The kinetics and mechanisms of alkali metalhyphen;promoted dissociation: A time resolved study of NO adsorption and reaction on potassiumhyphen;precovered Rh(100)

摘要

The adsorption, desorption, and dissociation of NO on potassiumhyphen;precovered Rh(100) have been characterized forTthinsp;ge;100 K and potassium coverages thgr;Kle;0.5 ML (1 MLequiv;1 NO/surface Rh) with low energy electron diffraction (LEED), Auger electron spectroscopy (AES), work function change (Dgr;fgr;) measurements, temperature programmed reaction spectroscopy (TPRS), and time resolved electron energy loss spectroscopy (TREELS). The saturation NO coverage increases monotonically with thgr;Kfrom 0.4 ML with a Khyphen;free surface to 0.8 ML when thgr;K=0.41 ML, and the initial sticking probability increases by up to ap;30percnt;. K precoverage promotes the dissociation of NO; the percentage of a saturation NO coverage that dissociates during TPRS increases monotonically with thgr;Kfrom 75percnt; to 100percnt;. Combined with the larger saturation NO coverage, this effect increases nearly threefold the maximum amount of NO that can be dissociated on the surface. The adsorption of NO is characterized by two molecular species, the sidehyphen;on bonded (or highly inclined) dissociation precursor agr;1NO (ngr;Nndash;Oap;115 meV on the Khyphen;free surface) and the vertically bonded desorption precursor agr;2NO (ngr;Nndash;Oap;200 meV on the Khyphen;free surface), the populations of which have been determined as a function of both thgr;NOand thgr;Kvia TREELS. The thgr;NOand thgr;Kdependence of the adsorption and reaction of NO on both Khyphen;free and precovered surfaces, as evidenced by the agr;1NO and agr;2NO populations and Nndash;O loss energies, can be qualitatively understood in terms of a competition for surface electrons; electrophilic agr;1NO is stabilized (destabilized) with respect to agr;2NO by an increase (decrease) in the surface electron density caused by coadsorbed K (NO, N, or O). In the presence of K, which is observed to perturb all NO adsorption sites including the nexthyphen;nearest neighbor, the stabilization is enhanced when the surface electron cloud extends towards the vacuum (thgr;K0.1 ML), but diminished by strong dipole fields emanating from charged K adatoms (thgr;K0.1 ML). The heating rate variation method has been combined with TREELS to determine the agr;1NO dissociation kinetics in the low coverage limit. Surprisingly, coadsorption with 0.14 ML Kreducesthe dissociation rate; although the activation energy is reduced from 10.5plusmn;0.7 to 4.5plusmn;0.4 kcalthinsp;molminus;1, there is a compensating decrease in the effective first order preexponential from 1011.8plusmn;0.7to 102.9plusmn;0.5sminus;1. Similar results are observed when thgr;K=0.17 ML. In light of the measured kinetics, the K promotion of dissociation is attributed to the stabilization of the dissociation precursor agr;1NO; when coadsorbed with 0.1 ML K essentially all the NO dissociates for all initial NO coverages, independent of the agr;1NO dissociation rate (which may actually be reduced), because the conversion of agr;1NO to agr;2NO is hindered and the reorientation of agr;2NO is facilitated. We conclude that the stabilization of sidehyphen;on bonded (or highly inclined) adsorption states and its effect on precursor mediated dissociation is a general phenomenon. Contrary to previous assumptions, we find that the promotion of dissociation is not necessarily due to an increase in the dissociation rate.