Elsevier

Carbon

Volume 37, Issue 9, 1999, Pages 1379-1389
Carbon

Modification of the surface chemistry of activated carbons

https://doi.org/10.1016/S0008-6223(98)00333-9Get rights and content

Abstract

A NORIT activated carbon was modified by different chemical and thermal treatments (including oxidation in the gas and liquid phases) in order to obtain materials with different surface properties. Several techniques were used to characterize these materials including nitrogen adsorption, chemical and thermal analyses, XPS, TPD and DRIFTS. The results obtained by TPD agree quantitatively with the elemental and proximate analyses of the oxidized materials, and qualitatively with the observations by DRIFTS. A simple deconvolution method is proposed to analyse the TPD spectra, allowing for the quantitative determination of the amount of each functional group on the surface. A multiple gaussian function has been shown to fit the data adequately, the parameters obtained for each fit matching very well the features observed in the experimentally determined TPD spectra.

It is shown that gas phase oxidation of the carbon increases mainly the concentration of hydroxyl and carbonyl surface groups, while oxidations in the liquid phase increase especially the concentration of carboxylic acids.

Introduction

Carbon materials are finding an increasing number of applications in catalysis, either as supports for the active phases, or as catalysts on their own. Their performance is determined both by their texture and surface chemistry [1].

In the case of activated carbons, the texture may be adapted to suit the situation by adequate choice of the activation procedure. In particular, it is possible to prepare carbons with different proportions of micro, meso and macropores [2].

On the other hand, the nature and concentration of surface functional groups may be modified by suitable thermal or chemical post-treatments. Oxidation in the gas or liquid phase can be used to increase the concentration of surface oxygen groups, while heating under inert atmosphere may be used to selectively remove some of these functions. Carboxyl, carbonyl, phenol, quinone, and lactone groups, have been identified on carbon surfaces [3], [4].

A variety of experimental techniques has been used to characterize these functional groups, such as chemical titration methods [3], [5], [6], [7], temperature-programmed desorption (TPD) [8], [9], [10], X-ray photoelectron spectroscopy (XPS) [11], [12] and infra-red spectroscopy methods (FTIR, DRIFTS) [8], [13], [14], [15].

The chemical titration methods, such as proposed by Boehm [3], [5], are especially useful when used in combination with other techniques [16]; however, they are not practical when dealing with small samples. In addition, these methods fail to account for a large proportion (as high as 50%) of the total oxygen content of carbon materials [17].

XPS is a surface technique which will provide an estimate of the chemical composition of the few uppermost layers of the material. In order to get further insight into the nature of the functional groups on the surface, a reconstruction of the O1s peak can be performed [18]. The reconstruction of the C1s region is generally more difficult, not only as a result of the broadness of the peak, but also because it is necessary to make an assumption concerning its nature after oxidation, namely if the surface graphite-like structure remains unchanged or not. This will affect the curve fitting procedure, an asymmetric peak shape being needed for the graphite-like structure, while a gaussian peak is required for an aliphatic structure [19].

Infra-red spectroscopic methods can be applied only to highly oxidized carbons, otherwise the intensity of the absorption bands is not sufficient. Diffuse Reflectance FTIR (DRIFTS) is preferable, to avoid the problems caused by sample dilution, and a recent study has shown the usefulness of the technique to monitor the surface functionalities that develop under oxidizing conditions [14]. The interpretation of the spectra is complicated by the fact that each group originates several bands at different wavenumbers, therefore each band may include contributions from various groups (cf. Table 1).

Temperature-programmed methods have become rather popular. Surface oxygen groups on carbon materials decompose upon heating by releasing CO and CO2 at different temperatures. There is some confusion in the literature with respect to the assignment of the TPD peaks to specific surface groups, as the peak temperatures may be affected by the texture of the material, the heating rate and the geometry of the experimental system used [20], [21], [22]. However, some general trends can be established as summarized in Fig. 1. Thus, a CO2 peak results from carboxylic acids at low temperatures, or lactones at higher temperatures; carboxylic anhydrides originate both a CO and a CO2 peak; phenols, ethers, and carbonyls (and quinones) originate a CO peak. In general, the TPD spectra obtained with carbon materials show composite CO and CO2 peaks which must be deconvoluted before the surface composition can be estimated.

Thus, in general the quantitative analysis of the surface functional groups is not straightforward. In the present work, activated carbons were modified by thermal or chemical post-treatments in order to obtain materials with different surface properties which were then characterized by different techniques, in an attempt to determine quantitatively the amount of each chemical function on the surface. A simple deconvolution method is proposed to analyse the TPD profiles, and the results obtained seem to validate this procedure.

Section snippets

Experimental

A NORIT ROX 0.8 activated carbon (pellets of 0.8 mm diameter and 5 mm length) was used as supplied and after different oxidation treatments: in the gas phase, with 5% O2 (in N2) at 698 K for different times in order to achieve the desired burn-off (B.O.), or with 50% N2O (in N2) for 8 h at 773 K; and in the liquid phase, with 5M nitric acid at the boiling temperature for 6 h, or with 10 M H2O2 at room temperature. The materials oxidized in the liquid phase were subsequently washed with

Results and discussion

The results obtained for all samples by the various characterization techniques are shown in Table 2, Table 3, Table 4, Table 5, Table 6, and will be analysed and discussed in three stages: (a) General characterization, with emphasis on the different oxidation methods (samples A1, A4, A9, A11, A12); (b) Heat-treatments at different temperatures (samples A4, A6, A7, A8); and (c) Different degrees of oxidation by the same treatment (samples A1, A2, A3, A4, A5).

Conclusions

It has been shown that gas phase oxidation of activated carbons increases mainly the concentration of hydroxyl and carbonyl surface groups, while oxidation in the liquid phase increases especially the concentration of carboxylic acids.

In terms of total oxygen content, the results obtained by TPD agree quite well with the elemental analyses of the oxidized materials. Higher amounts of oxygen are determined by XPS, indicating that the concentration of the functional groups is higher at the

Acknowledgements

This work was carried out with support from the PRAXIS XXI Programme (contract numbers 2/2.1/BIO/34/94 and PCEX/C/QUI/98/96). M.F.R. Pereira and M.M.A. Freitas acknowledge the grants received under the same programme. The authors are indebted to Dr. Carlos M. Sá (CEMUP) for assistance with XPS analysis, and to NORIT N.V., Amersfoort, The Netherlands, for providing the activated carbon.

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