The Formation of Mixed Phases

In fact, the support surface is not inert. The oxide/water system is thermodynamically metastable and a realistic description of its interface should take into account the oxide hydration and its possible dissolution. The kinetic factor, which is often overlooked for the two types of interactions described above, is here a parameter of prime importance, because the transformations involving the support are far from being instantaneous and should be favored by a long contact time between support and impregnation solution ( aging ). 

Upon contact with water, an anhydrous oxide can transform into superficial oxyhydroxide or hydroxide phases detectable by X-ray diffraction, such as in the case of MgO or AI2O3, or gel-Kke hydrated layers, such as in the case of Si02 [68-70]. Temperature accelerates the diffusion and fixation of the ions inside the superficial hydrated layer for instance, it was observed that the quantity of Ni ions eliminated from alumina by washing after drying diminishes as the temperature of drying increases [71]. 

Metal ions that are embedded inside a mixed phase at the support surface can be used as seeds for the subsequent growth of the active phase particles [61]. Unless the mixed phase decomposes during calcination, these ions are not accessible for catalysis. Subsequent thermal treatments trigger their further diffusion into the support bulk, where they can hardly be reduced. The resulting oxidic phases can, nevertheless, be useful for example, nonreduced NiAl204 is reported to inhibit the sintering of nickel particles at high temperatures [85]. 

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In very dilute HCl solutions, specifically those with a pH above 5-48, the 4 1 5 phase was found to be insoluble. By contrast, addition of concentrated HCl to the 4 1 5 phase was shown to lead to formation of the 1 1 2 phase (Sorrell, 1977). Below 35wt% HCl, the 4 1 5 phase was found to dissolve congruently. Since the 1 1 2 phase was also found to dissolve congruently in hydrochloric acid solutions with concentrations above 23 wt %, it follows that there is a range of concentrations over which both phases are soluble in aqueous HCl. This behaviour explains why the zinc oxychlorides have proved to be unsatisfactory in attempts to use them as dental cements. The preparation of such cements from concentrated aqueous solutions of ZnClj results in the formation either of the 1 1 2 phase alone or of mixtures of the 4 1 5 and 1 1 2 phases, neither of which is stable in the presence of water. Preparing dental cements from less concentrated solutions also results in the formation of mixed phases, unless the bulk composition has excessive amounts of ZnO present. In these latter cases the cement stability is acceptable but it lacks both a workable consistency and a reasonable working time. 

Water is a polar solvent so has different solvation properties that discriminate between polar and non-polar molecules. Chemical discrimination results in the formation of mixed phases, such as membranes, microenvironments and compartmentalisation. 

Conrad et al. (S2) studied in detail the mutual interaction of coadsorbed O and CO on a Pd(l 11) surface. Some of their relevant results are summarized here. Oxygen adsorption is inhibited by preadsorbed CO. At coverages below Oco 1/3, LEED patterns show that O and CO form separate surface domains. However, the behavior is different when O is preadsorbed. CO can be adsorbed on the Pd(lll) surface covered with O which is less densely packed than a saturated CO layer. The O adatom islands are then suppressed to domains of a (v 3 x y/l)R30° structure (0 = 1/3), with a much larger local coverage than can be reached with O alone, which orders in a (2 x 2) structure (ff = 0.25). After further exposure, the LEED patterns s uggest the formation of mixed phases of Oads and CO ads (with local coverages of ffo = Oco = 0.5) which are embedded in CO domains. When these mixed phases are present, CO2 is produced even at temperature lower than room temperature. Coadsorption studies of other noble metal surfaces are consistent with this scenario preadsorbed CO inhibits the dissociative adsorption of oxygen, whereas CO is adsorbed on a surface covered with O. 

It has become dear in recent years that the formation of mixed phases, containing both the transition metal and an element from the support, did not necessitate high caldnation temperatures indeed it occurs very commonly during the deposition step itself. In particular, talc- and nepouite-... 

Physical Equilibria and Solvent Selection. In order for two separate Hquid phases to exist in equiHbrium, there must be a considerable degree of thermodynamically nonideal behavior. If the Gibbs free energy, G, of a mixture of two solutions exceeds the energies of the initial solutions, mixing does not occur and the system remains in two phases. Eor the binary system containing only components A and B, the condition (22) for the formation of two phases is... 

The ore is ordinarily ground to pass through a ca 1.2-mm (14-mesh) screen, mixed with 8—10 wt % NaCl and other reactants that may be needed, and roasted under oxidising conditions in a multiple-hearth furnace or rotary kiln at 800—850°C for 1—2 h. Temperature control is critical because conversion of vanadium to vanadates slows markedly at ca 800°C, and the formation of Hquid phases at ca 850°C interferes with access of air to the mineral particles. During roasting, a reaction of sodium chloride with hydrous siUcates, which often are present in the ore feed, yields HCl gas. This is scmbbed from the roaster off-gas and neutralized for pollution control, or used in acid-leaching processes at the mill site. 

