The Kinetics and Mechanisms of Alkali Metal-Promoted Surface Reactions.
More than one third of all commercial chemicals are produced using catalysts that require alkali metal additives to achieve optimum efficiency; however, it is not known how the alkali metal promotes these reactions. In this thesis the adsorption and reaction of diatomic molecules on single crystal transition metal surfaces precovered with submonolayers of potassium have been studied in ultra -high vacuum with electron energy loss spectroscopy (EELS) and a host of complementary surface characterization techniques in order to elucidate the kinetics and mechanisms of alkali metal-promoted surface reactions. A study of CO adsorbed on K-precovered Ni(110) reveals that surface order can be important in determining the properties of molecules coadsorbed with alkali metal atoms; attractive interactions may lead to the formation of islands of fixed adsorbate:alkali metal stoichiometry. The adsorbate molecules within the islands are strongly perturbed, with distinctive characteristics that are insensitive to changes in coverage, while those outside the islands are only weakly perturbed, with properties characteristic of the alkali metal-free surface. In studies of both N_2 and K on Fe(111) and NO and K on Rh(100) it is found that K precoverage promotes the adsorption of and stabilizes the parallel bonded (or highly inclined) dissociation precursor. These effects can account for the K-induced enhancement of the overall N_2 dissociation rate on Fe(111) independent of any reduction in the intramolecular bond strength; an increase in the rate of the dissociation step is not necessary. Using time resolved EELS (TREELS), the effects of K on the NO dissociation kinetics have been determined; although the activation energy is reduced by a factor of two, a compensating decrease in the preexponential factor results in a small net decrease in the dissociation rate. This is surprising since it is also found that K -precoverage increases nearly three-fold the amount of NO that will dissociate upon heating. It is concluded that alkali metals promote the dissociation of diatomic coadsorbates via long range interactions that stabilize the dissociation precursor with respect to terminally bonded adsorption and desorption precursors, so that during dissociative adsorption or upon heating an adsorbate-covered surface the dissociation reaction is more facile than competing reactions independent of the kinetics of the dissociation step.
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- Physics: Condensed Matter; Chemistry: Physical