Research Article pubs.acs.org/journal/ascecg
Carbon Nitride Encapsulated Nanodiamond Hybrid with Improved Catalytic Performance for Clean and Energy-Saving Styrene Production via Direct Dehydrogenation of Ethylbenzene Zhongkui Zhao,* Weizuo Li, Yitao Dai, Guifang Ge, Xinwen Guo, and Guiru Wang State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, Peoples’ Republic of China S Supporting Information *
ABSTRACT: In this work, the unconsolidated carbon-nitridelayer close-wrapped nanodiamond (H-ND) hybrid has been successfully synthesized by a facile two-step approach including the mechanical milling of ND powder and hexamethylenetetramine and the followed pyrolysis of hexamethylenetetramine. The unique microstructure and surface chemistry characteristics of the nanohybrid have been identified by employing diverse characterization techniques including field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), N2 adsorption desorption (BET), X-ray diffraction (XRD), Raman spectroscopy (Raman), and X-ray photoelectron spectroscopy (XPS) analyses. Benefiting from the intensified synergistic effect between carbon nitride and nanodiamond, the as-synthesized H-ND hybrid carbocatalyst shows remarkably higher catalytic activity for oxidant- and steam-free direct dehydrogenation (DDH) of ethylbenzene than the nanodiamond (ND) and the previously developed mesoporous carbon nitride, which endows it to be a promising candidate for clean and energy-saving synthesis of styrene through DDH of ethylbenzene. Furthermore, this work also opens a new avenue for fabrication of diverse unconsolidated carbon nitride layers close wrapped nanocarbon hybrids with potential applications for diverse transformations owing to the intensified synergistic effect between carbon nitrides and the nanocarbons. KEYWORDS: Carbocatalysis, Nanodiamond, Carbon nitride, Hybrid, Direct dehydrogenation, Styrene production
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INTRODUCTION
The nanodiamond (ND) is beyond the shine. The ND powder consisting of individual diamond nanoparticles of 5 nm of average diameter were first produced in the 1960s in the USSR from soot formed in explosions.12 Recently, the ND particles have attracted more and more interest owing to their physical and chemical properties. The desirable properties of ND coupled with a large surface area and a modifiable surface chemistry have suggested the use of ND in various applications including catalysis, energy storage, fuel cell, drug delivery, optoelectronic devices, bioimaging and tissue engineering, data storage systems, biosensors, etc.13−27 It is highly desirable to improve their application properties by modifying their microstructure as well as the physical and chemical properties.28−38 Heteroatom doping and the fabrication of ND-based nanohybrids are considered as promising strategies for enhancing their properties.39−44 Recently, a graphitic-like carbon nitride material, referred to as g-C3N4, has attracted extensive attention owing to the incorporation of nitrogen atoms into a carbon matrix which can
Direct dehydrogenation (DDH) of ethylbenzene has attracted considerable attention owing to the growing demand for styrene in the chemical industry. The commercially available K−Fe catalyst has some disadvantages like quick deactivation due to potassium loss, an unstable Fe3+ state, and coke deposition, besides the health injuries to human beings caused by the used Cr in this catalytic system. Moreover, the introduction of superheated steam into the feed is indispensable, which gives the thermodynamic driving force as a heat due to its endothermic character, and shifts the chemical equilibrium to higher styrene conversion, besides inhibiting quick deactivation caused by coke deposition. However, the used excess steam (about 2−3:1 for current technology) leads to high energy consumption. From the viewpoint of sustainable development of chemical industries, the carbocatalysts are being considered as fascinating and green alternatives to Febased catalysts for energy-saving, clean, and safe styrene production through direct dehydrogenation of ethylbenzene.1−11 The development of excellent metal-free DDH nanocarbon catalysts is highly desirable but remains a rigorous challenge. © 2015 American Chemical Society
Received: September 6, 2015 Revised: October 27, 2015 Published: October 29, 2015 3355
