Accepted Manuscript Title: Effect of pyrolysis temperature and thermal oxidation on the adsorption properties of carbon cryogels Author: Gabriela Hotov V´aclav Slov´ak PII: DOI: Reference:
S0040-6031(15)00239-7 http://dx.doi.org/doi:10.1016/j.tca.2015.06.001 TCA 77248
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Received date: Revised date: Accepted date:
25-11-2014 27-3-2015 1-6-2015
Please cite this article as: Gabriela Hotov, V´aclav Slov´ak, Effect of pyrolysis temperature and thermal oxidation on the adsorption properties of carbon cryogels, Thermochimica Acta http://dx.doi.org/10.1016/j.tca.2015.06.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of pyrolysis temperature and thermal oxidation on the adsorption properties of carbon cryogels Gabriela Hotová*, Václav Slovák University of Ostrava, Faculty of Science, Department of Chemistry, 30. dubna 22, 701 03 Ostrava, Czech republic E-mail addresses: [email protected]
(G. Hotová), [email protected]
(V. Slovák) Tel.: +420597092188
Higher pyrolysis temperature can improved adsorption properties of carbon cryogels. Low-temperature oxidation can improved adsorption properties of carbon cryogels. Presence of graphene in carbon cryogels does not improve the adsorption properties.
Abstract The effect of pyrolysis temperature (500 °C and 930 °C) and partial oxidation by oxygen (250–450 °C) of resorcinol-formaldehyde cryogels (RF) and their graphene derivatives (RFGO) on adsorption capacity for Pb(II) and methylene blue were investigated. Increasing temperature of partial oxidation causes increase in adsorption capacity for Pb(II) from 0.09 to 0.20 (RF) and 0.10 to 0.29 mmol g–1 (RFGO) for samples pyrolysed at 500 °C resp. from 0.20 to 0.23 (RF) and 0.16 to 0.27 mmol g–1 (RFGO) for samples pyrolysed at 930 °C. Partial oxidation does not affect adsorption capacity for methylene blue. The adsorption capacity of carbon cryogels for both adsorbates increased with higher pyrolysis temperature about 90% for RF and 60% for RFGO. The presence of graphene in the structure does not improve the adsorption properties of prepared carbon cryogels.
Keywords Pyrolysis; Low-temperature oxidation; Carbon cryogels; Adsorption; Pb(II); Methylene blue
1. Introduction Adsorption of heavy metal ions  or ionic dyes  from aqueous solution is one of the intensively studied applications of carbon aerogels, cryogels and xerogels. Affinity of polar species to non-polar carbon surface is generally low . Therefore physical or chemical activation is often needed before adsorption application. Physical activation of carbon surface is based on partial oxidation (e.g. by H2O or CO2) at high temperature and leads to development of microporosity often interconnected with enlarging the surface area, but without significant changes in surface chemistry [4,5]. Formation of oxygen containing surface groups is typical for chemical activation with hydroxides  and for partial oxidation of carbonaceous matter either in gas or liquid phase [e.g. 6]. Gas phase surface oxidation by air or oxygen was tested on various activated carbons. It was found that increasing temperature of oxidation leads to higher amount of surface acidic oxygen containing groups [7,8]. Available temperature range for the oxidation is determined by reactivity of the carbonaceous material and for activated carbons is about 200 °C – 400 °C [7–9]. Partially oxidized
activated carbons show higher adsorption capacity for adsorption of dibenzothiophene , water vapour , ammonia  or heavy metal ions . Excellent adsorption properties of graphite oxide  demonstrate the fact that more oxidized carbons act as better adsorbents. Aim of this work was to explore the possibility of gas phase oxidation of carbon gels by oxygen and to test the effect of oxidation on adsorption properties of these materials. Resorcinol-formaldehyde carbon cryogels and their derivative containing graphene were selected as substrates for partial oxidation.
