Adsorbed species, mobile

The sequence of events in a surface-catalyzed reaction comprises (1) diffusion of reactants to the surface (usually considered to be fast) (2) adsorption of the reactants on the surface (slow if activated) (3) surface diffusion of reactants to active sites (if the adsorption is mobile) (4) reaction of the adsorbed species (often rate-determining) (5) desorption of the reaction products (often slow) and (6) diffusion of the products away from the surface. Processes 1 and 6 may be rate-determining where one is dealing with a porous catalyst [197]. The situation is illustrated in Fig. XVIII-22 (see also Ref. 198 notice in the figure the variety of processes that may be present). 

The flux of flie adsorbed species to die catalyst from flie gaseous phase affects die catalytic activity because die fractional coverage by die reactants on die surface of die catalyst, which is determined by die heat of adsorption, also determines die amount of uncovered surface and hence die reactive area of die catalyst. Strong adsorption of a reactant usually leads to high coverage, accompanied by a low mobility of die adsorbed species on die surface, which... 

In order for the reaction to proceed, hydrogen adsorption must be followed by its diffusion over the surface amongst other mobile adsorbed species... 

When the temperature of the analyzed sample is increased continuously and in a known way, the experimental data on desorption can serve to estimate the apparent values of parameters characteristic for the desorption process. To this end, the most simple Arrhenius model for activated processes is usually used, with obvious modifications due to the planar nature of the desorption process. Sometimes, more refined models accounting for the surface mobility of adsorbed species or other specific points are applied. The Arrhenius model is to a large extent merely formal and involves three effective (apparent) parameters the activation energy of desorption, the preexponential factor, and the order of the rate-determining step in desorption. As will be dealt with in Section II. B, the experimental arrangement is usually such that the primary records reproduce essentially either the desorbed amount or the actual rate of desorption. After due correction, the output readings are converted into a desorption curve which may represent either the dependence of the desorbed amount on the temperature or, preferably, the dependence of the desorption rate on the temperature. In principle, there are two approaches to the treatment of the desorption curves. 

Immobilization by adsorption onto a surface such as activated carbon or to an ion-exchange resin gives a reversible and relatively weak bond, but this can be sufficient to increase the retention time in a flow system to acceptable levels. Recall Section 10.6 where it is shown that the residence time of an adsorbed species can be much larger than that of the mobile phase, in essence giving more time for catalysis. 

Often the adsorbed species are bound rather strongly and can be considered immobile at the bottom of a vibrational well. The transition state may, however, have several possibilities, being, for example, a precursor that is highly mobile in two... 

Chemisorption of benzene at 297°C on Ni(110) occurred in a rather different manner. Several patterns, some streaked, were observed, and they followed the same sequence and showed the same behavior as those obtained when acetylene was chemisorbed on this surface (29). These structures have not been fully elucidated, but the streaked patterns suggest (i) that the mobility of adsorbed species along the "furrows of the (110) face is easier than their mobility across them, and (ii) that dissociation of the carbon skeleton of benzene and the formation of other structures occurs. 

The nature of the adsorbed species can be inferred from the usual chemical parameters, i.e. chemical shifts, linewidths and relaxation times. These latter allow the study of the mobility on the surfaces. As an analytical tool, C-NMR spectroscopy can also be used to determine the concentration of reactants or products as a function of time and hence kinetic constants can easily be determined. As a conclusion, a rather complete kinetic study can be carried out involving the nature of interaction between the admolecule and the surface and eventually the nature of the surface active centers. One can finally arrive at the proposition of a reaction mechanism. 

Suppose that we attempt to devise a model so that log L = 15, an acceptable value, for Example 4. Let mobile atoms be the adsorbed species. The value of 19 for Step 2 must be decreased by four units. According to Table II, the gas must lose 36.8 e.u. (that is, 4 x 9.2 e.u.) more than is postulated for Step 2. But S for Nj at 690 K and 0.16 atm [Eq. (79)] is only 56.6 e.u. A loss of only 19.8 e.u. (that is, 56.6 e.u. — 36.8 e.u.) upon adsorption seems impossible, since the rotational loss alone (which must be included since the model calls for dissociation into atoms) is 12.9 e.u. The difficulty with Example 4 is that an activation energy of 52 kcal mole is extremely large. We cannot choose a possible rate-determining step from the data. 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

A significant advantage of the methodology of NMR spectroscopy is that it allows application of pulse sequences for the discrimination of nuclei in specific local structures, if these nuclei are characterized by a coupling with other nuclei. Examples are dipolar-dephasing techniques such as those used in TRAPDOR experiments (Section II.C). CP experiments can be applied for the discrimination of nuclei in various structures, for example, for the identification and investigation of strongly adsorbed species with low mobility. 

No cross ozonide was formed from unsymmetrical alkenes. The authors theorized628 that the carbonyl oxide zwitterionic species formed on wet silica gel immediately adds water followed by rapid decomposition of the intermediate hydroxyalkyl hydroperoxide to carboxylic acid and water. It means that water on silica gel acts as participating solvent. In the absence of adsorbed water, rapid recombination of the adsorbed aldehyde and carbonyl oxide due to a favorable proximity effect gives normal ozonide. The low mobility of adsorbed species on the silica surface accounts for the absence of cross ozonides. 

The other extreme of adsorbate behavior is a very mobile surface species. In this case there is only a very shallow potential well surrounding each adsorption site. The adsorbed species are basically free to traverse the surface as a 2D gas. ... 

Sorption capacity is one of the major properties used for industrial applications of zeolites. H. Lee reviews the aspects of zeolites used as adsorbents. The other papers in the section deal with the theory of sorption and diffusion in porous systems, the variation of sorption behavior upon modification, and the variation of crystal parameters upon adsorption. NMR and ESR studies of sorption complexes are reported. H. Resing reviews the mobility of adsorbed species in zeolites studied by NMR. 

There is, therefore, much evidence that the constituents of many solids attain mobility during participation in heterogeneous reactions and this mobile material may enter directly [e.g., (104)], or possibly indirectly, into the steps required for the conversion of reactants to products. The absorption of gaseous reactants is expected to modify the electronic structure of the solid, thus influencing surface properties, including both quantities and reactivities of adsorbed species. 

Immobile, mobile. These terms are used to describe the freedom of the molecules of adsorbate to move about the surface. Adsorption is immobile when kT is small compared to AE, the energy barrier separating adjacent sites. The adsorbate has little chance of migrating to neighbouring sites and such adsorption is necessarily localised. Mobility of the adsorbate will increase with temperature and mobile adsorption may be either localised or non-localised. In localised mobile adsorption, the adsorbate spends most of the time on the adsorption sites but can migrate or be desorbed and re-adsorbed elsewhere. In non-localised adsorption the mobility is so great that a small fraction of the adsorbed species are on the adsorption sites and a large fraction at other positions on the surface. 

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