Isomorphic substitution of ions

Because of isomorphic substitution of ions in the crystalline lattice of layer silicates, many clay surfaces have a net negative charge which results in the abi-... 

In view of the above said, it is difficult to realize that the factors considered are connected with formation of new phases, chemical compounds or isomorphic substitutions of ions in the crystalline lattice, etc. and aimed at substantial changing the specific surface areas of synthesized samples. Therefore, without any extensive hypotheses, we will formulate conditions that should be satisfied by the initial binary system providing the most extensive surface and a high sorption capacity of the samples. 

II. Non-indifferent electrolytes are capable of changing the value of the surface potential, (p0. These electrolytes usually contain ions that are capable of entering the crystal lattice of the solid, e.g. by isomorphic substitution of ions forming the solid phase. The following characteristic cases can be outlined. 

FIGURE 5.1. The principle sources of surface charge in solids include (a) differential ion solubility phenomena, (h) direct ionization of surface groups, (c) isomorphous substitution of ions from solution, and (d) speciflc-ion adsorption from the solution phase (e) anisotropic crystal lattice structures. 

The surface charge characterization of clay minerals, when permanent charges from isomorphic substitutions of ions in a clay crystal lattice are present besides the variable edge charges, is more complicated than that of metal oxides. In this case, the intrinsic surface charge density, Cin, can be defined as the sum of the net permanent structural charge density, oq, and the net proton surface charge density, ffo.H, i-C-, [2,... 

This diffuse double-layer approach can be applied to describe the EDL of particles, if charges on particle surface are only permanent structural surface charges originating from isomorphic substitutions of ions in a clay crystal lattice (e.g., montmorillonite, which is a typical example of infinite flat plates with a constant charge density [19]) or they form by the adsorption of potential determining ions (e.g., Ag+ ions on a Agl surface is an example of the case of charged particles with constant potential [1,33,38]) and the diffuse swarm of indifferent electrolyte ions compensates surface charges. 

The key to any approach is knowing the electrical charge and potential on the surface of the mineral particle in an aqueous suspension. The following four phenomena contribute to the development of the surface charge specific adsorption of surface-active ions preferential dissolution of lattice ions dissociative adsorption of water molecules isomorphous substitution of ions comprising the mineral lattice (Fuerstenau and Herrera-Urbina, 1989). 

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

Selected OSC values are reported in Table 8.1 for ceria and cerium-zirconium mixed oxides. These results confirm that the isomorphous substitution of Ce4+ by Zr4+ ions clearly improves the catalyst stability. BET (Brunauer, Emmett, Teller) area of ceria treated at 900°C is close to 20m2g 1 while it amounts to 35 15 m2g 1 for most mixed... 

We have described above the evolution of the magnetic properties of the [Cp2M (dmit)]AsFg salts upon isomorphous Mo/W substitution. Another possibility offered by this attractive series is the isomorphous substitution of the counter ion, that is PFg- vs AsF6 vs Sbl- fi. Electrocrystallization experiments conducted with [Cp2Mo(dmit)] and the three different electrolytes afforded an isomorphous series, with a smooth evolution of the unit cell parameters with the anion size [32], This cell expansion with the anion size leads to decreased intermolecular interactions between the [Cp2Mo(dmit)]+ radical cation, as clearly seen in Table 2 from the decreased Curie-Weiss temperatures and Neel temperatures (associated with the transition they all exhibit to an AF ground state). 

Figure 3.4. Two types of isomorphous substitution. The middle structures are two-dimensional representations of clay without isomorphous substitution. On the left is an isomorphous substitution of Mg for A1 in the aluminum octahedral sheet. On the right is isomorphous A1 substitution for Si in the silicon tetrahedral sheet. Clays are three-dimensional and -OH on the surface may be protonated or deprotonated depending on the pH of the surrounding soil solution. There will be additional water molecules and ions between many clay structures. Note that clay structures are three-dimensional and these representations are not intended to accurately represent the three-dimensional nature nor the actual bond lengths also, the brackets are not intended to represent crystal unit cells.

The surfaces of colloidal particles are often charged and these changes can arise from a number of sources. Chemically bound ionogenic species may be found on the surface of particles such as rubber or paint latex particles. Charged species may be physically adsorbed if ionic surface active materials, for example, have been added. A charged surface may occur on a crystal lattice. An example is the isomorphous substitution of lower valency cations such as aluminium for silicon in the lattice structure of clays. A further example is the adsorption of lattice ions... 

The particular combinations of ions and molecules that will form precipitates in a given solution can be predicted from equilibrium thermodynamics. However, this often gives a misleading picture because there are kinetic limitations or there is inhibition, particularly in soil solutions. There may also be isomorphous substitution of one cation for another in the precipitate, resulting in a solid solution with a different solubility to the pure compound. 

The other way to introduce heterometals is their isomorphous substitution for Si in the framework, in a similar manner to the isomorphous substitution of Al. The heteroatoms should be tetrahedral (T) atoms. In hydrothermal synthesis, the type and amount of T atom, other than Si, that may be incorporated into the zeolite framework are restricted due to solubility and specific chemical behavior of the T-atom precursors in the synthesis mixture. Breck has reviewed the early literature where Ga, P and Ge ions were potentially incorporated into a few zeolite structures via a primary synthesis route [9]. However, until the late 1970s, exchangeable cations and other extraframework species had been the primary focus of researchers. 

In a quantum-chemical MNDO-PM3 level study of the hydration of the Mg2+ cation located in a ditrigonal cavity of the basal surface of clays [99], the most favorable area of Mg2+ cation location was predicted to be in the vicinity of the AIO4 tetrahedron formed by the isomorphic substitution of Si for Al in the silica-oxygen sheet. The authors have showed the important role of the hydrogen bond formation between the water molecules and the oxygen atoms of the silica-oxygen sheet in the Mg-ion hydration. This was confirmed in several MC simulation studies [65, 66]. 

Solid particle surfaces develop charge in two principal ways either permanently, from isomorphic substitutions of component ions in the bulk structure of the solid, or conditionally, from the reactions of surface functional groups with adsorptive ions in aqueous solution. A surface functional group is a chemically reactive molecular unit bound into the structure of an adsorbent at its periphery, such that the reactive portion of the functional group can be exposed to an aqueous solution contacting the adsorbent [3]. 

Fe with the template ion. DTA studies indicate that Fe-faujasites have lower thermal stability than their Al—analogs.The (OH) vibration frequency shifts from 3540 and 3630 to 3570 and 3643 cm respectively on isomorphous substitution of Al by Fe. Relative changes in the intensity of the ESR peak at g = 4.3 at low temperatures also support the conclusion that iron can be inserted in the fauja-site lattice positions. 

Whereas the surfaces discussed so far have been generated from the bulk by a simple cut, leading to a decrease in the coordination number of the surface atoms, catalytically important acidic surfaces can also be generated in microporous or layered materials by isomorphous substitution of lattice cations. This occurs in zeolites and smectite clays. Zeolites and clays can be considered as aluminosilicates. Their lattice compositions can vary significantly. In zeolites the Al3+ ion can be substituted by many other trivalent cations. Si4+ can be partially substituted by Ti4+ or Ge4+. 

Isomorphous substitution of Si4+ by a trivalent ion (this is often an Al3+ ion) results in a negative lattice charge. This negative charge can be compensated by a cation located in the zeolite cage or micropore or on the clay layer. When a... 

---