Nitrogen-doped graphene nanosheets as metal-free catalysts for dehydrogenation reaction of ethanol

Abstract

The catalytic performance of nitrogen-doped graphenes for the dehydrogenation of ethanol is investigated in this study. N-Doped graphene is synthesized by using urea and graphene oxide as nitrogen source and precursor, respectively. XRD, Raman spectra, XPS, SEM and HRTEM are utilized to characterize the structure of the synthesized N-doped graphenes. XPS reveals that the doped nitrogens on the surface exist in three forms of nitrogen atom: pyridinic, pyrrolic and graphitic. Moreover, it shows that the contents of the three states of doped N obtained from various nitrogen doped contents are different. Dehydrogenation results show that the N-doped graphene efficiently catalyzes the dehydrogenation of ethanol, and acetaldehyde is obtained as the only product without any byproduct in all the cases. In addition, the nitrogen-doped content and reaction temperature obviously have significant effects on the catalytic performance of NG. The dehydrogenation mechanism for ethanol under the catalysis of NG has also been proposed.

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1. Introduction

Graphene, a monolayer of carbon atoms arranged in a honeycomb network, has stimulated wide interest due to its properties associated with the two-dimensional (2-D) crystal structure formed by sp² hybridized carbon. It possesses excellent mechanical, electrical, thermal and optical properties.^1,2

In addition, it has been reported that the physical, chemical and mechanical properties of graphene could be modified by doping with other atoms or molecules (N, B, P, NH₂–, and S).^3–7 To date, N-doping has attracted the considerable interest among these types of doping, and N-doped graphene has been applied in various fields^8–11 due to its high efficiency, recyclability, high stability, and biocompatibility. To the best of our knowledge, N-doped graphene has been used as an electrocatalyst for oxygen reduction in fuel cells,^8–10,12 an electrode in supercapacitors,⁵ a sorbent material¹³ and a molecular sensor.¹⁴ In addition to the abovementioned applications, N-doped graphene also has recently attracted the attention of some researchers for its unique catalytic properties in organic synthesis. However, there are only a very few relevant reports in this field. Yoshimura et al. reported that a nitrogen-functionalized graphene-gold nanocrystal hybrid showed enhanced catalytic reduction of benzaldehyde.¹⁵ Wang et al. reported that multilayer N-doped graphene nanosheets could efficiently catalyze the aerobic selective oxidation of primary alcohols to aldehydes or ketones with >99% selectivity via an adduct mechanism under mild conditions.¹⁶ It was found that the nitridation temperature greatly influenced the N doping concentrations and forms. Su et al. used N-graphene to catalyze the oxidation of arylalkanes in aqueous conditions, suggesting that the N doping greatly enhanced catalytic activity as well as selectivity.¹⁷ The reduction of 4-nitrophenol to 4-aminophenol catalyzed by N-doped graphene was reported by Chen et al., who concluded that the N-graphene catalytic process was found to follow zero order kinetics, which was different from the first order kinetics observed using all the traditional metallic catalysts.¹⁸

In the present study, the preparation of a N-doped graphene (NG) using urea as the nitrogen source and the application of NG as a metal-free catalyst for the dehydrogenation of ethanol are investigated and reported. It is well documented that the dehydrogenation of ethanol is an extremely useful organic reaction, by which some valuable chemicals, including acetaldehyde, ethyl acetate, butanol and acetic acid, can be synthesized under different conditions.^19–22 To date, metal-based compounds are the most used catalysts in this reaction, but promoters and supports are necessary to prevent metal sintering and improve the selectivity.^19,21–24 Furthermore, in some cases, it is difficult to produce a given product with 100 percent selectivity when using metal-based catalysts, and some by-products, essentially deriving from acetaldehyde, can also be produced simultaneously.^20,25,26

