Co catalytic nano-monolayer

Journal of Catalysis 376 (2019) 228–237

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Controlled distribution of active centre to enhance catalytic activity of ordered Pd/Co catalytic nano-monolayer Weili Shang a, Xiangfeng Zeng a, Tiesheng Li a,⇑, Wenjian Xu a, Donghui Wei a, Minghua Liu b,⇑, Yangjie Wu a a b

College of Chemistry and Molecular Engineering, Zhengzhou University, Kexuedadao 100, Zhengzhou 450001, PR China Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing 100190, PR China

a r t i c l e

i n f o

Article history: Received 14 February 2019 Revised 23 June 2019 Accepted 1 July 2019

Keywords: Heterogeneous bimetallic catalyst Self-assembly film Coupling reaction Synergistic effect

a b s t r a c t Ordered imine phenanthroline Pd/Co self-assembled monolayers immobilized on graphene oxide (called as GO@Fmp-PdxCo1-x) designed was fabricated and characterized. Their catalytic properties were systematic investigated, in which the obtained [email protected] monolayer (TON = 46075, TOF = 23038 h
1) exhibited higher catalytic performance than that of monometallic counterparts for Suzuki-Miyaura coupling reaction with water as solvent due to the synergistic effects, the molar ratio and the physical chemistry properties of GO. Especially, distribution of active sites formed on the surface and inner of monolayer (denoted as 3D catalytic active centres called likely as MOFFs (Metallic Organic Frame-Films)), in which relationship between actives and structure of the self-assembly monolayer played an important role on their catalytic activity. The oriental and special structure confirmed can be utilized not only help for easily insight into what happened during the catalytic process, supplying exposure more controllable active sites precisely in certain manner and stability by introducing different ligands and fabricating process. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction The organic reaction catalyzed by transition metal catalysts has become one of the most powerful synthetic ways in organic synthesis [1]. Palladium-catalyzed various coupling reaction are widely used in a wide range for chemical and pharmaceutical industries [2–7], in which mild reaction conditions, less dosage of catalyst and good applicability of the substrate are presented [8–10]. However, palladium as noble metal is higher costly, scarce in nature, and the necessity required by green chemistry. Therefore, a new type of catalysts which has the effective reduction of precious metals and optimum catalytic activity is highly wanted. In this regard, alloying with a second metal is regarded as an effective strategy for obtaining high catalytic activity compared to mono-metal counterparts, especially for noble Pd-based catalysts [11–18]. Therefore, these have great promise to be newgeneration catalysts by mixing noble metals with non-precious metals [19–21]. Among non-noble metals, the Co-based catalyst has received highly concerned due to its low-priced, low toxic, the abundant resources in nature. To some extent, the dilution of

⇑ Corresponding authors. E-mail addresses: [email protected] (T. Li), [email protected] (M. Liu). https://doi.org/10.1016/j.jcat.2019.07.007 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

precious noble metals by cheap non-noble metals in bimetallic catalysts is of the great importance because of these new catalysts exhibit eminent performance except save materials, reduce cost [22–24]. As we know, the catalytic performance of bimetallic catalysts is superior to their monometallic counterparts due to the synergistic effects [25,26]. You et al. studied the synergistic effect of Pd and Co within the complex as a highly efficient catalyst for Suzuki-Miyaura reaction in water, which exhibited excellent catalytic activity compare to Co-free counterpart [27]. Atsushi et al. reported that a 1:1 ratio of Pd and Ni bimetallic NPs was more active than that of Pd NPs, but recycling should be improved [28]. From the reports, the conventional homogeneous reaction usually was performed by using palladium complexes [29,30] and ligands under homogeneous catalytic conditions. However, it was difficult to handle and recovery of catalyst in reaction system made it necessary to replace it with heterogeneous ones [31–35]. Thus, the conventional heterogeneous catalysis had aroused great attention [36,37]. The main form of heterogeneous catalyst was to immobilize it on different supports in a certain way, which facilitates separation and easily recycling. Although great achievements in heterogeneous catalytic researches had been achieved [38–40], the application of ordered selfassembled catalytic monolayer for catalyzing coupling reaction

