Surface oxygen transport

The MD and first-principles studies revealed that the diffusion mechanism is critically dependent on the structure at the surface and therefore such predictions are unlikely to be universally applicable to all ceria surfaces. In particular, some surfaces, for example, dipolar, will undergo more substantive relaxation (even faceting) compared to other surfaces and therefore one needs to simulate O transport for each surface individually before trends can be identified. Clearly, computer simulation is well placed to explore such diffusion and determine the activation energy barriers associated with O transport to particular surfaces. 

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Slime is a network of secreted strands (extracellular polymers) intermixed with bacteria, water, gases, and extraneous matter. Slime layers occlude surfaces—the biological mat tends to form on and stick to surfaces. Surface shielding is further accelerated by the gathering of dirt, silt, sand, and other materials into the layer. Slime layers produce a stagnant zone next to surfaces that retards convective oxygen transport and increases diffusion distances. These properties naturally promote oxygen concentration cell formation. 

The resulting unstable molecular ion Oj) rapidly adds another electron and protons to yield hydrogen peroxide. In alkaline solutions the same pathway is followed, but owing to the much lower polarization, the reaction becomes practically reversible (b = 0.03 V) its rate then is determined by oxygen transport to the surface, and polarization is of the concentration type (Bagotsky and Yablokova, 1953). 

As the CH4 component of the fuel reforms to CO and H2, H2O is consumed. This favors the reversal of Equation (8-16), which allows Si02 to be deposited downstream, possibly on exposed nickel surfaces. Oxygen-blown coal gas, however, has an H2O content of only -13%, and this is not expected to allow for significant Si transport. 

Corrosion attributable to oxygen is deemed to result from the solution or oxygen by a thin film of liquid adjacent to the metallic surface, the transportation of the oxygen through the film, and the subsequent reaction at the surface of the metal. This explains why there is corrosive action even in relatively arid land. In a very dry atmosphere corrosion Is. however, markedly reduced. 

Interphase inhibition [52] occurs where the inhibitive layer has a three-dimensional structure situated between the corroding metal and the electrolyte. The interphase layers generally consist of weakly soluble compounds such as oxides, hydroxides, carbonates, inhibitors, etc. and are considered to be porous. Non-porous three-dimensional layers are characteristic of passivated metals. The inhibitive efficiency depends on the properties of the three-dimensional layer, especially on porosity and stability. Interphase inhibition is generally encountered in neutral media, either in the presence or absence of oxygen. In aerated solutions, the inhibitor efficiency may be correlated with the reduction in the oxygen transport limited current at the metal surface. 

Figure 63. The kinetics in Lao. Sro.iCoOj.x> under the conditions given, is strongly influenced by the surface reaction. For pure diffusion control the normalized surface concentration would be unity.207 (Reprinted from R. A. De Souza, J. A. Kilner, Oxygen transport in La,.xSrxMni.yCoy03is. , Solid State Ionics, 106, 175-187. Copyright 1998 wih permission from Elsevier.)...

Sant and Varma (183) found that low concentrations of zirconium lowered the temperature required to reach the maximum yield. Various interpretations of this observation have been put forward either the increase in surface area or the increase in oxygen transport rates can be sufficiently altered by the zirconium to result in high yields of MA at lower temperatures. The studies generally agree that aroimd 1.5% zirconium has the most beneficial effect on the activity, and good catalytic performance could be achieved at lower temperatures (172). One of the reasons for this that has been proposed is that zirconium and titanium both create acidic surface sites on the vanadium phosphate surface. These sites prevent the desorption of reaction intermediates (butene, butadiene, and furan) while facilitating the desorption of the acidic MA. 

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