Size distribution carbon-nitrogen ratio

Pye, K., Blott, S. J., Croft D. J., and Carter, J. F. (2006). Forensic comparison of soil samples Assessment of small-scale spatial variability in elemental composition, carbon and nitrogen isotope ratios, colour, and particle size distribution. Forensic Sci. Int. 163, 59-80. 

Lately, potassium hydroxide (KOH) activation has been used to produce activated carbons from cheap and available natural precursors, such as coals and pitch-derived carbonaceous materials. This method efficiently develops the micropores, and aUows to get various pore size distributions depending on the kind of precursor and activation conditions [82—85]. The nitrogen adsorption isotherms (Fig. 23.11) at 77 K of activated carbons obtained from coal (C), coal semicoke (CS), pitch semicoke (PS), pitch mesophase (PM), and a commercial activated carbon (AC), using KOH and carbon in the 4 1 ratio, reveal that a... 

The nitrogen adsorption/desorption isotherms allow the specific surface area, pore size distribution as well as the micro/meso ratio to be estimated. The total surface area is quite similar for the investigated samples and it ranges from 329 to 403 m /g being the most developed for the Nt+3M+F composite as shown in Fig. 9.6. The nitrogen adsorption isotherms showed that the carbon materials are typically mesoporous (apart from the material M+F, that is, without nanotubes), and the amount of micropores is very moderate. The micropore volume values for all the samples are comparable var3dng from 0.152 to 0.174 cm /g. The porosity characteristics of all the composites are illustrated in Table 9.1. 

Carbon molecular sieves (CMS) adsorbents are produced by pyrolysis of carbonaceous materials followed by carefully controlled deposition of carbon within the pores [43]. In contrast to activated carbons which have a broad distribution of micropore size (generally in the 10 - 100 A range) the pores of a carbon molecular sieve are very small (< 10 A) and the pore size distribution in narrow. As a result the adsorption behavior is similar to that of a zeolite. Carbon molecular sieves are widely used for production of nitrogen from air (by selective adsorption of oxygen). There is little difference between the equilibrium isotherms of O2 and N2 on CMS but as a result of its slightly smaller molecular size oxygen is adsorbed very much faster (diffiisivity ratio 10 - 100). The sorption kinetics show some interesting features. 

Considering that different models are used to describe the adsorbate, namely, singles spherical particles in the DFT calculations, and two center Lennaid-Jones molecules with point quadrupole in the simulations, it is interesting that both determinations show similar distributions. Probably the most important difference is observed for the carbon obtained with a low KOH/lignin ratio, which has a small total pore volume and only small pores are present. In this case, the PSD obtained from simulations predicts pores of considerably smaller size than those obtained from DFT. This may be closely related to the different models used in both methods. It must be pointed out that even when the two-center Lennard Jones with point quadurlpole model is a more realistic representation of the nitrogen molecule, in both cases the carbon is considered as a smooth wall. For very small pores, as the ones where differences are observed with the two methods, it is possible that the smooth wall approximation is not valid. 

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