd 3d

XPS spectrum of the STOI shows two broaden overlapping peaks

for Pd 3d_5/2at ~336.02 eV and Pd 3d_3/2at ~339.85 eV. The

deconvolution of Pd 3d_5/2XPS gives 335.00 eV assigned to Pd⁰

and 337.02 eV to Pd^II, featured as higher BE values (∆E = ~0.2

eV) and much higher Pd^IIproportion (45.5%) than the fresh one

(6.9%), resulting from the presence of the Pd-I in the formed

STOI [11]. The I 3d XPS shows two sharp peaks for I 3d_5/2at

618.96 and I 3d_3/2at 630.36 eV, respectively, assigned to the Pd-I

bond [43]. The C 1s XPS shows two lines at 284.99 and 288.36

eV, which may be assigned to a carbon impurity and Pd-C,

Fig.

9

H₂TPR of 1.0-PdAu_0.11/Fe₃O₄@LDH (a), 1.0-

PdAu_0.16/Fe₃O₄@LDH (b), 1.0-PdAu_0.48/Fe₃O₄@LDH (c),

monometallic catalysts, and the support Fe₃O₄@LDH.

Fig. 9 presents H₂TPR of the PdAu alloy NCs catalysts 1.0-

PdAu_r/Fe₃O₄@LDH (r = 0.11, 0.16 and 0.48) compared to

Pd/Fe₃O₄@LDH, Au/Fe₃O₄@LDH, and Fe₃O₄@LDH. The

monometallic Au/Fe₃O₄@LDH only shows a weak broadened

hydrogen consumption peak at 267 ^oC, which may be ascribed to

reduction of Au⁺to Au⁰species [52], implying the presence of

small amount of Au⁺state, in line with the XPS data. The

Pd/Fe₃O₄@LDH displays a single negative peak at 72 ^oC

resulting from dissociation of β-Pd hydride formed by the

hydrogen adsorbing on Pd⁰when the pressure of hydrogen

surpasses 0.02 atm [45,53], and a strong broad hydrogen

consumption peaks at 180 ^oC with H₂consumptions of 30 µmol/g

attributed to the reduction of Pd^IIto Pd⁰[45]. The broad shoulder

at 245 ^oC can be ascribed to the reduction of the ferric species on

the magnetic support, which occurred at much lower temperature

respectively [11]. The TGA-DTA-MS study of CH₃Ph-PdAu_0.16

-

I/Fe₃O₄@LDH gives m/z values of 91 and 127 amu

corresponding to CH₃Ph and iodide species, respectively (Fig.

S9), while the m/z signal corresponding to CH₃-PhI cannot be

detected, indicating that the pyrolysis products are obtained from

this intermediate. The above results further demonstrate that the

Heck reaction indeed occurs on the heterogeneous surface of the

magnetic PdAu alloy NCs catalysts. It should be mentioned that

at this stage, both the neighboring Au atoms and the hierarchical

structured Fe₃O₄@LDH support contribute to the increase of the

electronic density of Pd centers to facilitate the oxidative addition

of aryl halides on the Pd⁰, which is responsible for the higher

Heck reaction activity. Secondly, the reaction proceeds by

coordination of alkene to the STOI, followed by its syn-

migratory insertion. Subsequently, the newly generated

organometallic species undergoes syn β-hydride elimination to

form the Heck coupling product. Finally, base-assisted

elimination of H-X from H-PdAu_r-X/Fe₃O₄@LDH species occurs

to regenerate the Pd⁰catalyst, and the intrinsic basic sites of the

LDH phase may promote this elimination to some extent, as

revealed in previous report [58].

o

than normally expected (350 – 950 C ) [54-56], ascribed to that

Pd⁰species resulted from initial reduction facilitates dissociative

chemisorption of H₂which has enough reducibility for the ferric

species [57]. After alloying with Au, the catalysts 1.0-

o

PdAu_r/Fe₃O₄@LDH show negative peaks around 75 C with a

slight shift compared with the monometallic Pd/Fe₃O₄@LDH,

and the peak gradually weakens with increasing Au/Pd ratio and

even disappears at

r = 0.48. The two catalysts 1.0-

PdAu_r/Fe₃O₄@LDH with r = 0.11 and 0.48 show H₂consumption

peaks owing to the reduction of Pd^IIto Pd⁰at varied temperatures

o

of 150 and 183 C, with lower H₂consumptions of 17 and 23

µmol/g than Pd/Fe₃O₄@LDH, respectively, suggesting various

interactions occurring between Pd and the added Au. However,

the catalyst 1.0-PdAu_0.16/Fe₃O₄@LDH displays a very weak H₂

o

consumption peak at ~180 C with much lower H₂consumption

(4.7 µmol/g) corresponding to its lowest Pd²⁺fraction (XPS

indicated) associated with its proper Au doping protecting Pd⁰

species from being oxidized to Pd^IIand stronger Pd-Au

interaction.

