N-doped graphene confined Pt nanoparticles for efficient semi-hydrogenation of phenylacetylene

Carbon 145 (2019) 47e52

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N-doped graphene confined Pt nanoparticles for efficient semihydrogenation of phenylacetylene Lixin Xia a, Dan Li a, Jun Long b, Fei Huang c, Lini Yang a, *, Yushu Guo a, Zhimin Jia c, Jianping Xiao b, **, Hongyang Liu c, *** a b c

Department of Chemistry, Liaoning University, Shenyang, Liaoning, 110016, China School of Science, Westlake University, Hangzhou, Zhejiang, 310012, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning, 110016, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 November 2018 Received in revised form 2 January 2019 Accepted 3 January 2019 Available online 4 January 2019

N-doped graphene (N-graphene) confined Pt nanoparticles (NPs) with core-shell structure supported on carbon nanotubes (CN@Pt/CNTs) are prepared by a facile two-step process. The obtained N-graphene nanoshell ranging from 2 to 4 graphene layers and the Pt NPs covered within N-graphene are uniformly dispersed on the CNTs. The as-prepared CN@Pt/CNTs exhibits much higher styrene selectivity and robust recycle ability in selective hydrogenation of phenylacetylene, compared with that of traditional CNTs supported Pt NPs (Pt/CNTs). DFT calculation reveals that the high styrene selectivity is derived from the confinement effect of N-graphene, which facilitates desorption of styrene from Pt NPs surface, avoiding the over hydrogenation of styrene to benzylethane. The present method paves a new way to design high selective Pt based hydrogenation catalyst. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction Selective hydrogenation of acetylenic compounds to alkenes in liquid phase is a critical process in the petrochemical production [1]. Semi-hydrogenation of phenylacetylene is a facile process for evaluating the hydrogenation catalysts since it can be carried out under mild condition. Generally, for improving alkene yield, semihydrogenation alkyne to alkene should be maximized and full hydrogenation to alkane should be prohibited [2,3]. To-date, Pd-based catalysts with additives are most widely used for this reaction, but the poor selectivity for high conversion hinders their further applications. Recently, Pt based catalysts (e.g., Pt/CNTs catalyst, Ru@Pt/CNTs catalyst, Pt colloid catalyst) have been successively reported for hydrogenation reaction [4e7]. However, relatively low selectivity and poor stability of Pt based catalysts have raised challenge to design the catalyst with both remarkable activity and selectivity in semi-hydrogenation reaction. In general, one strategy to regulate the catalytic hydrogenation

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (L. Yang), [email protected] (J. Xiao), [email protected] (H. Liu). https://doi.org/10.1016/j.carbon.2019.01.014 0008-6223/© 2019 Elsevier Ltd. All rights reserved.

selectivity over noble metal catalysts (e.g., Pd and Pt) is depositing a second metal or metal oxide on pure metal catalysts or fabricating intermetallic compounds [8e11]. The other way to enhance selectivity is creating a metal-organic interface on Pt or Pd surface [12,13]. Although the approaches have been applied for decades in industry, the poor activity, the presence of toxic additives and the loss of active components still keep driving researchers to search new strategies. Recently, metals nanoparticles (NPs) encapsulated within CNTs and graphene as catalysts have been employed in catalysis [14e19]. The collected results suggest that the confining environment makes it possible to modify the electronic properties of metal NPs, resulting in the dramatic promotion of catalytic performance [20,21]. Moreover, the hollow structure of the CNTs and/or the planar construction of the graphene could serve as a nanoreactor, which may provide a confined space for reactants. Thus, adsorbates (e.g. reactant, intermediates or product) may present novel physical and chemical properties under the confining environment [22,23]. Considering the demand for improving both selectivity and stability for hydrogenation catalysts, in this paper, we developed a facile two-step process for fabricating Pt NPs encapsulated within N-doped graphene (N-graphene) with core-shell structure supported on carbon nanotubes (CN@Pt/CNTs). The as-prepared CN@Pt/CNTs catalyst has a unique structure with Pt NPs underlying the N-graphene nanoshell to form a confinement environment

