**3. Results and discussion**

## **3.1. Characterization of ZnO-SEP catalysts**

## *3.1.1. XRD analysis*

washings and centrifugations (at 4000 rpm) were applied. The resulting catalysts were dried at 100°C for 12 h and then calcined at 500°C with a rate of 10°C min-1 for 5 h. Finally, the catalysts were ground into fine powder and named as 0.125 M ZnO-SEP, 0.25 M ZnO-SEP and 0.5 M

X-ray diffraction was used to monitor the formation of crystal planes and measure the crystalline size of ZnO nanoparticles. The analyses were recorded on a Rigaku-D/MAX-Ultima diffractometer using Cu-Kα radiation (λ=1.54 Å) operating at 40 kV and 40 mA and scanning rate of 2 min-1. Nitrogen sorption analysis was used to measure the surface areas, pore volumes

The nitrogen adsorption/desorption isotherms were obtained at 77 K using Quantachrome Nova 2200e automated gas adsorption system. The specific surface areas were determined using multi-point BET analysis and the pore sizes were measured by the BJH method of

The surface morphologies were determined using scanning electron microscopy (SEM) in combination with energy-dispersive X-ray analysis on an ESEM-FEG/EDAX Philips XL-30 instrument operating at 20 kV. The catalysts were fixed with carbon tape prior to the metalli‐

In X-ray photoelectron spectroscopy (XPS) tests, Thermo Scientific K-Alpha X-ray photoelec‐ tron spectrometer equipped with electron analyzer and Al-Kα micro-focused monochromator was used. The areas of peaks were estimated by calculating the integral of each peak after subtracting a Shirley background and fitting the experimental curve to a combination of Lorentzian/Gaussian lines. Binding energy shifts were observed in the samples and the instrument was calibrated using the carbon peak (C-1s) at 285 eV as in the other studies [20,21].

The UV-vis diffuse reflectance spectra (UV-vis DRS) of the supports, 0.25 M ZnO and sup‐ ported catalysts were obtained using UV-vis spectrophotometer (UV-2450, Shimadzu) equipped with an integrating sphere reflectance accessory. The baseline correction was done by BaSO4. The spectra were recorded in the range 200–600 nm for the catalysts and the FA

For the photocatalytic experiments, the details of reaction systems were given in our previous studies [22,23]. Briefly, a Pyrex flask reactor was located in an "irradiation box" equipped with eight black light lamps (Philips TL 15W/5BLB) with an emission maximum at λ=365 nm. The lamps were positioned to surround the flask with an incident photon flux of 4.7×1015 photons s-1. Then, 0.2 g of catalysts with 200 mL of 3.27 mg L-1 MO (unless the concentration effect of MO was controlled) was used as reaction solutions. The flask had an inlet for the air circulation and an outlet for the collection of aliquots. Prior to irradiation, suspensions were magnetically stirred in the dark for 30 min. UV-vis spectrophotometer (UV-2450, Shimadzu) was used to monitor the absorbance spectra of MO at 464 nm as a function of irradiation time. All experi‐ ments were performed at room temperature and without concerning the degradation inter‐ mediates. Also, measurements were conducted at least twice and the average value was

recorded. MO decolorization percentages were calculated by the following equation:

ZnO-SEP.

adsorption.

and pore diameters of the catalysts.

234 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

using BaSO4 as a reference.

zation process with gold (Sputter Coater-Balzer SCD050).

The phase identification of the raw SEP and the supported catalysts were performed by XRD analysis from 0° τo 70° (2*θ*) (Figure 1). The characteristic d 110 reflection of the SEP was noticed with a basal spacing of 12.19 Å at 7.24° (2*θ*). ZnO loadings decrease the intensities of all SEP reflections. For the supported catalysts, alteration in ZnO concentrations did not modify the position of the SEP peaks due to the non-expandable nature of the SEP. Supported catalysts also exhibited wurtzite ZnO structure with d 100, d 002, d 101, d 102, d 110, d 103 and d 200 crystal planes at 31.9°, 34.6°, 36.4°, 47.7°, 56.7°, 63.1° and 66.6° (2*θ*), respectively. The broad and less intense ZnO peaks got sharper and more intense with the increments in ZnO loadings.

