Multiple adsorption sites

Haaland was also able to resolve the ca. 3040-cm 1 vCH absorption into separate components at 3074, 3048, 3032, and 3011 cm 1. He attributed these to the presence of multiple adsorption sites, although such additional component bands were not observed for the other absorptions. It should be recalled, however, that the MSSR can break down for adsorbates on small-particle Pt catalysts (no mean particle size was given), as in the case of 77-bonded ethene (Part I, Section VI.B.c), leading to all the i>CH modes becoming active. 

The effect of alkyl chain length on the structure of alkanethiols on Au(lll) was studied with CH3(CH2) iSH, where n = 2,4, 6, 8, 10, 11, 12, 14, 15, 16, and 18).i The results, in terms of HREEL spectra, are displayed in Figure 11. It is most interesting to note that the intensity of CH3 a-deformation mode at 1380 cm (171 meV) is profoundly dependent on the number of carbons in the alkyl chain It is present only when the number of carbon atoms is even (cf, the spectra labeled Cio, C12 and C le) it is absent when the number is odd (cf, the spectra labeled Cu andCis). This odd-even trend is caused by the fact that the orientation of the CH3 head is parallel to the surface for odd number of carbon atoms but perpendicular when the number is even (cf, the inset in Figure 11). As dictated by the dipole selection rules, only the oscillator that has a component perpendicular to the surface (as in the even number chain) would show HREELS activity. It can also be seen in the frequency region below 220 cm (27.3 meV) that more than one peak, separated by about 30 cm (3.7meV) are present this indicates the existence of multiple adsorption sites for the subject alkanethiols on Au(lll). 

On metal oxide and metal chalcogenide semiconductor surfaces, multiple adsorption sites are accessible. On titanium dioxide, for example, there are acidic, basic, and surface defect sites available as likely targets for adsorption. Although the applicable adsorption isotherms differ at each site, selective activation toward a desired product on a specified semiconductor surface may indeed depend on photocatalyst preparation, which may in turn influence the relative fraction of each type of adsorption site. Although the number of basic sites can be determined by titration, the total number of acidic sites is more difficult to establish experimentally because of competitive water adsorption. Roughly, there are 2.4 times more acidic than basic binding sites on several of the commercially available Ti02. 

McQuarrie [96] has extended the stochastic theory to the case of multiple adsorption sites and to a column with a single type of site, but with various input distribution functions. 

Monomer and excimer fluorescence decays of Py, 1Py(3)1Py and the alkylpyrenylsilanes PPS and PDS, adsorbed on silica surfaces have been reported in the literature (21, 31-43). However, whereas for inter- and intramolecular excimer formation in homogeneous solution the rate constants of excimer formation and dissociation could be determined from the fluorescence decays (11,15,23), a considerably more complex situation is encountered on the silica surfaces (c.f. Section 3.2.1). This is not surprising, as the multiple adsorption sites at the inhomogeneous surfaces ma)ce different pathways in the excimer formation process li)[Pg.61]

Although the non-exponentiality of a fluorescence decay may be clear, the distinction between [9] (or even more complex multiexponential forms) and [10] can be rather difficult. This problem is alleviated when the experiments are performed at low, so that components of the form [10] arising from energy transfer within the adsorbed dye layer need not be considered. However, the fits are not unique even then For example, a reasonably good fit with [9] will often be improved if a sum of three exponentials is used instead (25). Whatever the details of the functional form used in the fit, there is clear evidence for more than one adsorption site in the fluorescence decays of dye molecules adsorbed on a variety of surfaces (12,18,25). At low e the emitting species are monomers with lifetimes which vary from site to site. The existence of multiple adsorption sites on the same surface raises the concern that kj may not be surface independent. 

Octanethiol SAMs on Au(lll) have been found to undergo an adlattice transition from a c(4 x 2) to a (6 X. 3) structure after long-term storage. HREELS was one of the techniques employed to examine the cause for the transitions. It was established that the structural transitions were caused by the dynamic surface diffusion of the sulfur anchor group between multiple adsorption sites. The adsorption-site exchange also resulted in orientational changes in alkyl chains. ... 

Lu W, Verdegaal WM, Yu J, Balbuena PB, Jeong H-K, Zhou H-C (2013) Building multiple adsorption sites in porous polymer networks for carbon capture applications. Energy Environ Sci 6 3559-3564... 

The catalytic reaction system containing NO, NO2, NH3, O2, and H2O on Fe-exchanged zeolites is quite complex as it involves multiple reaction pathways to several products (N2, N2O, NH4NO3), on catalysts with multiple adsorption sites (Bronsted acid sites, metal-exchanged sites), complicated by rate inhibition... 

For a single adsorption site, n = 1, when K changes one himdred folds on both sides of Ko, from C = 10 to = 10, the rate varies by a factor of less than two. Here the compensation effect plays a very important role. But for multiple adsorption site, when multiplicity n of the sites goes up, e.g., n = 6, a hundred folds change in K on the both sides on Ko modifies the rate by a factor of 10 folds. It is easy to prove, since K = exp(j4°/i T), that a variation in by 10" is due to a change in the standard affinity by an amount of 2nkcalmol at temperature of 453K. This cases is famiiiar for cataiytic study in practice. 

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