Porous oxide

It seems to be possible that most transition metal oxides can be made in porous crystals with different morphologies using various mesoporous silicas as templates. It is expected that these materials have potential in applications such as catalysis, fuel cell, gas sensors and Li-batteries. Their physical properties would fall in between nanoparticles and bulk specimens, although our knowledge about these properties is still very limited. 

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If a compact film growing at a parabolic rate breaks down in some way, which results in a non-protective oxide layer, then the rate of reaction dramatically increases to one which is linear. This combination of parabolic and linear oxidation can be tenned paralinear oxidation. If a non-protective, e.g. porous oxide, is fonned from the start of oxidation, then the rate of oxidation will again be linear, as rapid transport of oxygen tlirough the porous oxide layer to the metal surface occurs. Figure C2.8.7 shows the various growth laws. Parabolic behaviour is desirable whereas linear or breakaway oxidation is often catastrophic for high-temperature materials. 

The results obtained when the adsorbents are non-porous oxides are somewhat different. The values of a (Ar) referred to a,n(N2) = 16-2 A are... 

The models derived for continuous oxide layers remain valuable when porous oxides are formed they provide a frame of reference against which deviations may be examined and give a basis for understanding the factors governing the location of new oxide. In many cases, however, the experimentally derived rate laws no longer have a unique interpretation. For example, the linear rate law relating the thickness of oxide, x, to the time, t... 

In certain circumstances even the parabolic rate law may be observed under conditions in which the oxide is porous and permeated by the oxidising environment". In these cases it has been shown that it is diffusion of one or other of the reactants through the fluid phase which is rate controlling. More usually however the porous oxide is thought to grow on the surface of a lower oxide which is itself growing at a parabolic rate. The overall rate of growth is then said to be paralinear - and may be described by the sum of linear and parabolic relationships (see equations 1.197 and 1.198). 

Table 1.27 Metals which form porous oxides in dry oxygen...

The Role of Metal Dissolution or Volatiiisation in the Formation of Porous Oxides... 

As we have seen, a consequence of the formation of porous oxide is that the rate-controlling step reverts to that of a phase boundary reaction and... 

Porous oxide (silica) matrix infiltrated with phenolic resin... 

The anodic oxidation of sheet aluminum has been used for a long time to protect aluminum against corrosion by a well-adhering oxide layer. Porous oxide layers are formed if acid electrolytes are used that can redissolve the aluminum oxide (mostly sulfuric or phosphoric acid). A compact oxide layer is formed at the beginning of the electrolysis (Fig. 20.3). Simultaneously, the current decreases, due to the electric resistance of the oxide. Subsequently follows a process in which the oxide is redissolved by the acid, and the current increases until it reaches a steady state. The electrochemical oxidation continues to take place with formation of pores. At the end of a pore, where it has the largest curvature, the electric field has its largest gradient and the process of redisolution is fastest. 

Figure 3. A model of a porous oxide film formed at 120 V in a phosphoric acid solution, according to Heber.10...

In the above considerations, the O/S interface was taken to be a clear-cut boundary between the oxide and the electrolyte. In reality, however, the outer part of the oxide is likely to be hydrated and penetrated by the electrolyte. Hence, the true O/S interface is likely to be withdrawn from the surface to a sufficient depth such that some oxide is left without any electric field imposed across it. This is especially true of thick porous oxide layers, but it can occur with compact layers as well. For example, Hurlen and Haug35 found a duplex film in acetate solution (pH 7-10), composed of a dry barrier-type part and a thicker hydrated part consisting of A1203 H20. Although the hydrated part becomes thinner with decreasing pH and seems to practically vanish at low pH, even a thickness of less than a nanometer is sufficient for the surface oxide to stay outside the electrochemical double layer. 

When the oxide is formed by anodizing in acid solutions and the sample is then left to rest at the OCP, some dissolution can occur. This process has been studied by a numbers of authors,70-75 especially in relation to porous oxides [cf. Section 111(4)]. It was found that pore walls are attacked, so that they are widened and tapered to a trumpet-like shape.70 71 Finally, the pore skeleton collapses and dissolves, at the outer oxide region. The outer regions of the oxide body dissolve at higher rates than the inner ones.9,19 The same is true for dissolution of other anodic oxides of valve metals.76 This thickness dependence is interpreted in terms of a depth-dependent vacancy concentration in the oxide75 or by acid permeation through cell walls by intercrystalline diffusion, disaggregating the microcrystallites of y-alumina.4... 

The steady-state potential (or current density) is related to a steady growth of the porous oxide into the solution, maintaining a constant number of pores and a constant pore radius. This scheme is supported by electron microscopic observations reported by Xu et a/.102... 

Data on anion incorporation into a growing porous oxide were obtained Fukuda and Fukushima.165,166 Their study was the first to demonstrate a correlation between the kinetics of accumulation of oxalate165 or sulfate166 anions and the change of porous oxide growth stages. The results of galvanostatic and potentiostatic... 

The true role of incorporation of anions in the formation of anodic alumina is being intensively discussed. Baker and Pearson183 have considered the anion effect in modifying the structure of anodic oxides to be due to the coordinative ability of anions to replace alumina tetrahedra in the body of the oxides. Dorsey184,185 has postulated that in porous oxides, anions stabilize the network of alumina tetrahedra and octahedra. 

As for the porous oxides formed in alkaline solutions, there is evidence that they are heavily hydrated. Hurlen and Haug35,216 have recently shown that the chemical composition of the nonbarrier... 

Figure 33. Hydration capacity of porous oxides formed in various electrolytes. The capacity to absorb various anions is also shown.

Numerous publications have been devoted to the investigation of the morphology of porous oxides of aluminum. Pores of virtually... 

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