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Metallic cobalt nanoparticles imbedded into ordered mesoporous carbon: A non-precious metal catalyst with excellent hydrogenation performance
Journal of Colloid and Interface Science 505 (2017) 789–795
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Metallic cobalt nanoparticles imbedded into ordered mesoporous carbon: A non-precious metal catalyst with excellent hydrogenation performance Jiangyong Liu a,⇑, Zihao Wang a, Xiaodong Yan b, Panming Jian a a b
School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225002, China Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110, USA
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 17 May 2017 Revised 19 June 2017 Accepted 23 June 2017 Available online 24 June 2017 Keywords: Catalysis Hydrogenation Nitroaromatic compounds Carbon monoxide Cobalt nanoparticles Ordered mesoporous carbon
a b s t r a c t Ordered mesoporous carbon (OMC)-metal composites have attracted great attention owing to their combination of high surface area, controlled pore size distribution and physicochemical properties of metals. Herein, we report the cobalt nanoparticles/ordered mesoporous carbon (CoNPs@OMC) composite prepared by a one-step carbonization/reduction process assisted by a hydrothermal pre-reaction. The CoNPs@OMC composite presents a high specific surface area of 544 m2 g1, and the CoNPs are uniformly imbedded or confined in the ordered mesoporous carbon matrix. When used as a non-precious metalcontaining catalyst for hydrogenation reduction of p-nitrophenol and nitrobenzene, it demonstrates high efficiency and good cycling stability. Furthermore, the CoNPs@OMC composite can be directly used to catalyze the Fischer-Tropsch synthesis for the high-pressure CO hydrogenation, and presents a good catalytic selectivity for C+5 hydrocarbons. The excellent catalytic performance of the CoNPs@OMC composite can be ascribed to synergistic effect between the high specific surface area, mesoporous structure and well-imbedded CoNPs in the carbon matrix. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding author. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jcis.2017.06.081 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.
Ordered mesoporous carbons (OMCs), with uniform pores, tunable pore size, large surface area and chemical inertness, have
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aroused great attention owing to their wide applications in catalysis, adsorption, energy conversion and storage, etc. [1–7]. One of the most traditional ways for the synthesis of OMCs is the ‘‘hard-template” method, which involves the utilization of ordered mesoporous silicas as hard templates and sacrificial scaffolds [8–10]. However, the tedious procedure and high cost impede the large scale application of this hard-template strategy. Recently, a flexible and effective method known as the ‘‘soft-template” method has been developed for the synthesis of OMCs, which generally involves the self-assembly of amphiphilic triblock copolymers as soft templates and carbon precursors followed by the carbonization [11–15]. For the purpose of catalysis application, OMCs have been functionalized by heteroatoms doping or incorporation with metal components [15–18]. The conventional way for the preparation of metals/OMCs catalysts is the impregnation method with metals supported on OMCs by the typical procedures of impregnation, drying and calcination [2,17,19–21]. Although the impregnation techniques have great practical simplicity, the metals always distribute uncontrollably and randomly on the surface or within the channels of OMCs accompanying the presence of pore blockage. For the obtainment of confined reaction environment, we have recently developed a new-type cobalt imbedded zeolite catalyst with conventional Co/SiO2 as the precursor [22–24]. However, in these catalysts, the metallic cobalt particles can only be obtained through the high-temperature and time-consuming hydrogen reduction process. In this study, we report a simple one-pot synthesis of metallic cobalt nanoparticles/ordered mesoporous carbon (CoNPs@OMC) composite by combination of hydrothermal and carbonization reactions. The as-synthesized CoNPs@OMC composite is utilized as a bifunctional catalyst for the hydrogenation reduction of nitroaromatic compounds (p-nitrophenol and nitrobenzene) and the high-pressure CO hydrogenation process (Fischer-Tropsch synthesis, FTS). The CoNPs@OMC composite exhibits high catalytic activities and cycling stabilities in both applications. 2. Experimental 2.1. Materials preparation Pluronic F127 (poly(ethylene oxide)-b-poly(propylene oxide)b-poly(ethylene oxide) triblock copolymer, EO106PO70EO106) with an average molecular weight of 12,600 was used as the structure-directing agent. In a typical procedure, 2.5 g F127 was dissolved in 10 mL of deionized water and 10 mL of ethanol under continuous stirring for 1 h. Then 1.65 g of resorcinol was added into the solution and stirred for 30 min. Afterwards, 0.4 g of hydrochloric acid (HCl, 37 wt%) was added and stirred continuously for 1 h, followed by the addition of 1.11 g of Co(NO3)26H2O with stirring for another 1 h. Thereafter, 1.41 mL of formaldehyde (HCHO, 37 wt%) was added dropwise, and the mixture was further vigorously stirred for 1.5 h before the hydrothermal process was performed at 100 °C for 10 h. The polymeric monolith was obtained by filtration and washed thoroughly with deionized water and ethanol, followed by curing at 50 °C for 4 h and 80 °C for 15 h. Carbonization was conducted in a tubular furnace under Ar atmosphere at 600 °C for 3 h with a ramping rate of 3 °C min1. For comparison, the pure OMC was prepared under similar conditions without Co(NO3)26H2O. 2.2. Physical characterization The small-angle X-ray diffraction patterns (SA-XRD) were performed on a Rigaku D/max-2500VB2+/PC diffractometer (40 kV,
50 mA) using Cu Ka radiation (k = 0.154056 nm) within the 2h range of 0.5–10°, while the wide-angle XRD (WA-XRD) measurements were scanned at 40 kV and 200 mA between the 2h range of 5–90°. Thermogravimetric analysis (TGA) was conducted on a Netzsch STA 449C thermogravimetric analyzer. The morphology was investigated with a field-emission scanning electron microscope (SEM) (JEOL JSM-7800F). The Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-3010 microscope. N2 isothermal adsorption-desorption measurement at 77 K was performed with a Micromeritics ASAP 2010 system. Prior to the test, the samples were outgassed at 473 K for 5 h. The surface area of the samples was determined by the Bru nauer–Emmett–Teller (BET) method, while the pore volume was calculated at a relative pressure (P/P0) of about 0.99, where P and P0 are the measured and equilibrium pressures, respectively. The pore size distribution curves were obtained with the Barrett–Joy ner–Halenda (BJH) method from the desorption branches of the isotherms. The XPS data were collected on a Thermo, Fisher Scientific ESCALAB 250 spectrometer in the constant analyzer energy (CAE) mode, with an Al Ka monochromatized X-ray source (1486.6 eV). The survey spectra were measured at 30 eV pass energy.
