Defective ionic surfaces

Important questions of metal adsorption at ionic metal oxides are (i) preferred adsorption sites (ii) strength and nature of metal-support interactions on regular and defect-rich surfaces (iii) charge redistribution between deposits and supports (iv) geometric and electronic structure as well as magnetism of small metal particles and deposition-induced alteration of these features (v) implications for the reactivity. We will also discuss accuracy improvements due to more precise xc functionals as well as more realistic cluster models of oxide supports. 

Ionic materials - There have been several studies of adsorption on oxide surfaces, largely concerning the relatively simple reaction of dissociation on a defective MgO surface. The effects of the lattice have been treated in a variety of ways, including ignoring them totally. Experimentally it has been... 

The observed bulk polarisabUity of a solid, a, will arise from the sum of a number of separate terms such as electronic polarisability, ionic polarisability and so on. As the total polarisability of a material is made up several contributions, the relative permittivity, r, can also be thought of as made up from the same contributions. In a static electric field, all the various contributions will be important and both a and r will arise from electrons, ions, dipoles, defects and surfaces. 

The qualitative picture which emerges from the above introductory remarks is that the surface of an ionic crystal at equilibrium may have a variety of defects associated with it and possess both localized and extended dipolar layers. We shall discuss each of the effects listed above in some detail, review some of the experimental observations on ionic surfaces and finally mention a few applications of ionic materials where the surface defects play an important role. 

Structure Modification. Several types of stmctural defects or variants can occur which figure in adsorption and catalysis (/) surface defects due to termination of the crystal surface and hydrolysis of surface cations (2) stmctural defects due to imperfect stacking of the secondary units, which may result in blocked channels (J) ionic species, eg, OH , AIO 2, Na", SiO , may be left stranded in the stmcture during synthesis (4) the cation form, acting as the salt of a weak acid, hydrolyzes in aqueous suspension to produce free hydroxide and cations in solution and (5) hydroxyl groups in place of metal cations may be introduced by ammonium ion exchange, followed by thermal deammoniation. 

The ESR signal due to 02 demonstrates that the heterolytic activation of R-H has occurred. An interesting feature of the H-D exchange reactions over the MgO surface is the low activation energy, i.e., Ea 2 kcal/mol. This is lower than the gas phase for the reaction H + D2 —> 2 D + HD. The high activity of the ionic oxides has been attributed to the presence of basic sites and in particular to defect sites that are formed during the oxide preparation that persist even at elevated temperatures.41 42... 

Lattice defects in ionic crystals are interstitial ions and ion vacancies. In crystalline sodium chloride NaCl a cation vacancy Vn - is formed by producing a surface cation NaJ, (Nal - NaJ + Vua ) this is called the Schottky defect. On the other hand, in crystalline silver chloride AgCl a pair of cation vacancy Va,. and interstitial cation Ag is formed, (Ag - Agj + ) this is called the Frenkel... 

Fig. 3 -13. (a) A ion levels at the surface and in the interior of ionic compound AB, and (b) concentration profile of lattice defects in a surface space charge layer since the energy scales of occupied and vacant ion levels are opposite to each other, ion vacancies accumulate and interstitial ions deplete in the space charge layer giving excess A ions on the surface. 

The same theoretical examination as applies to the Frenkel defect may also apply to the Schottl defect in ionic compounds, in which an accumulation layer of vacancies is formed with a surface ion excess. 

The relative solubilities reported are very crude estimates based on equilibrium solubility products. These estimates do not take into account variations in solubility as a function of pH, ionic strength, activities of various solution species (e.g., HCO "), redox state, particle size, surface defect types and concentrations, the concentration of various types of adsorbates, including natural organic matter, on mineral surface, or the presence of different types of bacteria or microbial biofilms on mineral surfaces. 

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