物 理 化 学 学 报
Acta Phys. -Chim. Sin. 2021, 37 (10), 2001025 (1 of 8)
Received: January 7, 2020; Revid: February 10, 2020; Accepted: February 26, 2020; Published online: February 28, 2020. *
Corresponding authors. Emails: jinshui.zhang@ (J.Z.); zhanwc@ (W.Z.). Tel.: +86-137******** (J.Z.); +86-138******** (W.Z.). †
The authors contributed equally to this work.
The project was supported by the National Natural Science Foundation of China (21972022, U1805255), the 111 Project (D16008), Natural Science Foundation of Fujian Province, China (2018J01681), and the Independent Rearch Project of State Key Laboratory of Photocatalysis on Energy and Environment (SKLPEE-2017A03).
国家自然科学基金(21972022, U1805255), 111项目(D16008), 福建省自然科学基金(2018J01681)和能源与环境光催化国家重点实验室自主研究项目(SKLPEE-2017A03)资助项目
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[Article]
doi: 10.3866/PKU.WHXB202001025次开头的成语
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Hollow Nitrogen-Rich Carbon Nanoworms with High Activity for Metal-Free Selective Aerobic Oxidation of Benzyl Alcohol
Ping An 1,†, Yu Fu 2,†, Danlei Wei 1, Yanglong Guo 2, Wangcheng Zhan 2,*, Jinshui Zhang 1,*
1 State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University,
Fuzhou 350108, China.
2 Key Laboratory for Advanced Materials and Rearch Institute of Industrial Catalysis, School of Chemistry and Molecular
Engineering, East China University of Science and Technology, Shanghai 200237, China.
Abstract: Carbon materials have become one of the rearch hotspots in the field of catalysis as a typical reprentative of non-metallic catalytic materials. Herein, a facile synthetic strategy is developed to fabricate a ries of hollow carbon nanoworms (h-NCNWs) that contain nitrogen up to
9.83 wt% by employing graphitic carbon nitride (g-C 3N 4) as the sacrificing template and solid nitrogen source. The h-NCNWs catalysts were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscope (HR-TEM), N 2 adsorption-desorption, Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric (TG), Raman spectra, and X-ray photoelectron spectroscopies (XPS). The catalytic activities of the h-NCNWs catalysts for lective oxidation of benzyl alcohol with O 2 were also evaluated. The characterization results revealed that the h-NCNWs catalysts
displayed a unique hollow worm-like nanostructure with turbostratic carbon shells.
The nitrogen content and shell thickness can be tuned by varying the relative ratio of resorcinol to g-C 3N 4 during the preparation process. Furthermore, nitrogen is incorporated to the carbon network in the form of graphite (predominantly) and
pyridine, which is critical for the enhancement of the catalytic activity of carbon catalysts for the lective oxidation of benzyl alcohol. At a reaction temperature of 120 o C, a 24.9% conversion of benzyl alcohol with > 99% lectivity to benzaldehyde can be achieved on the h-NCNWs catalyst prepared with a mass ratio of resorcinol to g-C 3N 4 of 0.5. However, the catalytic activities of the h-
NCNWs catalysts were dependent on the amount of N dopants, in particular graphitic nitrogen species. The conversion of benzyl alcohol markedly decread to 13.1% on the h-NCNWs catalyst prepared with a mass ratio of resorcinol to g-C 3N 4 of 1.5. Moreover, the h-NCNWs catalyst showed excellent stability during the reaction process. The conversion of benzyl alcohol and the high lectivity to aldehyde can be kept within five catalytic runs over the h-NCNWs0.5 catalyst. The results indicate that rationally designed carbon materials have great potential as highly efficient heterogeneous catalysts for oxidation reactions.
