In the Laboratory
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Chemical Characterization of Activated Carbon Fibers and Activated Carbons J. M. Valente Nabais* and P. J. M. Carrott Centro de Química de Évora and Departamento de Química, Universidade de Évora, Rua Romão Ramalho, nº59, 7000-671 Évora, Portugal; *
[email protected] The objective of this laboratory is the chemical characterization of carbon materials using several methods to obtain information without using expensive instruments. During this lab the students determine the potential of zero charge (pzc) by mass titrations and quantify the acidic and basic sites by acid–base titrations. This lab is useful in two aspects: • In specific disciplines about carbon materials, the methods proposed in this article are important for the chemical characterization of the materials. For instance, the data are crucial to explain the results in studies of liquid-phase adsorption, such as those previously published in this Journal (1–3) or to analyze samples produced by chemical modification methods such as oxidation in the liquid phase with nitric acid (4), sulfuric acid (5), hydrogen peroxide (5), or by thermal treatments under controlled atmosphere (6, 7). • In general chemistry disciplines, the lab can interest students in the basic acid–base concepts and titrations as they are applied to materials that have a great importance for environmental protection and a significant number of daily life applications.
Activated carbons (AC) and activated carbon fibers (ACF) are examined. Both activated products have well developed porosity and surface chemistry but also have some interesting differences. A scanning electron micrograph clearly shows that the ACF is in the form of filaments (Figure 1). In this material the porosity is predominantly microporosity (pore width, dp, < 2 nm) and opens directly to the exterior. In AC, the microstructure is normally spherical and the porosity decreases from macro (dp > 50 nm) to meso (2 nm < dp < 50 nm) to microporosity from the external surface towards the interior of the material (Figure 2). These materials have a tremendous number of applications, which include antibacterial wound dressings, disposable gas masks, methane storage, polarizable electrodes, catalysis applications, SOx and NOx removal, gas separation, cigarette filters, skin substitutes, and adsorption of pollutants such as volatile organic compounds, hydrocarbons, nickel, copper, lead, and mercury. The applications in the gas phase are mainly due to the porous structure whereas the applications in the liquid phase are mainly dependent on the chemical structure of the materials. In this sense, the complete characterization of the materials, including the chemical characterization, is of great importance. Experimental Procedure
Materials This lab has been applied to a variety of different carbon materials, which include commercial activated carbons from Norit N.V. (Norit S51) and Merck (p.a., refª 2186) and activated carbon fibers from Nipon Kynol Inc. (ACF-160315) and samples produced in our laboratory from acrylic textile fibers. Acid–Base Titrations The carbon material, 300 mg, was equilibrated with 60.0 mL of 0.01 mol L᎑1 NaOH for 48 h with stirring at 25.0 ⬚C.
macropores fiber surface
micropores
A Figure 1. Activated carbon fiber from commercial textile acrylic fibers (8).
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Figure 2. Representation of the porous structure of activated carbon fibers (A) and activated carbons (B).
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In the Laboratory
The suspension was filtered using Whatman #1 paper. The concentrations of acidic sites were determined by back titration of the filtrate with 0.01 mol L᎑1 HNO3. The concentrations of basic sites were determined in a similar manner. These procedures should take place in a glove box under an inert atmosphere to avoid carbonation of the NaOH solutions by atmospheric CO2.
Mass Titrations The curves were obtained by preparing three series of suspensions with initial pH of 3, 6, and 11. The three initial pH values were obtained by adjusting the pH of a solution 0.1 mol L᎑1 NaNO3 with NaOH or HNO3 solutions. For each series, five suspensions of 50.0 mL of pH-adjusted solutions with 0.05, 0.1, 2.0, 4.0, and 7.0% (w兾v) in carbon material were prepared. The suspensions were placed in a temperature-controlled bath with stirring (Grant Model SS40D) for 48 h at 25.0 ⬚C, and then filtered with Whatman #1 paper, and the equilibrium pH measured with a Crison potentiometer (model 2002). For didactic objectives it is preferable to determine the complete curves although the pzc can also be obtained using only the three suspensions with 7% (w兾v) (9).
tions (5–10% for carbons) the curve tends towards a plateau, the corresponding pH value being independent of the initial pH value of the aqueous phase. The analytical proof of this principle can be found in ref 12. In adsorption processes the surface of the carbon materials can be positive, neutral, or negatively charged depending on the pH at which the adsorption takes place. If the adsorption pH is equal to pzc the surface will be neutral, but if the adsorption pH is greater than the pzc the surface is positively charged. On the other hand, if the pzc is greater than the pH of the suspensions used in the adsorption process the surface is negatively charged. The charge of the surface can be important for understanding the adsorption of ionic species. The graphics obtained for a commercial activated carbon from Merck and an activated carbon fiber from Kynol are shown in Figure 3.
