High-surface-area materials, model

Figure 1. Model of high-surface-area materials a, small particles b, reverse system, small pores and c, porosity generated by aggregation of small particles...

EDLCs store energy within the variation of potential at the electrode/electrolyte interface. This variation of potential at a surface (or interface) is known as the electric double layer or, more traditionally, the Helmholtz layer. The thickness of the double layer depends on the size of the ions and the concentration of the electrolyte. For concentrated electrolytes, the thickness is on the order of 10 A, while the double layer is 1000 A for dilute electrolytes (5). In essence, this double layer is a nanoscale model of a traditional capacitor where ions of opposite charges are stored by electrostatic attraction between charged ions and the electrode surface. EDLCs use high surface area materials as the electrode and therefore can store much more charge (higher capacitance) compared to traditional capacitors. 

The surface of a metal oxide consists of exposed cations, oxide ions, and hydroxyl groups. It is clear that these cations and anions in the surface of an oxide cannot be coordinatively saturated and, hence, that they must develop characteristic properties. The degree of unsaturation of individual surface atoms will be determined by the requirement for retaining stoichiometry in the crystal, and the type of crystal lattice will determine the local symmetry of surface vacancies. A detailed knowledge of the properties, structural and electronic, at an atomic level would be required for an in-depth understanding of the surface chemistry of oxides at a molecular level. This information, however, is almost impossible to obtain experimentally for high-surface-area materials of practical importance in adsorption and catalysis. The descriptions of these surfaces at an atomic level are based almost exclusively on model surfaces and the assumption that certain well-defined crystal planes (preferentially those providing the lowest... 

These results can again be related to studies of high-surface-area materials. VHien CO is adsorbed on Ti02 after thermoevacuation at elevated temperatures (>770 K), a carbonyl stretching frequency is observed near 2180 cm". Calculations based on an electrostatic model suggest that the frequency must arise from a CO ligand coordinated to Ti sites [20,21]. 

Fig. 8. Steady-state model for the earth s surface geochemical system. The kiteraction of water with rocks ki the presence of photosynthesized organic matter contkiuously produces reactive material of high surface area. This process provides nutrient supply to the biosphere and, along with biota, forms the array of small particles (sods). Weatheriag imparts solutes to the water, and erosion brings particles kito surface waters and oceans.

Finally, it should be mentioned that highly crystalline material and/or doped single crystals can be obtained from the elements by vapor transport techniques (36) or by impregnation of the highly crystalline binary sulfide (37). These materials serve as important models for higher-surface-area materials and often yield characterization information that is clear compared to more complicated catalysts. 

As mentioned in Section 10.2 above, both ceria and ceria-zirconia contain relatively weakly-bound oxygen when freshly prepared, e.g., in high-surface-area form. The thermal stability of this oxygen may differ in the two materials, however, as shown in steady-state CO-oxidation measurements performed by Bunluesin et al. [11] on model planar catalysts. In these experiments, films of ceria and ceria-zirconia were subjected to calcination treatments over a wide range of temperature before noble... 

The long effective pathlength and high surface area afforded by these colloidal semiconductor materials allow spectroscopic characterization of interfacial electron transfer in molecular detail that was not previously possible. It is likely that within the next decade photoinduced interfacial electron transfer will be understood in the same detail now found only in homogeneous fluid solution. In many cases the sensitization mechanisms and theory developed for planar electrodes" are not applicable to the sensitized nanocrystalline films. Therefore, new models are necessary to describe the fascinating optical and electronic behavior of these materials. One such behavior is the recent identification of ultra-fast hot injection from molecular excited states. Furthermore, with these sensitized electrodes it is possible to probe ultra-fast processes using simple steady-state photocurrent action spectrum. 

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