Photocatalytic systems

Molecular photocatalytic systems for solar energy conversion catalysts for the evolution of hydrogen and oxygen from water. K. I. Zamaraev and V. N. Parman, Russ. Chem. Rev. (Engl. Transl.), 1983,52,817-836(114). 

Photocatalytic systems with light-sensitive coordination compounds and possibilities of their spectroscopic sensitization — an overview. H. HennigandD. Rehorek, Coord. Chem. Rev., 1985,61,1 (281). 

Generation of nanoparticles under Langmuir monolayers and within LB films arose from earlier efforts to form nanoparticles within reverse micelles, microemulsions, and vesicles [89]. Semiconductor nanoparticles formed in surfactant media have been explored as photocatalytic systems [90]. One motivation for placing nanoparticles within the organic matrix of a LB film is to construct a superlattice of nanoparticles such that the optical properties of the nanoparticles associated with quantum confinement are preserved. If mono-layers of capped nanoparticles are transferred, a nanoparticle superlattice can be con-... 

Fig.3. Decomposition of acetic acid and ammonia over Ti02 and AI-Ti02 in three-phase fluidized photocatalytic system, a) For acetic acid decomposition and b) For ammonia decomposition...

Most of the so far designed photocatalytic systems can be divided into three categories simple molecular ones, organized molecular assemblies and semiconductor systems. In this section typical photocatalytic behaviour and reaction mechanisms will be discussed for photocatalysis with systems of all these types. 

To this category belong, e.g., homogeneous photocatalytic systems based on soluble metal complexes or organic dyes as photocatalysts. Instructive examples are photoreactions assisted by heteropolyacids (HPAs), transition meal complexes with carbonyl, phosphine or some other ligands, and metal porphyrins. 

A prominent example here are photocatalytic systems based on lipid vesicles for watersplitting into H2 and O2. Design of such systems is based on both functional and structural mimicing of natural photosynthesis. 

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. 

The photocatalytic oxidation of alcohols constitutes a novel approach for the synthesis of aldehydes and acid from alcohols. Modification of Ti02 catalyst with Pt and Nafion could block the catalyst active sites for the oxidation of ethanol to CO2. Incorporation of Pt resulted in enhanced selectivity towards formate (HCOO ad)-Blocking of active sites by Nafion resulted in formation of significantly smaller amounts of intermediate species, CO2 and H2O, and accumulation of photogenerated electrons. The IR experimental teclmique has been extended to Attenuated Total Reflectance (ATR), enabling the study of liquid phase photocatalytic systems. 

Develop photocatalytic systems with efficiencies great enough to use for chemical processing on a significant scale. 

At present, several stable photocatalytic systems for production of hydrogen from water and organic compounds are made of semiconducting oxides and suitable proton reducing catalyzer. An efficient electron transfer between inorganic semiconductor and bacterial hydrogenase was shown to result in hydrogen photoproduction. 

Much interest has recently been shown in artificial photosynthesis. Photosynthesis is a system for conversion or accumulation of energy. It is also interesting that some reactions occur simultaneously and continuously. Fujishima et al. [338] pointed out that a photocatalytic system resembles the process of photosynthesis in green plants. They described that there are three important parts of the overall process of photosynthesis (1) oxygen generation by the photolysis of water, (2) photophosphorylation, which accumulates energy, and (3) the Calvin cycle, which takes in and reduces carbon dioxide. The two reactions, reduction of C02 and generation of 02 from water, can occur simultaneously and continuously by a sonophotocatalytic reaction. 

Of the photocatalytic systems and structures composed of a single active material, eventually coupled with redox catalysts and/or metals, only a wide band gap oxide semiconductor, like Pt/Ti02, requiring UV irradiation, showed some photoactivity for water photosplitting. Water splitting with visible light requires the irradiation of multiple band gap photoelectrochemical cells (PEC) or Z-scheme systems (like the photosynthesis system of plants etc.). 

A similar heterogeneous photocatalytic system was applied for the study of the decomposition of the anthraquinone dye, Acid blue 25 (AB25). The chemical structure of the dye and those of the first intermediates tentatively identified by HPLC-MS are shown in Fig. 3.55. RP-HPLC-DAD analysis of AB25 was carried out in a C4 column (250 X 4 mm i.d. particle size 5 //m) at ambient temperature. The isocratic mobile phase was composed of ACN (solvent A)-water (pH adjusted to 4.5 with acetic acid and ammonium acetate) (42 58, v/v). 

Litter, M.I. 1999. Heterogeneous photocatalysis transition metal ions in photocatalytic systems. Appl Catal B Environ 23 89-114. 