**Figure 1.** XRD patterns of SEP and supported catalysts (Z: ZnO).

The crystalline sizes (DZnO) calculated using Scherrer's equation for the broadening of d 101 reflection of ZnO were found as 16.1 nm (for 0.25 M ZnO), 13.4 nm (for 0.125 M ZnO-SEP), 12.3 nm (for 0.25 M ZnO-SEP) and 15.1 nm (for 0.5 M ZnO-SEP) (Table 1). The reduction in the ZnO crystalline sizes and the decrements in the intensities of SEP reflections may suggest highly dispersed ZnO nanoparticles over the surface and bulk.


a Calculated from the (101) diffraction peak of ZnO using the Scherrer equation.

b Determined from nitrogen adsorption-desorption isotherms using BET equation.

c Determined from cumulative adsorption pore volume using BJH method.

d Determined from adsorption pore size using BJH method.

**Table 1.** Crystalline sizes (DZnO), surface areas (BET), total pore volumes (Vpore) and pore radius (Rpore) of 0.25 M ZnO, SEP and supported catalysts.

## *3.1.2. Nitrogen adsorption desorption isotherms*

The textural characterization points out that 0.25 M ZnO had type II isotherms, which refer to non-porous materials (Figure 2A and 2B). However, the raw SEP and the supported catalysts had type IV nitrogen adsorption-desorption isotherms typical of mesoporous structures with hysteresis loops [24]. Almost similar pore radius was detected for all catalysts and the SEP (Table 1). The capillary condensation taking place within the mesopores in the high P/P0 range became more difficult with the formation of narrower loops, owing to the higher ZnO loading concentrations. Accordingly, 0.125 M ZnO-SEP shows almost similar surface area and pore volume with respect to the SEP, whereas 0.25 M ZnO-SEP and 0.5 M ZnO-SEP reveal lower values. The partial blockage of the pores by the ZnO nanoparticles on the supports' surface sites, edges and corners was expected to induce an easier attraction among MO molecules and the catalyst particles.

## *3.1.3. SEM (EDX) analysis*

Figure 3 illustrates the SEM and elemental mapping images of 0.25 M ZnO-SEP. The charac‐ teristic fibrous structure of SEP and pseudo-spherical shape of the as-prepared ZnO catalyst were reported previously [17,22]. The heterogeneously dispersed ZnO nanoparticles within the SEP matrix resulted in a different morphology in comparison to the raw SEP and unsup‐ ported catalyst. In the EDX analysis of 0.25 M ZnO-SEP (not shown), the reduction in the peak intensities and percentages of SiO2 and MgO (from 69.25% to 16.08% for SiO2 and from 28.92% to 10.93% for MgO) and also simultaneous detection of ZnO peak signified the in situ build<sup>236</sup> Advanced Catalytic Materials - Photocatalysis and Other Current Trends 2 ZnO/Sepiolite Catalysts – Characterization and Photoactivity Measurements http://dx.doi.org/10.5772/61973 237

The crystalline sizes (DZnO) calculated using Scherrer's equation for the broadening of d 101 reflection of ZnO were found as 16.1 nm (for 0.25 M ZnO), 13.4 nm (for 0.125 M ZnO-SEP), 12.3 nm (for 0.25 M ZnO-SEP) and 15.1 nm (for 0.5 M ZnO-SEP) (Table 1). The reduction in the ZnO crystalline sizes and the decrements in the intensities of SEP reflections may suggest

 **g-1)**

0.25M ZnO 16.1 7.58 0.012 17.7 SEP - 104.5 0.140 14.9 0.125M ZnO-SEP 13.4 91 0.151 19.1 0.25M ZnO-SEP 12.3 47 0.077 20.2 0.5M ZnO-SEP 15.1 51 0.091 14.8

**Table 1.** Crystalline sizes (DZnO), surface areas (BET), total pore volumes (Vpore) and pore radius (Rpore) of 0.25 M

The textural characterization points out that 0.25 M ZnO had type II isotherms, which refer to non-porous materials (Figure 2A and 2B). However, the raw SEP and the supported catalysts had type IV nitrogen adsorption-desorption isotherms typical of mesoporous structures with hysteresis loops [24]. Almost similar pore radius was detected for all catalysts and the SEP (Table 1). The capillary condensation taking place within the mesopores in the high P/P0 range became more difficult with the formation of narrower loops, owing to the higher ZnO loading concentrations. Accordingly, 0.125 M ZnO-SEP shows almost similar surface area and pore volume with respect to the SEP, whereas 0.25 M ZnO-SEP and 0.5 M ZnO-SEP reveal lower values. The partial blockage of the pores by the ZnO nanoparticles on the supports' surface sites, edges and corners was expected to induce an easier attraction among MO molecules and

