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ScienceDirect Materials Today: Proceedings 3S (2016) S252 – S257

Proceedings of the International Conference on Diamond and Carbon Materials

Chemical and electrochemical studies of carbon black surface by treatment with ozone and nitrogen oxide Marius Ciobanua, Ana-Maria Lepadatua, Simona Asafteib* a

b

Institute of Chemistry of New Materials, University of Osnabrueck, Barbarastr. 7, 49076 Osnabrueck, Germany Institute of Dual Studies, Faculty Management, Culture and Technology, University of Applied Sciences Osnabrueck, Kaiserstr. 10c, 49809 Lingen (Ems), Germany

Abstract In this study it was investigated the surface chemistry of furnace carbon black (CB) particles after exposure to air-oxidative treatments, with ozone and nitric oxide respectively. The functional groups on the surface were determined by several complementary methods such as water up-take experiments in vapour saturated atmosphere, Boehm titrations and electrochemical methods. The treated carbon blacks were compared with two model references: one having high polarity and high specific surface area and another one having low-polar surface and graphitized structure. It was found that air-oxidative post-treatments induce formation of phenols, lactones and quinones and implicitly an increment in the electric capacitance. Moreover, the increment of the surface oxides is more pronounced after treatment with ozone, compared with the nitric oxide one. © 2014 The Authors. Published by Elsevier Ltd.

© 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the chairs of the International Conference on Diamond and Carbon Materials 2014. This is an (http://creativecommons.org/licenses/by-nc-nd/3.0/). underunder the CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/3.0/). open access Selection and article Peer-review responsibility of license the chairs of the International Conference on Diamond and Carbon Materials 2014. Keywords: carbon black; surface oxidation; oxides; functional groups; Boehm titration; electrochemistry1.

1. Introduction Carbon black (CB) is the generic name for carbon particles which are produced by incomplete combustion of gaseous or liquid hydrocarbons [1]. CB is commercially available in a huge number of forms, varying by particle

* Simona Asaftei. Tel.: +4959180094296. E-mail address: [email protected]

2214-7853 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Diamond and Carbon Materials 2014. doi:10.1016/j.matpr.2016.02.042

Marius Ciobanu et al. / Materials Today: Proceedings 3S (2016) S252 – S257

size, aggregate structure, porosity and surface chemistry. They consist of large aggregates which have significant effect upon bulk and surface properties [2]. CB is widely used in a variety of fields, such as electronics [3], electrochemistry [4], fuel cell devices [5], catalysis [6], colouring agent for ink and toner [1, 4] as well as filler mixed in plastics, elastomers, films, adhesives and paints [7]. CB is most commonly used as reinforcing agent in rubber products, especially tires [8]. Addition of CB increases hardness, modulus, tensile and tear strength of the unfilled rubber [9, 10]. A good dispersion and adhesion between the filler and the rubber matrix is required for optimum mechanical properties. The filler surface energy is also of great importance: it should be greater than, or equal to the surface energy of the polymer. The untreated CB exhibits a small surface energy and is apparently unable to form strong adhesive bonds with the polymer [11]. However, this issue can be achieved by surface modification of carbon black particles employing air-oxidative post-treatments. Already in the 60’s, Heckmann et al. [12] have been reported the oxidation of CB particles by hot air. Several oxidation treatments on carbon black have been described using oxidizing agents such as acid medium (HNO 3) [13], ozone [14, 15], nitric oxide [16], ammonium peroxydisulfate [17], hydrogen peroxide [18], plasma [19] and so on. Although all these treatments increase successfully the level of oxygen on the surface, they produce changes on the surface structure and morphology of CB. Oxidative post-treatments of CB reduce the time needed for the dispersing into the rubber matrix. Additionally, such treatments improve dispersion stability and wettability of carbon surfaces by promoting the formation of hydrophilic groups. Nevertheless, the surface modification of CB by oxidation process changes the surface energy, which has been interpreted by acid-base interactions [20, 21]. The present work presents the effect of two different oxidative post-treatments (i.e. with ozone and nitric oxide respectively) applied to furnace carbon black particles. The study includes detailed surface characterization of the untreated particles (CB-Basic), treated particles by ozone (CB-O3) and nitrogen oxide (CB-NO2) and two reference models: CB particles with low-polar and graphitized surface (CB-G) and particles with high polarity surface (CBNS). Determination of the water content, water uptake in the vapour saturated atmosphere, pH, acid-base properties by Boehm titration and electrochemical behaviour were employed to assess the changes of surface property of oxidized CB. 2. Results and Discussion Preliminary, the water content of the carbon black samples resulted from the adsorption of water was analysed in the storage conditions. The content (W1) of the adsorbed water for each sample is depicted in Table 1. The samples CB-G and CB-Basic have the lowest content of water which suggests the lowest surface polarity. Table 1. Water content and pH values of the carbon black samples Samples Characteristics W1 (wt %)

a)

