Plasma synthesis of nitrogen-doped porous graphene supporting Pd nanoparticles as a new catalyst for C–C coupling reactions

Abstract

We report an environmentally-friendly approach to the synthesis of hybrids based on porous graphene and metal nanoparticles. The nitrogen-doped porous graphene (N-PG) and Pd nanoparticles decorated N-PG (Pd/N-PG) was synthesized by a plasma method. The N-PG was characterized by X-ray photoelectron spectroscopy and Raman spectroscopy, and the results clearly indicate that the amount of nitrogen doping was 6.65 wt%. The synthesized Pd/N-PG hybrid materials were confirmed by transmission electron microscopy, X-ray diffraction and energy-dispersive X-ray spectroscopy mapping. The hybrid material based on Pd/N-PG as a new catalyst was applied in Suzuki reaction. This catalyst offers a number of advantages such as high stability, easy removal from the reaction mixture and reusability with minimal loss of activity, showing better performance than the well-known commercial Pd/C catalyst.

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Introduction

Graphene is a novel two-dimensional material with atoms arranged in a honeycomb lattice, and has exhibited fascinating exceptional electronic, mechanical, and chemical properties due to its unique physical structure and dimensions.¹ Moreover, graphene has a high surface to volume ratio and a theoretical mass-specific surface area, which is significantly higher than that of the other carbon nanomaterials such as carbon nanotubes, etc.² Therefore, it is widely believed that graphene has a unique advantage as an ideal metal nanoparticle support compared with the other carbon nanomaterials.³ However, the chemical inertness and easy aggregation of graphene results in a weak interaction between graphene and metal nanoparticles and a weak utilization of layer for metal nanoparticles storage, which has been confirmed by experimental and theoretical studies.⁴ To fully utilize graphene layers, doping graphene with heteroatoms can effectively modify its intrinsic properties, including electronic characteristic, surface and local chemical character. Although numerous methods have been developed to produce graphene (e.g., mechanical exfoliation of graphite,⁵ mild exfoliation of graphite,⁶ chemical vapor deposition,⁷ chemical,⁸ electrochemical,⁹ thermal,¹⁰ solvothermal,¹¹ reduction of graphite oxide, etc.), only a few routes have been explored to synthesize nitrogen-doped graphene, including direct synthesis: CVD, solvothermal, arc-discharge approach in the presence of C, N-containing precursors; and postsynthesis treatment: thermal and plasma treatment of synthesized graphene or graphene oxide in an N-containing environment.¹²

The theoretical specific surface area (SSA) of a single graphene sheet is 2630 m² g⁻¹ but the stacking of graphene sheets can lead to their agglomeration, which severely reduces the SSA to less than 700 m² g⁻¹.¹³ In order to make full use of graphene SSA as a heterogeneous catalyst support, the synthesis of graphene based materials always uses graphite oxide as a starting material.^3b However, harsh oxidation conditions of long duration are commonly applied when an acid mixture is used for the intercalation of graphite materials to produce graphite oxide and then graphene oxide (GO).¹⁴ Another alternative method for the synthesis of graphene-based metal nanoparticles is to use nitrogen doped graphene (N-G) as support, which could avoid using GO; N-G usually shows distinctive properties.^2c Recently, a new graphene nanostructure called porous graphene (PG) or graphene nanomesh with unique structural property was prepared by a CVD approach, and the SSA of the as-obtained PG reached 2038 m² g⁻¹, showing great promise as a catalyst support.^2c In this work, we performed a simple plasma method to homogeneously deposit Pd nanoparticles (Pd NPs) on nitrogen doped PG (N-PG), which provides a green route for the synthesis of PG and Pd NPs hybrid materials. In addition, the new catalyst based on N-PG was proven to be stable, and showed high catalytic performance in the Suzuki and Heck reactions.