Figure 3 illustrates the SEM and elemental mapping images of 0.25 M ZnO-SEP. The charac‐ teristic fibrous structure of SEP and pseudo-spherical shape of the as-prepared ZnO catalyst were reported previously [17,22]. The heterogeneously dispersed ZnO nanoparticles within the SEP matrix resulted in a different morphology in comparison to the raw SEP and unsup‐ ported catalyst. In the EDX analysis of 0.25 M ZnO-SEP (not shown), the reduction in the peak intensities and percentages of SiO2 and MgO (from 69.25% to 16.08% for SiO2 and from 28.92% to 10.93% for MgO) and also simultaneous detection of ZnO peak signified the in situ build-

**<sup>b</sup> Vpore (cm3**

 **g-1)**

**<sup>c</sup> Rpore (Å)d**

highly dispersed ZnO nanoparticles over the surface and bulk.

a Calculated from the (101) diffraction peak of ZnO using the Scherrer equation. b Determined from nitrogen adsorption-desorption isotherms using BET equation. c Determined from cumulative adsorption pore volume using BJH method.

d Determined from adsorption pore size using BJH method.

*3.1.2. Nitrogen adsorption desorption isotherms*

ZnO, SEP and supported catalysts.

the catalyst particles.

*3.1.3. SEM (EDX) analysis*

**Catalysts DZnO (nm)a BET (m2**

*figure\_2* **Figure 2.** (A) Nitrogen adsorption/desorption isotherms and (B) pore size distribution plots of 0.25 M ZnO, SEP and supported catalysts.

up of ZnO nanoparticles in the SEP structure. In mapping images, ZnO existence was proven by the dominating Zn signals within less dense regions of Si and Mg constituents.

**Figure 3.** SEM and mapping images of 0.25 M ZnO-SEP.

## *3.1.4. XPS analysis*

In order to control the presence of ZnO on the SEP host, the surface chemical composition was analyzed by the XPS method, focusing in particular on the binding energies of the typical lines of Zn and O (Figure 4). The survey scan of 0.5 M ZnO-SEP reveals Zn peaks in addition to the Mg (49.5 eV), Al (75.6 eV), Si (103 eV), O (531.47 eV) and some Auger peaks of the SEP (Figure 4A). Figure 4B depicts the presence of Zn ion with a doublet matching to Zn 2p3/2 and 2p1/2 core levels (Figure 4B). This shows the presence of Zn2+ ions in an oxide environment [25,26]. The O1s photoelectron peak is deconvoluted by three subspectral parts (Figure 4C). The peak at 527 eV is attributed to the lattice oxygen in a Zn-O-Zn network with 8.7% spectral area. The shift in the peak position with respect to ZnO binding energy of 530.5 eV can be due the complex configuration in the ZnO-SEP matrix. For the other two components, 64.7% spectral area refers to MgO and Al2O3 at 531 eV and 26.5% spectral area corresponds to SiO2 at 532.81 eV.

## *3.1.5. UV-vis DRS analysis*

UV-vis absorption spectroscopies of the SEP and supported catalysts are presented in Figure 5A. The raw SEP shows an extended profile in between 200 and 600 nm. In contrast, 0.25 M ZnO reveals its distinctive edge below 400 nm. The supported catalysts resemble this charac‐ teristic edge, which is more pronounced for the catalyst having the highest ZnO concentration. The UV activities of the supported catalysts are evidenced by the evaluated band-gap energies based on Kubelka-Munk transformed reflectance spectra as 3.08 eV for 0.125 M ZnO-SEP, 3.09 eV for 0.25 M ZnO-SEP and 3.16 eV for 0.5 M ZnO-SEP (Figure 5B).

ZnO/Sepiolite Catalysts – Characterization and Photoactivity Measurements http://dx.doi.org/10.5772/61973 239

**Figure 4.** XPS analysis of 0.5 M ZnO-SEP: (A) survey spectrum, (B) Zn 2p spectra and (C) O 1s spectra.

**Figure 5.** A) Diffuse reflectance spectra of SEP, 0.25 M ZnO and supported catalysts and (B) Kubelka-Munk trans‐ formed reflectance spectra of 0.25 M ZnO and supported catalysts.

