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Mesoporous carbon-xerogels films obtained by microwave assisted carbonization
Materials Letters 141 (2015) 135–137
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Mesoporous carbon-xerogels films obtained by microwave assisted carbonization David Espinosa-Iglesias, Carmen Valverde-Sarmiento, Agustín F. Pérez-Cadenas n, Ma. Isidora Bautista-Toledo, Francisco J. Maldonado-Hódar, Francisco Carrasco-Marín Carbon Materials Research Group, Department of Inorganic Chemistry, Faculty of Sciences, University of Granada, Campus Fuentenueva s/n, 18071 Granada, Spain
art ic l e i nf o
a b s t r a c t
Article history: Received 1 June 2014 Accepted 15 November 2014 Available online 26 November 2014
This work describes a new preparation method of carbon-xerogel films with well-developed mesoporosity from resorcinol – formaldehyde polymerization. The particular effect that the microwaves have during the assisted carbonization process shows that with low microwave carbonization degrees, high mesopore volumes can be obtained. Carbonization assisted by microwaves yields carbon-xerogels films whose aromatic and chemical characteristics are clearly different than those obtained by conventional thermal carbonization with similar carbonization degrees. & 2014 Elsevier B.V. All rights reserved.
Keywords: Carbon xerogel Microwaves Mesoporosity Carbon film Carbonization
1. Introduction These days, the development of carbon-based films applied as coatings on different structural materials like concretes [1] or ceramic monoliths [2], is reaching high interest due to their potential environmental applications [3]; biological treatments, adsorption, or catalysis. Carbon-xerogels are very good candidates for this type of coatings. Although, carbon-xerogels are typically microporous materials with low mesopore volumes [4] and the presence of mesopores is specially poor when carbon xerogels are formed in film shape [2]. The mesoporosity is very appreciated for many applications. Recently, several works have been published in which microwave radiation is used during the synthesis process of resorcinolformaldehyde organic gels [4,5]. On the other side, the use of microwave radiation as a heating source for pyrolysis has already been shown as a very rapid method when microwave receptors are present in contact with the sample [6]. As far as we know, the microwave carbonization of resorcinol-formaldehyde organic gels has not been described yet, which might be due to the fact that these organic polymers hardly absorb the microwaves produced by standard ovens, they normally operate at a frequency of 2.45 GHz [7]. This work collects the preparation method of carbon-xerogel films with well-developed mesoporosity which is obtained by microwave assisted carbonization and also by conventional carbonization. Besides, the particular effects that the microwaves produce
n
Corresponding author. Tel.: þ 34 958243316; fax: þ 34 958248526. E-mail address: [email protected] (A.F. Pérez-Cadenas).
http://dx.doi.org/10.1016/j.matlet.2014.11.052 0167-577X/& 2014 Elsevier B.V. All rights reserved.
during the carbonization process on the final chemical characteristic of these materials are shown; both, mesopore volumes and final oxygen contents of the carbon films do not depend on the carbonization degree developed using microwaves.
