Formation and reaction of allylic species on silver surfaces: bond-order conservation Morse-potential analysis

Abstract

The reaction energetics, particularly the intrinsic activation barriers for possible reactions involving allylic species C 3H 5X, X = H, OH, O, Cl on clean and oxygen-preadsorbed Ag surfaces, have been calculated by using the bond-order conservation Morse-potential (BOC-MP) method. The calculations were made for low coverages of C 3H 5X with qualitative corrections for higher coverages. On clean Ag surfaces, propylene C 3H 6 and allyl alcohol C 3H 5OH are projected to desorb without dissociation, in contrast to allyl chloride C 3H 5C1, which is projected to desorb only at high coverages but to dissociate at low coverages forming a stable π-allyl (and atomic chlorine). It is found that the intrinsic activation barrier for dimerization of π-allyl into 1,5-hexadiene is very small and the apparent barrier should be mainly of diffusional character. In the presence of preadsorbed hydroxyl OH s, π-allyl is projected to undergo various transformations producing allyl alcohol, allyl alkoxide, acrolein, and propylene, when most recombination and disproportionation reactions have low intrinsic activation barriers. The BOC-MP model projections are in good agreement with experiment, particularly with the recent HREEL and TPD studies of C 3H 5C1.