Abstract
The kinetics of the dissociation of carbon monoxide on clean and modified molybdenum (110) surfaces has been investigated by means of isothermal and temperature-ramped photoelectron spectroscopy (UPS and XPS). In contrast to most previous studies both the activation energy E and the prefactor A were determined, in the expression for the rate constant k r = ν N m = A exp(− E/kT) , where ν is the rate of the reaction and N m is the amount of molecular adsorbate. Results were obtained on unmodified Mo and in the presence of C, O, S, or K. The results show that co-adsorbates strongly influence E and A, which vary from about 0.82 eV and 10 12 s −1 in the presence of sulfur to about 0.37 eV and 10 4 s −1 in the presence of carbon, but the effects on k r tend to compensate. Also, the reaction poisons itself since the combined effect of atomic (C + O) is to lower the rate constant. As expected, sulfur poisons the reaction. However, this is not because k r for dissociation is reduced but because the desorption probability of undissociated CO increases. Potassium increases the adsorption probability but is not a very effective promoter on Mo(110), probably because CO dissociation precedes desorption on this surface even in the absence of potassium. A phenomenological rate equation is introduced in order to account for the observed kinetics, including the compensation effect.
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