### **3.2. Decolorization of MO by ZnO-SEP catalysts**

up of ZnO nanoparticles in the SEP structure. In mapping images, ZnO existence was proven

In order to control the presence of ZnO on the SEP host, the surface chemical composition was analyzed by the XPS method, focusing in particular on the binding energies of the typical lines of Zn and O (Figure 4). The survey scan of 0.5 M ZnO-SEP reveals Zn peaks in addition to the Mg (49.5 eV), Al (75.6 eV), Si (103 eV), O (531.47 eV) and some Auger peaks of the SEP (Figure 4A). Figure 4B depicts the presence of Zn ion with a doublet matching to Zn 2p3/2 and 2p1/2 core levels (Figure 4B). This shows the presence of Zn2+ ions in an oxide environment [25,26]. The O1s photoelectron peak is deconvoluted by three subspectral parts (Figure 4C). The peak at 527 eV is attributed to the lattice oxygen in a Zn-O-Zn network with 8.7% spectral area. The shift in the peak position with respect to ZnO binding energy of 530.5 eV can be due the complex configuration in the ZnO-SEP matrix. For the other two components, 64.7% spectral area refers

to MgO and Al2O3 at 531 eV and 26.5% spectral area corresponds to SiO2 at 532.81 eV.

eV for 0.25 M ZnO-SEP and 3.16 eV for 0.5 M ZnO-SEP (Figure 5B).

UV-vis absorption spectroscopies of the SEP and supported catalysts are presented in Figure 5A. The raw SEP shows an extended profile in between 200 and 600 nm. In contrast, 0.25 M ZnO reveals its distinctive edge below 400 nm. The supported catalysts resemble this charac‐ teristic edge, which is more pronounced for the catalyst having the highest ZnO concentration. The UV activities of the supported catalysts are evidenced by the evaluated band-gap energies based on Kubelka-Munk transformed reflectance spectra as 3.08 eV for 0.125 M ZnO-SEP, 3.09

by the dominating Zn signals within less dense regions of Si and Mg constituents.

**Figure 3.** SEM and mapping images of 0.25 M ZnO-SEP.

238 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

*3.1.4. XPS analysis*

*3.1.5. UV-vis DRS analysis*

Control experiments were performed under dark conditions for 0.25 M ZnO and 0.25 M ZnO-SEP (Figure 6). The remaining MO percentage was found as 83% in the presence of 0.25 M ZnO after 60 min, and then no significant variation was detected. The supported catalyst, however, shows a lower percentage (73%) in 80 min. The interaction between the SEP matrix and ZnO nanoparticles facilitated the adsorption of MO molecules. The low isoelectric point of the SEP (IEPSEP~2) increased the contact possibility of positively charged Zn2+ ions or ZnO (with high isoelectric point ~9) [18]. The attraction between positively charged Zn2+ ions and negatively charged MO moiety decreased the intensity of absorption band at 464 nm, which is responsible from the destruction of the chromophoric (-N=N-) group. The negligible photolysis of MO proves the active role of ZnO nanoparticles within the supported catalyst system. It is also noted that the raw SEP does not participate in the decolorization of MO (not shown).

The photoactivities of the supported catalysts were examined under irradiation (Figure 6, inset). The highest MO remaining percentage (43%) was obtained in the presence of 0.125 M ZnO-SEP within 120 min. The percentage of MO decreased upon the loading concentration of ZnO. The synergy established on the mixed structures of ZnO and SEP improved the catalyst performance. Accordingly, 0.5 M ZnO-SEP shows the best activity with the lowest MO remaining percentage (19.8%) in solution.

**Figure 6.** Photolysis and dark adsorption experiments. Inset: Photocatalytic activities of the supported catalysts.

The whole mechanism starts with the illumination of ZnO nanoparticles and production of electron-hole pairs (Eq. (1)).

$$\text{ZnO} + \text{h}\gamma \text{(UV)} \rightarrow \text{e}\_{\text{CB}} + \text{h}\_{\text{VB}} \text{ \textdegree \tag{1}$$

The main active species in such processes are known as hvb+ , ⋅ OH and O2 ⋅− [27,28]. The radicals may form either by the reactions of photogenerated holes or electrons. The photogenerated electron (ecb<sup>−</sup> ) can be easily transferred to the O2 molecules promoting the O2 ⋅− formation and then converted to the active <sup>⋅</sup> OH via hydrogen peroxide and hydroperoxyl radical generations (Eqs. (2)–(5)).

nanoparticles facilitated the adsorption of MO molecules. The low isoelectric point of the SEP (IEPSEP~2) increased the contact possibility of positively charged Zn2+ ions or ZnO (with high isoelectric point ~9) [18]. The attraction between positively charged Zn2+ ions and negatively charged MO moiety decreased the intensity of absorption band at 464 nm, which is responsible from the destruction of the chromophoric (-N=N-) group. The negligible photolysis of MO proves the active role of ZnO nanoparticles within the supported catalyst system. It is also

The photoactivities of the supported catalysts were examined under irradiation (Figure 6, inset). The highest MO remaining percentage (43%) was obtained in the presence of 0.125 M ZnO-SEP within 120 min. The percentage of MO decreased upon the loading concentration of ZnO. The synergy established on the mixed structures of ZnO and SEP improved the catalyst performance. Accordingly, 0.5 M ZnO-SEP shows the best activity with the lowest MO

noted that the raw SEP does not participate in the decolorization of MO (not shown).

