DEUTERIUM DISSOCIATION ON ORDERED SN PT(111) SURFACE ALLOYS/

We have explored the effect of alloying an unreactive metal, Sn, on th e dynamics of D-2 dissociative chemisorption at Pt(111). By comparing D-2 sticking and recombinative desorption on Pt(111) with that on the ordered p(2 X 2) Sn/Pt(111) and (root 3 X root 3)R30 degrees Sn/Pt(111 ) surface alloys, we examine the influence of the local surface compos ition on reactivity. The energy dependence of D-2 sticking S(E) has be en measured for all three surfaces using a hyperthermal beam. We find that the activation barrier for dissociative chemisorption is low on t he p(2 x 2) alloy, but the sticking probability is reduced, compared t o Pt(111), by an increase in the steric constraint on the dissociation site. Sticking on the (root 3 X root 3)R30 degrees alloy is inefficie nt at thermal energies with a threshold of similar to 280 meV, below w hich the sticking probability falls exponentially. The increase in the barrier to D-2 dissociation occurs as the stable, high coordination P t-3-D binding sites are lost by formation of the (root 3 X root 3)R30 degrees alloy. Despite the large activation barrier, sticking is domin ated by the vibrational ground state with the barrier occurring in the entrance channel, before the D-2 bond has stretched. Departures from a normal energy scaling indicate that the dissociation site is localiz ed in the unit cell and we suggest favorable dissociation sites on the alloy surfaces. Estimates for the heats of adsorption, obtained by co mparing activation energies to adsorption and desorption, indicate an abrupt decrease in the D binding energy as the Pt-3 sites are lost. We show that sticking and desorption parameters are consistent with an i ncreasing steric constraint for adsorption/desorption on the alloy sur faces as the Sn content is increased and an increase in the barrier to dissociation as the stable Pt-3 sites are lost by alloying. (C) 1998 American Institute of Physics. [S0021-9606(98)02031-5]