Grzegorz S. Szymański

The characterization of activated carbons with oxygen and nitrogen surface groups

Abstract The physicochemical properties and the surface chemical structure of the carbon materials obtained by the modification of the commercial activated carbon D43/1 (Carbo-Tech, Essen, Germany) were studied. The previously de-ashed activated carbon was subjected to the following modification procedures: high-temperature treatment (1000 K) under vacuum; oxidation with conc, nitric acid; and ammonia-treatment of annealed and oxidised carbons at high temperatures. The porous structure and the surface area of the five different carbon samples obtained were estimated by means of mercury porosimetry and from low-temperature nitrogen adsorption data. The thermogravimetric analysis and the quantitative determination of surface functional groups by selective neutralisation of bases and pH-metric titration were carried out. FTIR spectra (transmission) and X-ray photoelectron spectra (Cls, Ols and Nls) were obtained for all the carbon samples and compared with one another. The changes in the porosity and the chemical properties of the carbon surface caused by the modification were analysed. Some possible surface functional species, their structure and surface state are discussed.

The effect of the gradual thermal decomposition of surface oxygen species on the chemical and catalytic properties of oxidized activated carbon

Abstract The physicochemical properties, surface chemical structure and some catalytic properties of a series of carbons prepared by nitric acid oxidation of an activated carbon and subsequent heat treatment under vacuum and mild temperature conditions (423–573 K) were studied. The porous structure characteristics of the partially evacuated samples were estimated from low-temperature nitrogen adsorption data. The thermal analysis and the quantitative determination of surface functional groups by selective neutralization of bases and pH-metric titration were carried out. The dehydration of 2-methylpropan-2-ol was used as a test reaction. While gradual annealing in vacuum alters the surface only slightly, it does differentiate strongly the number and the acidic strength of the surface groups. Progressive heating under mild conditions removes mainly those surface groups that are located in macropores or on the outer surface of the carbon. According to TPD results, the decomposed surface groups are single carboxylic groups, as expected. The decomposition of single, strong carboxylic groups is accompanied by rearrangements of other carboxylic groups with the simultaneous formation of additional cyclic structures like anhydrides, lactones or lactols. Catalytic tests support our previous findings that oxidized carbons have a high dehydration activity. This activity is controlled not only by the number and the strength of acidic groups, but also by their accessibility. There exists an optimum concentration of surface acidic groups above which the activity decreases due to steric restrictions.

Modified porous carbon materials as catalytic support for cathodic reduction of dioxygen

Activated carbon materials were modified by generating several functional groups containing oxygen and/or nitrogen atoms on their surfaces. Surface properties of obtained carbon samples were investigated. The point of zero charge was determined by different methods. The catalytic properties of these materials in the decomposition of hydrogen peroxide and in the electrochemical reduction of dioxygen in aqueous electrolytes have been studied. The catalytic activity for O2 reduction correlates with that for HO2− decomposition. A linear relationship was derived for the dioxygen reduction peak potential and the decomposition rate constant. Thus, the selection of active catalysts for the heterogeneous decomposition of HO2− is a good starting point for the design of a carbon-based oxygen cathode.

Carbon surface polarity from immersion calorimetry

AbstractA series of carbons with various oxygen- and/or nitrogen-containing surface groups has beenprepared and analyzed by water vapor adsorption, acid–base titration, immersion calorimetry, andXPS. These techniques led to coherent description of the surfaces and of their chemistry. It has beenfound that the total concentration of surface oxygen and nitrogen correlates with the acid–baseproperties. At the same time, a simple correlation exists between the enthalpy of immersion intowater for such modified activated carbons and specific interactions with the solid. These interactionscorrespond to the acidic (with oxygen) and basic (without oxygen or with nitrogen) sites. The valueof water interaction with the carbon surface is ca. 21 kJ mol 1 of surface species.D 2002 Elsevier Science B.V. All rights reserved. Keywords: Activated carbon; Surface modification functional groups; Immersion enthalpy 1. IntroductionImmersion calorimetry is a useful technique which provides information on thehydrophobic/hydrophilic nature of carbons, as well as their structures [1–3]. The surfaceof a carbon can be considered as a combination of basal planes and polar sites at the edgesof the carbon layers that form graphite microcrystals. The edge carbon atoms arefrequently combined with oxygen, forming various functional groups with differentacid–base properties [1,3]. Carbon surfaces also contain basic sites which are thoughtto be oxygen-free Lewis sites located at regions rich in k electrons within the basal planesof the graphitic microcrystals away from the edges [3,4].

Adsorption from Aqueous Solutions of Selected Heavy Metal Cations on Chemically Modified Activated Carbon

The adsorption of selected cations (chromium/III/, iron/III/, copper/II/, and silver/I/) from aqueous solutions on modified activated carbons was studied. Carbon-oxygen and carbon-nitrogen surface species were formed on the activated carbon (previously de-ashed) by treating it with concentrated nitric acid or ammonia. The modification procedures differentiated the chemical nature of carbon surface rather, than its porous structure. The surface chemistry of the carbons was characterized by selective neutralization techniques and the XPS method. The number of adsorbed ions depends on the surface acid-base functional groups and the pH equilibrium in the aqueous external solution. The adsorbed species were eluated with water, dilute nitric acid, and ethanol and then identified (in bulk solution). The form of the metal bond to the carbon surface was studied by XPS. The adsorptive behaviour of these modified carbons is explained on the basis of their surface acidity and/or basicity and their redox properties.