rGO@CoAl-LDH via

Subscriber access provided by Eastern Michigan University | Bruce T. Halle Library

Article

Nanosheet Array-Like Palladium-Catalysts Pdx/ rGO@CoAl-LDH via Lattice Atomic-Confined in Situ Reduction for Highly Efficient Heck Coupling Reaction Yanna Wang, Liguang Dou, and Hui Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11695 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Nanosheet Array-Like Palladium-Catalysts Pdx/rGO@CoAl-LDH via Lattice Atomic-Confined in Situ Reduction for Highly Efficient Heck Coupling Reaction

Yanna Wang, Liguang Dou, and Hui Zhang*

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

*Correspondence should be addressed to Hui Zhang. Email: [email protected] Tel: +8610-6442 5872 Fax: +8610-6442 5385.

1 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: A series of novel nanosheet array-like catalysts Pdx/rGO@CoAl-LDH (x = 0.0098 - 1.9, refers to Pd loading in wt% on ICP, rGO: reduced graphene oxide, LDH: layered double hydroxide) were first prepared via a simple and green lattice atomic-confined in situ reduction of oxidative Pd precursors by the evenly atomic-dispersed reductive Co2+ sites on LDH layers of a nanohybrid rGO@CoAl-LDH with hexagonal LDH nanoplates (~73 nm × 7 nm) interdigitated vertical to the surfaces of rGO layer in both sides, fabricated through a simple citric acid-assisted aqueous-phase coprecipitation method. The as-obtained Pd catalysts possess clean Pd nanoclusters (NCs) with tunable sizes in 1.3 - 1.8 nm on varied Pd loadings. All the Pdx/rGO@CoAl-LDH catalysts show excellent activities for the Heck reaction, and the Pd0.0098/rGO@CoAl-LDH with the ultrafine Pd NCs of 1.3 ± 0.2 nm yields a maximum turnover frequency of 160000 h-1 over a heterogeneous catalyst so far. The excellent activities can be attributed to the ultrasmall Pd NCs with high dispersion and clean Pd surfaces, increased electron transfer capacity and surface area, and remarkable Pd—CoAl-LDH—rGO three-phase synergistic effect of the present unique nanosheet array-like Pd NCs catalysts. Moreover, the catalyst Pd0.33/rGO@CoAl-LDH shows a broad range of substrate applicability and can be reused more than five runs without obvious loss of activity, giving the present catalysts long-term stability. These findings make the rGO@CoAl-LDH hybrid prepared by a facile and scalable synthesis route a universal green platform to support other noble or non-precious metal NCs via lattice atomic-confined in situ reduction strategy to construct more desired heterogeneous catalysts. KEYWORDS: nanosheet array-like Pdx/rGO@CoAl-LDH catalysts, lattice atomic-confined in situ reduction, Heck reaction, excellent activity, Pd—CoAl-LDH—rGO three-phase synergistic effect


Page 2 of 40

Page 3 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION The transition metal Pd-catalyzed Heck coupling reaction of aryl halides with alkenes is one of the most important routes for C–C bond forming processes in the large-scale production of dyes, pharmaceuticals and functional materials.1-6 The impact of Heck coupling reaction on the way to build molecules today was soundly recognized with the 2010 Nobel Prize in Chemistry, enjoying enthusiastic attention from the academic and industrial world.7 Nowadays, the catalysts of Heck coupling reaction have been an ongoing research hotspot.8 Supported heterogeneous Pd catalysts, commonly as immobilised Pd nanoparticles (NPs) on supports including polymers,9,10 carbon,11 metal-organic frameworks (MOFs),12 metal oxides,13 and layered double hydroxide (LDH),14,15 have caught wide interests due to their straightforward separation from the reaction solution and good recyclability,5,16 and the nature of supports usually has a significant influence on the catalytic activity. Choudary et al.14 first reported Mg-Al LDH supported Pd0 catalyst by an exchange of Cl- in LDH for PdCl42- followed by hydrazine hydrate reduction. The as-obtained LDH-Pd0 catalyst (4 - 6 nm) exhibited a higher activity compared with other acidic or weakly basic supported Pd catalysts in an order of LDH-Pd0 > resin-Pd0 > Pd/C > Pd/Al2O3 > Pd/SiO2 in Heck coupling of 4-chloroanisole with styrene, which was attributed to the basic LDH providing sufficient electron density to Pd0 species to promote the oxidative addition with 4-chloroanisole. Goodman et al.17 studied the size effects in the synthesis of vinyl acetate on Pd/SiO2, and the Pd-1 (2.5 nm) showed a relatively superior activity and selectivity compared with Pd-5 (4.0 nm). Partha et al.12 prepared a series of Pd@MOF catalysts by NaBH4 reduction presenting increased iodobenzene conversion (62 - 74%) with decreased Pd NPs (2.8 to 1.5 nm) during the initial 6 h of the Heck reaction. Obviously, the small-sized Pd NPs will greatly facilitate the improvement of its catalytic activity. However, the above



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 40

mentioned Pd catalysts hardly possess highly dispersed tiny Pd species with clean surfaces due to the addition of reducing agents in the preparation process, which is not beneficial to fully expose the catalytic active sites. Consequently, it is highly desired to develop a support with homogeneously distributed reductive sites for in situ reduce oxidative Pd2+ to obtain well dispersed tiny Pd0 nanoclusters. LDH,

also

known

as

hydrotalcite,

has

been

widely applied

in

adsorption,18

electrochemistry,19 drug delivery,20 and catalysis areas14,15,21,22 due to their adjustable compositions, high thermal stability and intrinsic basicity. Particularly, Co-based LDH becomes a great candidate for creating Pd nanoclusters via in situ reduction by its evenly distributed reductive Co2+ sites on the LDH layers.15,23 While enhancing the Pd electron density is critical to improve the catalytic activity. However, the electron transfer capability from pure Co-based LDH to Pd has a large room for improvement. Graphene is preferentially chosen as a substrate to hybridize with LDH owing to its superior electrical conductivity, chemical stability and extremely large specific surface area, which can endow the hybrids with versatile properties.24-26 In recent years, the LDH/rGO hybrids have been widely used in electrochemistry27-31 and heterogeneous catalysis areas.32,33 For instances, Cai et al.27 prepared NiCo-LDH/RGO hybrids via one-pot solvothermal route showing excellent supercapacitive properties owing to the presence of graphene-based conductive network. Ma et al.32 reported Ni2/3Fe1/3-LDH/rGO superlattice composites via heteroassembling exfoliated NiFe LDH nanosheets and rGO layers, which exhibited much better performance than LDH nanosheets alone in oxygen evolution reaction. Nevertheless, the LDH in these hybrids reported usually presented randomly horizontal orientation to the surface of rGO layers. Generally, hierarchical structure of hybrids with high surface area and mesoporous may greatly enhance not only the electron transfer but also mass


Page 5 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transportation and diffusion in the reaction process.15,34 Most recently, our group33 reported a series of hierarchically structured nanoarray-like hybrids CuxMg3-xAl-LDH/rGO with ~2-fold specific surface area of pure Cu-LDH, via aqueous-phase coprecipitation, exhibiting much higher catalytic activity than Cu-LDH in reduction of 4-nitrophenol. Based on above reported studies, highly dispersed tiny Pd nanoclusters can be expected via a simple lattice atomic-confined in situ reduction upon the atomic-dispersed reductive Co2+ sites on LDH layers of the rGO@CoAl-LDH hybrid. As far as we know, no work has yet been devoted to preparing hierarchically structured nanoarray-like catalysts Pdx/rGO@CoAl-LDH, far from their use in heterogeneous catalysis and further revelation of structure-activity relationship. Herein, a series of gram-level nanoarray-like catalysts Pdx/rGO@CoAl-LDH were first prepared via a simple and green lattice atomic-confined in situ reduction strategy (Figure 1), and systematically characterized. We deeply study the size control effect of varied Pd loadings on Pd nanoclusters (NCs) and the influence of morphology, Pd loading and dosage of the present Pd NCs catalysts on Heck reaction activity. The substrate applicability, stability and essential correlation between high activity and structure of the as-synthesized catalysts are also profoundly explored. We choose a hierarchically structured hybrid support, rGO@CoAl-LDH, as a choice of the carrier, not only to stabilize the Pd NCs but also to afford a well chemical environment of nucleation and growth of the active Pd species with a clean surface by abundant Co2+ sites, which can facilitate the adequate exposure of Pd(0) active sites. Therefore, the as-synthesized hierarchically structured nanoarray-like catalysts Pdx/rGO@CoAl-LDH with highly dispersed tiny Pd NCs are expected to exhibit excellent activity in desired catalytic reaction. Figure 1.