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was rarely investigated [41–43]. To solve the problems of poor uniformity, repeatability and difficult to explore catalytic mechanism, ordered catalytic monolayer should be utilized for further exploring [44,45]. In recent decades, many efforts have been made to develop appropriate supported materials, with which it is possible to reach the most favorable stability, activity, selectivity, and excellent catalytic activity, such as carbon materials [46–48]. Among of them, graphene-oxide that was considered as appropriate support for heterogeneous catalysis has gained strong interest due to its the general, special physical and chemical properties [49–53], which are beneficial to promotes effective electron transport to improve catalytic activity and stability. However, the graphene-oxide supported bimetallic complex self-assembly monolayer for C-C coupling reaction has rarely been reported. Moreover, this type of catalyst could be a new kind of the robust material through tuning the electronic properties of the catalyst by different ligand designs. Ligands containing Schiff-base [54– 57] and 1,10-phenanthtoline [58–60] groups had the advantages of easy preparation, good thermal stability and the stronger ability to coordination with numerous transition metals (Pd Co Ni Cu. . .) which exhibited outstanding catalytic performance in various reactions [61–63]. In order to control the distribution of the active sites on the surface and inner of catalytic monolayer, an imine phenanthroline ligand acting as the binding site for Co/Pd were designed and grafted onto graphene-oxide to fabricate the ordered GO@FmpCo/Pd molecular monolayer. Its catalytic performances, catalytic mechanism and synergistic effect to enhance catalytic activity were systematic investigated.

2. Experimental section 2.1. Chemicals All chemicals were derived from commercial sources. Solvents were purified before used.

2.2. Characterizations with equipments The equipments used for characterizing were presented in Supporting information.

229

2.3. Synthesis of ligands Fmp Ligand Fmp was prepared as reported previously [64–66] and H NMR of Fmp (See Supporting information of Fig. S1).

1

3. Results and discussion 3.1. Preparation and characterization of GO@Fmp-Pd/Co monolayer The preparation route of GO@Fmp-Pd/Co self-assembly monolayer was depicted in Scheme 1. 3.2. Contrast experiment It is well known that the location of the metal coordination with some functional groups play an important role in determining the structure of catalyst, which contributes to identify the effect factors of catalytic properties. To identify whether or not cobalt complex with imine group of ligand, p-Bromo benzaldehyde (called as BBr) was chosen as ligand. The preparation route of GO@BBr-Co was similar to GO@Fmp-Pd/Co self-assembly monolayer. The test results as displayed in Fig. S2. The Co2p XPS spectrum of GO@BBr-Co displays typical bands at 781.47 eV, 786.52 eV, 796.5 eV, and 801.5 eV, which denoted to the bonding energy of Co (II). Results showed that Co was complexed with imine. 3.3. Characterization GO@Fmp-Pd/Co self-assembly monolayer was characterized by FTIR, Raman, XRD, SEM, TEM and XPS (see Supporting information). 3.3.1. FTIR FTIR is one of the most efficient approaches of determining the structure of functional groups in catalytic materials. The distinct characteristic peaks of GO (Fig. S3) at about 3422 cm
1 and 1736 cm
1 for AOH and C@O were observed (black line). The stretching vibration peaks of SiAO bond at 1112 cm
1 after grafted with APTES could be observed (red line). When the imine phenanthroline ligand was grafted, a strong absorption band around 1661 cm
1 corresponding to C@N appeared, indicating that ligand grafted onto the surface of GO (blue line).

Scheme 1. Fabrication route of GO@Fmp-Pd/Co.