2. Materials and methods 2.1.
Preparation of carbon cryogels
Preparation of used materials was derived from literature [11,12] and modified. Graphite oxide (GO) was produced by Hummer´s method [12,13]. Briefly, graphite powder (0,5 g) was added to 23 ml of 96% H2SO4, followed by the addition of NaNO3 (0,5 g) and solid KMnO4 (3 g) under stirring. The temperature of the mixture was kept at 0 °C for 1 h. Then, the temperature was increased to 40 °C and the mixture was stirred for 1 h. Distilled water (40 ml) was slowly added to the suspension which caused increase of the temperature to 90 °C – 100 °C. After 30 min, the suspension was diluted with distilled water (100 ml) and 30% H2O2 was added until the colour of the mixture changed from brown to yellow. The suspension was washed with aqueous HCl (1:1) to remove metal ions and then with water to remove the acid. The resulting graphite oxide was isolated by centrifugation (4000 G, 10 min) and freeze dried (52 Pa, 72 h). The graphene oxide was obtained by ultrasonic treatment (3 h) of suspension of prepared graphite oxide in water. Obtained suspension was used for cryogel preparation below. Synthesis of organic cryogels (RF and RFGO, the second doped with graphene oxide) was based on sol-gel polymerization of resorcinol (R) and formaldehyde (F) in aqueous solution with sodium carbonate (C) as catalyst. The molar ratio of R:F in initial mixture was 1:2 and R:C was equal to 200:1. The content of R+F in the reaction mixture was 4 wt%. GO was added to the initial mixture (1 wt%) for sample RFGO. The reaction mixture was mixed with intensive stirring, poured into glass vials and then transferred to an autoclave container for 72 h at 85 °C. After gelation the wet organic gels were freeze dried at 52 Pa for 24 h. Dry organic cryogels were pyrolysed at 500 °C or 930 °C under inert atmosphere (nitrogen) for 3 h. After this thermal treatment the graphene oxide sheets should be reduced to graphene sheets. Four types of cryogels were prepared for this study, concretely resorcinol-formaldehyde cryogel marked as RF-P500 or RF-P930 according to pyrolysis temperature and resorcinol-formaldehyde cryogel containing graphene marked as RFGO-P500 or RFGO-P930. The total amount of prepared carbon cryogels reached several milligrams (cca 200 mg).
Characterization of carbon cryogels
Infrared spectroscopy measurements were carried out at room temperature in ambient atmosphere using a Nicolet 6700 (ThemoScientific) spectrometer. FT-IR spectra of the prepared sample were recorded in the range of 4000–400 cm–1 using the ATR technique equipped with a diamond crystal. The spectra were obtained from 124 scans with the resolution of 4 cm–1. Surface characteristics of the carbon cryogels were determined from physical adsorption of N2 (-196 °C) and CO2 (30 °C) at absolute pressure range from 0 to 1 bar measured by volumetric analyser PCTPro E&E (Setaram). The weight of the dried sample for testing was approximately 0.20–0.27 g. The N2 adsorption isotherms were used to calculate the specific surface area (SBET). The CO2 adsorption data were evaluated according to Dubinin-Radushkevich and Medek equation to determine micropore surface area (Smicro) and micropore volume (Vmicro). Although the CO2 adsorption measurement provides information about surface area of micropores (pore diameter < 2 nm), the N2 adsorption gives information about surface area of mesopores and perhaps even coarser pores (external surface area).
The morphology of samples was examined by scanning electron microscopy (JEOL, JSM-6610LV) after coating the samples by Au film (120 s).
Thermoanalytical study of pyrolysis was conducted using simultaneous TG-DSC (SetsysEvolution, Setaram) coupled with mass spectrometer QMG 700 (Pfeiffer) by SuperSonic system (Setaram). Pyrolysis of organic cryogels (RF and RFGO) and graphite oxide (GO) was performed in crucible from α-Al2O3 (without standard) with 10–11 mg of sample under an argon (flow rate 20 ml min–1) from 15 °C to 1000 °C with heating rate 10 K min–1. Simultaneous TG-DTA (NETZSCH, STA 449 C) experiments was used for studying of the oxidation of the prepared samples in atmosphere of 80% O2 and 20% N2 (100 ml min–1) from 30 °C to 1000 °C, heating rate 10 K min–1. The experiments were performed with 8–13 mg of the sample in crucible from α-Al2O3 without standard.
