Vacancies transition metal oxide surfaces

Let us refer to Figure 5-7 and start with a homogeneous sample of a transition-metal oxide, the state of which is defined by T,P, and the oxygen partial pressure p0. At time t = 0, one (or more) of these intensive state variables is changed instantaneously. We assume that the subsequent equilibration process is controlled by the transport of point defects (cation vacancies and compensating electron holes) and not by chemical reactions at the surface. Thus, the new equilibrium state corresponding to the changed variables is immediately established at the surface, where it remains constant in time. We therefore have to solve a fixed boundary diffusion problem. 

The oxides containing cations in octahedral coordination, such as MgO and TiOz, seem to suffer little or no reconstruction of the crystal structure at the surface, but there are major changes in electronic structure. These effects have important implications for surface reactivity, especially when oxygen vacancy defects are considered and particularly in transition-metal oxides. The example of TiOj was discussed above more complex behavior is shown by species such as TijOj and NiO (Henrich, 1987). The latter, like other transition metals, shows increasing [Pg.415]

N2 ligand is not able to induce appreciable surface mobility or relaxation. The tendency toward strong relaxation in the presence of adsorbates differentiates the chemistry of transition metal ions on silica from the chemistry of the same ions on crystalline oxides (on which relaxation and mobility are definitely smaller). This property is likely to play a fundamental role in determining the properties of Cr2+ (Ni2+) on silica in catalytic processes (e.g., ethene polymerization) for which a large number of coordination vacancies are needed. 

The surface oxidation state of the transition metal and the availability of vacancies and/or adsorbed oxygen play a central role in defining the catalytic behavior of the oxides for this and other reactions. Particularly enlightening is fig. 25 which shows how the catalytic activity increases with the increasing average oxidation number (AON) of copper. 

Pyrochlores - The pyrochlores are a group of materials with the general formula A2B2O7. They have been mentioned as a material for catalytic combustion. The structure allows vacancy at the A site and the O sites to some extend. The A position can be a rare earth metal or an element with lone pair of electrons and the B position can be a transition metal or a post-transition metal. This make the structure rather flexible as the oxidation state of the transition metal B can be varied as well as the nature of the A and B metal ions. Subramanian and Castro et al. have prepared several pyrochlores. When studying the thermal stability of different complex oxides, Zwinkels et al. have shown that La2Zr207 pyrochlores have a surface area lower than 5 m g , already after calcination at 1000 °C. Hence, such materials are probably not suitable for high temperature applications unless the preparation method is improved. However, pyrochlore compounds have been patented for catalytic combustion applications, see Section 5.5. 

The pores of friendly nanomaterials could be used to store strong adds, even super acids, in some cases. Likewise, weak bases or strong bases could be stored for use as needed in killing or destroying advanced enemy toxins. In addition, the nanomaterial itself could be produced with acidic sites (metal ions and/or certain proton donors) built into the pore walls and crystal faces. For example, titanium or zirconium ions can serve as acid sites if adjacent to sulfate species. Likewise, the proton forms of some transition-metal oxygen-anion clusters (polyoxometalates or POMs ), like some metal oxides, are effective superacids in commercial processes. Polyoxometalates could be physically held within the pores or could be grafted onto the pore walls or onto the outer nanocrystal faces. Basic sites can also be built into the nanostructure, such as oxide anions near a metal cation vacancy. There are many other possibilities, such as sulfide substitution for oxide anions on the surface of the nanocrystals. 

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