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.
Highlights
Modified adsorbents have been for a long time the subject of interest of research groups all over the world [1,2,3,4,5,6,7,8,9] mainly because of a wide spectrum of their application
The increase in nitrogen content observed for urea at the precursor stage (UPA) and PUA samples is most probably related to the nitrogen building into the carbon structure during the reaction with urea, in the form of amines, amides, imines, lactams or nitriles [35, 36]
The above-discussed results have proved that it is possible to obtain from brown coal a large gamut of nitrogen-enriched activated carbons of very well-developed porous structure, showing different content of nitrogen and acid– base properties of the surface, depending on the sequence of particular processes and variant of thermochemical treatment
Summary
Modified adsorbents have been for a long time the subject of interest of research groups all over the world [1,2,3,4,5,6,7,8,9] mainly because of a wide spectrum of their application They are used both in protection of the natural environment as adsorbents of toxic gases [10,11,12,13,14] or liquids [15, 16] and in modern industry as catalyst or catalyst supports [17,18,19] as well as electrode materials for electrochemical capacitors [20, 21]. It was followed by the period of intense studies on
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