Adsorption experiments of Pb(II) ions and methylene blue from aqueous solutions were conducted in the batch mode. Sample (10–12 mg) was added to 10 ml of adsorbate solution with initial concentration of 1 mmol dm–3. The experiments were carried out with contact time 24 h (Pb(II) ions) or 30 min (methylene blue) for equilibration of the system. The Pb(II) ions concentration before and after adsorption tests was determined by AAS (Varian FS 240). Quantification of methylene blue in solutions was performed by VIS spectrophotometry (Spectronic 200, Thermo-Scientific) at 665 nm.
3. Results and discussion 3.1.
Pyrolysis of organic cryogels
Pyrolysis of GO, RF and RFGO was studied to assess possible effect of GO on pyrolytic step of carbon cryogels preparation. Termoanalytical curves (TG-DSC/MS) of pyrolysis of graphite oxide (Fig. 1a) show that two steps can be supposed. The first step is (observable to 150 °C) related to endothermic evaporation of water. The mass loss of this step is 14%. The second step (above 150 °C) is accompanied by more than 70% mass loss. During this step, graphite oxide quickly exothermically decomposes and releases CO2. The evolution of CO2 over 150 °C is connected with breaking of oxygen functional group away from the surface. Presence of hydrogen in the structure of GO is proved by evolution of water during this step (Fig. 1a).
Fig. 1. TG-DSC/MS curves of pyrolysis of GO (a), RF (b) and RFGO (c). Simultaneous TG-DSC/MS study of pyrolysis of organic cryogels (RF and RFGO) is illustrated in Fig. 1b, c. From the TG curves (up to 150 °C) are evident that RF and RFGO samples endothermically evaporate water (confirmed by DSC and MS analysis). The humidity content increases in the range RF < RFGO < GO, which corresponds to increasing content of oxygen functional groups. Intensive decomposition of graphite oxide above 150 °C was not observed in the case of RFGO cryogels. It can be assumed that reducing environment of organic cryogel probably prevents decomposition of graphene oxide and enables its reduction to graphene. This corresponds to the finding that further heating (up to 930 °C) of the RFGO sample shows less mass loss than the RF sample. Total mass loss during pyrolysis was 53% for RF and 50% for RFGO cryogel. Thus, the presence of GO in the structure of RFGO cryogel increases humidity content and carbon yield. Based on the pyrolytic measurements, two temperatures of pyrolysis for the preparation of RF resp. RFGO carbon cryogels were chosen. All pyrolytic processes including CO2 evolution are significantly in progress above 500 °C, so this temperature represents a possible minimum. The second chosen temperature (930 °C) is a maximal available temperature in our laboratory oven.
