Solid photocatalyst

The selective oxidation and, more generally, the activation of the C-H bond in alkanes is a topic of continuous interest. Most methods are based on the use of strong electrophiles, but photocatalytic methods offer an interesting alternative in view of the mild conditions, which may increase selectivity. These include electron or hydrogen transfer to excited organic sensitizers, such as aryl nitriles or ketones, to metal complexes or POMs. The use of a solid photocatalyst, such as the suspension of a metal oxide, is an attractive possibility in view of the simplified work up. Oxidation of the... 

In contrast, the analogous 1-methylnaphthalene gives a completely different product if the reaction is conducted in homogeneous solution compared with that found on the irradiated solid photocatalyst. Exclusive side chain oxidation is observed, eqn 4,... 

All the above considerations allow to conclude that the electronic surface modifications created imder irradiation eventually induce photoadsorption both in gas-solid and in liquid-solid regimes, being the photoadsorption phenomenon strictly related with the photoactivity of the solid photocatalysts. When light is switched off, a reversal of the process could be observed, although rarely the reversibility is complete and the achievement of a new equilibrium depends mainly on kinetics rather than thermodynamic factors. 

Photocatalytic reactions are promoted by solid photocatalyst particles that usually constitute the discrete phase distributed within a continuous fluid phase in the reactor. Therefore, at least two phases, that is, liquid and solid, are present in the reactor. The solid phase could be dispersed (SPD) or stationary (SPS) within the reactor. SPD photoreactors may be operated with the catalyst particles and the fluid phase(s) agitated by mechanical or other means. Depending on the means of agitation, the photoreactor resembles that of slurry or fluidized bed reactors. In numerous investigations, an aqueous suspension of the catalyst particles in immersion or annular-type photo reactors has been used. However, the use of suspensions requires the... 

Some diverse VOCs (halocarbons, isoprene (CH2C(CH3) CH2CH2), monoterpenes, ethanol, and methyl tert-butyl ether, (CHslsCOCHs)) were found to be photooxidized efficiently on solid aerosols. Solid photocatalyst particles, such as Ti02, ZnO, and Fe20s, were here of special importance, but the VOC oxidation was photoassisted also by dessert sand, volcanic ash, or even by chalk particles (23-25). Similarly, sulfur dioxide was found to... 

NOa can also undergo photoredox reactions on solid photocatalysts modified by coordination compounds, for example, on Ti02 modified by H2PtCl6 127) or on bismuth tungstate photocatalysts 128). 

Turnover numbers and turnover rates are relatively easily measured in homogeneous (photo)catalysis however, their determination has eluded several attempts in heterogeneous (photo)catalysis because of the required knowledge of the number of (photo)catalytically active sites on a (solid) photocatalyst. These turnovers appear to depend on how the (photo)catalytic process is described (Serpone et al, 2000). 

It was also assumed that the light irradiance I obeys exponential decay characteristics due to absorption of light by the solid photocatalyst. To describe the concentration of surface charge carriers for the above model, the continuity equation (eq. 5.124) was rewritten under steady-state conditions as expressed by eq. 5.128... 

Figures 5.42 and 5.43 give two more examples that demonstrate, respectively, the band-like and step-hke dependencies of the quantum yields for the photostimulated adsorption of oxygen, hydrogen and methane on dispersed MgO and Zr02 solids. The case of strong light absorption nsnally corresponds experimentally to the fnndamental absorption of light by the solid photocatalyst. Accordingly we have analysed the spectral variations of the qnantum yields for the case of intrinsic fnndamental absorption, and in particular at the fundamental absorption edge of the solids.

An important problem is identification of the products that are actually formed from various chemicals (in particular from complex organic compounds such as pesticides or refrigerants) on solid photocatalysts under conditions similar to those in nature. The available data here are also quite scarce. However, the possibility is not excluded that under certain unfavorable conditions in the presence of oxygen, such compounds are oxidized not only to CO2 and H2O, but also to more toxic or environmentally unfriendly compounds. This may be particularly true for compounds that contain such elements as Cl, P, or F. 

FIGURE lU Scheme of photon-induced reaction for overall water splitting by a solid photocatalyst. (See insert for color representation of the figure.)... 

At present the most efficient solid photocatalyst for CO2 reduction by H2 is based on Ti02, a semiconductor that is suitably modified to increase the efficiency under solar light irradiation and contain co-catalysts to increase production rate and selectivity of the process. One of the most efficient photocatalysts has been reported by Grimes and coworkers who have used anodized titania nanotubes in the anatase phase doped with nitrogen and platinum and copper nanoparticles as photocatalyst for the gas phase CO2 reduction by H2O production rates to methane as high as 75 ppm cm h have been achieved for this type of titania photocatalyst under solar light illumination [23, 25]. 

Fig. 2.1 (a) Overall water-splitting reaction on a solid photocatalyst and (b) Process of electron transport in overall water-splitting reaction... 

Keller et al. used the photocatalytic reactor which is a Vigreux-like Pyrex tubular reactor for photocatalytic inactivation of Escherichia coli as the model bacteria in airstream [92]. This reactor was made of Pyrex glass. Four 8 W blacklight tubes were located outside the reactor. The photocatalyst was coated on the inside of the tube. This technical solution of the reactor allowed better contact between the solid photocatalyst and flowing bacterial contamination. The reactor consisted of an aerosol generator and a bacterial cultivation medium. 

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