Determination of the Surface Oxidation Degree of the Carbonaceous

Subscriber access provided by University of Newcastle, Australia

Article

Determination of the surface oxidation degree of the carbonaceous materials by quantitative TG-MS analysis Gabriela Hotová, and Vaclav Slovak Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

Determination of the surface oxidation degree of the carbonaceous materials by quantitative TG-MS analysis Gabriela Hotová*, Václav Slovák University of Ostrava, Faculty of Science, Institute of Environmental Technologies, Department of Chemistry, 30. dubna 22, 701 03 Ostrava, Czech republic ABSTRACT: The developed quantitative TG-MS analysis was used for the determination of the surface oxidation degree of activated carbon cryogels. The surface chemistry of a prepared carbon cryogels pyrolysed at 400 °C and 500 °C was modified using H3PO4, Fenton-like reaction, (NH4)2S2O8 with H2SO4 and HNO3 with H2O2 into a different surface oxidation degree. The influence of activation method and the amount of oxygen surface groups were characterized by elemental analysis, immersion calorimetry, water vapor adsorption and Boehm titration. The obtained results from these methods were compared with the amount of surface oxygen determined by TG-MS. It was found out, that with the more intensive oxidation method (Fenton < (NH4)2S2O8 with H2SO4 < HNO3 with H2O2) the concentration of oxygen-containing surface groups increases, which lead to considerably higher parameters of immersion heat, amount of adsorbed water and acidity of the sample surface. The H 3PO4 treatment of carbon cryogels causes no significant changes in the surface chemistry. The results obtained from the TG-MS analysis imply the good agreement with the results obtained from other used methods. It was proved, that the quantitative TG-MS analysis could be a useful tool for the characterization of the surface of the carbonaceous materials.

Introduction Numerous techniques can be used for the characterization of oxygen surface complexes on carbonaceous materials. However, the most accurate description of the surface groups can be obtained by a combination of different techniques. Some of them provide suitable information about the composition and concentration of surface functional groups such as spectroscopic methods (FTIR or DRIFT, XPS), thermal and calorimetric techniques (TPD, immersion calorimetry), Boehm and potentiometric titration, zeta potential, water vapor adsorption or electrochemical methods [1, 2]. Unfortunately, some of these methods are limited only to a few oxygen functional groups and some of them did not provide quantitative information about the surface functionalities. For example, the Boehm´s method provides qualitative and quantitative information about the oxygen functional surface groups, but it is limited only on some kind of surface groups (carboxyl, phenolic and lactonic) and for that reason, this method is encumbered by a certain mistake. On the other hand, the TPD method is able to detect the more oxygen surface groups then the Boehm´s method. In contrast to Boehm´s method, the TPD method is not affected by the heteroatoms which do not contain the oxygen [3, 4]. The TPD is now the most useful method for the quantitative determination of the oxygen surface groups especially for the porous carbonaceous materials. This method is based on the decomposition of the oxygen surface groups to CO and CO2 gases during the heating of the sample. On the basis of released gases (CO or CO2) and the decomposition temperature the type of the surface groups can be determined and quantified [5]. The results obtained by TPD method is also in good correlation with the qualitative results from DRIFT method or potentiometric titration. Generally, the IR method is not so suitable for the carbonaceous

materials, because of their black color, which is too un-transparent for the analysis. This problem can be resolved by the use of the reflexive techniques (such as DRIFT method), but still, the quantitative analysis is too complicated because of the continuous background absorption and the presence of different functional groups [1, 4]. Among the other disadvantages of some of these methods belong the long-time analysis and the large sample consumption. Thus, the aim of this study was to test the new way for the characterization of the surface (oxygen), which will be comparable to other available techniques. The determination of the surface oxidation degree of carbonaceous materials is in this work based on quantitative TG-MS analysis, which was recently developed [6]. The framework of this study is also to show that it is possible to obtain similar information by the common equipment (TG-MS) as from the TPD-MS. The main advantage of the TG-MS method is that the actual calibration measurement is not necessary due to measuring of the sample mass loss. Theoretical Basis The recently published quantitative TG-MS method (described in more detail in [6]) is based on two assumptions. The first supposes that the amount of evolved gases during the thermal decomposition of the sample is proportional to the peak areas of fragment ions via constant of proportionality. The second assumes that the constant of proportionality (K) is the same for all evolved gases. Both assumptions were proved in the recent paper [6]. Based on these expectations, the calibration measurements is not necessary for each substance due to the confirmation that K is almost constant. The actual calibration measure-