2.3. Catalytic reactions The CoNPs@OMC composite was employed for the hydrogenation reduction of p-NP (p-nitrophenol) or NB (nitrobenzene). Typically, 5 mL of NaBH4 aqueous solution of (0.04 M) was added to 20 mL of p-NP or NB aqueous solution (0.1 mM). Then the reaction was initiated by adding 0.5 mL of CoNPs@OMC catalyst suspension (1.0 g/L) into the mixed solution. The hydrogenation reduction process was monitored by measuring the absorption spectra via a UV–vis spectrophotometer. For the FTS reaction, the CoNPs@OMC catalyst was directly employed in a continuous-flow-type fixed-bed reactor without the conventional hydrogen reduction process. The reaction temperature and pressure of the FTS reaction were 523 K and 2.0 MPa, respectively. The molar ratio of H2/CO was 2.0, and the Wcatalyst/Fsyngas was 10 g h mol1. The product was analyzed by online and offline gas chromatography [22–24]. All the analysis results were summed up to obtain the CO conversion and the hydrocarbon selectivity (in terms of carbon mol percentage, c-mol %).
3. Results and discussion 3.1. Structure and morphology characterization The ordered mesoporous structure of the OMC was examined by the SA-XRD analysis. The intense single diffraction peak indexed to the (1 0 0) plane of carbon in the SA-XRD profile of OMC (Fig. 1a) suggests a highly ordered mesoporous structure [16]. The weaker peak intensity for CoNPs@OMC indicates a decrease in the structure regularity possibly owing to the introduction of cobalt salts in the precursor solution. The WA-XRD patterns (Fig. 1b) show that both OMC and CoNPs@OMC have two broad peaks centered at 22 and 43°, indicating the amorphous state of the carbon framework [25]. Three other peaks in the XRD pattern of CoNPs@OMC locating at 44.3, 51.6 and 75.8° correspond to the (1 1 1), (2 0 0) and (2 2 0) crystal planes of face-centered Co (JCPDS 15-0806), respectively. This suggests that Co2+ has been reduced to metallic cobalt (Co0) during the carbonization process [26,27]. Obviously, no diffraction peaks of cobalt oxide or cobalt carbide was detected by XRD measurement. The grain size of CoNPs was calculated using Scherrer
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Fig. 1. (a) SA-XRD and (b) WA-XRD patterns of OMC and CoNPs@OMC.
equation at 2h = 44.3°. The grain size calculated from the (1 1 1) peak is 28.9 nm. Fig. 2a shows the SEM image of CoNPs@OMC. It is composed of nanoparticles. The TEM image (Fig. 2b) of OMC presents its highly ordered mesoporous structure with orderly aligned mesopores. The CoNPs@OMC composite shows deteriorated ordered mesoporous structure (Fig. 2c and d), which is consistent with the XRD analysis. The metallic cobalt nanoparticles were homogeneously imbedded or confined in the ordered mesoporous carbon matrix. The particle size distribution (PSD) of the CoNPs in the CoNPs@OMC composite obtained from the TEM image is in the
range of 30–75 nm, and the average particle size is 49.4 nm. This is larger than the grain size calculated by the Scherrer equation, implying that the Co nanoparticles are polycrystalline. Additional information regarding the mesoporous structure of CoNPs@OMC can be obtained from the N2 adsorption-desorption isotherm (Fig. 3a). It can be observed that the CoNPs@OMC composite shows a type IV isotherm, confirming the mesoporous structure [28]. The specific surface area of the CoNPs@OMC composite is 544 m2 g1, and the corresponding pore size distribution obtained from the desorption branch by the BJH model demonstrates a narrow distribution with an average pore size of 4.7 nm. Interestingly,
Fig. 2. (a) SEM image of CoNPs@OMC, TEM images of (b) OMC and (c and d) CoNPs@OMC. The inset in (d) is the particle size distribution of the CoNPs in CoNPs@OMC.
