Research Paper
Journal of Chemical Engineering of Japan, Vol. 47, No. 1, pp. 21–27, 2014
Thermal Stability of Oxygen-Containing Functional Groups on Activated Carbon Surfaces in a Thermal Oxidative Environment Liqing Li, Xiaolong Yao, Hailong Li, Zheng Liu, Weiwu Ma and Xin Liang School of Energy Science and Engineering, Central South University, Changsha, Hunan 410083, China Keywords: Activated Carbon, Oxygen-containing Functional Group, Thermal Stability, Thermal Oxidative Environment The thermal stability of oxygen-containing functional groups on activated carbon surfaces in a thermal oxidative environment was studied. The raw activated carbon (AC0) was first treated with nitric acid, and the resulting nitric acid-treated activated carbon (ACn) was further oxidized under 2.5% O2 (in N2) atmosphere at different temperatures. The types and the amount of oxygen-containing functional groups were analyzed by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Boehm titration, and X-ray photoelectron spectroscopy (XPS). Both oxygen- and nitrogen-containing functional groups were introduced onto the ACn surface. Under thermal oxidative conditions, hydroxyl was oxidized to the corresponding carboxyl group in the temperature range of 378–473 K, and epoxy groups and lactones were generated between 573 to 773 K via oxidation reactions between graphitized carbon and oxygen. In contrast, carboxyl decomposition occurred at around 573 K. Lactones, ketones, and quinones exhibited better thermal stability, undergoing decomposition between 773 to 973 K. Ether and epoxy groups exhibited the best thermal stability, decomposing only at temperatures above 973 K.
Introduction On account of the advantages held by unique porous structures and their surface performances, activated carbon (AC) is widely used as an adsorbent and has found application in the fields of sewage and gas purification (Ma et al., 2008), solvent and gas separation (Sun et al., 2012), air dehumidification (Qi and LeVan, 2005), and fuel storage (Najibi et al., 2008). The functional groups on activated carbon, especially oxygen-containing functional groups, play an important role in adsorption processes because they can change the chemical properties of the activated carbon surface. For example, Li et al. (2002) reported that oxygencontaining functional groups could enhance the hydrophilicity of an AC surface. Karanfil and Kilduff (2000) found that acidic surface groups increased the polarity of the AC surface. Therefore, the detailed knowledge of the nature and amount of oxygen-containing functional groups on the activated carbon surface is of great interest. There are various methods for introducing oxygencontaining functional groups on AC surfaces (Tangsathitkulchai et al., 2009; Mine et al., 2011). Oxidation is one of the most conventional and popular methods to modify AC materials; it includes wet oxidation (Cañizares et al., 2006; Lee et al., 2010) (i.e., utilization of oxidizing solutions such as nitric acid, hydrogen peroxide, etc.) and dry oxidation (Chiang et al., 2002; Zhu et al., 2010) (i.e., use of oxidizing Received on August 20, 2013; accepted on October 4, 2013 DOI: 10.1252/jcej.13we193 Correspondence concerning this article should be addressed to L. Li (E-mail address:
[email protected]). Vol. 47 No.©1 2014 Copyright 2014The Society of Chemical Engineers, Japan
gases, such as oxygen, ozone, etc.). After oxidative modification, a variety of oxygen-containing functional groups can be generated on the AC surface. The nature and concentration of oxygen-containing functional groups can be modified by thermal treatments (Biniak et al., 2009). The thermal stabilities of oxygencontaining functional groups are different under different atmospheric conditions. Kundu et al. (2008) used carbon nanotubes as a catalyst support to study the thermal stability of oxygen-containing functional groups under ultra-high vacuum and in diluted hydrogen. They reported that the oxygen-containing functional groups could be decomposed at a lower temperature in diluted hydrogen. Consequently, a heat-treatment atmosphere was employed to influence the stability of oxygen-containing functional groups. A variety of experimental techniques have been used to identify oxygen-containing functional groups on carbon surfaces, such as thermogravimetric analysis (TGA) (Haydar et al., 2000), Fourier transform infrared spectroscopy (FTIR) (Martinez et al., 2002), Boehm titration (Boehm, 2002), and X-ray photoelectron spectroscopy (XPS) (Lakshminarayanan et al., 2004). However, as the complexity of the type and nature of functional groups on the AC surfaces vary, each of these techniques can only provide partial knowledge of the oxygen-containing functional groups. For example, Boehm titration can only detect the amount of hydroxyl, carboxyl, and lactone groups on AC surfaces (Zhou et al., 2007). FTIR can only be used for highly oxidized carbon surfaces, the absorption band intensities are very poor otherwise (Kundu et al., 2008; Park and Kim, 2005). As a result, a more comprehensive analysis is required in which all of these experimental techniques are 21
combined to study the oxygen-containing functional groups on AC surfaces (Toebes et al., 2004). As mentioned above, both wet and dry oxidation can introduce various oxygen-containing functional groups onto the AC surfaces, and the thermal stability of these functional groups under ultra-high vacuum, inert gas, and nitrogen conditions have been widely studied (Kundu et al., 2008; Manchester et al., 2008). However, the thermal stability of the oxygen-containing functional groups on AC surfaces in an oxygen atmosphere has not yet been studied in detail. Thus, in this study, the thermal stability of the oxygencontaining functional groups on the AC surfaces in a thermal oxidative environment was investigated using TGA, FTIR, Boehm titration, and XPS methods. The amount and type of oxygen-containing functional groups was identified, and the mechanism of their generation was studied. Transformation of each kind of oxygen-containing functional group upon heating at high temperatures in a thermal oxidative environment was discussed as well.
