Diffusion of oxygen ions

The HTE technology requires that the system operate at 800°C for the rapid diffusion of oxygen ions through a solid membrane. This requires heating the steam to 800°C. There are two strategies to heat steam to this temperature ... 

Conversely, reaction conditions that maintained a rapid reoxidation and a small number of Mo5+ centers in the catalyst resulted in an increased selectivity. Hence, it may be concluded that in a process that involves diffusion of oxygen ions in the catalyst bulk and a prolonged lifetime of partially reduced V4+—Mo5+ metal sites, total oxidation of propene dominates. On the other hand, catalytic oxidation of propene proceeding on an oxidized V4+—Mo6+ active site at the surface of the catalyst yields an improved selectivity for partial oxidation products. 

In this section, a model is presented for solid oxide electrolytes based upon two consecutive steps for oxygen permeation one for the surface exchange process at the oxide surface on both sides of the membrane, and another for the joint diffusion of oxygen ions and electron-holes through the solid. 

Ally alcohol oxidation into acrolein on the rhombic phase of molybdenum oxide modified with vanadium oxide has been studied by the kinetic molhod ami Ijy ESR of ions in situ. II was shown, that active sites for this reaction are V ions situated in the bulk of the catalyst, or near its surface, but not at the surface. Fast diffusion of e.lectrons and a more slower diffusion of oxygen ions between the surface and the bulk occur during the reaction. 

The sintering of a compound MO is governed by diffusion of oxygen ions. If this compound is cation-deficient, propose a method by which the sintering rate may be enhanced. 

A problem with SOFCs is that diffusion of oxygen ions in the electrolyte is too slow at room temperature to make the cells viable. At present, satisfactory cell operation is accomplished only when the electrolyte is held at temperatures in excess of 650 °C, although intensive research is continually lowering this temperature. 

The SOFC unit has a three-layer sandwich structure two porous electrode, anode and cathode, serving as the chemical reaction, and the electrolyte, serving as the diffusion layer of oxygen ions but electrically nonconductive. A typical SOFC structure is shown in Figure 5-1. The anode and cathode should be porous to allow the diffusion of oxygen ions. A single SOFC unit cannot provide enough power therefore, the interconnection between stacks of cells is required. 

The diffusion of Pb ions through the PbO layer requires an activation energy of 66 kcal moP (2.86 eV) [10]. The energy of activation of the diffusion of oxygen ions is only 22.4 kcal moP (0.97 eV) [7]. This marked difference between the two activation energies indicates that it is oxygen that is transported through the PbO layer. 

In some ceramics the only species that can move in an applied electric field are the ions in the structure. Generally, the movement of ions is slow, but in a class of ceramics called fast ion conductors, they can move very rapidly. In cubic zirconia the diffusion of oxygen ions at high temperature is particularly fast, and this ceramic is used as the electrolyte in solid oxide fuel cells. Fuel cells are becoming a key part of a diverse energy plan for the twenty-first century. 

An important application of the data shown in Figure 4.4.21 and of the equation for the Warburg coefficient [Eq. (75)] is in the calculation of the diffusivity for anion vacancies within the film. In the case of passive polycrystalline nickel in borate and phosphate buffer solutions, Chao et al. (1982) computed a value of 1.3 X 10" cmVs for the diffusivity of oxygen ion vacancies. In a later study by Liang et al. [1984], a somewhat higher (and possibly more reliable) value of 1.5 x... 

Fig. 2. Mechanisms of oxide formation and growth (a) slow metal ion diffusion (b) slow diffusion of oxygen ion (c) slow electron transfer.

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