Adsorption frequency

Figure 8.7 RAIRS spectra show that lateral interactions force CO to leave the twofold adsorption sites on palladium (IR frequency of about 1920 cm 1) when NO is coadsorbed, and push it to the on top site (adsorption frequencies above 2000 cm-1). Adsorbed NO gives rise to the absorption peaks below 1800 cm 1 (from Raval et al. [22]).

Ichikawa ( ) also observed the lowering of the CO adsorption frequency on Rh-Mn/SiO catalysts and ascribed it to this type of interaction. Recently, Blyholder et al. (40), by theoretical calculations, showed that such interaction of the oxygen atom with Lewis acid centers would have a stabilizing effect on the formyl species. This stabilization could cause the high selectivity to oxygenates by a reaction pathway like the following ( ). ... 

This formidable formula is not so forbidding as it seems, and has been analysed in great detail, reduced to tractable forms that make sense in [3]. Pairwise summation emerges as a very special case, valid for gases only, and even then is a bad approximation to the full many-body interaction. Calculation of the interaction free energy for particular cases is not difficult [3-10] and requires a knowledge of measured dielectric properties and adsorption frequencies in the infrared, visible and ultraviolet, all known, in principle. 

Equations (92) and (93) show that the presence of a solvent medium other than a free space much reduces the magnitude of van der Waals interactions. In addition, the interaction between two dissimilar molecules can be attractive or repulsive depending on refractive index values. Repulsive van der Waals interactions occur when n3 is intermediate between nx and n2, in Equation (92). However, the interaction between identical molecules in a solvent is always attractive due to the square factor in Equation (93). Another important result is that the smaller the n - nj) difference, the smaller the attraction will be between two molecules (1) in solvent (3) that is the solute molecules will prefer to separate out in the solvent phase which corresponds to the well-known like dissolves like rule. However there are some important exceptions to the above explanation, such as the immiscibility of alkane hydrocarbons in water. Alkanes have nx = 1.30-1.36 up to 5 carbon atoms, and water has a refractive index of n = 1.33, and very high solubility may be expected from Equation (93) since the van der Waals attraction of two alkane molecules in water is very small. Nevertheless, when two alkane molecules approach each other in water, their entropy increases significantly because of the very high difference in their dielectric constants and also the zero-adsorption frequency contribution consequently alkane molecules associate in water (or vice versa). This behavior is not adequately understood. 

Here e is the static (zero frequency) relative dielectric constant hP is Planck s constant, i.e., 6.626 10 34 J s ve is the main UV adsorption frequency, which equals for most substances involved 2.9 — 3.0 1015 s 1 and n is the refractive index for visible light (generally taken at a wavelength of 589 nm). The first term in the equation is due to dipole-dipole and dipole-induced dipole interactions, and the second term is due to London dispersion forces (unretarded). The first term is always smaller than (3/4)kpT the second term can be much larger. 

Infrared adsorption frequencies and line broadening (X5,1 6) have been interpreted to show that water is bound 0 downwards to the HO M. The lifetime, and population, of aggregates and hydration sheaths is strongly affected by the rotational freedom of the water molecules. In bulk water this is restricted by hydrogen bond breaking. In adsorbed layers the interaction with the adsorbate dominates reorientation for all molecules within six to ten molecular dieurneters. 

Figure 2. Gas Adsorption frequency Characteristics of Yangquan coal samples.

With the increased availability of extensive data on the adsorption of reactants and products on well-defined surfaces it is now possible to contemplate the development of more mechanistically based kinetic models. Two approaches have been adopted the one described in the previous section, which has much in common with the LH approach, and the more general approach that requires a rate expression, incorporating heats of adsorption, frequency factors etc., for each of the individual synthesis reaction steps [Eqs. (9.18-9.25)]. These rate expressions are then solved simultaneously to give the overall reaction rate. 

An infrared spectrum of CO adsorbed on a ruthenium-palladium catalyst has a set of adsorption frequencies different in their position from the bands typical of pure ruthenium and palladium [11]. This fact indicates the absence of pure ruthenium and palladium particles, i.e., the formation of alloyed particles. 

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