Support dissolution

Let us assume that the total surface of an electrode is in an active state, which supports dissolution, prior to anodization. The application of a constant anodic current density may now lead to formation of a passive film at certain spots of the surface. This increases the local current density across the remaining unpassivated regions. If a certain value of current density or bias exists at which dissolution occurs continuously without passivation the passivated regions will grow until this value is reached at the unpassivated spots. These remaining spots now become pore tips. This is a hypothetical scenario that illustrates how the initial, homogeneously unpassivated electrode develops pore nucleation sites. Passive film formation is crucial for pore nucleation and pore growth in metal electrodes like aluminum [Wi3, He7], but it is not relevant for the formation of PS. 

The dissolution of titania supports was also investigated. Titania also (partially) dissolves in aqueous solutions and silica modification, using TEOS, effectively inhibited support dissolution [4]. 

Powder X-ray dififiaction patterns were recorded using Cu Ka (1.5418 A) radiation on a Philips 1050/81 vertical goniometer, fitted with a diffracted beam graphite monochromator. X-ray photoelectron experiments were carried out in a Escalab 200R (Fisons Instrumrats) using monochromatic A1 Ka radiation and operating at 10" Torr base pressure. Transmission electron microscopy analysis was performed in a JEOL lOOCX either by direct observation or after extractive replication of samples (support dissolution). 

Where solubility alone is the issue, simply changing solvent to permit all species to be dissolved allows the chemistry to proceed essentially as it would in aqueous solution were species soluble. Typical molecular organic solvents used in place of water include other protic solvents such as alcohols (e.g. ethanol), and aprotic solvents such as ketones (e.g. acetone), amides (e.g. dimethylformamide), nitriles (e.g. acetonitrile) and sulfoxides (e.g. dimethylsulfoxide). Recently, solvents termed ionic liquids, which are purely ionic material that are liquid at or near room temperature, have been employed for synthesis typically, they consist of a large organic cation and an inorganic anion (e.g. lV, lV,-butyl(methyl)-imidazolium nitrate) and their ionic nature supports dissolution of, particularly, ionic complexes. 

However, during impregnation of the support, dissolution of alumina followed by surface precipitation has been evidenced [1]. This phenomenon is increased when solubilized aluminum reacts with species such as cobalt, to form hydrotalcite-like coprecipitates, or molybdenum to form Andemon-lype heteropolyanion AMo6024H3 [2,3]. After calcination, these species do not lead to easily sulfiding species and thus decrease the overall yield in CoMoS active phase [4,5], which is of course detrimental for catalytic activity. 

Up to 0.5 Pt per nm [2] and to limit the support dissolution [1]. This procedure enables to prepare hi y dispersed Pt°/silica catalysts with relatively hi ... 

This test was performed in absence of any support. Dissolution of PdCl2 was obtained using 10" M HCl. According to literature [14], H2PdCl4 is formed in this conditions. As given by Chariot [15], the dissociation constant (pIQ) of PdCU is 13.2 and the solubility product (pKg) of Pd(OH)2 is 24. Thus precipitation of Pd-hydroxide should occur when pH becomes superior to 6.1. A rapid precipitation is indeed observed at pH 10 and room temperature. 

Coke deposited on HY zeolite during n-heptane reaction at 450°C, was completely dissolved by CI2CH2 after dissolution of the support with HF . This is not usual, since in most studies there is a fraction of insoluble coke. The coke formed on PtUSHY and PtHMOR catalysts during benzene hydrogenation at 80°C was also completely solubilized in CI2CH2 after support dissolution with 40% HF °. In this case, since the coke was deposited at low temperature, such a high solubility could be expected. 

Coke structure can be characterized by X-Ray diffraction analysis. This technique makes it possible to determine if there is coke with crystalline structure on the catalyst. However, the sensitivity of this type of determination is rather low, being it difficult to determine the fraction and/or amount of coke in the crystalline form. Support dissolution procedure was also used to analyse the coke free from support by XRD. Support dissolution procedure for coke XRD analysis, is more appropriate when the coke content on the catalyst is high, and as long as the strong acidic media used in the dissolution does not alter the coke structure. 

A membrane with the same topology formed on a porous alumina substrate also showed high separation factor (220) at 28 °C and the permeation properties were slightly influenced by water addition in the feed. The water adsorbed in the zeolite pores blocks the CO2 and CH4 permeation. The DD3R presents an all silica structure, furthermore, it is expected to be affected less by water adsorption. However, the membrane used in this work was not perfectly hydrophobic (Si/Al = 980), probably owing to the support dissolution in the synthesis gel. 

Apart from soluble support precursors, preformed sohd supports can be introduced during coprecipitation. As discussed in Section 7.2 coprecipitation is too fast for quantitatively depositing metals onto the internal surface area of a support. Nevertheless, preformed supports are heing used, mostly because via a support dissolution/redeposition mechanism, catalysts are obtained quite similar in catalytic performance to those based on soluble support precursors. 

The deposition-precipitation method using a base has been applied for the preparation of various catalysts. Upon raising the pH of the solution, the precipitation of a hydroxide onto the support is expected. In fact, it was shown in several cases (Table 14.2) that mixed compounds such as phyllosilicates for silica support or hydrotalcite for alumina support formed, involving support dissolution and neoformation of a mixed compound with a layered structure. 

Examples of Supported Oxide Samples Prepared by Deposition-Precipitation, Involving Support Dissolution and Neoformation of a Mixed Compound... 

---