Surface oxide ions

Alternative mechanisms which have been proposed which involve die surface oxide ion vacancies such as... 

The ratioed, scale-expanded insert shows these more clearly. There is a negative band caused by the gaseous SO2 upon which a positive doublet is superimposed. Pumping at room temperature caused the doublet to disappear. The 1375 and 1358 cm l bands are attributed to SO2 weakly coordinated to surface oxide ions... 

The dissolution reaction under acid conditions requires protons, which may become bound to the surface oxide ions and weaken critical bonds thus, detachment of the metal species into the solution results. Another part of the consumed protons replaces the metal ions, leaving the solid surface and thus maintaining the charge balance. 

Figure 9. Simplified model of the (111) surface of the corundum-type structure, (a) A view of the surface from a direction slightly shifted from <111>. Only metal ions of the zeroth, first, and second layers are shown, (b) A section of the surface along the arrows depicted in part a. Hexagonally close-packed oxide ion layers are shown with lines. Surface protons are not shown, (c) A divalent Co-57 or pentavalent Sb-119 ion on the zeroth metal ion layer, (d) Aquo or hydroxyl complex of divalent Co-57 or pentavalent Sb-119 hydrogen-bonded to the surface oxide ion layers of hematite.

Carbonate ions can be formed from the slow reaction of formate with surface oxide ions ... 

From the amount of hydrocarbon reacted, a very small proportion (probably < 1%) of the surface oxide ions are involved in the above reactions. The electrons resulting from these reactions were thought to be trapped at oxide... 

The mono-oxygen species on the surface fall into two main types O ions formed either by adsorption or from surface oxide ions of the lattice and O2 ... 

Magnesium oxide is considered to exhibit basic properties (20). It is thus not unexpected that neither Br nsted nor Lewis acid sites could be detected by ammonia adsorption (180,181). Hydrogen-bonding is the only type of interaction that ammonia probably undergoes with surface oxide ions on dehydroxylated surfaces (180) and with surface OH groups on hydroxylated surfaces (181). 

For this reason, the present review is restricted to mononuclear oxygen species, such as the O ion, but it has also been broadened in scope to include the nonparamagnetic surface oxide ion O2 . Recent spectroscopic measurements have shown that O2 has unusual properties when it is in a situation of low coordination on the surface. Other mononuclear forms of oxygen such as charged O+ and O2 + ions are known in the gas phase, but in view of the high ionization potential of the O atom (2) given below, these species are unlikely to be stabilized on the surface ... 

If Of is formed by interaction with oxide ions of the lattice, then the spectra of OJ on an oxide lattice where the surface oxide ions are enriched with 17Q should be different from the normal. This experiment has been carried... 

A detailed study of the oxidation of alkenes by O on MgO at 300 K indicated a stoichiometry of one alkene reacted for each O ion (114). With all three alkenes, the initial reaction appears to be the abstraction of a hydrogen atom by the O ion in line with the gas-phase data (100). The reaction of ethylene and propylene with O" gave no gaseous products at 25°C, but heating the sample above 450°C gave mainly methane. Reaction of 1-butene with O gives butadiene as the main product on thermal desorption, and the formation of alkoxide ions was proposed as the intermediate step. The reaction of ethylene is assumed to go through the intermediate H2C=C HO which reacts further with surface oxide ions to form carboxylate ions in Eq. (23),... 

A few moments thought about the nature of the surface of an oxide leads to the conclusion that the surface oxide ion should have quite different properties than the bulk lattice ions. For example, consider a simple cubic oxide such as MO with a sodium chloride structure where each ion is sixfold coordinated if this is cleaved along a <100) plane, then the coordination of the ions in this plane is reduced from six- to fivefold. This new surface will not be ideal, and ions of still lower coordination will also be present where higher index planes are exposed at the surface. However, for MgO prepared by thermal decomposition of the hydroxide or carbonate, evidence from electron microscopy (130) indicates that these have high index planes that... 

It is useful at this point to define more precisely what is meant by ions of low coordination. The normal coordination of the ion in the bulk lattice is defined by the number of nearest neighbor ions of the opposite sign, and for the octahedrally coordinated alkaline-earth oxides this will be six ions of lower coordination than this will be designated by the subscript LC to denote low coordination. For example, the surface oxide ions on an MgO< 100) plane can be described as Ojc with coordination 5, or Ofc, and more generally, L can take the values 5, 4, 3, etc., for different coordinations as shown in Fig. 11. The use of the subscript LC is recommended in preference to CUS (coordinatively unsaturated) because of its simplicity. 

The coordination of surface oxide ions has been explored further by following the adsorption of halogens on MgO 83). All the halogens are strongly adsorbed, and chlorine reacts to form chloride ions with no evidence... 

In conclusion, there is now good evidence that oxide ions of unusually low coordination, Olc and OjJ for the alkaline-earth oxides, are present on the surface and their chemical and spectroscopic properties have been characterized. It is clear that they can act as electron transfer sites when the normal surface oxide ions (02c for alkaline-earth oxides) do not, and their possible role in catalytic reactions will be discussed in the next section. 

Measurements of the spin concentration indicate that relatively high concentrations of A (equivalent to about 0.5% of the surface oxide ions) can be formed. Other sources of electrons such as TMI impurities may be ruled out because of their low concentrations. The formation of NB radical ions was also found to be much greater on MgO smoke that had been etched by water vapor than on normal MgO smoke (65, 72). Etching of the regular cubic particles composing MgO smoke led to a considerable increase in the lowest coordination surface oxide ions, i.c., O c-... 

The coordination of surface oxide ions has also been explored by using the adsorption of halogens on MgO (73). Oxygen was found to evolve as surface halide ions were formed according to the following reaction ... 

---