Ultra high performance N-doped carbon catalysts for the ORR derived from the reaction between organic-nitrate anions inside a layered nanoreactor

Abstract

The extensive research on the synthesis of nitrogen-doped carbon materials (NCMs) as non-precious metal catalysts (NPMCs) has shown a promising future for applying the NPMCs to catalyze the slow oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) and, therefore, the widespread use of devices based on PEFCs. However, the main reasons for the delay in starting the practical use of NCMs in PEFCs are the use of very specific organic chemicals as well as multiple stages of the catalyst synthesis, lack of stability, activity and/or selectivity limitations. Here we show that the NCMs can be produced by a simple route on a very large scale using any organic anions for the first time. The synthesized carbon catalysts showed highly porous structures, tunable nitrogen content and high electrochemical performance. As the best performing catalyst it had an open circuit potential (OCP) of as high as 1.04 V, and a Tafel slope of as low as 38.0 mV per decade with an exchange current density of as high as 1.34 × 10⁻⁴ A cm⁻², which is a much higher performance compared to other NPMCs.

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Introduction

NPMCs can really diminish the issues accompanied with the use of the noble metals Pt and Pd. These metals are the preferred electrocatalysts for the ORR due to their high electrochemical performance and durability.^1,2 On the other hand, they have significant drawbacks such as high costs and scarcity as well as cathode catalyst oxidation, catalyst migration, loss of surface area of electrocatalysts, etc.^3,4

In general, NCMs as NPMCs are mostly produced by coating macrocycles, chelates, polymers (also monomers and oligomers) or other organic compounds containing nitrogen with or without some transition metals, in particular Ni, Co and Fe, onto carbon nanoparticles and then heat-treating the obtained composites in an inert or nitrogen-rich atmosphere (N₂, NH₃, or HCN).^5–7 The carbonic layers consisting of M–N sites onto carbon nanoparticles are formed by the coating process. It is known that due to a radical enhancement, these active electrocatalyst centers facilitate the ORR and improve the performance and durability of the cell.^5,8,9 Accordingly, NCMs with high electrocatalytic activity and durability have been produced by various methods such as the heat-treatment process, arc discharge, chemical vapor deposition, plasma treatment and ball milling.⁵ In almost all cases, the type of the organic materials, the heat-treatment temperature, synthetic conditions, etc. play important roles in determining the efficiency of the NCMs as catalysts for the ORR.^9–16 Consequently, the significant factors to improve the electrocatalytic activity are a high nitrogen content within the carbon structure (as active sites), high porosity to increase the surface area, and the presence of functional groups such as oxide, hydroxide, carbonyl and carboxide in addition to some transition metals on the carbon surface.^5,15,17,18 To this end, there are still serious problems due to the production of NCMs in a large scale and cost-effective way. For instance, multiple stages are needed to obtain the product in addition to a low nitrogen content in the produced carbons.^5,6,16 Meanwhile, most macrocycles, chelates and other organic chemicals used for producing NCMs are poisonous and expensive.

Herein, we present an easy method for a large scale production of highly efficient nitrogen-doped carbon nanosheet catalysts for the ORR. In this method, an α-phase layered zinc hydroxide (αLH) was used as a layered nanoreactor to prepare the carbon nanosheets. αLH, typically zinc hydroxide nitrate (Zn₅(OH)₈(NO₃)₂·2H₂O) with card no. 24-1460, is a synthetic mineral with positively charged brucite-type layers constructed solely with one type of cation (Zn²⁺).^19–21 These positive charges can be neutralized by anions, which together with water molecules are located inside the interlayer spaces of the inorganic layers. The α-phase zinc hydroxide nitrate (ZHN) was chosen as the inorganic layered host and succinate/salicylate anions (without and with benzene rings) as the organic guests, which were encapsulated into the intergallery of the host to produce the organic–inorganic layered nanohybrids by a simple ion-exchange process. Heat-treatment of the nanohybrids resulted in the production of the nitrogen-doped carbon catalysts.