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Fig. 3. (a) Nitrogen adsorption-desorption isotherm of CoNPs@OMC with the inset of the corresponding pore size distribution. (b) XPS survey and (c) TG profile of CoNPs@OMC and OMC.
the Co nanoparticles were not detected by XPS, and only carbon and oxygen were observed in the XPS survey spectra of both OMC and CoNPs@OMC (Fig. 3b). This could be due to the excellent imbedding of CoNPs into the mesoporous carbon matrix as XPS is a surface-sensitive technique [29]. The cobalt content was examined by thermogravimetric (TG) analysis (Fig. 2c). The Co content in CoNPs@OMC was estimated to be 5.2% provided metallic Co was completely converted into Co3O4. 3.2. Hydrogenation reduction of p-nitrophenol and nitrobenzene The ubiquitous organic pollutant p-nitrophenol in the industrial effluents is also the raw material for the synthesis of paminophenol (p-AP). The latter is an important intermediate for the production of various medicines in the pharmaceutical industry, along with many other applications [30–32]. Preciouscontaining materials (such as Au, Ag, Pt and Pd) have been widely used for the catalytic hydrogenation reduction of p-NP into p-AP [33–37]. However, the high cost and scarcity of the noble metals restrict their practical applications. Therefore, it is of great importance to minimize the usage of noble metals or even replace them with nonprecious metal based catalysts. In this study, the CoNPs@OMC composite was used as a non-precious metal-containing catalyst for the liquid-phase hydrogenation reduction of p-NP. The catalytic process was monitored by UV–vis spectrophotometer. After addition of freshly prepared NaBH4 solution, the typical peak centered at 317 nm from the p-NP molecules in the UV–vis spectrum (Fig. 4a) disappeared, and a new peak located at 400 nm was observed accounting for the generation of pnitrophenolate ion. At the same time, the color of the solution changed from light yellow to deep yellow. It should be noted that the p-NP reduction reaction is unable to proceed in the absence of
catalyst [31]. Upon the addition of the CoNPs@OMC catalyst, the intensity of the absorption peak at 400 nm gradually reduced with time. Meanwhile, the deep yellow color gradually faded away and turned into colorless as the reaction progressed. In addition, a new absorption peak centered at 298 nm, indicative of the generation of p-AP as the final product, began to appear and increased gradually with time. After 36 min, the peak ascribed to the nitro compound vanished, indicating the complete reduction of p-NP. Moreover, two isosbestic points at 278 and 314 nm can be observed from the spectrum. This implies that no side reactions occurred with p-AP as the only product [38]. Fig. 4b presents the profile of C/C0 versus reaction time for the hydrogenation reduction of p-NP. Previous reports showed that the hydrogenation reduction of p-NP with excessive NaBH4 followed the Langmuir-Hinshelwood model, which suggested that the catalytic reaction rate can be measured with the pseudofirst-order kinetics [31,38]. As a result, the kinetic equation can be presented as the following:
lnðC=C0 Þ ¼ kt where C0 and C are the initial and instantaneous concentration of pNP, respectively. The inset in Fig. 4b shows the liner relationship of ln(C/C0) versus reaction time. From the slope of the fitted line, the apparent rate constant was calculated to be 0.1030 min–1. Furthermore, the turnover frequency (TOF) defined as the mole number of p-NP per mole of catalyst per hour was calculated to be 7.6 h1 for the CoNPs@OMC catalyst. This TOF value is comparable to those of the reported noble metal catalysts, such as Au/Graphene (11.4 h1) [39] and Pd-CNT-RGO (25.8 h1) [40]. This proves the high catalytic activity of the CoNPs@OMC composite. What’s more, the hydrogenation reduction of p-NP with CoNPs@OMC as the catalyst at different temperatures was conducted as well (Table 1). It can be
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Fig. 4. (a) and (c) are the evolution of UV–vis absorption spectra for the hydrogenation reduction of p-NP and NB with CoNPs@OMC as the catalyst, respectively; (b) and (d) are the corresponding C/C0 and ln(C/C0) (inset) versus reaction time.
lnðkÞ ¼ ðEa =RÞ þ lnðAÞ
Table 1 Reaction results of the hydrogenation of p-NP at different temperatures. t (°C)
T (K)
1000/T (K1)
k (min1)
lnk
20 25 30 40
293.15 298.15 303.15 313.15
3.411 3.354 3.299 3.193
0.0832 0.1030 0.2039 0.3957
2.487 2.273 1.590 0.927
where k is the apparent rate constant, R is the molar gas constant and A is the pre-exponential factor. The linear fitting of lnk versus 1000/T was presented in Fig. 5, and the Ea value was calculated to be 62.5 kJ mol1, which is comparable to those of the precious metal catalysts [41,42]. On the other hand, cycling stability is a key factor for the actual applications of the catalysts. Herein, the CoNPs@OMC catalyst demonstrates a good catalytic stability without obvious loss in catalytic activity during nine cycles (Fig. 6). It’s generally considered that the hydrogenation reduction of pNP into p-AP with NaBH4 is thermodynamically feasible but kinetically impeded by the high kinetic barrier [43]. Based on the char-
Fig. 5. Arrhenius profile for the hydrogenation reduction of p-NP performed with the CoNPs@OMC catalyst at different temperatures.
concluded that a higher reaction temperature gave rise to a higher reaction rate. The apparent activation energy (Ea) can be evaluated by the Arrhenius equation:
Fig. 6. The cycling test of the CoNPs@OMC catalyst for the hydrogenation reduction of p-NP.
