Oxygen diffusion, electronically

Either the and the two e s diffuse outward through the film to meet the 0 at the outer surface, or the oxygen diffuses inwards (with two electron holes) to meet the at the inner surface. The concentration gradient of oxygen is simply the concentration in the gas, c, divided by the film thickness, x and the rate of growth of the film dx/dt is obviously proportional to the flux of atoms diffusing through the film. So, from Pick s Law (eqn. (18.1)) ... 

Fig. 12. Auger electron spectroscopy (AES) sputter-depth profile of CAA-treated titanium after various exposure.s in vacuum (a) as anodized, (b) 450°C for 1 h, and (c) 7(X)°C for 1 h. The sputter etch rate is 1.5 nm/min. The line indicates the original interface. The arrow denotes oxygen diffused into the substrate. Adapted from Ref. [51].

SRB, a diverse group of anaerobic bacteria isolated from a variety of environments, use sulfate in the absence of oxygen as the terminal electron acceptor in respiration. During biofilm formation, if the aerobic respiration rate within a biofilm is greater than the oxygen diffusion rate, the metal/biofilm interface can become anaerobic and provide a niche for sulfide production by SRB. The critical thickness of the biofilm required to produce anaerobie conditions depends on the availability of oxygen and the rate of respiration. The corrosion rate of iron and copper alloys in the presence of hydrogen sulfide is accelerated by the formation of iron sulfide minerals that stimulate the cathodic reaction. 

The material would be expected to be a hole (p-type) semiconductor. However, in this compound the interstitial oxygen ions can diffuse fairly quickly, and the oxygen diffusion coefficient is higher than normal, so that the compound shows both high oxygen diffusivity and electronic conductivity, a situation referred to as mixed conductivity (Section 8.7). 

Overall, the rib effects are important when examining the water and local current distributions in a fuel cell. They also clearly show that diffusion media are necessary from a transport perspective. The effect of flooding of the gas-diffusion layer and water transport is more dominant than the oxygen and electron transport. These effects all result in non-uniform reaction-rate distributions with higher current densities across from the channels. Such analysis can lead to optimized flow fields as well as... 

The important processes occurring in a catalyst layer include interfacial ORR at the electrochemically active sites, proton transport in the electrolyte phase, electron conduction in the electronic phase (i.e., Pt/C), and oxygen diffusion through the gas phase, liquid water, and electrolyte phase. 

Heterogeneous catalysis is a surface phenomenon, therefore the overall kinetic parameters are dependent on the real exposed catalyst surface area. In the supported systems only a part of the photocatalyst is accessible to light and to substrate. Besides, the immobilized catalyst suffers from the surface deactivation since the support could enhance the recombination of photogenerated electron-hole pairs and a limitation of oxygen diffusion in the deeper layers is observed. 

Three dominant processes in the reaction diffusion in biofilms and cellular systems are (1) diffusion in a continuous extracellular phase B, (2) transport of solutes across the membrane, and (3) diffusion and reaction in the intracellular phase A. Consider aerobic growth on a single carbon source. The volume-averaged equations of a substrate S and oxygen O (electron acceptor) transport are... 

Figure 32. a) Impedance measurements on the 02, Pt SrTi03 SrTi03 Pt, O2 as function of the superimposed d. c. bias. Electrodes are parallel to the bicrystal boundary ( 5 tilt grain boundary, iron content 2 x lO cm 3). Both bulk and boundary resistances are predominatly electronic resistances.100 b) Measurements of oxygen diffusion into a SrTi03 bicrystal (interface perpendicular to... 

Finally, one of the most highly studied acid-base radical pairs is the superoxide/perhydroxyl (H0 /0 ) radical. In the presence of oxygen, the electron is scavenged in a diffusion-controlled reaction ... 

The reduction of oxygen to takes place at the interface of the three phase boundary. The mixed cathode which allows both electron and to travel along the surface and bulk extends the interface of the three phase boundary to an active zone for the reduction of O2 to 0 . The extension of the active zone depends on the oxygen diffusion, surface exchange coefficient, the microstructure and porosity of the cathode layer, shown in Fig. 2 (a). ... 

Such a unified correlation strongly indicates that the reduction of HRP compound I by thioanisoles proceeds via electron transfer rather than direct oxygen transfer. Electron transfer from the sulfide to HRP compound I in the protein cage might be followed by two competitive processes (i) oxygen rebound to afford the sulfoxide and (ii) diffusion of a sulfenium radical from the protein cage to enable the observation of HRP compound II as shown in Scheme 6 [108]. 

These microbially mediated redox processes utilize electron acceptors and produce reduced species. This will generate more reduced environments as long as there are electron donors available. The microbial population thus strongly affects their environment in the core of the plume. At the boundaries of the plume, complex microbial communities may exist, and steep redox gradients are created when dissolved electron acceptors are consumed. In addition, reoxidation of sulfides or ferrospecies by oxygen diffusing into the plume may increase the concentration of sulfate and ferric iron, which can stimulate sulfate and iron reduction in these zones as observed at Norman Landhll (Cozzarelli et al., 2000). 

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