Influence of doping nitrogen, sulfur, and phosphorus on activated carbons for gas adsorption of H₂, CH₄ and CO₂

Abstract

Hexagonally packed mesoporous silica (HMS) was used as a kind of template-like material to prepare heteroatom doped porous carbons from the polymerization reactions of resorcinol and formaldehyde in the HMS pore channels. Different acids (HNO₃, H₂SO₄, H₃PO₄) were used to catalyze the polymerization reactions to obtain N-doped, S-doped, and P-doped porous carbons, respectively. The adsorption properties of H₂, CH₄, and CO₂ were investigated on these doped carbon materials systematically. For the hydrogen adsorption, the adsorption amount was 11.8%, 21.1%, and 33.3% higher on N-doped, S-doped, and P-doped porous carbons, respectively, than the carbon sample without doping, and the highest hydrogen uptake reached 14.88 mmol g⁻¹ on the P-doped sample; regarding the CO₂ adsorption, the adsorption amount reached 3.60 mmol g⁻¹ and 3.16 mmol g⁻¹ on the S-doped sample and N-doped sample, respectively, which is 39.5% and 22.5% higher than the sample without doping; it also shows that methane uptake amount was significantly increased by 27.1% on the S-doped sample as compared to the sample without doping, i.e., 3.52 mmol g⁻¹ of CH₄. Therefore, it is predicted that these doped carbon materials with high specific surface area are of great potential for the gas adsorption of H₂, CH₄, and CO₂.

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1. Introduction

Porous carbon materials have been studied for a long time as promising gas adsorbents because of their remarkable advantages such as strong stability, high surface area and pore volume, low cost and possibility of mass production.^1–3 The adsorption of CO₂, H₂ and CH₄ could be characterized as physisorption with weak van der Waals interaction forces. Many efforts have been devoted to increasing the specific surface area, since the highly porous structure is indispensable to high adsorption capacity. Meanwhile, it is required to improve the adsorption efficiency and enhance the interactions between gas and carbon material, which is of great importance for the adsorption of carbon materials.

Heteroatom doping is widely used to modify the carbon material.⁴ The presence of a heteroatom leads to remarkable performances in different fields such as gas adsorption,^5,6 lithium batteries,^7,8 fuel cells,^9,10 supercapacitors^11,12 and so on. As for the adsorption of different gases, the surface polarity created by the electronegativity discrepancy between the heteroatom and carbon could enhance the interactions between gas molecules and the carbon surface, an improvement of adsorption properties could be expected. Effects of different doping elements including sulfur,^13–16 phosphorous^17,18 and nitrogen^19,20 on applications of carbon materials have been carefully investigated. For the adsorption of different gases, compared with sulfur and phosphorus doping, nitrogen is still the most studied doping element. For example, Wang et al. synthesized a series of templated carbons with various specific surface area and with/without N-doping, their results demonstrated that nitrogen doping led to an enhancement of CO₂ adsorption capacity.²¹

Two different methods are commonly used to incorporate the heteroatoms. The first strategy is the post-treatment of carbons with different chemicals. For instance treating carbon material with ammonia at high temperature yields N-containing carbon.²² However this method has drawbacks such as low element content and the doped element only exists on the carbon surface.^22,23 On the other hand, the direct carbonization of N/P/S containing precursors can also introduce heteroatoms into carbon framework. N-containing carbons with ordered mesoporous structure have been prepared with silica as the hard template through the CVD process.²⁴ Sulphur doped carbon could also be prepared by using a thienyl-based polymer network as the precursor.²⁵

The gas adsorption performance of nitrogen doped carbons has been reported recently, the results vary greatly from one report to another, probably due to the difference in carbon precursors and doping methods,^6,26,27 on the other hand the adsorption properties of sulfur and phosphorous doped carbons are much less investigated. Herein, we chose hexagonally packed mesoporous silica (HMS) as the hard template and resorcinol–formaldehyde resin as the carbon precursor. Different heteroatoms (N/P/S) doped porous carbons were synthesized in the same way by adding different acids as catalysts and heteroatoms source. The adsorption properties of H₂, CH₄ and CO₂ were investigated systematically and the effects of different doping elements were also discussed.

2. Experimental

2.1 Chemicals

Dodecylamine as a template for the preparation of HMS was purchased from Aladdin Industrial Corp. Resorcinol, formaldehyde (37 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd. A super activated carbon (AC) with specific surface areas of 3180 m² g⁻¹ was supplied by Shenhua Group Company. All chemicals were used without further treatment.

