Cobalt Graphitic Carbon Nanoparticles for Catalytic Hydrogenation of 2,4-Dinitrophenol

Cobalt carbon nanoparticles CoCNPs were prepared by pyrolysis of cobalt phenanthroline complex at different pyrolysis temperature and time of pyrolysis and used for the catalytic hydrogenation of 2,4-dinitrophenol. CoCNPs ( 1 ) and ( 3 ) were prepared by heating at 600 º C and 800 ° C respectively, while ( 2 ) was prepared by heating at 600 ° C with an additional intermediate stage at 300 ° C. The structures and chemical properties of the three catalysts were correlated with their catalytic activities. Among the three studied catalysts, the highest rate constant was obtained for ( 2 ) while the highest conversion was achieved by ( 3 ). Our data show that an increase in oxygen content of the cobalt carbon nano-catalyst reduces the catalytic activity, while an increase in pyrolysis temperature improves the conversion yield.


Introduction
Cobalt carbon nanostructures CoCNs are of great interest because of their wide important applications in batteries 1 , supercapacitors 2 , oxidation 3 and hydrogenation 4 catalysis and water treatment 5,6 .
Many methods have been developed so far in order to prepare CoCNs. For example, carbon nanotubes CNTs were prepared with cobalt catalyst by Arc discharge 7 , pulsed laser vaporization 8 and chemical vapor deposition CVD 9 . However, these methods face the challenge to produce carbon nanostructures in large amount with a specific morphology that serve a unique application. They are also sophisticated and expensive requiring catalyst and continuous flow of inert gas, carbon source gas like acetylene and reducing hydrogen gas. However, pyrolysis methods are simple and cheap. They involve pyrolysis in inert atmosphere of transition metal complexes which provide the metal catalyst for graphitization and carbon source from the ligands. Depending on precursors structures and heating conditions several nanostructures can be obtained. While pyrolysis of cobalt carbonyl phenyl-alkynes complexes gave carbon cobalt nanorods or nanospheres, pyrolysis of cobalt carbonyl graphene-alkyne complexes gave multi-walled carbon nanotubes MCNTs 10 . Also, pyrolysis of polyphenylene dendrimer/cobalt complexes can produce CNTs, carbon cobalt nanorods, or carbon/cobalt nanospheres 11 .
Nowadays, it is of great importance to find a suitable, abundant, and low-cost catalyst for reduction of nitrophenols into anilines. Since anilines have wide applications in industry such as preparation of pesticides, pharmaceuticals, dyes, pigments, agrochemicals, plastics, and rubbers 12 . Palladium 13 and gold-based catalyst 14 were used for selective hydrogenation of nitrophenols, however they are expensive for industrial applications.
Cobalt oxide N-graphene/activated carbon nano-materials Co3O4-NGr@C prepared by pyrolysis of cobalt pyridines derivative acted as a selective catalyst for hydrogenation of nitroarene derivatives under demanding conditions (at 100 º C for 11-17 hours) in THF solvent using Triethylamine Et3N and formic acid as hydrogen source 15 . However, when hydrogen gas was used at high pressure 5 bar and temperature 110 º C, the reaction time was 4 to 12 hours in THF 16 . Cobalt(0) nitrogen doped carbon Co@N-C 700 catalyst obtained by pyrolysis of Co metal organic framework showed remarkable performance for hydrogenation of p-nitrophenol to p-aminophenol using NaBH4 as reducing agent in water 17 .
Thus, there is still a need to seek nano-catalysts that can achieve hydrogenation of nitrophenol derivatives with high recyclability in short time at mild conditions. In the present study, we investigated the catalytic reduction of 2,4-dinitrophenol using cobalt carbon nanoparticles CoCNPs prepared by pyrolysis of cobalt phenanthroline complex under different pyrolysis temperature in presence of additional carbon source.