2. EXPERIMENTAL SECTION



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2.1 Chemicals. Natural graphite powder (325 mesh) was purchased from Qingdao Huatai Lubricant Sealing S&T Co. Ltd. Co(NO3)2∙6H2O and Al(NO3)3∙9H2O were purchased from Xi Long Chemical Limited Corporation (Guangzhou, China). NaOH, Na2CO3, K2CO3, KCl and N, N-Dimethyl Formamide (DMF) were purchased from Beijing Chemical Works (China). PdCl2 and styrene were obtained from Tianjin Fuchen Chemical Reagents factory (China). Iodobenzene bromobenzene, chlorobenzene, methyl acrylate and ethyl acrylate were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China). The deionized water with resistivity > 18.25 MΩ·cm (25 o

C) was used in all experiments. 2.2 Synthesis of rGO@CoAl-LDH Hybrid Support. Graphite oxide (GO) colloid was

obtained by the modified Hummers method.35,36 A three-dimensional (3D) hierarchical hybrid support rGO@CoAl-LDH (rGO: reduced graphene oxide, LDH: layered double hydroxide) was fabricated by an aqueous-phase coprecipitation route developed in our Lab.33 Typically, 50 mg of GO colloid (6.18 mg/mL) was first ultrasonically dispersed in 100 mL water for 20 min at room temperature to get an exfoliated GO suspension. Then, 0.2 g citric acid (CA) was dissolved in the above exfoliated GO suspension by ultrasonication for another 5 min to give a CA-modified GO suspension. Two hundred milliliter basic solution with NaOH (64 mmol) and Na2CO3 (20 mmol) was slowly added into the above CA-modified GO suspension under mechanical stirring until the pH = 10.0 ± 0.1 and kept for 10 min for stabilization. Subsequently, 100 mL salt solution containing Co(NO3)2·3H2O (15 mmol) and Al(NO3)3·9H2O (5 mmol) and the above mixed basic solution were simultaneously added into the above suspension under mechanical stirring at 25 oC keeping constant pH at 10 ± 0.1. Finally, the resulting slurry was aged at 65 oC for 4 h, then centrifuged and washed with water for several times, and then by freeze-drying, resulting in a hierarchically structured hybrid rGO@CoAl-LDH. Pure CoAl-LDH


Page 6 of 40

Page 7 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Co/Al = 3) support was obtained by a similar method with no addition of GO and CA. A non-hierarchically structured hybrid rGO@CoAl-LDH-h (h refers to horizontal) was obtained by a common coprecipitation method reported by Li et al.37 with slight modification. Typically, 100 mg of the initial GO colloid (6.18 mg/mL) was first added to 50 mL mixed salt solution of Co(NO3)2·3H2O (22.5 mmol) and Al(NO3)3·9H2O (7.5 mmol) followed an ultrasonication of 30 min to get a uniform suspension. Then, a certain amount of basic solution with NaOH (0.8 M) and Na2CO3 (0.2 M) was added into the suspension drop by drop under mechanical stirring until pH = 10.0 ± 0.1. Finally, the resultant was aged at 65 oC for 4 h, then centrifuged and thoroughly washed with water, and followed by freeze-drying, giving a non-hierarchically structured hybrid support rGO@CoAl-LDH-h. 2.3

Synthesis

of

Pdx/rGO@CoAl-LDH

Catalysts.

A

series

of

nanoarray-like

palladium-catalysts were prepared by lattice atomic-confined in situ reduction strategy. Briefly, the 0.5 g rGO@CoAl-LDH hybrid was added into 300 mL water under vigorous stirring and kept for 30 min to give a uniform suspension. Then, proper volume of K2PdCl4 aq. solution (0.0564 M, obtained by dissolving 1 g PdCl2 and 0.84 g KCl into 100 mL H2O) based on nominated Pd loadings of 0.02, 0.5, 1, 1.5 and 2.5 in wt% was added into the above suspension with mechanical stirring. After reaction of 10 h at room temperature, the resultant was centrifuged, washed with water for four times, and followed by freeze-drying, giving a series of Pdx/rGO@CoAl-LDH catalysts (x = 0.0098, 0.33, 0.6, 1.2, 1.9, is the Pd loading in wt% upon ICP). The control catalysts Pd0.56/rGO@CoAl-LDH-h, Pd0.92/CoAl-LDH and Pd2+0.78/GO (Pd2+ existed due to lack of reducing agent during the synthesis) were prepared with similar procedure according to the nominated Pd loading of 1.0 wt%.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2.4 Characterization. Characterization analysis methods including X-ray diffraction (XRD), Raman spectra, Fourier transform infrared (FT-IR) spectra, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) with fast Fourier transform (FFT), atomic force microscopy (AFM), inductively coupled plasma atomic emission spectroscopy (ICP-AES), N2 adsorption-desorption isotherms, surface area measurements, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with EDS and X-ray photoelectron spectroscopy (XPS) were the same as reported in detail in our previous work.33 NMR spectra were recorded on a Brucker AV600 NMR spectrometer (Germany) operating at 600.13 MHz. All of the 1H NMR and

13

C NMR spectra were measured in CDCl3 with TMS as the internal

standard. Pd dispersion degrees of the catalysts were estimated from CO-chemisorption tests on Micromeritics Auto Chem II 2920, which were conducted at 323 K by a dynamic pulse way on samples pre-reduced in H2 at 393 K.38 Typically, the samples of ca. 100 mg are first pre-treated at 393 K with the heating rate of 10 K/min in a 10% (v/v) H2/Ar stream of 40 mL/min and activated for 1 h. Then the sample was cooled down to 323 K in He flow (40 mL/min) with ~7 min and kept for another 23 min to remove the physisorpted H2. Then, a given amount of 5% CO/He mixture were passed over the sample from a 52 µL loop per 5 min until the intensity of the peak reached a constant value. A CO/Pd average stoichiometry of 1 was assumed for calculation of the dispersion.39 The Pd dispersion degree D (%) =

V  M  103  100 22.4  W  P

where V is the consuming volume of CO (mL), M is the relative molecular mass of Pd (106.42 g/mol), W is the catalyst mass (g), and P is the Pd mass fraction of the catalyst. 2.5 Activity Test. The Heck reaction of aryl halide and styrene or its derivative was


Page 8 of 40

Page 9 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


performed in a similar route like our previous work.22 DMF (12 mL), H2O (4 mL), K2CO3 (3 mmol), aryl halide (1 mmol), styrene or its derivative (1.2 mmol) and a proper amount of catalyst were added in the flask and then heated to setting reaction temperature at atmospheric pressure. The products were analyzed by GC. And all the coupling products were characterized by 1H NMR and 13C NMR spectra, which were attached in the end of the Supporting Information. 2.6 Recycling Procedure. After the Heck reaction, the catalyst was reclaimed by centrifugation, washed sequentially and thoroughly with ethanol and water followed by freeze-drying, then reused in the next run without further purification. 3. RESULTS AND DISCUSSION 3.1

Structure

and

Morphology

Characterizations.

The

XRD

patterns

of

Pd0.6/rGO@CoAl-LDH, compared to Pd0.56/rGO@CoAl-LDH-h, Pd0.92/CoAl-LDH, Pd2+0.78/GO and corresponding supports are shown in Figure 2A. The GO (Figure 2A(d0)) presents a sharp diffraction at ~11o indexed to (001) plane with a interlayer spacing of ~0.77 nm, much larger than pristine graphite of ~0.34 nm, being ascribed to the formation of oxygenated groups on the graphite sheets.40 The hybrids rGO@CoAl-LDH and rGO@CoAl-LDH-h as well as pure CoAl-LDH (Figure 2A(a0-c0)) show well-defined diffractions at ~11o (003), 23o (006), 34o (012), 39o (015), 46o (018), 61o (110) and 62o (113) of a typical hexagonal Co6Al2CO3(OH)16·4H2O phase (JCPDS no. 51-0045) without other peaks, suggesting the presence of pure LDH phase. The clear (110) and (113) peaks demonstrate well-dispersed Co2+ and Al3+ metal cations on the LDH layers,33 which may facilitate lattice atomic-confined in situ reduction of the oxidative palladium precursor by such atomic-level dispersed active Co2+ sites. However, the d003 values of the hybrids rGO@CoAl-LDH (0.7425 nm) and rGO@CoAl-LDH-h (0.7454 nm) are slightly smaller than pure CoAl-LDH (0.7456 nm) (Table S1 in the Supporting Information), which may



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 40

be related to the effective hybridization between CO32-_intercalated CoAl-LDH and rGO layer. Meantime, no diffraction indicative of GO or rGO detected in the hybrids clearly indicates an effective prevention of the GO from restacking during the hybrid synthesis process and a complete exfoliation of rGO in the obtained hybrid.41 It is worth noting that the intensity ratio of (110) and (003) peaks (I110/I003) of the rGO@CoAl-LDH (0.29) is higher than that of rGO@CoAl-LDH-h (0.18) and CoAl-LDH (0.24), suggesting the possibly oriented growth of LDH nanosheets with the ab-face perpendicular to rGO layer on both sides.33 Moreover, rGO@CoAl-LDH has broader diffractions compared to rGO@CoAl-LDH-h and CoAl-LDH, indicating the smaller size of the LDH crystals in the former, which is in agreement with its Scherrer dimension D003 (6.29 nm) and D110 (17.59 nm), smaller than those of rGO@CoAl-LDH-h (15.08, 18.79 nm) and CoAl-LDH (10.27, 21.64 nm) (Table S1 in the Supporting Information). This phenomenon may be due to the nucleation process of LDH is strongly affected by multiple interactions between LDH and rGO including electrostatic attraction and van der Waals force as well as the complexation of citric acid, further giving a small-sized LDH nanocrystals.33,41 All the obtained Pd catalysts (Figure 2A(a-d)) display similar XRD characteristics to the corresponding supports without diffractions of Pd crystal phase, implying the presence of small-sized and highly dispersed Pd species in the as-synthesized Pd catalysts. The microstructural characteristics of the as-synthesized Pd0.6/rGO@CoAl-LDH compared to Pd0.56/rGO@CoAl-LDH-h, Pd0.92/CoAl-LDH, Pd2+0.78/GO and corresponding supports are further explored by the Raman spectra (Figure 2B). For pure CoAl-LDH (Figure 2B(c0)), two Raman bands can be observed at ca. 455 and 528 cm-1 assigned to the Co-O and Al-O symmetric stretching vibrations, respectively.42 However, no bands indicative of the LDH phase detected in


Page 11 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rGO@CoAl-LDH and rGO@CoAl-LDH-h (Figure 2B(a0, b0)) indicate the high dispersion of LDH nanosheets on rGO layers. Two strong bands at ca. 1350 and 1590 cm-1 can be clearly seen in rGO@CoAl-LDH and rGO@CoAl-LDH-h as well as GO (Figure 2B(d0)), corresponding to the D and G bands that associated with the defects in the carbon materials and in-plane vibration of sp2 carbon atoms, respectively.43,44 Generally, the intensity ratio of the D to G band (ID/IG) is a measure of disorder and the average size of the in-plane sp2 region in graphitic materials.In our work, the higher ID/IG value (1.02) of both hybrids compared to GO (0.94) can be attributed to the decrease of sp2 regions and the existence of unrepaired defects created on reduction of GO to rGO with the removal of oxygen functional groups during the synthesis process of the hybrids.41 These sufficient defective sites may facilitate the growth of CoAl-LDH onto rGO layer and therefore prevent the LDH crystals from self-agglomeration, leading to the hybrids with LDH nanosheets uniformly grown on rGO layer. All the Pd catalysts (Figure 2B(a-d)) exhibit the similar Raman features to their supports, indicating considerably stable microstructural characteristics of the Pd catalysts by lattice atomic-confined in situ reduction strategy. Figure 2.

