IntroductionActivated carbons (ACs) are very important in many applications due to their superior physical/chemical properties.1 Production of ACs from raw materials (like biomass) consists of two steps: pyrolysis and activation.2 The activation process usually creates porosity by reacting carbon with CO2, H2O, KOH, or ZnCl2 at elevated temperatures.3It is widely believed that the activation step requires carbon oxidation, while carbons activated by reductant have been rarely reported.Recently, N-doped carbons have attracted much attention due to their improved properties compared with their pure carbon counterparts.4 For preparation of N-doped carbons, one approach is to use nitrogen-containing precursors for pyrolysis. CVD has also been used to synthesize N-doped CNTs or graphene. As another strategy, reducing graphene oxide with ammonium salts under hydrothermal conditions or hydrazine can also form N-doped reduced graphene oxides. Other methods, including solvothermal reactions, arc discharge and plasma treatment, have also been employed to form N-doped graphene materials. Another important approach is to treat carbons under NH3 at high temperatures. Reactions between NH3 and carbons at temperatures lower than 1000 �C can introduce a certain level of nitrogen doping. However, a carbon burn-off or significantly increased specific surface has not been reported for annealing carbon under NH3.NH3 is usually used as a reaction gas during fabricating metal nitrides. Interestingly, much less carbon residue remains after the pyrolysis under NH3 than under other inert gas, which suggests activation process occurs under NH3. This is very much different from the case of treating pure carbon under NH3. Herein, we study the pyrolysis of cellulose under NH3 to study the activation mechanism. We demonstrate that N-doped activated carbon (NAC) membranes with large surface areas can be prepared by simply heating cellulose filter paper (CFP) under NH3. The specific surface area and nitrogen content of NACs can be readily tuned by varying the experimental conditions. Moreover, we experimentally confirmed the formation of CH4 from the NH3 activation reaction with N-doped carbon. We also discovered the doped nitrogen plays an important role on the activation reaction of carbon under NH3.ExperimentalCommercial CFP (Whatman) was adopted as the cellulose precursor. The as-received CFP was washed by water and ethanol several times followed by dried at 80 �C overnight. Then, the clean CFP was stabilized in air at 240 �C for 8 hrs following a thermal treatment in a tube furnace under ammonia gas (550-1000 �C for 2 or 6 hrs, heating rate: 2.5 �C/min) with a flow rate of 54 cc/min. In the control experiment, the stabilized CFP was heated in a tube furnace under Ar (1000 �C for 2 hrs, heating rate: 2.5 �C/min).Results and DiscussionA series of NAC membranes with different specific surface areas and nitrogen contents have been synthesized by annealing CFP under NH3 (Figure 1). When CFP was heated under NH3 at T �C for A hrs, the obtained NAC is referred to as NAC-T-Ah. N2 sorption measurements for NACs were carried out to investigate the surface area of NACs. For the purposes of comparison, a reference carbon, designated as Ar-1000-2h, was also prepared by annealing CFP in Ar at 1000 �C for 2 hrs. It is evident that specific surface areas and pore volumes rise along with higher heating temperatures and longer durations during pyrolysis. As summarized in Figure 1, calculations based on the isotherms give Brumauer-Emmett-Teller (BET) surface areas of 93.3 and 91.8 m2/g for NAC-550-2h and NAC-550-6h, respectively. The low specific surface areas suggest that little activation occurs at this temperature. At 700 �C, the specific surface areas rise to 390.6 and 544.4 m2/g for NAC-700-2h and NAC-700-6h, respectively. Note that Ar-1000-2h exhibits a lower BET surface area of 302.2 m2/g than NAC-700-2h. This clearly indicates that carbon activation process under NH3 occurs at as low as 700 �C. At higher temperatures of 850 and 1000 �C, the surface areas reach 889.2, 1326.5 