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4.2. SELECTIVITY BETWEEN CL2 AND O2 59 values of the volcano plot (Figure 2.5)[108] that should be kept in mind. The first one is the optimal descriptor value for the OER, of ∆E (Oc ) = 2.4 eV. The activity for ClER is also maximized at that descriptor value. The second is the descriptor value for maximized ClER activity and selectivity, at ∆E (Oc ) = 3.2 eV. There are two descriptor values that result in optimal activity for ClER since there are two stable surface intermediates that can result in chlorine evolution at U = 1.37 eV. Figure 4.10 indicates the descriptor value for doped TiO2 with the dopant placed either in the active site (the “1cus” case) or in the bridge site next to the active site. In the first case, the dopant is the cation in the active site, whereas in the second case, the active site is a Ti cation that might be activated by the nearby dopant. In the left half of Figure 4.10, it is seen that the descriptor value for a number of dopants, including As, Mn, Co and Pd, is close to optimal for selective and active ClER (∆E (Oc ) = 3.2 eV). Sb or V in the same position results in a material with optimal activity for OER, while Ru or Ir in that position results in a material with electrocatalytic properties similar to those of pure RuO2. Turning next to the situation of dopants located next to a Ti CUS site (the right hand side of Figure 4.10), it is seen that a number of dopants, including Ru, Ir, As and V all result in a descriptor value close to the optimal one for ClER. These dopants all activate Ti sites. Finally, a number of dopants, including Sn, Ag and Au, hardly affect the electrocatalytic properties of TiO2. The surface of a real mixed oxide electrode presents a multitude of sites, with slightly varying geometry and with different arrangements of the oxide compo- nents. Some active CUS might be occupied by a dopant, and others by Ti. Thus, the overall electrocatalytic activity of a doped TiO2 material is described by the combination of the activity of dopant CUS sites and Ti CUS sites activated by dopants. The sensitivity to dopant position can be found by comparing the de- scriptor value for the cases when the dopant occupies CUS sites or bridge sites. As an example, on a real Ru-doped TiO2 electrode (such as a conventional DSA), Ru CUS sites will exhibit a similar activity as pure RuO2, that is it will be active for both ClER and OER. However, Ru bridge sites will activate Ti sites for optimum ClER activity and selectivity. Ideally, the dopant should provide the same effect independent of site since it is difficult to see how the dopant could be steered to the desired site. Arsenic is thus a very interesting dopant for chlorine-evolving electrodes, as TiO2 doped with As should exhibit optimum ClER activity and se- lectivity regardless of whether As occupies the CUS or the bridge site. Figure 4.10 shows that the electrocatalytic properties of TiO2 can be modified sig- nificantly by changing the dopant. Furthermore, by performing fundamental stud- ies of other electrochemical reactions on rutile oxides, it is possible that the results in Figure 4.10 could be applied also to understand the electrocatalytic activity of doped TiO2 in other reactions, such as e.g. direct hypochlorite or sodium chlorate production. The recent study of Viswanathan et al. [220], indicating that rutilePDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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