Study of Active Carbons

Innes, Raymond William (1988) Study of Active Carbons. PhD thesis, University of Glasgow.

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The intention of the present work was to examine a series of active carbons using a number of techniques. The series of carbons was produced from a commercial charcoal which was first carbonised and the activated at 850 C using carbon dioxide. The activation time was varied to produce a series of carbons possessing different degrees of burn-off. Histograms showing the micropore size distributions were obtained from both high resolution electron microscopy and immersion calorimetry. Immersion calorimetry is a macrotechnique whereby the sample is immersed in a variety of organic liquids and the quantity of heat released is correlated with a value of pore volume. Using this technique with molecular probes of increasing critical dimensions results in a set of values showing the pore volumes accessible to each of the probes which can then be used to construct a histogram of the micropore size distribution. High resolution electron microscopy on the other hand, is a technique whereby individual pores are imaged and then measured directly from the micrographs. This technique also results in a histogram showing the micropore size distribution. As the immersion results are averaged over many pores they are more statistically sound, microscopy however, because of its very nature, results in a clearer picture of pore structure. In order for the results obtained from these two methods to be compatible, certain assumptions had to be made based on theoretical calculations. These calculations had suggested that the interaction radius of an adsorbate molecule would be reduced during adsorption in a micropore. This reduction had to be accounted for, as did the finite widths associated with the carbon atoms constituting the micropore walls and this was undertaken by subtracting a value of 0.2nm from those results obtained from high resolution electron microscopy. When these corrections had been made, it was found that for most of the carbons studied, the agreement between the two techniques was reasonable, especially in the region below around 0.6nm, seeming to validate the Everett and Powl model on which the corrections were based. The disagreement encountered in the region of larger pore size also had interesting implications, suggesting that rather than slit-pores, most of the pores in this range were of a different nature. Microscopical evidence was also provided showing the structure of these larger pore types to be quite varied. It was also shown that the quantity of such porosity increased with the increasing degree of activation of the sample under investigation, a property reflected quantitatively from immersion results and qualitatively from electron microscopy. The microscopical evidence presents a structure for the higher burn-off carbons where these larger pore types are spread through the specimen interconnected by a network of smaller pores. It also appears that these smaller pores must undergo exfoliation in order for the quantity of vapour that was adsorbed to occur. In addition to these investigations, a variety of methods were also employed in order to discover the average pore size of some of the samples from their adsorption isotherms. The results of this investigation showed that for the carbons in this work at least, reliable results on average pore sizes could not be deduced from the adsorption isotherms. A final area of investigation, in addition to that of the active carbons, was a brief examination of some coke samples. These coke samples were activated in a similar manner to the carbons, however the resulting information that was obtained for the densities and immersion values of the series of cokes that had been produced showed little variation with the degree of activation, so no detailed assessment of these samples was undertaken.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry, Physical chemistry
Date of Award: 1988
Depositing User: Enlighten Team
Unique ID: glathesis:1988-77727
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 11:53
Last Modified: 14 Jan 2020 11:53

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