Bulk and Nanocatalysts Applications in Advanced Oxidation Processes

Advanced oxidation processes (AOPs) are considered to be vital methods for treating the contaminations produced mainly by the human activations. In present-day, UV light or solar light, bulk and nanophotocatalysts are often used to enhance this technology by creating the highly reactive species such as the hydroxyl radicals. Extreme hydroxyl radical is considered as a key to start the photoreaction. Photoreaction is widely used in treatment of Lab and industrial contaminations, preparation of compounds and produced the renewable energy, so it’s classified as green technique. In order to improve the efficiency of this reaction with fabrication the surface of the used photocatalyst such as metal doped, sensitized and produced a composite as bulk catalyst or nano catalyst.


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 (O 2 •− ), hydrogen peroxide (H 2 O 2 ) 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.

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 Essential mechanism for generating the ROSs under illumination of photo-catalyst particles [1]. 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].   In this kind, the transition occurrs as a radiative transition in direct band gap semiconductor. It is created when the Free photo electron in CB drops directly into free photo hole (an unoccupied state) in the VB and associated together. Note Figure 5

(A).
ZnO have a direct band gap.

Volume recombination
Centers recombination or Trap-assisted recombination This case obtains, when defect of semiconductor by impurities that given a new levels (as traps of photoelectron and photohole). It leads to liberate heat as phonon in indirect band gap semiconductor. Note Figure 5(B).
Pure TiO 2 and defect of TiO 2 by metal, which had given an indirect band gap.

Recombination of an exciton
This case occurs at low temperature, when the traps at or near the surface or interface of the semiconductor, capture the photo electron-hole as exciton.
That attitude to dangling bonds caused by the sudden discontinuation of the semi-conductor crystal with energy just below the band gap value. Note Figure 5(C).
It happed in solar cells and light emitting diode (LED) containing shallow levels.
Auger recombination -This recombination involves three carriers: Free photo electron, free photo whole recombine, and the emitting the energy as heat or as a photon (non-radiative process). The transition of energy deals with as intra-band transitions, which resulting when either electron elevates in higher levels of conduction band or hole deeper push into the valence band. Note     Table 3.

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, TiO 2 ZrO 2 , ZnS, WO 3 , CdS and Mn 3 O 4 . 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.

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.  © 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.