The above results strongly demonstrate that the catalyst 1.0-

PdAu_0.16/Fe₃O₄@LDH possesses the most negatively charged Pd⁰,

the maximum Pd⁰percentage and Au-rich surface and the

10

Journal of Organometallic Chemistry

attributable to the highly dispersed ultrafine PdAu NCs,

ACCEPTED MANUSCRIPT

increased surface Pd⁰electron density and remarkable enhanced

PdAu NCs – LDH – Fe₃O₄three-phase synergistic effect.

Particularly, 1.0-PdAu_0.16/Fe₃O₄@LDH with the smallest PdAu

alloy NCs and the largest surface Pd⁰electron density exhibits

the highest Heck activity. Moreover, the obtained catalysts can be

applied for Heck reaction of a variety of aryl halides with alkenes,

and can be simply separated by an external permanent magnet

and reused after ten runs without significant loss of activity,

rendering its long-term stability. The present hierarchically

core@shell structured magnetic PdAu alloy nanoclusters

catalysts may open

a

new vision to develop novel

environmentally benign magnetic bimetallic alloy catalysts for

varied catalytic applications.

Acknowledgements

The work was supported by the National Natural Science

Foundation of China (21576013).

Scheme 2. Plausible mechanism for the Heck reaction of aryl

halides with alkenes over the x-PdAu_r/Fe₃O₄@LDH catalysts.

Appendix A. Supplementary material

Supplementary data associated with this article can be found in

the online version, at http://dx.doi.org/

3.6 Recyclability

The recyclability of the magnetic PdAu alloy NCs catalysts is

further explored in the Heck reaction of PhI (1 mmol) and styrene

(1.5 mmol) over 1.0-PdAu_0.16/Fe₃O₄@LDH (Pd: 0.3 mol%) and

K₂CO₃(3 mmol) in DMF-H₂O solvent. After completion of the

reaction, the catalyst was magnetically separated from the

reaction mixture by an NdFeB magnet (0.15 T), thoroughly

washed with DMF and H₂O, and then reused under identical

conditions and the results show 95% PhI Conv. and 90% trans-

stilbene Sel. even in the 10th run (Fig. S10a). The SEM image of

the recovered catalyst distinctly depicts the well-kept

honeycomb-like core@shell structure (Fig. S10b). Moreover,

only 3.5 wt% of total Pd and 3.3 wt% of total Au leached into the

supernatant as confirmed by ICP (detection limit of 0.01 ppm) of

the used catalyst after 10 runs. The Pd⁰/(Pd^II+ Pd⁰) ratio

calculated from the Pd 3d XPS of the used catalyst (Fig. S10c) is

69.0%, significantly lower than that of the fresh one (93.1%),

which can be ascribed to the high reactivity of PhI molecules that

results in the high number of surface species, similar to

Choudary’s report on LDH-Pd⁰[11]. The surface Au/Pd ratio of

~0.77 (Fig. S10c and d) is close to that of the fresh one (0.73),

demonstrating that the surface metal composition of PdAu_0.16

alloy NCs was well-kept during the Heck reaction. These results

clearly suggest the remarkable structure stability and recycling

efficiency of the as-obtained magnetic PdAu alloy NCs catalysts.

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ACCEPTED MANUSCRIPT

Facile synthesis of magnetic recyclable palladium-gold alloy

nanoclusters catalysts PdAu_r/Fe₃O₄@LDH and its catalytic

applications in Heck reaction

Jin Li^‡, Yajuan Wang^‡, Shunwang Jiang and Hui Zhang^*

Highlights

· Magnetic PdAu nanoclusters catalysts via facile polyol reduction-sol immobilization

· The ultrafine 1.0-PdAu_0.16/Fe₃O₄@LDH showing the highest Heck activity

· Electron-rich suface Pd⁰species and the strongest PdAu NCs−LDH−Fe₃O₄synergy

· Remarkable recycling efficiency upon magnetic separation

Article Doi

Facile synthesis of magnetic recyclable palladium-gold alloy nanoclusters catalysts PdAur/Fe3O4@LDH and its catalytic applications in Heck reaction

DOI: 10.1016/j.jorganchem.2018.10.007

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Article abstract of DOI:10.1016/j.jorganchem.2018.10.007

Full text of DOI:10.1016/j.jorganchem.2018.10.007

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