In the abovementioned studies, the mechanisms of dehydrogenation of ethanol on various catalyst surfaces are also proposed. For instance, Beller et al. considered that the catalyst (ruthenium-based PNP pincer) is first activated by a base (NaOEt). Then, ethanol is oxidized to acetaldehyde through an outer-sphere dehydrogenation and hydrogen is released from the ruthenium center.²³ Bhan et al. applied γ-Al₂O₃ to catalyze an ethanol dehydrogenation and proposed that acetaldehyde synthesis proceeds through the cleavage of the C_α–H bond of a surface ethoxy species that was formed from an absorbed ethanol molecule.²⁶ Chen et al. deduced that ethanol was first adsorbed on the surface of the Cu-modified Mo₂C/Mo with at least a fraction of molecules having the O–H bond intact, then the O–H bond was then broken to form an ethoxy species, and the α-C–H bond of the ethoxy intermediate was cleaved to form acetaldehyde.²⁷

In this study, it is found that NG can efficiently catalyze the dehydrogenation of ethanol to produce acetaldehyde as the only liquid product under conventional conditions. The effects of the preparation conditions of NG and the N-doped contents on the catalytic properties are investigated. In addition, a possible mechanism of the reaction is also proposed in this study.

2. Experimental

2.1 Synthesis of NG

Graphite oxide (GO) was prepared by the modified Hummers method. Typically, 2.5 g of natural graphite and 7.5 g of KMnO₄ were slowly added to 50 mL of concentrated H₂SO₄ with vigorous stirring below 5 °C. Then, the mixture was stirred continuously for 1 h at 35 °C to oxidize graphite. Subsequently, 100 mL deionized water was added to the mixture, the temperature was increased to higher than 90 °C, and the suspension was maintained at 95–100 °C for 15 min. The mixture was then poured into 300 mL of deionized water. Next, 20 mL of H₂O₂ was added to the suspension. After being cooled to room temperature, the solid products were filtered, subsequently washed with 5% HCl aqueous solution and water, and finally dried to obtain graphite oxide.

The N-doped graphenes with different nitrogen contents were synthesized through a one-pot hydrothermal process with urea as the chemical dopant in the presence of a GO aqueous dispersion. Typically, 60 mg of GO was dispersed in 30 mL of deionized water, and then a given amount of urea was added to the GO dispersion under sonication for 3 h. Subsequently, the solution was sealed in a 50 mL Teflon-lined autoclave and allowed to stand at 180 °C for 16 h. The solids were filtered and washed with distilled water several times. For comparison, reduced GO (G) was also prepared under the same experimental parameters but without adding urea into the GO aqueous dispersion.

2.2 Characterizations

The structure and purity of the products were identified by XRD, which was carried out using Cu Kα₁ radiation on a D8 Advance X-ray generator (Bruker AXS Company, Germany). The X-ray intensity was measured over a 2θ diffraction angle from 20° to 80° with a step size of 2° min⁻¹. The morphologies of the products were observed by SEM images (JSM-6360LV, JEOL Tokyo, Japan), which were obtained at an accelerating voltage of 10 kV. TEM was carried out on a FEI Tecnai F20 microscope and a G2 microscope, operating at an accelerating voltage of 300 and 120 kV, respectively. XPS analysis was conducted with an ESCALAB250 surface analysis system (Thermo VG). Raman spectroscopy was performed on a Renishaw invia basis 532 LE instrument with a 633 nm excitation laser at a power of around 0.8 mW.

2.3 Catalytic tests

The catalytic activity of NG was determined using a fixed-bed Quartz tubular reactor (i.d. = 8 mm and length = 50 cm) with an online gas chromatograph. A total of 0.1 g of catalysts was used for each experiment and first activated by pretreating at a temperature of 300 °C for 2 h in a flow stream of Ar at 20 mL min⁻¹. After the pretreatment, the feed gas containing 60% EtOH and 40% Ar was allowed to flow through the catalyst bed. The reactivity tests were conducted at 300 °C. The as-obtained products were separated into liquid and gas phases through an ice-water trap. Both liquid and gas products were analyzed and quantified by an online GC system (Agilent GC7980) equipped with a TCD detector and a Porapak Q packed column (3 mm × 2 m). The GC oven was set at a constant temperature of 150 °C. For liquid products, hydrogen and acetone were used as the carrier gas and internal standard, respectively. For gas products, argon and hydrogen standard gas were used as the carrier gas and external standard, respectively.

