Adsorbates on Surfaces

So far in this chapter we have only looked at clean surfaces, that is, surfaces with the same chemical composition as the bulk material from which they are made. Many of the interesting things that happen with surfaces, of course, occur when other chemicals are present on them, so we now turn to that situation. We will only look at situations where atoms or molecules are chemisorbed on surfaces—this can loosely be defined as the situations where a chemical bond of some kind exists between the atom or molecule and the surface. 

As an example, let us perform some calculations to understand how H atoms bind on Cu(100), the metal surface we looked at earlier in the chapter. Many chemical reactions on metals involve the creation of individual H atoms 

TABLE 4.3 Results of Calculations for H Adsorption on Cu(100) as Described in Text  

Initial H Site Adsorption Energy Relative to Hollow Site Adsorption Energy Relative to H2(g) Local Minimum  

We can avoid this symmetry-induced trap by deliberately breaking the symmetry of our atom s coordinates. One easy way to do this is to repeat our calculations after moving the H atom a small amount (say, 0.2 A) in some arbitrary direction that does not coincide with one of the symmetry directions on the surface. What we find, if we run calculations in which we start the H atom at a point about 0.2 A away from each of the high-symmetry sites mentioned above is that the H atom relaxes to the fourfold hollow site even if it is started quite near the top and bridge sites. This shows that the top and bridge sites are not minima for this system. 

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SERS. A phenomenon that certainly involves the adsorbent-adsorbate interaction is that of surface-enhanced resonance Raman spectroscopy, or SERS. The basic observation is that for pyridine adsorbed on surface-roughened silver, there is an amazing enhancement of the resonance Raman intensity (see Refs. 124—128). More recent work has involved other adsorbates and colloidal... 

Estimating Minimum Sample Quantity for Moisture Measurement Estimates of material quantity for testing moisture content depend on mechanisms of moisture distribution in the material. Moisture is physically retained on particle surfaces, chemically adsorbed on surfaces and within pores of particulate solids, and contained as an internal constituent of solids. Significant internal moisture is most often encountered in organic and agricultural source materials. 

The width and shape of the energy loss peaks in HREELS are usually completely determined by the relatively poor instrumental resolution. This means that no information can be obtained from HREELS about such interesting chemical physics questions as vibrational energy transfer, since the influence of the time scale and mechanism of vibrational excitations at surfaces on the lifetimes, and therefore the line widths and shapes, is swamped. (Adsorbates on surfaces have intrinsic vibra-... 

Reflectometry is a useful probe with which to investigate the structure of multilayers both in self-supporting films and adsorbed on surfaces [51]. Specular X-ray reflectivity probes the electron density contrast perpendicular to the film. The X-rays irradiate the substrate at a small angle (<5 °) to the plane of the sample, are reflected, and are detected at an equal angle. If a thin film is present on the surface... 

Let us consider that particles are adsorbed on surface sites whose activation energies of desorption form a continuous spectrum between certain limits. The problem now consists of finding the distribution of initial surface populations ne0i according to the energies EA<-... 

Fig. 6 —Detect boundary films by means of the interferometry technique (a) h-v curve follows the Hamrock and Dowson line all the way down to a couple of nanometres when there are few molecules adsorbed on surfaces (b) the curve exhibits a downward turn if the adsorbed layer completely disappears and direct contacts occur (c) the curve levels off due to the existence of thick adsorbed layers.

Figure 2. NMR spectra of C6D6/(Na)X at room temperature. SW = 500 kHz. denotes benzene-dg adsorbed on surface.

Surface composition and structure of bimetallic nanoparticles are crucially important for their catalytic property as well as their optical property. IR measurement of CO adsorbed on surface metals (CO-IR) is utilized for this purpose. CO is adsorbed on metals not only on-top sites but also in two-fold or three-fold sites, depending on the kinds of metals and their surface structures. The dramatical changes of wavenumber of adsorbed CO occurs depending on the binding structure [177-181]. 

Unless complexes are adsorbed on surfaces, the successive reduction will not occur so that nanoclusters might not be grown on the surface. The third one is the direct deposition of metal from solution by the reduction of the reducing agent, as shown in Figure 2. 

Th do not reach constant concentrations over any reasonable distances, Ra does over several hundred meters, and all the other radionuclides require less than a few meters. Constant concentrations are achieved when the inventories of a radionuclide in both groundwater and adsorbed on surfaces decay at the same rate at which this radionuclide is... 

Equation (1) can be ignored. Assuming further that the desorption rate is fast compared to the Ra half lives, then the ( Ra/ Ra) ratios in the groundwater and adsorbed on surfaces (and so in the mobile Ra pool) are equal. In this case, the measured groundwater ( Ra/ Ra) ratio reflects the ratio of the supply rates of Ra and Ra, which is equal to the ratio, adjusted for any differences in the distributions of and... 

The elusive nature of binary gold carbonyls - known only from earlier matrix-isolation studies and from CO adsorbed on surfaces of bulk gold - has prompted quantum-chemical studies of the hypothetical species Au(CO) with n = 1, 2, 3. Two independent investigations (on the HF and ab initio level, MP2, CCSD(T)) have consistently... 

S = surface site of adsorbens A = adsorbate in solution SA = adsorbate on surface sites... 

In summary, these experiments demonstrate that Ba2 partitioning does not depend on the concentration of Ba2 over the broad range tested and that it is unaffected by moderate amounts of Sr2. Substantial quantities of NaCl decrease kcBa and rapid precipitation increases kcBa. These results suggest little or none of the Ba2 is incorporated in Ca 2 lattice sites in calcite, but instead adsorbs on surfaces and defects and is trapped by growth of the crystal. 

Bound moisture. This is water retained so that it exerts a vapour pressure less than that of free water at the same temperature. Such water may be retained in small capillaries, adsorbed on surfaces, or as a solution in cell walls. 

D. R. James, Y-S. Liu, P. DeMayo, and W. R. Ware, Distributions of fluorescence lifetimes Consequences for the photophysics of molecules adsorbed on surfaces, Chem. Phys. Lett. 120, 460-465 (1985). 

Butterfly-type folding has also been discussed in the context of rhodamine dyes adsorbed on surfaces/53 In this case, too, nonexponential fluorescence decays are observed. A comparison of the consequences of the different reaction mechanisms with respect to free volume sensing is given in Section 5.4. 

Cu and Zn enter sedimentary material in substantial proportions, both in the structure of minerals (carbonates, clays) and adsorbed on surfaces. Boyle (1981) showed that foraminiferal tests may contain Zn in excess of a few ppm. Partitioning of Cu and Zn between water and carbonates has been investigated by Rimstidt et al. (1998). The crystal chemistry of Cu and Zn in goethite has been investigated by EXAFS by Manceau et al. (2000). Typical Zn and Cu concentrations in FeMn nodules and encrustations are 500-1000 ppm and 800-6000 ppm, respectively (e.g., Albarede et al. 1997b). 

H —> "Si CPMAS has been widely applied to zeolites to identify the presence of silanol species [176]. CP was shown to be very useful in understanding probe molecules adsorbed on surfaces and their dynamics [177]. Mobile and immobile phases can also be easily distinguished by CP [178]. 

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