Photocatalysts modification

Ni et al. [5], in a review on the developments in photocatalytic water splitting using Ti02 for hydrogen production, divided the techniques used to enhance the photocatalytic efficiency in two broad groups photocatalyst modification techniques,... 

Many studies have been dedicated to catalyst immobilization strategies that are suitable for use in slurry reactors or membrane reactors or both. " Photocatalyst modification and doping ion order, with a view to enlarging the photonic activation of catalysts to the visible region, is another area of very active recent development. 

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]. 

Photobleach mechanism, 19 203 Photobleach reversal grains, 19 201 Photocatalysis, 19 73-106. See also Photocatalysts Photoreactors aqueous pollutants eliminated and mineralized by, 19 89t catalyst modifications in, 19 94-95 catalysts in, 19 75-76 challenges in, 19 101-102 fate of photo-holes in titania, 19 82-85 in fine chemistry applications, 19 102 influence of oxygen pressure in, 19 82 ion doping in, 19 94-95 mass of catalyst in, 19 77-78 noble metal deposit in, 19 94 parameters governing kinetics in, 19 77-82... 

The addition of a second species can cause a decrease in charge recombination and an increase in the TiOz photocatalytic efficiency. Such behavior was examined by loading a series of species on the surface or into the crystal lattice of photocatalysts inorganic ions [148-152], noble metals [153,154], and other semiconductor metal oxides [155], It was thus proven that modifications produced by these species can change semiconductor surface properties by altering interfacial electron-transfer events and thus the photocatalytic efficiency. 

Practical application of the results of photocatalysis will depend much on the capability of the catalysts to work under visible fight conditions. Therefore, the design of new photocatalysts either by modification of the existent TiOz or by discovery of new materials will be a key step in developing... 

It is well-known that nano-TiO is one of the suitable semiconductors for photocatalyst and has been applied in various photoeatalytic reactions (Fujishima et al., 2000). However, its properties, not only the photoefficiency or activity but also the photoresponse, are not sufficient (Kawai and Sakata, 1980). Meanwhile, the high recombination ratio of photoinduced electron-hole pairs also reduces its catalytic efficiency. Therefore various modifications have been performed on nano-TiO to promote its catalytic ability and develop new photoeatalytic functions (Ohno et al., 1996 Litter, 1999 Nawio et al., 1999 Choi et al., 1994 Nishikawa et al., 2001 Amiridis et al., 1999). 

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. 

Figure 15 Schematic diagram of photocatalyst surface modification by the addition of cocatalyst to facilitate the hydrogen (a) or oxygen (b) evolution in water splitting.

Upon stirring a suspension of titania powders in an aqueous solution of H2[PtCl6] in the dark, different maximum amounts were adsorbed. For the anatase hydrate and anatase/rutile samples TH (Kronos) and P25 (Degussa), there were taken up 4.0% and 1.1%, respectively, whereas only traces were adsorbed onto the rutile material Aid. The almost four times larger amount adsorbed by TH corresponds with the about four times larger surface area as compared to P25 (vide infra). Subsequent heat treatment at 200 °C afforded the desired photocatalyst (21). Surface modification may be performed also by simple grinding with PtCU, but the resulting powders are of lower photocatalytic activity and less stability (18,20). 

To find out whether or not sensitization of titania as described above is feasible also with halides of other d metals, sruface modification was also attempted with RhXs (X=C1, Br) (45). The novel hybrid photocatalysts x%RhCl3/TH containing 0.5,... 

Increasing the temperature may influence the slowest chemical reaction step and the adsorption equilibria of substrates and products. Depending on the relation of the two latter steps and on the temperature range investigated, the measured activation energy, E, may be zero or larger or smaller than zero [74-79]. Since these factors depend on the detailed nature of the photocatalyst, minor modifications may induce variations of Fa- Accordingly, values of 1.7 and 4.2 kcal mol" were reported for two different CdS samples used in the photodimerization of A-vinylcarbazole [80]. 

Photocatalytic reduction of carbon dioxide on TiO-, nanocrystals embedded in Si02 matrices (Q-Ti02/Si0,) and bulk CdS particles with and without surface modifieation by several thiol compounds was investigated in various kinds of solvents. Formate and carbon monoxide were obtained as the major reduction products and the ratio of the former to the latter was increased with an increase in the dielectric constant of the solvents used for the use of Q-Ti02/Si0, and bare CdS particles as photocatalysts. The surface modification of CdS particles with thiol compounds was effective in enhancing the ratio of formate to carbon monoxide. The observed selectivity of CO2 reduction products was explained well in terms of the stabilization of reaction intermediates on the photocatalyst surface. 

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