2.2 Synthesis

Porous carbons were synthesized with HMS as hard template, while HMS was synthesized according to the reported general method. HMS was added into the mixture of resorcinol and formaldehyde and the polymerization reactions occurred in the pore HMS channels. Different acids (HNO₃, H₂SO₄, H₃PO₄) were used to catalyze the polymerization reactions and to obtain N-doped, S-doped, and P-doped porous carbons (denoted as NC, SC and PC) respectively. The pure templated carbon (TC) material without doping was prepared with HCl as the catalyst. Table 1 showed the addition amount of different reactants. In a typical synthesis (N2), 0.935 g of resorcinol was dissolved in 21.25 ml of ethanol to form a transparent solution under stirring, followed by slow addition of 1.4875 g of HNO₃ (65 wt%). Then 1 g of HMS were added into the above solution and 1.7 ml of formaldehyde (37 wt%) was added dropwise within 1 h. The mixture was kept stirring until ethanol was evaporated completely. The obtained solid mixture was then heated at 100 °C for 24 h for further polymerization, the color of the sample turned into brown. After being grinded into powder, the sample was carbonized at 900 °C for 2 h in N₂ in a tube furnace. The nitrogen flowrate was kept at 50 ml min⁻¹ and the heating rate was 3 °C min⁻¹. The tube furnace was then cooled down to room temperature. HMS template was removed by dissolution in 80 ml of HF solution (10 wt%) for 4 h at room temperature. The remaining carbon was collected by centrifuge, washed with deionized water repeatedly and dried at room temperature.

Table 1 The adsorption amount of different reactants with different acid catalyst, the BET surface area and the gases uptakes of corresponding samples

Sample	Acid (g)		BET surface area (m^2 g^−1)	CH4	H2	CO2
				400 kPa	400 kPa	100 kPa
				RT	77 K	RT
TC	HCl (36.5%)	1.535	1236	2.77	11.16	2.58
N1	HNO3 (65%)	0.744	924	2.79	11.14	2.83
N2		1.488	1247	2.94	12.48	2.88
N3		2.231	902	2.97	11.78	3.16
P1	H3PO4 (85%)	0.295	1470	2.85	13.07	2.53
P2		0.590	1807	2.92	14.88	2.56
P3		0.885	1566	2.77	13.43	2.59
S1	H2SO4 (98%)	0.376	755	2.33	9.29	2.43
S2		0.752	962	3.37	12.99	3.19
S3		1.128	943	3.52	13.51	3.60

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2.3 Characterization

Powder X-ray diffraction (XRD) patterns were analyzed on a Rigaku D/MAX-2500/PC from 10° to 60° (2θ). Scanning electron microscopy (SEM) images were obtained on Hitachi S-4800. The specific surface area was measured on F-Sorb 2400 analyzer. Before each test, all samples were degassed at 175 °C for 3 h. X-ray photoelectron spectroscopy (XPS) was performed on the ESCALAB 250Xi apparatus of Thermo Fisher. A specially designed Sieverts-type apparatus was used to measure H₂ adsorption at 77 K, CH₄ and CO₂ adsorption at room temperature (298 K). Prior to adsorption measurements, all samples were heated at 200 °C for at least 3 h under vacuum.

3. Results and discussion

3.1 Characterization of carbons

Two broad diffractions peaks at around 24° and 43° are observed for the XRD patterns of different porous carbons in Fig. 1, corresponding to a typical amorphous carbon structure. The SEM images of carbons prepared with different acids as catalyst are shown in Fig. 2. The morphology is influenced by the type and addition amount of acid. HMS template consists of plate-like particles, while the carbons do not maintain this feature, regardless of the acid used. The carbons show irregular sponge-like or three-dimensional network structure, which might be due to the polymerization of the outside HMS. As for the sample TC, the particles are formed by stacking of small flakes; the sample N2 shows a sponge-like morphology with relatively smooth surface; when phosphoric acid is used, carbon material consists of many small interconnected particles with irregular shapes, macropores are formed due to the void between particles. For the sample SC materials, the morphology is also influenced by the addition amount of sulfuric acid. Compared with three-dimensional network structure of the sample S2, sponge-like morphology is obtained with a less amount of sulfuric acid (the sample S1), while rod-like particles are formed in the sample S3.

Fig. 1 XRD patterns of the pure templated carbon (TC) and the doped porous carbons. (N2) N-doped, (P2) P-doped, (S2) S-doped materials.

Fig. 2 SEM images of HMS and the prepared carbons with various amount of acid catalyst. (HMS) hexagonally packed mesoporous silica, (TC) pure templated carbon, (N2) N-doped, (P2) P-doped, and (S1)–(S3) S-doped carbons prepared with various amount of acid catalysts.

Table 1 shows the BET surface areas of various samples obtained from different acids. Generally the value increases with the increasing carbonization temperature. Although high carbonization temperature means high surface areas, it also reduces the doping element content.⁵ Therefore, 900 °C is set as the final carbonization temperature. The results also indicate that moderate addition amount of acid favors carbons with a high specific surface area. Sample P2 shows the highest BET surface area (1807 m² g⁻¹), which is probably – caused by the activation effects of phosphoric acid.^17,18,28 The specific surface area of the sample SC (below 1000 m² g⁻¹) is lower than that of the sample PC and NC.