关于植树的故事Key Words: Hollow nanostructure; Nitrogen doping; Metal-free catalyst; Selective oxidation; Benzyl alcohol
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富氮空心蠕虫状碳材料的合成及其苯甲醇非金属选择性催化氧化的高活性
中国人寿养老保险
安平1,†，付宇2,†，韦丹蕾1，郭杨龙2，詹望成2,*，张金水1,*
1福州大学化学学院，能源与环境光催化国家重点实验室，福州350108
2华东理工大学化学与分子工程学院，工业催化研究所，上海200237
摘要：作为非金属催化材料的典型代表，碳材料已经成为催化领域的研究热点之一。本文采用石墨化氮化碳(g-C3N4)为硬模板，通过先包覆树脂后碳化的简便方法合成中空碳材料(h-NCNW)。采用X射线衍射(XRD)、高分辨率透射电子显微镜(HR-TEM)、N2吸附-脱附、傅立叶变换红外光谱(FT-IR)、热重(TG)、拉曼光谱和X射线光电子能谱(XPS)对h-NCNWs 碳材料进行表征，并且研究了h-NCNWs催化剂对O2选择性氧化苯甲醇的催化活性。结果表明，通过在g-C3N4纳米片外围包覆由间苯二酚和甲醛前驱体缩合形成的薄层树脂，并在惰性气氛下进行高温热处理，去除g-C3N4硬模板，最终得到富氮(9.83%)空心蠕虫状碳材料，并且可通过改变制备过程中间苯二酚与g-C3N4的相对比例来调整氮含量和壳厚度。在h-NCNW中，杂原子N主要以石墨型和吡啶型的形式化学地结合到碳骨架中，其中石墨型N占主导地位，使得该材料在苯甲醇氧化反应中表现优异的催化活性。在120 o C时，采用间苯二酚与g-C3N4质量比为0.5制备的h-NCNWs催化剂，苯甲醇转化率为24.9%，苯甲醛选择性> 99%。但是N掺杂量会显著影响h-NCNWs催化剂的催化活性，采用间苯二酚与g-C3N4质量比为1.5制备的h-NCNWs催化剂，苯甲醇转化率降低至13.1%。另一方面，h-NCNWs催化剂在反应过程中显示出优异的稳定性，在五次循环使用过程中催化剂的活性保持不变。综上所述，理性设计和合成的碳材料作为多相氧化反应的催化剂具有巨大的潜力。
关键词：中空纳米结构；N掺杂；非金属催化剂；选择氧化；苯甲醇
中图分类号：O643
1 Introduction
Transition and noble metals are widely ud as active components for most of the heterogeneous catalytic redox reactions, owing to their empty d or f orbits that can be combined with the substrate molecules to form a low barrier transition state. Nevertheless, it was generally believed that nonmetallic substances are quite difficult to participate in the catalytic reactions due to the abnce of empty orbits. In the last two decades, carbon nanomaterials (e.g., activated carbon, fullerene, carbon nanotubes, graphene, nanofibers, nanodiamond) have aroud extensive concern owing to their unique physical and chemical properties 1–6, which give birth to moderate catalytic activity in many reactions, such as hydrogenation of acetylene and nitrobenzene 7,8, oxidation of benzylamine and aromatic aldehyde 9,10, hydroxylation of acetylene and benzene 11,12, and as well as electrochemical oxygen reduction reaction (ORR) 13–16. However, the activity of carbon materials in the reactions is typically veral orders of magnitude lower than that of metallic materials. To improve the catalytic performance, carbon materials are generally modified with hetero-atom dopants, and then their catalytic behavior is cloly depended on the kinds of hetero-atom dopants. For example, doping carbon materials with more electronegative nitrogen atoms (3.04 for N vs 2.55 for C) can induce positively charged carbon atoms, leading to novel functions for redox reactions 17
–20. Therefore, the control synthesis of carbon materials with doped foreign atoms is facile strategy to advance carbon materials for heterogeneous catalysis. Hollow nanostructured carbon materials have gained significant interest for a variety of applications, including drug relea, fuel cells, energy storage, gas storage and paration, lithium-ion batteries, and as well as catalysis 21–25, becau of their high surface area and surface-to-volume ratio. Recently, the preparation of hollow carbon materials with controlled size, wall thickness and geometry is extremely appealing, and veral strategies have been adopted for their synthesis. For example, hard template methodology is a straightforward approach using silica or polymer nanoparticles to direct the formation of hollow nanostructured carbons 26–29. However, it is formidable to combine the removal of hard template with the doping of hetero-atoms during the preparation of hollow nanostructured carbon materials, and thus multi-step process is employed generally. Fortunately, using graphitic carbon nitride (g-C3N4) as the template is a feasible approach to achieve two things at one stroke. g-C3N4 is a conjugated organic compound with N content of > 50% 30,31. Therefore, g-C3N4can be utilized as both sacrificing template and solid nitrogen source for the preparation of hollow carbon nanostructures with high content N dopants. Herein, a hollow carbon material doped with N atoms was prepared by a facile approach bad on carbonization of resins coated onto g-C3N4, in which g-C3N4was employed as the sacrificial template and solid nitrogen source. In Scheme 1, coating g-C3N4cores with resorcinol and formaldehyde
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precursors results in a thin layer of resins on their surfaces. The following thermal treatment under inert atmosphere decompos g-C3N4 to generate hollow carbon nanoworms with thin carbon shells containing nitrogen atoms up to 9.83% (w, mass fraction). The lective aerobic oxidation of benzyl alcohol molecules was lected as the model reaction to better evaluated their metal-free catalytic performance.