2. Experimental Three carbon xerogel films with different carbonization degrees, were prepared from an original organic xerogel film (XO). The organic xerogel film was prepared by dissolving resorcinol (R) and formaldehyde (F) in water and using Cs2CO3 as polymerization catalyst (C). The stoichiometric R/F and R/C molar ratios were 0.50 and 2300 respectively. The mixture was stirred to obtain homogeneous solutions that were cast into cylindrical glass molds. The molds were closed in order to avoid an evaporation process during the later curing period which included one day at room temperature, one day at 50 1C and five days at 80 1C. After the curing, the films were thermally dried at 110 1C and atmospheric pressure until constant weight. The thickness of the dry organic films was around 0.1 cm approx. The carbonization process was carried out using an oven microwave model Savoid MSG-20810-S, which was adapted with an entry of gases. Besides, the carbonization of organic xerogel films was performed with 800 W of power, in a stream of argon, using radiation periods of ten cumulative minutes. After each period there was a pause in the radiation, also under an argon stream. For the first period of treatment the organic xerogel films were situated between two sheets of graphite as a microwave receptor in order to induce a greater warming on the surface of the
136
D. Espinosa-Iglesias et al. / Materials Letters 141 (2015) 135–137
organic film. During the second and subsequent periods the top graphite sheet was removed. The cooling of the carbonized material was also performed in a stream of argon. In these conditions, carbon xerogel films with a percentage of weight loss of 13, 17 and 28 wt% were obtained using 3, 4 and 7 treatment periods, respectively. Carbon xerogel films are referred as XMW13, XMW17 and XMW28; the digits indicate the weight loss percentage after carbonization in each case. In order to know properly the particular role of the microwave during the carbonization process, other three carbon films with the same carbonization degree were obtained from the same original organic xerogel film. This was done by conventional carbonization in a thermal oven. In these last cases the carbonization was carried out in an argon flow at 340, 400 and 520 1C during 30 min using a heating rate of 10 1C/min and therefore obtaining the corresponding 13, 17 and 28 wt% of weight loss, respectively. The carbon films obtained by conventional carbonization are referred as XF13, XF17 and XF28; the digits indicate the weight loss after carbonization in each case. The porous characteristics were studied by N2 and CO2 adsorption at 77 and 273 K, respectively, using an equipment Autosorb-1C Quantachrome. The samples were also characterized by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics ESCA 5701 equipped with a MgKα X-ray source, magnetic nuclear resonance (NMR). This was done by using a Bruker Advance 500 instrument equipped with a standard-bore 11.74 T superconducting magnet operating at 125.76 MHz for 13 C, Raman, using a Micro-Raman JASCO NRS-5100 dispersive spectrometer with a 532 nm laser line, and elemental analysis (EA) using a Thermo Scientific – Flash 2000 elemental analyzer. Finally, pH of point of zero charge (PZC) measurements were obtained suspending 250 mg of each sample on 4 mL of degasified and distilled water. Suspensions were stirred and thermostated at 25 1C measuring the pH periodically until the reading was constant. The obtained final pH was considered as the PZC for each sample. 3. Results and discussion BET areas (SBET), micropore volumes (W0) and mean micropore width (L0) obtained by applying DR and Stoeckli equations to the CO2 adsorption isotherm, mesopore volumes (VBJH) and mean mesopore widths (LBJH) obtained by applying the BJH method to the N2 desorption data, elemental analysis (EA) data, surface oxygen content of the films (OXPS) and PZC data, are compiled in Table 1. All the carbonized films are mainly mesoporous materials with mesopore volumes up to 0.6 cm3/g. However, the mean mesopore widths (LBJH) were very similar among the films when microwaves were used. In contrast to this, narrower mesopores are progressively obtained by conventional carbonization. Very interesting is the fact that low microwave carbonization degrees, only 13 wt%, developed high mesopore volumes. In this line, all samples showed type IV N2-isotherms (Fig. 1); micropore volumes are generally very small and only sample XF28 has a remarkable BET area, which seems to be logical in terms of conventional carbonization processes. The pore size distributions (PSD) of Table 1 Textural characterization, chemical composition and PZC data of the films. Film
SBET (m2/ g)
XO 14 XF13 73 XF17 86 XF28 498 XMW13 98 XMW17 111 XMW28 121
W0 (cm3/ g)
L0 VBJH (nm) (cm3/ g)
LBJH CEA (nm) (wt %)
HEA (wt %)
OEA (wt %)
OXPS PZC (wt%)
0.00 0.08 0.10 0.19 0.10 0.09 0.14
– 0.7 0.6 0.6 0.6 0.7 0.5
– 10.2 6.1 2.2 15.7 15.5 15.4
6.3 5.2 5.4 4.7 3.7 3.6 3.4
34.8 30.4 24.5 17.8 35.3 33.3 33.9
– 30.5 23.2 14.7 23.9 24.1 24.3
0.05 0.24 0.39 0.62 0.50 0.58 0.40
58.9 64.4 70.1 77.5 61.0 63.2 62.7
3.6 4.3 5.6 6.8 3.4 3.8 3.3
0
Fig. 1. N2 adsorption and desorption isotherms at 77 K of samples XMW17 and XF17 (up); Pore size distributions of XMW17 and XF17 obtained by the application of QS-DFT to the N2 isotherms (down).
