Fundamentals of TiO2 Photocatalysis. Consequences for Some Environmental Applications

Abstract

This chapter considers the fundamental phenomena occurring when TiO 2 is excited by photons. The focus is first on the formation and fate of the charges generated by the excitation. Then, the roles in photocatalytic reactions of water and oxygen which are almost always present are presented and discussed; the effects of adding ozone or hydrogen peroxide are also briefly indicated. Regarding the photocatalytic degradation of organic compounds -which is involved in potential applications such as self-cleaning materials and air or water purification -the following issues are examined: the hole-induced and hydroxyl radical-induced pathways and the predictability of the nature of the intermediate products. Some of the material aspects of photocatalysis are dealt with through (1) the influence of the structural and textural characteristics of pristine TiO 2 , (2) the effects of modifying TiO 2 by metal deposits or ion doping, and (3) the basic outcomes of combining TiO 2 with either another oxide (insulating or semiconducting) or a supplementary adsorbent of high surface area. Conclusions are drawn from these fundamental topics about the applicability of TiO 2 photocatalysis.

Experimental measurement and modelling of reactive species generation in TiO2 nanoparticle photocatalysis

“…The absorption of photons of appropriate wavelength (λ<390 nm) leads to the formation of electron-hole pairs, which can migrate to the surface of the photocatalyst. When in the presence of water and oxygen molecules, this results in the production of reactive oxygen species (ROS), such as hydroxyl radicals and superoxide radical anions [4,5]. The potential of these highly reactive, non-selective species has been reported in previous research for the inactivation of pathogens and the degradation of persistent organic pollutants, such as solvents, dyes, pesticides, pharmaceuticals and personal care products, inter alia [6,7].…”

Integral Design Methodology of Photocatalytic Reactors for Air Pollution Remediation

“…According to which source of radiation will be used, the geometry of the reactor may differ greatly. Because of the TiO 2 bandgap, it cannot profit from a large portion of the solar spectrum [ 18 ]; thus, efforts are constantly directed towards the doping of TiO 2 for extending the absorption at larger wavelengths (>390 nm) towards the visible range. Doping with C, N, Ce, etc.…”

Solar or UVA-Visible Photocatalytic Ozonation of Water Contaminants

“…The former, ozonation, implies direct ozone-organic reactions and ozone decomposition reactions due to the appearance of hydrogen peroxide, increase in pH, and ozone photolysis, at least with radiation wavelengths of up to 320 nm. Photocatalytic oxidation involves hydroxyl radical formation from holes in the catalyst valence band and probably from the superoxide ion radical formed from excited electrons of the catalyst conduction band [ 24 , 25 , 26 , 27 ]. Detailed explanations of these mechanisms are available elsewhere [ 56 , 85 ].…”

“…Photocatalytic oxidation takes place when photons, with an energy equal to or higher than the band gap energy of the catalyst (or semiconductor) used, excite electrons from the highest occupied molecular orbital (HOMO) or the valence band to be transferred to the lowest unoccupied molecular orbital (LUMO) or conduction band of the catalyst. In this way, two charge carriers occur: an oxidizing point in the HOMO or hole, and electrons able to trigger reduction reactions in the LUMO [ 24 , 25 , 26 , 27 ]. The main problem with this process is that the electron-hole pair recombination inhibits the oxidation-reduction steps.…”

“…In fact, a search in the Web of Science database from 2000 to the present day, with the keywords ozonation and water or photocatalytic oxidation and water, produces 5656 and 10,434 papers published on the two processes, respectively. Furthermore, their main characteristics, possible applications, and experimental and practical results (with regard to ozonation) have been the subject of different reviews [ 21 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ].…”