The FT-IR spectra of the as-obtained Pd catalysts and corresponding supports (Figure S1) are shown in the Supporting Information. The GO (Figure S1B(d0)) shows clear stretching vibrations at ~1725, 1620, 1421 and 1049 cm-1, which can be assigned to C=O (carboxylic acid), C=C/C-C (carbon skeleton), C-OH (carboxyl) and C-O (alkoxy), respectively.33 However, both hybrids rGO@CoAl-LDH and rGO@CoAl-LDH-h (Figure S1B(a0, b0) in the Supporting Information) display even disappeared peaks corresponding to the above-mentioned oxygenated groups of C=O, C-OH and C-O, demonstrating the effective reduction of GO.43 While the peaks at ~3465, 1365 and 867 cm-1 (Figure S1B(a0-c0) in the Supporting Information) can be assigned to the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 40

typical v(OH), strong v3(symmetrical stretching) and weak v2(out-of-plane deformation) vibrations of CO32- in LDH phase, respectively, and the bands appeared at low-frequency region below 800 cm-1 belong to the vibrations of M-O and M-OH in LDH lattice.33,44,45 Note that the peak of carbon backbone observed in rGO@CoAl-LDH is redshifted to 1551 cm-1 compared to GO (1620 cm-1), implying a possibly strong interaction between LDH and rGO. All the Pd catalysts (Figure S1A(a-d) in the Supporting Information) exhibit the similar peaks to their corresponding supports , but the v(OH) at ~3393 cm-1 occurs obvious red shift compared to the supports (~3465 cm-1), which may be due to the strong interaction of -OH and Pd particles (later XPS confirmed). The morphological features of the fabricated Pd catalysts upon SEM, TEM and AFM results are shown in Figure 3. The SEM images show that all the Pd catalysts (Figure 3(A, D, F, G)) present well-maintained morphology compared to their supports (Figure S2 in the Supporting Information). In detail, the Pd0.6/rGO@CoAl-LDH (Figure 3A) clearly shows the well-crystallized LDH nanoplates (~73 nm × 7 nm) grown orderly with ab-plane staggered perpendicular to the both sides of rGO layer, giving a hierarchical nanoarray-like hybrid with an average thickness of ~123 nm (yellow arrows marked) and many interstitial caves of ~35-45 nm. The TEM of Pd0.6/rGO@CoAl-LDH (Figure 3B) further confirms the hierarchically structured nanoarray-like morphology of the sample. Moreover, compared with the light grey rGO substrate (blue arrow pointed), the vertically oriented LDH nanoplates of ~65 nm × 7.5 nm can be frequently observed in both sides of rGO layer as solid and dotted red arrows indicated. The AFM 3D phase image (Figure 3C) shows two distinct phase features with the upper bright LDH nanoplate domains and the bottom deep coloured rGO layer. And many distinguished dark caves with diameter of ~30 nm can be seen in the 2D phase image (inset in Figure 3C ) owing to the


Page 13 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interdigitated vertical LDH nanosheets on rGO layer, in line with the SEM and TEM results. The AFM topography image of the exfoliated GO on a silicon wafer displays a thickness of ~1 nm (Figure S3(a) in the Supporting Information), as shown in the whole scanning line, indicative of the single-layer feature of GO,36,46 demonstrating the single-layer feature of GO in the present synthesis process of the hybrid. This result also implies the single-layer feature of rGO in the hybrid support and the followed Pd catalysts, as the AFM of the rGO layer obtained by treating rGO@CoAl-LDH with 2 M HCl for 2 days indicated (Figure S3(b) in the Supporting Information),

similar

to

our

previous

work

on

Cu1Mg2Al-LDH/rGO.33

As

for

Pd0.56/rGO@CoAl-LDH-h (Figure 3(D, E)), it displays a very different morphology featured as most LDH nanoplates (~47 nm × 11 nm) horizontally distributed on the surface of rGO layer in a random manner. While the Pd0.92/CoAl-LDH (Figure 3F) shows a typical “sand-rose” morphology with much bigger and thicker LDH nanoplates (~145 nm × 15 nm) compared to the hybrid catalysts, implying that the rGO layer can greatly weaken the particle–particle interactions often occurred between the LDH nanoparticles.47 As expected, Pd2+0.78/GO only shows the thin and curled GO layers (Figure 3G). The EDS of Pd0.6/rGO@CoAl-LDH, Pd0.56/rGO@CoAl-LDH-h and Pd0.92/CoAl-LDH all display the signals of Co, Al, C, O and Pd elements with Pd contents of 0.5, 0.7, 1.0 wt%, respectively, similar to their ICP data. While Pd2+0.78/GO shows the signals of C, O and Pd (0.73 wt%). Above results clearly suggest the successful

synthesis

of

hierarchical

nanoarray-like Pd0.6/rGO@CoAl-LDH by lattice

atomic-confined in situ reduction strategy, which may provide more accessible active Pd sites. Figure 3.

The microstructure of the serial nanoarray-like catalysts Pdx/rGO@CoAl-LDH (x = 1.9, 1.2, 0.6, 0.33, 0.0098) is further disclosed by HRTEM. All the Pdx/rGO@CoAl-LDH catalysts (Figure 4(A, C, E, G, I)) show that Pd nanoclusters (NCs) with size < 2 nm are highly dispersed



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 40

on CoAl-LDH nanoplates though extremely few Pd NCs may be located on rGO layer. And the high-magnification HRTEM images (Figure 4(B, D, F, H, J)) of all the catalysts display typical fcc Pd crystal with lattice fringes of ~0.223 and 0.195 nm indexed to (111) and (200) planes, respectively. Meanwhile, the lattice fringes of 0.253 and 0.203 nm corresponding to the (012) and (018) planes of LDH crystal are also observed. For the samples with Pd loadings in the range of 1.9 - 0.6 wt%, the mean diameters of Pd NCs are ~1.8 ± 0.5 nm with narrow distribution, possibly owing to the far excessive number of reductive Co2+ sites on the hybrid compared to oxidative Pd2+ precursors, facilitating the uniform reduction of Pd2+ species in this loading range. Further reducing the Pd loadings to 0.33 and 0.0098 wt%, the mean diameters of Pd NCs decrease to 1.6 ± 0.6 and 1.3 ± 0.2 nm, respectively, which may be due to the greatly improved dispersion of Pd with the even low Pd loadings. Especially for Pd0.0098/rGO@CoAl-LDH, the the FFT image (inset in Figure 4J ) show a weak dispersive ring instead of several bright diffraction spots for other Pdx/rGO@CoAl-LDH catalysts (x = 1.9, 1.2, 0.6, 0.33, insets in Figure 4(B, D, F, H)), also

indicating

the

presence of

ultrafine Pd NCs

in

this

catalyst.

While

Pd0.56/rGO@CoAl-LDH-h and Pd0.92/CoAl-LDH (Figure S4(A-D) in the Supporting Information) show much bigger Pd nanoparticles of 3.5 ± 1.2 and 2.7 ± 0.4 nm, respectively. Then for Pd2+0.78/GO, no Pd particles are observed owing to the lack of reducing agents in its synthesis process (Figure S4E in the Supporting Information). Above results clearly demonstrate that the hierarchically structured support rGO@CoAl-LDH is quite in favour of producing tiny Pd NCs. Intriguingly, it can be seen that Pd NCs are preferentially tend to distribute on the border of LDH nanoplates (blue dotted ellipse marked in Figure 4(A, C, E, G, I)) possibly owing to numerous pending M-OH groups at the border of the LDH nanoplates.22 Particularly, the significant edge effect with almost all Pd NCs distributed on the edge of the LDH can be seen when the Pd


Page 15 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


loadings decreased to 0.0098 wt%, implying a possible existence of strong metal-support interaction. Meanwhile, the tiny Pd NCs in these catalysts may expose more active sites. Both of the above features may significantly promote the catalytic performance of the serial hierarchically structured nanosheet array-like catalysts Pdx/rGO@CoAl-LDH, especially Pd0.0098/rGO@CoAl-LDH. Figure 4.