XMW17 and XF17 films obtained by the application of QS-DFT to the N2 isotherms are collected in Fig. 1, showing a very good agreement with the results obtained by the application of BJH and DR methods. It should be mentioned that the different PSD obtained at similar carbonization degree, depends on the type of applied carbonization method. Chemically, all the samples are mainly composed of carbon, although the oxygen contents clearly depended on the carbonization technique. Samples carbonized in a conventional oven contain logical oxygen amounts in relationship with the corresponding carbonization degrees, decreasing the oxygen contents when the carbonization degree increases. However all samples which are carbonized using microwave, contain very high oxygen contents and are very similar among them. In solid materials the microwave energy is supplied at molecular level normally by a dielectric polarization mechanism resulting in a very efficient internal heating [7]. Therefore a possible explanation for the above mentioned high oxygen contents may be found in the heating mechanism, which could lead to the formation of active centers in the materials that, once exposed to an air atmosphere are stabilized by oxidation [5]. In this line, the XMW films also have very similar oxygen contents among them on the non-porous external surface (OXPS) (Table 1), whereas in the case of XF films the oxygen contents determined by XPS were very similar to those determined by EA indicating a homogeneous oxygen distribution throughout the material. On the contrary, XMW films showed the lowest oxygen concentration on the external non-porous surface areas, indicating that the above mentioned re-oxidation process would be more powerful taking place in the micro- and mesopore
D. Espinosa-Iglesias et al. / Materials Letters 141 (2015) 135–137
XMW28 XF28
137
fact that both films had the same weight loss during carbonization. This interpretation is in agreement with the Raman analysis whose spectra are collected in Fig. 3. The biggest relative intensity of the lower wavenumber band (alternating ring stretch vibration in benzene or condensed benzene rings [10]) regarding to the band centered at 1580 cm 1 indicates that the development of the graphenic character of these materials follows the order XF284XMW284XO.
4. Conclusions
290 270 250 230 210 190 170 150 130 110 90
70
50
30
10
-10 -30
ppm (δ) 13
C RMN spectra of the carbon films XMW28 (black) and XF28 (gray).
Raman Intensity (AU)
Fig. 2.
XO
The preparation of carbon xerogel films with well-developed mesoporosity has been described, showing that with low microwave carbonization degrees, only 13 wt%, high mesopore volumes can be obtained. Therefore it has been shown to be a new and shorter process to obtain mesoporous carbon-based films. This microwave assisted carbonization method will be especially useful when carbon-based films have to be prepared on ceramic materials that cannot be treated at high temperatures during long periods of time or throughout all the ceramic material. On the other side, carbonization processes assisted by microwaves yield carbon-xerogels films with a different carbonaceous structure, as well as much higher oxygen contents than those obtained by conventional carbonization at a similar carbonization degree.
Acknowledgments This work has been supported by the project P12-RNM-2892 and the contract CDTI No. IDI-20111005.
XMW28
Appendix A. Supporting information
XF28
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.11.052.
1000
1200 1400 1600 1800 -1 Wavenumber (cm )
2000
Fig. 3. Raman spectra of the organic xerogel film XO, and the carbon xerogel films XMW28 and XF28.