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ACS Sustainable Chemistry & Engineering efficiently enhance the catalysis, mechanical, field emission, energy storage, drug delivery, separation, and polluted water purification properties.45−51 Furthermore, the carbon nitridecontaining carbon nanostructures like graphitic C3N4@Carbon and GO/g-C3N4 have been fabricated and exhibited improved properties for diverse applications including catalysis.52−57 Therefore, we envision that the combination of ND and gC3N4 into a hybrid can hopefully improve the application properties of ND-based materials through the possible synergistic effect. However, rarely can reports on the synthesis of ND-carbon nitride hybrid be found. Generally, the carbon nitride-containing nanohybrids can be prepared by immobilizing the carbon nitride on the surfaces of the parent materials via an in situ polymerization of carbon nitride precursor at a temperature less than 600 °C,52−56 or mixing the as-formed gC3N4 with the other carbon structures.52 The former can cause the nanocarbon to be complete covered by a compact carbon nitride layer, but the latter may lead to the maldistribution of carbon nitride on the surface of carbon nanostructures or unformed synergistic effect between carbon nitride and carbon nanostructures. Therefore, to develop a facile and efficient strategy for constructing the carbon nitride-containing hybrids with an appropriate amount and distribution of a carbon nitride layer on the surface of nanostructures is highly desirable, but it still remains a challenge. In the previous report from our Advanced Catalytic Materials Research Group (ACM-RG), DUT,39 we presented a facile approach for the synthesis of the nanodiamond/carbon nitride hybrid nanoarchitectures with a controllable carbon nitride amount. Furthermore, the nanohybrid demonstrated higher catalytic activity than the ND and the mesoporous carbon nitride catalysts, which had been ascribed to the synergistic effect between ND and carbon nitride in the hybrid. However, the carbon nitride in the hybrid exists in the form of carbon nitride nanosheets, which inevitably depresses the synergistic effect between carbon nitride and ND and, therefore, affects its application properties. It can be envisioned that the synergistic effect can be intensified through the fabrication of an unconsolidated carbon-nitride-layer close-wrapped ND nanohybrid. Furthermore, not like the previously reported compact carbon nitride layers formed in the pyrolysis process at 650 °C,38 the unconsolidated carbon nitride layers allow the mass transport to take place smoothly. Therefore, the application properties of the ND-based materials can be significantly improved. In order to construct the novel unconsolidated carbonnitride-layer close-wrapped ND nanohybrid, the finding of an appropriate carbon nitride precursor is indispensable. By adopting the extensively used melamine as the precursor, the formed carbon nitrides are nanosheets but not close wrapped layers on ND.38 We first demonstrated in our previously reported work that the highly ordered mesoporous carbon nitride with ultrahigh specific surface area and ultralarge pore volume can be prepared by using hexamethylenetetramine, a new but low-cost nitrogen-containing organic compound, as a carbon nitride precursor.58 The finding inspires us to think that the hexamethylenetetramine might be an appropriate carbon nitride precursor for fabricating an unconsolidated carbonnitride-layer close-wrapped ND nanohybrid, and at least a new carbon-nitride−ND hybrid may be achieved. Therefore, we try to do this kind of hybrid. Once the designed unconsolidated carbon-nitride-layer close-wrapped ND nanohybrid can be
successfully fabricated, the application properties of the NDbased hybrid can be significantly enhanced. Herein, we report a facile strategy for constructing a novel loose carbon-nitride-layer close-wrapped ND (H-ND) nanohybrid through a two-step method including the mechanical milling of ND powder and hexamethylenetetramine and the following pyrolysis of hexamethylenetetramine. For comparison, the classical ND−carbon-nitride (M-ND) was also prepared by using a similar process as above except for with the replacement of hexamethylenetetramine with melamine. From the characterization results from HRTEM, nitrogen absorption experiments, the structural feature of H-ND with loose carbon-nitride layers close-wrapped on ND was identified. The direct dehydrogenation (DDH) of ethylbenzene has been established as a clean, highly efficient, and energy saving approach for styrene production,16,39−42,58−61 which herein was employed as a model reaction to illustrate the intensified synergistic effect between ND and carbon nitride layers in the as-synthesized nanohybrid. As a result, although the as-synthesized H-ND nanohybrid has a similar surface C O content, it demonstrates significantly higher catalytic activity than the as-synthesized M-ND, suggesting intensified synergistic effect between ND and carbon nitride. The fabricated novel H-ND nanohybrids, as efficient ND-based hybrids with enhanced properties, can be applied to the above various fields including catalysis, energy storage, fuel cells, drug delivery, optoelectronic devices, bioimaging and tissue engineering, data storage systems, biosensors, etc.13−27 Furthermore, this work opens a new avenue for fabrication of diverse loose carbonnitride-layer close-wrapped nanocarbon hybrids with excellent applications from the intensified synergistic effect between carbon nitrides and the nanocarbons.