1. Experiment 1.1 Materials A type of commercial AC material with diameters of 1–3 mm was obtained from Liming Port Co. It was first washed with distilled water to remove impurities and was then dried at 378 K under vacuum for 12 h. After that, it was further heat-treated under He atmosphere at 1073 K for 1 h to remove the existing functional groups on the AC surface. This sample is designated as AC0. In order to introduce the oxygen-containing functional groups on the AC surface, AC0 was immersed in 5 M nitric acid (HNO3) solution at 298 K for 48 h. The sample was washed with distilled water to remove the residual solution existing in the AC porous spaces and dried at 378 K for 12 h under vacuum. This sample is designated as ACn. ACn was then further modified under 2.5% O2 (in N2) atmosphere for 30 min at different thermal oxidative temperatures (taking a sample per 100 K from 473 to 1073 K, 378 and 423 K). After that, the samples were cooled to room temperature under N2 atmosphere. These samples were denoted as ACn-m, where m denotes the modification temperature. For example, ACn-573 represents an ACn sample that was further oxidized by O2 at 573 K. 1.2 Characterization The mass change for both AC0 and ACn in the thermal oxidative environment was detected using a Setaram TG analyzer (Labsys Co.). Before experiments, the samples were dried at 378 K for 12 h. The experiments were carried under a 2.5% O2 (in N2) atmosphere and the heating rate was controlled at 10 K/min during the temperature-programmed process. Infrared spectroscopic analyses of AC0, ACn, and ACn-m were performed on a 6700 FTIR spectrometer (Nicolet Inc.). The samples were grinded to a powder and dried for 24 h at 378 K before experiments. Then, the dried samples were 22
mixed with finely divided KBr at a ratio of 1 : 100. Samples for FTIR measurements were obtained over the frequency range of 400–4000 cm−1. Boehm titration was used to detect the amount of acidic functional groups (i.e., hydroxyl, lactone, and carboxyl groups) existing on the AC surface. Standard solutions of NaOH, Na2CO3, and NaHCO3 were prepared before the experiments. NaOH solution was used to neutralize the total amount of acidic groups on the AC surface. Na2CO3 solution was used to neutralize lactone and carboxyl groups, whereas NaHCO3 solution was used to neutralize only carboxyls. The types and amount of oxygen-containing functional groups on ACn-m surfaces were measured using a K-Alpha 1063 XPS analyzer (Thermo Fisher Scientific Inc.). A K-α X-ray source (anode operating at 12 kV and 6 mA) was used as incident radiation. The base pressure in the measurement chamber was around 1.0×10−9 mbar. XP spectra were recorded in the fixed transmission mode. The analyzer used a 180° double-directional gathered hemispherical analyzer whose slit width was set to 0.4 mm and its pass energy was set at 200 eV; this resulted in an overall energy resolution better than 0.5 eV.