surfaces. Besides, PZC values which depend inversely on the oxygen content of this type of materials follow this tendency in the XF series, whereas in the XMW series similar PZC values were obtained. Finally, 13C RMN and Raman analysis of the samples were carried out. Fig. 2 collects the 13C RMN spectra of samples XMW28 and XF28 where significant differences are shown. RMN signals have been assigned according to the literature [8,9]. XMW28 spectrum shows the following signals: one on 117 ppm associated with carbon atoms placed in the aromatic ring contiguous to those directly bonded to –OH groups; a second signal on 160 ppm corresponding to aromatic carbons directly bonded to –OH groups. On the other side, the XF28 spectrum shows its aromatic carbon signals at 130 and 155 ppm clearly shifted regarding to the corresponding signals in the XMW28 spectrum, being the signal at 130 ppm typical of aromatic carbons in carbon materials [9]. Therefore, XF28 is a material with less phenolic groups and higher graphenic character than XMW28 in spite of the
References [1] Alquier M, Kassim C, Bertron A, Sablayrolles C, Rafrafi Y, Albrecht A, et al. Halomonas desiderata as a bacterial model to predict the possible biological nitrate reduction in concrete cells of nuclear waste disposals. J Environ Manag 2014;132:32–41. [2] Morales-Torres S, Maldonado-Hodar FJ, Perez-Cadenas AF, Carrasco-Marin F. Structural characterization of carbon xerogels: from film to monolith. Microporous Mesoporous Mater 2012;153:24–9. [3] Moreno-Castilla C, Perez-Cadenas AF. Carbon-based honeycomb monoliths for environmental gas-phase applications. Materials 2010;3:1203–27. [4] Gallegos-Suarez E, Perez-Cadenas AF, Maldonado-Hodar FJ, Carrasco-Marin F. On the micro- and mesoporosity of carbon aerogels and xerogels. The role of the drying conditions during the synthesis processes. Chem Eng J 2012;181–182:851–5. [5] Calvo EG, Ferrera-Lorenzo N, Menéndez JA, Arenillas A. Microwave synthesis of micro-mesoporous activated carbon xerogels for high performance supercapacitors. Microporous Mesoporous Mater 2013;168:206–12. [6] Monsef-Mirzai P, Ravindran M, McWhinnie WR, Burchill P. Rapid microwave pyrolysis of coal: methodology and examination of the residual and volatile phases. Fuel 1995;74:20–7. [7] Mingos DM, Baghurst DR. Tilden Lecture. Applications of microwave dielectric heating effects to synthetic problems in chemistry. Chem Soc Rev 1991;20:1–47. [8] NMRShift Data Base; 2014 〈http://nmrshiftdb.nmr.uni-koeln.de/portal〉. [9] Freitas JCC, Bonagamba TJ, Emmerich FG. 13C high-resolution solid-state NMR study of peat carbonization. Energy Fuels 1999;13:53–9. [10] Schwan J, Ulrich S, Batori V, Ehrhardt H, Silva SRP. Raman spectroscopy on amorphous carbon films. J Appl Phys 1996;80:440–7.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Mesoporous carbon-xerogels films obtained by microwave assisted carbonization David Espinosa-Iglesias, Carmen Valverde-Sarmiento, Agustín F. Pérez-Cadenas n, Ma. Isidora Bautista-Toledo, Francisco J. Maldonado-Hódar, Francisco Carrasco-Marín Carbon Materials Research Group, Department of Inorganic Chemistry, Faculty of Sciences, University of Granada, Campus Fuentenueva s/n, 18071 Granada, Spain
art ic l e i nf o
a b s t r a c t
Article history: Received 1 June 2014 Accepted 15 November 2014 Available online 26 November 2014
This work describes a new preparation method of carbon-xerogel films with well-developed mesoporosity from resorcinol – formaldehyde polymerization. The particular effect that the microwaves have during the assisted carbonization process shows that with low microwave carbonization degrees, high mesopore volumes can be obtained. Carbonization assisted by microwaves yields carbon-xerogels films whose aromatic and chemical characteristics are clearly different than those obtained by conventional thermal carbonization with similar carbonization degrees. & 2014 Elsevier B.V. All rights reserved.
Keywords: Carbon xerogel Microwaves Mesoporosity Carbon film Carbonization
1. Introduction These days, the development of carbon-based films applied as coatings on different structural materials like concretes [1] or ceramic monoliths [2], is reaching high interest due to their potential environmental applications [3]; biological treatments, adsorption, or catalysis. Carbon-xerogels are very good candidates for this type of coatings. Although, carbon-xerogels are typically microporous materials with low mesopore volumes [4] and the presence of mesopores is specially poor when carbon xerogels are formed in film shape [2]. The mesoporosity is very appreciated for many applications. Recently, several works have been published in which microwave radiation is used during the synthesis process of resorcinolformaldehyde organic gels [4,5]. On the other side, the use of microwave radiation as a heating source for pyrolysis has already been shown as a very rapid method when microwave receptors are present in contact with the sample [6]. As far as we know, the microwave carbonization of resorcinol-formaldehyde organic gels has not been described yet, which might be due to the fact that these organic polymers hardly absorb the microwaves produced by standard ovens, they normally operate at a frequency of 2.45 GHz [7]. This work collects the preparation method of carbon-xerogel films with well-developed mesoporosity which is obtained by microwave assisted carbonization and also by conventional carbonization. Besides, the particular effects that the microwaves produce
n
Corresponding author. Tel.: þ 34 958243316; fax: þ 34 958248526. E-mail address: [email protected] (A.F. Pérez-Cadenas).