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EXPERIMENTAL SECTION
Material Preparation. The commercially supplied nanodiamond synthesized by a detonation method followed by acid washing from Beijing Grish Hitech Co. (China) was finely ground. The nanohybrid was synthesized by a facile two-step approach including mechanical milling and the pyrolysis of hexamethylenetetramine. The detailed preparation process is given as follows: the ground nanodiamond and hexamethylenetetramine (1:10 of mass ratio) were mechanically milled in an agate mortar to obtain the mixture, followed by pyrolysis of the mixture at 750 °C in a N2 atmosphere to obtain the final sample, a novel unconsolidated carbon-nitride close-wrapped nanodiamond hybrid, which is denoted as H-ND. To replace hexamethylenetetramine with melamine, the other sample M-ND was prepared by a similar process, described in a previous report.39 The commercially available nanodiamond was heated under the same conditions as those for H-ND and M-ND except for no hexamethylenetetramine or melamine was added to obtain the ND for comparison. Material Characterization. X-ray diffraction (XRD) profiles were collected from 10 to 90° at a step width of 0.02° using a Rigaku Automatic X-ray Diffractometer (D/Max 2400) equipped with a Cu Ka source (λ = 1.5406 Å). Field emission scanning electron microscopy (FESEM) experiments were performed on a JEOL JSM5600LV SEM/EDX instrument. Transmission electron microscopy (TEM) images were obtained by using a Tecnai F30 HRTEM instrument (FEI Corp.) at an acceleration voltage of 300 kV. The XPS spectra were carried out on an ESCALAB 250 XPS system with a monochromatized Al Ka X-ray source (15 kV, 150 W, 500 μm, pass energy = 50 eV). Raman spectra were measured using a laser with an excitation wavelength of 532 nm at room temperature on a Thermo Scientific DXR Raman microscope. Nitrogen adsorption and desorption isotherms were determined on a Beishide apparatus of model 3H-2000PS1 system at −196 °C. The specific surface areas were calculated by the BET method, and the micropore and mesopore 3356
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ACS Sustainable Chemistry & Engineering size distributions were calculated from the adsorption branch of the isotherm by H−K and BJH models, respectively. Catalytic Performance Measurement. The oxidant- and steamfree direct dehydrogenation of ethylbenzene was performed over the developed catalyst, and the experimental details are as follows: the reaction was performed at 550 °C for 20 h in a stainless steel, fixed bed flow reactor (6 mm O.D.). A total of 25 mg of catalyst was loaded at the center of the reactor with two quartz wool plugs at its two sides. The system was heated to 600 °C and kept for 30 min in Ar for pretreating the catalyst. After the system was cooled down to 550 °C and kept for 10 min, the feed containing 2.8% ethylbenzene with a flow rate 10 mL min−1 and Ar as a balance was then fed into the reactor from a saturator kept at 40 °C. The effluent from the reactor was condensed in two traps containing a certain amount of ethanol connected in a series. The condensed material was cooled externally in an ice water bath. Quantitative analysis of the collected reaction products (ethylbenzene, styrene, toluene, and benzene) was performed on a FULI 9790 II GC equipped with HP-5 column, 30 m × 0.32 mm × 0.25 μm, and FID detector. The resulting carbon balance was above 100 ± 4% in all reactions. The steady-state styrene rate (20 h of time on stream), selectivity of styrene, and styrene rate vs time on stream are employed as the evaluation standard for the catalytic performance of the fabricated catalysts. The styrene rate is calculated as the formed styrene molar amount per gram of catalyst per hour, and the selectivity of styrene is denoted as the percentage of the desired styrene relative to the total products including the desired styrene and the byproducts that contain benzene and toluene.
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Figure 1. FESEM (a−c) and TEM (d−f) images of the as-synthesized H-ND (a,d) and M-ND (b,e) as well as the parent ND (c,f) samples.