Results and discussion

The catalyst synthesis for nitrogen-doped carbon catalysts derived from a zinc hydroxide succinate nanohybrid as a schematic diagram is shown in Fig. 1. The as-synthesized ZHN with nitrate anions between its layers was dispersed in a solution comprising succinate anions. The organic–inorganic nanohybrids were produced by replacement of nitrate with succinate via the ion-exchange process at various times. Here, partial ion-exchange was done due to the existence of both nitrate (as a nitrogen source) and succinate (as a carbon source), simultaneously between the α-phase layers. After the ion-exchange process, the resultant nanohybrids were subjected to heat-treatment under an argon atmosphere. In fact, during the heat-treatment process, the basal spacing of the nanohybrids acts as a layered nanoreactor for the reaction between the organic and nitrate moieties to form N-doped carbon catalysts. The inorganic layers shield the content of the basal spacing (organic/nitrate) from direct heat and thus prevent the vaporization and burning of the anions. Finally, the obtained carbon catalysts were added in an HCl solution to remove any remaining zinc oxide.²⁰

Fig. 1 Schematic diagram of the synthesis of carbon catalysts. Synthesis of the initial zinc hydroxide nitrate (ZHN), and the incomplete ion-exchange process of nitrate between the layers with succinate anions of various times to produce nanohybrids with different nitrate/succinate ratios (a). Heat-treating the nanohybrids under an argon atmosphere and finally acid etching of the obtained zinc oxide (b).

XRD patterns for the as-synthesized ZHN and also its resultant nanohybrids with succinate and salicylate anions with different ion-exchange times (ZHSu and ZHSa) are shown in Fig. 2. As observed, all the samples show the α-phase brucite-like structures with the basal spacing peaks around 9.7 Å.^19–21 The inorganic–organic nanohybrids with succinate (ZHSu) and salicylate (ZHSa) anions have a new sharp peak at 11.8 and 15.6 Å, respectively. This shows that the organic anions could be intercalated into the interlayer of the parent material via the ion-exchange process. Due to the bigger size of the organic anions compared with that of nitrate, the interlayer space has expanded to accommodate succinate/salicylate, resulting in the expansion of the basal spacing from 9.7 Å for the host material to 11.8 and 15.6 Å for the ZHSu and ZHSa nanohybrids, respectively. The basal spacing peak at 9.7 Å due to the parent material (ZHN) can also be seen owing to the incompleteness of the ion-exchange process for all the nanohybrids. However, with the increase of the ion-exchange times, the peak intensities of the basal spacing for the nanohybrids increased at the expense of that of the as-synthesized ZHN. It indicates that both the anions of succinate (or salicylate) and nitrate with different mass ratios are simultaneously present inside the interlayer space of the nanohybrids. The nitrogen-doped carbon catalysts with various nitrogen percentages could be produced using the heat-treatment of the nanohybrids via a chemical reaction between the nitrate and organic groups which are encapsulated between the inorganic layered nanoreactors.

Fig. 2 XRD patterns for the as-synthesized zinc hydroxide nitrate (ZHN), its resulting nanohybrids with succinate (ZHSu) and salicylate (ZHSa) anions and the resultant CSu3 and CSa6 carbon catalysts.

Fig. 2 also shows XRD patterns for the CSu3 and CSa6 carbon catalysts. A very broad peak around 26° can be seen in both patterns. This peak reflects the 002 plane of the disordered graphite structure within the c-planes. The carbon patterns also reveal that the catalysts consist of mainly amorphous carbon material without any remaining zinc oxide after acid etching.²⁰