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The C@N stretching frequencies are slightly red-shifted after mixed with transition metals (green line), suggesting that the phenanthroline and the Schiff-base groups have coordinated with cobalt and palladium in different manner. 3.3.2. Raman spectroscopy Raman spectroscopy was adopted to characterize the fabrication step of hetero-bimetallic catalyst monolayer. As for GO, GO@APTES, GO@Fmp and GO@Fmp-Pd/Co shown in Fig. S4, the peaks at 1343 cm
1 and at 1593 cm
1 were assigned to characteristic D-band and G-band, respectively. The D-band represents that the disorder vibration of carbon-heteroatom on GO surfaces and the G-band is attributed to the vibration of sp2 carbon atoms in graphene-oxide. The intensity ratio of ID/IG was used to reflect disorder degree on GO surfaces. The intensity ratio of ID/IG for grafted APTES (red line) was significantly changed compare with GO (black line). Meanwhile, the intensity ratio of ID/IG for grafting the imine compounds onto GO surface decreased (blue line), indicating that sp2 carbon atoms on graphene-oxide surface were increased due to grafting ligand. After coordinating with palladium or cobalt ions, the intensity ratio of ID/IG had a little change (from 1.02 to 1.04), which indicated that the ordered of self-assembled molecule film on GO surface was regular (green line). Besides, the step by step blue shift of G band observed when different functional groups were grafted. This phenomenon could be explained that the change in vibration on GO surface was caused by grafting ligand and coordinating with metals. It also suggested that the interaction between GO and hetero-bimetals could prompt electron transferring effectively. 3.3.3. Low-angle X-ray diffractograms (LAXRD) As displayed in Fig. S5, the peak at 11.37° and 42.43° corresponding to graphene-oxide (black line). However, the diffraction peak had almost no change after modified with APTES (red line), indicating that the morphology of graphene-oxide was not altered. When ligand was modified, the diffraction peak at 11.37° shift to at 10.83°, which indicated the lamellar spacing of graphene-oxide increased. It also illustrated the ligand to be grafted onto the surface of GO (blue line). After bimetallic cobalt-palladium coordinated with GO@Fmp, the diffraction peak of GO at 11.37° significantly shifted to 10.71° (green line). The reason was that the ligand coordinated with cobalt or palladium, leading to the interaction between each layer enhanced. Although the main peak position of graphene-oxide had markedly changes during the process, the surface of GO remained crystal microscopic structure. This phenomenon revealed that the hetero-metallic self-assembled monolayer was uniform on GO surface. 3.3.4. Scanning electron microscopy studies (SEM) The morphology in the fabrication process of catalytic monolayer was investigated by SEM. The two-dimensional layer structures of GO (Fig. S6, black line) could be clearly observed. It could be noticed that the original structure of graphene-oxide was not altered by silanization. The wrinkling surface of GO increased after modified with ligand, but the similar structure of GO stayed basically. The results revealed that the twodimensional layer structure of GO surface was uniform and smooth after complexing with cobalt-palladium, suggesting that heterobimetallic catalysts maintain orderly characteristic during preparation. 3.3.5. Transmission electron microscopes (TEM) TEM images of GO showed the two-dimensional thin-layer structure as shown in Fig. S7a. The GO retained its original structure when treated with APTES (Fig. S7b). Obvious twodimensional could be observed after reacting with ligand

(Fig. S7c), meaning that the two-dimensional structure of GO was not destroyed. It was observed that layer structure of GO was relatively uniform after coordinating with palladium-cobalt. This result implied that the ordered hetero-bimetallic catalyst was distributed evenly over graphene-oxide surface. 3.3.6. X-ray photoelectron spectroscopy (XPS) studies The elemental composition and chemical valence on the GO@Fmp- Pd0.25/Co0.75 self-assembled monolayers were investigated by XPS (Fig. S8). The characteristic peaks of Pd, Co, N, Si and Cl elements were observed in [email protected]/Co0.75 selfassembled monolayer shown in Fig. S8a. The peaks at 283.26 eV were denoted to Cl 2p and 102.11 eV to Si 2p after grafted with APTES (Fig. S8a). Then N1s at 400.18 eV with a shoulder peak appeared after modified with Fmp (Fig. S8b). The peaks at 337.91 eV and 343.16 eV were ascribed to Pd2+ (Pd 3d) (Fig. S8c). The binding energies at 780.82 eV and 785.34 eV attribute to Co (2p3/2) and the binding energies at 796.42 eV and 800.03 eV corresponding to Co (2p1/2) and the characteristic of satellite peak, illustrating that Co was mainly in the state of Co2+ [67,68] (Fig. S8d). The surface of [email protected]/Co0.75 self-assembled monolayer exhibited the presence of Pd2+ and Co2+, which was consistent with SEM-EDS analysis (Fig. S9). The results obtained above showed that ordered bimetallic catalytic ([email protected]/Co0.75) monolayer was fabricated. 3.4. Catalytic properties of [email protected]/Co0.75