RESULTS AND DISCUSSION Scheme 1 depicts the schematic illustration for the fabrication of the designed H-ND nanohybrid. The preparation route
deagglomeration roles of the formed loose carbon nitride layers from pyrolysis procedure of hexamethylenetetramine. The difference in the colors of the as-synthesized H-ND (Figure S1a) and the calcinated ND (Figure S1c) also implies different carbon characteristics. The different colors for H-ND (Figure S1a) and M-ND (Figure S1b) also suggest the different CNx features. Thermal analysis results show that the increasing weights for H-ND and M-ND are 7.5 and 10.0%, respectively, calculated on the basis of added ND. The formed CNx percentages in the H-ND and M-ND hybrids are estimated to be 7.0 and 9.1%, respectively. Besides the changes in CNx characteristics and surface chemistry between H-ND and MND, the CNx percentage may also affect their catalytic perfromance. However, both hexamethylenetetramine and melamine completely decompose, and nothing is left if they sufferred from the calcination under the same conditions as those for preparing hybrids. In order to identify the formation of the loose carbon-nitride layers close-wrapped on ND, we further compare the magnified TEM images of the assynthesized H-ND and the parent ND. From Figure 2, the loose carbon-nitride layers close-wrapped on ND can be clearly seen (herein, we just can see the loose layers wrapped on ND; the existence of N was intensified by the following XPS analysis). However, only the relatively smooth shell for the sp2/ sp3 core/shell structure of ND can be observed. This difference in the feature between H-ND and M-ND can be further demonstrated by the magnified regions of the TEM images shown in Figure 2e and f. From the observation, the loose carbon nitride layers in the H-ND hybrid may be proposed. That is to say, in this work, the loose carbon-nitride layer closewrapped ND hybrid may be formed, which differs from the MND with the carbon nitride nanosheets in the nanohybrid. In order to further identify the above proposed new structures of the H-ND hybrid, the nitrogen adsorption experiments on the as-synthesized H-ND, M-ND, and parent ND samples were performed. The nitrogen adsorption−
Scheme 1. Schematic Illustration for the Fabrication of HND Hybrid via a Facile Mechanical Milling of ND and Hexamethylenetetramine Followed by a Pyrolysis Approach
includes two major steps: first, the ND powder and hexamethylenetetramine were mixed and mechanically milled; second, the pyrolysis was performed at a desired temperature for 0.5 h to yield the H-ND. The M-ND was also prepared by using a similar method to the above except for the replacement of hexamethylenetetramine with melamine. Figure 1 presents the FESEM (a,b,c) and the HRTEM (d,e,f) images of the as-synthesized H-ND (a,d), M-ND (b,e), and the parent ND (c,f). From Figure 1a−c, we cannot clearly see much visible difference in the morphologies of the three samples. It was previously reported that the carbon nanosheets in the M-ND hybrid (Figure 1e) can be clearly observed, in comparison with the parent ND. However, as for the assynthesized H-ND, carbon nitride nanosheets similar to M-ND on the H-ND cannot be seen (Figure 1f). In comparison with the parent ND, the looser ND aggregation for H-ND (Figure 1d) can be observed, which may be resulted from the possible 3357
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similar surface area and pore feature to those of the parent ND. From HRTEM images of the M-ND shown in Figure 1e, no change in morphology can be observed on the M-ND sample in comparison with that of ND, except for the inserted carbon nitride nanosheets into the M-ND hybrid. This may be a reason for the similar texture feature for the M-ND and ND. Thus, the significant difference in the texture feature toward H-ND and ND further presents definite evidence that H-ND has a different morphology from ND. Moreover, from Figure 3, the increased micropores (0.23 mL g−1 for H-ND by T-plot method) and decreased mesopores (0.35 mL g−1 for H-HD by BJH method) in comparison with ND and M-ND can be observed. The former can be resulted from the formed loose CNx layers wrapped on ND, but the latter might be ascribed to the disappearance of accumulational pores of NDs due to the pyrolysis of hexamethylenetetramine. The structures of the as-synthesized H-ND, M-ND, and the parent ND were further investigated by XRD techniques (Figure 4a). As for the parent ND sample, besides the
Figure 2. HRTEM images of the as-prepared H-ND (a,b) and parent ND (c,d) samples. Parts e and f are the magnified zones of parts b and d, respectively.
desorption isotherms, specific surface areas, pore volumes, and the pore size distributions of the three samples are presented in Figure 3. As shown in Figure 3, there is a significant difference
Figure 3. Nitrogen adsorption−desorption isotherms. Inset: BJH pore size distribution from desorption branch, H−K micropore distribution, surface area, and total pore volume of the as-synthesized H-ND, MND, and the parent ND samples.