Acid–Base Titrations By analysis of the titration curves using the first or the second derivatives it is possible to perform the quantification of the basic and acidic groups. We can assume that the reaction of the acidic and basic groups present in the materi-
Hazards A
12
10
8
pH
No chemicals or procedures used by students present any significant hazards. The stock solutions of nitric acid and sodium hydroxide (0.01 mol L᎑1) and solid potassium bromide are irritating and thus require proper eye and skin protection. Sodium nitrate is harmful if swallowed and irritating for eyes and skin. Its contact with combustible material may cause fire and thus the powder should be kept away from heat and sources of ignition and requires eye and skin protection.
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Results and Discussion
Potential of Zero Charge The functional groups on the surface of the carbon materials can be represented using a very simple model with two kinds of functional groups, one basic (B) and one acidic (HA) (10). These groups can suffer ionization determined by the respective pKa or pKb (11) and suspension pH but independently from each other.
initial pH = 11 initial pH = 7 initial pH = 3
4
2 0
2
4
6
8
Mass Concentration (%) B
12
10
HA (s) + H2O (l)
A−(s) + H3O+ (aq)
(1)
B(s) + H2O(l)
BH+(s) + OH−(aq)
(2)
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A common indicator of the surface acidity is the value of the potential of zero charge (pzc) defined as the value of pX at which the net surface charge is zero. X is the ionic species that determines the surface charge of the materials. For carbon materials pX is the value of pH (or pOH). In the mass titration method the pH of various aqueous suspensions of adsorbent, with increasing mass兾volume percentages of adsorbent, in acid, base, or neutral solutions, are plotted as a function of the mass of adsorbent. For high mass concentrawww.JCE.DivCHED.org
pH
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4
2 0
2
4
6
8
Mass Concentration (%) Figure 3. Mass titrations of an activated carbon from Merck (A) and an activated carbon fiber from Kynol (B).
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In the Laboratory
als with NaOH and HNO3, respectively, have stoichiometry 1:1 (eqs 3 and 4).
HA(s) + NaOH (aq) B(s) + HNO3(aq)
−
+
A Na (s) + H2O(l) (3) BH+NO3−(s)
(4)
With these data it is possible to determine the concentration of the acidic and basic groups expressed as mmol兾(g of carbon material) Summary The execution of this lab allows the study of some aspects of acid–base chemistry, like back titrations and acid– base titrations, in a more fashionable way by applying these techniques to a novel system (carbon materials). Also, the students have chance to learn about a different kind of titration, mass titration, where the solid acts as the titrant because it will, for each initial pH, react with the H3O+ present using the equilibria in eqs 1 and 2. When the “titrant” is added in excess the suspension pH will be determined by the solid characteristics and be equal to the pzc value. The mass titration curves can be obtained theoretically by numeric simulations using a simple model and algorithm (10). This can be useful in discipline where the use of the computer is introduced to execute some kind of calculations. Acknowledgments The authors thank the students who performed the lab during the 2001–2002 and 2002–2003 terms and Norit N.V. and Nipon Kynol Inc. for the carbon samples.
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Supplemental Material
Notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Lynam, M. M.; Kilduff, J. E.; Weber, W. J. J. Chem. Educ. 1995, 72, 80–84. 2. Rybolt, R. T.; Burrell, E. D.; Shults, J. M.; Kelley, K. A. J. Chem. Educ. 1988, 65, 1009–1010. 3. Turk, A.; Karamitsos, H.; Mahmood, K.; Mozaffari, J.; Loewi, R.; Tola, V. J. Chem. Educ. 1992, 69, 929–932. 4. Noh, J. S.; Schwarz, J. A. Carbon 1990, 28, 675–682. 5. Pradhan, B.; Sandle, N. Carbon 1999, 37, 1323–1332. 6. Menéndez, J. A.; Carrott, P. J. M.; Valente Nabais, J. M.; Ribeiro Carrott, M. M. L. Process for the Manufacture of Carbon Molecular Sieves. Spanish Patent Application Number P200302133, 2003. 7. Carrott, P. J. M.; Valente Nabais, J. M.; Ribeiro Carrott, M. M. L.; Menéndez, J. A. Carbon 2004, 42, 227–229. 8. Carrott, P. J. M.; Nabais, J. M. V.; Ribeiro Carrott, M. M. L.; Pajares, J. A. Fuel Proc. Tech. 2002, 77–78, 381–387. 9. Carrott, P. J. M.; Nabais, J. M. V.; Ribeiro Carrott, M. M. L.; Menendez, J. A. Mic. Mes. Mat. 2001, 47, 243–252. 10. Carrott, P. J. M.; Ribeiro Carrott, M. M. L.; Candeias, A. J. E.; Ramalho, J. P. P. J. Chem. Soc. Faraday Trans. 1995, 91, 2179–2184. 11. Leon y Leon, C.; Radovic, L. R. Interfacial Chemistry and Electrochemistry of Carbon Surfaces. In Chemistry and Physics of Carbon; Thrower, P., Ed.; Marcel Dekker: New York, 1994; Vol. 24, pp 236–237. 12. Noh, J. S.; Schwarz, J. A. J. Coll. Int. Science 1989, 130, 157– 164.
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