Proton surface diffusion

Many catalyst layer models have appeared in the literature during the last few years [15, 16, 17, 18, 19,20, 21]. This observation partly explains the complications associated with this topic. Still, much work remains to be completed since many effects have not yet been included, such as proton surface diffusion (outside the ionomer, [22,23]) and ionomer density (water content effect), which effectively and respectively increases/modifies the reactive surface area. The surface-sensitive nature of Pt catalysts on the oxygen reduction reaction rate [24] and electrochemical promotion (a catalytic effect, [25]) represent other examples which can also affect the reaction rate and surface area. All these effects are further compounded by the potential presence of hquid water which effectively modifies the reaction front, access to speeifie eatalyst particles and surface properties. 

Mechanisms of micellar reactions have been studied by a kinetic study of the state of the proton at the surface of dodecyl sulfate micelles [191]. Surface diffusion constants of Ni(II) on a sodium dodecyl sulfate micelle were studied by electron spin resonance (ESR). The lateral diffusion constant of Ni(II) was found to be three orders of magnitude less than that in ordinary aqueous solutions [192]. Migration and self-diffusion coefficients of divalent counterions in micellar solutions containing monovalent counterions were studied for solutions of Be2+ in lithium dodecyl sulfate and for solutions of Ca2+ in sodium dodecyl sulfate [193]. The structural disposition of the porphyrin complex and the conformation of the surfactant molecules inside the micellar cavity was studied by NMR on aqueous sodium dodecyl sulfate micelles [194]. 

These measurements showed that in-plane lateral proton diffusion was facilitated at air-water interfaces on which stearic acid monolayers were formed, with a surface diffusion coefficient that depended critically on the physical state of the monolayer, and which was at most ca. 15% of the magnitude in bulk solution. These promising initial studies... 

Molecular-level studies of mechanisms of proton and water transport in PEMs require quantum mechanical calculations these mechanisms determine the conductance of water-filled nanosized pathways in PEMs. Also at molecular to nanoscopic scale, elementary steps of molecular adsorption, surface diffusion, charge transfer, recombination, and desorption proceed on the surfaces of nanoscale catalyst particles these fundamental processes control the electrocatalytic activity of the accessible catalyst surface. Studies of stable conformations of supported nanoparticles as well as of the processes on their surface require density functional theory (DFT) calculations, molecular... 

The detection of other molecules, such as ammonia, requires the use of a porous catalytic metal. To obtain a gas response from the NH3 molecule, it is believed that active sites of triple points are required where the molecules are in contact with the metal, insulator, and ambient [30, 31]. It has been shown that gas species such as hydrogen atoms or protons also diffuse out onto the exposed oxide surface in between the metal grains [Figure 2.1(b)] [32, 33]. Furthermore, Lofdahl et al. have performed experiments that provide clear evidence that hydrogen atoms or protons also diffuse under the metal from the triple point [34]. The hollow structure of the metal surface facing the insulator has been revealed by Abom et al. [35]. 

By fitting experimental curves to the numerically generated working curves, Unwin and Bard were able to measure the rate of adsorption of protons on the (001) surface of rutile (TiC>2 ) and (010) surface of albite. They showed that surface diffusion in these systems was too slow in comparison with the solution diffusion to be measurable. 

The rate of surface diffusion may be controlled by either the speed of the proton or of the electron. The proton-affinity of the surface sites must not be too small or adsorption will not occur, nor too large or hopping will be hindered comparisons can be made with the motion of protons in fluids." Co-adsorbates of high proton affinity and/or large size may stabilize the bound proton to the extent that its movement is slowed. ... 

It has been found that adsorption/desorption of anions such as Cl and CIO4 on soil constituents is very rapid. In fact, reequilibrium is too rapid to be observed using p-jump relaxation. Fortunately, the electric-field pulse technique can be used for such systems. This method was employed by Sasaki et al. (1983) to study CI and CIO4 adsorption on goethite. Two relaxations on the order of microseconds were observed in acidified aqueous suspensions of a-FeOOH with either NaCl or NaClO4. The fast relaxation was dependent on the applied electric field intensity and was attributed to a physical diffusion phenomenon. The slower relaxation was independent of the applied electric field intensity and was interpreted in terms of the association/dis-sociation reaction of counter ions with protonated surface hydroxyl groups as ion pairs... 