Experimental -Preparation of cobalt carbon nanoparticles catalyst CoCNPs
Preparation of CoCNP (1) [Co(phen)2Cl2] 1.5CH3CN] 0.2 g 18 and anthracene 0.064g, were mixed and placed in a crucible without lid cover. Then heated under reduced pressure of N2 gas in a furnace for 2 hours to reach 600 º C. This was followed by pyrolysis at 600 º C for 8 hours. Porous silver black flakes were obtained with a yield of 0.05g. (2) Co(phen)2Cl2 1.5CH3CN] 0.2 g and anthracene 0.3 g were heated in a crucible covered with lid for two hours to reach 300 º C. The mixture was left for 2 hours at 300 º C (intermediate stage). The heating was increased again for one hour to reach 600 º C. Finally, isothermal heat was used for 8 hours at 600 º C. Flaky silver black shiny foams (0.0737 g) were obtained after slow cooling. (3) [Co(phen)2Cl2 1.5CH3CN] 0.2 g and anthracene 0.064 g were mixed and heated in a crucible with lid. Two and a half hours of heating was used to reach 800 º C, then kept at 800 º C for 5 hours. After cooling 0.076 g of silver black flakes were obtained.

-Characterization of CoCNPs
The XRD patterns of CoCNPs were obtained on a Bruker AXS D8 Advance diffractometer (40 kV, 40 mA) employing a radiation source of Cu Kα in which λ equals 1.5418 Å. The 2θ range of diffractograms was recorded between 20° to 80° at a step size of 0.01° per minute.
Field-Emission Electron Microscope analysis FE-SEM images were measured by JSM 7600F, Jeol instrument, Japan.
High-resolution transmission electron microscopy (HRTEM) images were carried out on a Jeol JEM-2100F at an acceleration voltage of 200 kV using a tungsten field emission gun (FMG) as electron generator. A charge-coupled device was used (CCD) to collect the HRTEM images. Energy dispersive X-ray (EDX) was performed using an X-MaxN 80T detector that was attached to the JEM-2100F.
X-ray photoelectron spectroscopy (XPS) measurements were performed implanting a Kratos Axis Ultra DLD spectrometer at room temperature and under ultra-high vacuum (UHV) systems. The XPS was implemented with a monochromatic Al Kα (hv = 1486.7 eV) as an excitation light source in which the steps, collection time and pass energy were 0.1 eV, 0.5 s and 20 eV, respectively. The photoelectrons being detected at a 90° take-off angle. The spectra were referenced to the binding energy (BE) of the C 1s peaks (284.6 eV). The XPS data were fitted using the CasaXPS software.
-Kinetic studies -Kinetic procedure of the reducing catalytic activity Roughly 1 mg of nano-catalyst (1), (2) or (3) were dispersed in 5 ml of deionised water under sonication for 15 minutes. The dispersed mixture was used to record the baseline on the UV-Visible spectrophotometer before starting the catalytic activity measurements, in order to subtract the effect of the light scattering by the nanoparticles.
On the other hand, a fresh solution of 2,4-Dinitrophenol (DNP), was prepared by dissolving 11.7 mg of DNP in 25 ml of deionised water. A 0.18 M fresh solution of sodium borohydride (NaBH4) was also prepared for the reduction purpose.
3 ml of dispersed catalyst were introduced into a quartz cell (ℓ = 1 cm), followed by adding 100 µl of DNP solution and 50 µL of NaBH4 solution. The UV-2600 spectrophotometer (Shimadzu, Tokyo, Japan) was used to record the spectra range from 200 to 600 nm at defined times.
-Successive reduction of DNP Nano-catalyst (2) was selected for the successive reduction study in the quartz cell. Briefly, 50 µl of DNP solution (4.4  10 -3 M) were added into a quartz cell containing 3 ml (0.2 g/L) of dispersed catalyst by sonication. This is followed by adding 50 µL of fresh solution of NaBH4 (1M). The rate constant was calculated from the decrease in intensity of the absorption peak at 358 nm (max of DNP) over time for different experiments. At the end of each cycle of reduction, additional 50 µL of DNP were added to the solution and the reduction process was monitored as done in the previous cycle. After the sixth cycle the reduction started to be slow and the process was stopped.
-DNP reduction under sonication 1.3 mg of nano-catalyst (1), (2) or (3) were dispersed in 30 mL of deionised water under continuous sonication. For each cycle, 5.5 mg of DNP and 75 mg of NaBH4 were used, and the reduction was monitored using the UV-Visible spectrophotometer and the percentage of reduction for each cycle was calculated. The average time for completion of each cycle is around 30 min.

