Bulk and Nanocatalysts Applications in Advanced Oxidation Processes

1. Introduction

In this section, the advanced Oxidation Processes concepts will be related to use of the bulk and the nano- catalysts as vital materials for easily generating a highly oxidizing species and reactive oxygen species (ROSs) such as in aqueous or alcoholic solution [1]. ROSs are contains three primary kinds: superoxide anion (O₂^•−), hydrogen peroxide (H₂O₂) and the hydroxyl radical (HO^•) [2], which produced from reaction of adsorbed oxygen molecule on catalyst’s surface with one electron in conductive band under illumination by light as UV, or visible or solar light, this mechanism is useful to reduce the recombination process and increased the life time of hole in valance band [3, 4]. As explained in Figure 1.

The ROSs are having the electron configurations as tabled in Table 1 [5, 6, 7, 8].

Table 1.

Electronic configurations and chemical formulas for the ROSs types.

2. Advance oxidation process applications

In the last few years, several researches have predominated in many universities and research centers on the scientific ventures to mainly treat the contaminations that produced by textile factories [9, 10, 11], reduced the degradation of food’s dye [12], decolorization of colored organometallic complexes [13], degradation of toxic cyclic compounds [14] and produced a hydrogen from alcohol as renewable energy [15]. The effective materials for all above mention research are generated the hydroxyl radical in aqueous solution with maximum oxidation power equals to 2.8 V [1]. Based on to the AOPs, the common sources for creation of^.OH in AOPs are illustrated in Figure 2, which regards as power to star the dark or photo reactions [1, 16, 17, 18, 19].

Fortunately, the benefits of AOPs are more than those of drawbacks. The benefits of AOPs are summarized up as [1, 20] follows to:

1. Create a large number of free radicals species.

2. Have the appropriate potential to depress the hazardous organic pollutants by complete their mineralization and producing CO₂ and H₂O.

3. Reduce the time of dark or photoreaction.

4. Have low economic cost.

Whereas, the drawbacks of AOPs [1, 21] are quenching the reaction rate with increasing the scavenger contains (mostly peroxide ion) and may be generated the undesirable hazardous products that prevented the complete of mineralization process, hence, the altered of pH or using further cost steps may be essentially to treat their problems.

3. Bulk and nano-catalysts

In general, the catalysts may be metal or alloy or semiconductor. Semiconductor is wide used as catalyst and can be element or compound as amorphous or crystalline or rock salt crystal. Because of semiconductors have intermediate properties between metal and insulator, which has given them rescannable electronic and structural properties, hence, semiconductor is classified as a better-known kinds, as mentioned in Figure 3 [22, 23, 24].

The usages of the bulk and nano catalysts are increment with increasing the development of life activations. The catalysts were known for the long time to increase the rate of reaction with decreasing the time of reaction and the activation energy in dark reaction or photoreaction. In order to use the catalyst in photoreaction as photo catalyst, must have a band gap with raged about 1.1 eV to 5.0 eV [1, 24]. Referring to Figure 4, several band gap energy positions of some common photo catalysts can be displayed [1, 25, 26, 27].

The mainly problem in bulk and nano catalyst is recombination process, which results in diminishing the efficiency of used photocatalyst by returning the photoelectron from conductive band to valance band and reacting with photohole immediately. The recombination includes four kinds can be followed in Table 2 and Figure 5 [1, 28, 29, 30].

In order to improve the activity of photocatalysts must depress the recombination with modify their surfaces with three main methods: surface sensitization, metalized photocatalyst surface and coupled for two or more photocatalysts as Composite. The details of these modification methods are mention in Table 3 and Figure 6 [40].

Table 3.

The description of the methods for modifying photocatalysts [31, 32, 33, 34, 35, 36, 37, 38, 39].

4. Used of bulk or nano catalyst in AOPs

There are many common application of AOPs in environment fields by using the white photocatalyst or its modified such as ZnO, TiO₂ ZrO₂, ZnS, WO₃, CdS and Mn₃O₄. The efficiencies with used these photocatalysts are altered with using AOPs methods. The efficiency of the photoreaction depends mostly on the concentration of colored material, initial pH which affected on the surface of photocatalyst and the temperature. As shown in Table 4.

5. Conclusions

This chapter focuses on the source of hydroxyl radical which produces via the advance oxidation process. Indeed, this process interests in the forming the different species, which in the final step generates a hydroxyl radical. The photocatalyst enhances the generating of hydroxyl radicals (2.8 V) in aqueous solution under Uv- light or visible or solar. The photoexitation of photocatalyst leads to jump of electon to conductive band then return to valance band and liberates a hot this process called recombination. It is depressed the efficiency of photoreaction. However, some procedures used to modify the photocatalyst surface.

Acknowledgments

The author wants to thank his family for helping him in carrying out this work.