Equilibrium constants for surface reactions

Kr Kinetic equilibrium constant for surface reaction Exam. 10.3... 

R is a measure of the energy absorbed by a surface plasmon, K is the equilibrium constant for a reaction between two polymers, A and B... 

In the general case the value of K appearing in the driving force term is the product of the equilibrium constant for the surface reaction Kr and the product of the adsorption equilibrium constants for the reactants divided by the product of the adsorption equilibrium constants for the reaction products. 

The results found in the adsorption studies are given in Table II. From these results the surface equilibrium constants for the reactions given in Table III were determined using the SURFEQL program. In these calculations it is assumed that the Ca2+, Mg2+, Cl, SO,2, phosphate and silicate surface species are those found previously to occur on the a-FeOOH surface (33, 34). The phthalate and salicylate complexes =FeA and sFeAH (where A7 is phthalate or salicylate) were considered in these calculations. 

The equilibrium constant for the reaction between methanol on surface sites and internal sites, K, is the most complex in its temperature and acetylation dependence. In some coals temperature dependences shift about from exothermic to endothermic reactions, and no overall pattern for high rank and low rank coals seems to exist. 

Deriving equilibrium constants for surface complexation reactions based on experimental data can be facilitated with paramater optimization programs such as FITEQL (Westall, 1982). Ideally, surface site density is known however, it can also be adjusted simultaneously with LogK s to... 

Equilibrium constants for these reactions are indicated byK The total capacity of the surface for adsorption of H+ ions and the equilibrium constants are determined by potentiometric titration, which involves addition of a known amount of acid or base and then measuring the change in solution pH. The difference between the amount of titrant added and its concentration in the solution represents the amount of H+ that has adsorbed onto, or reacted with, the surface. 

There are two approaches to the equilibrium constants of surface reactions. Originally, the so-called intrinsic approach was developed [28]. For example, binding of a proton to a surface site was separated into two steps. In the first step the proton was transferred from the bulk of solution (aq) to the space in the vicinity of the surface site (int). This equilibrium was treated by Boltzmann statistics, so that the distribution was affected by the electrostatic potential of that space (( jnt). The second process was chemical binding with the surface site represented by so-called intrinsic equilibrium constant (Ki t) ... 

C , = total concentration of active centers, in moles per gram of catalyst = specific reaction-rate constant for surface reaction Kqo = adsorption equilibrium constant for CO... 

In triple layer approximations the location of each adsorbate with respect to the surface must be specified. Protons and all ions assumed bound as inner-sphere complexes (specifically or chemically adsorbed species) are assumed to lose part of their hydration sheaths, bonding directly to sites in the surface itself. Adsorbates assumed to remain hydrated, forming outer-sphere surface complexes, are assigned to the OHP. In the intrinsic equilibrium constants for adsorption reactions, K ", the activities of ions transferred from solution to the surface are corrected for the electrical potential they experience, % or (27). 

Table I. Reactions and Selected Intrinsic Equilibrium Constants for Surface Ionization and Electrolyte Adsorption...

On the far right of the numerator we have two parameters that may be difficult to obtain independently. They are the adsorption equilibrium constant for B and the surface equilibrium constant for the reaction Asurf Bsurf- Our goal is to clear these by reexpressing them in terms of something that is unchanging. After all both of these may be strong functions of the catalyst structure and composition. The overall reaction A B is, however, one which is fixed at any temperature and pressure by the overall equilibrium constant. This is independent of the catalyst. Therefore we want to use this in the reaction rate expression. Here is how we do it ... 

The dimensionless equilibrium constant for chemical reaction on the catalytic surface is given by the kinetic rate constant ratio, forward, smf. rx/ backward, surf, rx-When lEtHougen Watson 0 in equation (22-36), one arrives at a relation between all of the equilibrium constants ... 

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