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Surface concentration of intermediates

Rate equations for simple reversible reactions are often developed from mechanistic models on the assumption that the kinetics of elementary steps can be described in terms of rate constants and surface concentrations of intermediates. An application of the Langmuir adsorption theory for such development was described in the classic text by Hougen and Watson (/ ), and was used for constructing rate equations for a number of heterogeneous catalytic reactions. In their treatment it was assumed that one step would be rate-controlling for a unique mechanism with the other steps at equilibrium. [Pg.296]

There was a simple relationship between the rate of caprolactam formation over the range of modified aluminas studied and the surface concentration of intermediate strength acid sites, namely those from which ammonia desorbed in the temperature range 200-350°C. This relationship is shown in figure 6 and establishes a link between acidic sites of intermediate strength and caprolactam formation. Based on these data turnover frequencies were all in the range 0.8-1.8 x 10 3 molecules of caprolactam formed per surface site of intermediate acidity per second. [Pg.540]

Since SSITKA can decouple the apparent rate of reaction into the contribution from the intrinsic activity ( the reciprocal of surface residence time of intermediates) and the nrnnber of active sites ( surface concentration of intermediates), the cause of deactivation of a catalyst during reaction can often be revealed. SSITKA has been used in a number of studies for this purpose. Catalyst deactivation during n-butane isomerization and selective CO oxidation are good examples. Deactivation studies are conducted by collecting isotopic transient data at particular times-on-stream as deactivation occurs. [Pg.198]

The results showed that K promotion resulted in almost 50 times the increase in the reaction rate. TOF based on the amount of hydrogen chemisorption increased by about two orders of magnitude with K-promotion. However, the intrinsic TOF based on SSITKA increased only by a factor of 16. The increase in activity with K promotion was actually due to both a significant increase by a factor of 3 in the surface concentration of intermediates, and an increase by a factor of 16 in the average intrinsic site activity. [Pg.199]

In more complex organic reactions involving several charge transfer or chemical steps in the reaction sequence, it is often possible for one step to have a smaller rate constant than another. If this situation applies to the second or subsequent steps in a sequence, then the steps prior to the ratecontrolling step can usually be regarded as almost at equilibrium, and the surface concentrations of intermediates, 6, can then be expressed as a function of potential, so that there are two parts of the rate equation where potential-dependent terms are involved. [Pg.656]

Considering that the number of active sites on the surface is small compared to the number of reactant molecules in the gas phase, a dynamic steady state is readily established between gaseous and adsorbed species if the intermediate steps are reactive enough. Under conditions where the quasi- or pseudo-steady-state approximation is applicable, the distribution of active sites between occupied and unoccupied forms does not change with respect to time, and thus, surface concentrations of intermediate species can be related to their gas-phase concentrations ... [Pg.23]

Ethylbenzene and toluene are hydrogenated faster than benzene over Cu-ZnO, contrary to the general rule. Such behavior is evidence for a t bonded intermediate, the surface concentration of which increases with the increasing electron-donating ability of the system (5(5). [Pg.119]

Power law expressions are useful as long as the approximate orders of reactant concentration are constant over a particular concentration course. A change in the order of the reaction corresponds to a change in the surface concentration of a particular reactant. A low reaction order usually implies a high surface concentration, a low reaction order, and a low surface reaction of the corresponding adsorbed intermediates. In order to deduce (Eq. (1.17b)) the rate of surface carbon hydrogenation, the power law of Eq. (1.18) has been used. [Pg.14]

A PP sample after ozonization in the presence of UV-irradiation becomes brittle after 8 hrs of exposure, whereas the same effect in ozone is noticeable after 50-60 hours.Degradation of polymer chain occurs as a result of decomposition of peroxy radicals. The oxidation rapidly reaches saturation, suggesting the surface nature of ozone and atomic oxygen against of PP as a consequence of limited diffusion of both oxygen species into the polymer. Ozone reacts with PP mainly on the surface since the reaction rate and the concentration of intermediate peroxy radicals are proportional to the surface area and not the weight of the polymer. It has been found that polyethylene is attacked only to a depth of 5-7 microns (45). [Pg.197]

Inspection of the calculated surface coverage of the intermediate species finally reveals that the surface concentration of the species Rn is typically of the same order of magnitude as that of CH2, i.e., the Q species associated with CO adsorption/conversion. This implies that the coverage of catalytic sites by the synthesis products has a significant influence on CO conversion rate, which conflicts with the traditional approach of developing separate models for CO conversion and products distribution. [Pg.312]