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for adsorption of reactants and desorption of products, which exhibits a remarkable selectivity of styrene (~90%) at total phenylacetylene conversion (100%) in semi-hydrogenation of phenylacetylene. DFT calculations reveal that the confinement effect of N-graphene can facilitate desorption of styrene from Pt NPs surface and avoids the over hydrogenation of styrene to benzylethane. In addition, the obtained CN@Pt/CNTs performs superior stability in recycling tests, compared with that of traditional Pt NPs supported on CNTs (Pt/CNTs).

collected on a FEI Tecnai G2 F20 transmission electron microscope at an accelerating voltage of 200 kV. Raman spectroscopy was taken on a LabRam HR 800 with a 533 nm laser beam. The samples were characterized by XRD on a Bruker Smart APEX II powder diffractometer with monochromatic Cu Ka radiation at 40 kV and 200 mA. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Ka radiation. The base pressure was 3  109 mbar.

2. Experimental

2.5. Catalytic performance test

2.1. Materials

The hydrogenation reaction of phenylacetylene in liquid phase was carried out in a 50 mL autoclave (silica vessel) under 0.3 MPa H2 pressure at 50
C, and the stirring speed was set at 800 rpm. In a typical procedure, 10 mg catalyst, 1.85 mmol phenylacetylene and 10 mL ethanol were placed in the vessel. After the replacement of air with Ar for 3 times, the autoclave was filled with 3 MPa H2 and then magnetically stirred. The reaction products were monitored and analyzed by gas chromatographic analysis (Agilent 7890A, internal standard n-octane). The conversion, and selectivity were calculated on basis of the following equations:

The CNTs with an average diameter around 50 nm and length up to micrometre were supplied by the applied Science Ltd., USA. Ethylene glycol (C2H6O2) was purchased from Tianjin Yong Da Chemical Regent Company Limited. Concentrated nitric acid (HNO3 68 wt%) were purchased from Tianjin KeMiou Chemical Reagent. Hexachloroplatinic (IV) acid hexahydrate (H2PtCl6$6H2O) were purchased from Shanghai Hu Tai Reagent. 2.2. Functionalization of CNTs CNTs were treated by concentrated nitric acid (68 wt%). 1 g CNTs

Conversionð%Þ

Selectivityð%Þ

Phenylacetylene feed ðmolÞ  Phenylacetylene residue ðmolÞ  100% Phenylacetlene feedðmolÞ

Styrene product ðmolÞ  100% Phenylacetylene feed ðmolÞ  Phenylacetylene residue ðmolÞ

dispersed in 100 mL HNO3 and refluxed at 100
C for 5 h. Then, functionalized carbon nanotubes were washed by deionized water to neutral and dried at 60
C overnight. 2.3. Synthesis of Pt/CNTs and CN@Pt/CNTs 0.2 g functionalized CNTs and 0.275 mL aqueous solution of H2PtCl6 (0.79 mg/mL) were successively dispersed in a mixed solution of 40 mL ethylene glycol under vigorous stirring for 10 min. And then, NaOH/EG (ethylene glycol) solution (0.5 M) was added to the above suspension and adjusted its pH value to 11. Subsequently, the mixture was subjected to 130
C and refluxed for 3 h. Finally, the prepared sample was washed with deionized water, dried at 60
C overnight and marked as Pt/CNTs. The obtained Pt/ CNTs (2 wt% confirmed by ICP) was reduced in H2 at 200
C for 2 h before use. The synthesis strategy of Pt NPs encapsulated within N-graphene supported on CNTs (CN@Pt/CNTs) was illustrated in Scheme S1. The CN@Pt/CNTs was synthesized by a traditional CVD process using acetonitrile as a precursor. The process of coating the Ngraphene layer on Pt NPs at 750
C was just for 20 min, then the asprepared sample was cooled down to room temperature in Ar and denoted as CN@Pt/CNTs. The concentration of nitrogen in CN@Pt/ CNTs is around 2.3 wt% confirmed by the XPS analysis. 2.4. Catalyst characterization Transmission electron microscopy (TEM) micrographs were

