Surface functional groups Boehm method

The pH of MCM-41 was measured with ASTM D 3838. About l.Og of dry MCM-41 was added to 20 ml of distilled water, and the suspension was shaken overnight to reach equilibrium. Then the sample was filtered, and pH of the solution was measured. The surface functional groups of the samples were determined by Boehm s titration method [13]. In the case of acid value, about l.Og of the sample was added to 100 ml of O.IN NaOH solution and the mixture was shaken for 24 h. Then the solution was filtered through a membrane filter and titrated with 0. IN HCI. Likewise, the base value was determined by converse titration. 

Table 24.7 Characteristics of activated carbon after an oxidation reaction, Gi are the surface functional groups determined by Boehm s method, Gi for carboxylic acid, G2 for lactone, G3 for phenol, and G4 for carbonyl. These groups are differentiated by neutralization with solutions (0.05N) of NaHCOj, Na COj, NaOH, and CH CH ONa [50]. 100 g of activated carbon in 500 ml of water with or without an oxidant...

Concentration (meq/g) of Surface Functional Groups Calculated Using Boehm Titration Method... 

Thirdly, in order to improve the dispersion of platinum catalysts deposited on carbon materials, the effects of surface plasma treatment of carbon blacks (CBs) were investigated. The surface characteristics of the CBs were determined by fourier transformed-infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), and Boehm s titration method. The electrochemical properties of the plasma-treated CBs-supported Pt (Pt/CBs) catalysts were analyzed by linear sweep voltammetry (LSV) experiments. From the results of FT-IR and acid-base values, N2-plasma treatment of the CBs at 300 W intensity led to a formation of a free radical on the CBs. The peak intensity increased with increase of the treatment time, due to the formation of new basic functional groups (such as C-N, C=N, -NHs, -NH, and =NH) by the free radical on the CBs. Accordingly, the basic values were enhanced by the basic functional groups. However, after a specific reaction time, Nz-plasma treatment could hardly influence on change of the surface functional groups of CBs, due to the disappearance of free radical. Consequently, it was found that optimal treatment time was 30 second for the best electro activity of Pt/CBs catalysts and the N2-plasma treated Pt/CBs possessed the better electrochemical properties than the pristine Pt/CBs. 

It is obvious that the Boehm titration method is the most popular one for the determination of various types of acidic (and basic) surface functionalities in carbon materials. From 1966 until 2002, when Boehm himself published a critical assessment of the analysis of surface oxides on carbon [201], an exhaustive utilization of this method has been desaibed by many authors. They underlined its simplicity, but pointed out also the need for using other complementary methods such as potentiometric titration, tanperature-programmed desorption (TPD), spectroscopic methods (mainly XPS and FTIR), and thermodynamic approaches such as calorimetry. The case of TPD is of special interest, to identify oxygenated functionalities. However, the CO and CO2 peaks must certainly be deconvo-luted before the surface composition can be estimated. Thus, a quantitative TPD analysis of surface functional groups is sensitive to the deconvolution method and to experimental conditions. The results are generally discussed in relation to those of DRIFTS and XPS analysis, as can be seen from the references listed in Table 3.1. 

The authors pointed out the importance of the accessibility to the functional groups for the understanding of their behavior in adsorption and catalysis. The mere evaluation of the chemical functionalities (e.g., by using the Boehm method) and surface areas is not always sufficient to arrive at reliable conclusions. 

The usefulness of XPS in the study of the surface chemistry of carbonaceous materials is well established [120-123]. It is significant, however, that in 1994, while reviewing some aspects of the surface chemistry of carbon blacks and other carbons (in particular, active carbons), Boehm [48] devoted only a few lines to XPS and argued that XPS is not very useful for quantitative determinations. The selected examples included here are meant to give the reader an idea of the use of XPS in the last decade, often in combination with other methods such as FTIR and TPD, and more recently with solid-state nuclear magnetic resonance (NMR), for both qualitative and quantitative characterization of the surface chemistry (i.e., functional groups) either of raw carbonaceous materials or after their treatment (e.g., activation, thermal or chemical treatment, and pyrolysis). 

---