The

N2

adsorption-desorption

isotherms

of

the

catalysts

Pd0.6/rGO@CoAl-LDH,

Pd0.56/rGO@CoAl-LDH-h, Pd0.92/CoAl-LDH and hybrid support rGO@CoAl-LDH (Figure S5 in the Supporting Information) present similar type IV isotherms (the IUPAC classification) with broad H3 hysteresis loops (p/p0 > 0.4) due to the slit-shaped mesoporous structure resulting from the aggregates of plate-like particles.48 The SBET values of the catalysts Pd0.6/rGO@CoAl-LDH (173.3

m2

g-1,

similar

to

183.5

m2

g-1

of

its

nanoarray-like

support)

and

Pd0.56/rGO@CoAl-LDH-h (133.3 m2 g-1, much lower than the former due to its horizontal LDH nanoplates on rGO layer) are much higher than Pd0.92/CoAl-LDH (75.0 m2 g-1) owing to the highly dispersed LDH phase by hybridizing with rGO, in line with the SEM results. The corresponding BJH plots (Figure S5 insets in the Supporting Information) suggest that there are two kinds of mesoporous size distributions in these samples, i.e. typical bimodal distribution. In detail, the Pd0.6/rGO@CoAl-LDH exhibits a narrow peak at 1.9 nm and a little wide one at 4.8 nm along with a total pore volume of 0.66 cm3 g-1, similar to its support (1.9 and 6.2 nm; 0.58 cm3 g-1) upon its orderly nanoarray-like structure, while Pd0.56/rGO@CoAl-LDH-h and Pd0.92/CoAl-LDH show a similar narrow peak at 1.9 nm and much broader one at 14.9 nm probably due to the aggregation of LDH nanoplates in these two non-array-like samples though with varied pore volume of 0.80 and 0.33 cm3 g-1, respectively. Predictably, compared with Pd0.56/rGO@CoAl-LDH-h

and

Pd0.92/CoAl-LDH,

the


nanosheet

array-like


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 40

Pd0.6/rGO@CoAl-LDH with a much higher surface area and numerous narrow distributed mesoporous may provide more available channels for facilitating the mass transportation and diffusion during reaction, further giving a much higher catalytic performance. 3.2 Catalytic Activity of the Pdx/rGO@CoAl-LDH Catalysts. Inspired by these appealing properties including unique nanoarray-like morphology, well-dispersed tiny Pd NCs and high surface area, we choose the Heck reaction between iodobenzene and styrene as a probe to estimate the activity of the present Pdx/rGO@CoAl-LDH catalysts for the first time. As shown in Table 1, the Pd0.6/rGO@CoAl-LDH shows much better reaction activity with iodobenzene conversion of 98.1% at 20 min (Table 1, entry 1) than Pd0.56/rGO@CoAl-LDH-h (98.2%, 50 min), Pd0.92/CoAl-LDH (95.0%, 60 min) and Pd2+0.78/GO (98.1%, 270 min) (Table 1, entries 2-4) under the same conditions. The turnover frequency (TOF) values increase in an order of Pd0.6/rGO@CoAl-LDH (981 h-1) > Pd0.56/rGO@CoAl-LDH-h (393 h-1) > Pd0.92/CoAl-LDH (317 h-1) > Pd2+0.78/GO (73 h-1). The Pd2+0.78/GO also shows a certain catalytic activity due to the formation of part active species Pd0 by the DMF under alkaline conditions.49 Noted that the Pd0.6/rGO@CoAl-LDH exhibits much higher activity than Pd0.56/rGO@CoAl-LDH-h, which can be attributed to its much smaller Pd NCs (1.8 ± 0.5 nm), larger surface area and hierarchical nanosheet array-like structure. While the Pd0.56/rGO@CoAl-LDH-h (3.5 ± 1.2 nm) shows slightly higher activity than Pd0.92/CoAl-LDH (2.7 ± 0.4 nm) though possessing a larger Pd nanoparticles size, clearly indicating that the conductive rGO layer plays a crucial role in improving the catalytic activity. These results suggest that the 3D hierarchical nanosheet array-like hybrid rGO@CoAl-LDH can significantly enhance the


Page 17 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


catalytic activity of the catalyst Pd0.6/rGO@CoAl-LDH owing to the possible Pd—CoAl-LDH—rGO three-phase synergistic effect (later XPS confirmed). Table 1.

A series of Pdx/rGO@CoAl-LDH (x = 1.9, 1.2, 0.6, 0.33, 0.0098) catalysts are employed to further explore the influence of Pd loadings on the Heck reaction. Clearly, the TOF values increase from 846 h-1 to 2982 h-1 with the Pd loadings reduced from 1.9 to 0.33 wt%, in line with the gradually improved Pd NCs dispersion of 19.4 to 32.3% (Table 1, entries 5-8). Furthermore, the effect of the catalyst dosage of Pd0.33/rGO@CoAl-LDH on the Heck reaction is also investigated, and the TOF value increases by nearly 2 times (Table 1, entries 8-10) with the dosage of catalyst reduced from 0.1% to 0.01 mol%. This phenomenon can be ascribed to the equilibrium of “homeopathic” Pd catalyst between the Pd existed in the nanoclusters and the Pd participating in the catalytic cycle is moved from nanoclusters at lower concentration, resulting in a higher proportion of active Pd species.50,51 Based on above results, we test the Heck reaction performance of low Pd loading catalyst Pd0.0098/rGO@CoAl-LDH with a lower Pd concentration of 0.0001 mol% (Table 1, entry 11). The TOF value reached to 160000 h-1 with iodobenzene conversion of 16.0% at 1 h, which is the highest TOF value observed in the Heck reaction over the heterogeneous Pd-based catalysts so far (Table S2 in the Supporting Information).52-56 The Pd0.0098/rGO@CoAl-LDH catalyst is further analyzed by STEM technique (Figure 5). In detail, STEM mapping images clearly show that Co, C, O and Al elements are distributed evenly in the catalyst, demonstrating the intimate hybridization of CoAl-LDH and rGO. Though only a few of highly dispersed Pd can be observed in the mapping images due to the extremely low Pd loading, the EDS of one bright spot selected from HAADF-STEM image of Pd0.0098/rGO@CoAl-LDH confirms the presence of Pd NCs.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

Thus, the superior catalytic performance of Pd0.0098/rGO@CoAl-LDH can be ascribed to its large specific surface area, highly dispersed ultrafine Pd NCs (dispersion of 57.8%, 1.3 ± 0.2 nm) with a clean surface, and the possible Pd—CoAl-LDH—rGO three-phase synergistic effect. Figure 5.

To outreach the range of the reaction, we tested the Heck reaction of varied substituted aryl halides with styrene and its derivatives. As shown in Table 2, the Pd0.33/rGO@CoAl-LDH shows excellent reaction performance for all aryl iodides containing electron-withdrawing groups or electron-donating groups, giving conversions > 94.3% within 40 min. And the reaction TOF values for varied substituted aryl iodides are in an order of NO2 > CH3C=O > H > CH3 > CH3O (Table 2, entries 1-5), which is in connection with the nucleophilicity of the aromatic ring.14 Moreover, the catalyst also show good catalytic activities for Heck reaction between bromobenzene with styrene and methyl acrylate as well as chlorobenzene with styrene and ethyl acrylate (Table 2, entries 6-9), respectively, which is much better than the prereported LDH/Pd catalysts under similar reaction conditions.14,15 All the above findings indicate that the Pdx/rGO@CoAl-LDH catalysts have excellent substrate applicability. Table 2. The stability of catalyst Pd0.33/rGO@CoAl-LDH is further explored by the Heck reaction between iodobenzene and styrene. Typically, after reacting with iodobenzene, the catalyst was recovered by centrifugation, sequentially washed with ethanol and water followed by freeze-drying, then reused under similar conditions. The catalyst show iodobenzene conversion of 90.2% within 40 min after being reused for 5 times. Moreover, ICP analysis shows that the Pd content (0.32 and 0.33 wt% for the used catalyst and fresh catalyst, respectively) of


Page 19 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pd0.33/rGO@CoAl-LDH has no appreciable change. All the above results indicate that the serial Pdx/rGO@CoAl-LDH catalysts have good stability. 3.3 The Kinetics Analysis of the Catalysts. To reveal the reason for the varied Heck reaction performance of the catalysts Pd0.6/rGO@CoAl-LDH, Pd0.56/rGO@CoAl-LDH-h and Pd0.92/CoAl-LDH, we study the macroscopic kinetic process of the present catalysts, involving the effect of the reaction temperatures and varied supports on the reaction rate for the Heck reaction. The evolution of the conversion (C) of the reactant as the function of the reaction time is obtained. As shown in Figure S6 in the Supporting Information, the t-C plots are linear up to ca. 70% conversion of iodobenzene, except for the less active Pd0.92/CoAl-LDH (60%), implying the small product inhibition. As shown in Figure 6, the term -ln(1-C) increases linearly with t for all the Pd catalysts (R2 > 0.98), implying that the Heck coupling is first-order reaction with respect to iodobenzene. The k (rate constant) value is thus achieved from the slope of the -ln(1-C) curve versus t. The k for the Heck reaction in 110 - 140 °C is listed in Table S3, and the k values of the Heck reaction at the same temperature increase in an order of Pd0.6/rGO@CoAl-LDH > Pd0.56/rGO@CoAl-LDH-h > Pd0.92/CoAl-LDH. A good linear relation is gained by plotting lnk versus the inverse of the temperature, and the apparent activation energy, Ea, could be achieved from the slope (R2 > 0.99, Figure 6, Table S3 in the Supporting Information). The least-squares fit analysis gives Ea as 49.63 kJ mol-1 for the Pd0.6/rGO@CoAl-LDH,

which

is

smaller

than

the

corresponding

values

of

Pd0.56/rGO@CoAl-LDH-h (60.44 kJ mol-1) and Pd0.92/CoAl-LDH (64.88 kJ mol-1). The maximum k and the minimum Ea of Pd0.6/rGO@CoAl-LDH suggest its optimal electronic structure and the synergistic effect among tiny Pd NCs, nanoarray-like CoAl-LDH nanoplates



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 40

and conductive rGO layer, i.e. Pd—CoAl-LDH—rGO three-phase synergistic effect, in good agreement with its superior Heck reaction performance. Figure 6.