3. Results and discussion

Fig. 1 shows the XRD patterns of GO, graphene and NG with different nitrogen doped contents. The sharp diffraction peak at about 2θ = 11° is typically attributed to GO, which disappears in the reduced graphene and NG. There is a very weak and broad diffraction peak in the pattern of GO and graphene at 2θ = 15–35°, indicating the re-aggregation of graphene when it is dried. The diffraction peaks of the three NGs are very similar, indicating that the nitrogen doping content cannot influence the crystal phase structure of NG.

Fig. 1 XRD diffraction patterns of GO, graphene and NG with different nitrogen doping contents.

The nanostructure of the as-prepared NG synthesized at 180 °C for 16 h via hydrothermal reduction with a mass ratio between urea and GO of 300 : 1 is investigated by SEM and TEM. The SEM image (Fig. 2a) shows the complex three-dimensional (3D) NG with stacked and overlapped structures and pore walls cross-linked with each other. The TEM image (Fig. 2b) shows NG with a structure of stacked and wrinkled nanosheets. Moreover, NG is estimated to be composed of 16 layers, based on the analysis of the HRTEM image (Fig. 2c). Both the SEM and TEM images of the product indicate that the 2D structure of NG is efficiently retained after being doped with nitrogen.

Fig. 2 SEM (a), TEM (b) and HRTEM (c) images of NG (300 : 1).

The Raman spectra reflect structural changes occurring in GO, graphene and NG (Fig. 3). All the Raman spectra of GO, graphene and NG show the G band and D band at 1590 cm⁻¹ and 1350 cm⁻¹, respectively. The G band corresponds to the zone center E_2g mode related to phonon vibrations in sp² carbon materials, the D band corresponds to the occurrence of sp² C with defects and the 2D band (the peak at ∼2700 cm⁻¹) is the second order of zone-boundary phonons.^28,29 Fig. 3 shows that the ratio of D/G intensity slightly increases to some extent with the increase of the nitrogen doped contents from 30 : 1 to 200 : 1 and 300 : 1, mainly due to the different C–C and C–N bond distances in GO, graphene and NG.³⁰

Fig. 3 Raman spectra of GO, graphene and NG.

The typical XPS survey spectra of the as-synthesized NG are displayed in Fig. 4a. The peaks at about 285.0, 399.0 and 533.0 eV are attributed to the binding energy of C 1s, N 1s, and O 1s, respectively. The XPS results offer direct evidence that N from urea was doped into GO during the reductive process. The XPS results for the curve fitting of N 1s ranging from 396 to 406 eV are shown in Fig. 4b–d, where the N 1s peaks are split into three peaks for all the samples. The binding energies around 398, 399 and 400 eV are indexed to the pyridinic (N1), pyrrolic (N2) and graphitic (N3) nitrogen atoms, respectively.^16,31

Fig. 4 XPS survey spectra (a) and high-resolution XPS spectrum curve fitting of N 1s peaks of NG prepared with different w_urea/w_GO: 30 : 1 (b), 200 : 1 (c) and 300 : 1 (d).

Fig. 5a–c show the curve fitting of C 1s. The C 1s peak of NG (30 : 1) is split into four peaks of 284.5, 285.7, 287.1 and 289.7 eV, which are ascribed to graphitic C, –C–O/–C–N, –C=O and –COO, respectively.^15,16,32,33 However, the C 1s peak of NG (200 : 1) is split into three peaks of 284.5, 285.7 and 287.1 eV, while the C 1s peak of NG (300 : 1) can be split into only two peaks of 284.6 and 285.4 eV. This indicates that some oxygen still remains in NG, which is typical for chemically reduced graphene. In addition, the oxygen-containing groups are obviously decreased with the increase in the ratio of urea and GO, which illustrates that GO is more reduced under w_urea/w_GO = 300 : 1 than under 200 : 1 or 30 : 1.

Fig. 5 High-resolution XPS spectrum curve fitting of C 1s and O 1s peaks of NG prepared with different w_urea/w_GO: 30 : 1 (a and d), 200 : 1 (b and e) and 300 : 1 (c and f).

