Effect of heat treatment on the physicochemical properties of nitrogen-enriched activated carbons

Abstract

A series of functionalized carbonaceous adsorbents were prepared by means of reaction with urea and chemical activation of Polish brown coal. In order to obtain nitrogen groups bonded in different ways to the carbon structure, the reaction with urea was performed at two different stages of processing, with precursor or char. Additionally, the influence of annealing under nitrogen atmosphere at 600 and 800 °C or in hydrogen/nitrogen mixture at 600 °C on the textural, acid–base and thermal properties of activated carbons prepared was tested. The resulting materials were characterized by elemental analysis, low-temperature nitrogen sorption, determination of the number of surface oxygen groups as well as by thermogravimetric study in helium atmosphere. The final products were microporous nitrogen-enriched activated carbons of very well-developed surface area reaching from 1303 to 2004 m2 g−1, showing different content of nitrogen (from 0.5 to 2.3 mass%) and different chemical character of the surface (from weakly acidic to intermediate acidic–basic), depending on the sequence of particular processes and variant of further thermochemical treatment. According to the results of thermal study, the nitrogen functional groups introduced into the carbon structure by the reaction with urea (especially at the char stage) show moderate thermal and chemical stability. Under the effect of high temperature, some of these groups underwent decomposition or transformation into more thermally stable nitrogen species. It was also proved that the thermochemical treatment of modified activated carbons led to considerable deterioration of their textural parameters as well as caused significant changes in the acid–base character of their surface.