Experimental section

Materials

All the chemicals were used as received without further purification: bromobenzene, 4-bromoanisole, N-methyl-2-pyrrolidone, 1-butyl-3-methylimidazolium, tetrafluoroborate ([BMIM]BF₄) and palladium 10% on carbon (Aladdin industrial corporation); palladium acetate, 4-bromoacetophenone, benzeneboronic acid, 4-methylbenzeneboronic acid, 2-methoxylbenzeneboronic acid, iodobenzene and styrene (Tokyo Chemical Co., Ltd >40%); 4-bromotoluene, 4-bromochlorobenzene, 1-bromonaphthalene and 4-bromophenol (J&K Scientific Co., Ltd); ethanol, potassium carbonate, nitric acid and hydrochloric acid (Beijing chemical works).

Fabrication of the N-PG

The synthesis of nitrogen-doped porous graphene (N-PG) was prepared by a glow plasma method with nitrogen at room temperature for 90 min.¹⁵ 50 mg PG was dropped into a stainless steel chamber used as the bottom electrode. The glow plasma was generated between the top flat stainless steel (SUS) and bottom electrode by a DC power source (KIKUSUI PMC 500-0.1 A). Nitrogen was introduced and used as the plasma-forming gas. The chamber was a stainless steel chamber with an inner diameter of 70 mm and four glass windows, and the gap between the electrodes was 4 mm. A direct current (DC) power source negative bias prepared by V_DC = 350–400 V was applied to a stainless steel electrode in gas phase for the generation of an N₂ plasma, where the discharge current I was fixed to be 0.03 A, and nitrogen was introduced up to a pressure of 500 Pa. The nitrogen flow was fixed at 204 mL min⁻¹. The plasma treatment time was controlled in 90 min to obtain N-PG samples.

Fabrication of the Pd/N-PG

The Pd/N-PG catalyst was prepared by a GLIP method with Pd(OAc)₂ N-PG at room temperature for 10 min.¹⁶ The glow plasma was generated between the top flat stainless steel (SUS) and bottom ionic liquid electrode. Ar gas was introduced and used as the plasma-forming gas. V_DC = 200–230 V was applied to a stainless steel electrode in gas phase for the generation of an Ar plasma, where the discharge current I was fixed at 0.01 A and the Ar gas was introduced up to a pressure of 140 Pa. 24 mg N-PG was dispersed in 5 mL ethanol by sonication for 30 min. After that, different amounts of Pd(OAc)₂ ethanol solution were added to the dispersed N-PG in ethanol solution, and then the ethanol was evaporated at 40–50 °C. As a result, Pd compounds were well dispersed on the surface of N-PG. The obtained N-PG decorated with Pd compounds were dispersed in an ionic liquid [BMIM]BF₄. For the formation of Pd nanoparticles, electrons were irradiated toward the ionic liquid for 10 min, then the mixture was sonicated in ethanol to remove the excess impurities and extracted from the ionic liquid by a centrifuge process. Several kinds of Pd/N-PG hybrid materials with different Pd weight ratios were prepared, and the respective materials were called Pd-n, (n = 1, 2, 3 and 4).

Pd-2 catalyzed Suzuki reaction

The Pd-2 catalyst corresponding to a percentage of palladium of 0.5 mmol% with respect to 4-bromoacetophenone was used during the reaction process. 4-Bromoacetophenone (0.5 mmol, 100 mg) and phenylboronic acid (0.6 mmol, 73 mg) were mixed in a pressure vial. Then, 2 mL potassium carbonate solution (2 M) and 2 mL ethanol were added in the solution and the mixture was stirred at 60 °C for 12 h. After cooling to room temperature, the mixture was filtered through a polyvinylidene fluoride (PVDF) membrane with 0.2 μm pore size to isolate the catalysts, which were washed with ethyl acetate (10 mL), deionized water (10 mL), and dried at 110 °C. The alcoholic/aqueous solution was extracted with diethylether (10 mL) for three times. The organic extracts were dried with anhydrous magnesium sulfate, filtered, evaporated to dryness, and the mixture was purified with silica gel chromatography to afford 1-([1,1-biphenyl]-4-yl) ethanone (97 mg, 99%) as a white solid.