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Fig. 7. (a) CO conversion with time and (b) FTS product distribution in the presence of the CoNPs@OMC catalyst.
acterization and reaction results, the reaction mechanism for the hydrogenation reduction of p-NP on the CoNPs@OMC catalyst can be speculated. The electrons generated from BH 4 can create an electron-enriched region near the Co/C sites, and thus effectively transfer electrons to the p-NP molecules to facilitate their reduction. In addition, the mesoporous structure with abundant pore channels offers a large accessible surface area for the reactants, and the high surface area of the CoNPs@OMC catalyst can ensure a high adsorption capability for the capture of p-NP onto the surface of the CoNPs@OMC catalyst, which work synergistically with the Co nanoparticles to efficiently facilitate the hydrogenation reduction of p-AP. In order to prove its high catalytic activity, nitrobenzene (NB) was selected as another nitroaromatic compound to further investigate the hydrogenation reduction capability of the CoNPs@OMC catalyst. As revealed by Fig. 4c, the absorption peak of NB at 268 nm gradually weakened, meanwhile a new absorption peak at 230 nm attributed to the generation of aminobenzene (AB) appeared and progressively increased as the reaction proceeded. This confirms the conversion of NB to AB. The pseudo-first-order kinetics can also be applied in the hydrogenation reduction of NB, and the apparent rate constant derived from the slope of the fitted line in the inset in Fig. 4d is 0.0383 min1. This suggestes that the CoNPs@OMC catalyst is also an effective catalyst for the hydrogenation reduction of NB.
3.3. Hydrogenation of CO (Fischer-Tropsch Synthesis) The heterogeneous catalytic reaction of Fischer–Tropsch synthesis creates approximately 2% of the world’s fuel, and has attracted great attention both academically and industrially [44,45]. Unfortunately, the FTS hydrocarbon products (from CH4 to long-chain hydrocarbons) obey the Anderson-Schulz-Flory (ASF) distribution, with a theoretically limited value for the desired products such as lower olefins or gasoline [23]. To regulate the product distribution of FTS, various catalysts have been developed. Among them, cobalt-based catalysts have been considered as the most promising ones [46]. Herein, the CoNPs@OMC composite has two merits over conventional cobalt-based catalysts [22–24]: firstly, the one-step carbonization/reduction process greatly simplifies the preparation process; secondly, the carbon matrix can effectively protect the Co nanoparticles from oxidation during the FTS reaction or by oxygen in air. The CO conversion and product distribution from the FTS reaction in the presence of the CoNPs@OMC catalyst are shown in Fig. 7. The CoNPs@OMC catalyst presents a high CO conversion of
30.2% after a reaction time of 30 h with only 5.2% cobalt loading in the catalyst, indicating an impressively high catalytic activity. The CoNPs@OMC catalyst also showed a good stability during the reaction as revealed by Fig. 7a. The good catalytic activity and stability of the CoNPs@OMC catalyst can be attributed to the following reasons: first, the high porosity provides a large surface for the FTS reaction; second, the mesoporous structure highly favors the diffusion of the reactive molecules and hydrocarbon products; third, the Co nanoparticles are well protected by the carbon matrix from oxidation by water (FTS byproduct). The product distribution shows that methane has a low selectivity of 15.2% and C+5 hydrocarbon product achieve a high selectivity of 81.5%. Note that methane is the least desired FTS product while C+5 selectivity represents the chain growth capability and contain the most important FTS products such as gasoline or diesel [22,47]. These results indicate that the CoNPs@OMC composite is a very promising FTS catalyst. 4. Conclusions In conclusion, we has successfully developed a novel CoNPs@OMC catalyst with metallic cobalt nanoparticles imbedded in the ordered mesoporous carbon by a simple one-step carbonization/reduction process assisted by a hydrothermal process. The CoNPs@OMC composite can be applied as a non-precious metalcontaining catalyst with a high activity and good cycling stability for the hydrogenation reduction of p-nitrophenol and nitrobenzene. What’s more, the CoNPs@OMC composite can effectively catalyze the Fischer-Tropsch synthesis process, and shows a good catalytic activity, excellent stability and high C+5 product selectivity. The superb catalytic performances of the CoNPs@OMC composite in various hydrogenation reactions can be attributed to the synergistic effect between high specific surface area, ordered mesoporous structure, and well-imbedded CoNPs. The CoNPs@OMC composite can be a promising catalyst for hydrogenation reduction of nitroaromatic compounds and FTS reaction, and also potentially can be extended to other catalytic processes by selecting proper metals. Acknowledgements We gratefully acknowledge the financial support from the High-level Talents Research Fund of Yangzhou University (137011046) and the Prospective Joint Research Project of Jiangsu Provincial Science and Technology Department (BYE015061-10). X. Y. thanks the funds provided by the University of Missouri-Kansas City, School of Graduate Studies.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Metallic cobalt nanoparticles imbedded into ordered mesoporous carbon: A non-precious metal catalyst with excellent hydrogenation performance Jiangyong Liu a,⇑, Zihao Wang a, Xiaodong Yan b, Panming Jian a a b
School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225002, China Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110, USA
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 17 May 2017 Revised 19 June 2017 Accepted 23 June 2017 Available online 24 June 2017 Keywords: Catalysis Hydrogenation Nitroaromatic compounds Carbon monoxide Cobalt nanoparticles Ordered mesoporous carbon
