Thermal stability of oxygenated functions in activated carbons

Activated carbons with various degrees of oxidation were prepared by treatment with concentrated nitric acid at various temperatures. Selective removal of the oxygen complexes by heating under nitrogen flow at 573, 773 and 1073 K was also considered. The samples were characterised through temperature-programmed desorption (TPD), Fourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) techniques, FTIR and XPS techniques were found to be very useful for relating the shape of CO2 desorption profiles during pyrolysis with the nature of the functional groups desorbed. Three steps in TPD profiles were observed: (i) the desorption of carboxylic acid groups in the form of CO2 in the lower temperature range, 400-623 K; (ii) the decomposition of lactone groups created during HNO3 treatment and, to some extent, originated during heating rearrangements, in the temperature range 623-823 K; and (iii) the elimination of the more stable ether groups upon treatment at temperatures above 823 K. The combination of these three techniques (TPD, FTIR and XPS) gives a reasonable picture of the surface chemistry of the oxidised activated carbons. (C) 1997 Elsevier Science B.V.