The XPS survey spectra for the doped elements (Fig. 3) prove that the heteroatoms are successfully integrated into the porous carbons. For the N 1s spectra, four peaks are observed at 398.55 eV, 400.1 eV, 401.1 eV, 402.5 eV, corresponding to pyridinic-N, pyrrolic-N, graphitic-N and pyridine-N-oxide nitrogen respectively.^29,30 The single peak observed at 133.1 eV of the spectra of P 2p indicates that phosphorous is doped in the form of P–O.³¹ S 2p peaks, according to the studies,³² split into 2p_3/2 and 2p_1/2 with 2 : 1 intensity and 1.2 eV binding energy gap. Herein, peaks at 164 eV, 165.2 eV and 168.7 eV, 169.9 eV show the presence of thiophenic compounds and sulfones.

Fig. 3 XPS spectra of carbon materials with different heteroatoms. (a) N2, N-doped, (b) P2, P-doped, and (c) S2, S-doped carbons.

3.2 Adsorption of different gases

According to the previous reports, hydrogen uptake at 77 K is mainly decided by the surface areas as well as other physical and chemical surface properties such as the polarity of adsorbents,^2,16,33,34 a linear relationship between the specific surface area and adsorption capacity could be observed.² The hydrogen adsorption isotherms at 77 K and hydrogen uptake on different carbons at 400 kPa are shown in Fig. 4 and Table 1 respectively. Furthermore, the relationship between BET surface area and hydrogen uptake is given in Fig. 4d, and it is close to a linear relation except sample S2 and S3. It should be noted that sample P2 exhibits the highest hydrogen uptake of 14.88 mmol g⁻¹, which is 33.3% higher than the sample TC. This result is expectable since sample P2 also has the highest BET surface area. Hydrogen uptake of sample N2 increases by 11.8% over the sample TC, although the BET surface areas of these two samples are very close, i.e., 1247 m² g⁻¹ and 1236 m² g⁻¹, respectively. This result is different from the reports. Jiang et al.³⁵ considered nitrogen doping had no obvious positive influence on the hydrogen adsorption at 77 K; Xia and his colleagues³⁶ concluded the effect of N-doping on hydrogen uptake was only apparent when related to the surface area and pore volume associated with micropores rather than total porosity. The differences are probably due to the different carbon precursors and doping methods. Obviously, sulfur doping favors hydrogen adsorption for which sample S3 adsorbs 13.51 mmol g⁻¹ of hydrogen, 21.1% higher than that of sample TC. It is due to the increase of polarity of the carbon framework which enhances the interaction between hydrogen molecules and the polar carbon surface.¹⁶ The modification of sulfur increases the adsorption efficiency of porous carbons very obviously, thus leading to the deviation from the linear relationship. The hydrogen uptake of sample S1 is low probably because of the lowest BET surface area and sulfur content.

Fig. 4 (a)–(c) Hydrogen adsorption isotherms at 77 K and (d) the relationship between the hydrogen adsorption amount (at 77 K and 400 kPa) and BET surface area of different carbons.