a b s t r a c t Ordered mesoporous carbon (OMC)-metal composites have attracted great attention owing to their combination of high surface area, controlled pore size distribution and physicochemical properties of metals. Herein, we report the cobalt nanoparticles/ordered mesoporous carbon (CoNPs@OMC) composite prepared by a one-step carbonization/reduction process assisted by a hydrothermal pre-reaction. The CoNPs@OMC composite presents a high specific surface area of 544 m2 g1, and the CoNPs are uniformly imbedded or confined in the ordered mesoporous carbon matrix. When used as a non-precious metalcontaining catalyst for hydrogenation reduction of p-nitrophenol and nitrobenzene, it demonstrates high efficiency and good cycling stability. Furthermore, the CoNPs@OMC composite can be directly used to catalyze the Fischer-Tropsch synthesis for the high-pressure CO hydrogenation, and presents a good catalytic selectivity for C+5 hydrocarbons. The excellent catalytic performance of the CoNPs@OMC composite can be ascribed to synergistic effect between the high specific surface area, mesoporous structure and well-imbedded CoNPs in the carbon matrix. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding author. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jcis.2017.06.081 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.
Ordered mesoporous carbons (OMCs), with uniform pores, tunable pore size, large surface area and chemical inertness, have
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aroused great attention owing to their wide applications in catalysis, adsorption, energy conversion and storage, etc. [1–7]. One of the most traditional ways for the synthesis of OMCs is the ‘‘hard-template” method, which involves the utilization of ordered mesoporous silicas as hard templates and sacrificial scaffolds [8–10]. However, the tedious procedure and high cost impede the large scale application of this hard-template strategy. Recently, a flexible and effective method known as the ‘‘soft-template” method has been developed for the synthesis of OMCs, which generally involves the self-assembly of amphiphilic triblock copolymers as soft templates and carbon precursors followed by the carbonization [11–15]. For the purpose of catalysis application, OMCs have been functionalized by heteroatoms doping or incorporation with metal components [15–18]. The conventional way for the preparation of metals/OMCs catalysts is the impregnation method with metals supported on OMCs by the typical procedures of impregnation, drying and calcination [2,17,19–21]. Although the impregnation techniques have great practical simplicity, the metals always distribute uncontrollably and randomly on the surface or within the channels of OMCs accompanying the presence of pore blockage. For the obtainment of confined reaction environment, we have recently developed a new-type cobalt imbedded zeolite catalyst with conventional Co/SiO2 as the precursor [22–24]. However, in these catalysts, the metallic cobalt particles can only be obtained through the high-temperature and time-consuming hydrogen reduction process. In this study, we report a simple one-pot synthesis of metallic cobalt nanoparticles/ordered mesoporous carbon (CoNPs@OMC) composite by combination of hydrothermal and carbonization reactions. The as-synthesized CoNPs@OMC composite is utilized as a bifunctional catalyst for the hydrogenation reduction of nitroaromatic compounds (p-nitrophenol and nitrobenzene) and the high-pressure CO hydrogenation process (Fischer-Tropsch synthesis, FTS). The CoNPs@OMC composite exhibits high catalytic activities and cycling stabilities in both applications. 2. Experimental 2.1. Materials preparation Pluronic F127 (poly(ethylene oxide)-b-poly(propylene oxide)b-poly(ethylene oxide) triblock copolymer, EO106PO70EO106) with an average molecular weight of 12,600 was used as the structure-directing agent. In a typical procedure, 2.5 g F127 was dissolved in 10 mL of deionized water and 10 mL of ethanol under continuous stirring for 1 h. Then 1.65 g of resorcinol was added into the solution and stirred for 30 min. Afterwards, 0.4 g of hydrochloric acid (HCl, 37 wt%) was added and stirred continuously for 1 h, followed by the addition of 1.11 g of Co(NO3)26H2O with stirring for another 1 h. Thereafter, 1.41 mL of formaldehyde (HCHO, 37 wt%) was added dropwise, and the mixture was further vigorously stirred for 1.5 h before the hydrothermal process was performed at 100 °C for 10 h. The polymeric monolith was obtained by filtration and washed thoroughly with deionized water and ethanol, followed by curing at 50 °C for 4 h and 80 °C for 15 h. Carbonization was conducted in a tubular furnace under Ar atmosphere at 600 °C for 3 h with a ramping rate of 3 °C min1. For comparison, the pure OMC was prepared under similar conditions without Co(NO3)26H2O. 2.2. Physical characterization The small-angle X-ray diffraction patterns (SA-XRD) were performed on a Rigaku D/max-2500VB2+/PC diffractometer (40 kV,
50 mA) using Cu Ka radiation (k = 0.154056 nm) within the 2h range of 0.5–10°, while the wide-angle XRD (WA-XRD) measurements were scanned at 40 kV and 200 mA between the 2h range of 5–90°. Thermogravimetric analysis (TGA) was conducted on a Netzsch STA 449C thermogravimetric analyzer. The morphology was investigated with a field-emission scanning electron microscope (SEM) (JEOL JSM-7800F). The Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-3010 microscope. N2 isothermal adsorption-desorption measurement at 77 K was performed with a Micromeritics ASAP 2010 system. Prior to the test, the samples were outgassed at 473 K for 5 h. The surface area of the samples was determined by the Bru nauer–Emmett–Teller (BET) method, while the pore volume was calculated at a relative pressure (P/P0) of about 0.99, where P and P0 are the measured and equilibrium pressures, respectively. The pore size distribution curves were obtained with the Barrett–Joy ner–Halenda (BJH) method from the desorption branches of the isotherms. The XPS data were collected on a Thermo, Fisher Scientific ESCALAB 250 spectrometer in the constant analyzer energy (CAE) mode, with an Al Ka monochromatized X-ray source (1486.6 eV). The survey spectra were measured at 30 eV pass energy.