The photocatalytic system is shown in Scheme 5, where BNAH is oxidized by the ZnP + moiety in the radical ion pair ZaP -Ceo (ki) produced upon photoirradiation of ZnP-Ceo, whereas HV " is reduced to HV by the Ceo" moiety of ZnP +-C6o ki). These individual electron-transfer processes compete, however, with the BET in the radical ion pair (/cbet)- This pathway was experimentally confirmed by photolysis of the ZnP-Ceo/BNAH/HV and ZnP-H2P-C6o/BNAH/HV + systems with visible light (433 nm) in deoxyge-nated PhCN [70], For instance. Fig. 4 depicts the steady-state photolysis in deoxy-genated PhCN, in which the HV absorption band (X ax = 402 and 615 nm) increases progressively with irradiation time. By contrast, no reaction occurs in the dark or in the absence of the photocatalyst (i.e., ZnP-Ceo or ZnP-H2P-C6o) under photoirradiation [70]. Once HV+ is generated in the photochemical reaction, it was found to be stable in deoxygenated PhCN. The stoichiometry of the reaction is established as given by Eq. (3), where BNAH acts as a two-electron donor to reduce two equivalents of HV [70] ... 

Scheme 5 Photocatalytic system of ZnP-Ceo for uphill photo-oxidation of BNAH by hexyl viologen. (From Ref. 70.)...

The previous discussion refers mainly to the key factors affecting the performance of a photocatalytic system in which oxidative pathways are predominant. Oxygen,... 

Although photocatalytic oxidation is generally applied to only low concentrations of gas-phase contaminants, typically in the ppm range, several smdies have addressed higher concentrations as well. Augugliaro et al. [14] and Martra et al. [16] evaluated the photocatalytic oxidation of toluene at concentrations of up to 1.3 mol% in air. At these high concentrations, significant levels of gas-phase intermediate products were observed. Of the toluene removed from the gas phase in the photocatalytic system, nearly 20% was converted into benzaldehyde, as... 

Humidity has a significant influence on the photocatalytic oxidation of aromatic contaminants in the gas phase. This is of particular interest, because commercial photocatalytic systems will be required to operate under a broad range of relative-humidity levels. The specific influence of relative humidity on the photocatalytic reaction has generally proved rather difficult to quantify because water has a dual role It may compete with contaminants for surface adsorption sites (a negative influence) and it plays a role in the regeneration of surface hydroxyl groups during photocatalysis (a positive influence). 

Alternatively, some of the reaction intermediates generated during the photocatalytic oxidation of aromatic contaminants may be rather recalcitrant to chlorine radicals, leading to the apparent deactivation phenomenon. Thermodynamically, intermediate species that are recalcitrant toward hydroxyl radicals may be recalcitrant toward chlorine radicals as well (Table 3). The accumulation of these recalcitrant reaction intermediates on the catalyst surface may gradually reduce the effectiveness of chlorine radicals in the photocatalytic system, decreasing the chlorine-promoted enhancement in toluene oxidation to negligible levels [51]. 

Tire decline in catalyst activity seen in some continuous photocatalytic systems has prompted researchers to examine methods of restoring activity to used photocatalysts. Because the decline in catalyst activity is often attributed to the accumulation of recalcitrant intermediates or by-products on the catalyst surface, most catalyst regeneration techniques focus on the removal of these presumed species. Two such methods, thermal regeneration and photocatalytic regeneration, have been examined for use in association with the photocatalytic oxidation of aromatic contaminants. 

Photocatalytic oxidation over illuminated titanium dioxide has been demonstrated to be effective at removing low concentrations of a variety of hazardous aromatic contaminants from air at ambient temperatures. At low contaminant concentration levels and modest humidity levels, complete or nearly complete oxidation of aromatic contaminants can be obtained in photocatalytic systems. Although aromatic contaminants are less reactive than many other potential air pollutants, and apparent catalyst deactivation may occur in simations where recalcitrant reaction intermediates build up on the catalyst surface, several approaches have already been developed to counter these potential problems. The introduction of a chlorine source, either in the form of a reactive chloro-olefin cofeed or an HCl-pretreated catalyst, has been demonstrated to promote the photocatalytic oxidation of... 

However, unlike photosynthesis in green plants, the titanium oxide photocatalyst does not absorb visible light and, therefore, it can make use of only 3-4% of solar photons that reach the Earth. Therefore, to address such enormous tasks, photocatalytic systems which are able to operate effectively and efficiently not only under ultraviolet (UV) but also under sunlight must be established. To this end, it is vital to design and develop unique titanium oxide photocatalysts which can absorb and operate with high efficiency under solar and/or visible-light irradiation [9-16]. 

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