Figure 4. XRD patterns (a) and Raman spectra (b) of H-ND, M-ND, and the parent ND samples.
in specific surface area, isotherms, pore volume, and pore distribution between the H-ND and ND that is exhibited. The H-ND hybrid has a remarkably higher specific surface area and lower pore volume than the parent ND, implying the significant difference in their morphologies. This is consistent with the results from the HRTEM analysis. The wrapped carbon nitride layers on ND and its deagglomeration toward a ND aggregate cause a higher surface area in comparison with that of the parent ND. However, for the M-ND sample, it has a very
characteristic diffraction peaks located at 42.9, 43.9, 75.5, and 91.0 o corresponding to the (100), (111), (022), and (113) diffraction plane of nanodiamond, a peak at 27.2 o corresponding to the stacked graphitic sheets can also well resolved, ascribed to the wrapped sp2 C graphene shell on the sp3 C core.39,40,62−64 The above five diffraction peaks appearing on the XRD pattern toward M-ND can also be observed, suggesting no large change in the structure of M-ND in 3358
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Scheme 2. Schematic Illustration for the Formation of the Carbon Nitride Nanosheet in the M-ND Nanohybrid from Melamine and the Carbon Nitride Nanolayers in H-ND Nanohybrid from Hexamethylenetetramine
sheets.39,40,58−60,65 A shift of the G-band on the H-ND from 1629 to 1585 cm−1 and of the D-band from 1330 to 1352 cm−1 in comparison with those of the parent ND can be observed, suggesting the incorporation of nitrogen into carbon lattices.40,58,60 A similar trend in the two bands can be seen on the M-ND. The results show the presence of carbon nitride in both of the hybrids. Furthermore, similar Raman spectra characteristics on the M-ND and the parent ND can be observed, indicating no obvious difference in their surface structures. The existence of carbon nitride nanosheets in the MND does not change the surface characteristics of ND in the MND hybrid. However, the Raman spectroscopy of H-ND is definitely different from those of M-ND and the parent ND, which suggests a significant change in the surface structural characteristics of H-ND in comparison with those of the M-ND and the parent ND. Correlated to the HRTEM, BET, and XRD analyses, this is an indicator for the formation of loose carbon nitride layers close-wrapped on a ND hybrid, which is significantly different from the our previously reported M-ND hybrid.36 Moreover, the remarkably higher ID/IG for the H-ND hybrid than the other two indicates the surface disordered carbon nitride and/or formed surface structural defects,39,40,60,66 which gives further evidence for the presence of a great amount of carbon nitride. From FESEM images of the N-ND and the parent ND, no visible difference can be observed. Correlated to the observation of HRTEM images, the
comparison with that of the parent ND. In comparison with parent ND, the strengthened and widened peak corresponding to the (002) plane of graphitic layers can be observed, ascribed to the carbon nitride nanosheets in the M-ND hybrid.38,55,56 The widened and less-intense peaks at 40−50° toward (100) and (111) planes of nanodiamond on the M-ND hybrid indicates the increased ND dispersity caused by the pyrolysis of melamine.39 More interestingly, on the developed H-ND hybrid, the peaks corresponding to the (022) and (113) plane disappear, and the peaks toward (100) and (111) facets are further widened and weakened, implying further enhanced ND dispersity by the pyrolysis of hexamethylenetetramine.39,40 This is consistent with the results from HRTEM analyses. Correlated to the HRTEM results, the broadened and strengthened XRD peak toward the (002) facet on the developed H-ND hybrid is caused by the loose carbon nitride layers wrapped on the ND. Raman is a sensitive technique for probing the surface structures of the materials.39,40,58−60 The Raman spectra of the as-synthesized H-ND, M-ND, and the parent ND are presented in Figure 4b. As shown in Figure 4b, all three of the samples exhibit two feature peaks in the range of 1000−2000 cm−1 consisting of well-defined D (1353 cm−1, A1g mode, corresponding to defect and disorder graphitic layer) and G (1600 cm −1 , E2g mode, corresponding to graphene), suggesting the existence of exposed graphene nano3359
