Corrosion crystallites

Studies performed on CdS [282, 283] have revealed the importance of the microstructure, i.e., crystal structure, crystallite size, and geometrical surface area, in both the control of band structure and the concentration and mobility of charges, in relation to the photocatalytic performance of the photocatalyst. It has been shown also that the solubility product of CdS colloids prepared from acetate buffer aqueous solutions of suitable precursors increases from 7.2x 10 for large particles to about 10 for small (< 2.5 nm) particle colloids, this increase invoking a positive shift on the cathodic corrosion potential [284]. 

The dissolution of zinc in a mineral acid is much faster when the zinc contains an admixture of copper. This is because the surface of the metal contains copper crystallites at which hydrogen evolution occurs with a much lower overpotential than at zinc (see Fig. 5.54C). The mixed potential is shifted to a more positive value, E mix, and the corrosion current increases. In this case the cathodic and anodic processes occur on separate surfaces. This phenomenon is termed corrosion of a chemically heterogeneous surface. In the solution an electric current flows between the cathodic and anodic domains which represent short-circuited electrodes of a galvanic cell. A. de la Rive assumed this to be the only kind of corrosion, calling these systems local cells. 

On the supposition that the total number of unit cells keeps invariable and no aluminum atoms are lost during the boronation, the composition of unit cell and the population of vacancies can be estimated as listed in composition of unit cell (I) in Table 2. It can be seen that the vacancies occupy about 30-50% of total T sites after the boronation. However, it should be noted that the population of vacancies thus obtained by chemical analysis is only a bulk average result. The composition on the surface of crystallites is actually different from that in the bulk because the dissolution of silicon starts first from the outer surface, so that the vacancies on the surface are much more than those in the interior of crystallites. Such a large number of vacancies on the surface will result in corrosion and dissolution of the surface parts of crystal particles. Therefore, the number of unit cells in the sample after the boronation is actually less than that before the boronation, whereas boron atoms in each unit cell should be more than those shown in composition of unit cell (1) in Table 2. On the other hand, if all the 64 T sites are occupied by silicon and trivalent atoms, we can give another set of compositions as shown in composition of unit cell (II) in Table 2. The real composition of a unit cell should be between these two sets of compositions, that is, the 64 T sites are neither occupied completely nor vacated so severely that the collapse of the framework occurs. It can also be seen that the introduction of boron atoms is so limited that there are no more than 1.5 atoms per unit cell even though the repeated boronation is performed. 

The SEM pictures of samples are shown in Fig.5. They provide a direct evidence for the corrosion on the outer surface of crystallites. For the parent sample, the average size of the crystal particles is 200 nm (Fig.5a). After the boronation, the average particle size decreases since the corrosion and dissolution of the outer layer of particles occurs. In the case of [B]-Naft-2 with a more severe dissolution, the average particle size is only about half as large as that of the parent sample (see Fig.5b). 

From all the above information, we can describe the important modification during the boronation of zeolites (3 as follows very limited boron atoms are inserted into the (3 framework by treating the sample with an alkaline solution containing boron species. Accompanied by this insertion, a considerable amount of silicon atoms are extracted from the lattice, resulting in the micropores in crystallites are enlarged into the mesopores and the smaller mesopores are developed into larger intracrystalline mesopores. Meanwhile, the corrosion of outer layer of crystallite makes the size of crystal particle reduce. 

Heckman and Harling57 examined the gas-phase oxidation of carbon black micro-structures and showed that oxidative attack of carbon crystallites was concentrated on the small crystallites, at the edges of layer planes and at lattice defects. Partial graphitization of a carbon black, so that only the outermost surface layers are well-ordered, causes oxidative corrosion within the core of the carbon particle, leaving an outer shell . Consequently, similar behavior can be expected for ungraphitized and partially graphitized carbons in electrochemical environments. 

The foregoing discussion serves to show that disordered carbon structures are oxidized more readily than well-ordered graphite planes and that dislocations and active sites provide nucleation points for attack of the carbon crystallite. Another factor that must be considered is that dispersed electrocatalysts, such as platinum, on the carbon surface are not benign. The electrocatalysts interact with the carbon causing local oxidation or corrosion, i.e., the platinum catalyzes the corrosion of the carbon itself. In the presence of oxygen, which is the condition under which the electrocatalyst will operate, reduction intermediates from the oxygen (e.g., HOj) can have an accelerated corrosion effect. 

Growth of metaJ crystallites and interactions with the support occur more readily in reaction environments containing more than one gas, e.g. Hj, H2O and CH4 relative to the separate gases. Formation of corrosive free radicals such as HOj may play a role in accelerating removal and transport of metal atoms from crystallites. 

Preliminary results showed that skeletal Cu, after promotion with chromia, has a very high surface area and a stable stmcture. The Cu crystallite sizes and surface areas remained almost the same as the initial values after being exposed to 6.1M NaOH at 323K for 170 hours, at 373 for 120 hours and at 423K for 100 hours respectively. However, unpromoted skeletal Cu, under the same conditions, showed a significant loss of surface area. When the temperature was increased the BET surface area decreased even finther. This significant decrease of surface area was attributed to dissolution and reprecipitation of copper in this highly corrosive environment. 

In addition to loss of the platinum, the carlxm support that anchors the platinum crystallites and provides electrical coimectivity to the gas-diffusion media and bipolar plates is also subject to degradation. In phosphoric acid fuel cell, graphitized carbons are the standard because of the need for corrosion resistance in high-temperature acid environments [129], but PEM fuel cells have not employed fully graphitized carbons in the catalyst layers, due in large part to the belief that the extra cost could be avoided. Electrochemical corrosion of carbon materials as catalyst supports will cause electrical isolation of the catalyst particles as they are separated from the support or lead to aggregation of catalyst particles, both of which result in a decrease in the electrochemical active surface area of the catalyst and an increase in the hydrophUicity of the surface, which can, in turn, result in a decrease in gas permeability as the pores become more likely to be filled with liquid water films that can hinder gas transport. 

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