Characterization of the Pd/N-PG

Transmission electron microscopy (TEM, Tecnai G2, F20) combined with an energy dispersive X-ray spectroscopy (EDS) at an acceleration voltage of 200 kV was used to measure the size, morphology, size distribution and elemental content of Pd/N-PG. X-ray diffraction (XRD, Bruker D8 Advance Germany) was applied to characterize the crystal structure of the hybrid materials, and the data were collected on a Shimadzu XD-3A diffractometer using Cu Kα radiation. The X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha American with an Al Kα X-ray source) was used to measure the elemental composition of samples. The amounts of Pd nanoparticle on the N-PG samples were determined by inductively coupled plasma optical emission spectrometer (ICP-OES). ¹H NMR and ¹³C NMR spectra were recorded on a JNM-LA300FT-NMR for investigating the final product from the Suzuki reaction.

Results and discussion

The detailed synthesis process of PG is described in a previous report.^2b The nitrogen plasma was generated between the top and bottom stainless steel electrodes by applying DC power source (350–400 V) in nitrogen atmosphere at 500 Pa. The plasma treatment time was controlled in 90 min to obtain N-PG samples. The Pd/N-PG was prepared by a GLIP method using Pd(OAc)₂ as a precursor. The experimental configuration used for the synthesis of materials is shown in Fig. 1a. Several kinds of Pd/N-PG hybrid materials with different Pd weight ratios (0.79%, 6.8%, 9.1%, 15%) were prepared, and the respective materials were called Pd-n, (n = 1, 2, 3 and 4). The nitrogen plasma irradiation process was monitored in situ by optical emission spectroscopy (OES), the region of 275–450 nm corresponds to various excited N species (N₂* at 337.1 nm, N₂⁺ at 391.4 nm), as indicated in Fig. 1b. In the case of GLIP process, in addition to the emission of Ar lines in the region of 696–844 nm, several typical peaks, including OH radicals at 308 nm, NH at 336 nm, CN at 357.1 nm and CH at 430 nm are observed in Fig. 1c. These peaks are obviously generated by ion irradiation with high energy, which causes the dissociation of the ionic liquid or Pd(OAc)₂.

Fig. 1 The experimental setup for both nitrogen plasma and the gas-liquid interfacial plasma (a). The optical emission spectra (OES) of the plasma performed in N₂ (b) and Ar atmosphere (c).

To investigate the chemical composition and content of each element in the samples, XPS was carried out by employing a monochromatic Al Kα X-ray source. Fig. 2a shows the XPS survey spectra of pristine PG, N-PG and Pd/N-PG. The spectra reveal that the main chemical components are C 1s, N 1s, O 1s in N-PG sample. The amount of nitrogen incorporated in the N-PG samples is found to be 6.65 wt% for the sample treated by plasma for 1.5 h, indicating that doping is successful. Fig. 2b shows the high-resolution XPS for N 1s, which is split into three types of N-PG structure. One peak is observed at 398.4 eV related to the pyridinic N, which bonds with two C atoms at the edges or defects of graphene. The second peak at 400 eV can be attributed to the pyrrolic N, which refers to N atoms that contribute two p electrons to the π system. In addition, the structure of quaternary N is found at 401.9 eV, in which the N atom is inserted into the graphitic carbon layer and bonded to the three carbon atoms. Three kinds of N dopants would considerably affect the electronic state of PG. Namely, the charge distribution of carbon atoms in PG will be influenced by the doped nitrogen dopants, which further induces the activation region on the PG surface. Our results have demonstrated that N-PG can provide good anchoring sites for the deposition of Pd NPs, which has been proven by the XPS measurement in Fig. 2a, where a distinct peak for Pd 3d is observed from the sample of Pd/N-PG. The two peaks for Pd 3d centered at 335.98 and 341.38 eV are clearly seen in Fig. 2c and are assigned to Pd 3d_5/2 and Pd 3d_3/2, respectively, which indicates the presence of Pd metal Pd(0) in the sample.