Fig. 5d–f display the XPS spectra of O 1s curve fitting. The O 1s peak of the NG (30 : 1) spectrum is deconvoluted into three components of –C=O at 531.0 eV, –C–O at 533.0 eV and –COO at 534.4 eV^16,34 (Fig. 5d), which are consistent with those of C 1s. For NG (200 : 1), the O 1s peak can be divided into two peaks centered at 530.9 and 532.6 eV, belonging to the –C=O and –C–O, respectively. For NG (300 : 1), the O 1s peak is split into four peaks centered at 529.4, 531.4, 532.9 and 536.5 eV, mainly belonging to the –C=O and –C–O.

In addition, the catalytic reaction temperatures are higher than the hydrothermal preparation temperatures in this study. Therefore, the N-doped graphenes after aging at 300 °C are also characterized to investigate the structural and compositional information of the catalysts during catalytic reactions. The XRD and XPS results are shown in Fig. 6. The XRD patterns in Fig. 6a shows that there is no obvious difference in the crystal structure between NG before and after aging (comparing with Fig. 1). The HRXPS spectrum curve fittings of N 1s, C 1s and O 1s in Fig. 6b–d after aging at 300 °C also look similar to those of Fig. 4. The further quantitative analysis for C, O, and N elements and three states of nitrogen atoms are listed in Table 1. The results show that there is no big difference in C contents between the three NG samples (∼86.5%). For the O element, NG (30 : 1) has the highest oxygen content (6.85%), while NG (300 : 1) possesses the lowest amount of oxygen (6.54%), probably due to the sequentially increased w_urea/w_GO. As anticipated, the NG (300 : 1) sample contains the highest amount of nitrogen (6.98%) as compared with NG (200 : 1) (6.74%) and NG (30 : 1) (6.61%). In addition, it is apparent that the contents of the three states of N doped graphene obtained from the various ratios of urea and GO are different. NGs prepared at ratios w_urea/w_GO = 30 : 1 and 300 : 1 have a relatively high pyridinic (N1) content (∼40%), while the sample prepared at a ratio of 200 : 1 has a higher pyrrolic (N2) content (38.40%).

Fig. 6 XRD diffraction patterns (a) and high-resolution XPS spectrum curve fitting of N 1s (b), C 1s (c) and O 1s (d) peaks of NG after aging at 300 °C.

Table 1 Quantitative analysis of XPS data of NG under various ratios of urea and GO after aging at 300 °C

No.	wurea/wGO	C%	O%	N total%	Pyridinic (N1)		Pyrrolic (N2)		Graphitic (N3)
					Position (eV)	%	Position (eV)	%	Position (eV)	%
1	30 : 1	86.54	6.85	6.61	398.4	41.73	399.6	34.46	400.4	23.80
2	200 : 1	86.45	6.81	6.74	398.4	31.21	399.7	38.40	401.0	30.39
3	300 : 1	86.48	6.54	6.98	398.3	40.09	399.4	30.47	400.2	29.44

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In the dehydrogenation reaction of ethanol, the NGs with different nitrogen doping contents were used as the catalyst for investigating the catalytic performance. For comparison, graphene without N-doping was also investigated. The results in Fig. 7 show that acetaldehyde is the only liquid product for the dehydrogenation of ethanol and the N-doped graphenes exhibit good catalytic activity, while graphene without N-doping cannot catalyze the reaction. In addition, the conversion of ethanol is obviously influenced by the content of nitrogen doped. It can be seen from Fig. 7 that the conversions of ethanol obtained at the nitrogen doped contents of 30 : 1 and 300 : 1 are significantly higher than that of 200 : 1. According to Table 1, the change in C, O and N amounts appears to have no obvious and inevitable impact on the catalytic results. In addition, it is reported that the pyridine-like N component can determine the catalytic activity of N-doped graphene.^35,36 Therefore, the better catalytic performance of NG (30 : 1) and (300 : 1) is attributed to their higher pyridinic (N1) content (40.09–41.73%, shown in Table 1) than that of NG (200 : 1) (31.21%). Fig. 7 also indicates that the reaction temperature also has a great effect on the catalytic performance of NG. For NG (200 : 1), 6.21% of the feedstock was converted to acetaldehyde at 200 °C, while 9.11% of conversion was obtained when the temperature was 300 °C. Correspondingly, hydrogen was detected as the only gas product in this reaction. Fig. 7 shows that the yields of hydrogen are 7.98–10.71%, 6.19–8.88% and 8.73–10.82% over NG with w_urea/w_GO ratios of 30 : 1, 200 : 1 and 300 : 1, respectively, which mainly agree with the conversions of ethanol.