Characterization of carbon cryogels
The values of surface characteristics of RF and RFGO carbon cryogels are presented in Table 1. The results show that the development of micropores surface area (Smicro) increased with increasing pyrolysis temperature especially for RF cryogels in comparison to RFGO cryogels where the micropore parameters are similar. Because the penetration of micropores by N2 is very low at -196 °C, the specific surface area determined from adsorption measurement can be considered as the specific area of mesopores and coarser pores (external surface) [14,15]. In the case of RF cryogels the specific surface area increased with higher pyrolysis temperature due to development of new pores that become accessible for N2 molecule. On the other hand, increase in pyrolysis temperature for RFGO cryogels leads to decrease of SBET values. This can be caused by pores widening and coalescence of neighbouring pores . It can be assumed that prepared material are especially microporous, because of
predominance of Smicro values over SBET. However, the presence of graphene in carbon cryogels negatively influenced textural parameters (see Table 1). Table 1 Surface parameters of carbon cryogels. Sample
RF-P500 RF-P930 RFGO-P500 RFGO-P930
957 1549 777 800
0.345 0.575 0.278 0.297
649 1344 472 114
The FT-IR spectra of carbon cryogels are presented in Fig. 2. In the spectra obtained for the carbon cryogels pyrolysed at the lower temperature, the well-defined vibrations are seen. RF-P500 and RFGOP500 cryogels exhibit absorption at approximately 3400 cm–1 (–OH vibration), 3055 cm–1 (unsaturated –CH stretching vibration), 2918 cm–1 (–CH2 stretching vibration), 1560 cm–1 (–COO– vibration), 1423 cm–1 (–CH bending vibration), 1260–1000 cm–1 (–C-O stretching in lactones, ethers, phenols and carboxylic anhydrides) and 900–700 cm–1 (low intensity out-of-plane –CH bending). However, for higher pyrolysed carbon cryogels the spectra are very simple without any significant absorption peak. FT-IR spectra of RF-P930 and RFGO-P930 cryogels are more similar to activated carbon [16,17], bituminous and subbituminous coal . Carbon cryogels pyrolysed at 930 °C indicates strong absorption increase with decreasing wavenumber. Their FT-IR spectra show some absorption peak (artefact) around 2200–2000 cm–1 caused by diamond crystal. The results show that the surface of the materials pyrolysed at higher temperature is carbonaceous and contains minimum of functional groups in comparison to materials pyrolysed at lower temperature.
Fig. 2. FT-IR spectra of RF-P500/930 (a) and RFGO-P500/930 (b) cryogels. Black curves illustrated FT-IR of carbon cryogels pyrolysed at 500 °C, while grey curves are for carbon cryogels pyrolysed at 930 °C. The typical SEM photographs of carbon cryogels pyrolysed at different temperatures are illustrated in Fig. 3. Pyrolysis temperature does not causes changes in the morphology of the prepared samples. Both RF-P500 and RF-P930 cryogels show a smooth surface morphology (Fig. 3a, c) while RFGOP500 and RFGO-P930 cryogels are more crumpled and rough (Fig. 3b, d). However, there is a considerable difference between the RF and RFGO cryogels. Presence of 1% of graphite oxide in the reaction mixture causes significant change in the morphology of carbon cryogels.
Fig. 3. SEM images of RF-P500 (a), RFGO-P500 (b), RF-P930 (c) and RFGO-P930 (d) cryogels (magnification 15000x, accelerating voltage 30 kV).
Oxidation of carbon cryogels
TG-DTA study of thermal oxidation of carbon cryogels pyrolysed at 500 °C and 930 °C in atmosphere of 80% O2 and 20% N2 is illustrated in Fig. 4. Basic thermoanalytical experiments have shown that both types of carbon cryogels pyrolysed at 930 °C are relatively resistant against oxidation and the presence of graphite oxide in reaction mixture during their preparation influenced the temperature of oxidation. The temperature of maximal oxidation rate (peak temperature at DTG) was shifted in the presence of GO to higher temperatures from 519 °C (RF-P930) to 534 °C (RFGO-P930). Higher thermal stability of the material containing graphene (RFGO-P930) is also evident from a broader temperature range of oxidation. The higher oxidation stability of RFGO cryogels can be caused by presence of graphene layers. The carbon atoms of the graphene layer in the basal plane are less reactive than the edge carbon atoms . Therefore, carbon materials containing basal plane sites are less reactive than materials with a preponderance of edge or active sites [19,20]. DTA curve of the oxidation of RF-P930 (Fig. 4a) showed a small exothermic peak around 330 °C which is not accompanied by a mass loss. This effect was not observed for RFGO-P930 and its reason is not clear. Carbon cryogels pyrolysed at 500 °C are more reactive than carbon cryogels pyrolysed at 930 °C (Fig. 4). The TG curves of these materials (RF-P500 and RFGO-P500) show an evident exothermic (DTA curves; Fig. 4) mass increase (about 3%) in the temperature range from 150 °C to 300 °C. This step can be related to chemisorbing of oxygen on the surface of these materials and creation of oxygen containing groups. This phenomenon is well known for other reactive carbonaceous materials, e.g. coals . However, the carbon cryogels pyrolysed at 500 °C were easily burned out when the temperature was higher than 300 °C. It is about 100 °C less than carbon cryogels pyrolysed at 930 °C due to their higher reactivity.
