Characterization of materials

In this part of the work, a special multilayer material produced by Ahlström Paper Group Company was used. The methods used for preparation of these materials and their characteristics were discussed in details in chapter II.

Our study was carried out using three different media; AC media, TiO2/AC media (tissue) and TiO2 media (Figs.II-1, 2 and 3, respectively, in Chapter II) and then compared to the original granular AC made from coconut which used in all studied media containing carbon. The compositions of these different media are summarized in chapter II.

The specific surface area of samples was determined using a Micrometrics ST-2000 automated apparatus based on the Brunauer–Emmett–Teller (BET) method at 77K with N2 as adsorbent and H2 as the carrier gas. The surface structure and particle size of the used media were investigated using Scanning Electron Microscopy (SEM). Additionally, the composition of AC in the tissue (AC/ TiO2) and the original AC powder was analyzed by EDX. The thermogravimetric analysis, TGA was used to identify the thermal loss of the original AC sample. This TGA analysis will be later used to compare the thermal behaviour of the used and virgin AC samples.

2.1. Surface area and porosity

The analyzed values of specific surface areas (SBET) and pore size distribution for the original granular AC and those of TiO2 media are presented in Table IV-1.

As expected, the TiO2 media exhibits very small SBET (75 m².g^-1) as compared to that of AC powder (1065 m².g^-1). The original activated carbon has a microporous volume of0.46 cm³.g^-1, and a mesoporous one of 0.046 cm³.g^-1. For TiO2 media, a significant decrease in microporous volume was observed and it was mainly mesoporous.

118 Table IV-1

Apparent surface area and pore volume distribution of AC powder and TiO2 media Material BET Surface

(m².g^-1) *Microporous

Volume (cm³.g^-1) *Mesoporous

Volume (cm³.g^-1) Size of grains (mm) Media TiO2

75 0.010 0.094 0.005 - 0.01

Granular AC 1065 0.460 0.046 0.25 - 0.60

*Microporous volume: pore size < 2nm, Mesoporous volume: 2nm < pore size < 50 nm.

2.2. Electron microscopic analysis

The SEM morphology of the granular AC, TiO2 media and the AC in TiO2/ AC tissue is presented in Figures IV-1, IV-2 and IV-3, respectively. Selected SEM images of these media are compared. As it is clearly shown in Figure IV-1, the pores are well developed in the granular AC. Figure IV-3 shows the binding of TiO2 to the surface media.

Fig. IV-1. SEM image of the original granular AC (x350, left and x 3000, right).

Fig. IV-2. SEM image of AC in TiO2 / AC tissue (x350, left and x 3000, right).

119 Fig. IV-3. SEM image of the surface of TiO2 media (x 1000, right) (Thevenet et al., 2005).

2.3. Energy dispersive X-ray microanalyses (EDX)

EDX analysis was used to determine qualitatively the elemental composition of the original AC powder and the AC in the TiO2/AC tissue in the selected regions shown in Fig.

IV-4a and IV-4b. Figure IV-5a shows the main peaks of carbon and oxygen in the original granular AC sample. For the AC in the TiO2/AC tissue, two regions are analysed, showing very different results concerning the presence of significant amounts of Ti, Si and O in Region I and the absence of Ti in Region II (Fig. IV-5b). So it is considered that the sample of AC in the TiO2/AC tissue contained these three main elements (Ti, Si, and O) beside C. The elemental composition percentages of these samples are given in Table IV-2.

Table IV-2

EDX elemental composition of the original granular AC and AC in TiO2 /AC tissue, in two different regions as indicated in Fig. IV.4

Composition

C O Ti Si

Sample

Wt % *At% Wt% *At% Wt% *At

% Wt% *At %

Original granular

AC 95.20 96.35 4.80 3.65 - - - -

AC in TiO2/AC

tissue (region I) 0.18 0.09 60.16 76.47 17.61 7.48 22.05 15.96 AC in TiO2/AC

tissue (region II) 78.98 83.55 20.39 16.19 - - 0.37 0.17 *At: atomic percentage.

120 Fig IV-4. Cross sectional SEM of the original granular AC (a) and AC in the TiO2/AC tissue (b; Region I & Region II) in which two different regions are pointed out.

Fig. IV-5. EDX diagrams of the original granular AC (a) and AC in the TiO2/AC tissue in two different regions (b; Region I & Region II).

a

b

Region II

Region I Region II

a b

121 It must be emphasized that the elemental percentages obtained by EDX analysis in this table give qualitatively and not quantitavely an idea about the proportion of certain elements in specific region on the sample surface and not on the whole surface.

2.4. Thermogravimetric analysis of the original activated carbon

According to the literature, the thermogravimetric analysis (TGA) allows the thermal decomposition of acidic groups on the activated carbon surface (De la Puente et al., 1997;

Figueiredo et al., 1999). It shows also the thermal desorption of adsorbed species after adsorption and oxidation steps.

The protocol of the thermogravimetric analysis is detailed in the chapter II. These measures were made under atmosphere of nitrogen for overcoming the combustion of activated carbon.

The results of TGA of AC are represented in Figure IV-6. A first loss of weight (3.92

%) is observed at nearly 100 °C corresponding to the rate of humidity of AC. There is then a second weight loss (4.39 %) much slower and progressive until approximately 630 °C. Then, there is an inflexion point and another weight loss (12.74 %) at 700 °C corresponding to the calcination of the surface functions formed during the activation of carbon.

Fig. IV- 6. Thermogravimetric analysis of the original granular AC.

122 De la Puente et al. (1997) and Figueiredo et al. (1999) cited that the successive weight losses by increasing the temperature corresponds to the decomposition of the carboxylic groups (150-400 °C), lactones functions (350-600°C) and phenols groups (which are decomposed between 600 and 700 °C). Moreover, Stratakis and Stamatelos (2003) assumed the first decrease up to about 150°C to be mainly due to the water evaporation– desorption. A second loss observed between 200 and 420°C was attributed to desorption of organic compounds.