J. Phys. Chem. C 2008, 112, 20057–20064
20057
Activated Carbon Fibers Prepared from a Phenolic Fiber by Supercritical Water and Steam Activation Francisco Salvador,* M. Jesu´s Sa´nchez-Montero, Jessica Montero, and Carmen Izquierdo Departimento Quı´mica-Fı´sica, Facultad de Quı´mica, UniVersidad de Salamanca, 37008 Salamanca, Spain. ReceiVed: October 08, 2008; ReVised Manuscript ReceiVed: October 29, 2008
Supercritical water (SCW) and steam were used to prepare activated carbon fibers (ACFs) to study the different behaviors of these two agents. The results showed that activation with SCW is much faster than with steam and that the lower activation energy observed when SCW is used suggests that the mechanism of gasification with SCW and with steam is different. All the ACFs prepared showed a highly developed micropore structure, with a large specific surface area. The ACFs prepared with SCW had a smaller micropore volume with a broader pore size distribution. In contrast, the ACFs prepared with steam had many small micropores. With SCW, external gasification was greater than with steam, fibers with smaller diameters being obtained, although these latter maintained their structure and their mechanical properties were scarcely altered. However, with steam, external gasification was less marked, but the fibers became fragile and friable. 1. Introduction Activated carbon fibers (ACFs) are adsorbent materials that have important advantages over other traditional activated carbons and have therefore been investigated in considerable depth. Such advantages include their high adsorption capacity and rate and their minimum resistance to mass transfer owing to their small diameter. They are highly microporous adsorbents with little or no mesoporosity, and they have a high specific surface area. Although their textural characteristics depend on the raw material used and the way in which they are prepared, ACFs are generally characterized by a relatively narrow pore size distribution. The most common procedure for the preparation of ACFs is physical activation.1 This comprises carbonization of the raw material and its later activation with an oxidizing agent, normally steam or CO2 at atmospheric pressure. The effect of the these two agents has been investigated in depth,1-6 and it has been concluded that at the same temperature, steam shows greater reactivity than CO2 and elicits a more pronounced widening of the microporosity as activation progresses than that achieved with CO2. In contrast, CO2 affords a very narrow microporosity, a slight widening occurring with burnoff, thus providing a greater micropore volume than that attained with steam. ACFs have been prepared from different raw materials, such as cellulose,7,8 rayon,9,10 polyacrylonitrile (PAN),11-13 pitch,6 nomex,14 phenolic resin,15-17 etc. Those prepared from phenolic resins show the greatest specific area.18 Another important advantage of ACFs is that they can be confined in different physical forms, such as fiber tows, fabrics and felts, aiding their handling.19 However, the main drawback of ACFs is the difficulty involved in maintaining the mechanical and physical properties of the original carbon fiber. These properties are intimately related to the experimental conditions used in preparation and to the type of activating agent employed. Generally, fiber diameter and mechanical strength decrease with activation. When the activation temperature is very high, the * Corresponding author. Phone: +34 923 294478. Fax: +34 923 294574. E-mail:
[email protected].
fibers become fragile and friable, and they are less resistant when they are activated with steam than with CO2. Recently, the authors have reported the use of supercritical fluids (water and CO2) to perform the activation of adsorbent carbonaceous materials,20-22 with the finding of greater reactivity and a different evolution of the porosity. In the present work, we report a comparative study of the preparation of ACFs with supercritical water (SCW) and with steam. The greater reactivity observed with SCW will allow lower activation temperatures to be used, thus improving the mechanical resistance of ACFs. Furthermore, the different development of porosity will allow ACFs to be obtained with different textural characteristics, which is more appropriate for certain applications. 2. Experimental As the raw material, we used a phenolic fiber, NoVoloid, in spun yarn form, supplied by Kynol (Japan). Prior to its activation, the fiber was carbonized in an inert atmosphere of N2 (100 cm3/min) in a horizontal furnace at 5 °C/min to 700 °C with a residence time of 90 min at 700 °C.Further experimental details of the installation used in the activation process can be found in ref 21. Samples (3.5 g) of carbonized fiber were activated at a 4.0 g/min flow of water at 29 MPa and at atmospheric pressure at different temperatures, 630-750 °C. For each of the temperatures, different experiments were carried out in which the activation time was varied to obtain ACFs with different burnoffs. Burnoff was determined from the loss of the mass undergone by the carbon fiber during the activation process. The nomenclature used to refer to the different ACFs obtained is as follows: The activation temperature is followed by “SCW” for the samples activated with supercritical water or “S” for those activated with steam and, finally, the percentage of burnoff. For example, the carbon fiber activated at 630 °C with supercritical water and 49% burnoff would read 630SCW49. All the ACFs prepared were characterized texturally from N2 adsorption-desorption isotherms at 77 K and from CO2 adsorption isotherms at 273 K in two volumetric devices from Micromeritics (ASAP 2010 and Tristar, respectively). Before
10.1021/jp808898k CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2008
20058 J. Phys. Chem. C, Vol. 112, No. 50, 2008
Figure 1. Installation for the phenol adsorption experiments.
the adsorption tests, all samples were outgassed at 350 °C under a vacuum for 24 h. Comparison of the changes occurring in the porosity of the ACFs prepared with the two activating agents was based on the following parameters: (i) Specific surface area SBET(N2), calculated by applying the BET equation to the N2 adsorption isotherm;23 and specific surface area SDR(CO2), calculated by applying the Dubbinin-Raduskhevic equation24 to the CO2 adsorption isotherm. (ii) micropore volume, deduced from application of the Dubbinin-Raduskhevic equation to the adsorption isotherms of N2, V0(N2), and CO2, V0(CO2). The adsorption of the two gases provides complementary information. Whereas N2 is adsorbed into micropores larger than 0.7 nm, the adsorption of CO2 at higher temperature takes place in smaller micropores (