Fig. 2 XPS survey spectra of (a) pristine PG, N-PG with nitrogen plasma treatment for 90 min, and Pd/N-PG with Ar plasma treatment for 10 min. (b) The high-resolution of N 1s XPS which is split into three types of N-PG. (c) XPS of Pd 3d_5/2 and Pd 3d_3/2 in the Pd/N-PG sample.

The Raman spectra of PG, N-PG and Pd/N-PG showing several feature peaks can be seen in Fig. 3. The G peak at 1590 cm⁻¹ is due to the normal vibration mode, which relates to the relative motion of C sp² atoms. The D band which can be seen at 1330 cm⁻¹ is related to a disorder-induced mode. The observed strong D band in PG is possibly related to the existence of graphene nanocrystalline with domain boundaries in the sample. At a region with high wavenumbers, the 2D peak at 2657 cm⁻¹ and the combined G + D modes at 2917 cm⁻¹ are observed. The enlarged image of normalized G peak and D band is shown in the inset of Fig. 3; the relatively increased intensity of the D band for N-PG indicates that the content of disordered carbon increases after plasma treatment, mainly by nitrogen doping.

Fig. 3 Raman spectra of pristine PG, N-PG and Pd/N-PG with plasma treatment.

The TEM images of different kinds of Pd-n are shown in Figs 4a–d, indicating that the number of Pd NPs is significantly dependent on the Pd weight ratios during the formation process. The results demonstrate that all the synthesized Pd NPs on N-PG exhibit uniform morphologies, and the particle diameters for Pd-1, Pd-2, Pd-3 and Pd-4 samples are in the range of 5–9 nm. It is important to mention that there is no content of Pd NPs on the surface of PG without N-doping due to the chemical inertness of graphene surface. The crystallinity of the Pd NPs on N-PG is further examined by high-resolution TEM (HRTEM), as shown in Fig. 4e. A typical interfringe distance of 0.223 nm is observed in the HRTEM image, which is the similar to the lattice spacing of the (111) planes of fcc palladium. Fig. 4f represents the selected-area diffraction (SEAD) pattern of Pd/N-PG, indicating the existence of single crystalline Pd grains, which exhibit the (111), (200), (220) and (311) directions. In addition, the Pd content on the N-PG was investigated by ICP-OES, the Pd weight ratios for Pd-1, Pd-2, Pd-3 and Pd-4 are 0.79%, 6.8%, 9.1% and 15%, respectively, indicating a clear increase on increasing the concentration of Pd solution. Therefore, our results confirm that the GLIP method is capable of growing highly-crystalline Pd NPs depositing on graphene without any further thermal process or using a reduction agent.

Fig. 4 TEM images of Pd-1 (a), Pd-2 (b), Pd-3 (c) and Pd-4 (d), corresponding to Pd/N-PG prepared with different Pd weight ratios. An HRTEM image of Pd/N-PG (e) and the SEAD pattern of Pd/N-PG (f).

Moreover, the samples were characterized by XRD, as indicated in Fig. 5. The XRD patterns show characteristic diffraction peaks at 26.5° and 43.2°, which correspond to (002) and (100) reflections of graphite, respectively. The abovementioned peaks weakened in N-PG due to the presence of N- doping. In the case of Pd/N-PG sample, in addition to peaks for graphite, distinct peaks are observed for the Pd at 40.1°, 46.7°, 68.7° and 82.0°, which could be indexed as the (111), (200), (220) and (311) reflections of crystalline Pd(0), respectively. This result is well-consistent with the HRTEM and SEAD characterizations.

Fig. 5 XRD spectra of pristine PG, N-PG, Pd/N-PG.

