Proton hopping

Apart from the problems of low electrocatalytic activity of the methanol electrode and poisoning of the electrocatalyst by adsorbed intermediates, an overwhelming problem is the migration of the methanol from the anode to the cathode via the proton-conducting membrane. The perfluoro-sulfonic acid membrane contains about 30% of water by weight, which is essential for achieving the desired conductivity. The proton conduction occurs by a mechanism (proton hopping process) similar to what occurs... 

The data points in Fig. 6 show a slow decrease of Cq for aluminosilicate zeolites with increasing proton affinity of the probe molecule. Experimental data of methanol were not included in the illustration because of possible proton hopping which... 

The mechanism of conduction is believed to be that of proton hopping. Under the influence of an electric field a proton can be transferred from one OH- group to an adjacent O2- ion. The electric field ensures that the H+ ion rotates in the field direction, making the next jump easier. 

The mechanism of long-range proton transfer processes in solutions is complex becanse seqnential proton hops from initial proton donors to proton acceptors are mediated by water (or solvent) molecules or other groups capable of ionization. 

After a proton hop, the leading rotor residue is neutralized [16, 20] and the electrostatic potential, V, between the two pairs of oppositely charged point charges in the resulting charge configuration can be written as ... 

A second important application of CMD has been to study the dynamics of the hydrated proton. This study involved extensive CMD simulations to determine the proton transport rate in on our Multi-State Empirical Valence Bond (MS-EVB) model for the hydrated proton. = Shown in Fig. 4 are results for the population correlation function, (n(t)n(O)), for the Eigen cation, HsO, in liquid water. Also shown is the correlation function for D3O+ in heavy water. It should be noted that the population correlation function is expected to decay exponentially at long times, the rate of which reflects the excess proton transport rate. The straight line fits (dotted lines) to the semi-log plots of the correlation functions give this rate. For the normal water case, the CMD simulation using the MS-EVB model yields excellent agreement with the experimental proton hopping... 

It should be noted, however, that the changes in the voltammetric response are conditioned by the nature of the extent of the redox reaction across the solid. Thus, for organic solids in contact with aqueous electrolytes, and using the aforementioned model of Lovric, Scholz, Oldham, and co-workers [115-118], the propagation of the redox reaction should involve proton hopping coupled with electron hopping between adjacent immobile molecules [119-125]. Chronoamperometric data... 

Table 4 The ranges of temperature where tunneling, protonation, hopping, or recombination are dominant in irradiated DNA samples...

FIGURE 2-14 Proton hopping. Short "hops" of protons between a series of hydrogen-bonded water molecules effect an extremely rapid net movement of a proton over a long distance. As a hydronium ion... 

In this reaction, water not only is a substrate but also functions in proton transfer by forming a network of hydrogen-bonded water molecules through which proton hopping occurs (Fig. 2-14). 

This paper, accompanied by an editorial comment in the same Science issue, describes the model for H+ translocation by proton hopping. 

Theory suggests that the electrical conductance A exhibits an anomalous contribution A oc f . With regard to the critical exponent 0, one may think of several scenarios. Scaling behavior with 0 = 1 — a is expected for short-range fluctuations [127] and also for a proton-hopping mechanism [128]. A... 

Advances were still being made in the 1990s concerning the mechanism of these reactions. In the first reaction (Stage A), the mechanism has been called the proton-electron mechanism and is portrayed in Fig. 13.42. The oxide is a mixture of Mn4+, Mn3+, O2", and OH" ", and the proton hops along between O2- and OH", whereas the electron leaps onto Mn4+ from Mn3+. This picture is a simplification and more details have been elucidated in terms of the Mn02 structure, but our scope will be limited to the above. 

Proton hopping -> proton mobility, see - proton transfer... 

Structural diffusion is provided by various complexes bare hydronium ion, Eigen complexes, and - Zundel complexes. Structural diffusion of bare hydronium ion and Eigen complexes occurs by proton hops between two water molecules. Two or more protons and several water molecules are involved in the structural diffusion of Zundel complexes. The contribution of mechanisms to the overall mobility depends on the temperature. Eigen and Zundel complexes prevail at room temperature, whereas bare hydronium ions dominate at high temperatures. Excess proton mobility of water has Arrhenius-like (-> Arrhenius equation) temperature dependence with the - activation energy about 0.11 eV. 

Proton transfer proton transfer is a translocation of the protons in condensed media and molecular systems by means of any microscopic mechanism of proton transfer. The latter may involve (1) vehicle motion of the hydrogen ion, (2) proton hopping between two molecular species, (3) a shift of the proton within a molecular structure. The motion of the hydrogen ion is a diffusion process. Two other mechanisms represent elementary reaction steps involving breaking and formation of chemical bonds and may be characterized by the reaction rate constant... 

Proton conducting copolymers of vinylphosphonic acid and 4-vinylimidazole have been reported recently by Bozkurt et al. [7]. Since the imidazole ring can act as a proton hopping site in the polymer matrix [8-10], these copolymers had ionic conductivity of about 10 S cm at 60°C without solvent or salt. To realize fast proton transport in copolymer systems, it is essential to design an ion conductive paths that uses the IL domain. 

FIGURE 16.4 The figures depict the dynamical nature of the H02-water interaction. The structures in (a) and (b) are related by one single concerted (cyclic) proton hop. This is also the case for the structures in (c) and (d). See Ref. [62] for details. 

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