Adsorbent forms

Returning to more surface chemical considerations, most literature discussions that relate adhesion to work of adhesion or to contact angle deal with surface free energy quantities. It has been pointed out that structural distortions are generally present in adsorbed layers and must be present if bulk liquid adsorbate forms a finite contact angle with the substrate (see Ref. 115). Thus both the entropy and the energy of adsorption are important (relative to bulk liquid). The... 

Adsorption usually increases as pH and temperature decrease. Chemical reactions and forms of chemicals are closely related to pH and temperature. When pH and temperature are lowered many organic chemicals are in a more adsorbable form. The adsorption process is also influenced by the length of time that the AC is in contact with the contaminant in the water. Increasing contact time allows greater amounts of contaminant to be removed from the water. Contact is improved by increasing the amount of AC in the filter and reducing the flow rate of water through the filter. 

It is noteworthy that even a separate treatment of the initial data on branched reactions (1) and (2) (hydrogenation of crotonaldehyde to butyr-aldehyde and to crotyl alcohol) results in practically the same values of the adsorption coefficient of crotonaldehyde (17 and 19 atm-1)- This indicates that the adsorbed form of crotonaldehyde is the same in both reactions. From the kinetic viewpoint it means that the ratio of the initial rates of both branched reactions of crotonaldehyde is constant, as follows from Eq. (31) simplified for the initial rate, and that the selectivity of the formation of butyraldehyde and crotyl alcohol is therefore independent of the initial partial pressure of crotonaldehyde. This may be the consequence of a very similar chemical nature of both reaction branches. 

In all cases the adsorbate forms a dipole with the metal. The adsorbate is overall neutral as it is always accompanied by its compensating (screening) charge in the metal.5 7 Thus the presence of an adsorbate on a metal surface will affect, in general, the work function of the surface.5... 

This kinetic relationship provides the necessary link between the gas-phase concentration ai and the concentration of A in its adsorbed form, which is denoted as [AS]. The units for surface concentration are moles per unit area of catalyst surface. S denotes a catalytically active site on the surface, also with units of moles per area of catalyst surface. 

The global design equations for packed beds—e.g.. Equations (10.1), (10.9), (10.39), and (10.40)—all have a similar limitation to that of the axial dispersion model treated in Chapter 9. They all assume steady-state operation. Adding an accumulation term, da/dt accounts for the change in the gas-phase inventory of component A but not for the surface inventory of A in the adsorbed form. The adsorbed inventory can be a large multiple of the gas-phase inventory. 

Solution The surface is estimated to contain 0.067mol of A in adsorbed form. The inlet gas contains 1.6 mol of A per cubic meter and is flowing at 150cm /s so that A is entering the reactor at a rate of 0.00024 mol/s. Five minutes are needed to supply the surface if all the incoming gas were adsorbed. Fifteen to thirty minutes would be a reasonable startup time. Recall that the reactor has a gas-phase residence time VjQout of only 1.5 s The residence time of the adsorbed species is 700 times larger than the average (nonadsorbed) molecule. 

In the majority of cases where adsorbates form ordered structures, the unit cells of these structures are longer than that of the substrate they are referred to as superlattices. Two notations are used to describe the superlattice, the Wood notation and a matrix notation.18 Some examples of overlayer structures at an fcc(llO) surface are as follows ... 

A COH species requires three adjacent adsorption sites. Under conditions of large degrees of coverage, the site requirement should be an important factor in determining the kind of adsorbate formed. 

It has been shown in Section 2.1.4 that methanol adsorbate formed from dilute solutions on a porous Pt surface, consists of COad and COHad in a ratio CO COH of ca. 20-30% [14]. The results of isotopic exchange with bulk CO seem to indicate that only the fraction present as COad can be desorbed and replaced by bulk CO. Probably the same arguments as in the case of pure COad can apply. COHad seems to be more strongly bound to the Pt surface and cannot be desorbed. 

The temperature dependence of the extent of adsorption was not interpreted, except that the results were considered to be consistent with the magnetic measurements of Selwood (see Section II,C) which indicate that the number of carbon-metal bonds between adsorbed species and the surface increases threefold between 120°and 200°C due to extensive dissociative chemisorption. The authors proposed that two forms of chemisorbed benzene exist at the nickel surface, (i) an associatively adsorbed form which can be displaced by further benzene, and which may be w- or hexa-dissociatively adsorbed form that requires the presence of hydrogen to bring about its removal from the surface. 

Thus, the spectroscopic work proved that the two forms of adsorbed hydrogen are very different. The weakly adsorbed form is definitely adsorbed above the Pt surface plane and so able to interact with the solution. In contrast, the strongly adsorbed form cannot interact with the solution, lying in the plane of the surface Pt atoms with its electron in the conduction band of the metal. This conclusion was supported by work that showed the strongly adsorbed form was not formed on close-packed surfaces. 

