PDF Publication Title:
Text from PDF Page: 051
2.4. ELECTROCATALYSISFROMTHEORY 39 ∗+2Cl−(aq) −−→ Cl∗+e− +Cl− −−→ ∗+Cl2(g)+2e−, (2.48) will be considered as an example. Different solid materials bind the Cl* interme- diate with different binding energies. The fundamental reason for why this is the case has been explained using the d-band model for transition metals[109, 194], and similar models have been developed also for TM oxides[165]. To achieve a high reaction rate, the catalyst needs to bind the adsorbate strongly enough to sta- bilize the adsorbate sufficiently to facilitate the discharge of Cl–, but on the other hand, if the Cl adsorbate is bound too strongly, the thermodynamic barrier for the next step in the reaction, where the Cl adsorbate is to react with solution-phase Cl– to form Cl2 which diffuses away from the surface, becomes too high. These con- siderations lead naturally to the Sabatier principle, which says that the activity for a certain catalyzed reaction shows a volcano-shaped dependence on the adsorption energy of the intermediates (as we shall see in the next section, this is also valid for multistep reactions). In other words, an optimal catalyst should exist for a cer- tain heterogeneously catalyzed reaction. As was described in the previous section, such information alone can give an understanding of both thermodynamics and kinetics (barrier heights) of a reaction[109]. The Sabatier principle is illustrated in Figure 2.3. In studies of electrocatalysis, the Sabatier principle is often used to construct volcano plots based on the following precondition for activity based on the electrode potential[108]. The precondition, in this case for the ClER involving a Cl adsorbate, is that U > Ueq + |∆G (Cl∗) |/e (2.49) for the catalyst to become active (if the precondition is fulfilled, the reaction free energy, ∆Gr, of each elementary reaction step is negative). In equation 2.49, U is the electrode potential, Ueq is the reversible electrode potential for the reaction and e is the charge of the electron. The expression indicates that the electrode potential has to be higher than the sum of the reversible potential and the potential required to either strengthen or weaken the surface-adsorbate bond so that the thermody- namic barrier is minimized. The optimal catalyst for the ClER involving Cl* thus has an adsorption free energy ∆G (Cl∗) = 0 eV. 2.4.3 Scaling relations The Sabatier analysis turns out to be applicable also to multistep reactions. The reason is that, fundamentally, similar adsorbates bind the same way to a certain type of surface (according to the same mechanism of charge transfer between ad- sorbate and surface)[108, 109, 194]. An example, relevant for the ClER on oxide surfaces, is seen in Figure 2.4. The Figure indicates how the adsorption energies of a number of adsorbates (including e.g. Cl) depend on the adsorption energyPDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
PDF Search Title:
Studies of Electrode Processes in Industrial ElectrosynthesisOriginal File Name Searched:
electrosynthesis.pdfDIY PDF Search: Google It | Yahoo | Bing
Salgenx Redox Flow Battery Technology: Power up your energy storage game with Salgenx Salt Water Battery. With its advanced technology, the flow battery provides reliable, scalable, and sustainable energy storage for utility-scale projects. Upgrade to a Salgenx flow battery today and take control of your energy future.
| CONTACT TEL: 608-238-6001 Email: greg@salgenx.com | RSS | AMP |