Adsorption sites active

A. Surface Sites, Adsorption Sites, Active Sites. 187... 

Mention was made in Section XVIII-2E of programmed desorption this technique gives specific information about both the adsorption and the desorption of specific molecular states, at least when applied to single-crystal surfaces. The kinetic theory involved is essentially that used in Section XVI-3A. It will be recalled that the adsorption rate was there taken to be simply the rate at which molecules from the gas phase would strike a site area times the fraction of unoccupied sites. If the adsorption is activated, the fraction of molecules hitting and sticking that can proceed to a chemisorbed state is given by exp(-E /RT). The adsorption rate constant of Eq. XVII-13 becomes... 

Another limitation of tire Langmuir model is that it does not account for multilayer adsorption. The Braunauer, Ennnett and Teller (BET) model is a refinement of Langmuir adsorption in which multiple layers of adsorbates are allowed [29, 31]. In the BET model, the particles in each layer act as the adsorption sites for the subsequent layers. There are many refinements to this approach, in which parameters such as sticking coefficient, activation energy, etc, are considered to be different for each layer. 

There are a number of causes of peak asymmetry in both gas and liquid chromatography, including heat of adsorption, high activity sites on the support or absorbent, and nonlinear adsorption isotherms. Assuming that good quality supports and adsorbents are used, and the column is well thermostatted, the major factor causing peak asymmetry appears to result from nonlinear adsorption isotherms. 

It should be noted that within the context of the Langmuir isotherm (energetically equivalent adsorption sites, no lateral interactions) Eq. (6.28), which relates two surface properties, i.e. aj and 0j, remains valid even when the surface activity of Sj, aj, is different from the gaseous activity, pJ5 i.e. when Pj(g) Pj(ad). 

We can think of a heterogeneous catalyst as a collection of active sites (denoted by ) located at a surface. The total number of sites is constant and equal to N (if there is any chance of confusion with N atoms, we will use the symbol N ). The adsorption of the reactant is formally a reaction with an empty site to give an intermediate I (or more conveniently R if we explicitly want to express that it is the reactant R sitting on an adsorption site). All sites are equivalent and each can be occupied by a single species only. We will use the symbol 6r to indicate the fraction of occupied sites occupied by species R, making N6r the number of occupied sites. Hence, the fraction of unoccupied sites available for reaction will be 1 - 0r The following equations represent the catalytic cycle of Fig. 2.7 ... 

Clearly, the sticking coefficient for the direct adsorption process is small since a considerable amount of entropy is lost when the molecule is frozen in on an adsorption site. In fact, adsorption of most molecules occurs via a mobile precursor state. Nevertheless, direct adsorption does occur, but it is usually coupled with the activated dissociation of a highly stable molecule. An example is the dissociative adsorption of CH4, with sticking coefScients of the order 10 -10 . In this case the sticking coefficient not only contains the partition functions but also an exponential... 

The prefactor at 500 K is Sjj = 1.49 x 10 , illustrating the reduction in entropy when a molecule from the gas phase becomes restricted in motion on an adsorption site on the surface. The activation energy has been measured to be slightly negative in this case, i.e. AErneasured = —0.034 eV which should be compared with... 

It is now assumed that the adsorption of NO on Rh(lOO) is associative and not activated. The sticking coefficients S(T) can be set to unity for all temperatures. Furthermore shall we assume that the activation energy for the desorption of NO from Rh(lOO) is 140 kj mol and that two Rh atoms constitute an adsorption site. 

Water exerts both a deactivating and inhibiting influence on Cu and Fe samples, while the reaction over Co is only inhibited. The deactivation of Fe- and Cu-ZSM-5 is clearly due to migration and the sintering of the active component in H2O atmospheres [34]. The Co-ZSM-5 catalyst is much more hydrotheimally stable in wet gas conditions [34,35]. The inhibition by water can be accounted for in a similar way as for CO via competitive adsorption on active sites, like in selective NO reduction studies [34]. For N2O decomposition this yields an expression like eq. (12). At 793 K Kn amounts to about 0.7 kPa". ... 

Kim and Somorjai have associated the different tacticity of the polymer with the variation of adsorption sites for the two systems as titrated by mesitylene TPD experiments. As discussed above, the TiCl >,/Au system shows just one mesitylene desorption peak which was associated with desorption from low coordinated sites, while the TiCl c/MgClx exhibits two peaks assigned to regular and low coordinated sites, respectively [23]. Based on this coincidence, Kim and Somorjai claim that isotactic polymer is produced at the low-coordinated site while atactic polymer is produced at the regular surface site. One has to bear in mind, however, that a variety of assumptions enter this interpretation, which may or may not be vahd. Nonetheless it is an interesting and important observation which should be confirmed by further experiments, e.g., structural investigations of the activated catalyst. From these experiments it is clear that the degree of tacticity depends on catalyst preparation and most probably on the surface structure of the catalyst however, the atomistic correlation between structure and tacticity remains to be clarified. 

The creation of additional sites with an enhanced adsorption of active forms of the oxygen-containing species involved in the slow oxidation step of the organic species chemisorbed on the platinnm snrface (bifnnctional mechanism of catalytic action) ... 

To shed hght on the origin of the enhanced ORR activity, Xu and co-workers performed extensive DFT calculations to investigate the reactivity of the Pt skin [Xu et al., 2004], in particular how oxygen interacts in vacuum with the ordered PtsCo alloy and with a monolayer of Pt formed on the alloy as a model for Pt skin. Figure 9.10 identifies the various adsorption sites for O and O2. Experiments have shown that up to four layers of Pt could sustain a 2.5% compressive strain without creating any surface... 

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