TiO2 , surface steps

The photodegradation rate dependence on phenol concentration in the TiOi/F system shows a plateau in the 3 X IO " M to 3 X 10 M range, whereas for naked TiO2, a maximum is reached around 2 X 10 M of phenol followed by a decrease. This behavior is rationalized by the possibility of reductive back reactions of intermediates formed after the first oxidation step. The presence of fluoride could limit the occurrence of this detrimental effect, reducing the interaction of the formed intermediates with the surface. Moreover, also the change of the oxidation pathway changes the amount of the products. 

It is comforting that the elemental rules for predicting surface terminations outlined in section 3.1.1. work so well for predicting the structure of the (1x1) terraces and the step edges of all the orientations of both rutile and anatase. The extensive theoretical work has helped to refine the understanding of surface relaxations, and the level of detail on the atomic geometry of the TiO2(110) surface is certainly comparable to that of certain elemental semiconductors or metals. 

The catalytic dehydration reaction of formic acid on TiO2(110) is suggested to involve the unimolecular decomposition of formate ions (HCOO (a)) as rate-determining step. The formate-surface interaction activates the unimolecular decomposition of formate to preferentially yield CO(g) and OH (a). An acidic proton of a HCOOH molecule, which encounters the surface in a steady state, reacts with the resultant OH (a) to form H2O as shown in Scheme 1. 

For the formation of ketones, a close examination of the data of Figure 2 reveals non negligible differences in the regioselecttivity for the different catalysts. In particular, the ratio of the amount of 2-one with respect to 4-one is higher for the modified Ti02. Since position 4 is less hindered, this behaviour may indicate that, for the formation of ketones, the interaction of methylcyclohexane with the surface is a more critical step with unmodified TIO2 than with derivatized T1O2. 

These wire-like structures contain sites which are very efifective for anchoring gold nanoparticles (see Fig. 6.2B). On CeO / TiOgCllO) surfaces, the dispersion of Au nanoparticles is substantially larger than seen on a pure TiO2(110) surface where Au mainly binds to the steps. 

On the other hand, Muryn et al. (1991) have observed that the presence of steps and point defects on TiO2(100) does not change the ability of the surface to dissociate water molecules. The relationship between the local electronic structure and the reactivity will be made explicit in Chapter 6. 

Liang Y, Gan S, Chambers SA, Eltman El (2001) Surface structure of anatase TiO2(001) reconstruction, atomic steps, and domains. Phys Rev B 63 303-306... 

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