To obtain direct evidences of the elemental composition and distribution, we carried out EDS mapping of samples during TEM measurement. Fig. 6a shows the dark-field TEM image for the mapping area. Different elements including C and O originating from PG, Cu element attributed to Cu grid for TEM, N from doping and Pd considered to be from Pd NPs, as seen in Fig. 6b. The elemental mapping images for different elements are shown in Fig. 6c–f, where different colors correspond to different elements. It is obvious that the N and Pd NPs are well dispersed on the N-PG surface.

Fig. 6 (a) Dark-field TEM image showing where the elemental maps were obtained, (b) EDS spectrum of Pd/N-PG, (c) EDS C-K map, (d) EDS N-K map, (e) EDS Pd-L map; (f) EDS O-K map.

The abovementioned results demonstrate that Pd/N-PG was successfully prepared, and its application as a new catalyst for Suzuki reaction was explored, as summarized in Table 1. 4′-Bromoacetophenone was reacted with phenylboronic acid in the presence of 0.01 mol% Pd-n using 2 M K₂CO₃ solution and ethanol as solvents. The cross coupling products have a high yield of 99% with Pd-1, Pd-2, Pd-3, Pd-4 as catalyst in 30 min, 15 min, 30 min, 60 min (entries 1–4). The turnover number value of Pd-2 is up to 9900, and the turnover frequency value reaches up to 39 600 h⁻¹. It is obvious that the Pd-2 catalyst is much more active than the Pd/C catalyst (entry 5); even when compared with our previous catalyst based Pd NPs and carbon nanotubes,¹⁵ the Pd-2 catalyst has comparable activity (entry 6).

Table 1 Suzuki coupling reaction under different conditions^a

Entry	Cat.	Time (min)	Yield^b (%)
a Reaction conditions: 4′-Bromoacetophenone (0.5 mmol), Phenylboronic acid (0.6 mmol), and Cat. (0.01 mol%) in 2 M K2CO3 (2 mL) and EtOH (2 mL) at 80 °C.b Isolated yield.
1	Pd-1	30	99
2	Pd-2	15	99
3	Pd-3	30	99
4	Pd-4	60	99
5	Pd/C	105	99
6	Pd/fWCNTs	20	99

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In addition, the scope of the highly reactive Pd-2 catalyzed Suzuki coupling was also examined, as summarized in Table 2. The reaction of bromobenzene 1a with phenylboronic acid 2a was carried out in the presence of Pd-2 of 0.5 mol%, using 2 M K₂CO₃ solution and ethanol as solvent, giving the desired product with 99% yield in 12 h at 60 °C (entry 1). The reaction proceeded well in the presence of Pd-2 of 0.5 mol% using 1.5 equiv. K₂CO₃ in 10 min at 80 °C, showing higher activity than Pd/graphene.¹⁷ All of 1b, 1c, 1d bearing an electron-donating and electro-withdrawing aromatic bromide react with 2a were subject to the same conditions, and excellent yields (up to quantitive) were obtained (entries 3–5). The reactions with the substrates 1-bromonaphthalene 1e, 4-bromophenol 1f and 2a also work well, giving the corresponding product 3ea and 3fa in high yields (entries 6 and 7). Similarly, excellent yields (entries 8 and 9) are also obtained with reactions of 4′-bromoacetophenone 1g, p-tolylboronic acid 2b and 2-methoxyphenyl boronic acid 2c.

Table 2 Pd-2 catalyzed Suzuki coupling reaction^a

Entry	Ar–Br (1)	Ar′–B(OH)2 (2)	Time (h)	Yield^c (%)
a Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), and Pd-2 (0.5 mol%) in 2 M K2CO3 (2 mL) and EtOH (2 mL) at 60 °C.b Reaction conditions: 1 (0.4 mmol), 2 (0.44 mmol), Pd-2 (0.5 mol%), K2CO3 (0.6 mmol), EtOH (0.5 mL), H2O (0.5 mL) at 80 °C.c Isolated yield.
1	C6H5Br (1a)	PhB(OH)2 (2a)	12	99
2^b	C6H5Br (1a)	PhB(OH)2 (2a)	0.17	99
3	4-MeC6H4Br (1b)	PhB(OH)2 (2a)	12	99
4	4-MeOC6H4Br (1c)	PhB(OH)2 (2a)	12	99
5	4-ClC6H4Br (1d)	PhB(OH)2 (2a)	12	99
6	1-C10H7Br (1e)	PhB(OH)2 (2a)	12	99
7	4-HOC6H4Br (1f)	PhB(OH)2 (2a)	12	99
8	4-CH3COC6H4Br (1g)	4-MePhB(OH)2 (2b)	12	99
9	4-CH3COC6H4Br (1g)	2-MeOPhB(OH)2 (2c)	12	99

