Surface reactions coverage-dependent activation energy

Oscillatory surface reaction models were first proposed in the second half of the 1970s and developed in two parallel strains. The buffer-step models originated with the work of Eigenberger (274) and models with coverage-dependent activation energies were first proposed by Belyaev et al. (154). 

The activation energy of reaction (19) was assumed to be linearly dependent on the coverage of the catalyst surface. Physically this can be interpreted either as an effect of the adsorbed species, which interact with one another, or as an effect of surface heterogeneity, which would also cause coverage-dependent activation energies. Analysis of this model revealed regions where bifurcations and oscillations were predicted, and it thus became one of the first successful mathematical models in this field. 

A similar model was analyzed by Pikios and Luss (283). They analyzed the same set of reaction steps with the coverage-dependent activation energy interpreted in terms of surface heterogeneity. They derived criteria for the occurrence of oscillations as did Belyaev et al. (154,162). They also found a singular steady state, which became a limit cycle for values of the surface heterogeneity lying above a certain threshold value, and they performed numerical analyses of these oscillatory states. 

There are some oscillatory surface reaction models that contain neither buffer steps nor coverage-dependent activation energies. The first model able to predict oscillatory behavior without these mechanisms was introduced by Takoudis et al. (275,279) and later was analyzed in great detail by McKarnin et al. (286). The model consists of the following three steps, the third of which is strongly autocatalytic ... 

Ivanov et al. (69) worked with a model involving a coverage-dependent activation energy so that the rate constant for the surface reaction is an exponential function of a reactant coverage. Predictions obtained by the numerical application of both this model and that involving a buffer provide fairly good representations of the experimentally measured oscillations (63). 

With the aid of (B1.25.4), it is possible to detennine the activation energy of desorption (usually equal to the adsorption energy) and the preexponential factor of desorption [21, 24]. Attractive or repulsive interactions between the adsorbate molecules make the desorption parameters and v dependent on coverage [22]- hr the case of TPRS one obtains infonnation on surface reactions if the latter is rate detennming for the desorption. 

We have measured the kinetics of ethylidyne formation from chemisorbed ethylene over Pt(lll) surfaces. The rates of reaction display a first order dependence on the ethylene coverage. There is an isotope effect, since the reaction for CjH is about twice as fast as for CjD. We obtain values for the activation energy of 15.0 and 16.7 Kcal/mole for the normal and deuterated ethylene, respectively. These values are lower than those obtained from TDS experiments, but the differences can be reconciled by taking into account the hydrogen recombination when analyzing the thermal desorption data. 

The simultaneous desorption peaks observed at 560-580 K in TPR are of reaction-limited desorption. The peak temperatures of these peaks do not depend on the coverage of methoxy species, indicating that the desorption rate (reaction rate) on both surfaces has a first-order relation to the coverage of methoxy species. Activation energy (Ea) and the preexponential factor (v) for a first-order process are given by the following Redhead equation [12] ... 

Potentiometric techniques have been used to study autonomous reaction rate oscillations over catalysts and carbon monoxide oxidation on platinum has received a considerable amount of attention43,48,58 Possible explanations for reaction rate oscillations over platinum for carbon monoxide oxidation include, (i) strong dependence of activation energy or heat of adsorption on coverage, (ii) surface temperature oscillations, (iii) shift between multiple steady states due to adsorption or desorption of inert species, (iv) periodic oxidation or reduction of the surface. The work of Sales, Turner and Maple has indicated that the most... 

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