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4.2. SELECTIVITY BETWEEN CL2 AND O2 61 oxides with a descriptor value of ∆E (Oc) =3.8 to 3.9 eV2 should be selective for electrochemical H2O2 production, is one example. Another way of controlling the activity of TiO2 is to use more than one dopant ele- ment. Industrial DSAs often contain not only Ru and Ti, but also other components such as Ir[221]. An example of the effect on the descriptor value of combining two dopants, where dopants are arranged in the same surface layer, to either side of the Ti CUS, is seen in Figure 4.11. It is seen that by surrounding a Ti active CUS site with two dopants, the descriptor value of that site can be modified to a value that is close to the mean value of the descriptor values of the respective binary dopant-Ti oxides. In this way, two dopants can be combined to further control the activity of doped TiO2. However, how to accurately control the atomic-scale arrangement of these dopants is an open question. If dopants are simply combined in a coating solution that is then calcined to yield a final oxide, the final result is most likely an oxide which exposes a variety of different active sites with different electro- catalytic activities and selectivities. New preparation methods where the atomic structure can be controlled are needed to fully profit from the results shown in Figure 4.10. Unless the arrangement of the oxide components is controlled on the atomic level, the dispersion of dopants in the material might be more or less random, and dopants might thus be present further down in the oxide layer rather than only in the surface layer. Furthermore, on real electrode surfaces, the coverage of adsor- bates will vary with the process conditions of the electrolysis (e.g. current density and reactant concentration). The effect of altering surface coverage, as well as the effect of moving a Ru dopant further from the surface of a doped TiO2 slab, is seen in Figure 4.12. It is clear that surface coverage has a strong effect on the electrocatalytic process. However, there are still arrangements of Ru and Ti that result in optimal selectivity for ClER, although the exact arrangement depends on the surface coverage. It is also seen in the figure that Ru dopants in the second and third layer also activate Ti surface sites, but that the effect has essentially dis- appeared once the dopant has been moved past the fourth layer. The effect is thus short ranged, and dopants must be present in the near surface region. This trend is likely similar for the other dopants considered in Karlsson et al. [17]. We also found that this activation effect might be specific to TiO2, or possibly to oxides of cations with low d-electron numbers (such as e.g. ZrO2 or TaO2). For- mally, Ti in TiO2 is d0. A comparison between the change in descriptor value with dopant position for two oxide models, either for Ru-doped TiO2, as has already been shown in Figure 4.12, or for Ti-doped RuO2 is seen in Figure 4.13. Also seen in the figure is the same trend for a model system with an overall Ru:Ti stoi- chiometry close to that in DSA. The DSA model system behaves similarly to the Ru-doped TiO2 system, indicating the applicability of our results also to materi- 2Heine A. Hansen, personal communication, 2015.PDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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