Fig. 3 shows the FTIR spectra for the as-synthesized ZHN, succinic and salicylic acids, the ZHSu3 and ZHSa6 nanohybrids and the CSu3/CSa6 carbon catalysts. In the ZHN sample, lattice vibrations of the metal–oxygen bonds Zn–O and Zn–OH are observed at 436 and 468 cm⁻¹, respectively.^20–22 Two weak bands at 837 and 1018 cm⁻¹ and an intense band at 1384 cm⁻¹ can be ascribed to ν₂, ν₁ and ν₃ of the intercalated anion, nitrate.²¹ The water-bending vibration (δ_H2O) of the adsorbed/intercalated water molecules is observed at 1623 cm⁻¹.^20,21 The FTIR spectrum of succinic acid shows two bands at 918 and 1417 cm⁻¹, which are due to the C–O–H out-of-plane and in-plane bending vibrations, respectively.²³ The band at 1202 cm⁻¹ is attributed to the C–O stretching vibration of the acidic group.²³ Two bands at 1687 cm⁻¹ (asymmetric) and at 1314 cm⁻¹ (symmetric) are due to C=O stretching vibrations.²³ The FTIR spectrum of ZHSu3, the zinc hydroxide succinate nanohybrid, is composed of spectral band features of both salicylic acid and ZHN.^23,24 The bands at 440 and 500 cm⁻¹ are due to metal oxide and hydroxide vibrations. The bands at 1035 and 1060 cm⁻¹ are attributed to C–O–H vibrations in the nanohybrid. The band at 1230 cm⁻¹ indicates the presence of a C–O group. Two bands at 1350 and 1550 cm⁻¹ are assigned to the symmetric and asymmetric vibrations of the carboxylic group. These red-shifted bands reveal that hydrogen bonding in the intercalated succinate with hydroxide and oxide groups within the brucite-like layers is much stronger than that for free succinic acid.^20,22 The intense peak at 1384 cm⁻¹ is due to the nitrate anion. It means that both nitrate and succinate anions are simultaneously between the nanohybrid layers. Fig. 3 also shows the FTIR spectrum for the CSu3 catalyst obtained by heat-treating the ZHSu3 nanohybrid followed by acid etching. There are two bands around 1334 and 1605 cm⁻¹ due to C=N/C–C and C=C/C=O stretching vibrations, respectively.^7,9,20 As can be seen, the spectrum doesn’t contain any bands in the range of 400–600 cm⁻¹ due to M–O vibrations, confirming that the N-doped carbon catalyst is free of the metal oxide after the acid washing process.²⁰ Similar results can be observed for the ZHSa6 nanohybrid and its resultant CSa6 carbon catalyst in Fig. 3.

Fig. 3 FTIR spectra for the as-synthesized zinc hydroxide nitrate (ZHN), succinic and salicylic acids, the organic–inorganic nanohybrids with succinate (ZHSu) and salicylate (ZHSa) anions and the resultant CSu3 and CSa6 carbon catalysts.

Linear sweep voltammograms (LSVs) show the electrocatalysis activity on different carbon catalysts and also a Pt/C reference electrode (10 wt% E-TEK, 200 μg_Pt cm⁻²) in Fig. 4. Also, the related data are listed in Table 1. According to RDE measurements (Fig. 4a and b), all the samples are catalytically active toward oxygen reduction as observed in the ORR polarization plots. Also, the Tafel plots on different carbon catalysts are presented to compare their activity in Fig. 4c and f, according to their kinetic current densities by correcting the mass transport.²⁵

Fig. 4 Polarization curves for the ORR on different carbon catalysts and the Pt/C reference catalyst (10 wt% E-TEK, 200 μg_Pt cm⁻²) in O₂ saturated 0.5 M H₂SO₄ solution observed using a RDE electrode with a rotating speed of 900 rpm (a and b), and observed using a carbon paper electrode (d and e), at a potential scan rate of 5 mV s⁻¹ and 25 °C. (c and f) Tafel plots for the ORR in O₂ saturated 0.5 M H₂SO₄ solution on different carbon catalysts and the Pt/C reference catalyst deduced from the polarization curves in (a and b).

Table 1 Obtained data from the ORR kinetic and elemental analysis of the carbon catalysts

Catalyst	ORR onset potential (V)	Tafel slope (mV dec^−1)	i0^a (A cm^−2)	D^1/2c (mol cm^−2 s^−1/2)	C (%)	N (%)	O (%)
a The i0 has been calculated based on the geometrical areas of the electrodes.
CSu1	0.93	40.5	8.96 × 10^−5	1.27 × 10^−7	70.16	15.99	13.85
CSu2	1.02	38.3	9.61 × 10^−5	7.53 × 10^−8	72.62	13.86	13.52
CSu3	1.04	38.0	1.34 × 10^−4	1.36 × 10^−7	75.94	11.57	12.49
CSu4	0.97	42.2	1.12 × 10^−4	1.69 × 10^−7	80.50	9.20	10.30
CSa2	0.87	43.9	7.12 × 10^−5	8.36 × 10^−8	76.46	14.57	8.97
CSa4	0.97	50.6	6.67 × 10^−5	5.19 × 10^−8	80.85	9.64	9.51
CSa6	1.02	40.1	1.62 × 10^−4	1.68 × 10^−7	84.17	7.47	8.36
CSa8	0.97	40.6	9.85 × 10^−5	1.21 × 10^−7	87.43	4.84	7.73
Pt/C	0.96	44.2	4.73 × 10^−4	6.74 × 10^−6	90	—	—

