The kinetics and mechanisms of alkali metal-promoted dissociation: A time resolved study of NO adsorption and reaction on potassium-precovered Rh(100)
Authors: Whitman L. J. , Ho W. 
Published: July 2002

The adsorption, desorption, and dissociation of NO on potassium-precovered Rh(100) have been characterized for T ≥100 K and potassium coverages θK≤0.5 ML (1 ML≡1 NO/surface Rh) with low energy electron diffraction (LEED), Auger electron spectroscopy (AES), work function change (Δφ) measurements, temperature programmed reaction spectroscopy (TPRS), and time resolved electron energy loss spectroscopy (TREELS). The saturation NO coverage increases monotonically with θK from 0.4 ML with a K-free surface to 0.8 ML when θK =0.41 ML, and the initial sticking probability increases by up to ≈30%. K precoverage promotes the dissociation of NO; the percentage of a saturation NO coverage that dissociates during TPRS increases monotonically with θK from 75% to 100%. 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 side-on bonded (or highly inclined) dissociation precursor α1NO (νN–O ≈115 meV on the K-free surface) and the vertically bonded desorption precursor α2NO (νN–O ≈200 meV on the K-free surface), the populations of which have been determined as a function of both θNO and θK via TREELS. The θNO and θK dependence of the adsorption and reaction of NO on both K-free and precovered surfaces, as evidenced by the α1NO and α2NO populations and N–O loss energies, can be qualitatively understood in terms of a competition for surface electrons; electrophilic α1NO is stabilized (destabilized) with respect to α2NO 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 next-nearest neighbor, the stabilization is enhanced when the surface electron cloud extends towards the vacuum (θK &gt;0.1 ML), but diminished by strong dipole fields emanating from charged K adatoms (θK &lt;0.1 ML). The heating rate variation method has been combined with TREELS to determine the α1NO dissociation kinetics in the low coverage limit. Surprisingly, coadsorption with 0.14 ML K reduces the dissociation rate; although the activation energy is reduced from 10.5±0.7 to 4.5±0.4 kcal mol−1, there is a compensating decrease in the effective first order preexponential from 1011.8±0.7 to 102.9±0.5 s−1. Similar results are observed when θK =0.17 ML. In light of the measured kinetics, the K promotion of dissociation is attributed to the stabilization of the dissociation precursor α1NO; when coadsorbed with &gt;0.1 ML K essentially all the NO dissociates for all initial NO coverages, independent of the α1NO dissociation rate (which may actually be reduced), because the conversion of α1NO to α2NO is hindered and the reorientation of α2NO is facilitated. We conclude that the stabilization of side-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.