Prior Dissociation, Forward Reaction Direction

Thus to expand the potential dependence of the surface activity of the reactant (which is an intermediate) of the rds, and ultimately that of the forward, reductive reaction direction, we again build up progressively from the initial reactants. The potential dependence of Group I reaction steps would be exactly that which was evaluated previously (Section IV) for the simple consecutive reaction sequence, since there is no change in molecularity between [or stoichiometric coefficients (see footnote g) of] reactant and product for any of these reaction steps. Thus the potential dependence of the activity of the reactant of the dissociation step ] is given by Eq. (29) (but where i - 1 is used as the limit for the summation and product in that equation), i.e. 

Assuming the quasi-equiUbrium approximation, the surface activity of the product of the dissociation step (step i) is defined by equating the rates of the forward and revise directions of that reaction step  

To continue, we now consider Group II reaction steps (i.e., those between the dissociation step and the rds). The quasi-equilibrium expression for the first of these, step t -h 1, is 

When this relation is rearranged to solve for the surface activity of the product of this step, the stoichiometric number will cancel. Although this step and in fact all others in Group II individually occur v times, their potential dependence evidently does not involve v, and hence the expression for the Group II reaction steps, steps t-H 1 through rds - 1, will [analogously to Eq. (29)] be given by 

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Here, s represents a free surface site and subscripts a and g represent adsorbed and gas-phase species, respectively. It is assumed that CO adsorbs on two sites prior to dissociation on the same sites and that hydrogen competes for the same sites. It is assumed that all steps prior to the rate-determining steps are in equilibrium. Reactions (21) and (25) were assumed to occur only in one direction and to have the same rates. The assumption that equation (21) is not in equilibrium would appear to be the main difference between the approach of Van Meerten et al. and that put forward by Ross.181 Van Meerten et al proceeded to consider a series of different equations which they then attempt to fit to their data. They showed that the best fit to their data was obtained if the rate determining step was the combination of CHa and Ha... 

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