For simplicity we assume that the intermediate stays at the electrode surface, and does not diffuse to the bulk of the solution. Let (j>l0 and 0oo denote the standard equilibrium potentials of the two individual steps, and cred, Cint, cox the surface concentrations of the three species involved. If the two steps obey the Butler-Volmer equation the current densities j and j2 associated with the two steps are ... [Pg.143]

The second most apparent limitation on studies of surface reactivity, at least as they relate to catalysis, is the pressure range in which such studies are conducted. The 10 to 10 Torr pressure region commonly used is imposed by the need to prevent the adsorption of undesired molecules onto the surface and by the techniques employed to determine surface structure and composition, which require relatively long mean free paths for electrons in the vacuum. For reasons that are detailed later, however, this so-called pressure gap may not be as severe a problem as it first appears. There are many reaction systems for which the surface concentration of reactants and intermediates found on catalysts can be duplicated in surface reactivity studies by adjusting the reaction temperature. For such reactions the mechanism can be quite pressure insensitive, and surface reactivity studies will prove very useful for greater understanding of the catalytic process. [Pg.3]

Reaetion (31) suggests acrolein as a key intermediate in SCR of NO by propylene. The formation of nitro species by this reaetion was already diseussed in the literature and evideneed by a significant reduction in the surface concentration of propylene adspecies in the presenee of a NO2 + O2 mixture[133, 134]. Note that the role of organie nitro species as active intermediate in the SCR of NO over Cu-ZSM-5 was already diseussed by Hayes et al.[135]. In addition, in TPR experiments, we observe Cu forming predominantly on the surfaee of Cu-Al-MCM-41 after exposure to CsHg, and a redox of Cu and Cu during the reaetion. It is therefore possible to postulate that the divalent eopper ion is reduced to monovalent in the conditions of the reaetion between adsorbed propylene and NO2 species. [Pg.67]

The above silica-mediated syntheses are limited by the number of silanol groups on the surface, which control the maximum surface concentration of the intermediate [HOs3(CO)io(OSi j [89]. Only loadings up to 4wt% Os/Si02 can be used [90]. Despite this limitahon, circa 300 mg of cluster can be obtained in a single reaction using circa 10 g of silica, which can be recycled after completion of the reaction [13]. [Pg.661]

Determination of the mode of bonding of the allylic intermediate formed would provide direct evidence that an M-O-C species produces acrolein. This is difficult because formation of the allylic intermediate is rate-determining and its subsequent reaction is fast. Thus, the surface concentration of the intermediate is small and not amenable to standard spectroscopic observation. [Pg.23]

When the reaction has adsorbed chemical intermediates, the surface concentration of which is potential dependent, the situation is difficult and was first put into a quantitative theory by Conway and Gileadi in 1962 and in more detail by Srinivasan and Gileadi in 1967. However, these pioneer authors dealt with submonolayers of simple entities such as H. How to deal with the potential-dependent intermediates in such a (still fairly simple) reaction such as methanol oxidation is not yet in sight (It can be done in principle, but there is still no knowledge of the kinetics of the reactions of the radical intermediates and how they are connected to the sweep rate.)... [Pg.709]

The first difficulty involves steady state. In this book, it has been stressed that fundamental electrochemistry is not limited to simple redox reactions with no adsorbed radical intermediates, and that we must accept the indisputable fact that most electrode reactions involve intermediates, and their concentration depends not only on the electrode potential but also on time during the potential sweep. Thus, unless the surface concentration of the intermediates is negligible or their relaxation times much faster than those of the sweep rate, the steady-state value of 0j, 02, etc. (of the various adsorbed radicals) may not be felt by the current registered in the sweep. However, when one writes a reaction sequence ... [Pg.718]

A modification of this simple adsorption theory must be made if there are other surface levels present. If the surface concentration of these levels is very large compared to the concentration of ions to be adsorbed, one would expect the adsorption to more closely resemble that on a clean metal, as electron transfer between the various surface traps may predominate over transfer between the adsorbed ions and the bulk semiconductor. If the number of these traps is small compared to the amount of adsorption, one would expect the adsorption characteristics to resemble those for the theory discussed above. Intermediate cases are also possible. [Pg.266]