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The LSV tests were also performed using carbon paper as the working electrode (Fig. 4d and e). As presented in Table 1, the ORR onset is as high as 0.87 V for the CSa2 catalyst (versus a reversible hydrogen electrode, RHE), prepared in a 2 h ion-exchange process. By changing the ion-exchange time (changing the C/N ratio), more improvements in the performance of the catalysts are gradually observed as evidenced by the lower ORR overpotentials and higher current densities. Specifically, the onset potentials show a positive shift from 0.93 to 1.04 V for the N-doped catalysts of CSu1 to CSu3 and from 0.87 to 1.02 V for the CSa2 to CSa6 catalysts.

The Tafel slopes in the low current density region can clearly identify the activity difference of the samples as a function of ion-exchange time as listed in Table 1. The table summarizes the onset potentials, the Tafel slopes and the exchange current densities (obtained from the Tafel equation,²⁶ η = b log(i/i₀), where η is the overpotential, b is the Tafel slope and i₀ is the exchange current density) of the ORR on different catalysts. According to both the ORR tests (RDE and carbon paper), it is evident that the activity of the catalysts increases in the order:

CSu3 > CSu2 > CSu4 > CSu1

CSa6 > CSa8 > CSa4 > CSa2

The Tafel slopes of 38.0 and 40.1 mV per decade were measured for CSu3 and CSa6, as the best performing catalysts derived from succinate and salicylate anions, respectively. The Tafel slope values are lower than those for other NPMCs.^1,5,6,8,27 The small values of the Tafel slopes are ascribed to the very fast kinetics of the transfer of electrons on the catalyst surfaces. Regarding the large thickness of the carbon catalyst layers that cause high electrical resistance and also mass transfer resistance, the observed high ORR performance and the observed current densities for the carbon catalysts are valued.²⁵

Although the exchange current densities were changed slightly with varying the ion-exchange time, the i₀ values are nearly similar for all the catalysts. However, in comparison with other NPMCs, the i₀ values are at least three orders of magnitude higher.^7,26,28

Oxygen permeability on different electrodes was measured by chronoamperometry according to the modified Cottrell equation.²⁶ Permeability is the product of D_b^1/2c_b, where D_b is the diffusion coefficient and c_b is the concentration of oxygen. Fig. 5 shows I vs. t^−1/2 plots for oxygen reduction on different electrodes, indicating the existence of the linear relationship for all the catalysts. As observed in Table 1, the oxygen permeability values show a small increase with an increase in the ion-exchange time. This reflects the porous nature of each catalyst and the abundant presence of active sites on the catalyst surface, resulting in the faster permeation and, therefore, reduction of oxygen on the catalyst surface. In accordance with the electrochemical tests and the elemental analysis data presented in Table 1, one can deduce that higher activity is not directly related to higher nitrogen content. An optimum amount of nitrogen provides the highest activity toward the ORR.

Fig. 5 Plots of I vs. t^−1/2 for oxygen reduction on different N-doped carbon catalysts.

As stated earlier, the XRD patterns of the CSu3 and CSa6 catalysts (Fig. 2) reflect that the carbon is mainly in an amorphous form with a disordered graphite structure, so that it may enhance the active site numbers via facilitating the incorporation of nitrogen within the carbon structure.^5,7 The presence of C–N, C–C and C–O bonds in the carbon catalysts was confirmed via FTIR spectroscopy (Fig. 3). These carbon–nitrogen and carbon–oxygen bonds are active sites and/or facilitate the ORR activity.^5,15,17 As we stated earlier, the doping of carbon with nitrogen and oxygen (or sulfur, phosphorous, etc.) causes an enhancement in the ORR activity. For example, the authors^7,29 showed that pure carbon cannot show a high ORR activity. They proved that these carbon–nitrogen and carbon–oxygen bonds are active sites for facilitating the ORR. Nitrogen (and other doping agents) is an n-type carbon dopant which donates electrons to the carbon and facilitates the ORR.^7,30 By acid washing and thus from the presence of C–O groups in a nitrogen-doped carbon catalyst, Nallathambi et al.²⁹ showed higher ORR activity of the obtained carbon material.