(1)

(2)

2.6. DFT calculation DFT calculations were performed with VASP 5.3.5 code [24,25]. The revised Perdew-Burke-Ernzerhof (revPBE) [26] exchangecorrelation functional was adopted as the generalized-gradient approximation functional. The valence orbital of involved atoms was described by plane-wave basis sets with cutoff energies of 400 eV [27]. A Gaussian smearing with the width of 0.2 eV was used. The convergence criteria for the energy and force were set to 104 eV and 0.05 eV/Å. A 16.9  16.9 Å Pt (111) supercell was used to model the Pt nanoparticles and the CN@Pt is modeled by covering a single layer of N doped graphene to Pt (111). The relaxed structures of Pt (111) and CN@Pt (111) are shown in Fig. S4. In order to model the catalytic hydrogenation of phenylacetylene, the adsorption energy (Ead) of relevant intermediates and reaction energy (Er) of each elementary step were calculated. Ead ¼ Eslab*A-Eslab-EA, where the Eslab*A, Eslab and EA are the total energy of substrate with the adsorbate, clean slab and isolated intermediate, respectively. Er ¼ Efs-Eis, where the Eis and Efs represent the total energy of the corresponding initial state and final state, respectively. To evaluate the energy barriers (Ea) of some key steps, the transition states were computed with the climbing image nudged elastic band (CI-NEB) method [28,29]. Ea ¼ Ets-Eis, where Ets is the energy of transition states. In addition, the free energy change (DG) for the desorption of styrene was calculated as DG ¼ -Ead(Styrene) þ DZPE-TDS, where the Ead(Styrene) is the adsorption energy of styrene, DZPE and DS are the difference of zero point energy and entropy between free

L. Xia et al. / Carbon 145 (2019) 47e52

and adsorbed styrene molecule, respectively. Construction of thermodynamic model. As known, the linear scaling relation between the adsorption energy of complex intermediates and some certain molecules has been proved widely. Herein, the adsorption energy of intermediates in hydrogenation of phenylacetylene were described as a function of the adsorption energy of CH3 over Zn (0001), Ag (111), Pd (111) and Pt (111) surface. The results are shown in Fig. S7 and the function can be written as: Ead(Int) ¼ k$Ead(CH3) þ b

(3)

where the Ead(Int) and Ead(CH3) represent the adsorption energy of the intermediates and CH3, respectively. Then, the reaction energy of each elementary step can be calculated via Ead(Int). For example, for elementary step A*þB* /AB*, Er ¼ Ead(AB)-Ead(A)-Ead(B)

(4)

where the Ead(AB),Ead(A) and Ead(B) represent the adsorption energy of A, B and AB, respectively, and they are referenced to a same criterion. Combined (3) and (4), we can construct the linear scaling relation between Er of each elementary step and Ead of CH3. 3. Results and discussion The preparing process of CN@Pt/CNTs is illustrated in Scheme S1. Firstly, Pt NPs with average size around 2.6 nm (Figs. S1 and S2) were uniformly supported on CNTs by the wet chemical reduction method. Then, the obtained Pt/CNTs was encapsulated with N-graphene via a traditional CVD process and acetonitrile was employed as the precursor for the fabrication of well-structured layered N-graphene. TEM results as shown in Fig. 1A, B and 1C display that Pt NPs in CN@Pt/CNTs are still uniformly distributed on the surface of CNT and each Pt NP is mainly covered by 2e4 layers of N-graphene. The average size of Pt NPs in CN@Pt/CNTs is about 4.6 nm as displayed in Fig. S3, which is gathered slightly during the

Fig. 1. TEM (A) and HRTEM (B, C, D) images of the obtained CN@Pt/CNTs sample.