W2 (wt %) b) pH

c)

CB-Basic

CB-G

CB-NS

CB-O3

CB-NO2

1

0.5

12.1

2.2

1.1

2.5

2.3

51.2

14

3.3

8.5

8.6

2.6

3.4

4.2

a)

W1 is the content of adsorbed water in the CB particles from the storage conditions; W2 is the content of adsorbed water after 500 hours of exposure in vapour saturated atmosphere; c) pH values of the aqueous slurries. b)

The content of water of CB samples exposed to the oxidative treatments is increasing. The water content of CBO3 is two times higher compared with the CB-NO2 sample. The higher water content in the oxidized samples is attributed to the physical adsorbed water molecules by hydrogen bonding to the oxygen groups on the surface of the carboceneous materials [22, 23]. The highest water content found in CB-NS sample is attributed to the cumulative effect of the high specific surface and the oxide functional groups. For determination of water up-take in vapour saturated atmosphere, pre-weighted homogeneous carbon black samples CB-G, CB-Basic, CB-NO2, CB-O3 and CB-NS respectively, with weight in the range 0.5 to 3 g, were exposed at 21°C in a desiccator chamber in which the desiccant agent was replace by water. After certain periods of

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time, between 96 and 500 hours, the samples were intermediary weighted and forward exposed to the vapour saturated atmosphere for determination of the adsorption kinetic. The water content in CB sample from the adsorption experiment in the vapour saturated atmosphere (W2 in Table 1) follows the same trend as the water content resulted from the storage conditions (W1). The sample CB-G, CB-Basic and CB-NO2 respectively reach the saturation above 300 hours (Figure 1). Notably, the samples CB-O3 and CB-NS have the highest content of adsorbed water but do not reach the saturation even after 500 hours of exposure. This is due to the higher porosity of both samples. It is well known that a high vapour pressure determines the clustering of water molecules followed by capillary condensation in the micropores [24].

Fig. 1. Kinetic of water adsorption in vapor saturated atmosphere of carbon black particles

The pH of the particles CB-NO2, CB-O3 and CB-NS is located in acidic range 2.6-4.2 which suggests the existence of functional groups such as carboxyl, phenol, lactone or anhydride [14] [25-27]. In contrast, the pH of CB-Basic or CB-G samples is located in the basic range 8.5-8.6. In this case the basic sites are mainly associated with the basal planes of the graphene layers [27]. The corresponding pH values are presented in Table 1. The functional groups from the surface of carbon materials having acidic or basic properties are conveniently determined by titration methods. This type of titration was first developed by Boehm, who proposed to neutralize the surface functionalities based on their acid strength [25, 26]. The amount of acidic sites was calculated by assuming that NaOH neutralizes carboxylic, phenolic, and lactonic groups; Na2CO3 neutralizes carboxylic and lactonic groups, while NaHCO 3 neutralizes only carboxylic groups. The amount of basic sites was calculated from the amount of HCl that reacts with the carbon black. The results of the titration of acidic and basic sites corresponding to 1 g of dry carbon black sample are shown in the Table 2. Basic sites show only CB-Basic and CB-G samples. Phenol groups and lactone are present in the treated sample, i.e. CBNO2, CB-O3 and CB-NS. The presence of the carboxylic groups is observed only on the CB-NS. Table 2. Acidic and basic sites of the carbon black samples determined by Boehm titration Samples

Acidic sites (mmol/g)

Basic sites (mmol/g)

Phenol

Lactone

Carboxyl

CB-Basic

0.02

0.00

0.00

0.00

CB-G

0.08

0.00

0.00

0.00

CB-NO2

0.00

0.42

0.08

0.00

CB-O3

0.00

0.60

0.09

0.00

CB-NS

0.00

1.33

0.95

0.38

Marius Ciobanu et al. / Materials Today: Proceedings 3S (2016) S252 – S257

The electrochemical measurements of the carbon black particles immobilized on carbon paste electrode (CPE) were recorded in aqueous acidic electrolyte (H 2SO4 0.1M). For the samples CB-Basic, CB-G and CB-NO2 the response of the capacitive current is smaller in magnitude compared with the samples CB-O3 and CB-NS (Fig. 2a and 2b). This exponential difference in magnitude of the non-faradaic current is explained by the different surface density of the functional groups on the carbon black surface. It is known that the presence of oxide functional groups on the surface contributes to the capacitive current magnitude of the CB particles [28, 29]. In other words, the samples with high density of oxide functional groups on the surface (CB-O3 and CB-NS) exhibit the highest capacitive response. These results are in good agreement with the results obtained from Boehm titration. Noteworthy, the sample CB-NO2 (Fig. 2b) exhibits higher capacitive properties than the untreated sample CB-Basic and CB-G respectively, but does not reach the same exponential magnitude shown by the samples CB-O3 and CBNS.

