Thermal desorption methods for porosity characterization of carbons and chars

A novel approach is presented and explored for porosity characterization an d the study of porosity development in carbons and chars during activation processes. This approach involves the interpretation of post-activation tem perature programmed desorption (TPD) spectra. The procedure involves first forming oxygen complexes on a carbon surface by oxygen gasification. Subseq uently, the sample temperature is raised in a low-pressure inert carrier ga s (helium) at a programmed heating rate, during which the rates of evolutio n of CO and CO2 are measured. A comparison of gas evolution results and sur face areas determined from nitrogen adsorption isotherms using the alpha (s )-plot method, show that there exist strong correlations between the nature of the resultant porosity and its development, and post-reaction desorptio n features of oxygen surface complexes formed during the activation process . It is demonstrated that the surface area of the larger pores and the tota l CO2 evolved upon TPD are correlated. It is concluded that the GO-evolving complexes are formed over the entire surface area of the chars during acti vation, but that the surface area in the larger porosity is primarily respo nsible for the formation of CO2-evolving surface complexes. The relationshi p between micropore surface area and the CO evolution 'corrected' for the e xpected contribution from carboxylic acid anhydride surface complexes in th e larger porosity, indicates that these latter types of complexes may be re sponsible for much of the CO2 evolution under conditions when oxygen is the activating agent. (C) 2001 Elsevier Science B.V. All rights reserved.