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ment is not even necessary in the case that a mass loss measurable on TG curve corresponds to the MS signal of know gaseous product. The present quantitative TG-MS analysis can be described by following equation (1): 𝑛𝑥 = 𝐾 ×

𝐴𝑚/𝑧 𝑘𝑖

(1)

where nx is the molar amount of the gaseous product evolved during the thermal decomposition of the sample, K characterizes the constant of proportionality, which is independent on the type of ion, Am/z describes the area of the peak on MS signal and ki refers to some constant, which is different for various ions, but its value can be obtained from the mass spectra database. The equation (1) is valid only for the peaks on MS signals corresponding to simple ion (e.g. signal m/z = 18 for water). If the peak on MS signal is caused by more substances (e.g. contribution of CO and CO2 to signal m/z = 28), it is necessary to include the contributions of all substances into the final equation, which becomes more complicated. If the evolution of the certain amounts of (xi….xn) gases during the decomposition of the sample causes the mass loss on TG curve (Δm), the following Eq. (2) is valid. ∆𝑚 = ∑(𝑛𝑥𝑖 × 𝑀𝑥𝑖 )

(2)

where nxi is the molar amount of the evolved xi gases and Mxi corresponds to the molar mass of evolved xi gases. Based on the solution of the mentioned general Eqs. (1) and (2) it is possible to quantify the value of the proportionality constant (K) and determine the molar amounts of the released gaseous products (nx).

Page 2 of 9

in a freeze dryer (Labconco) at 52 Pa for 24 h. Dry organic cryogels were pyrolysed at 400 °C (RF-P400) or 500 °C (RF-P500) in the nitrogen flow. The furnace was heated to 100 °C and maintained at this temperature for 1h. Then it was heated to selected pyrolysis temperature (400 or 500 °C) and kept at this temperature for 3 h. The heating rate was 10 °C min −1. Relatively low pyrolysis temperature was chosen to preserve reactivity of carbonized material for following oxidation enabling higher degree of the surface oxidation. Activation of carbon cryogels Treatment with phosphoric acid The prepared carbon cryogels (RF-P400 and RF-P500) were activated with H3PO4 according to [11]. The dried samples were mixed (1 g of RF per 50 ml of solution) with a 50% solution of H3PO4 (prepared from 85%, p.a., Mach chemikálie) and left overnight at room temperature. After 24 hours, the sample was washed with distilled water to neutral pH and dried in the oven at 60 °C. The samples will be referred in the text as RF-P400H3PO4 and RF-P500-H3PO4. Oxidation using a Fenton-like reaction The Fenton-like oxidation of the carbon cryogels used in this work was adjusted and based on the Fenton-like reactions from the literature [12]. The oxidation with a Fenton´s reagent is based on the reaction between ferrous ions and hydrogen peroxide. The sample was mixed with the 1 mmol dm−3 solution of the (NH4)2Fe(SO4)2·6H2O (p.a., Lachema), 0.5 g of RF per 50 ml of the solution. The reaction mixture was then shaken at room temperature for 24 h. After 24 hours, the suspension was filtered and the sample was dried in the oven at 60 °C. Afterwards, the dry sample was treated with a 50 ml of the concentrated H2O2 (30%, Penta) at room temperature for 24 h. The obtained product was subjected to washing with distilled water to neutral pH and dried in the oven at 60 °C. The prepared samples were marked as RF-P400-Fenton and RF-P500-Fenton, respectively.