Fig. 2. CV’s of the carbon black samples: CB-Basic (1), CB-G (2), CB-NO2 (3), CB-O3 (4) and CB-NS (5): 10-4A current range (a) and 10-6A (b) current range

The reversible redox process in the range +0.1 to +0.5 V is attributed to the reduction of the quinone functional groups present on the surface of carbon black samples [30]. The quinone is expected to undergo two-electron twoprotonation reaction steps in the acidic media [31] as shown in Scheme 1.

Scheme 1. Two-electron two-proton reduction of quinone in acidic aqueous media

The samples CB-G and CB-Basic do not show any faradaic current response in the range of the quinone reduction (Fig. 2b). The interference of the higher capacitive current with the faradaic current resulted from the reduction of quinine, makes difficult the qualitative investigation of the electroactive couple quinone/hydroquinone. Nevertheless, cyclic voltammetry offers important information concerning the reversibility and the mechanism of the quinone/hydroquinone couple on the carbon black surface. The potential pulses with certain amplitude and step potential significantly diminish the capacitive currents in the differential pulse voltammetry (DPV). As a consequence, the resulted voltammogram predominantly contains the current response resulted from faradaic processes. This makes the method superior to the cyclic voltammetry (CV) technique regarding the sensitivity at very low concentrations of the electroactive species. However, a better understanding of the redox processes on the CB particles surface can be achieved by correlation of both methods. The DPV was performed with the same

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electrochemical setup and using the same CPE working electrode like in CV experiments. The potential pulses were applied starting from -0.1 V to +1.35 V. Fig. 3 illustrates the corresponding differential pulse voltammograms of the CB immobilized particles. The current peak in the range 0 V to +0.6 V in the voltammograms of the samples CB-O3 and CB-NS is attributed to the oxidation of the hydroquinone and is similar to the anodic peak found in CV experiments.

Fig. 3. DPV’s of the carbon black samples: CB-Basic (1), CB-G (2), CB-NO2 (3), CB-O3 (4) and CB-NS (5). Inset: CB-NO2 in the 10-8 A current range

The oxidation peak of hydroquinone appears for the sample CB-NO2 as well, but at much lower magnitude (inset Fig. 3). Notably, at such small current magnitude (bellow 0.075 µA), the observed shoulders in the corresponding DPV of sample CB-NO2 are caused by the noisy currents which become observable on the current scale. The samples CB-Basic and CB-G respectively do not show any peak current in the hydroquinone oxidation range. The exponential increase of the current, starting at +0.7 V is attributed to the anodic electrolysis of water on the working electrode surface. The magnitude of the peak current in DPV corresponding to the hydroquinone oxidation is directly proportional with the surface density of the electroactive species and the specific surface area of the carbon black particles. By assuming a similar active surface area of the CPE and a similar distribution of the immobilized particles on the surface of the electrode, the contents of quinones existing on different carbon black particles can be quantitatively compared by following the maximum anodic currents corresponding to oxidation of hydroquinone on the surface of CB (Table 3). It can be observed that the surface density of the quinone groups follows the same trend like the other oxides presented above. The air-oxidative treatment with O3 seems to be much more effective for insertion of quinones on the surface of CB than the air-oxidative treatment in the presence of NO2. The highest content of quinones was found on the surface of CB-NS particles and is attributed to the highest specific surface area in comparison with the other CB samples. Interestingly, the maximum anodic current of CB-NS sample is five times higher than the one of CB-O3 which is exactly the same difference in the specific surface area between both samples. Table 3. Maximum anodic current corresponding to the oxidation of the hydroquinone on carbon black samples CB Sample

CB-Basic

CB-G

CB-NS

CB-O3

CB-NO2

Maximum anodic current [μA]

