While the search for a high-performance photocatalyst intensifies, the technology at this stage is still remote from the commercial requirements. The major hurdles include:
FIGURE 8.35 Kinetics of E. coli bacteria disinfection for undoped and N-doped TiO2, according to Liu et al. . (Reproduced with permission from Y Liu, J Li, X Giu, C Burda, Novel TiO2 nanocatalyst for wastewater purification: Tapping energy from the sun, Water Science & Technology, 54 (2006) 47-54. Copyright IWA Publishing.)
FIGURE 8.36 Reflectance spectra for undoped and N-doped TiO2, according to Liu et al. . (Reproduced with permission form Y Liu, J li, X Giu, C Burda, Novel TiO2 nanocatalyst for wastewater purification: Tapping energy from the sun, Water Science & Technology, 54 (2006) 47-54. Copyright IWA Publishing.)
• Cost-Related Reasons. There is a need to develop cathodic active sites (alternative to Pt) for enhanced photocatalytic reduction of oxygen and effective removal of electrons.
• Compatibility of Data. There is a need to form photocatalytic systems that are well defined. Most of the systems reported so far are not
FIGURE 8.37 Effect of F-doping on photodegradation of acetaldehyde by TiO2 thin films (normalized scale), according to Kim and Choi . (Reprinted from Appl Catal B, Environ, 69, H Kim, W Choi, Effects of surface fluorination of TiO2 on photocatalytic oxidation of gaseous acetaldehyde, 123-132, Copyright 2007, with permission from Elsevier.)
reproducible and, therefore, even the data for similar systems are not compatible. Consequently, there is a need to form the systems that can be compared between laboratories.
• Standards. There is an increasingly urgent need to establish standards that are well defined in terms of their key performance related properties. Rutile, which is thermodynamically preferred form of TiO2, is the best candidate for this purpose. The rutile-based standard may address its requirements as the reference when its processing conditions and the properties are well defined in terms of the following:
• Titanium-to-oxygen ratio
• Electrical properties
• Impurity level
• Surface area
• Surface composition (this may differ from that of the bulk as a result of segregation)
• Quantitative Assessment of Light-Induced Properties. The effect of light on photocatalytic activity depends on a number of factors, such as dispersion, turbulency, and light access. The effect resulting from these factors may be relatively well defined when using standard photoreactors.
The effect of pH has been commonly considered in terms of chemical equilibria in the aqueous system. Figure 8.38 shows the effect of pH on the photodegradation of potassium hydrogen phtalate . The effect of pH may also be considered in terms of incorporation of protons in the oxide lattice leading to a change in defect disorder and the related photocatalytic activity. So far, little is known in this matter.
FIGURE 8.38 Effect of pH on photodegradation of potassium hydrogen phtalate by TiO2, according to Alhakimi et al. . (Reprinted from J Photochem Photobiol A, Chem, 154, G Alkahimi, LH Studnicki, M Al-Ghazali, Photocatalytic destruction of potassium hydrogen phthalate using TiO2 and sunlight: Application for the treatment of industrial wastewater, 219-228, Copyright 2003, with permission from Elsevier.)