49

CVD process, compared with that of initial Pt/CNTs (Fig. S2). However, the loading amount of Pt in CN@Pt/CNTs is still about 2 wt %, which was confirmed by the ICP analysis, indicating that there is no Pt NPs loss after the CVD process at 750
C. In addition, the HRTEM image of randomly selected Pt NPs displayed in Fig. 1D indicates that the obtained Pt NPs present the face-cantered cubic (FCC) Pt lattice with a Pt (111) interlayer spacing of 0.22 nm. Raman spectra were used to characterize the structure of Pt/ CNTs and CN@Pt/CNTs. As shown in Fig. 2A, two intensive peaks at 1358 cm1 and 1590 cm1 are attributed to D and G bands of carbon materials, respectively. The ratio of ID/IG is generally used to reflect the surface structure of carbon materials. And the ratio of ID/IG of CN@Pt/CNTs is higher than that of Pt/CNTs (0.79 via 0.68), which indicated that more defects were created on CNTs by covering Ngraphene layers. Thus, we proposed that hydrogen and small molecular reactants could diffuse directly through defective N-graphene shell and the hydrogenation reaction occurs on the surface of Pt NPs [31]. The XRD measurement was carried out to further investigate the structure of the Pt/CNTs and CN@Pt/CNTs samples. As shown in Fig. 2B, the XRD pattern of the Pt/CNTs display two distinct peaks at 2Ɵ ¼ 25.7
and 43.1
, which correspond to the graphite structure (002) and the graphite structure (101) of the carbon materials. While the diffraction peak of Pt (111) (JCPDS PDF#65-2868) was especially weak, confirming that the smaller Pt NPs are uniformly dispersed on the surface of CNTs (Fig. 2B). X-ray photoelectron spectroscopy (XPS) was employed to study the chemical states of Pt species and N species. As shown in Fig. 2C, the most intense peaks (71.3 eV and 74.6 eV) are attributed to the metallic Pt in Pt/CNTs and CN@Pt/CNTs, respectively [30]. Notably, Pt NPs on CN@Pt/CNTs tends to have more Pt0 species (77.2% Pt0 and 22.8% Pt2þ) than that on Pt/CNTs (68.8% Pt0 and 31.2% Pt2þ), and after the Pt NPs covered with N-graphene layers, the peak of Pt (0) shifted to the lower level [32]. These results suggest that the Pt electron density of CN@Pt/CNTs slightly increased by the electrons transferred from N to Pt species through the interaction between the metal and N-graphene [7,33]. As displayed in Fig. 2D, after the CVD process at 750
C, carbonization of acetonitrile takes place and signals from pyridinic (N 1s 398.4 eV), pyrrolic (N 1s 400.1 eV), graphitic (N 1s 401.1 eV) and oxidized (N 1s 403.1 eV) nitrogen are observed [19e21]. (Relative content of nitrogen forms calculated from N 1s XPS spectra were shown in Table S1). The dominated nitrogen species in CN@Pt/CNTs are graphitic and pyridinic nitrogen. All these characterizations reveal that Pt NPs were successfully encapsulated with N-graphene shell by our facile two-step method. The selective hydrogenation of phenylacetylene was used as a model reaction to evaluate the catalytic behavior of Pt/CNTs and CN@Pt/CNTs. Fig. 3 shows the conversion and selectivity (Fig. 3A and B) and the recyclability (Fig. 3C) of Pt/CNTs and CN@Pt/CNTs for selective hydrogenation of phenylacetylene, respectively. It can be observed that the Pt/CNTs catalyst shows lower selectivity and stability during the hydrogenation reaction. Notably, it is quite easy to over hydrogenation to produce benzylethane over the pure Pt/ CNTs catalyst. The selectivity toward styrene decreases dramatically from 100% to 18% at full conversion of phenylacetylene. As for CN@Pt/CNTs, it is hard to over hydrogenation after the conversion reaches 100%, the selectivity of styrene still remains at ~90% after 120 min reaction (Fig. 3B). We proposed that the excessive hydrogenation of phenylacetylene to phenylethane can be inhibited by encapsulating the Pt NPs within N-graphene. In order to understand the improved styrene selectivity of CN@Pt/CNTs, density function theory (DFT) calculations were carried out. First, we constructed a thermodynamic model for understanding the trend of phenylacetylene hydrogenation to styrene. To model this process, close-packed Zn (0001), Ag (111), Pd (111) and Pt (111) were chosen to establish the scaling relation between the