-

-

10.54

1.96

0.075

Marius Ciobanu et al. / Materials Today: Proceedings 3S (2016) S252 – S257

3. Conclusions A multi-analytical approach combining chemical surface analysis using determination of the water content, water uptake in the vapour saturated atmosphere, pH, acid-base properties by Boehm titration and electrochemical evaluation of carbon blacks was performed to provide comprehensive understanding of carbon black structure and morphology by post-oxidative treatments in the presence of NO2 respectively O3. Structure-to-property correlation analysis indicates that the post-oxidative treatment of carbon black samples induce large amount of surface oxides formation and implicitly an increment in the electric capacitance properties of the carbon black. Moreover, the increment of the surface oxides is more pronounced after treatment with ozone, compared with the nitric oxide one. The identification of CB surface properties can serve as guidance in type of modification that should be applied in accordance with the reinforcing applications. Acknowledgements The authors would like to thank to the Orion Engineered Carbons GmbH, special to Dr. Michael Heinz, for furnish the carbon black samples used in this study. We express our thanks to Professor Norbert Vennemann, University of Applied Sciences Osnabrück for his support and interesting discussions in this project. References [1] M. J. Wang, in: Encyclopedia of polymer science and technology, John Wiley & Sons Inc, NJ USA, 2003, pp. 52–91. [2] A. I. Medalia, F. A. Heckman, Carbon, 7(5) (1969) 567–568. [3] D. D. L. Chung, J. Mater. Sci. 39(8) (2004) 2645–2661. [4] A. G. Pandolfo, A. F. Hollenkamp, J. Power Sources 157(1) (2006) 11–27. [5] H. Chang, S. H. Joo, C. Pak, J. Mater. Chem. 17 (2007) 3078–3088. [6] T. N. Murakami, M. Grätzel, Inorg. Chim. Acta 361(3) (2008) 572–580. [7] “Application Examples of Carbon Black”, Mitsubishi Chemical, retrieved from web 01-14-2013. [8] D. Parkinson, J. Br. Appl. Phys 2(10) (1951) 273–280. [9] Y. Fukahori, J. Appl. Polym. Sci. 95 (2005) 60–67. [10] M. J. Wang, S. Wolf, E. H. Tan, Rubber Chem. Technol. 66(2) (1993) 178–195. [11] S. J. Park, M. K. Seo, C. Nah, J. Colloid Interface Sci. 291(1) (2005) 229–235. [12] F. A. Heckman, D. F. Harling, Rubber Chem. Technol. 39(1) (1966) 1–13. [13] Y. Otake, R. G. Jenkins, Carbon 31(1) (1993) 109–121. [14] I. Sutherland, E. Sheng, R. H. Bradley et al., J. Mater. Sci. 31 (1996) 5651–5655. [15] W. Shen, Z. Li, Y. Liu, Recent Patents on Chemical Engineering 1 (2008) 27–40. [16] F. Jacquot, V. Logie, J. F. Brilhac et al., Carbon 40(3) (2002) 335–343. [17] C. Moreno-Castilla, F. Carrasco-Marı́n, A. Mueden, Carbon 35(10-11) (1997) 1619–1626. [18] C. Moreno-Castilla, M. A. Ferro-Garcia, J. P. Joly et al., Langmuir, 11(11) (1995) 4386–4392. [19] T. Takada, M. Nakahara, H. Kumagai et al., Carbon 34(9) (1996) 1087–1091. [20] S. J. Park, J. P. Hsu J P., Interfacial Forces and Fields: Theory and Applications, Dekker, New York, 1999. [21] K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties, John Wiley & Sons, New York, 1988. [22] M. M. Dubinin, Carbon 18(5) (1980) 355–364. [23] I. P. O'koye, M. Benham, K. M. Thomas, Langmuir 13(15) (1997) 4054–4059. [24] I. I. Salame, T. J. Bandosz, J. Colloid Interface Sci. 210(2) (1999) 367–374. [25] H. P. Boehm, Carbon 32(5) (1994) 759–769. [26] H. P. Boehm, Carbon 40(2) (2002) 145–149. [27] P. Burg, D. Cagniant, in: Chemistry & Physics of Carbon, CRC Press - Taylor & Francis Group, 2007, pp. 129–175. [28] M. J. Bleda-Martinez, J. A. Macia-Aqullo, D. Lozano-Castello et al., Carbon, 43(13) (2005) 2677–2684. [29] O. Martinez-Alvarez, M. Miranda-Hernandez, Carbon – Sci. Tech. 1 (2008) 30–38. [30] M. Miranda-Hernandez, J. A. Ayala, M. E. Rincón, J. Solid State Electrochem. 7 (2003) 271–276. [31] P. S. Guin, S. Das, International Journal of Electrochemistry Article ID 816202 (2011) doi:10.4061/2011/816202 (2011).

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