3.4 Structure-Catalytic Performance Correlation. To reveal the relationship of structure

and

activity,

the

oxidation

state

of

the

surface

elements

in

the

Pdx/rGO@CoAl-LDH catalysts and related samples are carefully analysed by XPS technique. The Co 2p and Pd 3d XPS spectra are displayed in Figure 7 and the corresponding parameters summarized in Table S4, while C 1s and O 1s are shown in Figure S7 in the Supporting Information. It can be seen that the C 1s (Figure S7A) spectrum of GO can be deconvoluted into five types bonds: sp2 C (284.6 eV), sp3 C (285.6 eV), C-O (286.5 eV), C=O (287.5 eV) and O-C=O (288.9 eV).40 Compared with GO, the peak intensities of C–C group for rGO and rGO@CoAl-LDH obviously increased and that of the oxidized carbon (C–O, C=O and O–C=O) greatly decreased, demonstrating the effective reduction of GO to rGO in the hybrid. The peak area ratio of the oxidized carbon bands to the total carbon ones in GO is calculated to be 0.47, while in rGO (synthesis see the Supporting Information) and rGO@CoAl-LDH, it reduces to 0.21 and 0.31, respectively. Therefore, it can be deduced that GO has been predominantly reduced to rGO during the synthesis of the rGO@CoAl-LDH hybrid,37 in line with the Raman and FT-IR data. It is noted that the binding energy (BE) of C=C bond in rGO@CoAl-LDH is slightly lower than that of GO and rGO. It has been reported that the binding energy is related with the electronic structure. 57 Thus, it can be inferred that the electronic transfer in rGO@CoAl-LDH hybrid is intensive, suggesting the strong rGO—CoAl-LDH interaction.


Page 21 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The Co 2p core lines of the Pdx/rGO@CoAl-LDH catalysts and related samples (Figure 7A) can be split into Co 2p3/2 (~781.5 eV) and Co 2p1/2 (~797.6 eV) peaks along with the satellite bands at 786.8 and 803.4 eV, indicating the existence of high-spin Co2+ state in all the samples.15,58 It is noteworthy that the BE values of Co 2p3/2 and Co 2p1/2 in the hybrids rGO@CoAl-LDH (781.5 and 797.6 eV) and rGO@CoAl-LDH-h (781.5 and 797.5 eV) (Figure 7A(a0,b0)) are ~0.3 eV higher than those of CoAl-LDH (Figure 7A(c0), 781.2 and 797.3 eV), revealing that the introduction of rGO can enhance the electron density of the Co core level, further confirming the strong CoAl-LDH—rGO interaction. After the in situ reaction with PdCl42- precursors, compared with rGO@CoAl-LDH support, the BE values of the Co 2p3/2 and Co 2p1/2 peaks of the serial catalysts Pdx/rGO@CoAl-LDH (Figure 7A(a, a1, a2)) downshift to a lower energy levels (x = 0.6, 781.2 and 797.1 eV; x = 0.33, 781.3 and 797.2 eV; x = 0.0098, 781.4 and 797.6 eV, respectively), and the shift amount is increased with increasing Pd loadings. The similar phenomenon is also observed in Pd0.56/rGO@CoAl-LDH-h and Pd0.92/CoAl-LDH (Figure 7A(b, c)). Meanwhile, the intensities of the satellite peaks in Pdx/rGO@CoAl-LDH, Pd0.56/rGO@CoAl-LDH-h

and

Pd0.92/Al-LDH

slightly

decrease

compared

with

the

corresponding supports. Such observations clearly suggest a slight change of valence from Co2+ to Co3+ in LDH layers of the supports after in situ reaction with PdCl42- precursors, and a possible electron-transfer from Co—OH to Pd0 in the as-synthesized Pd catalysts. The Pd 3d XPS spectrum of Pd0.6/rGO@CoAl-LDH (Figure 7B(a)) clearly shows Pd 3d5/2 and 3d3/2 of the Pd0 at 336.0 and 341.3 eV, respectively, and the about 64.0% Pd existed in the metallic Pd0 state (Table S4 in the Supporting Information). With decreasing Pd loadings, the BE values of Pd0 3d5/2 and Pd0 3d3/2 in Pd0.33/rGO@CoAl-LDH (Figure 7B(a1)) and Pd0.0098/rGO@CoAl-LDH (Figure 7B(a2)) gradually downshift to 335.5 and 340.9 eV as well as



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 40

335.4 and 340.7 eV, respectively. While the Pd0.56/rGO@CoAl-LDH-h (Figure 7B(b), 336.2 and 341.5 eV) and Pd0.92/CoAl-LDH (Figure 7B(c), 336.1 and 341.4 eV) exhibit higher Pd0 3d BE values. As expected, only the peaks assigned to Pd2+ are observed in Pd2+0.78/GO (Figure 7B(d)). Above results indicate that the in situ redox reaction occurred between the reductive Co2+ on LDH layers and the oxidative PdCl42-, implying the strong metal-support interaction, especially the remarkable interaction between Pd NCs and rGO@CoAl-LDH in the nanosheet array-like Pdx/rGO@CoAl-LDH catalysts, in line with its obvious edge effect as HRTEM indicated. Enhancing the electron density of Pd NCs can greatly promote the oxidation addition step. The serial Pdx/rGO@CoAl-LDH catalysts have higher Pd NCs electron density compared with other catalysts, accordingly significant enhancement of the reaction activity, which is consistent with the catalytic performance of the nanoarray-like Pdx/rGO@CoAl-LDH catalysts. Figure 7.

To further prove the presence of strong metal-support interaction in the as-prepared nanoarray-like Pd catalyst through electron transfer between Pd NCs and M-OH of rGO@CoAl-LDH, the O 1s XPS spectra of the Pdx/rGO@CoAl-LDH catalysts (x = 0.0098, 0.33, 0.6) and the rGO@CoAl-LDH support were analyzed. As shown in Figure S7B in the Supporting Information, the curve fittings of O 1s reveal that there are three oxygen species, including adsorbed H2O and/or intercalated carbonate (OI, 533 - 534 eV), surface OH (OII, 531 532 eV) and lattice O2- (OIII, 529 - 531 eV) species.59 The curve fitting results of the O 1s spectra show that the BE values of the surface OH groups (OII) in the Pdx/rGO@CoAl-LDH shift to higher levels compared to rGO@CoAl-LDH, implying the decrease in the electron density on the OH groups after loading with Pd NCs, and the amount of the surface OH species are in an order of rGO@CoAl-LDH (76.4%) > Pd0.0098/rGO@CoAl-LDH (75.8%) > Pd0.33/rGO@CoAl-LDH (73.3%) > Pd0.6/rGO@CoAl-LDH (70.9%). Meantime, the Pd BE values decrease in the order of


Page 23 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pd0.6/rGO@CoAl-LDH > Pd0.33/rGO@CoAl-LDH > Pd0.0098/rGO@CoAl-LDH, indicating the increase in the electron density of Pd core level. Valero et al.60 studied the effect of the surface -OH of γ-Al2O3 on the stability and diffusion of single Pd atoms by density functional theory (DFT), and show similar findings that the BE value and hopping rate of Pd atoms decreased with the -OH coverage increased. Above findings clearly demonstrate the presence of interaction between Pd NCs and M-OH in the present Pdx/rGO@CoAl-LDH catalysts. Wei et al.23 reported the interaction between Pd clusters and CoAl-LDH by DFT, and found that the -OH groups on the surface of LDH played a key role in stabilizing the Pd particles via an effective Pd–HO interaction along with the electron transfer from LDH to Pd particles, thereby affording to enhanced ethanol electrooxidation activity. Combining the present excellent Heck reaction performance and systematic characterization analyses along with previous studies,23,60 the electron-rich Pd NCs in the present nanosheet array-like Pdx/rGO@CoAl-LDH catalysts are more likely to originate from the abundant -OH groups on the rGO@CoAl-LDH support, as revealed by the O 1s spectra (Figure S7B in the Supporting Information). In addition, the presence of rGO significantly enhances the electron density of Co2+ in the hybrid rGO@CoAl-LDH as C 1s spectra (Figure S7A in the Supporting Information) indicated, which may greatly facilitate the in situ reduction of Pd2+, further giving the electron-rich Pd NCs catalysts

through

Co—OH … Pd

linkages,

strongly

suggesting

the

remarkable

Pd—CoAl-LDH—rGO three-phase synergistic effect in Pdx/rGO@CoAl-LDH catalysts, which is the nature for excellent Heck reaction activity of the present hierarchical nanoarray-like Pdx/rGO@CoAl-LDH catalysts. To prove whether the Heck reaction follows a heterogeneous or homogeneous reaction route, heterogeneity study is conducted via hot filtration test.14,61 Two parallel



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

experiments are realized for the Heck reaction over the Pd 0.33/rGO@CoAl-LDH (0.1 mol%) catalyst. The first one is proceeded until the completion of the reaction (99.4%, 20 min) and conversion-time plot is shown in Figure S8(a) in the Supporting Information. As for the second experiment, the catalyst is recovered from the solution at 10 min with a iodobenzene conversion of 54.4% by hot filtration, and the iodobenzene conversion remain unchanged as time goes by (54.4%, 25 min, Figure S8(b) in the Supporting Information) though ~2.5% (based on ICP) of the total Pd leached into the reaction system. Above results clearly suggest that only the Pd bound with rGO@CoAl-LDH hybrid is active during the reaction and the reaction occurs on the heterogeneous surface. Based on all the obtained results and previous findings,1,7 we tentatively suggest a plausible mechanism for Heck coupling reaction over the hierarchical nanoarray-like catalysts Pdx/rGO@CoAl-LDH (Figure 8). First, the aryl halides undergo oxidative addition with Pdx/rGO@CoAl-LDH, forming a aromatic Pd(II) species. The generally accepted view for the Heck coupling reaction, catalyzed by Pd catalysts, the rate-determining step is the oxidative addition of Pd(0) into the C-X bond.62 The strong Pd—CoAl-LDH—rGO three-phase synergistic effect in present Pdx/rGO@CoAl-LDH catalysts promotes the production of electron-rich Pd(0), which will facilitate the oxidation addition step. Second, the styrene coordinates to Pd(II), followed by the C=C syn-migratory insertion into the C-Pd bond to form the Pd(II) complex. Then, β-hydride is eliminated to give the aryl alkene product and HPdX intermediate. Finally, Pd(0) was regenerated from HPdX by base assisted H-X elimination to complete the catalytic cycle, and the basic sites in LDH may promote this elimination to some extent. Moreover, the large amount of reductive Co2+ on LDH layers of the Pdx/rGO@CoAl-LDH catalysts can


Page 25 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


effectively capture and reduce the leached Pd 2+ species during the reaction progress,15 and further preventing the Pd species from leaching, endowing the good stability of the present nanoarray-like Pdx/rGO@CoAl-LDH catalysts. Figure 8.