http://dx.doi.org/10.1016/j.matlet.2014.11.052 0167-577X/& 2014 Elsevier B.V. All rights reserved.
during the carbonization process on the final chemical characteristic of these materials are shown; both, mesopore volumes and final oxygen contents of the carbon films do not depend on the carbonization degree developed using microwaves.
2. Experimental Three carbon xerogel films with different carbonization degrees, were prepared from an original organic xerogel film (XO). The organic xerogel film was prepared by dissolving resorcinol (R) and formaldehyde (F) in water and using Cs2CO3 as polymerization catalyst (C). The stoichiometric R/F and R/C molar ratios were 0.50 and 2300 respectively. The mixture was stirred to obtain homogeneous solutions that were cast into cylindrical glass molds. The molds were closed in order to avoid an evaporation process during the later curing period which included one day at room temperature, one day at 50 1C and five days at 80 1C. After the curing, the films were thermally dried at 110 1C and atmospheric pressure until constant weight. The thickness of the dry organic films was around 0.1 cm approx. The carbonization process was carried out using an oven microwave model Savoid MSG-20810-S, which was adapted with an entry of gases. Besides, the carbonization of organic xerogel films was performed with 800 W of power, in a stream of argon, using radiation periods of ten cumulative minutes. After each period there was a pause in the radiation, also under an argon stream. For the first period of treatment the organic xerogel films were situated between two sheets of graphite as a microwave receptor in order to induce a greater warming on the surface of the
136
D. Espinosa-Iglesias et al. / Materials Letters 141 (2015) 135–137
organic film. During the second and subsequent periods the top graphite sheet was removed. The cooling of the carbonized material was also performed in a stream of argon. In these conditions, carbon xerogel films with a percentage of weight loss of 13, 17 and 28 wt% were obtained using 3, 4 and 7 treatment periods, respectively. Carbon xerogel films are referred as XMW13, XMW17 and XMW28; the digits indicate the weight loss percentage after carbonization in each case. In order to know properly the particular role of the microwave during the carbonization process, other three carbon films with the same carbonization degree were obtained from the same original organic xerogel film. This was done by conventional carbonization in a thermal oven. In these last cases the carbonization was carried out in an argon flow at 340, 400 and 520 1C during 30 min using a heating rate of 10 1C/min and therefore obtaining the corresponding 13, 17 and 28 wt% of weight loss, respectively. The carbon films obtained by conventional carbonization are referred as XF13, XF17 and XF28; the digits indicate the weight loss after carbonization in each case. The porous characteristics were studied by N2 and CO2 adsorption at 77 and 273 K, respectively, using an equipment Autosorb-1C Quantachrome. The samples were also characterized by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics ESCA 5701 equipped with a MgKα X-ray source, magnetic nuclear resonance (NMR). This was done by using a Bruker Advance 500 instrument equipped with a standard-bore 11.74 T superconducting magnet operating at 125.76 MHz for 13 C, Raman, using a Micro-Raman JASCO NRS-5100 dispersive spectrometer with a 532 nm laser line, and elemental analysis (EA) using a Thermo Scientific – Flash 2000 elemental analyzer. Finally, pH of point of zero charge (PZC) measurements were obtained suspending 250 mg of each sample on 4 mL of degasified and distilled water. Suspensions were stirred and thermostated at 25 1C measuring the pH periodically until the reading was constant. The obtained final pH was considered as the PZC for each sample. 