Fig. 5 shows the methane adsorption results at room temperature. The isotherms of sample NC, PC and TC carbons are very close to each other. The CH₄ adsorption amount of sample NC carbons increases slowly with the increase of nitrogen doping level, the adsorption amount of sample N3 is slightly higher than that of sample TC. Considering the lower specific surface area of sample N3, it could be inferred that nitrogen doping still has positive influences on methane adsorption. However, the similar adsorption amount of sample P2 and sample TC indicates that the modification of phosphorous fails to improve the methane uptake amount. The surface area of sample P2 is 46% higher than that of TC, the adsorption results might be caused by the enlargement of pores due to the activation effects of phosphoric acid. Among the three doped elements, sulfur demonstrates the best results. The sample with higher sulfur content also has higher methane uptake. S3 adsorbs 3.52 mmol g⁻¹ of CH₄, 27.1% higher than sample TC does. Fig. 5d shows the relationship between BET surfaces area and adsorption capacity. Unlike the characteristics of hydrogen adsorption, the adsorption amount of methane increases very slowly with the increase of specific surface area despite the deviation of SC. The results suggest that sulfur doping is also a useful tool to improve the methane adsorption performance of carbon materials. Regarding the CO₂ adsorption capacity, the surface acidity³⁸ and the existence of strong pole–pole interactions¹⁶ of the doped functional groups are playing key roles for the adsorption of acidic gases such as CO₂ on the doped carbon materials. The CO₂ capture capabilities of carbons with and without doping are depicted at room temperature in Fig. 6. The doped elements improve the adsorption amount except for phosphorous. The adsorption of CO₂ in porous carbons is also a typical physical adsorption process and the adsorption amount is dominantly determined by the porous structure.⁵ Incorporation of nitrogen into the carbon framework creates basic sites, which are beneficial for adsorption of acidic gases such as CO₂.^22,37 Fig. 6a shows the improvement by nitrogen doping for sample NC. Sample N2 adsorbs 3.16 mmol g⁻¹ of CO₂ at room temperature and 100 kPa, 22.5% higher than that of sample TC. Nitrogen doping works better for the adsorption of CO₂ than the adsorption of H₂ and CH₄. The effects of phosphorous doping are similar to that in Fig. 6b. Generally speaking, for carbons activated with phosphoric acid, the existence of phosphorous is in the form of PO₄/(PO₃)_m rather than being integrated into the carbon framework.^4,28 Under such conditions the carbon materials possesses acidic sites on the surfaces, the existence of phosphorous groups will not be helpful for the adsorption of acidic gases such as CO₂.³⁸ In agreement with this prediction, the P-doped carbons have similar CO₂ uptakes as sample TC does. Sulfur doping is proved to be the most effective doping element for the adsorption of CO₂ in Fig. 6c. The adsorption amount of sample S3 reaches 3.60 mmol g⁻¹, 39.5% higher than that of sample TC, which is highest among all the doped samples. Similar improved adsorption performance has also been reported for sulfur doped zeolite templated carbons (2.41 mmol g⁻¹ of CO₂ at 25 °C and 1 bar with 1627 m² g⁻¹ of specific surface areas), which might be caused by the formation of strong pole–pole interactions due to the existence of sulfur functional groups.¹⁶ The CO₂ adsorption amount does not show any clear dependence on the BET surface area in Fig. 6d and Table 1, and it is more obviously influenced by sulfur doping.

Fig. 5 (a)–(c) Methane adsorption isotherms at room temperature and (d) the relationship between the methane adsorption amount (at room temperature and 400 kPa) and BET surface area.

Fig. 6 (a)–(c) Carbon dioxide adsorption isotherms at room temperature and (d) the relationship between the carbon dioxide adsorption amount (at room temperature and 100 kPa) and BET surface area.

To further clarify the different effects of sulfur doping and specific surface area on adsorption of different gases by these carbon materials, adsorption isotherms of sample S3 and sample AC are compared in Fig. 7. The sample AC is a super activated carbon sample with the much higher BET surface area of 3180 m² g⁻¹ from Shenhua Group Company. As showed in Fig. 7, sample AC delivers a much higher hydrogen uptake of 24.9 mmol g⁻¹ at 400 kPa, but this advantage is not observed in the low pressure range (<30 kPa). Sample AC also demonstrates better methane adsorption performance, but the difference is much smaller. The methane uptake increases much more slowly with the increase in their surface areas. For the adsorption of CO₂, the adsorption amount of the S-doped sample S3 is higher in the whole pressure range.

Fig. 7 Comparison of the gas adsorption properties of S3 and activated carbon with high surface area (3180 m² g⁻¹).

4. Conclusion

The N-doped, S-doped, and P-doped carbons are synthesized through hard template method with resorcinol–formaldehyde resin as the carbon precursor and HMS as the template, the existence of the doped elements is confirmed with XPS results. The P-doped carbon has the highest surface areas of 1807 m² g⁻¹ due to the activation of phosphoric acid. All the doped carbon materials are tested for gas adsorption of hydrogen, methane and carbon dioxide. Sulfur, nitrogen, phosphorus doping is found to be effective to enhance the gas adsorption capacity of the carbon materials, among them sulfur doping gives the most promising results, e.g., the CO₂ adsorption capacity reached 3.6 mmol g⁻¹ of CO₂ on the S-doped sample at 100 kPa and room temperature, which is the highest value among all the samples, and also much higher than the reported results for other sulfur doped carbons.¹⁶ Furthermore, as compared to a super activated carbon sample the hydrogen adsorption amount is mainly related with the specific surface area, while the adsorption amount of CO₂ is significantly influenced by sulfur doping. In conclusion, it is no doubt that sulfur, nitrogen, phosphorus doping is very effective to increase the adsorption amount of H₂, CH₄, CO₂ on the doping carbon materials, hence these doped materials with high specific surface area is of great potential for adsorption of these gasses.

Acknowledgements

This work was funded by the National Key Basic Research Program No. 2013CB933103 funded by MOST and the Program for the Fundamental Research Supported by Shenzhen Science and Technology Innovations Council of China (Grant No. JSF201006300047A, No. JC201105201126A, and No. ZDSY20120619140933512).

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