Surface reactions Activation energies

E Activation energy for reaction on the surface oxide, Kcal/mol... 

The affect of diffusion on catalyst selectivity in porous catalysts operating under non-isothermal conditions has been examined by a number of workers. The mathematical problem has been comprehensively stated in a paper [21] which also takes into account the affect of surface diffusion on selectivity. For consecutive first-order exothermic reactions, the selectivity increases with an increase in Thiele modulus when the parameter A (the difference between the activation energy for reaction... 

The activation energy for Reaction (2) is 18-19 kcal/mole. It has been assumed that this is the energy required to remove ammonia from the surface of the solid NH3 -NI3. After removal of the ammonia the Nl3 that is left is unstable and decomposes with the liberation of heat. The explosion then grows from this hot region the reactions proposed being either... 

A proper description of heterogeneously catalyzed oxidation reactions must treat several difficult problems simultaneously. First is the characterization of the solid surface in its reactive state. What oxygen species exist on this surface and what reactions does each species undergo What other sites for adsorption are present Second is the problem of reaction path. What steps are involved in the reaction What are the structures and relative energy contents of the intermediates Third is the problem of reaction velocity, a general and difficult problem in all chemistry. What are transition states, activation energies, and reaction probabilities for the various steps ... 

Two species of molecules reacting on the surface of a catalyst may both be bound by chemisorption forces, or it may be that only one of the reacting species is bound. In the latter case—which is known as the Rideal-mechanism—both sorts of molecules hit the surface of the catalyst, but only one of the species is chemisorbed. The molecules of the other sort hit the chemisorbed molecules and form an activated complex which leads to reaction. They may, however, also be adsorbed by van der Waals forces and react with the chemisorbed reaction partner from a van der Waals layer. It may be stated that entropy considerations show that such reactions will proceed more easily the smaller the mobility of the adsorbed molecules is, other quantities, such as the activation energy of reaction, being the same. 

The experimental data of Kowalsky on 2H2 + O2 mixtures have been analyzed by Semenov [59] on the basis of reactions (i)—(v), together with surface destruction of H atoms. Because of the lower activation energies of reactions (i) and (hi) compared with reaction (ii), the concentrations of OH and O were assumed to be small compared with H. The variation of H atom concentration could thus be deduced by the method of partial stationary state concentrations [60], giving the net branching factor 0 at pressure p as... 

The activation energy of reaction (19) was assumed to be linearly dependent on the coverage of the catalyst surface. Physically this can be interpreted either as an effect of the adsorbed species, which interact with one another, or as an effect of surface heterogeneity, which would also cause coverage-dependent activation energies. Analysis of this model revealed regions where bifurcations and oscillations were predicted, and it thus became one of the first successful mathematical models in this field. 

The mechanism and kinetics of the NO + CO reaction on Rh(lll) have been discussed in detail by Zhdanov and Kasemo (108). They showed that simulations based on surface science data obtained at low pressures reproduce the scale of the reaction rate at the pressure regime of interest for the TWC but fail to predict accurately the apparent activation energy and reaction orders. 

Absolute reaction rate theory considers the formation of an activated complex intermediate between reactants and products (Figure 25). The activated complex can decay to products or back to reactants. Heterogeneous catalysis considers the adsorption of reactants as a means of lowering the activation energy of reaction which occurs on the surface followed by desorption of the products. The mechanism of development can be considered as a combination of these ideas. 

Fig. 2. Dependence of the activation energies for reaction of electrophilic substitution of proton in surface silanol groups vs. the proton-accepting properties of atoms bound with silicon (a) and the values of 3750 cm" shifts (b) after adsorption of organosilicon compounds 1 - SitCHj) 2 - (CH3)3SiCl 3 - (CH3>3SiOCH3 4 -(CH3)3SiN(CH3)j.

It is assumed that this is also a rate determining step for the overall reaction. The activation energy of reaction (4) and the site density of oxygen active centers were the only adjustable parameters of the model. In general, a C-H bond scission for reactants and products of the methane dimerization process occurs by an Eley-Rideal (E-R) type mechanism to form a gas-phase alkyl radical and a hydroxyl surface site (HO ) ... 

It follows from Table 6 that the Eg values, calculated for a dissociative addition to siloxane bonds of silica surface at the interaction with (CH3)3SiX (X = N3, NCS and NCO) compounds, are by 10 to 20 kJ-mole lower than the activation energies of reactions of these reactants with the silica surface OH groups. The lowest Eg value in the Aji process is displayed by the reaction with X = NCS that features the weakest Si X bond and a substituent with the highest electron acceptor ability, while the reactions with X = NCO and X = N3 occur to the close activation energies, which, probably, is related to the close strength of Si-X bond in these reactants. 

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