Experimental Section Preparation of carbon cryogels Carbon cryogels were prepared according to the method proposed by Pekala and Tamon [7−10] with some modification. Synthesis of organic resorcinol-formaldehyde gels (RF) was based on polycondensation of the resorcinol (R, 98%, Mach chemikálie) and formaldehyde (F, 36% methanol stabilized, p.a., Mach chemikálie) in aqueous solution with sodium carbonate (C, 99.8%, Lachema) as a catalyst. The molar ratio of R:F in an 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%. The reaction mixture was mixed with intensive stirring, poured into glass vials (inner diameter = 1cm), which were then sealed by rubber stopper. RF mixtures gelled 2 days at ~25 °C (ambient temperature), 1 day at 55 °C and 4 days at 90 °C. After gelation, the wet organic gels were immersed into a t-butanol (p.a., Mach chemikálie) at 60 °C for 24 h. This treatment was repeated three times in order to displace the water contained in the RF gels with t-butanol. The samples of RF organic wet gels were then pre-frozen in the refrigerator at -25 °C and dried under vacuum

Oxidation with ammonium persulfate and sulphuric acid The treatment of RF carbon cryogels with a (NH4)2S2O8 (p.a., Lachema) and H2SO4 (96%, p.a., Mach chemikálie) was derived from the literature [13] and slightly modified. The samples of the carbon cryogels were treated with a saturated solution of (NH4)2S2O8 in 50% H2SO4 (1 g of RF per 50 ml of the solution) and left overnight at room temperature. After 24 hours, the oxidized samples were washed with distilled water until the pH of the filtrate was neutral and dried in the oven at 60 °C. The obtained samples were marked as RF-P400-(NH4)2S2O8+H2SO4 and RF-P500-(NH4)2S2O8+H2SO4, respectively. Oxidation with nitric acid and hydrogen peroxide The treatment with the HNO3 (65%, p.a., Mach chemikálie) and H2O2 was performed using a reflux apparatus. Thus, the sample of carbon cryogel was introduced together with the 50% solution of HNO3 into a 250 ml two necks round bottom flask (3 g of RF per 100 ml of the solution). The condenser and the separatory funnel filled with the 15% solution of H2O2 (50 ml of the solution per 3 g of the RF) were connected to the round bottom


Page 3 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


flask. The round bottom flask was heated to boiling temperature and then the H2O2 was slowly added to the solution. After the addition of the H2O2, the reaction mixture was boiled for a short time, cooled down and filtered through the fritted glass. The oxidized sample was washed repeatedly with distilled water to neutral pH and dried in the oven at 60 °C. The prepared samples were marked as RF-P400-HNO3+H2O2 and RF-P500HNO3+H2O2. Characterization of the prepared carbon cryogels The content of oxygen (Oa) in the prepared materials was directly determined. The analysis was based on pyrolysis of the sample at high temperature, separation of products by gas chromatography and their quantification by thermal conductivity detector (and recalculation to content of oxygen) by using of Flash EA 1112 (Thermo Finnigan). The samples were analyzed as prepared (according to previous section), without additional drying procedure to prevent the thermal decomposition of the surface complexes. The enthalpies of immersion, ΔiH (H2O), of the samples into the water were determined using a Tian-Calvet calorimeter C80 (Setaram). Measurements were performed at a temperature of 30 °C and atmospheric pressure. Prior to the experiments, the samples were dried at 60 °C under vacuum for almost 2 h. The oxygen-containing surface groups were determined according to the method proposed by Boehm. The total acidity of the carbon cryogels was estimated by mixing about 0.25 g of each sample with 25 ml (50 ml in the case of RF-P400(NH4)2S2O8+H2SO4 and RF-P400-HNO3+H2O2) of 0.05 mol dm−3 NaOH standardized solution. The flasks were sealed and shaken for 24 h and then 5 ml of each filtrate was pipetted and the excess of the base was titrated with 0.05 mol dm−3 HCl standardized solution. The number of acidic sites corresponds to carboxyl, phenolic and lactonic groups. Adsorption of water vapor was performed at a low relative pressure (p/p0 ≈ 0.3). The measurements were conducted with about 0.25 g of the sample in a desiccator filled with a saturated solution of the CaCl2 (p.a., Penta), which relative humidity content is equal to 32.7 % (measured by hygrometer, 608-H2, Testo). The weight of the sample was recorded every 24 h. The samples were dried at 60 °C under vacuum for 2 h before the measurement. TG-MS analysis Thermoanalytical experiments (TG-MS) were performed using SetsysEvolution (Setaram) with a quadrupole mass spectrometer QMG 700 (Pffeifer) directly coupled by a SuperSonic system (Setaram). The measurements for determination of the surface oxidation degree were carried out with three different amounts of the prepared sample ranging from 5 to 20 mg in the crucible from α-Al2O3. TG-MS curves were recorded under an argon atmosphere (flow rate 20 ml min−1) from 15 °C to 1000 °C with the heating rate 10 K min−1. The MS signals corresponding to H2O (m/z = 18), CO (m/z = 28), O2 (m/z = 32) and

