J. Phys. Chem. C 2008, 112, 4991-4999
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Reactivity and Porosity of a Carbon Fiber Activated with Supercritical CO2 M. Jesu´ s Sa´ nchez-Montero, Francisco Salvador,* and Carmen Izquierdo Dpto. Quı´mica-Fı´sica, Facultad de Quı´mica, UniVersidad de Salamanca, 37008 Salamanca, Spain ReceiVed: October 2, 2007; In Final Form: January 15, 2008
This work describes a comparative study carried out on the reactivity and on the development of the porosity of a carbon fiber when it was gasified with supercritical CO2 (SCCO2) and with CO2 at atmospheric pressure (ATCO2). The kinetic results show that for the same temperature the rate of gasification of the fiber with SCCO2 is much higher than when ATCO2 is used. The kinetic parameters of activation, the activation energy and the preexponential factor, obtained suggest a change in the reaction mechanism when SCCO2 is used. This change would be favored by the formation of high-density clusters as a result of the increase in pressure, and these would be responsible for the increase in reactivity. Moreover, with both activating agents activated carbon fibers (ACFs) were obtained that were fully microporous and had very high surface areas and micropore volumes. Nevertheless, the ACFs prepared with ATCO2 developed very narrow micropores, while those prepared with SCCO2 developed wider micropores.
1. Introduction Activated carbon fibers (ACFs) are adsorbent carbonaceous materials with a large number of advantages over the more conventional forms: powder and granular. Their preparation, characterization, and applications have been reported in many studies.1-8 Different types of natural and synthetic fibers have been used as precursors of ACFs, such as cellulose, pitch, phenolic resin, polyacrylonitrile (PAN), etc.9 It is known, for example, that phenolic resins produce ACFs with greater surface areas than other precursors.10 The main advantage of ACFs as compared with activated carbons is that they have very large surface areas and a high adsorption capacity and rate in the liquid and gas phases.11 The porosity of ACFs depends on the procedure and conditions used in their preparation and also on the starting material.12,13 The usual procedure for the preparation of ACFs is physical activation, which is based on the carbonization of the raw material and later gasification with an oxidizing agent at high temperature.1 CO2 and steam at atmospheric pressure are the activating agents most widely employed. The gasification of carbon fibers is a heterogeneous process that depends on (i) the nature of the precursor, (ii) the concentration of active sites on the surface of the fiber, (iii) the nature of the activating agent, and (iv) the transport of the activating agent and of the reaction products to and from the reaction site. The diffusion of the activating agent on the surface and inside the fiber is often a limiting factor for the gasification rate.4 The gasification of carbon with CO2 is an endothermic reaction which in the absence of catalysts requires temperatures close to 1000 °C for the rate to be appreciable. The main mechanistic studies on the C/CO2 reaction were performed by Gadsby et al.,14 Reif,15 Ergun,16 Menster and Ergun,17 Strange and Walker,18 and Biederman et al.,19 later compiled in Laurendeau.20 Although many studies have addressed this reaction, only a few have been carried out at moderate * To whom correspondence should be addressed. E-mail: salvador@ usal.es;
[email protected];
[email protected].