2.3. Catalytic reactions The CoNPs@OMC composite was employed for the hydrogenation reduction of p-NP (p-nitrophenol) or NB (nitrobenzene). Typically, 5 mL of NaBH4 aqueous solution of (0.04 M) was added to 20 mL of p-NP or NB aqueous solution (0.1 mM). Then the reaction was initiated by adding 0.5 mL of CoNPs@OMC catalyst suspension (1.0 g/L) into the mixed solution. The hydrogenation reduction process was monitored by measuring the absorption spectra via a UV–vis spectrophotometer. For the FTS reaction, the CoNPs@OMC catalyst was directly employed in a continuous-flow-type fixed-bed reactor without the conventional hydrogen reduction process. The reaction temperature and pressure of the FTS reaction were 523 K and 2.0 MPa, respectively. The molar ratio of H2/CO was 2.0, and the Wcatalyst/Fsyngas was 10 g h mol1. The product was analyzed by online and offline gas chromatography [22–24]. All the analysis results were summed up to obtain the CO conversion and the hydrocarbon selectivity (in terms of carbon mol percentage, c-mol %).
3. Results and discussion 3.1. Structure and morphology characterization The ordered mesoporous structure of the OMC was examined by the SA-XRD analysis. The intense single diffraction peak indexed to the (1 0 0) plane of carbon in the SA-XRD profile of OMC (Fig. 1a) suggests a highly ordered mesoporous structure [16]. The weaker peak intensity for CoNPs@OMC indicates a decrease in the structure regularity possibly owing to the introduction of cobalt salts in the precursor solution. The WA-XRD patterns (Fig. 1b) show that both OMC and CoNPs@OMC have two broad peaks centered at 22 and 43°, indicating the amorphous state of the carbon framework [25]. Three other peaks in the XRD pattern of CoNPs@OMC locating at 44.3, 51.6 and 75.8° correspond to the (1 1 1), (2 0 0) and (2 2 0) crystal planes of face-centered Co (JCPDS 15-0806), respectively. This suggests that Co2+ has been reduced to metallic cobalt (Co0) during the carbonization process [26,27]. Obviously, no diffraction peaks of cobalt oxide or cobalt carbide was detected by XRD measurement. The grain size of CoNPs was calculated using Scherrer
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Fig. 1. (a) SA-XRD and (b) WA-XRD patterns of OMC and CoNPs@OMC.
equation at 2h = 44.3°. The grain size calculated from the (1 1 1) peak is 28.9 nm. Fig. 2a shows the SEM image of CoNPs@OMC. It is composed of nanoparticles. The TEM image (Fig. 2b) of OMC presents its highly ordered mesoporous structure with orderly aligned mesopores. The CoNPs@OMC composite shows deteriorated ordered mesoporous structure (Fig. 2c and d), which is consistent with the XRD analysis. The metallic cobalt nanoparticles were homogeneously imbedded or confined in the ordered mesoporous carbon matrix. The particle size distribution (PSD) of the CoNPs in the CoNPs@OMC composite obtained from the TEM image is in the
range of 30–75 nm, and the average particle size is 49.4 nm. This is larger than the grain size calculated by the Scherrer equation, implying that the Co nanoparticles are polycrystalline. Additional information regarding the mesoporous structure of CoNPs@OMC can be obtained from the N2 adsorption-desorption isotherm (Fig. 3a). It can be observed that the CoNPs@OMC composite shows a type IV isotherm, confirming the mesoporous structure [28]. The specific surface area of the CoNPs@OMC composite is 544 m2 g1, and the corresponding pore size distribution obtained from the desorption branch by the BJH model demonstrates a narrow distribution with an average pore size of 4.7 nm. Interestingly,
Fig. 2. (a) SEM image of CoNPs@OMC, TEM images of (b) OMC and (c and d) CoNPs@OMC. The inset in (d) is the particle size distribution of the CoNPs in CoNPs@OMC.
