Boehm titration method

This approach was used to characterize carbons [203,215,290]. The results showed good correlation with the data obtained nsing the Boehm titration method (Figure 2.15).The advantage of the IGC approach is that it provides information about acidity in a nonaqneons environment, and the resnlts are obtained in a rapid and reproducible way. 

The importance of basicity was also shown when the density of basic groups on the surface determined using Boehm titration method was linked to the MM removal ctpachy. The majority of carbons used for that research followed the linear trend. The discrepancy in some case was caused by the differences in the content of catalytically active phase, iron oxide, which will be addressed later in this section. 

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

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. 

Oxidation and incorporation of nitrogen usually affect the surface chemistry of carbons [12, 20, 24]. The types and numbers of basic and acidic groups evaluated using Boehm titration [3, 4]. are collected in Table 2. The data reported indicate that the initial carbons differ in their acidity as a result of the activation method and the type of an organic precursor [1]. BAX is more acidic than BPL. This is expected for a phosphoric acid activated carbon [25]. After modification with urea and heat treatment at 723 and 1223 K, the overall surface chemistry... 

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. 

FTIR spectroscopy. Several absorption bands are mentioned in the literature [9-11] and attributed to vibrational properties of different surface groups. It is important to stress that this method does not make the difference between identical groups of different strength. In this section, we will only focus on the common features of the support and the catalyst. The phenolic groups seem to be more numerous than the others (quinonic, carboxylic, lactonic). The nitric treatment seems to increase the amount of quinonic groups on the support, and the quinonic and phenolic ones on the catalyst. Those results are in accord with these obtained in the Boehm titration. At this moment, we have no direct information on the basic surface groups. 

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. 

The Boehm and potentiometric titration methods provide useful information about surface species that behave as acids in aqueous solutions. Nevertheless, a significant number of other oxygenated functionalities—such as carbonyls, esters, and ethers—are not taken into consideration even though they can play a significant role in adsorption processes by virtue of their polar properties and hydrogen-bonding abilities. Other methods described below (e.g., TPD, XPS, FTIR, as well as linear solvation energy relationship approaches) are therefore useful complements. 

It is seen that the ZCP is strongly dependent on acid treatment. The results of titration by using Boehm s method are also given in Table 5.5. The untreated sample of this particular activated carbon (North American carbon, low ash , <2%) is basic, with ZCP = 10. Carboxyl is the most abundant group on aU acid treated carbons. The approximate distribution of different add groups is seen from the titration results. 

Selective neutralization experiments provide another method for evaluating the relationship between the absorption spectra and the identity of the surface species [6,16]- Samples of Monarch 1300 carbon black which had been back titrated with different strength bases as per the Boehm scheme (see Table I) were filtered, washed with neutral water and then reacidified with dilute HC1. Spectra were obtained on samples dried after each of these steps. In these samples, the reacidification process completely reversed the changes in the spectra which had been produced by the selective neutralizations. Figure 10 compares the result of the NaHC03 titration with that of the reacidified black. This weak base, which is used to measure the carboxylic acid content of the black, reduces the 1725 cm 1 band and increases the intensity of the 1595 cm 1 band. Futhermore, there... 

Boehm and Johnson Matthey titrations. Both titrations give information on the acidic or basic character. The procedures are described in [13] and [14]. Boehm method classifies the surface acidic sites in three types (called phenolic, lactonic, carboxylic), by growing strength of acidity. However, this arbitrary classification fails to attribute one surface acidic group to one type of acidity. Moreover, this method does not give very accurate results with solids containing few acidic sites. Nevertheless, it appears for our active carbon that "phenolic" sites are a majority. The amount of each type of acidic sites does not change very much with the nitric treatment. 

A quantitative comparison between direct potentiometric titration and the classical Boehm method for characterization of the acidity of activated carbons has been drawn by various researchers [85,215,218], Although a rather acceptable agreement is generally accomplished for both methods, when applied to a carbon snrface where oxygen is the major constituent, some discrepancies exist. Figure 2.7 shows a correlation between the total acidic groups detected on an activated carbon using both Boehm and potentiometric titrations. 

Summarizing, both Boehm and potentiometric titration provide information about the acidic and basic species on the carbon surface, excluding neutral functionalities, (ca. ketones, aldehydes, esters, ethers, etc.) from being detected on the surface. Thus, to account for these species, complementary analytical techniques, such as spectroscopic and thermal methods need to be used. 

The well-known Boehm method [47,48], based on the titration of acidic centers using several basic solutions of increasing strengths (EtONa, NaOH, Na2C03, and NaHCOj) has become very popular (see Table 3.1) owing to its simplicity its protocol is exhaustively repeated in the literature (see, for example. Reference... 

Boehm s titration This method was used to evaluate the amount of carboxylic acid, lactone and phenol groups present at the surface of caibon samples, by stirring 0.5 g of carbon in 50 mL of NaHCOs, Na2C03 or NaOH [2]. The carbon was filtered out after 24 h and the filtrate was titrated with hydrochloric acid. All solutions were prepared using freshly distilled decarbonated water and maintained under nitrogen. 

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