3.4.1. Optimization of the reaction conditions To explore the catalytic performance of [email protected]/ Co0.75, the coupling reaction of 4-bromoacetophenone and phenylboronic acid acting as a model reaction was carried out. In preliminary studies, various solvent was chosen as object to study for filtrating their optimal solvent in a certain quantity of catalyst (4 mg) and 2 M equivalents of K2CO3 for 12 h at 80 °C (presented in Table S1, entries 1–7). It was clear that excellent yields could be obtained using water as solvent. Optimization for the various bases was then investigated, in which the ideal product yields was about 95% with K3PO4 as base (Table S1, entries 8–11). Better yield was also obtained even that catalyst dosage was reduced to 3 mg, 2 mg and 1 mg (entry 12– 14). The 97% yield could be accomplished when the quantity of substrate was enlarged to three or four times (entry 15–16). Highest yield (entry 17–18) was performed in 2 h at 80 °C and the same yield was also at 70 °C for 2 h. A markedly decreased in yield could be observed due to the decreased temperature below 70 °C which led to difficult formation of intermediate (Table S1, entries 19–21). It was worth noting that the product yield still kept favorable catalytic activity and turnover number (TON) 31058 h
1 even tenfold in quantity of the substrate used (Table S1, entries 22). Based on screening results, the optimized reaction conditions are 0.4 mmol of 4-Bromoacetophenone, 0.44 mmol of phenylboronic acid, 0.6 mmol of K3PO4, 4 mL of water and 1 mg of catalyst in 70 °C for 2 h. 3.4.2. The applicability of [email protected]/Co0.75 In order to get insight into the wide applicability, the further research was extended to various substituted aryl halides and arylboronic acids using [email protected]/Co0.75 as catalyst (Table S2). Aryl bromide containing both electron withdrawing and electron donating substituents showed excellent yields up to about 90% (Table S2, entries 2–9). At the same time, influence of steric hindrance on catalytic properties also investigated, in which the yields lightly decreased in the case of ortho-

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substituted aryl bromides due to the steric hindrance (Table S2, entries 6–8). In the case of p-nitrochlorobenzene or 4chloroanisole, lower yield could be obtained (Table S2, entries 10, 11). Moderate yields (Table S2, entries 12, 13) was obtained by reacting different substituted phenylboronic acids with 4bromoacetophenone.

3.4.3. Studied on the influence of ratios of Co/Pd The effect of the ratio of Co/Pd on catalytic activities (Pd0.01Co0.99, Pd0.05Co0.95, Pd0.25Co0.75, Pd0.38Co0.62, Pd, Co) (Table S3) were investigated. The product yields increased firstly and then decreased with the increasing of the ratio of Pd to Co. The bimetallic catalyst of supported Co0.75/Pd0.25 was much more active compared to other different ratio of supported Co/Pd catalyst, suggesting that the overloading palladium in bimetallic catalyst could not improve its activity. It was also evidence that higher density of active sites resulted in losing of active because of aggregating easily. Moreover, the fabricated [email protected]/Co0.75 presented higher catalytic activity compared to monometallic Pd or Co catalysts, indicating that cobalt had crucial impact on the catalytic properties through the synergistic effect between Pd and Co. Therefore, the [email protected]/Co0.75 was utilized for catalyzing coupling reaction in following experiment under optimum condition.

3.4.4. The effect of supports and ligands In order to further investigate if different supports and the structure of ligand affect their catalytic performance, experiments were presented in Table S4. Blank experiments were carried out, which proved that the metals species were essential (entries 1, 2). To study the influence of GO on catalytic activity, we designed a variety of homogeneous catalyst as contrasting. It was found that mixture of CoCl2(H2O)6/Li2PdCl4, CoCl2(H2O)6/Li2PdCl4/Fmp (entries 3, 4) showed poor catalytic activity. However, the fabricated ordered palladium-cobalt self-assembled monolayer supported on GO exhibited higher catalytic performance. It was attributed to proper structure of ligand which could effectively avoid to agglomerating, indicating that ordered self-assembly had excellent impact on enhance the catalytic effect. Silica gel as support was used for preparing functional catalyst ([email protected]/Co0.75) (entries 8). The yields for [email protected]/Co0.75 and [email protected]/Co0.75 are 61% and 97%, respectively. The [email protected]/Co0.75 exhibits excellent yields due to readily dispersed in the solution. Then, the influences of self-assembly method were discussed and compared with the mixture of GO/ Fmp/Li2PdCl4/CoCl26H2O or Li2PdCl4/CoCl26H2O/GO. It could be seen that [email protected]/Co0.75 catalysts showed the best catalytic performance with the highest TONPd 23038 h
1. This phenomenon indicated that the electron transport occurred from GO to Pd/Co catalysts, which was beneficial to improve catalytic activity. Also, the 2D structure of GO contribute to facilitates reactants to contact with supported active sites and ascribed for increasing contact area of catalyst with reactant [69].