CO2 (m/z = 44) were monitored in MID mode (multiple ion detection). The monitoring of the released gaseous products was performed only up to the used pyrolysis temperature, because above this temperature the whole pyrolysis process continuous and comparison of the samples is not possible. Results and Discussion The effect of oxidation/activation on surface properties The changes in the oxygen content of the carbon cryogels after the oxidation are shown in Table 1. From the Table 1 it is apparent that the samples get different oxidation degree after the oxidation/activation treatment. The oxidation degree depends on activation/oxidation procedure in order: as prepared samples < H3PO4 < Fenton < (NH4)2S2O8+H2SO4 < HNO3+H2O2. The dependence is more evident for materials pyrolysed at lower temperature 400 °C. The differences between oxidation degree of samples pyrolysed at 500 °C is less distinct, which can be explained by lower reactivity after pyrolysis (higher degree of carbonization). The values of the immersion heat (ΔiH (H2O)) listed in Table 1 correspond to the number of the primary adsorption centres. The enthalpy of immersion of carbonaceous materials in water has been correlated with their oxygen content by different authors. Barton and Harrison showed a linear relationship between the immersion heat and the total oxygen desorbed as CO and CO2. Bradley found out that the immersion heat linearly increases with the surface oxygen content [14]. Based on that, it can be assumed that the water molecules interact only with the oxygen-containing surface groups and their higher adsorbed amount causes the higher heat of immersion. It was proved that the higher oxidation degree lead to the higher values of the immersion heat (Table 1) and thus the higher amount of the oxygen functional surface groups can be expected. The content of oxygen functional groups with various acidity strength (total acidity: carboxyl, phenolic and lactonic groups) are presented in Table 1. The results agree with those obtained by elemental analysis and immersion calorimetry (Table 1). Samples that have higher amounts of oxygen containing surface groups have the surface with the highest acidity. In the case of the RF-P400-HNO3+H2O2 sample, it was expected the highest surface acidity, but it was comparable to RF-P400(NH4)2S2O8+H2SO4 sample. It is probably caused by the fact that the Boehm titration is only suitable for determination of certain oxygen-containing surface groups (carboxyl, phenolic and lactonic groups) and not for any other groups like ethers or peroxo groups. The activation of the RF-P500 samples by Fenton-like reaction, (NH4)2S2O8 and H2SO4, and HNO3 with H2O2 causes the similar total acidity of the surface of the samples.



Page 4 of 9

Table 1. Characteristics of the RF carbon cryogels and their activated derivatives.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Oa

ΔiH (H2O)

sample

g−1]

total acidity

adsorbed H2O

[%]