Results and discussion
We have previously reported the preparation of porous cobalt carbon nanostructures and the study of their applications in water treatment. They were prepared by pyrolysis of cobalt phenanthroline chloride complex by pyrolysis at 600 º C in a quartz tube furnace under slow and continuous flow of nitrogen in presence and absence of anthracene 6 . The materials prepared in presence of carbon source gave a higher capacity for removing Malachite Green (492 mg/g). In a second study we have used silica as support in the pyrolysis experiment in a nitrogen box furnace at 850 º C and obtained cobalt carbon@silica nanostructures with durable and regenerative properties. The capacity for removing Crystal Violet was 214 mg/g 19 .
In the present study, we have focused on optimizing the experimental conditions in order to prepare cobalt carbon nanoparticles that can act as heterogenous catalysis for the hydrogenation of dinitrophenol at mild conditions, in short time and high recyclability.
-Material Preparation: [Co(phen)2Cl2]1.5CH3CN] was prepared and used as a catalyst precursor for the pyrolytic synthesis of cobalt carbon nanoparticles. In addition to the phenanthroline ligands that acts a carbon source, anthracene was additionally used as an external source of carbon. Three experimental conditions were used for the pyrolysis of cobalt phen complex with anthracene under reduced nitrogen pressure of 0.02 atm.
(1) was obtained in an open crucible at 600 º C for 8 h, where the volatiles from decomposing materials have less chance to redeposit on the metal nanoparticles. (2) was prepared similarly to (1) by pyrolysis at 600 º C for 8 h, however after heating for 2 hours at intermediate stage of 300 º C in a cover crucible, so that decomposition of complex occurs at longer time and favors cobalt nanoparticles formation. Finally, (3) was also prepared in a covered crucible but by heating at higher temperature 800 º C for only 5h, since higher temperature favors graphitic carbon formation.
The best method for catalyst preparation is the one that can generate highly active species of small size metal nanoparticles surrounded with well-developed graphitic structures [16]. This is supported by the catalytic hydrogenation results as can be seen in this work. Optimization of the pyrolysis step showed that the temperature gradient and the heating time at the maximum temperature significantly influence the activity and selectivity of the catalyst. All pyrolysis experimental conditions play a role in obtaining the best performing catalyst. These conditions include the presence of intermediate stage of heating necessary to prepare the metal nanoparticles which are the precursors for the formation of graphitic carbon, the time of heating and the value of maximum temperature in the final stage necessary to develop the graphitic carbon structures. Therefore, during pyrolysis, a part of the organic ligand decomposes and evaporates to the gas phase, whereas the other part becomes carbonized in the framework. The graphitic structure provides a robust shelter around metal nanoparticles and reduce their degradation during the catalytic process. Closed systems can ensure a long residence time of volatile carbons above the cobalt(0) nanoparticles which catalyses the graphitization surrounding carbon.
The graphitic shell can prevent cobalt nanoparticles from further agglomerations which reduce the catalytic efficiency. However, there is limitation for the use of catalysts with thick graphite shell which due to diffusion limitations render the active cobalt sites of nanoparticles hardly accessible by reactants.
In this work, the best catalyst structures were obtained by carrying out the pyrolysis at 800 º C or at 600 º C with an intermediate stage at 300 º C, as can be inferred from the following SEM and TEM results.
As shown in SEM of CoCNPs, (Fig. 1), (1) consists of graphite sheets surrounding cobalt nanoparticles of size average in the range of hundreds of nanometers. While (2) and (3) showed cobalt nanoparticles of 20-200 nm embedded within porous hierarchical graphene sheets.
Cobalt nanoparticles of 20 nm sizes were labeled in (2) but still there are larger particles up to 400 nm. While in the case of (3) the labeled particles were 30 nm in sizes. There are also smaller ones and the larger ones were up to 200 nm. The porous structure is ideal to attract analytes towards the cobalt nanoparticles and facilitate the catalytic process. (3) (1) High resolution transmission electron microscopy HRTEM of CoCNPs are shown in Fig. 2.
The catalyst (2) contains mainly small dark cobalt particles of 3-20 nm in size embedded in lighter graphitic web. The same nanostructure is found in (1), though the cobalt nanoparticles are clearly seen more blackish with well-defined interplanar lines and less surrounding graphitic carbon. This is expected because the pyrolysis was performed in an open flak that reduces the presence of gaseous carbon source. Regarding catalyst (3), it was prepared at higher temperature 850 ºC. Thus, bamboo multiwall carbon nanotubes and graphitic onion like nanostructures were obtained along with cobalt nanoparticles of the same size as in the other nanostructures.