3.4.5. The comparison of catalytic activity of other catalysts The catalytic activities of the [email protected] used in this work, as compared to other different supports and nano-catalysts reported were shown in Table S5. It is noted that You et al. reported Pd2Co(HBPDC)2Cl4(H2O)4 catalyst for catalyzing SuzukiMiyaura coupling reaction with the low amount of catalyst of 0.01 mol% Pd. For our [email protected] catalyst (0.00293 mol% Pd) was much higher TOF than other literatures reported, suggesting that [email protected] catalyst showed some advantage compare with other catalysts reported [27,70–74].

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3.4.6. Recycling ability The stability and recycling ability of [email protected]/Co0.75 was examined by using p-bromacetophenone and phenylboronic acid as the model substrates. The test results showed that the catalyst had favorable stability and the activity of catalyst remained stable after 6 times (Fig. 1). However, after 7 cycles, significantly decreased, which were also investigated by TEM (Fig. 2a–d) and ICP analyses. It was clear that the more recycle, the more aggregation of metal and average size of nano-clusters, in which great agglomeration with average size of 5 nm after 6 cycles. Loss of metal ions was also confirmed by analyzing the date of Pd content used for 6 times (4.01  10
7 mmol g
1 Pd) which was much less than that of fresh catalyst. 3.4.7. Investigation on the catalytic mechanism It is essential to study whether or not catalyst leaching during catalytic reaction, which can be of great help for solving the question of heterogeneity [75]. 3.4.7.1. Kinetic studies and hot filtration experiment. The hot filtration tests were also used to detect activity change of catalyst before and after removing the catalyst from the reaction solution, with which more evidence whether or not the leaching of catalyst occurred. The yield was raised from 0% to 43% during 10 min reaction time as in Fig. S10. Then the catalyst was isolated from this reaction solution and the filtrated solution continued to react at different times. The product yield could not obviously be enhanced, indicating that almost no leaching of catalyst occurred. 3.4.7.2. Catalyst poisoning experiment. To ensure whether heterogeneous or not, the catalyst poisoning experiment was performed displayed in Table S6. The catalytic activity could be suppressed effectively by adding mercury (Table S6, entry 2). However, catalytic active sites were not completely covered, because mercury is very difficult to disperse in reaction solution. Secondly, the catalytic activity was not affected with the addition of triphenyl phosphorus. It could be interpreted as triphenylphosphine possesses strong coordination ability, leading to ligand exchanging with chlorine (Table S6, entry 1). It was worth noting that when thiophene was used as poisoning additive, the activity of catalyst markedly decreased or almost fully inactive due to the active sites was covered by thiophene. These results suggested that the catalytic process mostly happened at the interface and the

Fig. 1. The recycle experiments (1–7 times for 2 h, recycled 8 times for 4 h).

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Fig. 2. TEM images of reused catalyst (a) unused run, (b) after the 1st run, (c) after the 4rd run, (d) after the 6rd run. (inset: histogram of the diameters of Pd (0) nanocluster after the 6th run).

Fig. 3. ReactIR plots via time for 4-bromoacetophenone formation by Suzuki coupling reaction (a) 3D map catalyzed by [email protected], (b) 3D map catalyzed by Li2PdCl4/CoCl26H2O, (c) Analysis on the dynamics in reaction catalyzed by [email protected] and Li2PdCl4/CoCl26H2O using the band of 754 cm
1.

