Potassium promoted decomposition of methyl bromide on Ag(111)

Abstract

Using TPD, ΔΦ, UPS, and XPS, the decomposition promoted by potassium of methyl bromide (CH3Br) on Ag(111) was studied. While adsorption of CH3Br on K-free Ag(111) lowers the work function, on K/Ag(111) it increases the work function. In contrast to the K-free surface, on which CH3Br adsorbs and desorbs with no detectable decomposition, it adsorbs both molecularly and dissociatively on both submonolayer and multilayer K covered Ag(111) at 100 K. A new adsorption state of CH3Br on K/Ag(111) desorbs at 160-165 K, higher than for desorption of CH3Br from the K-free surface (142-145 K). Most of CH3 radicals desorb upon dissociation. The small fraction retained reacts to form CH4 and C2H4 above 250 K. All Br atoms are retained and react with K to form KBr, which desorbs at 650 K. The dissociation, attributed to electron harpooning from K to CH3Br to form temporary CH3Br - ions which then dissociate, is not thermally activated. Mechanistic comparison is made with photon induced dissociation of CH3Br on K-free Ag (111). © 1991 American Institute of Physics.