3. Results and discussion BET areas (SBET), micropore volumes (W0) and mean micropore width (L0) obtained by applying DR and Stoeckli equations to the CO2 adsorption isotherm, mesopore volumes (VBJH) and mean mesopore widths (LBJH) obtained by applying the BJH method to the N2 desorption data, elemental analysis (EA) data, surface oxygen content of the films (OXPS) and PZC data, are compiled in Table 1. All the carbonized films are mainly mesoporous materials with mesopore volumes up to 0.6 cm3/g. However, the mean mesopore widths (LBJH) were very similar among the films when microwaves were used. In contrast to this, narrower mesopores are progressively obtained by conventional carbonization. Very interesting is the fact that low microwave carbonization degrees, only 13 wt%, developed high mesopore volumes. In this line, all samples showed type IV N2-isotherms (Fig. 1); micropore volumes are generally very small and only sample XF28 has a remarkable BET area, which seems to be logical in terms of conventional carbonization processes. The pore size distributions (PSD) of Table 1 Textural characterization, chemical composition and PZC data of the films. Film
SBET (m2/ g)
XO 14 XF13 73 XF17 86 XF28 498 XMW13 98 XMW17 111 XMW28 121
W0 (cm3/ g)
L0 VBJH (nm) (cm3/ g)
LBJH CEA (nm) (wt %)
HEA (wt %)
OEA (wt %)
OXPS PZC (wt%)
0.00 0.08 0.10 0.19 0.10 0.09 0.14
– 0.7 0.6 0.6 0.6 0.7 0.5
– 10.2 6.1 2.2 15.7 15.5 15.4
6.3 5.2 5.4 4.7 3.7 3.6 3.4
34.8 30.4 24.5 17.8 35.3 33.3 33.9
– 30.5 23.2 14.7 23.9 24.1 24.3
0.05 0.24 0.39 0.62 0.50 0.58 0.40
58.9 64.4 70.1 77.5 61.0 63.2 62.7
3.6 4.3 5.6 6.8 3.4 3.8 3.3
0
Fig. 1. N2 adsorption and desorption isotherms at 77 K of samples XMW17 and XF17 (up); Pore size distributions of XMW17 and XF17 obtained by the application of QS-DFT to the N2 isotherms (down).
XMW17 and XF17 films obtained by the application of QS-DFT to the N2 isotherms are collected in Fig. 1, showing a very good agreement with the results obtained by the application of BJH and DR methods. It should be mentioned that the different PSD obtained at similar carbonization degree, depends on the type of applied carbonization method. Chemically, all the samples are mainly composed of carbon, although the oxygen contents clearly depended on the carbonization technique. Samples carbonized in a conventional oven contain logical oxygen amounts in relationship with the corresponding carbonization degrees, decreasing the oxygen contents when the carbonization degree increases. However all samples which are carbonized using microwave, contain very high oxygen contents and are very similar among them. In solid materials the microwave energy is supplied at molecular level normally by a dielectric polarization mechanism resulting in a very efficient internal heating [7]. Therefore a possible explanation for the above mentioned high oxygen contents may be found in the heating mechanism, which could lead to the formation of active centers in the materials that, once exposed to an air atmosphere are stabilized by oxidation [5]. In this line, the XMW films also have very similar oxygen contents among them on the non-porous external surface (OXPS) (Table 1), whereas in the case of XF films the oxygen contents determined by XPS were very similar to those determined by EA indicating a homogeneous oxygen distribution throughout the material. On the contrary, XMW films showed the lowest oxygen concentration on the external non-porous surface areas, indicating that the above mentioned re-oxidation process would be more powerful taking place in the micro- and mesopore
D. Espinosa-Iglesias et al. / Materials Letters 141 (2015) 135–137
XMW28 XF28
137
fact that both films had the same weight loss during carbonization. This interpretation is in agreement with the Raman analysis whose spectra are collected in Fig. 3. The biggest relative intensity of the lower wavenumber band (alternating ring stretch vibration in benzene or condensed benzene rings [10]) regarding to the band centered at 1580 cm 1 indicates that the development of the graphenic character of these materials follows the order XF284XMW284XO.
4. Conclusions
290 270 250 230 210 190 170 150 130 110 90
70
50
30
10
-10 -30
ppm (δ) 13
C RMN spectra of the carbon films XMW28 (black) and XF28 (gray).
