KOH/anthracite ratio

According to these results, and many others published in the literature [20,23,35,38-44,49-97], to increase the adsorption capacity of an AC high hydroxide/carbon ratios need to be used. However, in addition to the increase in surface area and micropore volume, it is also important to analyze the effect on the MPSD. Eigure 1.11 presents the MPSD calculated by applying the Dubinin-Stoeckli (DS) equation [10,11] to the N2 adsorption data. The higher the KOH/anthracite ratio, the wider the pore size distribution and the higher the mean pore size. These MPSD curves agree with what can be deduced from the difference in the micropore volumes calculated from N2 and CO2 adsorption data. 

Thus, the KOH/anthracite ratio not only affects the micropore volume but also the micropore size distribution, which should be taken into account for the final use of the AC. If for a given application (e.g., for gas storage see Section V.D) both high adsorption capacity and narrow MPSD are required, the hydroxide/ carbon ratio has important limitations as a variable, and it cannot be useful for optimizing the preparation protocol of this type of AC. 

FIGURE 1.11 MPSD calculated by applying the Dubinin-Stoeckli (DS) equation to the N2 adsorption data for samples prepared with different KOH/anthracite ratios (redrawn from Lozano-Castello, D., Cazorla-Amoros, D., Linares-Solano, A., and Quinn, D.F. Carbon 40(7) 989-1002, 2002. With permission). 

FIGURE 1.5 KOH versus NaOH activation comparison. N2 adsorption isotherms at 77 K of (a) two activated carbons prepared from an anthracite and (b) two activated carbons prepared from a subbituminous coal. In both figures the same preparation procedure was employed (physical mixing, hydroxide/anthracite ratio 3/1 weight, heating rate 5°C/min to 750°C soaking time 1 h Nj flow rate during heat treatment 500 mL/min). 

FIGURE 1.9 Nj adsorption isotherms of samples prepared by chemical activation using either physical mixing or impregnation with a hydroxide/anthracite ratio of 3/1 (weight) (a) KOH and (b) NaOH. 

Figure 1.27 presents the FTIR spectra of a carbon-hydroxide mixture as a function of the heat treatment temperature [99]. This example corresponds to an anthracite activated by NaOH using a physical mixing ratio of 3/1 by weight. Similar results have been obtained for other ratios, for other precursors, and for KOH activation. The figure shows that, as activation proceeds, sodium hydroxide is converted to sodium carbonate and that this reaction starts, for the system studied, between 360 and 570°C. We observe that NaOH is only identified up to about 400°C, whereas above 600°C, sodium carbonate is observed [99]. 

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