Energy dispersive X-ray EDX of CoCNPs indicates the spreading of cobalt, carbon and oxygen elements within the structure. EDX for (2) is shown in Fig. 3, while EDX for (1) and (3)    The powder X-ray patterns of all CoCNPs show sharp peaks characteristic of metallic cobalt -Co (Fig. 4). The experimental 2 values for (1), (2) and (3)   Raman measurements were used to investigate the carbon defects and the degree of graphitization. In Fig. 5, two distinct peaks about 1350 and 1584 cm −1 are associated with D and G bands, respectively. These are present in pyrolytic graphite and carbon nanotube. They are sharper in the case of nanotubes and broader in the case of graphite, while they merge into a single broad peak in the case of cobalt amorphous carbons 20 . It can be inferred that (3) with sharper peaks contains more CNTs, than (1) and (2). This is expected since higher temperature of pyrolysis (850 º C) was used in the preparation of (3). The area ratio between D and G bands (SD/SG) were calculated as 7.3, 3.43 and 4.17 for (1), (2), and (3) respectively. Two characteristic peaks of Co were observed around 477.85, and 687 cm −1 by Raman spectroscopy, which correspond to classical vibration modes Eg, and A1g of Co, respectively 21 .
XPS survey spectra show that nano-catalyst (1) contains more oxygen and less carbon atoms in its elemental composition compared with the two other nanomaterials (SI. Fig. 2, 3 and 4). Indeed catalyst (1) comprises 68% of carbon compared to 77% and 84% for catalysts (2) and (3) respectively. By comparing the nano-catalyst elemental composition and its catalytic activity, one can suggest that there is an important role of the carbon skeleton in the catalytic reduction of DNP. The XPS survey and high-resolution spectra have shown the presence of cobalt in the composition of nano-catalysts, Fig. 6. The Co 2p spectrum could be fitted into two spin-orbit doublets, which are characteristic of Co 2+ and Co 3+ , and two shake-up satellites 22 . Only in the case of catalyst (1), a small quantity of metallic cobalt was observed represented by the additional peak of Co 0 in the cobalt 2P3/2 spin orbit. The XPS high resolution was also conducted on the elements C (1s), O (1s) and N (1s) existing in the three nanomaterials as shown in Fig. 7, 8, and 9. Evaluation of C 1s photoelectron spectrum of the three nanomaterials showed similar resolving into components for different carbon chemical forms (C sp 2 , C sp 3 , other oxygen groups like hydroxyl COH, carbonyl CO, carboxyl COOH and satellite) 23 . Regarding the peak of O 1s, we can observe that nano-catalyst (1) shows less oxygen bound to cobalt in its composition, which is in agreement with the presence of metallic cobalt in this material 24 . Additionally, the examination of N 1s peak shows that nano-catalyst (3) contains more graphitic nitrogen compared with the two other nanomaterials which could have a role in enhancing the catalytic activity. 25 (1) (2) (3) (3) Fig. 7. XPS high resolution spectra of C (1s) in nano-catalysts (1), (2) and (3) . (1) (3) (1) (3) Fig. 9. XPS high resolution spectra of N (1s) in nano-catalysts (1), (2) and (3).

-Kinetics
The DNP reducing catalytic activity of nano-catalyst (2) was examined at room temperature (25 °C) by measuring the UV-vis spectrum of DNP in the presence of NaBH4 in the range of 200-600 nm at various times, Fig. 10. The reduction of DNP occurs due to the self hydrolysis of NaBH4 and hydrogen generation according to equation (1). In absence of catalyst, the reduction of DNP is very slow, however the presence of our carbon nanomaterial makes the reduction occur rapidly at room temperature.
In the presence of catalyst, the peaks at 258 and 358 nm, significantly decrease, which is due to decreased DNP concentration and new peaks appeared at 275 and 437 nm indicating the formation of 2,4-dinitrophenolate ion in the solution. It is well known, that the reduction of nitrophenols with an excess of NaBH4 in the presence of catalysts, undergoes via the formation of phenolate ions which will be transformed into aminophenols 26 . The color of the solution has changed from yellow to pale orange. These new peaks showed their maximum absorption after 4 minutes as shown in Fig. 10 (a). However, after ~5 minutes from the beginning of the reduction process, the intensities of absorptions at 275 nm and 437 nm gradually decreased as the reduction proceeds in the presence of the carbon nanomaterial as shown in Fig. 10 (b). A new peak appears at 302 nm which is attributed to the formation of the reduced product 2,4-diaminophenolate.
