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1.2. SELECTIVITY 11 even to less than 20% Ru at high temperature (90 ◦C)[101]. This could indicate that the selectivity of the Ru-Ti mixed oxide, which can be likened to Ru-doped TiO2 at lower Ru concentrations, is altered due to an electronic effect, related to charge transfer between the Ru and Ti components. XPS measurements indeed indicate that charge transfer occurs[102, 103]. Furthermore, an increase in surface area does not seem to be enough by itself to account for the maintained activity of ClER (and OER) as the Ru percentage in the coating is reduced, as e.g. Burrows et al. [100] found a difference in overpotential between 80% and 20% Ru of 5 mV, while the change in overpotential according to Tafel kinetics for such a decrease in active site numbers (if Ru is the only active site) should be more than 20 mV (see Karlsson et al. [16]). Nevertheless, as there is a lack of experimental methods to fully account for changes in surface area when determining the activity of electro- catalysts, it is challenging to fully decouple true catalytic effects from effects of surface area[104]. The same problem exists in studies of other mixed oxide combinations. Several studies exist of selectivities of ternary Ru-Ti oxides (i.e. where an additional dopant has been added to the coating), or of RuO2 combined with other transi- tion metals, and there are many more studies that do not examine the selectivity specifically. For example, doping RuO2 with SnO2 has been found to result in an improved chlorine evolution selectivity[43, 77, 105, 106], and this has again been coupled with changes in Tafel slopes[43] indicating an electronic effect. During the past decade, a deeper understanding of the connection between elec- tronic effects and selectivity has been gained from studies using density functional theory. When it comes to the electrocatalysis of oxygen and chlorine evolution on oxide surfaces, the studies of Rossmeisl et al. [107] (focusing on the OER) and Hansen et al. [108] (focusing on ClER and OER) are possibly the most important theoretical contributions since the early 1970’s. The workers found likely reac- tion mechanisms for both ClER and OER on rutile oxide surfaces through first principles calculations. Furthermore, scaling relations (linear relations between adsorption energies of different adsorbates, e.g. intermediates in reactions[109], see Section 2.4.3) were identified that enabled the activity for both ClER and OER to be coupled to one single descriptor, the adsorption energy ∆E(Oc) of O on the rutile coordinatively unsaturated site (CUS). In this way, the relative activities of different oxides could be accounted for in a single theoretical framework. The present thesis will use these results as a basis for a consideration of the selectivity of mixed oxides. The results of Rossmeisl et al. [107] and Hansen et al. [108] were later reconsidered by Exner et al.[110–113], essentially confirming their results. A question that still remains regards the effect of solvation[107, 110, 114], a factor that is challenging to account for in first principles studies. While both Rossmeisl et al. [107] and Siahrostami and Vojvodic [114] found, by modeling water explic- itly, that the hydration itself should have a minor effect on activity trends for OER, Exner et al. [110] found large effects when using an implicit water model.PDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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