Proton vehicle mechanism

The total electro-osmotic coefficient = Whydr + mo includes a contribution of hydrodynamic coupling (Whydr) and a molecular contribution related to the diffusion of mobile protonated complexes—namely, H3O. The relative importance, n ydr and depends on the prevailing mode of proton transport in pores. If structural diffusion of protons prevails (see Section 6.7.1), is expected to be small and Whydr- If/ ori the other hand, proton mobility is mainly due to the diffusion of protonated water clusters via the so-called "vehicle mechanism," a significant molecular contribution to n can be expected. The value of is thus closely tied to the relative contributions to proton mobility of structural diffusion and vehicle mechanism. ... 

A more recent view of proton transport is that of Kreuer, who, compared with the Zundel-based view, describes the process on different structural scales within phase separated morphologies. The smallest scale is molecular, which involves intermolecular proton transfer and the breaking and re-forming of hydrogen bonds. When the water content becomes low, the relative population of hydrogen bonds decreases so that proton conductance diminishes in a way that the elementary mechanism becomes that of the diffusion of hydrated protons, the so-called vehicle mechanism . 

X 10 cm /s at room temperature) and that the diffusion of protonated water molecules makes some contribution to the total proton conductivity (vehicle mechanism " ). This is --"22% when assuming that the diffusion coefficients of H2O and H3O+ (or H502 ) are identical. However, as suggested by Agmon, " the diffusion of H3O+ may be retarded, because of the strong hydrogen bonding in the first hydration shell. 

Figure 16.1 Models of proton transfer process via (a) Grotthuss mechanism and (b) Vehicle mechanism.

Proton-carried migration occurs in oxonium p-alumina and hydrogen uranyl phosphate, HUP. Both are of the mss type and belong to the specific subclass of vehicle mechanisms in which is the mobile... 

Proton-carrying mechanism for water-rich materials the vehicle mechanism (V)... 

In this case, the Tj translocation (Section 1.2.1.1) is often accepted as the likely mechanism. In 1982, however, a new mechanism called the vehicle mechanism was proposed . At first sight, it resembles the previous proton-carrying process (Fig, 1.3a), It differs, however, in that the proton migrates in one direction as etc, bonded to a vehicle such... 

Kreuer et al. assume that the proton transport in zeolites is by a vehicle-mechanism . By such a mechanism protons are transported attached to a vehicle, for example, ammonia or water molecules. 

If proton conduction is verified, is the conduction mechanism then proton hopping between partially occupied proton sites accompanied by a reorganization of the anion lattice or is the mechanism proton transport by a carrier (e.g. water, hydrazine or ammonia) which possibly counter-difTuses when the proton is discharged at the cathode (vehicle mechanism) ... 

Kreuer et al. have proposed a proton conduction mechanism, where the proton is attached to a vehicle (water, ammonia, hydrazine etc.), and the complex moves as a whole. If, in a d.c. experiment the vehicle is not supplied together with protons at the anode, migration of the proton-vehicle complex will generate a vehicle deficit close to the anode. This will cause the conductivity to decrease, if no comparable alternative conduction path exists. The decline will be observed in both the d.c. and the a.c. conductivities. 

Unfortunately HU As is the only proton-conducting hydrate for which available oxygen diffusion coefficient data allow direct verification of the vehicle mechanism. For other compounds there is just an indication that this may be the mechanism. These are e.g. diffusion bottleneck considerations for ionic conduction in zeolites or the exceptionally high temperature factors for the water oxygen in and some heteropoly-... 

The presence of water in PVPA seems to contribute to the conductivity of PVPA, at a temperature below the boiling point of water, by proton transport in additional proton solvents. Hence, the proton movement in PVPA can be explained by rapid transfer of protons via hydrogen bond-forming and bondbreaking (hopping mechanism) and by self-diffusion (vehicle mechanism). Different conductivity values could be obtained due to a different water content. Also, recent studies have revealed that the behavior of PVPA as a polyelectrolyte is very similar to poly(actylic acid) in aqueous salt solution under identical conditions. ... 

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