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therefore the H-ND shows significantly greater surface in comparison with M-ND and the parent ND. The nature and coordination of the carbon, nitrogen, and oxygen in the as-synthesized H-ND, M-ND, and parent ND were examined by XPS.39,40,58−60,66,67 The XPS survey spectra (Figure 5a) of the two hybrids in comparison with that of the
disordered carbon nitride layers must be close wrapped on ND for the H-ND hybrid. Correlated to the HRTEM images, the change in ID/IG of the M-ND hybrid in comparison with that of the parent ND can be caused by the carbon nitride nanosheets in the M-ND hybrid.39 By combing the various characterization results containing FESEM, HRTEM, BET, XRD, and Raman analyses, the structure of the loose carbon nitride layers close wrapped on the ND hybrid can be unambiguously identified. The unique structure of H-ND with the loose carbon nitride layers on the ND allows it to exhibit intensified synergistic effect between carbon nitride and ND. This may significantly improve the application properties of the ND-based materials in the established fields including catalysis, energy storage, fuel cell, drug delivery, optoelectronic devices, bioimaging and tissue engineering, data storage systems, biosensors, etc. Furthermore, the developed novel H-ND hybrid may have a chance to be extended to the other possible new fields. The presented preparation method including the mechanical milling of the hexamethylenetetramine with parent nanocarbons followed by the pyrolysis of hexamethylenetetramine can be applied to the fabrication of the other loose carbon nitride layers close wrapped on the carbon nanostructures with intensified synergistic effect between carbon nanostructures and the carbon nitride layers. From above, the microstructures of the carbon nitride/ND nanohybrids (carbon nitride sheets vs layers for M-ND and HND, respectively) can be changed by adjusting the source of carbon nitride. Then, what is the mechanism behind this? Scheme 2 presents the possible reaction route for the formation of sheets and layers from melamine and hexamethtlenetetramine in the carbon nitride/ND nanohybrids. From refs 67 and 68, the rigid carbon nitride multinuclear planes can be formed by the dehydrogenation condensation of melamine and the subsequently formed melem (Scheme 2a), owing to the binding energy of N−H (389 kJ mol−1) being much lower than that of CN (615 kJ mol−1). Therefore, it is understandable that the nanosheets are formed in the M-ND nanohybrid, although the formed tri-s-triazine-containing planes may partially decompose and reorganize while it suffers from calcining at high temperature. Generally, the CNx containing hybrid only can be formed at a temperature less than 600 °C. However, it was previously demonstrated that the CNx sheets can be formed at above 600 °C in the presence of the solid substrate (ND).39 In fact, although appropriate CNx can promote the catalysis of the as-synthesized carbon materials,52−57 too many CNx sheets may depress the accessibility of active sites and therefore lead to a decrease in catalytic activity. The formed CNx sheet can decompose while it suffers pyrolysis processes at high temperatures, and the amount of CNx sheets can be adjusted by changing the pyrolysis temperature.39 Different from the melamine, the condensation of hexamethylenetetramine takes place through the C−N splitting. The formed units can be linked with CH2, and therefore the flexible carbon nitride structure can be formed, which is different from the rigid structure from melamine. The flexible carbon nitride units can be further polymerized to form the CNx layers, and therefore the CNx-layer-wrapped ND nanohybrid can be formed, identified by the FESEM, HRTEM, BET, XRD, and Raman analyses. Furthermore, the flexible CNx units can insert the crack of the ND aggregates, and the aggregates can be deagglomerated by the expansion effect of the formed gases (NH3, COx, etc.) from the pyrolysis of carbon nitrides, and
Figure 5. XPS spectra including survey (a), C 1s (b), N 1s (c), and O 1s (d) spectra of H-ND, M-ND, and the parent ND samples.
