Chemical activations H3PO4, activation with

The steps involved in the production of activated carbons by chemical activation with H3PO4 are ... 

Frank Derbyshire and coworkers at the University of Kentucky [7,41] who carried out an extensive study of the H3PO4 activation of different raw materials, proposed an activation mechanism for the activation of lignocellulosic materials, the only materials which display interesting adsorption characteristics with this chemical reagent. Wood or lignocellulosic materials (hard fruit stones and shells) are composed of cellulose (42-50%), hemiceUulose (19-25%) and lignin (16-25%). Wood is often compared to a composite material, where most of the cellulose forms microfibrils with a crystallite structure, whereas hemicellulose and lignin fonn the matrix, an amorphous paste that surrounds the microfibrils. 

The oxidized fiber can then be treated with a chemical activation reagent such as ZnQ2, H3PO4 or HCl at 700-1000°C or, alternatively, gaseous activation can be undertaken in CO2, NH3 or steam in the presence of N2 from 700-1300°C. The product has a high N2 content (up to 15%) with a specific surface area of some 300-2000 m g and a fiber... 

For chemical activation, the main reagents used are KOH, ZnCl2, and H3PO4 [1]. KOH activation is a complex process, involving several redox reactions with carbon, followed by potassium intercalation/insertion and expansion of the structure [2, 3]. In the particular case of H3PO4, carbonization and activation generally proceed simultaneously at temperatures lower than 600°C [4]. 

Chapter 6 describes methods of chemical activation of carbons involving the co-carbonization of a parent feedstock with a chemical. Three chemicals, which have seen frequent use, include zinc chloride, phosphoric acid and potassium hydroxide/carbonate. Section 6.1 is an overview of the comparative behaviors of these three agents, using very similar parent feedstocks, and concluding with the suitability of such activated carbons for methane storage. Section 6.2 is an in-depth analysis of the chemistry of activation by phosphoric acid (H3PO4). Section 6.3 is an in-depth analysis of the chemistry of activation by potassium salts, and discusses intercalation chemistry. Section 6.4 reviews additional relevant literature. Enjoy. 

Figure 6.2. Adsorption isotherms for N2 at 77 K for two series of carbons chemically activated with ZnClj, H3PO4 and KOH. The contents of Zn, P and K (gg precursor) are used to distinguish between die carbons Caturla et al., 1991 Molina-Sabio and Rodriguez-Reinoso, 2004).

When chemical activation is considered as a reaction between a solid precursor and the chemical, then concentration, intimacy of mixing, temperature and activation time determine the extent of reaction. For the three activating agents, ZnCl2, H3PO4 and KOH, optimized conditions lead to activated carbons with micropore volumes of 0.5, 0.6 and 0.4 cm g , respectively. 

All these lignocellulosic precursors produce a char upon carbonization with a yield ranging from 20 to 30 wt%, this meaning that after therm activation the overall yield may be around 10 wt%. Woods, as sawdust, are favored for the manufacture of powdered activated carbons by the chemical activation method (addition of H3PO4). The precursor is crushed and sieved to a coarse powder before impregnation, and the mixture is then subjected to heat treatment. 

P is an NMR active nucleus with a nuclear spin of 1/% Its sensitivity (relative to H) is only 0.066, but because of its 100% natural abundance, the overall relative sensitivity of 31P is about 375 times that of 13 C. Thus, acquisition of phosphorous NMR spectra is relatively easy. The internal standard generally used for measuring 31P chemical shifts is 85% H3PO4. 

Nuclei that are NMR active are listed in Table 14.2. Routinely, NMR spectroscopy is used in characterizing P-containing species see for example case studies 1, 2 and 4 and end-of-chapter problem 2.29 in Chapter 2. Chemical shifts are usually reported with respect to 5 = 0 for 85% aqueous H3PO4, but other reference compounds are used, e.g. trimethylphosphite, P(OMe)3. The chemical shift range for P is large. 

Figure 3.6. Cyclic voltammograms of Teflon-bonded electrodes (2 mg active material, 0.15 cm projected area) containing intact (panel A) and heat treated ClFeTMPP/BP (panel B) in 1 M H3PO4 (according to Figure 1 in ref. [45] reproduced with permission of the American Chemical Society).

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