Surface photocatalytic processes

In addition, the rate of Oz reduction, forming 02 by electron, is of importance in preventing carrier recombination during photocatalytic processes utilizing semiconductor particles. 02 formation may be the slowest step in the reaction sequence for the oxidation of organic molecules by OH radicals or directly by positive holes. Cluster deposition of noble metals such as Pt, Pd, and Ag on semiconductor surfaces has been demonstrated to accelerate their formation because the noble metal clusters of appropriate loading or size can effectively trap the photoinduced electrons [200]. Therefore, the addition of a noble metal to a semiconductor is considered as an effective method of semiconductor surface modification to improve the separation efficiency of photoinduced electron and hole pairs. 

Fig. 16.2 Simplified kinetic model of the photocatalytic process. ps represents the light absorbed per unit surface area of the photocatalyst, e b and h+b are the photogenerated electrons and holes, respectively, in the semiconductor bulk, kR is the bulk recombination rate constant and /R the related flux, whatever recombination mechanism is operating A is the heat resulting from the recombination kDe and kDh are the net first-order diffusion constants for fluxes Je and Jh to the surface of e b and h+b in the semiconductor lattice, respectively e s and h+s are the species resulting from...

The key requirement for a SET step in the photocatalytic process seems to be the surface complexation of the substrate, according to an exponential dependence of the probability of electronic tunneling from the distance between the two redox centers [66]. However, as was pointed out in the preceding section on the key role of back reactions, the presence of a SET mechanism could be a disadvantage from an applicative point of view. If the formed SET intermediate (e.g., a radical cation) strongly adsorbs and/or does not transform irreversibly [e.g., by loss of CO from a carboxylic acid or fast reaction with other species (e.g., superoxide or oxygen)], it can act as a recombination center, lowering the overall photon efficiency of the photocatalytic process. 

In this case, the amount of reagent ions adsorbed at the surface of a single colloidal particle in the course of the photocatalytic process MOkina(is, DWnads will be determined by both the current concentrations of reagents and the value of the photocatalyst particle surface not blocked by PAA. Assuming the values MO s. Dkinads and the effective rate constant kn to be proportional to Q, we find ... 

The photocatalytic processes that could have been involved in the synthesis of prebiotic molecules are not limited to iron sulphides. Photocatalytic processes occurring on the surface of other sulphide and oxide materials should be considered as well. 

Hidaka, H., J. Zhao, E. Pelizzetti and N. Serpone (1992). Photodegradation of surfactants. 8. Comparison of photocatalytic processes between anionic sodium dodecylbenzenesulfonate and cationic benzyldodecyldimethylammonium chloride on the Ti02 surface. Journal of Physical Chemistry, 96(5), 2226-2230. 

The overall reaction rate of photocatalytic processes is usually slow compared to conventional chemical reaction rates due to low concentration level of the pollutants, and therefore, there is a need to provide large amounts of active catalyst inside the reactor. Although the effective surface area of the porous anatase catalyst coating is high, there can only be a thin coating (about 1 pm thick) applied to a surface. Thus, the amount of active catalyst in the reactor is limited, and even if individual degradation... 

As mentioned before, the photocatalytic process was studied extensively for water treatment, air treatment, self-cleaning of surfaces, as well as in the context of medical applications and energy conversion. Throughout the years many insights on the fundamentals of photocatalysis were obtained. Some of the insights, closely related to air treatment, are summarized briefly hereby. 

Attaching photocatalyst particles to inert domains may assist in improving the efficiency of the photocatalytic process. The basic concept, termed as Adsorb Shuttle, is based on using the inert domains for adsorbing target compoimds that otherwise hardly adsorb on the photocatalyst. That way, a reservoir of the contaminants within a small distance from the photocatalytic sites is formed. Once adsorbed in the vicinity of the photocatalyst, the target molecules may surface-diffuse to the photocatalytic sites as shown schematically in Figure 5. 

---