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Fig. 3. (a) Nitrogen adsorption-desorption isotherm of CoNPs@OMC with the inset of the corresponding pore size distribution. (b) XPS survey and (c) TG profile of CoNPs@OMC and OMC.
the Co nanoparticles were not detected by XPS, and only carbon and oxygen were observed in the XPS survey spectra of both OMC and CoNPs@OMC (Fig. 3b). This could be due to the excellent imbedding of CoNPs into the mesoporous carbon matrix as XPS is a surface-sensitive technique [29]. The cobalt content was examined by thermogravimetric (TG) analysis (Fig. 2c). The Co content in CoNPs@OMC was estimated to be 5.2% provided metallic Co was completely converted into Co3O4. 3.2. Hydrogenation reduction of p-nitrophenol and nitrobenzene The ubiquitous organic pollutant p-nitrophenol in the industrial effluents is also the raw material for the synthesis of paminophenol (p-AP). The latter is an important intermediate for the production of various medicines in the pharmaceutical industry, along with many other applications [30–32]. Preciouscontaining materials (such as Au, Ag, Pt and Pd) have been widely used for the catalytic hydrogenation reduction of p-NP into p-AP [33–37]. However, the high cost and scarcity of the noble metals restrict their practical applications. Therefore, it is of great importance to minimize the usage of noble metals or even replace them with nonprecious metal based catalysts. In this study, the CoNPs@OMC composite was used as a non-precious metal-containing catalyst for the liquid-phase hydrogenation reduction of p-NP. The catalytic process was monitored by UV–vis spectrophotometer. After addition of freshly prepared NaBH4 solution, the typical peak centered at 317 nm from the p-NP molecules in the UV–vis spectrum (Fig. 4a) disappeared, and a new peak located at 400 nm was observed accounting for the generation of pnitrophenolate ion. At the same time, the color of the solution changed from light yellow to deep yellow. It should be noted that the p-NP reduction reaction is unable to proceed in the absence of
catalyst [31]. Upon the addition of the CoNPs@OMC catalyst, the intensity of the absorption peak at 400 nm gradually reduced with time. Meanwhile, the deep yellow color gradually faded away and turned into colorless as the reaction progressed. In addition, a new absorption peak centered at 298 nm, indicative of the generation of p-AP as the final product, began to appear and increased gradually with time. After 36 min, the peak ascribed to the nitro compound vanished, indicating the complete reduction of p-NP. Moreover, two isosbestic points at 278 and 314 nm can be observed from the spectrum. This implies that no side reactions occurred with p-AP as the only product [38]. Fig. 4b presents the profile of C/C0 versus reaction time for the hydrogenation reduction of p-NP. Previous reports showed that the hydrogenation reduction of p-NP with excessive NaBH4 followed the Langmuir-Hinshelwood model, which suggested that the catalytic reaction rate can be measured with the pseudofirst-order kinetics [31,38]. As a result, the kinetic equation can be presented as the following:
lnðC=C0 Þ ¼ kt where C0 and C are the initial and instantaneous concentration of pNP, respectively. The inset in Fig. 4b shows the liner relationship of ln(C/C0) versus reaction time. From the slope of the fitted line, the apparent rate constant was calculated to be 0.1030 min–1. Furthermore, the turnover frequency (TOF) defined as the mole number of p-NP per mole of catalyst per hour was calculated to be 7.6 h1 for the CoNPs@OMC catalyst. This TOF value is comparable to those of the reported noble metal catalysts, such as Au/Graphene (11.4 h1) [39] and Pd-CNT-RGO (25.8 h1) [40]. This proves the high catalytic activity of the CoNPs@OMC composite. What’s more, the hydrogenation reduction of p-NP with CoNPs@OMC as the catalyst at different temperatures was conducted as well (Table 1). It can be
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Fig. 4. (a) and (c) are the evolution of UV–vis absorption spectra for the hydrogenation reduction of p-NP and NB with CoNPs@OMC as the catalyst, respectively; (b) and (d) are the corresponding C/C0 and ln(C/C0) (inset) versus reaction time.
lnðkÞ ¼ ðEa =RÞ þ lnðAÞ
Table 1 Reaction results of the hydrogenation of p-NP at different temperatures. t (°C)
T (K)
1000/T (K1)
k (min1)
lnk
20 25 30 40
293.15 298.15 303.15 313.15
3.411 3.354 3.299 3.193
0.0832 0.1030 0.2039 0.3957
2.487 2.273 1.590 0.927
where k is the apparent rate constant, R is the molar gas constant and A is the pre-exponential factor. The linear fitting of lnk versus 1000/T was presented in Fig. 5, and the Ea value was calculated to be 62.5 kJ mol1, which is comparable to those of the precious metal catalysts [41,42]. On the other hand, cycling stability is a key factor for the actual applications of the catalysts. Herein, the CoNPs@OMC catalyst demonstrates a good catalytic stability without obvious loss in catalytic activity during nine cycles (Fig. 6). It’s generally considered that the hydrogenation reduction of pNP into p-AP with NaBH4 is thermodynamically feasible but kinetically impeded by the high kinetic barrier [43]. Based on the char-
Fig. 5. Arrhenius profile for the hydrogenation reduction of p-NP performed with the CoNPs@OMC catalyst at different temperatures.
concluded that a higher reaction temperature gave rise to a higher reaction rate. The apparent activation energy (Ea) can be evaluated by the Arrhenius equation:
Fig. 6. The cycling test of the CoNPs@OMC catalyst for the hydrogenation reduction of p-NP.
