Admolecules with Interactions

At low coverages approximate values are q heat of evaporation of the sorptive medium. 

Relations (7.37, 7.38) provide an implicit equation to calculate the mass adsorbed (m) from given values of the parameters (b, c, d, k, a, m ,) and sorptive gas pressure (p). Resulting curves can be of lUPAC Type I, IV, V depending on numerical values of parameters b 0, c 0, d 0, k 0, a 0. Here c, d describe admolecular interactions, b is the Langmuir parameter (7.2) and k, a describe the topology of the neighborhood of the admolecule. Hence these parameters are related to the pore structure of the sorbent material. 

To elucidate this difference we mention the mean square deviation (o) between measured (exp) and correlated (cal) data for both AIS  

More details about the statistical derivation and range of application of the AI (7.37, 7.38) are given in the literature [7.29, 7.30]. For the sake of 

---

A Generalization of Langmuir s Adsorption Isotherm to Admolecules with Interaction,... 

The specific interaction of the admolecule with the surface is then rather well established, while the geometry of the adsorbed species is only tentative. One important conclusion to be drawn from the study of the chemical shifts, is that they cannot by themselves indicate unambiguously the exact geometry of a "contact-type complex". Nevertheless the 7r-complex nature of the adsorbed species was also suggested by the dependence of the adsorption coefficient of n-butenes on their energy of ionization (4). 

Due to the complexity by which adsorbed molecules (admolecules) can interact with the atoms and molecules of the sorbent and with each other, a variety of phenomena can be expected to occur during an adsorption process. Depending on the strength or interaction energy by which admolecules are bound to sorbent s surface, one can distinguish physisorption, physicochemical adsorption and chemisorption phenomena [1.11, 1.12],... 

Usually the presence of coadsorbates will decrease the interaction strength of admolecules with surface atoms. This may occur for several reasons. The deactivation of surface atom reactivity due to adatom adsorption is an example discussed earlier in this section. 

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

CO to a Pd(l 1 l)-like thick film and a Pd monolayer supported on Ta(110) [30,31], The spectrum for a thick palladium film is in very good agreement with that observed for adsorption of CO on a single-crystal Pd(lll) surface. The features at 11 and 8 eV correspond to emissions fi om the 4o and (In + 5o) levels of CO, respectively [30,31], In the photoemission spectrum for the Pd monolayer the 4o and (In + 5o) peaks of CO appear at higher binding energy than in the ectrum for the Pd(lll)-like film, and there is also an extra shake-up satellite ( s peak) around 13.6 eV. The spectrum for CO on the Pd monolayer matches the ectrum seen for CO on Cu(lll) [30,31], where the bonding interactions between the admolecule and metal substrate are much weaker than on Pd(l 11). 

The metal-metal interatomic interaction is generally weaker than the strong interaction, with the adsorbates chemisorbed to the surface. Therefore, adatoms or admolecules tend to localize electrons into the surface complex fragment orbitals formed between the adsorbate and surface atoms. 

In physisorption systems admolecules are weakly bound, often by van der Waals- and/or dispersion forces due to induced dipole-dipole interactions. They also can be desorbed reversibly by lowering the sorptive gas pressure or increasing the temperature. Admolecules are basically preserved and not subject to chemical reactions i. e. changes in the character of their electron shells due to interactions with the atoms and/or molecules of the sorbent. 

Figure 7.8. Energetic scheme of a Langmuir adsorbate with admolecular interactions. At low coverages the adsorption energy (q = 2.5 r) is much larger than the interaction energy of the admolecules (q S r) with (r) being the heat of evaporation of the sorptive medium.

A considerable element of the model is the assumption connected with the possibility of the kinetic motion of adsorbed molecules. When the motion of molecules in the z direction is restricted but molecules are able to move freely in the (x,y) plane, the process is classified as mobile adsorption. However, if the lateral translation is also hindered, the process is classified as localized adsorption. The motion of admolecules is controlled by the energetic topography of the surface, molecular interactions, and thermal energies. The adsorbed molecule is considered as localized on a surface when it is held at the bottom of a potential well with a depth that is much greater than its thermal energy. Except for extreme cases, adsorption is neither frilly localized nor frilly mobile and can be termed partially mobile [8]. Because temperature strongly affects the behavior of the system, adsorption may be localized at low temperatures and become mobile at high temperatures. 

To investigate the effect of interaction between surface atoms and lubricants on the velocity profiles, simulation was made with the half L-J parameter 80.5. Figure 9 shows the results. Weak interaction between lubricant molecules and surface atoms allows slip at both the upper and lower surfaces except for the case of the lower surface with cyclohexane. The weak interaction also allows the molecules to move more fi ly, resulting m rather straight density profiles. Figure 10 shows the profiles in the case when both upper and lower surface have steps and the L-J parameter S0.5 is employed. No slip is seen at the surfaces, and the velocity profiles are continuous in all cases. Motions of admolecules on the stepped surfaces are examined. Figure 11 shows top views of trajectories of n-hexane molecules adsorbed on the upper surface with/without steps for the L-J parameter 80.5-... 

---