Similarly, using the same experimental conditions, the catalytic activity of reduction of DNP was studied in presence of nano-catalysts (1) and (3) as shown in SI. Fig. 5 and 6, and in absence of catalysts SI. Fig. 7.
In order to compare the catalytic activity of the different nano-catalysts, the rate constant of the reduction process was calculated from the decrease in intensity of the absorption peak (A) at 358 nm over time for the different experiments. As the concentration of sodium borohydride can be considered constant, the reaction follows a pseudo first-order kinetic reaction 27 . The apparent rate constants can be directly calculated from the linear relation between ln(A/A0) and time, where A0 represents the initial absorbance intensity of DNP at 358 nm. From these kinetic linear variation, the apparent rate constants (kapp, min −1 ) were calculated. The rate constants kapp (min -1 ) of DNP reduction were 0.34 for (2) > 0.17 for (3) > 0.04 for (1) > 0.005 (no catalyst). It can be noted that nano-catalyst (2) showed the faster catalytic activity among the tested catalysts. In order to demonstrate the efficiency of our catalysts, we have initially conducted cyclic catalytic reduction of DNP by continuously adding DNP in the quartz cell in presence of nanocatalyst (2). The reaction was followed by UV-visible spectrophotometry, where the presence of only one peak in the spectra, centred at 300 nm, indicates the completion of the reaction (Fig. 12). In our experimental conditions, six cycles of DNP reduction were completed with a 100% conversion rate in presence of nano-catalyst (2). After the sixth cycle, the reaction became much slower indicating the possible poisoning of the catalyst. After this reaction, the catalyst was recovered by centrifugation, washed with distilled water and was reused for a total of four successive reduction cycles before the start of catalyst inactivation.
On the other hand, the catalytic activities at larger scale, under continuous sonication, of the three nanomaterials (1), (2), and (3) were compared. Different cycles of DNP reduction were performed without any treatment of the nano-catalysts between cycles, and the yield of DNP conversion was determined each time ( Indeed, up to the sixth cycle, the conversion percentage of DNP by (3) was still more close to 70%. Starting the seventh cycle, the % conversion decreased until 30 % indicating a significant poisoning of the catalyst. In these experimental conditions, nano catalyst (3) which was prepared at highest temperature 800 °C is more efficient in the reduction of DNP compared with the two other catalysts. Conversion percentage increased with pyrolysis temperature of the complex, suggesting that the increase in graphitic structure formed at higher temperature played an important role in this catalytic system. The ratio of C=C to C-C in XPS is in the order of increase (1) < (2) < (3), beside that we have shown that (3) contains more graphitic nitrogen compared with the two other catalysts. It has been suggested that the presence of basic groups on CoCNP might facilitate the transfer of protons to Co nanoparticles to form Co metal hydride species, thus promoting the production of the desired hydrogenation products. 28 The turn over number TON for (1), (2) and (3) was calculated to be 11.10, 21.68, and 25.41 mmol DNP per mmol cobalt. The best performance is for (3) which contains small size CoCNP dispersed uniformly with less agglomeration compared to other samples.

Conclusion
Novel cobalt carbon nanoparticles (1), (2), (3) were prepared by pyrolysis of cobalt phenanthroline chloride complex at different temperature and period of time. They were characterized using different techniques and applied as catalyst for hydrogenation of 2,4dinitrophenol. Catalyst (3) showed a highest catalytic conversion, this can be attributed to its higher content of graphitic structures and more uniformly dispersed cobalt carbon nanoparticles compared to the other catalyst. Nano-catalyst (1) demonstrated less catalytic activity. It contains more oxygen and less carbon atoms in its elemental composition compared to the two other nanomaterials. The obtained results in this work have showed the effect of selected temperature for nano-catalysts' preparations on the hydrogenation catalytic activities. These results should pave the road towards future discoveries of cheap and efficient metal carbon based nano-catalyst for different applications.

Conflicts of interest
There are no conflicts to declare.