Bimolecular surface reactions reactants adsorption

Bimolecular surface reactions reactants adsorption, 29 111-112 with single reactant, 29 108-109 1,1 -Binaphthyl, dehydrocyclization, 28 318 Binary oxides, 32 119 Binding energy, 32 160-162 chemisorbed sulfur, 37 281 hydrogen, sulfur effect, 37 295-296 shift, Pd, 37 62-64 ZnO/SiOj, 37 21-22 Binor-S, see Norbomadiene Biological systems, hydrogen in, activation of, 11 301... 

Steps 10 and 11 of Table I give the appropriate expressions for C to be used in Eq. (10) when the adsorption of only one reactant is significant. If in a bimolecular surface reaction a significant amount of each reactant is adsorbed, then the number of unoccupied sites is consequently small. Therefore,... 

In catalysis the reasons for all kinetic behaviour lie in the behaviour of surface coverage by the reactants. This means that the kinetics of certain catalytic reactions -and the catalytic oxidation of CO via a bimolecular surface reaction is one of than - do not depend directly on gas phase concentrations. To understand Figures 12.1 and 12.2 we need to examine the behaviour of 6co and dca as expressed by equations 12.4 and 12.5. It is only by understanding the behaviour of the fractions of surface covered by adsorbed species that an understanding of any catalytic reaction can be gained. Since at present there is no way to measure the adsorption isotherms of reacting species at high temperatures, we need a reliable mechanistic rate expression to examine this aspect of the reaction. An appropriate mechanistic rate expression will permit us to reliably simulate the behaviour of the isotherms from kinetic data. 

When reaction occurs between molecules of A and B adsorbed on the same type of site, and when one or both reactants are not at adsorption desorption equilibrium with the gas phase, the kinetic data may show some unusual effects. The following treatment is for a bimolecular surface reaction with weak adsorption of the product ... 

In these expressions is the rate of adsorption of species j, which for A may be written as A + S AS, where A is the gas-phase molecule. S is an empty site on the surface, and AS is the adsorbed molecule. We can consider adsorption as a bimolecular chemical reaction that is proportional to the densities of the two reactants A and S to give... 

Many catalyzed surface reactions can be treated as a two-step process with an adsorption equilibrium followed by one rate-determining step (diffusion, surface reaction, or desorption). The surface reaction kinetics are usually discussed in terms of two limiting mechanisms, the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. In the LH mechanism, reaction takes place directly between species which are chemically bonded (chemisorbed) on the surface. For a bimolecular LH surface reaction. Aawith competitive chemisorption of the reactants, the rate of reaction is given by the following expression ... 

A classical example of LHHW equations is the bimolecular reaction case, in which the surface reaction between the adsorbed reactants is the rate-limiting step. The adsorption and desorption steps are presumed to be rapid enough to reach quasi-equilibria. For instance, the overall reaction A -h B C comprises the steps... 

When both reactants were coadsorbed on Rb-X at 308 K, indications for the formation of reaction products or bimolecular complexes were not found in the IR spectra. The spectra rather suggest that toluene and methanol are independently sorbed. It should be noted, however, that after equilibration with equal partial pressures of both reactants, toluene was the main sorbed species. Note that only part of the sites can be covered by toluene molecules due to steric reasons (theoretically 2/3 of the cations are accessible) and pore filling, while methanol achieved a coverage of approximately one molecule/cation at elevated partial pressures (p = 1 mbar). Coadsorption of toluene onto a surface preequilibrated with methanol resulted in the displacement of the main fraction of the methanol molecules (80 %) from the sorption sites [24].The same coadsorbed state was reached irrespective of the sequence of adsorption of the two reactants. If toluene was adsorbed first, coadsorption of methanol did not change the coverage of toluene. 

In contrast, toluene and methanol coadsorbed on Rb-X do not form a bimolecular precursor complex and both reactants seem to be independently adsorbed at the surface. It should be noted, however, that after equilibration of the catalyst with equal partial pressures of both reactants, toluene was the main adsorptive. During toluene methylation, sorbed toluene was again the main surface species, the reaction rate, however, was proportional to the surface concentrations of both chemisorbed species (toluene, formaldehyde). The onset of the reaction was observed at much higher temperatures than in the ring alkylation which is at large ascribed to the indispensable conversion of methanol to a formaldehyde (or formate) species. 

Negative reaction orders are sometimes observed for bimolecular reactions on solid catalysts. Increasing the partial pressure of one reactant. A, which is strongly adsorbed, can lead to a surface mostly covered with adsorbed A, leaving little space for adsorption of reactant B. However, the negative order for A would change to zero order and then to a positive order as the partial pressure of A is reduced to very low values. Reactions that show negative order because of competitive adsorption are discussed in Chapter 2. 

If adsorption of reactant A is the rate-limiting step of a bimolecular reaction, molecules of A will react almost immediately after being adsorbed, and there will be hardly any A on the surface. The other reactant, B, and the product, C, may be at equilibrium coverage. The reaction rate is then proportional to the partial pressure of A and the fraction of vacant sites ... 

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