2 Experimental ction
2.1 Chemicals
AR grade powders of urea (CH4N2O, 99.5%), ethanol (C2H6O, 99.5%), ammonia aqueous solution (NH4OH, 28% (w)) and formaldehyde solution (HCHO, 37% (w)) in stoichiometric ratio were ud without further purification.
2.2 Preparation of hollow nitrogen-rich carbon
nanoworms (h-NCNWs)
2.2.1 Preparation of g-C3N4
10 g of urea was placed in a crucible covered with a lid and then heated at 550 °C for 4 h at a heating rate of 4 °C∙min−1. The light yellow solid obtained (designated as g-C3N4) was ground into powder before usage.
2.2.2 Preparation of g-C3N4@RF and h-NCNWs Typically, 0.4 g of g-C3N4 was first disperd in the solution of 20 mL of deionized water and 8 mL of ethanol in a round-bottom flask. After sonicating for 2 h, the mixture was heated to 30 °C, and then 0.1 mL of ammonia aqueous solution (NH4OH, 28% (w)) was added followed by stirring at 600 r∙min−1 for 30 min. Subquently, 0.2 g of resorcinol was added and dissolved under stirring for 30 min. 0.28 mL of formaldehyde solution (HCHO, 37% (w)) was then added to the reaction solution and further stirred for 24 h. Then the mixture was transferred into a 50 mL Teflon-lined autoclave and heated at 100 °C for 24 h. The as-prepared solid sample was parated by centrifugation and washed with water and ethanol. Finally, the solid was dried at 100 °C for 24 h to obtain g-C3N4@RF0.5.
The solid sample was also prepared without g-C3N4 with the same procedures as mentioned above, the obtained sample was designated as RF0.5. Fig. S1 (in Supporting Information) shows the photos of the g-C3N4, g-C3N4@RF0.5 and RF0.5 samples, indicating that the color of g-C3N4 template changed from light yellow to orange after coating with resins.
The g-C3N4@RF0.5 sample was then heated to 900 °C at a heating rate of 5 °C∙min−1 under a gas flow of 30 mL∙min−1 N2 and kept at this temperature for 1 h. After cooled to room temperature, the sample obtained was designated as h-NCNWs0.5. Other g-C3N4@RF x and h-NCNWs x samples were prepared by the same procedure except different amount of resorcinol and HCHO, in which x reprents the mass ratio of resorcinol to g-C3N4. In detail, h-NCNWs1 was prepared with 0.4 g of resorcinol and 0.56 mL of HCHO, while 0.6 g of resorcinol and 0.84 mL of HCHO were ud for the preparation of h-NCNWs1.5. Carbon sample was also prepared from RF0.5 by the same procedure as a reference, and designated as C-RF.
2.3 Catalytic activity testing
The lective oxidation of benzyl alcohol was carried out in a 50 mL autoclave lined with polytetrafluoroethylene (PTFE). 0.5 mmol of benzyl alcohol, 50 mg of catalyst and 3 mL of ethanol were added into the reactor. After purging with O2for three times, the pressure of O2 was adjust to 0.1 MPa, and then the reactor was heated to a certain temperature under stirring at 500 r∙min−1. After the reaction, the reactor was quenched with ice water to avoid the loss of volatile organics. The reaction mixture was diluted with ethanol to completely dissolve the side products. After the catalyst parated by centrifugation, a certain amount of cyclooctane was added as an internal standard sub
stance. The reaction products were analyzed by Agilent gas chromatograph (GC) 7890B equipped with an HP-5 capillary column and a flame ionization detector (7890B, Agilent Technologies, USA). In addition, the side products were further identified using Agilent 7890A-5975C gas chromatograph-mass spectrometry (GC-MS, Agilent Technologies, USA). The conversion of benzyl alcohol (Conv.) and the lectivity to benzaldehyde (Select.) were calculated as follows:
Conv.=
C(benzyl alcohol)
before
-C(benzyl alcohol)
after
C(benzyl alcohol)
before
×100% Select.=
C(benzaldehyde)
C(benzyl alcohol)
before
-C(benzyl alcohol)
after
×100% where C(benzyl alcohol)before and C(benzyl alcohol)after are the concentrations of alcohol before and after reaction, respectively. C(benzaldehyde) is the concentration of benzaldehyde generated during the reaction process.
The recycling test of the catalyst was carried out and the catalyst was ud for five times in the reaction. After every run, the catalyst was parated from reaction solution by centrifugation, washed with ethanol, and then dried in the air at 80 °C to constant weight.
