Adsorption of n-molecules

Let qs be the molecular partition function for an adsorbed species. Consider the adsorption of N molecules on some portion of the surface containing a total of M possible adsorption sites. The system partition function Qs of the collection of N adsorbed species is... 

The chemical contribution to the free energy, per unit area, due to the adsorption of n molecules (per unit area) of charge q on each surface of potential ij s is4... 

The reactant is adsorbed on the catalyst s surface. As a reactant molecule attaches to the surface of the catalyst, its bonds are weakened and the reaction can proceed more quickly because the bonds are more easily broken (Fig. 13.36). One important step in the reaction mechanism of the Haber process for the synthesis of ammonia is the adsorption of N2 molecules on the iron catalyst and the weakening of the strong N=N triple bond. 

The entropy factor should also be considered since cyclization results in a more ordered structure. The C5 cyclization of n-hexane involves an entropy decrease of about 15-17 entropy units (e.u.). The corresponding values for cyclohexane and benzene formation are about 25 and 38-45 e.u., respectively. These values are comparable with calculated values of adsorption entropy (29). Thus, adsorption of a molecule to be cyclized may supply a considerable part of the entropy change in other words, adsorption should take place in a geometry favorable for cyclization. This is one of the main roles of the catalyst. 

Adsorbate Molecular Orientation at Electrode Surface. Adsorption of some molecules from solution produces an oriented adsorbed layer. For example, nicotinic acid (NA, or 3-pyridinecarboxylic acid, niacin, or vitamin B3) is attached to a Pt(lll) surface primarily or even exclusively through the N atom with the ring in a (nearly) vertical orientation (12) (Fig. 10.5a). 

Afanasev et al. [140] have analyzed the literature data concerning adsorption of N-derivatives of morpholine and cyclo-hexylamine at the uncharged mercury electrode. They have also determined free energy of adsorption intrinsic to the interactions of organic molecules with mercury. 

In this subsection we investigate the adsorption of O2 molecules on neutral Aun clusters with 5cluster substrate, which is chosen among the equilibrium structures of pure An clusters obtained in a previous work . We do not consider initial configurations with two separated O atoms. Thus, O2 dissociative adsorption is obtained only when that process occurs without any barrier. 

Fig. 6.110. AG /kT vs. electrode charge (a) for the adsorption of n-aliphatic alcohols on mercury from aqueous 0.5 M Na2S04. The number on each plot is that of the carbon atoms of the given alcohol (n). The dashed curves were calculated by the authors of the graph. (Reprinted from R. Guidelli, in Adsorption of Molecules at Metal Electrodes, J.

A molecule with several independently adsorbed units in a train can migrate only if all of them are free. Even for the simplest case of a two-unit polymer the k value becomes k = 0.88 0.11 = 8 under chromatographic conditions yielding k = 2 for an independent unit. For the adsorption of n units with a constant k for each unit the retention factor for the whole train becomes ... 

Kern and Findenegg measured the adsorption of n-docosane (C22H46) in heptane solution to graphite [403], They used a porous graphite with a specific surface area of 68 m2g 1 as determined from BET adsorption isotherms with N2. Tmax, which is assumed to correspond to monolayer coverage, is found to be 88.9 /.xmol/g. Can you conclude something about the structure of the adsorbed molecules What is the area occupied by one molecule compared to its size ... 

In these equations, a, n, a, and fi are constants. Adsorption of noninteracting molecules on top of each other to form multiple layers leads to the BET isotherm (Brunnauer, Emmett, and Teller). 

Still most dynamical simulations of reactions at surfaces are limited to rather simple systems, such as the adsorption of diatomic molecules on low-index single crystal surfaces. With the development of more efficient algorithms and the improvement of computer power, more and more complex systems will be able to be addressed. One recent example is the ab initio molecular dynamics simulation of the soft-landing of Pd clusters on oxide surfaces [127] where up to n = 13 Pd atoms have been taken into account in the calculations. 

Alkali-metals are frequently used in heterogeneous catalysis to modify adsorption of diatomic molecules over transition metals through the alteration of relative surface coverages and dissociation probabilities of these molecules.21 Alkali-metals are electropositive promoters for red-ox reactions they are electron donors due to the presence of a weakly bonded s electron, and thus they enhance the chemisorption of electron acceptor adsorbates and weaken chemisorption of electron donor adsorbates.22 The effect of alkali-metal promotion over transition metal surfaces was observed as the facilitation of dissociation of diatomic molecules, originating from alkali mediated electron enrichment of the metal phase and increased basic strength of the surface.23 The increased electron density on the transition metal results in enhanced back-donation of electrons from Pd-3d orbitals to the antibonding jr-molecular orbitals of adsorbed CO, and this effect has been observed as a downward shift in the IR spectra of CO adsorbed on Na-promoted Pd catalysts.24 Alkali-metal-promotion has previously been applied to a number of supported transition metal systems, and it was observed to facilitate the weakening of C-0 and N-0 bonds, upon the chemisorption of these diatomic molecules over alkali-metal promoted surfaces.25,26... 

A simple shake test with aqueous 1 vol % n-butanol solution has been developed to test the selective adsorption of organic molecules over water molecules by a hydrophobic and organophilic adsorbent (26). Table V gives the results for severed, fluorine-treated zeolites and their untreated counterparts. The data clearly indicate that fluorine treatment of LZ-105, H-zeolon and NH4Y materials substantially increases their selectivity for n-butanol over water. 

The low-coverage energy data for the adsorption of n-hexane and benzene on various non-porous solids in Table 1.4 illustrate the importance of the surface structure of the adsorbent and the nature of the adsorptive. Since n-hexane is a non-polar molecule, Em > Esp, and therefore the value of E0 is dependent on the overall dispersion forces and hence on the density of the force centres in the outer part of the adsorbent (i.e. its surface structure). Dehydroxylation of a silica surface involves very little change in surface structure and therefore no significant difference in the value of E0 for n-hexane. However, replacement of the surface hydroxyls by alkylsilyl groups... 

These procedures make use of the differences between (a) the strong specific interactions given by certain polar molecules (e.g. n-butanol) with polar surface sites and (b) the non-specific interactions given by the adsorption of hydrocarbon molecules (e.g. n-C32H66) on the graphitic basal plane. 

The initial studies demonstrating the feasibility of using probe molecules to study acid sites were conducted by Gay and coworkers from 1977 onwards [215, 216]. This approach was further developed by Dawson and coworkers who employed CP-MAS NMR to investigate the number and nature of acid sites on y-Al203 through the adsorption of n-butylamine and pyridine [217, 218]. In the case of... 

The adsorption of neutral molecules is discussed in detail in Chapter J. Here we show electrocapillary curves obtained in the presence of n-butanol and compare them to an electrocapillary curve taken in a solution of NaF. Looking at Fig. IlH we note that adsorption... 

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