Membrane photocatalytic systems

An attractive way to overcome this problem is to use microheterogeneous photocatalytic systems based on lipid vesicles, i.e. microscopic spherical particles formed by closed lipid or surfactant bilayer membranes (Fig. 1) across which it is possible to perform vectorial photocatalytic electron transfer (PET). This leads to generation of energy-rich one-electron reductant A" and oxidant D, separated by the membrane and, thus, unable to recombine. As a result of such PET reactions, the energy of photons is converted to the chemical energy of spatially separated one electron reductant tmd oxidant. 

T0507 Membrane Technology and Research, Inc., VaporSep Membrane Recovery System T0552 NEPCCO, Photocatalytic Oxidation Technology T0595 Peat/Compost Biofiltration—General... 

In recent years some studies were performed using submerged membrane modules coupled to photocatalytic systems for the removal of organic pollutants such as fulvic acid [84], bisphenol-A [85], para-chlorobenzoate [86]. 

Other types of membrane separation processes that can be useful when they are coupled with a photocatalytic system are the membrane contactors. 

Various configurations of membrane photoreactors described can be chosen to influence the performance of the photocatalytic systems and possible solutions can be found to solve some problems such as the control of the catalyst activity and the fouling, the selectivity and the rejection of the membrane. 

Colloids of semiconductors are also quite interesting for the transmembrane PET, as they possess both the properties of photosensitizers and electron conductors. Fendler and co-workers [246-250] have shown that it is possible to fix the cadmium sulfide colloid particles onto the membranes of surfactant vesicles and have investigated the photochemical and photocatalytic reactions of the fixed CdS in the presence of various electron donors and acceptors. Note, that there is no vectorial transmembrane PET in these systems. The vesicle serves only as the carrier of CdS particles which are selectively fixed either on the inner or on the outer vesicle surface and are partly embedded into the membrane. However, the size of the CdS particle is 20-50 A, i.e. this particle can perhaps span across the notable part of the membrane wall. Therefore it seems attractive to use the photoconductivity of CdS for the transmembrane PET. Recently Tricot and Manassen [86] have reported the observation of PET across CdS-containing membranes (see System 32 of Table 1), but the mechanism of this process has not been elucidated. Note, that metal sulfide semiconductor photosensitizers can be deposited also onto planar BLMs [251],... 

A membrane-induced structure-reactivity trend that may be exploited to achieve selective processes has been recently observed in polymeric catalytic membranes prepared embedding polyoxotungstates, W(VI)-oxygen anionic clusters having interesting properties as photocatalysts, in polymeric membranes [17]. These catalytic membranes have been successfully apphed in the photooxidation of organic substrates in water providing stable and recyclable photocatalytic systems. 

Development of Structurally Organized Photocatalytic Systems for Photocatalytic Hydrogen Evolution on the Basis of Lipid Vesicles with Semiconductor Nanoparticles Fixed on Lipid Membranes... 

FIG. 11 Schematic view of the designed photocatalytic systems with (a) transmembrane and (b) interfacial electron transfer, which is photosensitized by the CdS nanoparticles attached to the lipid membrane surface. Menaquinone (MQ) and heteropolyanions (HPA, SiWi20jo) are lipophilic molecular electron relays. Palladium particles are attached to CdS and operate as dark catalysts of hydrogen evolution from water. MV + methylviologen bication Gl glucose. 

Abstract This chapter reports the properties of semiconductor materials used in heterogeneous photocatalysis together with a comparison of heterogeneous photocatalytic systems and a brief description of the types of membranes that can be used. Some aspects of membrane operations, such as fouhng, separation of a photocatalyst and effectiveness of photodegradation on permeate quahty are discussed. 

Research in the field of photocatalytic synthesis in a PMR is still at a preUm-inary stage, despite the great potentiality of photocatalytic processes, especially when they are coupled with a membrane separation system. 

Benotti M J, Stanford B D, Wert E C and Snyder S A (2009), Evaluation of a photocatalytic reactor membrane pilot system for the removal of pharmaceuticals and endocrine disrupting compounds from water . Water Res, 43,1513-1522. 

In MPSs, the processes of oxidation and reduction carry out over different sides of membrane on the same photocatalyst. Therefore, the electrons must transfer from one side of the membrane to the other side, and four quanta are needed to evolve 2H2 + O2. The photocatalytic system should also have a path to equalize the charges. The examples of such systems are very scarce (Tsydenov et al., 2012). 

Despite the photocatalytic system coupled with membrane processes offering interesting advantages, its use in the industry is still limited for different reasons (Ni et al., 2007) ... 

A membrane-induced structure—reactivity trend, which may be exploited to achieve selective processes, was observed in polymeric catalytic membranes prepared by embedding decatungstate within porous membranes made of PVDF or dense polydimethyl-siloxane (PDMS) membranes. These photocatalytic systems are characterized by different and tunable properties depending on the nature of the polymeric microenvironment (Bonchio et al., 2003). The polymeric catalytic membranes prepared were used for the batch-selective photooxidation of water-soluble alcohols. Membrane-induced discrimination of the substrate results from the oxidations of a series of alcohols with different polarity, through comparison with the homogeneous reactions (Fig. 27.7). 

Figure 17.2 illustrates our model for splitting water by solar energy. I" is important that all the redox reactions involved in thf system be reversible. The quinone compound in the organic solvent combines the two photocatalytic reactions, and its function can be compared to the electron relaying molecules in thylakoid membranes of chloroplasts. Electron transfer reactions via quinone compouncs in artificia systems have been studied as a model of photosynthesis22-23 and in an electrochemical system for acid concentration.24 ... 

In another study, Tsum et al. [80] reported the use of porous Ti02 membranes having pores of several nanometers for a gas-phase photocatalytic reaction of methanol as a model of volatile organic component (VOC). In this system, the titanium dioxide is immobilized in the form of a porous membrane that is capable of selective permeation and also a photocatalytic oxidation that occurs both on the surface and inside the porous Ti02 membranes. In this way, it is possible to obtain a permeate stream oxidized with OH radicals after one-pass permeation through the Ti02 membranes. 

In the development of a photocatalytic membrane reactor it is important to take into account some parameters that influence the performance of the system and its applicability to the industrial level. 

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. 

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