Chemical activations porosity development

Besides CNTs, another ID carbon nanostructure is carbon nanoflbers. For example, Shen et al. [156] prepared a series of hierarchical porous carbon libers with a BET surface area of 2,231 m g and a pore volume of 1.16 cm g. In this synthesis method, the polyacrylonitrile (PAN) nanofibers (prepared by dry-wet spinning) were selected as precnrsors, and pre-oxidation and chemical activation were involved to get the developed porosities. This type of material contained a large amount of nitrogen-containing groups (N content >8.1 wt%) and consequently basic sites, resulting in a faster adsorption rate and a higher adsorption capacity for CO2 than the commercial zeolite 13X that is conventionally used to capture CO2, in the presence of H2O (Fig. 2.27). 

The development of ACs with tailored porosity is necessary to improve their performance in classical applications and to prepare better adsorbents to satisfy new and emerging applications [4-7,17,18]. The preparation of such ACs can be carried out by two different methods the so-called physical and chemical activations [1,19-28], The differences between them lie mainly in the procedure and the activating agents used. 

We should emphasize that in some cases (especially for graphite) these two processes (up to 40 to 50% reaction and accompanied by metal intercalation) do not necessarily result in porosity development (see Figures 1.4a and 1.4b). Additionally, if we recall the comparison between physical and chemical activation (see Section III), we can readily conclude that the BO of the carbon atoms (physical activation) or carbon atoms reacted (chemical activation) alone cannot explain the development of porosity, because much higher BO is necessary by gasification with CO2 to get similar porosity than by the liquid-phase reaction with NaOH or KOH. Consequently, much more work is needed before a full understanding of the chemical activation process can be achieved. 

In the previous sections, a detailed analysis of the chemical activation with hydroxides has been presented with special emphasis on the variables of the process and the final porosity of the ACs. Sample characterization by adsorption (combination of N2 at 77 K and CO2 at 273 K) has shown that hydroxide activation enables the preparation of ACs having both a highly developed MPV and a narrow MPSD. This section shows the importance of being able to control both these parameters to improve the performance of AC in existing and emerging applications. 

Jimenez, V., J. A. Diaz, P. Sanchez, J. L. Valverde, and A. Romero. 2009. Influence of the activation conditions on the porosity development of herringbone carbon nanofibers. Chemical Engineering Journal 155(3) 931-940. 

Gahan et al. (2004) High-porosity carbons were prepared from bituminous coal pitches by combining chemical and physical activation. Chemical activation used KOH impregnation (450 °C for 1 h) followed by carbonization in nitrogen (375-850 °C). The ratio of KOH pitch of 3 1 optimized the development of porosity. In the carbonization range 800-850 °C, a carbon was produced with a micropore volume of 1.02cm g" and a BET surface area of 2350 m g . 

Molina-Sabio M, Rodriguez-Reinoso F. Role of chemical activation in the development of carbon porosity. Colloid Surfaces A Physicochem Eng Aspects 2004 241 15-25. 

The presentation is concerned with the main characteristics of the well known adsorbent activated carbon, the rather high inertness of the surface, the slit-shaped microporosity, the flexibility in the porosity development and the flexibility in the modification of the chemical nature of its surface, and the effects that such characteristics have on the application of activated carbon in adsorption processes. Several examples are shown to highlight these effects, with special emphasis on the gas separation and gas storage processes. 

With phosphoric acid activation, the porosity development is different because essentially microporous carbons can be prepared. However, the use of higher concentrations of this chemical also develops meso- and macroporosity. Further, a controlled process leads to carbon monoliths in which the internal... 

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