4. CONCLUSIONS In summary, a series of gram-level nanoarray-like catalysts Pdx/rGO@CoAl-LDH were first fabricated via a facile lattice atomic-confined in situ reduction strategy. The size of Pd NCs (1.8 1.3 nm) can be easily tuned upon varied Pd loadings. All the hierarchical nanoarray-like Pdx/rGO@CoAl-LDH catalysts show excellent activities for the Heck reaction, which can be attributed to the ultrafine Pd NCs with high dispersion and clean surfaces, increased electron transfer capacity and surface area, and remarkable Pd—CoAl-LDH—rGO three-phase synergistic effect of the present unique nanosheet array-like Pd NCs catalysts. Moreover, the catalysts have a broad range of substrate applicability and long-term stability. All the findings show that the Pdx/rGO@CoAl-LDH catalysts have a promising application prospect and significant advantages in the industry relative to the traditional supported Pd catalysts. Furthermore, this work provides a novel platform to support other noble or non-precious metal nanoclusters via a facile and green lattice atomic-confined in situ reduction strategy to construct desired heterogeneous catalysts in electrocatalysis, photocatalysis and hydrogenation fields, etc.. ASSOCIATED CONTENT Supporting Information Available: FT-IR spectra of the as-prepared Pd catalysts and corresponding supports (Figure S1), SEM images of the supports (Figure S2), AFM images of exfoliated GO and treated rGO layer (Figure S3), HRTEM images of reference catalysts ((Figure S4), N2 adsorption-desorption isotherms of Pd0.6/rGO@CoAl-LDH and



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 40

related samples (Figure S5), conversion-time (t) plots for the Heck reaction of Pd0.6/rGO@CoAl-LDH and related samples (Figure S6), C 1s XPS of rGO@CoAl-LDH and O 1s of Pd0.6/rGO@CoAl-LDH and related samples (Figure S7), conversion-time (t) plots for the Heck reaction with the catalyst Pd0.33/rGO@CoAl-LDH and with filtrate (Figure S8), XRD parameters of the as-prepared Pd catalysts compared to related samples (Table S1), comparison of the catalytic activity of the catalyst Pd0.0098/rGO@CoAl-LDH to the previous reports in the Heck reaction (Table S2), kinetic fitting of the Heck coupling reaction of iodobenzene over the catalysts (Table S3), XPS data of the Pdx/rGO@CoAl-LDH catalysts compared to related samples (Table S4), attached 1H NMR and 13C NMR spectra of the Heck reaction products in the end. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Hui Zhang; Email: [email protected]; Tel: +8610-6442 5872; Fax: +8610-6442 5385.

ACKNOWLEDGMENTS The authors greatly appreciate the financial support by the National Natural Science Foundation of China (21576013).

REFERENCES (1) Heck, R. F. Palladium-Catalyzed Vinylation of Organic Halides. Org. React. 1982, 27, 345–390. (2) Wang, D. H.; Engle, K. M.; Shi, B. F.; Yu, J. Q. Ligand-Enabled Reactivity and Selectivity in a Synthetically Versatile Aryl C-H Olefination. Science 2010, 327, 315–319. (3) Lo, J. C.; Gui, J. H.; Yabe, Y.; Pan, C. M.; Baran, P. S. Functionalized Olefin Cross-Coupling to Construct Carbon-Carbon Bonds. Nature 2014, 516, 343–348. (4) Feng, Z. Y.; Jiao, L. J.; Feng, Y. M.; Yu, C. J.; Chen, N.; Wei, Y.; Mu, X. L.; Hao, E. Regioselective and Stepwise Syntheses of Functionalized BODIPY Dyes through Palladium-Catalyzed Cross-Coupling Reactions and Direct C–H Arylations. J. Org. Chem. 2016, 81, 6281–6291. (5) Bradshaw, M.; Zou, J. L.; Byrne, L.; Iyer, K. S.; Stewartc, S. G.; Raston, C. L. Pd(II) Conjugated Chitosan Nanofibre Mats for Application in Heck Cross-Coupling Reactions. Chem. Commun. 2011, 47, 12292–12294.


Page 27 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Feng, Z.; Min, Q. Q.; Zhao, H. Y.; Gu, J. W.; Zhang, X. G. General Synthesis of Fluoroalkylated Alkenes by Palladium-Catalyzed Heck-Type Reaction of Fluoroalkyl Bromides. Angew. Chem. Int. Ed. 2015, 54, 1270–1274. (7) Johansson Seechurn, C. C. C.; Kitching, M. O.; Colacot, T. J.; Snieckus, V. Palladium-Catalyzed Cross-Coupling: A Historical Contextual Perspective to the 2010 Nobel Prize. Angew. Chem. Int. Ed. 2012, 51, 5062–5085. (8) Greco, R.; Goessler, W.; Cantillo, D.; Kappe, C. O. Benchmarking Immobilized Di- and Triarylphosphine Palladium Catalysts for Continuous-Flow Cross-Coupling Reactions: Efficiency, Durability, and Metal Leaching. ACS Catal. 2015, 5, 1303−1312. (9) Zeng, M. F.; Wang, Y. D.; Liu, Q.; Yuan, X.; Zuo, S. F.; Feng, R. K.; Yang, J.; Wang B. Y.; Qi, C. Z.; Lin, Y. Encaging Palladium Nanoparticles in Chitosan Modified Montmorillonite for Efficient, Recyclable Catalysts. ACS Appl. Mater. Interfaces 2016, 8, 33157–33164. (10) Li, Y. Z.; Xu, L.; Xu, B.; Mao, Z. P.; Xu, H.; Zhong, Y.; Zhang, L. P.; Wang, B. J.; Sui, X. F. Cellulose Sponge Supported Palladium Nanoparticles as Recyclable Cross-Coupling Catalysts. ACS Appl. Mater. Interfaces 2017, 9, 17155–17162. (11) Moussa, S.; Siamaki, A. R.; Gupton, B. F.; El-Shall, M. S. Pd-Partially Reduced Graphene Oxide Catalysts (Pd/PRGO): Laser Synthesis of Pd Nanoparticles Supported on PRGO Nanosheets for Carbon-Carbon Cross Coupling Reactions. ACS Catal. 2012, 2, 145–154. (12) Gole, B.; Sanyal, U.; Banerjee, R.; Mukherjee, P. S. High Loading of Pd Nanoparticles by Interior Functionalization of MOFs for Heterogeneous Catalysis. Inorg. Chem. 2016, 55, 2345–2354. (13) Rathi, A. K.; Gawande, M. B.; Pechousek, J.; Tucek, J.; Aparicio, C.; Petr, M.; Tomanec, O.; Krikavova, R.; Travnicek, Z.; Varmac, R. S.; Zboril, R. Maghemite Decorated with Ultra-small Palladium Nanoparticles (γ-Fe2O3–Pd): Applications in the Heck–Mizoroki Olefination, Suzuki Reaction and Allylic Oxidation of Alkenes. Green Chem. 2016, 18, 2363–2373. (14) Choudary, B. M.; Madhi, S.; Chowdari, N. S.; Kantam, M. L.; Sreedhar, B. Layered Double Hydroxide Supported Nanopalladium Catalyst for Heck-, Suzuki-, Sonogashira-, and Stille-Type Coupling Reactions of Chloroarenes. J. Am. Chem. Soc. 2002, 124, 14127–14136. (15) Li, P.; Huang, P. P.; Wei, F. F.; Sun, Y. B.; Cao, C. Y.; Song, W. G. Monodispersed Pd clusters Generated in Situ by Their Own Reductive Support for High Activity and Stability in Cross-Coupling Reactions. J. Mater. Chem. A 2014, 2, 12739–12745. (16) Balanta, A.; Godard, C.; Claver, C. Pd Nanoparticles for C–C Coupling Reactions. Chem. Rev. 2011, 40, 4973–4985. (17) Han, Y. F.; Kumar, D.; Goodman, D. W. Particle Size Effects in Vinyl Acetate Synthesis over Pd/SiO2. J. Catal. 2005, 230, 353–358. (18) Lu, H. T.; Zhu, Z. L.; Zhang, H.; Zhu, J. Y.; Qiu, Y. L.; Zhu, L. Y.; Küppers, S. Fenton Like Catalysis and Oxidation/Adsorption Performances of Acetaminophen and Arsenic Pollutants in Water on a Multi-metal