Fig. 4. TG-DTA curves of oxidation of RF-P500/P930 (a) and RFGO-P500/P930 (b) cryogels.
Partial oxidation of carbon cryogels with an isothermal step
Based on thermoanalytical measurements in oxidation atmosphere (Fig. 4) temperatures for partial oxidation of carbon cryogels were chosen. Partial oxidation of RF and RFGO cryogels pyrolysed at 500 °C were conducted at temperatures selected from the mass increase temperature range (250 °C, 270 °C and 280 °C). Carbon cryogels pyrolysed at 930 °C were oxidized at four selected temperatures. The minimum temperature (330 °C) was based on small DTA peak during RF-P930 oxidation (Fig. 4a). The other three temperatures were selected from the beginning of the oxidation, when the extent of oxidation at these temperatures was below 3% (420 °C, 440 °C and 450 °C). Then, the samples were heated to the selected temperature and kept for 1 hour using TA (Fig. 5). Products of these isothermal experiments were used for the adsorption study, except of RF-P500 cryogel oxidized at 280 °C. It is obvious from TG curve (Fig. 5a), when the RF-P500 cryogel reached 280 °C, during the heating, it burnt out.
Fig. 5. TG curves of partial oxidation of RF-P500/P930 (a) and RFGO-P500/P930 (b) cryogels. Temperature of isothermal testing (250 °C, 270 °C and 280 °C) corresponds to RF and RFGO cryogel pyrolysed at 500 °C, while (330 °C, 420 °C, 440 °C and 450 °C) for RF and RFGO pyrolysed at 930 °C.
Under the measurement conditions, the mass loss of RF-P930 cryogel were 11.8% (420 °C), 23.1% (440 °C) and 34.0% (450 °C), while 13.8% (420 °C), 20.6% (440 °C) and 27.8% (450 °C) for RFGOP930 cryogel. Partial oxidation of RF-P500 and RFGO-P500 cryogels at 250 °C indicates mass increase which stabilizes at constant value (Δm = +3%). A further increase of the partial oxidation temperature (270 °C and 280 °C) results in slow decreasing of the sample mass.
Effect of pyrolysis and oxidation temperature on adsorption properties
The result values of adsorbed amount of Pb(II) ions and methylene blue on carbon cryogels are shown in Fig. 6 and Fig. 7. It is evident, that pyrolysis temperature affects the adsorption properties of carbon cryogels (Fig. 6). Adsorbed amount increases with increasing pyrolysis temperature. According to the values shown in Table 1, raising the pyrolysis temperature from 500 °C to 930 °C leads also to the increase of microporous parameter. This pore development is the main reason of increasing adsorption capacities regardless of presence of functional groups [22,23]. Other authors have reported the same trend, that with higher pyrolysis temperature increases Pb(II)  and methylene blue adsorption capacity by other adsorbents. According to , the adsorbed amount of methylene blue on carbon xerogels rises from 0.12 mmol g-1 (pyrolysis temperature 500 °C) to 0.19 mmol g-1 (pyrolysis temperature 700 °C). While  found out that, the adsorbed amount of methylene blue on activated carbon increases from 0.53 mmol g-1 (pyrolysis temperature 350 °C) to 0.67 mmol g-1 (pyrolysis temperature 450 °C) and then decreases when the temperature exceeds 450 °C.