pressure,21-23 and there have been no studies performed under supercritical conditions. The use of supercritical CO2 and water in the preparation of adsorbent carbonaceous materials was patented by Salvador et al.24 That patent describes the procedure and equipment required for the production of activated carbon using supercritical water and CO2 as activating agents. The patent only reports the advantages of the use of supercritical fluids as regards the speed and economy of the process. However, it does not provide any information about the reaction mechanisms and textural characteristics of the carbons prepared with these agents. Since then, however, the number of papers published on the topic has been very low.25-27 Only Li et al.25 carried out a brief study of the activation of a carbon fiber with supercritical water, reporting a small development in surface area and the creation of some mesoporosity. Supercritical fluids are of interest because their properties are intermediate between those of gases and liquids. Supercritical CO2 has an easily accessible critical point (31.0 °C and 73.8 bar). Its transport properties are similar to those of gases, namely, low viscosity, high diffusivity, and very low surface tension, although its solvatation properties are similar to those of liquids. Accordingly, SCCO2 is usually able to penetrate a solid sample faster than liquid solvents because of its high diffusion rates, and because of its low viscosity, it can rapidly transport dissolved solutes from the sample matrix. With these unique properties, increasing numbers of applications and successful commercial-scale processes involving supercritical fluid CO2 have been demonstrated in a variety of fields. Recent reviews include polymerizations,28 pharmaceutical applications,29 textile processing and dyeing,30 coatings,31 natural products/food extractions,32 specialized materials fabrications,33 cleaning,34 and chromatography.35 In most of these applications, SCCO2 is used as a reaction medium and as a solvent and/or extractant, and there are few applications in which it is used as a chemical reagent. In fact, no studies of the C/CO2 reaction under supercritical conditions have been made. In light of the foregoing, it seems necessary to further investigate the C/CO2 reaction at high pressure with a view to
10.1021/jp709647y CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008
4992 J. Phys. Chem. C, Vol. 112, No. 13, 2008
Sa´nchez-Montero et al.
Figure 1. Experimental setup.
understanding the behavior of SCCO2 as an activating agent. Here we address a comparative kinetic study of the C/CO2 reaction in the supercritical state (SCCO2) and at atmospheric pressure (ATCO2) and its repercussions on the textural characteristics of a carbon fiber activated with both activating agents. 2. Experimental Section All ACFs were prepared from a common precursor, in this case a phenolic fiber, Novoloid, supplied by Kynol. This fiber was carbonized in nitrogen (100 cm3/min) (25 °C; 1 bar) in a horizontal furnace at 5 °C/min with a residence time of 90 min at 700 °C. Gasification of the carbon fiber with SCCO2 and ATCO2 was accomplished in a flow reactor placed inside an oven. Figure 1 shows a scheme of the setup. Further experimental details can be found in ref 36. The CO2 is stored in a cylinder in the liquid state at a pressure of 50 ( 2 bar. It is impelled by a highpressure fluid pump (Thar P-50) such that it must previously be cooled to -2 °C, to ensure the liquid state. In all experiments, flow rate was 1.5 cm3 of liquid CO2/min. Samples of 2.5 g of carbonized fiber were placed in the gasification chamber and subjected to a stream of N2 (100 cm3/ min) until the reactor had reached the working temperature. Once this temperature had been reached, the stream of N2 was replaced by the CO2 stream at the working pressure and temperature. The gasification experiments were performed at 110 bar and at atmospheric pressure at different temperatures, 750, 775, and 800 °C, maintaining these temperatures for the appropriate time to obtain burnoffs within the range 8-67%. Burnoff was determined from the loss of mass undergone by the carbon fiber during the gasification process. The nomenclature used to refer to the different series of activated carbon fibers is as follows: the activation temperature, followed by “SC” for the series activated with SCCO2 and “AT” for those activated with ATCO2, and finally the percentage of burnoff. For example, the carbon fiber gassed at 750 °C with SCCO2 and 39% burnoff would read 750SC39. N2 adsorption isotherms (N2 99.999% pure) at 77 K and those of CO2 (99.98% pure) at 273 K were assessed using ASAP 2010 and TRISTAR devices, respectively, both from Micromeritics. Using the ASAP 2010 device it is possible to measure N2 adsorption isotherms from relative pressures of around 10-610-7, obtaining the so-called high-resolution N2 isotherms. The N2 adsorption isotherms were analyzed following the BET method to calculate the specific surface area, SBET, and with
Figure 2. Effect of temperature and the activating agent on the gasification of a carbonized fiber.
the Dubinin-Radushkevick equation to calculate the volume of the widest micropores (>0.7 nm), V0(N2). The method based on the density functional theory (DFT)37,38 allowed us to determine the pore size distribution over a wide range, from micropore to macropore, by means of a single analysis of the N2 adsorption isotherms. The CO2 isotherms were analyzed by means of the Dubinin-Raduskevich equation to calculate the volume of the smallest micropores (