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 40

Cu-Zn-Fe-LDH. ACS Appl. Mater. Interfaces 2016, 8, 25343–25352. (19) Wang, X.; Lin, Y. Y.; Su, Y.; Zhang, B.; Li, C. J.; Wang, H.; Wang, L. J. Design and Synthesis of Ternary-Component Layered Double Hydroxides for High-performance Supercapacitors: Understanding the Role of Trivalent Metal Ions. Electrochim. Acta 2017, 225, 263–271. (20) Bi, X.; Fan, T.; Zhang, H. Novel Morphology-Controlled Hierarchical Core@Shell Structural Organo-Layered Double Hydroxides Magnetic Nanovehicles for Drug Release. ACS Appl. Mater. Interfaces 2014, 6, 20498–20509. (21) Li, L.; Dou, L. G.; Zhang, H. Layered Double Hydroxide Supported Gold Nanoclusters by Glutathione-Capped Au Nanoclusters Precursor Method for Highly Efficient Aerobic Oxidation of Alcohols. Nanoscale 2014, 6, 3753–3763. (22) Wang, S.; Yin, S. T.; Chen, G. W.; Li L.; Zhang, H. Nearly Atomic Precise Gold Nanoclusters on Nickel Based Layered Double Hydroxides for Extraordinarily Efficient Aerobic Oxidation of Alcohols. Catal. Sci. Technol. 2016, 6, 4090–4104. (23) Zhao, J. W.; Shao, M. F.; Yan, D. P.; Zhang, S. T.; Lu, Z. Z.; Li, Z. X.; Cao, X. Z.; Wang, B. Y.; Wei, M.; Evans, D. G.; Duan, X. A Hierarchical Heterostructure Based on Pd Nanoparticles/Layered Double Hydroxide Nanowalls for Enhanced Ethanol Electrooxidation. J. Mater. Chem. A 2013, 1, 5840–5846. (24) Novoselov, K. S.; Fal´ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192–200. (25) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Graphene-Based Semiconductor Photocatalysts. Chem. Soc. Rev. 2012, 41, 782–796. (26) Bonaccorso, F.; Colombo, L.; Yu, G. H; Stoller, M.; Tozzini, V.; Ferrari, A. C.; Ruoff, R. S.; Pellegrini, V. Graphene, Related Two-Dimensional Crystals, and Hybrid Systems for Energy Conversion and Storage. Science 2015, 347, 1246501. (27) Cai, X. Q.; Shen, X. P; Ma, L. B.; Ji, Z. Y.; Xu, C.; Yuan, A. H. Solvothermal Synthesis of NiCo-Layered Double Hydroxide Nanosheets Decorated on RGO Sheets for High Performance Supercapacitor. Chem. Eng. J. 2015, 268, 251–259. (28) Zhong, Y. Y.; Liao Y. Q.; Gao A. M.; Hao J. N.; Shu D.; Huang Y. L.; Zhong J.; He C.; Zeng R. H. Supercapacitive Behavior of Electrostatic Self-Assembly Reduced Graphene Oxide/CoAl-Layered Double Hydroxides Nanocomposites. J. Alloys Compd. 2016, 669, 146–155. (29) Xu, J.; Gai, S. L.; He, F.; Niu, N.; Gao, P.; Chen Y. J.; Yang P. P. Reduced Graphene Oxide/Ni1-xCoxAl-Layered Double Hydroxide Composites: Preparation and High Supercapacitor Performance. Dalton Trans. 2014, 43, 11667–11675. (30) Masikhwa, T. M.; Madito, M. J.; Momodu, D. Y.; Dangbegnon, J. K.; Guellati, O.; Harat A.; Guerioune M.; Barzegar, F.; Manyala, N. High Performance Asymmetric Supercapacitor Based on CoAl-LDH/GF and Activated Carbon from Expanded Graphite. RSC Adv. 2016, 6, 46723–46732.


Page 29 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Zhang, R. K.; An, H. L.; Li, Z. H.; Shao, M. F.; Han, J. B.; Wei M. Mesoporous Graphene-Layered Double Hydroxides Free-Standing Films for Enhanced Flexible Supercapacitors. Chem. Eng. J. 2016, 289, 85–92. (32) Ma, W.; Ma, R. Z; Wang, C. X; Liang, J. B.; Liu, X. H.; Zhou, K. C.; Sasaki, T. A Superlattice of Alternately Stacked NiFe Hydroxide Nanosheets and Graphene for Efficient Splitting of Water. ACS Nano 2015, 9, 1977–1984. (33) Dou, L. G.; Zhang, H. Facile Assembly of Nanosheet Array-Like CuMgAl Layered Double Hydroxide/rGO Nanohybrids for Highly Efficient Reduction of 4-nitrophenol. J. Mater. Chem. A 2016, 4, 18990–19002. (34) Yang, L.; Cheng, S.; Ding, Y.; Zhu, X. B.; Wang, Z. L.; Liu, M. L. Hierarchical Network Architectures of Carbon Fiber Paper Supported Cobalt Oxide Nanonet for High-Capacity Pseudocapacitors. Nano Lett. 2012, 12, 321–325. (35) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339–1339. (36) Cote, L. J.; Kim, F.; Huang, J. X. Langmuir-Blodgett Assembly of Graphite Oxide Single Layers. J. Am. Chem. Soc. 2009, 131, 1043–1049. (37) Xie, R. F.; Fan, G. L.; Ma, Q.; Yang L.; Li, F. Facile Synthesis and Enhanced Catalytic Performance of Graphene-Supported Ni Nanocatalyst from a Layered Double Hydroxidebased Composite Precursor. J. Mater. Chem. A 2014, 2, 7880–7889. (38) Anderson, J. R.; Pratt, K. C. In Introduction to Characterization and Testing of Catalysts; Academic Press, Sydney, Australia, 1986, pp 1–53. (39) Agostini, G.; Lamberti, C.; Pellegrini, R.; Leofanti, G.; Giannici, F.; Longo, A.; Groppo, E. Effect of Pre-Reduction on the Properties and the Catalytic Activity of Pd/Carbon Catalysts: A Comparison with Pd/Al2O3. ACS Catal. 2014, 4, 187–194. (40) Xu, J.; Gai, S. L.; He, F.; Niu, N.; Gao, P.; Chen Y. J.; Yang, P. P. A Sandwich-Type Three-Dimensional Layered Double Hydroxide Nanosheet Array/Graphene Composite: Fabrication and High Supercapacitor Performance. J. Mater. Chem. A 2014, 2, 1022–1031. (41) Dou, L. G.; Fan, T.; Zhang, H. A Novel 3D Oxide Nanosheet Array Catalyst Derived from Hierarchical Structured Array-Like CoMgAl-LDH/Graphene Nanohybrid for Highly Efficient NOx Capture and Catalytic Soot Combustion. Catal. Sci. Technol. 2015, 5, 5153–5167. (42) Palmer, S. J.; Nguyen, T.; Frost, R. L. Synthesis and Raman Spectroscopic Characterisation of Hydrotalcite with CO32- and VO3- Anions in the Interlayer. J. Raman Spectrosc. 2007, 38, 1602–1608. (43) Abdelkader, A. M.; Vallés, C.; Cooper, A. J.; Kinloch, I. A.; Dryfe, R. A. W. Alkali Reduction of Graphene Oxide in Molten Halide Salts: Production of Corrugated Graphene Derivatives for High-Performance Supercapacitors. ACS Nano 2014, 8, 11225–11233.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 40

(44) Huang, S.; Zhu, G. N.; Zhang, C.; Tjiu, W. W.; Xia Y. Y.; Liu, T. X. Immobilization of Co-Al Layered Double Hydroxides on Graphene Oxide Nanosheets: Growth Mechanism and Supercapacitor Studies. ACS Appl. Mater. Interfaces 2012, 4, 2242–2249. (45) Braterman, P. S.; Xu, Z. P.; Yarberry, F. Handbook of Layered Materials, Edited by Auerbach, S. M.; Carrado, K. A.; Dutta, P. K. Marcel Dekker, New York, 2004, 373. (46) Li, D.; Müller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable Aqueous Dispersions of Graphene Nanosheets. Nat. Nanotechnol. 2008, 3, 101–105. (47) Gursky, J. A.; Blough, S. D.; Luna, C.; Gomez, C.; Luevano, A. N.; Gardner, E. A. Particle-Particle Interactions between Layered Double Hydroxide Nanoparticles. J. Am. Chem. Soc. 2006, 128, 8376–8377. (48) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, Academic Press, London, 1999. (49) Scheuermann, G. M.; Rumi, L.; Steurer, P.; Bannwarth, W.; Mülhaupt, R. Palladium Nanoparticles on Graphite Oxide and Its Functionalized Graphene Derivatives as Highly Active Catalysts for the Suzuki-Miyaura Coupling Reaction. J. Am. Chem. Soc. 2009, 131, 8262–8270. (50) Vries, A. H. M. D.; Mulders, J. M. C. A.; Mommers, J. H. M.; Henderickx, H. J. W.; Vries, J. G. D. Homeopathic Ligand-Free Palladium as a Catalyst in the Heck Reaction. A Comparison with a Palladacycle. Org. Lett. 2003, 5, 3285–3288. (51) Deraedt, C.; Astruc, D. “Homeopathic” Palladium Nanoparticle Catalysis of Cross Carbon-Carbon Coupling Reactions. Acc. Chem. Res. 2014, 47, 494–503. (52) Fan, H.; Huang, X.; Shang, L.; Cao, Y. T.; Zhao, Y. F.; Wu, L. Z.; Tung, C. H.; Yin Y. D.; Zhang, T. R. Controllable Synthesis of Ultrathin Transition-Metal Hydroxide Nanosheets and their Extended Composite Nanostructures for Enhanced Catalytic Activity in the Heck Reaction. Angew. Chem. Int. Ed. 2016, 55, 2167–2170. (53) Yamada, Y. M. A.; Yuyama, Y.; Sato, T.; Fujikawa, S.; Uozumi, Y. A Palladium-Nanoparticle and Silicon-Nanowire-Array Hybrid: A Platform for Catalytic Heterogeneous Reactions. Angew. Chem. Int. Ed. 2014, 126, 131–135. (54) Young, J. N.; Chang, T. C.; Tsai, S. C.; Yang, L.; Yu, S. J. Preparation of A Nonleaching, Recoverable and Recyclable Palladium-Complex Catalyst for Heck Coupling Reactions by Immobilization on Au Nanoparticles. J. Catal. 2010, 272, 253–261. (55) Hagiwara, H.; Sugawara, Y.; Isobe, K.; Hoshi, T.; Suzuki, T. Immobilization of Pd(OAc)2 in Ionic Liquid on Silica: Application to Sustainable Mizoroki-Heck Reaction. Org. Lett. 2004, 6, 2325–2328. (56) Okamoto, K.; Akiyama, R.; Yoshida, H.; Yoshida T.; Kobayashi, S. Formation of Nanoarchitectures Including Subnanometer Palladium Clusters and Their Use as Highly Active Catalysts. J. Am. Chem. Soc. 2005, 127, 2125–2135. (57) Yin, S. T.; Li J.; Zhang, H. Hierarchical Hollow Nanostructured Core@Shell Recyclable Catalysts γ-Fe2O3@LDH@Au25-x for Highly Efficient Alcohol Oxidation. Green Chem. 2016, 18, 5900–5914.