and 1973.3 m2/g for NAC-850-2h, NAC-850-6h and NAC-1000-2h, respectively. This further demonstrates the activation effect under NH3.X-ray photoelectron spectrometer (XPS) was further employed to characterize the chemical composition of NACs obtained under various conditions (Figure 1). As a reference, Ar-1000-2h consists of 97.8 at% carbon and 2.2 at% oxygen, which indicates an effective carbonization. Under NH3, N-doping occurs through the reaction of NH3with the oxygenated species in the cellulose.44,47,48 At 550 �C, nitrogen constitutes 9.0 and 10.3 at% in NAC-550-2h and NAC-550-6h, respectively. At 700 �C, nitrogen content is stabilized at around 10.0 at% in NAC-700-2h and NAC-700-6h. However, at even higher temperatures of 850 and 1000 �C, nitrogen content drops to 4.5 at% for NAC-850-2h, 4.2 at% for NAC-850-6h, and 1.9 at% for NAC-1000-2h, which should be due to the effective removal of oxygenated species at a high temperature. For example, oxygen in NAC-850-6h is only 2.5 at%, comparable to that of Ar-1000-2h.It is important to reveal the activation mechanism of cellulose-derived carbon under NH3. As listed in Figure 1, a longer annealing duration (6 hrs vs. 2 hrs) at 850 �C results in a higher burn-off (89.4 wt% vs. 84.2 wt%) and a larger specific surface area (1326.5 m2/g vs. 889.2 m2/g). We did a simple experiment to test whether it is the doped nitrogen that results in the activation process. Both N-free carbon (Ar-1000-2h) and N-doped carbon (NAC-850-2h, 4.5 at% of N) were annealed under NH3 at 850 �C for one hour. A weight loss of ~ 5% was observed for Ar-1000-2h, while the burn-off is about 50 wt% for NAC-850-2h. The vast burn-off difference unequivocally confirms that doped nitrogen in NAC-850-2h plays an important role on the activation reactions between carbon and NH3.In order to further investigate the mechanism, mass spectrometry (MS) was used to analyze the exhaust gas collected after the furnace temperature had risen to 850 �C for 2 hrs when the formation process of NAC-850-2h is nearly finished. The spectra of the exhaust gas, background and methane standard are shown in Figure 2. In contrast to the background, exhaust gas and methane standard share similar peaks, particularly, at mass/charge ratios of 15 and 29 that are assigned to CH3+ and CH3CH2+ from the CH4 plasma. This proves the CH4 production during the activation process. Our results, for the first time, experimentally confirm the CH4 formation from the reaction between NH3and N-doped carbon. Here, we postulate an activation mechanism that can be described by the following equation:                    Doped-N in carbon3C (s) + 4NH3 (g)    ?     2N2 (g) + 3CH4 (g)  Figure 1.  Burn-off, specific surface area, and chemical compositions of NACs and Ar-1000-2h.  Figure 2. Mass spectrometry study on exhaust gas from the activation reaction. A spectrum of exhaust gas collected after the reaction temperature had risen to 850 �C for 2 hrs during the formation of NAC-850-2h (up blue), a background spectrum of air (middle red) and a standard methane spectrum (down black).ConclusionsIn conclusion, we have introduced the following discoveries in this study: i) N-doped activated carbon membranes with tunable nitrogen contents and surface areas can be formed by annealing cellulose filter paper under NH3; ii) Doped nitrogen in carbon lattice plays an important role on the activation reaction of carbon under NH3; iii) Methane is formed as a product in the reactions between N-doped carbon and NH3. References1, Simon, P.; Gogotsi, Y. Acc. Chem. Res. 2012 , 46, 1094.2, Rodr�guez-Reinoso, F.; Molina-Sabio, M.; Gonz�lez, M. T. Carbon 1995 , 33, 15.3, Lv, Y.; Zhang, F.; Dou, Y.; Zhai, Y.; Wang, J.; Liu, H.; Xia, Y.; Tu, B.; Zhao, D. J. Mater. Chem. 2012 , 22, 93.4, Zhao, L.; Fan, L.-Z.; Zhou, M.-Q.; Guan, H.; Qiao, S.; Antonietti, M.; Titirici, M.-M. Adv. Mater. 2010 , 22, 5202.

Pyrolysis of cellulose under ammonia leads to nitrogen-doped nanoporous carbon generated through methane formation

248th National Meeting (2014)

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