[J

RF-P400

12.14

31.5

0.7

2.8

RF-P400-H3PO4

12.70

32.3

1.8

3.6

RF-P400-Fenton

18.11

49.0

2.5

4.3

RF-P400-(NH4)2S2O8+H2SO4

23.37

75.3

6.8

7.2

RF-P400-HNO3+H2O2

43.31

80.5

6.9

7.4

RF-P500

4.48

21.5

0.5

2.0

RF-P500-H3PO4

6.56

21.5

1.6

3.0

RF-P500-Fenton

13.12

54

2.2

5.8


13.81

52

3.1

5.7

RF-P500-HNO3+H2O2

18.22

50

3.2

5.9

The surface groups on carbon cryogels are considered as the primary adsorption sites. It is assumed that at low relative pressures (p/p0 ≈ 0.3) the adsorbed water molecules are related to the amount of surface oxygen groups [15, 16]. The results (for relative pressure p/p0 ≈ 0.3) presented in Table 1 shows that with the higher concentration of the oxygen-containing functional groups on the surface of carbonaceous sample the amount of adsorbed water molecules increases. However, it should be mentioned that for the RF-P400-HNO3+H2O2 sample the adsorbed amount of water on the surface groups of the sample was a little bit smaller than expected and it is almost the same as for the RF-P400-(NH4)2S2O8+H2SO4 sample. It should be caused by the pre-treatment of the sample, concretely some surface groups could decomposed during the vacuum drying. From the obtained results, it is evident, that the oxidation/activation treatment leads to obtaining the samples oxidized into a different oxidation degree and that the oxygen-containing functional groups are created on their surface. Surface oxidation degree determined by TG-MS analysis Figure 1 summarizes the thermal behaviour of the original carbon cryogels (RF-P400 and RF-P500) during the heating up

[mmol

g−1]

[%]

to 1000 °C. The obtained results show that heating of the nonactivated carbonaceous samples up to the used pyrolysis temperature (400 °C or 500 °C) leads only to the evolution of the H2O (m/z = 18) and small amount of the CO2 (m/z = 44). The signal corresponding to CO (m/z = 28) shows very low changes in the intensity and simultaneously these changes are related to CO2 evolution (which partially ionizes to the CO+ ion with the m/z = 28). However, above the temperature of pyrolysis (400 °C or 500 °C) the whole carbonization process still continuous, the studied materials are decomposed and evolve a large amount of gases (H2O, CO and CO2). On the basis of these measurements, it is obvious that for the quantitative TG-MS analysis of decomposition of oxidized samples it will be easier to analyse only the signals (data) up to the used pyrolysis temperature. The main reason is mixing of the carbonization process together with the decomposition of more stable oxygen containing groups created during activation/oxidation (above the pyrolysis temperature). Therefore the following analysis is based on evaluation of experimental data up to appropriate pyrolysis temperature.

(a)

(b)

Figure 1. TG-MS curves of the thermal decomposition of the original RF-P400 (a) and RF-P500 (b) carbon cryogels.


Page 5 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

(a)

Figure 2. TG-MS curves of the thermal decomposition of the RF-P400 (a) and RF-P500 (b) based carbon cryogels.

MS signals corresponding to H2O (m/z = 18), CO (m/z = 28), O2 (m/z = 32) and CO2 (m/z = 44) were monitored during the heating of all studied samples in the inert atmosphere. In the case of the samples oxidized by HNO3 and H2O2 (RF-P400HNO3+H2O2 and RF-P500-HNO3+H2O2). MS signals corresponding to the nitrogen compounds (e.g. NO, NO 2, N2O etc.) were also detected. Form the obtained results, it was found, that only the signals corresponding to H2O (m/z = 18) and CO2 (m/z = 44) evolved in the studied temperature region (up to the used pyrolysis temperature). The results of TG-MS study of the thermal decomposition of the activated carbon cryogels are shown in Fig. 2. The different course of the TG curves is caused by the decomposition of groups originated during oxidation process. The total mass loss caused by the thermal decomposition of prepared carbon cryogels increases with higher content of the oxygen-containing surface groups.

