Defects on Oxide Surfaces

So far we have restricted our discussion on oxide surfaces to the geometry of terraces. However, any description of surface properties has to include defects. 

Steps For the formation of steps on oxide surfaces and their stability, the same concepts, namely, charge neutrahty and autocompensation, as apphed for the stability of regular surfaces, may be apphed. Ions exposed at step sites have lower coordination than the respective terrace sites. The coordinative unsaturation leads also to slight structural relaxation at step sites. Experimentally, the step direction can be measured by quantitative J-V LEED, or may be inferred from microscopic data, where the principal crystal directions are directly seen. However, with the complexity of crystal structures that are possible for oxide surfaces, the determination of step structures is not straightforward. 

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Thermodynamic considerations imply that all crystals must contain a certain number of defects at nonzero temperatures (0 K). Defects are important because they are much more abundant at surfaces than in bulk, and in oxides they are usually responsible for many of the catalytic and chemical properties.15 Bulk defects may be classified either as point defects or as extended defects such as line defects and planar defects. Examples of point defects in crystals are Frenkel (vacancy plus interstitial of the same type) and Schottky (balancing pairs of vacancies) types of defects. On oxide surfaces, the point defects can be cation or anion vacancies or adatoms. Measurements of the electronic structure of a variety of oxide surfaces have shown that the predominant type of defect formed when samples are heated are oxygen vacancies.16 Hence, most of the surface models 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. 

Anion defects on oxide surfaces are important centers for adsorption and acbva-bon of molecules in catalysis. For example, oxidabon reacbons following a Mars-van Krevelen mechanism require oxygen bansfer from a surface to an... 

Figure 15.24 Scanning probe microscopic images of point defects on oxide surfaces (a) MgO(OOl) thin films (5 x 5 nm ) (from Sterrer, 2006 [107]), (b) TiO2(110) (10 x lOnm ) (from Fukui, 1997 [109]), and (c) CeOj] ) (4x3.5nm ) (from Esch, 2005 [115].

We have noted when discussing the geometric structure of defects on oxide surfaces that, depending on the type of oxide, there are two different mechanisms of trapping the two electrons that remain if an oxygen atom is removed from the... 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

The surface diffusion of defects and adsorbates is of obvious importance in heterogeneous catalysis, as this process brings the reactants together. Understanding the dynamics of molecules on oxide surfaces is also a key step toward the realization of working molecular electronics. We note here that diffusion of Ob-vacs really means diffusion of Ob into the vacancy, which leaves another Ob-vac in the position vacated by the Ob- Similarly, diffusion of OHb occurs by diffusion of the H atom. 

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. 

Alumina, silica and many other metal oxides are insulators. However, recent experiments indicate that the surfaces of these insulators are mainly ionic (Masel, 1996). The pristine or freshly cleaved surfaces of single crystals of these oxides (cleaved under ultrahigh vacuum) are fairly inert and do not have significant adsorption capacities for even polar molecules such as CO and S02 (Masel, 1996 Henrich and Cox, 1994). However, the surface chemistry and adsorption properties are dominated by defects on real surfaces. For example, oxide vacancies on alumina expose the unsaturated aluminum atoms, which are electron acceptors, or Lewis acid sites. 

Through the above series of examples, it is clear that EPR offers many advantages for the characterization of paramagnetic species on oxide surfaces. The obvious Umitation of the technique is of course that it only detects paramagnetic centers. However, if paramagnetic centers, such as defects, radicals or transition metal ions, are involved in a heterogeneous process, then EPR is the ideal spectroscopic technique. To date most of the studies applied to oxides have used the traditional cw-EPR method. Modern pulsed techniques offer far more sensitivity and resolution than cw-EPR, and it is certainly hoped that these pulsed techniques will be more widely used as commercial spectrometers become more numerous in research laboratories. Compared to cw-EPR, the numerous hyperfine techniques... 

Clearly, the presence of O species at the surface of MgO is not disconnected from the existence of other defects, low-coordinated anions or O vacancies. In fact, the complex interconversion of one center into another one is one of the reasons for the difficult identification of defect centers on oxide surfaces. 

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