Cluster-like oxygen vacancies

More complex permeation models can be generated for cluster-like oxygen vacancies. In such a situation, two different oxygen vacancies can be proposed in the MIEC material. The exchange between these vacancies can be described... 

The results presented in the previous sections demonstrate the importance of point defects at the surface of oxide materials in determining the chemical activity of deposited metal atoms or clusters. A single Pd atom in fact is not a good catalyst of the cyclization reaction of acetylene to benzene except when it is deposited on a defect site of the MgO(lOO) surface. A detailed analysis of the reaction mechanism, based on the calculation of the activation barriers for the various steps of the reaction, and of a study of the preferred site for Pd binding, based on the MgO/Pd/CO adsorption properties, has shown that the defects which are most likely involved in the chemical activation of Pd are the oxygen vacancies, or F centers, located at the terraces of the MgO surface and populated by two (neutral F centers) or one (charged paramagnetic F centers) electrons. 

The topic of defect sites at oxide surfaces therefore becomes crucial in order to fully understand the metal-oxide bonding. This subject has been addressed theoretically only recently. In this review we have shown how defect sites at both MgO and Si02 surfaces play a fundamental role in both stabilization and nucleation, but also that they modify the cluster electronic properties. In particular, some defect centers that act as electron traps like the oxygen vacancies at the MgO surface are extremely efficient in increasing the electron density on the deposited metal atoms or clusters, thus augmenting their chemical activity toward other adsorbed molecules. Understanding the metal-oxide interface and the properties of deposited metal clusters also needs a deeper knowledge of nature, concentration and mechanisms of formation, and conversion of the defect sites of the oxide surface. 

In this chapter, we have concentrated on MgO, one of the most studied and better understood oxide materials. We have shown that even on such a simple nontransition metal oxide about a dozen of different surface defect centers have been identified and described in the literature. Each of these centers has a somewhat different behavior toward adsorbed metal atoms. It becomes immediately clear that the precise assignment of the defect sites involved in the interaction, nucleation, and growth of the cluster is a formidable task. Nevertheless, thanks to the combined use of theory and experiment, the progress in this direction has been particularly significant and promising. For instance, a lot of evidence has been accumulated that points toward the role of the oxygen vacancies, the F centers. At the moment, these sites seem the most likely sites for nucleation and growth of small metal clusters. 

The reactive centre in such a process is neither the surface oxygen Os nor the oxygen vacancy Vqs, but rather some cluster corresponding to oxygen vacancies either with or without oxygen in the cluster. By analogy, in photocatalysis there are surface centres (defects), which can be in a state with trapped carriers (Le. an intermediate, just like surface oxygen in the previous example) or without trapped carriers. These centres are in fact the centres of photocatalysis. 

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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