2. Results 2.1 Characterization of surface functional groups on ACn After nitric acid modification, the physicochemical properties of ACn appeared distinctly different from those for AC0, especially those with regard to the surface chemical properties. Figure 1 shows the infrared spectra of AC0 and ACn. One absorption peak around 1380 cm−1 appeared in the spectrum of AC0, which indicates the –C–H stretching vibrations (Cañete et al., 2011). However, the spectrum of ACn showed absorption peaks around 3440, 1710, 1530, 1190, and 960 cm−1. Pradhan and Sandle (1999) attributed the absorption peak at 3440 cm−1 to the –O–H stretch for hydroxyl or carboxyl groups. Li et al. (2012) analyzed AC surface functional groups, and they observed strong absorbance at 1210–1130 cm−1 and 1010–950 cm−1, which they
Fig. 1 Infrared spectra of AC0 and ACn Journal of Chemical Engineering of Japan
Fig. 2 TGA curve of ACn and AC0
attributed to –C–O–C– and –C–O– vibrations, respectively. Besides, the weak peak at 1710 cm−1 suggests that –C= O was introduced on the ACn surface (Xiao and Thomas, 2004; Tangsathitkulchai et al., 2009). Kohl et al. (2010) considered the complex peak pattern around 1530 cm−1 to be an overlap of different functional groups such as –C–N–, –C= N–, and –C= C– double bonds. This indicates that ACs modified with nitric acid also had –C–N– or –C= N–groups introduced on the AC surface. Swiatkowski et al. (2004) reported that the amounts of some functional groups on the nitric acid-treated AC surfaces (i.e., –OH, –C–O–C–, –C–O–R, –C–N–, and –C= O) were notably increased. At the same time, the amounts of other functional groups, such as –C= N–, –N= C–O–, –COOC–, and –COO–, were increased as well. Thus, it is assumed that large amounts of functional groups were introduced after the nitric acid-modification of AC0.
Fig. 3 FTIR spectra of ACn-m
2.2 Changing characteristics of oxygen-containing functional groups on ACn-m surface The weight losses and weight-loss rates (dM/dT) of both ACn and AC0 in a thermal oxidative environment as measured by TG analysis are shown in Figure 2. Compared with AC0, the weight of ACn decreased noticeably in the temperature ranging from 378–660 K. It has been reported that some oxygen-containing functional groups, such as carboxyl and hydroxyl groups, were decomposed at temperatures below 613 K (Haydar et al., 2000; Kundu et al., 2008). Weights of both ACn and AC0 began to decrease at temperatures above 767 K; this may be attributed to the oxygenolysis of activated carbon. Besides, the weight of ACn and AC0 increased with the temperature in the range from 600–770 K. This suggests that new oxygen-containing functional groups were probably generated via oxidation reactions, which may include the reactions occurring between oxygen and graphitized carbon, and those between oxygen and surface-functional groups existing in ACn. In order to elucidate the reactions occurring on the ACn surface in a thermal oxidative environment, the samples obtained at different temperatures were analyzed by FTIR, Boehm titration, and XPS. FTIR spectra of the samples
are shown in Figure 3. As noted in Figure 3(a), only a characteristic absorption peak around 3440 cm−1 could be observed in the infrared spectrum of ACn-423. The peak disappeared when the temperature reached 573 K. This suggests that the peaks corresponding to the functional groups containing –OH (i.e., hydroxyl and carboxyl) vanished as the temperature rose above 573 K. Figure 3(b) shows that the peak around 1530 cm−1 diminished when the temperature reached 473 K; this indicates that –C–N– and –C= N– were decomposed between 423 and 473 K. The absorption peak at 1710 cm−1 vanished in ACn-573 and seemed to appear again in ACn-673 and ACn-773, which indicates that the new functional groups containing –C= O might be generated through oxidation reactions. Additionally, another peak appeared around 1130 cm−1 at a temperature below 973 K. Figueiredo and Pereira (2010) suggested that some oxygen-containing function groups, such as anhydride, lactones, and epoxy groups containing –C–O–C–, exhibited better thermal stability and were only decomposed at a higher temperature. No characteristic peaks were apparent in the infrared spectrum of ACn-1073, indicating that no new functional group was generated and almost all of the surface functional groups in ACn had been
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decomposed. Figure 4 shows the Boehm titration results for ACn-m. The total amount of the acidic functional groups decreased with temperature. The variable regularities of disappearance of the three functional groups were different from each other. For hydroxyl, its content falls off quickly as the
Fig. 4
Variation of the amounts of three different functional groups on ACn-m surfaces