Raman Intensity (AU)
Fig. 2.
XO
The preparation of carbon xerogel films with well-developed mesoporosity has been described, showing that with low microwave carbonization degrees, only 13 wt%, high mesopore volumes can be obtained. Therefore it has been shown to be a new and shorter process to obtain mesoporous carbon-based films. This microwave assisted carbonization method will be especially useful when carbon-based films have to be prepared on ceramic materials that cannot be treated at high temperatures during long periods of time or throughout all the ceramic material. On the other side, carbonization processes assisted by microwaves yield carbon-xerogels films with a different carbonaceous structure, as well as much higher oxygen contents than those obtained by conventional carbonization at a similar carbonization degree.
Acknowledgments This work has been supported by the project P12-RNM-2892 and the contract CDTI No. IDI-20111005.
XMW28
Appendix A. Supporting information
XF28
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.11.052.
1000
1200 1400 1600 1800 -1 Wavenumber (cm )
2000
Fig. 3. Raman spectra of the organic xerogel film XO, and the carbon xerogel films XMW28 and XF28.
surfaces. Besides, PZC values which depend inversely on the oxygen content of this type of materials follow this tendency in the XF series, whereas in the XMW series similar PZC values were obtained. Finally, 13C RMN and Raman analysis of the samples were carried out. Fig. 2 collects the 13C RMN spectra of samples XMW28 and XF28 where significant differences are shown. RMN signals have been assigned according to the literature [8,9]. XMW28 spectrum shows the following signals: one on 117 ppm associated with carbon atoms placed in the aromatic ring contiguous to those directly bonded to –OH groups; a second signal on 160 ppm corresponding to aromatic carbons directly bonded to –OH groups. On the other side, the XF28 spectrum shows its aromatic carbon signals at 130 and 155 ppm clearly shifted regarding to the corresponding signals in the XMW28 spectrum, being the signal at 130 ppm typical of aromatic carbons in carbon materials [9]. Therefore, XF28 is a material with less phenolic groups and higher graphenic character than XMW28 in spite of the
References [1] Alquier M, Kassim C, Bertron A, Sablayrolles C, Rafrafi Y, Albrecht A, et al. Halomonas desiderata as a bacterial model to predict the possible biological nitrate reduction in concrete cells of nuclear waste disposals. J Environ Manag 2014;132:32–41. [2] Morales-Torres S, Maldonado-Hodar FJ, Perez-Cadenas AF, Carrasco-Marin F. Structural characterization of carbon xerogels: from film to monolith. Microporous Mesoporous Mater 2012;153:24–9. [3] Moreno-Castilla C, Perez-Cadenas AF. Carbon-based honeycomb monoliths for environmental gas-phase applications. Materials 2010;3:1203–27. [4] Gallegos-Suarez E, Perez-Cadenas AF, Maldonado-Hodar FJ, Carrasco-Marin F. On the micro- and mesoporosity of carbon aerogels and xerogels. The role of the drying conditions during the synthesis processes. Chem Eng J 2012;181–182:851–5. [5] Calvo EG, Ferrera-Lorenzo N, Menéndez JA, Arenillas A. Microwave synthesis of micro-mesoporous activated carbon xerogels for high performance supercapacitors. Microporous Mesoporous Mater 2013;168:206–12. [6] Monsef-Mirzai P, Ravindran M, McWhinnie WR, Burchill P. Rapid microwave pyrolysis of coal: methodology and examination of the residual and volatile phases. Fuel 1995;74:20–7. [7] Mingos DM, Baghurst DR. Tilden Lecture. Applications of microwave dielectric heating effects to synthetic problems in chemistry. Chem Soc Rev 1991;20:1–47. [8] NMRShift Data Base; 2014 〈http://nmrshiftdb.nmr.uni-koeln.de/portal〉. [9] Freitas JCC, Bonagamba TJ, Emmerich FG. 13C high-resolution solid-state NMR study of peat carbonization. Energy Fuels 1999;13:53–9. [10] Schwan J, Ulrich S, Batori V, Ehrhardt H, Silva SRP. Raman spectroscopy on amorphous carbon films. J Appl Phys 1996;80:440–7.