parent ND show that the stronger signal corresponding to N can be observed. Correlated to the surface N content from XPS analyze listed in Table 1, the formation of carbon nitride layers on H-ND and carbon nitride nanosheets can be identified. The C 1s peak region in the XPS of the H-ND and N-ND hybrids were deconvoluted to five peaks at around 284.6, 285.2, 286.0, 287.0, and 288.6 eV corresponding to CC, C−N, C−C/C− O (14.7%), CO/CN, and OC−O, respectively.69,70 But the C 1s peak region in the XPS of the parent ND is fitted into four peaks, and no peak at 285.2 eV can be seen. This shows the change of surface chemical properties of ND-based hybrids in comparison with the parent ND. The N 1s XPS peak (Figure 5c) illustrates the N atoms in all samples. From Table 1, the increased surface N content in H-ND and M-ND hybrids in comparison with that in parent ND further indicates the N incorporation into a carbon matrix of the hybrids. The N 1s spectrum for H-ND (Figure 5c) is deconvoluted into four peaks with binding energies of 398.0, 399.4, 400.7, and 403 eV that correspond to pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively,69,70 which is different from the N 1s spectra of the previously reported M-ND and the parent ND materials that display an absence of pyridinic N and graphitic N.39 The results provide further evidence that the carbon nitride in H-ND is different from that in M-ND and the parent ND, identified by the above HRTEM, BET, XRD, and Raman analyses. The N in parent ND can come from its preparation process by the detonation method. From our proposed mechanism for the formation of carbon nitride layers but not sheets in the H-ND hybrid (Scheme 2), the sp3 N atoms are linked by CH2 groups. However, XPS identifies that the N atoms exist in the form of sp2 N, which may be ascribed to the further polymerization of the formed flexible carbon nitrides structure. From Table 1, the surface N percentage is 2.3 and 3.0% for H-ND and M-ND, respectively; however, the increasing weights for H-ND and M-ND based on the added 3360
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Table 1. Relative Integrated Intensity of Deconvoluted N 1s and O 1s XPS Spectra for H-ND, M-ND, and the Parent ND Samples samples
Na (%)
N−1b (%)
N−2b (%)
N−3b (%)
N−4b (%)
Oa (%)
CO (%)
C−OH/CO−C (%)
OC−O (%)
H-ND M-ND ND
2.3 3.0 1.5
6.2
59.5 89.7 82.2
20.9
13.4 10.3 17.8
5.8 5.1 9.4
30.5 34.1 31.7
34.4 5.3 6.0
35.1 60.6 62.3
a
N and O atom content measured by XPS analysis. bPercentage of various nitrogen species occupying the total N content; N−1, N−2, N−3, and N−4 are denoted as pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively.
parent ND are 7.5 and 10.0%. This presents a further demonstration that the CNx is formed in the two hybrids but not just the possible N-doping. Based on recently reported research,42,58,59,71 the O 1s XPS spectra of the three samples can be deconvoluted into three peaks with binding energies of 531.5, 532.4, and 533.6 eV, assigned to CO, OC−O, and C−O−C/C−OH containing groups (Figure 5d and Table 1). Owing to the pyrolysis process, the decreased surface O content of the hybrids can be observed in comparison with that of the parent ND. Moreover, the C−OH/C−O−C percentage for the H-ND increases in comparison to that of the other two. Surface ketonic CO has been considered as the catalytic active species for activating the C−H bond.39−42,58−61 From Table 1, the surface ketonic CO contents for H-ND, M-ND, and the parent ND are 1.8, 1.7, and 3.0%, respectively. Both of the two hybrids have comparable ketonic CO content. However, the surface ketonic CO amount of the two hybrids is less than that of the parent ND, ascribed to the suffered pyrolysis temperature. The slightly higher CO of H-ND in comparison with M-ND can be seen, which might be ascribed to the prohibiting effect of the wrapped CNx layers on the O escaping from the hybrid or newly formed CO on the identified surface defect sites of the H-ND nanohybrid from Raman analyses. In order to illustrate the advantages of the as-synthesized loose carbon-nitride-layer close-wrapped ND hybrid catalyst, the catalytic performance of the H-ND, M-ND, and the parent ND in the DDH reaction of ethylbenzene is evaluated. Figure 6 presents the reaction results, and the as-synthesized mesoporous carbon nitride catalysts are also included for comparison. As shown in Figure 6a, the two as-synthesized carbon-nitride-containing ND-based hybrids exhibit higher catalytic activity (4.8 and 4.0 mmol g−1 h−1 steady-state styrene rate over H-ND and M-ND, respectively) than the parent ND (2.7 mmol g−1 h−1). From XPS data, both of them have lower surface ketonic CO groups, active sites for DDH reaction, than the parent ND, suggesting the existence of a synergistic effect between carbon nitride and ND. From our previous report,58 the 2.0 mmol g−1 h−1 of styrene rate with 96% selectivity can be obtained over the mesoporous carbon nitride (MCN-1) under the same conditions, even if the developed mesoporous carbon nitride (DUT-1) with the ultrahigh surface area and pore volume was used as a dehydrogenation catalyst, only a 3.4 mmol g−1 h−1 steady-state styrene rate with 94% selectivity can be achieved. The nonporous CNx material must show a much worse catalytic performance for the DDH reaction. The as-synthesized ND-based hybrids (H-ND and MND) show a remarkably higher catalytic performance than the sole ND or carbon nitrides, which further proves the existence of a synergistic effect between ND and carbon nitride. The electron-rich N atom can increase the nucleophilicity of CO, which may improve the catalytic activity of CO active sites. The synergistic effect may result from the improved
Figure 6. Steady-state styrene rate and selectivity to styrene at 20 h of time on stream (a) and the styrene rate as a function of time on stream (b) of the as-synthesized H-ND, M-ND, MCN-1, DUT-1, and the parent ND for direct dehydrogenation of ethylbenzene to styrene under oxidant- and steam-free conditions.