Fig. 7 The conversion of ethanol (, and ) and the yield of H₂ (, and ) over NG at different temperatures (200–350 °C).

On the basis of the XRD, TEM and XPS results, the reaction mechanism underlying the dehydrogenation of ethanol catalyzed by N-doped graphenes is elucidated and described in Fig. 8. As Fig. 8 shows, EtOH is first introduced onto the surface of the NG layer under suitable conditions. The H atom of the –OH is then attacked by the electrons of the N atom, and a new bond is made between the hydrogen and nitrogen atoms. Moreover, another bond is formed between the oxygen atom and an NG carbon atom. Subsequently, another new bond of N–H is made when the N atom attacks the H atom of methylene. Furthermore, a double bond between the O and C atoms of methylene is formed by obtaining the electrons conducted by NG, and acetaldehyde is produced as a result. On the other hand, H₂ is also released by the homolysis of two N–H bonds.

Fig. 8 Schematic diagram of dehydrogenation reaction mechanism catalyzed by NG.

4. Conclusion

In summary, the use of N-doped graphene catalysts for ethanol dehydrogenation is reported in this paper. XPS characterization reveals that the N species are doped on the carbon sheet as pyridinic, pyrrolic and graphitic nitrogen atoms. Catalytic reaction results show that the N-doped graphenes exhibit good catalytic activity for the dehydrogenation of ethanol and selectivity for acetaldehyde, while graphene without N-doped cannot catalyze the reaction. Acetaldehyde is the only product of the reaction with NG as the catalyst. Moreover, both nitrogen doping content and reaction temperature have irregular influence on the catalytic performance of NG.

Acknowledgements

The authors gratefully acknowledge the partial financial supports by the Natural Science Foundation of Liaoning Province, China (No. 2015020245), Liaoning BaiQianWan Talents Program, China (No. 2013921046) and the Scientific Research Fund of Liaoning Provincial Education Department, China (Grant No. L2015423).