The considerations in Section 18.3 apply to a single adsorbed atom. At higher coverages the interaction of the adatoms with each other is important. We consider the case of adsorbed alkali ions. If the adsorbates form a regular lattice, this interaction takes the form ... 

Experimental confirmation that alkali adsorbates form positive ions on metallic... 

Gas adsorption (physisorption) is one of the most frequently used characterization methods for micro- and mesoporous materials. It provides information on the pore volume, the specific surface area, the pore size distribution, and heat of adsorption of a given material. The basic principle of the methods is simple interaction of molecules in a gas phase (adsorptive) with the surface of a sohd phase (adsorbent). Owing to van der Waals (London) forces, a film of adsorbed molecules (adsorbate) forms on the surface of the solid upon incremental increase of the partial pressure of the gas. The amount of gas molecules that are adsorbed by the solid is detected. This allows the analysis of surface and pore properties. Knowing the space occupied by one adsorbed molecule, Ag, and the number of gas molecules in the adsorbed layer next to the surface of the solid, (monolayer capacity of a given mass of adsorbent) allows for the calculation of the specific surface area, As, of the solid by simply multiplying the number of the adsorbed molecules per weight unit of solid with the space required by one gas molecule ... 

Unlike absorption, in which solute molecules diffuse from the bulk of a gas phase to the bulk of a liquid phase, in adsorption, molecules diffuse from the bulk of the fluid to the surface of the solid adsorbent forming a distinct adsorbed phase. 

Experimentally, the kinetics of this reaction has been monitored by STM as reproduced in Fig. 4.11. [28] A Pt(lll) surface precovered with a substantial amount of Oad (a) was exposed to a very low partial pressure (5 x 10-8 mbar) of CO. The latter molecules were adsorbed within the 2 x 2 O unit cells as well as in the empty space between the Oad patches. The progress of the reaction is reflected by the continuous shrinking of the Oad islands. Since the two adsorbates form different ordered phases... 

The existence of multiple peaks for molecular desorption has been attributed to lateral interactions among adsorbed species 62-64). As discussed previously, adsorption onto the surface lattice may occur preferentially in next nearest neighbor sites to form p(2 x 2) structures. Even at low coverages, attractive forces may cause adatoms to occupy next nearest neighbor positions, so that clusters of adsorbate form which have local twofold periodicity 65) with respect to the surface. Such effects are entirely consistent with the perturbations of the surface electronic wave functions due to adsorption 66-68) which show that these binding sites represent the... 

As follows from the initial condition (2.148) no adsorbed form is present on the electrode surface prior to the voltammetric experiment. The boundary condition (2.151) shows that the current is produced by the flux of the dissolved reactant as well as by its adsorbed form. The solution for the R form at the electrode surface is ... 

The dissolved form of O decays to the final electroinactive product via a volume chemical reaction occnrring in the diffusion layer with the volume rate constant (kv), whereas the adsorbed form participates in the surface chemical reaction confined to the electrode surface, characterized by a surface rate constant (kg). These two chemical reactions proceed with different rates due to significant differences between the chenucal nature of dissolved and adsorbed forms of O. Obviously, the mechanisms (2.172)-(2.174) and (2.177) are only limiting cases of the general mechanism (2.178). 

A system of adsorbed particles is often treated as a two-dimensional gas covering the adsorbent surface. Such an approach is quite justified and fruitful, as long as we are dealing with physical adsorption when the influence of the adsorbent on the adsorbate can be regarded as a weak perturbation. In case of chemical adsorption (the most frequent in catalysis), the concept of a two-dimensional gas becomes untenable. In this case the adsorbed particles and the lattice of the adsorbent form a single quantum-mechanical system and must be regarded as a whole. In such a treatment the electrons of the crystal lattice are direct participants of the chemical processes on the surface of the crystal in some cases they even regulate these processes. 

This is not the only subject of debate the nature of the adsorbed forms of oxygen and reaction products on the electrode surface has been widely discussed, as well as the various steps of ORR. Very often, for conditions apparently similar, for example, the same electrode and solution, observations made by two research groups are different and so are, of course, the deductions and the pathways for the electrode reaction. There are so many possible steps, reactions and species that various combinations can be envisaged moreover, as other methods for observing the species or intermediates involved in the electrode reaction are rare, electrochemistry is often the only source of experimental facts. As for other multiparametric problems, when a plausible explanation is found, there is no certainty that this is the only possibility and the correct solution. Moreover, experimental conditions that look identical are not really exactly similar the problem... 

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