Based on the thermal decomposition measurements of oxygencontaining surface groups, the surface oxidation degree of prepared carbonaceous samples was determined using the quantitative TG-MS analysis [6]. The amount of gases evolved during the heating of the samples was experimentally determined using the total mass loss (Δm) from TG curve and Eqs. (1) and (2). The H2O released from the sample during the thermal decomposition can be attributed to the moisture and water formed during the decomposition of the oxygenated functional groups. The amount of water corresponding to the moisture of the sample is small (up to 1%, because the samples were dried at 60 °C before the measurement) and it is very difficult to separate it from the decomposition itself. So for that reason, the total amount of the released H 2O (together with the moisture of the sample) was used for the determination of the surface oxidation degree. The release of gaseous product from the sample during the heating is accompanied with some mass loss according to following equation (Eq. 3).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

∆𝑚 = 𝑚𝐻2 𝑂 + 𝑚𝐶𝑂2

(3)

The above mentioned equation (3) can be rewritten as follows (Eq. 4). ∆𝑚 = 𝑛𝐻2 𝑂 𝑀𝐻2𝑂 + 𝑛𝐶𝑂2 𝑀𝐶𝑂2

(4)

The molar amount of evolved H2O and CO2 can be expressed by the relations (5) and (6), which can be transformed from the original Eq. (1). Derivation process of the mentioned equations (5) and (6) is in more detail described elsewhere [6]. 𝐴18

𝑛𝐻2 𝑂 = 𝐾

0.8135

𝑛𝐶𝑂2 = 𝐾

0.8649

(5)

𝐴44

Page 6 of 9

Combination of the Eqs. (4), (5) and (6) together leads to the Eq. (7). 𝐾=

∆𝑚

(7)

𝐴18 𝐴 𝑀 + 44 𝑀 0.8135 𝐻2 𝑂 0.8649 𝐶𝑂2

Solution of the Eq. (7) leads to the quantification of the proportionality constant (K). Thus, with the use of the calculated value of K, the molar amount of evolved H2O and CO2 can be determined by assuming the Eqs. (5) and (6). The amount of the surface oxygen can be determined from the amount of evolved H2O and CO2. The surface oxidation degree was expressed as the amount of evolved oxygen (mass %) from the oxidized carbonaceous materials and is summarized for studied carbon cryogels in Table 2.

(6)

Table 2. The surface oxidation degree of RF carbon cryogels (expressed as the amount of oxygen evolved from the sample) determined by quantitative TG-MS analysis (3 repetitions).

sample

the amount of evolved oxygen [%] I.

II.

III.

ø

RF-P400

3.15

4.60

2.42

3.39

RF-P400-H3PO4

3.96

4.03

3.66

3.88

RF-P400-Fenton

12.06

12.44

11.01

11.84


19.09

19.57

20.37

19.68

RF-P400-HNO3+H2O2

26.46

30.94

28.55

28.65

RF-P500

2.90

2.69

2.76

2.78

RF-P500-H3PO4

5.20

4.81

4.39

4.80

RF-P500-Fenton

16.60

16.66

15.91

16.39


16.40

15.87

15.46

15.91

RF-P500-HNO3+H2O2

17.30

19.50

19.19

18.66

The comparison of the determined surface oxidation degree by TG-MS with other methods (elemental analysis, immersion calorimetry, Boehm titration and adsorption of water vapour) is shown in Fig. 3 (for RF-P400 carbon cryogels) and Fig. 4 (for RF-P500 carbon cryogels). For all the samples the good agreement between the studied parameters was found, except of the samples oxidized by (NH4)2S2O8+H2SO4, where the quite dissimilarity occur. The higher established acidity of the mentioned samples is probably caused by the technical problems

during the Boehm titration experiment. The separation of the sample from the NaOH solution was difficult, the filtrate had darkly brown colour (the sample maybe partially dissolves) and the determination of the equivalence point was quite difficult. Despite this discrepancy, the other obtained results proved that the quantitative TG-MS method is suitable for the determination of the surface oxidation degree.


Page 7 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

(a)

Figure 3. Comparison of determined surface oxidation degree by TG-MS with the immersion calorimetry, water vapour adsorption (a) and elemental analysis, Boehm titration (b) for RF-P400 carbon cryogels and their activated derivatives. Full circles display immersion calorimetry and elemental analysis, while empty circles display water vapour adsorption and Boehm titration.