Measurements of the rates of surface reactions on insulator surfaces, such as dissolution, adsorption, and surface diffusion, are possible (Chapter 12). For example, proton adsorption on an oxide surface can be studied using the tip to reduce proton and induce a pH increase near the surface (22). Then, by following the tip current with time, information about proton desorption kinetics is obtained. Studies of corrosion reactions are also possible. Indeed, work has been reported where a tip-generated species has initiated localized corrosion and then SECM feedback imaging has been used to study it (28). In these types of studies, the tip is used both to perturb a surface and then to follow changes with time. 

For the investigation of adsorption/desorption kinetics and surface diffusion rates, SECM is employed to locally perturb adsorption/desorption equilibria and measure the resulting flux of adsorbate from a surface. In this application, the technique is termed scanning electrochemical induced desorption (SECMID) (1), but historically this represents the first use of SECM in an equilibrium perturbation mode of operation. Later developments of this mode are highlighted towards the end of Sec. II.C. The principles of SECMID are illustrated schematically in Figure 2, with specific reference to proton adsorption/desorption at a metal oxide/aqueous interface, although the technique should be applicable to any solid/liquid interface, provided that the adsorbate of interest can be detected amperometrically. 

When the adsorption/desorption kinetics are slow compared to the rate of diffusional mass transfer through the tip/substrate gap, the system responds sluggishly to depletion of the solution component of the adsorbate close to the interface and the current-time characteristics tend towards those predicted for an inert substrate. As the kinetics increase, the response to the perturbation in the interfacial equilibrium is more rapid and, at short to moderate times, the additional source of protons from the induced-desorption process increases the current compared to that for an inert surface. This occurs up to a limit where the interfacial kinetics are sufficiently fast that the adsorption/desorption process is essentially always at equilibrium on the time scale of SECM measurements. For the case shown in Figure 3 this is effectively reached when Ka = Kd= 1000. In the absence of surface diffusion, at times sufficiently long for the system to attain a true steady state, the UME currents for all kinetic cases approach the value for an inert substrate. In this situation, the adsorption/desorption process reaches a new equilibrium (governed by the local solution concentration of the target species adjacent to the substrate/solution interface) and the tip current depends only on the rate of (hindered) diffusion through solution. 

The membrane of Stuchebrukchov s model is an infinite surface, where the multitude of proton binding sites (carboxylates with pK = 5) is represented by a density function (cr). The dwell time of a proton on any of the sites is determined by the pK (Tdweii = l disfeon), but dufing this time interval the proton can diffuse on the surface with a diffusion coefficient that is 10% of the bulk value (Dg 0.1 Db), screening an area with a radius Lg. On the surface, there is a proton-channel acting either as an absorbing sink, or a source which affects the immediate proton concentration, both at the surface and in the solution. The bulk phase in this model is an infinite reservoir, which is sufficiently far from the proton-consuming cluster to satisfy the demand AC/Ax = 0, a definition that is based on a chemical function... 

The rate constants for the proton transfer between surface groups and the bulk, as well as the collisional proton transfer between the protonated surface sites and the soluble proton acceptors, are of the order of diffusion controlled reactions, with no indication as to an energy barrier that retains the free proton near the surface. 

The scarcity of free proton in the cytoplasmic space of bacteria, eukaryotic cells or the mitochondrial matrix imposes a time limitation on the rate at which a free proton can diffuse towards the enzyme s active sites, so that the system appears to be rate-limited by the availability of free protons. However, the measured rates stUl seem to exceed the predicted values, for a review see Ref. [26], indicating that the protein s surface participates in channeling the proton to the orifice of the protonconducting channels. This case was first demonstrated with bacteriorhodopsin, a membranal protein which utilizes the energy of a photon, absorbed by its chromo-phore, to drive protons from the cytoplasmic space of the bacteria to the external space. 

Robin R, Cooper AR, Heuer AH (1973) Application of a nondestructive single-spectrum proton activation energy to study oxygen diffusion in zinc oxide. J Appl Phys 44 3770-3777 Robertson WM (1969) Surface diffusion of oxides. J Nucl Mater 30 36-39... 

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