Photogenerated electron

Excited-state wavefunction analyses arc carried out in the framework of the Intermediate Neglect of Differential Overlap/Single Configuration Interaction (INDO/ SCI) technique to characterize the properties of the photogenerated electron-hole pairs. The SCI wavefunction writes ... 

Recently, it is reported that Xi02 particles with metal deposition on the surface is more active than pure Ti02 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e ) in the conduction band to the external sjistem, and decrease the recombination with positive hole (h ) in the balance band of Xi02, i.e., less defects acting as the recombination center[l,2,3]. Xhe catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Xhus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. Xhe XiOa were prepared with two different methods i.e., hydrothermal method and a sol-gel method. Xhe prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. Xhey were characterized by UV-DRS, BEX surface area analyzer, and XRD[4]. 

Fig. 5.18 Suppression of charge recombination in coupled Sn02/CdSe system. The photogenerated electrons in CdSe quickly migrate to the lower lying conduction band of SnOa. As a result, they escape recombination with photogenerated holes in CdSe and are collected in greater number at the back contact OTE producing a larger photocurrent. (Reproduced from [333])...

Photocatalytic dehydrogenation of 2-propanol to acetone over An/Ti02 exhibited a higher activity for the generation of photogenerated electron than the photocatalytic inactive Pd/Ti02 and PCI/AI2O3 at 298 K. 

The Au/Ti02 catalyst shows activity for photocatalytic dehydrogenation of 2-propanol at 298 K. The activity of Au/Ti02 is attributed to its unique capability for producing photogenerated electrons evidenced by the featureless IR adsorption during UV-irradiation. 

Photocatalytic oxidation is a novel approach for the selective synthesis of aldehyde and acid from alcohol because the synthesis reaction can take place at mild conditions. These reactions are characterized by the transfer of light-induced charge carriers (i.e., photogenerated electron and hole pairs) to the electron donors and acceptors adsorbed on the semiconductor catalyst surface (1-4). Infrared (IR) spectroscopy is a useful technique for determining the dynamic behavior of adsorbed species and photogenerated electrons (5-7). 

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. 

In conclusion, nanorods are a potentially interesting material, but present results still do not allow understanding of whether the nanostructure leads to an improvement of the intrinsic photocatalytic behaviour, or whether other factors (accessible surface area, enhanced adsorption, etc) are responsible for the observed differences. In ZnO nanorods have been shown quite recently by surface photovoltage spectroscopy that the built-in electrical field is the main driving force for the separation of the photogenerated electron-hole pairs.191 This indicates that the nano-order influences the photophysical surface processes after photogeneration of the electron-hole pairs. A similar effect could be expected for Titania nanorods. However, present data do not support this suggestion, mainly due to the absence of adequate photo-physical and -chemical characterization of the materials and surface processes. 

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...

Explain the origin of photogenerated electrons and holes in a semiconductor irradiated with light of energy greater than the band... 

A semiconductor can act as a photosensitiser, which is characterised by its ability to absorb photons, and then, by utilising the photogenerated electrons and holes, cause the simultaneous oxidation and reduction of reactants. 

The photogenerated electrons and holes may recombine in the bulk of the semiconductor or on its surface within a very short time, releasing energy in the form of heat. However, electrons and holes that migrate to the surface of the semiconductor without recombination can respectively reduce and oxidise the reactants adsorbed by the semiconductor. Both surface-adsorption and photochemical-reaction rates are enhanced by use of nano-sized semiconductor particles, as a greatly enhanced surface area is made available. 

At the surface of the semiconductor, photogenerated electrons can reduce an electron acceptor, A ... 

Photogenerated electrons reduce oxygen to water, initially generating superoxide (02), which is further reduced to intermediate species in the formation of water. Hydroxide radicals ( OH) formed as intermediates are also involved in the mineralisation of the organic pollutant. 

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