**Figure 6.** Photolysis and dark adsorption experiments. Inset: Photocatalytic activities of the supported catalysts.

( ) - + ZnO + h UV e + h CB VB

g

The main active species in such processes are known as hvb+

The whole mechanism starts with the illumination of ZnO nanoparticles and production of

may form either by the reactions of photogenerated holes or electrons. The photogenerated

) can be easily transferred to the O2 molecules promoting the O2

® (1)

OH and O2

⋅− [27,28]. The radicals

⋅− formation and

, ⋅

remaining percentage (19.8%) in solution.

240 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

electron-hole pairs (Eq. (1)).

electron (ecb<sup>−</sup>

$$\bullet \bullet\_{2} \text{+} \text{e}^{\cdot} \rightarrow \bullet \text{\_{2}}^{\cdot \text{-}} \tag{2}$$

$$\text{O}\_2\text{"} \text{"} \text{H}^+ \rightarrow \text{HO}\_2^\cdot\text{"}\tag{3}$$

$$\text{2HO}\_2^\* \to \text{H}\_2\text{O}\_2 \text{\textquotedblleft O}\_2 \tag{4}$$

$$\mathrm{H\_2O\_2} + \mathrm{O\_2}^- \rightarrow \mathrm{"OH} + \mathrm{O\_2} + \mathrm{OH}^- \tag{5}$$

At the same time, the photogenerated hvb+ can be captured on the catalyst surface undergoing charge transfer with adsorbed water molecules or with surface-bound hydroxide species to generate active <sup>⋅</sup> OH (Eqs. (6) and (7)) and results in the degradation of MO molecule (Eq. (8)).

$$\rm H\_{\rm vb}^{\cdot} + H\_2O \to H^{\prime} + \rm {}^{\bullet}OH \tag{6}$$

$$\text{CH}\_{\text{vb}}\text{}^{\text{+}}\text{OH}\text{'}\rightarrow\text{'OH}\tag{7}$$

$$\text{"OH} + \text{MO} \rightarrow \text{MO} \left( \text{decolorization and degradation} \right) \tag{8}$$

Thus, the separation of the charge carriers also enhanced the yield of hydroxyl radicals (highly reactive electrophilic oxidants) and improved the photocatalytic activity of the supported catalysts. The addition of the hydroxyl radical to the double bond of the azo group is referred to as the main reaction pathway, with the disappearance of the color. The second route followed the addition of hydroxyl radical to the aromatic rings [29,30]. Alternatively, hydroxyl radicals may attack the carbon atom bearing the azo bond [30]. Further attacks caused the formation of sulfonated intermediates, aromatic amine and phenolic compounds and ring open fragments [31].

Kinetic analysis was performed by varying the initial ΜΟ concentration from 16.2 to 3.27 mg L-1 in the presence of 0.5 M ZnO-SEP (not shown). The linearity obtained between ln(*C*0/*C*) versus *t* plot indicates pseudo-first-order kinetics, where *C*0 is the initial concentration of ΜΟ (mg L-1) and *C* is the concentration of ΜΟ (mg L-1) at irradiation time *t* (min) (Figure 7). The rate constants *k* (min-1) hence calculated (slopes of the lines) were found to decrease with increasing concentration of ΜΟ (from 0.01 (3.27 mg L-1) to 0.005 min-1 (16.2 mg L-1)). This can be a result of blocking of the photocatalytically active sites on the supported catalyst and reducing the interaction of photons with these sites.

**Figure 7.** Effect of initial MO concentration on the photoactivity of 0.5 M ZnO-SEP.

The stability of 0.5 M ZnO-SEP catalyst was examined by recycling experiments (Figure 8). For each run, 0.5 M ZnO-SEP was filtrated, washed and calcined at 500°C for 2 h. After four cycles, the percentage of MO remaining in solution was found to increase by only approximately 3% (from 19.8% to 22%). The slight increment in the percentage can be attributed to the catalyst loss during each collection and rinsing steps.

**Figure 8.** Reuse properties of 0.5 M ZnO-SEP.
