Lattice cations

With the semiconducting oxides, we expect anionic chemisorption to occur over the lattice cations, and our simple molecular orbital theory will be adequate if the conduction band is associated mainly with the cation lattice. This is certainly the case with AI2O3, where there is direct evidence in the soft X-ray emission spectra that the highest filled band is the oxygen 2p band 16). 

Rare earth consumption could, however, be affected by a change in host lattice system, with Eu3+ still being retained as activator. Either cost or luminescence efficiency could drive such a change. Costs could be decreased either by eliminating rare earth host lattice cations or by a decrease in the required concentration of Eu. However, in spite of considerable research effort, new host systems that accomplish these objectives have not been found. 

Depending on niobium location, the Nb-containing catalysts can reveal Bronsted acid, Lewis acid, or redox properties. Niobium oxide cationic species (NbOn(5-2n)+), which occupy the extra lattice cation positions, play the role of the Lewis acid sites and may exhibit the redox properties. Nb localized in the framework of mesoporous MCM-41 sieves provides the Lewis acidity [3,4] and the oxidizing properties [5,12]. 

Electronic Properties. When the silver aggregate grows, its properties become more like those of bulk silver. These calculations for Ag atoms added at lattice positions show that a single atom on the crystal has a charge of +0.52, which is comparable to the average AgBr lattice cation charge of +0.55 calculated by... 

In these calculations averaged charges on the intra-tetrahedral lattice cation positions were used. The difference between the two heats of formation due to ionic bonding is added to the heat of formation due to covalent bonding resulting from the simple Extended Huckel Method for zeolitic silicas in order to arrive at the total heat of formation of the zeolite structure as a function of the amount of aluminum. 

The O- species on the surface will be further stabilized by coulombic interactions with the lattice cations. For the sake of completeness, the review also includes a brief section on oxygen ions which are closely bonded to transition metal ions (M=0) and which may play an important part in selective oxidation [Weiss et al. (4) The coverage of the mononuclear oxygen species is restricted to those papers where there is direct evidence on the nature of the oxygen species concerned. 

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+. 

In the zeolites each lattice cation is tetrahedrally coordinated to four oxygen anions (see Fig. 4.57). Each oxygen anion shares two lattice cations. In smectite clays an octahedral layer, usually containing Al3+ or Mg2+, connects two tetrahedral layers. The tetrahedral layer can be considered neutral when the tetrahedral site contains a four valent cation. This is usually a Si4+ ion. A top view and side view are shown in Fig. 4.59. 

In montmorillonite, similar to other minerals, when the size of the exchanged cation is similar to the pore sizes in the crystal lattice, cations can build into the crystal lattice and, consequently, they reduce the negative layer charge (Chapter 1, Section 1.3.3.2). Other neutral molecules or cationic substances (Chapter 1, Sections 1.3.3.1 and 1.3.3.2) can also be sorbed in the interlayer space and on the external surfaces as well. They play an important role in defining the internal and total surface area and catalytic properties, and they may have an effect on the hydrophobicity of the mineral, as well as playing an important role in the production of pillared materials, etc. 

The overlap of the excess electron wave function with the charge clouds of surface ions creates further hyperfine interactions with the lattice cations... 

Table III shows for a series of borates how the Stokes shift (i.e., AQ) increases if the size of the host lattice cation increases (100). In ScBOa the rare-earth ions are strongly compressed and the surroundings are stiff. Small Stokes shifts result for Ce, Pr, and Bi, but not for the smaller Sb . Note, however, that the Stokes shift of the 4/ -5d transitions is less sensitive to the surroundings than that of the 5s-5p transitions. If the data of Table III are extrapolated to, for example, borate glasses, it can be concluded that we find no efficient Sb or Bi emission, but for or Pr this may still be the case. This is what has been observed experimentally.

Nevertheless the orthoborates show the same increase of the Stokes shift of the Sb + emission for increasing host lattice cation (2 76). This suggests that the model proposed for Bi " is also valid for Sb. ... 

Cation-exchanged aluminosilicates can also act as bases, particularly when the extra lattice cation is large. A low Si Al ratio produces a more basic catalyst. A Cs+ exchanged amorphous aluminosilicate can catalyze the liquid phase aldol condensation of benzaldehyde with cyclooctanone (Eqn. lO.lb). Both mono-and di-benzylidene products are formed over this amorphous basic catalyst. As discussed later, with the crystalline aluminosilicates, the zeolites, selectivity for raono-benzylidene product formation is increased. 

The widespread application of zeolites as catalysts has directed attention towards enhancing and extending catalytic activity through the introduction of catalytically active metal species principally by cation exchange with lattice cations. In zeolites such as the X, Y and A type zeolites, the sodalite cages provide possible sites for accommodating reactive metal species. 

A language has grown up around the science of phosphors. Most phosphors consist of a host composition plus the activator, added in carefully controlled quantities. Tlie activator itself is a substitutioiial defect and is subject to lattice phonon perturbations. Therefore, it is essential that the charge on the substitutional cation is equal to that of the host lattice cations. Otherwise, an efficient phosphor does not result. 

For a number of the common scale forming species such as calcium sulphate and carbonate, the rate of crystallisation follows a parabolic relationship with supersaturation, and the value of the effective order of reaction is 2. An explanation may involve the dehydration of lattice cations at the crystal surface. For other examples when the value of n > 2, i.e. where the growth rate varies more strongly with supersaturation, the explanation may involve the interaction between several nuclei and spread of growth [Nielson 1964, O Hara and Reid 1973]. 

The catalytic activity of zeolites is generated by converting them to an "acid form" by heating the "ammonium form" (where the cations are NH4 ) to 450°C. This treatment causes decomposition of the ammonium ions and yields a material where the extra-lattice cations are, at least formally,. The process is sketched out in Eq. (2). The acid form of the zeolite will now act as a very pK)werf ul acid catalyst in reactions of hydrocarbons such as isomerizations, alkylations, and hydrogen transfers ... 

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