[email protected]/Co0.75 served as a heterogeneous catalyst. Meanwhile, the [email protected]/Co0.75 self-assembled monolayer was stable in the condition of the test. 3.4.7.3. In situ FTIR monitoring. ReactIR can be used to real-time online monitoring catalytic reaction process and exploring catalytic mechanisms. With time prolonging, ReactIR 3D maps catalyzed by [email protected]/Co0.75 (Fig. 3a) and homogeneous Li2PdCl4/CoCl26H2O (Fig. 3b) displayed significantly different results. For the [email protected]/Co0.75, the peak intensity trend presented ‘‘S shaped” with increasing reaction time. However, for homogeneous Li2PdCl4/CoCl26H2O displayed a sharply increasing trend in 10 min and then slowly increasing (Fig. 3b). The significantly different results illustrated different catalyzing mechanisms for Li2PdCl4/CoCl26H2O and [email protected]/Co0.75, respectively. For homogeneous Li2PdCl4/CoCl26H2O, the metal salt catalyst could be dispersed in reaction solution and created an isotropy reaction environment, which contributed to the reactants fully contact with the catalyst. In other words, reaction intermediates were produced rapidly at early stage of reaction. For this reason, within 10 min, the peak intensity trend increased rapidly could be observed on the ReactIR 3D map. Reaction intermediates were gradually converted to the coupling product as the reaction progresses, therefore, the peak intensity increased rapidly and then the peak intensity increased slowly. However, for heterogeneous [email protected]/Co0.75, a limited amount of catalytic active sites was on catalyst surface. So, the reaction mechanism was considered at interface reaction. The reason could be regarded as that the reactants first contacted with specific active sites on catalytic surface, then formed intermediates and gave product which diffused into reaction system by desorption from the catalytic surface. Therefore, for Suzuki reaction catalyzed by [email protected]/Co0.75, the ReactIR 3D maps displayed that characteristic peak intensity trend of product presented ‘‘S shaped” curve. To further verify

our conclusion, the FTIR spectra catalyzed by [email protected]/ Co0.75 at different time were selected and the trend change was analyzed as displayed in Fig. 3c. The Kinetic plots via time by describing the intensity of peak at 754 cm
1 (Fig. 3c) due to the strong intensity would decrease error greatly. For the reaction catalyzed by [email protected]/Co0.75, the peak intensity trend showed three stages (namely induction, acceleratory, stabilization periods) with increasing reaction time. Meanwhile, the kinetic curves of reaction follow ‘‘S shaped” curve (Fig. 3c, black line). Instead, for the homogeneous Li2PdCl4/CoCl26H2O (Fig. 3c, red line), the peak intensity trend went rapidly within 10 min and then increased slowly, suggesting that different mechanisms from [email protected]/Co0.75. 3.4.7.4. Quartz crystal microbalance (QCM)monitoring. Quartz crystal microbalance (QCM) is a highly sensitive technique for mass change in the molecular biology and the microchemistry field because it is simple, highly sensitive and real-time online detection. The detection principle by detecting the mass change of adsorbed substances on the surface of a quartz wafer to correspond to the frequency change, namely, the Sauerbrey formula: 4F =
2F20(qq lq)
1/24m/A,
4F/4m. In order to further study the reaction mechanism, [email protected]/Co0.75 self-assembled monolayer was fabricated by modifying [email protected]/Co0.75 onto the surface of the quartz crystal, and detected the frequency change of the quartz crystal during the catalytic Suzuki reaction by QCM. As shown in Fig. 4, the frequency of [email protected]/ Co0.75 catalyzed Suzuki template reaction shows a trend of ‘‘minus and increase”. At the same time, the change in frequency in the first half (700–1300 s) is more significant than in the second half (1300–1700 s). Correspondingly, in the catalysis process, the quality change of the quartz crystal surface showed a trend of ‘‘increase and decrease”. This showed that the adsorption and desorption of substances existed on the surface of quartz crystal during the

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9971350

D band

G band

9971300 9971250 ID/IG=1.07

120 min

ID/IG=1.06

60 min

ID/IG=1.04

0 min

Intensity (a.u.)

9971200 9971150 9971100 9971050 9971000 9970950 0

1000 2000 3000 4000 5000 6000 7000 8000

Time(s)

60 min

Intensity (a.u.)

0 min

30

40

50

1500

60

70

2000 -1

2500

)

Fig. 6. Raman spectra of [email protected]/Co0.75 at different time.