(b)

(a)

Figure 4. Comparison of determined surface oxidation degree by TG-MS with the immersion calorimetry, water vapour adsorption (a) and elemental analysis, Boehm titration (b) for RF-P500 carbon cryogels and their activated derivatives. Full circles display immersion calorimetry and elemental analysis, while empty circles display water vapour adsorption and Boehm titration.

Conclusions It has been shown that the data obtained from the quantitative TG-MS analysis follow the same trend and were in good correlation with the ones obtained by the immersion calorimetry, Boehm titration and water vapour adsorption at p/p 0≈0.3. The main advantages of the quantitative TG-MS method in comparison with others characterization methods are the short analysis time and the small sample consumption (mg). From the results of the present work, it can be concluded that the developed quantitative TG-MS method is a useful tool for the characterization of the carbonaceous surface, e.g. oxidation process, the amount of the oxygen presented on the surface of the sample and determination of the surface oxidation degree.

AUTHOR INFORMATION Corresponding Author * Phone: +420597092188, E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. The authors declare no competing financial interests.

ACKNOWLEDGMENT 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 “TEWEP”. The paper was also supported by the students grant no. SGS01/PŘF/16 (University of Ostrava).

REFERENCES [1] Bandosz, T. J.; Ania, C. O. Activated carbon surfaces in environmental remediation; Elsevier, Amsterdam, 2006, 159−229. [2] López-Ramón, M. V.; Stoeckli, F.; Moreno-Castilla, C.; CarrascoMarin, F. Carbon 1999, 37, 1215−1221. [3] Goertzen, S. L.; Thériault, K. D.; Oickle, A. M.; Tarasuk, A. C.; Andreas, H. A. Carbon 2010, 48, 1252−1261. [4] Burg, P.; Cagniant, D. Chemistry and physics of carbon; CRC Press, 2007, 129−175. [5] Figueiredo J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orfão, J. J. M. Carbon 1999, 37, 1379−1389. [6] Hotová, G.; Slovák, V. Thermochim. Acta 2016, 632, 23−28. [7] Hamano, Y.; Tsujimura, S.; Shirai, O.; Kano, K. Mater. Lett. 2014, 128, 191−194. [8] Fang, B.; Binder, L. J. Power Sources 2006, 163, 616−622. [9] Tamon, H.; Ishizaka, H.; Yamamoto, T.; Suzuki, T. Carbon 2000, 38, 1099−1105.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[10]Babić, B.; Kaluderović, B.; Vračar, Lj.; Krstajić, N. Carbon 2004, 42, 2617−2624. [11]Budinova, T.; Ekinci, E.; Yardim, F.; Grimm, A.; Bjornbom, E.; Minkova, V.; Goranova, M. Fuel Process. Technol. 2006, 87, 899−905. [12]Ramirez, J. H.; Maldonado-Hódar, F. J.; Pérez-Cadenas, A. F.; Moreno-Castilla, C.; Costa, C. A.; Madeira, L. M. Appl. Catal. BEnviron. 2007, 75, 312−323.

Page 8 of 9

[13]Ania, C. O.; Parra, J. B.; Pis, J. J. Fuel Process. Technol. 2002, 79, 265−271. [14]López-Ramón, M. V.; Stoeckli, F.; Moreno-Castilla, C.; CarrascoMarín, F. Carbon 2000, 38, 825−829. [15]Bandosz, T. J.; Jagiello, J.; Schwarz, J. A. Langmuir 1996, 12, 6480−6486. [l6] Do, D. D.; Do, H. D. Carbon 2000, 38, 767−773.


Page 9 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Authors are required to submit a graphic entry for the Table of Contents (TOC) that, in conjunction with the manuscript title, should give the reader a representative idea of one of the following: A key structure, reaction, equation, concept, or theorem, etc., that is discussed in the manuscript. Consult the journal’s Instructions for Authors for TOC graphic specifications.


9

Determination of the Surface Oxidation Degree of the Carbonaceous

Recommend Documents