Fig. 6. Effect of pyrolysis temperature on the adsorbed amount of Pb(II) ions and methylene blue for RF-P500/P930 (a) and RFGO-P500/930 (b) cryogels. The effect of partial oxidation of RF and RFGO carbon cryogels pyrolysed at different temperatures is illustrated in Fig. 7. The influence is more evident for Pb(II) adsorption on carbon cryogels pyrolysed at lower temperature. However, the effect of partial oxidation on adsorption of Pb(II) on RF-P930 and RFGO-P930 cryogels is not so clear. It can be supposed that carbon cryogels pyrolysed at 500 °C are more reactive and contain more heteroatoms. This might be proved by detected mass increase (about 3%) in the temperature range from 150 °C – 300 °C. Up to 350 °C, the oxygen chemisorbing on the surface of the samples and oxygen containing groups are created, while above 350 °C the oxygen surface complexes decompose and evolve as CO and CO2 . The higher temperature of partial oxidation results in higher oxygen contents and the higher adsorbed amount of Pb(II), respectively (see Fig. 7a, b). In the case of less reactive carbon cryogels (pyrolysed at 930 °C) the responding mass increase was not detected during partial oxidation. Carbon cryogels (RF-P930 and RFGO-P930) were partially oxidized in temperature range from 330 °C – 450 °C. Thus, the samples are partially oxidized and decomposed and this might be the main reason of irregular increase of adsorbed amount of Pb(II) (see Fig. 7c, d). An increase in the temperature of partial oxidation does not affect the adsorption capacity of methylene blue on RF-P500/930 and RFGO-P500/930 cryogels as it was expected. It is obvious that the
presence of graphene is concerned with decreasing adsorption capacities and does not improve the properties of the adsorbents.
Fig. 7. Effect of oxidation temperature on the adsorbed amount of Pb(II) ions and methylene blue for RF-P500 (a), RFGO-P500 (b), RF-P930 (c) and RFGO-P930 (d). In comparison with different adsorbents, carbon cryogels prepared in this study have a relatively high adsorption capacities. Nevertheless, adsorption capacity of Pb(II) for our samples was quite the same to 0.17 mmol g-1 for carbon aerogel . In the case of methylene blue adsorption, the other authors have reported a lower adsorption capacities by other adsorbents, such as 0.13 mmol g-1 for carbon aerogel  or 0.12 mmol g-1 for carbon xerogel . But it has to be note that these data have been obtained at quite different conditions. According to the adsorption tests, carbon cryogels exhibit quite well adsorption properties after activation and have a significant potential in the treatment of wastewater containing heavy metals and organic substances.
4. Conclusions Thermoanalytical experiments of carbon cryogels have shown that some relation among pyrolysis/oxidation temperature and adsorption properties exists. The low-temperature oxidation treatment of carbon cryogels modifies their surface and causes creation of oxygen containing groups. The higher temperature of partial oxidation of carbon cryogels leads to increase in adsorption capacity of Pb(II) ions, concretely from 0.09 to 0.20 mmol g–1 (RF-P500), 0.10 to 0.29 mmol g–1 (RFGO-P500), 0.20 to 0.23 mmol g–1 (RF-P930) and from 0.16 to 0.27 mmol g–1 for RFGO-P930 cryogels. It is obvious that this phenomenon is more evident for carbon cryogel pyrolysed at 500 °C, which are more reactive and after oxidation contains more oxygen functional groups. However, increase in the temperature of partial oxidation does not affect adsorption capacity of methylene blue. The higher pyrolysis temperature result in higher adsorption capacities of Pb(II) ions and methylene blue for both type of carbon cryogels. The adsorbed amount of Pb(II) increases from 0.09 to 0.20 mmol
g–1 for RF cryogel and from 0.10 to 0.16 mmol g–1 for RFGO cryogel, while from 0.23 to 0.42 mmol g– 1 (RF) and from 0.15 to 0.23 mmol g–1 (RFGO) in the case of methylene blue. The presence of graphene in the structure of RFGO cryogels does not improve the adsorption properties of prepared carbon cryogels. Nevertheless, graphite oxide in the reaction mixture causes increase in humidity content, carbon yield and oxidation stability.
Acknowledgement This work was financially supported by the Ministry of Education, Youth and Sports of the Czech Republic in the "National Feasibility Program I", project LO1208 "Theoretical Aspects of Energetic Treatment of Waste and Environment Protection against Negative Impacts".
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