Defects on MgO

Point defects in oxides determine the optical, electronic, and transport properties of the material and usually dominate the chemistry of its surface. A detailed understanding and a control at atomistic level of the nature (and concentration) of point defects in oxides are therefore of fundamental importance to s mthesize new materials with well-defined properties. Of course, before to be created in controlled conditions point defects have to be known in all aspects of their physicochemical properties. The accurate theoretical description of the electronic structure of point defects in oxides is essential for the understanding of their structure-properties relationship. 

MgO is a particularly well-studied oxide the structure of the (100) single crystal surface is extremely flat, clean, and stoichiometric. Both relaxation, —0.56 0.4%, and rumpling, 1.07 0.5%, are extremely small [103]. However, no real crystal surface consists of only idealized terraces. A great effort has been undertaken to better characterize the MgO surface, in particular for polycrystalline or thin film forms, which in some cases exhibit a heterogeneous surface due to the presence of various sites. All these sites can be considered as defects. 

In the following, we provide a summary of the most important kinds of defects at the MgO surface (Table2.1). In a broad classification, one can recognize at least 12 major kinds of point defects  

Low-coordinated cations (Sect. 2.3.1). These are Mg ions with a number of neighbors lower than on the fiat (100) terraces. To this category belong four-coordinated ions located at step and edge sites, and the three- 

Low-coordinated anions (Sect. 2.3.2). The sites exhibit a completely different chemistry when located in the bulk, at the five-coordinated terraces, 0 5c, or at irregular sites with lower coordination, 0 4c and 0 3c, where the MP (and the basicity) of the site change dramatically. Hydroxyl groups (Sect. 2.3.3). H2O is almost ubiquitous and easily reacts with low-coordinated sites to form OH groups at the surface of MgO. These centers induce asymmetries in the surface electric field and exhibit a classical Brpnsted acid behavior. 

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The D.C. conductivity measurements are used to identify the most abundant and the most mobile charged defects on MgO. In this study, the effect of oxygen and water on the D.C. conductivity of MgO catalysts were measured. These gases are the main components in the OCM reaction. The D.C. conductivities of the sample with oxygen gaseous... 

Point defects on MgO surfaces are catalytically active for H /D exchange and hydrogenation reactions, although a pure perfect surface seems inactive. Chemisorption and surface electronic structure of Mg oxide were recently reviewed by Kunz (1985), Fujikoka et al, (1985) propose a model for H /D exchange at step sites on the (001) surface of MgO. 

Fig. 4 Osmium clusters supported on MgO(OOl) a OssC/MgisOs and b OS5C at a surface point Vs defect site [33] these were represented by density functional theory, and the samples were characterized by EXAFS spectroscopy, transmission electron microscopy, and other techniques [15]...

A detailed study of the C02- species on MgO has been carried out by Lunsford and Jayne 26). Electrons trapped at surface defects during UV irradiation of the sample are transferred to the CO2 molecule upon adsorption. By using 13C02 the hyperfine structure was obtained. The coupling constants are axx - 184, am = 184, and a = 230 G. An analysis of the data, similar to that carried out in Section II.B.2 for N02, indicates that the unpaired electron has 18% 2s character and 47% 2p character on the carbon atom. An OCO bond angle of 125° may be compared with an angle of 128° for CO2- in sodium formate. 

Oxidative catalysis over metal oxides yields mainly HC1 and C02. Catalysts such as V203 and Cr203 have been used with some success.49 50 In recent years, nanoscale MgO and CaO prepared by a modified aerogel/hypercritical drying procedure (abbreviated as AP-CaO) and AP-MgO, were found to be superior to conventionally prepared (henceforth denoted as CP) CP-CaO, CP-MgO, and commercial CaO/MgO catalysts for the dehydrochlorination of several toxic chlorinated substances.51 52 The interaction of 1-chlorobutane with nanocrystalline MgO at 200 to 350°C results in both stoichiometric and catalytic dehydrochlorination of 1-chlorobutane to isomers of butene and simultaneous topochemical conversion of MgO to MgCl2.53-55 The crystallite sizes in these nanoscale materials are of the order of nanometers ( 4 nm). These oxides are efficient due to the presence of high concentration of low coordinated sites, structural defects on their surface, and high-specific-surface area. 

Theory indicates that supported nanoclusters typified by Os5C on MgO are bonded more strongly at surface defect sites than at defect-free sites. 

Abbet S, Riedo E, Brune H, et al. Identification of defect sites on MgO(lOO) thin films by decoration with Pd atoms and studying CO adsorption properties. J Am Chem Soc. 2001 123 6172-8. 

The positron lifetimes for different defects in MgO are calculated using the insulator model of Puska and co-workers. In this model, the annihilation rates are determined by the positron density overlapping with the enhanced electron density that is proportional to the atomic polarizability of MgO [8, 9]. Based on comparison between experimental and calculated values [5, 6], the positron lifetime of the embedded Au nanoparticle layer, 0.41 ns, suggests that positrons are predominantly trapped in clusters consisting of... 

The study of the cyclyzation reaction of acetylene to benzene on Pd atoms supported on MgO allows one to clarify some of the experimental aspects and to draw some general conclusions about the role of defects on oxide surfaces. The results show that only in the presence of surface defects a single Pd atom becomes an active catalyst for the reaction in fact, an isolated Pd atom is not capable to add and activate three acetylene molecules. 

We have reviewed theoretical approaches concerning perfect surface. Defects modify the adsorption. Hydration can strongly perturb the conclusions. If water saturates the adsorption sites, the admolecule should either displace an adsorbed molecule (and has to be more strongly adsorbed than water itself) or adsorb by hydrogen bonding on an ice layer. Such layers are formed on MgO(lOO) surfaces[3, 24, 80]. 

In the same work, Gribov et al. (124) also investigated the effect of surface defects on the reactivity of MgO toward FI2 splitting. The IR spectra of hydrogen adsorbed... 

In relation to the surface defects on the group II alkaline earth oxides, EPR has been instrumental in unraveling the electronic structure of the defects on both polycrystalline and well defined single crystal surfaces. These trapped electron centers can be formed in a number of different ways. The most convenient means on powders is by exposure of the alkaline earth oxide, such as MgO, to hydrogen atoms [22]. Spontaneous ionization of the H atoms occurs with the subsequent formation of excess electrons on the surface ... 

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