nucleophilicity of CO and basic properties of materials caused by the presence of CNx. More interestingly, the H-ND demonstrates remarkably higher catalytic activity in the DDH reaction than the M-ND hybrid. Correlating to characterization results from FESEM, HRTEM, BET, XRD, Raman, and XPS shown as above to the reaction results, we can safely say that the unique microstructure of H-ND and the high surface area increased defects, and the intensified synergistic effect between carbon nitride and ND through the formation of the unique structure endows the H-ND to demonstrate notably higher catalytic performance in DDH reaction than the other samples. Moreover, besides the effect of the different CNx features between H-ND and M-ND, the percentage of CNx may also affect the catalytic perfromance of the hybrids for the DDH reaction. From Figure 6b, the catalytic activity over diverse carbocatalysts decreases to some degree at the original period and then keeps at the steady state activity. The initial decrease can be ascribed to the reduction of the highly active CO by the formed hydrogen in the reaction process.39,59 Although the 3361
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ACS Sustainable Chemistry & Engineering initial decrease in catalytic activity can be observed as the time on stream is prolonged, the activity reaches the steady state after the reaction runs for 17 h. From Figure S2, the catalytic activity can be well maintained after the time on stream is further extended, and no visible decrease can be observed even if the time on stream reaches 50 h. Moreover, it was previously demonstrated that the decreased activity in direct dehydrogenation over diverse carbocatalysts can be completely recovered after being exposed to air at an appropriate temperature.16,59 The notable steady-state catalytic activity and the easy regeneration endows it to be a promising candidate for future industrial application for clean, highly efficient, and energy saving styrene production through the sustainable nanocarbon catalyzed DDH reaction of ethylbenzene under oxidant- and steam-free conditions. Furthermore, the opening of the gap of CNx makes it a graphene-like semiconductor with a HOMO−LUMO gap to activate molecular oxygen, not only in catalysis but also in photocatalysis where the light-trigged electron and hole induce a surface redox process for relevant chemical reactions like water splitting, CO2 reduction, and even oxidative coupling of amines. This work presents a facile method for the synthesis of the other novel loose carbon-nitride-layer close-wrapped nanocarbon hybrids with unique microstructures and an intensified synergistic effect for diverse applications.
ACKNOWLEDGMENTS
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REFERENCES
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CONCLUSIONS In summary, the novel carbon-nitride-layer close-wrapped nanodiamond hybrid with unique microstructure has been successfully synthesized by a facile two-step approach. The unique microstructure and surface chemistry characteristics of the nanohybrid have been identified by employing diverse characterization techniques including FESEM, HRTEM, BET, XRD, Raman, and XPS analyses. The developed H-ND nanohybrid demonstrates remarkably higher catalytic activity in the DDH reaction than the as-synthesized M-ND, MCN-1, DUT-1, and ND, ascribed to the unique microstructure and the intensified synergistic effect between carbon nitride and ND from the microstructure of the H-ND hybrid. The fabricated novel H-ND nanohybrid, as efficient ND-based functional hybrids with intensified synergistic effect, can be applied to the above various fields including catalysis, energy storage, fuel cell, drug delivery, optoelectronic devices, bioimaging and tissue engineering, data storage systems, biosensors, etc. This work also opens a new avenue for fabrication of the other carbonnitride-layer close-wrapped nanocarbon hybrids with excellent application properties in diverse fields. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01032. Photos of the as-synthesized H-ND and M-ND samples as well as the calcined and pristine ND and a graph of the long-term stability of the developed catalyst (PDF)
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This work is financially supported by the National Natural Science Foundation of China (grant no. 21276041), the Joint Fund of Coal, set up by National Natural Science Foundation of China and Shenhua Co., Ltd. (grant no. U1261104), and also sponsored by the Chinese Ministry of Education via the Program for New Century Excellent Talents in University (grant no. NCET-12-0079), the Natural Science Foundation of Liaoning Province (grant no. 2015020200), and by the Fundamental Research Funds for the Central Universities (grant no. DUT15LK41).
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Research Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 3362
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