Thermal desorption methods for porosity characterization of carbons and chars

Abstract

A novel approach is presented and explored for porosity characterization and the study of porosity development in carbons and chars during activation processes. This approach involves the interpretation of post-activation temperature programmed desorption (TPD) spectra. The procedure involves first forming oxygen complexes on a carbon surface by oxygen gasification. Subsequently, the sample temperature is raised in a low-pressure inert carrier gas (helium) at a programmed heating rate, during which the rates of evolution of CO and CO 2 are measured. A comparison of gas evolution results and surface areas determined from nitrogen adsorption isotherms using the α s-plot method, show that there exist strong correlations between the nature of the resultant porosity and its development, and post-reaction desorption features of oxygen surface complexes formed during the activation process. It is demonstrated that the surface area of the larger pores and the total CO 2 evolved upon TPD are correlated. It is concluded that the CO-evolving complexes are formed over the entire surface area of the chars during activation, but that the surface area in the larger porosity is primarily responsible for the formation of CO 2-evolving surface complexes. The relationship between micropore surface area and the CO evolution ‘corrected’ for the expected contribution from carboxylic acid anhydride surface complexes in the larger porosity, indicates that these latter types of complexes may be responsible for much of the CO 2 evolution under conditions when oxygen is the activating agent.