Mobility bimolecular surface reactions

Bimolecular surface reaction mobile reactants rotating activated complex (42)... 

Microkinetic modeling assembles molecular-level information obtained from quantum chemical calculations, atomistic simulations and experiments to quantify the kinetic behavior at given reaction conditions on a particular catalyst surface. In a postulated reaction mechanism the rate parameters are specified for each elementary reaction. For instance adsorption preexponential terms, which are in units of cm3 mol"1 s"1, have been typically assigned the values of the standard collision number (1013 cm3 mol"1 s 1). The pre-exponential term (cm 2 mol s 1) of the bimolecular surface reaction in case of immobile or moble transition state is 1021. The same number holds for the bimolecular surface reaction between one mobile and one immobile adsorbate producing an immobile transition state. However, often parameters must still be fitted to experimental data, and this limits the predictive capability that microkinetic modeling inherently offers. A detailed account of microkinetic modelling is provided by P. Stoltze, Progress in Surface Science, 65 (2000) 65-150. 

The pre-exponential term (cm mol s ) of the bimolecular surface reaction in case of immobile or mobile transition state is 10 The same number holds for the bimolecular surface reaction between one mobile and one immobile adsorbate, producing an immobile transition state. 

One can, assuming the mechanism is known, turn the equation around and then calculate the site density. We and others have done just that ( > ). Calculations have been made for both saturated and unsaturated surfaces, for both unimolecular and bimolecular reactions, for reactions in which the adsorbed molecule is immobile and also where it is mobile, and for reactions carried out at both low-conversion and high-conversion conditions. 

Since the rate constants of bimolecular diffusion-limited reactions in isotropic solution are proportional to T/ these data testify to the fact that the kt values are linearly dependent on the diffusion coefficient D in water, irrespective of whether the fluorophores are present on the surface of the macromolecule (human serum albumin, cobra neurotoxins, proteins A and B of the neurotoxic complex of venom) or are localized within the protein matrix (ribonuclease C2, azurin, L-asparaginase).1 36 1 The linear dependence of the functions l/Q and l/xF on x/t] indicates that the mobility of protein structures is correlated with the motions of solvent molecules, and this correlation results in similar mechanisms of quenching for both surface and interior sites of the macromolecule. 

The expression for the rate of a bimolecular reaction of adsorbed particles when the latter have a rapid surface mobility can be obtained in the traditional way for chemical kinetics if the law of mass action is used for large quasi-particles. Let us consider the reaction AZ+BZ on a homogeneous surface containing three species of particles (s = 3) A, B, and Y (the real properties of the vacant sites Y are taken into account in the final expressions). Particles of A and B are at neighboring sites of a lattice and enter the c.s. of one another. 

Cordes [21] extended the theoretical analysis to consider intracrystalline reactions including bimolecular processes. The treatment assumed that the energy of a molecule was not altered when a nearest neighbour became an activated complex, and that the only influence of the product species is to define an interface. In qualitative terms, low values of A were associated with strongly held ("tight") surface complexes and higher values with more mobile complexes. 

It is generally accepted that acid aldol condensations proceed bimolecularly between an activated carbonyl molecule adsorbed on a Br0nsted acid site and a mobile species and that formation of enols is involved in the reaction sequence. However, the way these species form and their nature, for example, surface or gas-phase species or the extent of the proton transfer in the activated molecule, is still a matter of controversy. 

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