Internal lattice oxygen

Figure 163 Schematic illustration of the reversed loss-and-gain of one atom of oxygen from the superlatb ce Sr4pe40,2 Sr4Fe40n + O.5O2 (Eq. (163)). The vacancy created and designated by II is due to loss of internal lattice oxygen. The vacancy created and...

Heterogeneous catalysis has, until recently, been exclusively the preserve of the surface chemist. Detailed study of the bulk structural features has become in oartant with the advent of shape selective catalysts, notably zeolites, where the distinction between external and internal surface is difficult to make, but surface studies have been considered most appropriate for other systems. However, in many real catalysts, where the catalytic action undoubtedly occurs on the external surface, it does so by means of intermediate structural states, and the catalytic efficiency is then dependent upon the relative stability and interactions of such intermediate states with the bulk material. Consequently an understanding of the structural chemistry and structural modification possible in the parent catalyst phase is still essential to understanding the catalytic action. This is especially true in the case of oxidation catalysts, where it can be shown (l) that lattice oxygen plays a part in the catalytic process. 

By modeling r in this way, we have tacitly assumed that the atomic oxygen has free access to the vacancy sinks which are the internal sites of repeatable growth for the lattice molecules. Properly spaced dislocations with fast oxygen diffusion could be a prototype of those sinks. 

These models consider the mechanisms of formation of oscillations a mechanism involving the phase transition of planes Pt(100) (hex) (lxl) and a mechanism with the formation of surface oxides Pd(l 10). The models demonstrate the oscillations of the rate of C02 formation and the concentrations of adsorbed reactants. These oscillations are accompanied by various wave processes on the lattice that models single crystalline surfaces. The effects of the size of the model lattice and the intensity of COads diffusion on the synchronization and the form of oscillations and surface waves are studied. It was shown that it is possible to obtain a wide spectrum of chemical waves (cellular and turbulent structures and spiral and ellipsoid waves) using the lattice models developed [283], Also, the influence of the internal parameters on the shapes of surface concentration waves obtained in simulations under the limited surface diffusion intensity conditions has been studied [284], The hysteresis in oscillatory behavior has been found under step-by-step variation of oxygen partial pressure. Two different oscillatory regimes could exist at one and the same parameters of the reaction. The parameters of oscillations (amplitude, period, and the... 

Fig. 8. All the partial RDFs for ASW, HGW, and LDA agree within the experimental error, which suggests that ASW, HGW, and LDA all represent the same structural state at 77 K and 1 bar. The basic short-range order structural motif is the Walrafen pentamer (i.e., a central oxygen atom, which is surrounded tetrahedrally by four oxygen atoms c.f. inset Fig. 8.). However, whereas their structures appear identical, it was suggested from inelastic incoherent neutron scattering that the dynamics of lattice and internal vibrations of water molecules differ significantly in HGW and LDA [176].

Bulk sapphire has rhombohedral symmetry, which is usually treated as hexagonal (space group R3c), with 30 atoms (six AI2O3 units) per primitive unit cell. The lattice parameters (a=fe=4.7570 A, c=12.9877 A) and the internal coordinates (x=0.3063, z=0.3522) are taken from ref. [56]. The bulk unit cell consists of an alternated stacking, along the c-axis, of two Al planes (twelve in the unit cell) with one atom per plane, and one oxygen plane (six in the unit cell) with three O ions arranged with a threefold symmetry. 

Static simulations of perfect lattices give the lattice energy and crystal structure of the garnets at 0 K. In the static limit, the lattice stmcture is determined by the condition 9 //9A = 0, where U is the internal energy, and the variables A define the structure (i.e., the lattice vectors, the atomic positions in the garnet unit cell, and the oxygen shell displacements). 

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