Frequency dependence Adsorption

Pt surface, as studied by Pajkossy [1994], almost ideally capacitative behavior can, in fact, be observed this is obviously a critical result indicating that it is not inseparable coupling between solution resistance and capacitance at a roughened (Pt) electrode surface (Pajkossy [1994]) that is the origin of dispersion effects. This led (Pajkossy [1994]) to the conclusion that it is ion (anion) adsorption that plays a crucial role in capacitance dispersion, because of frequency-dependent adsorption pseudocapacitance associated with anion chemisorption and associated kinetics of that process (Pajkossy [1994], Pajkossy et al. [1996]). 

Adsorption of carbon monoxide takes place all over the surface and there is distinct evidence that, at least on nickel, the CO stretching frequency depends upon the coordination number of the nickel atom to which it is attached. Hence, the adsorption of carbon monoxide yields information about the relative numbers of surface atoms with different coordination numbers. This information, howrever, is at best merely of a semiquantitative nature. Steric effects also play a role, as is evidenced by the fact that the subcarbonyl species can be formed only on nickel atoms with a lowr coordination number. 

Kubelkova and coworkers studied the interaction of CO with platinum in a NaX zeolite [141]. They decomposed PtlNHj I]] on the zeolite under various conditions and observed distinct CO vibrational spectra for PtO-CO, Pt -CO, Pti -CO and [Pt(CO)2] complexes after CO adsorption at room temperature. Subsequent reduction with hydrogen resulted in characteristic particle size distributions with distinct CO vibrational frequencies depending on whether the clusters (-1.2 nm) were inside the supercages or outside the cages (-4.0nm). 

In this section, the interpretation of interfacial admittance data in the case of an a.c. reversible reaction with adsorption of O is briefly described. The relationships expressing the frequency dependence were derived some time ago [15, 17], but the essential meaning of the parameters involved was fully explained only recently [143], The brief description here is derived from the latter reference. 

Precursors useful in the surface sol-gel process are not restricted to alkoxides. The requirements as precursors are chemisorption on surface hydroxyl groups and regeneration of the hydroxyl groups after hydrolysis. For example, TiO(acac)2 repeatedly adsorbs, when acid and alkali are added to the adsorption and hydrolysis media, respectively. Except for the case of Nb(0"Bu)5, all the compounds listed in Table 6.1 show linear frequency shifts. Adsorption conditions such as concentration, temperature, and immersion time are dependent on the solubility, reactivity, and the ease of hydrolysis of alkoxides. These conditions are varied as the structure of alkoxide units is changed. For example, Ti(01Pr)4, which exists as a monomer in solution [17], requires conditions different from Ti(0"Bu)4, which tends to form oligomer species. 

First, a typical power spectrum of capillary waves excited at the W/NB interface is shown in Figure 3.4a. The errors on the values of the capillary wave frequency were 0.1 kHz, obtained as the standard deviation of 10 repeated measurements. Capillary wave frequency dependence on CeHsONa is shown in Figure 3.4b. The frequency decreased significantly with increasing CeHsONa concentration. This indicated that interfacial tension was decreased by the interfacial adsorption of CeHsONa. 

Mixtures of hydrophilic Aerosil with the oligomer have been studied by H T and T2 relaxation experiments in a wide temperature range [9]. A two-phase model was used to describe the relaxation of ODMS molecules at the surface of Aerosil. According to this model [29], molecules at the surface of a solid co-exist in adsorbed and non-adsorbed states. In a certain period of time, molecules perform jumps between these two phases. The lifetime of a molecule in adsorbed and desorbed states is detected from peculiarities of the temperature dependence of T and T2 NMR relaxation times [29]. It was shown that, already at 240 K, the lifetime of ODMS molecules in the adsorbed state is about 50 ps, i.e., the frequency of adsorption-desorption at this temperature is close to 3 kHz [9]. At about 500 K, nearly all ODMS molecules are desorbed. 

The expression immediately gives an estimate of the enthalpy of adsorption in taking an atom from the gaseous (vacuum) state to a liquid, or to a composite medium like a zeolite, characterised by its measured dielectric frequency dependent response (o>). It is, exactly as for the electrostatic Bom self-energy in taking an ion from vacuum to water ... 

Rupprechter G., Freund H.-J. (2001) Adsorbate-induced restructuring and pressure-dependent adsorption on metal nanoparticles studied by electron microscopy and sum frequency generation spectroscopy. Topics Catal 14 3... 

Adsorbed gelatine molecules alone do not show a frequency dependence of surface elasticity (Fig. 6.19), which corresponds to a behaviour of an insoluble monolayers. The presence of surfactants changes the elastic and relaxation behaviour dramatically. With increasing SDS concentration the elasticity modulus (frequency independent plateau value of the elasticity) first increases and then decreases. The dynamic behaviour of the mixed adsorption layer changes from one completely formed by gelatine molecules to an adsorption layer completely controlled by surfactant molecules (Fig. 6.20). A similar behaviour can be observed for CTAB and a perfluorinated surfactant (Hempt et al. 1985). 

Cation Zeolites. Bertsch and Habgood (9) studied the adsorption of small amounts of water on Group IA X zeolites. They observed a sharp band between 3720 and 3648 cm"1 depending on the cation and broad bands near 3400 and 3200 cm1. Because the frequencies depended... 

We carried out the oxidation of methane over coat-R catalysts with the different R loading in order to examine their catalytic performance. Fig. 4 shows the turnover frequency in the oxidation of methane over coat-R catalysts. The turnover frequency was estimated based on the converted rate of methane into CO2 and the number of R atoms at the surface of R metal particles which was evaluated by CO adsorption on coat-Pt. As shown in Fig. 4, the turnover frequency for the oxidation of methane decreased with the lower Pt loadings, i.e. with thickness of silica layers which wrapped the catalytically active R metal. It is likely that the turnover frequency depends on the thickness of silica layers of coat-Pt because the diffusion rates of reactant molecules (CH4 and O2) in silica layers affect the reaction rates observed over coat-Pt catalysts. 

Fig. 4.8 AC admittance (a) and PMF responses (b) in TIR for the adsorption and transfer of ZnTMPyP(tosylate)4 from water to DCE. The continuous and dashed lines correspond to the real and imaginary components of the frequency-dependent signals, respectively. The behavior of the admittance and PMF significantly contrasts with that observed in the presence of Ru(bipy) because of the specific adsorption of the porphyrin at the interface (see Fig. 4.6). Reprinted with permission from Ref. [15]. Copyright (2000) American Chemical Society.

It was determined (see Fig. 1), that a square of the wave frequency in adsorption maximum of SPR (co ) linearly depends on a value of the average diameter d) Ag-NPs. 

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