The dissociation of CO on modified Mo(110) surfaces

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.