Fig. 6 shows the cyclic voltammograms in Ar-saturated 0.5 M H₂SO₄ for all the carbon catalysts with two separate oxidation and reduction peaks which can be attributed to the characteristic changes of oxidation state of the carbon catalysts (quinine–hydroquinone).^25,28,31

Fig. 6 The cyclic voltammograms of different carbon catalysts in Ar-saturated 0.5 M H₂SO₄ at a 50 mV s⁻¹ scan rate.

These high performances for our carbon catalysts are due to the preparation method which results in the production of the highest percentage of pyridinic nitrogen (ca. 398.5 eV) among other C–N bonds within the nitrogen-doped carbon structure, as observed in Fig. 7e. It is well known that the pyridinic nitrogen is positioned on the edge of the graphite plane and the lone pair of electrons from the nitrogen is due to the ORR performance.^13,32 The higher exposure of the smaller ion size of succinate (compared with the salicylate anion) to nitrate anions between the layers of ZHN is expected to render more pyridinic nitrogen, which enhances the ORR activity.

Fig. 7 Surface morphologies of the CSu3 (a and b) and CSa6 (c, d and f) carbon catalysts at different magnifications. High resolution XPS spectra of the N 1s signal of the CSu3 and CSa6 carbon catalysts after smoothing (e).

The morphologies of the catalysts were studied by FESEM as illustrated in Fig. 7. The images show highly porous nanosheets which result in high surface areas and greater access to active sites and, therefore, the catalysts obtain higher ORR performances.^10,12,13,15 The microstructure shown in Fig. 7f affords a velvet-like texture with micro/mesopores of 1–3 nm and with a narrow size distribution which are excellent sites to capture reactants for the ORR. Large pores in addition to pore tortuosity increase the material’s electrical resistance, resulting in lowering of the conductivity of the material and its ability to capture the reactants.³³ Also, the images show a higher surface area for CSu3 in comparison with that for CSa6. It is probably due to the lower heat-treatment temperature for the CSu catalysts (650 °C) in comparison with that for the CSa catalysts (850 °C). Moreover, as mentioned earlier, CSu3 has a greater percentage of pyridinic nitrogen than that for CSa6. Accordingly, it is expected that the ORR performance of CSu3 is higher than that of CSa6.

Conclusion

To conclude, we have demonstrated an easy, cost-effective and scalable method for producing nitrogen-doped carbon catalysts from an entirely new class of the organic–inorganic layered nanohybrids with an extremely high ORR performance. Obviously, more research is required to get better insight on the surface active sites for our samples and for making a variety of other carbon catalysts from this class of materials for use as catalysts for polymer electrolyte fuel cells.

Experimental

Catalyst synthesis

All solutions were prepared using distilled de-ionized water. The initial zinc hydroxide nitrate (ZHN) was synthesized from 0.2 M Zn(NO₃)₂ solution. The solution was kept at pH 7.0 ± 0.05 by dropwise additions of 0.5 M NaOH solution with vigorous stirring. The precipitate was filtered, washed with water and acetone, and dried in an oven overnight at 70 °C.^20,21 Zinc hydroxide succinate nanohybrids (ZHSu) were prepared using the ion-exchange method by contacting 1 g of the as-synthesized ZHN into a 250 ml solution of 0.1 M succinate at various times of 1–4 hours under nitrogen atmosphere.^19,20 In this study, the formation of succinate anions was done by dissolving succinic acid in distilled water, bringing the resulting solution to pH ≈ 7 by adding 1 M NaOH. The nanohybrids obtained after filtering, washing and drying were heated at 650 °C in an electric tubular furnace under argon atmosphere at a flow rate of 100 ml min⁻¹ for 1 h at a rate of 5 °C min⁻¹ and then cooled down naturally under argon atmosphere. To obtain the carbon catalysts and remove zinc oxide (zinc oxide is the product of heating ZHN),²⁰ the heat-treated products were added in 1 M HCl (1 g solid/200 ml) at 50 °C for 2 hours, and thoroughly washed in distilled de-ionized water and acetone and finally dried at 120 °C overnight. A similar procedure was repeated for the preparation of the zinc hydroxide salicylate nanohybrids (ZHSa). Zinc hydroxide salicylate nanohybrids were also prepared using the ion-exchange method by contacting 1 g of the as-synthesized ZHN into a 250 ml solution of 0.1 M salicylate at various times of 2, 4, 6 and 8 hours under nitrogen atmosphere. The nanohybrids were heated at 850 °C under argon atmosphere at a flow rate of 100 ml min⁻¹ for 1 h at a rate of 5 °C min⁻¹. The heat-treated products were added in 1 M HCl (1 g solid/200 ml) at 50 °C for 2 hours, and thoroughly washed in distilled de-ionized water and acetone to remove any remaining zinc oxide, although it is known that zinc oxide can be vaporized on a carbon surface at a temperature higher than 800 °C.³⁴ The nitrogen-doped carbon catalysts were labelled as CSu1–4 and CSa2–8 for carbon derived from the nanohybrids consisting of succinate and salicylate anions, respectively. The numbers indicate the ion-exchange times in hours for both the nanohybrids and the obtained carbon materials.