temperature reached 473 K. On the contrary, the amount of carboxyl increased when the temperature increased from 378 to 473 K. It has been reported that hydroxyl could be oxidized to carboxyl in a thermal environment (Li et al., 2003; Feng et al., 2006; Nicolaou et al., 2008). The carboxyl exhibited better thermal stability than hydroxyl, and the amount decreased rapidly at temperatures above 500 K, which indicates that the decomposition of carboxyl occurred above 500 K. The variation in the amount of surface lactone was considerably different from the other two groups; the amount was stable at temperatures below 573 K, which indicates that lactones exhibit the best thermal stability among the three functional groups. The amount of lactones increased in the temperature range of 673–773 K. Combined with the analysis results from TGA and infrared spectroscopy; this indicated that lactones were generated on the ACn surface between 673 and 773 K. Based on information obtained from the infrared spectroscopy and Boehm titration, the surface functional groups were further characterized by XPS. Figure 5 shows the O 1s spectra of ACn-m. The O 1s spectra revealed the presence of several peaks for each sample, and the magnitude for every peak differed from each other. The peaks at different positions in the XP spectrum represent various surface
Fig. 5 XP O 1s spectra of ACn-m at varying heat treatment temperatures Table 1 Relative content of surface functional groups present in XP O 1s spectra of ACn-m B.E. [eV] 530.6±0.2 532.3±0.3 533.5±0.2 534.3±0.2 536.3±0.4
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Assignment [%] –C=O (carboxyl) –C=O (ester, ketone, quinone) –C–O–C– (ether, lactones) –COH, –COOH, –N–O–N– H2Oads, O2ads
ACn-378
ACn-573
ACn-673
ACn-773
ACn-973
35.33 0.27 2.77 61.47 0.15
10.49 11.86 34.46 40.87 2.32
6.14 10.60 40.24 29.59 13.43
2.38 9.53 49.75 13.77 24.58
2.30 12.66 74.22 10.38 0.43
Journal of Chemical Engineering of Japan
functional groups (Pakula et al., 2002; Swiatkowski et al., 2002, 2004). The surface functional group compositions (in %) as well as the range of separate peak positions (B.E.) are shown in Table 1. The surface functional groups at 530.6 and 534.3 eV decreased significantly after 573 K, which is mainly due to the decomposition of the carboxyl group. On the contrary, the surface functional groups at 533.5 eV increased continuously with temperature, which can be attributed to the generation of lactones and ether groups. Besides, the surface functional groups at 532.3 eV visibly increased with the temperature increase, and maintained a relatively stable value above 573 K; this indicates that esters, ketones, and quinones have better thermal stability. These features are in agreement with the results obtained from the FTIR measurements and Boehm titration. Table 2 shows the relative contents of the surface functional groups present in the XP C 1s spectra of ACn-m (Burg et al., 2002; Pakula et al., 2002; Swiatkowski et al., 2002; Kohl et al., 2010). The content of graphitized carbon on the AC surface increased with temperature. The variations in carboxyl, lactone, ether, ester, ketone, and quinone groups obtained from Table 2 are in agreement with the results obtained from the XP O 1s spectra. The atomic group ratios of different functional groups to graphitized carbon (i.e., No/Ngrap×100%) were plotted using the relative contents of the surface functional groups (Table 2), and these are shown in Figure 6. The total atomic group ratio was observed to decrease with an increase in thermal oxidative temperature, which indicates that different sur-
Fig. 6
Variation of the atomic group ratios of different functional groups on ACn-m
face functional groups were decomposed at different temperatures. The atomic group ratio for –COOH at 289.3 eV decreased noticeably with the increase in temperature and reached a value close to zero above 573 K; this demonstrated the poor thermal stability of carboxyl groups and indicated that these groups were decomposed first in thermal oxidative environment. The atomic group ratio for –C= O at 287.7 eV decreased as well when the thermal oxidative temperature reached 573 K, and then decreased slightly above 573 K. This result indicates that other functional groups containing –C= O, such as ketones and quinones, exhibit better thermal stability than the carboxyl group. The atomic group ratio for –COOC– at 288.7 eV increased significantly with an increase in temperature and maintained a stable value at temperatures above 673 K; this is attributed mainly to lactone generation, which have good thermal stability. Besides, the ratio for –C–O–C– at 286.1 eV was maintained at a relatively stable value below 673 K; however, the value slightly decreased as the temperature rose from 673 to 973 K. This suggests that the thermal stabilities of ether and epoxy groups are the best among all oxygen-containing functional groups examined.