M-butane proceeds via an intermolecular mechanism with 2-butene involved intermediately.300-303 The role of the transition metal promoters such as Fe and Mn was shown to increase the surface concentration of the intermediate butene 304 The formation of butene is speculated to occur through an oxidative dehydrogenation on the metal site305 or by one-electron oxidation.306... [Pg.195]

After perturbation, the surface concentrations of the reactants, intermediates, and products attain new values. In a genuine stationary technique, these new values are maintained by providing a constant rate of mass transport (hydrodynamic techniques). The surface concentration, c[ of a species i is related to its flux, J, by... [Pg.210]

When a reaction occurs in an ideal system (i.e., in ideal gas mixture, ideal solution, or ideal adsorbed layer), then rs and r s in (44) are determined by simple mass action law. We shall call linear the stages whose rate, = rs - r s, depends linearly on the concentrations of intermediates (including free sites of the surface) the stages whose rate depends nonlinearly on the concentrations of intermediates (i.e., includes squares of concentrations of... [Pg.195]

The term reaction centre may be used to include both vacant and occupied catalytic sites. The sum of the surface concentrations of reaction centres on the surface of a catalyst is a constant L. Thus, if species m at a surface concentration Lm is the most abundant surface intermediate, Lm + Lv a L, where Lv is the surface concentration of vacant reaction centres. [Pg.375]

It follows from Equation 6.12 that the current depends on the surface concentrations of O and R, i.e. on the potential of the working electrode, but the current is, for obvious reasons, also dependent on the transport of O and R to and from the electrode surface. It is intuitively understood that the transport of a substrate to the electrode surface, and of intermediates and products away from the electrode surface, has to be effective in order to achieve a high rate of conversion. In this sense, an electrochemical reaction is similar to any other chemical surface process. In a typical laboratory electrolysis cell, the necessary transport is accomplished by magnetic stirring. How exactly the fluid flow achieved by stirring and the diffusion in and out of the stationary layer close to the electrode surface may be described in mathematical terms is usually of no concern the mass transport just has to be effective. The situation is quite different when an electrochemical method is to be used for kinetics and mechanism studies. Kinetics and mechanism studies are, as a rule, based on the comparison of experimental results with theoretical predictions based on a given set of rate laws and, for this reason, it is of the utmost importance that the mass transport is well defined and calculable. Since the intention here is simply to introduce the different contributions to mass transport in electrochemistry, rather than to present a full mathematical account of the transport phenomena met in various electrochemical methods, we shall consider transport in only one dimension, the x-coordinate, normal to a planar electrode surface (see also Chapter 5). [Pg.139]

The laws of conservation for the catalyst amount c3 + c4 + c5 = 6, = const, and the gas pressure cx + c2 = b2 = const, along with the natural conditions of non-negativity for c account for a convex polyhedron. This polyhedron determined by fixed values of the balances, in this case catalyst and pressure balances, is a balance polyhedron D0. Unlike the polyhedron D, the structure of the balance polyhedron D0 is, as a rule, rather simple (formally D0 is a particular case of reaction polyhedra). If there exists only one type of active site for the catalyst and accordingly one law of conservation with the participation of concentrations of intermediates, then D0 is a product of two simplexes D0(gas) and D0(surf). The dimensions of 2)0(gas) and D0 (surf) is a unit lower than the number of the corresponding substances, gaseous or those on the catalyst surface. Thus in the case under consideration, B0 consists of the vectors... [Pg.144]


See other pages where Surface concentration of intermediates is mentioned: [Pg.123]    [Pg.266]    [Pg.196]    [Pg.540]    [Pg.194]    [Pg.201]    [Pg.48]    [Pg.123]    [Pg.266]    [Pg.196]    [Pg.540]    [Pg.194]    [Pg.201]    [Pg.48]    [Pg.110]    [Pg.33]    [Pg.212]    [Pg.8]    [Pg.631]    [Pg.427]    [Pg.107]    [Pg.173]    [Pg.222]    [Pg.59]    [Pg.92]    [Pg.166]    [Pg.169]    [Pg.195]    [Pg.251]    [Pg.257]    [Pg.260]    [Pg.281]    [Pg.577]    [Pg.20]   
See also in sourсe #XX -- [ Pg.194 , Pg.198 ]




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