Page 31 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(58) Ma, R. Z.; Liang, J. B.; Takada, K.; Sasaki, T. Topochemical Synthesis of Co-Fe Layered Double Hydroxides at Varied Fe/Co Ratios: Unique Intercalation of Triiodide and Its Profound Effect. J. Am. Chem. Soc. 2011, 133, 613–620. (59) Deng, J. G.; Zhang, L.; Dai, H. X.; He, H.; Au, C. T. Strontium-Doped Lanthanum Cobaltite and Manganite: Highly Active Catalysts for Toluene Complete Oxidation. Ind. Eng. Chem. Res. 2008, 47, 8175–8183. (60) Valero, M. C.; Raybaud, P.; Sautet, P. Influence of the Hydroxylation of γ-Al2O3 Surfaces on the Stability and Diffusion of Single Pd Atoms: A DFT Study. J. Phys. Chem. B 2006, 110, 1759–1767. (61) Zhang, L. L.; Wang, A. Q.; Miller, J. T.; Liu, X. Y.; Yang, X. F.; Wang, W. T.; Li, L.; Huang, Y. Q.; Mou, C. Y.; Zhang, T. Efficient and Durable Au Alloyed Pd Single-Atom Catalyst for the Ullmann Reaction of Aryl Chlorides in Water. ACS Catal. 2014, 4, 1546–1553. (62) Li, H. B.; Johansson Seechurn C. C. C.; Colacot, T. J. Development of Preformed Pd Catalysts for Cross-Coupling Reactions, Beyond the 2010 Nobel Prize. ACS Catal. 2012, 2, 1147–1164.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 40

Figure Captions Figure 1. Schematic synthesis strategy of the hierarchically structured nanoarray-like Pd catalysts Pdx/rGO@CoAl-LDH. Figure 2. (A) XRD and (B) Raman spectra of Pd0.6/rGO@CoAl-LDH (a), Pd0.56/rGO@CoAl-LDH-h (b), Pd0.92/CoAl-LDH (c), Pd2+0.78/GO (d) and corresponding supports (a0-d0). Figure 3. SEM-EDS (A, D, F, G), TEM (B, E) and 3D phase AFM (C) with 2D phase image (inset) of Pd0.6/rGO@CoAl-LDH (A, B, C), Pd0.56/rGO@CoAl-LDH-h (D, E), Pd0.92/CoAl-LDH (F) and Pd2+0.78/GO (G). Figure

4.

HRTEM

images

of

nanoarray-like

catalysts

Pd1.9/rGO@CoAl-LDH

(A,

B),

Pd1.2/rGO@CoAl-LDH (C, D), Pd0.6/rGO@CoAl-LDH (E, F), Pd0.33/rGO@CoAl-LDH(G, H) and Pd0.0098/rGO@CoAl-LDH (I, J) (insets: histogram of particle size distribution and FFT). Figure 5. STEM mapping images of the catalyst Pd0.0098/rGO@CoAl-LDH. Figure 6. -ln(1-C) against time (t) and Arrhenius plots for the Heck reaction of iodobenzene and styrene on Pd0.6/rGO@CoAl-LDH, Pd0.56/rGO@CoAl- LDH-h, Pd0.92/CoAl-LDH at varied temperatures (110, 120, 130, and 140 oC). Reaction conditions: iodobenzene (1 mmol), styrene (1.2 mmol), Pd: 0.3 mol%, K2CO3 (3.0 mmol), solvent (VDMF:VH2O = 12 mL:4 mL). Figure 7. (A) Co 2p and (B) Pd 3d XPS spectra of Pd0.6/rGO@CoAl-LDH (a), Pd0.33/rGO@CoAl-LDH (a1),

Pd0.0098/rGO@CoAl-LDH

(a2),

rGO@CoAl-LDH

(a0),

Pd0.56/rGO@CoAl-LDH-h

(b),

rGO@CoAl-LDH-h (b0), Pd0.92/CoAl-LDH (c), CoAl-LDH (c0), Pd2+0.78/GO (d). Figure 8. Plausible mechanism for the Heck coupling reaction of aryl halides with styrene on Pdx/rGO@CoAl-LDH catalysts. Table 1. Comparison of the catalytic activity of the Pdx/rGO@CoAl-LDH catalysts for the Heck reaction of iodobenzene and styrene. Table 2 The catalytic activity of the Pd 0.33/rGO@CoAl-LDH for the Heck reaction of varied substituted aryl halides with styrene and its derivatives.


Page 33 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1.

Figure 2.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3.


Page 34 of 40

Page 35 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5.

Figure 6.


Page 36 of 40

Page 37 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7.

Figure 8.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

Table 1 Comparison of the catalytic activity of the Pdx/rGO@CoAl-LDH catalysts for the Heck reaction of iodobenzene and styrene a Entry

Catalysts

1

Pd0.6/rGO@CoAl-LDH

2

a

DPd / nm b Dispersion / % c 1.8 ±0.5

Pd0.56/rGO@CoAl-LDH-h 3.5 ±1.2

Pd / mol% d

t / min

Conv. / % e

TOF / h-1 f

25.6

0.3

20

98.1

981

-

0.3

50

98.2

393

3

Pd0.92/CoAl-LDH

2.7 ±0.4

-

0.3

60

95.0

317

4

Pd2+0.78/GO

-

-

0.3

270

98.1

73

5

Pd1.9/rGO@CoAl-LDH

1.8 ±0.5

19.4

0.1

70

98.7

846

6

Pd1.2/rGO@CoAl-LDH

1.8 ±0.5

23.8

0.1

40

96.3

1445

7

Pd0.6/rGO@CoAl-LDH

1.8 ±0.5

25.6

0.1

30

98.5

1970

8

Pd0.33/rGO@CoAl-LDH 1.6 ±0.6

32.3

0.1

20

99.4

2982

9


32.3

0.03

40

94.8

4740

10


32.3

0.01

70

99.9

8563

11 Pd0.0098/CoAl-LDH/rGO g 1.3 ±0.2

57.8

0.0001

60

16.0

160000

Reaction conditions: Iodobenzene (1.0 mmol), styrene (1.2 mmol), K2CO3 (3.0 mmol), mixed solvent: VDMF:VH2O = 12 mL : 4

mL, 120 oC under atmospheric conditions. b Based on HRTEM.

c

Based on CO-chemisorption tests. d Pd mol% based on the

amount of initial iodobenzene. e Determined by GC, products were identified via corresponding standard substance. f Turnover frequency (TOF) calculated by the formula: TOF = nIobenzene/(nPd·t), where nIobenzene represents the converted amount of iobenzene, nPd is the dosage of catalyst upon Pd content, and t is the reaction time that corresponding conversion costs. g Reaction conditions: Iodobenzene (50 mmol), styrene (60 mmol), K2CO3 (100 mmol), mixed solvent: VDMF:VH2O = 120 mL : 40 mL, 140 oC under atmospheric conditions.


Page 39 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 The catalytic activity of the Pd0.33/rGO@CoAl-LDH for the Heck reaction of varied substituted aryl halides with styrene and its derivatives a

a

Entry

Aryl halides (1)

Alkenes (2)

(3)

t / min

Conv. / % b

TOF / h-1 c

1

1-Iodo-4-nitrobenzene

Styrene

3a

5

97.7

11724

2

4-Iodoacetophenone

Styrene

3b

5

94.3

11316

3

Iodobenzene

Styrene

3c

20

99.4

2982

4

4-Iodotoluene

Styrene

3d

20

96.0

2880

5

4-Iodoanisole

Styrene

3e

40

95.2

1428

6

Bromobenzene

Styrene

3c

300

88.5

177

7

Bromobenzene

Methyl acrylate

3f

300

90.2

180

8

Chlorobenzene

Styrene

3c

900

15.6

10

9

Chlorobenzene

Ethyl acrylate

3g

900

17.0

11

Reaction conditions: Aryl halide (1.0 mmol), styrene or styrene derivatives (1.2 mmol), K2CO3 (3.0 mmol), mixed solvent:

VDMF:VH2O = 12 mL : 4 mL, Pd mol% (0.1) based on the amount of initial aryl halide, 120 oC under atmospheric conditions.

b

Determined by GC. c TOF = naryl halide/(nPd·t), where naryl halide refers to the converted amount of aryl halide, nPd the dosage of catalyst upon Pd content, and t the reaction time that corresponding conversion costs.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents (TOC)


Page 40 of 40

rGO@CoAl-LDH via

Recommend Documents