Oxide surface/oxygen thermodynamic

The theoretical surface density of oxygen ions was evaluated by Madier el al. for different crystallographic planes of Ce02 and Ce Zr Oj oxides [14], For ceria, the theoretical O density would be of 13.7, 9.7 and 15.8at.Onm 2 for (100), (110) and (111) surfaces respectively, which gives a mean surface density of 13.1 at.O nm 2 if one assumes an equidistribution of the three crystallographic planes. This figure leads to a theoretical OSC of 5.4 p,mol O m-2. The hypothesis of equidistribution may be not valid in all cases, which can explain some difference in the reported results. Note that the (111) surface is thermodynamically the most stable [34,35],... 

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 oxidation of Fe(II), V02+, Mn2+, Cu+ by oxygen is favored thermodynamically and kinetically by hydrolysis and by specific adsorption to hydrous oxide surfaces. As suggested in formula (IV) of Fig. 9.1 Fe(II) and the other transition elements Mn(II), VO(II), Cu(I) may more readily associate (probably outer-spherically) with... 

In the methanol synthesis, a fast hydrogenation of the adsorbed formaldehyde would overcome arguments invoking any thermodynamic limitation to its formation. Formaldehyde adsorbed through both the oxygen and the carbon ends has been characterized in homogeneous catalysis (43), on oxide surfaces (44) and more recently on ruthenium metal (45). 

A simple example of a catalytic solid is metallic silver. It is the best known catalyst for oxidizing ethylene to ethylene oxide. Under the conditions of use, oxidizing silver to silver oxide is not thermodynamically possible, but oxygen is rapidly and strongly adsorbed to form up to a monolayer on the surface. There is considerable evidence of both adsorbed oxygen atoms and oxygen molecules. Thus, some of the conceivable simple structures on the three low-index planes are as shown in Table I, where M denotes a silver (or metal) atom in a surface plane. 

Oxidation can be viewed as the chemisorption of oxygen. For example, nickel and silicon are oxidized at ambient conditions. The resulting oxide layer is thermodynamically more stable and passivates the pure material below it. Another important example is the oxidation of aluminum which provides the metal with a very hard roughly 100 nm thick aluminum oxide (AI2O3) layer. To stabilize the aluminum surface even more and to passivate it against reactive chemicals the thickness of the oxide layer can be increased electrochemically. This procedure is called the eloxal process (efectrolytical oxidation of a/uminum). 

Given the complexity of the reaction possibilities and the little structural knowledge we still have about such systems, it may be asked where the specificity of the reaction pathway comes from and what makes the reaction stop at the level of two oxygen additions instead of continuing all the way to deep oxidation that is thermodynamically so favorable. Model experiments with isolated clusters combined with high-level theoretical analysis and reactivity studies of surface-science grade well-defined oxide surfaces lead to several conclusions. 

Fig. 5.17. Cartoon sideviews illustrating the effect of an increasingly oxygen-rich atmosphere on a late TM surface. Whereas the clean surface prevails in perfect vacuum (left), finite O2 pressures in the environment first lead to oxygen adsorption phases. Apart from some bulk-dissolved oxygen, the lower deformation cost will at increasing pressures lead to a preferential accommodation of oxygen in the near-surface fringe. Thin surface oxide structures are the salient consequence before eventually thickening of the oxide film and formation of an ordered bulk compound set in. A thermodynamic or kinetic stabilization of the nanometer thin surface oxide could lead to novel functionalities, different from both bulk metal or bulk oxide surfaces...

We have reviewed some of the recent simulations of oxide surfaces by First Principles methods. Our emphasis throughout is on the reliability of simulation - how suitable are the models which are built and how accurate are the quantities calculated Practical aspects of the calculation are outlined, but also the thermodynamic framework in which energies can be interpreted and linked to experiment. Computational studies of surfece relaxation and reconstruction are surveyed and compared against experiment. The link between surface structure and dynamics is illustrated for the cases of reactivity with oxygen and water. 

Limited studies have been made to determine if an added oxidant could replenish the surface oxygen removed in aldehyde oxidations, when they occur. Thermodynamic calculations show the oxidation of aldehyde using water as an oxygen source would be favorable. 

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