Cation-site distribution

Cations siting, distribution, mobility, and metal segregation have been found to strongly influence catalytic activity and the stability of Co,Ag-FER catalysts in lean CH4-SCR of NOx. 

The hydrous aluminosilicates and Zr, Th, and Ti phosphates have significant ion exchange capacity (6). Further interpretive study of charge development in such systems will require recognizing the ion exchange properties and measuring trivalent cation site distribution as well as adsorption and mobility. 

Cation Site Distribution, Thin-film EDS analysis can also be used to quantitatively determine the site occupancy of atoms in a known crystal structure. Atom Location by Channeling Enhanced Microanalysis (ALCHEMI) is a technique which utilises electronchanneling enhanced X-ray emission for specific atoms in a crystal when appropriately oriented relative to the incident beam [43]. The method involves no adjustable parameters, can be used on relatively small areas of sample and provides fractional occupancies of atom positions [44] Unlike X-ray diffraction which has had limited success with adjacent elements in the periodic table [e.g. 45], ALCHEMI can provide site occupancies for adjacent elements and is relatively insensitive to sample thickness or the precise electron beam orientation [44] ... 

The predicted location of extra-framework cation sites in zeolites can also be studied by MC methods, although the problem is further complicated by the disorder in both framework aluminium location (and associated charge) and partial occupancy of cation sites. In this case a large number of possible extraframework cation site distributions has to be considered, and a model assumed for the location of framework charge. 

The site preference of several transition-metal ions is discussed in References 4 and 24. The occupation of the sites is usually denoted by placing the cations on B-sites in stmcture formulas between brackets. There are three types of spinels normal spinels where the A-sites have all divalent cations and the B-sites all trivalent cations, eg, Zn-ferrite, [Fe ]04j inverse spinels where all the divalent cations are in B-sites and trivalent ions are distributed over A- and B-sites, eg, Ni-ferrite, Fe Fe ]04 and mixed spinels where both divalent and trivalent cations are distributed over both types of sites,... 

The vacant sites will be distributed among the N lattice sites, and the interstitial defects on the N interstitial sites in the lattice, leaving a conesponding number of vacancies on die N lattice sites. In the case of ionic species, it is necessaty to differentiate between cationic sites and anionic sites, because in any particular substance tire defects will occur mainly on one of the sublattices that are formed by each of these species. In the case of vacant-site point defects in a metal, Schottky defects, if the number of these is n, tire random distribution of the n vacancies on the N lattice sites cair be achieved in... 

Many of the spinel-type compounds mentioned above do not have the normal structure in which A are in tetrahedral sites (t) and B are in octahedral sites (o) instead they adopt the inverse spinel structure in which half the B cations occupy the tetrahedral sites whilst the other half of the B cations and all the A cations are distributed on the octahedral sites, i.e. (B)t[AB]o04. The occupancy of the octahedral sites may be random or ordered. Several factors influence whether a given spinel will adopt the normal or inverse structure, including (a) the relative sizes of A and B, (b) the Madelung constants for the normal and inverse structures, (c) ligand-field stabilization energies (p. 1131) of cations on tetrahedral and octahedral sites, and (d) polarization or covalency effects. ... 

The refinement performed by Gualtieri et al. [5] evidenced that ECR-1 is formed by a strict alternation of mazzite (MAZ) and mordenite (MOR) sheets, with 4-, 5-, 6-, 8- and 12-membered tetrahedral ring forming a three dimensional ring. In the Na- form, sodium cations are distributed over 5 different extra-framework sites. The thirty-five water molecules are distributed over eleven sites. It is worth noting that the position of the cations found in ECR1 does not correspond with the site found for mazzite and mordenite. In the NH4 -exchanged form, the NH4 ions occupy three distinct extraframework sites, whereas the water molecules are distributed over the same eleven sites found for the Na-form [5],... 

Twin geometry may refer to just one set of atoms in the crystal rather than all. This type of twinning is met, for example, in cases where a fraction of octahedral or tetrahedral cation sites in a close-packed array of anions are occupied in an ordered fashion. The close-packed anion array remains unchanged by the twin plane, which applies to the cation array alone (Fig. 3.24). These boundaries are of low energy and are often curved rather than planar. In oxides such as spinel, MgAl204, in which cations are distributed in an ordered fashion over some of the octahedral and tetrahedral sites, boundaries may separate regions that are twinned with respect to the tetrahedral cations only, the octahedral cations only or both. 

The spinel family of oxides with composition AB2O4 has the A and B cations distributed in octahedral and tetrahedral sites in a close-packed oxygen structure (Supplementary Material SI). Impurity doping can take place by the addition of a dopant to octahedral or tetrahedral sites. In this, the spinel family of compounds is quite different from the A2B04 perovskite-related phases of the previous section in that both cation sites are similar in size and can take the same cations. 

In a crystal of overall composition MX, suppose that ns is the number of Schottky defects in the crystal at a temperature T (K), that is, there are ns vacant cation sites and ns vacant anion sites present, distributed over N possible cation sites and N possible anion sites. The configurational entropy change, ASs, due to the distribution of the... 

The typical unit cell content of zeolite L is (K,Na)gAi9Si27072.nH20 and its Si/AI ratio varies in the range of 2.6 - 3.5 [1-4]. Takaishi recently determined the distribution of Al atoms in the framework of zeolite L ly analyzing Si-MAS-NMR spectra. He thereby deduced five kinds of extra-framework cation sites as shown in Fig. 1., and estimated the relative strengths of their cation affinities [5]. 

The trajectory and radial distribution of the methane molecule were predicted from the calculations. There are many ways the trajectory of a particle may be quantified the authors used the number of times the methane molecule passes through the volume element bounded by r and Sr (where Sr = 0.1 A and r has a value between 0 and 5 A). At high temperatures, it was found that the methane molecule was delocalized over the whole cage. As the temperature was decreased, the cage center was never visited, and the molecule sweeps around the walls in the region of the Sill cation, finally locating itself between an Sill and SII cation site. 

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