3. Discussion The treatment of activated carbon with nitric acid introduces a significant amount of oxygen-containing functional groups onto the surface. Infrared spectroscopic analysis indicated that the oxygen-containing functional groups including hydroxyl, lactones (ester), ketones, quinone, ether, and epoxy groups were introduced on the activated carbon surface. Simultaneously, nitrogen functional groups (–C–N– or –C= N–) were also introduced on the activated carbon surface. Because of the influences from both heat treatment and O2 oxidation, the variation in each kind of oxygencontaining functional group on the activated carbon surface becomes very complex under the thermal oxidative conditions. The changing characteristics of oxygen-containing functional groups in a thermal oxidative environment analyzed by TGA, infrared spectroscopy, Boehm titration, and XPS indicated that the variation in these functional groups are mainly attributed to two aspects: the generation of new functional groups through oxidation reactions, and the decomposition of the existing functional groups through heat treatment. The main oxidation reactions that could introduce new functional groups on the activated carbon surface in this study are summarized and shown in Figure 7.
Table 2 Relative content of surface functional groups present in XP C 1s spectra of ACn-m B.E. [eV]
Assignment [%]
ACn-378
ACn-573
ACn-673
ACn-773
ACn-973
284.6±0.1 286.1±0.1 287.7±0.2 288.7±0.2 289.3±0.1 290.4±0.4
Graphitized carbon –C–OH (hydroxyl), –C–O–C– (ether, epoxy group) –C–N–, –C=O (carbonyl, ketonic, quinone) –C=N–, –COOC– (ester, lactones) –COOH (carboxyl acid) Carbonate groups
66.54 9.66 12.55 3.62 6.11 1.51
71.26 10.83 7.17 4.45 1.81 4.48
73.94 10.04 6.45 7.47 1.22 0.88
76.86 7.56 5.89 7.70 0.81 1.17
80.89 6.24 4.87 7.13 0.41 0.46
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but it decomposed at a low temperature. With a continuous increase in temperature, the ketone and quinone (–C= O) appeared to decompose in the temperature range between 773 to 973 K. Lactones or esters (–COO–) began to decompose in this temperature range as well. Ether and epoxy groups (–C–O–C–) showed the best thermal stability among the oxygen-containing functional groups and could not be decomposed until they were exposed to the thermal oxidative temperatures above 973 K.
Conclusion Fig. 7
Oxidation reactions between existing functional groups/graphitized carbon and oxygen in a thermal oxidative environment
Fig. 8
Thermal stability of different oxygen-containing functional groups in a thermal oxidative environment
First, an oxidation reaction can occur between hydroxyl and oxygen to form carboxyl groups in the temperature range of 378–473 K. As the thermal oxidative temperature rises above 573 K, the graphitized carbon can be involved in the oxidation reactions; this may generate various new functional groups at different temperatures. In the range of 573–673 K, new ether groups can be generated as the graphitized carbon is oxidized. Following this, new lactones can be generated on the activated carbon surface between 673 and 773 K. In addition, new ketone and quinone groups may be generated in the thermal oxidative environment at temperatures above 573 K, although this reaction is infinitesimal and remains uncertain. A summary of the analysis results pertaining to the thermal stabilities of different oxygen-containing functional groups, as mentioned above, is discussed and the comparison results are shown in Figure 8. The hydroxyl (–OH) exhibits the worst thermal stability among all oxygen-containing functional groups examined, and participates in the oxidation reaction with oxygen. The thermal stability of the carboxyl (–COOH) group is better than that of the hydroxyl, 26
Both oxygen- and nitrogen-containing functional groups were introduced onto a nitric acid-treated activated carbon surface. After exposure to a thermal oxidative environment, the amounts and types of the oxygen-containing functional groups were identified by FTIR, Boehm titration, and XPS. The changes in the oxygen-containing functional groups were mainly attributed to two aspects: (i) generation of new functional groups due to oxidation reactions occurring between the existing functional groups and oxygen, and between the graphitized carbon and oxygen; (ii) the decomposition of the functional groups through heat treatment. Experimental results indicated that hydroxyl groups were oxidized to carboxyl groups in the temperature range of 378–473 K. Epoxy groups and lactones were generated between 573 to 773 K when the graphitized carbon was oxidized. On the other hand, the carboxyl decomposition temperature was demonstrated to be about 573 K. Lactones, ketones, and quinones exhibited better thermal stability, decomposing at 773–973 K. The thermal stabilities of ether and epoxy groups were the best, and they were not decomposed until above 973 K. Acknowledgements This work was supported by the National Natural Science Foundation of China (21376274, 20976200, 51206192).
Nomenclature T M dM dT No Ngrap
= = = = = =
temperature weight loss unit weight loss unit temperature amount of surface functional group amount of graphitized carbon
[K] [%] [%] [K] [%] [%]
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