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Fig. 7. (a) CO conversion with time and (b) FTS product distribution in the presence of the CoNPs@OMC catalyst.
acterization and reaction results, the reaction mechanism for the hydrogenation reduction of p-NP on the CoNPs@OMC catalyst can be speculated. The electrons generated from BH 4 can create an electron-enriched region near the Co/C sites, and thus effectively transfer electrons to the p-NP molecules to facilitate their reduction. In addition, the mesoporous structure with abundant pore channels offers a large accessible surface area for the reactants, and the high surface area of the CoNPs@OMC catalyst can ensure a high adsorption capability for the capture of p-NP onto the surface of the CoNPs@OMC catalyst, which work synergistically with the Co nanoparticles to efficiently facilitate the hydrogenation reduction of p-AP. In order to prove its high catalytic activity, nitrobenzene (NB) was selected as another nitroaromatic compound to further investigate the hydrogenation reduction capability of the CoNPs@OMC catalyst. As revealed by Fig. 4c, the absorption peak of NB at 268 nm gradually weakened, meanwhile a new absorption peak at 230 nm attributed to the generation of aminobenzene (AB) appeared and progressively increased as the reaction proceeded. This confirms the conversion of NB to AB. The pseudo-first-order kinetics can also be applied in the hydrogenation reduction of NB, and the apparent rate constant derived from the slope of the fitted line in the inset in Fig. 4d is 0.0383 min1. This suggestes that the CoNPs@OMC catalyst is also an effective catalyst for the hydrogenation reduction of NB.
3.3. Hydrogenation of CO (Fischer-Tropsch Synthesis) The heterogeneous catalytic reaction of Fischer–Tropsch synthesis creates approximately 2% of the world’s fuel, and has attracted great attention both academically and industrially [44,45]. Unfortunately, the FTS hydrocarbon products (from CH4 to long-chain hydrocarbons) obey the Anderson-Schulz-Flory (ASF) distribution, with a theoretically limited value for the desired products such as lower olefins or gasoline [23]. To regulate the product distribution of FTS, various catalysts have been developed. Among them, cobalt-based catalysts have been considered as the most promising ones [46]. Herein, the CoNPs@OMC composite has two merits over conventional cobalt-based catalysts [22–24]: firstly, the one-step carbonization/reduction process greatly simplifies the preparation process; secondly, the carbon matrix can effectively protect the Co nanoparticles from oxidation during the FTS reaction or by oxygen in air. The CO conversion and product distribution from the FTS reaction in the presence of the CoNPs@OMC catalyst are shown in Fig. 7. The CoNPs@OMC catalyst presents a high CO conversion of
30.2% after a reaction time of 30 h with only 5.2% cobalt loading in the catalyst, indicating an impressively high catalytic activity. The CoNPs@OMC catalyst also showed a good stability during the reaction as revealed by Fig. 7a. The good catalytic activity and stability of the CoNPs@OMC catalyst can be attributed to the following reasons: first, the high porosity provides a large surface for the FTS reaction; second, the mesoporous structure highly favors the diffusion of the reactive molecules and hydrocarbon products; third, the Co nanoparticles are well protected by the carbon matrix from oxidation by water (FTS byproduct). The product distribution shows that methane has a low selectivity of 15.2% and C+5 hydrocarbon product achieve a high selectivity of 81.5%. Note that methane is the least desired FTS product while C+5 selectivity represents the chain growth capability and contain the most important FTS products such as gasoline or diesel [22,47]. These results indicate that the CoNPs@OMC composite is a very promising FTS catalyst. 4. Conclusions In conclusion, we has successfully developed a novel CoNPs@OMC catalyst with metallic cobalt nanoparticles imbedded in the ordered mesoporous carbon by a simple one-step carbonization/reduction process assisted by a hydrothermal process. The CoNPs@OMC composite can be applied as a non-precious metalcontaining catalyst with a high activity and good cycling stability for the hydrogenation reduction of p-nitrophenol and nitrobenzene. What’s more, the CoNPs@OMC composite can effectively catalyze the Fischer-Tropsch synthesis process, and shows a good catalytic activity, excellent stability and high C+5 product selectivity. The superb catalytic performances of the CoNPs@OMC composite in various hydrogenation reactions can be attributed to the synergistic effect between high specific surface area, ordered mesoporous structure, and well-imbedded CoNPs. The CoNPs@OMC composite can be a promising catalyst for hydrogenation reduction of nitroaromatic compounds and FTS reaction, and also potentially can be extended to other catalytic processes by selecting proper metals. Acknowledgements We gratefully acknowledge the financial support from the High-level Talents Research Fund of Yangzhou University (137011046) and the Prospective Joint Research Project of Jiangsu Provincial Science and Technology Department (BYE015061-10). X. Y. thanks the funds provided by the University of Missouri-Kansas City, School of Graduate Studies.
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