3.4.7.7. SEM images of [email protected]/Co0.75 in catalyzing process. The changes of surface topography of [email protected]/Co0.75 catalyst applied for catalyze Suzuki reaction at different times was measured by SEM. The original wrinkling structures on catalyst surface remained during the reaction (from 0 h to 2 h) as shown in Fig. 7, indicating that the structure of the [email protected]/Co0.75 catalyst was stable during the catalytic process.

120 min

20

1000

Raman shift (cm

Fig. 4. Resonance frequency changes in the catalytic process of the catalyst [email protected]/Co0.75.

10

500

80

2θ (degree) Fig. 5. XRD patterns of [email protected]/Co0.75 at different time.

catalytic process. In addition, the adsorption rate in the first half was greater than the desorption rate, desorption rate in the second half was greater than the adsorption rate, and the final adsorption and desorption are in equilibrium. 3.4.7.5. XRD patterns of [email protected]/Co0.75 for catalysis. X-ray diffraction (XRD) patterns of [email protected]/Co0.75 catalyst used for catalyzing were tested during the catalytic process (Fig. 5). The XRD spectrum of [email protected]/Co0.75 catalyst showed diffraction peaks at 2h = 11.37° assigned to the characteristic peak of GO before and after reacting. The diffraction peak shifted to a lower angle with increasing reaction time (from 1 h to 2 h), which indicated the interlayer spacing of the GO increased obviously. However, the structure of fresh and used catalysts was not damaged, demonstrating that the [email protected]/Co0.75 catalyst maintained excellent stability. 3.4.7.6. The Raman spectrum of [email protected]/Co0.75 during catalytic process. The Raman spectrum of [email protected]/Co0.75 obtained during catalytic process was shown in Fig. 6. The catalyst revealed two characteristic bands have not disappeared during catalyzing (from 1 h to 2 h). No shifting of characteristic peak was observed, demonstrating that the catalyst possessed good stability. However, Increased D/G intensity ratio could be observed after different reaction times due to the environment changes of the catalyst surface.

3.4.7.8. TEM images of [email protected]/Co0.75 during reaction process. Fig. 8 showed the TEM images of [email protected]/Co0.75 catalyst during the catalytic process. Compare to fresh [email protected]/Co0.75 catalyst (Fig. 8a), the surface basically kept the thin layer with two-dimensional structure after 1 h and 2 h and the shape and arrangement of [email protected]/Co0.75 at different time displayed that similar way which were determined by the lattice fringes (Fig. 8b, c). The lattice spacing of Pd (0) nano-clusters at 0.22 nm. Meantime, it revealed that the surface of catalyst had some nanoparticles, which might be partly formed by cobalt and palladium particles. The results were confirmed by TEM-EDS, in which the uniform distributions of Pd and Co were clearly observed during the reaction progress (Fig. 8d–f). The EDS spectra of [email protected]/Co0.75 catalyst after 1 h and 2 h were shown in Fig. 8d–i, from which the presence of Pd, Co elements in [email protected]/Co0.75 catalyst was demonstrated. 3.4.7.9. XPS of [email protected]/Co0.75 in catalyzing process. XPS is one of the effective ways to study further on catalytic mechanism. For fresh [email protected]/Co0.75 catalyst, the bimetallic Pd/Co was mainly in the form of Pd2+/Co2+. As displayed in Fig. 9a, after 1 h, the peaks at 340.98 eV and 335.64 eV appeared in the Pd (0) spectrum, and corresponded to Pd (0), suggesting that the reduction of Pd (II) to Pd (0) and the real catalytic activity in the reaction was Pd (0). At the end of reaction, it was clearly observed that the bonding energy of Pd decreased during catalyzing (from 0 h to 2 h), and the bonding energy of Pd also decreased even if the catalyst was reused for 4 times, the results indicated that palladium as an electron acceptor, Pd took part in the reaction after receiving electrons. The XPS spectrum of Co2p during the catalytic process is shown in Fig. 9b. After 1 h, the appearance of Co (III) at 779.8 eV and 794.9 eV [76–78], indicating that Co (II) was oxidized to Co (III). The Co (III) still exists after reaction finished and the catalyst repeatedly cycle 4times. From the above XPS spectrum of Co 2p, the Co (II) electrons are transferred during reaction progress. Based on the results obtained above, the [email protected]/Co0.75 catalyzed Suzuki reaction by the synergistic effect of Co (II) transfers electrons to Pd (II) which was made more negative. These active

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Fig. 7. SEM images of the process of catalyst (a) 0 h, (b) 1 h, (c) 2 h.

