Reactive systems surface reaction

How are fiindamental aspects of surface reactions studied The surface science approach uses a simplified system to model the more complicated real-world systems. At the heart of this simplified system is the use of well defined surfaces, typically in the fonn of oriented single crystals. A thorough description of these surfaces should include composition, electronic structure and geometric structure measurements, as well as an evaluation of reactivity towards different adsorbates. Furthemiore, the system should be constructed such that it can be made increasingly more complex to more closely mimic macroscopic systems. However, relating surface science results to the corresponding real-world problems often proves to be a stumbling block because of the sheer complexity of these real-world systems. 

Elementary steps on surfaces and in condensed phases are more complex because the environment for the elementary reactions can change as the composition of the reaction mixture changes, and, in the case of surface reactions, there are several types of reactive sites on solid surfaces. Therefore, the rate constants of these elementary steps are not really constant, but can vary from system to system. Despite this complexity, the approximation of a single type of reaction step is useful and often generally correct. 

In the second class of systems the reaction is such that it involves little or no change of the molecular geometry in the vicinity of the reacting sites, nor of the external shape of the crystal. The concept of the reaction cavity is useful in this context (184). This cavity is the space in the crystal containing the reactive molecule(s), and its surface defines the area of contact between this molecule and its immediate surroundings. Only if the shape of this cavity is little altered as reaction proceeds will the activation energy for the process be reasonably small and the rate of reaction nonzero. 

In this article, we present an ab initio approach, suitable for condensed phase simulations, that combines Hartree-Fock molecular orbital theory and modem valence bond theory which is termed as MOVB to describe the potential energy surface (PES) for reactive systems. We first provide a briefreview of the block-localized wave function (BLW) method that is used to define diabatic electronic states. Then, the MOVB model is presented in association with combined QM/MM simulations. The method is demonstrated by model proton transfer reactions in the gas phase and solution as well as a model Sn2 reaction in water. 

We have studied above a model for the surface reaction A + 5B2 -> 0 on a disordered surface. For the case when the density of active sites S is smaller than the kinetically defined percolation threshold So, a system has no reactive state, the production rate is zero and all sites are covered by A or B particles. This is quite understandable because the active sites form finite clusters which can be completely covered by one-kind species. Due to the natural boundaries of the clusters of active sites and the irreversible character of the studied system (no desorption) the system cannot escape from this case. If one allows desorption of the A particles a reactive state arises, it exists also for the case S > Sq. Here an infinite cluster of active sites exists from which a reactive state of the system can be obtained. If S approaches So from above we observe a smooth change of the values of the phase-transition points which approach each other. At S = So the phase transition points coincide (y 1 = t/2) and no reactive state occurs. This condition defines kinetically the percolation threshold for the present reaction (which is found to be 0.63). The difference with the percolation threshold of Sc = 0.59275 is attributed to the reduced adsorption probability of the B2 particles on percolation clusters compared to the square lattice arising from the two site requirement for adsorption, to balance this effect more compact clusters are needed which means So exceeds Sc. The correlation functions reveal the strong correlations in the reactive state as well as segregation effects. 

We conclude, therefore, that the identification of A and E with the concentration of the surface precursor to product formation and the energy barrier to a bond redistribution process in the dominant step of a surface reaction, respectively, is not always or necessarily justified and may not be a realistic representation of the kinetics of a surface change. More direct information concerning the concentrations and reactivities of surface intermediates is required to substantiate meaningfully the kinetic properties of reactions proceeding on surfaces. Such considerations also call into question the application of the transition state theory to systems for which the transition complex has not been characterized unambiguously. 

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