2.4 Characterization
蒲城美食
X-ray diffraction (XRD) data were collected on a D8 diffractometer (Bruker, Germany) using Cu Kα radiation (40 kV, 40 mA) at room temperature. The high-resolution transmission electron microscope (HR-TEM) analysis were performed on the JEM-2100 microscope (JEOL, Japan) at an accelerating voltage of 200 Kv. N2 adsorption-desorption isotherms of samples were obtained at −196 °C on a Micrometrics ASAP 2020M Sorptometer (Micromeritics, USA) using static adsorption procedures. Prior to the measurements, the sample was degasd at 120 °C for 12 h. The pore size distribution curves were calculated from the desorption branch by the Barrett-Joyner-
Scheme 1 Formation process of hollow nitrogen-rich
carbon nanoworms.
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Halenda (BJH) method. The FT-IR spectra of samples were recorded on a Nexus 670 FT-IR spectrometer (Nicolet, USA), and the samples were ground with anhydrous KBr and presd into thin wafers. Elemental analysis of carbon was measured on a Vario EL III Analyzer (Elementar, Germany). The Raman spectra of samples were recorded on an IuviaRefl equipment (Renishaw, England). X-ray photoelectron spectroscopies (XPS) were analyzed on an Escalab 250Xi photoelectron spectrometer (Thermo Scientific, USA) equipped with Al K α radiation as the excitation source. Thermal gravimetric (TG) analysis and the corresponding mass spectrometric (MS) signals were performed at a heating rate of 10 °C∙min −1 from 40 °C to 800 °C in air using a Pyris 1 TGA thermogravimetric analyzer (PerkinElmer, USA) coupled with Hiden HPR 20 mass spectrometer.
3 Results and discussion
3.1 XRD
The powder XRD patterns of the h-NCNWs samples are depicted in Fig. 1. All the samples exhibited two broad diffraction peaks corresponding to the (002) and (100) diffraction modes at 24.1° and 43.9°, indicating the typical disordered carbonaceous structure. However, the interlayer spacing
(d 002) of all samples is about 0.368 nm, which is wider than that of classical graphite (0.336 nm). As a comparison, the precursor g-C 3N 4@RF0.5 sample, showed the typical diffraction peaks of g-C 3N 4 at 27.4° and 13.1° (Fig. S2, in Supporting Information). The results revealed that the thermal treatment can effectively decompo and carbonize the g-C 3N 4@RF0.5 sample. 3.2 TEM
The morphology and nanostructure of h-NCNWs were characterized by TEM experiments. In Fig. S3 (in Supporting Information), the g-C 3N 4@RF sample exhibited a typical core-shell structure, indicating that g-C 3N 4 has been coated with resins. After being subjected to the thermal treatment, h-NCNWs samples displayed a unique hollow worm-like nanostructure with turbostratic carbon shells (Fig. 2). It should be pointed out that the preparation method makes it available to tune the shell thickness of h-NCNWs through adjusting the ratio of resorcinol to g-C 3N 4. The thickness of weakly ordered shell is determined
to be ca. 6 nm, 9 nm and 14 nm for the h-NCNWs0.5, h-NCNWs1 and h-NCNWs1.5, respectively. 3.3 N 2 adsorption-desorption
怎么设置桌面壁纸Fig. 3 shows N 2 adsorption-desorption isotherms and the corresponding pore size distribution curves of the h-NCNWs samples. All isotherms exhibited a typical IV type isotherm of mesoporous m
aterials. The textural information of the h-NCNWs were summarized in Table 1. The Brunauer-Emmett-
Fig. 1 XRD patterns of the h-NCNWs samples.
1020304050
607080
(1 0 0)
h -NCNWs1.5
h -NCNWs1
I n t e n s i t y (a .u .)
2θ / (o
)
h -NCNWs0.5
(0 0 2)
Fig. 2 HR-TEM images of the h-NCNW0.5 (a, b), h-NCNW1 (c, d),
h-NCNW1.5 (e, f) samples
Fig. 3 N 2 adsorption-desorption isotherms (a) and the corresponding
pore size distribution curves (b) of the h-NCNWs samples.