The new phases were discovered by the combination of exploratory synthesis and a phase compatibility study. As commonly practised, the new studies were initially made through the chemical modification of a known phase. Inclusion of salt in some cases is incidental, and the formation of mixed-framework structures can be considered a result of phase segregation (for the lack of a better term) between chemically dissimilar covalent oxide lattices and space-filling, charge-compensating salts. Limited-phase compatibility studies were performed around the region where thermodynamically stable phases were discovered. Thus far, we have enjoyed much success in isolating new salt-inclusion solids via exploratory synthesis. 

Induced precipitation is a collective name for processes accompanying the formation of solid phase, such as occlusion, adsorption, compound formation, formation of isomorphous mixtures, mixed crystals, colloidal solutions, etc. In... 

The IR spectra in Fig.7 indicate the preferential adsorption of NO on the Co sites. It may be conjectured that the Mo sulfide species are physically covered by the Co sulfide species or that Co-Mo mixed sulfide species are formed and the chemical natures of the Co and Mo sulfides are mutually modified. The Mo K-edge EXAFS spectra were measured to examine the formation of mixed sulfide species between Co and Mo sulfides. The Fourier transforms are presented in Fig.8 for MoSx/NaY and CoSx-MoSx/NaY. The structural parameters derived from EXAFS analysis are summarized in Table 1. The structure and dispersion of the Mo sulfides in MoSx/NaY are discussed above. With the Co-Mo binary sulfide catalyst, the Mo-Co bondings are clearly observed at 0.283 nm in addition to the Mo-S and Mo-Mo bondings. The Mo-Co distance is close to that reported by Bouwens et al. [7] for a CoMoS phase supported on activated carbon. Detailed analysis of the EXAFS results for CoSx-MoSx/NaY will be presented elsewhere. It is concluded that the Co-Mo mixed sulfides possessing Co-S-Mo chemical bondings are formed in CoSx-MoSx/NaY. 

The potassium/caesium phase diagram is an example of a system involving the formation of mixed crystals with a temperature minimum (Fig. 4.4). The right and left halves of the diagram are of the same type as the diagram for antimony/bismuth. The minimum corresponds to a special point for which the compositions of the solid and the liquid are the same. Other systems can have the special point at a temperature maximum. 

Another phenomenon to be detected by X-ray crystallography is the formation of mixed crystals, as observed in the mixed coupling of azo pigments or the solid solutions of quinacridone pigments. A change in the angles of the reflected X-rays of a mixed crystal indicates a transition from one crystal phase to another. If, how-... 

The most complicated equations presented in this chapter are of the types shown by Eq. (4.89), where a series of complexes are formed in the organic phase and, at the same time, one or a series of complexes in the aqneons phase. More complicated equations would be obtained if mixed complexes and/or polynn-clear complexes are present, and if varying activity factors are introdnced, bnt these cases are not discussed here. Thus the solvent extraction eqnations can be expressed by Eq. (4.89), where x and y are independent variables and and are unknown independent parameters. The polynomial in the denominator refers to the formation of aqueous phase complexes in the case that the metal forms several series of complexes with different ligands A, L, etc., the denominator contains several polynomials co + Ciz + , etc. The polynomial in the nnmera-tor always refers to the formation of organic phase complexes. 

Overall, it appears likely that the films contained chalcopyrite CuInSi mixed with other phases with similar diffraction patterns. Separate microstrac-tural characterization (EDS) of films with varying composition (ca. 10% excess Cu or In) showed the formation of separate phases of CuiS and IniSs, respectively, along with the CuInSi [39]. The best films were obtained at high deposition temperatures (80°C) and with stirring. Lower deposition temperature resulted in poorer stoichiometry (less S), and stirring improved fihn uniformity. Grain size, measured by TEM (which does not necessarily show crystal size) was 100 00 nm. 

Support-bound alcohols and thiols can be used to immobilize aldehydes and ketones as acetals. Mixed acetals of carbonyl compounds with support-bound alcohols can be prepared by transacetalization of a symmetric acetal under acidic conditions [719]. The formation of mixed acetals on solid phase is, however, not always easy to perform and control, and so prior preparation of a mixed acetal in solution followed by loading onto a support is often the preferred protocol [626,637]. Carbohydrates can be linked to resin-bound alcohols or thiols as glycosides (Table 3.40). 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

Seantier B, Breffa C, Felix O et al (2004) In situ investigations of the formation of mixed supported lipid bilayers close to the phase transition temperature. Nano Lett 4 5-10... 

Dependences of 2R on Tcaic for several pure or mixed semiconductor oxides are presented in Fig. 8.4. Iron doped titania photocatalysts with different iron contents at Tcalc below 400°C had iron ions uniformly distributed in the anatase-Ti02 phase [114]. At Tcalc > 400°C, 1 wt % Fe samples performed the same behaviour of 2R as without iron, and at Tcalo > 600°C in the samples with 10 wt % Fe content, the formation of hematite phases interacted with the titania phases was observed in XRD experiments. The crystalline structure of Ti02 phases was distorted at high Tcalc which also resulted in 5-fold decrease of 2R as compared to 1 wt % Fe case (Fig. 8.4). 

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