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Fig. 2. Raman spectra (A), the XRD patterns (B), Pt4f XPS spectra (C) of Pt/CNTs and CN@Pt/CNTs, and N1s XPS spectra (D) in CN@Pt/CNTs. (A colour version of this figure can be viewed online.)

reaction energies (Er) of each elementary steps and the adsorption energies of CH3 (Ead(CH3)), as shown in Fig. 4A. Step 1 to 4 are the adsorption of phenylacetylene, hydrogenation of phenylacetylene to C6H5C¼CH2, hydrogenation of C6H5C¼CH2 to styrene and desorption of styrene, respectively. Step 40 represents the over hydrogenation reaction of styrene, which is the competing reaction of Step 4. As shown in Fig. 4, the Step 4 is the rate-determining step for hydrogenation of phenylacetylene to styrene as the Ead(CH3) on catalysts is stronger than 1.55 eV, and the activity increases with weakening Ead(CH3). However, once the Ead(CH3) shifts over 1.55 eV, Step 1 will limit the total reaction rate, and it decreases gradually. Hence, the highest activity occurs at Ead(CH3) with 1.55 eV. The catalysts with a high styrene selectivity require desorption of styrene easier than its hydrogenation. To analyse the selectivity, the energy barrier of Step40 are calculated explicitly, denoted as Ea(Step40 ). With weakening the Ead(CH3) on catalysts, the DG(Step4) decreases more sharply than Ea(Step40 ), implying that the styrene selectivity of catalysts will increase with the Ead(CH3) weakening. Thus, with the scaling relation in Fig. 4A, the activity and selectivity of catalysts to hydrogenate phenylacetylene can be easily predicted by calculating the adsorption energy of CH3. Then, we calculated the adsorption of CH3 on Pt (111) and CN@Pt (111) (Fig. S4), with the results of Ead(CH3) equalling to 2.53 and 2.27 eV, respectively. The weaker Ead(CH3) on CN@Pt (111) implies a better styrene selectivity according to the analysis above, which is consistent with our experimental results, demonstrating the practicability of this method. Although the scaling relation can indicate the trend of activity and selectivity of catalysts to some extent, it is not sufficiently accurate when there are little energetic

differences at crossing points of two elementary steps. Take Pt as an example, the present experiment shows that the styrene selectivity is only 18% at full conversion (Fig. 3A), implying that hydrogenation of styrene on Pt is easier than its desorption. However, the scaling relation shows that Ea(Step40 ) and DG(Step4) of Pt are comparable, even Ea(Step40 ) is a slightly higher than DG(Step4). Hence, for accurate analysis, the DG(Step4) at experimental temperature and Ea(Step40 ) are calculated explicitly, over Pt (111) and CN@Pt (111), respectively. The potential energy diagram and geometry structures of involved intermediates are displayed in Fig. 4B. DG(Step4) (0.80 eV) is higher than Ea(Step40 ) (0.63 eV) on Pt (111), revealing that the over hydrogenation of styrene is slightly favourable than desorption on Pt. On the contrary, for CN@Pt (111), the DG(Step4) (0.16 eV) is much lower than Ea(Step40 ) (0.61 eV), implying that the styrene desorption is highly preferred. In this way, the high styrene selectivity of CN@Pt/CNTs can be explained well. According to above analysis, one of the principles to improve the styrene selectivity of catalysts in hydrogenation of phenylacetylene is to weaken the adsorption of styrene on Pt catalysts. It has been proved in previous works that confinement effect is able to weaken the binding energies of molecules or intermediates on catalyst surfaces [34e36], hence, that is a promising strategy to improve the selectivity of hydrogenation of phenylacetylene to styrene. The recyclability of Pt/CNTs and CN@Pt/CNTs was investigated under the same condition as displayed in Fig. 3C. Notably, the conversion of phenylacetylene over Pt/CNTs catalyst is seriously decreased (from 100% to 10%) after four Runs. While, the conversion of phenylacetylene over CN@Pt/CNTs catalyst still remains 100% after 4 cycles, revealing a preferable stability of the CN@Pt/ CNTs catalyst. Fig. S5 shows a remarkable loss of Pt NPs for the used

