Photocatalysis separation

In this section, the possibility of isolating products during sonophotocatalysis is discussed. In order tc isolate H2 and 02, an attempt was made to carry out sonolysis and photocatalysis separately one after the other. 

Other examples of organized molecular assemblies of interest for photocatalysis are (1) PC-A, PC-D or D-PC-A molecules where PC, A and D fragments are separated by rigid bridges (2) host-guest complexes (3) micelles and microemulsions (4) surfactant monolayers or bilayers attached to solid surfaces, and (5) polyelectrolytes [19]. 

Recently, we have shown that the combination of barium tetratitanate, BaTi40g and sodium hexatitanate, NagTigOis, with ruthenium oxides leads to active photocatalysts for water decomposition[1,2]. The unique feature of these photocatalysts is that no reduction of the titanates is required to be activated this is intrinsically different from conventional photocatalysts using TIO2 which are often heat-treated in a reducing atmosphere. Such different photocatalytic characteristics suggest that efficiency for the separation of photoexcited charges (a pair of electrons and holes) which is the most important step in photocatalysis is... 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

Nanomaterials can also be tuned for specific purposes through doping. Specifically, the effect of the presence of manganese oxides on photocatalysis involving primarily titanium dioxide will be considered in this section. Titanium dioxide is a well-known photocatalyst and will be considered separately. K-OMS-2, which has a cryptomelane structure, is illustrated in Figure 8.4. Not all the literature discussed in this section, however, involves OMS tunnel structure materials. For example, amorphous manganese oxide (AMO) is also discussed as a photocatalyst. Manganite (MnOOH) is also included in battery applications. 

The absorption of light by semiconductors cremes electron-hole pairs e h+) which can be separated because their components diffuse in different directions. The energies of these moieties can be stored by several mechanisms or used in photocatalysis or photosynthesis for nitrogen fixation, formation of amino acids, methanol, etc. The efficiencies of such conversions depend almost entirely upon the semiconductor material, and as yet these efficiencies are too low for significant application. Currently the most promise is demonstrated by the use of titania on a platinum substrate or single crystals of strontium titanate. See also Photoelectric Effect. 

There are several factors caused by the change of the particle size that affect the activity of particulate photocatalysis (1) surface area, (2) band energy shift, (3) accessibility to the surface, and (4) space for charge separation. 

Various efforts to apply photocatalysis to photoenergy conversion are described in Part III. Synthetic chemistry utilizing photocatalysis by semiconductors has been attracting attention as discussed in Chapters 11 and 12. The merits of the photocatalysts for synthetic chemistry are (a) multiple processes are possible, (b) catalysts can be separated easily and re-used, and (c) the reactions can proceed under ambient conditions, etc. (Chapter 11). In Chapter 12 photolysis and sonolysis are combined to obtain specific effects in addition to photochemical reactions. 

A very promising method to solve this problem is coupling the photocatalysis with membrane techniques, obtaining a very powerful process with great innovation in water treatment. In fact, membrane processes, thanks to the selective property of the membranes, have been shown to be competitive with the other separation technologies for what concerns material recovery, energy costs, reduction of the environmental impact and selective or total removal of the components [77]. 

To this purpose, in a study on the photocatalytic degradation of 4-chlorophenol, Camera-Roda and Santarelli [89] proposed an integrated system in which photocatalysis is coupled with pervaporation as process intensification for water detoxification. Pervaporation represents a useful separation process in the case of the removal of VOCs and in this study it is used to remove continuously and at higher rate the organic intermediates that are formed in the first steps of the photocatalytic degradation of the weakly permeable 4-CP. 

Membrane distillation - photocatalysis To solve the problem of membrane fouling observed in the pressure-driven membrane photoreactor, Mozia et al. [90] studied a new type of PMR in which photocatalysis was combined with a direct contact membrane distillation (DCMD). MD can be used for the preparation of ultrapure water or for the separation and concentration of organic matter, acids and salt solutions. In the M D the feed volatile components are separated by means of a porous hydrophobic membrane thanks to a vapor-pressure difference that acts as driving force and then they are condensed in cold distillate (distilled water), whereas the nonvolatile compounds were retained on the feed side. 

One of the main objectives in the use of a membrane process coupled to a photocatalytic reaction is the possibility of recovering and reusing the catalyst. Moreover, when the process is used for the degradation of organic pollutants, the membrane must be able to reject the compounds and their intermediate products, while if the photocatalysis is applied to a synthesis, often the membrane have to separate the product(s) from the environment reaction. Therefore, in a PMR the choice of a suitable membrane is essential to obtain an efficient system. 

In this context, hybrid systems based on photocatalysis coupled with separation process could represent an useful solution to these problems. 

Theron P, Pichat P, Guillard C, Guillard C, Chopin T. Degradation of phenyltrifluoromethylketone in water by separate or simultaneous use of Ti02 photocatalysis and 30 or 515 kHz ultrasound. Phys Chem 1999 1 4663-4668. 

Keywords Kinetics, mechanism, charge separation, Titanium dioxide, nanocrystals, photocatalysis, semiconductors, Magnetic resonance... 

Most of the efforts of researchers, working in the field of semiconductor colloids, are directed to the chemical modification of the system to make photoproduction reactions more efficient. EPR spectroscopy will certainly contribute to the development of new nanoscale materials which can effectively undergo charge separation for wide application in photocatalysis and solar energy conversion. 

However, due to the inherent complexity of this minute photoelectro-chemical system, details of the underlying reaction mechanisms of photocatalysis are even today still far from being understood. In contrast to an ordinary photoelectrochemical cell which employs an external bias voltage to deliberately separate oxidation and reduction processes in different compartments of the reactor, in photocatalysis both processes occur on the surface of the same semiconductor particle, usually only separated by a distance of a few angstroms. Moreover, as is evident from basic principles, the reaction rate of the overall process will be limited by the... 

In a single-crystal semiconductor (n-type) based photoelectrochemical cell, the problem of achieving charge separation is easily overcome by applying an anodic bias as was first demonstrated by Honda and Fujishima [263]. Using a single crystal Ti02, they were able to carry out the photoelectrolysis of water under the influence of an anodic bias. This concept to manipulate the photocatalytic reaction by electrochemical method can be extended to nanostructured semiconductor thin films [39,116]. The principle of electrochemically assisted photocatalysis is illustrated in Fig. 10. 

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