References

1. V. Singh, D. Joung, L. Zhai, S. Das, S. I. Khondaker and S. Seal, Prog. Mater. Sci., 2011, 56, 1178–1271.
2. T. Wu, G. Ding, H. Shen, H. Wang, L. Sun, D. Jiang, X. Xie and M. Jiang, Adv. Funct. Mater., 2013, 23, 198–203.
3. X. Wang, G. Sun, P. Routh, D. H. Kim, W. Huang and P. Chen, Chem. Soc. Rev., 2014, 43, 7067–7098.
4. X. K. Kong, C. L. Chen and Q. W. Chen, Chem. Soc. Rev., 2014, 43, 2841–2857.
5. L. Sun, L. Wang, C. Tian, T. Tan, Y. Xie, K. Shi, M. Li and H. Fu, RSC Adv., 2012, 2, 4498–4506.
6. C. Zhang, R. Hao, H. Liao and Y. Hou, Nano Energy, 2013, 2, 88–97.
7. Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen and S. Huang, ACS Nano, 2012, 6, 205–211.
8. Z. S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng and K. Mullen, J. Am. Chem. Soc., 2012, 134, 9082–9085.
9. Q. Li, S. Zhang, L. Dai and L. S. Li, J. Am. Chem. Soc., 2012, 134, 18932–18935.
10. B. P. Vinayan, K. Sethupathi and S. Ramaprabhu, Int. J. Hydrogen Energy, 2013, 38, 2240–2250.
11. S. Yu, W. Zheng, C. Wang and Q. Jiang, ACS Nano, 2010, 4, 7619–7629.
12. J. Bai, Q. Zhu, Z. Lv, H. Dong, J. Yu and L. Dong, Int. J. Hydrogen Energy, 2013, 38, 1413–1418.
13. H. Bi, X. Xie, K. Yin, Y. Zhou, S. Wan, L. He, F. Xu, F. Banhart, L. Sun and R. S. Ruoff, Adv. Funct. Mater., 2012, 22, 4421–4425.
14. R. Lv, Q. Li, A. R. Botello-Mendez, T. Hayashi, B. Wang, A. Berkdemir, Q. Hao, A. L. Elias, R. Cruz-Silva, H. R. Gutierrez, Y. A. Kim, H. Muramatsu, J. Zhu, M. Endo, H. Terrones, J. C. Charlier, M. Pan and M. Terrones, Sci. Rep., 2012, 2, 586.
15. K. Sanjeeva Rao, J. Senthilnathan, J. M. Ting and M. Yoshimura, Nanoscale, 2014, 6, 12758–12768.
16. J. Long, X. Xie, J. Xu, Q. Gu, L. Chen and X. Wang, ACS Catal., 2012, 2, 622–631.
17. Y. Gao, G. Hu, J. Zhong, Z. Shi, Y. Zhu, D. S. Su, J. Wang, X. Bao and D. Ma, Angew. Chem., Int. Ed. Engl., 2013, 52, 2109–2113.
18. X.-K. Kong, Z.-Y. Sun, M. Chen, C.-L. Chen and Q.-W. Chen, Energy Environ. Sci., 2013, 6, 3260.
19. J. Janlamool and B. Jongsomjit, Catal. Commun., 2015, 70, 49–52.
20. E. Santacesaria, G. Carotenuto, R. Tesser and M. Di Serio, Chem. Eng. J., 2012, 179, 209–220.
21. Q.-N. Wang, L. Shi and A.-H. Lu, ChemCatChem, 2015, 7, 2846–2852.
22. A. Resta, J. Gustafson, R. Westerström, A. Mikkelsen, E. Lundgren, J. N. Andersen, M.-M. Yang, X.-F. Ma, X.-H. Bao and W.-X. Li, Surf. Sci., 2008, 602, 3057–3063.
23. M. Nielsen, H. Junge, A. Kammer and M. Beller, Angew. Chem., Int. Ed. Engl., 2012, 51, 5711–5713.
24. W. H. Cassinelli, L. Martins, A. R. Passos, S. H. Pulcinelli, A. Rochet, V. Briois and C. V. Santilli, ChemCatChem, 2015, 7, 1668–1677.
25. C. Ampelli, R. Passalacqua, C. Genovese, S. Perathoner, G. Centi, T. Montini, V. Gombac, J. J. Delgado Jaen and P. Fornasiero, RSC Adv., 2013, 3, 21776.
26. J. F. Dewilde, C. J. Czopinski and A. Bhan, ACS Catal., 2014, 4, 4425–4433.
27. T. G. Kelly and J. G. Chen, Green Chem., 2014, 16, 777–784.
28. W. Qian, X. Cui, R. Hao, Y. Hou and Z. Zhang, ACS Appl. Mater. Interfaces, 2011, 3, 2259–2264.
29. D. Yang, A. Velamakanni, G. Bozoklu, S. Park, M. Stoller, R. D. Piner, S. Stankovich, I. Jung, D. A. Field, C. A. Ventrice and R. S. Ruoff, Carbon, 2009, 47, 145–152.
30. Y. Zheng, Y. Jiao, L. Ge, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed. Engl., 2013, 52, 3110–3116.
31. H. Wang, T. Maiyalagan and X. Wang, ACS Catal., 2012, 2, 781–794.
32. M. R. Gao, X. Cao, Q. Gao, Y. F. Xu, Y. R. Zheng, J. Jiang and S. H. Yu, ACS Nano, 2014, 8, 3970–3978.
33. E. Asedegbega-Nieto, M. Perez-Cadenas, M. V. Morales, B. Bachiller-Baeza, E. Gallegos-Suarez, I. Rodriguez-Ramos and A. Guerrero-Ruiz, Diamond Relat. Mater., 2014, 44, 26–32.
34. J. S. Lee, G. S. Park, S. T. Kim, M. Liu and J. Cho, Angew. Chem., Int. Ed. Engl., 2013, 52, 1026–1030.
35. Z. H. Sheng, L. Shao, J. J. Chen, W. J. Bao, F. B. Wang and X. H. Xia, ACS Nano, 2011, 5, 4350–4358.
36. K. R. Lee, K. U. Lee, J. W. Lee, B. T. Ahn and S. I. Woo, Electrochem. Commun., 2010, 12, 1052–1055.

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