Electrochemical tests

Electrochemical tests were carried out using an EG&G potentiostat (PARSTAT 2273) connected to a computer running the power suite software. First, the ORR activity of the carbon catalysts was evaluated using a rotating ring-disk electrode (RDE) setup with the standard three compartment electrochemical cell (a glassy carbon disk as a working electrode). Afterward, to make the ORR tests closer to the real conditions of a fuel cell cathode, we used carbon paper as the working electrode to evaluate the ORR again.^26,35 To this end, for the linear sweep voltammetry tests, the fabricated electrode (the working electrode) was mounted into a Teflon holder that contained a pyrolytic graphite disk as a current collector with the provision for feeding oxygen on the back of the electrode. A graphite rod was used as the counter electrode and a saturated calomel electrode (SCE) as a reference was inserted close to the working electrode to preserve the working electrode’s potential at a fixed value. The LSV polarization plots were recorded in an O₂-saturated 0.5 M H₂SO₄ electrolyte in a potential range of −0.3 to 1.2 V (NHE) at a scan rate of 5 mV s⁻¹. Cyclic voltammetry measurements were carried out in an Ar-saturated 0.5 M H₂SO₄ electrolyte in the potential range of −0.3 to 1.2 V (NHE) at a scan rate of 50 mV s⁻¹ and were recorded after 15 scans for each sample. Chronoamperograms were obtained by holding the potential of the electrodes at 0.3 V for 10 s with oxygen flowing on the back of the electrodes after holding the potential of the electrodes at 1.2 V for 60 s.² This approach was done for eliminating any oxygen from the electrode’s surface at relatively high potentials in the first step and measuring the pure diffusion of oxygen on the electrode’s surface at the diffusion region.

The catalyst inks were prepared by ultrasonic dispersion (45 min) of the carbon catalysts in an alcohol solution containing Nafion solution (5 wt% solution), distilled water (2 ml) and isopropyl alcohol (1 ml). The ink was then put into an oven at 90 °C for about 20 min to obtain a wet gel. The gel was painted onto the carbon paper (TGPH-0120T, Toray) by a brush, and the electrode was dried in an oven at 90 °C for 30 min to obtain a catalyst loading of 4 mg cm⁻² (with a mass ratio of Nafion/carbon = 30/70), after which the electrode was also put in an oven at 120 °C for 30 min to ensure the removal of the solvents. Finally, the electrode was sintered at 200 °C for 1 h.^26,35 For RDE measurements, the ink was deposited onto the glassy carbon disk electrode and dried in an oven at 50 °C for 1 h. The catalyst loading, the counter and reference electrodes and also the electrolyte and the potential scan rates were the same as those used in the LSV tests using the carbon paper. The RDE tests were measured at a 900 rpm rotating speed.

Physical characterization

X-ray diffraction patterns were collected on a Siemens D5000 powder diffractometer unit using Cu K_α (λ = 1.54 Å) at 40 kV and 35 mA. FTIR spectra were recorded using a Perkin-Elmer RXI spectrophotometer in the range of 400–2000 cm⁻¹. A CHNS instrument (Leco, USA) was used to determine the mass percentage of nitrogen and carbon in the obtained catalysts. The catalyst morphologies were characterized by a field emission scanning electron microscope (JEOL JFM-6700F), and energy-dispersive X-ray spectroscopy (EDX, MIRA, TESCAN) was used to determine the elemental composition of the bulk catalysts. X-ray photoelectron spectroscopic (XPS) measurements were performed on a Specs EA10 X-ray photoelectron spectroscope.

Acknowledgements

The authors would like to thank Malayer University for supporting in part this work.

Notes and references

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