Fig. 8. TEM and element mapping images of the process of catalyst (a) 0 min, (b) 1 h, (c) 2 h, (d) 1 h, (g) 2 h. Inset: HRTEM. Bottom: Histogram of the Pd (0) nano-cluster diameters during the reaction.

Fig. 9. High resolution XPS spectra of Pd 3d and Co 2p at different times.

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235

Scheme 2. Three -dimension of active centers in catalytic monolayer.

centres formed as 3D image in which the active sites were distributed on the surface, inner of monolayer was presented in Scheme 2. It could enhance catalytic activity because of its benefi-

cial for active sites to contact with substrates in all direction to form intermediates. On the basis of experimental results, as showed that Fig. S11, the DFT calculation of their REDOX potential

Scheme 3. Structure of [email protected]/Co0.75.

Scheme 4. Proposed mechanism.

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shows that Pd (II) is easily reduced, and the Gibbs free energy barrier of the oxidative insertion catalyzed by Pd (0) showed that only Pd (0) as the catalytic active center is favorable for oxidative addition (Fig. S12) [79–81]. These results are consistent with the results of XPS analysis. In addition, there were no results were calculated using Co and Pd (II). In order to determine whether the catalyst ([email protected]/ Co0.75) play an important role in catalytic activity with interior, the experiments was designed and was shown in Scheme 3. Compared with [email protected]/Co0.75, 87% of the catalyst ([email protected]/Co0.75) yield was obtained under the same reaction conditions, indicating that the inner activation center plays a major role. 3.4.7.10. Plausible mechanism for catalyzing Suzuki reaction by [email protected]/Co0.75. The catalytic activity depends on distribution of active sites, electron transferring between bimetals. In the graphene oxide supported catalysts, the graphene oxide possesses such advantages as high conductivity and facilitates electron transfer, which shows a variety of electron transfer routes during catalysis. Based on above analysis, an appropriate reaction mechanism is presented Scheme 4. First, the reactants contact with the catalytic active site through the mass transfer process on the surface and interior surface in different direction, then, part of the Co (II) transfer electron to Pd (II), result in that it became Pd (0) and as rich electron center under reaction conditions. Subsequently, the oxidative addition reaction of Pd (0) with the halogenated aromatic hydrocarbons to form intermediates Br-Pd-Ph. Finally, coupling product produced by reductive elimination desorped from the surface to completed one cycle. 4. Conclusion An ordered imine phenanthroline palladium-cobalt selfassembled monolayer immobilized on GO (called as [email protected]) was fabricated and characterized. Its catalytic activity of Pd to Co with different ratios was investigated, in which [email protected] monolayer as an example was much more active than that of Pd or Co monometallic monolayer for Suzuki coupling reaction in water, which was attributed to better disparity of catalyst, the synergies between Pd and Co and the electron donor properties of GO. Especially, distribution of active sites formed on the surface and inner of monolayer (denoted as 3D catalytic active centres (called as MOFFs) played an important role on their catalytic activity. The heterogeneous catalytic mechanism was systematic studied, in which substrate absorption, catalytic active centre intermediates, and product desorption process mainly occurred at the interface of catalytic monolayer. Acknowledgement The authors gratefully acknowledge the Chinese Academy of Sciences (Grants XDA09030203) for their financial support. The authors thank Prof. Zhi Ma (Institute of Chemistry, Chinese Academy of Sciences, Shanghai) for Raman measurement. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.07.007. References [1] Z.H. Fu, T.S. Li, B. Mu, L.Y. Mao, G.Q. Li, W.J. Xu, Y.J. Wu, J. Mol. Catal. A-Chem. 363 (2012) 200–207. [2] S. Gmouth, H.L. Yang, M. Vaultier, Org. Lett. 5 (2003) 2219–2222. [3] S. Rostamnia, S. Kholdi, J. Phys. Chem. Solids 111 (2017) 47–53.

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