TiO2, photocatalyst

In order to circumvent the addition of a hazardous air pollutant like TCE or PCE to generate enhanced removal of branched aromatics, d Hennezel and Ollis explored a catalyst pretreatment intended to introduce chlorine atoms into the surface structure of TiO2 photocatalysts. 

Figure 5 Shifts in the absorption spectra of various types of TiO2 photocatalysts (shown in Table 1) implanted with the same amounts of V ions, (a) original unimplanted pure P-25, (b) V/F-6, (c) V/F-4, (d) V/P-25, (e) V/F-2, (f) V/F-1. Amount of V ion implanted was 6.6 X 10 mol/g (3.4 x 10 wl%).

Figure 6 UV-vis absorption spectra (diffuse reflectance) of the original undoped pure TiO2 (a) and TiO, chemically doped with Cr ions (b -e ). Cr ions chemically doped in 10" mol/g (a) undoped original pure TiOi (P-25), (b ) 16, (c ) 200, (d i 1000, (e ) 2000. The TiO2 photocatalysts chemically doped with Cr ions did not exhibit any photocatalytic reactivity.

Figure 7 Reaction-time profiles of the photocatalytic decomposition of NO on the Cr-ion-implanted TiO2 photocatalyst under visible-light (X> 450 nm) irradiation at 295 K. The unimplanted original pure TiO2 photocatalyst did not show any photocatalytic reactivity under the same reaction conditions.

A few studies did address chemical selectivity in these oxidations. Pattenden and coworkers, for example, showed that primary alcohols could be selectively oxidized to the corresponding aldehydes, without appreciable overoxidation, when the platinized TiO2 photocatalyst was suspended in benzene, Eq. (11). Poor yields were obtained... 

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... 

A variation of this process, using an aqueous-phase treatment of TiO2 with 3 hydrochloric acid solution, was used by d Hennezel and Ollis for prechlorination of a titanium dioxide photocatalyst [68] ... 

Figure 9 Effect of the Cr- and V-ion-implantation on the photocatalytic reactivity of TiO2 under outdoor solar beam irradiation for the photocatalytic reaction of CH3CCH with H2O leading a hydogenolysis reaction of CH3CCH with H2O at 295 K. A fine, B cloudy weather (solar intensity during fine weather 12 inW/cm amount of photocatalyst 6.0 g).

As it appears from the scheme, this process is stoichiometric in light, well different from the above mentioned chain processes. The use of the term photocatalyst in this meaning is not common, since this term is rather referred to the oxidation and mineralization of organic pollutants caused by irradiation in the presence of semiconductor (e.g. TiO2, ZnS, CdS) powders. However, the mechanism in Scheme 4 seems strictly analogous to the usual concept of catalysis, with the additional requirement of photo activation and thus joining the two words and defining the process as photocatalytic seems to us natural, as illustrated in a recent review. ... 

Fig. 3 UV-VIS spectra of the salicylic acid solution during UV-irradiation using TiO2/M/H50 photocatalyst (csa,o = 0.15 mM, 0.1 wt/vol% catalyst content)...

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