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Reusability is one of the most important feature of a heterogeneous catalyst, which is superior to a homogenous one. To confirm the reaction catalyzed by solid Pd-2 rather than homogenous Pd species, we first carried out the reaction, then the catalyst Pd-2 was removed by filtration and the solvent was evaporated and treated with nitrohydrochloric acid. The Pd content was examined by ICP-OES analysis, and no leaching of Pd NPs was observed during the reaction. To assess the recyclability of Pd-2, multiple coupling 1g and 2a coupling cycles were carried out by filtration for the separation of the catalyst from the reaction mixture. The catalyst was repeatedly used for four times, and the product 3ga was obtained nearly quantitatively every time (Table 3, entries 1–4).

Table 3 Recycling experiments of Pd catalyst^a

Entry	Catalyst	Time (min)	Yield (%)^c
a Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), and Pd (0.5 mol%) in 2 mL 2 M K2CO3 and 2 mL EtOH at 60 °C.b Reaction carried out at 80 °C.c Isolated yield.
1	fresh	10	99
2	recycle1	60	98
3	recycle2	720	96
4^b	recycle3	480	99

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After the reaction, the catalyst was again examined by TEM, and the image indicates that particle size and morphology of Pd-2 have no significant changes, as indicated in Fig. 7a.

Fig. 7 A TEM image of Pd after recycling experiments (a) and Pd-2 catalyzed Heck coupling reaction (b).

Furthermore, the Pd-2 catalyst was also examined for the Heck reaction between iodobenzene and phenylethylene in the presence of 1.2 equiv. of K₂CO₃. Using N-methyl-2-pyrrolidone (NMP) as the solvent, the product 1,2-diphenylethene was obtained in 93% yield (Fig. 7b). This indicates that such new catalysts show promising applications for the Heck coupling reactions.

Conclusions

In summary, we provide an environmentally-friendly approach to the synthesis of hybrids based on PG and Pd nanoparticles, during which the synthesis of both N-PG and Pd/N-PG is performed by a plasma method. N-PG was characterized by XPS and Raman spectroscopy, and the results clearly indicated that the amount of nitrogen doping was found to be 6.65 wt%. The synthesized Pd/N-PG hybrid materials were confirmed by TEM, XRD and EDS mapping characterizations. Furthermore, the hybrid material as new catalyst was applied for Suzuki reaction. This catalyst offers numerous advantages, such as high stability, easy removal from the reaction mixture and reusability with minimal loss of activity, showing better performance than the well-known commercial Pd/C catalyst. Moreover, the catalyst demonstrated excellent catalytic activity for a remarkable turnover frequency (39 600 h⁻¹) in the Suzuki reactions. This outstanding reactivity and recyclability of the catalyst is attributed to high concentration of Pd(0) NPs dispersed well on the surface of the N-PG. In addition, such new catalysts show promising applications for Heck coupling reactions. Further work is in progress to extend this green approach to synthesize N-PG supported metal nanoparticles as catalysts for organic molecular transformations.

Acknowledgements

We gratefully thank the National Natural Science Foundation of China (Nos. 21106184 and 21202203, 21322609), the Science Foundation Research Funds Provided to New Recruitments of China University of Petroleum, Beijing (Nos. YJRC-2011-18, YJRC-2013-31, YJRC-2013-40), and Thousand Talents Program.

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Footnote

† These authors contributed equally.

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