Q u a n t i t y A d s o r b e d (c m 3ꞏg -1 S T P )
Relative Pressure (P/P 0)
d V /d l o g (D ) P o r
e V o l u m e (c m 3ꞏg -1⋅n m -1)
Pore Diameter (nm)
新郎讲话
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Teller (BET) surface areas of the h-NCNWs0.5, h-NCNWs1 and h-NCNWs1.5 samples are 440, 351, 459 m 2∙g −1, respectively. The pore size of the h-NCNWs1 sample decread from 9.4 to 5.5 nm co
mpared to the h-NCNWs0.5 sample (Fig. 3b). The distribution at 3.8 nm for the h-NCNWs1 sample is due to the tensile strength effect 32. However, the pore size was truly decread to about 3.8 nm for the h-NCNWs1.5 sample. Meanwhile, the h-NCNWs0.5 sample exhibited a large pore volume (1.40 cm 3∙g −1), which is more than two times of that of the h-NCNWs1 sample. In addition, the pore volume of the h-NCNWs1.5 sample further declined to 0.59 cm 3∙g −1. 3.4 FT-IR Spectroscopy
Fig. 4a shows FT-IR spectra of the g-C 3N 4, g-C 3N 4@RF0.5 and RF0.5 samples. The spectrum of g-C 3N 4@RF0.5 sample is similar with that of the g-C 3N 4 sample. The strong absorption peaks between 1100 and 1700 cm −1 obrved from g-C 3N 4 and g-C 3N 4@RF0.5 samples are attributed to the characteristic stretching modes of aromatic CN heterocycles in the polymeric
melon network. Meanwhile, the strong peaks at 805 cm −1 can be
attributed to the typical stretching mode of the triazine units 33,34. However, the FT-IR spectra of the h-NCNWs samples are rather
different from that of g-C 3N 4@RF samples (Fig. 4b). Compared with the spectrum of the C-RF sample, two new peaks centered at 1596 and 1200 cm −1 were prent for all h-NCNWs samples, which can be assigned to the characteristic stretching modes of aromatic CN heterocycles in the net
works. Significantly, the intensities of the two peaks decread with an increa in the thickness of the shell, suggesting the different content of N species in the h-NCNWs samples. 3.5 Raman spectroscopy
Raman spectra of the h-NCNWs samples are shown in Fig. 5. All h-NCNWs displayed two conspicuous bands centered at 1350 and 1594 cm −1, corresponding to the dispersive defect-induced vibrations (D band) and the vibration of graphitic sp 2-bonded carbon atoms (G band), respectively. The G -band indicates the graphitic structure of the samples, while the D -band demonstrates the prence of structural defects in the samples. Generally, the ratio of the intensity of D to that of G band (I D /I G ) can reprent intrinsic disorder of the carbon materials, and a higher I D /I G value suggests the prence of more structural defects 35,36. The I D /I G value of h-NCNWs0.5, h-NCNWs1 and h-NCNWs1.5 is determined to be 0.88, 0.87, and 0.87, respectively, which was clo to that of C-RF (0.90). The results revealed that N-doping cannot affected the degree of graphitization. 3.6 XPS
XPS spectra of the h-NCNWs samples are shown in Fig. 6. The XPS survey spectra of h-NCNWs samples displayed a strong signal relative to N 1s photoelectron excitation peak (Fig. 6a). The N content on the surface of the h-NCNWs samples is 8.14%, 7.13% and 2.76% (atomic fraction) for h-
NCNWs0.5, h-NCNWs1 and h-NCNWs1.5 respectively, which is consistent with the FT-IR results. Fig. 6b shows the C 1s XPS
Fig. 4 FT-IR spectra of g-C 3N 4, g-C 3N 4@RF0.5 and RF0.5
samples (a) as well as the h-NCNWs samples (b).
4000350030002500200015001000500
RF0.5
g -C 3N 4@RF0.5
T r a n s m i t t a n c e (a .u .)
Wavenumbers (cm -1)
g -C 3N 4
a
4000350030002500200015001000500
h -NCNWs1.5h -NCNWs1h -NCNWs0.5
T r a n s m i t t a n c e (a .u .)
Wavenumbers (cm -1)
C-RF
1596
1200
b
Table 1 Textural properties and the concentration of C, N and O atoms on the surface of the h-NCNWs samples detected by XPS.
Sample S BET /(m 2·g −1)
Pore volume/(cm 3·g −1)
Concentration (%, atomic fraction)
Graphitic N/Pyridinic N
Total C Total O Total N Graphitic N Pyridinic N h-NCNWs0.5 440 1.40 88.37 3.49 8.14 4.96 3.18 1.56 h-NCNWs1 351 0.66 89.37 3.50 7.13 4.68 2.45 1.91 h-NCNWs1.5
459
0.59
93.52
3.72
2.76
2.02
0.74
2.73
土豆卷的做法Fig. 5 Raman spectra of the h-NCNWs samples.
800100012001400160018002000
0.880.87
1594
G
I n t e n s i t y (a .u .)
Raman shift (cm -1)
C-RF
h -NCNWs0.5 h -NCNWs1 h -NCNWs1.5
D
1350
I D /I G 0.870.90
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