L. Xia et al. / Carbon 145 (2019) 47e52

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Fig. 3. The conversion and selectivity (A, B) and the recyclability (C) of Pt/CNTs and CN@Pt/CNTs samples or selective hydrogenation reaction of phenylacetylene. (Reaction conditions: 10 mg of catalyst, 1.85 mmol of phenylacetylene, 0.3 MPa H2, and 50
C). (A colour version of this figure can be viewed online.)

Fig. 4. (A) Scaling relation between Ead(CH3) and reaction energies (Er) of each elementary steps (black line), the energy barrier of Step 40 (blue line) and free energy change of Step 4 (red line). Scattered points represent the corresponding value calculated via Ead(CH3) on Pt(111) (triangles) and CN@Pt(111) (circles), respectively. (B) Potential energy diagram of desorption and hydrogenation of styrene on Pt (red line) and CN@Pt (black line). Inserts are geometry structures of involved intermediates. C8H6, C8H7, C8H8 and C8H9 represent phenylacetylene, C6H5C¼CH2, styrene and C6H5CHCH3, respectively. Asterisks represent active sites. The blue, gray, green and white balls represent Pt, C, N and H atom, respectively. (A colour version of this figure can be viewed online.)

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Pt/CNTs. But there is almost no change of the distribution of Pt NPs in the used CN@Pt/CNTs catalyst (Fig. S6). These results prove that the stability of CN@Pt/CNTs catalyst is dramatically improved after encapsulating within N-graphene layer.

[10]

[11]

4. Conclusions [12]

In summary, we present a new way to design highly selective Pt based hydrogenation catalyst by encapsulating supported Pt NPs within the N-graphene shell (CN@Pt/CNTs). The as-prepared CN@Pt/CNTs exhibits remarkably high selectivity (~90%) for the hydrogenation of phenylacetylene to styrene with a 100% conversion rate. In addition, the CN@Pt/CNTs show quite stable catalytic performance in recycling test. The superior hydrogenation performance is linked to the unique N-graphene encapsulating structure. DFT calculations reveal that the confinement effect of N-graphene is responsible to the high styrene selectivity of CN@Pt/CNTs, which promotes the desorption of styrene from the catalyst and avoids the over hydrogenation of styrene to benzylethane.

[13]

[14]

[15]

[16]

[17]

Acknowledgements [18]

The present work is supported by the National Natural Science Foundation of China (21773101, 21671089), the Scientific Research Fund of Liaoning Provincial Education Department (LZD201601, LZ2014001, LT2015012) and the Scientific Research Fund of Liaoning University (LDQN2015008). This work is also supported by the Ministry of Science and Technology (MOST) 2016YFA0204100, the National Natural Science Foundation of China (91845201, 21573254, 91545110) and Youth Innovation Promotion Association, Chinese Academy of Sciences. J.X. acknowledges the support from Westlake Education Foundation and the Supercomputing Clusters at Westlake University.

[19]

[20]

[21] [22]

Appendix A. Supplementary data

[23]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.01.014.

[24]

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