Rock-salt-type surface

The influence of Zn-deposition on Cu(lll) surfaces on methanol synthesis by hydrogenation of CO2 shows that Zn creates sites stabilizing the formate intermediate and thus promotes the hydrogenation process [2.44]. Further publications deal with methane oxidation by various layered rock-salt-type oxides [2.45], poisoning of vana-dia in VOx/Ti02 by K2O, leading to lower reduction capability of the vanadia, because of the formation of [2.46], and interaction of SO2 with Cu, CU2O, and CuO to show the temperature-dependence of SO2 absorption or sulfide formation [2.47]. 

Surface Hydroxyl Croups of Rock-Salt-Type Metal Oxides... 

ZnO surfaces are more complex than those of the rock-salt type oxides Uke MgO and NiO. ZnO crystalhzes in the wurtzite structure in which each Zn cation is tetrahedrally coordinated to four O anions and vice versa [105]. This crystal structure has no inversion center. The most important low-index surface planes are two polar planes, the Zn-terminated ZnO(OOOl) and 0-termi-nated ZnO(OOO-l) plane, and two neutral planes, ZnO(lO-lO) and ZnO(l 1-20). According to Nosker et al. [106] and Tasker [107], the two polar surfaces are thermodynamically unstable, however, they can be easily prepared and characterized experimentally, and do even show rather regular (1x1) LEED patterns [108]. This indicates that they are not stabilized by major reconstructions or other modifications. Therefore, it was believed for a long time that both polar surfaces exist in an unreconstructed bulk-Hke trimcation. Several contradicting proposals have been made to explain how the stability of the polar un-... 

Excess pressure is also observable in pressure-driven phase transformations. Tolbert and Alivisatos [2,110, 111] showed that a significant increase in pressure induced wurtzite to transform into rock salt (from the less dense to the denser phase) in CdSe nanocrystals, following a scaling law of the type Pnansf 1 + CjR. They observed an increase of 35% in transition pressure in 10 nm nanoparticles in relation to 21 nm nanoparticles (3.6-4.9GPa). The transformations were found to be fully reversible, albeit with some hysteresis, showing an energy barrier to direct transformation, which is coherent with the excess pressure at the surface. 

As we have seen, the perfect bulk termination with a dipolar stacking sequence is inherently unstable but there are examples in which surfaces with Miller indicies that give type III surfaces can be observed. For example, we have seen that MgO has the rock salt structure. This means that any surface formed from a face of the cube will contain an equal number of anions and cations and so will be a non-polar type I surface. These are the (100), (010), and (001) surfaces that we noted had been experimentally identified on samples of MgO prepared by various means (8). It can also be seen from the unit cell that other surfaces, such as the 111, will have a dipolar stacking sequence and so is fundamentally unstable. However, the surface is observed, at least as psuedo- ), in samples prepared by thermal decomposition of the basic carbonate. These samples are also more catalytically active than samples of MgO without 111 expressed. This may tie in with the suggestion that the surfaces are stabilized by the formation of local defects, which remove the dipole from the stacking sequence since these... 

The surface of, for example, the rock salt (100) or (110) plane contains equal numbers of cations and anions and possesses, therefore, no net charge (Q = 0). The repeat unit indicated by the square bracket in Figure 15.8a exhibits no dipole moment perpendicular to the surface (/r = 0), yielding a nonpolar type 1 surface. 

On type 2 and type 3 surfaces, on the other hand, the individual planes contain only one particular type of ion and are, therefore, not charge neutral (Q O). On a type 2 surface, however, the bulk repeat unit may be constructed such that the net dipole moment perpendicular to the surface vanishes (/t = 0), resulting in a nonpolar surface. This is shown in Figure 15.8b. Typical examples of type 2 surfaces are the (0001) plane of corundum and the (110) surface of rutile. In the case of type 3 surfaces, the building block always possesses a net dipole moment perpendicular to the surface (/ryfO), for example, the (111) surfaces of the rock salt structure. These are polar surfaces (Figure 15.8c). 

Uncompensated Storage Hard rock caverns and a few bedded salt caverns do not use brine for product displacement. This type of storage operation is referred to as pumpout or uncompensated storage operations. When the cavern is partially empty of liquid, the void space is filled with the vapor that is in equihbrium with the stored hquid. When liquid is introduced into the cavern, it compresses and condenses this saturated vapor phase. In some cases, vapor may be vented to the surface where it may be refrigerated and recycled to the cavern. 

The lithological parameter is only one of several parameters that control groundwater quality. Other factors include evaporation at the surface prior to infiltration, transpiration, wash-down of sea spray, and reducing conditions in the aquifer, connected to H2S production. Water moves underground, and its salt or mineral content is determined by all soil and rock types it passes through. Thus, occasionally, saline water may be encountered in rocks that by themselves do not contribute soluble salts. 

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